Soil Erosion and Control Measures

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March 26, 2023

Table of Contents

The removal and transportation of soil particles as a result of water, wind, or other physical disturbances is known as soil erosion. Water erosion tends to be more significant and problematic than wind erosion and usually occurs as sheet erosion, which is the periodic removal of thin sheets of soil over an area, or as gully erosion that forms incised channels.

Significant loss of soil due to erosion can create geohazards and affect civil engineering structures and infrastructures if not properly controlled or managed.

gully erosion geohazard — Gully erosion geohazard

Furthermore, cropland productivity and agriculture can be harmed by soil surface erosion, and eroded soil sediments can negatively affect streams, lakes, and reservoirs. The erosion of roadside embankments can result in rills and gullies on the embankments, increased surface runoff, more soil erosion, and ultimately slope failure.

The US Department of Agriculture (USDA) developed the universal soil loss equation (USLE) to predict the soil loss caused by surface erosion. This equation has been widely used to predict the average annual rate of sheet and rill erosion in agricultural fields. The equation, which takes into account terrain, crop system, rainfall pattern, and management techniques, is written as follows:

A = R × K × (LS) × C × P

where:
A = average annual soil loss; it is conventionally expressed in tons/ac/yr,
R = rainfall and runoff factor; it depends on the rainfall intensity and duration,
K = soil erodibility factor; it represents a soil’s ability to resist erosion and is determined by the soil texture, soil structure, organic matter content, and soil permeability,
L = slope length,
S = steepness factor,
C = cover and management factor; it is the ratio of soil loss in an area with specified cover and management to the corresponding soil loss in a clean-tilled and continuously fallow condition. For bare ground, C = 1.0,
P = support practice factor; it is the ratio of soil loss with a support practice such as contouring, strip-cropping, or implementing terraces compared to up-and-down-the-slope cultivation. For construction sites such as roadside embankment, P is not used in the equation.

The USDA published the Revised Universal Soil Loss Equation (RUSLE) in the year 1996 to replace the original USLE. The RUSLE still uses the same form and factors as the original USLE, but additional data were added and the factor assessment technique was changed.

The factor K was changed to be time-varying to reflect freeze-thaw conditions and consolidations; the topographic factors for slope length and steepness, LS, were changed to reflect the ratio of rill to interrill erosion; the cover-management factor C was changed from the seasonal soil loss ratios to a continuous function that is the product of four subfactors; and the factor P was expanded to consider environmental factors.

Control of Surface Erosion

In field practice, various surface erosion control techniques are used. Innovative, cost-effective, and sustainable techniques are always emerging as research and technology progresses.

Several of the most widely used surface erosion management techniques are described in the sections below.

Riprap

Riprap is “a flexible channel or bank lining or facing consisting of a well-graded mixture of rock, broken concrete, or other material, usually dumped or hand-placed, which provides protection from erosion,” according to the US Federal Highway Administration (1989).

Riprap is frequently used to protect and stabilize embankments, slope drains, storm drains, and side slopes of rivers, channels, lakes, and dams. Rock riprap, wire-enclosed rock, grouted rock, precast concrete block revetments, and paved lining are some examples of riprap revetments.

The most popular surface erosion prevention technique for river and channel banks is rock riprap. The figure above provides an illustration. The individual stones for riprap are preferably angular in shape and well-graded to allow for interlocking. Together with the weight of the stone, this interlocking property creates a strong stone layer that can withstand erosion.

On steep slopes, rock riprap can be unstable, particularly when rounded rock is used. Other materials, such as geosynthetics matting, should be taken into consideration for slopes steeper than 2:1. The following factors may be taken into account while designing rock riprap:

Rock size
Rock gradation
Riprap layer thickness
Filter design
Material quality
Edge treatment
Construction considerations

The following design recommendations can be adopted for rock riprap;

• Gradation: Rather than using one consistent size of rock, use a well-graded combination of rock sizes.
• Stone quality: Use hard riprap material (most igneous stones, including granite, have appropriate durability) so that freeze-thaw cycles do not quickly disintegrate it.
• Riprap depth: The thickness of the riprap layer should be double or more than the largest stone diameter.
• Filter material: Before laying the riprap, apply a filter material, often a layer of gravel or geosynthetic fabric. This keeps the earth beneath the riprap from penetrating it.
• Riprap limits: Place riprap so that it reaches the maximum depth of the flow or until enough vegetation has grown to effectively control erosion.
• Curved flow channels: Ensure that riprap reaches upstream and downstream of the beginning and end of the curve as well as the entire curved section to five times the bottom width.
• Riprap size: The riprap material’s size varies from an average of 5 cm to 60 cm in diameter depending on the shear stress of the flows to which it will be subjected.

Compost

Composting can slow erosion and hasten the development of vegetation in highly erodible environments. During composting, microorganisms transform organic matter into a stable amendment using an aerobic process. Composting prevents waste from going to landfills and transforms waste into valuable resources with significant economic and environmental advantages.

Composts of many kinds have been used to reduce surface erosion on embankments and naturally occurring slopes. Some of them are;

(1) organic stuff composted from grass clippings and agricultural waste
(2) manure compost made from animal and plant waste,
(3) co-composting material made from biosolids and green waste, and
(4) composts made from wood chips and forestry waste.

Composts made from municipal solid waste and leftover food have both been used to control erosion. Compost’s characteristics, including pH, soluble salts, moisture content, organic matter content, maturity, stability, particle size, pathogens, physical contaminants, and other factors, can differ greatly depending on where it comes from and how it is made.

Various organizations may specify compost that is appropriate for technical uses. To assure the quality of the finished dry compost, the USDA and the United States Compost Council (USCC) have established the “Test Methods for the Examination of Composting and Compost.”

Vegetation Cover

Another popular erosion control strategy is vegetation. It protects the soil surface from raindrop impact, shields the soil surface from overland flow’s scouring effect, and reduces the velocity of the flowing water to decrease its erosive capacity. In addition to strengthening the soil’s ability to withstand erosion and enhance the rate of infiltration, vegetation roots can also reduce runoff.

Vegetative cover is more aesthetically pleasing and is relatively easy to achieve and maintain. It is frequently combined with other erosion control techniques such as mulches, geosynthetic coverings, and blankets made of compost.

The following elements should be taken into account while designing and installing vegetative covers:

• Acidity, moisture retention, drainage, texture, organic matter, and fertility of the soil.
• The state of the site, specifically its slopes and vegetative cover.
• Climate factors like wind, precipitation, and temperature.
• Species choice, which is influenced by the local climate, planting seasons, water demand, soil preparation, weed management, post-construction land usage, and anticipated maintenance requirements, such as irrigation and cost.
• Methods of the establishment.
• The maintenance techniques.

Geosynthetics

Geosynthetics can be conveniently employed for slope stabilization, erosion control, and sediment management. Geosynthetics can be used in many different ways, and new ways are always being developed. Examples include hard armors, nondegradable RECPs, and degradable rolling erosion control products (RECPs).

On rehabilitated lakeshores, riverbanks, and alongside recently built roadsides, degradable materials can be utilized to improve the development of flora. When vegetation alone can adequately protect a site after an erosion control material has deteriorated, then degradable materials can be utilized.

Soil Erosion and Control Measures 7 — Geosynthetics in slope erosion protection

However, products that are not biodegradable offer vegetation long-term reinforcement. They are used in more difficult erosion control situations when immediate, high-performance erosion protection is required. By permanently bolstering the vegetative root system, the materials increase the erosion resistance of soil, rock, and other materials.

The use of geosynthetic erosion control measures frequently performs multiple functions, such as collecting and draining surface runoff, filtration, separating, strengthening, and establishing and maintaining vegetation coverings.

Soil Binders

Soil binders (materials with cementation properties) can be used to increase the resistance of a soil matrix to water and pressure. Soil binders are used to hold soil particles together and can be used for soil stabilisation and erosion control. The usage of stabilized soil and the local environmental conditions have a big impact on how well conventional soil binder applications work.

Cement is a common binder used in construction despite its several environmental disadvantages. Quicklime, hydrated lime, and lime slurry are the compounds used as lime-soil binders. Due to its weak resistance to the abrasive effect of traffic, fly ash is primarily employed to stabilize subbase or subgrade; it is not among the soil binder products suited for surface stabilisation.

Erosion Control Structures

Numerous structural measures can be used as either temporary or permanent installations for erosion control. There are many different forms of grade control structures, such as diversions, grassed or paved streams, buried pipe outlets, bench terraces or berms, retarding structures, debris basins, and many others. In the sections that follow, it will be explained where and how some of these measurements might be employed, as well as the placement and design criteria;

Diversions

Diversions are constructed over a slope in the form of graded channels with a supporting ridge on the lower side. They provide the function of collecting surface or subsurface water and directing it to a discharge point where it can be safely disposed of. These structures can be level or graded, temporary or permanent. While level terraces have closed ends and hold runoff, graded terraces transport water in a predetermined direction at a non-erosive velocity.

Diversions are helpful above borrow zones, gully heads, and cut slopes. They can be used to divert runoff water away from crucial construction sites and can be built across cut slopes to reduce slope length into non-erosive segments. They could be constructed on-site or off-site.

Water should empty from diversion points into prepared individual outlets, established disposal zones, or natural outlets. Individual outlets could be constructed as chutes, paved or grassed streams, or underground pipes. Diversion should be designed to handle rainwater with a return period of 10 years.

Berms

Steps or benches found on steep slopes are called berms. They are a variation of diversions and accomplish essentially the same thing. They shorten slope lengths and divide runoff water volume into manageable slugs when placed and designed properly.

Grading the berm in such a way that the outside edge is higher than the edge next to the cut slope gives it the capacity to carry the volume of water. By burying a pipe or using paved canals, runoff water from the berm can be removed.

Waterways

Waterways act as outlets for berms, diversions, and other constructions. They can be constructed or naturally occurring, shaped to the necessary size, and covered in vegetation or asphalt to collect runoff water. They are often built into one of the following common cross sections: rectangular, parabolic, trapezoidal, or V-shaped. Parabolic waterways are most frequently used in areas that will be vegetated and it is the shape that commonly occurs in nature.

constructed waterway — Constructed rectangular waterway

Protection from erosion is essential for the efficient operation of a waterway. This can be achieved by paving with concrete or rock or by establishing flow velocities that are benign for the grass that will be employed. Waterways ought to be built at least to handle the runoff from a rain event with a 10-year frequency.

Debris Basin

A debris basin is used to temporarily hold runoff water that contains substantial amounts of silt, sand, or gravel. Typically, it consists of a primary spillway made of perforated pipe and an earth embankment. Upon entering the impoundment, runoff water slows. The water is automatically pushed out through the main spillway as a substantial portion of the debris settles out.

Measures should be provided for periodic cleanup of the drainage basin, or the structure should be built to store the anticipated debris yield over the duration of the construction. There are both permanent and temporary debris basins. Those used to contain construction-related debris are often temporary and are demobilised when the project is finished and the region has stabilized.

Retarding Structures

A retarding structure is made up of earth embankment, a main spillway, and an emergency spillway structure. They could be built on-site or off-site. Their function is to temporarily retain runoff and then slowly release it to protect the land below. They can lower runoff peaks, allowing for the use of smaller bridges and culverts.

It is possible to do away with bridges and possibly save money if retarding structures are built on-site and the roadbed is filled. Cooperation from locals and groups like soil and water conservation districts will be required because any one of the structures has a variety of community benefits, including flood mitigation, grade control, and water for cattle, recreation, and irrigation. This kind of collaboration is encouraged by memorandums of understanding that a number of state transportation departments have with respective conservation organizations.

The main spillway typically comprises an earth fill exit and a horizontal pipe or monolithic concrete outflow made of reinforced concrete. The retarding storage space is typically intended to be empty in 10 days or fewer. The emergency spillway should, at the very least, be able to manage the routed runoff from a storm with a 25-year frequency or from a storm with a frequency equal to the structure’s design life, whichever is higher.

The runoff anticipated to occur at a frequency consistent with the level of protection to be offered should be able to be contained by the retarding storage.

Grade Stabilisation Structures

Structures for grade stabilization maintain grade or manage head cutting in both natural and man-made channels. They literally step runoff water down a hill at a regulated rate while lowering channel grade and flow velocity.

Earth embankments with pipe spillways or mechanical structures made of concrete, masonry, steel, aluminium, or treated wood are examples of grade stabilization systems. Although the design is dependent on site conditions, these structures should typically be able to manage storm runoff with a 25-year return period.

Gabions

Gabions are riprap (natural stones) contained in cages or cylinders made of galvanized steel-wire mesh. These are employed to prevent erosion on slopes, stream banks, or shorelines. Instead of being stacked vertically, they are typically positioned on slopes at an angle—either damaged or stepped back. The wireframe (cage) of a gabion determines how long it will last, and quality gabions have a fifty-year structural stability guarantee.

Conclusion

Erosion is the loss of soil due to the dislodgement and transportation of soil from one site to another. As soil erodes, soil nutrients are lost, rivers become clogged with dirt, and finally, the area becomes a desert or in the case of deep gullies, the area becomes uninhabitable.

Though erosion occurs naturally, human activity has the potential to significantly worsen it. Land that was once healthy and alive can become parched and lifeless due to erosion, which can also lead to mudslides and landslides.

When the proper techniques, equipment, and approaches are applied at the appropriate times, erosion can be easily managed on a construction site. Planting vegetation is the most natural and efficient technique to control erosion. Plant roots, particularly those of trees, cling to the earth and can effectively stop excessive soil movement.

Ultimate Bending Strength Calculation of Prestressed Girders

By

Ubani Obinna

-

March 29, 2023

A precast prestressed girder is an alternative economical solution to the construction of long-span bridges. This solution allows for quick bridge construction by using girders that are fabricated off-site, transported to the job site, and then installed into place.

In a construction site where efficiency or minimal traffic interruption is necessary, prestressed bridge girders are recommended because they require little to no falsework. Prestressed girders are very cost-effective and can be used for spans of up to 50 metres (166 ft).

A 164 feet long precast prestressed girder

In prestressed concrete design, structures are generally designed at the serviceability limit state (SLS). However, it is equally important to ensure that the designed structure has the sufficient moment and shear capacity at the ultimate limit state (ULS). In this article, checking for bending capacity at the ULS is discussed.

In order to determine the ultimate moment capacity of a given section which has been designed to the serviceability limit state of bending, it is necessary to determine the position of the neutral axis at the ultimate limit state. This usually involves a trial-and-error calculation procedure.

PRESTRESSED BRIDGE GIRDER DESIGN — On-site installation of a prestressed girder

Initially, a value for the neutral axis depth x is assumed. For the assumed depth, compressive forces in various parts of concrete and tensile force in all steel reinforcement are calculated. For equilibrium, the algebraic sum of total tensile and compressive forces must be zero. If the sum is not zero, calculations are repeated for a new value of neural axis depth and the calculations are continued till the correct value of x is found.

typical construction of a prestressed girder — Typical construction details of a prestressed girder

Ultimate Bending Strength Calculation of a Prestressed Girder

Calculate the ultimate bending strength of a bridge beam girder shown below. The calculation is adapted from Bhatt (2011).

The details of material properties, prestress details, etc. are follows.
Concrete: Concrete grade 40/50, f_ck = 40 MPa, material safety factor, γ_m = 1.50, f_cd = f_ck/γ_m= 26.7 MPa
Using a rectangular stress block: λ = 0.8, η = 1.0, f_ck ≤ 50 MPa, ηf_cd= 26.7 MPa
Maximum strain in compression ε_cu3 = 3.5 × 10^-3; σ_c≤ 50 MPa

Steel: 12.7 mm nominal diameter 7-wire strand with an effective cross-sectional area of 112 mm².
f_pk = 1860 MPa, f_p0.1k ≈ 0.88 × f_pk = 1637 MPa, γ_m = 1.15
f_pd = f_p0.1k /γ_m = 1424 MPa, E_s = 195 GPa

Prestressing details: 20 strands are located at the following distances from the soffit: six each at 50, 100 and 200 mm and two at 1100 mm from the soffit. The cables are spaced horizontally at 50 mm c/c.

Total prestress at service P_s = 2037 kN
Stress σ_pe due to prestress force in the strand = P_s/Area of 20 stressed strands = 2037 × 10³/ (20 × 112) = 909 MPa
Prestrain ε_pe= σ_pe/E_s = 909 / (195 × 10³) = 4.66 × 10^-3

The calculations consist of the following steps.

Step 1: Estimate a value for neutral axis depth:
i. Total tensile force:
Assume all 18 strands in the bottom three rows only ‘yield’ i.e. stress is a maximum of f_pd equal to 1424 MPa. The area of each strand = 112 mm².
Therefore total tensile force T = 18 × 112 × 1424 × 10^-3 = 2871 kN

ii. Total compressive force:
Assuming a rectangular stress block, the uniform compressive stress = ηf_cd = 26.7 MPa, λ = 0.8
(a). Total compressive force in top flange = 26.7 × 200 × 400 × 10^-3= 2136 kN
(b). Total compressive force in the top trapezium = 26.7 × 0.5 × (200 + 400) × 120 × 10^-3 = 961.2 kN
Total compressive force C = 2136 + 961.2 = 3097.2 kN
C > T. Therefore stress block λx < (200 + 120) or x < 400 mm

Step 2: For an assumed value of neutral axis depth x, calculate the total tensile and compressive forces and check for equilibrium.

Iteration 1: Assume x = 350 mm.
Rectangular stress block depth s = λx = 0.8 × 350 = 280 mm.
The uniform compressive stress = ηf_cd = 26.7 MPa

ii. Total compressive force:
(a) Compressive force in top flange = 2136.0 kN
(b) Total compressive force in part of the trapezium:

Depth d_x of stress block inside the trapezium = s – depth of top flange = 280 – 200 = 80 mm
Width B_x of the trapezium at the bottom edge of the stress block:
B_x = width of web + (width of top flange – width of web) × (1 − d_x /depth of trapezium)
B_x = 200 + (400 – 200) × (1− 80/120) = 267 mm
Compressive force in the trapezium = 26.7 × 0.5 × (400 + 267) × 80 × 10^-3 = 712.4 kN
Total compressive force C = 2136.0 + 712.4 = 2848.4 kN

iii. Total tensile force:
Bending strain ε_b in strand at d from the compression face = ε_cu3 × (d – x)/x
Calculations are shown in the Table below.

Distance from soffit (mm)	d (mm)	ε_b × 10³	ε_s = (ε_b + ε_pe) × 10³	σ_s, MPa	No. of strands	T (kN)
50	1150	8.0	12.66	1424	6	957
100	1100	7.5	12.16	1424	6	957
200	1000	6.5	11.16	1424	6	957
1100	100	-2.5	2.16	421	2	94
				Total	20	2965

T = 2965.3 kN
The difference between the total tensile force and compressive force = 2965 – 2848 = 117 kN
The compressive force is too small, indicating that the neutral axis depth is larger than 350 mm.

Step 3: Assume x = 450 mm. Stress block depth s = λx = 0.8 × 450 = 360 mm

Iteration 2:
ii. Total compressive force:
(a). Total compressive force in top flange = 26.7 × 200 × 400 × 10^-3 = 2136 kN

(b). Total compressive force in the top trapezium:
Depth d_x of stress block inside the trapezium = s – depth of top flange = 360 – 200 = 160 mm > 120 mm
The entire trapezium is in compression.
Compressive force in the trapezium = 26.7 × 0.5 × (400 + 200) × 120 × 10^-3 = 961.2 kN

(c). Compressive force in the web:
Depth of web in compression = s – 200 – 120 = 40 mm
Compressive force in web = 26.7 × 40 × 200 × 10^-3 = 213.6 kN
Total compressive force C = 2136.0 + 961.2 + 213.6 = 3310.8 kN

iii. Total tensile force:
Bending strain ε_b in strand at d from the compression face = ε_cu3 × (d – x)/x

Calculations are shown in the Table below.

Distance from soffit (mm)	d (mm)	ε_b × 10³	ε_s = (ε_b + ε_pe) × 10³	σ_s, MPa	No. of strands	T (kN)
50	1150	5.44	10.10	1424	6	957
100	1100	5.06	9.72	1424	6	957
200	1000	4.28	8.94	1424	6	957
1100	100	-2.72	1.94	378.0	2	85
				Total	20	2956

T = 2955.6 kN

The difference between the total tensile force and compressive force
= 2956 – 3311 = −355 kN

Step 4: Interpolation
From the two values of x and the corresponding difference between the total tensile and compressive forces, the value of x for which the difference is zero can be determined by linear interpolation.

x	T – C
350	117
450	-355

x = 350 + (450 – 350) × 117/ [117 − (− 355)] = 375 mm

Step 5: Final calculation
Assume x = 375 mm.
Stress block depth s = λx = 0.8 × 375 = 300 mm

ii. Total compressive force:
(a). Total compressive force in top flange = 26.7 × 200 × 400 × 10^-3 = 2136 kN

(b). Total compressive force in the top trapezium:
Depth d_x of stress block inside the trapezium = s – depth of top flange = 300 – 200 = 100 mm < 120 mm
Width B_x of the trapezium at the bottom edge of stress block = width of web + (width of top flange – width of web) × (1−d_x /depth of trapezium) = 200 + (400 – 200) × (1 − 100 /120) = 233 mm
Compressive force in the trapezium = 26.7 × 0.5 × (400 + 233) × 100 × 10^-3 = 845.5 kN
Total compressive force C = 2136.0 + 845.5 = 2981.5 kN

iii. Total tensile force:
Calculations are shown in the Table below;

Distance from soffit (mm)	d (mm)	ε_b × 10³	ε_s = (ε_b + ε_pe) × 10³	σ_s, MPa	No. of strands	T (kN)
50	1150	7.23	11.89	1424	6	957
100	1100	6.77	11.43	1424	6	957
200	1000	5.83	10.49	1424	6	957
1100	100	-2.57	2.09	408.0	2	91
				Total	20	2962

T = 2962 kN

The difference between the total tensile force and compressive force
= 2962 – 2982 = −20 kN
This is small enough to be ignored. In fact the correct value of x = 372 mm. The ‘error’ is because of linear interpolation.

Step 6:
Ultimate moment: The ultimate moment is obtained by calculating the moment of all forces, tensile as well as compressive about the soffit.

(a). Compressive force in top flange = 2136 kN
Lever arm from the soffit = 1200 – 200/2 = 1100 mm

(b). Compressive force in the top trapezium = 845.5 kN
Depth of stress block inside the trapezium d_x = 100 mm
Width of the trapezium at the bottom edge of stress block B_x = 233 mm
Lever arm = 1200 – 200 – (d_x/3) × [1 + B_x / (400 + B_x)] = 954 mm
iii. Tensile force forces at various level act at their position from the soffit as shown in the Table above.

The ultimate bending moment M_u is given by
Mu = [(2136 × 1100) + (845.5 × 954) – (957 × 50) – (957 × 100) − (957 × 200) – (91.4 × 1100)] × 10^-3 = 2721.1 kNm

Calculation adapted from:
Bhatt P. (2011): Prestressed Concrete design to Eurocodes. Spon Press, Taylor and Francis. ISBN: 978-0-203-84725-1

Applications of Geosynthetics in Civil Engineering

By

Ubani Obinna

-

March 12, 2023

Table of Contents

Geosynthetics are planar products manufactured from a polymeric material (polypropylene, polyester, polyethene, polyamide, PVC, etc) or natural materials and are used with soil, rock, earth, or other geomaterials as an integral part of a civil engineering project, structure, or system.

Geonaturals are products made from natural fibres (such as jute, coir, cotton, wool, etc.) that are utilized mostly in temporary civil engineering applications. Due to the fact that these materials biodegrade quickly when utilized with earth materials, they don’t have as many field applications as geosynthetics do.

During the production of geosynthetics, planar textile structures are created by combining elements such as fibres or yarns. Depending on their characteristics and purposes, many of such products fall under the general term ”geosynthetics”.

For instance, a permeable geosynthetic made of textiles is called a geotextile. Geogrids are a regular network of tensile elements with holes large enough to interlock with surrounding fill material and are generally utilized for soil reinforcement.

96CF4549 DE29 4847 91AF 27FDC6A27B9D — **Figure 1:** Geogrid as sol reinforcement in road construction

Low permeability geosynthetics called geomembranes are employed as fluid barriers in engineering constructions such as sanitary landfills. To benefit from the properties of each component, geotextiles and related products like nets and grids can be mixed with geomembranes and other synthetic materials.

Geotextile-geonet composites, geotextile-geomembrane composites, geotextile-geomembrane-grid composites, geotextile-polymeric core composites, and even three-dimensional polymeric cell structures can be combined to create these products, which are known as geocomposites.

The process utilized to assemble the filaments or tapes into the planar textile structure determines the type of geotextile. Geotextiles fall into one of two categories: woven or nonwoven. Polyolefin sheets are extruded and oriented to create rigid geogrids with integral joints. Flexible geogrids are constructed from polyester strands that have been knit or weaved together at their crossover points and then coated with a polymer.

The use of geosynthetics to replace customary construction methods has a wide range of applications. Subgrade stabilization, ground improvement, slope stabilization, reinforced earth, a system for lining and cover of landfills, retaining walls, and erosion and scour protection measures for river banks are some of the potential applications. The materials have been around for thirty years, but they haven’t been extensively utilised in all the possible areas of civil engineering construction.

The article discusses the current state of geosynthetics application in various civil engineering projects as well as its potential in the future. It also discusses the challenges of using this material, along with suggestions for how to get over them.

Applications and Functions of Geosynthetics

Geosynthetics have the following primary functions in civil engineering construction works:

Separation
Filtration
Drainage
Reinforcement
Barrier
Protection

Separation

A geosynthetic material is referred to as performing a “separation function” if it must prevent the mixing of adjacent dissimilar soils and/or fill materials during construction and over the anticipated service life of the application under consideration. Synthetic geotextiles are utilized for this. Depending on the manufacturing process, the following categories of geotextiles are currently available:

Woven Geotextile
Nonwoven Geotextile
Knitted Geotextile
Stitched Geotextile

Monofilament, multifilament, or fibrillated yarns, as well as slit films and tapes, are used to create woven geotextiles. Although nonwoven textile production is a recent industrial development, the weaving method has been around for quite some time. Continuously extruded synthetic polymer fibres or filaments are spun, blown, or otherwise placed onto a moving belt.

The mass of filaments or fibres is then either heat bonded, in which the fibres are welded together by heat and/or pressure at their sites of contact in the nonwoven mass, or needle punched, in which the filaments are mechanically entangled by a succession of small needles.

Instead of using a weaving loom, a knitting machine can be used to interloop one or more yarns (or other components) to create a knitted geotextile. Geotextiles that have been stitched together or sewn together using fibres, yarns, or both. Figure 2 illustrates how the geotextile layer keeps the structural integrity and functionality of both materials intact by preventing the mixing of soft soil and granular fill.

geotextile as a separator — **Figure 2:** Geotextile as a separator

A geosynthetic layer is typically placed at the interface between soft foundation soil and the overlaying granular layer in many geosynthetic applications, particularly in roadways, rail tracks, shallow foundations, and embankments. In this case, it becomes challenging to pinpoint if the primary role of the geosynthetic is reinforcement and/or separation.

When the ratio of the applied stress (s) on the subgrade soil to the shear strength (c_u) of the subgrade soil has a low value (less than 8), separation can be a dominant function over reinforcing, and it is largely independent of the settling of the reinforced soil system.

Filtration

A geosynthetic may serve as a filter that allows for enough fluid flow with minimal soil particle migration across its plane during the course of the application’s anticipated service life. Figure 3 demonstrates how a geosynthetic allows water to escape from a soil mass while limiting uncontrolled soil particle mobility. When a geosynthetic filter is positioned next to base soil (the soil to be filtered), a gap forms between the geosynthetic’s structure and the original soil structure.

drainage filter — **Figure 3**: Use of geosynthetics as filter fabric in a retaining wall

Certain soil particles, especially those nearest to the geosynthetic filter and with diameters smaller than the filter opening size, can go through the geosynthetic under the influence of seepage flow thanks to this discontinuity. The retention of the base material and permeability are the two parameters on which the design of the geotextile filter is based.

Soil Retention Criterion
O₉₅ < αD_w
O₉₅ = Apparent opening size of geotextile
α = 0.5 to 5.0 (a constant)
D_w = Representative soil particle size
= D₉₀ for unidirectional flow
= D₁₅ for cyclic flow Permeability Criterion

Permeability criterion
• Less critical applications and less severe conditions K_gt > K_soil
• Critical applications and severe conditions K_gt > 10K_soil

Traditional granular filters can be replaced by geosynthetic filters since they are less prone to clog up regularly over the course of a structure’s service life. Table 1 provides a comparison of granular and geotextile filters. The geotextile filters finds application in earth dams, embankments, engineered landfill etc.

Granular Filter	Geotextile Filter
Quality control can be difficult	Quality control can be easily assured
Installation is more complex	Installation is easy
There can be discontinuities in the filter media	The filter media is continuous
Clogging can be frequent	Clogging takes a very long time
Intimate contact with surface	Surface preparation is required
Large volume of granular material	Conserves granular materials

Table 1: Comparison between granular filter and geotextile filter

Drainage

A geosynthetic is said to perform the drainage function if it permits adequate fluid flow with little soil particle movement within its plane from the surrounding soil mass to different outlets. Figure 4 demonstrates how water from the pavement surface is captured and carried to the embankment drainage built by the geosynthetic layer placed in the pavement.

It should be mentioned that a geosynthetic dissipates the surplus pore water pressure while carrying out the filtering and drainage tasks by allowing water to flow in plane and across its plane. This can be accomplished by combining the geocomposite with geotextile (for filtering action) and geonet (for drainage function). It also considers load distribution.

Reinforcement

A geosynthetic serves as reinforcement by increasing the mechanical characteristics a soil mass as a result of its incorporation. A composite material known as “reinforced soil” is created when soil and geosynthetic reinforcement are mixed. This material has strong compressive and tensile strengths and is conceptually comparable to reinforced concrete.

In actuality, the primary function of any geosynthetic used as reinforcement in geotechnical structures is to withstand applied stresses or prevent unacceptable deformations. By acting as a tensioned part in this process and being joined to the soil/fill material via friction, adhesion, interlocking, or confinement, the geosynthetic preserves and enhances the stability of the soil mass.

The soil is reinforced using geogrid. It is a planar polymeric product made of crossing tensile-resistant ribs that are integrally joined at the joints to form a mesh-like or net-like regular open network. Extruded geogrid, bonded geogrid, and woven geogrid are the three different terms for the geogrid that results from joining the ribs together.

A regularly punctured polymer sheet is stretched longitudinally to create an extruded geogrid, which has a substantially higher tensile strength in the longitudinal direction than the transverse direction. a geogrid created by stretching a polymer sheet that has been punctured regularly in both the longitudinal and transverse directions; as a result, it has an identical tensile strength in both directions.

When building embankments with a constrained width and requiring that the face of the wall stay nearly vertical, reinforced earth (RE) techniques are typically used. The use of expensive retaining walls is completely eliminated by the RE wall technique. When ground conditions are unfavorable and adequate backfill materials are locally accessible, savings of between 25 and 50 percent have been documented. Figure 5 shows the various parts of the RE wall.

**Figure 5:** Components of a reinforced earth wall

The proper establishment of the friction connection between the soil and the reinforcement, which will depend on the composition, gradation, density, and properties of the fill material and geosynthetics, is essential for the stability of RE structures. RE structures are best suited to cohesionless soil that has been compacted to densities that cause volumetric expansion during shear.

Conversely, cohesive fill material produces short-term structural instability as a result of pore water pressure buildup, creep behavior, and frost actio. Early in the 1970s, because to worries about potential corrosion of metallic reinforcement, geotextiles were first used as reinforcing materials. While steel-reinforced walls have been built to heights exceeding 40 m, geosynthetic reinforced walls have been built to maximum heights of less than 20 m.

Barrier

In the lining and cover of an engineered landfill, geomembrane is employed as an impermeable barrier. It is a planar, comparatively impermeable synthetic sheet used as a barrier or liner in construction projects to restrict fluid flow. The material could be polymeric, asphaltic, or a combination of both. For leachate migration under subsoil conditions, natural or compacted clay liners are insufficient to provide the necessary level of environmental protection or necessitate additional thickness.

geosynthetics in landfill — **Figure 6:** Geoynthetics as a liner in a landfill

The use of geomembrane as a suplimentary barrier provides an economical approach to achieve the regulatory standard. Depending on the need, it can be employed in a variety of ways, including GCL (geosynthetic clay liner), HDPE (high density poly ethylene), and LDPE (low density poly ethylene). An engineered landfill offers the chance to use geosynthetics for all of their purposes in various locations. Figure 6 illustrates a typical application of geosynthetics in constructed landfills in all of its forms and purposes.

Protection

When a geosynthetic is applied on a soil surface, it stabilizes the surface by preventing soil particles from moving and dispersing due to wind and rain erosion, frequently while permitting or encouraging the growth of flora. Due to the hydraulic pressure of the soil, the conventional design of cementing the banks is not an inexpensive solution. To prevent erosion of river banks and seashores, geotextile gabions, tubes, and bags are utilized.

Conclusion

The usage of geosynthetic materials in civil engineering projects has a lot of promise. Although the use of geosynthetic materials in the building of landfills and embankments has gained widespread recognition, additional applications are still lacking. By providing the necessary knowledge at the undergraduate level and scheduling short-term courses for field engineers, certain confidence-building tactics might be implemented. The creation of a uniform code of conduct for use in India is essential, and testing facilities may be improved to make them easily accessible to all users within appropriate travel distances.

High Strength Concrete and Global Warming Potential

By

Ubani Obinna

-

March 1, 2023

In today’s construction industry, high strength concrete is becoming more popular. Using high-strength concrete has a number of advantages, including reducing the volume of concrete sections required in structural elements, increasing building occupancy rates, and extending the building’s service life (Papadakis, 2000).

When high-strength concrete (HSC) is employed in building construction, there is a greater chance of having smaller concrete sections compared to when normal-strength concrete (NSC) is used. However, will the use of less volume of HSC translate to a reduction in CO₂ emission for which the construction industry is famed for? Well, the answer depends.

Due to their inherent performance qualities, high strength concrete (50–100 MPa) and ultra high strength concrete (UHSC) (>100 MPa) are being utilized more frequently in the construction industry. As a result of their significant global usage, these concrete mixes have a higher carbon footprint. It is therefore important to take into account the embodied carbon of the HSC and UHSC.

**Figure 1:** High strength concrete is usually adopted in highrise building development

When compared to normal strength concrete, high-strength concrete has a lower water-to-binder ratio. As a result of this, a lot of cement or binder is needed and a lot of CO₂ is emitted during the production of high strength concrete.

However, as stated above, the volume of concrete structures can be considerably reduced by adopting concrete systems with high mechanical properties. Because less material is required for the same structural performance, this “performance strategy,” which includes the use of high-performance concrete, reduces carbon dioxide emissions (Fantilli et al, 2019).

Summarily, high concrete strength has a higher carbon footprint, especially during the production stage (Habert and Roussel, 2009), because more binder (and sometimes fibres) is required. As a result, material reduction may not always be sufficient to offset the rise in CO₂ emissions caused by higher concrete classes.

Studies on the Environmental Impacts of High Strength Concrete

The Global Warming Potential (GWP) of ultra-high performance concrete and normal-strength concrete was compared quantitatively for bridge structures at the construction stage by various researchers (Bertola et al., 2021; Dong, 2018; Rangelov et al., 2018; Sameer et al., 2019).

According to a comparison made by Aqib and Ma (2022), it was observed that the global warming potential of ultra-high-performance concrete was higher than that of normal-strength concrete by more than 60%. This can be attributed to the increased cement usage in ultra-high-performance concrete and the presence of steel fibres. As the cement content is much lower in conventional concrete, the GWP is also lower.

Reference	GWP of normal-strength concrete (kgCO₂eq)	GWP of ultra high-strength concrete (kgCO₂eq)	Percentage difference (%)
Dong, 2018	348	877	60%
Bertola et. al 2021	7217	10914	66%
Sameer et al., 2019	390	1700	77%
Rangelov et al., 2018	434	1930	78%

However, at the maintenance stage or during the service life of the structure, it was observed that ultra-high-performance concrete had less GWP compared to normal-strength concrete. Due to its superior durability and high strength, the maintenance needs of ultra-high-performance concrete are very low as compared to conventional concrete and hence, the life cycle GWP of ultra-high-performance concrete is very low as compared to normal strength concrete (Aqib and Ma, 2022).

In another study by Larsen et al (2017), a 40m long pedestrian bridge was designed with normal strength concrete (f_ck = 30 MPa) and ultra high performance concrete (f_ck = 150 MPa). The bearing structure is divided into two spans of 20 m, and consists of two simply supported T-beams as shown below.

**Figure 2.** Cross-sections for normal strength concrete and ultra high performance concrete bridges (left side) and the longitudinal structural model (right side). (Larsen et al, 2017)

Using life cycle analysis (LCA), the authors determined the environmental impact of each alternative. It is important to note that due to the fact that ultra high strength concrete contains about two to three times the cement content of normal strength concrete, it was important to reduce the volume of the section made with UHSC. After the design, the total amount of concrete used in the UHPC alternative was reduced by 36.7% compared to the NSC alternative.

**Figure 3.** Contribution analysis to overall global warming potential (GWP), expressed in tons CO₂ equivalents (Larsen et al, 2017)

The contribution of the major life cycle phases to the total GWP in tons CO₂ equivalent across the lifetime of each alternative is shown in Figure 3. The NSC alternative has lifetime CO₂ equivalent emissions of 81.7 tons, compared to 68.6 tons for the UHPC option.

Material manufacturing is the major source of CO₂ equivalent emissions for both alternatives at 58% of the overall emissions for the NSC alternative and 50% of the total emissions for the UHPC alternative.

The major findings of the study suggest that using UHPC in pedestrian bridges may have some environmental and design viability. Over a 200-year period, the UHPC alternative had overall lower consequences than the NSC alternative, which were much less. This suggests that the UHPC alternative is better in terms of the environment.

However, the results of this study need to be critically analyzed in light of methodological decisions made regarding the selection of functional units and life cycle phases, the uncertainty surrounding future emissions from material production, and the restrictions of the common T-beam bridge design in a comparative analysis.

In another study, Fantilli et al, (2019) evaluated three RC buildings with 14, 30, and 60 floors, respectively, whose structures are developed using finite element software for four different concrete classes (C25, C40, C60 and C80). The entire amount of concrete and reinforcement required to meet static and dynamic requirements was determined in this manner. When these quantities are multiplied by the materials’ unit carbon dioxide emissions, the global warming impact is calculated.

**Figure 4:** The RC structures of the existing building meshed with CDM Dolmen (Fantilli et al, 2019)

The parametric CO₂ content per cubic metre of the concrete classes are provided in the Table below;

Material	Parametric CO₂amount (kg/m³)
C25	215
C40	272
C60	350
C80	394

Indicators that link the carbon footprint of concrete to both the cement volume per unit and the compressive strength can be found in the literature. To estimate the unit CO₂ emissions as a function of the cylindrical compressive strength of concrete, Habert and Roussel (2009) presented the following empirical relationship:

kg of CO₂ per cubic meter of concrete = δ√Strength of concrete

Where δ = 46.5 kgCO₂

**Figure. 5:** Carbon footprint of steel and concrete versus concrete strength in the case of: a) structure #1; b) structure #2 and c) structure #3. (Fantilli et al, 2019)

From the study, it was observed that Building #1, which has the lowest number of storeys, exhibited a gradual increase in CO2 with concrete class (Fig. 5a). In contrast, the carbon footprint in the 60-story skyscraper (i.e., Structure #3 – Fig. 5c) reduces as concrete strength increases. At Structure #2, which has 30 stories (see Fig. 5b), emissions increase from classes C25 to C40 but fall for higher classes (especially from C60 to C80). As would be expected, each structure uses less structural material as concrete strength rises.

From the study, the lowest effect of CO₂ emission may be ensured for low-rise buildings (14 storeys) by employing normal strength concrete (e.g., C25). In rare circumstances, even when low-strength concrete is present, the theoretical cross-sectional area can be less than the minimum. As a result, the use of high strength concrete in these elements does not rule out the possibility of using less concrete and steel rebar. In comparison to normal strength concrete, it causes an increase in CO₂ emissions, particularly in low-rise structures.

The use of high-strength concrete, on the other hand, reduces the environmental impact of high-rise buildings (60 floors). As a result, in tall buildings, the use of the performance strategy to reduce the environmental impact of concrete structures becomes particularly successful, because the volume of structural elements can be significantly reduced.

References

Aqib S. M. and Ma Z. J. (2022): A Review on Carbon Emissions of Ultra-High-Performance Fiber Reinforced Concrete as a Building Construction Material. International High Performance Buildings Conference. Paper 426. https://docs.lib.purdue.edu/ihpbc/426

Bertola, N., Küpfer, C., Kälin, E., & Brühwiler, E. (2021). Assessment of the environmental impacts of bridge designs involving UHPFRC. Sustainability (Switzerland), 13(22), 1–19. https://doi.org/10.3390/su132212399

Dong, Y. (2018). Performance assessment and design of ultra-high performance concrete (UHPC) structures incorporating life-cycle cost and environmental impacts. Construction and Building Materials, 167, 414–425. https://doi.org/10.1016/j.conbuildmat.2018.02.037

Fantilli A. P., Mancinelli O. And Chiaia B. (2019): The carbon footprint of normal and high-strength concrete used in low-rise and high-rise buildings, Case Studies in Construction Materials, 2019(11)e00296, https://doi.org/10.1016/j.cscm.2019.e00296.

Habert G. and N. Roussel (2009): Study of two concrete mix-design strategies to reach carbon mitigation objectives, Cem. Concr. Compos. 31 (6) (2009) 397–402.

IngLarsen I. L., Aasbakken I. G., O’Born, Vertes K. and Thorstensen R. T. (2017): Determining the Environmental Benefits of Ultra High
Performance Concrete as a Bridge Construction Material. IOP Conf. Series: Materials Science and Engineering 245 (2017) 052096 doi:10.1088/1757-899X/245/5/052096

Papadakis V. G. (2000): Effect of supplementary cementing materials on concrete resistance against carbonation and chloride ingress, Cement and Concrete Research, 30(2):291–299

Rangelov, M., Spragg, R. P., Haber, Z. B., & H, D. (2018). Life-cycle assessment of ultra-high performance concrete bridge deck overlays (1st Edition).

Sameer, H., Weber, V., Mostert, C., Bringezu, S., Fehling, E., & Wetzel, A. (2019). Environmental assessment of ultra-high-performance concrete using carbon, material, and water footprint. Materials, 16(6). https://doi.org/10.3390/ma12060851

Carbon Footprint of Reinforced Concrete Structures

By

Ubani Obinna

-

February 26, 2023

Table of Contents

Reinforced concrete is one of the most popular construction materials in modern building construction, which is known to be responsible for significant amounts of steel and cement consumption. Due to its flexibility, strength, wide availability, and high adaptability, reinforced concrete is widely utilised all over the world. However, the high carbon footprint of concrete from ‘cradle to grave’ remains a big concern for our environment.

The term “sustainable concrete” refers to concrete that has been optimized in terms of material and technology, as well as technical, economic, and environmental factors. Cement is a major contributor to the high environmental impact values of concrete production (Marinkovi et al., 2008), accounting for more than 5% of annual CO₂ emissions worldwide (Kurda et al, 2018), as well as other environmental impact categories, such as energy usage (Paris et al., 2016).

According to UNEP (2019), building construction and operations accounted for the largest share of both global final energy use (36%) and energy-related CO₂ emissions (39%) in 2018.

global share of energy — **Figure 1:** Global share of buildings and construction final energy and emissions, 2018 (UNEP, 2019)

To achieve net-zero CO₂ emissions in the construction industry, material efficiency must be improved to minimize the primary demand for these materials. Furthermore, higher-value recycling and reuse of waste materials from the construction industry must be implemented, and the production of concrete/cement must be decarbonized. The most effective strategy to reduce steel and concrete emissions is to utilize them only when they are absolutely necessary for new structures with an emission reduction potential of 25 to 50%.

This necessitates design innovation in the areas of material substitution, extended product and structural lifetimes, ease of deconstruction, component reuse, and high-value recycling at the end of their useful lives. This will necessitate collaboration with national organizations responsible for a variety of issues, including architecture, design, civil engineering, construction, trades, and the development and enforcement of building codes.

The carbon footprint is the cumulative quantity of GHG emissions generated by a person, firm, company, activities, or items, measured in CO₂e, and expressed in tons of carbon dioxide emissions per year. CO₂ equivalent is a statistical scale that is used for the evaluation and measurement of different GHGs emissions on the basis of their global warming potential (GWP). The CO₂e of a particular gas can be obtained by the multiplication of its weight by its related global warming potential as described in the equation below;

“kgCO₂e = (weight of the gas in kg) × (GWP of the gas)”

Carbon Footprint of Different Construction Materials

Building structures are influenced by construction practices, regulations, and the accessibility of building materials in the area where they are built. However, the load-bearing and secondary construction elements of a structure are usually created using reinforced concrete, masonry (bricks), wood, steel, concrete, and a combination of these materials.

Due to the wide range of materials available in the construction market, the exact construction materials for any structures may not be known. However, for reinforced concrete, it is common knowledge that concrete (Hoxha et al., 2017) and structural steel (Thiel et al., 2013) are the materials that have the biggest effects on the environment.

Reduction of carbon footprint can be achieved through recycling and green construction

When compared with residential buildings built with other common construction materials, concrete structures have a bigger environmental impact than timber structures (Skullestad et al, 2016), masonry structures have a greater impact than steel (Rossi et al., 2012), and steel has a greater impact than wood (Carre, 2011).

The analysis of various structures (concrete, steel, and timber) for commercial buildings reveal a variation in total life cycle emissions of 9%. It has also been observed by researchers that for commercial/office buildings, concrete has a higher environmental impact than steel (Acree and Arpad, 2005). These two structural materials are the main contributors to the embodied impact of office buildings (Kofoworola and Gheewala, 2008).

The building envelope, in addition to structural elements, makes up a sizeable component of the overall life cycle effect (Hoxha et al., 2017; Stephan et al., 2017), and non-structural materials are the main source of uncertainty in LCA results (Hoxha et al, 2017).

Many studies in the literature have focused on maximizing the life cycle of a building envelope by examining various materials and elements because it encompasses not only the production of materials, as is the case with the load-bearing structure, but also replacement, maintenance, refurbishment, energy use processes, and energy efficiency measures/strategies (Ferdyn-Grygierek and Grygierek, 2017).

The variation and dependence of the envelope composition on a large number of materials, however, necessitates an extensive breakdown and normalisation analysis. This is because “a single building could comprise over 60 basic materials and 2000 separate products” (Haapio and Viitaniemi, 2008).

The embodied energy of various construction materials (cradle to gate) according to the ICE database (2011) are as follows;

Construction Material	Embodied Carbon (kgCO₂e/kg)
Gravel or crushed rock	0.0052
Average CEM I (Portland cement 94% clinker)	0.74
Sand	0.0051
Mortar 1:3 (Cement:Sand mix)	0.221
Mortar 1:4 (Cement:Sand mix)	0.182
Mortar 1:5 (Cement:Sand mix)	0.156
Mortar 1:6 (Cement:Sand mix)	0.136
Steel (average recycled content)	1.46
Steel (virgin)	2.89
Steel (recycled)	0.44
Concrete C16/20	0.1
Concrete C20/25	0.107
Concrete C25/30	0.113
Concrete C28/35	0.120
Concrete C32/40	0.132
Concrete C40/45	0.151
Reinforced concrete 25/30 (with 110 kg/m³of steel)	0.198
Rammed soil	0.024
Expanded polystyrene	3.29

sustainable materials in construction — Sustainable materials should be employed in construction

Carbon Footprint of Reinforced Concrete

The relationship between the production of the raw materials from which structural elements such as slabs, beams, columns, etc. are formed influences the embodied carbon (EC) of those structural elements and the finished structures they eventually produce (Hacker et al., 2008; Harrison et al., 2010).

As a result, there has been a lot of interest lately in the embodied carbon of structural materials (Purnell, 2013). In both scientific and quasi-technical literature, comparisons of the EC of concrete, steel, and timber (or structures made primarily of these materials) have become more common, purporting to show one or more of these materials as “the greenest.”

It can be difficult and complex to evaluate embodied carbon. The quantity of CO₂ emitted per unit of production by the three main structural materials — steel, wood, and concrete — is difficult to quantify (Purnell, 2013). In general, such generalizations ought to be avoided, and any structural design or analysis ought to go through a thorough life-cycle analysis (LCA, as defined by ISO 14040, 2006), taking into account CO₂ emissions generated during all production, processing, installation, maintenance, demolition, and disposal stages for the particular components of the structure under study (Purnell, 2013).

According to studies on CO₂ emissions over the course of a building’s life cycle, the stages of construction, operation, and demolition are responsible for roughly 13%, 85%, and 2% of CO₂ emissions, respectively (Li and Chen, 2017).

The optimization of reinforced concrete structural designs can help reduce the embedded carbon or CO₂ footprint in reinforced concrete structures in addition to using novel construction materials like low-carbon cement and clinker substitutes (WBCSD-IEA 2009).

In current practice, structural designs are usually optimized for total cost or total weight. Yet, optimal designs for embodied energy or CO₂ footprint are also preferred from a sustainability perspective (Yeo and Potra, 2015). It is important to remember that the CO₂ footprint embodied in the reinforced concrete used in a building is only a small portion of the total CO₂ footprint included in that building, even if the CO₂ footprint decrease addressed in the research only relates to the RC structure. Yet, the RC’s reduced carbon footprint adds significantly to the overall reduction in carbon emissions.

Optimisation of Reinforced Concrete Structures for Environmental Impacts

Interest in optimizing RC structures by taking environmental factors into account has been seen in a lot of research efforts by scholars.

Two objective functions were minimized by Paya-Zaforteza et al. (2009) using a simulated annealing-based approximate optimization method:

(1) total CO₂ emissions embodied in the structure and
(2) total structural cost.

The dimensions of the cross sections of the columns and beams, the type of concrete and steel reinforcement used, and the details of the longitudinal and shear reinforcement in the columns and beams were all design considerations. The methodology was tested using six standard building frames with up to four bays and eight stories. From the study, it was observed that the optimal structure for reducing emissions is just slightly more expensive (2.8%) than the optimum structure for minimizing cost.

Similar research was done by Villalba et al. (2010) for cantilever earth-retaining walls that ranged in height from 4 to 6 m, and they also observed that the optimum design for reducing embedded CO₂ emissions is just slightly (1.4%) more expensive than the optimum for minimizing cost. Interestingly, the authors observed that, despite the latter requiring around 2% more steel, walls designed for the lowest embedded CO₂ emissions needed about 5% more concrete than those optimized for least cost. Moreover, the concrete grade is greater in the case of walls with reduced emissions.

Yeo and Gabbai (2011) investigated the cost implications of optimizing a simple RC structural element (a rectangular beam with fixed moment and shear strengths) in order to reduce embodied energy. The results show that, as compared to a cost-optimized design, optimizing structural member designs for the least embodied energy led to a 10% reduction in embodied energy at the expense of a 5% increase in cost.

The actual reduction in embodied energy is determined by the cost ratio of steel reinforcement to concrete, which must account for both the material costs of the steel and concrete as well as construction costs like those associated with placing the concrete and installing the reinforcement.

Moreover, the concrete sections optimised for embodied energy contain more reinforcements than concrete compared to the members that were optimised for minimum cost. This supported the conclusions reached by Villalba et al (2010). To ensure that ductility is adequate for design purposes despite the increase in the amount of steel, the optimization technique’s constraints included a restriction on the strain in the reinforcing bars.

Yeo and Potra (2015) conducted exploratory research to evaluate the feasibility of optimising RC design for CO₂ emissions. The CO₂ footprint optimization increased the proportion of steel in the member cross sections, but the requisite ductility was guaranteed by constraints set during the optimization process.

Depending on the parameter values taken into account, the CO₂ footprint reduction achieved by optimizing the design to achieve minimum carbon emissions as opposed to optimizing the design to achieve minimum cost, ranges from 5% to 15%.

The study took into account an RC frame that was subjected to lateral and gravitational loads. It was shown that the design optimized for CO₂ footprint achieved a smaller CO₂ footprint (by 5 to 10%) than the one optimized for cost, depending on the parameter values used in the computations.

Smaller reductions may be observed in low-rise structures and other buildings with predominantly tension-controlled members. The reduction, however, might be greater for structures like high-rise buildings whose members often withstand extremely high compressive stresses. This category may also include certain concrete members that have been prestressed or post-tensioned.

In a case study involving a commercial-residential complex in South Korea, Paik and Na (2019) examined and contrasted the carbon dioxide emissions of a normal reinforced concrete slab against a voided slab system. A process-based life-cycle assessment (LCA) was employed to determine the carbon dioxide emissions during the construction phase, which includes all operations from the manufacture of materials through completion.

The results show that the normal reinforced concrete slab and the voided slab system have total CO₂ emissions of 257,230 and 218,800 kg respectively. The main source of CO₂ reduction comes from the embedded carbon dioxide emissions of building materials, which has a total of 34,966 kg of CO₂. The transportation of building materials ranks as the second biggest source with 3417 kg of CO₂. The study mentioned above shows that decreasing the amount of concrete used can aid in reducing embodied carbon.

Gan et al. (2017) looked into the effects of material selection, recycled content, building heights, and structural forms on the embodied carbon in high-rise buildings. The findings showed that while steel buildings release 25% to 30% more embodied carbon than composite and RC buildings, they are 50%–60% lighter overall.

This is due to the fact that the lateral load-resisting system of a steel building typically calls for sizable quantities of steel sections that are particularly carbon-demanding. If more than 80% of the steel is recycled, the steel building’s embodied carbon can be reduced to the lowest level of the three structures.

When compared to a steel building composed completely of recycled steel, the RC structure has the lowest embodied carbon, even when employing cement substitutes like fly ash and powdered granulated blast furnace slag (Gan et al, 2017). The unitary embodied carbon (kg CO2-e/m² GFA) vs. building height graph displays an upward concavity, showing that there is a recommended height range for each structural type where embodied carbon is at its lowest level.

Conclusion

The results from the literature surveyed showed that the embodied carbon in buildings and structures varies considerably depending on the building materials and structural designs employed. Depending on structural performance, material cost and availability, and other factors, structural engineers may in fact have a variety of options for construction materials and structural designs, which can lead to significant variations in the embodied carbon of high-rise buildings.

These studies can help decision-makers consider sustainability concerns when making choices about structural forms and materials in particular. First off, for highrise buildings, selecting the most effective lateral load-resisting system for a particular height can result in significant embodied carbon savings because it accounts for 70% to 80% of the embodied carbon in high-rise buildings.

Engineers can, for instance, select the less carbon-embedded core-outrigger structural form to withstand lateral stress for a 60-story high-rise building. Once the structural form has been decided upon, engineers can select a low-carbon alternative by taking into account the readily available materials (concrete, rebar, and structural steel) and their recycled content.

The following recommendations can be followed to reduce the carbon footprint of reinforced concrete structures:

(1) Floor slabs account for the majority of the mass in load-bearing building structures, and improving/optimising them can result in significant cost/carbon savings. Voided slab solutions, thinner sections, and slabs with high-performance concrete can be adopted.

(2) For load-bearing building structures with columns of smaller spacing (say equal to or less than 4m), only concrete of lower strength classes (e.g., C20/25 and C25/30) should be used. The environmental implications of employing higher-strength concrete are exacerbated by increased cement use.

(3) Variants of load-bearing building structures with columns of higher spacing (say about 8 m) are best designed from a concrete of strength class C50/60, which has the lowest construction costs and environmental impact.

(4) It is advantageous to design a building with more storeys by recalculating the construction costs and environmental impacts per m² of usable area. This is due to the foundation structure, which is the most expensive for a building with fewer storeys.

References

Acree G. A., Arpad H. (2005): Comparison of environmental effects of steel- and concrete-framed buildings, J. Infrastruct. Syst. 11 (2): 93–101.

Carre A., (2011): A Comparative Life Cycle Assessment of Alternative Constructions of a Typical Australian House Design, Final Report, Project No: PNA147-0809 Forest & Wood Products Australia, Melbourne, Victoria, 2011.

Ferdyn-Grygierek J. and Grygierek K. (2017): Multi-variable optimization of building Thermal design using genetic algorithms, Energies 10 (10) (2017) 1570.

Gan V. J. L, Chan C. M., Tse K. T., Irene M.C.L, Cheng J. C. P. (2017): A comparative analysis of embodied carbon in high-rise buildings regarding different design parameters. Journal of Cleaner Production 161 (2017) 663e675 http://dx.doi.org/10.1016/j.jclepro.2017.05.156

Haapio A. And Viitaniemi P. (2008): Environmental effect of structural solutions and building materials to a building, Environ. Impact Assess. Rev. 28(8):587–600.

Hacker J. N., De Saulles T. P., Minson A. J., Holmes M. J. (2008): Embodied and operational carbon dioxide emissions from housing: A case study on the effects of thermal mass and climate change. Energy Build. 40(3):375- 384. doi:10.1016/j.enbuild.2007.03.005.

Harrison G. P., Maclean E. J., Karamanlis S., Ochoa L. F. (2010): Life cycle assessment of the transmission network in Great Britain. Energy Policy 38(7):3622-3631. doi:10.1016/j.enpol.2010.02.039.

Hoxha E., Habert G., Lasvaux S., Chevalier J. and Le Roy R. (2017): Influence of construction material uncertainties on residential building LCA reliability, J. Clean. Prod. 144 (2017):33–47.

Kofoworola O.F. and Gheewala S.H. (2008): Environmental life cycle assessment of a commercial office building in Thailand, Int. J. Life Cycle Assess. 13 (6) (2008):498–511.

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Construction Measurement Criteria: Understanding the Benefits of Site Inspection

By

Ubani Obinna

-

February 27, 2023

Table of Contents

The measurement of works is an essential step of every Civil Construction project. Undoubtedly, for construction to be carried out successfully and safely, it is necessary to follow some criteria for measuring work. However, to bring benefits, the measurement of work must be done correctly.

There are many doubts about how this measurement is made, mainly because there is no standard to be followed, and it may depend on the company or agency. However, in order for the measurement criteria for works to be defined correctly without any calculation errors, it is necessary to be careful when performing this task.

Check out the following article presented by Sky Marketing and see what are the benefits of carrying out the inspection of works in civil construction.

What is Construction Inspection?

Inspection is certainly a fundamental task in the execution of projects in the field of Civil Construction. However, it is necessary to have defined criteria for measuring work. In this way, it is possible to carry out the management of works and, thus, avoid errors that could compromise the execution of the project. Construction inspection is a significant feature of Blue World City.

However, for the inspection to be carried out effectively and safely, it is necessary to follow some essential steps. In addition, the inspection of works must be carried out by a professional qualified to perform this task.

It is important to point out that, despite the need to follow these steps, there is no defined standard for measuring work. This means that the criteria must suit the needs of each project.

Importance of Construction Inspection

As previously mentioned, the inspection of works in Civil Construction is a job that, if done correctly, can bring many benefits to professionals. Check some important points below when carrying out the inspection of works.

Increase Workplace Safety

When carrying out the inspection of works, it is necessary for the professional in charge of this task to observe all aspects present at the construction site, especially due to the possibility that certain situations may pose risks to the team responsible for the project.

It is also essential that, during the inspection, the organization and conditions of the construction site are observed. A thorough inspection of the site will ensure that staff is working safely.

This inspection will also ensure that the team is complying with the necessary standards and using personal protective equipment (PPE).

Identify Risks

Certainly, any project in progress is subject to unforeseen events; however, it is extremely important to identify the risks, thus preventing the progress of the work from being compromised.

Considering that Civil Construction is one of the sectors that most causes accidents at work, it is necessary to pay attention to the risks to which the team will be exposed. This is a criterion that, when followed, can also avoid unnecessary expenses, especially since the risks will be identified early.

Increase Productivity

The inspection of works has a direct connection with productivity in the execution of projects. This is because, during the inspection, the professional responsible for this activity must analyze possible problems that may have a negative impact on the productivity of the team.

By observing the organization of the construction site, for example, it is possible to better visualize and monitor the progress of the work. In addition, it is also possible to make changes to the pace of production if necessary.

Monitor the Progress of the Work

Maintaining an inspection routine can make it possible to identify in advance aspects such as a drop in productivity or other factors that could compromise the progress of the work and, consequently, compromise the established deadlines.

By regularly analyzing the progress of the work, it is possible to think of mechanisms to solve failures or delays.

Constructive Quality

Inspections can bring many benefits to projects, especially when done frequently. Following a work progress control routine allows the professional to observe the team’s performance and the quality of the service to be provided. When deficient construction is observed, the supervisor can issue site instructions for remedial works.

Maintaining quality can avoid customer complaints and, thus, more profit for the company responsible for the work.

Progress Documentation

Of course, documenting the activities that are carried out also fulfils the function of monitoring the progress of the work. With this, the person responsible for the inspection can find information more easily.

In addition, the documentation can make it possible to renegotiate deadlines with customers in case there is any unforeseen event that compromises the work schedule.

Steps for an Efficient Construction Inspection

The inspection of works in the area of Civil Construction can bring several benefits to the company responsible for the project. However, to be carried out effectively, it is necessary to follow some important steps. Below are the main steps to carry out an efficient site inspection.

Plan the Inspection

The inspection can be carried out on a scheduled or random basis; however, if the person in charge chooses a scheduled inspection, care must be taken with the planning.

In addition, the person responsible for this function must list all the aspects that must be analyzed in work, thus preventing any important factor from going unnoticed.

Collect the Data

Data collection should be carried out at all points of construction. For this, it is necessary for the person responsible for the inspection to document the information using the appropriate tools and to carry out the records in a practical way.

Evaluate the Results

The previous task is of extreme importance for the evaluation of the results to be carried out, mainly because the data collection allows the visualization of the obtained results. With this, it is possible to compare these results with the initially planned project.

This step prevents results from deviating from what had been agreed upon when signing the contract.

Propose Corrective Actions

In order to propose corrective actions, it is indispensable that the results are all raised, thus allowing us to visualize all the problems found in work. In this way, it is possible to think of strategies to solve problems quickly and effectively, avoiding greater risks.

Inspection is a Fundamental Tool in Civil Construction

As seen in the article above, carrying out the inspection of projects in Civil Construction, following criteria for measuring works, is an extremely important task. With it, it is possible to identify risks and/or problems that could compromise the progress of the project.

To ensure that the process is done in a more elaborate way, the professional in charge of this function can make use of specific tools for the Civil Construction area.

Strengthening of Concrete Slabs | Retrofitting of RC Slabs

By

Ubani Obinna

-

February 25, 2023

Table of Contents

Different techniques can be used for strengthening concrete slabs that have been deemed structurally unsound or inadequate to withstand a specified floor loading. Some of the methods usually adopted for slab strengthening are; concrete overlay, adding steel sections, adding reinforced concrete beam sections, or FRP reinforcing.

These techniques for slab strengthening have been developed due to a number of reasons such as poor maintenance of structures, overloading of reinforced concrete members, corrosion of the steel reinforcement, and other deteriorating conditions that develop over time in reinforced concrete structures.

Generally, reinforced concrete slabs may need to be repaired or strengthened in the following circumstances;

a) Repairing damaged/deteriorated concrete slabs to restore their strength and stiffness.
b) Corrosion of the reinforcement.
c) Limiting crack width under increased (design/service) loads or sustained loads.
d) Retrofitting concrete members to enhance the flexural strength and strain to failure of concrete elements requested by increased loading conditions such as earthquakes or traffic loads.
e) Rectifying design and construction errors such as undersized reinforcement.
f) Enhancing the service life of the RC slabs.
g) Shear strengthening around columns for increasing the perimeter of the critical section for punching shear.
h) Changes in the structural system such as cut-outs in the existing RC slabs.
i) Changes in the design parameters.
j) Optimization of structure regarding the reduction of deformations and of stresses in the reinforcing bars.

Depending on the architectural requirements, functionality, and convenience of construction, a combination of two or more strengthening techniques may also be utilized, where one approach is used at the top and another technique is used to strengthen the slab bottom soffit.

In order to strengthen a slab, it may also be necessary to reduce the straining actions by incorporating new structural components, such as concrete or steel beams or columns. A reinforced concrete slab may also be strengthened for the purpose of improving the flexural moment or punching shear resistance.

By increasing the stiffness, slabs can be strengthened to improve their serviceability limit states by reducing deflection, controlling crack width, and improving the behaviour in vibration. Furthermore, strengthening might be added to increase the slab’s fire resistance.

Concrete Overlay for Slab Strengthening

Depending on which areas of the slab need strengthening, concrete overlays are constructed on either the top, bottom, or both surfaces. Although the top of the slab’s concrete overlay is simpler to construct, the bottom of the slab’s overlay could also be done while concrete is cast using shotcrete to ensure that there are no honeycomb or voids in the overlay.

The most important concern with the concrete overlay method is ensuring a proper bond between the “new” concrete used to strengthen the structure and the “old” concrete in the existing structure. The shrinkage of these two concretes must be taken into particular consideration.

However, it is generally acknowledged that strengthening by adding a new layer of reinforced concrete is considerably simpler to do when the operation is done on the top surface of the slab. Experience has shown that it is usually necessary to add new reinforced concrete to the member’s bottom face, particularly in the areas where they experience positive bending moments. Shotcrete or specific formwork must be used to pour concrete on the bottom face.

For general construction purposes, using a concrete overlay at the top and strengthening the slab’s positive moment section with steel plates or CFRP reinforcement may be more cost-effective. When doing a concrete overlay, steel dowels are inserted to transfer the interfacial shear forces between the old and new concrete when the slab is propped up to support both its own weight and the weight of the overlay.

**Figure 1:** Slab strengthening by concrete overlay (Abdelrahman, 2023)

According to Abdelrahman (2023), the total area of steel shear dowels planted in one-quarter of the slab panel (0.5l_x × 0.5l_y), as per the Figure above can be calculated based on the bending moments in the x and y directions integrated within half the slab length/width (Ṁ_x and Ṁ_y) as shown in the equations below.

The induced forces on the shear dowels in the x and y directions (F_x, F_y), can be calculated using the equations below for both the positive and negative bending moments independently. The total force on the shear dowels in one-quarter of the slab panel (F_s) is calculated after adding the forces in both directions (x and y) resulting from the positive and the negative moments. After which, the area of steel shear dowels can be calculated.

Slab strengthening by External Reinforcement

External steel plates, externally bonded FRP laminates, or near-surface-mounted FRP reinforcement can all be used to strengthen concrete slabs. Depending on the slab’s aspect ratio and the need for strengthening, the external reinforcement may be applied in one direction or two directions.

This method also entails strengthening slab systems in an approach that combines the actions of steel plates and steel bolts. In some cases, steel bolts can be arranged in a manner that is similar to how shear studs are arranged, when they are to be used as vertical shear reinforcements. Steel plates are then attached to the concrete surface at the upper and lower sides of the slab using epoxy glue and tightened with steel bolts and nuts.

strengthening of slab using steel plate 2 — **Figure 2:** Strengthening of slabs using steel plate (Source: https://www.horseen.com/)

So, in addition to providing vertical shear reinforcement, the purpose of the steel bolts is to ensure complete contact between the steel plates and concrete slab by transmitting horizontal force between the two materials and applying confinement pressure to the concrete. Hence, the suggested reinforcing method consists of integrating steel plates, steel bolts, and applying pressure to the slab to confine them.

Steel plates may be mounted to the top of the slab and FRP strips may be fastened to the slab’s bottom soffit using various reinforcing techniques on the same slab. As opposed to working overhead for the bottom surface, installing the steel strips and inserting the dowels from the top of the slab is easier. The area of steel dowels needed is calculated using the formula provided for slabs reinforced with concrete overlay.

To prevent corrosion of the exterior steel plates in the event that they are applied in two directions, care should be taken to fill the space behind the steel plates with filler material, such as grout or epoxy grout.

**Figure 3:** Slab strengthening with steel plates (Abdelrahman, 2023)

Slab strengthening by adding structural member

Furthermore, concrete or steel structural members, such as a column or beam, can be added to strengthen slabs. In order to divide the slab into smaller portions and increase its stiffness, the structural arrangement of the slab is altered by the addition of columns or beams.

After the new member is added, the straining actions and deformations of the slabs will be reduced. As shown in Figure 4, installing concrete beams to support pre-existing slabs needs drilling in the slabs for the longitudinal reinforcement and the supporting elements for the transverse steel stirrups.

**Figure 4:** Slab strengthening by the introduction of concrete beam (Abdelrahman, 2023)

To reduce the interfacial shear stresses between the old and new concrete caused by concrete shrinkage, low-shrinkage admixtures are added to the concrete before it is cast after the steel cage has been assembled. In this case, the slab should either be jacked up to release the load off the slab or propped so that the props hold the weight of the concrete and the weights above.

**Figure 5:** Typical details of the new beam (Abdelrahman, 2023)

Figure 5 shows the necessary steel reinforcement for the new beam in detail. Since the new beam will share the whole weights with the slabs, including the weight of the slab and the weights above, releasing the loads from the slab at the time of casting the new supporting beam would minimize the overall stresses in the slab.

Conclusion

Each of the techniques discussed in this article has a number of benefits and drawbacks. Some, such as concrete overlay, significantly increase the dead load of the structure and may necessitate additional strengthening of the other structural parts. On the other hand, the external plate bonding technique and is susceptible to corrosion damage which may lead to failure of the strengthening system.

However, the load-carrying capacity of reinforced concrete slabs can be increased using any of the strengthening methods, or the structural performance of the concrete parts can be at least partially restored. The magnitude of strengthening needed, the area where strengthening is needed, architectural requirements, ease and speed of application, and the overall cost will all affect the choice of the best technology to apply.

Reference

Abdelrahman A. (2023): Strengthening of Concrete Structures: Unified Design Approach, Numerical Examples and Case Studies. Springer Nature Singapore Pte Ltd. https://doi.org/10.1007/978-981-19-8076-3

The House Built with Precast Concrete Culverts

By

Charity Mmuo

-

February 27, 2023

In this article, Sky Marketing presents a creation of the Japanese architecture studio Nendo which has created a house and warehouse out of pieces of precast concrete sewers in central Japan.

The Prefab House with Concrete Tunnels

Architecture with prefabricated elements is nothing new. In fact, the architectural future – if not already – goes through this industrialized architecture to save on costs and build better.

We have seen how they transform freight containers into houses and play with their shapes, as well as how they build prefabricated exterior walls that are placed like a Tetris game or how large concrete pipes become micro-apartments and rooms, in short, a thousand ideas!

The imagination of some architects to innovate in prefabricated houses is causing really interesting architectural projects to appear.

NENDO 1 — Nendo architecture studio – Photos Daici Ano, Takumi Ota and Toru Shiomi

The famous Japanese architecture studio Nendo also presents us with a magnificent house built with precast concrete sewer tunnels. Called the Culvert Guesthouse, a construction between warehouse and residential housing that was built from four huge precast concrete tunnels that were stacked.

The unique building is located among the dense forests on the outskirts of the city of Miyota in Japan. In a quiet environment rich in nature where streams intertwine through a thick forest of red pines.

NENDO 4 — Nendo architecture studio – Photos Daici Ano, Takumi Ota and Toru Shiomi

Its elongated concrete forms were constructed from joining pieces of precast concrete culverts, commonly used to enclose utilities such as water and electricity underground.

As they are elements destined for infrastructures and the architectural project had other needs. The study used the prestressing method to achieve a more coherent stacking and tightness between pieces.

The prestressing method is a technique used in civil engineering structures, such as bridges, in which members are aligned and then tensioned with steel wires to connect the members. In this way, a smooth and seamless surface finish is achieved, obtaining a hermetic seal and greater durability.

A total of 63 square-shaped pieces of approximately 12 tons each were used in the prefabricated house. By connecting these pieces, a slender tunnel-like space is created with an internal dimension of approximately 2 x 2.3 meters.

According to the Nendo studio… “The space is not so architectural, but a project that combines civil engineering concepts with product design details.”

On the ground floor, the main space of the warehouse is located in a 40-meter-long tunnel that has large glass windows at each end for filing furniture, products, and works of art. Parallel to this space, there is another tunnel that houses the residential area that includes a kitchen, bathroom, and toilet.

NENDO 6 — Nendo architecture studio – Photos Daici Ano, Takumi Ota, and Toru Shiomi

NENDO 7 — Nendo architecture studio – Photos Daici Ano, Takumi Ota, and Toru Shiomi

Both ground floor tunnels are located in parallel. They are joined by a flat roof creating an interior space for the dining room. This is closed with large glass windows that occupy the entire wall on both sides.

The floor of the living room is finished with gravel hardened with a resin base, which facilitates the passage and avoids the glassy surface that would result from the poured resin. The windows were made without metal frames as far as possible, and the high-transparency glass is up to 10 meters in length.

Stacked over the two tunnels, two other perpendicular caissons form the top floor of the building. They contain a bedroom and a secondary archive with a study space.

The smallest details of the house are meticulously designed, from a bathtub carved into the ground, where the water is flush with the ground, suggesting a continuous and uninterrupted surface or the design of the door handle that integrates perfectly into the narrow tunnel opening.

All the concrete elements on the outside are painted white to give the warehouse and residential area a minimalist look. In addition to offering a contrast with the exterior landscape and its vigorous green.

NENDO 10 — Nendo architecture studio – Photos Daici Ano, Takumi Ota, and Toru Shiomi

In its interior spaces, the line of white colours is followed in all the rooms and the use of wooden furniture with simple and straight lines.

In the garden, there is a fifth concrete tube next to the main warehouse building to house additional living space that will be used in the future.

If you liked this article, share it! Moreover, you can also read about Lahore Smart city, a real estate jewel in Pakistan.

Design of Post-Tensioned Slabs

By

Ubani Obinna

-

February 23, 2023

Table of Contents

In a practical medium- to long-span structures, post-tensioned slabs are economically competitive with reinforced concrete slabs and make up a sizable share of all prestressed concrete construction. The numerous drawbacks of reinforced concrete slabs (especially for long-span structures) are eliminated by prestressing.

In post-tensioned concrete construction, fresh concrete is cast around hollow ducts that are fitted to any desired profile. Normally, the steel tendons are unstressed in the ducts during the concrete pour. Nevertheless, they can also be threaded through the ducts at a later date. The tendons are tensioned (stressed) after the concrete reaches the desired strength at a later date. Tendons can either be stressed from both ends or from one end while the other end is anchored. At each stressed end, the tendons are then anchored.

After the tendons are anchored, the prestress is maintained by bearing the end anchorage plates onto the concrete, which places the concrete in compression during the stressing operation. Every time the cable’s direction changes, the post-tensioned tendons place a transverse force on the member.

post tensioned slab 1 — **Figure 1:** Construction of post-tensioned slab

Post-Tensioned Slabs

In post-tensioned slabs, deflection, which is usually the controlling design consideration for concrete floors is controlled better. With an appropriate selection of prestressing/post-tensioning solutions, a designer is able to minimize or even completely eliminate deflection. As a result, thinner slab systems are possible, which can increase headroom or decrease floor-to-floor heights.

Additionally, prestress prevents cracking and can be utilized to create flooring that is watertight and devoid of cracks. Steel reinforcement layouts and fixes for prestressed slabs are often fewer and less complicated. As a result, pouring concrete and fixing steel reinforcements are simpler and faster. Additionally, prestressing reduces formwork costs and stripping times while enhancing punching shear. Conversely, prestressing frequently results in substantial axial shortening of slabs, necessitating special attention to the details of movement joints.

Post-tensioning is usually done on prestressed slabs using draped tendons. Flat-ducted tendons with five or fewer strands in a flat sheath and fan-shaped anchorages are frequently used, as shown in Figure 2. The use of small, portable hydraulic jacks allows for the one-at-a-time stressing of individual strands. The flat ducts allow for maximal tendon eccentricity and drape while being structurally effective. In order to provide a bond between the steel and the concrete, these ducts are usually grouted after stressing.

**Figure 2:** Details of typical flat-ducted tendons and anchorages (Gilbert et al, 2017)

Pre-stressed concrete slabs are typically thin in proportion to their span. A concrete slab may experience a high magnitude of deflections at full load or show excessive camber after transfer if it is too thin. The serviceability criteria for the member often dictate the initial choice of slab thickness.

The decision on the thickness of a slab is usually made using prior experience or suggested maximum span-to-depth ratios. Note that the initial choice of slab thickness, while useful as a starting point for design, does not always guarantee that serviceability requirements will be met.

Unbonded tendons are frequently used in a majority of post-tensioned slabs simply because they lower the cost of construction. Post-tensioned slabs provide a number of important advantages over reinforced concrete slabs, such as:

• Longer spans with fewer columns leading to flexibility in the positioning of partitions.
• Thinner slabs lead to saving in construction costs and reduced height of the building.
• Especially in the case of car parks, the virtually crack-free slabs are a great advantage to limit damage due to seepage of water with de-icing salts from melting snow.

Balanced Load Stage for Two-Way Slabs

Two-way panels bend into dish-shaped surfaces when subjected to transverse loads, as depicted in Figure 3. Because the slab is curved in both major directions, bending moments exist in each of those directions. Additionally, twisting moments that develop in the slab at all points other than the lines of symmetry resist a portion of the applied load.

**Figure 3:** Deformation of interior two-way slab panels. (a) Edge-supported slab. (b) Flat slab. (Gilbert et al, 2017)

Each prestressing tendon resists a portion of the applied load, and they are typically arranged in two directions parallel to the panel edges. The transverse force that is applied to the slab by the tendons in one direction adds to (or deducts from) the transverse load that is applied in the opposite direction by the tendons. The amount of the load to be borne by tendons in each direction for edge-supported slabs is more or less arbitrary; the only strict condition is that statics be met.

For flat slabs, tendons must carry the entire load from column line to column line in both directions. The concept of using the transverse forces produced by the draped tendons’ curvature to balance a portion of the applied load is advantageous from the perspective of managing deflections. Load balancing not only offers the foundation for creating a proper tendon profile but also makes it possible to calculate the prestressing force necessary to achieve zero deflection in a slab panel under the chosen balanced load.

It is important to know that the slab is essentially flat (has no curvature) at the balanced load and is only affected by the prestressing forces applied at the anchorages. Only uniform compression (P/A) in the directions of the orthogonal tendons is applied to a slab of uniform thickness. When the condition of the slab under the balanced load is confidently known, any suitable method can be used to determine the deflection caused by the unbalanced portion of the load.

Since only the unbalanced portion of the total service load must be taken into account and, unlike reinforced concrete slabs, prestressed slabs frequently do not crack at service loads, the calculation of the deflection of a prestressed slab is typically more accurate than that of a conventionally reinforced slab.

The external load that needs to be balanced typically makes up a significant amount of the sustained or permanent service load in order to minimize deflection issues. The sustained concrete stress (P/A), if all the permanent loads are balanced, is constant throughout the slab depth. There is little long-term load-dependent curvature or bending due to uniform creep strain caused by uniform compressive stress distribution.

Of course, bonded reinforcement limits creep and shrinkage and, if the steel is eccentric to the slab centroid, results in a change in curvature with time. Prestressed slabs typically contain a limited amount of bonded steel, therefore the time-dependent curve this restraint produces rarely results in appreciable deflection.

Potential serviceability issues may be indicated by the average concrete compressive stress after all losses. The prestress may not be enough to avoid or control cracking brought on by shrinkage, temperature changes, and unbalanced loads if P/A is too low. The average concrete compressive stress after all losses is subject to minimum limits that are outlined in some standards of practice. Prestressing levels for a two-way slab are typically in the range P/A = 1.2 – 2.6 MPa using flat-ducted tendons with four or more strands.

Axial deformation of the slab may be significant and cause distress in the supporting structure if the average prestress is high. Regardless of the typical concrete stress, the rest of the structure must be able to withstand and accommodate the shortening of the slab, but when P/A is high, the issue is made worse (Gilbert et al, 2017). It could be essential to use movement joints to separate the slab from rigid supports.

In particular, for slabs containing less than minimum quantities of conventional tensile reinforcement (i.e. less than around 0.15% of the slab’s cross-sectional area), the spacing of tendons in at least one direction shall not exceed the smaller of eight times the slab thickness and 1.5 m.

General Approach to the Design of Post-Tensioned Slab

After making an initial selection of the slab thickness, the second step in slab design is to determine the amount and distribution of prestress.

Usually, load balancing is employed to achieve this. The transverse stresses induced on a slab by the draped tendons in each direction balance out a portion of the load. The slab stays flat (without curvature) under the balanced load and is only susceptible to the resulting longitudinal compressive P/A stresses.

The remaining imbalanced load is taken into account when calculating service load behaviour, especially when estimating load-dependent deflections and determining how much cracking has occurred and how much crack control has been applied.

The complete factored design load must be taken into account at ultimate limit state conditions, where the slab behaviour is non-linear and superposition is no longer valid. The factored design moments and shears at each critical section must be computed and compared with the design strength of the section. Slabs are typically fairly ductile, and as the slab’s collapse load approaches, moments redistribute. Secondary moments can typically be disregarded in these circumstances.

Load Balancing

**Figure 4:** Interior edge-supported slab panel (Gilbert et al, 2017)

Consider the interior panel of the two-way edge-supported slab shown in Figure 4. The panel has parabolic tendons in both the x and y axes and is supported by walls or beams on all sides. The upward forces per unit area that the tendons in each direction exert if the cables are uniformly spaced in each direction are;

w_px = 8P_xz_d.x/l_x² ——— (1)

and

w_py = 8P_yz_d.y/l_y² ——— (2)

where P_x and P_y are the prestressing forces in each direction per unit width, and z_d.x and z_d.y are the cable drapes in each direction.

The uniformly distributed downward load to be balanced per unit area w_bal is calculated as:

w_bal= w_px + w_py ——— (3)

In practice, perfect load balancing is not possible, since external loads are rarely perfectly uniformly distributed. However, for practical purposes, adequate load balancing can be achieved (Gilbert et al, 2017). Any combination of w_px and w_py that satisfies Equation (3) can be used to make up the balanced load. The smallest quantity of prestressing steel will result if all the loads are balanced by cables in the short-span direction, i.e. w_bal = w_py.

DF2C1361 D03A 4A4A B41C 500835A55AB8 — **Figure 5**: Post-tensioned slab construction

However, under unbalanced loads, serviceability problems in the form of unsightly cracking may result. It is often preferable to distribute the prestress in much the same way as the load is distributed to the supports in an elastic slab, i.e. more prestress in the short-span direction than in the long-span direction. The balanced load resisted by tendons in the short direction may be estimated by:

w_py= [l_x⁴/(δl_y⁴ + l_x⁴)] × w_bal ——— (4)

where δ depends on the support conditions and is given by:

δ = 1.0 for 4 edges continuous or discontinuous
= 1.0 for 2 adjacent edges discontinuous
= 2.0 for 1 long-edge discontinuous
= 0.5 for 1 short edge discontinuous
= 2.5 for 2 long + 1 short edge discontinuous
= 0.4 for 2 short + 1 long edge discontinuous
= 5.0 for 2 long edges discontinuous
= 0.2 for 2 short edges discontinuous

Equation (4) is the expression obtained for that portion of any external load which is carried in the short-span direction if twisting moments are ignored and if the mid-span deflections of the two orthogonal unit-wide strips through the slab centre are equated.

With w_px and w_pyselected, the prestressing force per unit width in each direction is calculated using Equations (1) and (2) as:

P_x = w_pxl_x²/8z_d.x ——– (5)

and

P_y = w_pyl_y²/8z_d.y ——– (6)

Equilibrium dictates that the downward forces per unit length exerted over each edge support by the reversal of cable curvature (as shown in Figure 4) are:

w_pxl_x (kN/m) carried by the short-span supporting beams or walls
w_pyl_y (kN/m) carried by the long-span supporting beams or walls

The total force imposed by the slab tendons that must be carried by the edge beams is, therefore:

w_pxl_xl_y + w_pyl_yl_x= w_ball_xl_y ——– (7)

and this is equal to the total upward force exerted by the slab cables.

Therefore, for this two-way slab system, to carry the balanced load to the supporting columns, resistance must be provided for twice the total load to be balanced (i.e. the slab tendons must resist w_ball_xl_y and the supporting beams must resist w_ball_xl_y). This requirement is true for all two-way floor systems, irrespective of construction type or material.

At the balanced load condition, when the transverse forces imposed by the cables exactly balance the applied external loads, the slab is subjected only to the compressive stresses imposed by the longitudinal prestress in each direction, i.e. σ_x = P_x/h and σ_y = P_y/h, where h is the slab thickness.

Post-Tensioned Slab Design Example

It is required to design an exterior panel of a 175 mm thick two-way floor slab of a commercial building. The rectangular panel is supported by monolithic beams at all edges and discontinuous on two adjacent sides. It supports an additional dead load of 2.5 kN/m² apart from its self-weight and a live load of 4 kN/m². The slab is post-tensioned in both directions using the draped parabolic cable profiles shown below. The level of prestressing required to balance a uniformly distributed load of 4 kN/m² is required. Relevant material properties are f_ck = 40 MPa, f_ctm = 3.5 MPa, E_cm = 35,000 MPa, f_pk = 1,860 MPa and E_p= 195,000 MPa.

Self weight of the slab = 25 × 0.175 = 4.375 kN/m²
Superimposed dead load = 2.5 kN/m²
Total dead load = 6.875 kN/m²
Live load = 4 kN/m²
The total load on the slab = 6.875 + 4 = 10.875 kN/m²

Solution

Load Balancing

Load balancing:
Flat-ducted tendons containing four 12.5 mm strands are to be used with a duct size of 75 mm × 19 mm.

With a 25 mm concrete cover to the duct, the maximum depth to the centre of gravity of the short-span tendons is:

d_y = 175 − 25 − (19 − 7) = 138 mm (refer to the Figure above)

Eccentricity at the mid-span (short span) e_x= 175/2 – 38 = 50 mm (say)
Eccentricity at the mid-span (long span) e_y= 175/2 – 38 – 13 = 37 mm (say)

The cable drape in the short-span direction is, therefore:
z_d.y = (50 + 0)/2 + 50 = 75 mm

The depth d_x of the long-span tendons at mid-span is less than d_y by the thickness of the duct running in the short-span direction, i.e. d_x = 143 − 19 = 124 mm. The cable drape in the long-span direction is given by:

z_d.x= (50 + 0)/2 + 37 = 62 mm

Self-weight of the slab = 25 × 0.175 = 4.375 kN/m²
If 40% of the live load is assumed to be sustained, then the total sustained load is:
w_sus = 4.375 + 2.5 + (0.4 × 4.0) = 8.475 kN/m²

In this example, the effective prestress in the tendons in both directions balances an external load consisting of the self-weight and 10% of the superimposed dead load;

w_bal = 4.375 + 0.1(2.5) = 4.625 kN/m². The transverse load exerted by the tendons in the short-span direction is determined using Equation 4:

w_py= [l_x⁴/(δl_y⁴ + l_x⁴)] × w_bal
Where δ = 1.0 for two adjacent edges discontinuous.

w_py = 9⁴/(1.0 × 8⁴ + 9⁴) × 4.625 = 2.85 kN/m²

and the transverse load imposed by the tendons in the long-span direction is calculated;
w_px = w_bal− w_py= 4.625 − 2.85 = 1.775 kN/m²

The effective prestress in each direction is obtained from Equations (1) and (2):
P_i = w_pil_i²/8z_d.i

Therefore,
P_y = (2.85 × 8²)/(8 × 0.075) = 304 kN/m
P_x = (1.775 × 9²)/(8 × 0.062) = 290 kN/m

Maximum force per tensioning = 137 kN
Assuming a 25% loss over the long term, the force per strand = 0.75 × 137 = 102.75 kN
For four strands = 4 × 102.75 = 411 kN

The number of strands required:
x-direction: n_x = 290/411 = 0.7 (say 1 duct/m; comprising of 4 strands per duct),
Prestress provided = 102.75 × 4 = 411 kN/m
Load balanced, q_x = (8 × 411 × 0.062)/9² = 2.516 kN/m²

y-direction: n_y = 304/411 = 0.73, say (say 1 duct/m; comprising of 4 strands per duct)
Prestress provided = 102.75 × 4 = 411 kN/m
Load balanced, q_y = (8 × 411 × 0.075)/8²= 3.853 kN/m²

Total load balanced = q_x + q_y = 2.516 + 3.853 = 6.369 kN/m²

Alternatively;

Both the time-dependent losses and friction losses must be estimated in order to determine the jacking forces and cable spacing in each direction. Assuming a 25% loss is used to account for all the losses;

P_y = 304/0.75 = 405.3 kN/m
P_x = 290/0.75 = 386.7 kN/m

Using four 12.5 mm strands/tendon, A_p = 372 mm²/tendon and the maximum jacking force/tendon is 137 × 4 = 548 kN, and the required tendon spacing in each direction (rounded down to the nearest 10 mm) is therefore:

s_y = (1000 × 548)/405.3 = 1352 mm and
s_x = (1000 × 548)/386.7 = 1417 mm

However, it is recommended that the spacing of tendons should not exceed 8h in the span. Therefore, the maximum spacing of the tendons can be taken as = 8 × 175 = 1400 mm.

We will select a tendon spacing of 1000 mm in each direction. This simply means that the tendons in the y-direction will balance slightly more load than previously assumed. With one tendon in each direction per metre width, the revised prestressing forces at the jack per metre width are P_y = P_x = 548 kN/m and at mid-span, after all losses, are:

P_m,t.y= 0.75 × 548 = 411 kN/m
and
P_m,t.x =0.75 × 548 = 411 kN/m

The load to be balanced is revised using Equations (1) and (2):
w_py = (8 × 411 × 0.075)/8² = 3.853 kPa
and
w_px = (8 × 411 × 0.062)/9² = 2.516 kPa

and therefore w_bal= 3.853 + 2.516 = 6.369 kPa.

Estimate maximum moment due to unbalanced load

The maximum unbalanced transverse load to be considered for short-term serviceability calculations is:

w_unbal = w_sw+ w_G + ψ₁w_Q − w_bal= 4.375 + 2.5 + (0.7 × 4.0) − 6.369 = 3.306 kPa

Under this unbalanced load, the maximum moment in the slab can be investigated;

The aspect ratio of slab = l_y/l_x = 9/8 = 1.125

The negative moment at the continuous edge (short span) = 0.056 × 3.306 × 8² = 11.84 kNm/m
The positive moment at midspan (short span) = 0.042 × 3.306 × 8² = 8.886 kNm/m
The negative moment at the continuous edge (long span) = 0.045 × 3.306 × 8² = 9.521 kNm/m
The positive moment at midspan (short span) = 0.034 × 3.306 × 8²= 7.193 kNm/m

Obviously, in this case, the maximum moment is 11.84 kNm/m

Check for cracking

The concrete stresses in the top and bottom fibres caused by the maximum moment after all losses are:

σ_c.top = − P_m,t.y/A + M_CD/Z = [(411 × 10³)/(175 × 10³)] + [(11.84 × 10⁶)/(5.104 × 10⁶)] = −2.35 + 2.319 = -0.031 MPa (compression)
σ_c.btm = − P_m,t.y/A – M_CD/Z = −2.35 – 2.319 = -4.669 MPa (compression)

where A is the area of the gross cross-section per metre width (A = bh = 175 × 10³ mm²/m) and Z is the section modulus per metre width (Z = I/y = 5.104 × 10⁶ mm³/m). Both the top and bottom stresses are relatively low. Even though the moment used in these calculations is an average and not a peak moment, if cracking does occur, it will be localised and the resulting loss of stiffness will be small. Deflection calculations may be based on the properties of the uncracked cross-section.

Maximum total deflection

The deflection at the mid-panel of the slab can be estimated using the so-called crossing beam analogy, in which the deflections of a pair of orthogonal beams (slab strips) through the centre of the panel are equated. The fraction of the unbalanced load carried by the strip in the short-span direction is given by an equation similar to Equation (4).

w_unbal.x= 9⁴/(1.0 × 9⁴ + 8⁴) × 3.306 = 0.615 × 3.306 = 2.035 kN/m

and with the deflection coefficient β taken as 3.5/384, the corresponding short-term deflection at mid-span of this 1 m wide slab strip in the short-span direction through the mid-panel (assuming the variable live load is removed from the adjacent slab panel) is approximated by:

v₀= 3.5w_unbal,yl_y⁴/384E_cmI = (3.5 × 2.035 × 8000⁴)/(384 × 35000 × 446.614 × 10⁶) = 4.86 mm

The sustained portion of the unbalanced load on the slab strip is:
[l_x⁴/(δl_y⁴+ l_x⁴)] × (w_sw + w_G + y₂w_Q – w_bal) = 0.61 × [4.375 + 2.5 + (0.4 × 4.0) – 6.369] = 1.28 kPa

and the corresponding short-term deflection is:

v_sus.0 = 1.28/2.035 × v₀ = 3.056 mm

Assuming a final creep coefficient φ(∞,t₀) = 2.5 and conservatively ignoring the restraint provided by any bonded reinforcement, the creep-induced deflection may be estimated using:

v_cc= 2.5 × 3.056 = 7.64 mm

The final shrinkage strain is assumed to be ε_cs= 0.0005. The shrinkage curvature κ_cs is non-zero wherever the eccentricity of the steel area is non-zero and varies along the span as the eccentricity of the draped tendons varies.

A simple and very approximate estimate of the average final shrinkage curvature is;
κ_cs= 0.3ε_cs/h = (0.3 × 0.0005)/175 = 0.857 × 10^-6 mm^-1

The average deflection of the slab strip due to shrinkage is given by Equation 12.8:
v_cs= 0.090 × 0.857 × 10^-6 × 8000² = 4.936 mm

The maximum total deflection of the slab strip is therefore:
v_tot = v₀ + v_cc+ v_cs = 4.86 + 7.64 + 4.936 = 17.436 mm = span/456 < Span/250

Check flexural strength

It is necessary to check the design strength of the slab. As previously calculated, the dead load is 4.375 + 2.5 = 6.875 kN/m² and the live load is 4.0 kN/m². The factored design load is:

w_Ed = 1.35(6.875) + 1.5(4) = 15.28 kN/m²

The negative moment at the continuous edge (short span) = 0.056 × 15.28 × 8² = 54.763 kNm/m
The positive moment at midspan (short span) = 0.042 × 15.28 × 8² = 41.07 kNm/m
The negative moment at the continuous edge (long span) = 0.045 × 15.28 × 8² = 44kNm/m
The positive moment at midspan (short span) = 0.034 × 15.28 × 8²= 33.25 kNm/m

A safe lower bound solution to the problem of adequate strength is obtained if the design strength of the slab at this section exceeds the design moment. The resistance per metre width of the 175 mm thick slab containing tendons at 1000 mm centres (i.e. A_p = 372 mm²/m) at an effective depth of 138 mm can be obtained by considering the figure above.

The stress in the tendon caused by the effective prestressing force P_m,t= 411 kN is:

σ_pm,t = P_m,t/A_p = (411 × 10³)/372 = 1105 MPa

EN1992-1-1 permits the design stress in the tendon at the strength limit state to be taken as:
σ_pud = σ_pm,t+ 100 = 1205 MPa
and therefore the tensile force in the steel is F_ptd = 448.26 kN (= F_cd).

x = F_ptd/ηf_cdλb = (448.26 × 10³ )/(1.0 × 26.67 × 0.8 × 1000) = 21 mm

Such an analysis indicates that the cross-section is ductile, with the depth to the neutral axis of x₁ = 21 mm (or 0.152d).

M_Rd1 = σ_pudA_p × [d_p− (λx)/2)]
M_Rd1 = 1205 × 372 × [138 − (0.8 × 21)/2)] × 10^-6 = 58.09 kN/m

This shows that no additional reinforcement is required. However, 12 mm diameter non-prestressed reinforcing bars in the y-direction at 450 mm centres can be provided all over the slab.

Reference

Gilbert R.I, Mickleborough N.C. and Ranzi G. (2017): Design of Prestressed Concrete to Eurocode 2 (2nd Edition). CRC Press, Taylor and Francis,

Biocementation Method of Soil Improvement

By

Ubani Obinna

-

February 28, 2023

Table of Contents

Biocementation is an environmentally friendly, cost-effective alternative to imported granular fills, concrete, costly hauling of materials or export to a landfill. In-service performance of soils, foundations, and other earthen structures and their required maintenance is highly dependent on methods of stabilisation, ranging from expensive mechanical stabilisation to chemical processes.

As such, many alternative materials originating from bio-based sources are being explored as potential stabilising additives to improve weak subgrade soils (i.e., dispersive soils, erodible and collapsible soil, and soft or expansive clays). In order to prevent geotechnical failures induced by water infiltration and/or erosion, some relevant alternatives include the use of bio-derived enzymes, microorganisms, and polymeric additives.

Bio-based Innovations in Geotechnical Engineering

For sustainable road infrastructure development, various microorganisms and other bio-based materials, including secondary metabolites, enzymatic materials, and polymeric materials, have been studied as viable substitutes for traditional chemical stabilisers (Ikeagwuani and Nwonu, 2019). Thus, increasing the adoption of these bio-based materials and technologies in the building industry requires a deeper understanding.

The use of bacteria in bio-based admixtures produced through industrial fermentation processes is gaining popularity because of a number of positive aspects, including their quick biosynthesis rates, high yields of desired products, quick growth on cheap culture media that is readily available in large quantities, openness to genetic manipulation, and lack of pathogenic traits (Pei et al., 2015). By the outcomes of the microbial treatment of soil, at least eight different types of biotechnological processes associated with construction can be categorized as shown in Figure 1 (Ivanov et al., 2015).

**Figure 1**: Construction-related microbial biotechnology (Ramdas et al, 2021)

Of these, two biological processes (as far as the fundamental characteristics of research techniques) stand out;

(a) bioclogging, the production of pore-filling materials through biological means, to significantly reduce the hydraulic conductivity of soil or porous matrix and,

(b) biocementation, the generation of particle-binding materials through microbial processes in situ so that the shear strength of soil can be increased.

This is further reinforced by DeJong et al. (2010) stating that these methods allow a holistic view and meaningful characteristics in the area of soil stabilisation. The micro-organisms best suited for these techniques (i.e., soil bio-clogging, biocementation, and bio-aggregation) are facultative anaerobic and microaerophilic bacteria, although anaerobic fermenting bacteria have been suggested (Ivanov et al., 2015).

Microbial Biocementation

One of the most researched techniques is microbial-induced calcium carbonate precipitation (MICCP), also known as microbial biocement, which occurs when biological action increases the pH of soils to create supersaturated conditions (Oyediran and Ayeni, 2020).

Bio-cementation of soils — **Figure 2:** The process of bio-cementation

Recently, biocementation has gained attention as a practical method for soil improvement, notably because of its superior performance and environmental sustainability. For instance, an essential approach called microbially induced calcite precipitation (MICP) uses bacteria to create calcium carbonate precipitates and introduce “biocementation” between the sand grains (Cheng et al., 2013).

For instance, the alkalophilic soil bacteria Bacillus pasteuri uses the highly active urease enzyme to consume urea and break it down into carbon dioxide (CO₂) and ammonia (NH₃). In the presence of water, NH₃ is changed into NH₄⁺, while CO₂ equilibrates with carbonic acid, carbonate, and bicarbonate ions in a pH-dependent manner. The alkaline environment and carbonate needed for the reaction with Ca²⁺ and precipitation of calcite (CaCO₃) are provided by the rise in pH, as schematically depicted in Fig. 2.

**Figure 3.** Biocementation is two-step process a: Phase I, bacteria ingest nutrients such as, sugar, nitrogen, & proteins with the growth of the bacterial population and enzymes produced from the bacterial species they hydrolyse urea components in the presence of water to form ammonium and carbonate ions which leads to b: Phase II, again the addition of nutrients such as calcium chloride, in the presence of calcium ions and nucleation sites on the soil particles, the carbonate ions react spontaneously with the calcium ions to form calcium carbonate, **c-d**: the calcite precipitates/the cementing agent (produced by the bacteria) used to bind the soil particles together to increase strength and stiffness of the soil

Another significant but understudied example of biocementation and bioengineered soil structures is termite mounds (anthills). Ecologists, entomologists, architects, and soil chemists are very interested in these above-ground mounds, particularly those made by the termites known as Macrotermitinae that grow fungi.

According to Kandasami et al (2016), a critical component for termite societies with millions of individual termites is the stability of termite mounds, which are bioengineered granular ensembles. Termites are skilled engineers, as shown by an experimental investigation on the mechanobiology of mounds and mound soil of the fungus-growing termite Odontotermes obesus (Rambur). The physical and mechanical characteristics of mound soil were notably different from those of the nearby or “control” soil. Yet, the clay mineralogy of the mound and surrounding soils was the same.

To build anthills, termites combine the finer soil fraction with their secretions to form boluses in the presence of water, thereby significantly altering the soil. In order to efficiently mould the soil, they control the amount of water close to the soil’s plastic limit when producing these boluses (Kandasami et al, 2016).

As a result of these processes, the strength of the soil can be increased tenfold as a result of the cementation caused by termites utilizing their excretions and/or secretions, which may not have been possible otherwise. The modification of the soil by termites reduces the likelihood of erosion and collapse of the mounds.

Studies have also shown that termites effectively bonded foreign objects, indicating that they have a variety of cementation skills. When compared to reconstituted soil, a slope stability analysis with intact mound soil showed a much higher safety factor for the mound.

Applications of Biocementation in Soil Improvement

According to Cheng et al (2013), the MICCP method is most efficient at a particle contact right as cementation begins, and it becomes less effective as cementation spreads outward around a particle contact. Due to the increased inter-particle interactions, reallocating the CaCO₃ crystals to two contact places (connection points) as opposed to one would be more beneficial. As a function of the particle radius squared, the contact stress also decreases simultaneously. As a result, smaller particles have two complementary advantages: improved MICCP and reduced particle contact stresses.

Ng et al. (2012) used the B. megaterium MICCP procedure to evaluate shear strength, and they observed that the ratio of treated to untreated shear strength rose from 1.40 to 2.64. Van Paassen et al. (2010) reported a minimum of 300 kPa compressive strength. Similar results were published by Nafisi (2019), who observed that the soil strength varies between 210 kPa and 710 kPa depending on the type of sand and carbonate mass.

Van Paassen et al. (2010) also observed a 60% reduction in the permeability of treated soils at less than 100 kg/m³ CaCO₃ precipitation; however, as solution concentration increased, calcite crystal clogged pore spaces and reduced permeability between 50 and 99% using 1 M cementation solution.

In triaxial testing, Cheng et al. (2013) reported that the mechanical behaviour of the bio-cemented sand increased the effective shear strength parameters (i.e., cohesion, angle of internal friction) with an increase in CaCO₃ concentration at all saturation levels. By increasing the frictional angle at a lower saturation level, the precipitated crystals improved the cohesion of coarse sand. Under the same saturation level and similar CaCO₃ content as fine sand, coarse sand showed a greater friction angle than fine sand.

Here is how the researchers addressed this: The primary consequences of tiny soil particles include:

MICCP method can be improved by;

(a) adding additional inter-particle contact points and
(b) lowering the stress per particle contact.

An exponential relationship between the UCS value and the CaCO₃ content of the treated soils, according to van Paassen et al. (2010), demonstrates that even though the same amount of CaCO₃ precipitated, the mechanical response can vary depending on the CaCO₃ precipitation/crystals’ mechanism of action. Additionally, the process requires significant urease activity in the microorganisms, and the mode of action (MICCP) may be strain-dependent (van Paassen et al., 2010).

Several documented research works show several Bacillus spp. have the ability to precipitate calcium carbonate with structural properties, but a limited selection of micro-organisms highlights the triaxial and California Bearing Ratio (CBR) tests for potential road/pavement applications (Table 1).

*Preferable microbes come from Bacillacae* family**	Key micro-organisms showing mechanical improvement of construction material
B. flexus	Laboratory strength tests showed an increase in compressive (>40%), flexural (>30%) & split tensile (>10%) strength. X-ray powder diffraction (XRD) shows ureolytic properties of the organism (Roa et al., 2015).
B. sphaericus	Experimental results indicate polyurethane immobilised bacteria-induced higher strength regain (60%) and lower water permeability coefficient (10 –11 m/s) (Wang et al., 2012, 2014)
Sporosarcina pasteurii formerly known as B. pasteurii	Biocalcification mechanism showing potential construction applications such as dust control and strengthening of brick masonry (Sarda et al., 2009; Meyer et al., 2011). Triaxial tests indicate that the treated specimens exhibit a non-collapse strain softening shear behaviour, with a higher initial shear stiffness and ultimate shear capacity than untreated specimens. Samples treated showed a great increment in unsoaked and soaked CBR gained more strength when soaked for hours compared to unsoaked (Oyediran and Ayeni 2020). CBR obtained in a study showed poorer outcome from CBR index treated (0.6) compared to untreated samples (2.5). These investigations are of interest to improve soil stability, to build roads and paths, and to restore monuments (Morales et al., 2019).
B. licheniformi	Calcium carbonate precipitation from the strain has the potential for sealing cement-based materials (Vahabi et al., 2015).
B. subtilis	Strength performance of microbial concrete mechanism on the principle of calcite mineral precipitation by alkali-resistant spore-forming bacteria. Its response showed autonomous improvement in the self-healing process due to microcrack formation (Rao et al., 2017).

Table 1: Bacillus species related to microbial bio-cement

According to Mujah et al (2016), the factors affecting the bio-cementation of soils are;

Temperature
pH level
Urease activity/bacterial concentration
Degree of saturation
The concentration of cementation solution

Limitations of MICP for soil biocementation

The fact that MICP can only be applied to particular soil sizes is one of its key disadvantages. The approach is currently only effective for treating sands with particle sizes comparable to 0.5-3 mm, according to a review by Fragaszy et al (2011). Hence, expanding the use of MICP for improving and stabilising fine-grained soils like silt and clay would be a huge challenge.

Furthermore, some researchers have conducted field trials and upscaled studies to test the viability of MICP for in-situ bio-cementation deployment in construction. For instance, van Paassen (2009) conducted an experiment to treat 1-100 m³ of sand in the lab and observed that, after MICP treatment, the strength of bio-cemented sand was greatly increased; however, distinct spatial variability was observed. The most important factor that still needs further focus is treatment homogeneity or the homogeneous distribution of CaCO₃ across the treated soil matrix from top to bottom.

Conclusion

The engineering, mechanical, and physical properties of biocemented soils can potentially be enhanced by MICP treatment. The intended uses of MICP include slope stabilization, settlement reduction, erosion management, soil self-healing, and prevention of liquefaction.

The benefits of MICP for soil improvement include cheaper costs compared to chemical grouting and other man-made materials, treatment dependability, and an overarching idea that encourages sustainability in tandem with future needs. Before the MICP method is directly applied in the field, further study should be concentrated on optimizing it at both the micro and macro levels.

Investigations must be done into specific issues such as medium flow and transport via heterogeneous media, treatment durability, mixing procedure permanence, and particle-level stratigraphic mapping. Further potential directions in MICP technology include the use of seawater as a salt alternative as well as self-healing in terms of pre- and post-shearing of biotreated soils after significant earthquakes and related aftershocks.

References

Cheng L, Cord-Ruwisch R, Shahin MA. (2013): Cementation of sand soil by microbially induced calcite precipitation at various degrees of saturation. Can Geotech J 2013;50:81–90. https://doi.org/10.1139/cgj-2012-0023

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Mujah D., Mohamed A. Shahin M.A, and Cheng L. (2016): State-of-the-Art Review of Biocementation by Microbially Induced Calcite Precipitation (MICP) for Soil Stabilization. GEOMICROBIOLOGY JOURNAL http://dx.doi.org/10.1080/01490451.2016.1225866

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Control of Surface Erosion

Riprap

Compost

Vegetation Cover

Geosynthetics

Soil Binders

Erosion Control Structures

Diversions

Berms

Waterways

Debris Basin

Retarding Structures

Grade Stabilisation Structures

Gabions

Conclusion

Ultimate Bending Strength Calculation of a Prestressed Girder

Applications and Functions of Geosynthetics

Separation

Filtration

Drainage

Reinforcement

Barrier

Protection

Conclusion

Studies on the Environmental Impacts of High Strength Concrete

References

Carbon Footprint of Different Construction Materials

Carbon Footprint of Reinforced Concrete

Optimisation of Reinforced Concrete Structures for Environmental Impacts

Conclusion

References

What is Construction Inspection?

Importance of Construction Inspection

Increase Workplace Safety

Identify Risks

Increase Productivity

Monitor the Progress of the Work

Constructive Quality

Progress Documentation

Steps for an Efficient Construction Inspection

Plan the Inspection

Collect the Data

Evaluate the Results

Propose Corrective Actions

Inspection is a Fundamental Tool in Civil Construction

Concrete Overlay for Slab Strengthening

Slab strengthening by External Reinforcement

Slab strengthening by adding structural member

Conclusion

Reference

The Prefab House with Concrete Tunnels

Post-Tensioned Slabs

Balanced Load Stage for Two-Way Slabs

General Approach to the Design of Post-Tensioned Slab

Load Balancing

Post-Tensioned Slab Design Example

Load Balancing

Estimate maximum moment due to unbalanced load

Check for cracking

Maximum total deflection

Check flexural strength

Reference

Bio-based Innovations in Geotechnical Engineering

Microbial Biocementation

Applications of Biocementation in Soil Improvement

Limitations of MICP for soil biocementation

Conclusion

References

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