Traffic Loading Analysis for Airfield Pavements

By

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April 12, 2023

Table of Contents

Airfield pavements are designed and constructed to provide durable all-weather travel surfaces for the safe landing, takeoff, and quick movement of passengers and aircraft while maintaining an appropriate level of comfort for users. During the design and construction phases, considerable consideration is given to the following factors in order to meet these functional requirements for pavements:

(a) selection of pavement type,
(b) selection of materials to be used for various pavement layers and treatment of subgrade soils,
(c) structural thickness design for pavement layers,
(d) subsurface drainage design for the pavement system,
(e) surface drainage and geometric design, and
(f) rideability of the pavement surface.

Realistic methods for evaluating the loading characteristics of aircraft and the structural reaction of the pavement are necessary for the design of an airfield’s pavement. It has long been understood that the gross weights of the aircraft utilizing the pavement, as well as the arrangement, spacing, and tyre pressures of their undercarriage wheels, determine the severity of load-induced stresses in a pavement and subgrade.

Due to changes in traffic operations and functional applications of the pavements, the traffic loading analysis process for airport pavements differs slightly from that for highway pavements. The steps in the traffic loading analysis of airfield pavements include;

Estimation of expected initial year traffic volume
Estimation of expected annual traffic growth rate
Estimation of traffic stream composition
Computation of traffic loading
Estimation of design traffic loading for different functional areas

The information on the first two steps may be obtained from the planning forecast of the relevant airport authority.

Traffic Stream Composition

The nose gear and main landing gear of an aircraft transfer the weight of the aircraft to the pavement. The typical wheel arrangements on the main landing gear legs of civil aircraft are shown in Figure 2. For the purpose of pavement design, it is necessary to identify the types of aircraft, landing gear details, and their relative frequencies of arrival. This is because different aircraft have varied gross weights and wheel configurations.

**Figure 2:** Typical wheel configurations of a main leg of the aircraft landing gear.

Computation of Traffic Loading

The maximum takeoff weights of the aircraft are typically taken into account when designing pavement. It is also typical to assume that the main landing gears carry 95% of the aircraft’s gross weight and the nose gear carries 5%. Both the equivalent load idea and Miner’s hypothesis have been used in the analysis of mixed traffic loading.

aircraft wheels — **Figure 3:** Typical landing gear of an aircraft

For instance, the FAA method [Federal Aviation Administration, 1978] uses the variables in Table 1 to convert the annual departure of all aircraft into the equivalent departures of a selected design aircraft. In thi case, the designer should multiply the annual departures of a given aircraft type by the conversion factor to obtain annual departures in design aircraft landing gear.

Aircraft Type	Design Aircraft	Conversion Factor F
Single-wheel	Dual-wheel	0.8
Single-wheel	Dual-tandem	0.5
Dual-wheel	Dual-tandem	0.6
Double dual-tandem	Dual-tandem	1.0
Dual-tandem	Single-wheel	2.0
Dual-tandem	Dual-wheel	1.7
Dual-wheel	Single wheel	1.3
Double dual-tandem	Dual-wheel	1.7

Table 1: Conversion factor for computing annual departures

The FAA uses the equivalent single-wheel load (ESWL) concept to create the thickness design curves for flexible airfield pavements. When designing airfield pavement, the concept of ESWL is frequently used to evaluate the impact of multiple-wheel landing gears.

The control response chosen for ESWL computation, pavement thickness, and the relative stiffness of pavement layers all affect how much ESWL a particular landing gear has. ESWL calculations based on equal deflection (at the surface or at the pavement-subgrade interface) or equal stress (at the bottom face of the bound layer) are frequently used for airport pavement design.

Equal Stress ESWL

A detailed method for calculating the ESWL would require both a correct analytical solution for the required stress produced by the target wheel assembly and a process of trial-and-error to determine the magnitude of the single wheel that will produce the same value of stress. Because of how time-consuming this process is, simplified approaches have been used in practice.

A simplified method for calculating the equal subgrade stress ESWL of a pair of dual wheels for flexible pavement design is shown in Figure 4.

**Figure 4:** Equal subgrade stress ESWL of dual wheels for flexible pavement design

If the pavement thickness is less than or equal to d/2, where d is the lowest edge-to-edge distance between the tire imprints of the dual wheels, the ESWL is equal to one wheel load P with the specified 45° spread of applied pressure.

The method further assumes that ESWL = 2P for any pavement equal to or thicker than 2S, where S is the center-to-center spacing of the dual wheels. For pavement thicknesses between d/2 and 2S, ESWL is determined, as shown in Figure 4, by assuming a linear log–log relationship between ESWL and pavement thickness. Note that d is equal to (S – 2a), where a is the radius of the tyre imprint given by;

a = √(P/πp)

where p is the tyre pressure.

This simplified procedure provides an approximate ESWL estimation for flexible pavements.

In the case of rigid pavements, computation of stresses for equal stress ESWL should be based on rigid slab analysis such as the well-known Westergaard formulas, which give the maximum bending stress σ_max and the maximum deflection δ_max.

an aircraft — **Figure 5:** An aircraft landing on an airfield pavement

Solved Example

The total load on a set of dual wheels is 45,000 lb. The tire pressure of the wheels is 185 psi. The centre-to-centre spacing of the wheels is 34 in. Calculate the equal stress ESWL if the thickness of the pavement structure is (a) h = 30 in. and (b) h = 70 in.

Solution
Load per wheel = 22,500 lb., radius of tire imprint a = √(22500/185π) = 6.22 in., S = 34 in., and d = (S – 2a) = 21.56.

By means of a log–log plot as shown in Figure 4, ESWL is determined to be 33,070 lb. for h = 30 in. For h = 70 in., since h > 2S, ESWL = 2(22,500) = 45,000 in.

Airfield Pavement Loading in the UK

In the UK, Aircraft Classification Number (ACN) and Pavement Classification Number (PCN) are the two parameters often used in airfield pavement design.

The ACN of an aircraft describes the relative severity of its loading on a pavement supported by a given subgrade. Two mathematical models—one for rigid pavements and the other for flexible pavements—are used to compute ACNs.

For a given limiting stress or the number of load repetitions, the ACN of an aircraft is mathematically defined as twice the single wheel load (expressed in thousands of kilograms) at a standard tyre pressure of 1.25 MPa, which requires the same pavement thickness as the actual main wheel gear of the aircraft. The standard thickness is the pavement’s thickness.

ACN Calculation for Rigid Pavement

The method of calculating ACNs for aircraft on rigid pavements is set out below with reference to Figure 1:

(i) Calculate the reference thickness (t_c), the thickness of concrete slab which when loaded at the centre by one main wheel gear of the actual aircraft gives a maximum flexural stress of 2.75 N/mm² (f_ct) on a subgrade whose Modulus of Subgrade Reaction (k) is one of the standard values (see (iv)). The mathematical model for the stress calculation is the Westergaard solution for an elastic slab on a dense liquid subgrade (Winkler Foundation). The modulus of elasticity for concrete is taken as 27.6 x 10³ MN/m² and Poisson’s ratio as 0.15.
(ii) Calculate the single wheel load (W_R) which at a tyre pressure of 1.25 MPa induces a flexural stress of 2.75N/mm², in slab of thickness t_c.
(iii) The ACN = 2 x (W_R/1000) = W_R/500 where W_R is in kgs.
(iv) Calculate ACNs for each aircraft for the following four categories of subgrade characterised in terms of a standard k.

Subgrade Category	k
High	150 MN/m²/m
Medium	80 MN/m²/m
Low	40 MN/m²/m
Ultra-low	20 MN/m²/m

ACN Calculation for Flexible Pavement

The method of calculating ACNs for aircraft on flexible pavements is set out below with reference to Figure 2:

(i) Calculate the reference thickness (t_f), the thickness of conventional flexible pavement which allows 10,000* load repetitions by one main wheel gear of the actual aircraft on a subgrade whose CBR is one of the standard values (see (iv)). The method of calculation is based on the CBR Equation and Boussinesq deflection factors.
(ii) Calculate the single wheel load (W_F) which at a tyre pressure of 1.25 MPa allows the same 10,000 load repetitions on a flexible pavement of total thickness t_f. The calculation is carried out using the following formula:
W_F = 0.005t_f²/(0.878/CBR – 0.01249)
(iii) The ACN=2 x W_F/1000 = W_F/500
where WF is in kgs.
(iv) Calculate ACNs for each aircraft for the following four categories of subgrade characterised in terms of a standard CBR.

Subgrade Category	CBR
High	15%
Medium	10%
Low	6%
Ultra-low	3%

Pavement Classification Number (PCN)

By the definition of the ACN-PCN method, the PCN is the ACN of the aircraft which imposes a severity of loading equal to the maximum permitted on the pavement of unrestricted use.

PCNs are reported as a five-part code as follows:

Part i: The PCN Number: The highest permitted ACN at the appropriate subgrade category.

Part ii: The type of pavement: R = rigid, F = flexible. If the actual pavement is of mixed construction the engineer will need to decide whether the behaviour and mode of failure of the pavement are likely to be those of a rigid or flexible one, then classify accordingly.

Part iii: The pavement subgrade category:
A = High
B = Medium
C = Low
D = Ultra Low
The ranges of subgrade strength covered by these categories are shown in the Tables above. Note that these strength ranges are not equivalent for rigid and flexible pavements.

Part iv: The maximum tyre pressure authorised for the pavement:
W = High, no limit.
X = Medium, limited to 1.5 MPa (217 psi)
Y = Low, limited to 1.0 MPa (145 psi)
Z = Very low, limited to 0.5 MPa (73 psi)

Part v: Pavement design/evaluation method:
T = Technical design or evaluation
U = By experience of aircraft actually using the pavement

The Design ACN

The design ACN is based on the Design Aircraft; which is normally the aircraft with the highest ACN on the actual subgrade.

The actual weight of aircraft when using the pavement must be considered in determining the design ACN. The Maximum All-Up Weight figure will normally be used, but lighter weights are appropriate where:

(i) the runway length imposes restrictions on the operating weights,
(ii) the pavement is only used by landing aircraft (e.g. fast turn offs) and
(iii) the pavement is only used by unladen aircraft (e.g. the accesses to maintenance hangars).
To compute an ACN at a weight between the published values it is assumed that ACNs vary linearly with weight.

The design ACN should also relate the actual value of the subgrade under a pavement. The ACNs listed in B are for four standard subgrade categories. If the value of actual subgrade is not the same as that of a standard subgrade, the design ACNs are to be calculated by linear interpolation or extrapolation of ACNs for the standard subgrades.

The high category subgrade for flexible pavements is for CBR 15%. When designing pavements for subgrades with CBR greater than 15% the following rules may be applied:

(i) Single and Dual Main Wheel Gears
Take the ACN for CBR >15% to be the same as the ACN for CBR 15%.

(ii) Dual-Tandem Main Wheel Gears
Take the ACN for CBR ≥ 20% as equal to 0.95 x the ACN for CBR 15%. Values for CBRs between 15% and 20% can be obtained by linear interpolation e.g.
ACN for CBR 17% = 0.98 x the ACN for CBR 15%.

(iii) Tridem Main Wheel Gears
Take the ACN for CBR ≥ 20% as equal to 0.97 x the ACN for CBR 15%.
Values for CBRs between 15% and 20% can be obtained by linear interpolation.

For rigid pavements, the effect of the higher subgrade values is less significant and it is, therefore, acceptable to assume that: ACN for k >150 MN/m²/m = ACN for k of 150 MN/m²/m.

For pavements which would subsequently be difficult to strengthen, it may be appropriate to design for a higher ACN e.g. for aprons adjacent to hangars and terminal buildings. Where a design ACN of less than 10 is being considered a check should be made to ensure that the pavement is strong enough for the expected use by aircraft servicing vehicles.

Provided the PCN for a pavement is equal to or greater than the ACN of the aircraft and the operating tyre pressure does not exceed the PCN limitation, unrestricted use of the pavement by that aircraft (or those with lower ACNs) is permitted. The term ‘unrestricted use’ of a pavement is not specifically defined. However, it is a pavement design parameter which should reflect current and forecast use over an appropriate design life before major maintenance is required.

Design of Timber Columns

By

Ubani Obinna Uzodimma

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April 3, 2023

Axially loaded timber members are usually employed in building construction. Sometimes, these members are also subjected to simultaneous axial compression and bending. Some of these members include timber columns or beams, truss members, vertical wall studs, and bracing components. The design of timber columns involves the selection of adequate timber grade and section that will safely carry the anticipated load without undergoing excessive deflection or failure by squashing, buckling, or shear.

The limit states associated with the primary design effects for axially loaded members must also comply with the pertinent design rules and requirements of EN 1995-1-1:2004 (EC5). The equilibrium states and strength conditions are ultimate limit states because they are related to failure scenarios. The displacement condition pertains to situations encountered in typical usage, but EC5 offers no guidance on the state’s limiting criteria.

**Figure 1:** Old timber column structure

Where lateral instability of a member may occur, Section 10 of EN 1995-1-1 sets limitations on the maximum permissible deviation from straightness. For strength-related conditions, the design stress is calculated and compared with the design strength, which is modified as necessary by strength factors. When using the partial factor technique, the design stress cannot be greater than the design strength in order to comply with code reliability requirements.

When members or structures are subjected to combined stresses, such as those caused by the combined effects of axial and bending actions, extra design effects will need to be considered.

In this article, we are going to present an example of how we can design axially loaded timber columns according to the requirements of Eurocode 5.

Timber Columns Subjected to Axial Compression

These are members that are subjected to a compressive axial force acting perpendicular to the grain and along the member’s centroidal x-x plane. Such members serve as posts, columns, wall studs, or struts in pin-jointed trusses.

For timber sections subjected to axial compression, the failure strength is dependent on several factors such as:

strength/stiffness – compressive strength and modulus of elasticity of the timber;
the geometry of the member – cross-sectional sizes and length;
support condition – the amount of lateral support and fixity at its ends;
geometric imperfections – deviations from nominal sizes, initial curvature and inclination;
material variations and imperfections – density, the effect of knots, the effect of compression wood and moisture content.

When a timber column is subjected to an axial load, there is a tendency for it to displace laterally and to eventually fail by buckling because of imperfections in the geometry of the member or variations in its properties, or a combination of both, as the slenderness ratio, λ, of the member increases.

buckling of a column — **Figure 3:** Buckling of timber columns

The slenderness ratio is defined as the effective length of the member, L_e, divided by its radius of gyration, i,;

λ = L_e/i

where the radius of gyration about an axis i = √I/A, I is the second moment of area about the axis, and A is the cross-sectional area of the member. For any member, there will be a slenderness ratio, λ_y, about the y–y axis and, λ_z, about the z–z axis and when using a rectangular section, as shown in section A–A in Figure 3.

Design Example

Verify the capacity of 200 mm x 100 mm timber section of strength class C18 to satisfy ultimate limit state requirements under the following conditions:

Service Condition: Class 2
Characteristic permanent action G_k = 10 kN
Variable medium-term action Q_k = 25 kN
Height of column = 3.0m
Support condition: Effectively held in position but not in direction in both axes.

Solution
Geometric properties
Depth of member h = 200 mm
Width of member b = 100 mm

Area of member A = bh = 200 x 100 = 20000 mm²
Second moment of area about the y-y axis I_y = bh³/12 = (100 x 200³)/12 = 6.667 x 10⁷ mm⁴
Radius of gyration about the y-y axis i_y= √(I_y/A) = √(6.667 x 10⁷/20000 ) = 57.74 mm

Second moment of area about the z-z axis I_z = hb³/12 = (200 x 100³)/12 = 1.667 x 10⁷ mm⁴
Radius of gyration about the z-z axis i_z= √(I_z/A) = √(1.667 x 10⁷/20000 ) = 28.86 mm

Effective length about the y-y axis L_E,y = Effective length about the z-z axis L_E,z = 1.0 L = 3000 mm

Therefore slenderness ratio about the y-y axis;
λ_y = L_E,y/i_y =3000/57.74 = 51.957

The slenderness ratio about the z-z axis;
λ_z = L_E,z/i_z =3000/28.86 = 103.95

Timber Properties (Table 1 BS EN 338:2003(E))
For strength class C18;
Characteristic compression strength parallel to the grain f_c.0.k = 18 N/mm²
Fifth percentile modulus of the elasticity parallel to the grain E_0.05 = 6.0 kN/mm²

Partial safety factors
UKNA to BS EN 1990:2002, Table NA.A1.2(B)) for the ULS
Permanent actions, γ_G = 1.35
Variable actions, γ_Q = 1.5

Material factor for solid timber, γ_M = 1.3 (UKNA to EC5, Table NA.3)

Design Action
Design Action at ultimate limit state N_Ed = 1.35G_k + 1.5Q_k
N_Ed = 1.35(10) + 1.5(25) = 51 kN = 51000 N

Modification factors
Factor for medium duration loading and service class 2, k_mod.med = 0.8 (EC5, Table 3.1)
System strength factor, (not relevant) k_sys = 1.0

Therefore, the design compression stress α_c,0,d = N_Ed/A = 51000/20000 = 2.55 N/mm²

The design compression strength f_c,0,d = (k_mod.med k_sysf_c.0.k)/ γ_M
f_c,0,d = (0.8 x 1.0 x 18)/1.3 = 11.08 N/mm²

Buckling Resistance (clause 6.3.2 EC5)
Relative slenderness about the y-y axis
λ_rel,y = (λ_y /π) x √(f_c.0.k/E_0.05) = (51.957/π) x √(18/6000) = 0.9058 (equation 6.21 EC5)

Relative slenderness about the z-z axis
λ_rel,z = (λ_z /π) x sqrt(f_c.0.k/E_0.05) = (103.95/π) x √(18/6000) = 1.81 (equation 6.21 EC5)

As both relative slenderness ratios are greater than 0.3, the conditions in clause 6.3.2(3):

Maximum relative slenderness ratio of the column = λ_rel,z = 1.81

Factor β_c for solid timber β_c = 0.2 (EC5, equation (6.29))

The factor k_z (equation 6.28 EC5)

k_z = 0.5[1 + 0.2(1.81 – 0.3) + 1.81²] = 2.289

Instability factor (equation 6.26 EC5)

k_zc = 1/[2.289 + √(2.289² – 1.81²)] = 0.27
Design buckling strength k_zc .f_c,0,d= 0.27 x 11.08 = 2.9916 N/mm²
Therefore, design stress/design buckling strength ratio = 2.55/2.9916 = 0.852 < 1.0 Ok

Therefore, the section is satisfactory.

Geotechnical Investigation for Offshore Platforms Design

By

Ubani Obinna

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March 27, 2023

Fixed offshore platforms are used by the petroleum industry for a variety of operations, including drilling, production, housing, and storage. These constructions are situated offshore but closer to the coast than regular offshore platforms as a result of the recent growth in demand for LNG terminals. With regard to loading and foundation design, LNG terminals and offshore platforms are very similar, and the majority of what is covered here also applies to LNG terminals.

Steel constructions that are supported by piles or suction foundations or gravity platforms (GBS = “Gravity Based Structure”) that rest on the ocean floor can be used for permanent, fixed offshore platforms. Typically, open-ended tubular piles are driven for this purpose. Instead of driven piles, a few steel platforms are supported by steel suction caissons. The gravity platforms are made of steel or concrete and have skirts that reach the seafloor.

gravity offshore platform — Gravity-based offshore platform

The Ekofisk Oil Storage Tank, which is situated in approximately 60 meters of water, has short skirts that only penetrate about 0.6 meters into extremely dense sand, in contrast to the tallest GBS to date, which is located in the Troll field in the North Sea in about 330 meters of water. All of this serves to illustrate how important a site’s geotechnical properties are to the design of any structure.

Offshore platforms frequently have large deck loads and, in exposed places, will be subject to powerful storm loads. High dynamic loads are imposed by earthquakes in seismic regions, which may weaken the soil. Because of the loads that the foundation must support, foundation engineering is an important step in the design process.

Jack-up platforms can only be used in shallow waters, are often only installed for a short time (months rather than years), and are usually supported by spudcans.

jack up offshore platform — Jack-up offshore platform

The installation of various kinds of big floating structures is one of the most recent technologies for oil and gas exploration in the deep sea. These floating structures must be properly anchored.

Most offshore oil and gas fields have a small number of buildings. This is in contrast to an offshore wind farm, which is made up of numerous independently created wind turbines placed throughout the wind farm region. Suction caissons, monopiles, jacket piles, tripods, and gravity-type foundations are available for wind energy towers.

Foundation Design Issues for Offshore Platforms

For offshore platforms, some significant foundation design concerns include:

Bearing Capacity

Regardless of the structure, the subsoil needs to be able to support the static and cyclic loads with a safe enough margin against excessive displacements or failure. Potentially distinct from the bearing capacity under monotonic loading is the capacity under cyclic loading. Design considerations such as the soil’s bearing capacity under cyclic loads are also very important.

The parameters required for the evaluation of the bearing capacity are;

− Monotonic shear strengths under different stress paths
− Cyclic shear strength under combined average and cyclic shear stresses for triaxial and simple shear stress paths.
− Undrained angle of shearing resistance (sand).

Permanent Displacement (e.g. settlement)

Static actions will result in initial settlements as well as settlements in the soil outside and beneath the platform as a result of consolidation and creep. Due to increasing shear stresses and the dissipation of cyclically produced pore pressure, the cyclic loading from waves and/or earthquakes will result in extra permanent displacements.

The permanent displacements will decrease the freeboard—the distance between the deck of the platform and the water’s surface—and may put mechanical connections to things like oil wells, risers, and pipelines attached to the platform under stress.

The parameters required for the evaluation of the permanent displacement are;

− Compressibility
− Permeability
− Permanent shear strain and pore pressure under combined average and cyclic shear stresses for triaxial and simple shear stress paths
− Compressibility after cyclic loading

Cyclic displacements

The soil and the platform are displaced cyclically as a result of cyclic loads. Cyclic displacements of the platform could make it difficult for the people and equipment on the platform to function optimally. Cyclical displacements, like permanent displacements, can lead to stresses in the structural components and any connections to the structure.

The parameters required for the evaluation of the cyclic displacements are;

− Cyclic shear strain as a function of cyclic shear stress under combined average and cyclic shear stresses for triaxial and simple shear stress paths.
− Initial shear modulus.

Foundation Stiffness

For structural dynamic assessments under wave and/or earthquake loads, equivalent soil spring stiffness data are required. For platforms in deep water, the resonance period may be close to the period of the wave load, resulting in a large amplification of the wave load. Therefore, it’s crucial to either calculate and design for the wave load amplification factor or make sure the resonance period is sufficiently far from the wave load period.

The parameters required for the evaluation of the foundation stiffness are;

− Cyclic shear strain as a function of cyclic shear stress under combined average and cyclic shear stresses for triaxial and simple shear stress paths
− Initial shear modulus
− Damping

Reaction stresses in the soil

GBS constructions need to be built to withstand the stresses that the soil will experience from both static and cyclic loads. The soil reaction stresses may be particularly severe in some areas of an uneven seabed that is made up primarily of dense sand. Due to creep and the cyclical erosion of the soil modulus, the soil response stresses may also redistribute over time.

The parameters required for the evaluation of the reaction stresses are;

− Monotonic and cyclic shear strengths.
− Compressibility under virgin loading and reloading.
− Cyclic and permanent shear strains and permanent pore pressure under combined average and cyclic shear stresses for triaxial and simple shear stress paths.
− Seabed topography, objects on the seafloor.

Skirt penetration

To ensure that GBSs with skirts will reach the intended penetration depth, the skirt penetration resistance must be computed.

The parameters required for the evaluation of the skirt penetration are;

− Undrained anisotropic monotonic shear strengths.
− Remoulded shear strength (or sensitivity).
− Drained angle of shearing resistance (sand).
− Residual interface angle of shearing resistance (sand).
− CPT resistance (sand).
− Seabed topography and objects on the seafloor.
− Boulders in the soil within the skirt penetration depth.

Pile driveability

The selection of pile driving equipment must be carefully considered in order to guarantee that the pile can withstand the driving stresses and that an acceptable penetration depth is achieved.

The parameters required are;

− Axial and lateral response.
− Shear strength.
− Soil modulus or strain at 50% ultimate strength
− CPT cone resistance

Punch-through

The legs of a jack-up rig are often preloaded with ballast water before the footings (spudcans) are placed. Due to the rising ballast burden, these will pierce the seabed. In soils with a limited increase in soil strength with depth and in soils where a strong layer of limited thickness lays on top of a weaker layer, rapid, uncontrolled penetration may happen.

Analyses of liquefaction potential

This is necessary for locations with sands or silty sands, as well as seismically active regions and areas with the potential for high wave loading.

Liquefaction potential can be assessed using the following parameters;

− Initial shear modulus
− Cyclic shear modulus degradation curves
− Damping
− Coefficient of reconsolidation (sand)

Erosion and scour

Around offshore sites, erosion and scour may be brought on by waves and currents. In comparison to clay, sand has a higher propensity for scouring, which normally rises as water depth falls. A graded gravel fill may be necessary around the perimeter of gravity platforms constructed on sand in order to prevent erosion. Skirts around the edge will assist stop the base from scouring. Scour can be accommodated in the case of piles by including a specific scour depth in the design.

Permeability tests can be used to assess erosion and scour potential.

Soil and Rock Parameters Required

To address these design issues highlighted for offshore platforms, geotechnical and geological information needs to be collected. The sections below list the basic and specific soil and rock parameters needed for a typical offshore platform’s foundation design.

Clays

− General description
− Layering
− Grain size distribution
− Water content
− Total unit weight
− Atterberg (plastic and liquid) limits
− Indicative shear strength (miniature vane, torvane, pocket penetrometer, fall cone, UU, etc.)
− Remoulded shear strength
− Sensitivity
− Soil stress history and overconsolidation ratio
− Organic material content

Sand, silt, or gravel

− General description
− Layering
− Grain size distribution
− Water content (silt)
− Maximum and minimum densities
− Relative density
− Drained angle of shearing resistance
− Soil stress history and overconsolidation ratio
− Angularity
− Carbonate content
− Organic material content

Rock

− General description
− Rock Quality Designation (RQD)
− Water absorption
− Total unit weight
− The unit weight of solid blocks
− Unconfined compression strength
− Mineralogy
− Carbonate content

Typical Scope of Geophysical Investigations

The following are the typical scope of work for the geophysical investigation.

Seabed Topography
Minimum survey area: Usually 1 km x 1 km in shallow water, 2 km x 2 km in deep water. Possible extension to 5 km x 5 km in areas with geohazards to incorporate possible platform location shifts etc.
Means of survey: Swath bathymetry, preferably multibeam

Seabed features
Minimum survey area: Usually 1 km x 1 km in shallow water, 2 km x 2 km in deep water. Possible extension to 5 km x 5 km in areas with geohazards to incorporate possible platform location shifts etc.
Means of survey: Sidescan sonar, line spacing 100-200 m depending on water depth, with sonar range set to provide 200 % coverage from line overlap.

Subsurface Information
Minimum survey area: Usually 1 km x 1 km in shallow water, 2 km x 2 km in deep water. Possible extension to 5 km x 5 km in areas with geohazards to incorporate possible platform location shifts etc.
Means of survey: High-resolution / ultra high-resolution seismic survey for shallow geology and fault offset analysis. Line spacing: 100 m to 200 m depending on water depth. May be performed simultaneously with sidescan sonar. Also, 3D exploration seismic data (where available) to approximately 1.5 milliseconds for regional geohazard analysis and drilling hazard analysis to approximately 1000 m depth.

Typical Scope of Geotechnical Investigation

Gravity Platforms

The typical scope of work for the geotechnical survey of a gravity platform includes the following;

(1) 1 no. BH with continuous sampling down to 15 m, thereafter sampling with less than 0.5 m gaps to 0.5 – 0.7 times the platform diameter, followed by alternate sampling and CPT (preferably (P)CPT) with less than 0.5 m gaps. The minimum depth of penetration should be 1.5 times the platform diameter.

(2) 3 nos. BHs with continuous sampling to 15 m, thereafter sampling with less than 0.5 m gaps. The minimum depth of penetration should be 50 m.

(3) 10 nos. continuous (P)CPTs. The minimum depth of penetration should be 50 m or 1.5 times the platform diameter.

Sample testing

The samples retrieved from the subsurface exploration should be subjected to the following engineering tests;

Index testing,
Triaxial tests
Oedometer tests.
Permeability tests.
Simple shear tests and anisotropically consolidated compression and extension triaxial tests, monotonic and cyclic.
Shear wave velocity measurements by bender elements to determine initial shear modulus.
Resonant column tests.
X-ray photographs to determine soil layering within the tube, inclusions, sample quality.
Radioactive core logging (optional).

Piled Platform

The typical scope of work for the geotechnical survey of a piled offshore platform includes the following;

(1) 1 no. BH with samples every metre down to 15 m, thereafter sampling with less than 0.5 m gaps to 30 m, followed by alternate sampling and CPT (preferably (P)CPT) with less than 0.5 m gaps or 2 nos. BHs: one with sampling only and one with near-continuous CPT. The minimum penetration should be 4 pile diameters or pile penetration plus pile group, diameter, whichever is greater.

(2) Continuous (P)CPTs at a location 5-10 m from the main borehole. The penetration depth should be 30m.

Sample Testing

Index testing,
Testing for pile capacity and drivability, and for (static mudmat) bearing capacity

Jack-up Rig Platform

The typical scope of work for the geotechnical survey of a jack-up offshore platform includes the following;

(1) 1 no. BH with samples at every metre down to 15 m, and thereafter gaps less than 0.5 m. The minimum depth of penetration should be 30 m or anticipated spudcan penetration + 1.5 times spudcan diameter, whichever is deeper.

(2) 1 no. Continuous (P)CPT at a location 5 to 10 m from the main borehole and/or at each leg location. The minimum depth of penetration should be 20 m.

Sample Testing

Index testing
Testing for static bearing capacity

The scopes of work listed above are representative of common platforms and uniform soil types. The size of the platform and the soil’s heterogeneity over the platform’s footprint determine how many boreholes are needed. Additionally, one should account for potential adjustments to the footprint’s size or precise placement.

Once the design is underway, it’s also feasible that the platform’s type will be changed. The intended use of the proposed platform and the implications of risk analysis for potential subpar performance of the structure or the soil also influence the number of test sites.

The number of borings, (P)CPTs, and other in-situ tests are also based on the site’s ground variability as identified by preliminary geophysical investigations. Additional borings or (P)CPTs are especially required close to geophysical anomalies or close to discontinuities in the layers of the soil or rock.

Therefore, depending on size and soil variability, a GBS may require one or two deep boreholes in addition to shallow boreholes/CPTs. For a larger structure, more shallow boreholes are likely to be required. Depending on the size and soil variability, between one and four deep boreholes may be needed for piled construction.

For the measurement of horizontal soil resistance on piles and the design of mudmats, further CPTs or sample borings may be necessary. For axial pile design at sand sites utilizing existing CPT-based design approaches, near-continuous CPT profiles are necessary. It has been recognized that deeper penetration of piles leads to an increase in the axial capacity for piles, hence it’s important to characterize these deeper soils properly. For instance, one wouldn’t want to overlook clay bands, at the intended point elevation in a deep sand.

Conclusion

Site investigations for offshore platforms should ideally be carried out in stages. The number of in-situ testing may be limited in the early stages of a project to the data required to assess the viability of various construction forms. In this case, extra testing will be needed before moving on to the detailed design step.

The chance that the platform location will change due to changes in the field layout, the platform’s qualities, and the mooring leg properties should be taken into account throughout the site investigation. A more trustworthy evaluation of in-situ strength and stiffness parameters would include penetration tests (cone, T-bar, or ball), vane shear tests, and pressuremeter testing in addition to sample borings.

Soil Erosion and Control Measures

By

Ubani Obinna

-

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 39 — 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 21, 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

-

February 28, 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.

Kurda R., Silvestre J.D., de Brito J., and Ahmed H. A. (2018): Optimizing recycled concrete containing high volume of fly ash in terms of the embodied energy and chloride ion resistance. Journal of Cleaner Production 194:735–750 DOI: 10.1016/j.jclepro.2018.05.177

Li, L. and Chen, K. (2017): Quantitative assessment of carbon dioxide emissions in construction projects: A case study in Shenzhen. J. Clean. Prod. 2017(141):394–408.

Marinković S., Radonjanin V., Malesev M., and Lukic I., Life cycle environmental impact assessment of concrete. ed. by L.; Koukkari Em: Bragança, H.; Blok, R.; Gervásio, H.; Veljkovic, M.; Plewako, Z.; Landolfo, R.; Ungureanu, V.; Silva, L.S.; Haller, P. (eds.), Sustainability of constructions – Integrated approach to life-time structural engineering. COST Action C25 (Proceedings of seminar: Dresden, 2008. Addprint AG, Possendorf, Herstellung: 2008).

Paik I. and Na S. (2019): Comparison of Carbon Dioxide Emissions of the Ordinary Reinforced Concrete Slab and the Voided Slab System During the Construction Phase: A Case Study of a Residential Building in South Korea. Sustainability 2019, 11, 3571; doi:10.3390/su11133571

Paris J. M., Roessler J. G, Ferraro C. C., DeFord H. D., and Townsend T. G. (2016): A review of waste products utilized as supplements to Portland cement in concrete’, Journal of Cleaner Production, 121 (2016):1-18

Paya-Zaforteza, I., Yepes, V., Hospitaler, A., and González-Vidosa, F. (2009): CO₂-optimization of reinforced concrete frames by simulated annealing. Eng. Struct., 31(7):1501–1508.

Purnell P. (2013): The carbon footprint of reinforced concrete. Advances in Cement Research 25(6):362-368 DOI: 10.1680/adcr.13.00013

Rossi B., Marique A.-F., Reiter S. (2012): Life-cycle assessment of residential buildings in three different European locations, case study, Build. Environ. 51 (2012):402–407.

Skullestad J.L., Bohne R.A., Lohne J. (2016):High-rise timber buildings as a climate change Mitigation measure – a comparative LCA of structural system alternatives, Energy Procedia 96 (2016):112–123.

Stephan A., Jensen C.A. and Crawford R.H, (2017): Improving the life cycle energy performance of apartment units through façade design, Procedia Eng. 196 (2017):1003–1010.

Thiel C., Campion N., Landis A., Jones A., Schaefer L., Bilec M., (2013): A materials life cycle assessment of a net-zero energy building, Energies 6 (2) :1125–1141.

Villalba, P., Alcala, J., Yepes, V., and González-Vidosa, F. (2010): CO₂ optimization of reinforced concrete cantilever retaining walls. 2nd Intl. Conf. Eng. Optim., Technical Univ. of Lisbon, Lisbon, Portugal.

World Business Council for Sustainable Development—International Energy Agency (WBCSD-IEA). (2009): Cement Technology Roadmap 2009: Carbon emissions reductions up to 2050, Geneva, Switzerland.

Yeo D. and Gabbai R. D. (2011): Sustainable design of reinforced concrete structures through embodied energy optimization, Energy Build. 43(8):2028–2033.

Yeo D. and Potra F. A. (2015): Sustainable Design of Reinforced Concrete Structures through CO₂ Emission Optimization. J. Struct. Eng., 2015, 141(3): B4014002

Construction Measurement Criteria: Understanding the Benefits of Site Inspection

By

Ubani Obinna

-

February 25, 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