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 (fck = 30 MPa) and ultra high performance concrete (fck = 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 CO2 amount(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
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.
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/m3 of steel)
0.198
Rammed soil
0.024
Expanded polystyrene
3.29
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).
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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.
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.
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 (0.5lx × 0.5ly), 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 (Ṁx and Ṁy) as shown in the equations below.
The induced forces on the shear dowels in the x and y directions (Fx, Fy), 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 (Fs) 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.
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 theintroduction 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
In this article, Sky Marketing presents a creation of the Japanese architecture studio Nendo which has created a house and warehouse out of pieces of precast concrete sewers in central Japan.
The Prefab House with Concrete Tunnels
Architecture with prefabricated elements is nothing new. In fact, the architectural future – if not already – goes through this industrialized architecture to save on costs and build better.
We have seen how they transform freight containers into houses and play with their shapes, as well as how they build prefabricated exterior walls that are placed like a Tetris game or how large concrete pipes become micro-apartments and rooms, in short, a thousand ideas!
The imagination of some architects to innovate in prefabricated houses is causing really interesting architectural projects to appear.
Nendo architecture studio – Photos Daici Ano, Takumi Ota and Toru Shiomi
The famous Japanese architecture studio Nendo also presents us with a magnificent house built with precast concrete sewer tunnels. Called the Culvert Guesthouse, a construction between warehouse and residential housing that was built from four huge precast concrete tunnels that were stacked.
The unique building is located among the dense forests on the outskirts of the city of Miyota in Japan. In a quiet environment rich in nature where streams intertwine through a thick forest of red pines.
Nendo architecture studio – Photos Daici Ano, Takumi Ota and Toru Shiomi
Its elongated concrete forms were constructed from joining pieces of precast concrete culverts, commonly used to enclose utilities such as water and electricity underground.
As they are elements destined for infrastructures and the architectural project had other needs. The study used the prestressing method to achieve a more coherent stacking and tightness between pieces.
The prestressing method is a technique used in civil engineering structures, such as bridges, in which members are aligned and then tensioned with steel wires to connect the members. In this way, a smooth and seamless surface finish is achieved, obtaining a hermetic seal and greater durability.
A total of 63 square-shaped pieces of approximately 12 tons each were used in the prefabricated house. By connecting these pieces, a slender tunnel-like space is created with an internal dimension of approximately 2 x 2.3 meters.
According to the Nendo studio… “The space is not so architectural, but a project that combines civil engineering concepts with product design details.”
On the ground floor, the main space of the warehouse is located in a 40-meter-long tunnel that has large glass windows at each end for filing furniture, products, and works of art. Parallel to this space, there is another tunnel that houses the residential area that includes a kitchen, bathroom, and toilet.
Nendo architecture studio – Photos Daici Ano, Takumi Ota, and Toru Shiomi
Nendo architecture studio – Photos Daici Ano, Takumi Ota, and Toru Shiomi
Both ground floor tunnels are located in parallel. They are joined by a flat roof creating an interior space for the dining room. This is closed with large glass windows that occupy the entire wall on both sides.
The floor of the living room is finished with gravel hardened with a resin base, which facilitates the passage and avoids the glassy surface that would result from the poured resin. The windows were made without metal frames as far as possible, and the high-transparency glass is up to 10 meters in length.
Stacked over the two tunnels, two other perpendicular caissons form the top floor of the building. They contain a bedroom and a secondary archive with a study space.
The smallest details of the house are meticulously designed, from a bathtub carved into the ground, where the water is flush with the ground, suggesting a continuous and uninterrupted surface or the design of the door handle that integrates perfectly into the narrow tunnel opening.
All the concrete elements on the outside are painted white to give the warehouse and residential area a minimalist look. In addition to offering a contrast with the exterior landscape and its vigorous green.
Nendo architecture studio – Photos Daici Ano, Takumi Ota, and Toru Shiomi
In its interior spaces, the line of white colours is followed in all the rooms and the use of wooden furniture with simple and straight lines.
In the garden, there is a fifth concrete tube next to the main warehouse building to house additional living space that will be used in the future.
If you liked this article, share it! Moreover, you can also read about Lahore Smart city, a real estate jewel in Pakistan.
In a practical medium- to long-span structures, post-tensioned slabs are economically competitive with reinforced concrete slabs and make up a sizable share of all prestressed concrete construction. The numerous drawbacks of reinforced concrete slabs (especially for long-span structures) are eliminated by prestressing.
In post-tensioned concrete construction, fresh concrete is cast around hollow ducts that are fitted to any desired profile. Normally, the steel tendons are unstressed in the ducts during the concrete pour. Nevertheless, they can also be threaded through the ducts at a later date. The tendons are tensioned (stressed) after the concrete reaches the desired strength at a later date. Tendons can either be stressed from both ends or from one end while the other end is anchored. At each stressed end, the tendons are then anchored.
After the tendons are anchored, the prestress is maintained by bearing the end anchorage plates onto the concrete, which places the concrete in compression during the stressing operation. Every time the cable’s direction changes, the post-tensioned tendons place a transverse force on the member.
Figure 1: Construction of post-tensioned slab
Post-Tensioned Slabs
In post-tensioned slabs, deflection, which is usually the controlling design consideration for concrete floors is controlled better. With an appropriate selection of prestressing/post-tensioning solutions, a designer is able to minimize or even completely eliminate deflection. As a result, thinner slab systems are possible, which can increase headroom or decrease floor-to-floor heights.
Additionally, prestress prevents cracking and can be utilized to create flooring that is watertight and devoid of cracks. Steel reinforcement layouts and fixes for prestressed slabs are often fewer and less complicated. As a result, pouring concrete and fixing steel reinforcements are simpler and faster. Additionally, prestressing reduces formwork costs and stripping times while enhancing punching shear. Conversely, prestressing frequently results in substantial axial shortening of slabs, necessitating special attention to the details of movement joints.
Post-tensioning is usually done on prestressed slabs using draped tendons. Flat-ducted tendons with five or fewer strands in a flat sheath and fan-shaped anchorages are frequently used, as shown in Figure 2. The use of small, portable hydraulic jacks allows for the one-at-a-time stressing of individual strands. The flat ducts allow for maximal tendon eccentricity and drape while being structurally effective. In order to provide a bond between the steel and the concrete, these ducts are usually grouted after stressing.
Figure 2: Details of typical flat-ducted tendons and anchorages (Gilbert et al, 2017)
Pre-stressed concrete slabs are typically thin in proportion to their span. A concrete slab may experience a high magnitude of deflections at full load or show excessive camber after transfer if it is too thin. The serviceability criteria for the member often dictate the initial choice of slab thickness.
The decision on the thickness of a slab is usually made using prior experience or suggested maximum span-to-depth ratios. Note that the initial choice of slab thickness, while useful as a starting point for design, does not always guarantee that serviceability requirements will be met.
Unbonded tendons are frequently used in a majority of post-tensioned slabs simply because they lower the cost of construction. Post-tensioned slabs provide a number of important advantages over reinforced concrete slabs, such as:
• Longer spans with fewer columns leading to flexibility in the positioning of partitions. • Thinner slabs lead to saving in construction costs and reduced height of the building. • Especially in the case of car parks, the virtually crack-free slabs are a great advantage to limit damage due to seepage of water with de-icing salts from melting snow.
Balanced Load Stage for Two-Way Slabs
Two-way panels bend into dish-shaped surfaces when subjected to transverse loads, as depicted in Figure 3. Because the slab is curved in both major directions, bending moments exist in each of those directions. Additionally, twisting moments that develop in the slab at all points other than the lines of symmetry resist a portion of the applied load.
Figure 3: Deformation of interior two-way slab panels. (a) Edge-supported slab. (b) Flat slab. (Gilbert et al, 2017)
Each prestressing tendon resists a portion of the applied load, and they are typically arranged in two directions parallel to the panel edges. The transverse force that is applied to the slab by the tendons in one direction adds to (or deducts from) the transverse load that is applied in the opposite direction by the tendons. The amount of the load to be borne by tendons in each direction for edge-supported slabs is more or less arbitrary; the only strict condition is that statics be met.
For flat slabs, tendons must carry the entire load from column line to column line in both directions. The concept of using the transverse forces produced by the draped tendons’ curvature to balance a portion of the applied load is advantageous from the perspective of managing deflections. Load balancing not only offers the foundation for creating a proper tendon profile but also makes it possible to calculate the prestressing force necessary to achieve zero deflection in a slab panel under the chosen balanced load.
It is important to know that the slab is essentially flat (has no curvature) at the balanced load and is only affected by the prestressing forces applied at the anchorages. Only uniform compression (P/A) in the directions of the orthogonal tendons is applied to a slab of uniform thickness. When the condition of the slab under the balanced load is confidently known, any suitable method can be used to determine the deflection caused by the unbalanced portion of the load.
Since only the unbalanced portion of the total service load must be taken into account and, unlike reinforced concrete slabs, prestressed slabs frequently do not crack at service loads, the calculation of the deflection of a prestressed slab is typically more accurate than that of a conventionally reinforced slab.
The external load that needs to be balanced typically makes up a significant amount of the sustained or permanent service load in order to minimize deflection issues. The sustained concrete stress (P/A), if all the permanent loads are balanced, is constant throughout the slab depth. There is little long-term load-dependent curvature or bending due to uniform creep strain caused by uniform compressive stress distribution.
Of course, bonded reinforcement limits creep and shrinkage and, if the steel is eccentric to the slab centroid, results in a change in curvature with time. Prestressed slabs typically contain a limited amount of bonded steel, therefore the time-dependent curve this restraint produces rarely results in appreciable deflection.
Potential serviceability issues may be indicated by the average concrete compressive stress after all losses. The prestress may not be enough to avoid or control cracking brought on by shrinkage, temperature changes, and unbalanced loads if P/A is too low. The average concrete compressive stress after all losses is subject to minimum limits that are outlined in some standards of practice. Prestressing levels for a two-way slab are typically in the range P/A = 1.2 – 2.6 MPa using flat-ducted tendons with four or more strands.
Axial deformation of the slab may be significant and cause distress in the supporting structure if the average prestress is high. Regardless of the typical concrete stress, the rest of the structure must be able to withstand and accommodate the shortening of the slab, but when P/A is high, the issue is made worse (Gilbert et al, 2017). It could be essential to use movement joints to separate the slab from rigid supports.
In particular, for slabs containing less than minimum quantities of conventional tensile reinforcement (i.e. less than around 0.15% of the slab’s cross-sectional area), the spacing of tendons in at least one direction shall not exceed the smaller of eight times the slab thickness and 1.5 m.
General Approach to the Design of Post-Tensioned Slab
After making an initial selection of the slab thickness, the second step in slab design is to determine the amount and distribution of prestress.
Usually, load balancing is employed to achieve this. The transverse stresses induced on a slab by the draped tendons in each direction balance out a portion of the load. The slab stays flat (without curvature) under the balanced load and is only susceptible to the resulting longitudinal compressive P/A stresses.
The remaining imbalanced load is taken into account when calculating service load behaviour, especially when estimating load-dependent deflections and determining how much cracking has occurred and how much crack control has been applied.
The complete factored design load must be taken into account at ultimate limit state conditions, where the slab behaviour is non-linear and superposition is no longer valid. The factored design moments and shears at each critical section must be computed and compared with the design strength of the section. Slabs are typically fairly ductile, and as the slab’s collapse load approaches, moments redistribute. Secondary moments can typically be disregarded in these circumstances.
Load Balancing
Figure 4: Interior edge-supported slab panel (Gilbert et al, 2017)
Consider the interior panel of the two-way edge-supported slab shown in Figure 4. The panel has parabolic tendons in both the x and y axes and is supported by walls or beams on all sides. The upward forces per unit area that the tendons in each direction exert if the cables are uniformly spaced in each direction are;
wpx = 8Pxzd.x/lx2 ——— (1)
and
wpy = 8Pyzd.y/ly2 ——— (2)
where Px and Py are the prestressing forces in each direction per unit width, and zd.x and zd.y are the cable drapes in each direction.
The uniformly distributed downward load to be balanced per unit area wbal is calculated as:
wbal = wpx + wpy ——— (3)
In practice, perfect load balancing is not possible, since external loads are rarely perfectly uniformly distributed. However, for practical purposes, adequate load balancing can be achieved (Gilbert et al, 2017). Any combination of wpx and wpy that satisfies Equation (3) can be used to make up the balanced load. The smallest quantity of prestressing steel will result if all the loads are balanced by cables in the short-span direction, i.e. wbal = wpy.
Figure 5: Post-tensioned slab construction
However, under unbalanced loads, serviceability problems in the form of unsightly cracking may result. It is often preferable to distribute the prestress in much the same way as the load is distributed to the supports in an elastic slab, i.e. more prestress in the short-span direction than in the long-span direction. The balanced load resisted by tendons in the short direction may be estimated by:
wpy = [lx4/(δly4 + lx4)] × wbal ——— (4)
where δ depends on the support conditions and is given by:
δ = 1.0 for 4 edges continuous or discontinuous = 1.0 for 2 adjacent edges discontinuous = 2.0 for 1 long-edge discontinuous = 0.5 for 1 short edge discontinuous = 2.5 for 2 long + 1 short edge discontinuous = 0.4 for 2 short + 1 long edge discontinuous = 5.0 for 2 long edges discontinuous = 0.2 for 2 short edges discontinuous
Equation (4) is the expression obtained for that portion of any external load which is carried in the short-span direction if twisting moments are ignored and if the mid-span deflections of the two orthogonal unit-wide strips through the slab centre are equated.
With wpx and wpy selected, the prestressing force per unit width in each direction is calculated using Equations (1) and (2) as:
Px = wpxlx2/8zd.x ——– (5)
and
Py = wpyly2/8zd.y ——– (6)
Equilibrium dictates that the downward forces per unit length exerted over each edge support by the reversal of cable curvature (as shown in Figure 4) are:
wpxlx (kN/m) carried by the short-span supporting beams or walls wpyly (kN/m) carried by the long-span supporting beams or walls
The total force imposed by the slab tendons that must be carried by the edge beams is, therefore:
wpxlxly + wpylylx = wballxly ——– (7)
and this is equal to the total upward force exerted by the slab cables.
Therefore, for this two-way slab system, to carry the balanced load to the supporting columns, resistance must be provided for twice the total load to be balanced (i.e. the slab tendons must resist wballxly and the supporting beams must resist wballxly). This requirement is true for all two-way floor systems, irrespective of construction type or material.
At the balanced load condition, when the transverse forces imposed by the cables exactly balance the applied external loads, the slab is subjected only to the compressive stresses imposed by the longitudinal prestress in each direction, i.e. σx = Px/h and σy = Py/h, where h is the slab thickness.
Post-Tensioned Slab Design Example
It is required to design an exterior panel of a 175 mm thick two-way floor slab of a commercial building. The rectangular panel is supported by monolithic beams at all edges and discontinuous on two adjacent sides. It supports an additional dead load of 2.5 kN/m2 apart from its self-weight and a live load of 4 kN/m2. The slab is post-tensioned in both directions using the draped parabolic cable profiles shown below. The level of prestressing required to balance a uniformly distributed load of 4 kN/m2 is required. Relevant material properties are fck = 40 MPa, fctm = 3.5 MPa, Ecm = 35,000 MPa, fpk = 1,860 MPa and Ep = 195,000 MPa.
Self weight of the slab = 25 × 0.175 = 4.375 kN/m2 Superimposed dead load = 2.5 kN/m2 Total dead load = 6.875 kN/m2 Live load = 4 kN/m2 The total load on the slab = 6.875 + 4 = 10.875 kN/m2
Solution
Load Balancing
Load balancing: Flat-ducted tendons containing four 12.5 mm strands are to be used with a duct size of 75 mm × 19 mm.
With a 25 mm concrete cover to the duct, the maximum depth to the centre of gravity of the short-span tendons is:
dy = 175 − 25 − (19 − 7) = 138 mm (refer to the Figure above)
Eccentricity at the mid-span (short span) ex = 175/2 – 38 = 50 mm (say) Eccentricity at the mid-span (long span) ey = 175/2 – 38 – 13 = 37 mm (say)
The cable drape in the short-span direction is, therefore: zd.y = (50 + 0)/2 + 50 = 75 mm
The depth dx of the long-span tendons at mid-span is less than dy by the thickness of the duct running in the short-span direction, i.e. dx = 143 − 19 = 124 mm. The cable drape in the long-span direction is given by:
zd.x = (50 + 0)/2 + 37 = 62 mm
Self-weight of the slab = 25 × 0.175 = 4.375 kN/m2 If 40% of the live load is assumed to be sustained, then the total sustained load is: wsus = 4.375 + 2.5 + (0.4 × 4.0) = 8.475 kN/m2
In this example, the effective prestress in the tendons in both directions balances an external load consisting of the self-weight and 10% of the superimposed dead load;
wbal = 4.375 + 0.1(2.5) = 4.625 kN/m2. The transverse load exerted by the tendons in the short-span direction is determined using Equation 4:
wpy = [lx4/(δly4 + lx4)] × wbal Where δ = 1.0 for two adjacent edges discontinuous.
wpy = 94/(1.0 × 84 + 94) × 4.625 = 2.85 kN/m2
and the transverse load imposed by the tendons in the long-span direction is calculated; wpx = wbal − wpy = 4.625 − 2.85 = 1.775 kN/m2
The effective prestress in each direction is obtained from Equations (1) and (2): Pi = wpili2/8zd.i
Maximum force per tensioning = 137 kN Assuming a 25% loss over the long term, the force per strand = 0.75 × 137 = 102.75 kN For four strands = 4 × 102.75 = 411 kN
The number of strands required: x-direction: nx = 290/411 = 0.7 (say 1 duct/m; comprising of 4 strands per duct), Prestress provided = 102.75 × 4 = 411 kN/m Load balanced, qx = (8 × 411 × 0.062)/92 = 2.516 kN/m2
y-direction: ny = 304/411 = 0.73, say (say 1 duct/m; comprising of 4 strands per duct) Prestress provided = 102.75 × 4 = 411 kN/m Load balanced, qy = (8 × 411 × 0.075)/82 = 3.853 kN/m2
Both the time-dependent losses and friction losses must be estimated in order to determine the jacking forces and cable spacing in each direction. Assuming a 25% loss is used to account for all the losses;
Using four 12.5 mm strands/tendon, Ap = 372 mm2/tendon and the maximum jacking force/tendon is 137 × 4 = 548 kN, and the required tendon spacing in each direction (rounded down to the nearest 10 mm) is therefore:
sy = (1000 × 548)/405.3 = 1352 mm and sx = (1000 × 548)/386.7 = 1417 mm
However, it is recommended that the spacing of tendons should not exceed 8h in the span. Therefore, the maximum spacing of the tendons can be taken as = 8 × 175 = 1400 mm.
We will select a tendon spacing of 1000 mm in each direction. This simply means that the tendons in the y-direction will balance slightly more load than previously assumed. With one tendon in each direction per metre width, the revised prestressing forces at the jack per metre width are Py = Px = 548 kN/m and at mid-span, after all losses, are:
Under this unbalanced load, the maximum moment in the slab can be investigated;
The aspect ratio of slab = ly/lx = 9/8 = 1.125
The negative moment at the continuous edge (short span) = 0.056 × 3.306 × 82 = 11.84 kNm/m The positive moment at midspan (short span) = 0.042 × 3.306 × 82 = 8.886 kNm/m The negative moment at the continuous edge (long span) = 0.045 × 3.306 × 82 = 9.521 kNm/m The positive moment at midspan (short span) = 0.034 × 3.306 × 82 = 7.193 kNm/m
Obviously, in this case, the maximum moment is 11.84 kNm/m
Check for cracking
The concrete stresses in the top and bottom fibres caused by the maximum moment after all losses are:
where A is the area of the gross cross-section per metre width (A = bh = 175 × 103 mm2/m) and Z is the section modulus per metre width (Z = I/y = 5.104 × 106 mm3/m). Both the top and bottom stresses are relatively low. Even though the moment used in these calculations is an average and not a peak moment, if cracking does occur, it will be localised and the resulting loss of stiffness will be small. Deflection calculations may be based on the properties of the uncracked cross-section.
Maximum total deflection
The deflection at the mid-panel of the slab can be estimated using the so-called crossing beam analogy, in which the deflections of a pair of orthogonal beams (slab strips) through the centre of the panel are equated. The fraction of the unbalanced load carried by the strip in the short-span direction is given by an equation similar to Equation (4).
and with the deflection coefficient β taken as 3.5/384, the corresponding short-term deflection at mid-span of this 1 m wide slab strip in the short-span direction through the mid-panel (assuming the variable live load is removed from the adjacent slab panel) is approximated by:
The sustained portion of the unbalanced load on the slab strip is: [lx4/(δly4 + lx4)] × (wsw + wG + y2wQ – wbal) = 0.61 × [4.375 + 2.5 + (0.4 × 4.0) – 6.369] = 1.28 kPa
and the corresponding short-term deflection is:
vsus.0 = 1.28/2.035 × v0 = 3.056 mm
Assuming a final creep coefficient φ(∞,t0) = 2.5 and conservatively ignoring the restraint provided by any bonded reinforcement, the creep-induced deflection may be estimated using:
vcc = 2.5 × 3.056 = 7.64 mm
The final shrinkage strain is assumed to be εcs = 0.0005. The shrinkage curvature κcs is non-zero wherever the eccentricity of the steel area is non-zero and varies along the span as the eccentricity of the draped tendons varies.
A simple and very approximate estimate of the average final shrinkage curvature is; κcs = 0.3εcs/h = (0.3 × 0.0005)/175 = 0.857 × 10-6 mm-1
The average deflection of the slab strip due to shrinkage is given by Equation 12.8: vcs = 0.090 × 0.857 × 10-6 × 80002 = 4.936 mm
The maximum total deflection of the slab strip is therefore: vtot = v0 + vcc + vcs = 4.86 + 7.64 + 4.936 = 17.436 mm = span/456 < Span/250
Check flexural strength
It is necessary to check the design strength of the slab. As previously calculated, the dead load is 4.375 + 2.5 = 6.875 kN/m2 and the live load is 4.0 kN/m2. The factored design load is:
wEd = 1.35(6.875) + 1.5(4) = 15.28 kN/m2
The negative moment at the continuous edge (short span) = 0.056 × 15.28 × 82 = 54.763 kNm/m The positive moment at midspan (short span) = 0.042 × 15.28 × 82 = 41.07 kNm/m The negative moment at the continuous edge (long span) = 0.045 × 15.28 × 82 = 44kNm/m The positive moment at midspan (short span) = 0.034 × 15.28 × 82 = 33.25 kNm/m
A safe lower bound solution to the problem of adequate strength is obtained if the design strength of the slab at this section exceeds the design moment. The resistance per metre width of the 175 mm thick slab containing tendons at 1000 mm centres (i.e. Ap = 372 mm2/m) at an effective depth of 138 mm can be obtained by considering the figure above.
The stress in the tendon caused by the effective prestressing force Pm,t = 411 kN is:
σpm,t = Pm,t/Ap = (411 × 103)/372 = 1105 MPa
EN1992-1-1 permits the design stress in the tendon at the strength limit state to be taken as: σpud = σpm,t + 100 = 1205 MPa and therefore the tensile force in the steel is Fptd = 448.26 kN (= Fcd).
x = Fptd/ ηfcdλb = (448.26 × 103 )/(1.0 × 26.67 × 0.8 × 1000) = 21 mm
Such an analysis indicates that the cross-section is ductile, with the depth to the neutral axis of x1 = 21 mm (or 0.152d).
This shows that no additional reinforcement is required. However, 12 mm diameter non-prestressed reinforcing bars in the y-direction at 450 mm centres can be provided all over the slab.
Reference
Gilbert R.I, Mickleborough N.C. and Ranzi G. (2017): Design of Prestressed Concrete to Eurocode 2 (2nd Edition). CRC Press, Taylor and Francis,
Biocementation is an environmentally friendly, cost-effective alternative to imported granular fills, concrete, costly hauling of materials or export to a landfill. In-service performance of soils, foundations, and other earthen structures and their required maintenance is highly dependent on methods of stabilisation, ranging from expensive mechanical stabilisation to chemical processes.
As such, many alternative materials originating from bio-based sources are being explored as potential stabilising additives to improve weak subgrade soils (i.e., dispersive soils, erodible and collapsible soil, and soft or expansive clays). In order to prevent geotechnical failures induced by water infiltration and/or erosion, some relevant alternatives include the use of bio-derived enzymes, microorganisms, and polymeric additives.
Bio-based Innovations in Geotechnical Engineering
For sustainable road infrastructure development, various microorganisms and other bio-based materials, including secondary metabolites, enzymatic materials, and polymeric materials, have been studied as viable substitutes for traditional chemical stabilisers (Ikeagwuani and Nwonu, 2019). Thus, increasing the adoption of these bio-based materials and technologies in the building industry requires a deeper understanding.
The use of bacteria in bio-based admixtures produced through industrial fermentation processes is gaining popularity because of a number of positive aspects, including their quick biosynthesis rates, high yields of desired products, quick growth on cheap culture media that is readily available in large quantities, openness to genetic manipulation, and lack of pathogenic traits (Pei et al., 2015). By the outcomes of the microbial treatment of soil, at least eight different types of biotechnological processes associated with construction can be categorized as shown in Figure 1 (Ivanov et al., 2015).
Figure 1: Construction-related microbial biotechnology (Ramdas et al, 2021)
Of these, two biological processes (as far as the fundamental characteristics of research techniques) stand out;
(a)bioclogging, the production of pore-filling materials through biological means, to significantly reduce the hydraulic conductivity of soil or porous matrix and,
(b) biocementation, the generation of particle-binding materials through microbial processes in situ so that the shear strength of soil can be increased.
This is further reinforced by DeJong et al. (2010) stating that these methods allow a holistic view and meaningful characteristics in the area of soil stabilisation. The micro-organisms best suited for these techniques (i.e., soil bio-clogging, biocementation, and bio-aggregation) are facultative anaerobic and microaerophilic bacteria, although anaerobic fermenting bacteria have been suggested (Ivanov et al., 2015).
Microbial Biocementation
One of the most researched techniques is microbial-induced calcium carbonate precipitation (MICCP), also known as microbial biocement, which occurs when biological action increases the pH of soils to create supersaturated conditions (Oyediran and Ayeni, 2020).
Figure 2: The process of bio-cementation
Recently, biocementation has gained attention as a practical method for soil improvement, notably because of its superior performance and environmental sustainability. For instance, an essential approach called microbially induced calcite precipitation (MICP) uses bacteria to create calcium carbonate precipitates and introduce “biocementation” between the sand grains (Cheng et al., 2013).
For instance, the alkalophilic soil bacteria Bacillus pasteuri uses the highly active urease enzyme to consume urea and break it down into carbon dioxide (CO2) and ammonia (NH3). In the presence of water, NH3 is changed into NH4+, while CO2 equilibrates with carbonic acid, carbonate, and bicarbonate ions in a pH-dependent manner. The alkaline environment and carbonate needed for the reaction with Ca2+ and precipitation of calcite (CaCO3) are provided by the rise in pH, as schematically depicted in Fig. 2.
Figure 3. Biocementation is two-step process a: Phase I, bacteria ingest nutrients such as, sugar, nitrogen, & proteins with the growth of the bacterial population and enzymes produced from the bacterial species they hydrolyse urea components in the presence of water to form ammonium and carbonate ions which leads to b: Phase II, again the addition of nutrients such as calcium chloride, in the presence of calcium ions and nucleation sites on the soil particles, the carbonate ions react spontaneously with the calcium ions to form calcium carbonate, c-d: the calcite precipitates/the cementing agent (produced by the bacteria) used to bind the soil particles together to increase strength and stiffness of the soil
Another significant but understudied example of biocementation and bioengineered soil structures is termite mounds (anthills). Ecologists, entomologists, architects, and soil chemists are very interested in these above-ground mounds, particularly those made by the termites known as Macrotermitinae that grow fungi.
According to Kandasami et al (2016), a critical component for termite societies with millions of individual termites is the stability of termite mounds, which are bioengineered granular ensembles. Termites are skilled engineers, as shown by an experimental investigation on the mechanobiology of mounds and mound soil of the fungus-growing termite Odontotermes obesus (Rambur). The physical and mechanical characteristics of mound soil were notably different from those of the nearby or “control” soil. Yet, the clay mineralogy of the mound and surrounding soils was the same.
To build anthills, termites combine the finer soil fraction with their secretions to form boluses in the presence of water, thereby significantly altering the soil. In order to efficiently mould the soil, they control the amount of water close to the soil’s plastic limit when producing these boluses (Kandasami et al, 2016).
As a result of these processes, the strength of the soil can be increased tenfold as a result of the cementation caused by termites utilizing their excretions and/or secretions, which may not have been possible otherwise. The modification of the soil by termites reduces the likelihood of erosion and collapse of the mounds.
Figure 4: Termite mounds are a product of biocementation
Studies have also shown that termites effectively bonded foreign objects, indicating that they have a variety of cementation skills. When compared to reconstituted soil, a slope stability analysis with intact mound soil showed a much higher safety factor for the mound.
Applications of Biocementation in Soil Improvement
According to Cheng et al (2013), the MICCP method is most efficient at a particle contact right as cementation begins, and it becomes less effective as cementation spreads outward around a particle contact. Due to the increased inter-particle interactions, reallocating the CaCO3 crystals to two contact places (connection points) as opposed to one would be more beneficial. As a function of the particle radius squared, the contact stress also decreases simultaneously. As a result, smaller particles have two complementary advantages: improved MICCP and reduced particle contact stresses.
Ng et al. (2012) used the B. megaterium MICCP procedure to evaluate shear strength, and they observed that the ratio of treated to untreated shear strength rose from 1.40 to 2.64. Van Paassen et al. (2010) reported a minimum of 300 kPa compressive strength. Similar results were published by Nafisi (2019), who observed that the soil strength varies between 210 kPa and 710 kPa depending on the type of sand and carbonate mass.
Van Paassen et al. (2010) also observed a 60% reduction in the permeability of treated soils at less than 100 kg/m3 CaCO3 precipitation; however, as solution concentration increased, calcite crystal clogged pore spaces and reduced permeability between 50 and 99% using 1 M cementation solution.
In triaxial testing, Cheng et al. (2013) reported that the mechanical behaviour of the bio-cemented sand increased the effective shear strength parameters (i.e., cohesion, angle of internal friction) with an increase in CaCO3 concentration at all saturation levels. By increasing the frictional angle at a lower saturation level, the precipitated crystals improved the cohesion of coarse sand. Under the same saturation level and similar CaCO3 content as fine sand, coarse sand showed a greater friction angle than fine sand.
Here is how the researchers addressed this: The primary consequences of tiny soil particles include:
MICCP method can be improved by;
(a) adding additional inter-particle contact points and (b) lowering the stress per particle contact.
An exponential relationship between the UCS value and the CaCO3 content of the treated soils, according to van Paassen et al. (2010), demonstrates that even though the same amount of CaCO3 precipitated, the mechanical response can vary depending on the CaCO3 precipitation/crystals’ mechanism of action. Additionally, the process requires significant urease activity in the microorganisms, and the mode of action (MICCP) may be strain-dependent (van Paassen et al., 2010).
Several documented research works show several Bacillus spp. have the ability to precipitate calcium carbonate with structural properties, but a limited selection of micro-organisms highlights the triaxial and California Bearing Ratio (CBR) tests for potential road/pavement applications (Table 1).
Preferable microbes come from Bacillacae family
Key micro-organisms showing mechanical improvement of construction material
B. flexus
Laboratory strength tests showed an increase in compressive (>40%), flexural (>30%) & split tensile (>10%) strength. X-ray powder diffraction (XRD) shows ureolytic properties of the organism (Roa et al., 2015).
B. sphaericus
Experimental results indicate polyurethane immobilised bacteria-induced higher strength regain (60%) and lower water permeability coefficient (10 –11 m/s) (Wang et al., 2012, 2014)
Sporosarcina pasteurii formerly known as B. pasteurii
Biocalcification mechanism showing potential construction applications such as dust control and strengthening of brick masonry (Sarda et al., 2009; Meyer et al., 2011). Triaxial tests indicate that the treated specimens exhibit a non-collapse strain softening shear behaviour, with a higher initial shear stiffness and ultimate shear capacity than untreated specimens. Samples treated showed a great increment in unsoaked and soaked CBR gained more strength when soaked for hours compared to unsoaked (Oyediran and Ayeni 2020). CBR obtained in a study showed poorer outcome from CBR index treated (0.6) compared to untreated samples (2.5). These investigations are of interest to improve soil stability, to build roads and paths, and to restore monuments (Morales et al., 2019).
B. licheniformi
Calcium carbonate precipitation from the strain has the potential for sealing cement-based materials (Vahabi et al., 2015).
B. subtilis
Strength performance of microbial concrete mechanism on the principle of calcite mineral precipitation by alkali-resistant spore-forming bacteria. Its response showed autonomous improvement in the self-healing process due to microcrack formation (Rao et al., 2017).
Table 1:Bacillus species related to microbial bio-cement
According to Mujah et al (2016), the factors affecting the bio-cementation of soils are;
Temperature
pH level
Urease activity/bacterial concentration
Degree of saturation
The concentration of cementation solution
Limitations of MICP for soil biocementation
The fact that MICP can only be applied to particular soil sizes is one of its key disadvantages. The approach is currently only effective for treating sands with particle sizes comparable to 0.5-3 mm, according to a review by Fragaszy et al (2011). Hence, expanding the use of MICP for improving and stabilising fine-grained soils like silt and clay would be a huge challenge.
Furthermore, some researchers have conducted field trials and upscaled studies to test the viability of MICP for in-situ bio-cementation deployment in construction. For instance, van Paassen (2009) conducted an experiment to treat 1-100 m3 of sand in the lab and observed that, after MICP treatment, the strength of bio-cemented sand was greatly increased; however, distinct spatial variability was observed. The most important factor that still needs further focus is treatment homogeneity or the homogeneous distribution of CaCO3 across the treated soil matrix from top to bottom.
Conclusion
The engineering, mechanical, and physical properties of biocemented soils can potentially be enhanced by MICP treatment. The intended uses of MICP include slope stabilization, settlement reduction, erosion management, soil self-healing, and prevention of liquefaction.
The benefits of MICP for soil improvement include cheaper costs compared to chemical grouting and other man-made materials, treatment dependability, and an overarching idea that encourages sustainability in tandem with future needs. Before the MICP method is directly applied in the field, further study should be concentrated on optimizing it at both the micro and macro levels.
Investigations must be done into specific issues such as medium flow and transport via heterogeneous media, treatment durability, mixing procedure permanence, and particle-level stratigraphic mapping. Further potential directions in MICP technology include the use of seawater as a salt alternative as well as self-healing in terms of pre- and post-shearing of biotreated soils after significant earthquakes and related aftershocks.
References
Cheng L, Cord-Ruwisch R, Shahin MA. (2013): Cementation of sand soil by microbially induced calcite precipitation at various degrees of saturation. Can Geotech J 2013;50:81–90. https://doi.org/10.1139/cgj-2012-0023
Dejong J. T, Mortensen BM, Martinez BC, Nelson DC. (2010): Bio-mediated soil improvement. Ecol Eng 2010;36:197–210. https://doi.org/10.1016/j.ecoleng.2008.12.029.
Fragaszy RJ, Santamarina JC, Amekudzi et al 2011. Sustainable development and energy geotechnology—potential roles for geotechnical engineering. KSCE J Civ Eng 15(4):611–621.
Ikeagwuani CC, Nwonu DC (2019): Emerging trends in expansive soil stabilisation: A review. J Rock Mech Geotech Eng 2019;11:423–40. https://doi.org/10.1016/j. jrmge.2018.08.013.
Ivanov V, Chu J, Stabnikov V. (2015): Basics of construction microbial biotechnology. In: Pacheco Torgal F, Labrincha JA, Diamanti MV, Yu CP, Lee HK, editors. Biotechnologies and biomimetics for civil engineering biotechnologies and biomimetics for civil engineering. Switzerland: Springer; 2015. p. 21–56.
Kandasami RK, Borges RM, and Murthy TG (2016): Effect of biocementation on the strength and stability of termite mounds. Environmental Geotechnics April 2016 Issue EG2 Pages 99–113 http://dx.doi.org/10.1680/jenge.15.00036
Meyer F, Bang S, Min S, Stetler L, Bang S. (2011): Microbiologically-induced soil stabilization: application of Sporosarcina pasteurii for fugitive dust control. In: Geo-frontiers 2011 Advances in geotechnical engineering; 2011. p. 4002–11.
Morales L, Garzon ´ E, Romero E, Sanchez-Soto PJ. (2019): Microbiological induced carbonate (CaCO3) precipitation using clay phyllites to replace chemical stabilizers (cement or lime). Appl Clay Sci 2019;174:15–28. https://doi.org/10.1016/j.clay.2019.03.018
Mujah D., Mohamed A. Shahin M.A, and Cheng L. (2016): State-of-the-Art Review of Biocementation by Microbially Induced Calcite Precipitation (MICP) for Soil Stabilization. GEOMICROBIOLOGY JOURNAL http://dx.doi.org/10.1080/01490451.2016.1225866
Ng WS, Lee ML, Hii SL. 2012. An overview of the factors affecting microbial-induced calcite precipitation and its potential application in soil improvement. World Acad Sci Eng Technol 6(2):683–689.
Oyediran IA, Ayeni OO. (2020): Comparative effect of microbial induced calcite precipitate, cement and rice husk ash on the geotechnical properties of soils. SN Appl Sci 2020;2: 1–12. https://doi.org/10.1007/s42452-020-2956-0
Ramdas VM, Mandree P, Mgangira M, Mukaratirwa S, Lalloo R, Ramchuran S. (2020): Establishing miniaturised structural testing techniques to enable high-throughput screening of microorganisms and microbial components for unpaved road stabilisation application. J Adv Res 2020;21:151–9. https://doi.org/10.1016/j. jare.2019.11.002.
Rao MVS, Reddy VS, Sasikala C. (2017): Performance of microbial concrete developed using Bacillus subtilus JC3. J Insti Eng (India): Series A. 2017; 98(501–510). https://doi. org/10.1007/s40030-017-0227-x.
Roa R, Kumar U, Vokunnaya S, Paul P, Orestis I. (2015): Effect of Bacillus flexus in healing concrete structures. IJIRSET 2015;4:7273–80. https://doi.org/10.15680/ IJIRSET.2015.0408106
Sarda D, Choonia HS, Sarode D, Lele S. (2009): Biocalcification by bacillus pasteurii urease: A novel application. J Ind Microbiol Biotechnol 2009;36:1111–5. https://doi.org/ 10.1007/s10295-009-0581-4.
Vahabi A, Ramezanianpour AA, Sharafi H, Zahiri HS, Vali H, Noghabi KA. (2015): Calcium carbonate precipitation by strain Bacillus licheniformis AK01, newly isolated from loamy soil: A promising alternative for sealing cement-based materials. J Basic Microbiol 2015;55:105–11. https://doi.org/10.1002/jobm.201300560
Van Paassen LA, Ghose R, van der Linden TJ, van der Star WR, van Loosdrecht MC (2010):. Quantifying biomediated ground improvement by ureolysis: large-scale biogrout experiment. J Geotech Geoenviron 2010;136:1721–8.
van Paassen L. 2009. Biogrout: ground improvement by microbially induced carbonate precipitation. PhD Thesis, Delft University of Technology, Delft, Netherlands, p203.
Wang J, Soens H, Verstraete W, De Belie N. (2014): Self-healing concrete by use of microencapsulated bacterial spores. Cem Concr Res 2014;56:139–52. https://doi. org/10.1016/j.cemconres.2013.11.009.
Wang J, Van Tittelboom K, De Belie N, Verstraete W. (2012): Use of silica gel or polyurethane immobilized bacteria for self-healing concrete. Constr Build Mater 2012;26:532–40. https://doi.org/10.1016/j.conbuildmat.2011.06.054.
Lattice truss structural systems have been employed in constructing railway and highway bridges with great success for so many years. The design of truss bridges involves the analysis of the structure to obtain the internal forces due to moving traffic and permanent loads (self-weight), selection of adequate steel members, design of the connections, and check for fatigue. The availability of numerous commercial design software has made the analysis and design of 3D truss bridges easier than it was in the past.
The Warren truss, the Modified Warren truss, and the Pratt truss are the three major truss configurations in use today, and they can all be employed as an underslung truss, a semi-through truss, or a through truss bridge.
In an underslung truss, the live loading caused by the passage of automobiles or trains is carried directly by the top chord. In situations where the depth of construction or clearance under the bridge is not critical, underslung trusses can be conveniently used.
In semi-through trusses, vehicles travel on the bottom chord of the truss, but the transient live load projects above the top chord members due to the height of the vehicles relative to the top chord of the truss. As a result, the top chords in semi-through trusses cannot be braced laterally, and these chord elements must rely on U-frame action for lateral stability. However in a through truss bridge, vehicles travel through the centre of the bridge on the bottom chord, and the space between the live load and the top chords is sufficient that the top chord members can be braced laterally. Through truss bridge appears to be the most common type of truss bridge.
Figure 1: Major types of truss bridges (Parke and Harding, 2008)
Internal Forces in Truss Bridges
The members of a truss bridge will predominantly carry axial tension or axial compression stresses if the structure is designed and detailed so that live loading is effectively applied at the nodes. The global bending moment acting on the bridge may be resolved into a couple made up of the compression forces in the top chord and axial tension forces in the bottom chord. Similarly, the diagonal web elements carry the global shear force exerted on the truss bridge, either in axial tension or compression, depending on the configuration of the truss.
As an example, the diagonal web elements of a Warren truss alternately carry compression and tension over the bridge. The internal diagonal web members of a Pratt truss, on the other hand, are all loaded in tension, while the shorter vertical web members are loaded in compression.
Members of Truss Bridges
The chords and web members of truss bridges can be made out of a variety of steel sections. For the tension and compression chords as well as the web members of short-span (30–50 m) highway trusses, rolled “H” sections and square hollow sections are suitable. Larger fabricated sections, such as a “top hat” section or box section, will be needed for the chords of longer highway truss bridges or trusses bearing railway loads.
Built-up through truss bridge
Analysis of Truss Bridges
Truss bridges transmit imposed loads to the foundations through the axial tension and compression forces in the members. Therefore, these structures can be analyzed as pin-jointed members, either as a two-dimensional truss or, more preferably, as a three-dimensional space truss.
This form of analysis assumes that member connections are pinned, which means that none of the truss members may attract moment or torsion. By hand, a two-dimensional plane truss analysis can be solved by utilizing equilibrium equations to resolve the forces at each joint in turn, or by employing the method of sections to free-body segments of the bridge truss, again using equilibrium equations to derive member forces.
The stiffness method can also be used to calculate node displacements first, and then member forces. Nowadays, truss bridges do not have pinned joints; instead, the connections are welded or bolted; yet, analyzing the structure as a two- or three-dimensional pin-jointed assembly allows for an accurate assessment of member axial stresses but overpredicts truss node displacements.
However, since the joints are not pinned in real-life construction, it is necessary to analyze the truss as a three-dimensional space frame with six degrees of freedom at each node in order to obtain a more realistic prediction of node displacements as well as an assessment of the secondary bending and torsion moments, which will be small but still present.
Secondary moments and torsions acting on the structure can affect the bridge’s fatigue life, particularly if the truss is continuous and spans multiple supports. By guaranteeing that the neutral axes of all members meeting at a node intersect at a single location in three-dimensional space, secondary forces and hence stresses can be reduced.
Worked Example: Design of Truss Bridge
A 9.0m wide through-truss bridge is to be designed to carry normal traffic across a river. The total height of the bridge is 5m, and I-sections are to be utilised in the top and bottom chord members of the truss, while square hollow sections will be utilised for the web members. The vertical members of the web are spaced at 2.5m each and the total length of the bridge is 25m.
Modified Warren Truss Bridge
The deck of the bridge is composed of primary and secondary steel beam members. The floor beams consist of UB 457x191x161 members supported by the UB 610x305x179 bottom chord rail of the trusses and spaced at 2.5m intervals. The stringers are the secondary UB 305x102x28 members running parallel to the bottom chord and spaced at 1.5m. A 200mm thick reinforced concrete deck is expected to sit on the beams.
The truss bridge has been modelled on Staad Pro software as shown below. The top of the truss bridge (top chord) will be braced using UB 254x146x37 members in a K-truss arrangement (see below) to restrain the top chord from sway under wind action.
Structural Model of a Truss Bridge
Loading
In this article, the truss bridge will be analysed for the self-weight (all
dead and superimposed loads) and traffic load. All other environmental loads and indirect actions will not be considered.
Dead Load (1) Self-weight of steel members (to be calculated automatically by Staad Pro) (2) Self-weight of 200mm thick reinforced concrete deck = 0.2 × 25 = 5 kN/m2 (3) Self-weight of 75mm thick asphalt wearing course = 0.075 × 23.5 = 1.8 kN/m2
Total pressure dead load = 6.8 kN/m2
Live Load According to the requirements of Load Model 1 (LM 1), the carriageway width of 9m can be divided into three notional lanes as shown below;
In essence, the traffic on the bridge will be represented by the UDL as specified above and the tandem system. The worst effect of the wheel load on the bridge deck will be considered. However, it is important that the influence line analysis of the bridge be carried out, in order to determine the wheel load location that will produce the worst effects on the structure.
Bottom Chord Analysis Results
The result below depicts the internal stresses induced in the bottom chord at the load combination 1.35gk + 1.5qk (where qk represents the UDL component of the traffic load only).
The result below depicts the internal stresses induced on the bottom chord under the unfactored moving tandem wheel load only.
A little consideration will show that the following results are applicable for the bottom chord;
It is obvious that these maximum forces do not interact on the same point in the section. However, for the sake of simplicity, let us assume they interact at the same point in the structure. The design verifications are as follows;
Design of the Bottom Chord
Section s1 results summary
Unit
Capacity
Maximum
Utilisation
Result
Shear resistance (y-y)
kN
1441.9
339.1
0.235
PASS
Shear resistance (z-z)
kN
2047.7
3.6
0.002
PASS
Bending resistance (y-y)
kNm
1470.0
579.2
0.394
PASS
Bending resistance (z-z)
kNm
303.1
4.9
0.016
PASS
Compression resistance
kN
5598.0
914.2
0.163
PASS
Comb. bending and axial force
0.571
PASS
Section details Section type; UB 610x305x179 (BS4-1) Steel grade – EN 10025-2:2004; S275 Nominal thickness of element; tnom = max(tf, tw) = 23.6 mm Nominal yield strength; fy = 265 N/mm2 Nominal ultimate tensile strength; fu = 410 N/mm2 Modulus of elasticity; E = 210000 N/mm2
PASS – Combined bending and compression checks are satisfied.
Design of the Web Members (Verticals and Diagonals)
The result below depicts the internal stresses induced in the bottom chord at the load combination 1.35gk + 1.5qk (where qk represents the UDL component of the traffic load only).
The result below depicts the internal stresses induced on the bottom chord under the unfactored moving tandem wheel load only.
Design Axial compression : 1181.948 + 1.5(761.358) = 2324 kN Design axial tension = 928.940 + 1.5(623.655) = 1864 kN Design major bending moment: (ignored for brevity) Design minor axis bending moment = (ignored for brevity) Design Shear (major axis) = (ignored for brevity) Design Shear (minor axis) = (ignored for brevity)
Internal compression parts subject to compression – Table 5.2 (sheet 1 of 3) Width of section; c = b – 3t = 212.5 mm c / t = 17 = 18.4ε <= 33ε; Class 1
Internal compression parts subject to compression – Table 5.2 (sheet 1 of 3) Width of section; c = h – 3t = 212.5 mm c / t = 17 = 18.4ε <= 33ε; Class 1 Section is class 1
Slenderness ratio for y-y axis flexural buckling – Section 6.3.1.3 Critical buckling length; Lcr,y = Ly_s1 = 5590 mm (considering the length of the diagonal web members)
For completeness, the section should be checked for shear, torsional buckling, and axial/moment interaction.
Conclusion
The method adopted in this article is suitable for draft/preliminary designs. The approach can be extended and used to design and select all the members of the truss bridge. For instance, a little review of the design of the bottom chord shows that there is still room for reduction of the member size, while the same cannot be said for the web members.
The floor beams of the section will need to be designed as a composite beam, taking into account the interaction of the concrete deck. After all the members have been selected and checked, a detailed/final analysis and design can be carried out, to verify the suitability of the selected members.
The general theory of composite beam design calculations is well known among structural engineers, however, the execution of composite beam design in practice necessitates taking into account a number of factors in addition to structural calculations, such as fire engineering, constructability, and more. This article discusses the structural design of composite beams and some of the factors that must be taken into account while designing composite beams.
The construction industry in the United States of America now uses two basic approaches to composite beam design – The LRFD and ASD methods. The method featured in the 3rd Edition LRFD Manual of Steel Construction is both simpler in design and more cost-effective than the method described in the 9th Edition Manual of Steel Construction (ASD).
In the ASD method, the moment capacity is computed from the superposition of elastic stresses, while in the LRFD approach, the moment capacity is computed from the distribution of plastic stresses.
It is usually possible to produce an economical design with partial composite action in the beam. In many design situations, increasing the beam size can satisfy the design moment while significantly reducing the number of studs needed. The design of composite beams is almost always carried out using computer software or design tools like those found in the AISC Manual.
The deck size should, within reason, be chosen to allow for the beam spacing. For un-shored construction, the Steel Deck Institute (SDI) offers tables that show the maximum span permitted for a specific deck and slab arrangement. In general, the economy of the steel floor system is improved by maximizing the span for a given deck size, for un-shored construction. It is advised that you choose a deck assuming a 2-span un-shored condition and avoid single-span situations as much as possible.
The ponding of concrete as well as how the slab is poured must be taken into account. You might want to factor in an extra 1/2 inch of concrete to accommodate for ponding when estimating the amount of concrete needed to construct level slabs. Since the wet weight of lightweight concrete has been reported in the field to range up to 125 pcf, it is crucial to consider this.
Materials for Composite Beam Construction
All of the approved ASTM material specifications for the construction of composite floors are included in Section A3 of the AISC Specification. During the design of composite beams, it is pertinent to always specify ASTM A992 when broad flange beams are being used. However, HSS, pipes, and built-up shapes are also covered by the AISC requirements. The ASTM A108 shear stud, which has a tensile strength of 60 ksi, is frequently used in specifications. 3/4-inch diameter studs are the most typical size used in building construction.
In addition to reinforcing bars and welded wire, the composite slab may also be steel fibre reinforced in accordance with ASTM C1116 in specific circumstances. For normal-weight concrete and light-weight concrete, the minimum specified compressive strength of the concrete in the slab must be between 3 ksi and 10 ksi. Higher strengths should only be relied upon for rigidity. In order to comply with standard fire-rated assemblies, 3.5 ksi normal-weight concrete and 3 ksi light-weight concrete are typically specified.
Cambering in Composite Beams
Although there are several ways to obtain level steel-framed floors, cambering of beams is the technique of choice in the United States. Engineers in the field frequently misunderstand the purposes of proper beam cambering. Beam camber is just one component of a comprehensive floor levelness strategy that must take into account the slab pour method, building occupancy, and steel fabrication and installation procedure.
The main objective of cambering beams is to accurately predict how much the beam will actually deflect under the weight of the concrete. Correct camber is best attained between 75 and 80 percent of the estimated dead load deflection because of connection restraints and fabrication tolerances. Beams should never have excessive camber. Additionally, cambering is improper for a variety of beam types, including brace beams and very short beams.
Serviceability of Composite Beams
For composite floors, serviceability factors to be taken into account are long-term deflections from the superimposed dead load, short-term deflections from the live load, vibration control, and slab system performance. The acceptance standards relevant to the intended floor use, creep deflections under superimposed dead load, and partial composite action must all be taken into account when evaluating deflections.
Design and Detailing of Studs
The 1999 AISC Specification’s Section 15.6 addresses proper stud design and detailing. In the longitudinal and transverse directions, the minimum stud spacing is 6 times and 4 times the stud diameter, respectively. Two new factors—stud geometry and stud position inside the deck ribs—will need to be taken into account in accordance with the 2005 Specification.
Design Example of Composite Beams
Design a 25ft long secondary beam in a proposed commercial complex. The deck ribs are perpendicular to the beam, and the secondary beams are spaced at 10 ft intervals. The concrete for the steel deck has an overall depth of 6 inches with a compressive strength of 3 ksi. The following loadings are anticipated on the floor;
Weight of steel deck = 3.000 psf Additional dead load= 20.000 psf Weight of steel beam = 54.000 lb/ft Weight of construction live load = 20.000 psf Floor live load = 40.000 psf Lightweight partition load = 10.000 psf
Basic dimensions Beam span; L = 25.000 ft Beam spacing on one side; b1 = 10.000 ft Beam spacing on the other side; b2 = 10.000 ft Deck orientation; Deck ribs perpendicular to beam
Profiles are assumed to meet all dimensional criteria in AISC 360-16
Overall depth of slab; t = 6.000 in Height of ribs; hr = 1.500 in Centers of ribs; ribccs = 6.000 in Average width of rib; wr = 2.500 in
Material properties
Concrete Specified compressive strength of concrete; f’c = 3.00 ksi Wet density of concrete; wcw = 150 lb/ft3 Dry density of concrete; wcd = 130 lb/ft3 Modulus of elasticity of concrete; Ec = wcd1.5 × √(f’c × 1 ksi) /(1 lb/ft3)1.5 = 2567 ksi
Steel Specified minimum yield stress of steel; Fy = 50 ksi Modulus of elasticity of steel; ES = 29000 ksi
Loading – secondary beam
Weight of slab construction stage; wslab_constr = [t – hr × (1 – wr / ribccs)] × wcw = 64.062 psf Weight of slab composite stage; wslab_comp = [t – hr × (1 – wr / ribccs)] × wcd = 55.521 psf Weight of steel deck; wdeck = 3.000 psf Additional dead load; wd_add = 20.000 psf Weight of steel beam; wbeam_s = 54.000 lb/ft Weight of construction live load; wconstr = 20.000 psf Floor live load; wimp = 40.000 psf Lightweight partition load; wpart = 10.000 psf
Total construction stage dead load; wconstr_D = [(wslab_constr + wdeck + wd_add) × ((b1+b2)/2)] + wbeam_s = 924.625 lb/ft
Total construction stage live load; wconstr_L = wconstr × (b1 + b2) / 2 = 200.000 lb/ft
Required flexural strength; Mconstr_u = 111.684 kip_ft PASS – Beam bending at construction stage loading
Strength check at the construction stage for shear
Web area; Aw = d × tw = 3.737 in2 Web plate buckling coefficient; kv = 5.34 Depth to thickness ratio (h/tw);λw = 21.200 Web shear coefficient; Cv1 = 1.00 Resistant factor for shear; φv = 1.0 Design shear strength; Vconstr_n = φv × (0.6Fy × Aw × Cv1) = 112.110 kips Required shear strength; Vconstr_u = 17.869 kips PASS – Beam shear at construction stage loading
Design of shear connectors
Note – for non-uniform stud layouts a higher concentration of studs should be located towards the ends of the beam
Effective slab width of composite section; b = min(L/8, b1/2) + min(L/8, b2/2) = 75.000 in Effective area of concrete flange; Ac = b(t – hr) = 337.50 in2 Diameter of shear stud; dia = 0.750 in Length of shear stud after weld; Hs = 3.00 in Specified tensile strength of shear stud; Fu = 65 ksi Cross section area of one shear stud; Asc = π × dia2 / 4 = 0.442 in2 Maximum diameter permitted; diamax = 2.5 × tf = 1.537 in
PASS – Diameter of shear stud provided is OK
Point of max. B.M. from nearest support; LBM_near = 12.50 ft
No. of ribs from points of zero to max moment; ribnumbers = int(LBM_near /ribccs -1) = 24 No. of ribs with 1 stud per rib; Nr1 = 24 No. of ribs with 2 studs per rib; Nr2 = 0 No. of ribs with 3 studs per rib; Nr3 = 0
Total number of studs; Nprov = Nr1 + 2Nr2 + 3Nr3 = 24
Group effect factor for 1 stud per rib; Rg1 = 1.00 Group effect factor for 2 studs per rib; Rg2 = 0.85 Group effect factor for 3 studs per rib; Rg3 = 0.70
Value of emid-ht is less than 2 in (51 mm)
Position effect factor for deck perpendicular; Rp = 0.60
Nom. strength of one stud with 1 stud per rib; Qn1 = min(0.5 × Asc × √(f’c × Ec) , Rg1 × Rp × Asc × Fu ) = 17.230 kips
Nom. strength of one stud with 2 studs per rib; Qn2 = min(0.5 × Asc × √(f’c × Ec) , Rg2 × Rp × Asc × Fu ) = 14.645 kips
Nom. strength of one stud with 3 studs per rib; Qn3 = min(0.5 × Asc × √(f’c × Ec) , Rg3 × Rp × Asc × Fu ) = 12.061 kips
Total strength of provided shear connectors; Ssc = Nr1Qn1 + 2Nr2Qn2 + 3Nr3Qn3 = 413.51 kips
Moment of inertia of fully composite section; Il = Ix + A(d/2 + t – yl)2 + b(t – hr)3/(12nl) + b(t – hr)/nl (yl – (t – hr)/2)2 Il = 923 in4
Effective moment of inertia for partially composite; Il_eff = 0.75[Ix + √(Fshear / C)(Il – Ix)] = 563.6 in4
Proportion of live load which is long term; rL_l = 1 – rL_s = 33 %
Deflection due to long-term live load; DL_l = 5 × rL_l ´ wcomp_L × L4 / (384 × ES × Il_eff) = 0.0887 in
Dead load due to parallel wall & superimp. dead; wD_part = ww_par + (wserv(b1+ b2) / 2) = 0.0000 lb/ft
Long-term deflection due to superimposed dead load (after concrete has cured) Wall parallel to span and superimposed dead; D4 =5 × (wD_part) × L4 / (384 × ES × Il_eff) = 0.0000 in
Wall perpendicular to span; D5 =(ww_perp(b1+ b2) / 2) × L3 / (48 × ES × Il_eff) = 0.0000 in
Combined deflections Net total construction stage deflection; Dshort = Dshort_D + D2 – Dcamber = 1.125 in
Net total long-term deflection; Dlong = Dshort_D + DL_s + DL_l + D4 + D5 – Dcamber = 1.161 in
Combined short and long-term live load deflection; Dlive = DL_s + DL_l = 0.236 in
Net long-term dead and superimposed dead deflection; Ddead = Dshort_D +D4 + D5 – Dcamber = 0.925 in
Post composite deflection; Dcomp = DL_s + DL_l + D4 + D5 = 0.236 in
Household wastes are usually dumped in municipal solid waste landfills (MSWLFs). Landfills are sites that are designed for the dumping and management of municipal solid wastes. However, non-hazardous sludge, industrial solid waste, and construction and demolition waste can be dumped in landfills as well.
Modern landfills are well-engineered structures that are situated, developed, managed, and monitored to ensure they comply with the relevant environmental laws. The basic engineering design of landfills is to prevent the contamination of the ground and groundwater around the landfill. In essence, landfills for solid waste must be designed and constructed to safeguard the environment against contaminants that could be present in the waste stream.
Solid waste dump site
Many of the concerns with landfills in the past were caused by poorly managed and improperly engineered dump sites. The disposal of waste in landfills has a lot of possible environmental consequences. The potential for groundwater and surface water pollution, the unchecked movement of landfill gas, and the generation of odour, noise, and visual nuisances are just a few of the long-term problems that may arise.
The dangers to human health resulting from the disposal of waste will be prevented, or at least reduced, to the greatest extent practicable, by proper landfill site design. It is important that the designer embrace practices, standards, and operational frameworks that are based on best practices currently in use and that take into account advancements in management practices and containment standards. The design approach should take into account the need to safeguard both human health and the environment.
Designing a landfill is a collaborative process that takes into account conceptual design ideas, results of environmental assessments and environmental monitoring, risk assessment, and findings from site investigations. Sustainable development is the main goal of waste management. Therefore, it is implied that landfill development and operation, which are inextricably intertwined, should take this strategy into account.
Construction of a landfill
In addition to providing additional precautions, the landfill siting plan limits the placement of landfills in environmentally sensitive locations while on-site environmental monitoring systems look for any indication of groundwater pollution and landfill gas. Additionally, a lot of modern landfills capture potentially dangerous landfill gas emissions and turn them into electricity.
The main goal of landfill site design is to offer efficient control measures to prevent or reduce, as much as possible, adverse effects on the environment, in particular the contamination of surface water, groundwater, soil, and air, as well as the resulting risks to human health resulting from the landfilling of waste.
The soil properties, geology, and hydrogeology of the site, as well as any potential environmental effects, all affect a landfill’s architectural idea. A site-specific design should be able to be created with the help of the studies for a landfill.
Figure 1: Cross-section of a typical modern sanitary landfill (Megooda et al., 2006)
The philosophy of landfill design has changed recently from the dry storage concept to the bioreactor approach. Leachate is recirculated in the bioreactor approach to increase the moisture content of the municipal solid waste and speed up biodegradation. This is a financially viable solution because it would be costly to dispose of collected leachate securely. By recirculating leachate, one can avoid the costly treatment cost of leachate.
In addition, waste degrades quickly as a result of the high moisture content brought on by leachate recirculation. Consequently, bioreactor landfills offer a significant decrease in post-closure management time and operation expense (Reddy and Bogner 2003).
A bioreactor landfill is described by SWANA (2001) as “any permitted landfill or landfill cell, subject to new source performance standards/emissions guidelines, where liquid or air, in addition to leachate and landfill gas condensate, is injected in a controlled manner into the waste mass to accelerate or enhance bio-stabilization of the waste.”
There are three different types of bioreactor landfills:
anaerobic,
aerobic, and
hybrid.
In anaerobic bioreactor landfills, anaerobic microorganisms (those that do not need oxygen for cellular respiration) speed up biodegradation. These bacteria turn organic wastes into organic acids, which are then converted into methane and carbon dioxide (Sharma and Reddy 2004). Aerobic microorganisms, which need oxygen for biological respiration and create carbon dioxide, are used in aerobic bioreactor landfills. Hybrid bioreactor landfills combine the aforementioned two methods.
Types of Landfills
The Environment Protection Agency (EPA) reports that landfills are controlled under RCRA Subtitle D (solid waste) and Subtitle C (hazardous waste), or by the Toxic Substances Control Act (TSCA).
States and local governments are responsible for the principal planning, regulating, and implementing bodies for the management of nonhazardous solid waste, such as domestic waste and nonhazardous industrial solid waste (Subtitle D);
Subtitle D landfills include the following:
Municipal Solid Waste Landfills (MSWLFs) – Specifically designed to receive household waste, as well as other types of nonhazardous wastes.
Bioreactor Landfills – A type of MSWLF that operates to rapidly transform and degrade organic waste.
Industrial Waste Landfill – Designed to collect commercial and institutional waste (i.e. industrial waste), which is often a significant portion of solid waste, even in small cities and suburbs.
Construction and Demolition (C&D) Debris Landfill – A type of industrial waste landfill designed exclusively for construction and demolition materials, which consists of the debris generated during the construction, renovation and demolition of buildings, roads and bridges. C&D materials often contain bulky, heavy materials, such as concrete, wood, metals, glass and salvaged building components.
Coal Combustion Residual (CCR) landfills – An industrial waste landfill used to manage and dispose of coal combustion residuals (CCRs or coal ash). EPA established requirements for the disposal of CCR in landfills and published them in the Federal Register April 17, 2015.
Subtitle C establishes a federal program to manage hazardous wastes from cradle to grave. The objective of the Subtitle C program is to ensure that hazardous waste is handled in a manner that protects human health and the environment. To this end, there are Subtitle C regulations for the generation, transportation and treatment, storage or disposal of hazardous wastes. Subtitle C landfills including the following:
Hazardous Waste Landfills – Facilities used specifically for the disposal of hazardous waste. These landfills are not used for the disposal of solid waste.
Polychlorinated Biphenyl (PCB) landfills – PCBs are regulated by the Toxic Substances Control Act. While many PCB decontamination processes do not require EPA approval, some do require approval.
Typical section of a landfill
Design Considerations for Landfills
The designer should consider all environmental media that may be significantly impacted through the life of the landfill. The chosen design will have a major influence on the operation, restoration and aftercare of the facility. Aspects that must be considered in the design are briefly discussed below.
(1) Nature and quantities of waste The waste types accepted at the landfill will dictate the control measures required. The requirements at a landfill accepting inert waste will be different to those at one accepting non-hazardous biodegradable waste which in turn will be different from a facility accepting hazardous waste.
(2) Water control To reduce leachate generation, control measures may be required to minimise the quantity of precipitation, surface water and groundwater entering the landfilled waste. Contaminated water will need to be collected and treated prior to discharge.
(3) Protection of soil and water A liner must be provided for the protection of soil, groundwater and surface water. The liner system may consist of a natural or artificially established mineral layer combined with a geosynthetic liner that must meet prescribed permeability and thickness requirements.
(4) Leachate management An efficient leachate collection system may have to be provided to ensure that leachate accumulation at the base of the landfill is kept to a minimum. The leachate system may consist of a leachate collection layer with a pipe network to convey the leachate to a storage or treatment facility.
(5) Gas control The accumulation and migration of landfill gas must be controlled. Landfill gas may need to be collected with subsequent treatment and utilisation, or disposal in a safe manner through flaring or venting.
(6) Environmental nuisances Provisions should be incorporated in the design to minimise and control nuisances arising from the construction, operation, closure and aftercare phases of the landfill. Nuisances that may arise from landfilling include; noise, odours, dust, litter, birds, vermin and fires.
(7) Stability Consideration must be given to the stability of the subgrade, the basal liner system, the waste mass and the capping system. The subgrade and the basal liner should be sufficiently stable to prevent excessive settlement or slippages. The hydraulic uplift pressure on the lining system due to groundwater must be considered. The method of waste emplacement should ensure stability of the waste mass against sliding and rotational failure. The capping system should be designed to ensure stability against sliding.
(8) Visual appearance and landscape Consideration should be given to the visual appearance of the landform during operation and at termination of landfilling and its impact on the surrounding landforms.
(9) Operational and restoration requirements The designer must consider the manner of site development and the necessary site infrastructural requirements during landfill operation and restoration. Landfill sites should be developed on a phased basis. Site infrastructure should include for the provision of; site accommodation, weighbridge, waste inspection area, wheelwash, site services and security fencing.
(10) Monitoring requirements The designer should consider monitoring requirements at the design stage. These should be consistent with the requirements outlined in the Agency’s manual on ‘Landfill Monitoring’.
(11) Estimated cost of the facility The designer should estimate the cost of the total project (construction, operation, closure and aftercare) from commencement to completion. This should include the costs of planning, site preparation and development works, operational works, restoration/capping works, landfill aftercare, and monitoring. Consideration should be given to the financing of the facility at the design stage in order to ensure that sufficient funds can be generated to fund ongoing and potential liabilities.
(12) Afteruse The designer should consider the intended afteruse of the facility. It should be compatible with the material components and physical layout of the capping system, the surrounding landscape and current landuse zoning as specified in the relevant development plan.
(13) Construction Environmental effects during construction must be considered. These may include noise from machinery, dust from soil excavation and soil placement, disturbance, traffic diversion, and avoidance of pollution by construction related activities.
(14) Risk Assessment The design and engineering of a landfill should be supported by a comprehensive assessment of the risk of adverse environmental impacts or harm to human health resulting from the proposed development.
Conclusion
Modern landfills are well-engineered facilities which are situated, constructed, operated, and monitored in line with both federal and municipal laws. In the written word, there are three different kinds of landfills. Traditional dry landfills are the most popular choice. Dry landfills are being replaced by bioreactor landfills as a more environmentally friendly option. The newest entry on the list is sustainable landfills. Resources can be mined and refilled in sustainable landfills.
It is possible to think about landfills as a reliable and abundant source of materials and energy. This is widely recognized in the developing world, where waste pickers are frequently seen scouring the trash for useful stuff. Either landfilling is discouraged in underdeveloped nations or materials are recovered from landfills. In this framework, it is possible to see the idea of sustainable landfills as offering a universal remedy for waste disposal in both developed and developing countries.
