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B. Bhattacharjee Emeritus Professor Civil Engineering Department, IIT Delhi |
Introduction
Modern concrete composite is a versatile technological material. The versatility manifests itself in the form of: a) mould-ability ranging from driest roller compacted concrete (RCC) to self-compacting concrete (SCC); b) strength ranging from 5-10 MPa compressive strength for mass concrete to 200 MPa (and higher) grade for Reactive Powder Concrete (RPC). RPC with fibre reinforcement ensures pseudo ductility. Thus concrete can provide robustness of dam using mass concrete and slenderness of section with high strength that is desirable in tall buildings and long span bridges. It is durable with low life cycle cost having maintenance free long service life of sections, excellent fire resistance compared to other structural and construction materials. It is imperative in many respect vis-àvis other construction materials and thereby is the most popular construction material.
Concrete is a composite having a skeleton that is particulate aggregate phase. This phase acts as inclusions in a continuous cementing matrix phase that is also responsible for bonding the particulate system together to form a hard composite material once matured. To induce pseudo ductility and for enhancement of flexural tensile strength through resistance to crack propagation, high modulus fibre is incorporated in the overall composite matrix. The particulate materials in the skeleton matrix are inert, generally obtained from natural rocks or stone. The common cementing matrix is formed by hydration reaction of a class of inorganic materials known as cement with water. Organic binders in the form of resins or monomer in liquid form can also be used, which on polymerisation through use of appropriate chemical agent can harden to solid continuous binder phase. Being costly, such exclusive organic binders are restricted to special usages e.g., repair etc. Solid binder and liquid water, when mixed in appropriate proportions can produce desirable plastic mixture. The rheological characteristics of mentioned plastic mixture can be engineered appropriately with suitable chemical and some mineral additives or even with low modulus fibre addition. Engineered concrete composite therefore is made by mixing several component ingredients. Binders can be a single component or may be again from combinations; hence as of now, possibility of many binders exists. Similarly composite material may be formed using different materials in the skeleton matrix and lastly varieties of fibres can be used to enhance specific properties as desired. Fig.1 depicts a general 3-D representation of concrete or cement based composite i.e. a chemically combined ceramic that is the most consumed material by human being after water. The advantage of mould-ability allows it to be used in fabrication of structural element both cast in-situ, i.e., in place at site or precast the elements at a factory away from actual place in structure, to transport and place by erection. Both have their relative merits and disadvantages in terms of cost, quality, feasibility etc. This article focuses on appraisal of precast concrete vis-à-vis cast in situ concrete in the context of sustainability for use in construction, with special reference to buildings in India or other countries belonging to similar socio-economic and human conditions.
Sustainability Concepts and Affordable Housing Potential
The UN appointed Brundtland Commission in 1987 defined sustainable development as “Development that meets the needs of the present without compromising the ability of future generations to meet their own needs”. Sustainability is currently understood as ability to exist constantly. Total matter in the earth’s eco system is constant according to law of conservation of mass postulated by Lavoisier in 1789. “Nothing is created nor destroyed only transformed”. The energy received by the earth from the sole source, that is the sun, in a year is also transmitted back to cosmos and hence internal energy of the earth is also conserved. Materials are transformed from one state to another by expense of energy and there by energy remains stored in the material. Such energy e.g., chemical energy stored in fossil fuel is harvested by human being and consumed. The transformation of materials e.g., CO2 to biomass materializes through carbon cycle. The natural process of photosynthesis by plant enables capture of solar energy for conversion of CO2 to biomass. The CO2 is produced by living beings through respiration and by other anthropogenic activity such as use of fossil fuel for energy and calcination of lime stone for cement etc. Thus extra CO2 is produced by human beings by virtue of consumption of fossil fuel and cement.
The annual consumption by humanity is accounted in term of ecological foot print (EFP). This consumption includes food, livestock product, space used for built environment etc., and forest land required to absorb CO2 generated. The unit is global hectare (gha) [1]. Similarly bio capacity of the available space i.e. land, water etc., to generate the consumables is also accounted in gha. The ratio of ecological foot print to bio capacity is the number of earth required to sustain the current population with its presentday consumption pattern. Number of earth required for average consumption and living pattern of every country as well as for the entire humanity is calculated on annual basis using the mentioned accounting system. More than unity value of number of earth is unsustainable. The CO2 generation is the major issue. The average living pattern however, is not uniform across the globe. The living pattern is also related to life expectancy, education and overall quality of life. These aspects along with income are accounted in the statistic composite called human development index (HDI). Variation of HDI against EPF is for 2007/08 shown in Fig.2. Earths bio capacity/capita of 2.1gha is shown as red vertical line. A similar figure showing number of earths against HDI are shown in Fig.3 for 2016 data. Global sustainability zone is shown in both the figures. The HDI of India is lower than the minimum required for sustainability. The bio capacity and EPI per capita of India is also lower than respective global average. Hence there is scope to improve upon HDI and bio capacity maintaining EPI in a controllable level. Increasing built-up area, particularly shelter and housing of economically weaker section (EWS) and lower income group can improve HDI, although, at the cost of increasing EFP. To be sustainable, the above mentioned housing needs to be affordable i.e., within the expenditure capacity of a household. An analysis of housing affordability, based on 2011 census expenditure distribution data by author and team [2,3] demonstrate that market for affordable EWS housing are 11% and 20.5% of India’s rural and urban populations respectively, excluding land cost. House accommodation layout plan designated by Central Building Research Institute (CSIR-CBRI) and Building Materials Promotion Council (BMPTC) as economically weaker section (EWS) housing is considered in the analysis. The population below poverty line have been excluded in the above percentages, as they would not be able to sustainably retain the house in preference to purchase of minimum nutrition required. However, there is surplus supply for higher income population and a large number of houses are vacant according to an analysis carried out by JLL [4]. Such segment is also excluded in the above percentage computations. Thus there is a possibility of exploring the above scenario to enhance sustainable development in India through efficient and eco-friendly material, building and construction practise. This is true for many other countries in Asia, Sub-Saharan Africa and Latin America.
Fig. 2: Ecological Foot Print Per Capita and HDI
Industrialized Concrete Constructions for Sustainability
EFP of built-up area is equivalent to that of crop land which is most productive among land use, and is given maximum relative weightage in EFP accounting. Carbon foot print is the other major aspect related to infrastructure and building. Comparing materials and construction from sustainability angle shall look in to both these issues. For example mud as a construction material for EWS housing would consume large crop land as buildings can at best be 2 storeyed. Although the carbon footprint could be lower than concrete. Large volume of housing would favour concrete. Masonry construction with burnt clay bricks masonry unit as structural material is also likely to have higher EFP compared to concrete as building height would be restricted; also, carbon foot print would be more than that of mud and slightly less than that of concrete. Timber would have much higher EFP as equivalent carbon footprint would be much higher because of loss of forest land. Ecological footprint in general favours concrete. The advantage of concrete material system can be realized only when it is chosen judiciously from the discrete combinations illustrated in Fig.1. The construction element adopted along with associated design and construction technique implemented shall be able to minimize the sustainability concerns. The sustainability issues to be addressed in the context of concrete constructions are [5,6]: carbon foot print, life cycle embodied energy, natural resource consumption and energy implications in building during service condition. Besides, choice of materials and quality control during production, play a major role in eco-efficiency of material used. Such quality control can only be attained through mechanized and industrialized building construction. Like the concrete composite material system, the domain of industrialized building construction system with concrete is vast. The various industrialized concrete construction are illustrated in the next paragraph.
Fig. 3: Ecological Foot Print per Capita, Number of Earths and HDI (2016)
The potential industrialized concrete construction system may include Autoclaved aerated concrete panels, linear systems, insulated precast sandwich panel system, hollow core slabs and other planner systems, 3-D concrete modular system and 3-D Printed precast element [7,8]. Although not prefab, insulated form and tunnel form also are part of semi industrialized construction. Industrialized construction technologies yield thoroughly engineered concrete building products. As an example 3-D modular element is shown in Fig.4. Cement can be picked considering minimum carbon footprint and suitability with respect to exposure environment from the many options available, along with compatible admixtures. Manufactured aggregate with proper shape and packing characteristics can be selected so as to minimize paste content and hence cement content for given mould-ability, i.e. for appropriate rheological considerations. Recycled aggregate can also find a use in appropriate application. Treated waste water can be used for mix preparation as well as curing. One can also minimize water consumption by making use of curing compound. CO2 curing can help in sequestration. Overall the material system can be designed for required compressive strength. Smaller sections can be adopted for higher strength material, thereby reducing the material consumption. With lower standard deviation that can be achieved in a controlled production process, mean strength would be lower for a given characteristic strength, hence saving on costliest binder material. For enhancement of pseudo ductility and flexural strength fibres can be incorporated in the matrix by material design. Element can be designed for maintenance free service life compatible with the exposure environment and nature of loading, viz., static or fatigue loading. One can evaluate the ecological footprint of such system and minimize the same.
| In a nutshell, industrialized concrete construction can provide for sustainable housing solutions as opposed to conventional prevailing non-engineered or partially engineered construction practices. Planned implementation and encouragement can provide affordable housing to large population and also ensure employment to large workforce of skilled worker with enhanced higher income. As mentioned earlier volume of required housing is enormous in absolute numbers. This implementation stated above, thereby can result in improvement in HDI towards sustainable quadrant in Fig. 2, for India. It needs mention here that, there is dearth of such educated and skilled worker at present and would need action towards creation of such work force. |  Fig. 4: 3-D Modular Concrete Element |
Influencing Economic Factors
At present the preference of technology is mostly governed by cost, profit and market. Cost per unit area of building is directly proportional to total area, space and specification. For similar area and specifications two factors identified in literature with respect to implementation of advanced Industrialized Building constriction (IBC) are C-factor and P-factors involving labour cost and plant and machinery costs. These factors are given by [ 8]:
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These factors vary from country to country. The readily available C-factor and P-factors for some country and region are shown in Table 1. In table 1 percent cement consumption is also shown side by side. One interesting observation in the table is that higher the C-factor lower is the P-factor. Cement in precast concrete for buildings is high for lower P-factor. Further an inference is drawn from this analysis is shown in equation 3. (P − factor) × (C − factor) ≈ 10 (3) |
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It can also be inferred from the table -1 that where P-factors are low, which means countries where labour cost is high, i.e., average income of skilled work force is high, cement in precast concrete for Buildings is also high that means precast concrete is favoured. Another inference that can be drawn from Fig 2 and Fig.3 in conjunction with table 1 is that countries where precast concrete is favoured rank very high in HDI, but their EFP is also high.
Conclusions
Country like India which is quite below the sustainability quadrant in HDI needs well planned strategy to enhance HDI at least to lower limit of sustainability quadrant, and, at the same time needs control of EFP. The construction and housing can play a positive role in this direction by encouraging industrialized precast concrete construction. Improvement of skill of the workforce can enhance their income and their by amplify the proportion of eco-efficient construction practices.
References
1. https://www.footprintnetwork.org/ (2019): “Working Guidebook to the National Footprint and Biocapacity Accounts”
2. Mondal Darpagiri (2017): “Optimization Of Housing Affordability”, M.Tech thesis; IIT Delhi.
3. Mondal Darpagiri and Bhattacharjee B (2020): “Housing For All: Analysis Of Possibility And Potential” Current Science. Revised Paper under review
4. JLL (2012): Affordable Housing In India – An inclusive approach to sheltering the bottom of the pyramid. 2012
5. Bhattacharjee, B. (2011) Sustainability Performance Index for Concrete. International Concrete Sustainability Conference, August 9-11, 2011, Boston, MA, USA. www.concretesustainabilityconference. org
6. Bhattacharjee, B (2010): “Sustainability Of Concrete In Indian Context” Indian Concrete Journal. Vol 84 No.07 July 2010. pp. 45-51.
7. Warszawski Abraham ( 1990): “Industrialization and Robotics in Building: A Managerial Approach”. Harpercollins College Div.
8. Elliott, Kim S. and Hamid, Z. A (Eds): (2017) “Modernisation, Mechanisation and Industrialisation of Concrete Structures”. Wiley Blackwell.