With the increasing focus on the need to reduce the carbon footprints from buildings and construction, “material efficient design” is gaining importance. It is reported that steel and cement account for nearly 15% of carbon emissions globally [1]. Here, let’s consider the cement sector. With 4.2 billion tonnes of cement produced worldwide during 2020, the cement industry is responsible for almost 7% of the total carbon emissions. The industry is however conscious of its responsibility and has committed to achieve ‘net zero’ emissions by 2050 through concerted efforts on reducing emissions from the five “Cs” of its value chain – namely, clinker, cement, concrete, construction and (re) carbonation.
The production of ordinary Portland cement (OPC) and its different variants are mostly responsible for the carbon emissions from the cement sector. It is encouraging to witness that the cement industries from different countries have already initiated a variety of measures to reduce carbon emissions, involving reduction in energy-related emissions, use of renewable energy, reducing process emissions through new technologies, co-processing waste from other industries, etc. Simultaneously, some efforts are also being made in exploring the option of Carbon Capture, Storage and Utilization (CCSU). Obviously, these efforts initiated by the cement industry may not suffice to achieve the net zero target. The other three “Cs”, namely concrete, construction and (re) carbonation also need to supplement the efforts of cement industries to mitigate emissions.
With the massive 14 billion m3 of concrete produced globally during 2020, it happens to be largest man-made material on the earth. Several strategies can be adopted by the concrete industries from different countries to reduce the carbon footprints. A brief article included elsewhere in this issue by Mr. Lionnel Lemay, Executive Vice President, Structures and Sustainability at National Ready Mixed Concrete Association (NRMCA), USA, mentions the top 10 ways to reduce concrete’s carbon footprints. Incidentally, our readers can also download the NRMCA’s guidance document on this topic from www.nrmca.org/sustainability. In the present write-up, we would like to focus only on two areas mentioned by Mr Lemay – namely, concrete mix optimization and use of later-age criteria for acceptance of the compressive strength of concrete. The objective of the author is to explore how best we can adopt the two ideas in India, mainly for the concrete produced from ready-mixed concrete (RMC) plants.
Concrete Mix Optimization
One of the key strategies to reduce concrete’s carbon footprints is to optimize concrete mix design. A variety of supplementary cementitious materials (SCMs) and chemical admixtures are now available in India. An intelligent use of these materials can render great help in optimizing concrete mix design and achieving reduction in the OPC content, which in turn can lead to reduction in the carbon footprints.
It is easier to say this than to implement it. Concrete mix proportioning is a laborious, empirical exercise and the uncertainties in performance of the final product result in increasing the safety margins across the supply chain. The uncertainties stem from numerous factors, chief amongst which are the variabilities in the properties of different ingredients, inadequate control measures adopted during production, transportation, placing, curing, etc. and possible errors in sampling and testing.
In a typical commercial RMC plant or a batch plant at project site, the personnel from quality control and operational fields are faced with the difficult task of controlling the quality of their final product, taking due cognizance of the large day-to-day variations in the properties of ingredients. Take for example the use of OPC. Its supply and cost are dependent on market forces. Therefore, with a view to ensure continuity in the production of concrete and simultaneously achieving economy of the final product, the RMC producer or the contractor is constrained to use different brands of OPC. There is a mistaken belief in some quarters that OPC supplied from different manufacturers provide similar performance. OPCs of different brands do conform to the requirements of IS 269; yet their 1-day, 3-day, 7-day and even 28-day strengths may vary appreciably, which will get reflected in the compressive strength of concrete at various ages. The present writer has observed that even when the same brand of cement is being obtained from the same factory, there is an appreciable week-to-week variations in the compressive strengths of cement with a standard deviation of almost 1.5 MPa! This is not to cast aspersions on the cement manufacturer as the physical and chemical properties of the cement mainly depend up on the properties of the raw materials which are prone to variations. But this does highlight the difficult task faced by the personnel in RMC plant in delivering uniform quality of concrete on a consistent basis in actual practice.
The Indian standard specifies that concrete mix design should be carried out based on guidelines included in IS 10262 or other rational method and the mix so obtained needs be approved by the project authorities or client’s representative. Field practice indicates that the cement contents arrived from such mix design exercise generally tend to be on higher side. Further, the RMC producers have been facing strong resistance to the use of SCMs as partial replacements of OPC. Thanks to the efforts put in by the RMC industry during the past two decades, this resistance is slowly diminishing, Yet, when the use of SCM is permitted currently, the percentage replacement of OPC is usually lower than the maximum permissible limit specified in the IS standard – 35% for fly ash and 70% for GGBS. There are examples when the client/client’s representative specify arbitrarily high cementitious contents than necessary. This is possibly to have sufficiently higher ‘safety’ margins! Such practice defeats the very purpose of having optimized mix designs that help in reducing the carbon footprints.
On the positive side, the present author is aware of certain efforts done by a few enlightened RMC producers in conducting the optimization of concrete mixes by achieving dense packing of the aggregate skeleton. This helps in minimizing the voids, thus needing less cementitious paste to coat the aggregates. A simple procedure involving the use of dry-rodded bulk density testing can be done to achieve the best possible particle packing of the aggregate that provides the proportions of 20mm, 10mm and crushed stone sand. The cementitious content and the replacement levels of SCM are then decided based on the past performance of various alternative combinations of the cementitious contents
With a view to ensure that consistency in the quality of concrete is maintained throughout the production process, detailed guidelines on quality control and quality assurance as specified in IS 4926 are generally followed [1]. RMC plant certification scheme brought out by the Quality Council of India also provides useful criteria for production control of RMC [2]. Strict adherence to the above documents in practice would certainly help in streamlining the QA-QC exercise which can go a long way in ensuring that the mix optimization already achieved does not get disturbed during actual production.
Incidentally, the use of statistical quality control tools help in finding out the standard deviations and the coefficient of variations of concrete mixes. Using these, one can initiate corrective actions to minimise the cementitious contents, if excessively high strengths are being achieved.
It is observed on many occasions that the minimum cement criteria included in the prescriptive specifications of IS 456 become a hurdle to optimise the cement content. The adoption of performance-based specifications as advocated by NRMCA and many other organizations/standards is certainly one of the best tools to overcome the limitations posed by the prescriptive specifications. Unfortunately, in spite of the publications of a number of technical papers and technical reports and seminars/conferences held on the need to change from prescriptive to performance-based specifications, the practice of adopting the latter has not taken roots within the concrete industry.
In view of the uncertainties in getting the required compressive strengths in actual practice, there are cases in India when some structural consultants tend to specify one grade higher than necessary. This is unfortunate and it is tantamount to a shear waste of natural resources. It is therefore essential that the RMC producers need to set their practices right and produce good quality concrete on a consistent basis so as to win over the confidence of the structural consultants. It is also necessary that all stakeholders should encourage RMC producers to undertake the concrete mix optimization exercise on a routine basis. Such collective efforts can lead to the reduction in the carbon footprints on the one hand and the cost of concrete on the other. Thus, it’s a win-win situation for all stakeholders if they give priority to concrete mix optimization!
Specifying Later Age Compressive Strength
For reducing carbon footprints of concrete, another avenue is open for specifiers. This involves specifying later-age compressive strength of concrete (wherever possible) in place of the usual criteria of specifying the 28-day strength. A majority of the concrete produced from commercial ready-mixed concrete plants in India contain some SCM like fly ash or ground granulated blast-furnace slag (GGBS).
With the moderate level of OPC replacement of around 20-25% of fly ash and 45-50% of GGBS, a clear trend of increase in the 56-day compressive strength over the 28-day value is usually observed which is generally 10-15% higher in OPC+ fly ash mixes and 15-20% higher in OPC + GGBS mixes. Of course, the percentage may vary from case to case depending on the mix proportions; however the broad trend may prevail.
It would be advisable to take advantage of the gain in strength of concrete at 56 days. One can certainly reduce the OPC content in the concrete mix appropriately, which will permit to achieve the specified strength at 56 days. This would lead to saving in cost and also reduce the concrete’s footprints.
It is possible for the structural consultants to specify the 56-day strength for certain structural elements like foundations, basements, lower-level columns, and other structural elements which will be subjected to the full loading at a much later stage. A recent case study can bring more clarity to this issue.
The structural consultant specified M35 grade concrete for a raft foundation. The prescriptive specification of IS 456 specify 450 kg of OPC and a water-binder ratio of 0.45 for M35 grade of concrete. When the consultant agreed to relax the acceptance criteria to 56-day, the mix proportion suggested by the RMC producer was 130 kg OPC and 190 kg of GGBS and w/b ratio of 0.45. The OPC replacement level works out to 60%, which is well within the 70% permissible limit specified by the IS. With this mix, 40-plus MPa compressive strength was achieved at 56 days. Incidentally, the reduction of OPC helped in reducing the peak temperature within the raft foundation a maximum of 680C, which was lesser than the 700C limit specified by the consultant.
Of course, care needs to be taken to ensure that the sufficient early-age strength is available for de-shuttering so that the construction schedule does get affected adversely. This can be achieved by developing an intelligent mix design that can satisfy the early age strength requirement for de-shuttering as well as the 56-day compressive strengths for acceptance.
Before we conclude, we would like to underline the fact that the concrete mix having lower carbon footprints may also turn out to be the mix having lowest cost.
References
- Thomas Czigler, Sebastian Reiter, Patrick Schulze, and Ken Somers, Laying-the-foundation-for-zero-carbon-cement, Mckinsey & Company, May 2020.
- IS 4926: 2003,Ready-Mixed Concrete – Code of Practice (Second Revision), Bureau of Indian Standards, New Delhi.
- ‘Criteria for Production Control of Ready Mixed Concrete under Ready Mixed Concrete Plant certification Scheme’ (QCI), Building Materials & Technology Promotion Council, Ministry of Housing & Urban Poverty Alleviation, Government of India, New Delhi, 2013.