Engineers And Sustainable Concrete Construction

K P Abraham
Former Chief Engineer
CPWD

Introduction

Engineers play a vital role in human development by building infrastructure for the functional requirements of the human society and its development. An engineer’s role in making these development sustainable is all the more important. With ingenuity and innovations, along with sensitivity towards ecology and environment, they can implement an infrastructure development, which is cost-efficient, preserves natural resources and creates least disturbance to human and natural environment.

All the definitions of sustainable development point to the target of improving the quality of life indefinitely without degrading the quantity, quality and the availability of economic, environmental and social resources. In September 2015, UN General Assembly adopted the 2030 agenda for Sustainable Development emphasizing a holistic approach for achieving sustainable development for all. The responsibility of overseeing SDG implementation in India is assigned to National Institution for Transforming India (NITI) Aayog.

Achieving SDGs is a collective responsibility of Engineers of all disciplines, Economists, Scientists, Management experts, Financial expert and a host of other stakeholders. But one goal, which is of particular interest to Civil Engineers and construction Industry professional is “Goal 12: Responsible Consumption and Production”. This goal contains 12 targets to ensure sustainable consumption and production patterns, out of which the following three targets are of particular importance to all construction industry stakeholders:

  1. Achieve sustainable management and efficient use of natural resources, by 2030
  2. Substantially reduce waste generation through prevention, reduction, recycling and reuse, by 2030
  3. Achieve environmentally sound management of waste and pollutants and significantly reduce their release to air, water and soil, by 2030

In addition, concrete supports a sustainable society. Challenges of the future cannot not be solved without involving the concrete industry in the process. The most important properties of concrete that contribute to this support is its long life time, lesser maintenance requirements and the fact that concrete very often is based on locally available/manufactured material.

Considering the quantum of concrete being produced and consumed, any action to mitigate its adverse impacts will greatly contribute towards achieving Sustainable Development Goals. The ways and means by which concrete construction can be made more and more sustainable is described in this article to enable the construction engineers and construction industry professionals to effectively play their role in achieving the Sustainability Development Goals.

Sustainable Concrete Construction

The versatility of concrete as a material of construction makes it the largest consuming construction material worldwide. Such a large consumption leads to many environmental concerns including the fast depletion of natural resources like coarse and fine aggregates, environmental pollution, GHG gas emission, waste generation, etc.

Make Durable Concrete Structures

Concrete structure is expected to give a service life of 80-100 years. If a concrete structure shows premature distress, maintenance efforts will be much more. If the distress is in advanced stage, either it is has to be reconstructed or major retrofitting is called for to retain the structure in a safe condition. This necessitate greater deployment of material and energy sources and greater waste to dispose. Hence, a durable structure with long service life has to be ensured for sustainability. A concrete structure to be durable, it has to be designed properly, constructed carefully and maintained systematically.

Fig. 1: 6 cm Thick RC Conoidal Shell Roof Constructed in 1948

Deterioration of concrete takes place because of the presence of deleterious chemical elements in the concrete. Deteriorating agents may reside inside the concrete itself or may enter into the concrete from the service environment to which the structure is exposed. While the deleterious material present in the concrete can be controlled by restricting the deleterious elements of constituent materials within the limits specified in the codes and specifications, entry of damaging elements from the surrounding environment require care and control in all the stages of concrete construction process.

It is the inherent nature of concrete to crack. Hardened concrete is a brittle material and the tensile strength of concrete is very low. Whenever the induced tensile stress goes beyond the current tensile strength of concrete, it cracks. These cracks provide easy access for deleterious materials and therefore, must be avoided or controlled and minimized to improve the durability of the structure. While the cracks caused by the applied stresses can be taken care of by appropriate design interventions and by reinforcement detailing, shrinkage and temperature induced cracks are required to be controlled by the construction engineer by taking appropriate action during construction

Many defects do occur in concrete construction leading to honey combs, voids and other surface defects, especially in the Indian construction scenario. It could be due to defective shuttering, improper compaction, or by mere negligence of the site supervisory staff. All these defects are potential avenues for the damaging elements to enter into concrete. Care and sound construction practices are required to be adopted to get defect free concrete. If defects occurs, in spite of taking adequate care during construction, the defects are to be rectified by proper treatment to make it impermeable

A structure may contain many insertions or protrusions passing through concrete elements such as pipes, cables, ducts, left out holes and protruding steel reinforcements, all of which are potential damage initiation points and require careful treatments at the insertion points to avoid damage deep into the concrete.

Fig. 2: Typical Construction Defects

Fig. 3: Typical Insertions/Hole in the Concrete


Adopt More And More Precast Construction Technologies

The magnitude of construction done by precast methodologies is very low in our country. Presently, majority of the structures in the country is built by traditional cast-in-situ (CIS) construction methodology. While precast construction technology is extensively used in bridges and other infrastructure projects, this technology is not widely accepted by the building construction industry.

Precast construction involves casting concrete elements using reusable mould, curing it in a controlled environment, transporting to the site and erecting in position. Precast concrete construction is a proven construction methodology all over the world. It is a promising solution to meet the country’s infrastructural deficit. It offers several advantages such as fast and quality construction and enhanced health and safety. In spite of these advantages, the precast construction is not gaining momentum in India. Lack of standardization, limited knowledge, problems of joints and connections, non-availability of tools, technology and equipment, contractual issues, taxation, inadequate certification and testing facilities, etc. are some of the major challenges faced by the industry. Precast technology is also technically challenging, requiring lot of pre-engineering and close coordination between stakeholders.

Fig. 4: Precast Slab


The advantages of using precast technology include:

– Precast technology is technologically advanced than the traditional construction and proven to be safe, durable and very fast in construction as precast constructions are highly mechanized. Hence it not only speeds up the construction but also ensures improved quality, safety, productivity and hence more sustainable compared to traditional CIS construction.

– Precasting leads to leaner design reducing dead weight of the structure.

– Precast technology affords cost reduction due to standardization, increase in productivity and deployment of less number of labours.

– The quality control in a manufacturing environment is much better than that could be achieved at the site. Hence, pre-casting offers exceptional high quality as these components are cast in a controlled environment with state-of-the-art machinery and erected at site with requisite equipments.

– Precast construction is characterized by high degree of durability.

To popularize the adoption of precast technologies, many steps are needed and engineers play a crucial role. Some of the measures for popularizing the precast technologies are mentioned below:

– Standardization can help both manufacturer as well as the consumer. At present, we do not have standards for the size of elements, their transportation and handling, and erection process. Standardized designs enable coordination and design integration, which not only add value for money but also increase safety and defects reduction, enhancing confidence in owners and contractors.

Fig. 5: Precast Panel for Tunnel Lining

– Certification scheme is required for plants, personnel and erection process. Certified plant assures that a quality system is in place. Certification of personnel and erection process builds confidence to the client. Our precast industry lacks certification scheme to ensure quality and consistency of precast products, plants and erection process.

– Another major roadblock in growth of precast industry is the non-availability of testing facilities. Full scale testing facilities for the precast elements, joints and connections are essential for better understanding.

– One of the biggest factors hindering the growth of precast construction is the lack of knowledge and awareness among civil engineers in the country. Experienced engineers in conventional construction have many apprehensions and their lack of expertise in precast methodology is a major road block. Current academic curricula does not adequately cover the precast construction technologies. Shortage of engineers with requisite knowhow in the precast technologies is a major cause leading to poor design, production and erection practices. While engineers should come forward in acquiring knowledge and skill in precast technologies, professional associations and institutions should take a lead in fostering the precast knowledge and sensitize the working professionals.

Reduce The Consumption Of Cement And Water

Cement and water are the unavoidable constituents of cement concrete. Cement is the binder and water is required both at the time of mixing and curing. It is estimated that about 170 litres of good quality water is required for production of a 1 m3 of concrete and this water will be consumed totally by chemical reaction and possibly will be disappeared from nature. Curing supplements the water to the hydration process of cement and controls the moisture movement from the concrete structure. There are no quantitative methods to establish the actual water demand during the curing. Cement is the most expensive and energy intensive component and water is getting increasingly scarce. Hence, it is absolutely essential to reduce the consumption of cement and water for achieving sustainability

A tonne of Cement requires about 1.2 t of limestone and smaller quantities of Gypsum, Bauxite, Iron ore etc., in addition to about 750 KCL/Kg thermal energy and 85 KWh/t of electrical energy. CO2 emission is on an average about 0.8 t per tonne of cement with the present level of technology. In such a scenario, an Engineer’s role in reducing the cement content of concrete is crucial. He can use supplementary cementitious materials (SCM), which are mainly industrial byproducts/waste, either in the form of blended cements or as mineral admixtures in concrete. These SCMs not only reduce the OPC content, but also greatly enhance the durability of concrete at the same helping other industries to dispose of the waste.

Binary blended cement reduces the OPC content with durability advantages like reduced heat of hydration, reduced cracking, reduced reinforcement corrosion, etc. Ternary blended cement substantially reduces the OPC content in the concrete mix for a particular level of strength at the same time, increasing the durability because of the additional contribution of Fly ash or GGBS. Differences in the particle size distribution of binders leads to dense particle packing. Interfacial transition zone surrounding the coarse aggregates in the fresh concrete have relatively higher water cement ratio, making it the weakest link in the composite. Ternary blended cement leads to high degree of densification of the transition zone with little presence of calcium Hydroxide, leading to better strength and durability

Mineral admixtures can added at the time of batching and mixing instead of using blended cements. Using blended cement at factory has an edge over site mixing of mineral admixtures because of the uniformity of blend that can be achieved at the factory. The site mixed process may be economical, but the quality and durability can get affected depending upon the level of quality control at site. IRC: 15 allows use of fly ash at site and to ensure uniform mixing, batch mix plant with automated process control is specified. Code does not permit site mixing of GGBS and only factory made PSC with GGBS is permitted.

Advent of superplasticier has facilitated substantial reduction in water for getting the same workability. Workability in concrete is mainly the performance of cement in the presence of water. In a normal concrete both cement and water are not fully available for workability as water tends to become droplets around which cement particles coagulate due to their electric charge forming micro spheroids. When chemical admixture is added, cement is prevented from coagulation, thus releasing the entrapped water and total cement is available as individual particles. It causes repulsion among cement particles preventing the formation of spheroids making the cement totally available for binding and entrapped water made available for workability.

By judicious use of mineral admixtures and chemical admixtures, the quantity of cement and water in the concrete can be drastically reduced at the same time rendering the structures more durable with improved service life, making the construction sustainable.

Huge quantity of water is required for curing the concrete. Potable water is required for curing and potable water is getting scarce even for drinking and other household purposes. Hence, there is a dire need to reduce the potable water for curing purposes. One way to reduce the water requirement for curing is to use suitable curing compounds, wherever found feasible. Curing compounds can be either by internal curing compounds or external curing compounds. These curing compounds trap the moisture within the concrete by preventing the loss of water by evaporation, thereby reducing drastically the requirements of water for curing. These compounds have the advantage of one time application against water curing which has to be applied at specified intervals during the entire curing period. Moreover, negligence in proper water curing is a perennial issue in any construction site.

Fig. 6: Application of Curing Compound

Water from RMC plants and precast yards can be reused after removing the impurities. For major projects, reusing of curing water with suitable collection arrangements can be implemented. Use of recycled water from industrial sources and sewage treatment plants can be used, after making sure that such treated water will not affect the durability. In general primary treated wastewater from sewage treatment plants may not meet the codal requirements and secondary treatments including industrial RO systems could be effectively used to make it suitable for curing.

Replace Natural Aggregates With Manufactured Aggregates

Coarse and fine aggregates are major components of concrete and aggregates from natural sources traditionally used in concrete. But the huge consumption of concrete for developmental works is causing fast depletion of these resources and in many parts of the country, the availability of these aggregates is getting scarce. Coarse and fine aggregates manufactured from other than natural sources are available and aggregates from natural sources can be partly replaced with manufactured aggregates in concrete, which will contribute greatly towards sustainable construction. Engineers have a wide choice and the commonly available manufactured aggregates in our country are indicated below.

(i) Course aggregates: Iron slag aggregate, steel slag concrete, recycled aggregates and bottom ash are some of the manufactured coarse aggregates.

(ii) Fine aggregates: Iron slag aggregate, steel slag aggregate, copper slag aggregate and recycled aggregates are some of the manufactured fine aggregates.

Iron slag is a by-product of Blast Furnace. The molten slag is cooled to form different sizes of aggregates. Air-cooled slag is crushed to get coarse aggregates, while it can be quenched rapidly to get fine aggregates. Steel slag, a by-product of steel making process, is cooled by air and sprinkling water and made metal free by magnetic separation. The slag is then crushed and dried to get different sizes of aggregates. Copper slag is a by-product of copper smelting process and is quenched with water to produce granulated particles having size varying from 150 micron to 4.75 mm. Recycled aggregates are obtained from processing of construction and demolition wastes.

IS 383: 2016 specifies the extent of replacement of the manufactured aggregates along with their limitations and precautions to be taken while using the same in the concrete. Engineers should take initiative to use these manufactured aggregates, wherever it is found feasible and available, for the cause of sustainability.

Fig. 7: Different Types of Manufactured Aggregates

Use Industrial Waste-Based Materials In Concrete

Industrial waste like fly ash, ground granulated blast furnace slag (GGBS), silica fume, ultra-fine slag, rice husk, ash, etc. can be effectively used not only to reduce cement content, but also to improve various characteristics of concrete both in the fresh and hardened state. These wastes, otherwise dumped in land fill could be major sources of pollution of air and water. The benefits of using these waste material in concrete include reduced cost, improved workability, chemical resistance and durability, lower heat of hydration, increased aberration resistance and increased long term strength. This waste can be added either at the cement plant or at the batching and mixing plant at site as permitted by the codes and specifications. These materials posses little or no cementitious property, but can chemically react with calcium hydroxide in the presence of moisture at ordinary temperatures to form cementitious compounds.

Fly ash, a waste from thermal power station is being used in concrete making since long. Use of fly ash is covered under IS 3812- 2013 (Part I). Cement industry extensively uses it either in blending with cement or as a Kiln feed material. Finer the particle size of the ash, greater the area exposed to lime and greater the reaction rate. The slow pozzolonic reaction can be improved by increasing the curing temperature. Loss of ignition (LOI), which reflects the presence of unburnt carbon, reduces the pozzolonicity and hence, the maximum permitted value as per the code is 5%

For a given strength, fly ash concrete is less permeable than OPC concrete, as the porosity of the concrete get considerably reduced. The permeability of the fly ash concrete reduces with time as the pozzolonic reaction continues over a time, effectively blocking the ingress of deleterious materials in to the concrete.

GGBS is a waste product emanating from Blast furnace in the iron making process. IS 16714-2018 covers the requirements of GGBS. It has both hydraulic and pozzolonic properties. The hydration reaction of GGBS is slower than OPC resulting in slower strength development in concrete using blended cement. Finer the material, more the water demand and more reactive the material. More the amount of GGBS, more the workability retention of the concrete.

Micro silica is the byproduct of high temperature reduction of quartz in electric arc furnace. It contains more than 85% SiO2 and is collected as an ultrafine powder. IS 15388-2003 covers the requirements of Micro silica. The normal dosage of micro silica in concrete is in the range of 7-10% of weight of cement. Generally, adding silica fume lowers the slump and require effective superplasticiser and longer mixing time to deflocculate the micro silica particle and to ensure adequate dispersion. Micro silica is compatible with both fly ash and GGBS and is used in combination with these to make ternary blended cements, delivering the beneficial characteristics of both of them.

Fig. 8: Micro Silica

The fineness and high reactivity of silica fume results in very dense micro structure and virtual absence of weak Interfacial transition zone (ITZ). Along with fly ash/ GGBS, silica fume concrete shows better sulphate resistance than even Sulphate Resisting Concrete. For equivalent strengths of concrete, the initiation of chloride attack is delayed in a silica fume containing concrete. Leaching and efflorescence will occur when excess calcium hydroxide comes out to the concrete surface. This usually happens when the concrete surface is subject to either continuous water contact or intermittent wetting and drying. Addition of silica fume reduces leaching due to the refined pore structure and increased consumption of calcium hydroxide. Silica fume is also found to increase the abrasion resistance of concrete.

Ultra-fine slag is manufactured by drying and grinding blast furnace slag. It has both hydraulic and pozzolonic properties. Its requirements are covered in IS 16715:2018. The principle of particle size distribution (PSD) applies to binders as it applies to gradecombined aggregates. When more than two materials are added together, combined grading play an important role with regards to flowability, compactness and strength. Single size particles generally tend to coagulate. Paste made using material with narrow PSD like micro silica becomes less workable. Well-graded broader PSD material results in smooth flow of paste with good rheological properties. Controlled grinding process of ultrafine slag results in Broader and well graded PSD resulting in better concrete rheology. Because of the combination of fineness, broad PSD and dual reaction, ultra-fine slag produce higher amount of C-S-H gel, refines the pore structure of concrete resulting in concrete of high strength and high durability.

Fig. 9: Recycled Concrete Aggregates

Recycle Waste/Demolished Concrete

Large consumption of concrete leads to fast depletion of natural resources like coarse and fine aggregates, which constitute the major chunk of the concrete. In India, there is acute shortage of these natural resources in many states, forcing them to import from various foreign sources to meet the demand. The problem is made acute due to environmental regulations, which do not permit mining of rocks or dredging of sand at many locations. On the other hand, demolition of concrete structures creates large volume of concrete waste, which is traditionally disposed in landfills leading to not only many environmental problems but also locking up precious urban land parcels. Moreover, large quantity of waste concrete is generated in all construction related activities, be it original construction, repair, rehabilitation and retrofitting.

Concrete recycling gains importance because it protects natural resources as it uses the waste concrete as a source of aggregate for new concrete construction or other applications. Using recycled coarse and fine aggregate derived from concrete waste leads to conservation of natural resources, freeing the precious urban land which is otherwise used for waste filling, reducing environmental pollution and reduce the overall construction cost. Demolished Concrete is considered as resource not a waste in many countries and they consume all such materials in their new construction. In our country also many initiatives were taken to utilize the demolished concrete. Technology, plants and equipment are available to process the same. Codes, standards and guidelines are available and rules and regulations are also in place. Now it is the role of construction engineers to incorporate this waste stream into our concrete construction practices, wherever found feasible.

Conventionally, concrete is considered as a material difficult to recycle. But technology has been developed to process concrete to extract coarse and fine aggregate from it. The technology varies from simple crushing to extracting extremely clean aggregates, which can be used for making fresh concrete. The process involved is primary crushing of the rubble to a size of 40-50 mm using a Joe crusher, removing the other material like, wood, plastic, steel, etc.

After that, secondary crushing using an impact crusher is carried out followed by removal of adhered cement paste from the aggregate particles. Removal of hydrated adhered cement paste is essential, as its presence affects both workability and strength of fresh concrete made using these aggregates.

After carrying out different levels of processing, these aggregates can be used in variety applications like sub-base for roads, asphaltic concrete, lean concrete and structural and nonstructural concrete.

Fig. 10: Stack of Coarse and Fine Aggregates in a Recycling Plant

Nearly 95% of concrete waste is recycled in Japan. They allow recycled aggregates in concrete mixes up to M45 grade. UK permits recycled concrete aggregate up to M50 grade concrete. German construction industry recycles almost 85% of waste generated and DIN permits the use of recycled aggregate in concrete up to the strength 37.5 MPa. Norway recycle almost all of their concrete waste.

Use High Performance Concrete

To meet the special performance requirements like high strength, ductility, toughness, fluidity, blast resistance, etc. and increase of service life in severe environments, use of high performance concrete is required to be encouraged. One should remember that high performance concrete is not always the same thing as high strength concrete. New generation superplasticisers are rendering high fluidity to concrete and fibre reinforcement is extensively used for improved ductility and toughness. This led to the development of Ultra high performance (UHPC) concrete with strength level reaching 200 MPa and offering outstanding durability, tensile ductility and toughness. Because of the enhanced strength, structural members are relatively thinner with high ’performance –volume ratio’; thereby, contributing towards sustainability. Very high strength of concrete allow high degree of pre-compression helping to compensate high level of tensile stresses, optimizing the construction of long span structures and very high rise buildings.

Another high performance concrete is that can flow without segregation. Self-Compacting Concrete (SCC) is that which can fill the formwork without the application of vibration, while maintaining its homogeneity. It is suitable for applications where reinforcements are Fig. 10: Stack of Coarse and Fine Aggregates in a Recycling Plant congested, formwork geometry is complicated, pouring points are limited and accessibility to allow vibration is not available. It can be speedily placed, requiring no finishing and will obviate the noise due to vibration. It reduce wastage, improve productivity with improved durability, eventually contributing towards sustainability.

Adopt Emerging Nano Technologies

Nano technology is emerging and influencing the concrete technology like all other fields of science and technology. Research has shown that nano materials can improve various properties as well as impart new properties to concrete. Many nano materials like carbon nanotubes, nano-titanium oxide, nano-alumina, nano-iron, nano-silica are available, but it is nano-silica, which is used commonly in concrete to improve strength and durability. Research results indicates the 2-4% of Nano silica can substantially improve the mechanical and durability properties of concrete. This improvement is through its high pozzolonicity and filler effect.

Fig. 11: Placing of SCC

Nano-silica is available dry powder form or in colloidal form. Colloidal form is found to be more convenient for handling and application in concrete compared to the dry powder form. Its contribution to enhanced properties is through filler effect, Nucleation effect and pozzolonic effect. It acts as a nano filler filling the gel pores, thereby by increasing the packing density of C-S-H. During cement hydration nano-silica particles serve as nucleation sites accelerating the hydration of cement, increasing the rate of release of Calcium hydroxide, which is readily consumed by Nano- silica to form additional C-S-H. Its pozzolonic activity is higher than that of other SCMs especially at early stages.

Fly ash content in structural concrete usually limited to 30% by weight of cement. The pozzolonic reaction of fly ash is slow resulting in delay of strength development and consequent increase in construction time. Nano silica significantly increase the reactivity of fly ash in the cement matrix. Similarly, it will help in overcoming the delay in early age strength of high volume fly ash concrete. This would enable use of larger quantities of fly ash in structural concrete, thereby immensely contributing towards sustainability.

Fig. 12: Pier Cast with SCC

Current methods of nano-silica production are energy sensitive and time consuming. It has to be correctly dosed and dispersed to get the required improvements. High dosage of super plasticizer and appropriate curing methods may be required to reduce the shrinkage. The effect of nano-silica on deterioration of concrete due to carbonation is yet to be fully understood.

Conclusion

Environmental impact of a structure over its entire life cycle is to be reduced to achieve sustainability development goals. The objective of sustainable concrete construction is to achieve less emissions, less consumption of energy and natural resources, less waste generation and structures that requires reduced maintenance, rehabilitation and rebuilding. Engineers can contribute immensely towards sustainable concrete construction through informed intervention and sensitivity towards environmental protection. They can contribute towards sustainability by reducing the cement and water consumption, increased adoption of pre casting technologies, replacing natural resources with industrial waste based products in the making of concrete, encouraging the use of high/ultra high performance concrete and by encouraging the use of emerging nano technologies.

References

  1. M. Neville & J.J Brooks, Concrete Technology, Pearson Education Ltd.
  2. ICI, Handbook on Concrete Durability
  3. ICI, Handbook on Precast Concrete for Buildings

 

 

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