High potential for LWC growth in India
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M R Kalgal, Head-Technical Services, UltraTech Cement Ltd.
In an interview with Built Expressions, Dr. M R Kalgal, Head-Technical Services, UltraTech Cement Ltd. tells that the designers and suppliers of lightweight concrete products should convince customers about the cost-benefit ratio considering the life cycle costs.
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BE: What exactly is Light Weight Concrete?
MRK: Well, any concrete that is lighter than normal concrete (which weighs around 2200 kg/m3-2400kg/m3) can be called light weight concrete. The density could range from 500 kg/m3-2000 kg/m3. Reduction in weight is achieved either by using light weight aggregates or by incorporating a large volume of minute pores by foaming agents or other techniques. Lightweight aggregates used could be natural ones like pumice, aggregates/pellets made by thermally treating clay, slate or shale, aggregates made by processing industrial by-products like fly ash, furnace bottom ash, slag etc. Polystyrene beads are often used as aggregate to make LWC. Application of LWC can be for non-structural purposes like architectural finishes, facia elements, infill walls in framed structures, screeds etc. or for structural use.
BE: What are the advantages and disadvantages of LWC as compared to conventional concrete?
MRK: The advantages of LWC are reduction in deadweight (resulting in savings in foundations and reinforcement), improved thermal insulation, fire-resistance, and sound absorption. It is easy to lay and when hardened, easier to cut, drill, nail or chase using wood working tools. If used in precast units or blocks, LWC enables easier transportation and handling. If used for cast-in-place concrete there would be reduction in formwork and propping.
The bond between the aggregate and the matrix is stronger in light weight aggregate concrete (LWAC) in comparison to normal weight concrete. Because of the penetration of cement paste into the pores of the aggregate, the weakest zone viz. Interfacial Transition Zone (ITZ) between the aggregate and matrix is virtually absent.
The major disadvantage is that its strength is inversely proportional to the density. Lighter the concrete, weaker it becomes. It should be noted that as the density increases, the strength increases, but all other benefits enumerated above decrease. Hence to use it for structural purposes, certain trade-off between the advantages and disadvantages is inevitable. Typical strength ranges between 2MPa and 20MPa. With rich cement mixes and high range water reducers, researchers claim LWC strengths upto 80MPa.
BE: Can you briefly describe the role of LWC in Precast industry?
MRK: LWC is very useful in making non-structural elements which would be light and hence easy to transport and handle. Even as structural elements, LWC of moderate weight and strengths can be beneficially used to achieve overall economy.
BE: What is the difference between Foam Concrete, Cellular Concrete, Light Weight Aggregate Concrete and Autoclaved Aerated Concrete?
MRK: All of these are different varieties of LWC. Foam Concrete and Cellular Concrete are the same.
Foam(ed) concrete is a highly workable and can incorporate up to 50 per cent entrained air. Foamed concrete is made by injecting stable, pre-formed foam (manufactured on site) into cement sand slurry. It is generally self-leveling, self-compacting and may be pumped. Most popular application is for sub-screeds and filling under-floor voids. Since compressive strength is usually less than 5 MPa, it is generally not used as a structural material. Blocks are manufactured using this technique and are popularly called as Cellular Lightweight Concrete (CLC) blocks.
Autoclaved Aerated Concrete (AAC) is made from silica sand and/or fly ash, lime, cement and water, to which an expansive agent (Aluminium powder/paste) is added. If sand is used, it is ground to the required fineness in a ball mill. The mixing results in a chemical reaction that expands the mixture in the moulds to form small, finely-dispersed air bubbles (air content can be as high as 80%). The semi-solid material is transported to the cutting machine to cut it using steel wires, into the size required for the building elements like blocks and panels. The final phase in the production process is curing under steam pressure of 8 to 12 bar in autoclaves for up to 12 hours.
BE: How is Light Weight Concrete different from Controlled Low Strength Material (CLSM)?
MRK: CLSM generally consists of a mixture of cement, aggregate, water and flyash. Although it has much lower strength than normal concrete like LWC, it does not have many of the attributes of LWC. It is primarily used as structural fill or trench backfill.
BE: What different types of LWC are being used in India?
MRK: The lightweight blocks, especially the AAC blocks, with their superior properties are quite popular for multistoried buildings in many urban areas. CLC blocks are also slowly making inroads, but consistency in quality and supply is still an issue. Foamed Concrete is mostly used as thermal insulation screed or filler here and there. Since the availability of lightweight aggregate is very limited, the only type of LWAC is that using polystyrene beads in some urban areas.
BE: How do you rate the growth of LWC in India?
MRK: There is a very high potential for its growth in India. Supply and use of blocks is expected to increase rapidly. Precast industry is picking up and with its growth the use of LWC in precast is expected to pick-up. As regards the cast-in-place LWC, the ready mix manufacturers have to closely work with the designers and specifiers to understand the requirements, develop the appropriate mixes. Together, the designers and suppliers should convince the customers of the cost-benefit ratio considering the life cycle costs.