Using recycled concrete in construction

USING RECYCLED CONCRETE IN CONSTRUCTION

Dr J D Bapat

The concrete structures which have rendered economic service life are fully / partially demolished for repair or rebuilding. In such cases, large quantity of concrete rubble is required to be disposed off. The recycling of such concrete rubble as an aggregate in new construction is an economic and eco-friendly option. This practice is encouraged and prevalent in many countries. According to one report from the Federal Highway Administration (USA), concrete with recycled concrete aggregate (RCA) performs as good as the concrete with natural coarse aggregate (NCA). The reports on the comparison of strength properties of concrete with NCA and that from RCA show that RCA gives at least two-third of the mechanical properties, namely compressive strength and the elastic modulus in comparison to that with NCA. The plants for production of RCA are similar to those engaged in the production of crushed stone aggregate.

The recycling of concrete is a relatively simple process. It involves breaking, removing and crushing existing concrete into a material with specific size and quality. For example, unprocessed RCA with 50 mm or 37.5 mm size is useful for pavement sub-base. The ACI 555 (2001) gives more information on the subject. The processing of RCA involves removal of reinforcing steel and other embedded materials, contaminants like asphalt, soil, glass, gypsum boards, sealants, plaster, paper, wood and roofing materials. The processed RCA can be used in (a) pavements, shoulders, median barriers, sidewalks, curbs and gutters, bridge foundations, (b) structural grade concrete, (d) soil-cement pavement bases, (d) lean concrete and (e) bituminous concrete.

The crushing characteristics of RCA are similar to those of natural rock and are not significantly affected by the grade and quality of the original concrete. It can be batched, mixed, transported, placed and compacted in the same manner as in conventional concrete.

The use of RCA as coarse aggregate is common but using it as fine aggregate needs more care. The substitution of fine aggregate up to 10-20 % by the RCA is beneficial; optimal rate may be decided by trials.

The water absorption of RCA has been found to be higher due to higher porosity. Therefore it is recommended that RCA be batched in pre-wetted and close- to-saturated surface-dry condition. In order to achieve the same workability, slump and water-to-cement ratio, the paste content (i.e. the amount of cementitious) or the amount of superplasticiser may have to be increased.

It is generally accepted that, when natural sand is used, up to 30 % of NCA can be replaced with RCA, without significantly affecting the mechanical properties of concrete; replacing higher amounts may result in increased drying shrinkage.

With the growing construction activity, the shortage of aggregate is growing, especially in cities. At the same time, concrete waste such as rejected concrete elements and rubble from demolished concrete structures, is also growing at a fast rate, raising ecological and environmental threats. The RCA is a viable alternative to using natural aggregate in concrete, especially in situations where natural aggregate is not locally available or transported over a long distance. Although the processing cost of recycled aggregate is higher in some cases, the situation will change as the natural aggregate becomes scarce.

Question:

Do you think recycling the buildings waste in concrete is proper and effective utilisation of waste ?