By Dr. Dipak Kumar Sahoo, Professor, School of Engineering, CUSAT
It is learnt from unconfirmed sources that about 7000 tonnes of demolition wastes will be generated from the demolition of the four flats at Maradu. Of this about 1000 tonnes could be concrete wastes from the frame of the structures. It is felt that the good quality concrete waste can be recycled and reused completely as aggregates for fresh concrete or for manufacturing masonry blocks instead of throwing it as a waste or using as a land-fill. In this context, the following research findings of CUSAT are relevant.
The most economically gainful and ecologically sustainable use of concrete waste lies in its complete reuse as recycled concrete aggregates (RCA) for structural concrete applications because aggregates constitute the bulk of any concrete. Instead of using the waste as landfills, it is wise to reuse the waste as quality products.
When waste concrete is crushed, three structurally distinct fractions of crushed materials are obtained. The superfine fraction below 0.15mm can be used as pozzolanic material in fresh concrete or used in small proportion along with quarry sand to get better finish in plastering or pavements.
The fine RCA fraction in the range of 0.15 to 4.75 mm is quite similar to quarry sand used in Kerala these days for concrete and plastering, and international research over the past two decades has established that this fraction is an effective substitute for sand.
The major product of crushing of concrete waste , that is, coarse RCA in the size range of 4.75 to 20 mm, classified according to the ASTM Standard C 294 (2005) as an artificial aggregate, holds immense promise as an effective substitute for fresh natural coarse aggregate (NCA) for structural concrete applications, the only impediment being that the Bureau of Indian Standards is yet to explicitly specify its structural suitability in the Indian Concrete Code IS 456.
To ascertain the technical feasibility of coarse RCA (obtained by crushing of concrete waste) as a substitute for fresh natural coarse aggregate (obtained from crushed stone), a three-year long (2013-16) KSCSTE-sponsored research project was undertaken successfully by a team consisting of Professor (Dr.) Dipak Kumar Sahoo as the Principal Investigator, Dr. Glory Joseph as Co-Principal Investigator and two research scholars, Praveen Mathew and Biju Varghese at School of Engineering, CUSAT (Sahoo, 2016). The concrete waste was sourced from an old framed building which was demolished due to Kochi Metro Rail Project and crushed by a mini jaw crusher in the lab.
This study was focused on the complete utilisation of crushed concrete waste as a substitute for natural (crushed stone) coarse aggregate (NCA) for use in fresh structural concrete as well as masonry blocks. The research study showed that old concrete can be recycled in-situ or in a crusher plant and used as coarse aggregate in fresh structural concrete and as aggregates for manufacturing masonry blocks of acceptable quality.
An innovative concrete mix proportioning method was developed and successfully tested in this investigation to use recycled concrete coarse aggregate (RCA) in place of natural coarse aggregate (NCA) to the maximum extent possible (Mathew and Sahoo, 2019). The proposed method does away with the conventional percentage replacement which produces inferior mix. The equivalent mortar volume (EMV) method proposed in the literature was simplified and fused with the mix proportioning steps recommended in the Indian Standard IS 10262 : 2009, wherein the residual mortar attached to the coarse recycled concrete aggregate (RCA) particles was proposed to be treated as part of the total mortar volume in the equivalent fresh NCA concrete.
Test results from the laboratory experiments suggested that the quality of this well-proportioned RCA concrete was closely comparable with that of conventional NCA concrete. Experiments also suggested that overnight pre-soaking of RCA in super plasticizer- laden water could remarkably enhance the workability of RCA concrete. The modern self- compacting concrete, which flows on its own and does not require mechanical compaction, was also produced using 10 mm down coarse RCA.
Trial mixes of normal and self- compacting concrete were prepared for RCA concrete and equivalent NCA concrete and their fresh and hardened properties were determined and compared. RCA concrete was found to have a low workability than NCA concrete. This problem could be overcome by adjusting the proportion of super-plasticiser (HRWRA) and admixtures like silica fume and fly ash.
Compressive strength of RCA concrete was found to be slightly less than the equivalent NCA concrete, but the flexural and shear strengths were found to be comparable and at times better. The stress-strain patterns were similar although the peak was lower for RCA concrete. Plain and reinforced concrete panels, beams and short columns were tested for structural suitability of RCA concrete, Ultrasonic pulse velocity measurements showed comparable and good response in NCA as well as RCA concrete.
Solid masonry blocks of size 300×200×100 mm were manufactured with ordinary Portland cement, manufactured sand (as fine aggregate) and 12 mm down-graded coarse RCA in volumetric proportion of 1:1.5:5.5. The blocks, prepared in a hydraulic block manufacturing machine, and tested after four weeks of water curing, fulfilled all the technical requirements of the Indian Standard IS 2185 (Part 1) : 2005.
Recommendations regarding reuse of concrete waste from Maradu:
The concrete waste has to be segregated from other demolition wastes and crushed in-situ or in a nearby crusher to the required size range. The fraction between 0.15 and 4.75 mm can be used as quarry sand for producing fresh concrete or cement mortar for masonry and plastering works. No codal restriction is there on its use if it fulfills the particle size distribution specified for sand in IS 10262.
The small quantity of superfine fraction below 0.15 mm obtained from crushing can be used in cement mortar in allowed proportion to get smooth finish in plastering or in pavement or non-structural concrete for finishing works. The fraction between 4.75 and 10 mm can be used as coarse aggregate for manufacturing masonry blocks. The fractions, 4.75-10 mm and 4.75-20 mm, are suitable for producing conventional and self-compacting concrete (SCC), respectively, subject to approval by clients because their use as coarse aggregate for structural concrete is not yet permitted in Indian codes.
(Note that many countries including Japan have brought out national codes permitting the use of coarse RCA for structural concrete applications). Alternatively, the entire concrete waste can be crushed to below 4.75 mm size for use as fine aggregate (sand) or crushed to 4.75-10 mm size for use as coarse aggregate for manufacturing masonry blocks, because to our knowledge there is no codal bar on such uses.
