Studies on Concrete Made of Recycled Materials for Sustainability

Abstract

Construction industry uses Portland cement which is known to be a heavy contributor to the CO2 emissions and environmental damage. Incorporation of industrial wastes like demolished old concrete, silica fume (SF) and fly ash (FA) as supplementary cementing materials (SCMs) could result in a substantial reduction of the overall CO2 footprint of the final concrete product. However, use of these supplementary materials in construction industry especially in the making of concrete is highly challenging. Significant research efforts are required to study the engineering properties of concrete incorporating such industrial wastes. Present research is an effort to study the properties of concrete incorporating industrial wastes such as demolished concrete, SF and FA.
Recycled coarse aggregate (RCA) concrete construction technique can be called as ‘green concrete’, as it minimizes the environmental hazard of the concrete waste disposal. Indian standard recommends target mean compressive strength of the conventional concrete in terms of water cement ratio (w/c). The behaviour of RCA concrete, prepared from two samples of parent concrete having different age groups, is investigated, to propose the relationship of compressive strength with water cement ratios, in the present study. Number of recycling may influence the mechanical properties of RCA concrete. The influence of age and number of recycling on the properties such as capillary water absorption, drying shrinkage strain, air content, flexural strength and tensile splitting strength of the RCA concrete are examined. While the compressive strength reduces with number of recycling gradually, the capillary water absorption increases abruptly, which leads to the conclusion that further recycling may not be advisable.
Previous studies show that the properties of RCA concrete are inferior in quality compared to NCA concrete. The improvement of properties of RCA concrete with the addition of two ureolytic-type bacteria, Bacillus subtilis and Bacillus sphaericus to enhance the properties of RCA concrete. The experimental investigations are carried out to evaluate the improvement of the compressive strength, capillary water absorption and drying shrinkage of RCA concrete incorporating bacteria. The compressive strengths of RCA concrete are found to be increased by about 20% and 35% at the cell concentrations of 106 cells/ml for the two bacteria. The capillary water absorption as well as drying shrinkage of RCA are reduced when bacteria is incorporated. The improvement of RCA concrete is confirmed to be due to the bacterial mineral precipitation as observed from the microstructure studies such as EDX, SEM and XRD.
The mechanical properties, such as compressive, flexural and tensile splitting strength, of SF concrete considering the 10% additional quantity of cement as recommended by International codes, by partial replacement of slag cement on low to medium strength concrete, have not been investigated so far. The present study investigates the mechanical properties of medium strength SF concrete made as per this construction practice by partial replacement of slag cement. Effect of SF on compressive, flexural and tensile splitting strength of hardened concrete is examined. Seven concrete mixes are prepared using Portland slag cement (PSC) partially replaced with SF ranging from 0 to 30%. The mix proportions were obtained as per Indian standard IS: 10262-2009 with 10% extra cement when SF is used as per the above the construction practice. Optimum dosages of SF for maximum values of compressive strength, tensile splitting strength and flexural strength at 28 days are determined. Results of the present study are compared with similar results available in literature associated with Portland cement. Relationships, in the form of simplified equations, between compressive, tensile splitting and flexural strengths of SF concrete are proposed.
Several studies related to sustainable concrete construction have encouraged the usage of industrial waste products such as SF and FA. Design of structures, made using such SF and FA concrete, for an acceptable level of safety, requires the probabilistic descriptions of its mechanical properties. For this purpose, an extensive experimental programme was carried out on compressive strength, flexural strength and tensile splitting strength properties of SF and FA concrete. The probability distribution models are proposed based on the three goodness-of-fit tests such as Kolmogorov-Sminrov, Chi-square and log-likelihood tests. The proposed probability distributions are used to study performance of typical buildings made of SF and FA concrete through seismic fragility curves and reliability indices.