Abstract:
This study investigates mechanical properties of concrete made using stone chips (SC), crushed burnt clay brick chips (CBC), recycled brick concrete (RCB) and recycled stone concrete (RCS) as coarse aggregates. Compressive strength, splitting tensile strength, core strength, rebound hammer strength, modulus of elasticity and Poisson’s ratio of concrete are studied. The American Concrete Institute (ACI) guidelines are specifically intended for stone aggregate concrete. Hence, it is the aim here to compare the influence of other aggregates on early age behavior of concrete, identify any deviations and propose suitable correlations. Compressive strengths for this study range between 20 MPa to 45 MPa. Evaluations from the parametric studies are briefly discussed below.
In-situ strength of 100 mm, 75 mm and 50 mm cores taken from blocks are compared to the compressive strength of molded cylinders. Tolerance factor method was found to be conservative with more scatters than Alternate Method in predicting equivalent core strength. RCS and CBC cores show higher strength than SC and RCB cores, respectively. Relative core strength decreases as core diameter decreases or concrete strength increases with values between 70-100% for SC and RCS. Strengths for CBC and RCB cores are slightly overestimated. The standard deviation increases for smaller diameter cores leading to less reliable estimation of in-situ strength.
RCS concrete exhibits higher splitting tensile strength than SC concrete followed by CBC and RCB concrete. It varies as the square root of compressive strength for SC and RCS concrete whereas for CBC and RCB, the exponent is 0.40 and 0.70, respectively. ACI code tends to overestimate the strength and hence power based correlations were suggested through regression analysis. Using integral absolute error (IAE) as goodness of fit revealed similar percentage variation for SC and RCS concrete (9.5-10%) and CBC and RCB concrete (7-8.5%) inferring recycled aggregates inherit the properties of parent aggregates and demonstrate similar concrete properties.
Rebound hammer test was performed on concrete cubes to investigate the in-situ concrete quality. Correlations were developed taking rebound number and compressive strength of cylinders as parameters. Employing 95% confidence limits and then excluding outliers significantly improves the regression coefficient of the correlation.
Estimation of modulus of elasticity according to ACI 318 produced results for SC concrete that corresponds quite closely but deviates for other concrete types. Inclusion of unit weight of concrete seems to provide better fitted correlation with similar equation for SC and CBC concrete and RCS and RCB concrete. Comparison made with ACI-318, AIJ, EN 1992 which reveals overestimated values of modulus of elasticity. The Poisson’s ratio tends to be between 0.15-0.20 on average.
It is concluded that RCS can be a suitable alternative to stone and brick chips.
