A STUDY ON THE REUSE OF PLASTIC CONCRETE USING EXTENDED SET-RETARDINGADMIXTURES

The disposal of ready mixed concrete truck wash water and returned pla stic concrete is a growing concern for the ready mixed concrete indust ry. Recently, extended set-retarding admixtures, or stabilizers, which slow or stop the hydration of portland cement have been introduced to the market. Treating truck wash-water or returned plastic concrete wi th stabilizing admixtures delays its setting and hardening, thereby fa cilitating the incorporation of these typically wasted materials in su bsequent concrete batches. In a statistically designed experiment, the properties of blended concrete containing stabilized plastic concrete were evaluated. The variables in the study included (1) concrete age when stabilized, (2) stabilizer dosage, (3) holding period of the trea ted (stabilized) concrete prior to blending with fresh ingredients, an d (4) amount of treated concrete in the blended batch. The setting tim e, strength, and drying shrinkage of the blended concretes were evalua ted. For the conditions tested, batching 5% treated concrete with fres h material did not have a significant effect on the setting time, stre ngth, or drying shrinkage of the resulting blended concrete. Batching 50% treated concrete with fresh materials had a significant effect on the setting characteristics of the blended cocnrete, which in turn aff ected the water demand to maintain slump. The data suggests that for a known set of conditions, the stabilizer dosage can be optimized withi n a relatively narrow range to produce desired setting characteristics . The strength and drying shrinkage of the blended concretes were esse ntially a function of the water content at different sampling ages and the relationship followed the general trend of control concrete.