A Study on the Reuse of Plastic Concrete Using Extended Set-Retarding Admixtures

The disposal of ready mixed concrete truck wash water and returned plastic concrete is a growing concern for the ready mixed concrete industry. 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 with stabilizing admixtures delays its setting and hardening, thereby facilitating the incorporation of these typically wasted materials in subsequent 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 treated (stabilized) concrete prior to blending with fresh ingredients, and (4) amount of treated concrete in the blended batch. The setting time, strength, and drying shrinkage of the blended concretes were evaluated. For the conditions tested, batching 5 % treated concrete with fresh material did not have a significant effect on the setting time, strength, 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 affected the water demand to maintain slump. The data suggests that for a known set of conditions, the stabilizer dosage can be optimized within a relatively narrow range to produce desired setting characteristics. The strength and drying shrinkage of the blended concretes were essentially a function of the water content at different sampling ages and the relationship followed the general trend of control concrete.