Field Validation of Recycled Concrete Fines Usage

Field Validation of Recycled Concrete Fines Usage

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    Final Project 9/16/2013-2/15/2015
  • Abstract:
    The amount of recycled concrete fines permitted in concrete mixing water is limited by ASTM C 1602 to 5.0 percent of the mixing water, by mass, in order to avoid detrimental effects on concrete properties. Depending upon the exact nature of the recycles concrete fines, researchers have reported no detrimental effects at significantly higher fines contents in some cases, and unacceptably-lowered strengths at fines contents below the allowed limits in other cases. In practically all instances, concrete producers control the quantity of recycled concrete fines by measuring the specific gravity of the mix water containing the fines. This measurement, while providing an indication of the total amount of fines in the water, is unable to distinguish between dissolved and suspended solids. In addition, the effect of pH – significant in terms of the rate of cement hydration, is ignored. Recent work has looked at characterizing the fines in terms of both the conductivity of the mix water containing the fines and the pH of the mix water. Correlations relating performance of mortar mixtures and the conductivity and pH of the mix water have been developed. Performance characteristics included set time as well as compressive strength at 3 and 28 days. This report documents results of using revised performance correlations on concrete produced at a ready-mix concrete plant. An instrumentation assembly with conductivity and pH probes was placed into the tank used to weigh the mix water. Mixtures with either no recycled fines or two different levels of recycled fines content were then prepared in full-truck batches and compression specimens were prepared from concrete obtained from the trucks. This was repeated for a total of four separate sampling days, in order to achieve some variation in the exact nature of the recycled fines. Compression results indicated that all of the mixtures achieved at least 90 percent of the control 3-day strength and the only mixture to not achieve 90 percent of the control 28-day strength was correctly predicted. The occurrence of some false-negative predictions for mixtures with higher pH mixing water indicates that additional work is needed in order to refine the predictive equations so they are reliable for a larger range of recycled concrete-fines mixing water parameters.
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