Paper and Presentation by George Vorobieff & Graham Hennessy from ASCP 3rd Concrete Pavements 2015
The quality of the input of fine and coarse aggregates for concrete has an influence on the quality and consistency of the new concrete, and this is particularly applicable when recycled concrete aggregates (RCA) are incorporated in the concrete. The overseas literature indicates that where substitutions do not exceed 20%, the variability of the concrete aggregate does not appear to affect the quality of the output recycled aggregate concrete (RAC).
Where the content of coarse natural aggregates is replaced by coarse RCA by 20% to 30% in concrete production, the international literature indicates that detrimental impacts on the properties of concrete are possible. As the substitution levels approach 30%, adverse changes in key properties, such as increased permeability and shrinkage become more pronounced.
Appropriate process control measures must be developed by suppliers to supplement those required for the specification and handling of extracted quarried aggregate. For example, equipment used to process RCA is similar to that used to process aggregates but with additional features to remove contaminants, such as steel, soil and vegetation. Typical processes involve mechanical and magnetic separation, primary screening, primary crushing (e.g. jaw crusher), secondary and tertiary crushing (e.g. impact crusher), screening/air sifting/washing and pre-wetting to achieve saturation prior to mixing.
The fines fraction of RCA is not recommended for use as concrete aggregate as this results in significant reductions in concrete workability, strength and durability.
Further research work and monitoring of the application of these recommendations on highway pavements will be undertaken before the recommended changes are made to specifications R82 and R83.
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