Recycled aggregates in concrete: a performance related approach

The relative proportions of the three main constituents within recycled aggregates (unbound stone, crushed concrete and crushed brick) can vary widely and it is generally assumed that, as a result, the performance of concrete containing recycled aggregates can vary significantly. Thus, in order to ensure satisfactory concrete performance, specifications are strict on the composition of recycled aggregates that may be used. However, research has shown that it is possible in many circumstances to use recycled aggregates satisfactorily that do not meet the current specifications in BS 8500-2. A proposed approach, which may encourage wider use of recycled aggregates and promote sustainability, is to base the selection of recycled aggregates on performance-related characteristics that relate the properties of recycled aggregates to concrete performance across the whole range of recycled aggregate quality, independent of constituents and source. To develop this performance-related approach, concrete mixes were cast and tested using combinations of unbound stone, crushed concretes and crushed bricks. From the results, three classes of recycled aggregates have been derived based on Los Angeles coefficient, aggregate absorption, density and drying shrinkage of the combined coarse aggregate. The concept is that the highest quality recycled aggregates will be suitable for high-performance applications, meeting the relevant standards and specifications, while the two lower classes will be more appropriate for lower performance applications. Given this approach, material that is currently not fully specified for use in BS 8500 may be classified and considered for relevant applications. This should remove the main technical barrier that is preventing the uptake of recycled aggregates in concrete, and lead to greater confidence in specifying and using recycled aggregates.