Evaluation of conventional and equivalent mortar volume mix design methods for recycled aggregate concrete

The use of recycled aggregate concrete (RAC) is undoubtedly on the increase and further research is required for more appropriate mix design methods. This paper presents the result of investigations into the mix design of RAC. The study examined experimentally two conventional and one unconventional mix design methods. The conventional methods include the absolute volume approach according to the American Concrete Institute (ACI) standard and the Department of Environment (DoE) concrete mix design method given by the British specification. The unconventional method is a new technique dubbed "Equivalent mortar volume (EMV)" method developed to incorporate both fine and coarse recycled aggregates for RAC production. A total of four mixes were designed as follows: (1) natural aggregate concrete regarded as the reference mix using the ACI guide, (2) RAC using the ACI guide, (3) RAC using the DoE guide and (4) blended aggregate concrete using the EMV guide. Fresh concrete properties were tested, and twenty concrete cubes and eighteen cylinders were produced accordingly and tested after curing by water immersion for density, compressive strength, tensile splitting strength and water absorption. The results showed that the EMV mix design method used significantly lesser amount of cement to achieve a RAC of higher strength than those obtained from its comparable conventional methods even with original natural aggregates. This is the first work on mix design of RAC incorporating both coarse and fine crushed concrete aggregates carried out using the EMV method. This research highlights that the ACI and DoE mix design method guides should be reviewed and amended appropriately, before adopting them for RAC mix design.