Effect of absorption of limestone aggregates on strength and slump loss of concrete

Slump loss of concrete is very important in construction practice particularly with ready-mixed concrete. It is believed that slump loss occurs due to reduction of mixing water caused by aggregate absorption, evaporation, and cement hydration. In this study, the effect of absorption of limestone aggregates on the effective w/c ratio and on the strength and slump loss of concrete was investigated. It was found that the absorption of dry aggregates in all concrete mixes considered in this study occurred mainly in the first 15 min after the start of mixing and diminished substantially thereafter. The rate or slump loss or concrete was almost identical for mixes with dry and wet aggregates, even when setting time was extended. The results indicated that limestone aggregates used in this study with an absorption capacity of 1.3-1.9% by weight can absorb about 75% of their absorption capacity when mixed in concrete. The remaining 25% will add to the free water, thus increasing the effective w/c ratio. This phenomenon was demonstrated by the increased initial slump and decreased compressive strength of mixes with dry aggregates compared to those with wet aggregates. (C) 2009 Elsevier Ltd. All rights reserved.