Summary
Tests were made on damp and dry aggregates to discover how the definition of “effective water/cement ratio” is affected by aggregate absorption. Analysis of the results indicates that mixes with saturated river gravel aggregates give higher strengths and compacting factors than supposedly similar mixes made with dry aggregates. The coating of cement paste considerably reduces the rate of absorption, which ceases when the paste hardens.
The effective water/cement ratio for strength should be based on the free water content available when the paste hardens. This is known more accurately when saturated aggregates are used. For dry aggregates, crushed rock material becomes saturated to the same degree as after immersion in water for 30 min, but for uncrushed river gravels the degree of saturation depends on the amount and consistence of the cement paste.
The effective water/cement ratio for workability is more difficult to define. It can be assumed, provisionally, that initially dry aggregates will have achieved, at the time of the workability test, the same degree of saturation as they would have in water.
These effects of absorption only apply to high-strength mixes. Rich, uneconomical site mixes can be avoided if laboratory trials are based on the effective water/cement ratio as defined in this paper.
