Importance of aggregate grading
Ideally coarse and fine aggregate should be graded in such a way as to minimise the voidage. After compaction the volume of the cement paste must be more than the voidage between particles. Underfilling will result in entrapped air and an unworkable mix.
An extreme example of this is a no-fines concrete where the sand fraction is minimised, the course aggregate interlocks, but nothing fills the voids. This will not protect the reinforcement from corrosion or provide a weather tight structure. Similarly achieving a paste volume to ‘just’ fill the voids will result in a mix where the coarse aggregate will interlock but not necessarily in an optimum compacted state making placing difficult and leaving voids. Some overfilling of the void space between the coarse particles by the sand fraction and between the sand particles by a cement paste is necessary for workability, placeability and durability of the concrete.
Grading curve envelopes are given in BS 882, Specification for aggregates from natural sources for concrete.