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. Particle Size Distribution (gradings) are given in BS EN 12620, Aggregates for Concrete and is supported by PD 6682-1 for UK use.
At some aggregate sources (quarry/pit) a problem that can be encountered is the fraction availability when screened/crushed. For example a 0/4 (CP) may need to be supplemented with a finer ´fines´ graded such as 0/2 and then proportionally blended to provide an optimum grading say 0/4 (MP).