Generally we use well graded aggregate or continuous graded aggregate, which means representation of all the standard particle sizes in certain proportion. Assumption made in well gradation is that voids created by the higher size of aggregate will be filled-up by immediate next lower size of aggregate and again some smaller voids will be left out which will again be filled-up by next lower size aggregates.
Practically it has been found that voids created by a particular size may be too small to accommodate the very next lower size. Therefore the next lower size may not be accommodated in the available gap without lifting the upper layer of the existing size. Therefore, Particle Size Interference is created which disturbs the very process of achieving the maximum density.
In fact the size of voids created by a particular size of aggregate can accommodate the second or third lower size aggregates only i.e. voids created by 40 mm will be able to accommodate size equal to 10 mm or 4.75 mm but not 20 mm. This concept is called Gap Grading.
Advantages of Gap Grading
Requirements of sand is reduced by 26 to 40%.
Specific area of area of total aggregates will be reduced due to less use of sand..
Point contact between various size fractions is maintained, thus reducing the dying shrinkage.
It requires less cement as the net volume of voids to a greater extent.
A word of caution while using gap grading is that, sometimes it may lead to segregation and may even alter the anticipated workability. Therefore tests must be conducted before adopting this gradation.