Gap-graded aggregate and role of grading

Gap-graded aggregate is an aggregate in which one or more intermediate size fractions are missing or present only in small amounts; the grading curve shows a flat (horizontal) portion between two sizes. Example: 20-10 mm coarse aggregate plus sand, with the 10-4.75 mm fraction omitted.

% passing
100|              _______
   |             |
 50|  ___________|    <- gap (flat)
   | /
   +--------------------> log size

Features

• When the sand fraction is correctly chosen, it can give dense packing with less sand, giving good strength and economy; it is used in high-strength concrete and exposed-aggregate finishes.
• Requires a lower sand content (about 30 per cent compared with 40 per cent) for the same workability.
• Prone to segregation (large particles separate from the mortar), so it needs a low slump and careful handling; it is not suitable for pumped or highly workable mixes.

Role of grading in the strength of concrete

• Grading controls the void content, and thus how much paste is needed and the resulting w/c ratio for a given workability.
• A well-graded mix reaches the required workability with less water, so it has a lower w/c ratio and higher strength.
• A dense, well-packed aggregate skeleton reduces voids and permeability and gives uniform stress transfer, so there are fewer weak zones and honeycombs.
• Poor grading (too many fines or too coarse) increases water demand or causes bleeding and segregation, and the strength falls.