How to Choose the Maximum Aggregate Size for Concrete

Choosing the maximum aggregate size is one of the first decisions in mix design. The number affects both how concrete passes between reinforcing bars and how much cement paste is needed.

Geometric limits

A common rule found in ACI 318 says the nominal maximum size should not exceed:

• One-fifth of the narrowest dimension between form sides
• One-third of the slab depth
• Three-quarters of the minimum clear spacing between bars

For projects in Iran, the Iranian Concrete Code (ABA) and the project specification are the final references. The smallest of these values is the upper limit of the choice.

Why bigger is not always better

As aggregate gets larger, the total particle surface falls and less cement paste is needed to coat it. This can lower cement use and shrinkage. In high-strength concrete, however, smaller aggregate usually behaves better, because the transition zone between paste and stone is more limited and stresses are more evenly distributed.

Choosing by placement method

Conditions	Common option
Massive members with light reinforcement	Almond gravel
Ordinary columns and beams	Gravel with a maximum of 19 to 25 mm
Pumped concrete, dense reinforcement	Pea gravel, or a blend with coarser gravel
Thin sections	Fine coarse aggregate with a maximum of about 9.5 to 12.5 mm

A note on pumping

The pump line diameter also sets a limit, and maximum size must suit it. Coordinate with the concrete supplier or pumping contractor before ordering.

This table is a general guide and does not replace a mix design. To review material types, see the concrete requirements.