Technical Questions

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What nominal maximum sizes of aggregate are recommended?

Q. What nominal maximum sizes of aggregate are recommended?

A. According to (a)(5) of ACI 318-19, nominal maximum size of coarse aggregate shall not exceed the least of (i), (ii), and (iii):

(i) one-fifth the narrowest dimension between sides of forms
(ii) one-third the depth of slabs
(iii) three-fourths the minimum specified clear spacing between individual reinforcing bars or wires, bundles of bars, prestressed reinforcement, individual tendons, bundled tendons, or ducts

These limitations shall not apply if, in the judgment of the licensed design professional, workability and methods of consolidation are such that concrete can be placed without honeycombs or voids.

Similar information can be found in ACI 211.1. In concrete slabs, the nominal maximum size should not exceed three-fourths of the minimum clear spacing of reinforcing bars or one-third of the section depth (ACI 302.1R).

The commentary R26.4.2.1(a)(5) of ACI 318-19 states that the size limitations on aggregates are provided to facilitate placement of concrete around the reinforcement without honeycombing due to blockage by closely-spaced reinforcement. Because maximum aggregate size can impact concrete properties such as shrinkage, and also the cost of concrete, the largest aggregate size consistent with the requirements of should be permitted.

With the w/cm fixed, the larger the nominal maximum size of aggregates, the lower the cement content for a given consistency. Or, stated differently, for a given cement content, the larger the nominal maximum size of aggregate, the lower the water requirement. When strength requirements are high, however, smaller nominal maximum size aggregate will produce the highest strength for a given w/cm (ACI 363R, ACI 211.4R).


References: SP-1(02); ACI PRC-211.1-22; ACI 318-19; ACI 302.1R-15; ACI 363R-10; ACI 211.4R-08

Topics in Concrete: 318 Building Code; Aggregate; Codes; Slab; Specifications

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