According to 26.4.2.1 (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

This Bulletin describes types of aggregates normally used in concrete, aggregate properties affecting performance of the concrete, tests used to measure aggregate properties, and methods used to obtain test samples. Normalweight as well as lightweight aggregates are discussed. The measurement system used in this Bulletin is the Inter-

Maximum Size versus Nominal Maximum Size: The specification I prepared for a project calls for “Maximum coarse aggregate size to be 1 in. nominal.”

As the nominal maximum size of the aggregate is increased, the amount of water needed for the same workability is reduced. At the same cementitious material content, strength is therefore greater because the w/cm is lower.

ASTM C31 requires the cylinder diameter to be 3 times the nominal maximum size of the coarse aggregate. The nominal maximum size of an aggregate is the smallest sieve opening through which the entire amount of aggregate is permitted to pass.

Aggregate packing is one of several factors to consider in selecting aggregates. Aggregate gradings finer than maximum packing density are often preferred. The packing density, along with shape, is useful in determining amount of paste required. Consider the combined solid particle size distribution.

In general, a decrease in the maximum coarse aggregate size may have advantages when it comes to concrete compressive strength: “internal weak planes may be less likely to exist, and the smaller particles reduce stress concentration effects at paste-aggregate interfaces.”1 Also, smaller-sized aggregate having a higher surface area

Results indicate that increasing the maximum size of coarse aggregate may not necessarily be beneficial to con-crete strength. Drying shrinkage was not substantial1 increased by reduced size of aggregate down to about 8/4 in.

Maximum (coarse) aggregate size: Water Content, Bulk Volume of Coarse Aggregate, Required air content for Freeze-and-Thaw. Fineness Modulus: Bulk Volume of Coarse Aggregate. Use of round aggregate: Reduce water contents by 8%. ACI 302.1R Guidelines for the gradings of fine, coarse, and the combined aggregates for floor and

Depending on circumstances such as richness of mix, individual properties of the particular aggregate, and the magnitude of the size difference, an increase in maximum size may result in either an increase or decrease in concrete strength at a fixed cement content.