Question

You are working on a mix design and have been asked by the engineer to recommend the maximum size aggregate that can be used in the mix. The concrete is for a continuously reinforced concrete slab. The slab is 10 inches thick and will be reinforced with No. 6 bars (3/4-inch diameter) at 6 inch on-center (so a 5 1⁄4 inch space between the bars). The bars will be placed just above mid-depth of the slab so they have 4 inches of cover of concrete. What is the maximum size aggregate for this slab based on the absolute volume method of concrete design

Answer 1

According to ACI 318-19( clause 26.4.2.1 (a)(5),

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

If we take one third depth of slab, 10/3 = 3.33 inches

               three fourths clear spacing between bars = (5.25x3/4)=3.93 inches

Hence maximum size should be less than 3.33 inches