ACI 318-19

ACI 318-19 Section 24.3 | Crack Control & Maximum Bar Spacing

Even properly designed reinforced concrete will crack on the tension side. To keep crack widths narrow for aesthetic and corrosion protection reasons, the code limits the maximum spacing between individual rebar bars.

Code Equation

Governing Formula

s_{\max} = 15 \left( \frac{40000}{f_s} \right) - 2.5 c_c \le 12 \left( \frac{40000}{f_s} \right)

Standard: ACI 318-19Dual US Customary & SI Metric

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Notation & Variables

s_{\max}

Maximum center-to-center spacing of flexural reinforcement

f_s

Stress in reinforcement at service loads (permitted to be taken as 2/3 fy)

psi

c_c

Least distance from surface of reinforcing steel to the tension face

Worked Example

Estimate service stress

Take service stress fs = (2/3) * 60,000 = 40,000 psi.

Calculate equation limits

For an interior slab with 1.0 inch clear cover (cc = 1.0"), compute s_max.

s_{\max} = 15(1.0) - 2.5(1.0) = 12.5 \text{ in }

Check upper cap

Upper limit is 12 * (40000 / 40000) = 12.0 inches. Therefore, s_max = 12.0 inches controls.

Governing Criteria

When does this govern?

Frequently controls rebar detailing in thin two-way slabs, bridge decks, and water-retaining structures.

Common Pitfall

Common Mistake

Specifying 2 or 3 jumbo bars instead of distributing smaller bars evenly across the beam width, resulting in wide tension cracks under normal service loads.

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