Question 1

How is crack width checked per ACI 318-19?

Accepted Answer

ACI 318-19 Section 24.3 (Gergely-Lutz approach) controls crack width indirectly by limiting the maximum bar spacing: s ≤ 380(280/fs) – 2.5cc for exterior exposure, or 300(280/fs) for interior, where fs is the service steel stress.

Question 2

What is the service steel stress fs used in crack width checks?

Accepted Answer

fs = Ms/(As × jd) ≈ 2fy/3 at service load, where Ms is the service moment, As is steel area, and jd ≈ 0.875d is the moment arm. Alternatively, fs is computed directly from the cracked transformed section.

Question 3

What is the clear cover cc in the crack width formula?

Accepted Answer

cc is the clear cover from the concrete surface to the bar nearest the tension face, measured to the bar surface (not the centerline). ACI 318-19 allows cc as small as 40 mm (1.5 in.) in the bar spacing formula.

Question 4

Why is crack control important for structural performance?

Accepted Answer

Cracks wider than ~0.3 mm (exterior) or 0.4 mm (interior) allow moisture and chloride ingress, accelerating corrosion of reinforcing steel. ACI 318-19 limits crack width indirectly through bar spacing to maintain durability.

Question 5

Does ACI 318-19 give an explicit maximum crack width?

Accepted Answer

No. Unlike Eurocode 2 (w = 0.3 mm limit), ACI 318-19 controls crack width indirectly through maximum bar spacing requirements in Section 24.3. The implicit limits are approximately 0.33 mm for interior and 0.25 mm for exterior exposure.

Step	Formula	Substitution	Result
s,max	380(280/fs) − 2.5cc	380×(280/240)−2.5×40	343 mm
s,prov	s	150	150 mm	✓

Step	Formula	Substitution	Result
s,max	380(280/fs) − 2.5cc	380×(280/240)−2.5×40	343 mm
s,prov	s	150	150 mm	✓

Crack Control

Crack Control via Spacing

Parameters

Crack Control

Given:

Material Properties:

Max Spacing (ACI 24.3.2)

Crack Control

Crack Control via Spacing

Parameters

Crack Control

Given:

Material Properties:

Max Spacing (ACI 24.3.2)