ACI Joint Layout & Crack Control 3

Question 1

What is the recommended evaporation rate threshold (lb/ft²/hr) at which precautions against plastic shrinkage cracking should begin?

Accepted Answer

0.20

Answer

ACI 305R recommends taking precautions when the evaporation rate reaches 0.20 lb/ft²/hr to prevent plastic shrinkage cracking.

Question 2

Fiber reinforcement added to concrete mix primarily helps control:

Accepted Answer

Plastic shrinkage cracks during the early hydration period

Answer

Synthetic and steel fibers are most effective at limiting plastic shrinkage cracking by bridging micro-cracks before the concrete gains strength.

Question 3

When laying out control joints for a large rectangular slab, panels should ideally be:

Accepted Answer

As square as possible with length-to-width ratio not exceeding 1.5:1

Answer

Panels should be as square as possible; ACI recommends a maximum length-to-width ratio of 1.5:1 to reduce diagonal cracking.

Question 4

A premolded joint filler material in an expansion joint must be:

Accepted Answer

Compressible enough to accommodate slab expansion without buckling

Answer

Expansion joint filler must compress when the slab expands due to temperature rise so the concrete does not buckle or spall.

Question 5

Early-entry dry-cut saws allow joints to be cut sooner than conventional wet saws primarily because they:

Accepted Answer

Can operate before the concrete surface fully sets, reducing raveling

Answer

Early-entry saws are designed to cut within 1–4 hours of finishing before the concrete becomes hard, reducing the risk of random cracking.

(ACI) American Concrete Institute Concrete Finisher Practice Test