ACI Joint Layout & Crack Control 3 — Questions and Answers
Question 1: What is the recommended evaporation rate threshold (lb/ft²/hr) at which precautions against plastic shrinkage cracking should begin?
- 0.10
- 0.20 (Correct answer)
- 0.35
- 0.50
Correct answer: 0.20
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:
- Long-term drying shrinkage cracks after hardening
- Plastic shrinkage cracks during the early hydration period (Correct answer)
- Structural cracks caused by overloading
- Corrosion-induced cracks in reinforcing steel
Correct answer: Plastic shrinkage cracks during the early hydration period
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:
- Long and narrow to reduce the number of joints
- As square as possible with length-to-width ratio not exceeding 1.5:1 (Correct answer)
- At least twice as long as they are wide
- Exactly 10 ft × 10 ft regardless of slab thickness
Correct answer: As square as possible with length-to-width ratio not exceeding 1.5:1
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:
- Rigid and non-compressible to maintain joint width
- Compressible enough to accommodate slab expansion without buckling (Correct answer)
- Bonded to both sides of the joint faces
- Installed after the slab has fully cured
Correct answer: Compressible enough to accommodate slab expansion without buckling
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:
- Cut deeper grooves requiring less saw passes
- Use diamond blades that don't require water cooling
- Can operate before the concrete surface fully sets, reducing raveling (Correct answer)
- Produce wider joints that heal more easily
Correct answer: Can operate before the concrete surface fully sets, reducing raveling
Early-entry saws are designed to cut within 1–4 hours of finishing before the concrete becomes hard, reducing the risk of random cracking.
Question 6: Which condition most commonly causes 'map cracking' (crazing) on a concrete slab surface?
- Excessive troweling of bleed water into the surface (Correct answer)
- Insufficient depth of control joint cuts
- Too much aggregate at the surface
- Delayed installation of isolation joints
Correct answer: Excessive troweling of bleed water into the surface
Crazing results from overworking the surface, especially troweling in bleed water, which creates a weak, high water-cement ratio layer at the top.
Question 7: When should sawing of contraction joints begin after concrete placement?
- Immediately after screeding while concrete is still plastic
- As soon as the concrete can support the saw without raveling the cut edges (Correct answer)
- Only after the concrete has cured for 28 days
- After the first rain event following placement
Correct answer: As soon as the concrete can support the saw without raveling the cut edges
Sawing should begin as soon as the concrete is hard enough that the saw blade does not ravel or dislodge aggregate from the cut edges.
ACI Joint Layout & Crack Control 3
What is the recommended evaporation rate threshold (lb/ft²/hr) at which precautions against plastic shrinkage cracking should begin?