Joint Layout & Crack Control Flashcards
7 cards from real ACI practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Joint Layout & Crack Control flashcards as text
What is the recommended evaporation rate threshold (lb/ft²/hr) at which precautions against plastic shrinkage cracking should begin?
Answer: 0.20
ACI 305R recommends taking precautions when the evaporation rate reaches 0.20 lb/ft²/hr to prevent plastic shrinkage cracking.
Fiber reinforcement added to concrete mix primarily helps control:
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.
When laying out control joints for a large rectangular slab, panels should ideally be:
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.
A premolded joint filler material in an expansion joint must be:
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.
Early-entry dry-cut saws allow joints to be cut sooner than conventional wet saws primarily because they:
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.
Which condition most commonly causes 'map cracking' (crazing) on a concrete slab surface?
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.
When should sawing of contraction joints begin after concrete placement?
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.