ACI ACI Concrete Finishing Defects & Troubleshooting 1 — Questions and Answers
Question 1: What is the primary cause of scaling on concrete surfaces exposed to freeze-thaw cycles?
- Using deicing chemicals on concrete with low air entrainment (Correct answer)
- Over-vibrating the concrete during placement
- Using too much coarse aggregate in the mix
- Finishing the concrete too late
Correct answer: Using deicing chemicals on concrete with low air entrainment
Scaling is most often caused by applying deicing salts to concrete that lacks adequate air entrainment to withstand freeze-thaw cycles.
Question 2: Surface dusting of a concrete floor is most commonly caused by which finishing error?
- Troweling the surface while bleed water is still present (Correct answer)
- Using too coarse a broom for the final texture
- Applying the second trowel pass too late
- Over-curing the slab with water
Correct answer: Troweling the surface while bleed water is still present
Troweling over bleed water dilutes the surface paste, resulting in a weak, dusty wearing surface.
Question 3: Delamination of a concrete slab surface occurs because:
- Air or bleed water becomes trapped beneath a prematurely sealed surface (Correct answer)
- The slab was cured too quickly with a curing compound
- Too much coarse aggregate was used near the top
- The concrete was placed at too high a slump
Correct answer: Air or bleed water becomes trapped beneath a prematurely sealed surface
When the surface is closed by troweling before underlying bleed water and air escape, they become trapped, creating a thin shell that later separates.
Question 4: According to ACI, what finishing action most frequently leads to delamination in flatwork?
- Finishing the surface before bleed water has evaporated (Correct answer)
- Using a bull float before screeding
- Applying the curing compound before final troweling
- Placing concrete directly on a dry subgrade
Correct answer: Finishing the surface before bleed water has evaporated
ACI states that prematurely closing the surface traps bleed water and air beneath the mortar-rich layer, causing delamination.
Question 5: Which of the following best describes the ACI definition of surface scaling?
- Peeling or flaking of the near-surface mortar layer (Correct answer)
- Random shallow cracks across the slab
- White salt deposits on the surface
- Loss of surface paste due to abrasion
Correct answer: Peeling or flaking of the near-surface mortar layer
ACI defines scaling as the progressive loss of surface mortar or aggregate, exposing the underlying paste or coarse aggregate.
Question 6: Plastic shrinkage cracking differs from drying shrinkage cracking primarily because it occurs:
- Before the concrete has achieved initial set (Correct answer)
- After the concrete has fully hardened
- Only in slabs thicker than 6 inches
- Only when calcium chloride is used
Correct answer: Before the concrete has achieved initial set
Plastic shrinkage cracks form while concrete is still in a plastic state, when rapid evaporation exceeds the bleed rate.
ACI ACI Concrete Finishing Defects & Troubleshooting 1
What is the primary cause of scaling on concrete surfaces exposed to freeze-thaw cycles?