ACI ACI Concrete Finishing Defects & Troubleshooting 2 — Questions and Answers
Question 1: Crazing in a concrete surface is best described as:
- A network of fine random cracks confined to the surface mortar (Correct answer)
- Deep structural cracks caused by overloading
- Large isolated cracks from shrinkage
- Cracks that run parallel to the long axis of the slab
Correct answer: A network of fine random cracks confined to the surface mortar
Crazing is a map-cracking pattern of very fine surface cracks that do not penetrate deeply into the slab.
Question 2: What is the most common cause of crazing in concrete flatwork?
- Rapid drying of the surface before the concrete has set (Correct answer)
- Using air-entrained concrete in warm weather
- Placing concrete on a saturated subgrade
- Applying too much curing compound
Correct answer: Rapid drying of the surface before the concrete has set
Rapid surface drying shrinks the paste-rich surface layer faster than the body of the slab, producing the characteristic map-crack pattern.
Question 3: Surface bug holes (voids) on formed concrete walls are primarily caused by:
- Air trapped against the form face that was not released during vibration (Correct answer)
- Insufficient water in the concrete mix
- Over-vibration of the concrete
- Using aluminum forms instead of wood
Correct answer: Air trapped against the form face that was not released during vibration
Bug holes form when air pockets trapped between the fresh concrete and the form surface are not released by adequate vibration.
Question 4: ACI recommends that protective measures against plastic shrinkage cracking should begin when the evaporation rate exceeds:
- 0.20 lb/ft²/hr (Correct answer)
- 0.50 lb/ft²/hr
- 1.00 lb/ft²/hr
- 0.10 lb/ft²/hr
Correct answer: 0.20 lb/ft²/hr
ACI 305R states that when the evaporation rate exceeds 0.20 lb/ft²/hr, precautions such as windbreaks, sunshades, or evaporation retarders are required.
Question 5: Bleed water rises to the surface of fresh concrete because:
- Heavier cement and aggregate particles settle, displacing mix water upward (Correct answer)
- Air entrainment forces water to the top
- The cement hydration reaction generates water
- Retarders prevent water absorption by aggregates
Correct answer: Heavier cement and aggregate particles settle, displacing mix water upward
Bleeding is the natural settlement of solid particles under gravity, which displaces mix water upward to the surface.
Question 6: Finishing concrete before bleed water has evaporated traps water under the surface, leading to:
- Delamination and scaling during service (Correct answer)
- Increased compressive strength
- Reduced plastic shrinkage cracking
- Better surface texture and hardness
Correct answer: Delamination and scaling during service
Entrapment of bleed water creates a weak, porous plane just below the surface that later separates under traffic or freeze-thaw action.
ACI ACI Concrete Finishing Defects & Troubleshooting 2
Crazing in a concrete surface is best described as: