ACI Flatwork Placement & Leveling 2 — Questions and Answers
Question 1: When placing concrete in hot weather (above 90°F), which practice best prevents rapid moisture loss during flatwork placement?
- Increase the water-cement ratio
- Use sunshades, windbreaks, and mist the subgrade before placement (Correct answer)
- Add extra cement to the mix
- Pour concrete thicker than specified
Correct answer: Use sunshades, windbreaks, and mist the subgrade before placement
Sunshades, windbreaks, and pre-moistened subgrade reduce evaporation rate and prevent premature surface drying during hot-weather placement.
Question 2: What is the primary purpose of using a highway straightedge (10-foot screed) during flatwork leveling?
- To compact the concrete surface
- To detect and remove high and low spots across the slab (Correct answer)
- To create control joints
- To apply curing compound evenly
Correct answer: To detect and remove high and low spots across the slab
A highway straightedge spans across the slab to identify and correct high and low spots, ensuring a flat, even surface.
Question 3: During concrete placement, why should concrete not be moved horizontally with a vibrator?
- It wastes energy
- It causes segregation of aggregates and weakens the concrete (Correct answer)
- It creates surface cracks immediately
- It damages the vibrator head
Correct answer: It causes segregation of aggregates and weakens the concrete
Moving concrete horizontally with a vibrator causes aggregate segregation, pushing coarse particles away and creating weak zones in the slab.
Question 4: What does 'F-number' (FF/FL) system measure in flatwork?
- Concrete compressive strength
- Flatness and levelness of the slab surface (Correct answer)
- Fiber content in the mix
- Finishing time after placement
Correct answer: Flatness and levelness of the slab surface
The F-number system uses FF (flatness) and FL (levelness) values to quantify surface tolerance and waviness of concrete floors.
Question 5: Which condition indicates the concrete surface is ready for bull float finishing after screeding?
- Surface water (bleed water) has completely evaporated (Correct answer)
- Concrete begins to harden and resist a boot print
- Immediately after screeding while concrete is still very wet
- After 2 hours regardless of bleed water status
Correct answer: Surface water (bleed water) has completely evaporated
Bull floating should begin after initial bleed water disappears so the tool does not trap water beneath the surface, which would weaken it.
Question 6: When placing concrete for a sloped flatwork slab (e.g., a driveway approach), where should placement begin?
- At the top (high end) of the slope
- At the bottom (low end) of the slope (Correct answer)
- In the center and work outward
- At any point since slope doesn't matter
Correct answer: At the bottom (low end) of the slope
Placement begins at the low end of a slope so that fresh concrete is placed against already-placed concrete, preventing runoff and ensuring proper consolidation.
Question 7: A concrete slab shows 'blistering' during finishing. What is the most likely cause?
- Insufficient cement content
- Sealing or troweling the surface before bleed water and air escape (Correct answer)
- Using a too-stiff mix
- Screeding too aggressively
Correct answer: Sealing or troweling the surface before bleed water and air escape
Blistering occurs when the surface is sealed by premature finishing, trapping bleed water and air beneath a dense mortar layer that later delaminates.
ACI Flatwork Placement & Leveling 2
When placing concrete in hot weather (above 90°F), which practice best prevents rapid moisture loss during flatwork placement?