ACI Surface Preparation & Finishing Techniques 2 — Questions and Answers
Question 1: What is the primary purpose of screeding concrete after placement?
- To add color to the surface
- To strike off excess concrete and establish grade (Correct answer)
- To compact the subbase
- To cure the surface
Correct answer: To strike off excess concrete and establish grade
Screeding strikes off excess concrete and brings the surface to the desired grade or elevation.
Question 2: Which finishing tool is used immediately after screeding to embed aggregate and bring mortar to the surface?
- Steel trowel
- Fresno trowel
- Bull float (Correct answer)
- Edger
Correct answer: Bull float
A bull float is used right after screeding to embed coarse aggregate and bring cement paste to the surface.
Question 3: When should a finisher begin hand floating operations?
- Immediately after placing concrete
- After the concrete has fully hardened
- When the bleed water sheen has disappeared and the surface can support a finisher's weight (Correct answer)
- Only after the concrete reaches 3,000 psi
Correct answer: When the bleed water sheen has disappeared and the surface can support a finisher's weight
Hand floating begins when surface bleed water has evaporated and the concrete is firm enough to support a finisher without excessive indentation.
Question 4: What defect results from troweling concrete before bleed water has evaporated?
- Scaling
- Pop-outs
- Delamination (Correct answer)
- Crazing
Correct answer: Delamination
Troweling too early traps bleed water under the surface, causing delamination where the top layer separates from the base.
Question 5: In which direction should a darby or bull float be pushed across fresh concrete?
- Parallel to the screed direction only
- In overlapping arcs, keeping the leading edge slightly raised (Correct answer)
- Perpendicular to screed direction only
- In circular motions only
Correct answer: In overlapping arcs, keeping the leading edge slightly raised
Darbies and bull floats are worked in overlapping passes with the leading edge slightly raised to avoid digging into the surface.
Question 6: What is the purpose of edging concrete flatwork?
- To increase slab thickness at the perimeter
- To create a rounded, durable edge that resists chipping (Correct answer)
- To add a decorative color border
- To seal the concrete against moisture
Correct answer: To create a rounded, durable edge that resists chipping
Edging rounds and densifies the slab perimeter to produce a smooth, chip-resistant edge.
Question 7: What spacing is typically recommended for hand-jointing (grooving) a 4-inch thick concrete slab to control cracking?
- Every 1 foot
- Equal to the slab thickness in feet
- 10 times the slab thickness in inches (approximately) (Correct answer)
- Every 50 feet
Correct answer: 10 times the slab thickness in inches (approximately)
Control joints are typically spaced at intervals of 24–36 times the slab thickness, or roughly 8–12 feet for a 4-inch slab.
ACI Surface Preparation & Finishing Techniques 2
What is the primary purpose of screeding concrete after placement?