ACI Concrete Flatwork Finishing Questions and Answers 1 — Questions and Answers
Question 1: A concrete finisher is preparing to perform the initial floating of a freshly placed slab. Which of the following is the primary purpose of bull floating?
- To create a smooth, dense, hard-troweled surface.
- To embed large aggregate, level the surface, and remove screed marks. (Correct answer)
- To open the surface to allow bleed water to evaporate quickly.
- To cut control joints to a depth of one-quarter the slab thickness.
Correct answer: To embed large aggregate, level the surface, and remove screed marks.
The primary purpose of bull floating is to level ridges and fill voids left by screeding, while also embedding coarse aggregate particles just below the surface. This process prepares the slab for subsequent finishing operations like edging, jointing, and troweling.
Question 2: While finishing a concrete slab on a hot, windy day, the finisher notices the surface is drying and crusting over much faster than the underlying concrete. Continuing to trowel under these conditions is likely to cause which surface defect?
- Crazing
- Scaling
- Blistering or delamination (Correct answer)
- Discoloration
Correct answer: Blistering or delamination
When the surface of fresh concrete sets or seals before the underlying concrete, rising bleed water and entrapped air can get trapped beneath the densified surface. Finishing operations, especially troweling, can exacerbate this, causing blisters or larger delaminated areas.
Question 3: According to ACI standards, what is the proper depth for a saw-cut contraction joint in a 6-inch thick slab-on-ground?
- At least 1/2 inch
- At least 1 inch
- At least 1 1/2 inches (Correct answer)
- At least 2 inches
Correct answer: At least 1 1/2 inches
ACI recommends that the depth of a saw-cut or tooled contraction joint should be at least one-quarter (1/4) of the slab thickness. For a 6-inch slab, this calculates to 1.5 inches (6 inches / 4 = 1.5 inches).
Question 4: A finisher is working on an exterior slab that will be exposed to freeze-thaw cycles. After the bleed water has evaporated and the surface has been floated, what is the most appropriate next step to ensure durability without creating a dangerously slick surface?
- Perform multiple hard trowel passes with a steel trowel.
- Apply a dry shake hardener and trowel it into the surface.
- Provide a light broom finish.
- Immediately cover the slab with plastic sheeting. (Correct answer)
Correct answer: Immediately cover the slab with plastic sheeting.
For exterior surfaces exposed to weather, a hard, smooth trowel finish can be very slippery when wet. A light broom finish is applied after floating (and sometimes a single trowel pass) to create a slip-resistant texture, which is crucial for safety on sidewalks, patios, and driveways.
Question 5: Which of the following conditions is most likely to cause plastic shrinkage cracks to form on the surface of freshly placed concrete?
- Low ambient temperature and high humidity.
- Heavy rain during placement.
- Finishing operations beginning too late.
- High wind speed and low humidity. (Correct answer)
Correct answer: High wind speed and low humidity.
Plastic shrinkage cracks occur when the rate of moisture evaporation from the surface exceeds the rate at which bleed water rises to the surface. This condition is most common on hot, dry, and windy days, causing the surface to shrink and crack before it has hardened.
Question 6: To achieve a dense, smooth, and wear-resistant surface on an interior concrete floor, when should the first troweling operation begin?
- Immediately after bull floating, while the concrete is still soft.
- After the concrete has reached its final set.
- While bleed water is still visible on the surface.
- After the bleed water has evaporated and the concrete can support the finisher's weight with only a slight indentation. (Correct answer)
Correct answer: After the bleed water has evaporated and the concrete can support the finisher's weight with only a slight indentation.
Troweling should only begin after the concrete has stiffened enough to support the weight of the finisher and their tools, and after all bleed water has disappeared from the surface. Starting too early can trap bleed water, leading to defects like blisters or delamination, and will not produce a durable surface.
ACI Concrete Flatwork Finishing Questions and Answers 1
A concrete finisher is preparing to perform the initial floating of a freshly placed slab.
Which of the following is the primary purpose of bull floating?