Concrete Flatwork Finishing Flashcards
6 cards from real ACI Certification practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Concrete Flatwork Finishing flashcards as text
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?
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.
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?
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.
According to ACI standards, what is the proper depth for a saw-cut contraction joint in a 6-inch thick slab-on-ground?
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).
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?
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.
Which of the following conditions is most likely to cause plastic shrinkage cracks to form on the surface of freshly placed concrete?
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.
To achieve a dense, smooth, and wear-resistant surface on an interior concrete floor, when should the first troweling operation begin?
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.