ACI Certification Concrete Flatwork Finishing 2 — Questions and Answers
Question 1: What is the recommended minimum thickness for a concrete slab-on-grade used in a residential garage, per ACI 302.1R?
- 4 inches (100 mm) (Correct answer)
- 2 inches (50 mm)
- 6 inches (150 mm)
- 8 inches (200 mm)
Correct answer: 4 inches (100 mm)
ACI 302.1R recommends a minimum 4-inch (100 mm) thickness for residential garage slabs; thinner slabs are prone to cracking under vehicle loads.
A 4-inch slab provides adequate bending resistance for typical passenger vehicles and light trucks. Subgrade preparation is equally important: a uniform, well-compacted granular base minimizes differential settlement. ACI 302.1R also recommends that contraction joints be spaced at intervals of 24 to 30 times the slab thickness (8–10 feet for a 4-inch slab) to control random cracking.
Question 2: What is 'bleed water' in fresh concrete, and why must flatwork finishers wait for it to evaporate before finishing?
- Excess mix water that rises to the concrete surface; finishing before it evaporates traps water beneath the surface and weakens the wear layer (Correct answer)
- Water added during consolidation that improves surface hardness
- Chemical admixture water that retards setting time uniformly
- Water introduced by curing compounds applied immediately after screeding
Correct answer: Excess mix water that rises to the concrete surface; finishing before it evaporates traps water beneath the surface and weakens the wear layer
Bleed water migrates upward through the paste; finishing while bleed water is present dilutes the surface paste, reducing surface strength and causing scaling and dusting of the finished slab.
Bleeding is a form of segregation where mix water with fine particles rises to the surface due to gravity settlement of solids. If a finisher works the surface while bleed water is present, the water is sealed beneath the densified surface crust, creating a weak, high w/cm layer prone to delamination, scaling, and dusting. Finishers must wait until the bleed sheen disappears and footprints leave less than 1/4-inch indentation before beginning power troweling.
Question 3: When cutting contraction joints in a concrete flatwork slab, what is the recommended joint depth relative to slab thickness?
- One-quarter (1/4) of the slab thickness (Correct answer)
- One-half (1/2) of the slab thickness
- One-third (1/3) of the slab thickness
- One-eighth (1/8) of the slab thickness
Correct answer: One-quarter (1/4) of the slab thickness
ACI 302.1R recommends saw-cutting contraction joints to a depth of at least one-quarter (1/4) the slab thickness to create a plane of weakness that controls where cracking occurs.
Contraction joints work by reducing the cross-section so that when the concrete shrinks during drying and temperature drop, the crack initiates at the pre-cut joint rather than randomly. At 1/4 depth, the remaining cross-section is weakened enough to control crack location. Cuts shallower than 1/4 depth often fail to guide the crack, while deeper cuts reduce load transfer capacity.
Question 4: What type of surface finish is produced by dragging a burlap or broom across freshly screeded concrete, and where is it most commonly specified?
- A broom or burlap-drag finish, commonly specified for exterior flatwork such as sidewalks, driveways, and ramps to provide slip resistance (Correct answer)
- A smooth trowel finish for use in industrial warehouse floors
- An exposed aggregate finish for decorative interior applications
- A hardened steel-troweled finish for parking structures
Correct answer: A broom or burlap-drag finish, commonly specified for exterior flatwork such as sidewalks, driveways, and ramps to provide slip resistance
Broom or burlap-drag textures create surface ridges that improve traction on wet surfaces, which is why exterior flatwork that must handle foot and vehicle traffic requires this type of finish.
A broom finish is applied by drawing a stiff-bristle broom across the freshly floated concrete surface, creating parallel ridges perpendicular to the direction of traffic. Both broom and burlap-drag finishes increase the coefficient of friction compared to a smooth troweled surface, reducing slip-and-fall liability on exterior flatwork such as pool decks, driveways, and public sidewalks.
Question 5: What is 'curling' (or warping) in concrete slabs, and what is its primary cause?
- Upward or downward bending of slab edges due to differential shrinkage between the top and bottom of the slab (Correct answer)
- Lateral sliding of the slab due to inadequate subbase friction
- Heaving of the slab center caused by expansive subgrade soils
- Horizontal delamination of the surface mortar layer caused by bleed water
Correct answer: Upward or downward bending of slab edges due to differential shrinkage between the top and bottom of the slab
Curling occurs when the top of the slab dries and shrinks faster than the bottom, causing edges to lift (upward curling) or curl downward depending on the moisture gradient direction.
In typical outdoor conditions, the slab surface dries more rapidly than the bottom, creating a shrinkage gradient. The top contracts more than the bottom, bending the slab upward at edges and corners — upward curling. This can cause the slab to rock on its center and create step faults at joints under traffic. Minimizing curling involves reducing the w/cm, using shrinkage-reducing admixtures, maximizing subgrade moisture retention, and keeping the slab well cured.
Question 6: What is the purpose of placing a vapor retarder directly beneath an interior concrete slab-on-grade?
- To prevent moisture transmission from the subgrade through the slab into the building interior, protecting floor coverings and occupant comfort (Correct answer)
- To increase slab compressive strength by slowing curing
- To prevent upward migration of aggregate during vibration
- To serve as a bond breaker so the slab can slide freely over the subbase
Correct answer: To prevent moisture transmission from the subgrade through the slab into the building interior, protecting floor coverings and occupant comfort
A vapor retarder (e.g., 10-mil polyethylene film) limits moisture vapor transmission from soil beneath the slab, preventing moisture-related failures of adhesive-bonded flooring, hardwood, and carpet.
Subgrade soil contains moisture that migrates upward by capillary action and vapor diffusion. Without a vapor retarder, this moisture passes through the slab and can cause adhesive failure in vinyl tile, buckling of hardwood floors, mold beneath carpet, and high indoor humidity. ASTM E1745 classifies vapor retarders by permeance; a Class A retarder (≤ 0.01 perms) is suitable for sensitive flooring.
ACI Certification Concrete Flatwork Finishing 2
What is the recommended minimum thickness for a concrete slab-on-grade used in a residential garage, per ACI 302.1R?