ACI Concrete Construction Inspection Questions and Answers 1 — Questions and Answers
Question 1: An inspector is checking the placement of reinforcing steel in a 12-inch thick foundation wall prior to concrete placement. The project specifications reference ACI 318 for concrete cast against and permanently exposed to earth. What is the minimum required concrete cover over the reinforcing steel?
- 3/4 inch
- 1 1/2 inches
- 2 inches
- 3 inches (Correct answer)
Correct answer: 3 inches
ACI 318 specifies a minimum concrete cover of 3 inches for concrete cast against and permanently exposed to earth. This substantial cover is required to protect the reinforcing steel from moisture and corrosive elements in the soil, ensuring the long-term durability of the structure.
Question 2: According to ACI 347R, Guide to Formwork for Concrete, which of the following is the most critical factor in determining the safe stripping time for vertical formwork on walls and columns?
- The ambient air temperature at the time of stripping.
- The specified 28-day compressive strength of the concrete.
- The ability of the concrete to support its own weight and resist surface damage. (Correct answer)
- The type of cement used in the concrete mix.
Correct answer: The ability of the concrete to support its own weight and resist surface damage.
While factors like temperature and mix design influence strength gain, the primary criterion for safely removing vertical (non-load-bearing) forms is whether the concrete has achieved sufficient strength to be self-supporting without being damaged during form removal operations. Removing forms too early can lead to sloughing, damaged corners, and other surface defects.
Question 3: An inspector is overseeing a concrete pour during hot weather conditions as defined by ACI 305R. Which of the following is a recommended measure to mitigate the adverse effects of high temperatures and wind on the fresh concrete?
- Adding calcium chloride to the mix to accelerate setting time.
- Increasing the water-cement ratio at the job site to improve workability.
- Delaying the start of curing procedures until the concrete has cooled.
- Erecting sunshades and windbreaks around the placement area. (Correct answer)
Correct answer: Erecting sunshades and windbreaks around the placement area.
ACI 305R, Guide to Hot Weather Concreting, recommends measures to control concrete temperature and reduce moisture loss. Sunshades and windbreaks help lower the concrete temperature and, more importantly, reduce the rate of evaporation from the surface, which mitigates the risk of plastic shrinkage cracking. Adding an accelerator or extra water is detrimental in hot weather, and curing must begin immediately.
Question 4: An inspector observes a worker repeatedly inserting an internal vibrator into the same location in a deep form for an extended period. The inspector notes a large amount of cement paste and fine aggregate collecting at the surface near the vibrator. This practice is most likely to cause which of the following issues?
- Honeycombing due to insufficient consolidation.
- Segregation of the concrete mixture. (Correct answer)
- Cold joints between concrete lifts.
- Entrapment of excessive air voids.
Correct answer: Segregation of the concrete mixture.
Over-vibration causes the heavier coarse aggregates to settle to the bottom while the lighter components (cement paste, water, and fine aggregate) rise to the top. This separation of constituents is known as segregation and results in non-uniform, weaker concrete with poor surface durability.
Question 5: As part of a pre-placement inspection for a concrete slab-on-ground, which of the following items is most critical to verify is correct immediately before concrete discharge begins?
- The subgrade is moist but free of standing water, and all formwork is clean. (Correct answer)
- The laboratory has submitted the 7-day strength results from a previous pour.
- The finishing tools have been cleaned and staged.
- The curing compound has been delivered and is stored in a shaded area.
Correct answer: The subgrade is moist but free of standing water, and all formwork is clean.
A proper subgrade condition (moist to prevent rapid water loss from the concrete, but not puddled) and clean forms are essential for the quality and integrity of the concrete at the moment of placement. These must be checked and confirmed immediately prior to placement. The other items are important project tasks but are not immediate pre-discharge requirements that directly affect the concrete quality as it is being placed.
Question 6: When using a liquid membrane-forming curing compound compliant with ASTM C309 on a freshly finished concrete surface, what is the most important application requirement for it to be effective?
- The compound should be applied at double the manufacturer's recommended rate.
- The compound must be applied to achieve a continuous, uniform film. (Correct answer)
- The application must be delayed for at least 12 hours after finishing.
- The concrete surface should be lightly wetted with a fog spray just before application.
Correct answer: The compound must be applied to achieve a continuous, uniform film.
The effectiveness of a curing compound relies on its ability to form an unbroken membrane over the concrete surface to prevent moisture from escaping. Gaps, thin spots, or non-uniform coverage will compromise its function and lead to inadequate curing and potential surface defects. It should be applied at the manufacturer's specified rate as soon as the surface sheen disappears.
ACI Concrete Construction Inspection Questions and Answers 1
An inspector is checking the placement of reinforcing steel in a 12-inch thick foundation wall prior to concrete placement.
The project specifications reference ACI 318 for concrete cast against and permanently exposed to earth.
What is the minimum required concrete cover over the reinforcing steel?