ACI Certification Concrete Construction Inspection 2 — Questions and Answers
Question 1: During concrete placement, what is the maximum free-fall height typically permitted for concrete without causing segregation?
- 5 feet (1.5 m) (Correct answer)
- 15 feet (4.5 m)
- 20 feet (6 m)
- 25 feet (7.5 m)
Correct answer: 5 feet (1.5 m)
ACI 301 and general practice limit free-fall of concrete to about 5 feet (1.5 m) to prevent aggregate segregation; exceeding this can cause mortar and coarse aggregate to separate.
When concrete falls more than about 5 feet, the heavier coarse aggregate tends to separate from the mortar, causing segregation that weakens the hardened concrete. For tall forms, concrete should be placed through tremie pipes, elephant trunks, or pump lines directed to lower levels to maintain mixture uniformity. The inspector must verify that placement methods do not violate this limit.
Question 2: What is the primary purpose of consolidating concrete using internal vibration during placement?
- To remove entrapped air and ensure the concrete completely fills the form without voids (Correct answer)
- To increase the water-cement ratio
- To accelerate the initial set time
- To increase the slump of stiff concrete mixtures
Correct answer: To remove entrapped air and ensure the concrete completely fills the form without voids
Internal vibration consolidates fresh concrete by reducing entrapped air voids and ensuring the concrete flows around reinforcement and form edges, resulting in denser, stronger hardened concrete.
Entrapped air (not intentionally entrained air) can constitute 5–20% of the volume of unvibrated concrete. Internal vibrators liquefy the concrete temporarily, allowing air bubbles to rise and escape and the mixture to flow into form corners and around rebar. The vibrator should be inserted at regular intervals not exceeding 1.5 times the vibrator's radius of action and should penetrate 6 inches into the previous lift to knit lifts together.
Question 3: When inspecting reinforcing bar placement, the ACI concrete construction special inspector checks that cover to reinforcement meets the specified minimum. What is the typical minimum clear cover for cast-in-place concrete exposed to weather (bars #6 and larger)?
- 2 inches (50 mm) (Correct answer)
- 1 inch (25 mm)
- 3/4 inch (19 mm)
- 1.5 inches (38 mm)
Correct answer: 2 inches (50 mm)
ACI 318 Table 20.6.1.3.1 requires 2 inches (50 mm) of clear cover for #6 and larger bars in cast-in-place concrete exposed to weather.
Clear cover protects reinforcement from corrosion and provides fire resistance. ACI 318 specifies that cast-in-place concrete exposed to weather requires 2 in. (50 mm) cover for #6 and larger bars and 1.5 in. (38 mm) for #5 and smaller. The inspector verifies cover with a cover meter or by direct measurement before concrete is placed, checking that bar chairs, spacers, or tie wire configurations maintain specified dimensions.
Question 4: An inspector observes that the concrete delivered to the site has a slump 3 inches higher than specified. What is the correct course of action?
- Reject the load and notify the contractor and engineer of record before placement (Correct answer)
- Add dry cement to the truck to stiffen the mix
- Allow placement if the surface appears workable
- Add superplasticizer to normalize the measured value
Correct answer: Reject the load and notify the contractor and engineer of record before placement
A load that exceeds the specified slump tolerance must be rejected; only the engineer of record can authorize using out-of-spec concrete, and field additions of dry cement are not permitted.
Excessive slump typically indicates higher water content, which raises the water-cement ratio and reduces compressive strength, durability, and increases shrinkage cracking potential. Adding dry cement to the truck (retempering) is prohibited by ACI 301 because it alters the designed proportions unpredictably. The inspector's duty is to reject non-conforming loads and document the rejection, allowing the contractor to request an engineer's review if they believe the concrete is still acceptable.
Question 5: What does 'honeycombing' in hardened concrete indicate, and what is its primary cause?
- Voids in the concrete matrix caused by inadequate consolidation or segregation, leaving coarse aggregate exposed (Correct answer)
- Surface cracking from rapid drying of the surface
- Delamination of the surface mortar layer from thermal cycling
- Alkali-silica reaction gel pockets within the paste
Correct answer: Voids in the concrete matrix caused by inadequate consolidation or segregation, leaving coarse aggregate exposed
Honeycombing refers to voids or open texture in hardened concrete where mortar failed to fill spaces between coarse aggregate particles, typically due to inadequate vibration, over-stiff mixes, or reinforcement congestion.
Honeycombing is a consolidation defect that appears after form removal as rough, open-textured areas with visible aggregate. It reduces structural capacity and allows moisture and chloride penetration. Primary causes include insufficient vibration, concrete that is too stiff to flow around rebar, congested reinforcement preventing vibrator insertion, mix segregation, or leaking forms. Inspectors must document honeycombing extent and depth; structural repairs typically require engineering evaluation and may involve pressure grouting or concrete patching per ACI 301.
Question 6: According to ACI 305R, what is the maximum concrete temperature at point of discharge permitted without special hot-weather precautions?
- 95°F (35°C) (Correct answer)
- 75°F (24°C) ambient air temperature
- 85°F (29°C) ambient air temperature
- 100°F (38°C)
Correct answer: 95°F (35°C)
ACI 305R recommends that concrete temperature at point of discharge not exceed 95°F (35°C); higher temperatures accelerate hydration, reduce workability, and can lead to strength loss.
Elevated concrete temperatures during hot weather increase water demand, accelerate cement hydration, shorten the placement and finishing window, and can cause plastic shrinkage cracking. ACI 305R sets 95°F (35°C) as the maximum concrete temperature at point of discharge. The inspector monitors concrete temperature using a calibrated thermometer inserted at least 3 inches into fresh concrete and documents results on delivery tickets.
ACI Certification Concrete Construction Inspection 2
During concrete placement, what is the maximum free-fall height typically permitted for concrete without causing segregation?