ACI Concrete Repair & Restoration 3 — Questions and Answers
Question 1: Carbonation of concrete is most likely to cause:
- Expansion and cracking of the surface
- A reduction in pH that can lead to corrosion of reinforcement (Correct answer)
- Increased surface hardness and abrasion resistance
- Delamination of the top layer due to freeze-thaw action
Correct answer: A reduction in pH that can lead to corrosion of reinforcement
Carbonation lowers concrete pH from ~12.5 to below 9, destroying the passive oxide layer that protects steel reinforcement.
Question 2: When grinding concrete joints to restore load transfer in a slab, what is the primary goal?
- Removing old sealant material
- Reducing differential vertical movement (faulting) between adjacent panels (Correct answer)
- Widening the joint for new sealant installation
- Eliminating aggregate interlock at the joint face
Correct answer: Reducing differential vertical movement (faulting) between adjacent panels
Diamond grinding levels faulted joints to eliminate differential movement that causes rough riding surfaces and pavement damage.
Question 3: Hydrostatic pressure on a below-grade concrete wall is best countered by a repair that:
- Uses a crystalline waterproofing compound applied to the positive (wet) side only
- Applies a hydraulic cement plug from the negative (dry) side (Correct answer)
- Seals cracks with flexible polyurethane from the positive side
- Covers the interior surface with a vapor barrier membrane
Correct answer: Applies a hydraulic cement plug from the negative (dry) side
Hydraulic cement can be applied from the interior (negative) side to stop active water infiltration against hydrostatic pressure.
Question 4: Spalling along the soffit of a concrete bridge deck is most commonly caused by:
- Alkali-silica reaction in the coarse aggregate
- Corrosion of reinforcing steel expanding and fracturing the cover concrete (Correct answer)
- Freeze-thaw damage to the cement paste
- Improper air entrainment during original placement
Correct answer: Corrosion of reinforcing steel expanding and fracturing the cover concrete
Corrosion products occupy up to 6 times the volume of the original steel, creating expansive forces that spall the concrete cover.
Question 5: Prior to applying a thin-bonded overlay, a pull-off (tensile bond) test on the prepared substrate should yield at least:
- 50 psi
- 200 psi (Correct answer)
- 500 psi
- 1,000 psi
Correct answer: 200 psi
ACI 546 recommends a minimum substrate tensile strength of approximately 200 psi before placing bonded overlays.
Question 6: What is the function of saw-cutting a repair area to a defined perimeter before removing deteriorated concrete?
- It allows water to drain during hydrodemolition
- It prevents feathered edges and creates a defined vertical face for bonding (Correct answer)
- It eliminates the need for a bonding agent
- It reduces the amount of concrete that must be removed
Correct answer: It prevents feathered edges and creates a defined vertical face for bonding
Saw-cut perimeters create straight, vertical edges that prevent thin feathered patches which debond quickly.
Question 7: Silane and siloxane penetrating sealers are applied to concrete surfaces primarily to:
- Fill surface cracks and seal honeycombs
- Repel water and chloride ions by lining pore walls without forming a film (Correct answer)
- Increase the compressive strength of the surface layer
- Bond loose aggregate to the surface
Correct answer: Repel water and chloride ions by lining pore walls without forming a film
Silane/siloxane sealers penetrate the pore structure and chemically bond to create a hydrophobic lining, reducing water and chloride ingress.
ACI Concrete Repair & Restoration 3
Carbonation of concrete is most likely to cause: