ACI Concrete Repair & Restoration 2 — Questions and Answers
Question 1: What is the primary purpose of applying a bonding agent before placing repair mortar on existing concrete?
- To waterproof the repair area
- To improve adhesion between old and new concrete (Correct answer)
- To slow the curing process
- To add color to the repair patch
Correct answer: To improve adhesion between old and new concrete
Bonding agents create a chemical or mechanical link between the existing substrate and the repair material to prevent delamination.
Question 2: When using dry-pack mortar for repair, the water-to-cement ratio should be:
- As high as possible to aid workability
- Just enough to ball in your hand without crumbling or releasing water (Correct answer)
- Equal to that used for standard ready-mix concrete
- At least 0.60 to ensure full hydration
Correct answer: Just enough to ball in your hand without crumbling or releasing water
Dry-pack mortar uses a very low w/c ratio; the mix should barely hold together when squeezed, minimizing shrinkage.
Question 3: Alkali-silica reaction (ASR) in concrete is best identified by:
- Uniform surface scaling and popouts
- Map cracking with gel exudate around aggregate particles (Correct answer)
- Rust staining along crack lines
- Delamination of the top 1/4 inch of the slab
Correct answer: Map cracking with gel exudate around aggregate particles
ASR produces a characteristic map or crazing crack pattern with silica gel oozing around reactive aggregates.
Question 4: Which repair method is most appropriate for filling wide, dormant horizontal cracks in a concrete floor slab?
- Epoxy injection under pressure
- Routing and sealing with a flexible sealant
- Dry-pack mortar rodded into the crack (Correct answer)
- Shotcrete overlay
Correct answer: Dry-pack mortar rodded into the crack
Wide dormant cracks in horizontal slabs are best repaired by routing to a uniform shape and filling with dry-pack mortar or rigid filler.
Question 5: The minimum repair area depth recommended by ACI 546 for a partial-depth patch is generally:
- 1/4 inch
- 1/2 inch
- 1 inch (Correct answer)
- 2 inches
Correct answer: 1 inch
ACI 546 recommends a minimum partial-depth repair of 1 inch to ensure adequate bond and material integrity.
Question 6: Before placing a concrete overlay, the existing surface should be profiled to a minimum CSP (Concrete Surface Profile) of:
- CSP 1 (light brush-off)
- CSP 3–5 (medium scarification) (Correct answer)
- CSP 7–9 (heavy shot blast)
- CSP 10 (scarification to 3/4 inch depth)
Correct answer: CSP 3–5 (medium scarification)
Overlays require moderate surface profile (CSP 3–5) to provide mechanical interlock without excessively damaging the substrate.
Question 7: Which condition would cause a repair patch to fail most rapidly?
- Curing the patch with wet burlap
- Leaving the repair area slightly damp before applying bonding agent
- Placing repair material over a dry, dusty surface with no bond coat (Correct answer)
- Using a polymer-modified mortar
Correct answer: Placing repair material over a dry, dusty surface with no bond coat
A dry, dusty substrate without a bonding agent prevents adhesion and leads to early delamination of the patch.
ACI Concrete Repair & Restoration 2
What is the primary purpose of applying a bonding agent before placing repair mortar on existing concrete?