ACI Decorative Concrete & Stamping 3 — Questions and Answers
Question 1: What is the purpose of back-rolling a sealer coat applied to stamped concrete?
- To remove excess sealer and prevent puddle buildup in low areas (Correct answer)
- To embed aggregate into the sealer surface
- To increase the sealer film thickness uniformly
- To activate the UV inhibitors in the sealer
Correct answer: To remove excess sealer and prevent puddle buildup in low areas
Back-rolling after spraying or rolling distributes sealer evenly and prevents it from pooling in the stamped texture depressions, which can cause whitening or peeling.
Question 2: Acid staining relies on which chemical reaction to produce color in concrete?
- Oxidation of iron particles in the mix
- Reaction between acid and free lime (calcium hydroxide) in hardened concrete (Correct answer)
- Absorption of UV pigment into the cement paste
- Reaction between silica and the stain carrier
Correct answer: Reaction between acid and free lime (calcium hydroxide) in hardened concrete
Acid stains react with calcium hydroxide in cured concrete to form colored metallic salts that permanently bond with the concrete surface.
Question 3: Which of the following best describes 'integral color' in decorative concrete?
- Color applied to the surface after finishing with a dry shake
- Color mixed throughout the entire concrete batch before placement (Correct answer)
- Color acid-etched into the surface after curing
- Color sprayed on with a sealer coat
Correct answer: Color mixed throughout the entire concrete batch before placement
Integral color is a pigment added to the concrete mix so that color is uniform throughout the full depth of the slab, not just on the surface.
Question 4: What is the function of a texture mat (skin) in decorative stamping?
- To compact the concrete to the correct depth
- To impart a surface texture without creating a repeating joint pattern (Correct answer)
- To cut control joints into the decorative surface
- To apply release agent evenly across large areas
Correct answer: To impart a surface texture without creating a repeating joint pattern
Seamless texture skins or mats apply realistic surface texture — such as stone or slate grain — without the visible repeat lines that ridged stamp tools produce.
Question 5: Why is it important to keep stamp tool patterns aligned during a decorative pour?
- Alignment increases the compressive strength of the slab
- Misalignment creates visible breaks in the pattern that look unnatural (Correct answer)
- Aligned stamps reduce water-cement ratio in the mix
- Tool alignment prevents carbonation of the surface
Correct answer: Misalignment creates visible breaks in the pattern that look unnatural
Misaligned stamp tools disrupt the repeating decorative pattern, making joints and grout lines appear crooked or inconsistent, which compromises the appearance.
Question 6: What minimum concrete compressive strength is typically required before applying a chemical stain to a new slab?
- 500 psi
- 1,500 psi
- 2,500 psi or full cure (28 days recommended) (Correct answer)
- 5,000 psi
Correct answer: 2,500 psi or full cure (28 days recommended)
Staining should be done after the concrete reaches adequate strength and cure — typically after 28 days — to ensure the stain reacts properly and color is consistent.
Question 7: Which condition most commonly causes stamp tool impressions to lose detail and appear shallow?
- Concrete that is too stiff (over-set) at the time of stamping (Correct answer)
- Concrete that has too high a water-cement ratio
- Stamp tools that are too heavy
- Using liquid release agent instead of powder release
Correct answer: Concrete that is too stiff (over-set) at the time of stamping
When concrete has stiffened beyond the workable stamping window, stamp tools cannot penetrate deeply enough to transfer full pattern detail.
ACI Decorative Concrete & Stamping 3
What is the purpose of back-rolling a sealer coat applied to stamped concrete?