ACI Color & Texture Finishing 2 — Questions and Answers
Question 1: When using dry-shake color hardener on a concrete slab, at what stage should the first application be made?
- After the slab has fully cured
- After bleed water has evaporated and the surface can support foot traffic (Correct answer)
- Immediately after screeding, before any bleed water appears
- After the slab reaches initial set
Correct answer: After bleed water has evaporated and the surface can support foot traffic
Dry-shake hardener is applied after bleed water evaporates and the surface can just support a worker's weight without sinking more than about 1/4 inch.
Question 2: Which chemical compound is primarily responsible for the color produced by integral iron oxide pigments in concrete?
- Calcium silicate hydrate
- Iron oxide (Fe2O3 or Fe3O4) (Correct answer)
- Titanium dioxide
- Carbon black
Correct answer: Iron oxide (Fe2O3 or Fe3O4)
Iron oxide pigments in red, yellow, brown, and black shades are the most common and stable colorants used in integrally colored concrete.
Question 3: A contractor notices that a stamped concrete patio has 'ghosting' — faint impressions from the stamp that appear lighter than the surrounding surface. What is the most likely cause?
- Excess release agent trapped under the stamp
- Insufficient water in the mix
- Stamping performed too early while bleed water was present (Correct answer)
- Using a polyurethane stamp instead of a rubber stamp
Correct answer: Stamping performed too early while bleed water was present
Stamping while bleed water is still present traps water under the stamp, creating lighter, weaker areas called ghosting.
Question 4: What is the purpose of a 'release agent' when stamping concrete?
- To accelerate the set of the concrete surface
- To prevent the stamp from bonding to the concrete and add a secondary color accent (Correct answer)
- To seal the concrete after stamping is complete
- To increase the depth of the stamped pattern
Correct answer: To prevent the stamp from bonding to the concrete and add a secondary color accent
Release agents serve a dual purpose: they prevent stamps from sticking to the surface and add a contrasting antiquing color that highlights pattern details.
Question 5: A concrete finisher is applying an acid stain to a cured slab. Which safety precaution is MOST critical?
- Wearing steel-toed boots
- Using respiratory protection and chemical-resistant gloves and eye protection (Correct answer)
- Wearing a hard hat
- Using non-slip shoe covers
Correct answer: Using respiratory protection and chemical-resistant gloves and eye protection
Acid stains contain hydrochloric acid and metallic salts that can cause severe burns to skin and eyes and emit harmful fumes requiring respiratory protection.
Question 6: In exposed aggregate finishing, what determines the size and appearance of the aggregate that is revealed?
- The water-cement ratio of the mix
- The type of aggregate specified and how deeply the surface mortar is removed (Correct answer)
- The temperature during curing
- The slump value at the time of placement
Correct answer: The type of aggregate specified and how deeply the surface mortar is removed
Both the aggregate gradation specified in the mix design and the depth of surface retardation or washing control the final exposed aggregate appearance.
Question 7: What happens to the color intensity of integrally colored concrete if excess water is added to the mix on the job site?
- Color intensity increases due to better pigment dispersion
- Color intensity decreases and becomes uneven or washed out (Correct answer)
- Color is unaffected because pigment is bonded to cement
- The color shifts from warm to cool tones
Correct answer: Color intensity decreases and becomes uneven or washed out
Adding water increases the water-cement ratio, diluting the pigment concentration and producing lighter, less uniform color with reduced durability.
ACI Color & Texture Finishing 2
When using dry-shake color hardener on a concrete slab, at what stage should the first application be made?