A driver adds water to a ready-mix load at the site beyond what the mix design allows. What is the main effect on the hardened concrete?
- Higher strength and less surface dusting
- Faster set and higher early strength
- Better freeze-thaw resistance once fully cured
- Lower strength and more shrinkage cracking
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D. Lower strength and more shrinkage cracking
Extra water raises the water-cement ratio, which lowers strength and durability and increases drying shrinkage. It makes placing easier, but it does not speed the set or add strength.
Reference: ACI 318 durability and mix provisions; ASTM C94 (ready-mixed concrete)
Why is an air-entraining admixture specified for exterior concrete in cold climates?
- To raise the 28-day compressive strength
- To shorten the time before finishing can start
- To reduce the amount of cement in the mix
- To improve resistance to freeze-thaw damage
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D. To improve resistance to freeze-thaw damage
Billions of tiny air bubbles give freezing water room to expand, which prevents scaling and cracking. Entrained air slightly lowers strength rather than raising it.
Reference: ASTM C260 (air-entraining admixtures); ACI 318 freeze-thaw exposure provisions
Which type of portland cement is used when high early strength is needed, such as for fast form removal?
- Type I
- Type III
- Type II
- Type V
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B. Type III
Type III is ground finer and gains strength quickly. Type I is general purpose, Type II gives moderate sulfate resistance and Type V gives high sulfate resistance.
Reference: ASTM C150 (portland cement types)