CSLB General Building (B)Site Work, Foundations and ConcreteHard
A concrete supplier reduces the water content in a mix design to improve strength while keeping cement content constant. What is the most likely trade-off of this change?
- AIncreased compressive strength but reduced workability
- BIncreased workability but reduced compressive strength
- CNo change in workability or strength
- DIncreased curing time with no change in strength
Show answer & explanationAnswer & explanation
Correct answer: A. Increased compressive strength but reduced workability
Lowering the water-cement ratio (while keeping cement constant) generally increases compressive strength because there is less excess water to create voids after hydration, but it reduces the mix's workability, making it harder to place and finish.
Why the other options are wrong
- B. This describes the opposite effect; increasing water improves workability but weakens strength.
- C. Changing the water-cement ratio always affects both strength and workability, not neither.
- D. Curing time is influenced by conditions like temperature and humidity, not directly by mix water content.
Water-Cement Ratio
The ratio of water weight to cement weight in a concrete mix, which governs the trade-off between strength and workability.
- Lower water-cement ratio = higher strength, lower workability
- Higher water-cement ratio = better workability, lower strength
- Typical structural mixes range from 0.40 to 0.60 w/c ratio
Memory trick: Less water, more power — but harder to pour.