When designing a concrete mix for coastal construction in Hawaii, what is the maximum allowable water-cement ratio to resist chloride penetration?
Correct Answer
D) 0.40
Coastal exposure requires low water-cement ratios to minimize permeability and chloride penetration. 0.40 is the maximum allowable ratio for severe exposure conditions.
Why This Is the Correct Answer
For severe coastal exposure conditions, ACI 318 and the Hawaii Building Code limit the water-cement (w/c) ratio to a maximum of 0.40. A lower w/c ratio reduces concrete porosity, which limits the ingress of chloride ions that cause steel reinforcement corrosion and concrete deterioration in marine environments.
Why the Other Options Are Wrong
Option A: 0.50
A w/c ratio of 0.50 is acceptable for mild exposure conditions but is too high (too permeable) for coastal and marine environments in Hawaii. Higher water content creates more voids in hardened concrete, allowing chlorides to penetrate more easily.
Option B: 0.45
0.45 is more restrictive than 0.50 but still exceeds the 0.40 maximum for severe coastal exposure. It would not provide adequate resistance to chloride-induced corrosion over a structure's design life.
Option C: 0.35
0.35 is more restrictive than required (lower w/c = less permeable = better). While using a 0.35 ratio would provide superior durability, it is not the maximum allowable — it is stricter than the code requires. The question asks for the maximum, which is 0.40.
Memory Technique
Coastal concrete rule: '0.40 is the door — don't let more water in.' Water weakens coastal concrete by opening pathways for salt. 0.40 is the maximum gate; anything above 0.40 lets too much chloride through.
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