In Hawaii's tropical climate, what is the primary concern when selecting concrete mix designs for coastal construction?
Correct Answer
D) Chloride resistance and durability
Coastal construction in Hawaii requires concrete with high chloride resistance due to salt air exposure, which causes corrosion of reinforcement steel.
Why This Is the Correct Answer
Chloride resistance and durability are the primary concerns for coastal concrete mix design in Hawaii. Salt-laden sea air introduces chloride ions that penetrate concrete and attack the steel reinforcement, causing it to corrode, expand, and eventually crack and spall the concrete. A mix designed for high chloride resistance — typically with low water-cement ratio, supplementary cementitious materials like fly ash or slag, and adequate cover — protects the reinforcement and extends structure life.
Why the Other Options Are Wrong
Option A: High compressive strength only
High compressive strength alone does not address the primary threat of salt-air corrosion. While strength is always a consideration, a high-strength mix that is permeable to chlorides will still fail prematurely in coastal environments. Durability encompasses more than just compressive strength.
Option B: Fast setting time
Fast setting time is a workability and scheduling consideration, not a primary structural or durability concern for coastal construction. In Hawaii's warm temperatures, concrete already sets more quickly, making fast set time less of a priority than durability.
Option C: Low cost materials
Low cost materials prioritize economics over performance. In a coastal environment where chloride-induced corrosion is the dominant failure mechanism, using cheap materials without regard for chloride resistance would lead to premature structural failure and far higher long-term costs.
Memory Technique
Picture concrete on a Hawaiian beach: the biggest enemy is not a big wave (strength) but the salt slowly eating the steel inside (chloride corrosion). 'Chloride resistance' = protecting the rebar skeleton from salty air.
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