What is the primary reason Hawaii Building Code requires special consideration for concrete mix design in coastal areas?
Correct Answer
D) Chloride ion penetration causes reinforcement corrosion
Chloride ions from salt spray and seawater penetrate concrete and cause corrosion of reinforcing steel, requiring special mix designs with lower permeability and corrosion inhibitors.
Why This Is the Correct Answer
The correct answer is chloride ion penetration causes reinforcement corrosion. In Hawaii's coastal environment, chloride ions from salt spray, salt air, and seawater penetrate concrete through pores and microcracks. When chloride concentration at the rebar surface reaches a threshold level, it disrupts the passive oxide layer protecting the steel and initiates corrosion. The resulting rust expands volumetrically, cracking the surrounding concrete — a self-accelerating failure process. Special mix designs with low water-cement ratios, supplementary cementitious materials (fly ash, silica fume), and corrosion inhibitors reduce concrete permeability to resist chloride ingress.
Why the Other Options Are Wrong
Option A: Seismic activity requires higher strength concrete
While Hawaii is seismically active and higher-strength concrete does improve seismic performance, seismic activity alone does not drive the special coastal mix design requirement. Seismic demands are addressed through structural design (reinforcement, connection details), not primarily through concrete mix selection. The coastal exposure requirement is driven by durability, not strength.
Option B: Higher temperatures require accelerated curing
Hawaii's temperatures are warm but relatively moderate and consistent. Higher temperatures can accelerate hydration and affect curing, but this is managed through standard hot-weather concrete practices (chilled water, admixtures) rather than being the primary driver of Hawaii's coastal mix design code requirements.
Option C: Volcanic aggregates have different expansion properties
Volcanic aggregates from Hawaii quarries do have unique properties and require testing, but variable expansion due to volcanic aggregate is not the primary reason for special coastal concrete mix design requirements. The code focus on coastal areas specifically targets chloride-induced corrosion, which applies regardless of aggregate source.
Memory Technique
Coastal concrete = chloride corrosion. Salt (NaCl) breaks down to chloride ions (Cl⁻) in water. Those ions migrate through concrete like water through a sponge, eventually reaching the rebar and starting rust. Low permeability concrete = less chloride penetration = less corrosion. The whole coastal mix design story starts and ends with chloride.
More Hawaii Questions
A concrete mix design for coastal construction specifies a water-cement ratio of 0.40. If 650 lbs of cement is used per cubic yard, how many gallons of water should be added?
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