Cracked Marine Jetty Deck: ASR or Chloride-Induced Corrosion?
A 28-year-old reinforced concrete jetty deck in Hong Kong is grade 40 with 150 mm nominal cover and sits in the splash zone. Inspection shows map cracking with white exudate on soffits and some D-shaped cracks parallel to the reinforcement. Half-cell potentials average −380 mV CSE, with readings to −450 mV near cracks. Carbonation depth is 8 mm. Acid-soluble chloride is 0.42 % by mass of cement at 40 mm depth and 0.18 % at 70 mm. Two cores show reactive fine aggregate under petrographic examination, and a gel-like deposit is present in one crack. The owner wants a rapid patch repair to reopen the berth in three weeks and asks you to confirm that the cracking is only surface shrinkage.
1. Set out the diagnosis and the testing you would commission before agreeing to any repair method.
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Evidence review. Chloride at 0.42 % by mass of cement at 40 mm is above the usual ~0.4 % threshold for acid-soluble chloride in cracked concrete, and half-cell potentials of −380 to −450 mV CSE indicate a high probability of active corrosion. Carbonation at only 8 mm is not the driver. The map cracking and white exudate could be alkali–silica reaction gel, but calcium hydroxide leaching from corrosion cracks can look similar; the petrographic finding of reactive aggregate and gel in one crack makes ASR a credible co-cause.
Testing to commission.
- Petrography on cores for ASR gel, reaction rims and crack pattern.
- XRD/DTA for gel and ettringite.
- Full chloride profile at 10, 20, 30, 40 and 60 mm.
- Cover survey, half-cell contour mapping and resistivity.
- Crack width and depth mapping, and delamination sounding.
A patch repair without identifying and removing the chloride source would simply move the corrosion cell to the patch boundary.
2. Propose a repair strategy and a specification for the replacement concrete in the splash zone.
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Repair strategy. Remove all delaminated and chloride-contaminated concrete to at least 20 mm behind the rebar by hydrodemolition or controlled breakout, with square-cut edges. Clean the rebar to Sa 2.5 and replace with a low-permeability repair mortar containing silica fume or PFA and a migrating corrosion inhibitor. In the splash zone, patch repairs alone are inadequate; install galvanic anodes or an impressed-current cathodic protection system to protect the remaining chloride-contaminated concrete, and monitor half-cell potentials after repair.
Specification for new concrete. Grade C40/20, maximum w/c 0.40, 40–60 % PFA or GGBS, 60 mm cover in the splash zone, low-chloride aggregates, and a curing regime suitable for marine exposure. Verify by trial mixes, chloride diffusion testing and cube strength at 7 and 28 days.
3. The owner insists on patch repair only. How do you respond professionally?
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I would explain in writing that patch-only repair does not address the chloride-contaminated concrete outside the patch and is likely to accelerate corrosion at the patch boundary, producing a recurrence within a few years. I would set out the risks, the expected maintenance cost and the monitoring that would be needed if the owner proceeds. If the owner still instructs patch-only repair, I would record the instruction, confirm it is an informed owner decision, require a monitoring plan, and decline to certify the repair as a durable solution. Safety of the structure remains the overriding duty, so any structural weakening would require a separate assessment.