Effect of coupled deterioration by chloride and carbonation on chloride ions transport in concrete

Abstract

Carbonation is one of the factors that can influence the chlorides transport. Despite the combined action of chlorides and carbonation being a reality, there is no consensus on the effect of this coupled deterioration on chloride transport. Accelerated tests were used to simulate this environment. Concrete specimens were cast using 0.60 water/cement ratio and CEM I 42.5R. After curing, half of specimens was submitted to accelerated carbonation test (20ºC, 55% RH and 4% CO2) during 1 and 7 months. The other half was protected with plastic film during the same period. Then, the specimens were submitted to the accelerated chlorides attack using: migration test and immersion test. Finished the immersion test, powder samples were extracted from surface to bulk and analysed to obtain free and total chloride profiles and pH profile. According to migration test results, the carbonation increases chloride transport regardless the test period. According to the immersion test results, there is a decrease in chloride ions transport when the specimens are carbonated for 1 month and a noticeable increase when carbonation period increases to 7 months. The redistribution of pores in carbonated concrete and the difficulty of chlorides binding in carbonated concrete are related to these results.

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“…However, when the chloride concentration exceeds a critical threshold at the reinforcement level, the passivation film is destabilized, and the initiation of localized corrosion occurs. Furthermore, the process of carbonation reduces the concrete alkalinity and hinders passivation, accelerating corrosion in the presence of chlorides [39,40]. Carbonation generally fosters corrosion over a larger surface of the reinforcement, whereas chloride induced corrosion can be very localized for high performance concrete, especially in the presence of cracks [41].…”

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“…Carbonation occurs when carbon dioxide (CO2) in the air combines with hydration products dissolved in pore water, lowering the pH of the concrete solution, and speeding up uniform steel corrosion [19]. Furthermore, it appears to raise the density of the concrete's surface layer, lowering chloride ion permeability and, as a result, sorptivity [20,21]. Moreover, carbonation has both positive and negative impacts on the durability of concrete [13].…”