Curing effects on carbonation of concrete using a phenolphthalein indicator and Fourier-transform infrared spectroscopy

Abstract

One of the traditional ways of determining carbonation depth is to spray a phenolphthalein indicator onto the surface of a split concrete prism. This method captures the color change boundary between uncarbonated, partially carbonated or fully carbonated concrete, at which the pH is about 9. It is particularly useful as it gives a continuous carbonation front in a visual form. Many research findings have shown that there are drawbacks, as the carbon dioxide could react at depths much greater than those indicated by the phenolphthalein. The (phenolphthalein test) method used for this research, based on the visual judgement of a color boundary, did not indicate a significant change in color. The findings of this research study indicated that water-cured concrete is carbonated to 72% of the air-cured sample level after 3 months of accelerated curing. However, the estimation of carbonation rate, based on the phenolphthalein test, does not give a consistent result for either water-cured or air-cured samples. In this study, infrared spectroscopy was used to determine the carbonation depth by referencing the characteristic peak of the C–O stretching bonds. Carbonated samples at 30, 60, and 90 days were inspected with Infrared spectra detected using Fourier-transform infrared spectroscopy (FT-IR) and compared with results from the phenolphthalein method. The results indicated that the variation of the measured carbonation depth using the phenolphthalein test is larger. The discrepancy in the carbonation depth measured by IR for both air- and water-cured conditions reduced over time. Samples cured in water for 28 days became carbonated to only 53% of the level for air-cured samples, but then increased to 78% after three months. The findings indicated that carbonation increases with age and decreases with depth and that carbonation depths determined with the infrared spectra were 1.5 mm deeper than those determined using the phenolphthalein test. The carbonation rate constant determined by IR spectrum analysis was 23.9% higher than that obtained by using the phenolphthalein indicator. It is believed that the IR spectroscopic test can identify the partially carbonated front more readily than phenolphthalein indicator. Past estimations of carbonation depth based on phenolphthalein indicator may have underestimated the level of carbonation. The use of FT-IR provides a useful tool in providing early warnings of carbonation in structural concrete.