Evaluation of Carbonation Depth & its Impact on Corrosion of Reinforcement & Compressive Strength of Concrete

Abstract

One process which is responsible for the degradation of concrete is the carbonation of concrete. The reaction of one of the cement hydration products Ca(OH)2 with CO2 in the atmosphere produces CaCO3. Thus, the alkaline conditions of hydrated cement paste are neutralized by the carbonation process of concrete. This neutralization, by dropping the pH of concrete, affects the protection of reinforcing steel from corrosion. On the other hand, carbonation reduces the porosity of concrete as the carbonation product CaCO3 occupies a greater volume. As a result, hardness of concrete increases. In this study, experiments are conducted under severe environments for progression of carbonation for concrete and measurements are obtained for carbonation depth and corrosion potential for concrete. Combined with the measurement of carbonation depth, half-cell potential values are determined to find critical carbonation depth for corrosion initiation of reinforcement in concrete. Experiments are performed to evaluate the influence of carbonation depth on corrosion potential of reinforcement in concrete and on compressive strength of concrete. Three different beam specimens, i.e. beam specimen without rebar, beam specimen with rebar of 12 mm diameter with clear cover 15mm, and beam specimen with rebar of 12mm diameter with clear cover 25mm of size 100 × 100 × 200mm, for different concrete grades i.e. M15, M20, M25, M30, M35, M40, respectively are casted. All the beam specimens are exposed to Accelerated Carbonation in the Carbonation Chamber at CO2 content 4%, temperature 60°C and relative humidity 60% for 7 days. Carbonation depth measurements, as well as different non-destructive tests like rebound hammer, ultrasonic pulse velocity, electrical resistivity and half-cell potential are performed at the time interval of 2, 3, 4, 5, 6, and 7 days after placing all the beam specimens into the Accelerated Carbonation Chamber. It is observed that carbonation depth of concrete increases with increase in exposure period at Accelerated Carbonation Chamber. It is also noticed that for concrete grade M15 the carbonation depth is the highest and for the concrete grade M40 it is the least. Thus with increase in concrete strength the carbonation in concrete is found to be diminishing. From the rebound hammer test carried out on beam specimens, the increment in rebound number is found with increase in the carbonation depth. It is also observed that the increment in rebound number due to carbonation of concrete diminishes as the strength of concrete increases. To consider the effect of carbonation of concrete on rebound number, equations of strength reduction coefficient for rebound number as a function of concrete strength and carbonation depth is derived. The strength reduction coefficient is to be multiplied with the rebound number. The equations show good agreement with the experimental data. The ultrasonic pulse velocity test carried out on beam specimens shows that with increase in carbonation depth the increase in ultrasonic pulse velocity is found for different grades of concrete. The increment in pulse velocity is due to the reduction in porosity due to carbonation of concrete, which is indicative of increased concrete quality. It is observed from resistivity tests carried out on beam specimens, there is an increment in resistivity of concrete with increase in carbonation depth. It is also observed that resistivity of concrete grade M40 is higher as compared to other grades of concrete. Half-cell potential test carried out on the beam specimens shows that with increase in carbonation depth the corrosion potential to reinforcement in concrete increases. It is also observed that with increase in concrete strength the corrosion potential of reinforcement in concrete decreases, thus the critical carbonation depth for corrosion initiation increase with higher concrete strength. For lower concrete grades the carbonation depth is found to be larger as compared to higher grades of concrete. The carbonation in concrete causes the corrosion of reinforcement. The corrosion of reinforcement due to carbonation will not start until the carbonation has reached to a certain level from the reinforcement. The surface hardness of concrete is found to be increasing with the carbonation depth. This increase in surface hardness is limited up to the thickness of carbonated layer of concrete, which is not the real indicative of the core hardness of the concrete.