Service life prediction for different types of concrete using chloride diffusion coefficients

Abstract

Concrete durability has emerged as a significant design consideration for reinforced concrete (RC) structures in recent years. This design consideration includes the measure of the level of serviceability degradation of a RC structure due to various adverse phenomena and the countermeasures adopted to rectify the deteriorating situation throughout its service life. However, the prevalent construction practices of Bangladesh overlook the durability aspects and focus mainly on the strength gaining objectives. One of the major phenomena that affects the durability of RC structures is corrosion induced by the chloride ion ingress which in turn depends on the concrete mix quality. Therefore, it is crucial to observe and quantify the effects of different concrete mixes and design parameters of a RC element on the resistance to chloride ion ingress and thus on its service life. Hence, a study has been carried out to evaluate service life of RC elements, prepared following local construction practices and using commonly available material. The experimental program involved determination of concrete resistance to chloride ion ingress in terms of diffusion coefficient values. Consequently, the service life evaluation of the concrete mixes was based on chloride induced corrosion damage only. The other effects like carbonation were not considered for determination of service life. The observations show high resistance of stone aggregate concrete against chloride intrusion as compared to brick aggregate concrete. Furthermore, usage of PCC as alternative to OPC and inclusion of water reducing admixture improves the permeability of concrete to a great extent. These observed patterns were further validated by electrical resistivity values of the mixes considered and correlations between resistivity and diffusion coefficient values were established. Based on the diffusion coefficient data, the corrosion initiation time of the considered mix variations were evaluated for different exposure classes and concrete cover values. The mixes with high coefficients, such as brick with OPC and mix proportion of 1:2:4, yield least value of corrosion initiation time because of its lower resistance to chloride attack. On the other hand, usage of stone aggregate, PCC and admixture with low w/c ratio seems to yield higher corrosion initiation time. Besides, higher value of concrete cover slows down the chloride ingress to rebar level and thus improves the initiation time. However, in case of OPC, significantly higher cover has been observed to be required when the structure is exposed to severe environment. A similar impact pattern can be observed in the ultimate service life of the various mixes considered. The corrosion initiation time is followed by the time required to severe crack to occur which is summation of the crack initiation and crack propagation time. The attained values of service life of different mixes follow a similar pattern of variation as that of corrosion initiation time with changes in parameters. Based on these observed patterns, it has been found that usage of stone and PCC in concrete mixes for severe environmental exposure would result in satisfactory service life of 75 to 100 years for chloride induced corrosion. Any concrete mix with OPC and with typical cover results in significantly less service life under severe marine environment. However, for normal exposure category, the service life of OPC concrete mixes with the typically used cover do not appear to be adversely affected. The inclusion of water reducing admixture with low w/c ratio also proves to be effective in improvement of service life, especially under severe exposure conditions.