Effect of blended cement on service life enhancement of concrete structures in marine environment

Abstract

The concern for adequate durability is emerging as the foremost consideration in the aspect of serviceability assurance while designing Reinforced Concrete (RC) structures in modern construction practices. The escalating significance of this phenomenon is enrooted to the prolific infrastructural expansion in developing countries like Bangladesh. However, durability considerations have always been veiled by priority of strength consideration in prevalent design procedures which is further exacerbated by poor construction practices in this country. One of the prime determinants of durability degradation is corrosion of rebars due to ingress of chloride ions in marine environment. Loss of rebar constituents due to corrosion accounts to premature dilapidation of strength of associated structures with lowered service life. Ingress of chloride ions in concrete structures is habitually reliant on high inherent permeation characteristics of concrete which accounts for such elevated risks of service life degradation. It is, therefore, imperative to focus on exhaustive solutions to reduce the intrinsic high permeability of concrete as a construction material. Previous studies have deduced the importance of Portland Composite Cement (PCC) in service life enhancement of RC structures in marine environment. Hence, the outcomes imply the potential of using Supplementary Cementitious Materials (SCM) like fly-ash and slag as replacement of cement. This is expected to contribute in pore refinement of concrete and thereby improve the durability. Lower cost and reduced carbon footprint associated with the use of SCM also contributes to the beneficial impact of such supplementary materials. However, commercially available PCC consists of a certain proportion of SCM, based on strength consideration, without any regard to durability concerns in different construction exposures. This study has been undertaken to evaluate the ideal and typical proportion of SCM as cement replacement to optimize the durability performance of concrete without significant loss of strength. Different concrete mixes with a variation in mix and design parameters have been analyzed to infer the effect on resistance to chloride ion ingress. Concrete with various mix-proportions have also been investigated for compressive strength to consider its effect along with enhancing durability standards. Based on the obtained diffusion coefficients, corrosion initiation times of concrete mixes have been determined along with crack initiation and propagation times to deduce the total service life of structures for a range of cover values. The findings show a superior durability performance of fly-ash as SCM in terms of lower diffusion coefficients and higher service lives. It has, however, been demonstrated that use of slag as SCM enhance the service life values moderately. The results also represent the durability threats associated with mixes with no SCM in terms of low service lives. However, simultaneous consideration of compressive strength reflects an opposite trend. This signifies the usefulness of the combined generalized graphs developed in this study. It also connotes the importance of considering and balancing both the compressive strength and service life data in order to infer an optimum mix for particular exposure conditions. The results has also confirmed the higher degree of risks associated for RC structures in XS2 condition where the structures are subjected to cyclic wet and dry sessions. In this study, a service life guideline of RC structures using blended cement concrete has been developed for ensuring increased durability in marine environment.