2019 CSCE Annual Conference - Laval (Greater Montreal) Conference
Mr. Md. Jihan Hasan, Department of Civil Engineering, Bangladesh University of Engineering & Technology (BUET)
Mr. Bayezid Baten, Department of Civil Engineering, Bangladesh University of Engineering & Technology (BUET)
Ms. Tafannum Torsha, Department of Civil Engineering, Bangladesh University of Engineering & Technology (BUET)
Mr. Md. Fahim Azraf Khan, Department of Civil Engineering, Bangladesh University of Engineering & Technology (BUET)
Dr. Khandaker Hossain, Ryerson University
Concrete mix design in many parts of the globe is still primarily based on strength requirement. One of the main reasons for strength focussed mix design is the relative simplicity and promptness of the strength measurement procedure. Moreover, concrete of higher strength are generally presumed to possess better durability. However, under severe exposure condition like marine environment, typical durability concept fails to provide required serviceability of a Reinforced Concrete (RC) structure. This structural hazard is usually significant in concrete structures of low to moderate strength, particularly evident in regions where durability is not a prime concern over strength, like Bangladesh. Any RC structure in marine environment is susceptible to chloride ingress and hence, requires impermeable concrete to hinder diffusion of chloride ions within it. The impermeability of concrete can be measured through chloride diffusion coefficient which eventually, has significant effect on corrosion initiation of reinforcement. The typical strength focussed mix design practices cannot ensure reduced diffusion coefficients since pore refinement is necessary to disconnect continuous pore system within concrete. Refinement of pores can be warranted through use of composite or blended cement that contains supplementary cementitious material like fly ash. Nevertheless, use of such composite cement could hamper the strength gain of concrete. As a result, use of Ordinary Portland Cement (OPC) is preferred over blended cements in most cases of strength based mix design. Considering all the aforementioned aspects, an effort has been undertaken in this study to investigate the probable service life of RC structures of some common concrete mixes targeted for low to moderate strength (in the range between 20.7 and 34.5 MPa) with a workable slump value (around 100 to 150 mm). Concrete mixes, considered in this study, were proportioned for both OPC and composite/blended cement in such a way to satisfy the strength ranges and slump criteria. Chloride diffusion coefficients of the mixes were obtained by non-steady state rapid migration test. Finally, a probabilistic approach was followed to ascertain probable service life considering concrete covers typically used in RC structures constructed in marine condition. It has been obvious from the study that factors affecting durability of concrete should be given utmost importance in order to achieve satisfactory service life. Only strength based mix design without a proper choice of cement type could result in extremely poor serviceability of RC structures under severe chloride exposure.