Out of all the durability problems related to concrete structures, corrosion of reinforcing steel is the single most serious deteriorating process and this is likely to be more severe in the presence of defects (e. g. cracks)in the concrete surface. The review of published literature shows that at fixed w/c ratio, cement reduction is not detrimental to fresh and hardened concrete properties and may improve performance in some cases. However, cement content variations (in some cases) are relatively small and do not cover an adequate range around the specified minimum cement contents in standards. In addition, insufficient data exist to establish relationships between cement content and resistance of concrete to carbonation-andchloride- induced reinforcement corrosion. Moreover, no data were found on the combined influences of crack width and cement content on resistance of concrete to reinforcement corrosion.Given this background, this study was designed to fill in many areas where information was either inconclusive or not available. The experimental programme, which was carried out in three stages, examined the influence of variation in cement and water contents in equal proportion, Le. at fixed w/c ratio, on the fresh, engineering, permeation, durability properties of concrete and, particularly, resistance of concrete (both cracked and uncracked) to reinforcement corrosion at the limits of specifications and beyond, covering concrete mixes made with a wide range of mix constituents.Stage 1 of the study mainly showed that for a given set of constituents, at fixed w/c ratio, cement (and,therefore, water) reduction below the given minimum cement contents in current specifications did not adversely affect the fresh, engineering, permeation and properties of concrete. Indeed, many concrete properties were improved, particularly with the inclusion limestone filler (with cement reduction)to maintain the mix fines content.Stage 2 indicated that observations noted for permeation properties were not influenced by aggregate or cement type. However, for a given mix, the resistance of concrete to permeation was improved by using lower absorption aggregates and PC / PFA or PC / GGBS, instead of PC. Cement reduction at fixed w/c ratio below the given minimum cement contents in current specifications had a minor adverse effect on the carbonation resistance of concrete, particularly when the fines content was allowed to deplete, and this was not influenced by aggregate or cement type. For a given mix, concrete containing absorption aggregate showed higher resistance to carbonation and that containing PC / PFA or PC / GGBS showed lower resistance Furthermore, the effect of cement type on carbonation was more pronounced than that of cement content. However, cement content had no influence on the carbonation-induced reinforcement corrosion after initiation, regardless of aggregate and cement type. Moreover, the effect of cement content on corrosion rate was insignificant compared to the cement type and the use of PFA or GGBS with PC increased the corrosion rate compared to PC. Cement reduction at fixed w/c ratio below the given minimum cement cement contents in current specifications led to reduction in chloride diffusion and chloride content, particularly with the inclusion of limestone filler to maintain the mix fines content, and this was not affected by aggregate or cement type. Again, for corresponding mixes, concrete made with lower absorption aggregate and PC / PFA or PC / GGBS showed greater chloride resistance. Furthermore, the effect of cement type on chloride diffusion was more pronounced than that of cement content. Reflecting the chloride resistance,cement reduction, particularly with the inclusion of limestone filler to maintain the mix fines content,resistance of concrete to chloride-reinforcement regardless of aggregate and cement type and, for a given mix, PC / PFA or PC / GGBS showed greater resistance compared to PC. Stage 3 showed that for concrete with 0.3 mm surface crack (intersecting) width, cement reduction (below the specified minimum cement contents) at fixed w/c ratio slightly increased the chloride penetration through the cracked zone, but led to negligible influences on the resistance of concrete to chloride-induced reinforcement corrosion. Although there was a little influence of the surface crack (intersecting) width varying from 0.1 to 0.5 mm, on the chloride penetration through the cracked zones (not directly proportional to the width), it had no influence on chloride-induced reinforcement corrosion This study demonstrates that in situations, there is no clear need to specify minimum cement contents and, if specified, their values should be reconsidered. Indeed, the minimum cement content may be an unnecessary restriction on mix design. Furthermore, if a reduced or no minimum cement is specified,attention should be made to specify a minimum fines content appropriately to achieve a closed structure and,consequently, to improve the performance of concrete. Moreover, it is a mistake to pay too much attention to cement content in determining the resistance of concrete to reinforcement corrosion, which is likely to be mainly influenced by cement type.
|Date of Award||2003|
|Supervisor||Ravindra Dhir (Supervisor)|