Effect of microcracking on strength and durability of low water-to-cement ratio concretes

Concrete is prone to cracking, and thus, numerous studies have been conducted to determine the critical crack width, especially when concerns regarding serviceability are warranted. With most of these studies being conducted on single cracks, the effect of crack density has been largely ignored. However, the presence of a large number of microcracks could cause detriment, even if individual cracks are smaller than the critical crack width values for strength (3 mm) and corrosion (250μm). Thus, the appearance of unexplained microcracks in several in-service prestressed concrete girders with low water-cement ratios led to concerns of reduced service life arising from long-term strength and durability issues due to the presence of these microcracks. This research was aimed at determining the effect of microcrack density on key properties of concrete and estimating the likelihood of long-term strength and durability issues that might arise due to the presence of these microcracks. Non-destructive tests were conducted, and cracking, strain, stiffness, and resistivity measurements were taken from full-scale girders, some of which were in-service, over a two-year monitoring period. Cores were collected from some of the girders for further laboratory investigation of cracking, strength, stiffness, and durability properties. While the girders exhibited an increase in cracking and shrinkage, their present state mechanical properties and durability did not show significant losses. A service life model that can account for exposure conditions and crack density, along with material properties such as water-cement ratio and pulse velocity, was developed and compared to a commercially available service life prediction software, Life-365®. Reductions of up to 13 years in service life were calculated for the girders. The results show that crack density, in addition to crack width, is important to consider. Even cracks under 100 μm width can likely impair the long-term durability of concrete when crack density exceeds a critical value.