The influence of longitudinal cracks on the corrosion protection afforded reinforcing steel in high performance concrete

It is almost impossible to produce crack-free concrete and, therefore, codes of concrete structural design (such as ACI 318 [ACI 318, Building Code Requirements for Structural Concrete, American Concrete Institute: Farmington Hills, MI, USA]) take cracking into account and relate permissible crack widths to exposure conditions. Chloride ingress is significantly enhanced by cracks because the ions can penetrate the concrete cover from the walls of the crack as well as from the outer surface of the concrete [P. P. Win, M. Watanabe, and A. Machida, Penetration profile of chloride ion in cracked reinforced concrete. Cement and Concrete Research, 2004. 34(7): p. 1073–1079]. Thus, while the chlorides reach the steel very rapidly directly through the crack, they also reach adjacent areas of steel more rapidly than in uncracked concrete.The objective of the project was to ascertain whether high performance concrete (HPC) could provide superior protection to Ordinary Portland Cement Concrete (OPCC) even when the structure was cracked parallel to the reinforcing bars (longitudinal cracks) while being exposed to a de-icing salt environment. Two HPCs, as specified by the Ministry of Transportation of Ontario (MTO) [MTO, SSP 904 S13 High Performance Concrete, Amendment to OPSS 904 Construction Specification for Concrete Structures. 1995, Ontario Provincial Standard Specification] were tested: both used blended silica fume cement (Canadian Type 10 E-SF) and had 25% replacement of the cement by either fly ash or ground granulated blast furnace slag. A Class C-2 ordinary Portland cement concrete was used as control.In the case of cracks parallel to the rebar, HPC does not appear to have any beneficial influence on the corrosion of bars, which is not surprising in view of the fact that the whole length of the bar is directly exposed to the environment via the crack. In contrast, HPC has been found to provide better protection for steel exposed to transverse cracks than does OPCC. However, the benefits of HPC are not as great as they are for sound (uncracked) concrete. The reasons for the better protection are (i) the greater resistance of HPC to chloride penetration from the walls of the crack, (ii) its greater tendency to crack healing and (ii) the different crack path in HPC.