International Concrete Abstracts Portal

The Canadian association of large public real estate companies has initiated a 5-year research project aimed at determining the most cost-effective way to rehabilitate deteriorated parking structures. A sample of 49 garages includes office, retail, and residential buildings. The repair history of each building has been documented and, in some cases, a formal condition survey of the garages is undertaken yearly. The collected data serve as a basis for the evaluation of the effectiveness of the various repair techniques and strategies. The project is now in its third year. The investigation was carried out concerning the excessive cracking noted in some garages constructed with epoxy-coated reinforcing steel. The benefits of intensive maintenance and good housekeeping have been shown by the analysis of the case history of a garage. Various types of concrete sealers have been evaluated by testing in the laboratory 57 products applied to 8 types of concrete substrate. Preliminary results indicate waterproofing membranes are an effective means to reduce the moisture content in the slab.

A waste repository for the underground disposal of low-level radioactive waste, called IRUS (Intrusion Resistant Underground Structure), is planned at the Chalk River Nuclear Laboratories. It relies greatly on the durability of concrete for a minimum of 500 years of service life. A research program based on laboratory testing to design a durable concrete and predict its useful engineered service life is in progress. The durability of concrete depends on its resistance to deterioration from both internal and external causes. Since the rate of degradation depends to a major extent on the rate of ingress of aggressive ions into concrete, laboratory testing is in progress to establish the diffusion rates of chlorides and sulfate ions. A total of 1000 concrete specimens and 500 paste specimens are being exposed at 22 and 45 C to 25 different combinations of corrosive agents, including Co2. Procedures to measure the ionic penetration profile and to determine the factors controlling diffusion of ions in the various concretes have been developed. The paper presents initial results from the research program and the longevity predictions to qualify concretes for the IRUS waste repository, based on 16 months of diffusion testing on laboratory specimens.

Comparisons of the corrosion performance and pullout strength of black, hot-dip galvanized, and fusion-bonded epoxy-coated steel reinforcement in concrete have been undertaken. Accelerated exposure testing confirmed that zinc coating was able to delay considerably the onset of corrosion and that epoxy coating effectively eliminated corrosion, provided the coating was not damaged. Where coated reinforcement was left with cut ends unrepaired, the epoxy-coated bars showed early corrosion of the exposed steel, with corrosion progressing along the bar under the coating. Even where cut ends were repaired, the epoxy-coated bars showed many sites of breakdown of the repair and corrosion of the underlying steel. The sacrificial nature of the zinc coating provided positive protection to the underlying steel where the coating was damaged. Pullout testing revealed that there is no significant difference in the ultimate bond strength of black, epoxy-coated or galvanized deformed bars. For plain reinforcement, the ultimate bond strength of epoxy coated bars in some 17 percent less than that for black steel bars, while that for galvanized bars is some 31 percent greater than for black steel. The ultimate bond strength of deformed bars is up to 50 percent higher than that of plain bars. The passivation of galvanized plain bars by chromate additions to the concrete mix water in the range 15 to 150 parts per million could not be shown to significantly improve the bond strength. Further work is being done in this area with much larger sample populations to clarify this result.

Chloride intrusion and carbonation are the two major deteriorating processes that cause depassivation of steel embedded in concrete and lead to damage to concrete structures. If surface coatings to concrete are to be effective in protectin and preserving new as well as existing structures, they have to fulfill two important functions. First, they should prevent intrusion of chloride ions, and second, they should protect steel in chloride-contaminated concrete by preventing ingress of water, air, and other destructive agents. The paper presents extensive test data on the ability of an acrylic rubber-type coating, in fulfilling these two functions. Tests on coated and uncoated reinforced concrete prisms containing various amounts of added sodium chloride and exposed to seawater are reported. The results show conclusively that Aron Wall coating prevents the intrusion of air, water, and chloride ions, and provides excellent protection to steel in chloride-contaminated concrete while maintaining its adhesion to concrete, integrity, and continuity.

Silica fume and/or fly ash concretes were produced to determine the long-term effects of these pozzolans on chloride ingress, electrical resistivity, microstructure, and the subsequent effect on corrosion of embedded steel reinforcement. Initial results of these experiments showed that there was a good relationship between the 28-day compressive strength, rapid chloride permeability, and electrical resistivity. In this paper the long-term corrosion performance, electrical resistivity, chloride ingress, and microstructure are examined after 3 years of partial ponding in 3 percent NaCL. These results are compared to the initial concrete properties to determine how well early measurements of the properties relate to long-term corrosion resistance and chloride ingress. Also, long-term changes in the concrete microstructures are documented. The results show that concrete resistivity and rapid chloride permeability measurements are better indicators of corrosion resistance than concrete strength. In general, increasing silica fume and/or fly ash contents and/or reducing water-cementitious ratios improves corrosion resistance. However, for mixes with over 20 percent pozzolans by mass of cement, some carbonation has occurred next to cracks where the reinforcement exited (the nonsubmerged sections of the samples). The significance of these findings is discussed.