International Concrete Abstracts Portal

Roller-compacted concrete (RCC) pavements are now an economical alternative to those constructed from asphalt and conventionally placed portland cement concrete, particularly for those pavements experiencing heavy-duty, low-speed traffic. However, a major concern related to the use of RCC pavement is its frost resistance. RCC pavements can be constructed with aggregate that are not susceptible to frost, and can be cured to an appropriate degree of maturity so as to reduce the fractional volume of freezable water on saturation to limits that can be accommodated by elastic volume change and by the air-void system. However, the ability to effectively entrain proper air-void systems in RCC pavements has remained a question due to the low water contents required to place the mixtures. An investigation was conducted by the U.S. Army Engineer Waterways Experiment Station Structures Laboratory to determine if proper air-void systems can be entrained in RCC pavement mixtures proportioned with several types and dosage rates of air-entraining admixtures, and with various aggregate types and gradings. Results of the investigation indicated that air-void systems sufficient to protect critically saturated RCC pavement mixtures from deterioration due to cycles of freezing and thawing could be created in a wide range of the mixtures produced without following special mixing procedures.

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.

Most studies dealing with the problem of reinforcement corrosion have been carried out using exposure tests in which entire specimens (with their continuous reinforcement) were exposed under uniform conditions. The present paper describes the results of some of the experiments carried out to clarify the characteristics of corrosion occurring under a combination of different conditions. Experiments were carried out by subjecting reinforced concrete specimens to different exposure conditions in the laboratory and a marine exposure site. From the laboratory studies using continuous reinforcing bars in specimens subjected to diverse and simultaneous exposure conditions, it was found that corrosion occurs in those parts of the bars that are exposed to cyclic wetting and drying and at the interface of the submerged and aerially exposed parts. Some aspects of the corrosion monitoring using natural potential measurements under these conditions and the importance of water movement in determining the chloride concentration in concrete are also discussed. From the study of cracked and uncracked reinforced concrete marine environment, it was found that the presence of flexural cracks does marginally accelerate the initiation of corrosion at crack sites. Further, the presence of stirrups in the shear span of these beams was also found to have a significant restraining effect on the initiation and propagation of corrosion-induced longitudinal cracks along the main reinforcing bars. Once the width of these cracks exceeds a certain value, the corrosion of the bars and the widening of such cracks apparently becomes a vicious cycle.