Hydro=Quebec has undertaken a major study of shotcrete as a repair material for concrete dams. Six dry-mix shotcretes and one wet-mix shotcrete were shot on an old concrete slab and into sampling boxes using Types 10 and 30 portland cement, silica fume, superplasticizer, and polypropylene and steel fibers. An air-entraining agent (AEA) was added to all mixes except one to verify the effect of air entrainment on shotcrete. After wet-curing for 3 days at approximately 20 C and exposure to field conditions for 25 days, prisms and cylindrical specimens were taken from each of 600 x 600 x 150-mm shotcrete slabs. These specimens were then subjected to 500 freezing and thawing cycles, and tested for air-void parameters, compressive strength, and bond to old concrete. Freezing and thawing results showed that five of the six air-entrained mixtures yielded a durability factor (DF) over 100 after 500 freezing and thawing cycles, whereas one showed a DF of 84, although its spacing factor L was only 159 m. The one mixture with no AEA gave a DF of 64 with L of 272 m, although its corresponding air-entrained mix gave a DF of 102 and L of 234 m. The overall results showed that: 1) freeze- and thaw-resistant wet- and dry-mix shotcretes incorporating silica fume can be produced using proper proportioning and AEA; 2) silica fume shotcrete can be shot on a 70-deg inclined wall in thicknesses of up to 150 mm without sagging and can produce a good bond; 3) the wet-mix process produces better homogeneity than the dry-mix process.

The mechanisms underlying physical disintegration of concrete by crystallization of mirabilite (Na 2SO 4 10H 2O) and thenardite (Na 2SO 4) were studied by a series of laboratory experiments. In contrast to chemical sulfate attack, which manifests itself in the formation of gypsum or ettringite, the deterioration investigated in this study did not involve chemical attack on the cement paste in concrete. Rather, the damage was strictly of a physical nature, caused by phase changes within the sodium sulfate-water system. Several possible mechanisms of distress were investigated, including pressure caused by hydration, evaporation, and temperature effects. Rapid temperature changes were found to be the dominant mechanism of deterioration. In particular, rapid decreases in temperature resulted in supersaturation, rapid crystallization, and a net increase in the sodium sulfate-water system. Consequently, significant hydraulic pressure, similar to that observed in the classical freeze-thaw phenomenon, would develop if drainage conditions within the concrete were not adequate to allow for the volume increase of the sodium sulfate-water system.

Presents field data of up to 8 years on chloride penetration into concrete and consequent steel corrosion in a test structure exposed to an aggressive environment favoring rapid transportation of chloride ions into concrete. The structure consisted of reinforced concrete beams, slabs, and columns. Two types of concrete, one without salt and the other containing 0.5 percent NaCl by weight of concrete, were used in the construction. Parts of the structure were left exposed and unprotected, while the other half was protected with a highly elastic acrylic rubber coating previously subjected to intensive examination. The chloride contents in the structural members were determined regularly over a period of 8 years. In addition, the influence of the coating and the different salt concentrations on corrosion of the embedded steel were evaluated. It is shown that the acrylic rubber coating can almost totally protect the concrete from chloride penetration and consequent steel corrosion and maintain this protective effect for many years.

An overview of the scientific and technical literature published on the subject is presented. The first part of this report is devoted to the fundamentals of frost action in concrete. The mechanisms of freezing and hypotheses explaining the detrimental effects of deicing salts are discussed. Special attention is paid to the influence of the concrete curing temperature and moisture history on its frost durability. A critical appraisal of three different test procedures designed to assess the deicer salt scaling resistance of concrete is given in the second section. Each test method is evaluated on the basis of reproducibility, variability of test results, operating cost, and relationship to field exposure conditions. Finally, the influence of various parameters on the deicer salt scaling resistance of concrete is discussed. Topics such as mix composition, air entrainment, casting operations, curing, and use of concrete sealers are reviewed.

High-strength lightweight concrete is a promising material with many advantages over normal weight concrete in the construction of offshore structures that are submerged underwater for most of the time. In these structures, which are subjected to dynamic loading, the flexural fatigue strength and endurance limit of concrete submerged in water are important design parameters because these structures are frequently designed on the basis of fatigue loading. Presents the results of an experimental investigation to determine the flexural fatigue strength of lightweight concretes made using expanded shale aggregates. Six different concretes were investigated in this study. A total of 120 prisms (20 prisms of 76 x 102 x 406 mm, in size for each concrete) were tested in flexural fatigue loading of 20 cycles per sec (Hz) when they were submerged in water. The prisms that survived 2 million cycles of fatigue loading were tested in static flexure to demonstrate their residual strength (modulus of rupture). The test results are compared with the results of similar concretes tested in air in an earlier investigation. The static flexural strength (modulus of rupture) and the flexural fatigue strength were higher for the specimens tested underwater compared to similar specimens tested in air. In general, there was no reduction in the endurance limit (the ratio of the fatigue strength to the modulus of rupture) for the lightweight concretes when they were submerged in water. There was an increase in the residual static flexural strength for the prisms previously subjected to 2 million cycles of fatigue stress underwater.