International Concrete Abstracts Portal

The demand for high-performance concrete is steadily rising in the construction market. Whereas it may not be difficult to attain high compressive strength with these concretes, controlling the rheology in the fresh state can create problems. The composition and properties of several high-performance concretes in their fresh and hardened states, made with reground Type 50 (ASTM Type V) cement of Blaine fineness 650 mý/kg, and silica fume, slag, and fly ash at w/c 0.30 or lower are presented. All these high-performance concretes present long slump retention, combined with high elastic modulus, modulus of rupture, and splitting tensile strength. The actual compressive strength can be as high as 124 to 136 MPa at 1 year. These results are compared with a reference concrete made with the same cement at the same w/c, but without any mineral admixtures. The microstructural characteristics of these concretes at 1 year are described. The correation between the microstructure and the mechanical properties are discussed.

Discusses the relationship between the composition and the mechanical properties (compressive strength, modulus of elasticity, autogenous shrinkage) of high-strength concretes (HSC) in the range of 50 to 100 Mpa. The models proposed for each of these properties are based on an analysis of the hardened concrete as a composite material, making it possible to go from the properties of the concrete to those of its matrix. The properties of the matrix are related to the two main parameters of composition (water-cement and silica-cement ratios) by empirical models obtained by smoothing the experimental data. Eleven concretes were made using the same constituents; the parameters of composition were varied separately to determine their influence on the properties in question. These experimental data, together with other data taken from the literature, were used to evaluate the accuracy of the proposed models. It is finally shown that these models, which sum up the current knowledge of the material, can be useful in designing HSCs according to specifications.

The chloride-ion attack on low water-cement ratio pastes containing silica fume was studied by soaking small paste disks in four different pH-controlled sodium chloride solutions for periods of up to 12 months. The pastes were made using water-cementitious material ratios of 0.30 and 0.25. Three types of cementitious materials were used: an ASTM Type III cement (Canadian CSA Type 30), the same Type III cement with 6 percent silicafume, and a French CPA-HPR cement with 6 percent silica fume. The four solutions in which the paste disks were soaked were the following: 3 percent NaCl (by weight) at a pH of 13.0, 3 percent at 11.5, 0 percent at 13.0, and 0 percent at 11.5. The curing period was fixed at 28 days for all mixtures. Mercury intrusion porosimetry, x-ray diffraction, scanning electron microscopy, and electron microprobe measurements were the techniques used to study the various samples after removal from the solutions. The chloride-ion attack on these low water-cementitious material ratio pastes was always very small, even after 12 months of exposure to 3 percent NaCl solutions at pH values of 13.0 and 11.5. After several months of exposure at a pH of 13, only very small amounts of chloride ions ( 1 percent) were detected and only minor changes to the microstructure were noted. At a pH of 13.0, the penetration of chloride ions was not found to be a function of the paste characteristics [w/(c + sf), type of cement, silica fume content]. The major parameter controlling chloride-ion penetration in low water-cementitious material ratio pastes is the pH of the NaCl solutions. When the pH is 11.5, the penetration of chloride ions into the pastes is easier because of the leaching of calcium ions creating a very fine microporosity. For this relatively low pH, it was found that the use of lower water-cementitious material ratios and silica fume can reduce the amount of chloride ions that can penetrate the cement paste.

Corrosion of reinforcement is one of the major degradation mechanisms of reinforced concrete elements. The majority of studies published on concrete-steel corrosion have been conducted on unstressed specimens. Structural concrete, however, is subjected to substantial strain near the steel reinforcing bars that resist tensile loads, which results in a system of microcracks. Report presents the initial results of an investigation to determine the effect of applied load and microcracking on the rate of ingress of chloride on and corrosion of steel in concrete. Simply supported concrete beam specimens were loaded to give a maximum strain of about 600 æî on the tension face. Chloride ion ingress on cores taken from loaded specimens was monitored using energy-dispersive x-ray analysis techniques. Corrosion current and rate measurements using linear polarization electrochemical techniques were also obtained on the same loaded specimens. Variables investigated included two concrete types, two steel cover depths, three applied load levels, bonded and unbonded reinforcing steel, and the exposure to tension and compression beam faces to chloride solution. One concrete mixture was made with Type 10 portland cement, the other with 75 percent blast furnace slag, 22 percent Type 50 cement, and 3 percent silica fume. The rate of chloride ion ingress into reinforced concrete and hence the time for chloride ion to reach the reinforcing steel is shown to be dependent on applied load and the concrete quality. The dependence of corrosion process descriptors--passive layer formation, initiation period, and propagation period--on level of applied load is discussed.

Compressive fatigue strength of concrete in a submerged condition deteriorates drastically compared with concrete in an air-dried condition. One of the reasons for the lowering of fatigue strength in submerged or wet concrete appears to be the influence of the reduction of the bond at the interface between the aggregate and the cement paste. However, this reduction may be mitigated by reducing the calcium hydroxide content and filling the voids at the interface. In this study, compressive fatigue tests were performed in submerged conditions using concrete composed of blast furnace slag or silica fume. The 2-million-cycle fatigue strength of this submerged concrete improved up to 44 percent of its static strength in water compared to 31 percent for ordinary concrete in water. However, this was found to be smaller than 56 percent for ordinary concrete in air. During these tests, the pH of the water in the test tank and the strain of the specimens were measured, and the amounts of calcium hydroxide that oozed out from the specimen and the strain behavior were investigated. The increase in fatigue strength is due to an improvement in the aggregate interface bond and watertightness. However, the expansion of cracks just before failure, which is a distinct characteristic of fatigue in water, was not checked.