International Concrete Abstracts Portal

Chemical characteristics of the cement paste-aggregate interfacial zone have been considered to influence the durability and mechanical properties of concrete. Particularly, effects of mineral admixtures such as fly ash and slag on the microstructure of the interfacial zone deserve attention. An x-ray diffraction technique was used to evaluate the amounts of Ca(OH)2, ettringite, and the orientation of Ca(OH)2 crystals in the interfacial zone. Composite specimens with several types of rocks were broken to produce a fracture surface on the cement paste prism to which the x-ray diffraction analysis was applied. The analyses showed that the addition of fly ash or slag considerably affected the peak height and orientation of Ca(OH)2 crystals in the interfacial zone, which normally extends up to 50 to 100 æm from the interface. The formation of ettringite in the vicinity of the aggregate surface was restricted by the addition of the admixtures. These results also suggest that the addition of the mineral admixtures favorably affects the resistance of the interfacial zone against aggressive agents from the surroundings. The x-ray fluorescence analysis was conducted to quantify calcium and silicon in the zone. The results obtained complemented the conclusions described previously. 137-389

Many marine and hydraulic structures must be constructed and repaired while submerged under water. Frequently, this requires placement of relatively thin (0.5 m or less) layers of concrete to fill voids in exposed surfaces or submerged formwork. Concretes placed underwater should flow readily and with little segregation and resist erosion from underwater currents. The hardened concrete should achieve excellent adhesion to underlying surfaces and develop high strengths. To achieve the desired performance, the concrete should contain a moderate amount of anti-washout admixture, a cement content of approximately 350 kg/m3, 25 kg/m3 of silica fume to enhance durability, and 18 kg/m3 fly ash to improve workability of the fresh concrete. A hard, natural gravel, representing approximately 54 percent of the aggregate content, should be used for wear resistance, and with the lowest possible w/c (0.41 ñ 0.03, typically) consistent with placement requirements, to maintain strengths. Prior to the actual field placement, several rheological and mechanical properties should be determined to insure proper placability, homogeneity, and therefore increase the success probability and cost effectiveness of site trials and subsequent actual field placements.

Presents results of investigation to determine the combined effect of deicing salts and repeated cycles of freezing and thawing on condensed silica fume concrete. The concrete mixtures tested in this phase included mixtures incorporating silica fume as an 8 percent replacement by mass for cement, along with control mixtures (no silica fume), both covering a range of water-to-cementitious materials ratio of 0.40 to 0.65. All mixtures were air entrained and some contained a superplasticizer. For each mixture, freeze-thaw resistance and scaling resistance to deicing salts were determined using ASTM standard procedures. For some selected mixtures, scaling resistance was also determined using slight variations in the testing procedures. In general, concrete incorporating silica fume is slightly more susceptible to scaling than concrete without silica fume. Preliminary results clearly indicate that the methods of preparing and curing the test specimens has a significant influence on the scaling resistance of the concrete, but further investigations are needed to establish possible correlations between the degree of scaling, type of curing, method of specimen preparation, and percentage of silica fume in the concrete.

ASTM C 672 scaling tests were carried out on normal concretes and concretes containing 5 percent silica fume, with air-void spacing factors in the 100 to 200 æm range. Five curing methods were compared: a 24 hr heat cycle with a maximum temperature of 70 C, 2 and 14 days moist curing, and two different curing compounds. Results indicate that the use of silica fume does not improve the scaling resistance of concrete. Concretes cured with one particular curing compound were found to have a scaling resistance similar to that of those cured in water for 14 days, weight losses after 50 cycles being lower than 1 kg/mý for all mixes. Concretes cured with the other curing compound had a lower and more variable scaling resistance. As expected, specimens cured for only two days in water also had a lower scaling resistance. All mixes cured using the heat cycle exhibited a poor performance, although, in this case only, silica fume reduced very significantly, but not sufficiently, the damage due to the freeze-thaw cycles in the presence of deicing salts.

Durability properties of concrete and mortar based on a special type of alkali-activated slag called F-cement have been studied. The microstructure was found to possess a high occurrence of microcracks that had an obvious influence on the flexural strength and rate of carbonation. The rate of chloride-ion diffusion was about 30 times lower than in the portland cement concrete. Salt scaling was found to depend solely on the water-to-binder ratio and is independent of the air content. Early freezing takes place when the strength exceeds 5 MPa, and F-mortar shows high chemical resistance against solutions of sodium, calcium, and magnesium chloride.