International Concrete Abstracts Portal

Usability of rice husk ash was investigated as a siliceous material for calcium silicate products manufactured by hydrothermal reaction. It is concluded that rice husk ash can be used as a superior siliceous material for manufacture of calcium silicate insulating materials with good thermal durability up to 1000 C. Characteristics of rice husk ash, namely, high SiO2 content, reactive silica phase comprising amorphous silica, cristobalite and/or tridymite, and high surface area, are favorable to the formation of well-grown xonotlite which forms bodies of insulating materials. Trial products with bulk densities ranging from 0.11 to 0.41 g/cm3 prepared from rice husk ash using glass fiber for reinforcement not only satisfied all the requirements in the industrial standards (JIS A9510) but also gave 1.4 to 2 times higher bending strength than commercial products prepared from conventional siliceous materials, such as finely ground quartz sand, silica fume, and diatomaceous earth. A variety of rice husk ashes with different crystallinity are usable for manufacture of calcium silicate products, but the hydrothermal reaction condition should be optimized according to the crystallinity or amorphousness of the ash.

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.

Curing of concrete is impaired by exposure to drying at early ages. Removing water from the surface layers restricts the binder reactions and pore structure development. High porosity in the surface region will allow the ingress of deleterious agents, which can lead to durability problems. Present work reports results obtained by hydrating a flyash blended cement under drying conditions. Comparisons are made with similar results from a portland cement. Small samples of OPC/pfa (70/30) paste with a water binder ratio of 0.59, initially cured under saturated conditions for 7 days, were exposed at 20 C in a CO2-free environment, to various preselected relative humidities. After 28 and 91 days, the extent of reaction and the porosities of the samples were measured by thermogravimetry and methanol adsorption, respectively. Results show the extent of hydration falls when changing from saturated to 70 percent relative humidity (rh) conditions; below this rh, it is virtually constant. From the shape of the TGA curve, it seems that there is little change in the nature of the gel phase. The pozzolanic reaction appeared to cease below 80 percent rh. Total porosity only fell very slightly with increasing relative humidity even after 91 days exposure. Under drying conditions (70 percent rh) the large-diameter porosity was three times greater than large-diameter porosity obtained under saturated conditions. From these tests it is clear that to promote reaction and to effect a decrease in the volume of large pores, the relative humidity must be greater than 95 percent, at least during early-age curing.