International Concrete Abstracts Portal

Presents results of five-year exposure tests on the long term properties of concretes containing fly ash (FA), blast furnace slag (BFS), and silica fume (SF). Four kinds of concretes with and without a mineral admixture (OPC concrete, FA 30 percent concrete, BFS 50 percent concrete, and SF 10 percent concrete) were prepared. After 28 days of initial curing, they were exposed to different environments for five years. Compressive strength, pulse velocity, depth of carbonation, and chloride ion penetration of concrete were determined at various intervals of exposure time. From the results, it was found that under the indoor exposure condition, influences of initial curing conditions on the long term strength development of concrete were especially pronounced for FA 30 percent concrete and BFS 50 percent concrete, but that under the outdoor exposure conditions, its influence was considerably reduced due to the supply of rainfall during the outdoor exposure. On the other hand, SF 10 percent concretes showed some reduction in compressive strength when they were initially cured in water for seven days and then continuously air-dried indoors for a long period. The depth of carbonation of BFS 50 percent concrete and FA 30 percent concrete was much greater than that of the corresponding OPC concrete and SF 10 percent concrete when they were exposed indoors or outdoors for five years. Furthermore, all mineral admixtures used in this study were found to be equally efficient in preventing chloride ions from intruding into concretes under a marine environment.

It is necessary for concrete engineers to get more information on the ion movement through and in concrete for the development of new technologies, such as cathodic protection, desalination, and re-alkalization for reinforced concrete members. In concrete members with these treatments, various ions should be moved through and in concrete members. The movement of ions could influence concrete properties and steel reinforcing bars. Ground granulated blast furnace slag, fly ash, and silica fume have been recognized as high quality mineral admixtures for concrete. Since structures built with these materials might eventually be subjected to electro-migration processes, a set of experiments to assess the effects of these pozzolans were devised. In this study, considering the conditions mentioned above, the movement of several kinds of ions through hardened mortar with mineral admixtures was investigated. As ions, Na +, K +, and Cl - were selected because the ions were closely related to alkali-aggregate reaction or chloride attack. As mineral admixtures, ground granulated blast furnace slag, fly ash, and silica fume were used. Also, the influences of water-to-binder ratio on the movement of ions were investigated. Electrochemical cells were used for the experimental work; the current was applied to a cell in the range between 0.1 A/m 2 and 10.0 A/m 2. Analyzing the data from the experimental work, the following conclusions were obtained. 1. The electromigration of ions through mortar are reduced with the addition of admixtures. 2. The electromigration of ions increases with the water-to-binder ratio. 3. The electromigration of ions is closely related to the pore size distribution of mortar and paste.

Bagasse is the fibrous residue of sugar cane, which is burned for energy leaving various types of ashes as waste residue, of which grate ash is found to be the most suitable for use in concrete. Grate ash shows poor chemical reactivity with portland cement, making it not very effective as a pozzolan. It can, however, be used as a fine aggregate constituent of concrete. Five grades of concrete were tested, ranging from 20 to 60 MPa, to compare the performance of grate ash concrete with that of normal concrete. The use of the ash alone as fine aggregate gave harsh concrete with low workability and poor cohesion. This was improved by blending about 25 percent normal concrete sand with the ash. Bleeding was comparable with, if not generally less than, that of normal concrete. Grate ash concrete, in particular the lower strength mixes, had 10 to 18 percent higher initial drying rates and would, therefore, require more stringent curing precautions than normal concrete. Rates of strength development were comparable in the two concretes. Compressive strengths of over 80 MPa were achieved after one year with the high-strength ash concrete mixtures. But, for a given strength, the grate ash concrete requires more cement than normal concrete. In comparison with normal concrete, grate ash concrete had similar shrinkages, slightly lower modulus of elasticity, and about 40 percent lower creep deformations. For equivalent strengths, the two concretes showed similar durability properties, in terms of their resistances to mechanical abrasion, water absorption, chloride diffusion, and carbonation. However, due to the porosity of the grate ash particles, the concrete had a much better resistance to freezing and thawing attack than normal concrete, even though all concretes were non-air-entrained.

Presents the results of a study of the influence of ground fly ashes on workability and strength of mortars. Fly ash (T0) was obtained from the thermoelectric power plant of Andorra-Teruel (Spain). Samples of (T0) fly ash were ground using a laboratory ball mill for 10, 40, and 60 minutes (T10, T40, and T60). This process crushed spherical or spheroidal fly ash particles so that the morphology of the particles was substantially modified and the fineness notably increased. Mortars were prepared by replacing from 15 to 60 percent of cement by fly ash. Curing time, curing temperature, and fly ash amount influenced the strength of mortars. Curing times longer than seven days showed significant differences among fly ashes, with compressive and flexural strengths decreasing in the order T60 > T40 > T10 > T0. Increasing the curing temperature from 20 to 40 C produces a rise of compressive strength that exceeds control mortars when T60 and T40 fly ashes were used. It is concluded that the use of ground fly ashes improves the strength of mortars compared with strengths obtained with normal fly ash, but high replacement percentages of ground fly ash adversely affect workability.

Introduces some experimental results on the application of ASTM Class C (high lime) fly ash from Thailand to reduce shrinkage of cement paste. Measurement of both autogeneous and drying shrinkage were conducted on cement paste samples. Parameters varied in the autogeneous shrinkage test were type of fly ash (two samples of Class C fly ash, from the same source, with different chemical composition, and a sample of Class F fly ash), cement replacement percentage (0 percent, 30 percent, and 50 percent), curing condition (seal and submerged), and curing period. For autogeneous shrinkage specimens, flexural strength, compressive strength, and setting time were tested to compare the mixtures containing Class C fly ash with those containing Class F fly ash and with those without any fly ash, to derive some basic information for judging the suitability of the Class C fly ash as a pozzolan. From the test results, it was found that Class C fly ash was effective for reducing autogeneous shrinkage and improving flexural strength. The effect was due to expansion which occurred in the samples containing Class C fly ash. For the tested range, the higher the replacement percentage, the more effective the fly ash becomes. Class C fly ash which contained higher SO 3 content was more effective than that with the lower SO 3 content for reducing shrinkage. Water curing was more effective than sealed curing since the expansion process required water; specimens with longer water curing periods showed smaller shrinkages. The pozzolanic activity index of the tested Class C fly ashes was higher than that of the tested Class F fly ash; therefore, higher 28-day compressive strength was observed in paste with the Class C fly ash than in paste with the Class F fly ash. Setting times of pastes with the Class C fly ash were generally shorter than that of the paste with the Class F fly ash. The Class C fly ash also proved to be effective for reducing drying shrinkage, as indicated by comparison of specimens made with and without Class C fly ash. Longer curing periods reduced the drying shrinkage of specimens with and without fly ash.