International Concrete Abstracts Portal

The increasing construction of high-rise buildings in recent years had led to a demand for lightweight, high-strength concrete. In this study, the compositions of the matrix and the air void structure of aerated mortar containing silica fume were investigated as the basis for manufacturing lightweight, high-strength concrete. Mortars made with cement containing silica fume and fine or ultra-fine silica stone powder, having a particle size between that of cement and silica fume, were tested; the properties of cement paste in fresh and hardened conditions were improved. The compressive strength and the air void structure of prefoamed aerated mortars were determined and their relationship studied. Based on the results, it was confirmed that lightweight, high-strength concrete could be made with an effective combination of aerated mortar containing silica fume and lightweight coarse aggregate.

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.

Presents the results of research performed in developing and using flowable fly ash slurry which is classified as a Controlled Low Strength Material (CLSM) as defined by ACI Committee 229 for underground facility construction and abandonment. The mixture proportions for the CLSM described in this paper used fly ash as a primary ingredient. The fly ash was produced at Wisconsin Electric's Port Washington Power Plant as a byproduct of burning coal from Pennsylvania. Port Washington Power Plant has four 80 MW electric generating units that were brought in service between 1935 and 1949. Additional ingredients included portland cement, water, and conventional fine and coarse aggregates. Information is also included on the compressive strength, electrical resistivity, thermal conductivity, and compatibility with plastics used in the manufacture of underground electric cable jackets and natural gas lines. The results indicate that CLSM fly ash slurry is an excellent material for backfilling trenches and filling abandoned underground facilities.

The water-to-cementitious materials ratio (W/CM) is recognized as an important variable in understanding and controlling the quality of concretes containing pozzolan powder additives, such as fly ash. This paper presents part of a study to determine a correct way to evaluate the contribution of fly ash to concrete strength as is usually indicated by the W/CM ratio. A rational mathematical model of the form W/CM = W/(C + K * FA) is presented, in which W, C and FA are water, cement, and fly ash contents per m 3 of concrete, respectively. K is a pozzolan efficiency factor based on comparing the compressive strength of two concretes having the same workability and the same amount of cement. An experimental program was conducted to demonstrate the use of the proposed W/CM ratio model and corresponding K values. The variables studied were class and addition level of fly ash and slump, strength, and age of concrete. Results show that the model describes and considers the influence of fly ash on the rheological properties of fresh concrete and on the strength of hardened concrete. The model is also suitable for use as an equivalent to Abram's law to account for modern day powder additives.