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.

Small-angle neutron and X-ray scattering techniques are being used in a systematic study of the development of concrete microstructure on the nanometer scale (1 to 1000 nm) as a function of the addition of fly ash, silica fume, or other pozzolanic materials. These methods yield direct measures of the fractal aspects of the material microstructure, including volume- and surface- fractal exponents and structure parameters within the calcium-silicate-hydrate gel. These variables are being evaluated for use in a classification system of microstructures. In the first phase of the program, the emphasis has been on the characterization of silica fume products both as separate phases and after reaction in concrete. The combination of small-angle scattering with a fractal interpretation scheme has been found to provide a resilient and powerful probe of the undisturbed statistically-significant microstructures in cementitious systems.

The strength development of blends of five cements with various levels of a fly ash, two blast furnace slags, a ground limestone, and a dried chalk dust was determined using EN 196 mortars and, for selected materials, concretes. Three of the cements were based on normal portland cement (OPC) clinkers and two on a high-early-strength (HES) mineralized clinker. At the same specific surface area and SO 3 content, the HES clinker gave cements with strengths 5 to 10 MPa higher than those based on equivalent normal clinker at all ages from one to 56 days. This allows the use of significant levels of fly ash, slag, or other less reactive materials in blends giving similar early strengths to normal portland cements. The early strengths of the blends with the ground limestone and the lower surface area HES cement were higher than expected. The finer chalk dust gave significant contributions to strengths with all the base cements, particularly at early ages. The effect was greater with the lower surface area cements and those based on HES clinker. It is concluded that the acceleration of hydration by the fine calcium carbonate is particularly strong with cements based on the mineralized clinker.

The concept of highly-flowable concrete was developed from the transformation of underwater concreting ideas to the concreting of structures on land. Therefore, the general properties of highly-flowable concrete are similar to those for concreting under water. The viscosity of highly-flowable concrete is high so that segregation of the coarse aggregate from the concrete can be eliminated. The slump flow of highly-flowable concrete is greater than 600 mm so as to increase its flowability. The slump flow is defined as the diameter of slumped concrete. The distinctive feature of the mixture is that a larger proportion of fine material is used in it. The high viscosity and large amount of fines increase its resistance to segregation. In the method of mixture proportioning of highly-flowable concrete proposed by the authors, a high-range, water-reducing admixture (HRWRA) is used to increase the slump flow. Furthermore, a segregation-reducing agent is used to minimize the segregation, although a large proportion of fines somewhat increases the viscosity of concrete. Limestone powder, which is a relatively low reactive material, is used to reduce the heat of hydration and shrinkage. In the proposed method of mixture proportioning, it is possible to choose the required average strength, water content, and fine aggregate-total aggregate ratio to suit special and particular conditions of concrete structures under various environmental conditions.

Because of increasing annual production volumes of fly ash, large volume applications, such as aggregate production, are beneficial in solving the disposal problem of fly ash while making economical use of a mineral resource. Aggregates have been produced from high calcium fly ash of Soma Thermal Power Plant and their engineering properties measured. For high volume utilization of fly ash, aggregate production involving pelletization and pressing into specially designed molds has been carried out successfully. Mechanical property and durability tests were conducted on the cured lightweight fly ash aggregates; five percent by weight lime addition to fly ash showed the best performance. It was also shown that different shapes of aggregates can be produced using the pressing technique.