International Concrete Abstracts Portal

The freezing and thawing (F/T) durability of nonair-entrained cement pastes and mortars was evaluated by measuring the decrease in compressive strength. At the water-cementitious ratio (w/c) of 0.24, both the paste and mortar showed excellent F/T resistance at 0, 5, and 10 percent silica fume levels. When the w/c was higher than 0.24, the paste and mortar durability were significantly reduced. The F/T durability of nonair-entrained concrete was determined according to ASTM C 666, Procedure A. At a w/c of 0.24, the nonair-entrained concretes were F/T-durable regardless of the silica fume and total cementitious content, but concretes with a w/c greater than 0.24 had better overall durability. The salt-scaling of nonair-entrained concrete at different w/c was tested according to ASTM C 672. No scaling was found in the concrete at w/c of 0.24 and 0.27. The results from the paste, mortar, and concrete showed that w/c was the most important factor in evaluating F/T resistance of these concretes.

Presents the results of an investigation on the long-term deformation of steel fiber reinforced concrete containing silica fume. The influence of loading ages on the creep and ages of curing on the shrinkage of specimens was investigated. The volume fraction of steel fibers used in concrete is 0, 1, and 2 percent. The addition of silica fume is 0, 5, and 10 percent by weight of cement. Test results indicate that the combined effect of fibers and silica fume reduces the creep and shrinkage and enhances the development of compressive strength of concrete. At specific silica fume content (10 percent), the effect of increasing fiber content to reduce creep and shrinkage decreases gradually as the fiber content increases. This phenomena is similar to the addition of silica fume in concrete with 1 percent volume fraction of steel fibers.

Besides high workability (high filling capacity and high resistance to segregation), little drying shrinkage and high resistance to cracking are important properties required for high-performance concrete. Addition of expansive admixture by appropriate proportion is an efficient way to compensate drying shrinkage for conventional concrete. By experimental investigations, it is confirmed that there is a very similar effect for high-performance concrete (HPC). But the effects of different types of expansive admixture for improving properties of high-performance concrete are various. As distinguished from using silica fume to produce HPC, fly ash is adopted in tests. It is found that appropriate amount of fly ash can not only avoid segregation, but also helps regulate unfavorable expansion of concrete caused by excessive content of expansive admixtures. Four types of expansive admixture are used in tests. Their effects on workability, deformation, and strength of HPC are discussed in this paper.

The control of differential thermal stress or strain due to heat of hydration in a thick concrete section can be a requirement for a high-performance concrete. An investigation was carried out to study the use of ground granulated blast furnace slag (GGBFS) as partial replacement of cement to reduce the adiabatic temperature rise of concrete. By testing concrete mixes instead of cement pastes, this study includes the effects of not only the cement but also the presence of aggregates in their proportions and directly relates the mix to the job. A computer-controlled cell is designed to measure the adiabatic temperature rise in concrete with initial concrete temperature at 20, 30, or 40 C. Slag replacement up to 70 percent by mass of total cementitious binder content was studied. Other parameters studied include water-binder ratio ranging from 0.40 to 0.60, fineness from 300 to 400 kg/m 2, and binder content from 250 to 350 kg/m 3 of concrete. The results of the adiabatic temperature rise in concrete show that an increase in slag replacement reduces the temperature rise. The effect of higher fineness or higher total cementitious binder content leads to higher temperature rise. However, the influence of placing temperature on the temperature rise indicates a lower rise at higher placing temperature. It is also noted that at higher placing temperature, slag replacement greater than 55 percent by mass tends to reduce temperature rise to a greater extent than at lower replacement levels. The development of the heat of hydration with time of the concrete mixes under adiabatic condition is expressed in equation form.

Cement particles generally consist of micropores measuring 5 to several hundred. The micropores are too small to permit permeation of water due to water surface tension, but large enough to accommodate diffusion of steam under elevated pressure. The size of a water molecule has been scientifically determined. When dry cement particles are in contact with steam, heat immediately transfers from steam to cement, and part of the steam is forced into inner regions of the cement particles via the micropores. As a result, cement particles gain activation energy, and at the same time steam partially condenses due to energy dissipation to form moisture coating on the surfaces of cement particles as well as the interior surfaces of the micropores. Both the activation energy and condensation of steam enhance a rapid and complete hydration. Test results show that concrete made with the dry-mix/steam-injection procedure developed high CSH/CH ratios in paste and a high strength of 700 kgf/cm 2 (10,000 psi), approximately 2.5 times that of companion concrete made with the wet-mix procedure, in less than 1 day. Another test series demonstrated a 50 percent reduction of cement requirement in comparison with the wet-mixed concrete with an equivalent strength of 560 to 630 kgf/cm 2 (8000 to 9000 psi).