International Concrete Abstracts Portal

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).

Two Canadian aggregates, a reactive siliceous limestone and nonreactive crushed granite, were evaluated for their potential alkali reactivity (AAR) in high-performance concrete. The concretes were proportioned to have high strength and cement content greater than 400 kg/m 3. Concrete mixes were made using a silica fume blended cement and a cementitious system in which 25 percent of a CSA Type 20 low-alkali cement was replaced by ASTM Class F fly ash. Also, control mixes were made with a CSA Type 10 high-alkali cement. The susceptibility to AAR of these concrete mixes was evaluated by casting concrete prisms and subjecting them to various accelerated storage conditions in the laboratory. For comparison purposes, mortar bars were also made, and tested according to the ASTM P 214 (1990) accelerated mortar bar test procedure. The AAR concrete prism tests performed in this study have shown that none of the concrete prisms made with silica fume blended cement and low-alkali cement incorporating fly ash showed significant expansion after 18 to 24 months of testing either in 1N NaOH or in exposure conditions of 38 C and relative humidity greater than 95 percent. The accelerated mortar bar test results, however, suggest that long-term testing may be needed to evaluate the effectiveness of blended cements in reducing expansion due to AAR, especially for highly reactive aggregates.

The Hibernia offshore concrete platform is being constructed in Newfoundland, Canada, and will be used in hydrocarbon production on the Grand Banks off the east coast of Canada. The 111-m tall concrete structure will contain approximately 165,000 m 3 of high-strength concrete. Construction of the concrete platform through 1993 consisted of a 108-m-diameter base slab that rested on a series of precast and cast-in-place concrete skirts. Specified 28-day compressive strengths (cylinder) for the skirts and base slab were 49 and 69 MPa, respectively. Actual average compressive strengths achieved were73.8 and 81.7 MPa, respectively. The remaining construction will be completed by 1996. The use of two different concrete production systems and their results are described.

When a damaged concrete slab is repaired, new concrete will generally be placed in a downward direction. Form panels must be set up first under the damaged concrete slab, and then concrete must be placed between the form panels and the damaged slab. However, compaction by vibrator is needed in most concrete (slump: 8 to 21 cm), and therefore concrete placement under a damaged slab that needs repair is impossible realistically. A repair method using high-performance concrete without vibration or compaction was developed. The method was applied to the repair of concrete slabs that had been damaged by heat. It was confirmed that this repair method was easier and that repair was possible while the slab was in use. In addition, the costs and construction period were reduced greatly by this repair method.