International Concrete Abstracts Portal

SP-154 In 1995, The Canadian Centre for Mineral and Energy Technology (CANMET), in association with the American Concrete Institute and other organizations sponsored a second conference on Advances in Concrete Technology. The objectives of this conference was to bring together representatives from industry, universities, and government agencies to present the latest information and explore new areas of needed research and development. Thirty two papers from 20 countries were reviewed and accepted for inclusion in this new publication based on the symposium subject, advances in concrete technology. The range of subjects is varied due to the wide range of experts involved in this project.

Over the last 50 years, many researchers have investigated the subject of sulfate attack of concrete and special cements have been formulated to combat the problem. However, recent site investigative work and laboratory- based studies carried out by the Building Research Establishment have shown that a particular form of sulfate attack can proceed even in some concretes which were specifically designed to provide good sulfate resistance. In this type of attack, the main mechanism of deterioration is the breakdown of the calcium silicate hydrate phases in the hardened cement paste in the presence of an available supply of sulfate and carbonate ions to produce the mineral thaumasite (CaSiO3CaSO4CaCO315H2O). A microstructural overview incorporating three examples of the thaumasite form of sulfate attack is presented in this paper. The combination of optical and electron microscopy has proved to be a very powerful technique for examining the processes of deterioration in carefully selected site samples

Mechanisms of stratlingite (C 2ASH 8) formation in high-alumina cement (HAC)-siliceous material systems were investigated. Different siliceous materials (silica fume, fly ash, ground granulated blast furnace slag) and chemical admixtures (sodium silicate, sodium sulfate) were employed. Reactions between CAH 10 or C 2AH 8 and dissolved silica occur. Acceleration of silica dissolution by addition of chemical admixtures promotes the formation of stratlingite. The pH value of the HAC-siliceous materials system was also studied. The intrinsic relationship between the pH value and stratlingite formation is discussed in this paper. Mechanisms of stratlingite formation in preference to hydrogarnet (C 3AH 6) in HAC products are postulated. A method for prevention of strength reduction of HAC products due to the conversion of thermodynamically unstable hexagonal calcium aluminates to cubic hydrogarnet is described.

As a result of the very low water-cement ratio in a high-performance concrete, the rate of cement hydration at early ages is significantly different from that in a normal strength concrete. The ultimate degree of cement hydration is lower in a high-performance concrete; the hydration process will terminate earlier because of the rapidly diminishing water supply. Another characteristic of high-performance concrete is caused by the relatively high dosage of superplasticizer which delays the onset of the cement hydration. This paper presents the extension of the research on temperature and strength development in hardening concrete from normal strength concrete to high- performance concrete. It models the development of heat of hydration in high-performance concrete, taking into account the effects of water-cement ratio, superplasticizers, and temperature changes. General formulations of the rate of heat of hydration as functions of concrete maturity (hydration stage) and current temperature are provided. Comparison with some test results verifies the theoretical model.

Presents results of expansion tests carried out on concretes immersed in 1/10-M and 1-M sodium chloride solutions. The concretes were prepared using two reactive aggregates, cristobalite and a natural aggregate from the southwest of the U. K. Tests were carried out both at alkali levels which were known to induce expansion due to alkali-silica reaction (ASR) and alkali levels which would not normally induce expansion due to ASR. The concretes were, at the ages of one, three, and six months, immersed in a sodium chloride solution. The concretes were stored at 38 C, 20 C, and externally. For the concretes containing the natural aggregate, it was shown that immersion in a 1-M salt solution had no major adverse effects upon long-term expansion. This is attributed to the low available reactive silica content within the concretes. In the case of concretes containing cristobalite, it was shown that the immersion in 1-M salt solution had an adverse effect upon long-term expansion. This is attributed to the high available reactive silica content of the concrete.