International Concrete Abstracts Portal

This study investigated the accelerated test results of pozzolanic cement concretes. The accelerated test method used is the ASTM C 684-74 Boiling Water Method. The Portland-Pozzolan cement used contains 30% natural pozzolan. The laboratory work involved the production of concrete mixes with cement contents of 300 to 450 kg/m3, water-cement ratios varying between 0.3 and 0.63. Maximum agregate size was chosen to vary between 15 and 40 mm and both gravel and crushed aggregates were used. 28 day compressive strengths ranged from 14.2 to 35.5 MPa. From each concrete mix five 150x300 mm cylinder specimens were cast. Accelerated cure was applied to two specimens cast in covered molds. The other three specimens were cured under standard conditions for 28 days. The production, curing, capping and testing of specimens were in accordance with the relevant standards. Analysis of the results reveal a power type relationship between the accelerated and the standard strength results and the coefficient of correlation was found as 0.967. The linear relationship also gives a high coefficient of correlation (0.963). Using the linear relationship 28-day strengths can be estimated with 2.4 MPa accuracy within 90% confidence limits. The relationship is compared to and found to be in agreement with those proposed by other investigators. However accelerated strengths given by the pozzolanic cement seems to be a little higher than the others.

This paper reports the results of an investigation to develop heat-curing cycles for portland cement/fly ash concrete for use in the precast industry. The fly ash was incorporated into concrete not as a replacement for cement but as a partial replacement for fine aggregate. The variables considered were temperature of curing, preset time and duration of heating. The results of the investigation indicate that portland cement concrete (W/C = 0.40) in which 18% of the fine aggregate has been replaced by fly ash can be heat-cured to accelerate strength development at ages ranging from 12 to 24 h, and compressive strengths of the order of 30 to 45 MPa can be achieved at these ages. Two of the most promising heat-curing cycles for the materials under investigation consist of a 12-h cycle at 90°C with a preset time of 2 h or a 24-h cycle at 55° to 70°C with a preset time of 4 h. However, it is emphasized that each precast concrete producer will have to perform investigations to optimize the heat-curing cycle that best suits his materials and production needs.

As a pozzolan, fly ash has been used to replace some of the portland cement in concrete. Because of its many advan-tages, the use of fly ash in concrete will undoubtedly increase. Although many studies have been conducted on the physical properties of concrete containing fly ash, very little research has been reported on the fatigue properties of fly ash concrete. Since most concrete pavement design procedures currently in use are based on the consideration of concrete fatigue, the importance of fatigue properties of concrete pavement containing fly ash is evident. The purpose of this study was to evaluate the fatigue properties of concretes containing fly ash and compare these properties to those of concrete without fly ash. More than 350 concrete specimens at four levels of cement replacements (0, 25, 50 and 75%) and two types of fly ash (high-calcium and low-calcium) were cured 28 days and subjected to compressive fatigue loading in which the stress varied from essentially zero to a predetermined maximum stress as percentages of the compressive strength. The fatigue strength of concrete containing fly ash was found to vary with both type of fly ash and cement replacement ratio. Concretes with equivalent or higher compressive and fatigue strengths could be obtained with cement replacement of 25% by weight of low-calcium fly ash or 50% by weight of high-calcium fly ash.

The use of autoclaving processes in producing cement-based products is well established. The main reason for using autoclaving processes is often to increase production rate and/or to decrease sensitivity towards variations in humidity, such as reducing moisture movements. The latter effect is mainly believed to be caused by an improved crystallinity of the finished material. The use of fly ash as the siliceous component in the base mixture is also well known. However, the crystallinity is ignored by the use of a noncrystalline base material, such as fly ash. From productions of autoclaved cement-based products utilizing silica sand it is known, that additions of small amounts of gypsum to the base mixture may improve strength and reduce moisture move-ments. This paper deals with the production of autoclaved materials utilizing fly ash and gypsum or its derivatives. Materials produced are characterized according to their density and strength-characteristics as well as to their crystallinity. The use of gypsum or its derivatives may cause significant improvements in strength as well as in crystallinity, but the op-timum design is closely related to the actual production process as well as to the chemical properties of the base materials and to the physical properties of the fly ash.

Concrete subjected to elevated temperatures may suffer considerable loss in strength due to the development of microcracks or phase transformations in the matrix. The prevailing mechanism depends on the type of aggregate as well as on the moisture content of concrete. Experiments on different hydrated cement systems showed that under hydrothermal conditions phase transformations in neat cement paste lead to an increase in porosity and reduction in strength. In cement pastes containing fly ash or ground quartz in sufficient amounts, gel-like compounds are formed in pozzolanic reactions during hydrothermal exposure. An increased specific surface area as well as an increase in strength is observed. Concrete exposed to hydrothermal conditions is affected by these phase transformations of the matrix; a loss in strength can be prevented by addition of fly ash or ground quartz. Due to a higher shrinkage of the modified matrix which causes increased microcracking these concretes, however, show a loss in strength when drying during temperature exposure.