International Concrete Abstracts Portal

Focuses on the damaging implications of the daily temperature fluctuations in the aggressive climatic conditions of hot-arid regions due to strain incompatibility resulting from widely differing coefficients of thermal expansion of the local crushed limestone aggregate and the hardened cement paste. The data strongly indicate that temperature fluctuations cause microcracking in concrete, which increase its permeability and lower its tensile strength and cracking time. In this investigation, concrete specimens with water-cement ratios of 0.40, 0.50, and 0.65, with cement content of 550 lb/yd 3 were subjected to cyclic heating in programmed ovens which carried out 120 temperature fluctuations, each simulating the temperature regime of a typical summer day in eastern Saudi Arabia. The thermal regime was characterized by a temperature swing from 27 to 60 C within a 24 hr period. This included the effect of concrete surface heating by direct solar radiation. Pulse velocity, permeability, and time-to-cracking data were developed in reference to cyclic heat-treated specimens at 20, 40, 60, 80, and 120 heating cycles. The cyclic heat-treated specimens had a significantly reduced pulse velocity, a noticeably increased permeability, and, depending on water-cement ratio, a 55 to 70 percent reduction in cracking time due to reinforcing bar corrosion. This implies that a significant degree of microcracking is induced in concrete due to the thermal incompatibility of concrete components.

Presents the results of the effect of temperature on cathodic protection level needed for effective control of chloride corrosion of reinforcing steel in concrete structures. The chloride levels in the concrete were 8 and 32 lb/yd 3, and chloride gradients were 1.5 and 2.0. Chloride gradient was created by embedding in the concrete specimen a relatively higher chloride-bearing macrocell and thereafter connecting the macrocell steel and the main steel through an external resistor. Current reversal technique was used to establish the protection level needed for effective control of reinforcing steel corrosion. Two sets of specimens were used: the first set of reference specimens were kept at the controlled room temperature of 25 C, and the second set of temperature-treated specimens were kept in a temperature chamber with a peak value of 60 C. The corrosion activity of the reinforcing steel increased with an increase in the temperature to which concrete is exposed. Increased corrosion activity at a higher temperature exposure of 60 Required an increased level of cathodic protection as indicated by the higher protection current density, higher instant-off protection potential, and marginally higher decay potential at the beginning of the polarization period. The 60 C temperature effect requires about 20 percent higher level of protection in terms of current density and about 20 to 30 mV higher instant-off potential/delay potential for an initial polarization period of two months. Thereafter, no additional protection is required against the temperature effect. The subsequent reduction in the level of cathodic protection required at higher temperature is indicative of a dominant influence of the electromigration factor in the interactive relationship between corrosion activity and the beneficial effect of electromigration of ions caused by higher temperature.

Laboratory concrete made under different curing conditions was evaluated using electron-optical techniques. Differences in microstructure and strength were observed in relationship to water-cement ratio, wet/dry curing, temperature of curing, presence of supplementary materials, and mode of preheating. This presentation highlights the partial results of the microstructural evaluation.

The best time to place quality concrete is during cold weather, as long as the concrete is prevented from freezing. Why is it so hard to place quality concrete during hot climate conditions? The culprits are concrete temperature, air temperature, humidity, and wind velocity. There are secrets that can drastically improve the present quality of concrete placed in hot climates. This paper discusses how to cope with hot weather conditions and still produce high-quality concrete. Concrete in hot climates is affected by water demand, rapid setting times, and the resulting ultimate strength reduction. Understanding these detrimental effects and how to overcome them can result in high-quality, durable concrete. Many of these effects can be overcome by using the proper chemical or mineral admixture, but using techniques slightly different than the usual. There may be times when an accelerating admixture, or insulation, may be used effectively even in hot climates. Relatively high concrete temperatures may be appropriate to obtaining durable concretes.

Concreting of thick sections involves large volume placements for cases such as foundation rafts and beams of exceptional dimensions, which frequently occur in highrise construction in Singapore. The heat of hydration generated in using concrete of structural grade is much higher than that associated with mass concreting of dams. For any thick section, The temperature differential between the warmer interior and the cooler surfaces gives rise to different thermal strains which may be sufficiently high to result in cracking. In hot climates, it also leads to very high peak temperatures (often above 70 C). Typical case histories of such placements in tropical climates are presented, including the mixtures used and the overall dimensions of the members. For two of the cases, the measured temperature histories are compared with those from numerical simulation using finite elements. The requirements of concrete mix design and precautions to be considered in relation to the planning and execution of large placements as well as the use of insulation to control temperature differentials are discussed.