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.

Performance properties of mortars made with ordinary portland cement (OPC) and pozzolanic cements containing either fly ash (PFA) or granulated blast furnace slag (GBS) have been measured after exposing the mixes to laboratory-simulated hot dry environments. The simulated environments were: 20 C at 70 percent relative humidity; 35 C at 70 percent relative humidity; and 45 C at 30 percent relative humidity. Specimens were cured for different lengths of time before testing. The tests carried out to assess the performance properties and thus the durability of the mortars were: total porosity, pore size distribution, and gas permeability using oxygen. The tests showed that performance of the mortar mixes is enhanced by increased curing time. Uncured specimens subjected to hot dry environments (45 C at 30 percent relative humidity) were strongly affected by their durability characteristics as shown by the deterioration of the performance indicators. OPC mortars were severely affected by the hot dry environments independent of the length of curing. Pozzolanic mortars subjected to curing periods of 1 day or more in hot dry environments exhibited better properties than equivalent mortars cured at normal temperature.

Numerous methods are available to improve the surface durability of concrete. The most commonly used techniques are improved curing practices and the application of surface treatments. A new technique that employs a controlled permeability formwork liner (CPF) has been introduced in the U.K. This paper describes the results of an investigation to compare the effect of the controlled permeability formwork liner with that of various curing techniques and the absorption of silane in relation to the air permeability, sorptivity, water permeability, and strength of the cover concrete. Also, the resistance to carbonation has been studied. Results indicate that, in general, the use of CPF improves the surface properties compared with conventional steel formwork. The effect of variation of curing methods was marginal for concrete with CPF.