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.

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.

Specific problems associated with concrete and concreting in hot, particularly hot-dry, weather, have been recognized in Israel since the late 1950s. The effects of hot environments on properties and performance of concrete have been studied for 35 years at Israel's National Building Research Institute. This research has included laboratory tests as well as site experiments conducted in hot-dry and hot-wet climate regions, some of which were in marine environments. The effect of environmental factors on concrete properties has been studied for both fresh and hardened concrete. Studies related to fresh concrete dealt with climatic effects on water demand and slump loss in concretes with and without admixtures, and in concretes incorporating fly ash. Special attention has been given to workability and slump loss of concrete subjected to long hauling and mixing periods. The mechanism of plastic shrinkage and the factors affecting possible plastic shrinkage cracking have been extensively investigated. Studies on properties of hardened concrete include early and later age strength, drying shrinkage, and creep, and how these factors relate to the very early exposure of fresh concrete to hot environments has been investigated. The effect of hot environments on reinforcement corrosion has been studied both in the laboratory and in exposure sites.

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.