International Concrete Abstracts Portal

Concrete quality and strength are strongly affected by the curing procedures used in the initial days after concrete is cast. The relative advantage of any particular method and the strength-gain relations must be understood to assess concrete strength and quality adequately. In the present study, the strength gain with age of concrete up to 1 year with different compressive strengths and under different initial and subsequent curing conditions in warm and high-humidity climates was determined. The initial curing techniques evaluated were those most widely used in practice and were intended to represent actual, imperfect construction practice. The subsequent curing conditions were artificially modeled to simulate dry and rainy climates. Of the curing methods evaluated, ponding was found to be the most effective, followed by intermittent sprinkling, unsealed plastic covers, and curing compound. An initial period of three days can develop adequate concrete strength in Puerto Rico's climate; this has a good influence on concrete strength gain with age. Slabs that were kept indoors, and had no contact with water, showed in all cases a decrease in strength under saturated conditions at an age of 1 year, while those maintained outdoors with a rainy climate showed a continuous gain in strength with increasing slope at all times and developing strengths higher than the 28-day strength.

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.

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.

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.

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.