International Concrete Abstracts Portal

Cement-based composites are used in the construction of a wide range of structures. During their service life, many of these structures are exposed to various types of aggression; their durability is generally controlled by their diffusivity and permeability of the cement-based composite. Since the assessment of these two properties by laboratory or in situ tests is often difficult and generally time-consuming, a great deal of effort has been made towards developing microstructure-based models to predict them. A critical review of the most recent developments in this field is presented in this paper. The report begins with a survey of the various mathematical concepts developed to characterize the structure of porous media. Empirical and physical models are reviewed in separate sections. Special emphasis is placed on recent innovations in the field of numerical and digital image analysis based modeling. Each model is evaluated on the basis of its ability to predict the mass transport properties of a wide range of cement-based composites and its potential application to the study of other micro- and macro-structural properties.

Presents basic information on a newly developed electrically conductive concrete. The concrete differs from previous inventions in that both high conductivity and mechanical strength are simultaneously achieved. The electrical and mechanical properties of the conductive concrete developed at Institute for Research in Construction, National Research Council of Canada are given. The material has superior electrical conductivity values and excellent mechanical strength. Experimental results of a laboratory-scale study on the application of conductive concrete to deicing and/or snow melting are presented in this paper. The results indicate that heat can be uniformly produced by the conductive concrete heating element when the element is activated by an external electric power supply. The new method is effective for deicing purposes. Power output of the conductive concrete heating element is stable over a wide range of temperature. The minimum heater power output required for deicing at various air temperatures was determined. This value is linearly dependent on the air temperature, ranging from 150 to 855 W/m 2 as air temperature varies from -5 C to -30 C.

The durability of reinforced concrete structures can be improved by resorting to methods which insure a better resistance of concrete to various aggressive environments. Some commonly used methods include subjecting concrete to a better curing practice, the use of modified concretes, and the application of surface treatments on concrete surfaces. In addition to these, efforts have been made in the recent past to develop new techniques by which the water- cement ratio in the near surface region can be lowered and a dense matrix achieved. One way of achieving this is to use a controlled permeability formwork system (CPF), in which the surplus mixing water and entrapped air are removed from the fresh concrete via a fiber liner. This produces a surface layer of concrete with a very low permeability which is likely to be highly resistant to various forms of environmental attack. Relatively little information is available at present on the efficiency of CPF in improving the protection of the concrete against various mechanisms of deterioration and on how it compares with other techniques, such as the application of better curing practices. Therefore, an experimental investigation was carried out with three water-cement ratios, five different curing regimes (air curing, wet hessian curing, and the use of three different curing compounds), and the application of a CPF liner system. Measurements of gas permeability, sorptivity, chloride diffusivity, surface tensile strength, freezing and thawing resistance, and carbonation resistance have indicated that the use of CPF can enhance the durability of concrete and that the extent of this improvement is significantly more than that obtained for the various curing regimes. This paper details the experimental program and presents results which are used to evaluate critically the use of CPF for normal concrete.

In recent years, highly flowable concrete which can be placed without any consolidation has been widely studied. A basic study on this type of concrete incorporating limestone powder and a method for reducing shrinkage properties of the concrete are presented in this paper. In the mixture proportioning for the concrete, a high-range water-reducing admixture is used to increase the flowability of concrete. A small amount of viscosity-increasing agent is also added to minimize the segregation. Limestone powder, which is a low reactivity material, is used to reduce the heat of cement hydration and shrinkage of concrete. Although drying shrinkage of the highly flowable concrete incorporating limestone powder was smaller than that of ordinary concrete or other highly flowable concretes, shrinkage of the concrete needs to be further reduced so that it will be a crack-free concrete. To accomplish this, a method for reducing drying shrinkage of concrete by applying a shrinkage-reducing agent and an expansive additive was tested and good results obtained.

Presents the results of an ongoing experimental research program on creep and shrinkage behavior of high-strength concrete loaded at an early age (16 hours) and a normal age (28 days). The experiments were carried out on high-strength concrete with three types of aggregates (crushed gravel, granite, and limestone). The concretes were dried and loaded at ages of 16 hours and 28 days after casting. Loading levels with stress/strength ratios ranging from 0.15 to 0.70 were adopted in the experiments. The creep deformations were measured for a duration ranging from 90 to 210 days. The experimental results are compared in this paper with the predictions of CEB-FIP Model Code 1990, the modified MC90 model, and the model proposed by ACI Committee 209. The aging effect (in particular, at early ages) is emphasized and the influences of various factors on the aging effect are discussed.