International Concrete Abstracts Portal

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.

The resistance of rice hull ash (RHA) concrete to freezing and thawing in saline environment was studied in the laboratory, for non-air- entrained high performance and normal concrete. The Swedish standard test for concrete resistance to freezing and thawing in saline environment was used. Although the number of tests was limited, the results were very promising for the use of RHA in non-air-entrained normal or high performance concrete. The laboratory salt scaling for concrete with 15 to 30 percent replacement of portland cement with RHA indicated that RHA concrete without air entrainment would be fairly resistant to freezing and thawing in most applications except for in very severe climates. No indications on an accelerated scaling rate over time was observed for RHA concrete, as opposed to the accelerated scaling rate found for a non-air-entrained high performance silica fume concrete tested.

Electrochemical chloride removal was applied to a concrete test area of about 36 m 2 in a reinforced concrete hall which had been used for more than 10 years as a depot for deicing salt, in an attempt to extract the chloride that had penetrated into it. Since the salt had been stored loosely and the interior of the hall was frequently exposed to outside air, the concrete was heavily contaminated by chloride (up to about 15 percent Cl - in cement). Chloride removal was performed with an average current density of 1 A/m 2 for a period of 132 days. The studies were aimed at determining the changes in total chloride content and the Cl - and OH - concentrations of the pore solution at varying concrete depths. It was shown that the efficiency of chloride removal decreased in the concrete cover with increasing depth and that it was least efficient near the reinforcement. The factor that was identified as being responsible for this was the change in OH - concentration of the pore solution that had been caused by reactions at the electrodes. The OH - concentration of the pore solution decreased in the area close to the surface during treatment, while it rose dramatically around the reinforcement (up to approximately 2.5 mol OH -/L). This resulted in an increase of the Chloride Transference Number and, thus, the efficiency of chloride removal close to the concrete surface, as well as a drastic decrease close to the reinforcement. Hence, a reduction of the Cl - to "harmless" levels was not possible in this particular case. However, practice has shown that in many cases such a reduction can be achieved as chloride contamination is normally much less severe; thus, most of the chloride can be extracted from the reinforcement area before the rising Cl -concentration of the pore solution has diminished the efficiency of chloride removal. If, however, chloride has penetrated beyond the reinforcement, it can be removed to a limited extent only.