International Concrete Abstracts Portal

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.

Pier B in the port of Halifax, NS, Canada, was built in 1930-32 using 18,000 tons of calcium aluminate cement (CAC, Ciment Fondu), sea-dredged sand and aggregates and mixing water pumped from an inland freshwater lake. The climatic conditions at Halifax are extremely severe; it is estimated that exposed concrete is subjected to about 100 freezing and thawing cycles per year. Pier B is over sixty years old and in regular service as a container terminal for ocean-going ships. The main structure is permanently submerged in sea water. A protective layer of facing concrete made of both CAC and normal portland cement, cast over the outside faces in the tidal zone, has needed periodic repair. The CAC concrete displays excellent durability with cylinder compressive strengths of 29 to 49 MPa, modulus of elasticity of about 30 GPa, and a Poisson's ratio typical of normal weight concrete. Records indicate that the CAC concrete was cast with a water-cement ratio of 0.5 to 0.6 and a cement content of about 330 kg/m 3. Volume porosity is of the order of 10 percent. The investigation reported here of cores taken from Pier B in 1993 provides a broad characterization and guide for more detailed examinations in a collaborative program, the results of which will be reported as they become available. A particular focus of interest will be the speculative existence of zones of enhanced impermeability on the exposed faces of the concrete, an effect which has been observed in other old CAC structures.