International Concrete Abstracts Portal

This paper describes an investigation that was carried out to determine the within-test variability of various nondestructive test methods and their correlation with the corresponding compressive strength. The nondestructive test methods that were studied were the pin penetration, the ultrasonic pulse velocity, and the nuclear density gage. The tests were performed on solid concrete blocks, cylinders, and reinforced concrete slabs at different ages. The within-test variability of the direct readings of the ultrasonic pulse velocity was small compared to the pin penetration and nuclear density gage. However, when the readings were translated into strength, the within-test variabilities of the standard compression test were about the same as the other methods. The relationship between strength and the pin penetration and pulse velocity readings were good, while the relationship between the density of fresh and hardened concrete and the nuclear gage readings were satisfactory.

In repairing cracked concrete structures, excellent workability and repairing effects have been obtained by using epoxy resins that are liquid organic polymer materials. However, in most situations, the repairing effects are not long-term and when these organic materials are used and exposed outside for a few years, they will deteriorate. Therefore, a material for crack injection, utilizing inorganic material such as ultra-fine slag cement, has been developed recently, and has shown good performance in terms of workability and durability. In this study, plain concrete beams and reinforced concrete beams repaired by using epoxy resin or ultra-fine slag cement are tested. The results show that the repairing effect deteriorates because of bleeding or drying shrinkage when inorganic material is used with a great deal of water. However, satisfactory properties are obtained using superplasticizer added to the inorganic repair material.

In the present study, materials for repairing cracks that occur in concrete structures are discussed. Up to now, organic materials have been used mainly to repair cracks in concrete, but these have problems of heat resistance and durability, etc. Therefore, to overcome these problems, blast furnace cement, which is an inorganic material from the same group as the constituent element of concrete, is chosen and used as an impregnating material by pulverizing it to an ultra-fine powder. The specific gravity of this material is 2.96, the average particle diameter is 2.9 micrometers, and the specific area equals 9500 cmý/g, which is almost three times that of normal cement. It is found that the viscosity of the blast furnace cement paste with a water-cement ratio equal to 70 percent is less than 35 MPa/s, which is almost the same viscosity as epoxy resin. Consequently, the paste of ultra-fine powder will be able to penetrate cracks like epoxy resin and is expected to be a suitable repair material.

The results obtained from tests conducted on high-strength concrete columns are reported. The experimental program comprised ten axially loaded short columns and six slender columns subjected to axial compression and bending moment. The main test parameters in the case of short columns are the area and the type of lateral reinforcement, whereas the eccentricity of the axial thrust is the major variable in the slender column tests. The paper also includes methods of calculating the strengths of reinforced concrete columns made of high-strength concrete. The calculated strengths agree closely with the test values.

During a comprehensive research program on in situ determination of concrete strength, the influence of core dimensions on mean compressive strength results and their dispersion was studied. Tests were conducted on 14, 11, 9, and 5 cm diameter cores, usually with a length/diameter ratio l/d = 1; cores with l/d less than 1 were also tested. Cores were drilled from 20 cm cast concrete cubes, stored in a moist room. When using 5 cm diameter cores, the curing conditions of the cubes were varied. The 28-day strength results showed that dispersion increases with increases in concrete strength, maximum aggregate size, and reduction of core diameter. A good correlation was found between the mean compressive strengths of cubes and cores, both for the results of cores with l/d = 1 and l/d = 0.6. The mean compressive strengths of cores were the same as those for cubes and were not influenced by the core diameter. Thus, the use of small diameter cores may be possible. In this case, however, the concrete from the surface must be removed when preparing the cores, if the concrete was not moist-cured.