International Concrete Abstracts Portal

This paper reports a study on the effect of a high-reactivity meakaolin (HRM) on the sulphate resistance of mortars. Mortar bars containing 0-25% HRM were prepared in accordance with ASTM C 1012-89 with three cements of varying C3A content. After 6 months of exposure to a 5% Na2SO4 Solution, mortar bars incorporating any level of HRM (5-25%) as a partial replacement for a high-C3A cement (~10%C3A) may be considered "moderate sulphate resistant"; mortar bars made with HRM and a moderate or low-C3A cement (<6% C3A) may be considered "high-sulphate resistant". Beyond 6 months, however, it was found that insufficient levels of HRM may, in fact, reduce the sulphate resistance of some cements, and that 15% HRM, or more, may be required for long term sulphate resistance, depending on the C3A content of the cement.

A modified pull-off method, which was developed to assess the tensile strength of surface layers of concrete is described. The modified pull-off method enabled the measurement of tensile strength at an arbitrary depth form concrete surface using a circular steel probe with a hollow cylinder. In this paper, the basic characteristics of the modified pull-off method were investigated. As for the influencing factors to exert on the test values, the effects of depth of core slit, core diameter, maximum size of coarse aggregate, and the moisture content were examined. In addition, the relationships of the pull-off tensile strength with the splitting-tensile strength and the compressive strength which was also estimated from the rebound number, the pulse velocity and the pin-penetration depth, were also examined. The applicability of the test was also examined by applying it to the surface layer of concrete cast using mold lined with permeable sheets. When applied to the damage concrete due to freezing-thawing cycling in the laboratory, it was observed that the relationship between the depth of damaged concrete estimated form the pull-off tensile strength and freezing-thawing cycles could be expressed by a power expression. This suggests the possibility that the thickness of the damaged section can be predicted at the required freezing-thawing cycles.

Changes in the quality of concrete due to ambient conditions start from its surface immediately after the demolding, whereby the quality of cover concrete, particularly strength and durability, is likely to be lower than that of the interior parts. This study deals with effects of drying on the compressive strength of cement paste and concrete. Cement paste specimens with a very small cross section were prepared using normal portland cement and the relationship between the pore structure and the compressive strength was investigated. Next, the inhomogeneity of porosity of concrete was measured. Finally, the distribution of compressive strength from the surface layer to interior of concrete was estimated, using two equations relating pore volume to compressive strength. The results can be summarized as follows. (1) The relation between the total pore volume and the natural logarithm of compressive strength obtained from the hardened comment paste a liner compressive strength obtained from the hardened cement paste a liner correlation for each water-cement ratio. (2) The smaller was the distance from concrete surface, the larger was the total pore volume. This tendency was greater for the concrete of larger water-cement ratio and for the concrete of earlier drying initiation. (3) Compressive strength of concrete near the surface was lower in accordance with the increase of total pore volume. The difference of the strength between the surface layer and interior of concrete was approximately 20 MPa. (4) It was at most to the depth of 5cm from concrete surface that the compressive strength decreased due to drying in the case of concrete with water-cement ratios of .4 and .6, and it was at most to the depth of 10 cm in the case of concrete with a water-cement ratio of .8.

The work presented details a testing methodology whereby a range of properties of cover-zone concrete can be determined, these properties being ultimately linked to the long-term performance of concrete. Emphasis is placed on the application of a surface-applied, water absorption method in conjunction on the application of a surface-applied, water absorption method in conjunction with electrical measurements taken within the cover zone. In the current work, with electrical measurements taken within the cover zone. In the current work, the influence of cyclic wetting and drying of normal portland cement concretes, with and without blast-furnace slag replacement, were studied. Samples were subjected to both water and chloride solution. The study shows that the depth and rate o water penetration during absorption can be evaluated; when these data are combined with volumetric gain, the effective porosity of concrete in the cover region can be estimated. Conductivity profiles taken over cycles of wetting and drying can be used to provide information on the one of influence of drying action (convective zone), on-going hydration effects, and ionic ingress.

The importance of measuring, rapidly and reliably, the quality of the concrete cover as a mean to ensure a proper durability performance cannot be overemphasized. The "Torrent Permeability Tester" (TPT) is a method devised to that end, i.e. to measure the permeability to air of the covercrete, both in the laboratory and on-site, in less than 15 minutes. The paper presents test results obtained in two laboratories, Instituto Torroja in Spain and "Holdrbank" Management and Consulting in Switzerland, within the activities of RILEM's TC 116-PCD (Permeability of Concrete as a Criterion of its Durability). The tests included the determination of air-permeability (TPT Method), O2-permeability (Cembureau Method), capillary suction, "rapid chloride permeability test" (ASTM C 1202) and 1-year carbonation depth. The tests were performed on concretes of different compositions and curing regimes. The gas-permeability tests were repeated on the same specimens at the ages of 28d and 6 and 12 months. The results obtained indicate that the TPT is: Sensitive to changes in the quality of the concretes. Repeatable, for a sensitive property such as gas-permeability. Correlated to other relevant durability-related properties such as :O2- permeability, rate of capillary suction, chloride permeability and carbonation rate and , hence, suitable to characterize the quality of the concrete cover.