International Concrete Abstracts Portal

This paper deals with CANMET investigations on the performance of concrete, with and without supplementary cementing materials, in a marine environment. A series of more than 250 concrete prisms, 305 x 305 x 915 mm in size, were cast over a period of 16 years and installed at Treat Island, Maine, an outdoor exposure facility operated by the U. S. Army Corps of Engineers, U. S. A. The prisms of the first phase of the investigation were installed at the site in 1978, with the remaining specimens being installed at almost yearly intervals. The specimens of the latest phase were installed in 1994. The test prisms are installed at mid-tide level on a rack and are exposed to repeated cycles of wetting and drying and to about 100 cycles of freezing and thawing per year. The test specimens will be kept at the exposure site until at least the year 2005. The test prisms are evaluated annually. The evaluation includes visual examination and rating and ultrasonic pulse velocity testing. Also, a complete photographic record is kept. Some of the principal conclusions based upon up to 17 years of exposure of some of the test prisms are as follows. The use of non air-entrained concrete is not recommended for the exposure conditions experienced at Treat Island. For the exposure conditions experienced at Treat Island, the percentage of silica fume in concrete should be limited to 10 percent. Both the normal weight and semi-lightweight concretes incorporating fly ash or slag or silica fume or a combination of these materials are in good to excellent condition, provided water-to- cementitious materials ratio is kept below 0.50 and portland cement is kept at a certain minimum level. There is no significant difference in the performance of concrete made with ASTM Types I, II, and V cements.

The fire resistance of high-strength (greater than 60 MPa) concretes has been reported to be reduced when compared to normal strength concrete. This behavior has been attributed to the very dense concrete matrix usually associated with high-strength concrete. This dense matrix does not allow rapid transmission of water vapor within the concrete exposed to high temperatures, thus leading to disruptive vapor pressures. This problem is aggravated further when the fire is a hydrocarbon fire which reaches 880 C within three minutes. Offshore concrete platforms, which are typically built with high-strength concrete, are therefore at risk from hydrocarbon fires. This paper presents the results of two test programs involving hydrocarbon fires and high-strength concretes. Both lightweight aggregate concrete and modified normal density concrete (blends of normal density and lightweight aggregate coarse aggregates) were evaluated. Small beams of lightweight concrete, using different lightweight aggregate types, were evaluated for spalling resistance. Polypropylene fibers, used in some beams, were successful in greatly reducing spalling. Large wall sections of modified normal density concrete experienced significant spalling, but retained adequate concrete strength behind the reinforcing bars because of the low rates of heat transfer within the large sections.

Presents some results from a major research project carried out on corrosion of steel reinforcement in concrete. The performance of a range of portland and blended cement concretes containing fly ash and blast furnace slag exposed to simulated marine conditions was evaluated over a period of six years. A large amount of data relating to corrosion of embedded steel in concrete was obtained in this project. Long-term half-cell potential data on reinforcement within concrete slabs and resistivity data on the same concrete specimens are considered in detail in this paper. Rates of corrosion of steel in concrete were also measured using potentiodynamic anodic polarization procedures. Trends observed in the data were different for the portland and blended cement concretes investigated. It was found that concrete resistivity may influence the measured half-cell potential of steel in concrete. This factor needs to be taken into account for half-cell potential data interpretations.

Admixtures for concrete to be placed underwater are now widely used. The use of such admixtures allows concrete to be placed underwater with much less risk than has been previously possible. However, such concrete has to satisfy several requirements when fresh (workability, washout resistance), as well as when hardened (strength, durability). Workability and washout resistance are particularly important for underwater placing. Several test methods and related equipment have been designed to evaluate the required properties of the fresh underwater mixtures, including devices to evaluate their washout resistance. One of the problems facing both producers and users of underwater (UW) concrete admixtures up to now has been an absence of a standardized and realistic test method for the assessment of performance of such products in practical concrete construction. This research project aims to develop efficient new tests for the practical evaluation of the washout resistance and nondispersability of fresh concrete mixtures placed underwater. The paper briefly reviews existing washout tests, assesses the potential of alternative tests, and introduces the new tests designed by Ceza. The principle of the new test, the apparatus required, and the test procedure are described. Results of tests on practical underwater mixtures are shown and effects of UW admixtures discussed. The apparatus simulates well the actual washout condition; its performance and design satisfy requirements of a viable test suited for standardization.

Electrical potential technique was used as an accelerated testing method to determine the chloride diffusion coefficient of high-performance concrete. An electrical potential difference of 15 volts was applied to a disc-shaped concrete specimen of 50 mm in thickness. This direct current voltage increased the flux of chloride ions migrating through the concrete and permitted the determination of the chloride diffusion coefficient (D CL) within three to four weeks of the testing period. The water-to-cementitious materials ratio (W/C) had a significant effect on D CL, where the relationship between W/C and D CL was expressed as D CL={0.32EXP[5.47(W/C)]} x 10 -8 cm 2/s for normal concrete without any mineral additives. Concretes with five and 10 percent silica fume replacements for ordinary portland cement were also tested and had D CLs equal to 3.14 and 2.99 x 10 -8 cm 2/s, respectively, after a 28- day moist curing period. These D CL values were lower than that of concrete containing only portland cement, being 4.3 x 10 -8 cm 2/s for concrete with the same W/C of 0.45 and a moist curing time of 28 days. Furthermore, super- workable concrete which had a W/C of 0.3 and was moist cured for 28 days had a D CL of 1.97 x 10 -8 cm 2/s.