International Concrete Abstracts Portal

Assessment of durability of concrete motorway bridges in Northern Ireland is part of a program of collaborative research of the Civil Engineering Department with the Roads Services of the Department of Environment (Northern Ireland). This involves testing bridges on site to develop methods that can be used to investigate the condition of approximately 1300 reinforced concrete bridges in the province so that a planned maintenance scheme can be proposed. Describes results of comprehensive tests that were carried out on a severely delaminated motorway bridge both prior to and after repair. Five other motorway bridges that represent the general population in terms of age, construction form, and past maintenance were also tested and the results compared. It has been observed that concrete strength is not a reliable index of proneness to deterioration, but permeability results allied to the knowledge of exposure to salt are more likely to provide a useful guide to durability. The in situ tests for sorptivity indicated that, with the two silane-treated bridges, one keeps the moisture out, whereas the other fails to do so. High-permeability concrete combined with leaking expansion joints have been found to be the main cause of absorption of chloride and the corrosion of steel.

Reflects on the progress that has been made in the past 20 years regarding knowledge of steel corrosion in concrete and the disparity between the current level of understanding and practice. Two examples are taken from common practice: the measurement of the chloride ion content of concrete and the measurement of half-cell potentials, to illustrate the implications of a lack of understanding of basic principles. Paper concludes with an identification of the research needed to provide a more complete understanding of the mechanism and the factors influencing the corrosion of steel embedded in concrete.

One of the major problems with rehabilitation of frost damage in old non-air-entrained hydraulic structures is prevention of failure of the repaired surface at the new/old concrete interface or in the old substrate concrete below the repair due to trapped moisture and subsequent freezing of the critically saturated substrate. A survey of several 40- to 75-year-old, non-air-entrained concrete dams indicated that in many cases the concrete at the water line was not damaged due to freezing and thawing. This would appear to contradict conventional wisdom based on ASTM C 666 testing of non-air-entrained concrete. Instead, most of the deterioration had taken place on the inclined downstream faces of the gravity sections, away from direct exposure to water, but subject to many cycles of freezing in air. In some cases, water leaked through joints in the concrete and initiated progressive raveling. However, in other cases, it appeared that moisture was drawn to the exposed surfaces by capillary suction. As a result of these observations, it was decided to develop a one-sided freezing test, with the unfrozen side exposed to water and the freezing side exposed to air, to better simulate field exposure. The cylindrical concrete specimens are monitored with temperature, relative humidity, and moisture probes at various depths during testing. This should allow evaluation of non-air-entrained concrete and various repair materials on the freezing surface to observe whether moisture is building up to critically saturated levels that would result in deterioration. While the equipment for this test has been designed and built, testing is at a preliminary stage. Describes the nature of frost damage in hydraulic structures and then describes the new test procedure in detail.

The 100 m reinforced concrete open spandrel arch bridge over the Storms River Gorge was constructed in the mid-1950s, and in 1982 surface cracking of the concrete was noticed. Cores were obtained from the various members and laboratory testing confirmed that the concrete was suffering from the effects of alkali-aggregate reaction (AAR). In 1986, the decision was made to rehabilitate this bridge, consisting of two distinct stages, of which the first was widening and strengthening the superstructure, as well as strengthening the concrete arch rib itself. The second stage consisted of treating the concrete surfaces with a hydrophobic coating to halt any further effects of AAR. To assess the long-term effectiveness of the hydrophobic coating, the bridge was instrumented and strain gage readings were taken at regular intervals. The analysis of the readings show that the concrete has been shrinking since the strain readings were started, confirming that, to date, the silane hydrophobic coating is still effective.

The greatest threat to the durability of reinforced concrete structures is the reinforcement corrosion. The paper presents the importance of the concrete protective cover and the conditions causing the reinforcement corrosion under the action of chlorides and carbonic acid. Processes of absorption, diffusion and flow, i.e. of transport of media through concrete depend on the pore system and the amount of water in the pores.Physical laws describing the penetration of aggressive agents into concre-te can serve as a basis for engineering calculations of reinforcement durability in the concrete as well as for the designing of the concrete cover. Physical laws and corresponding material parametars are briefly reviewed in the paper. For engineering purposes, in calculating the durability, four typical tasks can be solved. The processes of degradation depend on the pore system in the concrete structure, and the paper indicates some possible technological measures of structure modifications.