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.

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.

Concrete prisms 500 x 100 x 100 mm were made using cement:aggregate proportions of 1:3, 1:6, and 1:9 with three different aggregates (flint gravel, limestone, basalt) and water-cement ratios in the range 0.35 to 0.90. Drilled-hole (Figg) permeability measurements were made after the prisms had been cured under water for 28 days and conditioned at 20 C and 65 percent relative humidity. The samples were exposed outdoors in trays 50 mm deep to accelerate damage from weathering in the expectation that the most permeable concretes would have the least durability. After 10 years, because the prisms were in remarkably good condition, they were dried and the air permeabilities were measured. The prisms were then returned to the exposure trays. The most permeable concretes (basalt) suffered damage after 12 winters of exposure. The permeability tests correctly predicted relative durability, but the unexpectedly good performance of the specimens is attributed to complete compaction, thorough curing, and continued access to water, allowing further cement hydration. Other test results on the fresh and hardened concretes are reported, and the permeability measurement procedures are briefly described.

Paper examines critically the role and effectiveness of mineral admixtures in counteracting the effects of ASR. Tests are reported on plain concrete prisms and reinforced concrete slabs incorporating a slowly reactive but moderately expansive reactive aggregate and containing either fly ash, slag, or microsilica. It is shown that control of expansive strains and consequent cracking are acceptable and satisfactory solutions in many instances, particularly in unreinforced concrete. However, there are many situations where additional factors such as preserving the strength and stiffness of the damaged structure and control of structural distortions are equally important if the safety, stability, and serviceability of ASR-affected structures are to be maintained. Judged on these five significant criteria, data are presented to show that mineral admixtures, when used correctly and at the required level, can control material damage and structural deterioration effectively and substantially, although they may not be able to eliminate all deleterious effects completely and at the same time. Mineral admixtures should not be expected to fulfill such a global and over-protective role, but they have an unequaled, positive, and promising function in contributing to the safety, stability, and durability of concrete materials and concrete structures affected by ASR.

Microscopical examination of hardened concrete is the only method for definitively identifying the presence of AAR and also providing valuable information regarding many additional criteria that are relevant to the durability of concrete. It is common, however, for the extent of such evidence to be indicated by the use of subjective terms only, which inhibits the comparison of different examples and does not allow correlations to be made between microscopical observations and any other data that indicate the quantity or severity of any damage to the concrete. A procedure is described for recording and quantifying the various microscopic features identifiable in thin section under a petrological microscope. It is shown by reference to example data that the procedure facilitates the objective comparison of different concretes and can also help to establish the relative extent and degree of deterioration that has occurred. The technique has been developed to improve the sensitivity of microscopy in the diagnosis of AAR but could be applied similarly to a range of threats to concrete durability.