International Concrete Abstracts Portal

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.

A laboratory study on the durability of OPC and trass-OPC mortars and concretes is presented. A natural pozzolan of volcanic origin obtained from the Demavend area in Iran and known as "trass" is used to substitute 20 percent of the OPC content of mortar and concrete. The performance of the material is evaluated by measurements of total porosity. Pore-size distribution and permeability are given in relation to their response to aggressive chloride and sulfate concentrated solution, and also CO 2 gas. The carbonation data is used to formulate a performance-time function, which is proposed as a model to assess durability of the mortars and concretes.

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.

The use of fly ash as a partial replacement for portland cement or as an admixture in concrete may provide many technical and economic advantages if properly utilized. One such technical advantage is to improve the resistance of concrete to sulfate attack. However, this benefit is not realized with all types of fly ash. Some fly ashes have been shown to significantly reduce the sulfate resistance of concrete, others have demonstrated no effect, while still other sources of fly ash improve the durability of concrete in sulfate environments. To clarify the effect of different types of fly ash on concrete exposed to sulfates, a research project is currently being conducted utilizing fly ash from 15 different sources. These fly ashes include sources from bituminous, sub-bituminous, and lignite coal-powered electric generating plants that generally conform to the requirements of ASTM C 618, Class F or C fly ash. The fly ashes are categorized by both chemical and mineralogical phase composition. A Type II portland cement is being used with a fly ash replacement level of 35 percent by volume. The sulfate exposure test consists of concrete cylinders soaking in a 10 percent sodium sulfate solution. Performance is measured by mass loss, expansion, and visual inspection of concrete.

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.