International Concrete Abstracts Portal

Abrasion resistance of 10 concrete mixes with compressive strength ranging from 0.24 to 0.42 MPa was evaluated. Mixes studied contained silica fume, fly ash or natural pozzolan, and addition of superplasticizer in some cases to reduce mixing water. Concrete workability remained constant. Tests were carried out following a Portuguese standard similar to a Brazilian standard and German Standard DIN 52108, using the Dorry apparatus. Porosity and compressive strength of concrete were also determined. The main conclusions are as follows: 1) cement replacement by mineral admixtures always reduced the abrasion resistance at rates between 10 and 25 percent, while less satisfactory results were obtained with condensed silica fume concretes; 2) addition of superplasticizer increased the abrasion resistance about 25 percent; 3) abrasion resistance varied inversely with water-cement ratio, cement paste volume, and concrete porosity; 4) general correlation was poor between abrasion resistance and compressive strength, indicating a strong influence from cementing material type, mainly in the case of silica fume; 5) there was evidence that poor performance of condensed silica fume concrete can be ascribed to self-desiccation; 6) even the worst results obtained in this test series were equivalent to abrasion resistance at least six times higher than that of ordinary concrete with 20 MPa compressive strength.

In 1980, dead-burned dolomite particles removed from a cement kiln were inadvertently distributed in aggregates that were later used in concrete. These particles were of coarse aggregate size (38 mm) and contained approximately 55 percent calcium oxide (CaO) and 35 percent magnesium oxide (MgO). When this contaminated aggregate was used to make concrete in 1980, it caused some relatively large popouts (up to possibly 230-mm-diameter). Subsequent periodic visual evaluations of this contaminated concrete were performed to verify the acceptability of the concrete and the durability of popout repairs. To the authors' knowledge, only one structure was removed and repaired. In 1989, another such incidence occurred, but this time the portland cement was contaminated with smaller (<9.5-mm) dead-burned dolomite particles with approximately the same proportions of CaO and MgO. Paper reports on how data developed from the 1980 incident was extended for use in evaluating the concrete contaminated in 1989, and how instrumentation was used to effectively determine the actual volume of dead-burned dolomite in the contaminated concrete and degree of hydration of the particles. Such information is being used to predict the long-term effects of the contamination.

The European prestandard for concrete, ENV 206, includes durability requirements framed principally in terms of maximum water-cement values for various exposure classes. Minimum cement contents are also given, but the values are relatively low. No minimum concrete strength grades are included, and no distinction is made between cements of different strength classes or different types. It is clear that the variety of cement types specified in the European prestandard for cement, ENV 197-1, will produce a wide range of concrete strengths when fulfilling the limiting specification requirements for a given exposure category. Previous work has suggested that, although water-cement ratio may be the most suitable specification parameter within a single cement type, strength should also be specified if comparable concrete durability is to be achieved with a range of cement types. To test this proposition, two series of concrete mixes were prepared with a variety of cements, including slag, fly ash, and limestone filler: one series had equal cement contents and water-cement-ratio and the other had equal strength grade and workability. The results to date show that durability, as measured by permeability, carbonation, and freeze-thaw resistance, is not equal for all cements at the same water-cement ratio, and suggest that concrete strength grade is a better specification parameter if similar durability is required from the wide range of cement types defined in ENV 197-1.

Paper describes a number of research programs on assessment of the effectiveness of practical on-site curing techniques; in addition, it reviews the need for curing of structural concrete, for which carbon-induced corrosion of reinforcement is the most likely cause of deterioration. It is demonstrated that, while compressive strength of small specimens such as 11- or 150-mm cubes are sensitive to curing, the same has not been shown true in larger elements. Carbonation depths from 4-year exposure tests for a range of concrete types suggest that the influence of ambient conditions in a temperate climate may be greater than that of most curing methods, even when the concrete is sheltered from rainfall. Limitations on the validity of the use of small specimens and water storage to assess the effect of curing, or predict the performance of concrete, are discussed, together with the possibility of developing a suitable in situ test for curing effectiveness. It is concluded that, except in conditions of very dry air, little evidence was found of easily measurable curing effects on durability-related properties of formed surfaces, in temperate climates, for concretes made with normal portland cement in conditions where carbonation-induced corrosion is the most likely form of deterioration.

Paper discusses the current approach for specifying concrete durability in structures. The shortcomings of the use of bulk parameters such as strength, water-binder ratio, and binder content to specify durability are discussed. Studies carried out over the last 10 years at Dundee University, using simple permeation tests, which are sensitive to curing, cement type, and concrete grade, have shown close association between permeation properties and concrete durability. Paper deals with the measurement of concrete durability by the Dundee-modified initial surface absorption test (ISAT). A wide range of concrete mixes made with ordinary portland cement and blends with pulverized fuel ash (PFA) and ground granulated blast furnace slag were designed. The duration of moist-curing was varied from 0 to 28 days, and the maximum aggregate size ranged from 5 to 40 mm. All mixes were tested for absorptivity and durability, including freeze-thaw resistance, carbonation, chloride ingress, and mechanical wear. Results show that the absorptivity of concrete, measured with the ISAT, can be used as an accurate specification for concrete durability, irrespective of curing, grade, or mix constituents. A tentative surface absorptivity classification for durability has been proposed.