International Concrete Abstracts Portal

A significant proportion of Australian infrastructure is located in a zone that is close to or in direct contact with seawater. At most of these locations, the coastal environment is coupled with high ambient temperatures and large diurnal temperature ranges, conditions that are conducive to promoting corrosion of steel reinforcement in concrete structural elements. Users of concrete are thus always looking for ways to maximize concrete performance for long-term use under these aggressive conditions. The options available in terms of binder systems for concretes in a marine environment have increased in recent years. There are currently available a range of cements and blended cements that include fly ash, slag, and silica fume, which have a place in specifications for marine concrete applications. To provide technical data for potential specifiers and users of such concrete types, a collaborative CSIRO-CSR research and development project was initiated to consider the performance of a range of concretes for marine environments. Concretes considered had a water-binder ratio of 0.35 and included both portland and blended cements. Paper reviews current standards on specifications of concrete for marine environments and goes on to present some recently produced Australian data for different concretes reflecting potential performance. Techniques considered include chloride-ion penetration of concrete based on charge transfer measurements, chloride-ion penetration through concrete, and some mechanical properties of concrete. Conclusions are drawn as to the suitability of certain concrete types under marine conditions.

The combined use of chemical and mineral admixtures has resulted in a new generation of concrete called high-performance concrete (HPC). Understanding the roles of mineral admixtures, such as silica fume, fly ash, and slag depends on in-depth microstructural investigation of HPC at different ages. What is of major interest concerning these materials is their contrasting hydraulic behavior. Whereas silica fume and fly ash are pozzolanic, slag is strictly cementitious. The early strength of concrete increases when silica fume is incorporated, but the activity of slag and fly ash starts much later, and therefore, manifestation of changes in concrete properties, such as strength enhancement, also appears to be delayed. It is in this light that the roles of these admixtures, both individually and in combination, are described in terms of the development of HPC moisture.

Report presents the data on the effects of silica fume on mechanical and durability-related properties of high-strength concrete. High-strength concrete had a compressive strength in the range of 90 to 100 MPa. The compressive strength of high-strength concrete containing 8 percent silica fume was 25 to 30 percent higher than that of a corresponding concrete without silica fume. Both the splitting tensile strength and the modulus of elasticity of high-strength concrete increased as the compressive strength increased, but at a slower rate. The pore structure both in the cement paste and at the cement paste-aggregate interface in high-strength concrete containing 8 percent silica fume was very dense and homogenous due to the microfiller and pozzolanic effect of silica fume, leading to an improvement of the bond between cement paste and aggregates. Durability-related properties such as the chloride-ion permeability, the resistance to freezing-thawing, and the depth of carbonation of high-strength concrete with and without silica fume were also investigated with a special interest in the influence of curing condition at early ages on their properties. From the results, it was found that the use of silica fume in high-strength concrete led to a significant improvement of chloride-ion permeability, and no negative influence on the carbonation. However, the resistance of non-air-entrained high-strength concrete with and without silica fume to the freezing and thawing cycles was very sensitive to the lack of moist curing at early ages, and a poorly cured nonAE high-strength concrete containing 8 percent silica fume deteriorated more seriously.

Reports the moisture-induced shrinkage and swelling of high-strength concrete with 28-day cube strengths ranging from 81 to 107 MPa. The concrete mixtures consisted of hydraulic and blends of ordinary portland cement with 35 percent blast furnace slag content, silica fume, or fly ash in different proportions. The results showed that after 460 days of air-drying, shrinkage of high-strength concretes with 3-day water-curing is between 545 and 775 microstrains, depending on the binder materials used. The incremental shrinkage strains between 28 and 460 days for the concretes range from 215 to 285 microstrains. The highest proportion of drying shrinkage recovered was 69 percent of the 460-day shrinkage for concrete with 35 percent slag content, whereas, the control concrete showed the lowest recoverable shrinkage of 57 percent. Drying shrinkage after 100 days for concretes, which are water-cured for 460 days prior to drying, ranged from 39 to 67 percent of the corresponding shrinkage for similar concretes that are initially water-cured for only 3 days. Shrinkage of mature concrete having blended cement with 35 percent slag content is 240 microstrains, which is 39 percent lower than that for the control concrete with ordinary portland cement, although both concretes had compressive strength of about 105 MPa at the beginning of drying. The effect of partially replacing ordinary portland cement with silica fume decreased or increased the shrinkage of concrete, having 3-day water-curing, depending on the silica fume content. However, the shrinkage of concrete, having 460 days' water curing decreased when ordinary portland cement was replaced partially with silica fume up to 15 percent or with 5 percent silica fume and 5 percent fly ash.

Recently, the study and assessment of the fracture parameters influencing concrete behavior have emerged as an important research field, especially in the context of the development of high-performance concrete. Deformation and failure behavior of concrete are affected by the fracture properties of the constituent materials and the properties of the interfaces between these constituent materials. In this paper, for concrete modeled as a two-phase composite, consisting of mortar with aggregate inclusions, fracture mechanics parameters influencing the properties of high-performance concrete are assessed. These include elastic moduli mismatch parameters between the mortar and the aggregate and the ratios of the interface fracture toughness to the fracture toughness of the aggregate or that of the mortar. From the tests on novel physical models, the roles of these parameters on the mechanical behavior of concrete are studied. On the basis of the established parameters, preliminary guidelines are proposed to manufacture high-performance concrete composites with improved mechanical properties.