International Concrete Abstracts Portal

The shear friction analogy is a valuable and simple tool that can be used to estimate the maximum shear force transmitted across a cracked plane in a concrete member. The expressions to determine the shear friction capacity up to now have been based on experiments on concretes with cylinder strengths of at most f' c = 60 N/mm 2. In such concretes, the aggregate particles normally do not break at the formation of cracks through the concrete. In high-strength concrete, however, the cement matrix is strong enough to cause fracture of the aggregate particles. As a result, the crack faces are relatively smooth, so that the shear friction capacity is expected to be reduced. In this paper, shear friction tests are described on concrete with a cylinder strength of f' c = 100 N/mm 2. The experiments are carried out on cracks in plain concrete and on reinforced cracks. It is shown that the reduction in shear friction capacity due to aggregate fracture is considerable.

The Strategic Highway Research Program (SHRP) awarded a contract to North Carolina State University (NCSU) to investigate the use of high-performance concrete (HPC) in highway pavements and bridge structures. The goals of the project were threefold. First, a number of HPC mixtures were developed for highway applications. Second, laboratory testing of the HPC mixtures was conducted. Finally, a number of field test sites were constructed and monitored. Three different classes of HPC were established for this research. These are very early-strength (VES), high-early-strength (HES), and very high-strength (VHS) concrete. Two types of VES and VHS concrete were developed. The VES mixture was developed for use primarily as a rapid repair material where time is critical and cost is a lesser concern. The HES mixture was developed for bridge deck construction where deterioration due to freezing and thawing and steel corrosion is a major problem. The HES mixture can also be used for repair where cost is important and time is a lesser concern. The VHS mixture was developed for use in bridge structures where high-long-term strength is needed rather than rapid strength gain characteristics. Paper summarizes the development of the mixture proportions for the three classes of HPC. Included in the paper are the strength and serviceability requirements for the mixtures. Recommendations are made for adapting the HPC mixtures for local conditions.

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.