The amount of heat developed in any concrete structure due to cement hydration is of concern for two reasons. First, concrete sets at a much higher temperature than ambient so its strength can be quite different than that of standard specimens and it shrinks as it cools. Second, cooling is not done at the same rate in all parts of the structure resulting sometimes in thermal gradients large enough to cause cracking. This paper describes briefly some features of an ongoing study of the thermal behavior of high performance concrete and presents some results. It reports measurements of temperature induced by the heat of hydration in a high performance concrete viaduct built near Montreal, Quebec, Canada. The experimental results are compared with the results of a finite element analysis with regard to early-age temperature developments in a concrete structure. The results obtained numerically are in good agreement with the experimental results. Once the validity of the numerical model is established, it becomes a powerful research tool which can be used to study different aspects related to the thermal behavior of concrete structures.

This investigation was performed to access the applicability of conventional maturity functions to high strength concretes incorporating silica fume and superplasticizer. Concrete specimens were allowed to cure under three temperatures simulating hot weather, laboratory, and cold weather conditions. Both linear and exponential strength-maturity functions predicted different values of strength for different concrete curing temperatures for the same value of maturity, as has been observed by other researchers. Of these two unctions, the exponential performed somewhat better in this regard for elatively low values of maturity. Concrete strength-gain behavior was influenced by the presence of silica ume and the high amount of super-plasticizer in the mixture. Strength-maturity equations already developed for normal strength concrete underestimated strength at low maturity ages and overestimated strength at high maturity. It is suggested that further studies should be done to evaluate such effects in those relationships.

Prestressed concrete frames are commonly used in bridge design. However, very little is known about their behavior under reversed cyclic loads, particularly when the frame is made of high strength prestressed concrete and is subjected to severe earthquakes. Most bridge codes do not provide the required design guidelines. Results from small scale models of eight prestressed concrete frames (divided into two groups), tested under various displacement histories simulating earthquake forces are presented. The primary curves (horizontal force-displacement relationships) and the hysteretic loops are determined experimentally. Concrete strength are 35 MPa and 52 MPa, for the two groups, respectively, and the effective prestress is 51 percent of the ultimate strength of prestressing steel. It is found that prestressed frames with high strength concrete provide greater ductility and dissipated energy than those with normal strength concrete. The effect of displacement history on the mechanical behavior is significant.

In order to enable rational and safe design with high performance concrete recommendations for this material are necessary. In the Netherlands an extended research program has been carried out focusing on aspects like behaviour in compression at various loading rates, shear friction in cracks, in-plane loading of cracked reinforced elements, splitting effects in the anchorage zone of prestressing strands, joints between precast columns, and creep. Furthermore trial casts have been carried out in order to get more experience with HPC at the building site. A four storey office building was completely built in HPC. During construction the temperature of the hardening concrete was measured at many locations, in order to investigate the development of temperature stresses and to get indications of the cracking probability. More-over a section of a box girder bridge was cast as an exercise for the construction of a 160 m span bridge in 1996. Both the labora-tory experiments and the site trials raised the confidence in suc-cessful applications of high performance concrete.

Reinforced concrete columns are typically made with higher strength concrete than are the floor slabs that they support. In construction, the slab is usually cast continuous through the region of the slab-column joint. As a result, load in the column above the slab must pass through a layer of weaker slab concrete before reaching the column below the slab. The column-slab joint may be viewed as a “sandwich” column, with high strength concrete above and below a layer of lower strength concrete. In design, the effective column concrete strength is based on a special weighted average of the column and slab concrete strengths. Because of confinement, the slab concrete in the joint region is assumed to be capable of carrying stresses well in excess of its specified strength. This confinement is, in turn, affected by gravity loading of the slab. Existing design procedures are based on tests of slab-column joints in which no load was applied to the slabs. This paper presents the results of a series of tests on interior column-slab joints in which service level loads were applied to the slabs prior to loading the columns. The major conclusions of this study are: (1) tests of sandwich slab-column joints with unloaded slabs consistently overestimate the strength of the connection and (2) the AC1 3 18-89 provisions for interior column-slab joints are unconservative for high ratios of column to slab strength and/or high ratios of slab thickness to column size.