International Concrete Abstracts Portal

SP-167 This ACI special publication is a collection of 15 symposium papers that were presented in three sessions at the ACI Fall Convention in Montreal in November of 1995. The authors were asked to present a view of activities on high-strength concrete from work done in their respective countries: Japan, France, Norway, Holland, Germany, Canada and the United States. The following titles are contained within this informative document.

High performance concretes (HPC) are increasingly used in high-rise building columns. Some creep and shrinkage data of HPC has been published based on laboratory tests. Very few long term field results are available. During the construction of the new library of Concordia University (Montreal, Canada) in 1990, two pairs of circular 850 mm diameter reinforced, concrete columns made of lOO-MPa silica fume concrete and of 80.MPa non silica fume concrete were instrumented with vibrating wire extensometers. One column (called active column) of each pair was part of the structure; the other (called mockup column) was never loaded and used for shrinkage and thermal strain measurements. Strains and temperatures at different locations in concrete and in reinforcing bars as well as load in concrete were recorded for about 5 years. Data obtained on these HPC columns is presented in this paper. Creep, shrinkage, thermal expansion coefficient of the two field concretes are discussed. Stresses in concrete and in reinforcing bars are analyzed. Axial loads in active columns are calculated from experimental data and compared with the specified loads used in the structural design.

The diffusion mechanisms of chloride ions into ordinary and high performance mortars were studied. Four different mortar mixtures were tested. Test parameters included the water/binder ratio (0.25 and 0.45) and the use of silica fume. An ASTM type III cement was used in the preparation of the 0.25 water/binder ratio mortars while the 0.45 water/binder ratio mixtures were prepared with an ASTM type I. For all mixtures, the sand volume fraction was maintained constant at 50%. The diffusion properties of the mortars were studied according to two different experimental procedures. In a first series of tests, apparent diffusion coefficients were calculated from chloride ion profiles measured after a 12-month immersion period. In a second series, a migration test (where the chloride ion penetration is accelerated by the application of an electrical potential of 10 volts) was used to investigate the transport properties of the four mortars. All test results clearly show that the reduction of the water/binder ratio and the use of silica fume contribute significantly to the reduction of the chloride ion penetration. The consequences of these results on the long-term durability of high-performance concrete structures and, more specifically, on their ability to resist to reinforcing steel corrosion are discussed. The ability of the accelerated migration test to reliably predict the penetration of chlorides in cement-based materials after only a 14-day test period is also discussed.

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.

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.