International Concrete Abstracts Portal

High-performance concrete has been made using different cementitious combinations: portland cement and fly ash; portland cement and silica fume, and portland cement, ground granulated slag, and silica fume. The use of a supplementary cementitious material like fly ash or ground granulated slag is not only interesting from an economical point of view but also from a rheological point of view. Replacing in some cases up to 20 percent of cement by a less reactive cementitious material like fly ash or up to 50 percent by ground granulated slag can solve the slump loss problem observed with some very reactive cements when used at water/cement ratios ranging from 0.25 to 0.30. Moreover, the use of a supplementary cementitious material results in a significant decrease in the superplasticizer dosage needed to achieve a given workability. In terms of rheology, compressive strength, and cost, one of the most promising combinations of cementitious materials for high-performance concrete is a mixture of ground granulated slag, silica fume, and portland cement, when ground granulated slag is available at a reasonable price.

Presents a summary of the results from a systematic in situ testing program on the concrete compressive strength in three Norwegian offshore platforms representing 460,000 m3 of high-strength concrete. The goal of the testing program was to document that the in-situ strength in these platforms is higher than assumed in relevant design codes and thus substantiates a higher utilization of the compressive design strength. The specified concrete quality for these projects has been in the range of C55 to C70. In a systematic manner, approximately one thousand 75 mm concrete cores have been tested and evaluated. Comparison has been made to the results of 100 mm cube specimens as reference. The main factor influencing the in situ strength proved to be the effective compaction applied to the fresh concrete. Thus, slipformed concrete shows systematically higher strength than conventionally placed concrete in stationary formwork. The in situ strength was significantly higher than presumed in relevant design codes. The results further indicate that the increase with time of the in situ strength was slightly higher than for the laboratory-cured reference specimens. For concrete platforms that are subjected to rigorous quality control programs and stringent working procedures, like the Condeep platforms, it is suggested that an increase in the compressive design strength should be allowed with 5 to 10 percent for slipformed concrete compared to the actual values given in the recently revised Norwegian design code.

Use of silica fume is important to produce high-strength concrete. Possible negative effects on long-term properties are, therefore, of vital interest for the future development of high-strength concrete. It has been reported that silica fume concrete stored in air showed strength loss from 90 days to 5 years, but courses are not discussed. The report was based on a limited number of results. Similar results are not found in high-strength concrete up to 10 years old either in laboratory tests or testing samples from existing structures in Norway. Results from two major research projects showed that, for laboratory-stored specimens, the strength increased or was constant for concrete stored in water or air, respectively. No difference was found between high- and normal strength concretes. The increase was somewhat higher for concretes without silica fume compared to concretes with up to 20 percent silica fume by weight of cement. Furthermore, the strength increase was somewhat higher for water-stored concretes than for air-stored. However, high-strength silica fume concrete was not more sensitive to early drying than concrete without silica fume. High-strength concrete from several existing structures did not exhibit the same consistent pattern in strength development, however. This is probably due to insufficient documentation at an early age. However, the results did not show any significant negative long-term strength development.

Two bridges, the Joigny and Pertuiset, have recently been built in France using high-strength concrete. It was necessary to measure the shrinkage and creep deformation of the concretes for their design. Two series of samples were taken, corresponding to the two kinds of concretes (one with and one without silica fume). The specimens were loaded at different levels and ages (including early ages). Some cylinders were carefully sealed to avoid any drying. Besides the mathematical equations deduced from these trials and detailed in the paper, the following results were discovered: the nonsilica fume high-strength concrete (HSC) is quite comparable to the normal strength concrete (NSC); during the setting, the silica fume HSC exhibits a certain autogenous shrinkage which is higher than that of the NSC concrete; for the silica fume HSC, the magnitude of the creep deformation is highly dependent on the age of concrete at loading, compared with elastic strains, so that the creep is much lower than for NSC (except when loading occurs at a very early age); regarding NSC, the theory of superposition applies fairly to the creep of high-strength concrete for nondecreasing loadings; and, finally, the desiccation creep is reduced for nonsilica fume HSC and entirely cancelled for silica fume HSC, meaning that creep does not depend on size for these materials. Some physical models are proposed at the paper's conclusion to explain these phenomena.

This investigation reports on three high-strength concretes produced by low-heat portland cement with silica, blast furnace slag cement, and rapid-hardening portland cement, and a normal strength control concrete produced by rapid-hardening portland cement. The weight losses of creep and shrinkage cylinders are compared to corresponding deformation values at 1 year. The porosities of creep and shrinkage concrete specimens were determined by mercury porosimeters at the age of 7 days when the tests started and, thereafter, at the ages of 14, 35, and 372 days. The microporosity of binder paste specimens was determined by nitrogen adsorption at the ages of 14 and 35 days. The creep and shrinkage values of the high-strength concretes are compared to the values obtained by CEB formulas. It is concluded that the initial creep and shrinkage rate of high and normal strength concretes is governed by the evaporable water amount lost to the external environment. The average pore radii of the test concretes emptied from water during the creep and shrinkage tests were calculated.