International Concrete Abstracts Portal

Briefly reviews five joint industry-research programs pertaining to offshore concrete structures. These programs were sponsored by the oil and gas industry and related construction industries. These studies, conducted in both North America and Norway, included the use of high-strength, lightweight aggregate concretes in both material and structural evaluations. Selected characteristics of the high-strength, lightweight aggregate concretes used in these studies (such as ductility in reinforced concrete elements, punching shear behavior, and fatigue characteristics) are summarized. Future research needs are discussed.

Pore structure, density, and strenght may vary within a wide range for different types of lightweight aggregate. Hence, not all types of lightweight aggregate are suitable for production of high-strength concrete. In the present work, the significance of various lightweight aggregates on the concrete strenght and density was studied. Twenty-eight-day compressive strengths up to 102 MPa, corresponding to a density of 1865 kg/m3, were obtained. The type of lightweight aggregate appears to be the primary factor controlling both the density and the strength. For high-strength lightweight concrete, it is difficult to predict the 28-day strengths from early strengths because of the influence of the aggregate.

Compressive strength and water penetration of three grades of high-strength concretes with cement contents ranging from 400 to 500 kg/m3 and a proprietary superplasticizer are reported. The control mixes were redesigned by adding a Class F-type fly ash at fly ash/cementitious ratios of 0.15 and 0.35. All concretes were designed for a similar workability. The strength development was monitored in three curing regimes. It is concluded that the superplasticized concrete developed a higher strength than that predicted from a reduction in the water/cement ratio. The curing conditions significantly influenced the strength development and the water penetration of the concretes. An optimum fly ash/cementitious ratio of 0.15 was found to be appropriate for the concretes; larger amounts of fly ash were found undesirable for higher strength development.

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.