International Concrete Abstracts Portal

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.

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.

Heat of hydration of a selection of high-strength concretes has been investigated by means of a so-called semiadiabatic calorimeter test. The temperature development within a hardening specimen enclosed in an insulated container is used as basis for a simulation of the adiabatic temperature increase and the specific heat development of the cement. The results indicate that the heat of hydration can be affected within a relatively wide range by the utilization of traditional mix design parameters. Heat of hydration is affected not only by the cement content but also by the water/cementitious ratio {w/(c + s)} and the silica fume content. The heat of hydration per cement unit decreases approximately 9 percent when the w(c + s) is reduced form 0.36 to 0.27. At w(c + s) 0.50, the replacement of cement by silica fume on a 1:1 basis induces a significant increase in the heat evolved per cement unit. The increase corresponds approximately to the reduction in cement content. Hence, the temperature rise in the concrete is not significantly affected. The ability of the silica fume to increase the heat evolvement of the cement decreases with decreasing w(c + s), and is negligible at w(c + s) 0.27. Hence, a replacement of cement by silica fume on a 1:1 basis at this w(c + s) leads to a lower temperature rise of the concrete.

Two test methods were used to investigate the frost resistance of high-strength concrete with and without air-entraining agents: a volume deterioration method (ASTM C 666) and a salt-scaling method (SwedishStandard SS137244) similar to ASTM C 672. In addition, low-temperature calorimetry was used to measure ice formation in concretes after a drying/resaturation treatment. For concretes with 0 and 10 percent silica fume contents and water-binder ratios from 0.40 to 0.25, the calorimetry results showed only very minor ice formation down to 20 C. The cement used was a high-strength type (Norwegian P30 4A). This result contrasts an earlier calorimeter result with ordinary portland cement, and indicates that the P30 4A cement produces a more finely divided capillary pore structure. The salt-scaling tests showed that the high-strength concrete with water-to-binder ratios less than about 0.37 exhibits acceptable resistance to salt-scaling, even without air entrainment. The ASTM C 666 test results showed relatively severe damage to concretes with water-to-binder ratios down to 0.28. No air-entrained concrete was tested with ASTM C 666. This result is in apparent conflict with the calorimetry results and suggests that the damage may be related not to ice formation but to thermal fatigue effects caused by differences that are too large between the thermal expansion coefficients of aggregates and binders.