International Concrete Abstracts Portal

Use of silica fume is an accepted technology in the Norwegian concrete environment and high-strength concrete incorporating silica fume is being used increasingly. However, in recent years, conflicting reports regarding the fire resistance of high-strength concrete have been published. This review indicates that eventual fire-safety problems seem to be linked less to the materials used than to physical properties of hardened concrete or conditions under which high strength concrete is used.

Reports performance of gravel concrete incorporating silica fume after 72 hr exposure at 150, 300, and 450 C. A total of eight concrete mixtures, each 0.09 m3 in volume, were made at water-cement ratios of 0.23, 0.35, 0.50, and 0.71. The mixtures at each w/c consisted of one control mixture and one incorporating 8 percent silica fume by weight of cement. The cylinders and prism specimens were cast for testing in compression and flexure before and after exposure to the elevated temperatures. Before exposure to the elevated temperatures, the test specimens were moist cured for 7 days and air dried for 21 days at ambient temperature and 50 percent relative humidity. After exposure, the test specimens were cooled to room temperature and tested in compression and flexure. The weight loss and pulse velocity determinations were made before and after heat exposure. The test results show that after 72 hr exposure at 150, 300, and 450 C, the performance of the control concrete in the compression testing mode is marginally superior to that of the silica fume concrete. The reverse is true when the two types of concretes are tested in the flexural mode.

Mixtures employing silica fume (SF) and superplasticizers with a worst-case combination of the available local gravel and cement sources were evaluated using 250 to 500 kg/m3 of Type I cement instead of Type III, and up to 15 percent silica fume. Many of the mixtures evaluated met or substantially exceeded the specified strength criteria at a cost below that of the concretes presently used. Hardened concrete air void parameters are also satisfactory for the SF concretes examined. A mixture using 300 kg/m3 of Type I cement with 10 percent SF comfortably met the present strength specifications at considerably reduced material cost when using accelerated curing. However, 20-hr strengths of 45 to 50 MPa and 28-day strengths over 50 MPa are possible with higher cement contents. A mixture using 450 kg/m3 of Type I cement with 10 percent SF can just meet the 20-hr strength requirement without accelerated curing, and its 28-day strength exceeds the current specification requirement by more than 50 percent. Slightly lower material costs and the saving associated with eliminating accelerated curing coupled with long-term strength in excess of 60 MPa make this concrete attractive in terms of both cost and strength if designers can fully utilize the higher strength.

Purpose was to throw light on the behavior of ultra-fine admixtures introduced with superplasticizer in high-strength concretes as partial portland cement replacement. Paper describes results from different pastes and mortars prepared with normal portland cement including 10 percent silica fume or alumina powder as replacement for cement. Conclusions are as follow. 1) Water demand is modified only by CSF. When associated with superplasticizer, all admixtures lower the water demand. No relationship was found between fineness and water-reducing effect. 2) Superplasticizer acts as a set-retarder and the admixtures as accelerators, but they cannot compensate for the superplasticizer's retardation except for one of the silica fumes used. 3) Two silica fumes showed an important pozzolanic effect; another silica fume and the alumina powder were inert. 4) All admixtures in the presence of superplasticizer increase the compressive strength of mortars. This is due not to the pozzolanic reaction, but is explained by the lowering of w/c.

Since 1971, a contracting firm has produced approximately 1.3 million m3 of high-strength concrete for offshore platforms in the North Sea. The major part of this concrete volume has been produced without condensed silica fume (CSF), fly ash, or any other pozzolanic material. For the company's latest project, moderate dosages of CSF in combination with high dosages of superplasticizing admixtures have been introduced to meet new demands in design and construction. The predominant requirements for concrete for extremely dense reinforcement (500 to 1000 kg/m3) are high workability (slump of approximately 250 mm) and negligible bleeding or segregation. Concrete mixes fulfilling these requirements and still having a moderate content of cement have been designed by adding 2 percent (by weight of cement) of CSF. Slipform construction and concrete pumping are vital elements in the construction procedures for very large offshore structures. CSF, through extensive full scale field tests and shaft slipforming, has been found to improve the pumpability and stability of high strength/high workability concrete mixes.