International Concrete Abstracts Portal

Discusses the utilization of silica fume concrete admixture to prevent reinforcing steel corrosion. The mechanism of steel corrosion in salt-impregnated concrete is described, along with laboratory test date showing how ordinary concrete's corrosion-prone characteristics are altered by the use of silica fume. The mineral admixture significantly lowers the concrete permeability to prevent chloride ingress to the reinforcing steel level, while simultaneously increasing the concrete's electrical resistance to corrosion currents. Test data from the FHWA 90 day Chloride Ponding Test indicates a 98 percent reduction in chloride penetration. The AASHTO T277 rapid chloride permeability test shows a 10-times impermeability and 25-times resistivity improvement with the use of 12 percent silica fume. T he Time-to-Corrosion FHWA/NCHRP 244 slab test is scaled-down steel-reinforced deck, from which macrocell corrosion current, AC resistance, half-cell potential, and chloride absorption are measured. Zero corrosion current was measured after NaCl was ponded in alternate soak/dry cycles for 48 weeks. The second phase test program evaluated the corrosion performance of full-sized concrete bridge sections, including beams, columns, piles, and bridge deck panels. The test members were subjected to environments simulating salt water and deicing agents for 370 days. Test results show that silica fume admixture prevents salt-induced corrosion of steel a reinforcing bar and tensioning strands.

The durability in seawater of high-strength concretes produced with the addition of silica fume replacing a part of the cement was investigated. The influence of the wet-curing time on the behavior in seawater of high-strength mortars (strength in excess of 60 MPa) in which a part of the cement was replaced by densified silica fume, was determined. The various curing times applied to the specimens, after mold removal, were 48 hr, 7 days, and 28 days at 100 percent relative humidity, followed by storage for 28 days at 20 C and 50 percent relative humidity before the start of tests for resistance to seawater for 1 year. Investigation of the porosity of these mortars shows that, just after curing, the silica fume, as expected, reduces the total porosity of the reference mortar (25 to 45 percent) and substantially alters the pore-size distribution--the shorter the curing time, the more marked this effect. However, as hydration continues at 50 percent RH, the porosity of the reference mortar decreases and the differences in total porosity with respect to the mortars containing silica fume become smaller--the longer the initial curing time and the higher the C3 A content of the cement, the greater this effect. This explains the results of resistance to seawater, where it is found that silica fume contents of less than 10 percent do not lead to any significant improvement in behavior in seawater. This shows that the type of curing and the ambient conditions under which strength increases may limit the beneficial effects of silica fume on durability, when the addition of the silica fume is accompanied by a corresponding reduction of the cement content. It is also found that the best curing method is the specimens in fresh water for the first 7 days, while a curing time of only 48 hr is highly detrimental in terms of the subsequent behavior of the mortars in seawater.

The biological effects and chloride penetration studies were carried out on a 10-year-old concrete structure of the arch bridge on the island Krk in the North Adriatic. Concrete cores of 100 mm diameter were drilled from three main locations of the structure and tested for chloride content at depths 0 to 10, 10 to 20, and 20 to 40 mm. The influence of aggressive organisms on the elements below sea level was determined. The biological life of the concrete below sea level was massive, the value being 1300 units per mý at a depth of about 18 m. The shell Recellaria Dubia can make depressions of 5 to 10 mm diameter.

A part of the Statpipe Development Project is a landfill for two gas pipelines on the exposed western coast of Norway. The pipelines are placed inside a submerged concrete tunnel that acts as an underwater protecting bridge over the rocky sea bed. The 590 m long tunnel was cast in five separate elements produced in two dry docks. The tunnel starts at a water depth of 30 m and ends up at water level. The tunnel elements were produced and placed during the summer of 1982. The splash zone element encompassed the following characteristics; 400 kg ordinary portland cement and 32.5 kg silica fume per m3 concrete. The water-cement-sand ratio was 0.36, the slump value was approximately 200 mm, and the 28-day cube strength was approximately 78 Mpa. After 7 years in service, cores were drilled from the splash zone element. The testing of the cores included compressive strength, capillary absorption, chloride profile, thin-section analyses, x-ray diffraction, scanning electron microscopy, and element analysis. The results indicate that in such a low-porous concrete, the reaction products between seawater and cement paste will fill up the original low porosity and tighten the concrete so that the ingress of chlorides will cease. For concrete exposed to seawater, ingress of clorides and risk of reinforcing bar corrosion represents the most severe problem. The tightening effect of seawater in such a high-performance concrete seems to reduce this problem to a minimum.

The purpose of this research is to examine the resistance to repeated freezing and thawing cycles of non-air-entrained and air-entrained concretes containing high dosages of condensed silica fume. Testing was carried out on a total of 76 air-entrained and non air-entrained cylindrical specimens. The compressive strength and the complete stress-strain curves of the specimens under uniaxial compression were determined from different freezing and thawing cycles. The influence of the treatment on the shape of the stress-strain curves was investigated. In addition, the dynamic modulus under the same cyclic conditions was determined. To investigate both the spacing factor and the specific surface, the air void and pore structure characteristics of hardened specimens were studied.