International Concrete Abstracts Portal

This paper presents the introduction of a steel fiber made by a pre-cast manufacturer suitable for plant-produced products and transit-supplied concrete. The fiber con-figuration allows fiber manufacturing to be done in-house as are the other concrete products. Toughness test results indicate equivalent or improved performance with lab mixtures compared with other steel fibers available and tested. Tests were conducted with both wet (laboratory and transit mixture) and dry cast techniques for testing samples and full-scale three-edge bearing tests for dry cast pipe. Performance issues were identifiable for the sample casting techniques, compression strength, maturity, and toughness tests with fiber reinforcement. Pipe tests were conducted for the first visible crack, the first 0.25 mm crack, and the ultimate load with fabric reinforcement only, fiber reinforcement only, and then with both fabric and fiber reinforcements. Concrete mixture proportions for the pipe were constant with three dosages of fiber used: 0.25, 0.50, and 0.75 percent by volume.

This state-of-the-art starts out with a short discussion of the methods for measuring autogenous deformation of cementitious paste and how influence of artifacts (e.g. segregation forming bleed water) can be avoided. The influence of all components of cementitious paste on autogenous shrinkage is reviewed; water-cement ratio, cement type, additives like silica fume and fly ash (including the pozzolanic reaction itself), and ad-mixtures like lignosulphonate and super-plasticizers. The mechanism of autogenous shrinkage is discussed, with emphasis on it's relation to setting time and pore pressure. Finally, remedies to reduce the amount of autogenous shrinkage are briefly mentioned in terms of mechanisms and effects.

Autogenous healing of cracks; and ingress of chloride and sulfate through the cracks in concrete were investigated utilizing 15 years old precracked prism specimens. The size of the specimens was100x 100x600 mm. The specimens were made with ordinary portland, slag (Types A, B and C), and fly ash (Type B) cements. A round steel bar of diameter 9 mm was embedded in each specimen. W/C were 0.45 and 0.55. Crack widths were varied from 0.1 to 5 mm. The specimens were exposed to the tidal and sub-merged zones. Deposits along the path of the healed cracks as well. as the de-bonded areas over the steel bars located at the root of the crack were investigated by scanning electron microscope (SEM) and X-ray diffraction (XRD). Mappings for chloride, sulfate, and magnesium oxide through the cracks in concrete were carried out by electron probe micro analyzer (EPMA). Autogenous healing is observed for narrower cracks (5 0.5 mm) irrespective of the cement types and exposure zones. Healing continues along the crack path. It extends to the debonded area over the steel bars at the cracked region. The deposits are con-firmed as calcium carbonate, ettringite, magnesium hydroxide, and rust. Accumulation of more chloride is found in the vicinity of the unhealed wider cracks (> 0.5 mm), especially for slag cements with a high amount of slag content. Sulfate ingress was limited over a very thin region from the crack plane. Interestingly, chloride concentration at the sulfate rich region is remarkably low. It indicates dissolution of chemically ad-sorbed chloride as well as the loss of ability of adsorption of chloride in the pore structures with the presence of sulfate.

Corrosion of stainless steel bars in concrete was investigated using sound and pre-cracked concrete specimens. Three types of stainless steel were investigated, such as 18Cr, 18Cr-8Ni and 18Cr-12Ni-2.5Mo. Concrete specimens were exposed to two environments, where wetting and drying alternately repeated. One was in the outdoor with atmospheric temperature, and the other was in a controlled chamber, where the temperature was 60°C during wetting and 15°C during drying. The detail investigation was carried out after two years. No corrosion was observed on stainless steel bar in both sound and pre-cracked concrete exposed to outdoor. The maximum chloride ion concentration was 7.0 kg/m3 for 18Cr-8Ni, 8.0 kg/m3 for 18Cr-12Ni-2.5Mo and 6.0 kg/m3 for 18Cr at the crack region of concrete. This result indicated that the chloride ion threshold level for stainless steel was larger than these values under marine environment with atmospheric temperature. No corrosion was observed on both 18Cr-12M-2.Wo and 18Cr in both sound and pre-cracked concrete exposed to controlled chamber. However, corrosion was observed only for 18Cr-8Ni at the crack region, even when the chloride ion concentration at the crack region was 6.0 kg/m3. This was considered to be due to the effect of a high temperature.

Lignosulphonate is a widely used plasticizing admixture in concrete. It is well documented that different qualities of this material give different performance in concrete. Depending on what kind of concrete that is needed, workability can be controlled by adding different amounts or qualities of the lignosulphonate. This investigation compares the adsorption of lignosulphonate on three different portland cements, to the rheological properties of cement pastes made from the same cements. The adsorption isotherms were calculated from depletion experiments. A rheometer with bob-cup geometry was used to measure the rheological properties of the cement pastes. The plasticizing effect of lignosulphonates in cement paste slurries was confirmed. Recent advances have given a novel lignosulphonate with superplasticizer performance. This investigation demonstrates these improved properties achieved by this novel lignosulphonate by determining the differences in adsorption of the different lignosulphonates, on cements with different chemical characteristics.