International Concrete Abstracts Portal

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.

This paper presents the effect of mixture composition on the compressive strength of fly ash-based geopolymer concrete. Test results show that water-to-sodium oxide (H2O-to-Na2O) molar ratio and the water-to-geopolymer solids ratio by mass influence the compressive strength of fly ash-based geopolymer concrete. The compressive strength decreases when these ratios increase. However, the sodium oxide-to-silicon oxide (Na2O-to-SiO2) molar ratio of the geopolymer mixture does not have any significant effect on the compressive strength within the range of 0.095 and of 0.120 of this ratio.

A long-term research study on anti-corrosion products for reinforced concrete exposed to aggressive environmental conditions was initiated in 1990. The performance of products was evaluated through accelerated laboratory testing and natural site expo-sure conditions as tidal zone, above ground and below ground. An exposure site on the Dubai creek shore is designated for long term performance testing at different ages ex-tending up to ten years. A series of physical and electrochemical testing were performed in three phases. The prime objective of the first phase was to assess the performance of the various products, and to assess the practical value of different electrochemical test methods. The focus of the second phase testing shifted towards a more comprehensive evaluation of the test methods. The interim results have been presented at different international conferences. The focus of third phase, which was performed in early 2000, was to observe the actual extent of corrosion sustained by the rebar. This paper presents the final data to substantiate conclusions relating to ingress of chlorides for the various exposure conditions (threshold values), provides recommendations for corrosion monitoring for new structures and test methods for evaluating products and future research requirements.