International Concrete Abstracts Portal

Inspection and service reports from concrete platforms up to 20 years old in the hostile environment of the North Sea are positive. The need for remedial work has been minimum and these high-quality concretes demonstrate excellent performance under marine conditions. To meet the demands for durability and development in structural design and construction methods, a continuous effort has been made to advance concrete materials. Improved concrete properties are predominantly a consequence of improved cement qualities, more efficient admixtures, and better controlled processing of aggregates. The soundness of the aggregates has been verified from the start. Examinations of concrete specimens drilled out from different elevations of some of the platforms have revealed rather high concentrations of chlorides close to the concrete surface. For most of the specimens, the chloride content is, however, negligible at the reinforcement. Epoxy coating applied to the concrete surface in the splash zone of some platforms has shown to be efficient in preventing ingress of chlorides.

Despite the excellent resistance of high-performance (HP) concretes in the presence of aggressive agents, instances of application have shown that the microstructure of the concrete surface can be greatly disturbed by the curing method, thereby compromising durability on the part covering the reinforcement. Paper presents results of a study on three concrete design mixes (one reference concrete and two HP concretes with and without silica fume), each subjected to three curing methods and three durability tests. Results on carbonation, variation in free lime, and microcracking indicate that HP concrete with silica fume is more sensitive to the curing method than the reference concrete or concrete without silica fume, as evidenced by increased carbonation and a larger reduction in alkalinity. The study of microcracking in the various concretes showed that desiccation causes more microcracking in the HP concrete with silica fume than in the HP concrete without silica fume. Results of microstructural inspection and physical and chemical tests explain these variations in mechanical properties and carbonation behavior of various concretes, depending on the curing method.

Investigates the effect of high temperatures (70 to 600 C) on the residual strength of ordinary and high-performance concretes made with the same cement and aggregates. Measurements of weight losses and residual strengths were carried out. Between 25 and 600 C, the mass loss was about 8 percent of the wet concrete weight. The tests showed that after being exposed to a temperature of 600 C and then cooled, the concrete retains 38 to 46 percent of its initial compressive strength. Experimental results indicated further that mercury porosimetry measurements were suitable for obtaining information about microstructural changes resulting from thermal exposure. The distribution function of the pore system indicated that no remarkable changes had taken place in its shape and location up to 120 C. The residual porosity increased with temperature, particularly after 300 C, and the pore size distribution was significantly modified. Approximately one-third of the high-strength concrete samples failed through explosion at about 300 C. With the results obtained, the authors were able to analyze the phenomenological aspects susceptible to explain the observed behavior. This behavior might be caused by a tension in the solid microstructure produced by thermal stresses and by the pore vapor pressure.

A comprehensive study was undertaken to determine the effect of sulfates and freezing and thawing on the durability of high-strength concrete with silica fume and lignite fly ash. The concrete mixes contained normal CSA Type 10 (ASTM Type I) portland cement, 10 percent silica fume, and different amounts of fly ash from 0 to 80 percent of the binder weight in the mix. The aggregate-binder ratio by weight was maintained at 5 and the superplasticizer weight was varied between 1.5 and 2.2 percent of the binder, while the water-binder ratio was maintained at 0.27. Results of the freeze-thaw tests, as well as those of the sulfate tests (using Na 2So 4), were encouraging. Concrete with either 20 or 35 percent fly ash replacement along with 10 percent silica fume gave satisfactory resistance to frost action, while concrete with up to 50 percent fly ash and 10 percent silica fume considerably suppressed the sulfate attack. Concrete strength at 28 days with up to 60 percent fly ash and 10 percent silica fume was equal to or greater than that of control concrete of 100 percent Type 10 cement. A study of matrix morphology and microstructure bonding using a scanning electron microscope showed that fly ash + silica fume concrete had a denser paste microstructure (compared to the control), and this played an important part in enhancing compressive strengths and resistance to frost and sulfates. However, increasing fly ash content beyond 50 percent weakened the matrix bonding due to the presence of too many unreacted fly ash particles, which adversely affected the compressive strengths and resistance to frost and sulfates.

Corrosion characteristics of steel embedded in hardened cement pastes and mortars were investigated by using data obtained from potentiodynamic anodic polarization and polarization resistance techniques. One normal portland cement and one fly ash were used. The dosages of fly ash as cement replacement material were 0, 20, 40, and 60 percent. The results indicate that there was no negative effect of pozzolanic reaction of fly ash on steel passivation, even at a high replacement dosage of 60 percent, and after 2 years of curing. In fact, with prolonged curing, steel embedded in fly ash-blended cement pastes was found to have a higher degree of passivation with greater stability than that embedded in plain cement paste. Chloride binding capacity of 40 percent fly ash-blended cement paste, as indicated by the measured corrosion rate of steel, was found to be very effective after 3 days of curing. In accelerated carbonation condition, corrosion rates of steel were initially high in fly ash-blended cement mortars. When fully carbonated, the results indicate that the corrosion rate of steel can be higher in plain cement in comparison to 40 percent fly ash blend. In the case of chloride penetration, the corrosion rate of steel was found to be consistently less than that of equivalent plain cement when compared on an equal water-binder ratio.