International Concrete Abstracts Portal

The decrease in relative humidity during hydration and the chemical shrinkage have been measured for different cement paste compositions. The amount of nonevaporable water per degree of hydration as found by NMR, pore size distribution by mercury intrusion, and total porosity to water have also been determined. The cement pastes were made form portland cement with 0, 8, and 16 percent condensed silica fume, with w/c + s of 0.20, 0.30, and 0.40. The relative humidity (RH) was found to decrease rapidly during the first 2 weeks and reach about 78 percent RH after more than a year for the lowest w/c + s, independent of the CSF dosage. The highest ratio gave about 87 percent RH. The nonevaporable water per degree of hydration depends on the NMR-based estimate of the degree of cement hydration, but it is most consistent (i.e., independent of w/c + s and CSF dosage) when it is assumed that the CSF dosage does not consume any water. The water porosity was found to increase with increasing CSF dosage, while the mercury intrusion results showed both a finer pore structure and smaller total porosity with increasing CSF dosage. Mercury intrusion into miniconcretes (dmax = 8 mm) with the same binders gave a much coarser pore size distribution, indicating that the paste-aggregate interface region is more open than the bulk paste. No evidence was found that increased CSF dosage improved the interface pore structure. This is in contrast to other evidence in the literature, and may be caused by partial dehydration and/or microcrack formation during the drying at 105 C.

Condensed silica fume (CSF) greatly influences not only the mechanical but also the physical properties of concrete. The most striking effect is the reduced permeability, caused by a change in the pore structure. Another sign of this alteration, though not as evident, is the change in the form of the water vapor isotherm. Preliminary results from an investigation concerning the first desorption isotherms of mortar with CSF-cement ratio varying between 0 and 25 percent and a water-cement ratio varying from 0.3 to 0.6 are presented. The results show that CSF influences the pore size distribution not only in the mesopore range, as shown in earlier studies, but also in the micropore range. The drying courses were also recorded in the project and it is clear that CSF significantly prolongs the time in reaching equilibrium, especially in relative humidities below 80 percent. This indicates that the continuous pore system is much narrower when CSF is incorporated. The question of when the "true" equilibrium is attained is discussed.

Results of an investigation of cement paste structure, and strength, permeability, and frost resistance of concrete with admixtures of silica fume type are given. The admixtures are waste materials from metallic silicon, low-grade ferrosilicon, ferrosilicon chrome production, containing SiO2 in the amount of 92, 70, and 66 percent, and surface area of 25.0, 44.9, and 18.5 mý/g, respectively. The influence of the admixtures on the cement paste microstructure results in an increase of gel porosity volume, decrease of capillarity porosity, and in an increase of strength. Thus, concrete strength increases and its permeability decreases. Physical and chemical properties of the silica fume-type admixtures insignificantly affect gel pore volume, whereas they have significant influence on capillary porosity. An increased dosage of high-range water-reducing admixture (HRWR) is a beneficent factor in increasing hydration degree and gel porosity, decreasing capillary porosity volume, and, consequently, increasing strength. Concrete frost resistance with silica fume dosages up to 10 percent of cement mass is not lower than the reference concrete with the same amount of binder.

Presents results of investigations carried out on high fly ash content concrete (HFCC) cores removed from several structures constructed in the U.K. since 1979. Structures investigated included a road pavement, a major road viaduct, water-retaining and industrial structures, and a slipway subjected to marine exposure. Concrete properties measured after 10 years of service include compressive strength, depth of carbonation, permeability, and chloride and sulfate penetration profiles. In addition, petrographic analysis of thin sections was also undertaken. The HFCCs studied were designed considering the fly ash to be just a further ingredient in the concrete rather than as a cement replacement. This led to higher fly ash contents and lower cement contents than is generally normal practice. The structures examined were in excellent condition after 10 years. Results show a durable concrete exhibiting increases in compressive strength beyond 28 days, little evidence of carbonation, low to average permeability, and resistance to chloride penetration. In this respect, it is significant that at the marine exposure sites, the chloride concentrations decreased significantly with depth. No evidence of alkali-silica reaction was detected in spite of reactive aggregates being present in some of the concretes.

The results of a great number of trial mixes for mix design of roller compacted concrete (RCC) are presented. This particular RCC used a local fly ash, rich in lime and sulfates, which does not meet any existing specification. This fly ash's performance in concrete has been studied for some time at the Laboratory of Reinforced Concrete of Aristotle University of Thessaloniki. Recently, this fly ash was used in the construction of a large RCC dam in northern Greece. Measurements of the strength development and the elasticity of RCC mixes showed that the 80 percent (by weight) of the cementitious material could be substituted for this fly ash. Therefore, it was proven that in RCC mixes this fly ash is more effective than in conventional concrete.