International Concrete Abstracts Portal

Presents results regarding the effects of various temperature and humidity environments on the compressive strength of concretes containing blast furnace slag cement (BFSC) and ordinary portland cement (OPC). Three types of blended cements containing 4.5, 35, and 68 percent slag weight replacements of portland cement were used. The specimens were stored in locations with controlled environments, such as 35 C (95 percent relative humidity), standard ambient temperature 23 C (lime water, sealed with polypropylene bag, 100 percent relative humidity fog room, and 50 percent relative humidity drying room), and 10 C. Test results indicate that the temperature effect on the initial rate of strength development of BFSC concrete is more sensitive than that of OPC concrete; high temperature accelerates the strength gain and low temperature suppresses the initial strength increase of BFSC concrete. Higher ultimate strength was achieved for the 4.5 and 35 percent BFSC well-cured concretes as compared to OPC concrete. However, the inadequate supply of reactive materials resulted in lower compressive strength for the 68 percent BFSC concrete. Under dry conditions, concrete with high slag content stopped its strength development as excess loss of moisture hindered the hydration process of cement. Strength degradation was also found for high slag content BFSC concrete.

Durability properties of concrete and mortar based on a special type of alkali-activated slag called F-cement have been studied. The microstructure was found to possess a high occurrence of microcracks that had an obvious influence on the flexural strength and rate of carbonation. The rate of chloride-ion diffusion was about 30 times lower than in the portland cement concrete. Salt scaling was found to depend solely on the water-to-binder ratio and is independent of the air content. Early freezing takes place when the strength exceeds 5 MPa, and F-mortar shows high chemical resistance against solutions of sodium, calcium, and magnesium chloride.

Presents results of investigation to determine the combined effect of deicing salts and repeated cycles of freezing and thawing on condensed silica fume concrete. The concrete mixtures tested in this phase included mixtures incorporating silica fume as an 8 percent replacement by mass for cement, along with control mixtures (no silica fume), both covering a range of water-to-cementitious materials ratio of 0.40 to 0.65. All mixtures were air entrained and some contained a superplasticizer. For each mixture, freeze-thaw resistance and scaling resistance to deicing salts were determined using ASTM standard procedures. For some selected mixtures, scaling resistance was also determined using slight variations in the testing procedures. In general, concrete incorporating silica fume is slightly more susceptible to scaling than concrete without silica fume. Preliminary results clearly indicate that the methods of preparing and curing the test specimens has a significant influence on the scaling resistance of the concrete, but further investigations are needed to establish possible correlations between the degree of scaling, type of curing, method of specimen preparation, and percentage of silica fume in the concrete.

ASTM C 672 scaling tests were carried out on normal concretes and concretes containing 5 percent silica fume, with air-void spacing factors in the 100 to 200 æm range. Five curing methods were compared: a 24 hr heat cycle with a maximum temperature of 70 C, 2 and 14 days moist curing, and two different curing compounds. Results indicate that the use of silica fume does not improve the scaling resistance of concrete. Concretes cured with one particular curing compound were found to have a scaling resistance similar to that of those cured in water for 14 days, weight losses after 50 cycles being lower than 1 kg/mý for all mixes. Concretes cured with the other curing compound had a lower and more variable scaling resistance. As expected, specimens cured for only two days in water also had a lower scaling resistance. All mixes cured using the heat cycle exhibited a poor performance, although, in this case only, silica fume reduced very significantly, but not sufficiently, the damage due to the freeze-thaw cycles in the presence of deicing salts.

Effects of alkalies in Class C fly ash on Alkali-aggregate reaction were studied by using two cements, a type I high-alkali cement and a type II low-Alkali cement, and three Class C fly ashes. Mortar bar expansion was measured according to ASTM C 441. Reaction products of alkali-aggregate reaction were examined n by XRD, SEM, and EDAX. were to study: The purposes of this research (1) the significance of the standard mortar bar test in determining the degree to which high and low-alkali cement could be replaced by Class C fly ashes, and (2) effect of fly ash alkali contents on alkali reactivity. Expansion of mortar bars prepared using high-alkali cement increased at low replacement levels but decreased at high replacement levels for curing periods up to 12 weeks at 38 C; whereas expansion of mortars prepared using low-alkali cement increased at all Levels of fly ash replacements up to 40% by volume. A critical equivalent Na20/Si02 mole ratio was identified and found to characterize alkali reactivity. No crys-talline reaction products could be identified by XRD. Results of SEM and EDAX showed that the reaction product was an alkali-silicate gel, composed mainly of silica, sodium, potassium, and calcium, with their relative amounts varying within the gel.