International Concrete Abstracts Portal

Tests were carried out on a series of concrete mixes, designed to equal workability and 28 day compressive strength and with a range of pulverized fuel ash (pfa) levels, to study the effect of curing on the strength and permeability of pfa concrete. Concrete specimens were subjected to a range of moist-curing periods prior to air storage. Compressive strength was determined at various ages and permeability to oxygen and water was determined at 28 days. Results confirm the importance of curing, with reductions in the curing period resulting in lower strength, more permeable concrete. The strength of the pfa concretes appears to be more sensitive to poor curing than ordinary portland cement (opc) concrete, the sensitivity increasing with increasing pfa content. However, despite exhibiting lower strengths, pfa concretes moist-cured for only one day were, generally, no more permeable to water and substantially less permeable to oxygen than similarly cured opc concretes. As the period of curing increased, the pfa concretes became considerably more impermeable to water and oxygen than the opc concretes. These results are discussed in the context of the minimum periods of curing and protection recommended in BS 8110. It is argued that although the increased curing periods suggested for pfa concrete are justified on the basis of concrete strength, pfa concrete may require no more curing than opc concrete to achieve equal durability, as measured by oxygen and water permeability.

Longitudinal thermal strains of high-strength, air-entrained, lightweight concrete containing silica fume were monitored during a single frost cycle between 15 and -157 C to measure the coefficients of thermal length changes at different temperatures. The immunity of such concrete to repeated exposures to liquified petroleum and natural gas temperatures was also evaluated. Scanning electron microscopy was employed to observe the deterioration of concrete prior to and after five frost cycles to -73 C. The coefficients of thermal length changes of air-dried and water-saturated concretes were calculated to be 3.2 and 4.1 x 10-6 cm/cm/C, respectively, before freezing, and 2.9 and 3.0 x 10-6 cm/cm/C at post-freezing temperatures. The strength deterioration study indicated that dried concrete cycled to -40 C can experience maximum drops in compressive and splitting tensile strengths of 7 and 3.5 percent, respectively. These reductions can be expected to increase to 15 percent if the extreme cooling temperature is lowered to -73 C. Water-saturated concrete can lose 12 to 17 percent of its initial strength after five freeze-thaw cycles to either temperature range.

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.

Purpose was to throw light on the behavior of ultra-fine admixtures introduced with superplasticizer in high-strength concretes as partial portland cement replacement. Paper describes results from different pastes and mortars prepared with normal portland cement including 10 percent silica fume or alumina powder as replacement for cement. Conclusions are as follow. 1) Water demand is modified only by CSF. When associated with superplasticizer, all admixtures lower the water demand. No relationship was found between fineness and water-reducing effect. 2) Superplasticizer acts as a set-retarder and the admixtures as accelerators, but they cannot compensate for the superplasticizer's retardation except for one of the silica fumes used. 3) Two silica fumes showed an important pozzolanic effect; another silica fume and the alumina powder were inert. 4) All admixtures in the presence of superplasticizer increase the compressive strength of mortars. This is due not to the pozzolanic reaction, but is explained by the lowering of w/c.