International Concrete Abstracts Portal

The effect of different curing conditions on the long-term compressive strength of concrete containing silica fume and fly ash was investigated. The project was carried out as a parallel test program at three different laboratories for a period of two years. Two mixes were chosen: one blended cement (MP30) containing 25 percent fly ash, and the other containing 25 percent fly ash and 10 percent silica fume. The cubes were exposed to six different curing conditions at different temperatures (20 to 70 C) and humidities (about 30 to 100 percent relative humidity). In one of the series, the cubes were allowed to dry on one side only. The compressive strength for both the cements containing fly ash and silica fume were sensitive to the early drying procedure. Good initial curing conditions in the first three days of curing gave an improvement in compressive strength compared to the samples dried immediately after demolding.

Studies were done on the physical, mechanical, and geochemical properties of candidate cementitious materials for sealing a geologic nuclear waste repository in a tuff host rock environment. One sanded cementitious grout contains substantial replacement of cement by low-calcium fly ash and silica fume, yet maintains a water-cementitious solid ratio of 0.32 for a fluid grout. The ash and fume were used to achieve a higher SiO2 and Al2O3 content more compatible with the tuff geochemistry than a plain portland cement. One possible application for such materials is fracture sealing near waste canister emplacement holes. The effects of temperatures from 15 to 300 C on the material properties were investigated. Initial compressive strengths of materials cured at 38 C for 7 to 900 days ranged from 100 to 125 MPa. Other properties investigated include bond strength (to tuff), water permeability, interfacial permeability, Young's modulus, density, porosity, expansive stress, and phase changes. Samples heated to 150 C for extended periods (28 days), either dry or hydrothermally, maintained their strength and well-bonded microstructure, while the results of heating at 300 C were mixed, with some strengths remaining high (95 to 110 MPa) and others diminishing (44 to 51 MPa). The water permeability did not increase much at 150 C but did increase at 300 C.

Anti-washout concrete has been proposed for recent construction projects such as foundation work for long-span bridges. For such applications, thermal cracking of concrete is one of the major concerns. For conventional underwater concreting, rich mixes are required to insure the quality of the concrete. With the intent of developing a low-heat type underwater concrete, high blast furnace slag content was used as replacement of cement to reduce the heat of hydration of cement. The characteristics of fresh concrete, adiabatic temperature rise, and compressive strength of the anti-washout concrete with slag were examined. The major variables in the experiments were slag content, fineness of slag, type of cement, etc. Adiabatic temperature rise of anti-washout concrete with a slag content of 50 to 80 percent was higher than that without slag. However, when the slag content was more than 80 percent, significant decrease in adiabatic temperature rise was observed. Compressive strength with up to 90 percent of slag content was higher than that without slag. From these results, it was proposed that a mix proportion of concrete with a slag content of 90 percent and fineness of slag 4000 cmý/g would be suitable for such underwater massive concrete.

Fly ashes having maximum particle diameters of 20, 10, and 5 æ, called classified fly ash (CFA), were investigated for their effect on concrete properties. The CFA-concrete containing 15 to 30 percent CFA by cement weight requires less water content per unit volume. Greater strength and watertightness, lower drying shrinkage, and higher resistance to alkali silica reaction are imparted to concrete by the addition of CFA. Grading or type of fly ash determines its effect on concrete.

The use of supplementary cementing materials for reducing harmful alkali-aggregate reactions (AAR) in concrete is being studied at the Canada Centre for Mineral and Energy Technology (CANMET). One investigation involves the use of three types of reactive aggregate and supplementary cementing materials that include fly ash, slag, silica fume, and natural pozzolans. This report covers the part in which ground granulated blast furnace slags (one from the U.S. and two from Canadian sources) were used to partially replace cement in concrete containing the three reactive aggregates. Test data include characterization of the materials used, their proportions in mixtures, concrete strengths, and 2-year expansion measurements of mortar bars and concrete prisms containing these materials. Test results show the effectiveness of the slags in reducing deleterious AAR and their optimum replacement levels. These slags are all effective in controlling such reactions, particularly with the highly reactive Kingston dolostone.