EFFECT OF SUPERPLASTICIZER DOSAGE ON MECHANICAL-PROPERTIES, PERMEABILITY, AND FREEZE-THAW DURABILITY OF HIGH-STRENGTH CONCRETES WITH AND WITHOUT SILICA FUME

This paper presents the results of a study on the effects of slump and super plasticizer dosage on the compressive and flexural strength, pe rmeability, and freezing and thawing durability of typical high-streng th concretes. Two types of cement were used: a Canadian Type 10 portla nd cement (ASTM Type I) and a Canadian blended cement containing 7.5 /- 0.5 percent silica fume. A water-to-binder ratio of 0.30 was select ed to produce air-entrained and non-air-entrained concretes having a c ompressive strength ranging between 60 and 100 MPa. The superplasticiz er dosages (ranging from 0.7 to 1.6 percent, expressed as dry mass of superplasticizer per mass of cement or blend) were selected to produce concretes having slumps ranging from 70 to 240 mm. Some physical and mechanical properties of high-strength concretes made with Type 10 por tland cement were found to be significantly reduced when the superplas ticizer dosage approximately, exceeded the saturation concentration of the cement-superplasticizer combination. Relatively high dosages of s uperplasticizer (> 1.1 percent; slump > 200 mm) can reduce compressive strength (1-, 28-, and 91-day) and flexural strength (28-day) by 15 t o 50 percent, and Increase all permeability by a factor of 3 to 5. The effect of such such relatively high dosages of superplasticizer on th e frost resistance (ASTM C 666, Procedure A) of concretes made with Ty pe 10 cement appears to be of less importance, since the durability of all concretes containing at least a small volume of entrained air was not influenced by slump or superplasticizer content. Compressive stre ngth, flexural strength, air permeability, and rapid chloride permeabi lity of high-strength concretes made with silica fume-blended cement ( HSF) were nor affected by the superplasticizer dosage. Silica fume app ears to be very effective in preventing strength losses and permeabili ty increases in high-slump, high-strength concretes containing high do sages of superplasticizer.