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Title: Coupled Effects of High Temperature, Prolonged Mixing Time, and Chemical Admixtures on Rheology of Fresh Concrete

Author(s): Moncef Nehdi and Samer Al-Martini

Publication: Materials Journal

Volume: 106

Issue: 3

Appears on pages(s): 231-240

Keywords: high temperature; high-range water-reducing admixture; mixing time; rheology; viscosity; yield stress.

DOI: 10.14359/56547

Date: 5/1/2009

The rheological properties of fresh concrete incorporating various chemical admixtures were investigated as a function of the mixing time, temperature, and admixture dosage. Three chemical admixtures were used, namely, polycarboxylate-, melamine sulfonate-, and naphthalene sulfonate-based high-range water-reducing admixtures. Concrete mixtures were continuously agitated up to 110 minutes after mixing using a low shear rate concrete mixer under controlled temperature ranging from 22 to 45 °C (72 to 113 °F). The rheological testing was performed using a BML rheometer. The test results indicate that the Bingham constants of concrete are significantly affected by changes in temperature, mixing time, and admixture dosage. The yield stress was found to have an inverse logarithmic curvilinear correlation with the slump, whereas no correlation was found between plastic viscosity and slump. The relationship between the yield stress and plastic viscosity was investigated in an attempt to develop a rational means for the objective assessment of the rheology of concrete mixtures in hot weather. The findings of this study are of practical significance and provide realistic recommendations on the use of chemical admixtures in hot weather conditions. Such admixtures have been generally developed in areas with mild climates and, therefore, current technical information on their use is often unreliable when transposed to extreme hot weather conditions.