Effect of Admixture Combination on Top-Bar Effect of Highly Flowable and Self-Consolidating Concrete Mixtures

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Title: Effect of Admixture Combination on Top-Bar Effect of Highly Flowable and Self-Consolidating Concrete Mixtures

Author(s): K.H. Khayat, E.K. Attiogbe, and H.T. See

Publication: Special Publication

Volume: 247

Issue:

Appears on pages(s): 33-44

Keywords: bond; reinforcing bars; self-consolidating concrete; top-bar effect; viscosity-modifying admixture

Date: 9/1/2007

Abstract:
This paper summarizes a testing program undertaken to evaluate the uniformity of bond strength between concrete and reinforcing bars positioned at various depths of experimental wall elements. In total, four self-consolidating concrete (SCC) mixtures and three conventional flowable mixtures were prepared with different combinations of viscosity-modifying admixtures and high-range water reducers. The concrete mixtures were used to cast experimental wall elements measuring 1.54 m in height, 1.1 m in length, and 0.2 m in width. Two of the walls were steam-cured, while the remaining four elements were air-cured. Each wall had 16 reinforcing bars, four per row positioned at four levels, that were subjected to pullout tests at 1 and 28 days of age. The concrete mixtures were prepared with Type III cement, 20% Class F fly ash substitution, and a low w/cm of 0.37, which is typical of structural precast concrete construction. The targeted 1-day compressive strength was 40 MPa. Uniform distribution of in-situ compressive strength and adequate bond to the reinforcing bars were obtained with relatively small variations along the experimental wall elements. The 1- and 28-day top-bar effect ratios varied between 1 and 1.4 for the majority of the test results. These values were lower for the air-cured mixtures compared to the steam-cured mixtures. The top-bar effect is shown to be sensitive to the type of VMA used in the SCC.