Methods of Preparing Horizontal Construction Joints in Mass Concrete. Report 2 - Minimizing Laitance

摘要

This report presents the results of a research program examining the effects of different methods of preparing horizontal construction joints in mass concrete construction. The purpose of the research program was to confirm existing guidance or, if necessary, update it. The joint moisture conditions at the time of concrete placement were: (a) continuously moist, (b) dry, and (c) dry and then remoistened. There was no cleaning of the joint surface prior to placement of a second lift. The concrete mixture used in the first lift was proportioned to minimize bleeding and the resulting laitance. Jointed specimens were tested for direct tensile strength and shear strength. The results indicated that an addition of a small quantity of silica fume does decrease bleeding and reduce loose, flaky laitance. However, a smooth, somewhat glassy surface appears to interfere with the bonding of the next lift of concrete. Bond strengths are not as good as those when the horizontal construction joints are cleaned by high-pressure water cutting or by air-water cutting, and then allowed to dry approximately 24 hr immediately prior to placement of the next lift of concrete. This technique is not recommended so current guidance concerning joint cleaning procedures should be followed.