Concrete Products

MAR 2016

Concrete Products covers the issues that attract producers of ready mixed and manufactured concrete focusing on equipment and material technology, market development and management topics.

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Page 58 of 75 March 2016 • 57 TECHNICAL TALK 2016 TRB MEETING area of 0.24 in. per sq. ft.," the authors write. "Two 0.25 in.-diameter longitudinal bars (lacer bars) were added to the overlapping mesh in order to develop the joint reinforcement." According to AASHTO, two longitudinal wires spaced at least 2 inches apart are sufficient to develop the deformed wire in the trans- verse direction. The longitudinal joint was then grouted the entire length of the girder using non-shrink grout with a specified min- imum compressive strength of 4,500 psi. Longitudinal joint con- struction required temporary plywood at the bottom while grout was being placed. The proposed specimen was tested with and without an option to restrain the rotation of the girders relative to one another. The restraint was accomplished by means of a diaphragm assembly placed between interior stems of adjacent girders. The assembly con- sisted of a 6- x 12-in. concrete cylinder strut to restrain the closing of the gap at the bottom of the 18 stem, and a 3/4-in.-diameter tie bolt to restrain widening of the gap between the stems. The cylin- der ends were chamfered to allow for a snug fit between the stems. A 1-in.-diameter PVC sleeve at the center of the concrete cylinder allowed for placement of the tie bolt through the cylinder. Galva- nized steel sleeves in the stems allowed for passage of the steel bolts. The diaphragm assemblies were placed at 5 feet on center along the length of the girder. "The proposed continuous joint with overlapping steel mesh reinforcement in a grouted shear keyway exhibited substantial- ly improved serviceability and strength performance characteris- tics over the conventional grouted joint with discrete welded con- nections," write Wehbe, Konrad and Breyfogle. "The proposed joint mitigated water leakage through the joint. Water leakage through the conventional joint started at 19,500 and 15,000 load cycles for Fatigue II and Fatigue I loads, respectively, whereas the proposed joint remained water tight under 800,000 cycles of combined Fatigue I and Fatigue II loading." The proposed joint almost eliminated stiffness degradation due to fatigue. Under fatigue loading, the conventional joint deteriorated rapidly resulting in significant stiffness degradation, while the pro- posed joint remained essentially intact and had negligible effect on stiffness. For Fatigue II loading, the stiffness degradation rate of the conventional specimen was 26 times that of the proposed specimen. "The proposed joint enhanced the continuity between adjacent girders," Wehbe, Konrad and Breyfogle conclude. "The enhanced con- tinuity was manifested by higher flexural strength, increased load transfer across the joint, and more uniform distribution of reaction forces at the girder stems. The flexural capacity of the proposed spec- imens was more than 1.5 times that of the conventional specimen." Continued on page 58 Proposed specimen joint details for double tee girders.

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