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Development, field testing, and implementation of improved bridge parapet designs.
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2015-03-01
[PDF-3.29 MB]
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Abstract:A total of 22 test parapets was constructed as part of this project to evaluate different approaches to address
premature cracking. The experiments included the use of deeper saw cuts through glass fiber reinforced polymer
(GFRP) reinforcement or field cut steel reinforcement, with the saw cut depth increased from 1 ½ inches (38 mm)
to 3 ½ inches (89 mm). Another experimental modification was the use of polypropylene fibers in the mixture at
two different dosage rates, 1 lb/yd3 (0.59 kg/m3) and 2 lb/yd3 (1.19 kg/m3). The research team monitored bridge
parapets during and immediately after construction. This included instrumentation with embedded maturity
sensors to capture temperature history of the parapets, using Command Center technology. Researchers also
observed and documented parapet construction. It was important to note any constructability issues that may
arise that may be related to the proposed solutions, since they would potentially impact the cost of
implementation. Crack surveys were performed once formwork was removed and sawcutting had been
completed. The stiffness gain of parapets and joint cracking were monitored using ultrasonic pulse velocity (UPV)
technology. While all of the eight control bridges showed mid-panel cracking, only four of the twelve experimental
bridges had mid-panel cracking. Both parapets with Vandal Protection Fence (VPF) posts showed cracks near the
posts. The main purpose of using GFRP or field cut steel reinforcement with the 3.5 inch (89 mm) deep saw cut is
to create a weakened plane at the control joint through the gap provided in the reinforcement, in order initiate
cracking at the joints and reduce the risk of cracking elsewhere on the parapet. From the data gathered, this
modification allowed more joints to crack at early stages, and prevented uncontrolled cracking. The parapets with
fibers showed no measurable improvement over the parapets without fibers in regards to preventing shrinkage
and temperature cracking. Reducing the joint spacing over negative tension areas was found to reduce
uncontrolled bridge parapet cracking. However, since the experimental parapets with the reduced joint spacing
were coupled with another modification of either the polypropylene fibers or the deeper saw cut, it was hard to
determine if this alone would solve uncontrolled bridge parapet cracking.
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