Becco Contractors Inc. had approximately 26,000 feet (7925 m) of barrier wall to slipform on U.S. Highway 169 in Tulsa, Oklahoma. Approximately 12,000 feet (3658 m) of that wall was variable, with height differences ranging from 42 to 60 inches (1067 to 1524 mm) on the outside wall. The company has slipformed almost one-half million feet (152,400 m) of barrier wall with their GOMACO machines on different projects around the state, but this was their first time slipforming variable barrier.
The profiles of wall on the project included a 42 inch (1067 mm) tall safety barrier with continuous #5 rebar fed through the front of the barrier mold. The second profile was a variable height safety barrier slipformed over a continuous cage of steel reinforcing. Both walls were nine inches (229 mm) wide across the top and 24 inches (610 mm) wide on the bottom.
The wall was slipformed on a new asphalt surface, within tight working conditions, and surrounded by live traffic. Setting stringline and keeping it in place proved to be difficult. Their solution was to create their own "stringless" system for their GOMACO Commander III utilizing three independent grade averaging ski systems. According to Becco Contractors, this is how their unique "stringless" system worked.
"Two skis were used to operate the mold. They were the length of the mold and attached directly to the left and right sides of the mold framework. They averaged the exact profile of the asphalt grade, specifically in and out of a superelevation and maintaining those elevations. The third averaging ski was built in two parts, with one attached to the left, rear leg barrel extending 10 feet (3 m) back. The other attached to the left, front leg barrel and extended 10 feet (3 m) forward.
"They were connected with a common stringline and averaged a 50 foot (15.2 m) length. The left-side machine control sensors traced this line, which controlled the top of the wall. The frame-mounted cross slope sensor kept the top of the wall level. The steering sensors traced off square tubing that was laid on grade and aligned with a pre-painted control line."
Their "stringless" innovation worked well. Barrier slipforming production averaged between 900 to 1200 feet (274 to 366 m) per day, depending on wall design. Their largest production day reached 3300 feet (1006 m).
The concrete was a fly ash mix with standard 57 stone. Slump averaged between 0.25 to 0.5 inches (6 to 13 mm). Ready-mix trucks carrying 10 cubic yard (7.6 m3) loads supplied the concrete to the Commander III. On average, the trucks emptied their load in 12 minutes onto the Commander III's conveyor belt.
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Becco slipformed 12,000 feet (3658 m) of single-sided variable barrier wall as part of a project on Highway 169 in Tulsa.
Becco Contractors averaged between 900 to 1200 feet (274 to 366 m) of barrier production per day. Their best day's production reached 3300 feet (1006 m).
Grade skis on the left side of the mold control the 42 to 60 inch (1067 to 1524 m) height variation for the single-sided variable barrier wall.
Becco Contractors equipped their Commander III with grade averaging skis to control the paver and the height and level of the barrier wall.