More than 10 miles of permanent concrete barrier wall was slipformed in place on Massachusetts Route 3 as part of the $385-million widening of the busy road linking Burlington, MA with Nashua, NH.

New England Specialty Concrete, Inc. (NESC) of Salisbury, MA installed the barrier under subcontract to Modern Continental Construction (MCC) of Cambridge, MA.

The barrier used three curb and gutter slipforming machines manufactured by Power Curbers, Inc. Two Model 8700s and a Model 5700 were used to slipform the barrier that varied in height from 32 inches to 54 inches.

The project was originally designed with stainless steel/epoxy rebar. Key personnel from Modern Continental called Pat Maillett of HarMac Rebar & Steel and asked if HarMac could help supply the product. Pat considered the amount of steel being tied - over 55,000 linear feet - and immediately thought of weldable and the time savings possible. However, currently there is no way to weld stainless steel/epoxy rebar. HarMac’s in-house application specialists convinced the Massachusetts Highway Department that galvanized rebar would meet or exceed the specifications currently outlined.

The barrier wall cages were produced in 20’ lengths with tolerances of +0/-1/4” to meet the requirements of the slipform machines. The cages were then transported to a nearby galvanizer and processed. The galvanized cages were delivered to the job site as needed.

An engineer from Power Curbers, Inc. told Pat Maillett that the tight tolerances and rigidity of the cages allowed the slipform machines to continuously produce. There was no “twisting” of the cages and therefore the machine did not jam up and have to stop for cage adjustment. The engineer estimated that productivity of the slipform machines increased 20-30% with the use of the welded cage. The cages used 600 tons of #706 weldable, galvanized steel.

NESC employs up to 10 workers on a slipforming crew, including the machine operator and two concrete finishers, one to a side. An estimated 10,600 cubic yards of ready mix was required for this project. The ready mix has a low water/cement ratio, a slump of between ¾ inch and 1 inch and a 28 day compressive strength of 4,000 psi.

The Power Curber machines are computer controlled and guided by a set of stringlines, so tight tolerances on the pre-fabricated cages were essential. Transit mix trucks deposit concrete onto a conveyor mounted on the tracked machine whose slipformer rides over the preset reinforcing cage. Typically, a separate crew of between five and 10 men is responsible for setting up the rebar cages, this was also reduced. Thanks to the rigidity of the welded cages, less manpower was needed to transport and place the cages. Crews were able to slipform between 700 linear feet and 1,000 linear feet a day depending on the height. At the end of each day, workers used a concrete saw to produce a 2 inch deep sawcut on each side of the barrier every 20 feet.

For more information on how HarMac can increase your productivity call us at 207-935-3531.