Being the second largest state in the country, as well as the second most populous state, Texans have a "do it up big" approach to life, but there are times when a finer approach is needed. That was the case this past summer when TexOp Construction opted to use a fine-toothed drum mounted on a Wirtgen W 2000 cold milling machine to precisely prepare the surface of the President George Bush Turnpike for a new thin overlay.
The Turnpike is a major east-west route in the northern half of the Dallas Metroplex, and is a 30.5-mile, six-lane, limited access toll highway which passes through or along the cities of Garland, Richardson, Plano, Dallas, Carrollton, Farmers Branch and Irving.
Near Carrollton, TX, the project owner — the North Texas Tollway Authority (NTTA), planned to overlay a section of the expressway with NovaChip high-performance, proprietary open-graded friction course. But such thin lifts — with maximum 3/8-in. stone — require extremely well-prepared and even surfaces for placement. Any irregularity in the existing pavement will reflect through the thin surfacing immediately. Conventional surface prep for worn pavements previously involved diamond grinding the pavement to precise tolerances, involving specialized mobile equipment, excessive lane closures and long project times.
But cold milling subcontractor TexOp Construction, LP, of Roanoke, TX, and Wirtgen America Inc. had a different idea. In conjunction with its general contractor APAC, TexOp proposed using a W 2000 with a 6.56-foot fine-texture drum to prepare and level the aged asphalt pavement surface prior to the thin surfacing.
The Wirtgen fine-toothed drum removed all imperfections and bumps, and leveled the driving surface side-to-side, so a NovaChip surface could be placed using a Vögele 1800 SP paver. The NovaChip paving process places an ultra-thin, coarse aggregate hot mix over a special asphalt membrane applied in one pass over the existing pavement surface.
"On this particular job, fine-texturing with a cold mill saved the authority as much as $500,000 over diamond grinding, and the most critical thing is that the diamond grinders were asking for a minimum 90 days, and the contractor was on track to finish the same area in 30 to 40 days," says Britt Barnes, regional sales manager, Wirtgen America Inc. "My early calculations said we could do it in one-tenth the time, and even if it took more than that, it will have been a big success."
"There was a very high cost to the diamond grinding," says Danny Simpson, managing partner, TexOp Construction. "Also, the time it would take to do the diamond grinding was excessively long. We proposed to do the job in a much shorter period of time at a lower cost, but with the same benefit as the diamond grinding, 40 days versus 90 days, giving them exactly the same ride in a third of the time."
Milling machines benefit from precise multi-sensing grade controls, which don't exist on diamond grinders.
Originally diamond grinders were specified for the work. "Their only option was diamond grinding," Barnes says. "But diamond grinders are slow, tedious and expensive. We felt Wirtgen mills could do that job a lot better. Our fine-texture drum gives the toll authority that certain striation of the grooved line that they are looking for. Now APAC and the toll authority are happy about the smooth ride, and quietness of the ride that fine texturing will bring. And they are anticipating that the NovaChip will adhere to the surface better and give them a smoother end product."
While premium overlays and thin-lift hot mix asphalt overlays provide a durable driving surface, their thinness makes them very vulnerable to variations in the pavement substrate on which they are placed. The evenness and smoothness of these thin surface treatments depends entirely on the smoothness of the prepared surface, and that's where the fine texturing process comes in.
During the project, precision cold milling control systems were keeping the cut "on the level."