A unique project in Missouri is currently in its second phase on I-64 through St. Louis. The design-build project, the city’s first, is a complete replacement of approximately 10 miles of the interstate right through the heart of the city. One-half of the 10 miles is completely shut down to traffic while work takes place. The unprecedented move is taking years off the completion date and creating a safer environment for the construction workers.
The project is being built by a consortium called Gateway Contractors, which involves contractors Granite Construction, Fred Weber Inc. and Millstone Bangert Inc. The two-year project involves 200,000 cu yd of concrete. All of the concrete will be slipformed.
Fred Weber and Millstone Bangert have brought their fleet of equipment onto the project. That fleet includes three GOMACO 9500 trimmer/placers, an RTP-500 placer, four Commander III four-tracks and a GHP-2800 paver. It is a lot of equipment, but the variety of applications the equipment is slipforming is impressive.
The Commander IIIs are slipforming shoulders, medians, variable-width ramps, inside median barrier walls, outside barrier walls, retaining walls, roundabouts, bridge parapets, truck lanes, half-shaped barrier walls against mechanically stabilized earth (MSE) walls and moment slabs for the sound wall.
10-12 on the smoothness scale
All of the material on the project was recycled. The concrete was crushed and used again for the base material. The base for the I-64 project consists of 10 in. of 6-in.-minus rock, capped with 2 in. of Type 5 rock. The top layer is trimmed to the accurate, final grade with a GOMACO 9500 with an 18-ft-wide trimmerhead.
Millstone Bangert is responsible for all of the mainline paving on the project and is using its four-track GHP-2800 paver. Project smoothness specifications require a reading of less than 30, and Millstone Bangert is consistently running between 10 and 12 on the zero-blanking band. Smoothness, according to Ron Dibler, Millstone Bangert’s paving superintendent, begins with the base.
“Good ride is a process that begins from the ground up,” Dibler said. “You have to have good string, consistent mix and try to keep the paver moving with minimum stops. Most important, though, is a good, solid trimmed base to pave on and run the paver’s tracks.”
Each paving pass is 9 or 10 in. thick and 25 ft wide. They are building four new lanes of interstate for both the eastbound and westbound sides. Paving production averages between 2,500 and 3,000 cu yd per day.
All of the concrete for the project is being supplied by two onsite batch plants. Concrete is delivered to the paving site by tandem-axle dump trucks. The concrete is a Missouri Department of Transportation (MoDOT)-approved mix with an average slump of 1.5 in.
Putting up walls
The profiles for the inside and outside barrier walls are very similar, but the outside barrier has sound wall mounted to the top of it. A Commander III is used to slipform a moment slab with a rebar grid. The steel for the wall is tied to the rebar grid in the moment slab. With the steel in place, the spacing for the sound wall is meticulously plotted, and the anchoring bolts for each post are carefully set.
“The sound wall posts have to be put in the exact location, because each panel is made to fit a certain area,” Dibler said. “After the cages are built, we dry run the steel to make sure everything is going to work and it’s all set to the right height.”
The concrete trucks dump their loads into an RTP-500, which then feeds the belt on the Commander III. The central mix concrete, according to Dibler, gives their concrete more consistency and allows them to run a drier mix for their barrier work.
Behind the Commander III, workers have to locate the anchoring bolts for the sound wall, dig them out and expose them so the posts can be installed after the concrete has cured.
A 9500 placer feeds another four-track Commander III as it slipforms the variable-height center median barrier. The height of the wall varies from 4 to 7.25 ft.
The non-variable-height barrier has 10 longitudinal bars fed into the front of the mold for wall reinforcement. The variable-height barrier is slipformed over a steel cage in the lower half of the wall. They are inserting six longitudinal bars into the front of the mold for the top section of the wall.
The consortium engineered a cost-saving measure on the project by stacking two slipformed walls on top of each other instead of putting in MSE wall. The first retaining wall varies in height between 3 and 6 ft. Two rebars are hydraulically inserted vertically into the 24-in.-wide top of the wall, and vibration is applied to the rebar during insertion. The wall is allowed to cure and then backfilled. The roadway is brought up to grade, and then another section of wall is slipformed on top of the existing wall.
“The slipformed retaining wall replaces MSE walls shorter than 9 ft in areas that would retain dirt,” Dibler explained. “Rather than building a costly MSE wall, we were able to slipform retaining walls.”
All of the shoulders and medians on the project are slipformed with a side-mounted mold on the Commander III. Medians are slipformed 6 ft wide. Shoulders vary from 4 to 10 ft, or 8.5 ft if there is a barrier wall on it. Zero-clearance paving against the MSE wall was accomplished after modifying the mold.
“We had an idea of moving the sideform cylinders and mounting structure from the outside of the mold to the inside of the mold and minimizing the thickness of the sideform,” Dibler said. “We were able to get that down to about 2 in., which allows us to slipform closer to the walls, and that’s been a big help on this job.”
Width changes on the ramps include transitions from 15 ft down to 12 ft wide and 18 ft down to 12 ft wide. The transitions are made on the go, with no need to stop and adjust the paver width.
Another cost-saving measure on the project included building two roundabouts instead of entrance and exit ramps. The 20-ft-wide, 8-in.-thick roundabouts, with integral curb on one side, had to be built around a 45-ft radius. Fred Weber decided to pave the first roundabout in two separate pours. The first was 10 ft wide and the second was the same width with a 3-in. mountable curb.
“The tight radius intimidated us a bit on the first roundabout,” Jackson said. “The second one, though, we decided to pour the full 20 ft with the curb on it. It worked out just fine, and we accomplished it all in one pour, instead of two.”
To finish out the roundabout, a lane was slipformed inside of its radius. The lane, which is used to assist trucks through the roundabout, was 9 in. thick and 10 ft wide with a 6-in. vertical curb.
The I-64 project is scheduled to have traffic back on the finished second phase by the end of this year. It is a deadline the consortium is confident they will make.
