Fly pattern?

Oct. 16, 2009

The city of Milwaukee is served by an old circumferencial arterial fondly known as STH (State Trunk Highway) 100.

The city of Milwaukee is served by an old circumferencial arterial fondly known as STH (State Trunk Highway) 100.

It serves the north, west and south perimeters of the city, has survived all the freeway construction done in the area over the past 50 years and runs concurrently on several freeway legs. As traffic and development increased, it was upgraded from a two-lane roadway to its present configuration (a six-lane divided arterial) in the early 1980s. But as all roads do, it just plain wore out. The concrete pavement is badly distorted, the ride is intolerable, maintenance is impossible and WisDOT programmed just over two miles of the north leg between IH 43 and Green Bay Road for reconstruction for this year. This segment serves the villages of Brown Deer, Bayside and River Hills.

The existing STH 100 consists of 36 in. of portland cement concrete pavement in each direction with typical urban features including storm-drain systems, curb and gutter, utilities and a number of very busy local road intersections. The old pavement is 8 in. thick (nonreinforced and doweled) over a 6-in. crushed aggregate base course and a subgrade which is essentially clay but varies in texture and bearing capacity over the entire length of the project. The soils report prepared by Andy Zimmer, WisDOT regional soils engineer, identified four areas in about half the project length where unstable soils would be encountered. The report identified several options to deal with the issue, but it did point out that the cost of cut and fill in select areas would be approximately the same as using fly ash to stabilize the entire two miles. The predictable performance and uniformity of the fly-ash stabilized (FAS) subgrade also is an advantage.

The new STH 100 will include three travel lanes in each direction with a bikeway adjacent to the outer lane. Median width will be modified to allow for the 5-ft bike lane. Total width excluding auxiliary lanes (turn, access, etc.) will be 40 ft. The new pavement will be concrete with a new mountable curb. Traffic projections are 33,900 AADT in 2029 (up from 27,700 in 2004). The project will be completed in two basic phases with traffic being accommodated on the south lanes, one eastbound, one westbound, and separated by the center lane for turning traffic. After construction was completed on the north side in late July, traffic was switched and the south side is currently under construction.

First for everything

WisDOT engaged AECOM to prepare the plans for the project. The project manager, A.J. Catalanotte, explained the process that led to the use of Class C fly ash to deal with the uncertainties of the clay subgrade soils. Lafarge had recently conducted workshops for the DOT and many area consulting firms including AECOM to inform potential users of the merits of FAS in pavement construction. The principal advantages include cost and time savings, significant improvement in CBR and the reuse of industrial products.

AECOM developed a number of cost and performance models including reclaiming the existing pavement for reuse as the base course and FAS subgrade, undercut and backfill the subgrade, impacts of each on traffic, completion times and several others.

Catalanotte said that while the results of the cost analyses were similar, the environmental advantages and improved construction times offered by the FAS option were compelling. WisDOT had some experience with FAS on prior projects, mostly as crisis remedies for soil conditions. And while there was some apprehension on the part of staff and contractors that had no experience with the process, WisDOT chose the FAS option and prepared plans and specs accordingly.

This is the first project designed, bid, let and now constructed on this basis. The final cross section in the plan calls for a 12-in. FAS subgrade extending 2 ft beyond the back of the curb, a 6-in. dense-graded crushed aggregate base course and an 8-in. nonreinforced, doweled and tined concrete pavement. The alternative would have been a 16-in. breaker run and/or geogrid subgrade replacement under the pavement.

FAS facts

The project was let earlier this year, with six bidders competing. Trierweiler Construction and Supply, the prime and paving contractor, was the successful low bidder. Other subcontractors included Tri-County Paving for mixing and blending the fly ash with the clay soils, American Road Reclaimers for hauling and spreading the fly ash prior to blending, Musson Brothers for the reclaiming and crushing of the existing pavement and placing the new base course, and Lafarge North America for furnishing the fly ash from the We Energies Oak Creek plant.

Basic quantities for the FAS were 91,028 sq yd of subgrade stabilization and 5,730 tons of Class C fly ash delivered and spread. WisDOT has adopted the two-part-pay bid for this method.

While both items are required, the two-part allows for variation in both ash and treated area. On the bid basis, the fly ash application rate is approximately 125 lb/sq yd.

Take as directed

As the project was planned, Phase 1 would run from May through July and Phase 2 from August to November. At press time, the work was on or ahead of schedule, even though a very wet, rainy spring pushed the start into June.

Prior to actually beginning the work, WisDOT conducted a meeting to assure that each contractor was clear about his mission. Often the blending contractor also provides compaction, grading and finishing operations. In this case, Trierweiler provided those operations after the ash was spread and mixed and was more comfortable with the arrangement since it also was doing the paving. Questions regarding sequencing, protecting loose fly ash from disruption, finishing, lag time for next operation, traffic on FAS materials and a host of others were resolved.

Work began with breaking and removing of the existing concrete pavement, hauling it to a crushing site at the west end of the project for conversion to base course, remodeling the storm-sewer system, modifying utilities as needed and preparing the subgrade for stabilizing.

Fly ash deliveries began in early June and the stabilizing began. As the work progressed, minor adjustments were made to the water content in the blend, the speed of the pulverizer to achieve desired results, compaction procedures and final grading. Once all the contractors were tuned in, work proceeded very well. Production rates were 800-900 sq yd per hour for the complete process. It took only eight days to complete the stabilizing on Phase 1.

The WisDOT soils unit was onsite and was pleased with the QA results. Densities determined by nuclear testing were 95-100% of Proctor and moisture contents fell within 1% of optimum. Cross traffic was accommodated, utility work was completed on time and construction of the new base course began immediately after the FAS process was complete. Base-course construction can begin the day after the FAS is complete.

It is important to follow proven protocols for the FAS method. To be confident in the mix design, a geotechnical engineer should provide the application rate and optimum moisture for the soils based on Proctor tests. The fly ash is spread on subgrade soil at the design rate. The pulverizer follows immediately with water supplied by a water truck tied directly to the pulverizer with a hose to mix the materials to the 12 in. depth and design moisture content. Proper moisture content control is crucial to performance.

Immediately after the mixing is complete, initial compaction follows with a 20-ton vibratory padfoot roller; usually two to three passes will produce the required results. Final grading follows, and to complete the operation the grade is rolled with a smooth drum roller in static mode so as not to disrupt the hydration.

Solid success

At press time, Phase 1 paving was nearly complete. A conversation with the project manager and inspector, Tony Minto, indicated that the FAS subgrade was a huge success. He mentioned that there were no disturbances or soft spots in the subgrade as the base course was completed and that during paving all the concrete trucks traveled on the newly constructed base with no displacements or deflections.

The Trierweiler project manager, Joe Matchey, stated that he had never seen such a successful method. This was his first experience with fly ash, but he will definitely support its use on future projects. Even with nonuniform soils, the stabilized subgrade performed extraordinarily well from his point of view. That view seems to be shared by all parties involved. The expectation is that Phase 2 will perform even better with new experience.

About The Author: Rosenmerkel is a consultant to Lafarge North America. He can be reached at [email protected]; 262/547-2585.

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