Somewhere between super and all-star

March 12, 2007

For several years prior to 2005, the Michigan Department of Transportation (MDOT) Metro Detroit Region had been searching for a viable project to perform its first thin concrete overlay. Since most MDOT Metro pavements were originally built with concrete, more than likely this would mean overlaying a composite (asphalt over concrete) or a concrete pavement base. This would require innovative thinking, since most thin concrete overlay work to date nationally has been done over asphalt bases.

For several years prior to 2005, the Michigan Department of Transportation (MDOT) Metro Detroit Region had been searching for a viable project to perform its first thin concrete overlay. Since most MDOT Metro pavements were originally built with concrete, more than likely this would mean overlaying a composite (asphalt over concrete) or a concrete pavement base. This would require innovative thinking, since most thin concrete overlay work to date nationally has been done over asphalt bases.

At first glance, many would not consider a stretch of Gratiot (Gra-shut) Avenue (M-3), through the city of Detroit, the location for an inaugural experimental project. The original underlying concrete pavement was known to date back to the early part of the 1900s, having been overlaid several times with hot-mix asphalt. The project required completion in 2005 before the city welcomed Super Bowl XL at Ford Field in February 2006. In addition, Detroit was hosting the 2005 Major League Baseball All-Star Game at Comerica Park while the road would be under construction. The Gratiot project limits are within a couple of miles of both stadiums.

After careful analysis, an open mind and the advantage of being able to leverage a strong MDOT/industry relationship, the decision was made to move forward. In the end, a new breath of life was given to a very old yet stable roadway. The project was even recently acknowledged by the American Concrete Pavement Association as the 2005 silver award winner in the Overlay Category.

Encouraging results

The project stretches from Gratiot’s intersection with I-75 northeasterly to its junction with I-94, approximately 3.2 miles in length all within the city of Detroit. The cross section consists of a very wide nine lanes; a parking lane and three through lanes in each direction plus a common center turn lane. In total, over 198,000 sq yd of pavement needed to be repaired.

The fix was originally set up as MDOT’s standard rehabilitation treatment consisting of cold milling and replacing with a two-course hot-mix asphalt overlay. Under the guidelines of a 2001 Michigan-legislated pavement-demonstration program, MDOT and the Michigan Concrete Paving Association (MCPA) agreed to switch the overlay pavement material to concrete.

Thepavement-demonstration program was conceived to encourage engineers to develop nonstandard Michigan fixes for enhanced pavement performance. Under current standard policy, rigid lowest life-cycle cost pavement selection requirements prevent the use of treatments without proven Michigan performance. The program balances projects from both paving industries.

After cold milling, the overlay section would consist of 4 in. of plain concrete over a 1-in. asphalt-separator layer. John Carlo Inc. was awarded the construction contract with a bid of $7.17 million; the engineers’ estimate was $6.91 million. The project began and was completed in one construction season with several traffic stages.

Stepping into the project, it was known there were several challenges that needed to be addressed to give the project its best chance for success. To accomplish this, a cross agency team was assembled including engineers from MDOT, FHWA, MCPA and Tetra Tech-MPS, the project design engineering firm.

Through a six-month time frame and three additional field investigations, the team came to agreement on how to handle 31 identified project issues. The result was the creation of six unique specifications, four plan typicals and notes and two experimental elements for study. Experimental sections using two different asphalt-separator mixes and sealed versus unsealed joints were included in the project.

Working with history

MDOT does not have a record of the original pavement construction for this portion of Gratiot. The first project on record is a 1939 resurfacing of the original concrete with “sheet asphalt.” It is believed the original underlying concrete is of the early 1900s vintage. In the outside parking or bus lanes, underlying brick pavers were discovered. Additionally, that same lane visually showed some localized areas of poor base support.

Because of underlying base concerns, the development team decided to employ the MDOT falling weight deflectometer (FWD) testing equipment to quantify underlying base conditions. The results indicated the center turn and through lanes had very good base support. In the outside parking or bus lanes the support values were lower, but better than expected. Every core through the brick paver locations revealed a solid concrete base underneath the brick.

With base concerns minimized, asphalt cold milling with standard MDOT rehabilitation pavement repair techniques was decided upon for pavement conditioning. Full-depth concrete repairs would be used in the poorest base locations. The strategy for less severe deterioration would be partial-depth pavement removal with hand patch asphalt replacement.

During construction, once the cold milling was complete, the stability of the underlying pavement and base observed through preliminary investigations was confirmed. In fact, plan repair quantities proved to be overestimated in comparison with what was needed, which is uncommon for an MDOT rehabilitation project.

Many of the main Detroit thoroughfares owned by MDOT, Gratiot included, used to serve dual purpose with trolley lines. When trolley service ceased, the rails were left in place and subsequently buried when overlaid with later asphalt rehabilitation treatments.

With the desire to continue rehabilitating these older pavements using milling methods, the existence of the rails has become a problem for several MDOT Metro projects. Adjusting cross slope to meet modern standards adds to the complexity. Through plan research, trolley rails in the center lanes were known to exist on Gratiot. MDOT made the mistake of not being prepared for these rails on another Gratiot project just northeast of this one; it did not want to make the same mistake twice.

Ground penetrating radar (GPR) was used to identify the location and depth of the rails during design. The information was displayed on the plans for contractor use. The designer also used the GPR data to adjust cross slope to avoid rails when feasible. A special specification was written to handle the identified need for removal in some locations.

Even with the additional GPR planning, the rails caused construction problems that required field adjustments. The removal quantity exceeded what was set up on the plans. Cross-slope manipulation proved very difficult with the way the traffic was staged. As the project progressed, the contractor ended up physically pre-locating the depth of the rails prior to milling to best plan for dealing with the rails in future stages.

More to deal with

Over the 3.2-mile project length there were approximately 350 utility access structures, on average one structure every 50 ft of paving length. From past projects in Detroit, it was anticipated that many of these structures were of similar vintage as the underlying pavement, if not older. Dealing with the poor condition of these structures would be a challenge in itself.

A specification and plan details were developed to detail isolation techniques from the adjoining pavement. The pavement base within these isolated areas was well thought out to provide adequate structural support.

One of the first tasks the contractor performed was separating the structures from the pavement by employing a large-diameter coring machine. When staged work began in an area as the project progressed, they would repair each structure as needed, place isolation materials and re-establish adequate base support prior to overlaying.

Attention to utility structure details with this project improved construction progression and performance as compared with some previous thicker urban unbonded concrete overlay projects in Michigan’s recent past.

With all slabs divided into 6-ft by 6-ft (maximum) panels, sawing was a critical operation. Adding to the complexity were numerous intersecting streets, several at offset angles from 90°. Plus, jointing needed to line up with the 350 utility structures.

During development of the project, because of jointing complexity, it was decided to require the contractor to develop jointing details prior to paving through any major intersection. The plan needed to be submitted to the engineer for review and approval. This aspect proved to be very worthwhile for the overall quality of the project. Contractor employees learned a lot about jointing challenging intersections, which resulted in fantastic joint layout for the final product. This element was such a success that it is now required for all MDOT concrete paving through major intersections.

More to prove

Upon construction completion, a wrap up session occurred in January 2006. The goal was to determine how to best proceed on future projects of similar nature. The result was a three-page document of lessons learned to be used by design engineers setting up projects like Gratiot. For the most part, the work initiated by the development team was value-added. There were a few things that could have been expanded upon during design, such as additional trolley rail GPR data collection.

Until thin concrete overlays of composite pavement have proven performance in Michigan, they will still be considered demonstrative. The Gratiot project will be observed and reported on through 2015. The MDOT Metro Region is very satisfied with the initial project outcome and is investigating the viability of building another project under the same demonstration program. Ultimately, the goal is to have a comparable concrete rehabilitation option to compare with asphalt rehabilitation options for the best fit for a given project.

It is no accident the Gratiot project has been a great initial success. Careful planning, a spirit of cooperation and, most importantly, a dedicated contractor made this project work. The challenges were many, but in the end MDOT believes a new and innovative chapter of life has been given to a very old Detroit thoroughfare.

About The Author: Grazioli is soils & materials managing engineer in MDOT’s Metro Detroit Region.

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