Continuing to grow in popularity across the U.S., thin concrete overlays—or whitetopping—was originally developed to rehabilitate deteriorated full-depth asphalt. In many parts of the U.S., agencies are faced with choosing an appropriate rehabilitation option to address an installed system of old concrete pavements or existing composite pavement sections.
Agencies in Michigan, including the Michigan Department of Transportation (MDOT) have developed a technique for combining whitetopping technology with unbonded or separated overlay experience to rehabilitate these existing concrete and composite roads. The technique involves placing a 1-in. asphalt separator on the old pavement or composite pavement and then building a 4-in. concrete overlay with short-joint spacing as the new pavement surface.
In southeast Michigan, a number of existing pavements consist of deteriorated asphalt on top of an old concrete base. This is especially true of many of the arterial roads in the Detroit metropolitan area where original concrete, sometimes up to 60 years old, still exists underneath many layers of asphalt. Over the years, thin asphalt overlays had been applied as routine maintenance with expectations of lasting 10 to 15 years, with many lasting much less than that.
The Outer’s edge
The first thin concrete overlay of a composite pavement in Michigan was constructed by Wayne County on approximately 1 mile of Outer Drive in the city of Dearborn Heights. This road is located in a residential area and receives a small percentage of commercial traffic. It was chosen because of its need for rehabilitation. The previous 3- to 4-in. asphalt overlay had lasted only eight years. Outer Drive proved to be suitable for this initial proj-ect because it had a relatively stable base of old concrete underneath, and it was a primary road with generally low truck traffic. A 4-in. concrete overlay was proposed that would be cost-competitive with a 4-in. asphalt overlay but with the expectation of lasting twice as long.
The first steps in scoping the Outer Drive project, as well as the other thin concrete overlay projects in the metro area, included taking pavement cores to determine existing asphalt and concrete thickness and the condition of the underlying concrete. Several cores were taken by the Wayne County Testing Lab. Results indicated that the current asphalt overlay thickness was approximately 5 in. and that the old pavement below was approximately 9 in. thick. A review of records also indicated that the original concrete pavement that was built in 1928 was in fairly good condition, but would require some full-depth repairs and partial-depth asphalt repairs prior to constructing the whitetopping.
Once this information was gathered, a design was proposed that consisted of 4 in. of concrete. This was determined based on the elevation constraints and the desire to compare the performance with other traditional thin conventional whitetopping projects.
In order to prevent reflective cracking in the new thin concrete overlay, a 1-in. bituminous separator layer was placed on top of the old concrete pavement. Because of the advanced deformation of the existing asphalt, the design called for completely removing it. The mix for the separator consisted of a typical base-course bituminous mix used by the county for its asphalt pavement sections. Prior to placing this separator layer, a minimum number of full-depth repairs were made to mainly mid-panel cracks in the old pavement.
Because this particular project consisted of a composite section and thus more base support was present, Wayne County decided to utilize a 6-ft x 6-ft joint spacing pattern. The joints were constructed utilizing a 1?8-in.-wide cut at a depth of 1 in. and were not sealed. The mix design used for the thin concrete overlay consisted of a standard Wayne County 5.6-sack concrete mix. In order to save costs, polypropylene fibers were not used on the Outer Drive project.
The first step in the construction phase of the project consisted of milling the existing deteriorated asphalt down to the old concrete pavement. Once the surface of the old pavement was exposed, a field determination was made as to what type and how many repairs were to be made. Surprisingly, a minimum number of full-depth repairs were needed on the old concrete. Many surface repairs of the old concrete were made with asphalt and were done in the same manner as if preparing for an asphalt overlay.
During this initial phase of construction, it was determined that the curbs, which were originally planned to be left intact, were in much worse condition than expected. It was decided that the curbs would be removed and that a new curb would be constructed, integrally with the construction of the thin concrete overlay. Due to the low cost of the concrete in the initial bid, the extra cost for the curb added minimally to the final cost of the overlay.
The bituminous separator layer was placed using standard asphalt paving equipment. Traffic was not allowed to run on the separator layer once it was placed. One issue that surfaced during the placement of the separator was dealing with grade corrections. It became evident once the milling took place that the pavement section was variable and did indeed require correction, especially near the crown. After discussions among the county engineer and the Michigan Concrete Paving Association staff, it was determined that the best approach would be to make corrections in the pavement section utilizing the concrete overlay not the bituminous separator layer. This in the end would provide a better section and be more cost-effective.
Prior to placing the overlay, the pavement structures were addressed by isolating them. Manhole covers were removed ahead of the paving operation and fill placed around the structure. A steel plate was placed on top so that the crews could go back after paving and refinish around the structure and place the new cover.
The thin concrete overlay was constructed part-width using a slipform paver. This allowed for traffic to move through the project while construction was in process. As per the county specification for surface texture on local roads, an AstroTurf drag was applied. Shortly after the concrete was placed and finished, the 6-ft x 6-ft joints were cut utilizing an early-entry saw because of the more rapid setting time of the thinner section. After joints were sawed, a curing compound was sprayed onto the pavement surface.
One of the key components of thin concrete overlays is that the construction costs remain competitive with other rehabilitation options. When the Outer Drive project was scoped, keeping costs as close as possible to asphalt overlays was a key factor in the design and construction. The original bid for the Outer Drive project was approximately $765,000. The expected life for the 4-in. concrete pavement is 15-20 years. The Wayne County Department of Public Services estimated that a 3.5-in. asphalt overlay would have cost approximately $650,000, with a service life of only 8-10 years.
