Starting at the bottom

Feb. 1, 2006

Some highway repaving projects require more preparation than the existing pavement can afford. Generally, the structural integrity of the roadway, for one reason or another, has been compromised to the point where only complete excavation of the existing paving and even part or all of the sub-base is an acceptable solution. Simply put, the road must be completely rebuilt and paved. Such road conditions exist on a section of I-80 near Milesburg, Pa.

This section of I-80 was built in 1968, and over the past 37 years it has endured a moderate to increasingly heavy traffic flow.

Some highway repaving projects require more preparation than the existing pavement can afford. Generally, the structural integrity of the roadway, for one reason or another, has been compromised to the point where only complete excavation of the existing paving and even part or all of the sub-base is an acceptable solution. Simply put, the road must be completely rebuilt and paved. Such road conditions exist on a section of I-80 near Milesburg, Pa.

This section of I-80 was built in 1968, and over the past 37 years it has endured a moderate to increasingly heavy traffic flow. At present, the combined average daily traffic (ADT) flow for both eastbound and westbound totals 22,000 vehicles. By some traffic standards, this might be considered a modest vehicle flow rate experienced on a four-lane highway. However, it is not so much the number of vehicles as it is the high number of tractor-trailers and other heavy transport vehicles that are using this highway. Interstate-80 is Pennsylvania’s main northern-tier east-west highway for serving eastbound traffic to northern New Jersey, New York City and New England; all westbound traffic coming from the northeastern states that travel through Pennsylvania likewise uses I-80. Since the highway is a major route to and from NYC, the truck traffic flow rate is proportionally very heavy compared with the passenger vehicle traffic.

Heavy trucks have a way of punishing any highway, but this can be particularly so when the subgrade of the roadway lacks good structural integrity. While the section of I-80 in need of complete reconstruction is much longer than the project now under way, PennDOT is going in the right direction by fully addressing the structural problem rather than attempting to patch and pave over the existing degrading concrete pavement.

The project has been divided into two phases. Phase I includes building temporary crossovers, alternate ramps at the exchange and reconstructing and paving the westbound two-lane highway. This all was carried out during 2005. Phase II commenced in early 2006. According to Patrick Hawbaker, superintendent of the paving division, the project currently is on schedule and he further projects that both phases will be completed on time.

Miles of Milesburg

The project includes the reconstruction of the Milesburg interchange, 6.3 miles of the westbound two-lanes and four miles of the eastbound two-lanes. Additionally, three westbound and three eastbound bridges are being replaced along this section of the highway.

PennDOT let the contract to Glenn O. Hawbaker Inc., State College, with its low bid of $43.4 million. Hawbaker is a major contractor in central Pennsylvania specializing in road and bridge construction with an emphasis on asphalt paving. It employs 1,000 people.

The challenge associated with carrying out this project is two-fold. First, the roadways have to be excavated down to and including part of the subgrade. Second, the thickness design of the new pavement is unusually massive.

Deep excavation is necessary because part of the subgrade, which was put in by a contractor during the original road construction, included a clay-like earth that when moist becomes somewhat plastic. The clay was added to the original road construction for bringing the subgrade to fine grade. However, clay is generally not an acceptable subgrade for a flexible pavement (such as hot-mix asphalt) even though it can be adequate for a steel-reinforced concrete pavement as it exists here.

Nevertheless, the clay-layer subgrade ultimately contributed to the rough ride qualities of the existing cracked concrete pavement. While the existing pavement has been in existence for 30 years, arguably, it should have been replaced at least five or more years ago.

The cuts being made along the eastbound and westbound roadway alignments are 32 in. deep to reach the clay layer. Removing the clay layer requires an additional 4- to 16-in.-deep cut. With the undercutting made to remove the clay, the total depth of the roadway cut ranges from 36 to 48 in.

Considering the length of highway under reconstruction, an inordinate quantity of construction materials and earth is being excavated including the materials excavated for building temporary access ramps at the Milesburg exchange. A total of 455,000 cu yd of concrete paving, road base and sub-base are being excavated with a Caterpillar excavator and hauled away with 40-ton-capacity Volvo and Caterpillar articulated trucks. The existing 10-in.-thick concrete pavement alone is 140,000 cu yd. A subcontractor is demolishing the concrete pavement with guillotine breakers.

While the concrete pavement could be recycled into an aggregate for use on the project, Hawbaker opted not to recycle it because it was not considered cost effective when compared to using crushed and screened quarry rock. Fortuitously, Hawbaker owns and operates the Pleasant Gap Limestone Quarry, which is only five miles south of the construction project. The quarry’s crushing and screening plant is already set up for processing mined rock into Superpave aggregates that comply with PennDOT’s aggregate specifications for Superpave mix designs.

Inches and inches of asphalt

An exceptionally thick pavement design was foremost on PennDOT engineers’ minds when they made up the specifications for this road reconstruction project. Two main reasons for the heavy-duty design were tied to the geotechnical conditions and the relatively high equivalent single-axle load (ESAL) values that were calculated.

Generally, ESALs are forecasted based on establishing the two-way average annual daily traffic; the two-way vehicle classification volumes; the flexible ESAL factors; and the design-lane ESAL distribution. This forecast essentially correlates different axle loads and axle configurations to the road damage experienced by a given number of 18,000-lb dual-tired single axles that are run over a pavement with a specified strength and its design life.

Based on the calculated forecast, the engineers decided on first covering a geotextile on top of the open cut. Depending on the ground conditions, either Class-4A (non-woven) geotextile is applied where no undercut was made or Class-4C (woven) was installed where an undercut was carried out. The function of the geotextile is to widely distribute the combined dead and live loads and to separate the in situ subgrade from the finish grade of shot rock and aggregates. This precludes the soil fines from migrating up into and filling the rock and aggregate voids that would eventually block the surface water inflow from draining out and away from the roadway.

Following the geotextile installation, the area is brought to the wanted fine grade by backfilling it with shot rock (deep-cut areas) and R-3 rock. The rock is then compacted with a rubber-tire roller. PennDOT Class 2A stone is then laid on top as the final grade sub-base in a 1?8-in. lift. A double-drum vibratory roller is used to compact the 2A stone.

Heading the paving train is a Roadtec shuttle buggy followed by a Stealth paver. The shuttle buggy is used for ensuring minimal aggregate segregation, more uniform HMA temperatures and smoother paving results.

The asphalt paving base is designed with a 30-million ESALs forecast in mind and thus PennDOT designed it 19 in. thick. The base mix aggregate size is 37.5 mm (1.47 in.). At 19 in. it is too thick in one lift to achieve a uniform compacted density throughout the base pavement by using a conventional roller, thus the base is being laid and compacted in three lifts. Lift one is a 10-in. pre-compacted lift that is compacted to 93 to 94% Maximum Theoretical Density (MTD), bringing it down to a finished 8-in. thickness. Lift two is 7.5 in. for compacting it to 6 in. and the third lift is 6 in. resulting in a compacted 5-in. thickness. The compacted density results with these two latter lifts are similar to the first lift compaction results.

Tender care

The roller used for breakdown on each of the three lifts is a new Bomag BW266 tandem vibratory roller. This roller features two 66-in.-wide x 48-in.-diam. drums for a 66-in. compaction width. A main attribute of this roller is the Multi-System Performance Indicator (MSPI) that calculates drum impact spacing based on the roller’s working speed and vibration speed. The results from using the MSPI are improved control over pavement densities and its smoothness while the operator controls the rolling patterns. The MSPI also can be programmed to start and stop vibration automatically.

The BW266 roller operating weight is 20,600 lb. Coupled with its working width, this roller has been very efficient for Hawbaker on this project despite the challenging three-lift base that features 37.5-mm aggregate in the HMA. As a breakdown roller, the vibratory feature is turned off and four passes are made to complete its compaction assignment. At the start of compacting the paving temperature is 240°F.

For compacting during or near the tender-zone temperatures, a new Hypac C560B pneumatic-tired roller was selected. It is strictly a static roller. Nevertheless, with water added as ballast the roller has a nominal operating weight of 40,000 lb. The compaction width of the roller is 78 in.

It is acknowledged by many paving contractors and compaction experts that a rubber-tired roller is superior to steel-drum rollers when operating in or near the tender-zone conditions. Rubber-tired rollers generally will not force the laid asphalt mix laterally as is often so with steel drums. Hawbaker’s Hypac C560B features a skirt attached around the lower perimeter of the roller to support the heat retention established in the tires that is derived from the heat expelled from the laid HMA. The purpose of keeping the tires hot is so asphalt does not adhere to them, accordingly reducing the uniformity of the pavement’s surface. It requires making four passes with the roller for reaching the wanted compaction at this point. The HMA is about 180°F.

Bringing up the rear of the paving train as the finish roller is a Hypac C784 double-drum vibratory roller. Its operating weight is slightly more than 28,000 lb and the drums are 84 in. wide. Five compaction passes are made with this roller. The first two are made while the roller is in the static mode and then followed by two passes made in the vibratory mode. Finally, the fifth pass is made in the static mode. The HMA is about 160°F with the advent of the finish rolling and gradually drops to 120°F by the time the compaction is completed. Upon completion, the pavement density reached is 93 to 94% MTD. So far, this MTD value has been achieved upon completing compaction for each of the three lifts.

Compaction of the binder and wear courses is being carried out somewhat differently. The major difference is the changed positions of the rollers in the paving train. Now the HMA breakdown roller is the Hypac C560B, followed by the Hypac C784 and finally for the finish compaction the Bomag BW266 is used. The reason for putting the Hypac C560B in the lead is, according to Patrick Hawbaker, it ensures a very smooth pavement because of its extraordinary roller span, which is 150 in.

With the 30-million+ ESALs assumed, the binder course features a 19-mm (3?4-in. aggregate) binder mix and the pavement is 3 in. thick. Accordingly, the 2-in.-thick wear course features HMA with 12.5-mm (1?2-in.) aggregate.

Four passes with the rubber-tired roller are made to complete breakdown. Four passes are made for the interim compaction and, finally, five passes are made for the finish rolling. The finish rolling is the only time the vibratory mode is used on the roller, and that is during the second and third passes.

Sweetened with lime

With the poor subgrade conditions found on the project, PennDOT has recently seriously considered using a soil stabilization method on Phase II that has been relatively successful on some Pennsylvania Turnpike projects. It would eliminate the clay excavation activities.

On the turnpike projects where roadway reconstruction is called for and the structural integrity of the subgrade is poor, soil stabilization is used. Here the existing subgrade includes a large measure of clay, making it unsuitable for paving a new roadway on top of it with an HMA pavement. While the existing reinforced concrete pavement was competent of spanning most of the subgrade deficiencies, this spanning phenomenon cannot be expected for the Superpave that is specified for the reconstructed highway.

It was evident to the turnpike engineers that a cost-effective method of stabilizing the subgrade was necessary. The predominantly clay-like ground conditions run unusually deep (i.e., 4 to 12 ft) along some sections of the highway’s alignment. Excavation costs of the massive quantities of clay would be prohibitive.

There is a parallel to be drawn between the turnpike and I-80 projects in that both are "blessed" with clay in the subgrade. However, that is where the parallel ends. The turnpike’s clay layer is at least 4 ft thick where the I-80 project’s clay layer is as little as 4 in. thick with some areas erratically going to 18 in. deep. Neal Fannin, P.E., materials/geotechnical engineer for PennDOT, said a 4-in. layer of clay is inadequate to stabilize effectively when using lime and pozzolan. He explained, "We have thoroughly investigated the possibility of using a lime/pozzolan mix for stabilizing the existing clay on Phase II and have concluded that at least a 12-in. layer of earth must be stabilized to reach an acceptable structural integrity. In order to treat a 12-in. lift with lime/pozzolan on this project, we would have to make a cut into rock. Such steps are not feasible. I feel our method of excavating the clay and laying geotextile plus bringing the subgrade to grade with crushed rock is as good if not better than stabilizing the clay."

Jeff L. Laninger, P.E., project manager for Hawbaker, concurs with the PennDOT engineers’ conclusion. He said, "Sure we [contractor] could excavate into the rock and then mix in the lime/pozzolan, but it would be very time consuming and far more costly to do than the procedure we are using."

Not many projects call for 24-in.-thick asphalt paving but it has been demonstrated on this project that it is possible by using a conventional paver and rollers, albeit the rollers feature some important current technologies for assuring a smooth roadway when completed. It is the thickest paving ever carried out by Hawbaker, who has laid millions of tons of HMA during recent years.

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