A Hot Wintry Mix

Feb. 17, 2004

When the schedule for Boston’s Central Artery/Tunnel (CA/T) project required asphalt paving operations during this past winter, the joint venture of Bechtel/Parsons Brinckerhoff (B/PB) prepared a cold-weather paving procedure that allowed the difficult work to proceed and helped ensure that the Liberty Tunnel on I-93 was ready for its scheduled opening in March 2003.

When the schedule for Boston’s Central Artery/Tunnel (CA/T) project required asphalt paving operations during this past winter, the joint venture of Bechtel/Parsons Brinckerhoff (B/PB) prepared a cold-weather paving procedure that allowed the difficult work to proceed and helped ensure that the Liberty Tunnel on I-93 was ready for its scheduled opening in March 2003.

Opening the 1.3-mile I-93 northbound tunnel to traffic was one of the biggest milestones completed to date on the CA/T project, a.k.a. the “Big Dig.” Construction of the tunnel, which began in 1995, involved nine construction contracts and used the slurry-wall construction method.

The new highway tunnel reduces the number of on and off ramps from seven to three in the 1.3 miles of new tunnel, which allows the approximately 200,000 vehicles per day that use the tunnel to move through the city more efficiently.

Paving in between

Most DOTs in the northern states of the U.S. halt bituminous paving during the winter months. Hot-mix asphalt needs to be properly compacted to a minimum of 95% of the Marshall density before the temperature of the material drops below 180°F. Once asphalt drops below that temperature, further compaction causes cracking and fragmentation of the aggregate in the mix, which causes the surface to be uneven and shortens the life of the pavement. In cold weather there is usually not enough time to place and compact asphalt before its temperature falls below 180°F.

The contract specifications for the CA/T project required paving work be halted between Nov. 15 and April 1. A change was approved to allow paving between those dates, provided the contractor exercised adequate controls to achieve a durable riding surface. B/PB, as the management consultant for the Massachusetts Turnpike Authority on the CA/T, prepared a cold-weather paving procedure that required the paving subcontractor and the asphalt supplier to meet minimum asphalt temperatures at various points between batching and placement. Asphalt that didn’t meet the following minimum material temperatures was rejected:

• Upon leaving the batch plant 310°F;

• Upon transfer to paving machine 280°F; and

• Upon achieving 95% compaction 180°F.

In addition, the general contractor monitored the National Weather Service’s temperature projections as part of its scheduling efforts. The paving subcontractor had to remove any ice or standing water on the concrete sub-base, provide insulated or heated delivery trucks and increase the number of rollers available for compaction. The asphalt supplier had to use non-frozen aggregates and keep the delivery trucks moving efficiently from the batch plant to the jobsite. B/PB had to either accept or reject each delivered load of asphalt and monitor asphalt temperatures through the entire batching, delivery and placement process.

The asphalt supplier used a batch plant located 10 miles north of the jobsite to mix all the asphalt for the tunnel roadway. The target batching temperature at the plant was 310°F. Batching temperatures above 380°F were not acceptable because liquid asphalt burns off and the asphalt mix loses its proportion at that temperature. Although Massachusetts allows a maximum of 10% recycled aggregate product (RAP) in wearing courses, no RAP was used for the I-93 wearing course because it would have stiffened the mix and made the asphalt more difficult to compact in cold weather.

B/PB lab personnel used PaveCool, a computer program developed by the Minnesota DOT, to decide each day whether paving should be allowed to proceed. The agreed-upon time limit needed to properly compact the asphalt was 20 minutes. Paving was only allowed to proceed when the program indicated that the temperature of the asphalt would stay above 180°F for 20 minutes or more.

Input data to the PaveCool program included base slab temperature, ambient temperature, wind speed, lift thickness and mix delivery temperature. B/PB lab personnel operated the computer program at three junctures: the afternoon of each day prior to paving, the morning of the paving and while the paving was under way.

Paving of the I-93 northbound tunnel began on Dec. 18, 2002. Over a period of 13 days, 9,071 metric tons (10,000 short tons) of asphalt was placed and compacted. The mix selected for this roadway was the Massachusetts Highway Department’s Modified Top Wearing Course—a maximum 1-in. mix that provides a good wearing surface and has good rock-to-rock contact that minimizes pavement rutting.

The planned sub-base consisted of 17.6 millipascals (4,000 psi) cast-in-place concrete. Before placing the asphalt, the subcontractor cleaned the concrete sub-base of all debris and dirt, then sprayed on a bituminous emulsion tack coat at a rate of 1 gal per 20 sq yd to improve the bond between the portland cement concrete (the sub-base) and the asphalt. The majority of the roadway surface was paved in one course with an average thickness of 3 to 5 in.

The paving subcontractor used a material transfer vehicle to efficiently move asphalt from delivery trucks to the paving screed. The transfer machine provided the added benefit of remixing the delivered asphalt prior to placing it in the screed. Compaction was performed via a 12-short-ton steel vibratory roller for breakdown rolling and a 10-short-ton steel static roller for finish rolling prior to density testing.

Longitudinal and transverse joints were allowed to cool before their adjacent courses were paved and treated with a hot rubberized asphalt to create a watertight bond between the two adjacent courses of pavement. Density testing was performed with nuclear density gauges.

During the 13 days of paving, the ambient temperatures in the tunnel ranged from 22°F to 48°F, which was considered moderate for Boston in wintertime. The paving subcontractor placed 450 to 1,700 short tons of asphalt per day.

9,800 passed

One lesson the B/PB team learned was that thin leveling courses (less than 2 in. thick) couldn’t be placed successfully during cold weather (less than 30°F) because the courses cooled off too quickly.

B/PB lab personnel used an infrared camera to monitor asphalt temperatures in the field, which were supposed to be maintained at the required 280°F at transfer and 180°F as compacted. The camera downloaded and recorded infrared pictures, and B/PB lab personnel identified color spots of low-temperature material in the delivered asphalt. When low-temperature spots were identified in a batch, the paving subcontractor was notified and moved to quickly spread and compact that batch before its temperature actually dropped below the minimum.

Of the 10,000 short tons of asphalt placed, 9,800 short tons met the contract requirements for temperature and density.

B/PB rejected 200 short tons of placed asphalt because of either low temperatures or compaction values below 95%. The 200 short tons were removed and replaced. Of the 160 in-place density tests, 155 met the minimum 95% compaction, and five tests were below the required 95% compaction. A total of 11 delivery trucks were rejected before leaving the batch plant because of either low temperature or material segregation.

B/PB personnel and contractor personnel inspected the road surface for planeness, surface texture and smoothness after each placement of asphalt and recorded areas of distress, if any, such as tearing, excessive liquid asphalt or exposed aggregate. The contractor then was required to repair the areas of distress. The contract requirement for planeness was a maximum 0.25-in. tolerance over 16 ft.

In-service assessments of the road surface after three months of use indicate the bituminous placed during winter is performing well. The demolition of the existing Central Artery and surface restoration remain to be finished on the CA/T project, which is expected to be complete in 2005. The I-93 southbound tunnel was open in December 2003.

About The Author: Donovan is a materials supervisor on the CA/T project. He has been working with Parsons Brinckerhoff on the CA/T project for the past 11 years.