During mid-summer evenings for the past three years, crews of R. Zoppo Corp. have been executing an efficient, well-organized routine to repair and resurface the decks of I-93 just north of Boston's Zakim Bridge under an $18 million contract with the Massachusetts Department of Transportation (MassDOT).  

The Stoughton, Massachusetts, contractor developed a nocturnal routine that meets MassDOT mandates to minimize traffic impacts on daily commuters while maximizing safety for its workers. Under terms of the contract Zoppo is limited to working on the interstate between 9 p.m. and 5 a.m. Monday through Friday, with extended weekend work allowed on Saturday and Sunday mornings as necessary.

Patching One Barrel, Paving the Other

The contract covers both north- and southbound barrels of the interstate highway on a two-mile section running from Exit 30 in Somerville to Exit 26 near Leveret Circle. It entails repairing the concrete decks and resurfacing them with SUPERPAVE bituminous concrete. Zoppo repaired the concrete deck of the southbound barrel in 2013 and 2014, and covered the deck with temporary wearing course bituminous concrete. Early in July, crews began milling off the temporary surface and overlaying the deck with permanent SUPERPAVE bituminous concrete.  Simultaneously, other Zoppo crews started repairing the concrete deck of the northbound barrel and overlaying the deck with temporary surface mix. In 2016, they will mill off the temporary northbound wearing course and replace it with SUPERPAVE mix.

Due to time constraints, work takes place in relatively short sections each night. Occasionally, it requires lane closures and even ramp closures on the southbound barrel, with signs, pavement markings and channelizing devices used to guide drivers safely through the work zone. Since about 65 percent of the project takes place on an elevated portion of I-93, travel speeds are affected. Also, this means construction crews are frequently dealing with elevated deck joints similar to those of highway bridges.

Franco Nicolazzo, Zoppo Project Superintendent, is supervising construction with assistance from two Project Foremen, Paul Pimental and Tony Furtado. Andy Greenlaw is Zoppo's Project Executive.

Milling Pavement to Access Deck

On the northbound barrel, once the location of deteriorated concrete deck has been determined by MassDOT, subcontractor Swank Construction Company of North Attleborough, Massachusetts, mills off the bituminous concrete to allow Zoppo's patching crew access to the damaged concrete.  Swank uses a Wirtgen W210i to remove the existing asphalt pavement, which is 2-1/4 inches thick on roadway sections and 1-1/4 inches thick on elevated sections. This machine, which has a 7-foot 3-inch drum, weighs approximately 64,000 pounds with a half tank of water and half tank of diesel, significantly heavier than the 50,000-pound machines usually allowed by MassDOT on elevated highways, according to Greenlaw.

"This weight limit would have limited the milling to be done by a lighter machine with just a 4-foot drum, and the job would take too long," he said. "They were concerned that the heavier machine would produce too much vibration on the elevated steel structure. I got permission from MassDOT to have a professional engineer conduct vibration tests using both machines and the larger machine produced no more vibration that the smaller, so they allowed us to use the bigger machine." 

One of the challenges facing crews as they mill pavement on the elevated portion of I-93 is the presence of so many deck joints. When the machine is very close to a joint, the milling head is raised until the machine passes over the joint. Then Zoppo uses a Bobcat 863H Skid-Steer equipped with a cold plane attachment to mill the asphalt pavement. A Caterpillar 246 Bucket Loader removes the millings.

Rapid Set Cement Speeds Patching

Once asphalt pavement has been milled off a designated patch area, Zoppo employs jackhammers powered by an Ingersoll Rand VHP 400 Compressor to demolish deteriorated concrete. Patch areas are squared off using concrete saws. The depth of a patch can vary anywhere from shallow to full depth - in the latter case it can involve cutting away and removing damaged rebar and installing new rebar. For deep patches in elevated deck sections, carpenters fabricate wood forms on the underside of the supporting steel to prevent patch material leakage.  

Rather than using Portland cement to make concrete for the patch, Zoppo employs Rapid Set calcium sulfoaluminate cement manufactured by CTS Manufacturing Corp. 

"We buy the Rapid Set in superbags and the aggregate in bulk, and mix the material on the job in a Cemen Tech mobile continuous mixer," said Greenlaw. He explained that the mobile mixer is supplied and operated by Sealcoating Inc. of Braintree, Massachusetts, but Zoppo personnel perform the concrete demolition and patching. Sealcoating Inc. is using a model MCD10-100 Cemen Tech volumetric proportioning mixer with a 10-cubic yard (cy) carrying capacity, a 4.5-cy cement bin and a maximum production rate of 30 cy per hour.

Greenlaw noted that using Rapid Set expedites the patching process.

"The initial set time for the material is about 45 minutes, and the final set takes about an hour and 10 minutes. Engineers have recorded four-hour breaking strengths of 4,000 psi for the concrete test molds," he said.

After the patching material has set, crews overlay the area with temporary asphalt concrete.

Fast Drying Tack Coat Proves Beneficial

On the southbound barrel, crews are milling off the existing asphalt pavement and resurfacing the entire width of the deck with SUPERPAVE mix. Aggregate Industries is manufacturing the SUPERPAVE at its Saugus, Massachusetts, plant and placing the hot mix at the job with a Cat 10-20B Paver. Two HAMM HD+120 Steel Drum Rollers operating in static mode are consolidating the material. An Elgin Pelican Vacuum Sweeper supplied by Wells Inc. picks up and disposes of millings in preparation for the application of an asphalt emulsion tack coat. 

Riverside Asphalt Services is applying the tack coat using a 3,000-gallon Etnyre liquid asphalt distributor mounted on a Mack chassis.  The Boston asphalt supplier is applying the rapid setting anionic asphalt emulsion (RS-1h), at the rate of 0.07 gallons per square yard. Ordinarily, the asphalt emulsion that would be used in this case to create a bond between the new asphalt concrete and the existing concrete or asphalt concrete surface would be an RS-1. The "h" in the designation for the tack coat actually being applied indicates the emulsion was manufactured from a "hard" asphalt - an asphalt with a low penetration grade designation (indicating a high viscosity).

In use, this emulsion "breaks" very fast, which means the asphalt and water contained in the emulsion will separate rapidly as evidenced by the short time it takes for its color to change from brown to black. When all the water is evaporated the emulsion has "set", but it's not necessary to wait till it sets before paving. The RS-1h emulsion applied by Riverside dries extremely fast, a feature that has the product playing a key role in deck resurfacing.

Protecting Boston's Icon

The paving train is heading south towards the Zakim Bridge, the cable-stayed asymmetrical bridge across the Charles River whose unique styling has made it an icon for Boston, often shown as a backdrop of national news channels to establish location. As the paving train proceeds, haul trucks laden with hot asphalt mix back up over the tack coat to the paver's hopper, discharge their load, then drive forward over the tack coat again as they leave, heading straight for Boston's signature bridge. And in all likelihood they would leave black tracks all over the surface of the majestic span if the tack coat was an ordinary one. 

But this tack coat is different.

"In 20 minutes, the tack coat is bone dry," said Greenlaw. "The truck tires don't pick up the asphalt when they run over it. So no tracks are left on the Zakim." He pointed out that the tack coat softens when it comes in contact with the hot mix and creates the required bond with the deck surface

New Guardrail for Added Safety

As part of its contract, Zoppo is retrofitting galvanized steel thrie-beam guardrail on both sides of the highway. Thrie-beam is fabricated so it presents a larger face to wayward vehicles than standard guardrail, providing greater impact surface and strength plus improved resistance to overturning vehicles. About 21,000 linear feet of guardrail is being installed, mostly by hand. On the elevated portions of highway, workers are drilling holes in the existing concrete parapet for attaching base plates that are bolted in place to support guardrail posts.

High Precision Teamwork

Executing many different operations during discrete 8-hour windows at night demands intensive logistics planning and high precision job performance. Crews have to be focused on task at all times, working as a team like a well-oiled machine while maintaining extraordinary safety standards for the protection of motorists and construction workers. In a recent interview Company President Dave Zoppo expressed his appreciation for the performances of all personnel involved in this complex project.

"They're doing a great job. I'm very proud of their work," Zoppo said. 

Barring any unforeseen circumstances, the contractor is expected to complete paving the southbound barrel and patching the deck of the northbound deck this year, and complete resurfacing the northbound barrel in 2016.