Laying concrete for Southern Beltway is a scientific process
That science got even more complex for this project, which is the first time the Pennsylvania Turnpike Commission is using what is called long-life concrete. That product, which uses a mix with more small stones and less water, is so intricate the association held what it calls Just in Time classes a month before paving began July 18 to make sure the turnpike and its contractors understood the procedures.
Concrete control center
Dave Weiland’s official title at Allega Concrete Corp. is plant manager, but in reality he’s a concrete chef. His kitchen is part of a trailer at the on-site concrete plant, an area about the size of a powder room with windows on three sides.
From here, Mr. Weiland uses two computer screens filled with numbers and specifications, two printers and a series of dials and buttons to control the ingredients. It’s Mr. Weiland’s job to prepare the mixture in a huge cylinder and pour it into a series of dump trucks that parade under him and carry the material from the plant to the first ribbons of prepared subsurface off Candor Road.
Allega built the on-site concrete plant on a flat field off Route 980 near McDonald. An on-site plant really isn’t a novelty for such a large project, but this one is a variation on the usual facility because it is producing long-life concrete.
Dump trucks arrive throughout the day to replenish piles of sand and two sizes of stone, larger No. 57 and the new ingredient, smaller No. 8. Instead of filling the usual two hoppers with material, a front loader fills three and the hoppers drop it on conveyor belts that pull it into the cylinder, where Mr. Weiland adds water from a chiller as well as cement and fly ash from storage containers.
As a delivery truck pulls up, Mr. Weiland mixes the batch and pours it, prints out identical copies of the delivery paperwork for the driver and the turnpike and sends the load on its way. As a hundred or more truckloads are filled every day, each move seems choreographed in a pattern that usually has two trucks at the plant, two dumping at the roadbed and two in transit.
But there’s more to it than that.
As many as four times a day, Mr. Weiland and others conduct tests of the ingredients to make sure the water/ aggregate ratio is just right and the temperature of the mix is between 50 and 80 degrees before it leaves the plant. The mix can change throughout the day as damp, cloudy mornings change to sunny, dry afternoons or vice versa.
In one test, technicians take a sample of stone from the storage pile and weigh it. Then they heat it to boil off all of the water and weigh it again to find out how much water had been absorbed by the stones. If the stones have too much water, Mr. Weiland reduces the amount he adds from the chiller; if there’s not enough, he activates sprinklers to wet the storage piles.
Another test measures the amount of air in the mixture. The ideal amount of 5.5 to 8.5 percent air bubbles provides room for the concrete to expand and contract without much cracking. If there’s not enough, Mr. Weiland can inject more.
The delivery trucks take the concrete to the construction site, where three large machines are laying the new roadway.
The trucks dump their load into a cylinder on a machine known as a placer, which uses a conveyor to carry the concrete to a large funnel that dumps it onto the subsurface in piles the way a baker drops roses on a birthday cake.
Here, the mixture becomes the responsibility of concrete technician Vince Fusco. Every 15 minutes or so, he uses a variation on a meat thermometer to make sure the concrete is still the right temperature. It almost always is.
Mr. Fusco also conducts a cone test in the field, filling a cone with concrete and dumping it to see if it leans or stands straight. Leaning means there is too much water in the mix.
After the piles are dumped, a machine that stradles the subsurface and is known as a slipform paver flattens the piles by vibrating across the top and forcing the concrete between the sides of the machine that serve as the frame to form a 12-inchthick layer of concrete. The paver inches along and if the mixture is made right, it is thick and stiff enough that it stands on its own as the machine inches past the finished area.
The slipform paver is followed a few yards behind by another machine straddling the new road surface. That machine sprays the curing agent, polyalphamethylsterene, from the front and drags tines from the rear to make lines in the concrete to help keep the surface smooth.
The polymer also seals the surface to prevent water from entering and provides a top layer for vehicles to ride on.
Within 24 hours, before the concrete cures completely, crews use a huge masonry saw to make cuts across the highway every 15 feet. This is a key step to control where the surface cracks as it expands and contracts.
Mr. Fusco’s work isn’t limited to the construction site. At the turnpike’s field office on Laska Road in McDonald, he has a trailer devoted to a series of quality control and assurance tests.
Every day, Mr. Fusco fills 18-inch plastic cylinders with samples from that day’s paving and takes them to the field office. One group is stored in a garage and given a pressure test after seven days and another is placed in a tub with a lime solution for 28 days.
At the proper time, the samples are put into a machine that basically squeezes them until they burst. They must withstand 3,000 pounds per square inch after seven days, 4,000 pounds after 28 days to be acceptable to the turnpike.
All that testing can provide a benefit for the contractor. The company can receive bonus payments for meeting or exceeding standards for the smoothness of the surface, depth of the concrete and the watercement ratio in the mix.
Tight schedule
Because of the region’s wet spring and summer, Allega and the turnpike are on a tight schedule to finish paving this section of the highway by the end of this construction season.
Crews wanted to begin paving by mid-June at the latest, but record rain in April delayed that process while workers removed the top 2 or 3 feet of dirt. It was too wet to lay the 10-inch subsurface of stones and permeable asphalt.
As a result, the first day of pouring concrete didn’t happen until July 18.
“The schedule has us finishing some time in late October,” Bob Kohlmyer, construction supervisor for CDR Maguire, the turnpike’s construction manager for this project, said during a site visit by the Post-Gazette 10 days ago.
That often means starting at 4 a.m. and working through periodic showers. During the site visit, rain interrupted work four times, causing crews to spread plastic over recently spread concrete to prevent additional water from marring the surface.
Despite the rain, crews laid about 1,000 yards of new highway on this day, 90 to 100 trucks bringing 11 yards of concrete per trip from the on-site plant.
Long-life concrete
Long-life concrete has been used in Pennsylvania before, first by the Pennsylvania Department of Transportation for a project on Interstate 95 near Philadelphia in 2012.
It has been used on five other projects since then and although it is not required, PennDOT district offices across the state have been “enthusiastic” about including it in bid specifications the past two years, spokesman Rich Kirkpatrick said in an email.
Mr. Becker of the pavement association said the Southern Beltway project is the first in Pennsylvania to use all of the elements of the process at the same time. In addition to less water and more stones, the process includes changes in the load transfer bars placed every 15 feet on the subsurface before paving, the new polymer for curing and tining in the direction traffic uses the road.
In the past, the 6-inch-tall, hollow load transfer bars were made of steel covered with epoxy, but the epoxy often got nicked and allowed the steel to rust. The new 1½inch dowels are stainless steel so they don’t corrode.
Overall, the cost of additional stone and different load transfer bars are offset by the use of less cement and water, Mr. Becker said, making the final cost of long-life concrete about $61 a square yard. That’s “very close” to the cost of the old method, he said.
Regardless of the cost, the expectation is the long-life concrete should last at least 40 years with minimal repairs. Standard concrete often requires substantial maintenance after 26 to 30 years, Mr. Becker said.
In an email, Terry Dreher, the turnpike’s materials management supervisor, was even more optimistic.
“Long Life Concrete Pavement is a system that address all of the items used in the final Concrete Pavement to be the best possible material that covers every aspect of known deterioration of our pavement failures. By providing the best materials, operation, training etc., the pavements are expected to last 50–75 years plus.”