Laying concrete for 3.2 miles of the Southern Beltway in Pittsburgh, Pennsylvania (Pa.) is a much more precise, scientific procedure than just building a wooden frame, mixing 3 shovels of stones, 2 shovels of sand, and 1 shovel of cement with a little water, pouring into the form, and waiting for it to dry! John Becker, President-Pennsylvania Chapter of American Concrete Pavement Association (ACPA) said, “There is a science to making good concrete.”
The Pennsylvania Turnpike Commission (PA Turnpike) is using “long-life concrete” for the first time—where the science is even more complex for this project than other on-site projects. Regardless of the cost, the expectation is that long-life concrete should last at least 40 years with minimal repairs vs. standard concrete that can last 26 to 30 years, Mr. Becker said. This particular product uses a mix with more small stones and less water, and is so intricate that the Association held “Just-in-Time” classes a month prior to paving to make sure the turnpike and its contractors understood the procedures.
This on-site project involves:
- A computer control center to create the product
- 2 sizes of stone—larger no. 57 and the new ingredient, smaller no. 8
- Fly ash
- 3 hoppers (vs. 2)
- New 1½-inch stainless steel load-transfer dowels (that don’t corrode)
- Daily lab tests to check the water & air ratio
- Daily field tests temperature of the mix
- A fleet of trucks to deliver the concrete from the on-site production plant to the roadbed
- 3 machines & a saw—including a slipform paving machine—that creates the frame as it spreads the concrete
- Polymer called “polyalphamethylsterene” to help it cure quickly and seal the surface—it prevents water from entering and provides a top layer for vehicles to ride
CONCRETE CONTROL CENTER
Dave Weiland, Plant Manager-Allega Concrete Corporation, is a “concrete chemist” where his “lab” 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. Built on a flat field off Route 980 near McDonald, Pa., the on-site concrete plant isn’t a novelty for such a large project, but this one is a variation on a normal facility because it is producing long-life concrete. Weiland uses 2 computers and 2 printers for all of his numbers and precise specifications to prepare the mixture in a huge cylinder, 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.
Dump trucks arrive throughout the day to replenish piles of sand and 2 sizes of stone—larger no. 57 and the new ingredient, smaller no. 8—(vs. the usual 1 stone). A front loader fills 3 hoppers with material (vs. the usual 2 hoppers), then drop it on conveyor belts that pull it into the cylinder, where Weiland adds water from a chiller as well as cement and fly ash from storage containers.
As a delivery truck pulls up, Weiland mixes the batch and pours it, prints out identical copies of the delivery paperwork for the driver and the turnpike, then sends the load on its way. As 100+ truckloads are filled every day, each move is choreographed in a pattern that usually has 2 trucks at the plant, 2 dumping at the roadbed, and 2 in transit.
FIELD TESTS:
WATER, AIR, TEMPERATURE, & LEAN OR STRAIGHT
More than that, as many as 4 times a day, 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 the weather conditions change—damp and cloudy mornings change to sunny and dry afternoons, or vice versa … “We adjust the moisture and there are small variations,” said Weiland.
In one test, technicians take a sample of stone from the storage pile and weigh it, heat it to boil off all water, then weigh it again to find out how much water had been absorbed by the stones. Too much water, Weiland reduces the amount he adds from the chiller; not enough water, he activates sprinklers to wet the storage piles. Another test measures the amount of air in the mixture—ideally 5.5% to 8.5% air bubbles provides room for the concrete to expand and contract without much cracking. If not enough, Weiland injects more. Every 15 minutes or so, Vince Fusco, Concrete Technician, uses a thermometer to make sure the concrete is still the right temperature (it almost always is). 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.
PAVING PROCESS: (See illustration at top of article, and ISCP home page photo)
The delivery trucks take the concrete to the construction site, where 3 large machines lay the new roadway. The trucks dump their load into a cylinder on a placer—a machine with a conveyor—to carry the concrete to a large funnel that dumps it onto the subsurface in piles. After the piles are dumped, a slipform paver—a machine that straddles the subsurface—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-inch-thick 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 a texture/cure machine straddling the new road surface that sprays the polymer curing agent from the front and drags tines from the rear to make lines in the concrete to help keep the surface smooth.
Within 24 hours, before the concrete cures completely, crews use a huge masonry saw to make cuts across the highway every 15 feet—a key step to control surface cracks as it expands and contracts.
FIELD OFFICE:
Mr. Fusco’s work isn’t limited to the construction site. At the PA Turnpike’s field office in McDonald, he has a trailer devoted to a series of quality control and assurance tests. Every day, he fills 18-inch plastic cylinders with samples from that day’s paving and takes them the field office:
Group 1 is stored in a garage-then given a pressure test after 7 days
Group 2 is placed in a tub with a lime solution-then given a pressure test after 28 days
The pressure machine squeezes them until they burst: To be acceptable to the PA Turnpike, Group 1 must withstand 3,000 pounds per square inch after 7 days, and Group 2 must withstand 4,000 pounds after 28 days. All of that testing can provide a benefit for the contractor who may receive bonus payments for meeting or exceeding standards for the mixture’s smoothness of the surface, depth of the concrete, and the water-cement ratio.
TIGHT SCHEDULE:
Because of the region’s unusually wet spring and summer, the job was delayed a month, so Allega and the PA Turnpike are on a tight schedule to finish paving this section of the highway. It began July 18, 2018, and according to Bob Kohlmyer, Construction Supervisor-CDR Maguire/Construction Manager-PA Turnpike project, “The schedule has us finishing some time in late October.”
That often means starting at 4 a.m. and working through periodic showers. During a mid-August site visit, rain interrupted work four times, causing crews to spread plastic over recently spread concrete to prevent additional water from marring the surface. Mr. Kohlmyer said, “Instead of calling it quits on rainy days, they sometimes say ‘Let’s go for it and see what happens.’”
LONG-LIFE CONCRETE
Long-life concrete has been used in Pennsylvania before, first in 2012 by the Pennsylvania Department of Transportation (PennDOT) for an Interstate 95 project near Philadelphia. 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, according to Rich Kirkpatrick, Spokesman.
Becker 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—new 1½-inch stainless steel dowels (that don’t corrode)—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, 6-inch-tall, hollow load transfer bars made of steel covered with epoxy were used, but the epoxy often got nicked, causing the steel to rust. Mr. Becker said overall, the cost of additional stone and new load transfer bars are offset by the use of less cement and water, making the final cost of long-life concrete “very close” to the cost of the old method. He added, “We’re looking to get at least 40 years out of the long-life concrete—an additional 10-plus years compared to regular concrete projects.”
In an email, Terry Dreher, Materials Management Supervisor-PA Turnpike, said, “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.”
For the full Pittsburgh Post-Gazette article titled “Concrete science: Laying Southern Beltway pavement isn’t just mixing sand, stone and cement”, please go to: http://www.post-gazette.com/news/transportation/2018/08/27/Southern-Beltway-Allegheny-County-Washington-County-paving-long-life-concrete-Pennsylvania-Turnpike-toll-road/stories/201808250007