The California Department of Transportation (Caltrans) and U.C. Berkeley may have discovered the fountain of youth for asphalt. While most asphalt concrete (AC) pavement lives only 10 to 15 years, their long-life pavement can last 30 to 40 years, with some minor plastic surgery every five to 10 years.
One of the first recipients of this asphalt elixir is the I-710 in California.
“The first segment of I-710 opened to traffic in 1952; the last segment opened in 1970,” said Maria Raptis, public information officer of Caltrans. “Today, in 2008, this heavily traveled freeway with 234,000 average daily traffic has one of the highest concentrations of deteriorated pavement in the state.”
And that’s no surprise. Not only is the freeway heavily traveled, but 18 percent of its vehicles are freight trucks carrying heavy material to and from the ports of Long Beach and Los Angeles.
Maintenance costs for the DOT are high, costing millions of dollars each time the pavement is replaced — and with traditional pavement, it is replaced often.
“On urban freeways, if they use conventional pavement, the life span is typically 10 to 15 years,” said E.B. Lee, associate researcher of U.C. Berkeley’s Institute of Transportation Studies (ITS). “With long-life pavement, it is 30 to 40 years. That’s a big improvement.”
The long-life pavement will save Caltrans millions of dollars over the pavement’s life.
“The bottom line is: whatever comes out the most cost effective for the taxpayer that’s what we should be going to,” said Chuck Suszko, chief of construction engineering for Caltrans.
In addition to saving taxpayers’ money, long-life pavement will save frustration. All motorists get annoyed when road maintenance slows their drive to work, and long-life pavement means this won’t happen nearly as often. Less delays means a faster commute for motorists and truckers, and that means greater productivity for businesses.
“We’d like to stress that point,” said Raptis. “Cost savings are not just for the DOT. They are for our users, as well.”
Mixing it Up
Kids that like running toy cars over tracks may have a future as researchers at U.C. Berkeley. Researchers used the school’s heavy vehicle simulator (HVS) to run a set of dual tires over different types of pavement — over and over again.
“It cost us $100,000 a month to run them,” said Suszko. “Of course, it saved a lot of money, so it was a good investment. It’s a way of taking something from the lab and testing it before using it on a road.”
But the experiments showed something unpleasant about the researchers’ top layer design: it was prone to rutting.
Rutting is when pavement isn’t tough enough to stand up to California traffic and it caves in under the pressure — literally. The pressure from thousands of heavy vehicles forms channels in the pavement.
The channels are dangerous. Water gets trapped in them, and unlike the water strider, cars were not designed to drive on water. Nor were people designed to handle hydroplaning cars.
So how could researchers solve this problem? Originally, they were using well-graded asphalt. The asphalt had big pieces, medium pieces and small pieces of aggregate in it.
However, it is commonly understood amongst those who study asphalt that gap-graded mixtures reduce rutting. Gap-graded mixtures:
• limit medium-sized aggregate. There are large pieces of stone and small pieces, but the medium pieces are removed.
• use a strong aggregate, such as granite.
• use more asphalt than other mixtures.
• use a tough binder.
• maximize stone-on-stone contact.
When stones touch other stones, they unite and form a stone fortress that is difficult to penetrate, and the rubber binder fills in the gaps where the medium sized aggregate used to be.
“The rubber is a thick binder, so you really pump the mix full of binder and that makes it crack resistant,” Suszko said. “The top layer we use now is a gap-graded rubber mix.”
Rubber? Isn’t that for bouncy balls and yellow duckies? In the case of asphalt, rubber takes on a whole new use.
“I believe the top two layers of most AC pavements in California will have rubberized asphalt in the next few years,” said Lee. “There are two benefits — less noise and also better performance in terms of the life span. Also, the rubber comes from recycled tires and recycling is good for the environment.”
But rubber in the top layer isn’t enough to make asphalt last for 30 plus years. In fact, the top layer gets replaced every 10 to 15 years anyway. Having strong material in the bottom two layers was even more important.
“The key [to long-life AC pavement] is a different layer design,” Lee said. “Typically one or two layers of the same mix are placed. In long-life pavement, we put three different AC mixes.”
So how did researchers make a bottom layer that wouldn’t crack under pressure?
“It has a rich bottom, which means you increase the asphalt content by 0.5 percent,” explained Suszko. “That prevents bottom up cracking.”
But asphalt wasn’t enough. The bottom layer also is thick — 3 in. (7.6 cm) to be precise. The thickness helps the pavement resist tensile strain from massive trucks that fly over it carrying heavy loads.
The intermediate layer is actually two layers, each layer 3.25 in. (8.2 cm) thick. The binder has a high viscosity and a hard aggregate.
In total, the pavement being placed for this job is 10.5 in. (26.7 cm) thick. In many long-life projects, the pavement is thicker, but the DOT had an important consideration in this case. It wanted to ensure tall freight trucks had enough clearance to safely travel under the overpasses.
“Putting 10.5 inches of asphalt will shorten the vertical clearance,” Raptis explained.
When the test results were in, researchers had a mixture that would last. There was just one important thing left to do: pave the roads.
Most paving is done at night in seven to 10 hour intervals. But Caltrans wanted to get the job done quickly, save taxpayers money and keep construction workers safe. By working only at night, Caltrans would have had to shut down spot locations for weeks, which would have been expensive and annoying for drivers.
Instead of limiting itself to nighttime closures, Caltrans could shut down two lanes continuously. The job would get done faster, but angry drivers would have a hard time getting to work.
And doing the work two lanes at a time had another disadvantage: there was no room for the equipment to spread out. Trucks, wheel loaders and paving machines would have to cram themselves into a small space or trickle onto the freeway on-ramps, slowing things down for drivers and construction workers alike. So what was the best strategy?
To analyze the situation, Caltrans turned to a program called Construction Analysis for Pavement Rehabilitation Strategies (CA4PRS). This program was developed by Berkeley’s ITS department. The Federal Highway Administration (FHWA) formally endorsed it in 2008, classifying it under “Priority, Market-Ready Technologies and Innovations.” The FHWA also arranged group licenses for all fifty state DOTs.
“It’s a way of looking at construction productivity. It’s been useful in making decisions on lane closures. Do you do them every night or do you do complete closures?” said Suszko.
The program has only been calibrated on a few projects, but Berkeley researchers are working to add more data. Using the program, project planners can type in how much they want to spend, what the road conditions are and other constraints and CA4PRS calculates the maximum amount of pavement that can be rehabilitated for a given set of constraints.
With help from CA4PRS, Caltrans determined that the best strategy was to simply shut down half of the freeway over the weekend.
“One direction of traffic — either northbound or southbound — will be detoured onto one side of the freeway while the contractor works on the other side of the freeway,” Raptis explained. “The result is lane reduction, rather than a full and complete freeway closure. One side of freeway will have two lanes going northbound and three lanes going southbound. This strategy allows the contractor to work continuously for 53 hours with minimal interruptions to traffic flow and eliminates worker exposure to traffic.”
Having one entire side of the freeway allows contractor Atkinson Construction to get its equipment on and off the road much more efficiently, because the asphalt concrete delivery trucks and removal off hauling trucks don’t have to compete with Corollas and Maximas.
It also makes the weekend paving safer for Atkinson, which no longer has to work on a road next to “lush-hour” drivers, who frequent the roads at around 2 a.m., when the bars close.
Heavy-duty equipment used on the job includes excavators, front loaders, motorgraders, milling machines, mechanical breakers for rubblizing PCC slabs, guillotine breakers for PCC slab cracking and, obviously, paving machines, including asphalt pavers, pneumatic-tired rollers, vibratory steel rollers, hauling trucks and AC delivery trucks.
“The guillotine breakers crack concrete. They come down on the concrete just like a guillotine,” Raptis said.
Getting the Word Out
Before beginning, Caltrans had to solve one last problem. Freight trucks often travel to and from the ports of Long Beach and Los Angeles. They carry important cargo that businesses need. If these trucks didn’t know about the freeway closures, they wouldn’t be able to get their shipments to businesses on time. Regular commuters also would be inconvenienced if no one told them about the closures. Caltrans needed to tell drivers.
“Fortunately there are alternate routes — the 110, the 605,” said Raptis. “People here know how to get around, but we need to alert them. We’re doing an extensive public outreach on the radio and on the air to alert the motoring public. Traffic reporters, CHP, the chamber of commerce and the port officials are being briefed, so that we can make sure no one is caught off-guard.”
The project is scheduled for completion in 2011 and it will be completed in several phases.
“We’re doing seven closures in 2008, paving the overpasses in spot locations between the 91 freeway and the 105,” Raptis explained, “and construction will continue next year on replacing all the pavement for 9 miles from I-405 to Firestone Boulevard.”
Even after the project’s completion, Caltrans will continue to improve pavement life for future projects.
“Renewing pavements on urban highways has become an important issue confronted by many transportation agencies today, as they are now showing signs of wear,” Raptis said. “Caltrans continues to work with the pavement industry and the University of California at Berkeley Pavement Research Center to improve pavement performance, extend the life of the pavement, and to provide the motoring public with smoother ride.” CEG