Freeway on a Flatbed: New Precast Panels Make Pavement Portable on I-5
District 7 is implementing a cutting-edge pavement technology on the new I-5/SR-14 connector â€“ precast, pre-stressed concrete panels, a first for the State of California.
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If you ever played with Hot Wheels cars and tracks as a kid, you’re already familiar with the precast panel concept. The Hot Wheels track came in pieces, allowing you to assemble an orange, plastic-paved freeway anywhere you wanted. Now, the same strategy that has enchanted car-loving 7-year-olds for generations is being applied to freeways, albeit on a larger scale and with sturdier materials. This month, for the first time ever in California, District 7 is installing precast concrete panels on a connector.
Technically, they’re known as Precast, Pre-stressed Concrete Panels, or PPCPs. (We’ll break that down shortly.) District 7 is using them on the northbound Golden State Freeway (I-5) carpool connector to the northbound Antelope Valley Freeway (SR-14). PPCPs have been used only once before in the district, on a mainline demonstration project in El Monte on the San Bernardino Freeway (I-10) in 2004.
To understand the beauty of PPCPs, you need some basic information about concrete pavement construction. A typical freeway in District 7 has three layers. The bottom layer is an aggregate base, which looks like dirt but is really engineered rock and small particles that become stable when compacted. On top of that there’s a lean concrete base – “lean” because it has less cement in it than regular concrete. The top layer is that old pavement standby, Portland cement concrete (PCC).
Normally, PCC is poured on-site, but with PPCPs, concrete panels are cast and cured at a plant off-site – in this case, in Perris – and then trucked to the construction location and installed using a crane. Part of what makes this pavement technology so promising is the pre-stressing that occurs when the panels are cast.
“There’s a seven-strand steel cable that’s stretched out within the concrete when it’s poured. After the concrete sets up, the strand tension is released, like a rubber band,” explained District Materials Engineer Kirsten Stahl. “Relaxing the steel pulls it tighter, putting the concrete under compression. Concrete performs best when it’s compressed.”
At the construction site, another set of cables are threaded through the panels, aligned and pulled tight – almost like the leaves in a dining room table. To ensure that the fit stays nice and tight, grout is pumped in around the cables. On the I-5/SR-14 project, crews are laying 43 panels over several days. Most are eight feet by 19 feet, with seven larger ones (29 feet) to be used where the connector widens.
PPCPs have a number of benefits. One, because precast panels are fabricated and cured off-site, the freeway can be opened to traffic almost immediately after installation. No waiting for the concrete to gain strength, no delayed closure openings. Two, since the panels are cast in a controlled environment, there’s more control over production. Three, pre-stressing the concrete means the slabs are more durable (they last 40 years instead of 20), resulting in savings on maintenance costs. And four, the panels are thinner.
“The thinness is one of the best reasons to use PPCPs,” said Stahl. “The problem with conventional concrete is you need extra thickness – about 14 inches – to get 40 years out of it, so you have to dig deeper. With PPCPs, we can get 40 years out of a nine-inch panel.”
Although PPCPs are a relatively new pavement technology, they’re increasingly being used in the U.S. and Canada. Caltrans has learned much from PPCP projects in other states, particularly Texas, Missouri, Delaware and Virginia, all early adopters, Stahl said. The I-5/SR-14 connector project is an excellent opportunity for District 7 to gain practical experience with PPCPs. Staff will be assessing every step of the process, including transport, installation and performance. Other transportation departments are paying close attention as well.
“The country’s infrastructure is aging. A lot of it was built in the ’50s and is now deteriorating. It needs to be replaced,” said Stahl. “The nation is watching, looking at our experience with PPCPs. They’re learning from us.”