When transportation agencies and contractors must reopen roads to traffic as soon as possible, precast concrete pavement is the best solution.
By Evan Gurley
You might say precast concrete pavement is a late bloomer. One of the earliest reported uses of precast concrete pavement technology in the United States occurred in South Dakota during the 1960s. In the 1970s, Michigan, New York, Florida and Virginia all performed additional experiments with precast concrete pavement. And although initial trials were generally successful, real momentum for broader implementation didn’t materialize until about a decade ago when the U.S. Federal Highway Administration (FHWA) introduced its Concrete Pavement Technology Program (CPTP) to improve the performance and cost-effectiveness of the systems.
Transportation engineers need to be convinced
Since that time, numerous jointed precast concrete pavement systems (PCPS) have been developed (both proprietary and nonproprietary) and are currently being installed on roadway projects all across North America.
The future is promising for jointed PCPS technology, but mainstream acceptance has been slow. Pavement engineers need to be convinced that precast slabs will provide a coherent pavement structure equal to, if not better than, a cast-in-place (CIP) installation. They recognize that load-transfer devices across transverse joints, ties across longitudinal joints, full and complete bedding of the slabs and 3-D surface geometry must be achieved for PCPS to be fully successful.
Engineers also need to be convinced that PCPS installs rapidly, in eight-hour or even five-hour work windows, and that the result will provide a service life at least as good as CIP pavements. In the past decade, extensive testing by state DOTs and other roadway specifiers has convinced them that PCPS can do all this and more, but it has been a case-by-case approach that has led to slow adoption.
“Precast pavement has a huge potential for speeding up and reducing the risk of overnight construction and getting the roadway returned to the public without them even knowing anything happened – except for now, the roadway is easier to drive on and it stays that way for years,” said Kirsten Stahl, senior transportation engineer/district materials engineer with Caltrans.
Fastest system out there
With recent improvements, most pavement engineers consider PCPS to be the optimal material for rapid, durable repair on heavily traveled roadways. Engineers understand that paving slabs manufactured off site under controlled conditions have the superior quality needed for long-term performance on busy roads. More importantly, engineers appreciate that fully cured slabs support traffic immediately after placement, thus eliminating the long delays and traffic disruption experienced with CIP curing.
“The speed and efficiency of a jointed precast concrete pavement system are actually the primary drivers in the way that we have used precast pavement for repairs,” said Scott Nussbaum, Region One materials engineer, Utah Department of Transportation.
All CIP pavements, even specialized, fast-track concrete, require on-site curing and finishing time. PCPS is especially beneficial for work windows of eight hours or less, because slabs can be placed right up to the end of work shifts. “We like to close the road for the least amount of time as possible and get it open to traffic. We want to do all of the work off-peak. Precast concrete pavement systems are really the only way we’re able to stay within the kind of windows that we want to have and still expect durability for the repair,” added Nussbaum.
The speed, durability and service to local business and the driving public make PCPS the optimal transportation system for the specifying and design community. “I would recommend this product to owners, contractors and colleagues,” said Erich Brown, project manager, The Lane Construction Group. “It’s definitely a good option when dealing with high traffic-volume areas and areas where you have limited hours of work. It’s an expeditious way of repaving the roadway especially where you do not have access to full closures of the roadway or traffic redirection.”
More durable, longer service life
Durability of precast pavement is enhanced, because slabs are fabricated in a controlled plant environment free from adverse temperature and weather-related conditions. Precast concrete manufacturing plants stock a wider selection of admixtures and aggregates needed to extend pavement life. Plant casting eliminates problems associated with job-site curing and CIP shrinkage. In addition, PCPS service life far exceeds that of fast-track concrete or asphalt overlays.
“When comparing precast pavement with the other high-early strength and rapid repairs using cast-in-place concrete in Utah’s environment with as many freeze/thaw cycles as we have and as much salt as we put on our roadways, we know that the majority of our high-early cast-in-place concrete repairs do not last for more than five years,” said Nussbaum.
“We have the potential with the precast system that’s properly cured, carefully constructed and carefully placed to see 10, 15, 40 years – we don’t know. But if properly placed with some good attention from the installers in the field, we have confidence that these repairs are going to last many years and significantly longer than some of the cast-in-place repairs that we’re seeing now.”
Consistent, certified, quality product
In states where NPCA certification is required, an independent engineering firm inspects precast plants, and approval is based on adherence to extensive manufacturing and inspection procedures.
Concrete is tested at frequent intervals in state-of-the-art facility labs, and every slab is thoroughly examined for defects and specification conformance. Quality control programs are specifically designed to prevent inferior concrete from reaching the job site, greatly enhancing the uniformity and quality of the finished pavement. The NPCA Plant Certification Program is accredited by ANSI (the American National Standards Institute).
“The biggest advantages of using precast pavement is that you get a quality product where you can control the manufacturing, design it to fit the needs of the project, improve the life-cycle cost of the product, and it’s safer and more durable. It’s not just one benefit – there are multiple benefits,” said Stahl.
Precast slabs last longer than alternative materials and enhance job-site safety, as fewer repair projects are required over time. Night work is dangerous at best, and every night shift avoided significantly improves overall safety for highway maintenance workers.
“I’d advocate the use of precast concrete pavement, not just for my peers but for anyone who will listen,” said Stahl. “Whenever I see a tool that is very beneficial returning the roadway back to the public, making it safer for the workers and for the public, and giving us good service for an extended period of time, I would advocate that product or technology in future projects.”
A real cost/benefit analysis
Ultimately, designers and specifiers need to compare the cost of precast pavement to the cost of comparable pavement alternatives that can be installed in the same amount of time and with the prescribed service life expectancy. The usable life of fast-track CIP concretes varies as much as the types of mix designs used. CIP durability depends on mix design, worker skill, rate of placement, weather conditions during curing and finishing, and regional climate. When comparing other materials with PCPS – with a life expectancy of 40 years or more – it is important to compare life cycles and costs.
A detailed life-cycle cost analysis includes all maintenance and traffic protection over a 20- to 30-year period.
Comparisons must also include costs associated with required replacement of the fast-track concrete for the same time period. In a recent 20-year life-cycle cost comparison of the two materials – on a project where both materials were installed side-by side – results indicate that PCPS is 11% more cost effective.
“You can increase your production for roadway rehabilitation and repair, as you’re not limited to an eight-hour work window in which you can only physically place concrete for four hours and then you have to let it cure for four hours,” said Brown. “Using jointed precast pavement, you have the entire time almost entirely for setting these slabs. We were actually able to install more square yards of precast pavement than we were with cast-in-place pavement with the same amount of workers.”
Serving the driving public
FHWA’s Highways for LIFE program, which promotes and supports the implementation of ready-to-use and proven technologies, has identified PCPS as a vanguard technology. Clearly, PCPS has demonstrated success in numerous projects and holds promise for use in the rapid repair and renewal of our aging infrastructure while minimizing maintenance and repair needs. For a detailed description of other PCPS benefits, visit precast.org/pavement.
Why choose precast concrete pavement systems?
PCPS has several advantages:
• Rapid installation
• Expeditious opening to traffic serves public, business and agency reputation
• More DOT control over project deadlines
• No weather-dependent pour/finishing delays
• Safer, quick repair methods
• Eliminates lost time for CIP placing, curing and finishing
2. High quality, long service life
• Proven durability
• Consistent quality – certified plant inspection/testing procedures
• Precise manufacturing meets specified project tolerances
3. Environment & site advantages
• Keeps job site clean, meaning less construction-related pollution to groundwater and streams/rivers
• No extra site space required for material laydown, washout or ready-mix trucks
• LEED points
• Fewer trades for less site congestion and safer work site
• “Buy America” – local materials and labor support local economy
4. Cost considerations
• No CIP concrete overrun expenses
• Less labor time and payroll
• Job completion incentives
• Higher initial cost balanced by less construction time, labor and longer service life
5. DOT experience with PCPS
• Proven DOT track record of successful projects
• Just-in-time delivery and installation for scheduling/planning
• Precast engineering assistance with on-site problems like connection questions
• Precast producer coordination with DOT engineers on best project-specific solutions
• Versatility – surface finishes, skew, superelevation and decking configurations
• Simple, long-lasting load-transfer mechanism
Evan Gurley is a technical services engineer with NPCA.