Jointed precast pavement slabs reach mainstream acceptance status.
By Peter J. Smith
With the recent emphasis on “shovel ready” stimulus projects that needed to get underway quickly, the use of precast concrete pavement slabs for rapid rehabilitation suddenly became a valuable tool in the pavement designers’ tool box.
Pavement engineers have long considered precast concrete pavement slabs as an optimal material for rapid repair of concrete pavements on heavily traveled roadways where durable and lasting repairs are needed. Engineers have always understood that casting concrete under controlled conditions results in slabs of superior quality necessary to provide long-term performance under high traffic volumes. Furthermore, they recognized that fully cured slabs can be put into service immediately after placement, eliminating the need for long curing periods and further traffic disruption. Both of these aspects of precast slabs make them especially attractive for rapid repair of concrete pavement in high-traffic areas (Figure 1).
The first experiments with precast pavement slabs were conducted in the 1970s in Michigan, New York, Florida and Virginia. Although initial trials were generally successful, real momentum for broader implementation never materialized until 2000 when the U.S. Federal Highway Administration (FHWA) introduced its Concrete Pavement Technology Program (CPTP) to improve the performance and cost-effectiveness of precast, prestressed postensioned and jointed precast pavement systems. At about the same time, a number of private sector companies initiated independent development efforts.
Two design concepts
Precast pavement developed from two distinctly different design concepts. One concept involved the use of precast, prestressed slabs that were post-tensioned to create interconnected assemblies of slabs 100 feet to 250 feet in length. The advantage of this technology, developed primarily for continuous installations, is that many joints are eliminated and slab cracking is minimized due to the compression induced in the slabs. An additional advantage is that slabs can be thinner than conventionally cast concrete pavement while providing the same, if not better, structural capacity. A full description of the development of this technology is provided in the January 2009 issue of the FHWA CPTP TechBrief.
The second design category of precast slab development is that of jointed precast pavement slabs. The development of jointed precast pavement technology emerged from both the public sector and from private sector companies at about the same time. The timing of the simultaneous efforts was triggered by the need for rapid repair techniques in different parts of the country. Most of the concrete pavement that now needs to be rehabilitated in the United States and Canada is jointed and will need to be repaired in an intermittent fashion simply because municipalities can’t afford to replace everything at once. The jointed design is often compatible with existing concrete pavement since slab lengths, as they relate to expansion and contraction needs, are similar.
The FHWA initiative
The FHWA CPTP initiative began with a study conducted at Michigan State University on I-675 in Zilwaukee, Mich., and M-25 in Port Austin, Mich. This study resulted in a drop-in-type repair slab that generally emulated the cast-in-place dowel-bar retrofit method of pavement repair, already in widespread use. In this method, the existing pavement and a thin layer of subgrade material are removed to accommodate flowable-fill bedding material and the new precast slab. Prior to placing the new slab, the flowable fill is screeded to the appropriate level to provide grade control and support to the new slab (Figure 2). Load transfer across transverse joints is established by sawing slots in the existing adjacent pavement to accommodate the load-transfer dowels. A full report of this study was provided in the October 2008 issue of a FHWA CPTP TechBrief.
Private sector initiatives
The Michigan Method, as it is now referred to, has been adopted in various forms by a handful of companies. The Uretek Co. of Tomball, Texas, uses proprietary urethane foam for the bedding material and a proprietary plate device for load transfer. Another system recently developed by the Roman Stone Corp., Long Island, N.Y., uses urethane foam bedding material but incorporates standard dowels for load transfer. A similar concept was recently adopted by the Illinois Tollway Authority as an acceptable standard for precast slabs to be used on the Tollway System.
In 2001 the Fort Miller Co. Inc. of Schuylerville, N.Y., introduced a proprietary jointed pavement system called the Super-Slab System that accomplishes load transfer by casting slots on the bottom of one slab that allows the slab to be lowered over dowels cast in the end of the previously set slab. For drop-in repair slabs, dowels are inserted and grouted in the adjacent existing slab before placing the new slotted Super-Slab. Slab support is accomplished by introducing a thin layer of fine granular bedding material that is precisely graded to provide grade control for the slabs. Complete slab support is assured by pumping bedding grout (under-sealing grout) into a grout-distribution system cast in
the bottom of the slab to fill any voids that might have resulted from the grading process.
In spite of numerous introductory projects of jointed precast pavements in several states, mainstream acceptance of the jointed precast pavement concept was slow in coming during these early years of use. Pavement engineers needed to be convinced that precast slabs could be installed to provide a coherent pavement structure equal to, if not better than, a cast-in-place concrete pavement. They recognized that load transfer devices across transverse joints, ties across longitudinal joints, full and complete bedding of the slabs and three-dimensional surface geometry must be achieved for precast pavement slabs to be successful. In addition, engineers needed to be assured precast slabs could be installed rapidly, in eight- or even five-hour work windows, and that the resulting pavement would provide a service life equal to or better than cast-in-place pavement slabs.
First large-scale convincing project
The first large-scale project that provided the assurance engineers were looking for was the replacement of the toll plaza for The New York State Thruway Authority at the Tappan Zee Bridge in Tarrytown, N.Y. This continuous installation project, completed in 2002, involved replacement of more than 17,000 square yards of pavement during eight-hour work windows while maintaining a traffic flow of more than 135,000 vehicles per day. The slabs were cast with standard load-transfer dowels across transverse joints and tie bars
across longitudinal joints.
Falling Weight Deflectometer tests on the finished pavement showed load-transfer efficiencies from average to excellent and further indicated the slabs were fully bedded. The fact that the slabs were installed at the rate of more than 330 square yards per day while maintaining full traffic flow was evidence that the “rapid” criterion had been met. Assurance of long pavement life continues to grow now that the Plaza pavement has been in place for eight years and appears to be in excellent condition.
Heavy Vehicle Simulator test provides evidence of long pavement life
Another important indicator of the life of jointed precast slabs for specifying engineers emerged from a Heavy Vehicle Simulator test that was performed on the Super-Slab System in California, by researchers from the University of California, Berkley, for California Department of Transportation (CALTRANS), from 2005 to 2006 (Figure 4). In that test, the slabs did not fail under normal highway traffic loads. The tests led researchers to conclude that the slabs would last at least as long as conventionally cast concrete pavement (38 years) under the design test traffic count of 155,000 vehicles per day.
Other key projects providing assurance
Acceptance of the use of precast slabs for rapid repair was further buoyed by the completion of a variety of projects in several states following the Tappan Zee Bridge project. Several projects involved overnight intermittent repair of interstate highways where slabs were used as a single drop-in repair or for repair of long stretches of a single or even multiple lanes. Most of the installation took place during an eight-hour nighttime window. In one shift, workers would set up traffic maintenance patterns, remove existing pavement, install dowels and tie bars, and set and grout the new precast slabs before re-establishing the original traffic pattern.
The intermittent repair project on the New York State Thruway (I-95) in New Rochelle, N.Y., built during 2007 and 2008, was another notable project that involved installation of more than 700 slabs (9,400 square yards) of full-depth repair in five-hour work windows. Approximately seven miles of this interstate highway was repaired using the precast slab technique while maintaining an average daily traffic flow of more than 140,000 vehicles. The success on this project provided irrefutable evidence that the use of precast slabs is truly a viable method for rapid repair.
Another key project that supported specifier acceptance of the jointed precast slab concept was the overnight replacement of the intersection of Watt Street and Route 7 in Schenectady, N.Y., in 2006 by The New York State Department of Transportation. The critical part of the intersection was replaced under live traffic with 158 slabs (3,200 square yards), placed continuously in 18-night work windows. Each morning the entire intersection was open to traffic. The important achievement on this project was that precast slabs were fabricated and placed to achieve the heavily contoured pavement surface of the intersection (Figure 3). This and other three-dimensional precast pavement projects provided engineers with assurance that precast slabs can be detailed, fabricated and installed to meet all vertical and horizontal geometric requirements.
Sidebar: Square Yards of Jointed Precast Slab Installation
Rate of Acceptance
The rate of acceptance of precast concrete paving slabs can be demonstrated by plotting the area of completed projects installed since 2001. The graph above shows the cumulative amount of installation for slabs of all types, for intermittent repair and for production application.
For the purpose of this comparison, the “Production Slabs Installed” curve (production slabs are defined as slabs installed on projects that were executed in a conventional design, bid and build fashion) is shown separately from “Total Slabs Installed” since the latter includes small demonstration or test projects that do not indicate market-driven implementation. The “Intermittent Repair Slabs Installed” curve is shown separately as this category is likely the most important application of precast slabs in the United States.
It is evident from the graph that jointed precast slab installations started off with a bang with the continuous-installation project at the Tappan Zee Bridge Toll Plaza in 2001. After that project, “demonstration projects” started to appear as represented by “bumps” in the “Total Slabs Installed” curve.
The overall trend shown in the “Total Slabs Installed” curve is important. Approximately half of the total installed square yards to date (40,000 square yards) occurred over a six-year period from 2001 to 2007. The remaining half (about 40,000 square yards) was installed in just 1.5 years since 2007, indicating a significant upward trend toward precast slab implementation. Projections for the next two years are even more convincing. Firm orders to be installed during the next two years exceed 56,000 square yards and an additional 157,000 square yards are expected to be awarded or go to bid within the next several months.
Acceptance in nine states and two provinces
Another indication that the jointed precast slab idea has developed meaningful traction is the number of states and provinces that are now using or planning to use precast slabs. Prior to 2009, precast slabs were installed on a production basis in New York, Colorado, New Jersey, Illinois, Minnesota, Ontario and Quebec. In 2009 alone, projects will be completed or will be on-going in New York, New Jersey, Utah, Pennsylvania, Nevada, Virginia and Ontario. California will be added to the list in 2010 to make a total of nine states and two provinces that use jointed precast pavement slabs.
A fi nal important indicator of precast pavement system acceptance is that a number of states have specifi ed jointed precast pavements multiple times. At least 13 projects have been specified and built in New York State alone. New Jersey has specifi ed slabs on at least five projects since its fi rst project in 2007. The Illinois Tollway Authority is about to install slabs on its fourth project and Ontario’s Ministry of Transportation is presently installing its third production project.
There can be no doubt that jointed precast pavement slabs are now a mainstream method of rapid rehabilitation of concrete pavements. It has been clearly demonstrated that precast systems work in any pavement confi guration and that they can be installed on a production basis in work windows as short as five hours. This should be truly good news for the concrete pavement industry that has thus far been limited to rapid repair materials that last for only up to 15 years at best.
Peter J. Smith, P.E., is vice president of Market Development and Product Engineering for The Fort Miller Co. Inc. of Schuylerville, N.Y., where he develops and promotes new precast concrete products for the highway and bridge industry. He is a graduate of Rensselaer Polytechnic Institute, where he received a B.S.C.E. and is a registered Professional Engineer in the State of New York.