Mehdi Parvini, Ph.D., PE, PMP
Concrete pavements are durable and long-lasting, but they need expensive equipment and time for curing. The need for costly equipment limits the application of concrete pavements in remote areas where access to a concrete paver is not available. On the other hand, the need for concrete curing limits the application of this type of pavement for high traffic volume urban areas where closing the road for a long time is not feasible.
Precast Concrete Pavement (PCP) is a relatively new technology which allows building durable and long-lasting pavements in a short time without the need for a paver. This technique also allows constructing pavements under harsh climate conditions (high or low temperature, wind, rain, etc.). In order to optimize the design by reducing the thickness of the precast panels, reinforcement and prestressing is utilized. Prestressing could be in the form of pre-tensioning or post-tensioning. When panels are prestressed, the strategy is referred to as Prestressed Precast Concrete Pavement (PPCP).
Precast Concrete Pavement Characteristics
Precast Concrete Pavement systems are defined as concrete panels fabricated off-site, transported to the project site, installed (connected and supported) on a prepared foundation (existing or re-graded foundation).
“Precast concrete pavement could be used for repair, reconstruction, or new construction of both Hot Mix Asphalt (HMA) and Portland Cement Concrete (PCC) pavements.”
Because of the fact that panels are fabricated off-site in a plant, they already gain strength before they are transported to the field and do not need any curing at the project site (Figure 1).
While PCP is considered a long-life and durable pavement strategy, it should be mentioned that it does not address failures due to subgrade or drainage problems. Also, this type of pavement has a higher initial construction cost (Figure 2).
Precast concrete pavement is used in two main applications; repair of an existing pavement, and a new construction.
Repair Existing Pavement
The first application is for the repair of an existing pavement. This application, which is commonly called individual slab replacement (ISR), is a corrective maintenance strategy with an expected service life of 5 to 10 years. Broken concrete slabs are removed and replaced with precast panels. Although the strategy is called individual slab replacement, it could be used for replacing a short number of adjacent slabs in a lane or different lanes (Figure 3). The thickness of the precast panels used for slab replacement is often equal to the thickness of the existing pavement. The existing base is often remained in place for a slab replacement job, unless it is found necessary to repair or replace it after removing the slab.
Replacement or New Pavement
The second application is for a complete reconstruction of an existing pavement, or building a brand new pavement. The former is commonly called lane replacement (LR). Both the lane replacement and the new pavement are expected to have a service life of over 40 years. There is technically no limit for the length of the PCP in this application. The precast panel thickness is designed to perform for its expected service life considering the traffic, climate, and the support/soil condition. In the case of the reconstruction, the existing base is also removed and replaced with a new base.
The main two categories of precast concrete pavements are jointed and continuous.
In a jointed system, panels are placed side-by-side and connected with dowel bars to provide a load transfer system (LTS) from one panel to another one. Often the dowels are cast in one side and receiving slots are provided on the other side of a panel. After placement, the dowels in a panel fit into the slots of an adjacent panel, and slots are filled with a special grout to bond the dowels and connect the two panels. This creates a transverse joint between the panels that is often between 6 to 12 mm wide.
In a continuous system, a series of panels that have key on one side and keyway on the other side are placed side-by-side and glued together. Then the row of the panels are post-tensioned at the ends to hold them together. This creates a long panel section that is 50 to 75 m long. There would be no active transverse joint between the panels in this section, however, there is an expansion joint with a width that depends on the length of the section and the climate condition at either end of this post-tensioned section.
Figure 4 illustrates the two categories of PCP.
Precast concrete pavement systems could be generic or patented. There has been an increasing trend in innovating new systems of PCP. The systems often differ in the way the panels are connected (load transfer system), the way panels are graded, and the way panels are supported underneath. Figure 5 shows a number of different configurations used for panel load transfer system.
PCP panels are reinforced, prestressed, or both. The main role of the reinforcement is to hold the panels intact during transportation and installation. On the other hand, prestressing (pre-tensioning or post-tensioning) is used to increase the strength of the panels under traffic loads. Jointed systems are often nominally reinforced or pre-tensioned, while continuous systems are reinforced and post-tensioned. The reinforcement and tensioning also depends on the panel dimensions. Panels longer or wider than 4.5 m are required to be pre-tensioned.
In addition to reinforcement and tensioning strands, dowel bars, tie bars (for some systems), and lifting/grading devices are installed in a precast panel. For post-tensioned PCP, ducts for post-tensioning strands are placed in concrete panels (Figure 6).
Precast Concrete Pavement Construction
The main steps of a PCP construction are fabrication, transportation, and installation of precast panels.
Fabrication of the panels involve setting reinforcement, concrete placement, pre-tensioning (if applicable), finishing, and curing.
Fabrication of precast concrete panels is often done in a precast plant. Most of the plants produce their own concrete mix. Since there are very tight tolerances for panel dimensions, it is recommended to use steel formwork that are preciously measured to cast the panels (Figure 7). Reinforcement, strands and other fixtures to be cast inside the panels need to be securely tied to avoid any movement during concrete placement. Attention should be paid to mix consolidation, and finishing and texturing the panel surface. After casting concrete, panels are often steam cured to expedite releasing the formwork, and hence increasing the production rate.
Panel Storage and Transportation
There might be cases when panels need to be stockpiled in the plant for a while. In such a case, panels need to be properly stored to avoid any damage or over stressing due to inadequate supports. When stacking panels vertically, proper dunnage placement needs to be considered (Figure 8).
Panels are loaded on a truck at the plant and moved to the job site to be installed in place. Due to the weight of the precast panels, often only one panel is transported with each truck (Figure 9). Depending on the size of the panels, the load might be oversized with or without a need for a permit. Similar to the loading phase, attention should be paid to the unloading phase to avoid any damage to the panels when lifting and placing them on the ground. If panels have different sizes or shapes, they need to be properly tagged at the source to make sure that the sequence of the panel arrival to the job site and their placement order is right.
Installation of panels consists of support preparation, placing panels, grading panels, underslab grouting, dowel grouting, and joint filling. If needed, grinding might be done to remove any vertical elevation difference at the joints.
After making sure that the subgrade soil is not agitated, or re-compacted if it was disturbed, the base layer that could be a lean concrete base (LCB), dry lean loncrete (DLC), or a bituminous type base is placed and properly graded (Figure 10).
After gaining strength (for the cementicious type base), panels are lifted and placed on top of the base (Figure 11). A critical operation in installation phase is grading the panels. This is done by using shims, leveling bolts, or other grading methods. It is important to make sure that joint widths (both for transverse and longitudinal joints) are uniform and within the specified tolerance.
The next process is grouting under the panels through the grout ports (small holes from the top to the bottom of a panel placed at couple of locations within the panel surface) to make sure that potential gaps beneath the panels are filled and a uniform panel support is provided. After underslab grouting, grouting of the dowel slots is done to connect the panels and provide an accceptable load transfer system (Figure 12). At the end, joints are sealed with the specified joint seal material.
Since the main contributing factors to a PCP performance are panel quality (strength and durability), load transfer at joints, and panel support condition, attention should be made to assure acceptable fabrication quality at the plant, and high quality installation workmanship in the field.
Similar to any other construction activity, production rate directly impacts the price of a precast concrete pavement. Currently, typical production rate per night shift closure (often 11 pm to 5 am) is as follows:
- For individual (separated) repair application: 15 to 20 panels
- For continuous application: 30 to 40 panels
- These rates are based on a typical panel size of 3.7 m wide and 4.5 m long. The production rate changes for smaller or larger panel sizes.
Precast concrete pavement is increasingly gaining popularity due to its advantages. Its rapid construction and anticipated long service life is the result of quality materials and fabrication. While the initial construction cost of a precast concrete pavement could be higher than a conventional cast-in-place pavement, its future low maintenance cost could make it the strategy of choice when a cost-benefit analysis is performed.
Innovation is an important aspect associated with precast concrete pavement. Quite a few new ideas have merged since the inception of this technology. This has allowed the contractors to customize this technology to fit their unique capability, equipment availability, and operational methods.