Text and illustrations by Chetana Rao, Ph.D.
For decades, civil engineers and architects have sought precast concrete construction in transportation, industrial, commercial, residential and other innovative structural applications. In recent years, however, precast concrete has revolutionized highway and airfield pavement construction and has allowed owner agencies to balance several needs: accelerated construction and rapid renewal, adequate structural capacity, durability, sound quality assurance procedures, geometric and aesthetic versatility, and reduced life cycle costs.
The increased use of this technology has been triggered by the development of improved precast concrete pavement systems with complete design details (such as joint design and post-tensioning options) and installation methods that have been proven in the field. Also, they have addressed the increasing demands for rapid construction and reduced lane closure times in pavement reconstruction and rehabilitation. Precast concrete pavement slabs, cast at an off-site location under tight quality assurance standards, can be installed overnight, on weekends, and at off-peak hours with minimal interruption to traffic. Moreover, precast slabs can be designed equivalent to jointed concrete pavements with or without prestressing and post-tensioning options.
Precast concrete slabs are fabricated to meet design strength and geometric requirements. The slab geometry is designed to meet project-specific requirements. While a new pavement alignment typically should consider a thickness design for the slab at the selected joint spacing, most rehabilitation projects tend to match the elevation and joint locations of the existing pavement adjacent to the repair area. Slab thicknesses also should accommodate changes in the vertical profile of the roadway alignment.
Concrete slabs, precast or otherwise, require a mechanism to transfer loads across transverse joints while also holding the slabs closely together. In PCPS, this has been achieved either through post-tensioning the installed slabs or through the use of load transfer devices across the joints (see Figure 2). Post-tensioning operations have been carried out either from the end or from the mid-slab location. Often, an interlayer between the base layer and the slab or immediately underneath the slab reduces the frictional restraint.
In slabs without post-tensioning, several innovative devices and technologies have been developed for achieving load transfer across slabs.
The most widely used are dowel bars, but several different means of field installation have been adopted. In addition, mechanical couplers or tie bars are used across longitudinal joints to tie adjacent lanes. A few projects also have attempted to post-tension across lanes. A joint design that allows for free expansion and contraction of the slabs is a critical factor for good pavement performance.
Finally, precast slabs also use grouting materials to fill encased hardware used in the installation process, including post-tensioning and load transfer devices. Some PCPS also use bedding materials or a bedding grout to provide a uniform and stable surface underneath the installed slab. The slabs are prefabricated with slots or ports for filling grout into the system.
The precast industry has incorporated some or all of the features discussed above into complete systems. The alternatives include both proprietary and nonproprietary products. Three widely used systems and their key attributes are listed in Table 1.
Each system makes use of a specialized design procedure and has unique design, fabrication and installation features. These include all methods to:
- Meet specification tolerances for geometry
- Match required elevations on field
- Accommodate installation on multiple lanes
- Achieve proper load transfer
- Provide uniform slab support
- Reduce frictional resistance of the base
- Distribute grout
- Control excessive stresses, deflections and distress levels
Additionally, the use of generic precast concrete slabs has also been explored on many projects. Innovative solutions for achieving load transfer have been designed. For example, precast full-depth replacement/dowel bar retrofit (PFDR/DBR) has shown good performance in the United States.
Design considerations in precast pavements
The design and construction process for precast pavements shares a common objective with cast-in-place pavement design, which is to provide a strong, smooth and safe-riding surface for the traveling public. The structural analysis and design criteria are the same in many respects, and both pavement types are designed to carry the estimated traffic and environmental loads over the design life without exceeding the allowable levels of distresses. The challenge, however, lies in the installation process that will ensure uniform slab support, efficient joints and good ride quality. Additionally, economic considerations, both initial construction costs and life cycle costs, play a key role in defining the pavement type selection or rehabilitation and repair strategy.
Rigid pavement design – the basics
A pavement with a portland cement concrete (PCC) surface layer is referred to as a rigid pavement. Pavement design takes a different approach from most structural engineering problems in that it involves the analysis of members with a relatively simple geometry but with complex support conditions and a high degree of material variability within the design life. The material properties and structural support conditions of the layers vary with time, temperature and moisture conditions (see Figure 3).
This implies that a given load application could cause different magnitudes of mechanical responses (stresses, strains and deflections) and accumulate different levels of damage at different times (summer, winter, spring freeze-thaw periods).
Furthermore, the temperature gradients in a PCC slab undergo cyclical changes on a daily basis, causing daily stress reversals and subsequent loss of support at the edges and corners of the slab as shown in Figures 4 and 5.
Daytime temperature gradients (positive temperature gradients resulting from higher temperatures at the top of the slab relative to the bottom of the slab) cause the top surface to expand relative to the bottom surface. Likewise, nighttime temperature gradients (negative gradients resulting from lower temperatures at the top of the slab) cause the bottom surface to expand relative to the top. When restraints from the dead weight of the slab and the frictional resistance of the base come into effect, high tensile stresses exist at the bottom and top of the slab for positive and negative temperature gradients, respectively.
Additionally, as can be observed in Figures 5 and 6, slabs cast in place will develop a permanent effective built-in gradient that accounts for the gradient locked into the pavement as a result of built-in temperature gradients, concrete shrinkage and creep. In other words, temperature gradients induced in the slab due to ambient conditions at time of placement, as well as shrinkage and creep deformations, are permanently locked into the slab. Precast pavements have a significant advantage over cast-in-place slabs because plant curing helps minimize built-in gradients shrinkage, which results in more uniform support to the slab.
Pavement design procedures
The American Association of State Highway Officials (AASHO) Road Test at Ottawa, Ill., conducted in the 1950s formed the primary source of data leading to the development of a formalized “empirical” design procedure. This design procedure, incorporated into the AASHTO Pavement Design Guide in the 1960s, was based on pavement serviceability. The dominant factor in assessing pavement serviceability was the pavement’s ride quality, which is a functional utility of the pavement but highly dependent on the structural conditions as well. The design exercise was a means to determine the thickness of the PCC slab for a given concrete strength, load transfer mechanism, design traffic, modulus of subgrade reaction, and drainage features at a selected level of design reliability and serviceability.
This procedure, which underwent major revisions in 1972, 1986 and 1993, supported highway agencies remarkably well for several decades. However, the 1990s and early 2000s have witnessed a shift in the industry’s perspective of pavement management – one from pavement designs for serviceability to designs that meet “performance” criteria. In other words, controlling structural distresses and maintaining an agency-specified level of ride quality defines the adequacy with which a pavement fulfills its purpose.
This shift to performance-based concepts has necessitated the adoption of mechanistic-empirical procedures over purely empirical procedures for pavement design. A recent series of national research studies resulted in the development of the Mechanistic-Empirical Pavement Design Guide (MEPDG), which has been adopted by states as an interim AASHTO Design Guide. Several other factors motivated the development of the MEPDG. Modern pavement technology withstands traffic levels and utilizes material types and construction practices far beyond those envisioned in the 1950s.
Highway agencies are faced with the growing challenge of undertaking more rehabilitation projects than new construction projects. Also, advances in theoretical analyses and computational abilities, improvements in pavement materials characterization, results from experimental investigations on test tracks, and data from national and regional level-pavement performance studies have lent themselves to calibrating mechanistically derived parameters (such as fatigue damage) with field observations of pavement performance.
The MEPDG procedure is integrated in a design software program and enables the user to iteratively evaluate multiple trial designs and narrow down the process to a feasible design option (see Figure 7). It considers the impact of both climate and aging on materials properties in a comprehensive manner and also accounts for certain construction aspects that greatly affect performance.
The procedure first divides the design life into time increments, monthly in the case of rigid pavements. The hourly, monthly and annual traffic load variations are considered and superimposed with appropriate material properties in each increment of time, bringing the analysis as close to reality as possible. The damage resulting in each time increment is accrued in the pavement over all time increments to compute the accumulated damage, which is the key parameter from the “mechanistic” calculations of the design process.
Next, the damage is used to predict distresses using the field-calibrated distress models. The distresses considered in rigid pavement design are transverse cracking, joint faulting and surface roughness (see Figure 8).
These distress levels are compared with agency-specified criteria to optimize and select a design.
Figure 9 shows the influence of built-in temperature gradient on slab cracking, illustrating the benefit of using PCPS over cast-in-place alternatives, as PCPS slabs can be fabricated with minimal built-in curvatures or stresses. In other words, the MEPDG concepts offer a framework to illustrate construction effects on pavement performance, and the plant QC process in precast slabs lends longer fatigue life in the slabs.
Figure 10 shows the sensitivity of joint faulting to load transfer efficiency, emphasizing the need for a good load transfer mechanism across joints and non-erodible base materials to achieve longer service lives.
Table 2 is a summary of the impact of several factors on pavement design and special considerations for precast concrete pavements.
Promising future for precast pavements
Over the past decade, stakeholders in the highway industry and the precast industry have been working with PCPS to provide cost-effective rehabilitation and reconstruction treatments for PCC pavements. Precast concrete pavements have been installed in heavily trafficked urban areas under tight construction windows. Construction planning, mobilization and scheduling are critical in such projects and, while it can be a challenge on the field, contractors nearly have this down to a formula to optimize production. Experience has shown that planning and communication are key to successfully completing a precast concrete pavement project.
FHWA’s Highways for LIFE (HfL) 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 future use in the rapid repair and renewal of our aging infrastructure while minimizing maintenance and repair needs. PCPS should be part of a pavement management toolkit.
Chetana Rao, Ph.D., is a Senior Research Engineer with Applied Research Associates, Inc. Her areas of expertise include concrete pavement materials, analyses, design, and precast pavement technology. She was a key member of the team that developed the MEPDG and serves as the PCPS expert on the HfL team for ARA. Dr. Rao also has led the development of precast concrete pavement specifications and implementation materials for Caltrans and is on the team developing generic specifications for the Illinois Tollway.