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Construction of the Iowa Highway 60 Precast Prestressed Concrete Pavement Bridge Approach Slab Demonstration Project
Chapter 1. Introduction
The 'bump at the end of the bridge,' caused by bridge approach slab settlement, is an ongoing problem for many State highway agencies. The bump not only degrades the ride quality of a roadway, but also presents a safety issue for drivers and increases impact loads on bridges. Approach slab settlement is generally caused by a loss of support due to consolidation or erosion of the underlying embankment material and may be accompanied by failure of the paving notch/paving seat at the abutment. While problems with the embankment material can be prevented with improved construction practices, a separate but equally important issue is how to reconstruct bridge approach slabs that have already failed. This is particularly challenging in urban areas where lane closures must be minimized to reduce the impact of reconstruction on the traveling public.
The Iowa Department of Transportation (IADOT) is addressing this issue by investigating the use of precast prestressed concrete pavement (PPCP) for expedited bridge approach slab reconstruction. Precast panels are fabricated and stockpiled at a precast plant, then delivered to the jobsite and quickly installed as needed. Precast panels can support traffic immediately after installation, thereby facilitating overnight or weekend construction operations. Prestressing the approach slab benefits performance by keeping the pavement in compression to minimize or even eliminate cracking. Further, prestressing gives the approach slab a 'bridging' ability to span voids in the embankment material that may redevelop over time, helping to achieve the ultimate goal of longer lasting pavements.
In August 2006, IADOT constructed a PPCP approach slab on a new bridge along Highway 60 near Sheldon, Iowa. This successful project allowed IADOT to evaluate and refine design and construction details for PPCP bridge approach slabs. IADOT is currently planning a second project in which failed approach slabs at either end of twin bridges will be reconstructed under traffic. In addition to utilizing precast concrete for the approach slab, IADOT will also be evaluating the use of precast concrete for replacement of deteriorated or poorly constructed paving notches on these bridges. The end result will be an innovative method for rapid reconstruction of bridge approach slabs with minimal disruption to the traveling public.
The Highway 60 PPCP bridge approach slab demonstration project was part of a recent effort by the Federal Highway Administration (FHWA) to evaluate the viability of PPCP construction for a variety of paving applications through a series of demonstration projects throughout the United States. These demonstration projects provide State highway agencies the opportunity to evaluate PPCP for pavement rehabilitation and reconstruction, and also help to familiarize local contractors with the technology. FHWA provides design and construction support for these projects and, if necessary, limited funding for construction.
The FHWA demonstration projects stemmed from an initial feasibility study completed in 2000 by the Center for Transportation Research at The University of Texas at Austin.(1) This feasibility study resulted in the development of a viable concept for PPCP that was subsequently evaluated through three demonstration projects in Texas, California, and Missouri between 2001 and 2006. The PPCP concept was adapted to the constraints of each particular project. The Iowa Demonstration Project, described herein, represents the fourth PPCP demonstration project and a fourth adaptation of the PPCP concept to meet the specific needs of State highway agencies. A summary of the four demonstration projects completed to date is provided below.
Interstate 35 Frontage Road - Georgetown, Texas(2)
Completed in spring 2002, the I-35 frontage road demonstration project, constructed by the Texas Department of Transportation, was the first project to demonstrate the viability of the PPCP concept. While not constructed under traffic and short time windows, the intent of this project was to evaluate the design details and construction procedures for PPCP. Approximately 700 m (2,300 ft) of PPCP were constructed the full width of the frontage road. Both full-width and partial-width panels were utilized. The full-width panels spanned the entire 11-m (36 ft) width of the roadway, including two traffic lanes and inside and outside shoulders. The partial-width panels were constructed in two adjacent sections, one 6 m (20 ft) wide and the other 5 m (16 ft) wide, to achieve the full 11-m (36 ft) roadway width. The adjacent sections were tied together with additional transverse post-tensioning. Many aspects of PPCP were demonstrated by this project:
- Overall fabrication and construction feasibility of PPCP.
- Use of an armored expansion joint cast into the precast panels.
- Use of central stressing for precast, post-tensioned pavement panels.
- Use of non-match-cast precast panels with interlocking keyways.
- Installation of precast panels over a hot-mix asphalt leveling course.
- Construction of precast pavement on a vertical curve.
- Lane-by-lane construction of precast pavement using 'partial width' precast panels.
The second PPCP demonstration project was constructed by the California Department of Transportation in April 2004. PPCP was incorporated into a project to widen eastbound I-10 near El Monte, California. A section of PPCP 76 m (248 ft) long was installed adjacent to the existing main lanes, adding 8 m (27 ft) of traffic lanes and a 3-m (10 ft) shoulder to the existing pavement. Several aspects of the demonstration project were unique:
- Incorporation of a change in pavement cross slope into the surface of the precast panels.
- Nighttime installation of precast panels during a 5-hour construction window.
- Installation of precast panels over a lean concrete base.
- Use of epoxy-coated strands for longitudinal post-tensioning.
- Use of a nonarmored, dowelled expansion joint.
- Diamond grinding of the finished surface to achieve pavement smoothness requirements.
The third PPCP demonstration project was constructed by the Missouri Department of Transportation on I-57 near Sikeston, Missouri, in 2005. PPCP was used for the reconstruction of 308 m (1,010 ft) of mainline pavement on I-57. The full 11.6-m (38 ft) pavement width was reconstructed with precast panels, including two main lanes and inside and outside shoulders. Unique aspects of this project include the following:
- Incorporation of a crowned pavement cross section into the precast panels.
- Post-tensioning from the joint panels as opposed to central stressing.
- Use of a 'header type,' nonarmored expansion joint.
- Use of noncontinuous keyways for the panel joints.
- Installation of precast panels over a permeable asphalt-treated base.
- Diamond grinding of the finished surface to achieve smoothness requirements.
Highway 60-Sheldon, Iowa(8)
The most recently completed demonstration project, constructed by IADOT on State Highway 60 near Sheldon, is described in more detail in this report. This project demonstrated several aspects of PPCP construction:
- Use of precast prestressed panels for bridge approach slab construction.
- Use of bi-directional post-tensioning.
- Lane-by-lane construction (partial-width panels) installed on a crowned pavement section.
- Installation of panels over an aggregate base.
- Diamond grinding of the finished surface.
While the benefits of PPCP have been documented more thoroughly elsewhere,(1,2,3,9) a summary of these benefits is provided below, including specific benefits for bridge approach slabs.
The primary benefit of PPCP is rapid construction. More accurately stated, PPCP permits faster opening of the pavement to traffic. Precast panels are cast and cured off site, allowing them to reach opening-to-traffic strength before installation. The need for rapid construction techniques is perhaps even more critical for bridges (and bridge approach slabs) than it is for pavements. Bridges are critical links within a highway system, and it is often not possible to divert traffic around a bridge during reconstruction without significant inconvenience and cost. Consequently, staged reconstruction is often required, permitting only part of the bridge to be reconstructed at any given time, squeezing traffic into fewer lanes, and often increasing user delays. With proper planning, PPCP will permit rapid reconstruction of bridge approach slabs during nonpeak travel times, minimizing lane closures and associated user delays.
Reduced User Delay Costs
State highway agencies are continually seeking new techniques for pavement reconstruction and rehabilitation that minimize disruption to the traveling public and the associated user delay costs caused by lane closures. In many urban areas, agencies are often limited to overnight or weekend windows for lane closures. These constraints necessitate solutions that will permit the pavement to be opened to traffic quickly. Because precast concrete panels are fabricated and cured off site, they can be hauled to the jobsite, installed quickly, and opened to traffic almost immediately after installation. A reduction in user delay costs is where the primary economic benefit of PPCP construction will be realized.
Reduced Disruption to Local Businesses
Roadway users are not the only people impacted by pavement construction. Construction of intersections and noncontrolled access roadways can also have a significant impact on local businesses by limiting access to these businesses by customers and suppliers. PPCP permits construction to be completed during nonpeak business hours, thereby minimizing inconvenience to both customers and local businesses.
Improved Safety and Reduced Traffic Control Costs
Rapid construction helps improve safety for both construction workers and vehicle drivers by limiting construction operations and associated lane closures to nonpeak travel times. During these limited operations, construction workers are less exposed to traffic, and drivers are less exposed to traffic control measures. Also, by eliminating the need to build temporary traffic lanes around the project and by avoiding lengthy detour routes, traffic control costs can be greatly reduced.
Savings in user delay costs from expedited construction are only beneficial if the solution for reconstruction is a durable, high-performance solution and not just a temporary 'quick fix.' Precast prestressed concrete has provided durable, high-performance solutions for the bridge and commercial building industries for decades, and can do the same for pavement. Precast concrete manufacturing plants have a very high degree of control over the concrete mixture and the production process, helping to ensure a very consistent, high-quality, final product. Further, by incorporating prestress, many additional performance benefits can be realized.
The precast concrete manufacturing process offers a great deal of flexibility in formulating the concrete mixtures used for precast products. Mixtures with very low water-to-cementitious materials ratios and various combinations of pozzolans and water-reducing and air-entraining admixtures permit the product to be tailored for the needs of the paving project. Concrete mixtures are produced in a very consistent manner and hauled only short distances from the batch plant to the forms, helping to minimize problems that are often encountered with cast-in-place pavement construction. Precast concrete manufacturing also permits the use of lightweight aggregates and hollow-core manufacturing processes to reduce the weight of the precast panels.
The precast manufacturing process also permits a great deal of flexibility in the curing process. Precast concrete products can be steam cured, wet-mat cured, cured under plastic sheeting, or simply coated with curing compound. The curing process can be selected to meet the requirements of both the concrete mixture and production rate. Proper curing will help to eliminate problems that can result from inadequate curing such as surface strength loss and will help to reduce other problems such as 'built in' curling.
Reduced Cracking and Number of Joints
Prestressing benefits the durability and performance of PPCP by inducing a compressive stress in the concrete through pretensioning and post-tensioning. This compressive stress helps to minimize or even eliminate the occurrence of cracking that can lead to further distress. Any cracks that do form will be held tightly closed by the prestressing force in the concrete. While cracking can be a maintenance issue for pavements, so can joints. Prestressing significantly reduces the number of joints required in a pavement slab by post-tensioning long sections of pavement. This benefit has been documented for both cast-in-place and precast concrete pavements.(1,10)
Another primary benefit of prestressing is a reduction in the required pavement slab thickness. By inducing a compressive stress in the pavement slab, tensile stresses caused by traffic loading and environmental effects (curling and warping) can be significantly reduced, permitting the use of thinner pavement slabs in situations where current designs would call for a much thicker slab. This not only provides savings in concrete material and transportation costs for precast panels, but also permits pavement reconstruction with in-kind slab thickness. For bridge approach slabs, this is an important benefit as it allows the pavement thickness to be dictated by the specific project requirements such as the slab depth at the bridge abutment or the pavement thickness at the adjoining pavement.
Another benefit of precast concrete pavement, particularly for construction in cold climates, is the potential it provides for extending the construction season. Precast panels can be installed in extreme cold (and warm) temperatures that would normally prohibit cast-in-place pavement construction.
The benefits presented above are inherent benefits of PPCP that can be realized through essentially any PPCP project. However, there are additional benefits of PPCP for bridge approach slabs. First, prestressing gives PPCP an improved ability to span voids and non-ideal base materials beneath the slab. This permits PPCP to be designed as a 'slab bridge' that is unsupported (or has poor support) over a certain length of the pavement. Prestress levels can be adjusted to account for the flexural stresses produced by traffic loading on an unsupported or poorly supported slab. For bridge approach slabs, where a loss of support due to erosion or consolidation of the underlying embankment might be anticipated in the future, this is a significant benefit.
Another benefit of PPCP for bridge approach slabs is the permissible slab length between expansion joints. By prestressing the approach slab, expansion joints can be moved further away from the abutment where water infiltration into the embankment can cause erosion and consolidation of the embankment material. Prestressing can also be used to permanently tie the approach slab to the bridge abutment, keeping the joint between the abutment and approach slab tightly closed.
The primary objective of this report is to summarize the construction of the PPCP demonstration project on Highway 60 near Sheldon, Iowa. This includes the design, fabrication, and panel installation processes for the project. The report also presents recommendations for future PPCP bridge approach slab projects based on lessons learned from this project. The following is a summary of the remaining chapters of this report:
- Chapter 2 presents the key features of PPCP that were developed through the feasibility study and refined through the course of previous demonstration projects.
- Chapter 3 presents an overview of the Highway 60 approach slab demonstration project, including the precast panel layout.
- Chapter 4 presents the design of the Highway 60 project, including the design procedure and design details.
- Chapter 5 discusses the fabrication of the precast panels for the Highway 60 demonstration project, including issues that should be addressed for future projects.
- Chapter 6 discusses the construction or installation of the precast pavement on site. This includes base preparation, transportation, panel placement, post-tensioning, grouting, and instrumentation.
- Chapter 7 presents an evaluation of all aspects of the Highway 60 demonstration project and some considerations for future projects.
- Chapter 8 contains a summary and recommendations for future implementations.
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