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Construction of the California Precast Concrete Pavement Demonstration Project
Chapter 1. Introduction
In the past decade, State transportation agencies have become desperate for construction techniques that will help them to “Get in, get out, and stay out!” To “get in, get out” implies rapid construction with minimal disruption to the motoring public. As if this is not challenging enough, agencies must also “stay out,” requiring pavement structures that are long lasting and durable, with minimal maintenance over a 30- to 50-year lifespan.
Precast prestressed concrete pavement is a solution for this need, providing both improved durability and rapid construction. In 2000, a feasibility study was completed by the Center for Transportation Research (CTR) at The University of Texas at Austin that examined the use of precast prestressed panels for expedited pavement construction.(1) This study was followed by an implementation study funded by the Federal Highway Administration (FHWA) and conducted by CTR, which resulted in the construction of a 0.7-km (2,300 ft) precast prestressed concrete pavement pilot project near Georgetown, Texas.(2)
FHWA Demonstration Projects
The Texas precast pavement pilot project demonstrated that precast prestressed concrete is a viable technique for rapid pavement construction. However, due to unfamiliarity with this new technology, it is difficult for agencies to evaluate this technology further. In response to this need, FHWA initiated an effort to construct additional precast pavement demonstration projects based on the concept developed from the CTR feasibility study. The purpose of these new demonstration projects was to further evaluate the viability of precast prestressed pavement for “real world” applications while also familiarizing contractors and agencies with this new technology. As part of this effort, the design, construction, and cost of each of these demonstration projects was evaluated.
This report discusses the first of these new demonstration projects, constructed on a section of Interstate 10 (I-10) in El Monte, California, in April 2004.
Benefits of Precast Concrete Pavement
One of the biggest benefits of precast concrete is the improvement in durability that can be realized by casting the panels in a controlled environment. Precast manufacturers are able to produce very consistent concrete mixes with a high degree of quality control.(3) Because the mixture is transported only a short distance from the batch plant to the forms, it permits the use of durable, low-permeability mixes with a low water-to-cementitious-materials ratio and reduces the probability of problems such as segregation and flash set. Other problems that are commonly encountered with cast-in-place concrete pavement, such as so-called “built-in curl” (from temperature and moisture gradients in the slab), surface strength loss (from insufficient curing), and inadequate air entrainment, can also be minimized or eliminated with precast concrete.(1)
Prestressing further benefits precast pavement durability by inducing a compressive stress in the pavement to greatly reduce or even prevent the occurrence of cracking.(1) This benefit was demonstrated by a cast-in-place prestressed pavement, 150 mm (6 in.) thick, constructed on I-35 near West, Texas, in 1985, that has required virtually no maintenance over its 19-year life.(4,5)
In addition to enhancing durability, prestressing also permits a significant reduction in slab thickness. With the introduction of a precompressive stress into thinner, precast pavement panels, tensile stresses caused by wheel loads can be limited to those of a thicker, non-prestressed pavement, resulting in a design life equivalent to a much thicker pavement. For example, the precast pavement in Georgetown, Texas, was designed, at 200 mm (8 in.) thick, to have a life equivalent to a continuously reinforced concrete pavement that is 355 mm (14 in.) thick.(2)
Perhaps the most obvious benefit of precast concrete for pavements is reduced construction time. One of the problems with conventional portland cement concrete pavement is the curing requirements. Portland cement concrete needs time to reach a specified strength before it can be opened to traffic. Although fast-setting concretes are available,(6) there is still some question regarding the early-age and long-term durability of these mixes.(7,8) Precast concrete panels, on the other hand, are cast and cured prior to placement, ensuring adequate time to reach the required design strength under controlled curing conditions. This means that the panels can be set in place and opened to traffic immediately, greatly reducing construction time by eliminating the cure time requirement. This benefit allows construction to take place during overnight or weekend operations.
Reduced construction time and improved durability will greatly reduce user costs associated with pavement construction and rehabilitation. User delay costs are accumulated when construction activities cause traffic congestion. These costs can be substantial, as demonstrated by the CTR feasibility study, which estimated daily user delay costs for a four-lane divided facility carrying 50,000 vehicles per day to be as high as $383,000 per day for 24-hour-per-day lane closure versus only $1,800 per day for nighttime lane closure only.(1) (It should be noted that 50,000 vehicles per day is very conservative. Many urban facilities carry in excess of 200,000 vehicles per day). Likewise, user costs resulting from poor pavement condition can be substantial. As a 2001 TRIP (The Road Information Program) report indicated, Americans spend over $41 billion annually on vehicle repair due to roads in poor condition.(9) Smoother and more durable pavements will substantially reduce these types of user costs.
Finally, another benefit of precast concrete that may not be immediately apparent is the potential for extending the construction season for paving. Because precast panels are cast and cured in a controlled environment, they are not as susceptible to on-site environmental conditions as conventional concrete. This will permit pavement construction to continue under adverse weather conditions, such as subfreezing or extremely hot temperatures, that would normally prohibit cast-in-place concrete paving.
While this report introduces the background and fundamentals of precast prestressed concrete paving, the primary objective of the report is to describe a demonstration project constructed by the California Department of Transportation (Caltrans) in El Monte, California. The design, fabrication, and construction procedures are discussed and evaluated for applicability for future projects. This report also presents recommendations for future precast pavement construction.
Following is a summary of the remaining chapters of this report: