Sectioned on

Oct. 6, 2011

California, like most states with densely populated areas, has many urban high-volume roadways, which are expected to last long, provide a smooth ride and require little maintenance, thereby minimizing inconvenience to the traveling public.

California, like most states with densely populated areas, has many urban high-volume roadways, which are expected to last long, provide a smooth ride and require little maintenance, thereby minimizing inconvenience to the traveling public.

Rehabilitation of these roadways poses a tremendous challenge to Caltrans, as current work windows for road construction crews are sometimes as little as four to five hours, and typically at night. Yet, in these short work windows, Caltrans is required to maintain its highways at a high level of service under the constant wear and tear from high traffic volumes and varying weather conditions.

Currently, Caltrans rehabilitates concrete pavement through the use of rapid-strength concrete (RSC). The use of RSC is typically focused on projects where the highway facility is in relatively good condition, requiring spot rehabilitation of isolated areas. Typical RSC mixes achieve compressive strengths of around 2,500 psi in only 1.5 hours. It is common to add water-reducing admixtures, plasticizers and accelerators to a mix in an effort to maintain the workability of the mixture, while expediting the strength gain. However, the durability and long-term performance of such materials has come into question recently on many rehabilitation projects.

In northern California, many projects completed over the past decade using RSC have underperformed and required further repairs and maintenance. There are instances where RSC was used on long-term rehabilitation projects and several sections failed in less than three years. Sections of the existing concrete pavement on these projects outlived the newly constructed RSC areas, and in many instances continue to do so. Caltrans was at a crossroads, particularly within the District 4 office in the San Francisco Bay Area. A better approach to concrete-pavement rehabilitation had to be identified, or steps were going to be taken to possibly overlay all of the existing concrete pavements with hot-mix asphalt (HMA). Unfortunately, HMA overlays are not considered by District 4 to be a long-life solution as they do not address the underlying issue, namely the distressed concrete pavement, and therefore do not last as long as full-depth reconstruction of the concrete pavement.

This led to a decision to evaluate precast concrete pavement as an alternative solution. A recent project on I-680 provided the opportunity to evaluate a prestressed precast concrete pavement solution to rehabilitate this major highway in the San Francisco Bay Area. The project incorporated a combination of jointed precast pretensioned concrete pavement (JPPCP) and precast post-tensioned concrete pavement (PPCP) on a large scale.

Pieces in place

The existing I-680 concrete facility ranges from 40 to 45 years of age and is located roughly 35 miles east of San Francisco between the towns of San Ramon and Danville. The entire project length is 12.5 miles, with the southernmost 7.5 miles utilizing the precast concrete pavement rehabilitation technique, and northernmost 5 miles utilizing a crack, seat and overlay rehabilitation technique with HMA and rubberized HMA (RHMA). The HMA/RHMA strategy was used on the northern portion due to the excessive distress on the existing concrete pavement. It is important to note that both the rigid and flexible pavement strategies were determined from life-cycle cost analysis and value analysis studies. The precast concrete pavement portion of the project involved replacing distressed concrete pavement in intermittent sections, as short as 12 ft and up to 1,600 ft in length, and either single or multiple lanes wide.

Some of the key features of the I-680 facility included:

Eight-lane concrete-barrier-divided freeway with concrete auxiliary lanes intermittently throughout the project; 10 interchanges; 44 ramps; Lane 1 is a 12-ft-wide HMA pavement with an 11-ft-wide HMA inside shoulder; and Lanes 2, 3 and 4 are typically 12-ft-wide concrete pavement over 40 years old.

The existing concrete pavement section was typically 8-9 in. of PCC supported by 4-5 in. of cement-treated base (CTB) and 12-14 in. of aggregate sub-base (AS).

Initial field condition surveys were performed to evaluate the distress levels of the roadway. Pavement evaluations were done by walking the project limits and through windshield surveys. The limits of distressed pavement for rehabilitation with precast pavement were identified on plan sheets prior to project bidding. Overall, approximately 5.5 lane-miles of precast concrete pavement were installed on this project.

The installation sequence is significantly different from RSC rehabilitation techniques, but still relatively straightforward. Once the limits of the distressed pavement locations are identified, those specific areas are saw-cut ahead of time for ease of removal.

Next, the demolition occurs with removal of all the existing CTB and PCC material using a lift-out (rather than rubblization) method. The remaining sub-base is then proof-rolled in preparation for installation of a rapid-setting lean concrete rapid base (LCB-RS) material. The LCB-RS is placed 6 in. thick and allowed to reach a compressive strength of 100 psi before precast panel installation can begin. Typically, this strength is achieved within two hours after placement, depending on the ambient air temperature. Once the desired strength is achieved a bond breaker (polyethylene sheeting) is placed over the LCB and precast panel installation commences.

For areas where the PPCP system is utilized, six seven-wire epoxy-coated post-tensioning strands are fed through ducts cast into the precast panels in the longitudinal direction for permanent post-tensioning. Two temporary strands also are fed through the panel assembly and stressed after each panel is installed to ensure a closed joint. Once the last panel of each section of post-tensioned panels is installed, the temporary strands are removed and the epoxy-coated strands are permanently stressed to 75% of the ultimate tensile strength.

A final “drop-in” panel is installed at either end of the precast panel section to connect with the existing pavement. Slots cast into these drop-in panels receive the dowels cast into the precast panels and drilled into the existing pavement in order to provide load transfer between the precast panels and drop-in panels and between the drop-in panels and existing pavement at either end of the section.

All precast panel installation, including removal of existing pavement, placement of the LCB-RS and post-tensioning (for PPCP sections) occurs entirely during nighttime lane closures. The following morning, the panels are opened to daily traffic and the traveling public doesn’t notice a thing, other than a new pavement and a smoother ride. Grouting of post-tensioning tendons and dowel slots and underslab grouting takes place during a subsequent nightly closure. Once all the precast panels are installed a final diamond grind is performed for high ride quality on all concrete pavement, and all new and existing joints are sealed. The end result is a roadway with precast concrete with compressive strengths upwards of around 8,000 psi that will be long-lasting and well suited for high traffic loads.

Positive precast

Although relatively new to the pavement industry, precast concrete has long been established as a durable, high-performance product for the bridge and commercial building industries. As demonstrated by the I-680 project, these benefits can be applied to pavement rehabilitation as well. Precast concrete overcomes the limitations of constructing a long-life pavement within short work windows during nighttime lane closures. Concrete is placed and cured in a controlled environment at a precast plant, eliminating the need for on-site curing and the durability issues that Caltrans has experienced with rapid-setting materials.

Prestressing further benefits pavement durability by putting the pavement slab in compression over its life, reducing or even eliminating the potential for cracking. Furthermore, prestressing permits the replacement of a concrete pavement slab with a precast panel of the same thickness that will have the equivalent design life of a much thicker nonprestressed pavement slab. For the I-680 project, panels were pretensioned in both directions at the fabrication plant. PPCP sections were further post-tensioned on-site in the longitudinal direction after installation.

Currently, there are several large precast projects on the horizon, one in southern California involving several miles of precast concrete pavement and another in northern California on a major truck route and transportation corridor between San Francisco and the state capital of Sacramento.

The future of precast pavement at Caltrans looks bright. The I-680 project demonstrates the tremendous benefit precast concrete pavement brings as a sound long-life rapid rehabilitation strategy. We are in an era where our agency is looking to preserve, rather than continually reconstruct, our existing infrastructure. Also the time is fast approaching where roadways throughout California, if not the nation, will need to be maintained at a much higher level of service than they are at present to effectively move people, goods and services. The I-680 rehabilitation project, along with many others, has shown how precast concrete pavements provide a viable option for PCC pavement rehabilitation.

Mishra is a district materials engineer for Caltrans, District 4. Merritt is a project manager for The Transtec Group, Austin, Texas.

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