Time to Wrap

April 2, 2018
New technologies like CFRP help strengthen bridge columns in seismic zones

About the author: White is director of marketing, repair and protection for Sika Corp. Isaac is a project sales representative for Sika Corp.

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Twenty-five deteriorated bridges along a 5-mile corridor of I-80 in Salt Lake City were in need of major repair. Most were showing signs of advanced corrosion due to prolonged exposure to the harsh environments and deicing salts. Of the 25 bridges, 12 were considered structurally deficient indicating that significant elements of each bridge needed repair. There was a considerable amount of spalling concrete from the decks, pedestals, bent caps and columns, and beams and bearing units also were in poor condition. Without repairs, many of these spans might have required weight restrictions, bracing, shoring or emergency replacement of decks and other components. All of the bridges did not meet current seismic design standards.

I-80 in this part of Salt Lake City crosses over the Wasatch Fault that runs along the east bench of the valley. The Wasatch Fault is one of the longest and most active faults of its type in the world and contributes to the Wasatch Front having the greatest earthquake risk in the interior western U.S. The bridge structures along this section of freeway were designed and built between 1964 and 1971, prior to the seismic design codes in place today. The violent shaking during an earthquake could collapse a number of these older and under-designed bridge structures along I-80 between State Street and the mouth of Parley's Canyon. Since it was not economically feasible to completely rebuild the bridges to current seismic standards, other options were considered. Ultimately, it was decided to use simple, low-cost techniques, including carbon fiber reinforced polymer (CFRP) fabrics and other structural repairs that could reduce the severity of damage from an earthquake.

The Utah DOT engineers and expert consultants from the University of Utah discussed and presented new technologies such as CFRP, emphasizing the importance of performing as much seismic retrofit work as possible. UDOT expressed its intent to complete as much research as funding would allow (in partnership with the university) on the rehabilitation techniques and materials to be used for the I-80 project. UDOT hoped to extend the life of these bridges for a minimum of 10 years at which time they would rebuild this section of freeway with new structures.

In 1998 and 1999, University of Utah engineers were presented with the unique opportunity of conducting full-scale, in-situ tests of the CFRP system on bridge bents similar to the ones on I-80 scheduled for demolition. Dr. Chris Pantelides, professor of civil engineering at the University of Utah, explained that in the tests several concepts and designs on retrofitting older bridges with FRP were tested and evaluated including:

1. Restoration of columns for confinement, lap splice control and shear strengthening;

2. Shear strengthening of the bent-cap column joints; and

3. Tensile anchorage of longitudinal column bars ending in the bent cap.

The test bents were wrapped with CFRP according to the design and then pushed to failure. The retrofitted columns and bent caps deflected 201/2 in. before reaching total failure. Pantelides explained, "All of the design concepts were verified in this testing, and the goal of improving the ductility of the bridge bent caps and columns was achieved." These proven design ideas were eventually implemented on the State Street and I-80 Bridge proj-ects, which needed flexural strengthening of the bent caps as well as shear reinforcement.

The second part of this research program was the development of the specifications. The specs were written with input from UDOT, the University of Utah and outside consultants who created special provisions for column and bent-cap wrapping. UDOT said that due to environmental concerns only CFRP systems would be considered. The composite wrap system was required to meet minimum initial properties for tensile strength, Naval Ordnance Laboratory (NOL) Ring strength, fiber volume and glass transition temperatures. In addition, the special provision called for the required design thickness to be based on an environmental durability rating factor, which would account for material property losses due to environmental aging over the projected life of the composite.

Test the panel, train the group

Another part of the specification required a great deal of field sampling and testing to ensure quality control on the project. This included flat panel samples, NOL Rings and core samples of the CFRP to verify strength, stiffness, fiber volume, resin/fiber ratio, thickness and glass transition temperature. An independent laboratory tested the panels and core samples and the results were sent to both the contractor and UDOT. Michael Fazio, P.E., UDOT, stated, "The benefit of this testing was an after-the-fact assurance of quality control on the project. The testing was specified in the beginning because the selection of the fiber and resins was unknown and testing was required to assure quality and uniformity of the product from our standpoint."

Gerber Construction, Lehi, Utah, UDOT, the Federal Highway Administration and Sika Corp., Lyndhurst, N.J., conducted a training seminar long before the first layer of CFRP was installed to ensure a smooth project. During the seminar, installation crews, inspectors, quality assurance, quality control and testing personnel were given training and hands-on experience with the CFRP. This training helped to coordinate the responsibilities of all parties and provided a perspective from the installer's point of view while focusing on how important QC was going to be. Excellent cooperation on the project was a key benefit of the training seminar and made everyone's job easier.

Column hugging

Five bridges were chosen to receive the CFRP for seismic upgrade. A total of 76 columns and four bent caps (beams) were wrapped. Some columns received as many as 17 layers of CFRP. During the project, 124,000 sq ft of fabric and 1,760 gal of epoxy were used. After weeks of concrete restoration and repair were completed, surface preparation began prior to the installation of the fabric.

The bent caps at the State Street location required the most extensive surface preparation and were the most difficult because of the horizontal surfaces. The Highland Drive Bridge required the most extensive concrete repair prior to application of the CFRP.

Traffic control also was a critical part of this project. Because some traffic lanes could only be closed during certain hours of the day, the staging of the work by the contractor was very important.

Using a typical installation crew of 10 workers, Gerber Construction was able to make quick progress on the bridges. Custom-built platforms lifted in place by a forklift allowed the workers 360° access to the columns, eliminating the need for scaffolding and providing a time and cost savings. The installation crews were responsible for making the required daily test specimens that had to be prepared with great care and precision while keeping detailed logs of the installation process and test samples.

Special tools and equipment were needed (like the custom-made NOL Ring forms) to produce the samples and cure and store them before delivery to the laboratory for testing. Approximately 342 flat panel specimens were prepared along with several dozen NOL rings and nearly 75 core samples during the project.

The application of fabric on the bridge columns was clear-cut, but the bent caps presented a new challenge for the installation crew. Because the bent caps would require flexural layers of CFRP on the bottom followed by diagonal wrap layers and then circumferential wraps, the crew spent a great deal of time cutting and preparing fabric for the special widths and angles. This part of the installation was further complicated by the fact that the bridge girders were offset from one another. Following the curing and testing of the CFRP it was coated with a textured acrylic coating for UV and abrasion protection, as well as long-term durability.

Not only was field inspection performed by UDOT for quality assurance, but the contractor also had to provide an independent quality control inspector to make daily site visits and prepare reports on the installation process. Every day during construction two site visits were conducted. During these inspections surface preparation, fabric saturation, application, curing, sample preparation and coatings were reviewed.

Currently, the State Street Bridge is being monitored by the University of Utah to determine in-situ environmental characteristics of the CFRP composite. This includes both non-destructive and destructive testing. NOL Rings (20-in.-diam. cylinders constructed from five layers of CFRP) exposed to the environment are currently being stored at the bridge site and will be tested on an ongoing basis. The data from these tests will provide needed information about the long-term effects of UV exposure, salt water and freeze-thaw cycles on the CFRP. Flat panels plus CFRP-wrapped concrete cylinders and beams will be tested for tensile, direct pull-off, flexural and axial compression strength as part of this study.

The deteriorated concrete was removed and replaced and the carbon fiber system did not alter the existing geometry or look of the bridge columns and bents. The strengthening materials were unobtrusive and were coated as well to blend in with the existing bridge structure.

In the past, alternative methods would have entailed either steel jacketing or concrete enlargement, but both would have noticeably changed the appearance of the bridge and would have been a reminder to the public that a rehabilitation project had taken place.

The installation of the lightweight, non-corrosive, high-strength CFRP allowed UDOT to seismically upgrade older bridges for a lot less tax-paying dollars than building new bridges, especially if the bridges are damaged by a seismic event.

Full-scale testing provided the needed proof of the FRP design giving the state confidence in the system they specified. Good planning and staging by a quality contractor minimized impacts on the driving public. Follow-up testing will provide needed data for strengthening projects in the future.

A major fault

When people in the U.S. think of earthquakes, places like California, Utah and Alaska come to mind. However, not many people are aware that the largest release of seismic energy in the continental U.S. occurred in the Mississippi Valley. The New Madrid earthquake of 1811-12 included three main tremors greater than 8.0 on the Richter scale. There also were two events equal to 8.0 and five more greater than 7.7. By comparison, the Northridge, Calif., earthquake of 1994 measured 6.7 and the recent devastation in Turkey measured 7.4.

The 150-mile New Madrid Fault crosses five state lines--Arkansas, Missouri, Tennessee, Kentucky and Illinois--and passes under the Mississippi River in at least three places. The Illinois DOT began a program in 1999 to seismically upgrade the bridges in this region.

The first project undertaken was the I-57 Bridge in Cairo, Ill., which is near the apex of the fault. A total of 50 piers and 158 columns were strengthened using the SikaWrap Composite System. The number of layers varied from four to 14 and the columns were then coated with two layers of Sikagard 670W, an acrylic, protective, anti-carbonation coating. A saturator was used to install 94,000 sq ft of fabric and 1,820 gal of Sikadur 300, a high-strength, high-modulus epoxy.

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