Current Practices In FRP Composites Technology
FRP Bridge Decks And Superstructures
Seismic Retrofit for Columns/Piers
Composite Bridge Column Retrofitting in California
- Los Angeles - SB5 to EB2 connector, NB5 to WB2 connector, and NB5 to Griffith Park Dr. Ramp.
These were the first composite column casings placed on bridges in California after testing was completed at U.C. San Diego in the early 90's. These jobs were done when we truly started to develop material and installation specifications, testing requirements and construction records and procedures. These first sites used our "system 1" which is a 2-part field mixed ambient curing epoxy matrix with an E-glass cloth. The cloth has a small amount of Kevlar filaments in the weave that can be used as tracers to verify proper orientation. E-glass becomes translucent once wetted out. Extensive experimentation for the epoxy mixture, additives and various installation techniques were done at these sites. For example, the plastic hinge zone of these single column bents (approx. 5' high) were wrapped using a "prestressed" bladder system that provides an "active" inward pressure that has to be overcome before the dilation of the column's concrete can occur. A rubber bladder was first wrapped around the hinge zone then the bladder was wrapped over with many layers of composites. After the composite hardens over the bladder, small injection ports that were cut through the lay-ups are pressure filled with Portland cement grout. At first they were pressure injected with epoxy but this was deemed too expensive and not necessary. As the grout fills the bladder it expands the composite casing thus placing an inward pressure to the column.
For many reasons this method of retrofitting columns is not used anymore and is not recommended for use in the future. One problem is that due to time constraints the work needs to proceed and the composite may not have cured all the way prior to grout injection. Also, as the composite expands, snapping and popping of the composite can be heard thus weakening the jacket. Many pressure jackets developed pinholes and a high-pressure spray of grout shot out causing safety issues to arise. A lot of the pressure jackets had to be removed and replaced making the economics of such a system suspect. Many "passive" layers will perform the same job. Since labor is a driving factor in project cost, the passive system was deemed by the vendor to be a better option. Some of the pressure jackets snapped (split apart) after a few years due to being under pressure (creep rupture) thus rendering them useless. These broken jackets were replaced at the vendor's cost with passive wraps. The rupturing of some jackets led to the demise of active wrap systems for use on Caltrans projects.
Outside the plastic hinge zones only passive wraps were used (the bulk of the job) and much was learned about documentation of the materials, jacket installation and what to look for during inspection. As with any new product installation, training for the workers and inspectors is very important.
Many aspects of composite jacket installations were refined during these installations such as proper field mixing of the epoxy, column surface preparation, acceptable porosity in the jackets (how many allowable air bubbles), repair of large voids by epoxy injection, acceptance testing criteria and so on. To say the least, the learning curve was steep for these first installations.
- Lompoc US-101/SR-246 Interchange
This next composite column retrofit project also utilized ambient curing 2-part epoxy glass cloth (System 1) to wrap the columns full height at this ramp structure. The columns were excavated down to the footing level and wrapped from the bottom upward using scaffolding as with the other projects. The material (glass cloth) was saturated with epoxy and run through an impregnating machine (two rolling cylinders spaced 1/8" apart) that completely saturates the fibers. The saturated cloth is wrapped around a cylinder and handed up by rope to be hand wrapped around the column. The cloth is worked by hand to smooth it down, adhere it to the column and to embed each subsequent layer and release all air bubbles. Protective equipment (respirator, gloves, etc. should be utilized.
One unusual facet of this job was the unexpected shape at the base of a few columns. The columns above ground were circular and were shown to be circular the full height on the as-built plans. In reality, the bottom 5' or so was rectangular is shape. The contractor's circular column forms may not have been as tall as the columns. Since the bottom of the column was buried, forming a rectangular shape at the base was allowed. When this project was designed, Caltrans was only permitting circular shapes to be wrapped because they are very efficient in distributing hoop stresses, therefore we had a mild dilemma. There were no approved rectangular casings at that point, so we basically used analytical techniques to conceive a system that conservatively put some thick layers around the rectangular shapes. The sharp corners of rectangular shapes should be rounded to at least ½ inch radius. Rectangular composite wrapped shapes have been successfully tested since that time.
- La Jolla, San Elijo Lagoon
This job used machine wrapped pre-preg carbon filaments (System 2) accomplished by raising and lowering a circular orbiting winding apparatus up and down the columns. Thus the jacket thickness was built up by applying multiple thin carbon filaments under tension on top of each other. The carbon spools should be kept cold (refrigerated) so they don't start to cure until the filaments are in place. Once all the fibers are placed and the jacket is the designed thickness, a heat blanket or oven system is placed around the jacket. A sustained (1 hour) high temperature cure (250 degrees F) is then carried out. All the composite column casings are painted to match the concrete color of the bridge and to protect the matrix from sunlight. The end product of the carbon wrap for this job was a bit rougher (bumpy) than the lab tested specimens but structurally this should not be a problem. Many columns were wrapped with carbon at these multi-column, multi-bent structures near the ocean.
- Los Angeles demonstration composite column retrofit project on the Santa Monica Freeway
This demonstration project was to allow all the vendors who were interested in bidding for Caltrans contracts at that time, to wrap one column on some "modern" multi-column bents (i.e. no retrofitting needed). Various systems were constructed. Two machine wrapped carbon filament systems were displayed, one pre-preg system and another that ran the fibers through an ambient cure epoxy bath just before winding onto the columns. Two systems of pre-formed jackets were adhered (glued) to the columns to form composite jackets. Also, some hand lay-up carbon sheets were placed on other columns. Most of Caltrans approved composite systems are demonstrated at this site.
- Santa Barbara County, Arroyo Quemado Arch Bridge
Some of the spandrels (upright supports off the arch) were retrofitted on this structure using System 1. These rectangular shaped members were retrofit for shear by wrapping the full height. This project went very smooth and was completed on time. All uses for Caltrans column retrofit are for seismic resistance upgrading. Access was made for the workers and small work platforms are erected as with most of the composite column casing retrofits. The composite work was a small part of this large bridge seismic retrofit scheme. Again, the cloth was saturated with epoxy at grade and then wrapped around a cylinder, handed up to the workers and walked around the spandrels. The layers are then worked by hand to push up against the spandrels to create a tight jacket system. The jackets were painted as the last order of work after a topcoat of resin is applied. This topcoat protects the outer fibers if light sand blasting is needed to remove the existing paint when re-painting is needed in the future.
- Sacramento, Yolo Causeway
Yolo Causeway is a bypass structure for the Sacramento River to divert large winter flows away from town. Most of the area beneath the structure is flooded for a few months out of the year. The bridge is actually a conglomerate of many types of structures due to it being widened so many times. The interior structure is fairly old and utilized small pile shaft extensions (15.5"dia) as it's supports. A few thousand of these pile shaft extensions were retrofit with partial height (2.5' high) composite shells to contain the lap splice where the exposed part of the column was formed on top of the cast-in-drilled-hole (CIDH) portion of the column. A series of pre-fabricated polyester/E-glass shells (System 4) were manufactured and then adhered (glued) to the columns in the lap splice region using a polyurethane adhesive. The shells are formed to a slightly smaller diameter than the column so that they fit tightly around the column once they are cinched up. These shells are cut vertically to allow for placement. Many layers are made and placed around the column, staggering the cut vertical seam, thus building up the casing to the proper thickness. Adhesive is applied to the inner surface of the shell and the shell is then simple opened wide and placed around the shaft and allowed to snap back around the column. After all the shells are in place (this operation needs to be done fairly quickly), straps are used to cinch all the shells tight up against the column. These straps remain in place until the adhesive hardens (24 hours). Many columns a day needed to be completed to stay on schedule because there were so many columns to wrap. The shells were then painted to match the concrete color. This project was competitively bid amongst various jacket systems including steel casings.
- Pasadena, Arroyo Seco Arch Bridge
This structure had the bottom part of the spandrels retrofit (4.5') using system 1. This unique seismic retrofit was to keep the bottom part of the spandrels concrete from breaking up and falling off and compromising its vertical bearing capacity. The wrapping went well and the regions to be retrofit were accessed by wooden stairways and platforms. The composite was prepared on the ground and either walked up or roped up to the working area. This project had the best documentation and inspection to date for a composite column casing installation. The raw materials (cloth, 2 components of epoxy, top coat admixture, etc. were inspected by Caltrans laboratory personnel and tagged prior to shipment to the job site. Specifications and testing requirements were a well-established part of the contract. A pre-construction meeting was held and installation training was given to the inspectors. Working drawings showing the locations and the required number of layers were submitted and approved prior to installation. Sampling and testing of the materials went smoothly. The project was finished on time. The use of composites on historic arch bridges allows Caltrans to maintain the same lines of the structures for aesthetic considerations.
Authored by: Pat Hipley, team member from Caltrans 3/21/01