In recent years, numerous reports have discussed the dilapidating state of bridges across the nation. The Federal Highway Administration has generally categorized these issues as either functional or structural in nature. In particular, many bridges in the Northeastern and Midwestern parts of the country have suffered significant damage due to many years of salt and deicing chemical use. Leaking and damaged bridge expansion joints have exacerbated the problem by compounding the damage on the bents beneath. This type of damage is also present in many other areas of bridges with inadequate drainage, and in the splash zone areas. The damage shown in Figure 1 is typical of that found in hundreds of bridges across the country. Thus, there is an urgent need for effective and economical techniques that can be utilized to mitigate degradation and to restore the structural integrity of these bridges.
A cost-effective method for seismic upgrading of concrete bridge columns developed at the University of Arizona more than 20 years ago can be equally effective in restoring the integrity of deteriorated bridge beams and columns. This method involves wrapping of columns with high-strength fiber reinforced polymer (FRP) sheets to confine the concrete and enhance its mechanical properties, ultimately increasing the shear and flexural capacities of the column.
FRPs are made of filaments of high performance fibers such as glass, carbon and Kevlar that are woven into a fabric, then saturated in an epoxy matrix and bonded
to the concrete substrate. Typical fiber tensile strength ranges from 500 to 700 ksi (kips/square inch). The specific gravity of the fibers is about one fifth that of steel. These ultra, high-strength, lightweight fabrics are very attractive for structural applications. The materials are not prone to electrochemical corrosion, and they are very pliable and conform to any substrate shape while the epoxy is uncured. Since its inception, this method of strengthening has been well accepted by the engineering communities and extensively used around the world.
The effectiveness of FRPs in upgrading structural member strength, such as concrete beams and columns, has been demonstrated in several studies at the University of Arizona, as well as in numerous other studies in laboratories and universities around the world. In a first of its kind study conducted at the University of Arizona and funded by the National Science Foundation, the behavior of seismically deficient concrete columns strengthened by wrapping FRP sheets in the plastic hinge regions was examined.
A column foundation subassembly was designed and constructed to simulate seismic deficiencies in existing columns, such as the lack of sufficient lateral ties, inadequate lap splice length, and/or poor
reinforcement detailing. The column was tested under reversed inelastic cyclic loading to failure. The column performed very poorly, with significant loss of strength and stiffness after the third cycle.
An identical column was then wrapped in the plastic hinge area at the column/foundation connection and then subjected to the same loading regime as that of the control column. The hysteretic behavior of the column after it was wrapped improved significantly when compared to that of the control specimen. The column developed stable hysteresis behavior with a positive envelope slope even beyond the sixth cycle. The test was stopped when the actuator reached its maximum stroke. This and other similar tests attest to the effectiveness of FRP for increasing the strength and ductility of concrete columns.
This technique has been used to retrofit concrete columns in buildings and bridges around the world to resist seismic forces. It has also been utilized to restore integrity and strength of columns damaged due to corrosion of reinforcement. Many such bridges have been retrofitted in the Northeast, where the effects of years of salt and deicing chemical use have caused significant structural damage to bridges. A recent application to two bents in an interstate highway in the Northeast is discussed below.
Due to the severe corrosion of the rebars and spirals, there was significant spalling and damage to both the columns and the cap beam of the bridge. The beam and columns were first prepared by chipping and removing all damaged concrete and cleaning of rebars. Following removal of damaged and spalled concrete, the area was formed and patched with polymer concrete.
In order to make up for the lost area of steel due to corrosion, flexural reinforcement in the form of carbon strips were bonded to the tension zones of the beam, as shown in Figure 2. The columns were strengthened by wrapping carbon fabric onto the column in the hoop direction to make up for the loss of spiral reinforcement and in the longitudinal direction along the height of the column to make up for the longitudinal and flexural reinforcements. All areas where carbon fabric had been used were then painted with a UV stable paint (Figure 3). This technique proved to be a versatile, effective and economical way of restoring structural integrity to bridge structures damaged due to extensive corrosion of rebars. In addition to restoring the bridge to its original condition, the carbon fabric shield helps protect the concrete from further detrimental effects of salt and deicing chemicals in the future.
Since its inception, this method of strengthening has been well accepted by the engineering communities and extensively used around the world.
Hamid Saadatmanesh, Ph.D., P.E., is a former member of ACI Committee 440 and is the director of FRP for Simpson Strong-Tie. He can be reached at firstname.lastname@example.org.