Seismic Repair and Strengthening of Concrete Columns with Glass or Carbon FRP

Reinforced Concrete columns or bridge piers can be efficiently strengthened with Glass FRP (GFRP) or Carbon FRP (CFRP). Older (pre-1970s) columns have two major shortcomings; they are inadequately confined (usually a No. 3 or 4 tie placed at a spacing of 12 inches) and the ends of the ties are not properly anchored in the core region. During an earthquake, the ties open and allow the longitudinal steel to buckle, leading to failure of the column.

Glass FRP and Carbon FRP can provide significant lateral confinement for concrete columns or bridge piers. While spiral columns have in general performed well in past earthquakes, the above shortcomings have resulted in failure of many tied columns such as the one shown on the right.

The solution is to externally confine the column. External confinement increases the strength of the concrete, but more importantly for seismic applications, the strain at failure of the concrete (i.e. ductility) increases significantly. Among the advantages of retrofitting columns with Fiber Reinforced Polymer (FRP) are:

• Increases Ductility
• Increases Shear Strength
• Improves Bond in Starter Bars
• Conforms to Various Cross Sections
• Requires Minimum Access
• Costs Less than Conventional Methods

Research and Development

QuakeWrap principals were the first research team in the U.S. to receive a 3-year grant from the National Science Foundation in 1991 to study the behavior of columns retrofitted with fiber composite jackets. Both repair and retrofit of circular and rectangular columns with active and passive confinement were examined. The findings of this extensive study have been published in several journal articles dealing with such topics as confinement effects on circular and rectangular columns and repair of earthquake-damaged columns. A number of other researchers have also studied this problems and a sample of those technical papers is also available.

Columns were subjected to an axial load of 100 kips that remained constant throughout the test while the top of the columns were subjected to reversed cyclic loading, simulating earthquake motions. Retrofitted specimens continued to resist additional lateral loads during consecutive loading cycles and testing usually had to be stopped because the maximum displacement of the testing equipment was reached.

Case Studies

• I-40 Bridge in Oklahoma City, OK
• The Phoenician Resort, Scottsdale, AZ