California’s major earthquakes at Loma Prieta in 1989 and at Northridge in 1994 clearly indicate how destructive seismic events can be to concrete structures. The major damage to concrete bridge columns occurs with these seismic events and occasional vehicular encounters, usually with the vehicle getting most of the damage. The repair of these damaged structures requires several factors be considered before the means and methods of repair are determined.
Although these seismic and impact events are significant in their degree of damage, the majority of structural concrete damage problems relate to rebar corrosion caused by improper cover, cracking, construction joints, and permeable concrete. Corrosion of this reinforcing steel leads to spalling of the surrounding concrete.
A damaged structure undergoing repair must have the repair methods determined by the appropriate engineer so the repaired concrete structure is strong enough to carry the specified design loads and durable enough to withstand the forces associated with weathering. These methods also need to be performed in a timely and cost-efficient manner. A critical factor in any repair of a highway bridge structure is the length of time the structure will be out of service.
The severity of damage to a cast-in-place structural column falls into two categories defined by the degree of damage to the concrete and the reinforcing within.
1) Damage to the concrete with apparent spalling of the concrete without any damage to the longitudinal or spiral reinforcing
2) Spalling of the concrete with damage to the longitudinal and/or spiral reinforcing
The degree of damage to the reinforcing also may go from simple buckling of the spiral reinforcing to complete crushing and fracture of the column reinforcement. The column damage limited to concrete spalling with no reinforcement damage often can be repaired with epoxy injections into the concrete cracks after removing the spalled concrete, followed by repair of the surface concrete.
In the case of damaged concrete structural components, where the removal of the existing concrete and the longitudinal and spiral reinforcing is necessary, a thorough analysis needs to be conducted to avoid construction conflicts. This includes determining the means and method for the attachment of the new longitudinal reinforcing with mechanical couplers onto the existing dowels after the column reinforcing has been sheared off.
The engineer specifying the couplers for the attachment of the longitudinal bars and the reinforcing subcontractor who will install them need to consider the congestion that may be created when the new reinforcing is coupled on to the existing reinforcing. The size of the longitudinal bars along with their placement within the column profile necessitates a coupler be used that easily attaches to the existing dowels, after the longitudinal bars have been severed off at the base. The results of this type of repair, as reported in an ACI Structural Publication reporting on a research program, indicates the repaired column developed stiffness, strength, and deformation capacities comparable to the original column.
In the event of significant reinforcing congestion where the removal and replacement of the column reinforcing is not practical, an alternate method is to cast a concrete jacket reinforced with headed longitudinal bars encasing the damaged region of the column. This type of repair also develops comparable strengths to the original column.
To learn more about structural repair, refer to ACI 98-S24, “Repair of Earthquake-damaged Bridge Columns” atwww.concrete.org.