Repairing and Strengthening Concrete Bridge Girders and Railings with Carbon Fiber Reinforced Polymers (CFRP) For Impact Damage

Repaired regions of impact-damaged concrete girders with installation of externally bonded (EB) or near surface mounted (NSM) carbon fiber reinforcements has been observed to improve structural resistance to subsequent impact events. This evidence led the Texas Department of Transportation to replace its standard prestressed concrete girder impact protection detail, a structural steel angle bolted to the bottom girder flange edge, with a detail that uses a CFRP wrap of the bottom flange over the travel lanes. These observations and implementation of CFRP to “harden” a concrete structural element for impact damage by a DOT leads to the question will this approach benefit DOTs with their concrete traffic railing, especially those railings that require frequent repair, such as on highly curved ramps. Research is needed to evaluate the performance of prestressed and reinforced concrete elements such as girder flanges/webs and concrete traffic barriers and to determine if the performance under impact improved with the application of CFRP wrapping.

DOTs have invested in developing techniques for repairing impact-damaged concrete structures. Given the susceptibility for structures at some sites to be re-impacted after being repaired, developing guidelines for repairs after initial impacts is an important task to help maintain the integrity and safety of the transportation system. If CFRP wrapping can improve the impact performance of concrete elements such as girders and concrete railings, for both new installations and maintenance repairs, this can benefit DOTs with their maintenance program and the safety of workers and the traveling public.

The objectives of the proposed research are to:

i) Identify when it is appropriate to adopt a CFRP-based system to repair concrete girders and railings subject to multiple impact events,

ii) Develop techniques to assess the extent of damage in repaired structures after a subsequent impact event,

iii) Develop guidelines for how to implement CFRP-based strengthening and/or repairs for impact events based on results from full-scale dynamic impact testing,

iv) Develop tools to predict the capacity of prestressed concrete girders and railings for initial and second (or subsequent) impacts and repair.

Impact damage to concrete girders and railings is an ongoing challenge facing state DOTs. The instances of repaired girders and railings being re-impacted are common. The lack of a comprehensive set of guidelines for repair after second impacts limits the ability of DOTs to implement effective solutions and will typically require extensive research in the event a second repair is needed. The availability of comprehensive guidelines for these repairs will help extend the life of the existing infrastructure system and reduce the need for costly emergency replacements and prolonged closures.

NCHRP has sponsored a synthesis study to assess the state of the art in repairing impact-damaged prestressed concrete bridge girders through NCHRP Project 20-07, Task 307 (Harries et al., 2012). In that study different repair systems were identified and assessed, and guidelines were provided on when to repair impact damaged elements and when to replace them. The North Carolina Department of Transportation (NCDOT) sponsored research to investigate the use of un-stressed externally bonded CFRP for flexural and shear repair of impact damaged AASHTO girders (Miller, 2006). The use of prestress externally bonded CFRP to repair impact damaged prestressed girders was demonstrated through a field application in Canada (Kim et al., 2008a). In a subsequent study a numerical model was developed to determine the live load distribution factors of the repair bridge (Kim et al., 2008b). While representing only a small subset of the existing literature, these studies demonstrate that the use of CFRP to repair impact-damaged girders can effectively restore the capacity of the elements to their original state. While the NCHRP study identified some structures which have been repaired multiple times using FRP composites, no comprehensive study or guidelines have been identified to facilitate the regular use of this repair approach.

Formal adoption of guidelines through AASHTO is anticipated as the primary outcome of the proposed research. To that end, it is expected that approval through AASHTO would be pursued and that that AASHTO Committee on Bridges and Structures, T-6 (Fiber Reinforced Polymer Composites), T-10 (Concrete Design), and T-7 (Bridge Railing) would play key roles in the development and approval of such a document. The availability of such an AASHTO guideline would facilitate implementation by state DOTs. Given the unpredictable nature of impacts and collisions, it is not possible to predict how soon after completion of the research implementation of these guidelines could be expected. The report on NCHRP Project 20-07, Task 307 (Harries et al., 2012) indicates that repairs of second impacts have been implemented by some DOTs. Implementation on a demonstration basis is anticipated as the first step towards broader implementation until confidence in the approach is gained.