FRP : FRP-concrete bond in structural strengthening and rehabilitation

Technical Committee FRP

General Information

Chair: Prof. Nemkumar BANTHIA
Activity starting in: 2003
Cluster E

Subject matter

Last two decades have seen a significant interest in the use of FRPs for civil engineering structures including bridges, buildings, parking garages and residential construction. On the one hand, their use has been driven by the desire to find non-corroding reinforcing materials alternate to steel, and on the other hand, for the purposes of developing cost effective and durable structural strengthening and rehabilitation solutions. In the latter category, seismic retrofit has appeared as one of the most effective uses of FRPs, and numerous projects around the world can be cited in this regard. FRPs are applied both in internal applications like ordinary steel bars in concrete and for external applications in the form of wraps, laminates or sprays applied for repair and strengthening of steel, concrete and timber structures. In the case of repair and strengthening, it is unfortunate that in spite of the ever-increasing use of FRPs in civil construction, the bond developed between FRP and the substrate material remains poorly understood. Very little is understood of the mechanisms that allow bonding to be developed and factors that can cause deterioration over time. This TC will focus on FRP-Concrete bond when FRP (in the form of a wrap, laminate or spray) is externally bonded to an existing concrete structure. This is a new subject matter and is not being addressed by any other international committee.

Terms of reference

It is proposed to create two task groups within the committee. Task Group 1 (TG1) will investigate the properties and durability of FRP-concrete interface as a function of fiber type, type of polymeric matrix, bonding agent, surface preparation, etc. Task Group 2 (TG2), will study existing test methods for the measurement of FRP-Concrete bond properties, and pursue the development of a recommended test procedure. The estimated time necessary is about three years. Two years after the start, two comprehensive state-of-the-art documents will be produced. One will highlight properties, durability and long-term performance of bond, and the other will deal with testing and measurement issues. Finally in the third year, a recommended test procedure for measurement of bond will be proposed along with a possible round robin test program. At the conclusion of the committee's work, an international workshop will be organized covering the work of the two task groups and the round-robin test program. The membership of the committee will be restricted to about 20 members with approximately equal numbers working on the two task groups. The membership is expected to be truly international with equal representation from academia, industry, and practice. A general call for membership will be issued along with more targeted recruitment.

Detailed working programme

This committee will focus on the following objectives: 1. Concrete-FRP Bond: Reviews of State-of-the-art and Identification of Knowledge Gaps: The first and foremost objective of the committee will be to create a state-of-the-art document on our current understanding of bond between FRP and concrete, when FRP is used as an externally bonded element to a concrete substrate as in strengthening and rehabilitation projects. An extensive review of existing literature will be carried out and various ways of bond measurements and the corresponding findings with respect to type of FRP, concrete surface preparation and use of bond enhancers will be detailed. Our understanding of possible bond degradation under various chemical and physical conditions is very limited. Only few chemically deleterious agents have been investigated so far and doubts surrounding the performance of FRP will remain until tests are extended to an extensive list of chemical environments. The effects of physically aggressive environments such as freezing and thawing, prolonged exposure to elevated or sub-zero temperatures and large number of thermal cycles are clearly not known. Most investigations are valid for the specific case being studied and the results cannot be extended to conditions that differ based on severity of the environment, length of exposure and/or mode of attack. Clearly comprehensive damage models are needed for such an extrapolation. 2. Development of Test Techniques for Measurement of Concrete-FRP Bond : There are presently no standardized tests methods available to measure the properties of bond between FRP and a concrete substrate. Most researchers have resorted to techniques that differ widely based on specimen dimensions and configuration, details of gripping, method of loading, rate of loading and environmental conditions. No comparisons, therefore, can be made between the findings of various studies and no general conclusions can be drawn. Clearly, we need test methods that can be used to rationally measure the bond between FRP and concrete substrate. We also need better in-situ tests to assess the field performance of FRPs. Furthermore, all studies in the past have focused on measuring the load that occurs at the time of debonding, and as such only the ultimate values of bond have been measured. Debonding is, however, a gradual process where slow growth of cracks occurs at the interface, and the characteristics of such growth including the coalescence of cracks is controlled by fracture-mechanics based parameters. Thus, fracture-based characterization of the interface will produce far more useful indicators of interfacial quality than the ultimate bond strength alone. This characterization may also be a better representation of interface damage and durability. The committee will also strive to develop fracture based bond tests. A round-robin test program will be carried out. . While the committee will focus on documenting the long term performance of bond under deleterious conditions of environment, development of accelerated aging tests on bond is beyond the scope of this committee.

Technical environment

The committee will deal with topics associated with repair and strengthening of concrete structures, a topic well within the scope of RILEM as an organization dealing with a range of construction materials. The work of the committee will have links with some other committees within RILEM, but no overlap is expected.

Expected achievements

The direct benefits include: - A better regrouping of experts working in the area of FRP for strengthening and retrofit of concrete structures with a focussed objective of understanding the properties and durability of the bond - Development of harmonized test methods for measurement of bond properties - A state-of-the-art report on FRP-concrete bond and bond durability - A round robin test program - Final report and recommendations - An international workshop

Group of users

Academics, test laboratories, industry and practicing engineers

Specific use of the results

As mentioned above the state of world infrastructure is rapidly deteriorating costing billions of dollars in lost revenue, traffic delays and perpetual maintenance. It is therefore imperative that we find cost-effective and above all durable repair and strengthening solutions that can be applied internationally. Use of FRPs in repair and strengthening of concrete structures is one such effective solution. Unfortunately, however, there is considerable discomfort at the moment on the issue of concrete-FRP bond durability and this has led to scepticism and lack of confidence towards the use of FRPs. It is clear that developing an understanding of bond between FRP and concrete and devising better ways of maintaining a durable bond will not only lead to a better acceptance of this technology, but also have major economic implications in terms of reduced frequency of repair, inspection and maintenance.