This manuscript presents a method for controlling the quality of installation for spike anchors designed to delay delamination of externally bonded fiber reinforced polymers. The protocol includes the preparation of the drill hole and the insertion process. The most influential parameters on the efficiency of the anchors are discussed.
Fiber reinforced polymer (FRP) anchors are a promising way to enhance the performance of externally-bonded FRPs applied to existing structures, as they can delay or even prevent debonding failure. However, a major concern faced by designers is the premature failure of the anchors due to stress concentration. Poor installation quality and preparation of the clearance holes can result in stress concentration that provokes this premature failure. This paper deals with an installation method that aims to reduce the impact of the stress concentration and to provide a proper quality control of the preparation of the drill hole. The method involves three parts: the drilling and cleaning of the holes, the smoothing of the hole edges with a customized drill bit, and the installation of the anchor itself, including the impregnation of the anchor dowel and its insertion. Anchor fans (the free length of the spikes) are then bonded to the external FRP reinforcement. For the end anchorage, and in the case of reinforcements with multiple plies, it is recommended that the anchor fan be inserted between two plies to assist the stress-transfer mechanism.
The proposed procedure is complemented with a design approach for spike anchors, based on an extensive database. It is proposed that the design follow a number of steps, namely: selection of the anchor diameter and subsequent tensile strength of the connector (that is to say, the anchor before fanning out the free end), evaluation of the reduction in the tensile strength due to bending, provision of enough embedment to prevent slippage failure, and consideration of the number and spacing of anchors for a given reinforcement. In this sense, it should be noted that further research is needed in order to obtain a general expression for the contribution of spike anchors to overall bond strength of FRP reinforcements.
FRP anchors offer a promising way to enhance the performance of externally bonded FRPs applied to existing structures, given that they can delay or even prevent debonding failure1,2. However, a major concern for designers entails the premature failure of the anchors in shear due to stress concentration in the bending region. Installation quality and preparation of the clearance holes are crucial to limit this stress concentration that provokes such premature failure.
This paper deals with an installation method that aims to reduce the impact of the stress concentration and to provide a proper quality control of the preparation of the drill hole and the installation of the anchors. The method involves four parts: drilling and cleaning the holes, smoothing the hole edges with a customized drill bit to avoid irregularities in the stress-distribution within the bending region, installation of the anchor itself, including the impregnation of the anchor dowel and its insertion, and adhesion of the anchor to the reinforcement.
From previously published research3,4,5,6,7, it can be concluded that spike anchors with a bending region (that is to say, with a certain angle between the free end and the embedded region), suffer stress concentration that is prone to provoke premature failure. This cannot always be avoided due to the geometry of the original members. In many cases, 90° dowel angles are broadly employed, although it is generally agreed that 135° dowel angles allow a reduction in stress concentration and lead to better performance of spike anchors. The main reasons for the use of 90° dowel angles are that they are simpler to execute and control in any direction and that they reduce the possibility to meet internal reinforcements.
Figure 1 shows a typical spike anchor with the most common dowel angles. Spike anchors installed with 90° dowel angles can, nevertheless, display a relatively good performance if proper control of the stress concentration is provided. Limiting stress concentration generally involves designing the anchors with a large inner bending radius, as the inner bending radius has been found to play a major role in fiber kinking8,9. In this sense, authors such as Orton et al.3 suggest that a bending radius of four times the anchor diameter should be used. This recommendation results in impractical bending radii, even for small anchor diameters, as increasing the bending radius involves diminishing the actual embedment length for a given hole depth.
The authors believe that the recommendation of large bending radius is related to the difficulty of controlling the real inner bending radius, from a geometrical point of view, when smoothing is done by hand. A customized drill bit has been consequently designed that allows an easy quality control of the installation and ensures that the bending radius is considered in the design.
Two different processes are considered in the paper. The first one is related to the installation procedure for the connectors (anchors, especially before fanning out the free end), whereas the second includes the proposed method for design with spike anchors and the verification needs.
1. Anchor Installation Method
NOTE: This method includes the hole drilling, cleaning, and smoothing of the hole edge, as well as impregnation and insertion of the anchor.
2. Design with Spike Anchors
NOTE: The design method is explained here for fan anchors, but similar procedures could be followed for different anchorage devices. This method consists of the evaluation of anchor capacity, bond strength, and contribution of the anchors to the overall strength of the reinforced member.
Tests were conducted on isolated connectors to evaluate the effectiveness of the smoothing method. In addition, two methods of impregnation and insertion of the connectors were compared. The wet method involved impregnating the anchors immediately before insertion, as in the presented protocol. The hardened (or pre-impregnated) method consisted of impregnation of the embedded region of anchors in advance, at least 24 h before insertion.
In tests conducted following the proposed method, an average increase of 27 MPa was achieved compared with non-smoothed specimens of the same embedment length and hole diameter. Of note is the difference in terms of the standard deviation, which was only 10.9 MPa for smoothed specimens following the method, whereas for an identical configuration and non-smoothed specimens it was 88.2 MPa. It should be noted that the tensile strength of the tested carbon fiber ropes was not achieved in any test, as all anchors exhibited premature failure due to being in shear.
The difference between the two impregnation and installation methods was not important in terms of the ultimate load, but it was significant in terms of the scatter. This has been related to the relative ease of quality control, which is critical for spike anchors. It should be noted that the handling of FRP anchors requires skillful workers. Nevertheless, given that the quality of the impregnation is difficult to control in the hardened method, this method is not recommended. Pre-impregnated connectors had higher standard deviations when embedment length was sufficient to prevent adherent failure (100 and 125 mm embedment lengths, hemb). Results obtained from wet and hardened anchors with smoothed holes are displayed in Figure 4.
Load-bearing capacity calculated according to the equations presented fits the available data in pull-out and shear. To find out more about this design modeling and the test results, please see previous works by the authors7,14.
Once the load bearing capacity of isolated spike anchors has been addressed, it is crucial to evaluate the contribution to the overall strength of externally bonded reinforcements. The existing data for anchored FRP joints in simple situations with spike anchors (i.e., single or double shear tests on concrete specimens) are quite limited. The proposed steps for design with FRP anchors were found to fit acceptably well to the existing database including tests by different authors5,19,20,21,22.
Figure 1: Configuration and dowel angles of FRP spike anchors. The dowel angle, together with the embedment length, plays a major role in stress concentration in spike anchors. The fan angle must be adapted to the width of the external reinforcement. (a) Typical FRP spike anchor. (b) Variables for design (fan angle, dowel angle, embedment length, and fan length) are summarized. The parameters d0 and da are the nominal diameter of the anchor and the hole diameter, respectively. This figure has been modified from Villanueva Llauradó et al. 201714. Please click here to view a larger version of this figure.
Figure 2: Customized drill bit. Generate customized drill bits with the desired radius. Diamond bits may be of choice for durability reasons. The proposed tool (a) has eight cutters. The smoothing process is terminated when the tool round plate of the tool touches the surface of the substrate. The resulting profile of the hole is shown in (b). Please click here to view a larger version of this figure.
Figure 3: Connection between spike anchor and external reinforcement. The main steps involved in the installation and connection between the external reinforcement and the spike anchor are pictured here. (a) The first ply (or layer) of the FRP reinforcement is applied to the substrate with resin. (b) The insertion of the anchor dowel in the hole. (c) The free length of the anchor is fanned out and bonded to the reinforcement with resin. (d) After each step of application of resin, the air voids must be removed with a bubble roller. Please click here to view a larger version of this figure.
Figure 4: Influence of the insertion method on the scatter of results. Results of tests performed by the authors (mean values and error bars representing the central 95% range). There is an almost linear increase of the anchor capacity with embedment length. This is displayed together with the influence of the installation method in the scatter of results. The horizontal and vertical axes represent, respectively, the embedment length (hemb) and the ratio between the actual performance of the anchors (Pa) and their tensile strength (Pu). Please click here to view a larger version of this figure.
A step-by-step protocol for installation and design of FRP spike anchors is presented. To the best of the authors' knowledge, no detailed protocols on spike anchors have been developed regarding the effect of installation parameters and process on anchor capacity.
The proposed smoothing drill bit is beneficial in the performance of spike anchors, by means of reducing the stress concentration, and has proven its efficacy in reducing the scatter of the tests performed on isolated anchors. This is related to the enhancement in the quality control of the installation. Also, the low scatter of the anchors executed following the proposed installation protocol allows a reduction of the standard deviation, thus contributing to a reliable design.
Regarding the test results presented, there are no significant differences between pre-impregnated and wet installation in terms of the ultimate load. However, for the proposed protocol it is recommended that the anchor dowels be impregnated immediately prior to insertion, to guarantee proper impregnation of the bending region. In addition, this prevents brittle failure due to hardened resin in the bending region. If pre-impregnated installation is employed, then it must be guaranteed that the hardened portion is shorter than the straight leg of the anchor dowel. Proper impregnation of the bending region after insertion is difficult to achieve for test conditions. It should be noted, however, that this problem is reduced when the splay region of the connector is fanned out.
The main disadvantage associated with the customized drill bit is that it was designed with a radius of 20 mm, resulting in a 25 mm inner bending radius for 10 mm diameter anchors in 20 mm diameter holes. When compared with a larger inner bending radius, there is lower connector efficiency. It is recommended that, if high bend strength is required, that the drill bit be designed with a non-constant radius able to minimize stress concentration (by means of maximizing the inner bending radius).
The proposed design procedure includes all the necessary steps for a complete design of anchored externally bonded FRP reinforcements. The contribution of the anchors must always be considered in terms of an addition to the bond strength. It should be noted that tests on isolated anchors must be conducted on specimens with the designed geometry (hole and anchor diameter, embedment length, and bending radius, in particular) for the anchors. Then, the contribution of the anchors can be calculated as an addition to the bond strength of the FRP reinforcement. To date, the main limitations for design are the maximum contribution of the anchors and the optimum arrangement of multiple anchors. The authors wish to indicate that, according to the available databases, spike anchors alone can carry as much load as the adherent mechanism alone, which would mean that the overall strength of the anchored joints could be expected to be twice that of unanchored bonded joints. However, this value cannot be confidently adopted as a design value given the limited number of existing data.
The presented protocol for installation and design aims to be a basis for future developments in the field of anchorage of externally bonded FRP reinforcements.
The authors have nothing to disclose.
The authors wish to express their gratitude to Sika SAU for their support and, particularly for their supply of the material for the anchors and for the reinforcements. Betazul is especially acknowledged for their help with the customized drill bit and with the preparation of the video.
Concrete | The concrete for support has a dosage made by the authors, and a strength class no lower than C40 | ||
SikaWrap anchor C | SIKA | This material has been used for the FRP spike anchors. SikaWrap Anchor C is a unidirectional, carbon fiber rope, sheathed in an elastic gauze. The gauze can be cut onsite to create a fan end that anchors CFRP fabrics and plates used in the structural strengthening of masonry and concrete. | |
Sikadur 330 | SIKA | Impregnating resin, apt for manual saturation methods. The product was used for impregnating the anchor dowel before insertion | |
Sikadur 30 | SIKA | Thixotropic, two part epoxy resin applied by spatula and therefore suitable for virtually any application, including overhead | |
Drill bit | Betazul | Drill bit employed to smooth the holes that was designed by the authors and developed by Betazul SA | |
Hammer drill | Hilti | Tool for the execution of anchor holes on masonry and concrete, for different drilling ranges | |
Wire brush | Hilti | Hit series | For the proper brushing of drilled holes of varying diameters and embedment depths |
Blow-out pump | Hilti | Hit series | Manual blow-out pump |
SikaWrap-230 C | SIKA | Unidirectional woven carbon fiber fabric for dry application process | |
Aluminium Bubble Roller | Fibre glast | For laminations where increased pressure is necessary to release air bubbles. They are straight across the width of the head and provide excellent air relief for nearly all applications. | |
Brush | For impregnation of FRP bundle and sheet | ||
600 kN testing machine | Proeti | DI-CP/S | This is used for the shear test of anchors, in order to evaluate the efficacy of the proposed insertion method |
Cable ties | Cable ties are needed to fasten the end of the anchor dowel in order to prevent fanning out of the fibers during insertion | ||
Measuring tape | The measuring tape is necessary to control the embedment length as well as the diameter of the drill bit and hole clearance | ||
Steel wire | Required to assist insertion | ||
Rigid (steel) bar | A rigid bar of any material (in this case, it was made with a steel bar) is needed to control the embedment length |