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Engineering
Procedia Engineering 14 (2011) 3176-3182
www.elsevier.com/locate/procedia
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DevvvC MvstvfinojeC1, MesvuC Shemoli
1 Department of Civil Engineering, Isfahan University of Technology, Isfahan, Iran (dmostofi@cc.iut.ac.ir) 2 Department of Civil Engineering, Isfahan University of Technology, Isfahan, Iran (sm.shameli@cv.iut.ac.ir)
Abstract
Externally Bonded Reinforcement (EBR) is known as a conventional method for flexural strengthening of concrete beams with Fiber Reinforced Polymer (FRP) composites. However, concrete beams strengthened with this method are often associated with premature debonding mechanisms. Recently, a new promising method has been introduced at Isfahan University of Technology (IUT) to postpone or eliminate debonding of FRP sheets from concrete surface in concrete beams strengthened for flexure, named Externally Bonded Reinforcement On Grooves (EBROG). Experiments have shown that the probability of debonding when attached to concrete substrate using EBROG method is much lower than the EBR method; and in some cases debonding is completely eliminated. The aim of the current study is to examine the efficacy of grooving method when used under multilayer FRP sheets. For this purpose, beam specimens with dimension of 120x140x1000 mm were cast and strengthened with both conventional surface preparation and EBROG method with different number of layers of FRP composite. Then, the strengthened beams were subjected to a four point flexural loading. The results showed that the EBROG method provides higher failure loads in the beams strengthened with multi FRP layers compare to those with conventional surface preparation.
Keywords: Fiber Reinforced Polymer (FRP); strengthening, debonding; Externally Bonded Reinforcement On Grooves (EBROG); Externally Bonded Reinforcement (EBR)
1. INTRODUCTION
Over the past decade, conventional materials such as concrete and steel are being replaced by Fiber Reinforced Polymer (FRP) Composites for repairing and rehabilitation of concrete structures. Numerous advantages such as high tensile strength, high durability and corrosion resistant, low weight and easy installation, and no limitation in size and configuration, have made FRP to be highly desirable and implemented in a large number of practical projects worldwide [1,2].
It has been shown through experimental and numerical studies that externally bonded FRP composites can effectively improve the load carrying capacity of concrete beams as well as their stiffness and
1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.07.401
durability, known as Externally Bonded Reinforcement (EBR) technique. According to EBR method, after surface preparation, FRP sheet is adhesively bonded to the tension face of the concrete beam. It should be noted that the purpose of surface preparation is to remove contamination and weak surface layers and regularize the concrete surface to promote the adherence capacity of concrete substrate.
Previous research efforts have investigated the behavior of these FRP strengthened beams when subjected to flexural loading. Results have shown that the ultimate load carrying capacity of the retrofitted elements is directly influenced by the interface bonding performance and EBR technique cannot mobilize the full tensile strength of the FRP composite due to their premature debonding from the concrete substrate [3-5]
Large numbers of studies have been conducted to improve the performance of EBR method with the aim of postponing or eliminating the FRP laminate from the concrete surface. Kamada and Victor (2000) [6], Toutanji and Ortiz (2001) [7], and Galecki et al (2001) [8], investigated the effect of surface preparation on the bond behavior of strengthened elements, also Oh and Sim (2004) [9] examined the effect of length, thickness and plate width on interface debonding failure and Pimanmas and Pornpongsaroj (2004) [10], conducted an experimental programme to examine the peeling behavior of strengthened beams under various end restrained conditions. Although these studies have helped to enhance the efficiency of EBR technique, experiments have shown that strengthened beams are still highly prone to debonding and this limitation has affected the efficiency and safety of this method.
Mostofinejad and Mahmoudabadi (2009) [11,12] have introduced an alternative method of surface preparation to postpone debonding of FRP laminates in concrete beams. In this method, first the longitudinal grooves on the concrete surface of the elements were cut using a diamond blade cutter in order to strengthen the element [Figure 1]. The grooves were then cleaned by jet air and filled with the epoxy resin [Figure 2]. FRP sheets were later installed on the concrete surface saturated with the epoxy resin and the resin in excess was removed. This method is called Externally Bonded Reinforcement On Grooves (EBROG). It should be noted that what is herein called EBROG technique, was previously named as Grooving Method (GM).
EBROG method has shown great promises in dealing with debonding problems. Experiments have revealed that in this technique, debonding is postponed and in some cases is completely eliminated. Generally the EBROG method is superior to EBR method due to a number of advantages: (1) as mentioned before, the EBROG technique postpones the debonding compare to the EBR method and in some cases provides the full elimination of debonding of FRP laminates and this is the most important aspect of this technique. (2) the amount of site installation work may be reduced since surface preparation other than grooving is no longer required. This leads to less economic losses due to less work activities; (3) due to elimination of surface preparation, EBROG technique has less environmental impacts.
So far, the performance of the EBROG method is examined only with single layer of fiber and there is no research when multilayer fibers are used to strengthen beams with EBROG method. The aim of the current study is to carry out some experimental data for debonding failure as well as ultimate load carrying capacity and efficacy of EBROG technique when multilayer FRP sheets are used.
2. SPECIMENS DETAILS AND MATERIAL CHARACTERISTICS
In order to carry out the tests, 14 concrete beam specimens with the dimension of 120*140x1000 mm were cast. The specimens have no internal flexural reinforcement but for preventing any undesired shear failure, they were reinforced for shear by internal stirrups. The dimensions of beams, as well as their steel reinforcement details, are shown in figure 3. The concrete specimens were removed from the mold just one day after casting and then were cured for 28 days in the moisture bath. The average values of concrete compressive strength at 28 days were evaluated from uni-axial compression test on cubic
specimens and are presented at table 2 at equivalent cylindrical values. The composite materials used for flexural strengthening were of the unidirectional FRP type with the length of 800 mm and width of 100 mm and were applied according to wet-lay-up procedure for all the specimens. Table 1 includes the values of the properties of the CFRP sheets. Two types of suitable epoxy resin, Sikadur C31 and Sikadur C300 were also used as filler and matrix phase of FRP composite respectively.
Table 1: Properties of FRP sheets
modulus of elasticity ultimate tensile strength thickness ultimate strain
3. EXPERIMENTAL PROCEDURE
Flexural tests were conducted on beams strengthened with one, two and three layers of FRP sheets. Two method of flexural strengthening, i.e. EBR and EBROG methods were also considered. The specimens were divided into 7 groups. Each group includes two identical beams to ensure test repeatability and to avoid any unforeseen issues. The specimens of group REFRENCE have no flexural strengthening and were served as control beams. The groups of beams which were strengthened with EBR method were designated by EBR-1L, EBR-2L and EBR-3L for one layer, two layers and three layers of FRP sheets respectively. Groups EBROG-1L, EBROG-2L and EBROG-3L include beams which were strengthened with EBROG method with 1, 2 and 3 layers of FRP sheets. In the EBR method, after surface preparation, epoxy resin Sikadur C31 was used as primer layer. After 24 hours the surface was saturated with epoxy resin Sikadur C300 and FRP sheet was installed on the surface. In EBROG method, the created grooves were first filled with Sikadur C31 and after an hour the FRP sheet was applied to the surface, using Sikadur C300. The specimens were allowed to be cured for at least 5 days prior to testing. The size of FRP laminates used for both EBR and EBROG methods is 800mm x 100mm with the thickness of 0.12mm. The groove size used in the EBROG method is 850mm long, 8mm wide, and 10mm deep. The edge to edge distance of each two grooves next to each other is 15mm. A description of the specifications of the specimens is provided in Table 2 and Figure 3.
Table 2: Specifications of strengthened specimens
Beam series average concrete compressive strength (MPa) number of FRP layers
EBR-1L 36.7 1
EBR-2L 35.9 2
EBR-3L 37.1 3
EBROG-1L 37.2 1
EBROG-2L 37.8 2
EBROG-3L 36.8 3
231 GPa 4100 MPa 0.12 mm 1.7%
Figure 1: Grooves at EBROG method; Figure 2: EBR and EBROG methods
P/2 P/2
100 mm
0>6(^50mm> s
140 mm
50 mm Figure 3: concrete specimen
900 mm
120 mm
Figure 4: Testing device
The strengthened flexural elements were tested under four point loading over a span of 900 mm (Figures 3 and 4). In order to obtain an accurate deflection reading, two Linear Variable Differential Transducers (LVDT) were also mounted at the mid-span and connected to a data logger. Crack imitation and propagation were also monitored by visual inspection during each test.
4. RESULTS
4.1. Load-Displacement diagrams
The load versus mid-span deflection curves for each series of tested beams is illustrated in Figures 5 and 6. Each curve represents the average of two tested beams.
■EBROG-1L -EBROG-2L
^ _T- EBROG-3L
DISPLACEMENT (mm)
DISPLACEMENT (mm)
Figure 5: Load-Displacement curves for strengthened beams: (a) beams strengthened with EBR method; (b) beams strengthened with EBROG method
Figure 6: Load-Displacement curves for all the beams
All the strengthened specimens, except specimens of group EBROG-1L, failed in the same manner. After a crack occurred on the bottom of the concrete beams near the mid-span, the FRP laminate essentially stressed until debonding occurred. The type of debonding in these groups was plate end debonding. In EBROG-1L beam series, debonding was completely eliminated and failure due to FRP rupture was occurred.
4.2. Stiffness of the strengthened beams
As it is shown in Figures 5 and 6, the deflection of the strengthened beams are reduced under the same load as number of FRP layers increase in both EBR and EBROG methods. Consequently, the stiffness of the beams increases with increasing the number of strengthening FRP layers.
4.3. Ultimate load capacity
The values of ultimate loads for each series of beams are presented at Table 3. It is concluded from the results that the specimens strengthened with EBROG method achieve higher ultimate loads than those strengthened with the conventional surface preparation method.
Table 3: Ultimate load carrying capacity
Beam Series Ultimate Load (KN) Percentage of ultimate load increase compared to the reference beam Percentage of ultimate load increase in EBROG method compared to EBR method with the same FRP layer
Reference 6.7 — —
EBR-1L 9.3 39% —
EBROG-1L 19.3 188% 107%
EBR-2L 12.8 91% —
EBROG-2L 31.5 370% 146%
EBR-3L 19.6 193% —
EBROG-3L 38.1 469% 94%
5. CONCLUSIONS
In this paper, the effect of increasing FRP layers on EBR and EBROG methods has been investigated experimentally. Based on the results, the following conclusions can be drawn:
1- Externally bonded FRP laminates to the concrete beams can effectively increase ultimate loads as well as ductility.
2- EBROG method is a very efficient technique even for beams strengthened with multi-layer FRP sheets and as it was shown, beams strengthened with this method can noticeably achieve higher ultimate loads (up to 146% in current experiments) and strains compared to EBR method.
3- Debonding failure of strengthened beams with multi-layer FRP sheets is of the plate end debonding type for both EBR and EBROG methods. However, the EBROG technique is advantageous as it can utilize higher tensile strength of FRP sheets and tends to achieve FRP rupture before debonding.
REFERENCES
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