PULLOUT CAPACITY BEHAVIOUR OF FRP-HEADED REBARS

Transcription

1 PULLOUT CAPACITY BEHAVIOUR OF FRP-HEADED REBARS Hamdy M. Mohamed NSERC Post-Doctoral Fellow University of Sherbrooke Sherbrooke, Quebec, Canada. Brahim Benmokrane Professor University of Sherbrooke Sherbrooke, Quebec, Canada. Abstract This paper presents an experimental investigation on the pullout capacity of the new developed fibre reinforced polymer (FRP)-headed rebars to be used in the reinforced concrete structures. A total of 40 specimens containing FRP-headed rebars embedded in a 200 mm concrete cube were tested under monotonic tension. The effect of two parameters and their interactions on the pullout capacity were investigated; namely, the bar size and concrete compressive strength. Glass FRP (GFRP) rebars of high Grad (tensile modulus over 60 GPa) are used. For each rebar, the test results include the pullout capacity and the mode of failure. The test results showed that the FRP-headed rebars are an efficient method of providing anchorage capacity to develop the ultimate tensile strength of the bar. As, the average stresses at the ultimate pullout capacity in the headed rebars reached to almost 1.5 to 1.8 times the yield strength of the steel bars. The developed FRP-headed rebars of this study can provide a suitable alternative to bent bars in some applications such as concrete bridge barriers. Keywords: Concrete; Fiber reinforced polymers; Bond; Pull-out resistance; Headed. 1. Introduction Electrochemical corrosion of steel is a major cause of the deterioration of the civil engineering infrastructure. It is becoming a principal challenge for the construction industry world-wide. Fibre reinforced polymer (FRP) rebars have been extensively used in different concrete structures as an alternative to the steel reinforcements due to their non-corrosive nature. FRP materials in general offer many advantages over the conventional steel, including one quarter to one fifth the density of steel, neutrality to electrical and magnetic disturbances, and greater tensile strength than steel. In the last decade, several field applications of FRP bars in marine structures, concrete bridge deck slabs, bridge barriers, parking garages, and concrete pavement, as well as several experimental studies have supported the suitability of FRP rebars for structural use, (Benmokrane et al. 2006) The bond characteristic of FRP rebar is one of the most important parameters that control the design of FRP-reinforced concrete members. Pullout and splitting are the two dominate bond failure modes expected between FRP rebars and concrete (ACI ; Benmokrane et al. 1996). The occurrence of each one depends on the confinement around the bars, concrete cover and strength, and the bar embedment length (Harajli and Abouniaj 2010). Page 1 of 8

2 FRP bent bars are being needed in many applications such as concrete bridge barriers (CSA S6-2006, El-Salakawy et al., 2005). In this case, the bond and the bar embedment length are become more critical. The problem is attributed to the significant reduction of the tensile strength at the bend portions of the FRP rebars. This paper presents an experimental investigation on the strength behaviour of FRP-rebars with new developed end anchorages to be used as alternative to the bent FRP-rebars. 2. Experimental Work An experimental investigation was carried out to investigate the pullout capacity of glass FRP-headed rebars. The test parameters introduced in this study are the bar size and concrete strength. The following sections provide detailed description of the FRP-headed rebars, specimens preparation, and experimental results. 2.1 New Developed FRP-Headed Rebars Sand-coated GFRP bars with new developed end anchorages were used in this study (see Figure 1). The FRP bars are made of continuous fibres (glass) impregnated in a vinyl ester resin using the pultrusion process, manufactured by a Canadian company [Pultrall Inc., Thetford Mines, Quebec]. Two different bar sizes were used; FRP bars No. 5 and 6 (diameter equal to 16 mm and 19 mm, respectively). The characteristic tensile strength and the mechanical properties are given in the Table 1. Headed-FRP No. 5 Headed-FRP No. 6 Figure 1. Overview of the FRP-bars with new developed end anchorages. The surface geometry of the heads employed in this study is new with special configuration of ribs to enhance the bond with concrete interface. Also, the FRP bar ends were prepared with rounded grooves on the surface before attaching the head to the bars to enhance the bond and increase mechanical interlocking between the bar end and the head, (see Figure 2). The headed anchorages are cast onto the deformed ends of the straight bar and hardened at elevated temperatures. The maximum outer diameter of the end heads is 3 times the diameter of the bar. The head length is approximately 100 mm. It begins with a wide wedge which helps to transfers a large portion of the load from the bar into the concrete and to develop the required uniform stress for equilibrium. Beyond this wedge, the head tapers in five steps to the outer diameter of the blank bar. This configuration is responsible to develop a stronger anchoring system and to avoid the splitting action in the vicinity of the head. Also, it can be used as a suitable alternative to bent bars in some applications for high strength grade FRP rods for reinforcing concrete frame structures. Page 2 of 8

3 Figure 2. Details and overview of the bar-head interface. Table 1. Mechanical properties of the straight FRP bars. Bar Size Cross-sectional area (mm2) Glass content % Vol ( % weight) Nominal bond strength Tensile strain (%) Nominal tensile modulus Guaranteed tensile strength No (83) ± No (83) ± Specimens Preparation The block specimens were constructed using two types (A and B) of normal-weight concrete with an average 28-day compressive strength of 36 and 47 MPa, respectively. The two concrete mixes were considered to address the effect of concrete strength on the pullout capacity of the FRP-headed rebars. At least six 150-by-300-mm cylinders were prepared for each concrete batch and cured under the same conditions as their concrete block specimens. The compressive and tensile strengths of concrete were evaluated at 28 days. The FRP bar anchors No. 5 and 6 were centred and adjusted vertically in a wooden formwork measured mm. Figure 3.a shows placing the FRP anchors in the centre of the formwork and adjusting their alignment. The load was intended to be resisted by the head of the GFRP anchor. So, debonding tubes were attached to the headed GFRP bars starting from the end of the head up to 200 mm on the bar. These tubes insure that the load is transmitted to the headed portion. Figure 3.a shows the test specimens after attaching the debonding tubes. Preliminary experimental tests conducted on these new developed FRP anchorages showed that the concrete blocks failed by splitting before achieving the full pullout capacity of the FRP-headed bars. Therefore, mild steel bars of diameter 3.2 mm are used as spiral reinforcement in the concrete blocks as shown in Figures 3.b. and 3.c. The spiral inserted in the formwork with pitch equal to 25 mm along the depth of the block. This spiral was used to avoid the splitting failure of the concrete blocks, as it was intended to obtain the maximum pullout capacity of the FRP anchorages. Also, the confining action provided by the spiral reinforcement should be adequate so as to minimize the risk of splitting the concrete by bond forces. However, ten concrete blocks were prepared without spiral reinforcement, as reference to address the effect of confinement on the pullout capacity. These specimens as presented in Table 2 and 3 are A5#1 to A5#5 and A6#1 to A6#5 for No. 5 and 6, respectively. The concrete was placed in three layers of approximately equal thickness and each layer was compacted 25 times with 16 mm diameter tamping rod. After molding, the specimens were initially cured by covering them with plastic sheet to prevent moisture loss for 24 h. Thereafter, the molds were removed, and for final curing the specimens were stored for 14 days in a moist room. During this period, they were sprayed daily with water so as to maintain moisture on the surfaces at all the times. Figure 3 shows the fabrication and casting of the test specimens at the laboratory of department of civil engineering, University of Sherbrooke. Page 3 of 8

4 a. bars attached with debonding tubes b. Steel spiral reinforcements c. Placing the bars and adjusting the alignment d. Concrete casting. Figure 3. Fabrication and casting the FRP-headed concrete blocks. 2.3 Test Setup and Instrumentation Before testing, the free end of the FRP bar was anchored for each concrete block specimen using steel tubes filled with Bristar 10 cement as adhesive. Also, the concrete block was attached with closed steel plate frame to confine and delay the splitting of the concrete. The specimens were tested on a BALDWIN machine of about 1000 kn capacity. Figure 4 shows the test setup for the GFRP anchors in the 200 mm concrete blocks. The load was increased until anchorage or concrete splitting failure occurred. The load was measured with the electronic load cell of the machine.the displacement of the free-end of the GFRP-headed anchor was measured using one LVDT and that of the loaded-end was measured using other LVDT. The BALDWIN machine and LVDTs were connected by a 10 channels Data Acquisition System and the data were recorded every second during the test. The loading rate range was 2.0 kn/s during the test by manually controlling the hydraulic pump. Figure 4. Test setup for the GFRP anchors. Page 4 of 8

5 3. Test Results and Discussion 3.1 Pullout Load Capacity Experimental results for the pullout capacity are given in Table 2 and 3, providing a comparison between the tested FRP-headed rebars. Failure was defined as the point of maximum pullout load during the test. Corresponding maximum nominal stress in the bar were then defined as those values occurring at the point of failure. The average pullout capacity of ten specimens for rebars No.5 for the two types of the concrete A and B were and kn, respectively. The corresponding values for the headed rebar No. 6 were and kn. The test results indicated that the pullout strength values developed in specimens with concrete type 36 MPa were lower than the ones developed in 47 MPa. This is attributed to the early splitting of the concrete blocks that didn t allow the anchors to reach its full strength. The previous test result showed that the load carrying capacity of the reference FRP-headed rebars No.5 was 95 kn, which failed by head breakout (Ahmed and Benmokrane 2009). The increase in the pullout strength value of the new developed FRP-headed appears to be more than 35%. This seemed to be an important attribute to the new materials used and the interface configuration which improved the mechanical interlocking between the heads and the FRP bars. The average stress at the failure in the FRP bars No. 5 for the two types of concrete A and B were 668 and 750 MPa, respectively, which represents 57 and 64%, respectively, of the strength of the GFRP bars. These stress values presents approximately 1.5 to 1.8 times the yield strength of the steel bars. The corresponding stress values for bars No. 6 were 568 and 639 MPa, of the strength of the FRP bars which represents 52 and 58%, respectively, of the strength of the FRP bars. On the other hand, specimens A5#1 to A5#5 and A6#1 to A6#5 as shown in Tables 2 and 3, respectively, for FRP bars No. 5 and 6 presented a significant lower pullout capacity than the other specimens, which failed by concrete breakout. These specimens were prepared without spiral reinforcement which leads to an early splitting failure of the concrete block, and the FRP anchorages didn t achieve the maximum pullout capacity. In contrast, specimens with spiral reinforcement achieved higher pullout strengths. This confirms the fact that the spiral reinforcement provided uniform confinement to the concrete along the anchorage which resists the splitting forces and prevents the concrete breakout. Table 2. Summary of the test results Specimens No. Average failure load (kn) Average stress in the bar at failure Mode of Failure *A5# CB Specimens No. FRP-Headed Bar No. 5 Average failure load (kn) Average stress in the bar at failure Mode of Failure + B5# BS A5# CB B5# CSH A5# CB B5# CSH A5# CB B5# BS A5# CB B5# HB A5# CSH B5# HB A5# CSH B5# HB A5# CSH B5# CSH A5# CSH B5# BS A5# CSH B5# HB Average Average CSH *A = Block with concrete type 36 MPa; + B = Block with concrete type 47 MPa; CB = Concrete Breakout; CSH = Concrete splitting followed by head breakout followed by bar slippage; BS = Bar slippage as a result of shear failure of the grooves; HB = Head breakout followed by bar slippage out from the attached head Page 5 of 8

6 Table 3. Summary of the test results Specimens No. Average failure load (kn) Average stress in the bar at failure Mode of Failure Specimens No. FRP-Headed Bar No. 6 Average failure load (kn) Average stress in the bar at failure A6# CB B6# HB A6# CB B6# HB A6# CB B6# BS A6# CB B6# BS A6# CB B6# BS A6# CSH B6# BS A6# CSH B6# HB A6# CSH B6# HB A6# CSH B6# HB A6# CSH B6# HB Average Average *A = Block with concrete type 36 MPa; + B = Block with concrete type 47 MPa; CB = Concrete Breakout; CSH = Concrete splitting followed by head breakout followed by bar slippage; BS = Bar slippage as a result of shear failure of the grooves; HB = Head breakout followed by bar slippage out from the attached head 3.2 Mode of Failure Mode of Failure Figures 5 to 8 show the FRP anchors after testing that failed in different modes of failure. The failure mode of all specimens was very explosive and suddenly a combined with a drop in the pullout strength and slippage of the FRP bar from the concrete block. Concrete breakout is the dominate failure mode for the ten concrete block specimens with FRP-headed bars No. 5 and 6, that have no spiral reinforcement. However, using steel spiral changed the mode failure for FRP-headed rebars No. 5 and 6 that were cast using concrete type 36 MPa, to concrete splitting followed by head breakout followed by bar slippage. On the other hand, FRP-headed rebars No. 5 and 6 that were cast using concrete type 47 MPa failed in three different modes. Specimens of maximum pullout capacities failed by bar slippage from the concrete block, as a result of shear failure of the grooves. The other specimens failed by head breakout followed by bar slippage out from the attached head. In conclusion, by comparing the different modes of failure of FRP-headed bars in this study, an important note was observed. When sufficient confinement is provided to a headed bar during pullout, shear stress develop between the bar ribs and the head before the bar fails in a pull-through mode. When this kind of failure happens, the pullout capacity of the FRP-headed rebars depends mainly on the split tensile strength of the concrete. Figure 5. Overview of the concrete block splitting. Page 6 of 8

7 Figure 6. Failure mode by bar slippage Figure 7. Failure mode by concrete block breakout Figure 8. Head breakout followed by bar slippage out from the attached head 4. Conclusions An experimental study was conducted to investigate the pullout capacity of new developed FRP-headed rebars. A total 40 concrete block specimens anchored with FRP-headed rebars No. 5 and 6 were prepared and tested. The test results showed that the FRP-headed rebar is an efficient method of providing anchorage capacity to develop the ultimate tensile strength of the FRP rebars. The FRP-headed rebars exhibited higher anchorage strengths in higher concrete strength (47 Mpa) than in lower concrete strength (36 MPa). The developed FRPheaded rebars of this study can provide a suitable alternative to the bent bars in some applications such as in case of the bridge concrete barriers. Whereas, the average stresses in the FRP-headed rebars at the ultimate pullout capacity ranged from 600 to 750 MPa. Theses stresses are almost 1.5 to 1.8 times the yield strength of the steel bars. Finally, the confinement action provided by the concrete block to the FRP-headed rebar should be considered in the future research studies. 5. Acknowledgements The writers wish to express their gratitude and sincere appreciation to the Natural Sciences and Engineering Research Council of Canada (NSERC), and Pultrall Inc. (Thetford Mines, Quebec, Canada), for financing this research work. The help received from the technical staff of the Structural Laboratory in the Department of Civil Engineering, Faculty of Engineering at the University of Sherbrooke is also acknowledged. Page 7 of 8

8 6. References [1] American Concrete Institute (ACI). (2006). Guide for the design and c onstruction of concrete reinforced with FRP bars, ACI 440.1R-01; 06, Farmington Hills, Mich. [2] Ahmed, E., and Benmokrane, B., Characterization and Strength Evaluation of the Headed GFRP Bars, University of Sherbrooke, Technical report submitted to Pultrall Inc., November 2009, pp [3] Benmokrane, B., El-Salakawy, E., El-Ragaby, A., and Lackey, T., Designing and Testing of Concrete Bridge Decks Reinforced with Glass FRP Bars. Journal of Bridge Engineering, Vol. 11, No. 2, 2006, pp [4] Benmokrane, B., Tighiouart, B., and Chaallal, O., Bond strength and load distribution of composite GFRP reinforcing bars in concrete. ACI Material Journal., Vol. 93, No. 3, 1996, pp [5] Canadian Standards Association (CSA). (2006 -Edition 2010). Canadian highway bridge design code Section 16, updated version for public review. CAN/CSA-S6-06, Rexdale, Toronto. [6] El-Salakawy, E.F., Benmokrane, B., Brière, F., (2005). Glass FRP Composite Bars for Concrete Bridge Barriers. Journal of Science and Eng. of Composite Materials, Vol. 12, No 3, pp [7] Harajli, M., and Abouniaj, M., Bond Performance of GFRP Bars in Tension: Experimental Evaluation and Assessment of ACI 440 Guidelines, Journal of Composites for Construction, Vol. 14, No. 6, November/December 2010, pp [8] Fibre reinforced polymers (FRP) V-ROD rebars Product technical specifications. Pultrall, Inc., ( (May. 2011). Page 8 of 8