The Joining Method for Permanent Formwork

Transcription

1 The Joining Method for Permanent Formwork Q. JIN 1, C.K.Y. Leung 2, and W.L. Chung 3 1,2,3 Department of Civil and Environmental Engineering of HKUST, HKSAR, China ABSTRACT: In this paper, the combined use of Fiber Reinforced Cementitious Composite (FRCC) and Fiber Reinforced Plastic (FRP) has been studied for the application related to the development of permanent formwork for durable concrete structures. Formwork elements, fabricated with pseudo-ductile cementitious composites (PDCC) and embedded glass fiber reinforced polymer (GFRP) reinforcement, need to be effectively joined together in practice. A novel jointing method, involving the embedment of GFRP in high-strength fiber reinforced cementitious composites (HSFRCC), is thus proposed. To find the joint width, it is necessary to obtain the interfacial parameters governing the bond capacity between HSFRCC and GFRP bars. Direct pull-out test is carried out to investigate the bonding capacity between the HSFRCC and GFRP reinforcement. From the experimental data, interfacial parameters are extracted for calculating the required embedded length of GFRP bars to ensure sufficient bonding capacity. According to the test results, the required embedded length of GFRP bars is about 21d (where d is the diameter of GFRP bar). 1 INTRODUCTION Nowadays, the durability of concrete structures is a major concern in our concrete community. For the new concrete structures, durability is often governed by concrete cover since it is the main protection for steel bar being exposed to the outside environment. To improve the durability of concrete member, the use of a new type of FRCC to fabricate permanent formwork has been proposed and developed in Leung and Cao (2010). This new FRCC is also referred as pseudo-ductile cementitious composites (PDCC). After pre-casting the PDCC formwork, the structure is made by casting normal concrete on the formwork. With high crack resistance under large deflection and low permeability of the material itself, PDCC permanent formwork acts as an effective surface cover for concrete members to resist the penetration of water and chemicals. As a result, the high durability of the concrete member can be achieved. Two different configurations have been developed: plain and U-shaped. In Yu et al. (2010), to make the formwork into a U shape is found to be more advantageous. However, it is impossible to precast the entire structure at one time (a continuous structure such as a footbridge), a number of separated PDCC formwork elements need to be prefabricated first and connected in-situ. An effective joining method to connect the permanent formwork elements thus becomes crucial. In this paper, a novel joining method involving the embedment of GFRP in high-strength fiber reinforced cementitious composites (HSFRCC) is proposed and demonstrated. The major issue for the joining method of permanent formwork is to determine the width of the joint. To find the joint width, it is necessary to obtain the interfacial parameters governing the bond capacity between HSFRCC and GFRP bars. These parameters can be back calculated from the load vs displacement curve of the direct pull-out test with the help of a theoretical model. Once the parameters are obtained, the bond capacity can be calculated for an

2 arbitrary combination of GFRP size and length. The width of joint for sufficient load transfer capacity can then be determined. In our testing program, to avoid the rupture of GFRP rebar at the hydraulic grip, a method to protect the end of the GFRP rebar is developed and verified with the direct tension test. Then the direct pull-out test will be performed with different embedded lengths of GFRP rebar and finally the results will be collected for numerical analysis in accordance with a theoretical model. 2 EXPERIMENTAL PROGRAMS 2.1 GFRP Grips Preparation of GFRP grips The GFRP rebar is very brittle and can rupture easily at the hydraulic grip of the testing machine. To ensure that the bar could be held securely by the testing machine without rupturing during the direct pull-out test, the two ends of the extruded GFRP bars were wrapped with GFRP fabric (Fig. 1) to form GFRP grips. To make sure the GFRP grips can serve the intended purpose; its fabrication procedure was carefully designed. To make a GFRP grip, three pieces of bi-directional GFRP fabric was glued on the GFRP bars with epoxy resin as the adhesive agent. Bi-directional fabric is used instead of fiber sheets with aligned fibers because the former has a much higher shear strength which is important for the grip. In the making of the GFRP grips, attention has to be paid to several important aspects. Firstly, sufficiently large gripping area should be provided for the testing machine to hold the specimen securely. This can be achieved by using GFRP fabric of width larger than the height of the machine grips. Secondly, to allow even distribution of stress to the gripping surface, the surface of the GFRP grip should be made sufficiently smooth by uniform distribution of GFRP fabric over the GFRP bar. Thirdly, excessive epoxy resin should be avoided when bonding the GFRP fabric on the bars, otherwise large pieces of hardened brittle epoxy between layers of GFRP fabric might affect the transfer of stress from the machine to the specimens and act as weak spots. Figure 1. GFRP Grip Verifying the Effectiveness of GFRP Grips All the bare GFRP bars failed by rupture under direct tension, at locations away from the grip. Thus the results verified the effectiveness of the GFRP grips. According to the material properties provided by manufacturer, the GFRP bars with diameter of 6 mm can resist ultimate tensile load of 26.2 kn. However, when the tensile load was calculated from first principle using the specified tensile strength (σ= 825 MPa) and radius (r = m), the ultimate tensile load becomes: F σa π kN (1)

3 The difference of the specified and calculated tensile load is due to the difference in the crosssectional areas (theoretical: mm 2 ; actual: mm 2 ). The calculated result of kn can serve as the theoretical reference. On the other hand, the specified result of 26.2 kn is close to the experiment result kn. To be conservative, the theoretical value kn was chosen as the tensile load capacity of GFRP bar in this paper. 2.2 Experimental Program of Direct Pull-out test After confirming that the GFRP grip is strong enough to ensure failure of the bar is away from the grip, the direct pull-out test was conducted to provide data for the determination of interfacial parameters. There are different tests to study the pull-out behavior and its bond capacity. Windisch (1985) performed the pull-out test according to recommendations of RILEM/CEB/FIP Committee and Chana (1990) conducted the eccentric pull-out test to demonstrate the realistic situation (pull-out under bending) of tensile reinforcement in concrete beam. To keep the testing method simple, Direct Tnesion Pullout Bond Test (DTP-BT) developed by Cheung and Leung (2008) is applied in this paper Specimen Preparation A rectangular HSFRCC plate with embedded GFRP bars was employed for pull-out test. The mix proportion of HSFRCC is provided in Table 1. The compressive strength of the HPFRCC is 150 MPa and its flexural strength is 20 MPa after 28 day curing. To represent the joint of PDCC formwork, the size of the HSFRCC plates was designed to be mm (width length depth), with the width and depth being the same as those for a typical PDCC formwork element. Table 1: Mix proportion of HSFRCC Cement Fly ash Silica fume Silica sand* Water SP# Steel fiber Type 1 Type 2 Type 3 (%) * Silica sand: Type 1 : µm Type 2: µm Type 3: µm # BASF Superplasticizer: a) ACE80: 80% b) B211: 20% (a) (b) Figure 2. Preparation for direct pull-out test (a) Wooden Formwork; (b) Spicemens

4 To make the specimens, a wooden formwork was designed as shown in Fig. 2a. GFRP bars were placed in proper position inside the wooden formwork before the casting of HSFRCC. The configuration of test specimen is shown in Fig. 2b. Bar 1 represents the GFRP bar with varied embedded length under investigation. To ensure the failure to occur at Bar 1, a longer GFRP bar (>25d, Bar 2), aligned along the same line as Bar 1, was embedded on the other side of the specimen. To prevent tensile failure of the HSFRCC plate, bars 3 and 4 were inserted near the edges along the longitudinal direction. It should be noted that Bar 3 and Bar 4 are only needed for the tested specimen. In the real situation, the lapping embedded bars can prevent the tensile failure of HSFRCC by themselves. In order to provide sufficient data for the extraction of interfacial parameters and verification of the theoretical bond model, four groups of specimens with different GFRP embedded lengths of 10d, 15d, 20d and 25d (where d is the diameter of the GFRP bars) were prepared. The range of embedded length chosen for this experiment was chosen according to the minimum lap length of steel reinforcement which is equal to 15d (Code of Practice for Structural Use of Concrete 2004). As the GFRP bars employed in this project was 6 mm in diameter, the embedded lengths were 60mm, 90mm, 120mm and 150mm accordingly (Fig. 2b). Three specimens were prepared for each case Experimental Setup The experimental setup for the pull-out test is shown in Fig. 3. After 7 days of curing of epoxy resin, an aluminum plate (serving as a reference point for the measurement of displacement during testing) was glued on the exposed GFRP bar of the specimen with hot melt adhesive. The plate was placed away from the GFRP grip and at a distance of 15 mm away from the edge of HSFRCC plate. Two Linear Variable Differential Transformers (LVDT) were placed vertically to measure the displacement of GFRP Bar 1 (Fig. 3). During the test, a loading rate of 0.1 mm/min was applied to the specimen. 15mm Figure 3. Test Setup for direct pull-out test 2.3 Test Results and Discussions The ultimate tensile strength for all four groups of specimens is summarized in Table 2, together with failure mode. It should be pointed out that cracking and peel-off of HSFRCC were observed near the junctions of GFRP bars and HSFRCC plates. Possible reason for the phenomenon was that the HSFRCC plates were not thick enough to provide enough confinement to resist the micro-cracking around the GFRP bar. The chosen thickness is consistent with that for common PDCC formwork elements. However, in real joint construction, peeling-off of HSFRCC may occur to a smaller extent because the cast concrete on one side of the formwork may limit the peeling to the other side only.

5 The typical force versus displacement curves of all groups are illustrated in Fig. 4. For the group with 150 mm embedded GFRP bar, the failure mode was observed to be GFRP bar rupture. The failure occurred on either the shorter bar of interest (Bar 1) or the longer one (Bar 2). However, it was interesting to find that the tensile loads achieved here were lower than that of a bare GFRP bar. Since the bar rupture was observed at the junction of the HSFRCC plate and GFRP bar, a plausible explanation is the stress concentration due to sudden change of cross-sectional areas at the junction, from a larger area within the HSFRCC plate to a smaller area of the GFRP bar. The concentrated stresses at the junction lead to premature failure of GFRP bar and thus a lower value is measured at failure. For group with 120 mm embedded GFRP bar, the ultimate load for this group of specimens was within the same range as that of the last group with 150mm embedded length. However, two out of the three specimens were observed to fail by bar rupture while the other one showed pull-out failure of the rebar. Again, for specimens exhibiting rupture failure, failure occurred at the junction of the HSFRCC plate and GFRP bar. Based on the observation from the data and specimens, it was suggested that the critical embedded length could be around 120 mm (20d) as the pull-out load capacity is similar to the rupture load at this embedded length. For the groups with 90 mm and 60 mm embedded GFRP bar, the pull-out failure of the shorter GFRP bar (Bar 1) occurred instead. Lower value of ultimate tensile load was measured. From the pull-out failure of GFRP bars, it can be concluded that the critical embedded length for GFRP rupture had not been reached at 90 mm (15d). Table 2. Summary of experimental results of the specimens Specimen batch Embedded Average measured Failure Mode Length (mm) ultimate tensile load (kn) HSFRCC/GFRP(60) 60 (10d) Pull-out of GFRP Bar HSFRCC/GFRP(90) 90 (15d) Pull-out of GFRP Bar HSFRCC/GFRP(120) 120 (20d) Rupture/Pull-out of GFRP Bar HSFRCC/GFRP(150) 150 (25d) Rupture of GFRP Bar (a) HSFRCC/GFRP(150) (b) HSFRCC/GFRP(120) (c) HSFRCC/GFRP(90) Figure 4. Typical Force against Displacement Curve (d) HSFRCC/GFRP(120)

6 3 DATA ANALYSIS FOR DETERMINATION OF INTERFACIAL PARAMETERS In order to connect two permanent formwork elements, load has to be transferred from one element to another through an effective joint. Therefore, its width has to be determined first. The joint width is controlled by the lap length of the reinforcement protruded from the permanent formwork. For the proposed joining method, the relationship between the embedded length of GFRP in HSFRCC and its load carrying capacity has to be determined. To find this relation, the properties at the HSFRCC/GFRP interface need to be obtained first with the help of a theoretical model for interfacial debonding. In this paper, a model developed by Stang et al (1990) is employed. The model is originally developed for the push-out test. However, as the general behavior in push-out and pull-out are similar (Li et al. 1991), it is also applicable to our tests. Figure 5. Pull out Problem with a Stress vs Slipping Curve For a GFRP rebar embedded inside a matrix, the geometry (debonding length a and embedded length L) and the three major interfacial parameters (k, τ y and τ f ) are illustrated in Fig. 5. The model in Stang et al. (1990) is based on the assumption that interfacial debonding starts to occur once the interfacial bond strength (τ y ) is reached. Before debonding, the interfacial shear stress increases linearly with fiber/matrix relative displacement and the proportionality constant is shear stiffness k. After debonding, the interfacial stress will drop immediately to the interfacial friction (τ f ). The Interfacial friction τ f is caused by the undulating surface of GFRP rebar and surrounding matrix and is assumed to stay constant on further interfacial sliding. In the model, three related parameters were introduced: the maximum shear force per unit length q y, frictional shear force per unit length q f and interfacial stiffness parameter ω, where q 2πrx τ 2 q 2πrx τ ω= k E f A 3 4 To illustrate the determination of interfacial parameters, we take the experimental curve of a specimen with 90mm GFRP embedment length (Fig.4c) as an example. It is first simplified into a multi-linear load vs displacement curve (Fig. 6) for the identification of key points in the curve. All the points are based on the average values of three tests in this group. In the post-peak region beyond point C, the loading is dropping slowly in a linear manner. The interface is then governed by friction alone, the q f can then be calculated from the load P at point C and the remaining embedded length of the GFRP bar, from:

7 Figure 6. Simplified Curve for Group with 90 mm embedded GFRP bar q P U where U is the displacement of embedded GFRP. 0.14kN mm 5 In Stang et al. (1990), expressions were derived to relate the displacement of GFRP bar (U) and the applied load (P). Specifically, when debonding just starts to occur (i.e., at the end of the elastic stage), U is related to the initial debonding load (P y ) through: U P E A ω coth ωl 6 Initial debonding is taken to occur at point A in Fig. 6, where the corresponding curve in Fig. 4c starts to deviates significantly from the initial linear behavior. At this point, the measured displacement (δ total ) consists of two parts, the displacement (U) of GFRP bar at the edge of the HSFRCC plate and the elongation of the bar for the 15mm length between the glued aluminum plate and the edge of HSFRCC plate. With P y obtained at point A, U is determined from: δ P l E A U 7 For the data shown in Fig. 6, P y = 12 kn and δ total = 0.4 mm. Also, with properties of the GFRP bar, A = 28.3 mm 2 and E a = 40.8 GPa, the value of U is found to be 0.24 mm, ω can be calculated by substituting L = 90 mm into equation 7. The value of ω is found to be After the onset of interfacial debonding, the load for a particular debonded length (a) can be calculated from the interfacial parameters through: P q a q tanh ω L a ω 8 To determine q y, an iterative procedure is employed. A value of q y is first assumed and used for the calculation of P in equation 8. The debonded length a is varied until a maximum value is reached. There will always be a maximum value for P if q y is greater than q f, which is one of the basic assumptions of the model. This maximum value is compared to the peak load measured in the pull-out test. Depending on whether the calculated value is larger or smaller, a smaller or larger q y will be employed in the next iteration. The procedure is repeated until the peak load calculated from the model is consistent with the test result. The corresponding value of q y is taken as the actual interfacial strength. Using the result in Fig. 6, q y was found to be 0.48 kn/mm. Due to the rough surface of GFRP bar and high strength of HSFRCC matrix, the value of shear force q y obtained from the data analysis is acceptable.

8 Table 3. Comparison between experimental and calculated values Specimen Ultimate Tensile Load (Experimental)(kN) Tensile Load (numerical analysis) (kn) Percentage Error* HSFRCC/GFRP(60) % HSFRCC/GFRP(90) % HSFRCC/GFRP(120) % HSFRCC/GFRP(150) % After determining the interfacial parameters (q y, q f and ω), the pull-out force P max for any embedded length can be calculated. In Table 3, the calculated results for all tested embedded lengths are compared to the experimental data. Except for the group with 150mm embedded length, the values generated from the numerical model are in close agreement to the experimental results (with the percentage error of less than 3%). When the embedded length is 150 mm, failure occurs by rupture. As expected, the calculated debonded load is higher than the failure load. To take GFRP rupture into consideration, the pull-out capacity is bounded by the ultimate tensile load capacity of GFRP rebar, which is kn. According to the pull-out capacity as well as the failure mode of GFRP, it is safe to say that, with a joint width larger than 126 mm (21d), there is sufficient stress transfer to reach the rupture load of the GFRP bar. 4 CONCLUSIONS In this paper, a joining method for permanent formwork elements, which involve the embedment of protruded GFRP reinforcements in HSFRCC matrix, is proposed and studied. The direct pull-out test was carried out to investigate the bonding capacity between the HSFRCC joint material and GFRP reinforcement. In order to predict a suitable and adequate embedded length for GFRP bars in the HSFRCC, interfacial parameters were first extracted from the experimental data according to a debonding model. After the interfacial parameters were successfully obtained, the recommended length of GFRP bars embedded in joint can be predicted. To fully utilize the capacity of GFRP rebar, the rupture failure has to be ensured and the recommended length should be greater than about 21d (i.e. 126 mm for 6mm-diameter GFRP bars). For the future study, the four point bending test in beam members can be carried out to verify the feasibility of the proposed joining method in practical applications. 5 REFERENCES Chana, P. (1990) A Test Method to Establish a Realistic Bond Stress, Magazine of Concrete Research, 24 (151), Cheung, A.K.F. and Leung, C.K.Y. (2008) Experimental study on the bond between steel reinforcement and self-compacting high strength fiber reinforced cementitious composites, Proceedings of Seventh International RILEM Symposium on Fibre Reinforced Concrete: Design and Applications BEFIB: , 135(2): Code of Practice for Structural Use of Concrete 2004 (Second Edition). Building Department, HKSAR Government. Section : Leung, C.K.Y. and Cao, Q. (2010) Development of Pseudo-ductile Permanent Formwork for Durable Concrete Structures, RILEM Materials and Structures, 43(7): Stang H, Li Z. and Shah, S. P. (1990) Pull-out problem: Stress versus fracture mechanical approach, Engineering Mechanics, ASCE, 116(10): Windisch, A. (1985) A modified pull-out test and new evaluation methods for a more real local bond-slip relationship, Materials and Structures, Springer Netherlands, 18 (3), Yu, C., Leung, C.K.Y. and Cao, Q. (2010) Behavior of concrete members constructed with SHCC/GFRP permanent formwork, Proceedings of 7th International Conference on Fracture Mechanics of Concrete and Concrete Structures, CD ROM Li Z., Mobasher B. and Shah, S. P. (1991) Characterization of interfacial properties in fiber-reinforced cementitious composites, J. American Ceramic Society, 74(9):