Grout Filled Steel Pipe Integrated With Shear Key for Precast Concrete Connection

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1 Grout Filled Steel Pipe Integrated With Shear Key for Precast Concrete Connection AHMAD BAHARUDDIN ABD. RAHMAN and ONG HERN YEE Department of Structure and Materials, Faculty of Civil Engineering Department Universiti Teknologi Malaysia, Skudai, Johor, MALAYSIA Abstract: - This paper presents the use of steel pipe integrated with shear keys as an alternative for splice connections in precast concrete construction. The experimental results involved 17 grout-filled splice sleeve with different configuration, in terms of reinforcement bar sizes, embedment length of reinforcement bar and embedment length of shear key were subjected to tensile test. All the specimens were subjected to increasing axial tension until failure. The effect of reinforcement bar sizes, embedment length of reinforcement bar and embedment length of shear key on the ultimate tensile load and mode of failure were analysed. The results shows that an inexpensive steel pipe combined with shear keys can be adopted as a connection for use in precast concrete construction. Key-Words: - Grouted splice connection, confinement, ultimate bond stress, precast concrete connection. 1 Introduction Precast concrete buildings have gained popularity worldwide, see Figure 1. Buildings which were previously constructed with cast-in-situ concrete could be constructed with precast concrete components prefabricated in the factories. These ready-made loose components such as precast concrete wall panels are installed on site. To facilitate the process of installation, connections such as special reinforced bar splicing systems are needed to join the loose precast concrete components together. Also, the connection has the ability to increase the structural integrity of precast concrete components. Fig. 1: Precast concrete wall building In Malaysia, there are currently two methods for connecting precast concrete structural members; ISBN:

2 first is the conventional lapping of reinforcement bar and second, the mechanical connections such as splice sleeve connections. A sleeve is a cylindrical shape mechanical coupler that is used to join steel reinforcement bars for joining precast concrete components, see Figure 2. This sleeve act as a reinforcement bar connection using non shrink high strength grout as the medium for load transferring and bonding material. The grout provides a continuity of compressive forces across the joints whereas the reinforcement bars provide continuity for tensile forces. The use of grouted sleeve connections in precast concrete structure reduces the lap length of reinforcement bars. Fig. 2: Precast concrete wall to wall connection In the late 60 s, Dr. Alfred A. Yee developed a grouted splice sleeve connection. This mechanical coupler can be embedded into the precast units and grouted by injection from the exterior, resulting in a fully continuous reinforcement steel splice with no pockets to patch during erection [1]. It utilizes grout to transfer the forces in one bar to another to achieve continuity of the reinforcement in the precast structural members. Bond between steel and concrete is essential for the integrity of any reinforced concrete structure. Nevertheless, bond is a complex problem and depends on many parameters. Due to its significance for practical design, the study of bond between steel and concrete has always been a popular issue in the field of research. According to Untrauer and Henry [2], bond can be defined as the adhesion of concrete or mortar to reinforcement bar or to other surfaces against which it is placed. Bond can also be defined simply as the gripping effect of an annulus, usually concrete or cement on an embedded length of a steel bar to resist the tendency of forces to slide the bar longitudinally [3]. Abrams [4], who first mentioned the deformability characteristics of bond-slip relationship and reported the results of about 1500 pull-out tests carried out in displacement said that there are two different mechanisms of load transfer between the bar and the surrounding concrete: 1. The adhesive resistance, developed before relative movement between bar and concrete (slip) begins, and 2. sliding resistance, when this slip takes place. Adhesive resistance is due to tangential adhesion, of a chemical nature, and static friction. As soon as the sum of these contributions is overcome, a relative movement takes place and bond stress develops with a frictional mechanism (sliding resistance); this mechanism has the same nature with respect to the static friction component. The quality of bond between the reinforcement bar and the grout to ensure the success of load transfer depends on many factors such as grout compressive strength and confinement. There have been a number of research studies on the effects of concrete confinement on the bond behaviour and the effective bond strength between reinforcing bars and the surrounding concrete. From a structural point of view, confinement is achieved by applying force in a direction perpendicular to the applied stress. Moreover, confinement can also be achieved by means of transverse reinforcement, by providing thick concrete cover to the main reinforcing bar, or by increasing the spacing between the reinforcing bars. One of the earliest investigation works on the effect of lateral pressure on bond was done by Untrauer and Henry [2]. They found out that the bond strength between steel and concrete increases linearly with normal pressure. They also derived an equation that represents the relationship between the compressive strength of concrete, normal pressure and reinforcing bond strength. The effects of confinement on bond behaviour can also be observed through the modes of bond failure. Typically, bond failure of deformed bars involves local crushing of concrete in front of the bar ribs, and splitting of the concrete due to radial cracks around the bar. When the confinement provided by either surrounding concrete or transverse reinforcement is large or the rib height is small, the local crushing occurs. This mechanism of bond failure tends to be ductile. However, splitting of the concrete dominates when the confinement is small or the rib height is large, the failure mechanism is brittle [5]. Lutz and Gergely [6] found out that in cases where confinement is not provided, deformed bars fail in bond by splitting, which depends mainly on the force on the concrete and not so significant on the bar stress and the bar perimeter (see Figure ISBN:

ultimate load per unit length depends increasingly on the bar perimeter.

The compacted crushed concrete creates a wedge that becomes lodged in front of the rib and moves along with it [6]. This produces a rib with a face angle of 30 to 40 degrees.

3 3(a)). In the present of confinement, normally by the use of stirrups or a large concrete cover, bond failure occurs by shear failure of the concrete keys between the steel ribs, see Figure 3(b), and the ultimate load per unit length depends increasingly on the bar perimeter. After adhesion is lost and ribs begin to bear on the concrete, slip occurs by progressive crushing of the porous concrete paste structure in front of the rib. The compacted crushed concrete creates a wedge that becomes lodged in front of the rib and moves along with it [6]. This produces a rib with a face angle of 30 to 40 degrees. Thus, as the load acting on the reinforcing bar increases, the angle at which the steel rib bears on the concrete changes. The consequence is radial splitting stresses tend to increase at a rate greater than the parallel bond stresses as tensile load in the bar rises. pipe which will be detailed as A Y16 represents Series A with 35 mm shear key length, pipe length of 210 mm and main bar diameter of 16 mm, in high yield grade steel, respectively. All the specimens were subjected to increasing axial tensile loads in order to determine the maximum capacity and failure modes of the connections. The purpose was to study the bond behaviour of the connections as well as to investigate the feasibility of the different configurations of the grouted pipe connection. Table 1 and Figure 4 show the specimen identification descriptions and details of the specimens with further dimensions and properties. All the specimens had the same inner pipe diameter of 52 mm. Specimens in A Series have shear key with length of 35 mm, whereas B series and C series had shear key of length 65 mm and 95 mm respectively. The shear keys consisted of high yield deformed steel reinforcement welded to the inside of steel pipes. Only B series was slightly unique as the three sizes of reinforcement bar of Y12, Y16 and Y20 were analysed together with the variation in shear key embedment length that made up a total of nine specimens in this series. Table 1: Specimen details (a) (b) Fig. 3: (a) Bond failure by splitting (b). Bond failure by shearing of concrete keys in between ribs [6] 2 Experimental Program The experimental program was conducted to study the behavior and performance of a proposed grouted pipe connection with integrated shear keys that involved 17 specimens with different parameters such as embedment length of reinforcement, diameter of reinforcement bar and length of shear key incorporated in the sleeve pipe. The specimens were divided into three series, namely A, B and C series. The names were based on the length of shear key incorporated in the sleeve ISBN:

Fig. 4: Details of connection In preparing the specimens, plywood frames were made to hold the specimens and reinforcement bars in position, to ease the process of pouring grout into the sleeve pipe.

Steel wires were used to tie the reinforcement bars and pipe sleeve in position and the bottom part of sleeve was covered with plywood filled with silicone to prevent grout from flowing out during

The grout was mixed using a mixer according to the specification as stated. The grouting process was done by pouring the grout into a cone connecting to the pipe.

4 Fig. 4: Details of connection In preparing the specimens, plywood frames were made to hold the specimens and reinforcement bars in position, to ease the process of pouring grout into the sleeve pipe. The sleeve pipe was first tied onto the frame before the reinforcement bar was inserted into the pipe and tied onto the frame. Steel wires were used to tie the reinforcement bars and pipe sleeve in position and the bottom part of sleeve was covered with plywood filled with silicone to prevent grout from flowing out during grouting. After the reinforcement bars and pipes were fixed on the plywood frames as shown in Figure 5, the grouting was performed. The grout was mixed using a mixer according to the specification as stated. The grouting process was done by pouring the grout into a cone connecting to the pipe. The cone was used to speed up the grouting process as the whole grouting process must be done within half an hour to prevent the grout from harden. All the specimens were loaded with increasing axial force to failure, see Figure 6. The tensile tests were carried out after 28 days, to allow the grout to achieve the target strength. The maximum applied load up to the point of failure was recorded using a computer, and the mode of failure was observed and documented. All the specimens were also equipped with strain gauges to measure the longitudinal strains in the pipe and the axial strains in the reinforcement bar. Fig. 6: Experimental setup of the specimen 3 Results and Discussion Table 2 summarizes the test results for all the specimens. The results are categorized according to the type of series, specimen labels, ultimate tensile load, displacement and mode of failure. In addition, the specimen series consisted of control specimens, i.e. 1. The upper boundary condition consisted of steel pipe with three shear keys which are fully integrated in the pipe ends and 2. The lower boundary condition, consisted of pipe without any shear key provided in the sleeve. Table 2: Test results Figures 7, 8 and 9 show the modes of failure that occurred to the specimens. Fig. 5: Assembly of the specimens on the plywood frame ISBN:

$35 mm is adequate to ensure the main bar fractured outside the sleeve.$ Therefore, in short a minimum of 3 shear keys with length of 35 mm is adequate to ensure that there is no grout-pipe failure. Fig. 7: Bar tensile failure for A35-410-Y16 Fig.

Therefore, in short a minimum of 3 shear keys with length of 35 mm is adequate to ensure that there is no grout-pipe failure. Fig. 7: Bar tensile failure for A35-410-Y16 Fig.

1 Effect of Shear Key Length The shear keys were provided to avoid groutpipe bond failure. Figure 10 shows the response of tensile load versus length of shear key.

5 35 mm is adequate to ensure the main bar fractured outside the sleeve. Referring to Table 2, similarly, for other specimens with shear keys of 65 mm and 95 mm and with main bar of Y12, Y16 and Y20 no groutpipe failure occurred. Therefore, in short a minimum of 3 shear keys with length of 35 mm is adequate to ensure that there is no grout-pipe failure. Fig. 7: Bar tensile failure for A Y16 Fig. 10: Tensile load versus the length of shear key Fig. 8: Grout-bar bond failure for B Y16 Fig. 9: Grout-pipe bond failure for control specimen Y20 bar without any shear key 3.1 Effect of Shear Key Length The shear keys were provided to avoid groutpipe bond failure. Figure 10 shows the response of tensile load versus length of shear key. From the A Series with shear key length of 35 mm, Specimens A Y16 with main bar embedment length of 100 mm and A Y16 with main bar embedment length of 150 mm failed at kn and kn respectively by grout-bar bond failure. On the other hand, Specimen A Y16 with similar 35 mm shear key length but with main bar embedment length of 200 mm failed by bar fracture. The results of A series with Y16 main bar showed that there was no grout-pipe bond failure. These results indicate that for Y16 main bar with embedment length of 200 mm, a shear key length of Another two control specimens were tested to further study the effect of shear key length on the mode of failure, namely the lower bound Y20 and upper bound Y20. The mode of failure for lower bound Y20 was grout-pipe bond failure whereas for the upper bound Y20 was grout-bar bond failure. These two specimens proved that lower bound Y20, without shear key, had caused the grout to be pulled out together with the reinforcement bar. Then, when shear key of length 65mm was provided as in specimen B Y20, the mode failure observed was grout-bar bond failure. This finding shows that the addition of shear key has provided good bond resistance between the pipe and grout and eventually provide enough resistance to avoid slippage of grout from the steel pipe. As pulling force was applied on reinforcement bar, the interlock between grout key and bar ribs resisted the slippage of main bar. The inclined surfaces of ribs caused resultant resistance force which can be derived into two components; normal and longitudinal to the reinforcement bar as shown in Figure 11. Shear resistance of grout keys between bar ribs resisted the longitudinal component resulting in reduction of slippage of reinforcement bar. On the other hand, the normal component caused the grout to move away from the reinforcement bar, which led to splitting force, where the grout moved outwards and split at all direction. The combination of these two components then caused splitting cracks onto the grout surrounding the reinforcement bar. ISBN:

11: The normal and longitudinal components acting from the rib surfaces [7] (a) (b) Fig. 12: a) Normal forces exhibited by bar ribs; b) Sleeve providing confinement to control crack splitting [7] 3.

6 However, the confinement by the steel pipe controls the action of splitting by the grout as a result of pulling action [7], [8]. It can be observed in Figure 12, the pattern of cracks on the side of the connection as a result of confinement by sleeve and shear key. Fig. 13: Tensile load versus the embedment length of reinforcement bar Fig. 11: The normal and longitudinal components acting from the rib surfaces [7] (a) (b) Fig. 12: a) Normal forces exhibited by bar ribs; b) Sleeve providing confinement to control crack splitting [7] 3.2 Effect of Size of Reinforcement Bar Three different sizes of reinforcement bar were tested and the results are as shown in Figure 13. The three sizes of reinforcement bar exhibited the same characteristics, which is as the embedment length of reinforcement bar increases, the ultimate tensile load increases. One clear finding from the figure indicates that only the series of reinforcement bar of Y20 show significant increase whereas the other two series of Y12 and Y16 show slight increase. Referring to Table 2, for larger bar size of 20 mm, the embedment lengths of 100 mm and 150 mm are not adequate to achieve bar fracture failure, as can be seen from Specimens B Y20 and B Y20 which failed by bar-grout bond failure. 3.2 Effect of Embedment Length of Main Reinforcement Bar From the test results shown in Figure 14, it can observed that as the embedment length of reinforcement bar increases, the ultimate tensile load increases for all three different length of shear key provided in the sleeve. All specimens with embedment length of reinforcement bar of 200mm failed by bar fractured. Amin Einea [10], concluded that the lap splice or embedment length as short as seven times the bar diameter can achieve bar development when the appropriate grout compressive strength and confinement are provided. From the results shown in Table 3, only three specimens failed by bar tensile failure which were A Y16, B Y16 and C Y16. These three specimens have adequate development and embedment length as short as seven times as required. Thus, judging from the ratio of L e /Ø for the three specimens which are for A Y16, 8.44 for B Y16 and 6.56 for C Y16, these results further clarify the Amin Einea s finding which stated that lap splice or embedment lengths as short as seven times the bar diameter can achieve bar development when the appropriate grout compressive strength and confinement are provided. ISBN:

7 Table 3: Ratio of embedment length/bar diameter L e /Ø results discussed in this paper show the performance of the connections under axial tension only. In real practice, the connections such as in the precast concrete wall-to-wall or column-to-column might be subjected to bending. Therefore further tests of the connections subjected to increasing flexural load could be studied. Acknowledgement The authors would like to thank the Universiti Teknologi Malaysia (UTM) for the financial support offered in conducting this experimental study. From the result, it is observed that embedment length of approximately 7 times is required for specimen C Y16 to achieve bar fractured failure. As the length of shear key increases, the ratio of L e /Ø decreases. 4 Conclusion Based on the results of this experimental study, the following conclusions can be drawn: 1. The incorporation of shear key changes the mode of failure from grout-pipe bond failure to grout-bar bond failure. All specimens with shear key did not showed any grout-pipe bond failure. The addition of shear key improves the performance of splice connection and only a minimum 35 mm length of shear key is needed to avoid grout-pipe bond failure. 2. As the diameter of reinforcement bar in the connection increases, the larger embedment length of main reinforcement is required to achieve bar fracture failure outside the steel pipe. 3. The effect of embedment length of reinforcement bar plays a significant role in determining the performance of the splice sleeve. A minimum embedment length of 200 mm is required to ensure Y12, Y16 and Y20 bars fracture failure outside the steel pipe. The experimental configuration showed convincing results as more than half of the connections fulfilled the requirement for a successful splice connection. However, the test References: [1] Yee, Alfred, A., Structural and economic benefits of precast/prestressed concrete construction, PCI Journal, [2] Untrauer, R.E., and Henry, R.., Influence of normal pressure on bond strength, ACI Journal, V.69, No. 5, [3] Mahdi, M., Ahmad, J., and Arash, K. 2003, Bond of cement grouted reinforcing bars under constant radial pressure, Master Project, Universiti Teknologi Malaysia, [4] Abrams, D. 1913, Test of bond between concrete and steel, Bulletin No.71, Univ. Illinois Bull. [5] Tepfers, R., A theory of bond applied to overlapped tensile reinforcement slices for deformed bars, Chalmers University of Technology, Division of Concrete Structures, 1973 [6] Lutz, L. A., and Gergely, P., Mechanics of Bond and Slip of Deformed Bars in Concrete, ACI Journal, American Concrete Institute, Vol. 64, No. 11, [7] Ling, J.H., Baharuddin, A., Karim, A., and Hamid, Z.A. 2008, Performance of CS sleeve under direct tensile load, Part I failure modes, Malaysian Journal of Civil Engineering, Vol. 20, No [8] Seyed Jamal Aldin Hosseini and Ahmad Baharuddin Abd. Rahman, Analysis of spiral reinforcement in grouted pipe splice connectors, Građjevinar, 65 (6), 1-10, [9] Seyed Jamal Aldin Hosseini, Ahmad Baharuddin Abd. Rahman, Mohd Hanim Osman, Aziz Saim, Azlan Adnan, Bond behavior of spirally confined splice of deformed bars in grout, Construction and Building Materials, Elsevier, Vol. 80, [10] Einea, A., Yamane, T., Tadros, M.K. 1995, Grout-filled pipe splices for precast concrete construction, PCI Journal ISBN:

World Engineering Congress 2010, 2 nd 5 th August 2010, Kuching, Sarawak, Malaysia Conference on Engineering and Technology Education

World Engineering Congress 2010, 2 nd 5 th August 2010, Kuching, Sarawak, Malaysia Conference on Engineering and Technology Education STRUCTURAL PERFORMANCE OF SPLICE SLEEVE CONNECTOR WITH VERTICAL AND SPIRAL REINFORCEMENT BAR UNDER DIRECT TENSILE LOAD Shuhaimi Shaedon 1, Ahmad Baharuddin Abd Rahman 2, Izni Syahrizal Ibrahim 3, Zuhairi