Quasi-Static Behaviour of Precast Wet Connections with Shear Key

R. Siva Chidambaram, Scientist, ASCD Division, CSIR-Central Building Research Institute, Roorkee, discusses the importance of shear resisting key based precast joints and the influence under cyclic loading. The hysteresis behaviour and energy dissipation show the enhanced seismic resistance over the control specimens.

Precast construction
The speed of construction and its durability makes the precast construction technique highly successful for mass housing and infrastructure development. The structural elements are made at a factory with better workmanship and eliminate common construction issues such as improper cover depth, stirrups spacing, stirrups shape, water cement ratio, etc, besides air and noise pollution at site. The precast structural elements are erected at site using mechanical cranes.

Connections
The performance of the precast structure subjected to gravity and seismic loading completely depends on the connection technique adopted. Connections are generally categorised as dry, wet, and hybrid. Each type is unique in terms of stress distribution and plastic hinge formation. In seismic resistant design, the connection is important in plastic hinge formation and energy dissipation to ensure the ductile behaviour. But it is not applicable in precast structural connections due to the mechanism involved in establishing the assembly.

Dry connections such as corbel based, bolted, welded, coupled, wet with fresh concrete, hybrid using partial wet and dry are being used to establish the link between the independent structural elements.

Connections play a crucial role in load transfer path and ductility. The connection for gravity loading and seismic loading are different in nature in addressing the demand from lateral load. In particular, the connection to resist the seismic loading needs to be ductile and able to dissipate more energy at the defined plastic hinge location.

Corbel-based connection integrity has been improved by introducing a dowel between the column and beam, but it fails to offer better hysteresis behaviour. Similarly, the bolted and welded connection shows pinching in the hysteric curve which shows the brittleness of the connections. The discontinuity and combination of different layers of materials affect the cyclic performance compared to monolithic connections. These actions lead to development of shear key-based connections in precast elements. Use of segmental precast elements with different shear key offers better inelastic behaviour and the performance varies with respect to the shear key size. This research work mainly focuses on the influence of different shear key on the cyclic behaviour of external precast beam column joints.

Experimental Work and Test Results
The experimental program consists of monolithic connection and precast connection with shear key provisions. All the joint specimens were tested under displacement controlled cyclic loading. The hysteresis behaviour, crack pattern and dissipated energy were used as the controlling parameters to quantify the performance of different connection mechanisms.

Specimen Details
The Ordinary Portland Cement (43 grade) with coarse and fine aggregate was used for the production of precast beam and column members, both of which were cast with extended reinforcements for connection purpose. The proposed connection consists of rebar coupler at the potential hinge region and interlinked by the grid confinement.

The shear key provision has been made in beam and column during casting. In the proposed connection, High Performance Fiber Reinforced Cementitious Composites (HPFRCC) (J2) made using PP and steel fiber and Steel Fiber Reinforced Concrete (SFRC) (J3) using steel fiber has been used at the hinge region to improve the ductile behaviour. Figure 1 shows the typical reinforcement details used in the control and precast specimens, in which 12 mm diameter rebars were used as longitudinal reinforcement in beam and column and 6 mm diameter stirrups.


Figure 1b depicts the typical connection details of precast joints. The protruding bars of precast beam and column were connected using the hybrid coupler as shown in Figure 1b followed by grid confinement. The connection region and shear key in the beam and column has been cast with HPFRCC monolithically. All the beam-column joints were tested under displacement controlled reversed cyclic loading as shown in Figure 2.


Hysteresis Behaviour
Figure 3 shows the hysteresis behaviour of all the three joint specimens. The hysteresis curve of control specimen shows enlarged loop compared to other precast joints. The behaviour of precast joints is varying with respect to the type of key used. The specimen with cast in-situ key shows diverse behaviour compared to the joint (J3) with precast key. It shows that the type of shear key also plays vital role in seismic resistance. The displacement ductility of J2 is higher than J1 and J3 but shows the pinched loop. The pinching in the curve shows reduction in energy dissipation. But the cumulative energy dissipation of J2 is 25% higher than control specimen J1. The lateral load resistance of J3 is 100% higher than J1 and J2 which shows the efficacy of the precast key made using HPFRCC in resisting the applied load.


But the joint specimen J3 also shows reduced loop area due to the key at the hinge region. Similar to the joint specimen J2, the cumulative energy of J3 is also higher than the control specimens. The post peak behaviour of the J2 and J3 shows reduced arte in post peak strength reduction and higher displacement compared to control specimen due to the higher rotation supported by the HPFRCC in the hinge region. It is also noted that the precast joint provides more geometrical stability to the specimen due to the presence of HPFRCC in the connection region.

Crack Pattern
Figure 4 shows the crack patterns observed and mapped over the specimens during the testing. The brittle nature of concrete at the joint region of control specimen J1 shows early cracks and start to wide at the interface region as shown in Figure 4a. It is noted that the J1 experienced flexural cracks in the beam region at the initial stage of loading a followed by the interface cracks. During higher rotation, the joint experienced diagonal cracks and led the longitudinal reinforcement to slip from the joint anchored region as depicted in Figure 4a.


The precast joint specimen J2 experienced lesser flexural cracks in the beam region compared to J1. Instead the cracks were initiated at the hinge region and followed by widening of crack at the potential hinge region as shown in Figure 4b. The placement of coupler and distance between the coupled regions led the joint to experience the crack at the hinge region. Unlike control specimen, J2 did not show any sign of joint shear cracks and the reinforcement did not experience any slip from the joint region. It proves that the provided hinge successfully yields during higher rotation and restricted the joint shear cracks.

The joint with precast key J3 also surveyed the foot path of J2 in the initial stage followed by few minor diagonal cracks at the joint region. There was no spalling of concrete and crushing of concrete in the precast joints at the joint or interface region. It proves that the provided HPFRCC and SFRC connection offers better resistance to load and provided higher rotation with more geometrical stability compared to the conventional concrete.

Conclusions
The proposed coupler may be a very effective and practically viable solution to connect the precast structural member and to restrain the load-carrying capacity of structural member. The combined action bolting, friction and grout increases the factor of safety and allows force transfer mechanism without slip.

The provided grid confinement in the hinge region over the coupled portion works well in confining the concrete in the coupled region and restricts buckling and shears failure. This also helps to address the concrete cover issues when coupler is engaged.

Hysteresis behavior of the joint specimen with proposed connection shows that there is a significant improvement in the pre and post-peak behavior of beam–column joint specimen under cyclic loading.

The performance is further elevated with HPFRCC and in particular the damage tolerance capacity and energy dissipation capacity enhanced much more compared to the existing techniques and conventional monolithic construction techniques.

References

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