The Diverse Techniques Of Precast Connections

R. Siva Chidambaram
CSIR-Central Building Research Institute
Roorkee, Uttarakhand

The fast and durable construction makes precast construction technique a successful technique for mass housing and infrastructural development. The structural elements are made at a factory with better workmanship thus eliminating common construction issues such as improper cover depth, stirrup spacing, stirrup shape, water cement ratio etc. It also helps eliminate air and noise pollution at the site. Dry connection methods such as corbel based, bolted, welded, coupled; wet connections methods with fresh concrete and hybrid connection methods incorporating partial wet and dry methods are being used to establish the link between independent structural elements. These connections play a crucial role in load transfer paths and ductility. The connections for gravity loading and seismic loading are different in nature to address the lateral load demand. In particular, the connection to resist the seismic loading needs to be ductile and be able to dissipate more energy at the defined plastic hinge location.

Failure Analysis
In the parking garage collapse caused by the Miami earthquake, the column collapsed from column-foundation connection due to absence of grouting and incomplete grouting at the footing under the column as shown in Fig. 1. The column base plate was deformed and bolts were bent in S-shape manner. The inadequate confinement in the column leads to severe damage at its base as shown in Fig. 2. Due to bolt failure in the double cantilever head, the entire structure collapsed as shown in Fig. 2.

1 (a) Incomplete Grouting

1 (b) Absence of Grouting on the Base Plate

2 (a) Failure of Bolts at Supports

2 (b) Inadequate Connement at the Column Base

Fig. 1: Parking Garage Collapse during Construction at Miami, 2013 (OSHA 2013) | Fig. 2: Structural Damage Kocaeli (Turkey) Earthquake, (Saatcioglu, 2001)

The plastic hinge formations at the column led to concrete spalling and rebar buckling as shown in Fig. 3. Due to insufficient seating and fixing of roof panels over the beam and walls, the floor diaphragm action was not adequate which led to the collapse of the roof panels in the structure as shown in Fig. 4b.

Fig. 3: Column Reinforcement Buckling Failure
(Saatcioglu et al. 2001)

4 (b) School Building Collapse
(Source: EERI Special Earthquake Report 2001)

4 (a) Precast Shear Walls
(Source: OSHA 2014)

Fig. 4: Different Types of Failure due to Absence of Integral Action

All the failure studies emphasise on the importance of integral action in transmitting the force and resistance of large drifts. Also, man-made mistakes such as improper grouting, improper anchoring and inadequate support (Fig. 4) led to the collapse of the structure. This makes precast construction more vulnerable compared to monolithic construction. There is need to ensure structural integrity in terms of connections and joints for sustainable and safer precast constructions.

Structural Integrity Of Precast Connections
The existing corbel-based connection is vulnerable to any lateral drift but its ease of construction and execution promotes its application. Many improvements have been made to improve the integrity of the corbel-based connection such as; introduction of single and double dowel bars to anchor the beam with column, welding the base plates of beams and columns together and the use of partial fresh concrete with continuous bars through the column as shown in Fig. 5 and 6. Dry connection energy dissipation capacity has been enhanced using external dampers as shown in Fig. 7. The use of dampers at the joint increases energy dissipation capacity and the additional use of pre-tension system offers re-centring capacity during large lateral drifts. Combination of dampers and pre-tension systems offers better lateral load resistance and improves the damping capacity. The use of multiple precast segments at the junction region significantly affects the hysteresis behaviour of these connections.

Fig. 5: Different Corbel Connection

Fig. 6: Corbel Connections with Fresh Concrete

Fig. 7: Precast Joint with External Dampers

Fig. 8: Precast Beam Shells to Support the Wet Connection

Fig. 8 shows a typical precast shell beam-based connection in which reinforcement gages will be kept over the shell and fresh concrete will be used. The use of precast shell eliminates the need for form work and also contributes in resisting tensile force. Steel plates as a connecting and yielding source have been used at potential hinge region of precast connections as shown in Fig. 9.

9 (a) JC 4 of Rong et al. (2020)

(b) PC2 of Ghayeb et al. (2017

In steel plate connections the role of web plate is predominant in transferring the force compared to similar connections with stiffeners (Rong et al. 2020). Bolted and welded connections show pinching in the hysteresis curve which indicates brittleness of the connections. The discontinuity and combination of different layers of materials affects the cyclic performance of precast connections compared to monolithic connections. These actions have led to development of diverse connection techniques such as pre-stressed, shear key based and filler with transition bar connection in precast element. These diverse connections and their structural performances are discussed.

Hysteresis Behaviour Of Precast Connections
The hysteresis curve is a representation of the cyclic behaviour of any specimen. The enlarged loop area is an indication of high energy dissipation and the reduced area indicates a brittle behaviour. Fig. 10 shows typical hysteresis curves in which the first curve stands for enlarged hysteresis curve and represents high energy dissipation whereas the second curve shows pinching effect. The pinching in the loop significantly reduces dissipated hysteric energy (Gusella et al. 2017). The third curve represents the re-centring behaviour. Based on the literature study, it is understood that the use of dry connections failed to show better hysteric behaviour compared to joints with wet concrete. Corbel connection with dowel bars provided better displacement ductility but the observed pinching shows inefficiency of the proposed configuration in energy dissipation (Vidjeapriya et al. 2014). The use of fresh concrete at the joint and through reinforcement enhances energy dissipation capacity of corbel-based connections whereas the use of dry joint results in pinching effect (Girgin et al. 2017). The use of dampers at the joint enhances dissipated hysteretic energy and the combined use of pre-tensioning system with dampers offers re-entering and energy dissipation capacity (Morgen et al. 2004).

Fig. 10: Typical Hysteretic Pattern of Different Precast Connections

11 (a) J4 (Rong et al. 2020)

11 (b) PC2 (Haider et al. 2017)

The partial use of wet concrete at potential joint regions with multiple precast units aects hysteresis behaviour. But relocation of plastic hinge eliminates pinching effect and shows enhancement in energy dissipation compared to the pinched curve observed with monolithic and precast joints without hinge relocation mechanism (Eom et al. 2016). The use of steel plates as a yielding source in precast connections offers diverse hysteresis behaviour. The use of web plates has shown enlarged hysteresis curve compared to the joint without web plates (Rong et al. 2020). The use of steel plate connection at the interface significantly affects the hysteretic energy as shown in Fig. 11 (Ghayeb et al. 2017).

It is well understood that the hysteric behaviour of every connection system is unique but the use of wet concrete at the joint region shows huge difference in hysteresis behaviour compared to joints with dry connections.

Experimental Work And Test Results
The experimental program consists of monolithic connection and precast connection with coupler provisions at the hinge region. All the joint specimens were tested under displacement controlled cyclic loading at Advanced Structural Composites and Durability Lab. The hysteresis behaviour, crack pattern and dissipated energy were used as the controlling parameters to quantify the performance of different connection mechanisms.

Specimen Details
Ordinary Portland Cement (43 grade) with coarse and fine aggregate was used for the production of precast beam and column members. Both beam and column members were cast with extruded reinforcements for connection purposes. The proposed connection consists of rebar coupler at the potential hinge region with interlinked grid confinement to avoid loss of cover depth.

Fig. 12 shows the typical reinforcement details used in control and precast specimens; in which 12 mm dia. rebars were used as longitudinal reinforcement in beam and column with 6 mm bar stirrups. Fig. 12b depicts the typical connection details of precast joints. The protruding bars in the precast beam and column were connected using hybrid couplers as shown in Fig. 1b followed by grid confinement. The connection region in the beam and column was cast with Fiber Reinforced Cementitious Composites (FRCC).

All the beam-column joints were tested under displacement controlled reversed cyclic loading as shown in Fig. 13.

Fig. 14 shows the hysteresis behaviour of two tested joint specimens. The hysteresis curve of the control specimen shows a higher rate of strength and stiffness degradation compared to the precast joint. The control joint experienced diagonal cracks and spalling of cover concrete at the joint region as shown in Fig. 14.

The hysteresis behaviour of precast joint is dissimilar to that of control specimen and shows an enlarged loop. The post yield portion of the curve shows absence of sudden degradation in strengthened stiffness as experienced with control specimens. Also, the absence of pinching in the curve shows enhanced dissipated hysteretic energy of the precast joint. The control specimen experienced shear failure at the joint whereas plastic hinges were formed at the beam region and allowed higher rotation in precast connection. The employed FRCC allows multiple crack formation with better geometric stability at the hinge region and restricts diagonal crack formation at the joint. The post failure analysis shows absence of reinforcement slip in the coupler and buckling of reinforcement at the hinge region. It proves the efficacy of the wet connection technique adopted and also proves the effectiveness of hybrid rebar coupler at the potential hinge region of precast connections in transferring the force at higher rotation without any slip failure.

Conclusions

1. The use of wet connection with proper plastic hinge relocation mechanism offers better ductile mechanism compared to the existing dry connection.

2. The corbel based dry connections with bolts and dowel bars are inadequate to resist the lateral forces. Whereas, the use of partial fresh concrete with proper welding between the base plates of corbel and beam withstands seismic forces and offers better hysteretic energy.

3. The use of dampers is also an alternate method to enhance the energy dissipation capacity of precast joints with proper anchoring mechanism.

4. The use of couplers at the potential hinge region successfully transfers the forces under cyclic loading without any slip. The space provided between the coupled region and the joint interface plays a crucial role in inelastic rotation and energy dissipation.

5. The coupled region has a stiffness which is higher than the rebar stiffness. This leads to yielding of the rebars at the plastic hinge region.

6. The presence of FRCC at the hinge region supports the inelastic rotation with minor cracks and controls the shear failure pattern.

Acknowledgements
Author would like to sincerely acknowledge the support of Mr. Baariu Kelvin, Mr. M. V. Tarun Kumar, Mr. Vellautham, Mr. Sakthivel V.G. and Dr. Poornima M.D.

Reference

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