5.8 Beam-Column Connections for Precast Concrete Frames Using High Performance Fiber Reinforced...

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G.J. Parra-Montesinos, H.W. Reinhardt, and A.E. Naaman (Eds.): HPFRCC 6, pp. 347354.

RILEM 2012

Beam-Column Connections for PrecastConcrete Frames Using High PerformanceFiber Reinforced Cement Composites

L.F. Maya and L. Albajar

Department of Continuum Mechanics and Structures,

Universidad Politcnica de Madrid, Spain

Abstract. Precast construction system interest has been growing given the

emphasis on improving work zone safety, reducing construction time andenvironmental impact, while maintaining the quality. The connections are the

more important singularity of precast construction systems, being the general

behavior of the precast structures related to their design, construction and

performance. A rigid beam-column connection typology for precast construction

is proposed. This typology takes advantage of the outstanding properties of the

High Performance Fiber Reinforced Cement Composites (HPFRCC) and in

particular its better bond conditions. The principal mechanism of connection is the

splice of longitudinal beam reinforcement, which takes place at the end of the

beam elements, outside the joint region. The configuration proposed avoids theinterference between longitudinal and transverse reinforcement, reduces the in situ

work and makes possible to define an efficient and safe construction process.

1 Introduction

Precast construction is regarded as an appealing alternative to be considered in a

wide range of construction projects. It is due among other factors to the

advantages related to the reduction in construction times, work force and in situlabors, as well as a more favorable cost-benefit relation, less environmental

impacts, and greater control and final quality of the elements. However, the use of

precast construction is sometimes limited by an inappropriate assessment of

several typical singularities of these construction systems. Being the connections a

critical factor and one of the most important singularities, they can determine the

general structure behavior. Besides, the connections represent the construction

process stage developed in situ,during which common problems at the structure

assemblage process have to be faced.

Several studies have been conducted over the last 25 years to evaluate theefficacy of using Fiber Reinforced Concrete (FRC) to improve the behavior of

beam-column connections. More recently, different proposals to incorporate High

Performance Fiber Reinforced Cement Composites (HPFRCC) in beam-column


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348 L.F. Maya and L. Albajar

connections have been successfully tested [1, 2]. This research deepens on the

same line by considering the incorporation of HPFRCC to develop continuity

connections among precast elements. In particular, the use of these materials in

reinforcement splices using short splice lengths is studied. A two stage

experimental study was carried out in collaboration with the TechnicalDepartment of PRAINSA. During the initial stage, four beam flexion tests were

performed to experimentally assess the behavior of short splices in simple flexural

elements. In the second stage, a new interior beam-column connection for moment

resisting frames was proposed and tested. This paper is devoted to describe the

experimental results of the second stage of the study. A detailed description and

theoretical analysis for first experimental stage is presented in [3].

2 Test Program

Four beam-column connections of precast elements were fabricated and tested.

The elements represent a typical interior beam-column joint of a multi-storey

frame bounded by the point of contra-flexure in the members. The tested elements

consisted of a single column element and two beam elements framing at opposite

sides of the joint, which were connected through the splice of the longitudinal

reinforcement in the beam elements. The design process was performed according

to the strong column and weak beam approach, as suggested in most of the

modern design codes [4, 5]. A design detail of the test elements is shown in Fig. 1.

The design of the elements was defined by the construction process proposed. The

beam elements had channels at the top and the bottom, where the splices of the

reinforcement took place. The channels were connected by two vertical ducts to

allow the pouring of the HPFRCC in situ. The beam ends at each side of the

column were different for practical erection-building purposes. The columns had

two windows at the level of the beam channels to allow the splice bars to pass

through the joint. Additionally, two other windows were left below the joint

section to fix provisional supports during the construction process. Once the

column had been erected, the beam with the longer channel was rested on a

provisional support and the splice bars were slid through the windows in thecolumn until the bar ends did not protrude from the column outer face. Then, the

second beam was rested on the support at the opposite side of the column and the

splice bars were slid to their final position, so that they spliced the longitudinal

reinforcement of both beams.


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Beam-column Connections for Precast Concrete Frames 349

Fig. 1. Test element configuration and reinforcement details (unit:mm)

The configuration and construction process proposed enable to overcome some

common drawbacks of beam-column connections for precast structures. Firstly,

connection takes place outside the joint region, which is usually subjected to high

stress demands. Furthermore, it is possible to provide closed stirrups all along the

beam, including the beam ends close to the column faces, and into the joint core,

where the confinement and shear strength are important to assure the proper

element behavior and to fulfill the ductility requirements. Likewise, the HPFRCCpoured in situ enables to establish some continuity among the precast elements,

not being the sections at the column faces completely discontinuous.

The test elements were designed such that the shear demand was high but it did

not limit the strong column and weak beam approach. Two main variables were

considered; the splice length and the transverse reinforcement spacing in the splice

region. The splice lengths tested were 200 mm (10db ) and 300 mm (15db), while

the spacing of the transverse reinforcement in the splice region was set as 100 mm

and 150 mm, according to the beam test carried out previously [3]. Table 1

summarizes the test units fabricated. The reaction frame used in this study is anadaptation from the test setup described in [2], Fig. 2. The beams and the column

were pin supported at their ends. The vertical displacements at the beam ends were

restricted by double hinged supports, which also acted as pseudo-load cells. The

top of the column was restrained in the horizontal plane but allowed vertical

displacements, while the loads were applied through the actuators at the bottom of

the column. A rail was set on the floor to prevent the test elements from moving

transversally. The test setup did not allow axial loads to be applied to the column.

The cyclic loading history used is illustrated in Fig. 3. Two cycles of each load

step were applied, starting at 0.5% and increasing by steps of 0.5% until a 2.0%

drift was achieved. Then, the step increment was set to 1%.All the precast elements were cast with self-compacting concrete, which

average compressive strength tested on 150 mm x 300 mm cylinders and the age

of testing are given in Table 1.


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Fig. 2. Experimental test setup

The reinforcement consisted of bars of quality B500SD. According to theSpanish Code [5], the characteristic yield stress and ultimate strength of the

reinforcement are 500 and 575 MPa, respectively. Compact Reinforced Composite

(CRC), a HPFRCC marketed byHi-Con, was poured in situ to configure the splice

regions. The average compressive strength tested on 100 mm side cubic specimens

and the age of testing are also given in Table 1.

Fig. 3. Loading history

The lateral drift at the bottom of the column was measured by a linear variable

displacement transducer (LVDT). Four LVDTs were located at the beam ends

close to the column faces to measure the width of the foreseen cracks at the beam-

column junctions. Additionally, other four LVDTs were located at the beam ends

to measure the average strain on a base length of 400 mm in the CVP-1A element

and 300 mm in the rest of the elements (see Fig. 5). A pseudo displacement

control was implemented for the load steps, according to the actuator and testing

facilities available. The load was first applied in one direction by means of an

actuator. Then, the unit was unloaded and loaded in the opposite direction by

means of another actuator to complete the cycle. Unfortunately, due to thecharacteristics of the actuators used, it was possible to register the load steps but

not the unloading steps. Although the load history was registered by the load cell

in the actuator, the vertical supports at the beam ends were also instrumented to

act as pseudo-load cells, since the friction on the rail was an uncertainty.


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Table 1. Element and material properties

Element

Splice configuration Material properties

Splice

length(mm)

Transversereinforcement

spacings (mm)

Precast Elements HPFRCC

fc(MPa)

Age of

testing(days)

fc,HPFRCC(MPa)

Age of

testing(days)

CPV-1A 300 100 53.6 203 147.1 190CPV-2A 300 150 52.6 216 145.0 205

CPV-1B 200 100 59.7 226 127.0 215CPV-2B 200 150 59.5 231 133.4 219

3 Test Results

The general behavior of the four tested elements corresponded with the expectedbehavior for rigid frame elements subjected to lateral forces. From the first

loading steps, the tested elements were cracked at the beam-column junction.

Additional flexural cracks were observed along the beam elements outside the

HPFRCC poured region, increasing their width and depth during the test.

Horizontal cracks were observed in the column elements with approximately the

same spacing of the transverse reinforcement, as well as a pair of horizontal cracks

that defined completely the joint region. Furthermore, diagonal shear cracks were

observed in the joint but they did not compromise the integrity of this detailed

region. The load-drift curves of all the elements are shown in Fig. 4.

Fig. 4. Load-drift curves

The story drift of the elements varied between 2.9% and 4.6%. The measured

and calculated maximum load Vcand the joint shear stress vjh are listed in Table 2.


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The calculated strength is based on the flexural strength of the reinforced beams,

assuming monolithic behavior of the frame element. The actual concrete

properties listed in Table 1 and reinforcement bars of quality B500SD were

considered. Furthermore, for the reinforcing steel an overstrength factor of 1.25

was considered.

Table 2. Experimental results

Element

(kN)

(MPa)

CPV-1A 174.7 1.10 7.6 1.04CPV-2A 172.2 1.09 7.5 1.03

CPV-1B 138.1 0.87 6.0 0.78CPV-2B 135.7 0.86 5.9 0.77

Elements CPV-1A and CPV-2A attained approximately 9% higher lateral load

than the calculated value, while elements CPV-1B and CPV-2B attained

approximately 85% of the strength calculated. The failure of elements CPV-1B

and CVP-2B was determined by the short splice length, 200 mm (10db). The crack

width at the beam-column junction increased due to the progressive loss of bond

capacity in the splices as the load and drift were also increased with the cycles.

Element CVP-2B, which had a lower transverse reinforcement ratio, exhibited a

more fragile failure than element CVP-1B, being observed an extensive cracking

in the splice region. A single but wider crack was observed at the beam-column

junction in the splice region of element CPV-1B, as is show in Fig. 5.

Fig. 5. Splice regions under tension. Element CVP-2B (left), element CVP-1A (right)

Element CVP-1A and CVP-2A tests were stopped once the loading process

became unsafe, given the type of actuators and the pseudo-displacement control

process employed. However, at the final stage of the tests there was no evidence

of bond failure in the splice regions. The aspect of element CPV-1A at the 3.7%

drift cycle is shown in Fig. 6. Both element CPV-1A and element CVP-2A did notexhibit fragile failure, which indicates the adequacy of the 300 mm (15db) splice

length for the proposed precast connection system. Other precast yielding

connections but with different typologies than the element tested in this research


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have been satisfactorily assessed for moment-resisting frames [2, 6, 7, 8]. Unlike

the cast in place systems, the complete formation of plastic hinges in the precast

beams was not observed, in agreement with the expected behavior of yielding

connections. Moreover, the continuity established among the precast elements by

the HPFRCC poured in situ and the better bond condition enable a more uniform

distribution of the steel strains in the critical region of the beam elements. The

tension chord behavior is defined by the composite action between bar

reinforcement and the HPFRCC, being the post-cracking behavior positively

influenced by the action of the HPFRCC.

Finally, beam-column joint behavior involves an interaction between strut and

truss mechanisms, where large shear and bond strength can be required [9]. The

better bond condition of the reinforcement in the splice regions and into the joint

core enabled to develop adequately the joint resistance mechanisms. The strut

mechanism of the forces flow through the joint was observed in the element CVP-

2A, as is shown in Fig. 6. However, the joint behavior did not limit the loadcapacity of the whole frame, despite the beam flexural overstrength.

Fig. 6. Element CVP-1A. c=3.7% (left), element CVP-2A, c=4.6% (right)

4 Concluding Remarks

A beam-column connection typology for precast construction is proposed and thefollowing conclusions were obtained from the experimental results:

The behavior of the precast connection proposed was adequate and correspondswith the expected behavior of yielding connections for moment-resisting frames.

The use of HPFRCC in the splice region enables to reduce the splice lengthrequired. As a result, it is possible to propose a simple and efficient

construction process.

The construction process proposed aims to reduce in situ labors andconstruction time, while overcoming some common drawbacks of beam-column connections for precast structures.

Closed stirrups can be provided all along the beam and into the joint core,where confinement and shear strength are important to assure the adequate

element behavior.


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Joint damage is not extensive, given the localization of the connection outsidethe joint core. However, the strut and truss mechanisms were observed at the

joint region, indicating adequate bond and shear strength.

A splice length of 15db (300 mm) was enough to provide the requiredconnection performance, preventing the occurrence of fragile bond failures.Other bonding materials will influence the ratio between splice length and bar

diameter. A theoretical model has been developed to assess this ratio.

Additional analysis of the complete experimental results is being performed.

Alternative proposals of bond connections for precast structures will be the subject

of subsequent studies.

Acknowledgements. The authors gratefully acknowledge the financial support of this

research from PRAINSA, Eduardo Torroja Institute for Construction Science, Hi Con,

ACHE, and ANDECE. The technical support and advice of the Technical Department ofPRAINSA andHi Con were fundamental in carrying out this study. The authors would also

like to extend special thanks to the technicians of the laboratory of the Eduardo Torroja

Institute for Construction Science for their invaluable assistance.

References

[1] Parra-Montesinos, G., Peterfreund, S., Chao, S.: Highly damage-tolerant beam-columnjoints through use of high-performance fiber reinforced cement composites. ACI

Structural Journal 102(3), 487495 (2005)[2] Brooke, N., Ingham, J.: Advanced fiber reinforced precast concrete beam-column

joints. In: Proceedings of the New Zealand Concrete Industry Conference,

Christchurch, New Zealand (2006)

[3] Maya, L.F., Albajar, L., Moran, F., Portabella, J.: Ultra High Performance FiberReinforced Concrete (UHPFRC) for connections between precast elements. Informes

de la Construccin 62(510), 2741 (2010) (in Spanish)

[4] American Concrete Institute, Building code requirements for structural concrete andcommentary. ACI 318-08. ACI Committee 318, Farmington Hills, MI, USA (2008)

[5] Comisin Permanente del Hormign, Instruccin de Hormign Estructural EHE 2008(2008)

[6] Castro, J., Yamaguchi, T., Imai, H.: Seismic performance of precast concrete beam-column joints. Journal of Structural Construction Engineering AIJ 455, 113126

(1994)

[7] Alcocer, S., Carranza, R., Perez-Navarrete, D., Martinez, R.: Seismic test of beam-to-column connections in a precast concrete frame. PCI Journal 47(3), 7089 (2002)

[8] Khaloo, A., Parastesh, H.: Cyclic loading response of simple moment-resisting precastconcrete beam-column connection. ACI Structural Journal 100(4), 440445 (2003)

[9] Paulay, T., Park, R., Priestley, M.: Reinforced concrete beam column joints underseismic action. ACI Structural Journal 75(11), 585593 (1978)

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