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,

81310 Skudai, Johor,

MALAYSIA

baharfka@utm.my, hyong3@siswa.utm.my http://www.civil.utm

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;

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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

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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.

(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

pipe which will be detailed as A35-210-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

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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. 5: Assembly of the specimens on the plywood

frame

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.

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Fig. 7: Bar tensile failure for A35-410-Y16

Fig. 8: Grout-bar bond failure for B65-210-

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 grout-

pipe 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

A35-210-Y16 with main bar embedment length of

100 mm and A35-310-Y16 with main bar

embedment length of 150 mm failed at 103.973 kN

and 127.097 kN respectively by grout-bar bond

failure. On the other hand, Specimen A35-410-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

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 grout-

pipe 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. 10: Tensile load versus the length of shear key

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 B65-310-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.

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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. 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 B65-210-Y20 and

B65-310-Y20 which failed by bar-grout bond

failure.

Fig. 13: Tensile load versus the embedment length

of reinforcement bar

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

A35-410-Y16, B65-410-Y16 and C95-410-Y16.

These three specimens have adequate development

and embedment length as short as seven times as

required. Thus, judging from the ratio of 𝐿𝑒/Ø for

the three specimens which are 10.31 for A35-410-

Y16, 8.44 for B65-410-Y16 and 6.56 for C95-410-

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.

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Table 3: Ratio of embedment length/bar diameter

𝐿𝑒/Ø

From the result, it is observed that embedment

length of approximately 7 times is required for

specimen C95-410-Y16 to achieve bar fractured

failure. As the length of shear key increases, the

ratio of 𝐿𝑒/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

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.

References:

[1] Yee, Alfred, A., Structural and economic

benefits of precast/prestressed concrete

construction, PCI Journal, 2001.

[2] Untrauer, R.E., and Henry, R.., Influence of

normal pressure on bond strength, ACI Journal,

V.69, No. 5, 1965.

[3] Mahdi, M., Ahmad, J., and Arash, K. 2003,

Bond of cement grouted reinforcing bars under

constant radial pressure, Master Project,

Universiti Teknologi Malaysia, 2003.

[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, 1967.

[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.1 2008.

[8] Seyed Jamal Aldin Hosseini and Ahmad

Baharuddin Abd. Rahman, Analysis of spiral

reinforcement in grouted pipe splice

connectors, Građjevinar, 65 (6), 1-10, 2013. [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, 2015.

[10] Einea, A., Yamane, T., Tadros, M.K. 1995,

Grout-filled pipe splices for precast concrete

construction, PCI Journal. 1995.

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ISBN: 978-1-61804-301-6 55