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
[email protected], [email protected] 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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ISBN: 978-1-61804-301-6 54
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:
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[5] Tepfers, R., A theory of bond applied to
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[8] Seyed Jamal Aldin Hosseini and Ahmad
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ISBN: 978-1-61804-301-6 55