Behavior of anchor systems in cracked concrete under tension loading (AMTaj)

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29 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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Behavior of anchor systems in cracked concrete under tension loading
(AMTaj)


1 Anchor load transfer mechanism
Fasteners transfer applied loads to the base material in various ways (
Fig. 1
). Load
transfer mechanisms are typically identified as mechanical interlock, friction or bond.



a)


b)


c)

Fig.
1

Anchor load transfer mechanism [1]



Mechanical interlock involves transfer of load by means of bearing interlock between the
fastener and the base material. Mechanical interlock is the load transfer mechanism
employed by headed anchors, screw anchors (e.g. Hilti HUS/ KWIK-HUS EZ), and
undercut anchors (e.g. Hilti HDA).
Friction is the load transfer mechanism employed by expansion anchors (e.g. Hilti HSL,
Hilti HSA and Hilti HST). During the installation process, an expansion force is
generated which leads to the rise to a friction force between the anchor and the sides of
the drilled hole. This friction force is in equilibrium with the external tensile force.
In the case of chemical interlock, the tension load is transferred to the base material by
means of bond i.e. some combination of adhesion and micro-keying. Chemical interlock
is the load transfer mechanism employed by bonded anchors e.g. HIT-HY-150 MAX,
HIT-RE- 500 (SD).




2 Cracked Concrete

In the design of reinforced concrete flexural or tension components, a cracked tension
zone is assumed because concrete possesses relatively low tensile strength, which may
be fully or partly used by internal or restraint tensile stresses not taken into account in
the design [1] (
Fig. 4
). Experiences has shown that crack widths resulting from primarily
quasi permanent loads (dead loads plus fraction of live loads) do not exceed the value of
w
95%
~ 0.3mm to 0.4mm [2,3,4] (
Fig. 2
). These crack widths are generally acknowledged
as permissible. Wider cracks are to be expected under maximum permissible service
loads, which according to [3] reach w
95%
~0.5mm to 0.6mm, see
Fig. 3
[2,3,4]



Fig. 2 Relative frequency of measured crack width
under quasi permanent load

[2,3,4]

Fig. 3 Relative frequency of measured crack width
under maximum service loads [2,3,4]


It has been observed that when cracks form in a concrete member, there is a relatively
high likelihood that they will intersect the anchor location directly or tangentially [1]. This
occurs because higher tensile stresses exist around the anchor as a result of: (a) hoop
stresses associated with the prestressing and loading of the anchor, (b) possible local
flexural stresses resulting from the concentrated load introduced by the anchor and (c)
the stress concentration caused by the presence of the anchor hole (notch effect), see
Fig. 5
.




Fig. 4 Crack types (a) by loads (b) by induced
deformations [1]
Fig. 5 Stress distribution around the anchors as a
result of hoop stresses associated with prestressing
and loading and stress concentration caused by
discontinuity (notch effect)


Fig. 6
shows the effect of cracking on anchor performance of headed studs by means of
schematically load displacement curves taken from [1] in non-cracked and cracked
concrete. Note that this behavior is similar to Hilti undercut anchor HDA which is suitable
for cracked concrete applications. Anchor failure is characterized by concrete cone
breakout, both cracked and non-cracked concrete. For crack width ∆w= 0.3mm general,
failure loads of headed studs and Hilti undercut anchor HDA range from 0.5mm to
1.0mm (on average 0.75) times the value in non-cracked concrete. This is based on the
fact, that both headed studs and Hilti undercut anchor HDA transfer the applied tensile
force to the concrete by means of mechanical interlock (provided by the undercut).






Non
-
cracked

cracked


Fig. 6 Load displacement curves of headed
studs tested in tension in cracked and non-
cracked concrete (comparable with Hilti
undercut anchor HDA)

[1]
.

Fig.7
Distribution of forces in the concrete
anchorage zone of a heade
d stud, undercut
anchor (e.g. Hilti HDA) located in tension zone [1]


The reduced failure load in cracked concrete must therefore be attributed to the
disruption of the stress field associated with cracks (
Fig.7
) [1]. In non-cracked concrete a
tension loaded HDA Hilti undercut anchor generates a rotationally symmetric stress
pattern around the anchor as schematically seen in
Fig.7
(headed studs). If the anchor is
located in cracks of sufficient width, the tensile stresses can no longer be transferred
across the crack plan and are not rotationally distributed (disturbance of the rotational
stress field). This reduces the failure load in case of concrete cone failure up to 25%.

Schematic load displacement curves for tests in non-cracked and cracked concrete
conditions associated with a torque controlled expansion anchor (e.g. Hilti HSL, Hilti
HST/ KB-TZ) that is suitable for applications in cracked concrete are shown in
Fig. 8
a.
Anchor failure is characterized by concrete cone breakout in both cracked and non-
cracked concrete. The effect of cracking on the load displacement behavior and peak
load is similar to that observed for headed studs. Torque controlled expansion anchors
that are not suitable for applications in cracked concrete (Hilti HSA/KB-3) can exhibit so
called uncontrolled slip when loaded in tension in cracks, since such anchors may not
develop follow-up expansion (necessary to reestablish anchorage in crack) or so only
after significant displacement (
Fig. 8
b) [1].


Fig. 8 Schematic load displacement curves of torque controlled expansion anchors tested in tension in
cracked and non
-
cracked concrete [1]

a)
Anchors suitable for use in cracked
concrete
b) Anchors not suitable for use in cracked
concrete (inadequate or non-existing
follow
-
up expansion)



In principle bonded anchors exhibit the same basic failure modes as expansion and
undercut anchors. However, the performance (bond strength) of bonded anchors is
primarily a function of the mortar and the sensitivity to hole cleaning, condition of the
borehole (dry, wet and water filled), drilling process (hammer drilled, diamond core
drilled), temperature and various other parameters, see [6]. Tests in [6] performed with
Hilti HIT-RE 500-SD indicated that if the bond strength of the mortar is high enough and
concrete cone failure occurs, the influence of cracks on the failure load is comparable
with the influence of cracks on the load displacement behavior of expansion and
undercut anchors by means of a reduction of ~25%.
Fig. 9
show presents the ratio of
tension failure loads for bonded anchors tested in cracks to their mean capacity in non-
cracked concrete, plotted as a function of crack width [1]. The tests were conducted
using both, capsule type anchors and injection systems and various mortar types. The
anchors were installed in hairline cracks that were subsequently opened to the desire
with. The anchors were then loaded to failure with
cracks open while pullout failure occurred. Pullout
failure is usually a consequence of loss of bond
between mortar and drilled hole, although with
some systems the bond between mortar and
anchor rod may fail. The scatter of the results of
tests in cracked concrete are rather large due to
the fact that all kind of mortar types were used and
it was not distinguished between the individual
behavior of the mortars. If the results are taken
together, the anchor capacity in cracked concrete
with a crack width of w= 0.3mm to 0.4mm is about
25% to 80% of the values valid for non-cracked
concrete. On average the ratio is about 50%.

In contrary to this, tests performed in [6] in cracked
concrete with Hilti HIT-RE 500-SD according to the
procedure mentioned above showed only a reduction of 25% for the value in non-
cracked concrete due to the significantly improved penetration behavior and high
material strength of the mortar compared to the systems given in
Fig. 9
. Note, that the
reduction is comparable to the behavior of undercut and expansion anchors.
The reduction of the tension capacity by cracks for bonded anchors systems in case of
pullout failure can be explained as follows. Owing to the high tensile strength of the

Fig. 9 Influence of crack width on the
failure load of bonded anchors [1]


mortar, crack opening after anchor installation results in a redirection of the crack around
the anchor along the interface between mortar and concrete, effectively causing bond
loss on one side of the anchor. Assuming that the crack trajectory as shown in
Fig. 10

occurs over the full embedment depth, the bond capacity is theoretically 50% of the
capacity in non-cracked concrete. Investigations in [6] indicated that this assumption is
not generally valid. Using Hilti HIT-RE 500-SD shows significant less bond loss
compared to other mortar types (~25% on one side). This is mainly based on the fact
that the mortar system influences the crack trajectory compared to the crack trajectory
given in
Fig. 10
due to its penetration behavior into the pores of the concrete.

As a structure responds to earthquake ground
motion (seismic event) it experiences
displacement and consequently deformation
of its individual members. This deformation
leads to the formation and opening of cracks
in members (
Fig. 11
). Consequently all
anchorages intended to transfer earthquake
loads should be suitable for use in cracked
concrete and their design should be predicted
on the assumption that cracks in the concrete
will cycle open and closed for the duration of
strong ground motion. During large
earthquakes, parts of the structures may be
subjected to extreme inelastic deformation. In
the reinforced areas yielding of the
reinforcement and cycling of cracks may result
in cracks width of several millimeters,
particularly in regions of plastic hinges [5].
Qualification procedures for anchors do not currently anticipate such large crack widths.
For this reason, anchorages in this region where plastic hinging is expected to occur,
such as the base of shear wall, joint regions of frames, spandrel beams, should be
avoided unless suitable design measures are taken [1].
For more details about seismic design of anchors, please refer to www.hilti.com/seismic
.


Fig. 10 Disturbance of bond between mortar
and concrete by crack [1]



Fig. 11 Mechanism for accommodating transverse motion of a building through member cracking
assuming a strong
-
column, weak girder design (lp=
plastic

hinge length)

[5]


[1] Eligehausen R.; Mallee, R.; Silva, J.F. (2006): Anchorage in Concrete
construction, Ernst & Sohn, Berlin 2006
[2] Schiessl, P. (1986): Crack influence of the durability of reinforced and
prestressed concrete components. Schriftenreihe des Deutschen Ausschuss
für Stahlbeton, No. 370, Ernst & Sohn, Berlin 1986 (in German)
[3] Bergmeister, K. (1988): Stochastic in fixing technology based on realistic
influenced parameters, Doctor Thesis, University of Innsbruck, 1988 (in German)
[4] Eligehausen, R.; Bozenhardt, A. (1989): Crack widths as measured in actual
structures and conclusions for the testing of fastening elements. Report No. 1/42-
89/9, Institute of Construction Materials, University of Stuttgart, August 1989
[5] Höhler, M, S. (2006): Behavior and testing of fastenings to concrete for use in
seismic applications, Doctor Thesis, University of Stuttgart, 2006
[6] Appl, J. (2008): Load bearing behavior of bonded anchors under tension loading,
Doctor Thesis, University of Stuttgart, 2008