Civil Engineering Reference
In-Depth Information
in non-aggressive environments where the con-
sequences of failure are significant. Both the bond
and unbonded lengths of the tendon or bar are
protected with either cement grout-filled encap-
sulation or an epoxy coating; the head of the
anchor is also protected.
Class II
protection is
used for temporary anchors in non-aggressive
environments; protection is limited to grout on
the bond length, a sheath on the unbonded
length, and protection of the head if exposed.
Figures 12.7 and 12.8 show typical Class I cor-
rosion protection systems for bar and strand
anchors respectively.
Based on these categories of corrosion, a
decision tree has been developed to assess the vul-
nerability of rock anchors to corrosion and loss
of anchorage capacity (Figure 12.9) (TRB, 2002).
High strength steel (ultimate tensile strength
σ
ult
>
1000 MPa) is vulnerable to attack from
hydrogen embrittlement and corrosion stress
cracking. Generally, high strength steel is used
to manufacture wire strand elements (
σ
ult
≈
1700-1900 MPa), and strand anchors are more
vulnerable than bar anchors because of the larger
surface area of steel.
It is also possible to estimate the service life of
anchors based on the rate of corrosion by calcu-
lating the loss of element thickness over time. The
service life
t
in years is given by
Approximate values for
K
are 35 for normal con-
ditions, 50 for aggressive conditions, and 340 for
very aggressive conditions, as defined in Table
12.6. Equation (12.5) assumes that the working
stress equals 0.6 times the yield stress.
Methods of protecting steel against corrosion
include galvanizing, applying an epoxy coating,
or encapsulating the steel in cement. Cement is
commonly used for corrosion protection, primar-
ily because it creates a high pH environment that
protects the steel by forming a surface layer of
hydrous ferrous oxide. In addition, cement grout
is inexpensive, simple to install, has sufficient
strength for most applications, and a long service
life. Because of the brittle nature of grout and its
tendency to crack, particularly when loaded in
tension or bending, it is usual that the protection
system comprise a combination of grout and a
plastic (high density polyethylene, HDPE) sleeve.
In this way, the grout produces the high pH envir-
onment around the steel, while the plastic sleeve
provides protection against cracking. In order
to minimize the formation of shrinkage cracks
that reduce corrosion resistance of the grout, it
is usual to use non-shrink grouts for all compon-
ents of the installation. Figures 12.7 and 12.8
show examples of three-layer corrosion protec-
tion systems in which the steel is encapsulated in
a grout-filled HDPE sheath, and the outer annu-
lus, between the sheath and the rock, formed by
the centering sleeves, is filled with a second grout
layer.
For anchors with unbonded lengths, it is partic-
ularly important that the head be protected from
both corrosion and damage. This is because loss
of the nut or wedges, or fracturing of the rock
under the reaction plate, will result in loss of ten-
sion in the anchor even if the remainder of the
anchor is entirely intact.
ln
(X)
−
ln
(K)
ln
(t)
=
(12.4)
n
where
X
is the loss of thickness or radius (
m),
and
K
and
n
are constants (Table 12.7). The
loss of thickness
X
is computed from the original
radius
r
o
, and the critical radius
r
crit
that is the
radius at which the yield stress is reached, at
constant load, due to the loss of cross-section
compared to the original cross-section
A
o
, i.e.
µ
0.6A
o
π
Step 4: Bond type
r
crit
=
(12.5)
Tensioned anchors comprise two portions—a
bond length and an unbonded length (Figures 12.7
and 12.8). In the bond length, the bar or strand
is bonded by one of a variety of means to the sur-
rounding rock. In the unbonded length, the bar or
and
X
=
(r
o
−
r
crit
)
(12.6)
