Expansion Joints
Expansion joints are
very important in
epoxy grout pours as
they serve to limit
the cracking that can
often occur in the grout.
Cracks are the result,
generally, of a curing stress
that develops in the grout as it
hardens.
Grouts generate heat after
they are mixed and the
chemical reaction starts. The
hotter the exothermic
chemical reaction (necessary
to develop a high compressive
strength grout) and the more
heat there is from external
sources (such as direct
sunlight) the greater the stress
that will develop in the grout.
If the stress does not
exceed the tensile strength of
the grout, no crack develops.
However, if additional stress
is added, as is very possible
with a change in temperature,
then the higher total stress is
enough to cause a crack. That
is why cracks sometimes
develop months after
installation. With a
coefficient of expansion of
more than twice that of
concrete or steel, it is easy to
see that temperature changes,
such as a cold snap, can
trigger enough additional
stress to allow a crack to
develop. Expansion joints
will serve to break up the
grout pours into smaller
sections, thus reducing the
probability of a crack.
Even with expansion
joints, cracks can start from a
stress concentration point,
such as a sharp corner, so in
addition to the use of expansion
joints, elimination or
treatment of all sharp corners
should be called for. How to
install expansion joints and
reduce stress concentrations is
the subject of this issue of the
Grouting Technology
Newsletter.
Location
Expansion joints,
perpendicular to the crank
shaft or long dimension of the
block, should be laid out
looking at a plan view of the
foundation. An obvious point,
such as a change in block
width, could serve as a
starting point, and additional
joints could be added to break
the pour into 3' to 4' sections.
The exact locations
should be dictated by the
equipment configuration
rather than a rigid adherence
to 3' to 4' distance criteria. Do
not call for an expansion joint
that will intercept a load
bearing area such as a sole
plate or chock, but instead
move it over so the loaded
section is equidistant between
expansion joints. If in doubt,
add an extra expansion joint.
Ends of long metal rails are a
logical place to put an
expansion joint, since a rail
that spans two or three anchor
bolts needs to have room to
grow thermally.
How to Construct
There are three ways
commonly used to construct
expansion joints in epoxy
grout pours:
- Waxed wooden strips that
are pulled out after the
grout hardens and the gap
then filled with an oil
resistant elastomeric
compound
- Styrofoam strips that are
partially dissolved after
the grout hardens and
then sealed with an oil resistant
elastomeric
compound
- Closed cell oil resistant
foam.
All three systems are easy to
install if added at the time the
forms are being constructed.
Expansion joints, in
conjunction with full bed
epoxy grouting, need only go from the edge of the block,
across the grout shoulder and
2" to 3" back under the
equipment frame. If the frame
is elevated on metal, epoxy
chocks or Rowan Tri-
Chocks™, the expansion
joints should go the full width
of the grout pour.
With fabricated steel
bases (skids) that are epoxy
grouted to a concrete slab,
common field practice has
been to put expansion joints
only in the shoulders, even if
the grout bed extends 8' to 10'
to the other side, unless there
is access to the interior areas
to install the elastomeric seal.
Cracking is more likely to
occur in the shoulder areas.
Pump bases are usually
grouted without any expansion joints because of their smaller
size and configuration.
Wooden Strips
Today, the most popular
method of installing joints in
epoxy grout caps is to use 1"-
wide wood strips (redwood is
preferred). The strips are
attached to the forms and
extend to the full depth of the
epoxy grout pour, or pours if
there is more than one pour.
The top ½" at the final
grout elevation should be
removable, although the 1/2"
depth required for the oil resistant
elastomeric seal can
be chiseled out with difficulty.
The wood should be sealed to
the chipped concrete substrate elastomeric seal.(See Figure
1).
If the epoxy grout is
going to be placed on the
vertical edge of the concrete
face (overpour) as well, the
removable wooden strip
should go down the sides.
This will enable the
elastomeric seal to go down
the sides as well, completing
the seal. (See Figure 2).
After the epoxy grout has
hardened, the 1/2" deep
removable wood strip is
pulled out and any wax
cleaned from the edges of the
grout. A new, single-package
caulking tube of oil- resistant
silicone (Rowan Blue™) is a
good sealing compound.
After cleaning the sides of the
grout, put a piece of masking
tape on the top of the remaining wood as a bond
breaker and on either side of
the grout cap to aid in clean
up. For the best possible
adhesion, prime the edges of
the epoxy grout that will be in
contact with the elastomeric
silicone with an approved
primer.
Next, seal the joint with
the elastomeric sealer,
carrying the seal down the
vertical edge of the foundation
as well. The single-package,
oil-resistant silicone will tool
off smoothly by using a little
water on the trowel or putty
knife.
The above provides an
optimum expansion joint
design where the elastomeric
seal has a cross section with
the depth being 1/2 of the
width and a bond breaker on
top of the wood left in place. Such a properly installed
expansion joint will allow for
proper expansion and
contraction of the grout
without pulling loose.
Styrofoam Strips
This really is a variation
of the step above. However,
since styrofoam can be
dissolved readily with acetone
or other solvents, the top 1/4"
or so can be dissolved and the
joint sealed with an appropriate
elastomeric compound
as described in the step above.
A word of caution, though:
some epoxy grout
formulations contain solvents
themselves and so cannot be
used with this method. Be
sure to check the compatibility
of the epoxy styrofoam ahead
of time.
A better variation of the
design is to set the foam 1/2"
low using a removable
wooden strip as in Method 1
and then completing the joint
with an oil-resistant,
elastomeric silicone.
This method is falling out
of favor, as the styrofoam
below can crush when
someone steps on the joint,
causing the seal to tear loose.
We do not recommend using
this method.
How to Install Closed Cell
Oil Resistant Foam
Suitable foam materials
of neoprene or urethane are
available in 3/4" to 1" width
stock. The depth should be
approximately that of the
grout depth and the length cut
to fit the distance from the
form edge to where the grout
terminates at the equipment
oil pan area.
The foam will tend to
"snake" after installation. To
control this, the foam should
be fastened to a wooden strip
that is nailed securely into the
form. (See Figure 2). The top
of the foam should be at the
finished elevation of the grout
with any excess being neatly
trimmed.
An alternative is to use
foam a 1/2" low with a
removable wooden strip on
top of the foam as in Method 1
and then completing the joint
with an oil resistant
elastomeric silicone.
A thicker 2" x 2" strip of
foam works very well for skid
mounted equipment. The
foam strips should be sealed to
the sharp corners of the
equipment frame, if full bed
grouted, or of a sole plate or
rail can cause a crack to start.
Good design calls for a 1 1/2"
radius, or larger, to help
alleviate the stress concentration.
Metal plates that will
be encased in the grout pour,
such as a jackscrew landing
plates, should be circular in
design or have well rounded
corners. If a sharp corner or
edge is found during the pregrouting
check, electrical
putty or duct seal can be used
to encase the sharp corner.
With a single pour of
epoxy grout, rebar dowels
should terminate 2" below the
top surface. With a multiple
pour, the rebar dowels should
extend through the bottom
layers and terminate 2" below
the top surface of the final
pour. Horizontal rebar should
also have 2" of grout cover
over them, or they, too, can
cause a crack in the grout.
Formed interior corners,
for example between compressor
cylinders, should also
have a generous 1 1/2"
contour or radius in the grout.
A piece of 3" PVC pipe can be
cut and used at the forming
stage to create the contour.
Finally, while not directly
related to cracking from stress
concentrations, the forms
should also have a chamfer
strip to break the sharp edges
of the final grout pour.
Following the above
recommendations will not
guarantee that cracks will not
develop but certainly will
greatly reduce cracking no
matter what brand of grout is
used. If a crack does develop,
it should be repaired,
preferably before the crack
becomes contaminated with
oil. Hints on how to repair
cracks will be provided in a
subsequent newsletter.
A Good Suggestion
Regardless of which of
the three methods is used,
some grout suppliers suggest
a layer of oil-resistant, elastomeric
material at the bottom
of the expansion joint as a
“secondary seal.” This needs
to be applied when installing
the expansion joint at the
form building stage.
