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Metzger/McGuire Technical Advisory for Floor Designers
Should An Architect or Engineer Approve Polyurea Floor
Joint Fillers
at Less Than Full Depth Over Backer Rod?
Contractors are increasingly submitting for approval
polyurea industrial floor joint fillers. The
tech data on some of these fillers calls for the use of a compressible backer
rod to limit the filler depth. Should you approve these polyureas, and thus approve their
recommended installation procedure? Here
are some facts to consider before you decide:
Non-Compliance
with ACI and PCA Filler Depth Criteria
The American Concrete Institute and the Portland Cement Association (PCA) are
the accepted authorities on concrete floor design and construction.
Documents from both mandate that saw cut joints in industrial floors be
filled full depth. This is because any compressible rod placed below the filler
or any other non-structural base offers inadequate filler support, leaving the
filler susceptible to deflection and exposing the joints edges to impact.
If the floor joint edges deteriorate and claims arise as a result of
less-than-full-depth filling, these documents will likely be used against the
designer.
Non-Compliance
with ACI and PCA Filler Hardness Criteria
Polyurea fillers that call for the use of compressible backer rod to control
depth generally have a hardness of Shore A75 or lower.
ACI and PCA documents mandate that industrial floor joint fillers must
have a “minimum” hardness of Shore A80.
In case of claims for deteriorated joints, ACI and PCA documents could
again be used against the designer.
Fillers
Cannot Be Both Stiff and Flexible
Why is backer rod recommended? The
manufacturer’s theory is that limiting the filler depth increases its ability
to expand laterally. This is true,
and is the reason that polyurethanes and elastomeric sealants, chosen for
lateral movement, are installed in this manner.
But it’s important to remember that industrial floor joint fillers are
designed to “fill” not “seal” a joint.
If a filler is flexible enough to expand substantially, it is also
flexible enough to deflect under hard wheel traffic, exposing the joint edges to
damage as loads cross the joint. If
the filler doesn’t protect the joint edges, it has no value in an industrial
floor. Result; designer
vulnerability.
The
Promise of Expansion is Implausible
If you specify sealants for vertical walls you know that a typical polyurethane
sealant (Shore A35) has a maximum expansion capability of approximately 30%,
with an optimum 2:1 width/depth ratio. An
industrial floor joint which is cut 1/8” wide will typically open to about
3/16” at the time of filling and about ¼” at about one year after pour.
This means that the post-filling expansion of the slab joints is about
33%. If a Shore A35 elastomer with
a 2:1 width/depth ratio can’t tolerate typical floor joint opening, how can a
Shore A75 polyurea with a 1:4 width/depth ratio?
Even If
It Works, It Fails
Let’s assume that an A75 polyurea does somehow handle the anticipated 33%
joint expansion. When a flexible
material expands it takes on an hourglass profile. In a horizontal joint this means that the top of the filler
will become concave in profile, leaving joint edges exposed and vulnerable to
hard wheel impact and damage.
Conclusion
When you approve an applicator’s request to use a polyurea over backer rod,
you have nothing to gain. But you
do expose your client to potentially severe joint damage and leave your firm
vulnerable to financial claims resulting from the damage.
Is it really worth the risk?
If you specify or are asked to approve a polyurea joint
filler for industrial floors, just make sure that your approval is contingent
upon two things:
1. A minimum Shore A80 hardness to comply with ACI & PCA
2. The polyurea is installed to the full depth of the saw cut and not placed
over a compressible base (per ACI and PCA)
Ensuring that these requirements are met provides maximum protection for both
the owner’s floor and your reputation.
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