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Better Industrial Floors
Through Better Joints
By
Steve Metzger
Twenty years ago I wrote "The Concepts of
Industrial Floor Joint Sealants" for Concrete
Construction. Now, I regret having used the word
"sealant." The goal of restoring a jointed surface
should be filling the gap, not sealing it.
A floor joint is an interruption in an otherwise smooth surface.
In an industrial floor, the joint needs to be protected from hard
wheels. By filling rather than sealing it, you protect the edges
of the joint from damage.
Over the past 20 years, material handling vehicles running on
smaller, harder wheels have handled heavier loads at higher
speeds. Has the concrete industry kept pace by developing more
durable joints to protect floors? Do architects, contractors, and
owners know current industry standards? Is the importance of a
durable joint system understood? I would like to answer a firm
yes to all these questions but I cannot.
American Concrete Institute (ACI) and Portland Cement Association
(PCA) publications tell how to design and build joint systems
with better-than-adequate edge protection. Too few specifiers or
contractors follow this advice. Many of the ineffective joint
designs and joint fillers from 10 years ago are still being used
today. And otherwise good industrial floors are still
deteriorating rapidly at the joints.
Joint Enemies
Hard wheels are the joints' worst enemy. Take a 4-inch-diameter
hard wheel, add a heavy vehicle with several thousand pounds of
load, and you have the perfect concrete destroyer. Floors must be
able to withstand abuse from impact, concentrated loads, and
abrasion at the most vulnerable part-the joint.
Delayed shrinkage is the other major enemy. Studies show that
major slab-on-grade shrinkage can occur long after concrete is
placed. Norman Henning, former president of Twin Cities Testing
in St. Paul, Minnesota, found that at the beginning of the
heating season only 20% of slab-on-grade shrinkage had taken
place after 30 days. Only about 60% occurred within the first
year. These findings show that some joints will open more after
filling, even if the filling is deferred for several months. To
perform well, a joint system must accommodate these late
movements.
Requirements for an industrial floor joint system that works
A good industrial floor joint system resists hard-wheel loadings
and tolerates delayed joint opening. To do this the system must
be:
Invisible
Forgiving
Durable
Practical
You can see invisible joints but vehicles never feel them. Filled
joints should be nearly flush and narrow to provide minimal
exposure to hard wheels. There's no impact as hard wheels cross
the joint and no deflection of the joint filler.
Forgiving joint systems allow the concrete to contract and
expand. Because of delayed shrinkage, joints will try to open
wider. If joint details restrict movement, the slab may crack
instead. Joints also must be able to accommodate some compression
as slabs expand. For instance, if a slab is poured in the fall
and joints are filled in winter, they'll get tighter in the
summer as the slab expands.
In durable joint systems. the concrete edge is supported by a
filler that provides long-term support. If there's no support,
concrete at the joint is almost certain to chip or spall. Using a
1-year filler in a 10-year floor is foolish and wastes the
owner's money.
To be practical, a joint system has to be compatible with
concrete, the construction process, and the service environment.
A joint filler isn't practical if using it requires joint
surfaces to be exceptionally clean. Joint forming systems aren't
practical if they require a lot of finesse in handling or if the
joints can't be easily filled. And a joint isn't practical if it
can't take the beating of hard-wheel traffic.
Joint Problems in
Industrial Floors
Despite heavy educational efforts by ACI, PCA, and others, some
floors are still built with joint systems not suited for
industrial use. These systems violate one or more of the
requirements mentioned above.
Left-in-place metal keys. Used for years to form construction
joints, metal keys have no place in floors exposed to hard-wheel
traffic. In theory a metal key makes sense; it should provide
load transfer from pour to pour. But when shrinkage occurs, the
lip of the female side is placed in a cantilevered position.
Material handling vehicles soon break off this lip.
Metal key construction joints aren't forgiving or durable in
industrial applications; they don't accommodate shrinkage or
stand up to hard-wheel traffic. They also are not practical
because they don't provide for use of a filler.
Plastic crack-inducing strips. In theory, these joints also make
sense. If they are inserted plumb, they can induce the desired
shrinkage crack and they offer minimum wheel exposure. But they
often end up out of plumb because of the finishing process. When
this happens, a cantilevered nose is created which is soon broken
off by hard-wheel traffic.
To avoid disturbing the strip, finishers may go easy when working
next to the joint. The strip also leaves an edge vulnerable to
hard-wheel traffic after the joint opens.
Joints made with plastic strips aren't forgiving or durable on
industrial floors. They don't accommodate shrinkage and the joint
edge may not be densified. Because the strip is so thin, it
requires finesse to keep it plumb, aligned, and butted end to
end. Many times this isn't practical. Also, this joint system
doesn't provide for subsequent filling.
Elastometric joint fillers. Elastometric materials (such as
polyurethane) at best only seal the joints by theoretically
expanding and contracting with joint movement. In an industrial
floor the joint must be protected from hard wheels, not just
sealed. Elastometric materials in any traffic-bearing industrial
floor are worthless. They permit joint edge deterioration.
Elastometric fillers deflect under load, so vehicles hit the
joint. The
elastometrics don't make durable joints because they provide no
edge support or protection. And they aren't practical because
they require a very clean bond surface inside the joint.
"Macho" epoxies. High strength and super-strong
adhesion are usually desired properties. In a floor joint,
though, they result in rapid and severe deterioration. The
purpose of a control joint is to induce a shrinkage crack, then
accommodate the shrinkage movement.
High strength epoxies restrain the movement, often causing a
crack next to the joint. Also, high-strength epoxy materials are
frequently brittle, not a good characteristic for a filler
subjected to frequent hard-wheel impact.
Hard epoxy joint systems aren't forgiving because they restrict
movement. And they aren't practical because they may be too
brittle to resist impact loads from hard wheels.
The
Industry Standard Joint System
Saw cut joints filled with semi-rigid epoxy best meet
requirements for joint performance. The saw cut is narrow. The
epoxy has a relatively low strength and a Shore D and A hardness
of about 50 and 80, respectively. This system performs better
than any other any other joint system on the market. But the best
isn't perfect. Problems can still occur.
Saw cuts can be made too early or too late. Cutting too early
causes raveling at the joint edges, an invitation to later
trouble. Floors cut too late may crack at an unplanned location.
Or a crack may form in front of the saw cut, creating an
irregular joint opening that can't be filled properly. If the saw
cut isn't deep enough to induce the crack, the crack forms
somewhere else and shifts maintenance worries to that location.
If the cut is too wide, more filler is exposed hard wheels and
vehicles bounce more as they cross the joint.
If the saw cut isn't filled to full depth, the joint won't resist
load as well. A semi-rigid epoxy poured over a soft backer rod is
worthless.
Separation nearly always occurs at some of the joints because
joints are almost always filled before the slab starts shrinking.
But when semi-rigid epoxy separates as shrinkage occurs, it
hasn't failed. It's doing precisely what its designed to
do-relieve stress on the slab.
Industrial Joints
To ensure top joint performance, designers need to address these
problems in the specifications.
Improving Joint System Quality
Specifications should tell the contractor the width of cut and
depth of cut needed. Make sure that the contractor times joint
sawing correctly, taking jobsite temperature into account. A good
contractor knows when to saw to avoid uncontrolled cracking.
All parties, including the owner, should understand that epoxy
separation doesn't constitute a joint failure. Have the
specifications call for a two-stage installation. The first
installation would be the normal filling procedure, deferred as
long as possible to allow for concrete shrinkage. A second stage
touch up would take place just before project turnover or at a
defined period after the first filling.
Another option is having the normal filling done just before
occupancy, then having the owner responsible for maintenance. The
owner, designer, and contractor can decide responsibility, as
long as the refill issue is addressed. Otherwise, the
unsuspecting applicator will get the headache as is often the
case now. When a floor is already in service it's foolish to let
edges spall while arguments over responsibility delay needed
touch-up work.
The semi-rigid epoxy(s) should be specified by properties and
name. Requiring joints to be filled with a nameless semi-rigid
epoxy is too vague. Be specific, ask for credentials and
references that verify, not imply, longevity. Remember that an
"or equal" clause in a specification almost always ends
up meaning "or cheaper." Consider the economic life of
the floor, not today's cost of epoxy.
The specifications and drawings also should call for the filler
to be installed full depth.
A first-class floor soon becomes a second-class slab if the
joints aren't properly designed, specified, and installed. Just
as with the floor itself, you have only one chance to do the
right job.
Copyright 2001 Metzger/McGuire
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