A Metzger/McGuire
Technical Advisory
The
Myth of "Flexible" Fillers
For Industrial Concrete Floors
We are all
familiar with the adage "If it sounds to good to be
true, it probably is." As many designers and
facility owners have discovered too late, it pays to
consider this adage when someone tells you they have a
"flexible" filler that will meet all
the needs of the joints in an industrial floor.
The More Things
Change...
A recent trend among manufacturers of industrial floor
joint fillers has been to label and promote their fillers
as "flexible" rather than
"semi-rigid," even though the characteristics
of their fillers have remained unchanged. This trend
seems to have developed in response to the industry's
desire for a "perfect" filler- one that's
flexible enough to stretch with the joint yet durable
enough to support the demands of load-bearing lift
traffic. If a filler matching this criteria sounds too
good to be true, it's because it is. The limitations of
semi-rigid fillers haven't changed, just the way they're
being sold.
This technical article will discuss
the difference between a truly "flexible"
filler and a "semi-rigid" filler, and why you
need to know the difference.
Understanding an
Industrial Floor
To understand the criteria for a floor joint filler, you
must first understand the floor itself, and its intended
use.
An industrial concrete floor is not
really one floor. It is a series of smaller floor panels
(usually 15'x15' or 20'x20') that are both separated by
and connected by joints. There are two basic joint types;
contraction (control) joints and construction (formed)
joints. Contraction joints are created to prevent/reduce
random cracking by inducing cracking along a straight
line (beneath the joint). Construction joints are the
ends of the pour sequences.
Joints almost always grow wider
than they were originally created. That's because
concrete shrinks. (See PCA Shrinkage Chart, below). Until
all of the shrinkage has taken place, each slab panel
must be allowed to shrink without restraint at the
joints. Unfortunately, an industrial slab will be
subjected to heavily loaded, possibly hard-wheeled
material handling vehicles (MHV's) long before all
significant shrinkage has taken place.
PCA
Slab Shrinkage Chart
Since MHV traffic creates the need
to fill joints in the first place, it follows that joints
will often need to be filled prior to ultimate shrinkage
as well. What effect the timing of joint filling will
ultimately have on the filler itself needs to considered
and predicted. But it's important not to lose sight of
the long term function and characteristics of the joint
filler you choose. Is it wise to trade off short-term
flexibility for long term durability? Can the filler
provide you with both properties, one, or neither?
The Ideal Joint
Filler
An ideal joint filler would be a material that would
achieve three basic functions;
1. Allow the joints to open without
restraint
2. Continue to totally fill the joint as it opens.
3. Protect the edges of the joint from damage (spalling)
caused by the passage of hard-wheeled vehicles.
When you read a data sheet for a
so-called "flexible" filler, the word flexible
would seem to imply that the filler is elastic, and will
stretch (expand) as the joint continues to open. In fact,
as evidence of its elasticity you may notice (or be told)
that the filler has a high elongation of 100%, 150%, etc.
Reading further, you will likely find that the filler
also claims to be sufficiently rigid to protect and
support joint edges in traffic conditions. In other
words, this flexible filler allegedly meets all three of
the ideal filler characteristics. Here's where the adage
comes into play.
The Myth of the
Flexible Filler
Avoiding Restraint
There are two ways a filler can avoid slab panel
restraint; by stretching with the joint or by separating
either adhesively or internally. Flexible fillers imply
they stretch (expand) with the joint.
Expanding with the
Joint
Flexible filler data sheets and the product's
representatives are quick to point out their high
elongation. But joints don't elongate; they open
laterally, side-to-side. Thus, the elongation percentage
can be misleading. The real questions to ask are;
1. How much, by percentage, will
the joint open.
2. What percentage of opening (expansion) can the filler
handle before separation.
A general rule of thumb for
conventional slabs is that they will shrink 1/8" in
length every 20', in both directions. Therefore, if you
start with 1/8" wide joints at 20' centers, each
joint may open to an eventual 1/4" width, a 100%
expansion. You can reduce this 100% if you use a low
water cement mix and closer joint spacing. And if you
delay the filling long enough for some of the shrinkage
to take place, you can also reduce the amount of
expansion that the filler must achieve to perhaps 25%.
But can a flexible filler with a hardness of Shore A
75-100 achieve even 25% expansion? The answer is almost
assuredly NO.
Look at a data sheet for a highly
elastic polyurethane "sealant," a product whose
only function is to achieve maximum expansion. These
sealants are half as stiff (Shore A 35-45), yet they
state they can accommodate only about 30-40% expansion,
if they are installed with a controlled depth. The common
width/depth ratio is 1:1 or 2:1. Thus, to accommodate
30-40% movement, these soft sealants must be installed
only 1/8" to 1/4" deep. If they are installed
deeper, this expansion capability is proportionately
reduced. Considering the limited expansion of a soft,
shallow sealant, can you really expect a twice-as-stiff
filler installed 1" or 2" deep to expand
20-30%? No.
Joint Edge
Protection
Now we must deal with the third criteria for an
"ideal" filler. The flexible filler data sheet
says the product will protect joint edges from hard wheel
damage. As verification, it cites its hardness of Shore A
75-100. But this raises an important question: if this
filler is soft enough to stretch as the joint opens,
won't it also deflect under load? The answer is clearly
yes, it will.
Consider this; the product
manufacturer (or applicator or distributor) is telling
you that the product is both hard and soft at the same
time. This is the myth of flexible filler, and why they
truly are "too good to be true."
What ACI and PCA
Say
Since 1978, both ACI (American Concrete Institute) and
PCA (Portland Cement Association) have established the
standard criteria for joint fillers for industrial
concrete floors. They both call for a filler to be a
"semi-rigid epoxy with a minimum hardness of Shore
A80. First, note that they say "semi-rigid,"
not "flexible" or even
"semi-flexible." ACI and PCA understand that
there is a difference. Second, refer to the recently
published ACI 302.1R-96 (Guide for Concrete Floor and
Slab Construction). This document states that "if
the joint should be filled before most of the slab
shrinkage has occurred, separation should be expected
between the joint edge and the joint filler, or within
the joint filler itself." In other words, ACI
tacitly acknowledges that a proper, supportive filler
won't adequately stretch with the joint. Third, the use
of Shore A hardness readings was established in the
1960-70 era (by Metzger/McGuire). At this time, the
material handling industry was not producing 10,000 lb.
vehicles carrying 5,000 lb. loads on 4" diameter
solid wheels. The criteria for joint fillers in future
years will need to expand its description to include
compressive resistance, etc.
Conclusion
We hope that as a result of this technical article you
will view filler claims with a little more caution and
skepticism than before. We will continue to provide you
with updated knowledge as the floor and filler industry
change. But the adage "sounds too good to be
true" remains as valid as ever. Please feel free to
contact us should you have any questions concerning joint
fillers for industrial concrete floors. We will continue
to give you the best, unbiased answers we can.
Copyright 2001 Metzger/McGuire
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