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How to Build
MORE DURABLE CONCRETE FLOORS
BY DANIELL A. GUNTIN*
M
ETHODS, MATERIALS AND WORKMANSHIP are
among the main ingredients of any concrete floor.
And the true test of how well these are applied usually
comes only with time. Nevertheless, there are basic procedures to be followed to achieve a good, hard-wearing
top surface of a concrete floor.
One of the first considerations is determining the essential wearing requirements of the floor. Naturally, the
durability needs of an industrial plant floor or concrete
airport runway are more extreme than those of a simple
basement slab. Nor will a basement slab have to take the
chemical abuse given to a floor in a paint factor or brewery. The ultimate end use is a prime factor affecting the
planning and construction of the floor. This bears directly on costs as well as workmanship.
The prime causes of a poor top surface are: excessively wet mixes, over-vibration of wet concrete, improperly
timed floating and finishing and failure to properly cure.
Ill-timed or hasty finishing operations are probably the
greatest cause of inadequate resistance to wear.
We are all aware of the importance of the water/ce-
ment ratio and how it dictates the ultimate strength of
concrete. We should all be equally aware of the fact that
the ultimate compressive strength of concrete has a direct relationship to its wear resistance. The lower the
strength of the concrete the poorer will be its wear resistance under equal conditions of finishing. Howe ve r, under the press of time, floors are often finished while
quantities of bleed water remain on the surface. The moment this occurs a complete change is made in the basic water/cement ratio of the surface cement grains. The
action of a float or trowel on the surface in effect combines the free water that has bled to the surface with the
cement paste that is already there. In an instant concrete
that may have originally been 3,000 psi receives more
water and its strength is promptly reduced to 300 psi, depending upon the amount of bleed water present. Even
with proper curing the surface is incapable of supporting even moderate traffic. The result: a dusty, crumbly
floor. This does not take into account the effects of excessive manipulation of a plastic surface that forces the
coarse aggregate down from the surface and away from
the area where it is vitally needed to support the heavy
loads and abrasive wear.
Improper or inadequate curing also ranks high as a
basic cause for poor- we a ring floors. Without the con-
tinued presence of moisture a very rapid hydration or
drying out of the cement gels results. This produces a
myriad of tiny shrinkage cracks on the surface and
around each piece of fine and coarse aggregate. The result is less adhesive power. This loss of adhesion is often
visible as aggregate popouts, scaling, pitting and other
damage to the finished surface.
There are two basic types of concrete floor slabs —the
monolithic and the two-course. Of the two systems, the
monolithic floor slab is the most popular for the majority of concrete wearing surfaces. It is also more economical. However, the monolithic method has one major disadvantage in terms of abrasion resistance. It is very
prone to all the evils of aggregate segregation due to its
greater bulk and weight. In addition, a higher water/cement ratio is needed for workability and with its greater
volume, more handling and manipulation are required.
As a result, the two-course slab is now given more consideration for heavy-duty floors. The concrete topping
is usually 3/4 to 1 inch thick which means less concrete
to handle. For the topping, a very low slump (0 to 1 inch)
can be used and at these volumes, bleeding and flotation
are minimal and segregation is nil. Thus, a topping
course can easily achieve compression strengths of 5,000
to 7,000 psi. The advantages may well be worth the additional cost of a two-course construction.
Slab design
All too frequently contractors use a rule-of-thumb figure when planning a slab—for example, 6 inches at
grade with a reinforcement of 40 pounds per 100 square
feet. This might suffice for a small shop or garage, but for
a heavily-used industrial floor, it invites trouble.
Proper slab design is a complicated subject involving
many factors, including soil-bearing capacity, strength
of mix, and the area and position of the static and dynamic loads applied. In practice, however, some of these
factors must be estimated. The overall controlling feature then becomes the heaviest concentrated load which
can be expected on the edges of the slab. With heavy individual pieces such as presses, lathes and similar machinery it may be more desirable to construct separate
foundations set off from the slab itself.
Theoretically, a thin slab on a uniform base will carry
a very heavy evenly distributed, live load. In practice,
howe ve r, the minimum thickness is approximately 5
inches with more thickness added for safety, if necessary,
at nominal construction cost. Definite figures on the expected load per square inch and the required thickness
and strength may be easily worked out from current data available.
The placement of reinforcement has an influence on
the basic durability of the slab and it will do much to prevent cracking. Reinforcement is highly desirable for
heavy-duty floors and the usual practices of setting the
rods and properly placing the concrete over the steel
should be observed.
An example of improperly cured concrete. Without the
presence of moisture, drying out of the cement gel occurs.
This produces a myriad of tiny shrinkage cracks on the
surface and around each piece of fine and coarse
aggregate.
Limitations of concrete floors
Even a properly designed and finished concrete floor
has its limitations. For instance, plain concrete will be affected by alternate cycles of freezing and thawing, even
if salts or chemicals are not present. Acid attack is another limitation with the degree of damage depending
on the specific acid. Certain types and concentrates of
alkalis will also attack concrete.
Although abrasion resistance is in direct relation to
compressive strength most floors will suffer if very hard
substances are continually ground into them. Concrete
is basically a very hard, brittle material but it does have
a tendency to shatter under pinpoint impact. Long exposures to salt can produce spalling.
Two other features that may require further consideration are the static electricity factor and dust-free environments. Raw concrete will generate considerable frictional electricity or sparking when struck by a metallic
object. In gas-laden atmosphere, this could spell disaster. The need for dust-free environments in certain experimental areas may also be a critical problem.
Concrete floor treatments
In order to overcome some of the natural limitations
of a concrete floor, the practicality of a specific surface
treatment to achieve the desired effect of wear resistance
has proven very successful. The value of such treatments
depends, of course, on the specific quality needed and
selecting the correct treatment to gain this feature in a
concrete floor. Naturally, the treatment will not prevent
or overcome the faults of a badly constructed slab. But it
can be an excellent help to the professional concrete
contractor who needs that final finishing touch to assure
a good finishing job.
Concrete surface treatments can perform a variety of
finishing tasks. For example, there is a selected iron aggregate treatment which “hardens” the surface by de-
The monolithic floor slab provides the most popular and
economical concrete wearing surface. However, greater
bulk and weight make it prone to aggregate segregation.
The two-course slab is recommended for heavy-duty floors. A
topping course can easily achieve compression strength of
5,000 to 7,000 psi.
positing soft or ductile iron on the top. This material is
comparatively malleable and it will laminate under impact. As a result of its ductility, the impact stresses are
absorbed by the iron aggregate rather than by the cement matrix. When a non-rusting metallic aggregate is
required, there are specific emery aggregates for this
purpose.
Another plus for the iron aggregate material is that it
will act as a thermal and electrical conductor. Since the
iron has a higher rate of temperature conductivity than
concrete, it will absorb some of the frictional heat when
a metal object strikes it. The iron can also help control
static electricity in hazardous areas. Of course, a certain
amount of special-grade iron must be present and the
floor must be adequately grounded. The iron aggregate
method is not recommended when a wet environment
is expected, such as outdoors.
There is also a floor treatment material that uses selected quartz particles as a major part of its ingredients.
The quartz, in effect, adds a top surface of excellent
hardness to the floor finish, and its abrasion resistance is
very good. Quartz is, by nature, a brittle aggregate, and
therefore it is not recommended where impact loads or
sharp falling objects are expected. Nor should it be applied where frictional heat or sparking are factors. The
quartz application is excellent where specifications call
for a moderate-duty, nonrusting surface.
If a nonslip surface is desired, the concrete can be given a broomed finish or the surface can be embedded
with sharp, coarse nonmetallic aggregate just before
troweling. Silicon carbide and aluminum oxide are usually used for this purpose. The end result is a relatively
smooth surface with particles placed in it to act as traction aids. This type of finish is particularly good for a
wet environment subject to impact abrasion and steelwheeled traffic.
Since imperfections may show up in even the very
best floors, the floor’s abrasion and wear resistance can
often be improved by using a chemically reactive liquid
floor treatment.
When applying a chemical floor treatment, there are
some general rules that must be observed. For instance,
the concrete must be clean and in some cases at least 10
to 14 days old. At this point it is dry enough to permit
the application. By various tables and data, the most effective solution can be determined and applied. The
floor is usually treated in two or three applications.
Another liquid treatment combines the functions of a
hardener and curing compound. By nature it is not as efficient as a pure hardener or a pure curing material used
separately. But it is meant to be used as a very low-cost,
medium-performance combination cure-and-seal compound with the appropriate relationship to cost.
Concrete floor sealers
A floor sealer deposits a surface film onto a concrete
floor much like varnish acts on a wood surface. This film
cover accomplishes several things. It penetrates and
binds porous cement and aggregates. Minor irregularities and surface pores are filled up. Once completed, the
sealer produces a surface that is very easy to clean compared to an untreated floor. And the material acts as a
dustproofer in that it functions as an adhesive binder for
loose powdery concrete.
Unlike the aggregate and the liquid hardeners, the
floor sealer does not become a permanent, physical adjunct to the concrete. But rather it performs as a vital
top coating for the surface. Certain types of floor sealers
are immensely valuable in providing chemical protection. Oil stains, for example, can be readily cleaned and
removed when the floors are properly sealed.
Liquid floor sealers come in a variety of types. Among
this group are epox y- e s t e r, catalyzed epoxy resin,
polyurethane and one made from synthetic rubber
resin. Each functions in a slightly different manner with
at least one special feature associated with each. The
e p ox y-ester type, for example, offers protection from
abrasion, oil and grease besides resisting mild chemical
attack. It is effective against many organic and mineral
acids, except oxidizing acids and ketonic solvents. Detergents or alkali cleaners may be safely used to wash it
down.
The new polyurethane sealer affords the same protection in a one-component product that an epoxy seal-
er provides in two-coat application. Since it is moisture
cured, the polyurethane compound need not be applied
to a dry surface. In fact, the moisture-cured urethane will
dry tack-free in one-half to one hour.
High abrasion resistance, good gloss retention and resistance to stronger chemicals are other advantages
which are making the polyurethanes increasingly attractive to contractors and builders.
The catalyzed epoxy resin type has great durability
and it has a high level of resistance to gasolines and aviation fuels. Protection from acid attack is good, except
with the acetic variety. Both the epoxy-ester and the catalyzed epoxy sealers are available in colors as well as a
transparent coating.
The synthetic rubber resin is essentially a combination cure and seal compound that accomplishes both in
one operation. It is applied while the concrete floor is
still damp and the curing efficiency is reported to be excellent. In this respect, the material also acts as a hardener since the finished floor is given an almost perfect
cure. The great advantage to the combination curing
and sealing compound lies in that it not only perf o rm s
its function, but saves time and expense. It cuts total
construction time as well as clean-up expenses. For jobs
with tight budgets it has proved to be an indispensable
tool for the knowledgeable contractor. Immediate sealing and dustproofing are additional major benefits.
Elementally, there is no one answer to cover all concrete floor requirements. And since inclusion of all the
technicalities involved would prove extensive, this information briefly highlights some of the major considerations in constructing and maintaining good, durable
concrete floors.
* The author is Technical Service Manager, Sonneborn
Building Products, Des Plaines, Illinois.
Iron aggregate treatment hardens the surface by depositing
soft or ductile iron on the top. This malleable material
laminates under impact. As a result of its ductility, impact
stresses are absorbed by the iron aggregate rather than by
the cement matrix.
PUBLICATION #C650114
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