Quality Concrete Slabs
A checklist to help avoid cracks and maximize life
By Joe Mehaffey
Many
people will tell you that there are two kinds of concrete. Concrete
that WILL crack and Concrete that HAS ALREADY cracked. Yet, reality is
far more complex. Properly mixed and installed concrete will normally
have FEW cracks and a long life while improperly installed and/or mixed
concrete will have many cracks and a short life. This checklist will
help you get the former.
There are many different mixes of
concrete and many methods of installing concrete. The Portland Cement
Association (PCA) is the concrete industry’s standards group. The PCA
has published large numbers of books and guides describing in detail
the proper methods of producing and installing concrete for virtually
all applications. These publications are recommended reading.
Many
work crews are not aware of the subtleties of the art and are happy
with less than what is achievable by just following industry standard
procedures. This checklist will help you get the quality concrete slab
floor you are looking for. A good motto to remember is: A
well INSPECTED job is necessary to get the job you EXPECT.
Note:
The general methods described here may have to be adjusted to suit
local requirements and conditions. However, in the majority of cases,
these construction methods, with proper attention to detail, will
achieve quality slabs in residential slab applications.
Slab Floors
Concrete
slabs are used in basements, garages, porches, floors, decks,
sidewalks, and driveways in homes and buildings all over America. We
are all used to seeing cracks, large and small in household concrete.
Some concrete installers would have us believe that whatever cracking
occurs in concrete is uncontrollable and nothing can be done about it
and DEFINITELY it is not their fault! It is true that cracks may occur
in large concrete slabs no matter what you do. It is also very true
that if you do it wrong, you may have 20 to even 100+ cracks in 1000
square feet and if you do it exactly RIGHT you will have perhaps 1
crack or less per thousand square feet. AND.. If you do it right, the
cracks will be small, limited in running distance and hopefully will
not pull apart.
How to minimize cracking
Like
many things, the easy, quick, and cheap way of making concrete slabs is
NOT going to produce high quality. Many of the methods to be described
add little or nothing to the cost of a slab. However, some of the
improved performance options suggested will add moderate cost above a
“normal” slab.
Much
household concrete is poured without expansion joints. For most
such applications, this is a good compromise. Even properly installed
concrete may crack every 15 to 30 feet with a shrinkage crack. In
properly poured concrete, the shrinkage crack thus formed will be small
and will generally be less noticeable than a sawed or otherwise
installed expansion joint. The only drawback of a “natural” expansion
joint is that it will typically meander across a floor in an
unpredictable manner The following items are ESSENTIAL to a
good concrete slab job and a purchaser should insist that they are
“standard”.
Prepare a firm soil base
A
major quality item to watch for is a well packed base. If the soil
under the slab is loose dirt or just not well packed, the concrete may
eventually crack as the soil under it shifts. Be sure to machine tamp
or roll the subsurface. Just running a rubber tired Bobcat or similar
tractor over the surface IS NOT enough, yet it is done every day.
Prepare an aggregate base
After
the base is prepared, a 4 inch (minimum) layer of aggregate (gravel)
should be placed and tamped. The aggregate base has as its principal
function to distribute the load uniformly over the soil surface. It
also lifts the vapor barrier above the soil surface and thus helps keep
moisture from getting into the basement through the floor. Under no
circumstances should an interior slab floor be poured directly on earth
without an aggregate layer.
Put a Vapor Barrier on top of the aggregate
Most
people abhor a damp basement. Some people may save a few dollars and
omit a vapor barrier under the slab. This is a mistake on two fronts.
First, it will result in additional dampness being let into the
basement through the slab. Second, the absence of a vapor barrier will
let moisture escape from the concrete during curing and allow the
concrete to set faster than it would with the bather. The standard
vapor bather is 6 mu Polyethylene. See~figure la, and lb.) It is tough
and resists tearing but it can be damaged by walking on it during
construction operations. A tougher but more expensive material such as
Grifflon has a nylon mesh sandwiched between two layers of
polyethylene. This material cannot be torn by ordinary working and
resists punctures better than 6 mil polyethylene.
The
vapor bather should ALWAYS cover the exposed footing and run up the
wall to prevent bonding of the floor to the wall. In addition, there
should be a layer of sand or gravel on top of the footing and under the
vapor barrier film. See figure Ia and lb. Any excess vapor barrier
protruding above the edge of the slab after pouring can be left or
trimmed away as desired.
Put aggregate or sand on top of the lip of the footing.
Standard
practice is to put about 4 inches of aggregate (gravel) under the
concrete and on top of the footing lip. However, some crews may dig
down 4 inches below the footing lip, and put in four inches of
aggregate to bring the aggregate level back up to even with the footing
lip. Then they pour the slab right on top of the edge of the footing.
This is NOT a good idea! One of the MAJOR ‘causes of
cracking in slab floors is bonding of the slab around the edges to the
footing. Normal 4 inch slump concrete shrinks about 1/16 inch per 10
feet of distance as it hardens. (Wetter concrete shrinks lots more,
dryer shrinks less.) If the slab is firmly bonded to the footing
or to the wall at the edges, it MUST crack when it shrinks as the
footing is unlikely to move! A proper slab will FLOAT and be
independent and isolated from the footing and ALL walls of the
basement. Don't let anyone tell you that a good slab is bonded to the
footing and/or walls though this is a common misconception!
The
easiest way to be sure that the slab is floating and not attached to
the footing or walls (see figure la below) is to a) use an expansion
joint connection at all walls, and b) use sand or gravel as an
isolating bearing surface on top of the footing with a vapor barrier on
top of the sand or gravel. The sand may be as little as 1/4 inch thick,
and if gravel is used, 2 to 4 inches above the footing lip is
recommended. (Use 4 inches of gravel total over the main slab area.)
Many
experts suggest using a designed for purpose 1/4 or 1/2 inch expansion
joint material between the slab and the wall all around the slab.
Others suggest just running the 6 mil vapor barrier film up the wall to
prevent the floor from bonding to the wall. If the slab is poured in
hot weather, either method should work. If the slab is poured in cold
weather, it conceivably could expand in hot weather and buckle. On
balance, the 35 cents per linear foot cost for the half inch expansion
joint material is a good buy. Many termite treatment firms require that
the floor to wall joint be sealed with an asphalt or silicone sealant
to prevent the possibility of insect intrusion. This is essential if
the joint is hidden behind a wall.
The
main point is to ISOLATE the floor from the walls and the footing. the
slab should float and be able to expand and contract without pulling or
pushing on any other structure.
Steel Reinforcement
A
heavy wire mesh is normally used to strengthen and reinforce concrete
in residential applications. Half inch Rebar on 18" to 24" centers is
stronger, but more expensive. Many experts suggest that the wire
mesh or rebar reinforcement has little effect on whether or not
concrete will crack. However,there is general agreement that the
reinforcement will limit the size of cracks if they start. Just
as important, the reinforcement will limit the vertical movement
of the two parts of the broken slab. Vertical displacement of a cracked
slab is perhaps the most objectionable kind of crack. Make sure to have
your concrete slab installer support the wire mesh or rebar off the
vapor barrier with a physical means such as the special wire supports
made for the purpose. The optimum position for the wire mesh or other
reinforcement material is about 1/3 of the way up from the bottom of
the concrete slab. Many crews will attempt to pull the
wire or rebar up off the vapor barrier into the concrete with
rakes or similar tools, but this is a haphazard technique at best and
cannot insure uniformity of location of the reinforcement within
the concrete slab. This “lift with a rake” procedure can also puncture
the vapor barrier.
In
heavy duty applications, such as garages and driveways, where
heavy loads are anticipated, half inch or larger steel reinforcing bar
is often used to strengthen concrete. This is usually not needed
in residential applications. However, installing a few reinforcing bars
at all corners of a slab, but especially interior corners (see figure 2
below) where concrete is most likely to have cracks to start is an
excellent technique. These rebars are in addition to the normal
heavy wire mesh.
The Concrete
The
concrete mix can make or break a slab floor. If the concrete is too
stiff, it is hard to work into a smooth flat surface. If the mix is too
wet, it is easy to work, but it shrinks a great deal more when it dries
and cracks much more extensively. A good compromise is 3500 to 4000psi,
maximum 3 inch slump mix. Note: The Portland Cement Association
recommends a slump of from 1 to 3 inches for slabs. They allow UP TO A
MAXIMUM SLUMP OF 4 inches for handworking IF REQUIRED. A three inch
slump mix is fairly easy to work and yet normally will have a
reasonable number of cracks. Keep concrete moist while it is setting up
if possible. It is a good idea to use a mist spray, moist burlap, or a
plastic film over the slab after initial setup and finishing to assure
minimum shrinkage and increased strength. For exterior slabs, air
entrained concrete is recommended as a) it is less inclined to crack in
cold/hot temperature swings and b) the air entrainment makes the
concrete easier for workmen to push around so they are less inclined to
water it down.
Note:
Concrete mixtures can vary widely and still produce quality concrete.
However, as a general rule, the less water added to concrete, the
stronger it will be and the fewer cracks you will have in the finished
product. Each 1% increase in water content increases concrete shrinkage
by 2%. Thus, it is extremely important to use an absolute minimum of
water (lowest slump) in the concrete mixture. For slab floors, concrete
slump more than 4 inches maximum should be avoided to minimize
shrinkage and consequent cracking of the slab. Coarse aggregate content
should be maximized in the mix as this will also minimize shrinkage and
cracking. Generally, the water/cement ratio by weight for 3000psi
concrete is 0.58 or lower for non air-entrained concrete.
For 3500psi concrete, the ratio would be .51 or lower.
Concrete should NOT be poured during freezing weather unless
special precautions are taken.
If
possible, have an independent expert watch the pour to insure that good
quality concrete is poured and that the crew adds NO water to the
specified mix which would bring the slump above
4 ABSOLUTE MAXIMUM. For best results, insure that you get nominal 3
inch slump concrete OUT OF THE HOPPER of the concrete truck. If there
is a question about the slump of the mix in the concrete truck, ask the
concrete provider for a slump test. Many concrete providers are
prepared to perform a slump test on the site, if requested in advance.
Note: Some concrete installers may try to make work easy for themselves
by adding too much water to the mix or by adding water to the surface
of the slab during finishing. Don’t do it! If you see loose water on
top of the mix as it is being poured or loose water on the vapor
barrier film, the mix is too wet and will result in excessive cracking
in the slab. If water does appear on top of the slab, do not continue
to finish without first removing any pooled water.
Warning:
One of the main causes of surface defects in concrete slab surfaces is
finishing the surface with bleed water on the surface of the concrete.
No finishing operation should be performed on any concrete surface
while water is present on the surface as this will cause serious
crazing, dusting or scaling of the surface. (Quoted from Portland
Cement Association Handbook)
Concrete mix EXTRAS
There are a number of additives to concrete that you should know about. Some are good and some are bad depending on conditions.
1)
Calcium is an additive used to accelerate the set up of concrete. It is
useful in cold weather to hasten set up if a freeze is expected.
Otherwise, you should avoid using it as the faster concrete sets up,
the more prone it is to cracking and crazing and other surface defects
as it ages.. Note: The addition of calcium to
concrete increases the apparent slump and care should be used to
maintain the recommended 3 inch slump in the delivered product. Also,
calcium additive will cause discoloration and non uniform appearance of
the finished concrete surface.
Generally
try to avoid Calcium or other “quick set” additives. Many slab crews
are in a hurry and want the concrete to set up as fast as possible.
This desire is the opposite of what you as a purchaser want. Concrete
that sets up fast shrinks more and over a shorter period of time. This
results in a much higher probability of crack formation.
2)
Fiber Fill is an additive (often glass or plastic fibers) designed to
add tensile strength to the concrete. Concrete has very little strength
in tension and the fiber improves the situation substantially. If
concrete does crack, the fiber fill tends to keep the crack from
enlarging. The extra cost for fiber fill concrete is perhaps $6 a cubic
yard and it adds nothing to the installation effort. It will result in
more durable concrete for most slab applications. Note: The finished
fiber filled concrete will have “hair” protruding from the surface
which will wear away with traffic or it can be easily cleared with a
buffing or sanding machine once the concrete is dry..
3)
Air Entrained concrete is a special mixture for use outdoors in areas
subject to freeze/thaw cycles. The air entrainment additives trap
millions of microscopic air bubbles in the mixture. These air bubbles
enable the concrete to survive freeze/thaw cycles much better than
ordinary concrete. Portland Concrete Association (PCA) outdoor samples
of air entrained concrete in Wisconsin showed little deterioration
after 20 years of freeze/thaw whereas the surface of normal concrete
was significantly deteriorated.
4)
Expansion Joints (if used) should generally be installed in a
symmetrical pattern. As a rule of thumb, joints are installed at
intervals of about 3 ft per inch of slab thickness. Thus 12 ft. for a 4
inch slab. I personally like a sawed control joint, but
others prefer the rounded pressed-in-with-a-tool loints. Either
is quite OK.
Slab Flatness
It
is unfortunate but true that many (most?) building codes do not have a
flatness requirement for household concrete. Many slab purchasers (even
builders) do not specify any measure of concrete flatness and take what
they get as long as the customer does not complain. Here are a few
rules of thumb.
1)
About the very best that can be achieved in flatness over large areas
using ordinary techniques is +/- 1/8 inch about a nominal. This is an
expensive slab and not achievable by most slab crews.
2)
Arbitration awards have been given homeowners when the slab elevation
changed by more - than 1/4 inch in an 8 foot distance. Many homeowners
accept variations considerably larger than this and are not even aware
of it.
3)
A standard specification for slab flatness in quality residential
applications is +/- 1/4 inch about a nominal. Over a large area, most
slab crews can maintain flatness to better than +/- 1/4 inch with just
a little extra care. The variation over any distance of 8 feet on the
slab should be less than 3/16 inch. When complete, there should be no
pools of water at any point on the slab that are more than 1/4 inch
deep. Any standing water should drain toward an outside portal or a
floor drain as appropriate.
4)
If you are looking for the cheapest slab, specify slab flatness at +/-
3/8 of an inch about a nominal. ANY slab crew should agree to a slab
with maximum flatness variation of +/- 3/8 of an inch over the entire
slab area. If your crew will not agree to this MAXIMUM, you should look
for another crew.
Note:
Many slab crews use metal or wood spikes in the gravel to aid in
maintaining a level surface. Once the markers are covered with
concrete, the crew must try to visually maintain flatness. This method
will result in considerably larger variations in elevation than if a
movable string or other above slab reference means is used. It is
important to discuss slab flatness with your potential slab installer
and specify a WRITTEN flatness requirement in your specification. If
you fail to ask this question and expect a flat slab, you may be in for
a surprise.
Surface Sealer
Finished
concrete has a surface which tends to turn to dust as it wears. A
concrete sealer is a material which is applied to the surface of the
concrete about 6 to 10 hours after pouring (and sometimes much later).
This sealer, if installed a few hours after the concrete surface has
set up can hold moisture inside the concrete and aid in curing the
concrete. The sealer will also bind the surface of the concrete and
produce a smoother surface which will not produce dust.For most home
applications, a sealer is a good investment. There are after cure
sealers that are applied as paint.’ The proper selection of a sealer is
dependent on the application and you would be well advised to consult a
building supply outlet or concrete materials supplier for options.
Drainage
The
soil subsurface under a slab floor should be sited and constructed to
freely drain and not collect water. A foundation drainage pipe system
should be laid outside the foundation to prevent moisture rising above
the lower aggregate level under vapor barrier. See figures la and lb.
The TOP level of the drain tube should not be higher than the BOTTOM of
the concrete slab surface to prevent moisture rising to the level of
the concrete slab. Four inches of aggregate is usually provided under a
slab to provide a capillary break between the subsurface (ground) and
the concrete. (See figures la and 1b). Caution: If the slab lays on top
of the footing without the four inches of aggregate on top of the
footing, then the drain MUST be put beside the footing and NOT on top
of the footing. The general rule is to MAKE SURE that the bottom of the
concrete slab is HIGHER than the TOP of the foundation drain pipe. If
the drain pipe is installed as shown in figure (1c), the installer is asking for a damp basement.
About
four inches (minimum 2 inches) of aggregate should be under the drain
pipe and perhaps 12 inches (minimum, 6 inches)of aggregate above the
drain pipe. This will facilitate long term drainage and help prevent
dirt clogging of the drain pipe over the years. The width of the gravel
about the drain pipe should be a minimum of 8 inches
.
Figures 1 and 2 above
Summary
Concrete
slab and wall design and installation is a mature science. Many volumes
have been written covering the manufacture of concrete and the
installation thereof. This short document can but scratch the surface
of the technology. For further information on specific applications or
problems, consult your library or the Portland Cement Association,
Skokie, Illinois USA. Much
of the information from which this document was derived can be found in
the following publications. Additional details and references may be
found in these publications as well.
References:
Design and Control of Concrete Mixtures, Kosmatka and Panarese, Portland Cement Association publication, Skokie, Ill.
Contracting Your Home, McGuerty & Lester, Betterway Books, Cincinnati, Ohio 45207 phone 800-289-0963