by Gabe Martel
Our industry prides itself on quality, both in materials and
workmanship. I doubt if any tradesman wakes up in the morning
and says to himself, "Today I think I'll include some defects
in my work." So why do we accept uncontrolled cracks in
concrete slabs as unavoidable? It's a result that no one wants
and no one wants to explain, especially to an angry client.
We even have a cliche we use to excuse the phenomenon:
"Concrete does two things — it gets hard, and it cracks."
But I disagree with that claim. In fact, as a commercial
construction superintendent, I have poured 15,000-square-foot
slabs containing 260 cubic yards of concrete without a single
crack. And I do nothing different than what thousands of other
commercial contractors do every day.
Residential concrete doesn't usually need to meet the same
quality requirements as most commercial concrete, but
nevertheless, you can eliminate uncontrolled cracking in your
slabs using the techniques and principles I will describe. If
you're pouring your own slabs, this information is essential.
If you are a general contractor or builder, you can use this
information to more effectively supervise and assist your
subcontractor. True, the concrete work may legally be within
the subcontractor's scope of work, but it's your project, and
there's precious little to be done if the slab comes out
poorly. Better to take care of problems before they happen.
If I had to summarize the secret to good concrete work in
one word, it would be "consistency." For a successful outcome,
you must exercise the same level of care at every stage of the
process — design, preparing the subgrade, placing,
finishing, and curing.
Prepping the Base
If the subgrade settles after you've poured the slab, you
increase the likelihood of a crack. It seems obvious, but this
is the cause of far too many problems. Several things can
contribute to subgrade settlement, but the two prime offenders
are soils with different bearing capacity across the area of
the slab and improper compaction of the subgrade.
Always make sure the soil is free of topsoil and organic
matter. If you have any doubt about the bearing capacity of the
soil you're building on, check with the local building
department or other area builders. If you're still not sure,
get the soil tested. The few hundred dollars you'll pay for
soil analysis and a compaction test is nothing compared to
going back and fixing a failed foundation.
As the subgrade elevation is being established, make sure
that the soil is uniformly and properly compacted (see
Basics," 3/94). Compaction will be expressed as a
percentage at optimal moisture content (usually 95% or
greater), and it's worth verifying. If you decide not to do a
compaction test, at least run the plate tamper or jumping jack
until there is very little impression left with each successive
pass. It is imperative that soil be compacted in lifts, ideally
about 6 inches at a time and never more than 10 inches. If you
attempt to compact greater depths, the surface will look good
but the soil underneath will remain uncompacted and will settle
Also, check for uniform bearing capacity of the subgrade.
Has the excavator tossed a few buckets of sand into one corner
because he excavated just a little too deep the first time
around? Will one corner of the slab rest on rock while the
remainder is over sand? Better to correct that now by replacing
questionable fill with suitable soils that will behave like the
rest of the subgrade when properly compacted. What constitutes
a suitable soil will vary with local conditions. If you are in
doubt, get a professional opinion.
Optimal moisture content happens in a narrowly
defined range, so while you may need to add water to dry soils
for compaction, take care not to add too much. Sometimes, due
to unpredictable weather conditions and routine difficulties
with grading and drainage, residential sites get too wet before
compaction can take place. What constitutes too wet will vary
with soil types, but it doesn't have to be muddy to be too wet.
If you see evidence of pumping action in the soil or
near-liquefaction of the soil in the vicinity of compaction
equipment, you must stop and correct this condition. Your
choices are limited at this point: Either wait for the soil to
dry out or over-excavate and replace it.
Flat on the bottom. It's helpful to picture
your slab upside down. If it's almost as smooth on the bottom
as it is on the top, the battle is half won. If it looks like a
crater on the moon, then your crack has already begun. Soil
must be graded to the same level across the full width of the
slab (see Figure 1).
A properly prepared subgrade should be
uniformly compacted across the full area of the slab, leveled
to within 1/2 inch.
Your subgrade should be level within 1/2 inch (that's a 12.5%
variation in your slab thickness on a 4-inch slab). Where
subgrade elevation is inconsistent, then the slab will vary in
thickness. This variation will cause it to cure unevenly, which
will stress the concrete, increasing the likelihood of a
Laying Out Contraction
The shape of the slab can greatly affect cracking. An inside
corner, for example, is a pressure point and, as far as the
concrete is concerned, a great place to crack. Try to divide
your slab into symmetrical squares using contraction joints
(also called control joints) rather than creating irregular
shapes with lots of re-entrant corners. Contraction joints can
be hand-tooled right after the bleed water leaves the surface,
cut with a saw right after finishing, or formed in the wet
concrete using one of the proprietary plastic or metal joint
materials (Figure 2).
2. Contraction joints control the location of shrinkage
cracks. They can be hand-tooled, cut with a saw, or formed with
proprietary joint material.
No matter how you make them, all contraction joints work the
same way. By creating an intentionally weakened point in the
slab, stress from the inevitable shrinkage (as a result of
drying or temperature changes) is relieved in a predictable,
A contraction joint should be one-fourth the depth of the
slab, or 1 inch in a 4-inch slab. Any less, and it may not
function as designed; any more, and you will unnecessarily
weaken the slab, risking vertical displacement of the concrete
if the subgrade should move.
When laying out a slab, keep in mind that squares are better
than rectangles. If you must use a rectangle, make sure the
long side is not more than 1 1/2 times the short side. Where it
isn't possible to avoid closely spaced re-entrant corners,
additional steel reinforcing may help control cracks.
When laying out contraction joints on a typical 4-inch
residential slab, 8- to 12-foot spacing is reasonable. Under
ideal conditions, spacing up to about 15 feet between joints
may be possible.