Tip 1: Overbuild your forms
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Diagonal “kicker” plus stake
The angled 2x4 reinforces the side stake that holds the form vertical.
Every builder has a horror story about forms that
bulged or even collapsed under the force of wet concrete.
To avoid a horror story of your own, build
strong forms. Use 1-1/2-in.-thick boards (2x4s, 2x6s,
etc.) except on curves. If you're using 2x4s or 2x6s,
place stakes no more than 3 ft. apart. If the forms
extend belowground, pack soil against them. If they
extend more than 6 in. aboveground, reduce the
spacing between stakes and brace each one with a
second stake and a diagonal “kicker.”
Tip 2: Form curves with hardboard
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Create smooth curves
Hardboard is inexpensive form material and is easy to flex into smooth curves.
Hardboard siding is intended for exterior walls, but it's also great stuff for
forming curves because it's flexible and cheap. A 12-in. x 16-ft. plank costs
about $10 at lumberyards and you can cut it to any width you need.
Because it's so flexible, hardboard needs extra reinforcement to prevent
bulging against the force of the concrete. If the forms are belowground,
place stakes no more than 3 ft. apart and pack soil against them. For aboveground
forms, space stakes 16 in. apart. To form consistent, parallel sides
for a curved sidewalk, build one side first. Then use a “gauge board”—a
1x4 with blocks screwed to it—to position the other side. In wet weather,
hardboard can swell and your perfect curves might become wavy. So if rain
is forecast, be prepared to cover your hardboard forms.
Tip 3: Keep stakes below the form tops
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Screed without obstructions
Drive or cut off stakes below the form top to make screeding easier.
Stakes that project above forms
create a hurdle for your screed
board—and screeding concrete is
hard enough without obstacles.
So before you pour, take five minutes
to cut off any protruding
stakes. If the tops of your forms
are near ground level, make sure
your screed board won't drag
against the ground; you may have
to skim off a little dirt to clear a
path for the board.
Top 10 Concrete Mistakes
1. Ordering just enough. If you're ordering concrete,
it's much better to pay $60 for an extra
half yard of concrete than to come up short. You
don't have to use it all. The driver will haul away
the leftovers.
2. Ignoring the forecast. A little rain can destroy
a freshly poured slab. Beware of hot, dry weather
too. The concrete may set faster than you can
finish it, especially if you're a beginner.
3. Working solo. Line up more help than you
think you'll need. Extra help not only lightens the
workload but avoids situations where the concrete
hardens faster than you can work.
4. Not being ready. A big concrete pour is a rush
job. Don't add stress by waiting until the last
minute to finish forms or gather tools. Have
everything done and all your tools handy long
before the truck pulls up.
5. Using wimpy wheelbarrows. A heavy load of
concrete can crush a garden wheelbarrow. Use
heavy-duty models only, even if you have to rent
them ($15 per day).
6. Relying on fiber. The tiny fibers added to
some concrete mixes may reduce surface cracking,
but fiber is no substitute for metal rebar or
wire mesh.
7. Finishing with extra water. It's tempting to
sprinkle a little water on the surface while you're
troweling to help you get a smooth finish. But it
weakens the surface and will lead to flaking
later.
8. Getting burned. Some people can tolerate
hours of skin contact with concrete. Others end
up in the emergency room with severe burns.
Don't risk it: Wear gloves and long pants, and
wash concrete off skin immediately.
9. Tearing off forms too soon. It's easy to break
off concrete edges while removing forms. So let
the concrete harden for at least two days first.
10. Forgetting your autograph. Be sure to
scratch your initials in the concrete before
it hardens.
Tip 4: Put down a solid base
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Pack the base
A plate compactor is a heavy vibrating machine that solidly packs a soil or gravel base.
A firm, well-drained base
is the key to crack-free
concrete. The best plan
for a solid base usually
includes compacted soil
followed by several inches
of a base material such
as gravel. But the best
base depends on climate
and soil conditions. So
talk to a local building
inspector who's familiar
with conditions in your
neighborhood. Packing
the soil with a rented
plate compactor is always a good
idea, but you may be able
to skip the gravel altogether
if you have sandy
soil.
Tip 5: Plunge out the bubbles
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Pack the concrete
Packing the concrete forces out air bubbles and voids that weaken a slab.
When you pour concrete, air pockets
get trapped against forms, leaving
voids in vertical surfaces. That usually
doesn't matter on sidewalks or driveways.
But aboveground, on stairs,
curbs or walls, the results can look like
Swiss cheese. To prevent that, just grab
a 2x4 and “plunge” all along the forms.
Then go all along the forms with a
hammer, tapping the sides.
Tip 6: Avoid too much water
When you have concrete delivered, the
first words out of the driver's mouth may
be “Should I add some water?” Unless
the concrete is too dry to flow down the
chute, your answer should be “No,
thanks.” The right amount of water is
carefully measured at the plant. Extra
water weakens the mix. More water
makes it easier to work with right away,
but will lead to a weaker slab.
If you're mixing your own concrete,
do this test: Plow a groove in a mound of
concrete with a shovel or hoe. The
groove should be fairly smooth and hold
its shape. If it's rough and chunky, add a
smidgen of water. If it caves in, add more
dry concrete.
Tip 7: Don't delay floating
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Bull float in action
Use a bull float to flatten large areas immediately after placing the concrete.
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Close-up of mag float
Use a magnesium or wooden float to flatten smaller areas.
After screeding concrete, the next step is to “float” it. Floating
forces the stones in the mix down and pulls the cement
“cream” to the surface so you can trowel or broom the surface
later without snagging chunks of gravel. If you wait too long
and the concrete begins to stiffen, drawing the cream up is difficult
or impossible. So the time to float is right after screeding.
On a long sidewalk or driveway, it's best to have a helper who
can start floating even before all the screeding is done. There is
one reason to delay floating, though: If puddles of water form
on the surface after screeding, wait for them to disappear before
floating.
On small projects, you can use a hand float made from wood or magnesium (both at home centers). The “mag” float glides easier for less
arm strain. But for bigger projects like driveways or patios,
don't mess around with a hand
float. Instead, rent a
bull float. The long
handle extends
your reach and
makes the job easier, while
the broad head covers the surface
quickly and flattens out any
bulges or depressions. To make the surface
as flat as possible, float it in both directions.
Recipe for a Lasting Driveway
Too many builders—pros and DIYers
alike—treat a driveway just like a
patio or sidewalk. But because they
support vehicles, driveways deserve
some extra effort and expense.
Whether you're hiring a contractor or
doing it yourself, here are three suggestions
for a stronger slab:
Thickness: Most driveways are 4 in.
thick, but consider a 5-in. slab. That
extra inch of concrete increases the
strength of the slab by about 50 percent
but increases the cost of a typical
driveway by only $200 to $300.
Reinforcement: There are two ways
to reinforce a concrete driveway: with
rods of rebar or with wire mesh. The
purpose of reinforcement is to
reduce cracking and to hold the slab
together if it does crack. Rebar does
both jobs better and costs only a few
bucks more for a typical driveway.
Base: What goes under your driveway
is just as important as the driveway
itself. The best base varies
according to climate and soil conditions.
It may be a 16-in. layer of special
base material or 6 in. of compacted
gravel. To get advice, talk to a
building inspector who's familiar with
soil conditions in your area. If you're
hiring a contractor to do the job, be
sure the bids describe the base in
detail. And don't choose a low bid
that skimps on the base work.
Tip 8: Cut deep control joints
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Cutting a control joint
Hand cut control joints with a special groover and straightedge while the concrete is still soft.
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Close-up of groover
A groover has a blade (typically 1-in.) on the underside that cuts the groove.
The grooves in concrete are called “control joints” because they control cracking.
Concrete shrinks as it dries, so cracks have to happen somewhere. Control joints
create straight breaks rather than an ugly spiderweb pattern. They also limit
cracks that form later. On a sidewalk, space joints 5 ft. apart or less; on a slab or
driveway, no more than 10 ft. apart.
There are two ways to make control joints: Plow them
in the wet concrete right after floating or cut them
the following day with a saw. You can buy a diamond
blade for your circular saw at home centers. Creating
joints with a “groover” in wet concrete is less work and
less mess. Joints should be at least one-fourth the depth of the
concrete. A groover that cuts 1-in.-deep grooves costs about $25. If you have an inexpensive version ($7) that doesn’t cut deep enough, use it to create
the initial joint, then deepen the cut with a stiff putty knife.
Tip 9: Finish with a broom
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Broom the finish
A stiff broom creates a slightly roughened concrete surface.
A smooth, steel trowel finish is too
slippery for outdoor concrete. Instead,
drag a broom over the concrete. You'll
get a nonslip texture and hide imperfections
left by floating or troweling.
You can use a plain old push broom,
but a special concrete broom cuts finer
lines. The sooner you start, the rougher
the finish. Make your first pass about
15 minutes after floating. If the texture
is too rough, smooth it over with a mag
float and try again in 15 minutes. Drag
the broom over the concrete in parallel,
slightly overlapping strokes. You
may have to rinse off the broom occasionally
to avoid a too-rough finish.
Tip 10: Slow the curing
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Cover with plastic
Plastic sheeting keeps the moisture in the concrete and aids curing.
Water is essential for the chemical
process that makes concrete harden—the longer concrete stays damp,
the harder and stronger it gets. One
way to slow down drying is to cover
concrete with 4-mil plastic sheeting.
When concrete is hard enough
so you can't make an impression
with your finger, gently spread the
plastic. Stretch it out to eliminate
wrinkles and weigh down the edges
to seal in moisture. When you see
signs of drying, lift the plastic and
gently sprinkle on more water. Keep
a sidewalk or patio damp for three
days. Seven days is best for a driveway.
Plastic can cause mottled coloring
on concrete, but the splotches
disappear in a month or two.
Pros often skip the plastic and
spray on a waxy liquid “curing
compound” to slow down evaporation.
Though not as effective as
plastic, curing compounds are easy
to apply with a garden sprayer.
Curing compounds are available at
home centers.
