Renovation 4th Edition
Page 60
2.32
3.09
3.86
4.63
5.40
6.17
7.00
7.72
8.49
9.26
600
1.85
2.78
3.70
4.63
5.56
6.48
7.41
8.33
9.26
10.19
11.11
700
2.16
3.24
4.32
5.40
6.48
7.56
8.64
9.72
10.80
11.88
12.96
800
2.47
3.70
4.94
6.20
7.41
8.64
9.88
11.11
12.35
13.58
14.82
900
2.78
4.17
5.56
6.95
8.33
9.72
11.11
12.50
13.89
15.28
16.67
1,000
3.09
4.63
6.17
7.72
9.26
10.80
12.35
13.89
15.43
16.98
18.52
* This table can be used to estimate the cubic content of slabs larger than those shown. To find the cubic content of a slab measuring 1,000 sq. ft. and 8 in. thick, add the figures given for thicknesses of 6 in. and 2 in. for 1,000 sq. ft.
† Courtesy of Bon Tool Company, © 2003, from statistical Booklet: Contractors, tradesmen, apprentices (see also www.bontool.com).
slabs consist of 4 in. of concrete poured over 4 in.
Concrete Work
of crushed rock, with a plastic moisture barrier
Concrete is a mixture of portland cement, water,
between. In addition, garage floors are often rein-
and aggregate (sand and gravel). When water is
forced with steel mesh or rebar to support greater
added to cement, a chemical reaction, called
loads and forestall cracking.
hydration, takes place, and the mixture hardens
To improve the building’s energy profile,
around the aggregate, binding it fast. Water makes www.GreenBuildingAdvisor.com advises installing
concrete workable, and cement makes it strong.
rigid insulation under slabs. Details will vary
The lower the water to cement ratio (w/c), the
regionally, but extruded polystyrene (XPS)
stronger the concrete.
panels 2 in. thick are typically recommended.
XPS panels are durable, waterproof, and rated
pouring a ConCrete slaB
R-5 per inch. But using more than 2 in. isn’t
Pouring (or placing) a concrete slab is pretty
recommended because too much insulation
much the same procedure, whether for patios,
under a slab will isolate it from the earth’s cool-
driveways, basements, or garage floors. Most
ing influence during summer months.
Foundations and Concrete
253
prep steps
As with any concrete work, get plenty of help.
Concrete weighs about 2 tons per cubic yard, so
if your slab requires 10 cu. yd., you’ll need to
move and smooth 40,000 lb. of concrete before it
sets into a monolithic mass. Time is of the
essence, so make sure all the prep work is done
before the truck arrives: Tamp the crushed stone,
spread the plastic barrier (minimum of 6 mil),
and elevate the steel reinforcement (if any) on
dobie blocks or wire high chairs so it will ride in
the middle of the poured slab. Finally, snap level
chalklines on the basement walls or concrete
forms to indicate the final height of the slab—
you’ll screed to that level.
To pour concrete with a minimum of wasted
energy, use a 2-in. (interior diameter) concrete-
pump hose. A hose of that diameter is much
lighter to move around than a 3-in. hose. Another
This 2-in. (interior diameter)
concrete-pump hose is easier to
handle than a 3-in. hose. But its
smaller diameter requires smaller
3⁄8-in. aggregate in the mix.
Although a 2-in. hose is much
lighter than a 3 in., tons of
Insulating a Shallow
Foundation
concrete pass through it—so
you’ll need helpers to support
zzzzzz insulating a shallow Foundation
the hose and move it to the
pouring points.
Continuous insulation under
the slab helps reduce heat
loss through floor.
Metal flashing
with drip leg
Protective cover over
above-grade foam
XPS or EPS rigid foam
Minimum 12 in.
(R-value needed depends on
below grade
local air-freezing index)
Compacted layer
6-mil plastic sheeting
of crushed stone
under slab
254 Chapter 10
advantage: It disgorges less concrete at a time,
allowing you to control the thickness of the pour
ordering Concrete: Be specific
better. And a 2-in. hose gives easier access to dis-
tant or confined locations. Important: As you
Concrete has so many different uses (such as floors, foundations, and counter-
place concrete around the perimeter of the slab,
tops) and so many admixtures (water reducers, retardants, accelerants, air entrain-
be careful not to cover up the chalklines you
ers, and so on) that the best way to get the mix you need is to specify its use and
snapped to mark the slab height. And as you
desired characteristics. that is, when ordering the mix, tell the supplier the quantity
place concrete in the slab footings, drive out the
you need (in cubic yards), how the concrete will be used (driveway, foundation,
air pockets by using a concrete vibrator.
patio slab), the loads it will bear, how far it must be pumped, how it will be fin-
establishing screed levels. If the slab is only
ished, and other such details.
10 ft. or 12 ft. wide, you can level the concrete by
if you’re pouring a slab that will have a smooth finish, you might specify “a
pulling a screed rail across the top of formboards.
2,500-psi mix but a true five-sack mix,” which will be “creamy” enough to finish
Otherwise, create wet screeds (leveled columns
with a steel trowel. if you specify a 2,500-psi mix but don’t describe the finish, the
of wet concrete) around the perimeter of the
slab and one in the middle of the slab to guide
supplier might use four sacks of cement and a water reducer to attain that strength.
the screed rails. The wet screeds around the
However, with less cement in it, the mix would be sandier and more difficult to finish.
perimeter are the same height as the chalklines;
Where the concrete will be placed can also
affect the mix. For example, concrete
pump concrete near those lines and level it with
for a second-story patio far from the street may require a smaller, 2-in. (inner diam-
a trowel. This technique is very much like that
eter) hose to pump it, so the supplier may specify smaller aggregate (3⁄8 in. versus
used to level tile mortar beds, as explained in
3⁄4 in.) to facilitate flow. aggregate size, in turn, affects load-bearing capacity, so a
chapter 16.
mix with 3⁄8-in. gravel is often bumped up, say, to 3,000 psi. if the patio slab will
The wet screed(s) in the middle should be
also be steel troweled, the mix thus becomes “a 3,000-psi, 3⁄8-in. aggregate, true
more or less parallel to the long dimension of the
six-sack mix.” Well, you get the point: Be specific.
slab. There are several ways to establish its
estimating the amount of concrete in cubic yards is straightforward: the calcula-
height, but the quickest way is to drive 18-in.
tion is width length depth (in feet) of the area you want to cover. you then
lengths of rebar into the ground every 6 ft. or so,
divide that result by 27 (because there are 27 cu. ft. in 1 cu. yd.). if you’re pouring
and then use a laser level or taut strings out from
a slab, see “Cubic yards of Concrete in slabs of various thicknesses” on p. 253 for
perimeter chalklines to establish the height of the
calculations of cubic yards based on the slab thickness (in inches).
rebar. In other words, the top of the rebar becomes
most concrete-mixer trucks can hold 9 cu. yd. to 11 cu. yd. so if your pour
the top of the middle wet screed. When you’ve
troweled that wet screed level, hammer the rebar
requires more than one truck, ask the supplier to time deliveries 90 minutes apart so
below the surface, and fill the holes later.
you have enough time to deal with each delivery. Finally, don’t shave the estimate
too close; far better to have too much concrete than too little.
sCreeding and Floating
Screeding is usually a three-person operation: two
to move the screed rail back and forth, striking
off the excess concrete, and a third person behind
them, constantly in motion, using a stiff rake or a
square-nose shovel to scrape down high spots or
to add concrete to low ones. You can use a mag-
nesium screed rail or a straight 2x4 to strike off,
but the key to success is the raker’s maintaining a
good level of concrete behind the screeders, so
the screed rail can just skim the crest of the con-
crete without getting hung up or bowed by trying
to move too much material.
Screeding levels the concrete but leaves a fairly
rough surface, which is then smoothed out with
a magnesium bull float, a long-handled float that
also brings up the concrete’s cream (a watery
cement paste) and pushes down any gravel that’s
near the surface. This creates a smooth, stone-free
surface that can be troweled and compacted later.
A bull float should float lightly on the surface.
As you push it across the concrete, lower the
Because of the loads it will bear, this garage floor slab is reinforced with rebar 12 in. on center.
handle, thereby raising the far edge of the float.
The stepped forms running along the sides of the slab will create foundation walls roughly 1 ft.
Then, as you pull the float back toward you, raise above grade.
Foundations and Concrete
255
the handle, raising the near edge. In this manner,
the leading edge of the bull float will glide and
not dig into the wet concrete.
FinisHing tHe slaB
After the bull float raises water to the surface,
you must wait for the water to evaporate before
finishing the concrete. The wait depends on the
weather. On a hot, sunny day, you may need to
Ready for concrete, this site has 4 in.
wait less than an hour. On a cool and overcast
of gravel over compacted soil, 6-mil
day, you might need to wait for hours. Once the
plastic atop that, and rebar elevated
water’s evaporated, you have roughly one hour to
by concrete dobie blocks so the steel
trowel and compact the surface. When you think
will lie in the middle of the slab.
the surface is firm enough, put a test knee board
atop the concrete and stand on it. If the board
Fill slab footings first, then vibrate
sinks 3⁄4 in. or more, wait a bit. If the board leaves
them to drive out any air pockets.
only a slight indent that you can easily hand float
Because placing a concrete slab
further and then trowel smooth, get to work.
usually entails standing in wet
concrete, wear rubber boots.
As the left photo on the facing page shows,
knee boards distribute your weight and provide a
mobile station from which to work. You’ll need
two knee boards to move across the surface,
moving one board at a time. Then, kneeling on
both boards, begin sweeping with a magnesium
hand float or with a wood float, if you prefer a
rougher finish. Sweep back and forth in 3-ft.
arcs, raising the far edge of the float slightly as
you sweep away and raising the near edge on the
return sweep. The “mag” float levels the concrete.
After you’ve worked the whole slab, it’s time for
the steel trowel, which smooths and compacts
the concrete, creating a hard, durable finish.
As the concrete dries, it becomes harder to
work, so it’s acceptable to sprinkle very small
amounts of water on the surface to keep it work-
able. Troweling is hard work, especially on the
back. When the concrete’s no longer responding
to the steel trowel, edge the corners and then
Screeding is a job for three people:
cover the concrete with damp burlap before call-
two to level the concrete by moving
ing it a day. If the weather’s hot and dry, hose
the screed rail from side to side and
down the burlap periodically—every hour, at
a third with a shovel or rake to fill
least—and keep the slab under cover for four or
low spots and pull away excess.
five days. At the end of that time, you can remove
the forms. Concrete takes a month to cure fully.
Damp Basement Solutions
Bull-floating the concrete pushes
To find the best cure for a damp basement, first
down gravel near the surface and
determine whether the problem is caused by
brings the concrete’s cream to
water outside migrating through foundation
the surface.
walls or by interior water vapor inside condens-
ing on the walls. To determine which problem
you have, duct tape a 2-ft.-long piece of alumi-
num foil to the foundation, sealing the foil on all
four sides. Remove the tape after two days. The
wet side of the foil will provide your answer.
Chapter 14 has more abou
t mitigating moisture
and mold.
256 Chapter 10
Finishing the slab. Start with magnesium or wood floats,
An edger compresses and rounds the slab’s edge, making it
and finish with a steel trowel for a smooth, hard finish.
stronger and less likely to chip than a square corner.
Knee boards distribute your weight as you work.
P R O T I P
Curing Basement Condensation
Waterproofing membranes
Water vapor is everywhere and the warmer the air, the more moisture it contains. Water vapor tends to
are relatively fragile, especially
move from higher to lower concentrations and from warm to cool. Consequently, when warm summer air
those that are sprayed on. after
comes in contact with the cool concrete of basement walls and floors, its water vapor reaches a dew point
they’re applied, they’re often
and condenses. (Basements are cool because they abut the earth, whose ambient temperature is roughly
covered with rigid insulation,
55°F year-round.)
drain board, protection board,
and the like. these rigid panels
to “cure” dampness resulting from condensation (flowing water is another issue altogether), you must
protect the membranes during
keep the warm air in living spaces from coming in direct contact with masonry masses by insulating base-
backfilling and insulate the foun-
ment floors and walls. three details will help you do that successfully:
dation walls to reduce heat loss.
Choose materials that can tolerate moisture, such as expanded polystyrene (EPS) panels, and won’t
be degraded by it. moisture migrates through concrete year-round, so moisture-sensitive materials such
as wood, paper-faced drywall, and fiberglass insulation should not be in direct contact with it.
accept that a small amount of moisture will migrate, and choose materials that allow that. again,
eps panels are a good choice because they are semipermeable, whereas polyethylene sheet plastic is not.
unable to migrate through plastic, moisture will condense and collect, which could lead to rot and mold.
make your insulation layer—your thermal barriers—as airtight as possible. Here, we’re not talking
about the migration of moisture but of the loss of heat. that is, tightly sealed eps panels will contain
conditioned air yet allow minuscule amounts of water vapor to move back and forth. thermal barriers are
not vapor barriers.
to see how one builder put these principles to the test in his own basement, see “installing eps
panels to Create a dry Basement” on p. 417.
Foundations and Concrete
257
cool surfaces on which water vapor might con-