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| >>Exterior
concrete | Exterior
paint | Foundation
parging
Concrete is a mixture of Portland cement, water and
aggregate. The strength of the concrete is determined
largely by the amount of cement in the mixture. Other
factors that affect the strength of the concrete are
the type of aggregate, the slump of the mixture, and
the absence of organic particles in the mixture.
The slump of concrete is a measurement of the amount
of water in the mix. To test the slump concrete is
put into a cone shape container and then dumped out.
The more water the more the cone shape slumps, the
weaker the concrete.
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Sand and gravel make up the aggregate in concrete.
The strongest concrete has a mixture of particles
from sand to -3/4" gravel.
The cement content is usually described as bags per
yard. A standard mix is 5 bag mix. Because exterior
concrete flatwork is exposed to the elements 6 bag
minimum mix is recommended.
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Dirt is a common organic matter that if mixed into
concrete will reduce it's strength. That is why only
washed aggregate is used in quality concrete.
The Quality of the concrete job is only as good as
the base. The soil (base) that the concrete is placed
on must be level drainable and compacted, If not the
base will give way causing the concrete to sink and
break.
In poor bearing soils (clay, silt and sand mixtures)
compacted road base is used to give the base extra
bearing strength. Where soil strength is not a concern,
pea gravel is often used because it give doesn't need
compaction and drains well. The gravel is usually
placed to a depth of 4"-6". Ask your concrete
contractor what he recommends for your particular
situation. 
Forms (usually 2 x 4's) are laid on edge to create
the mold. The concrete is laid in place, screeded
(leveled), floated, then toweled, and broom finished.
During the finishing process joints are placed in
the concrete.
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Control joints are placed no more than 10' apart in
both directions to control cracking. Concrete will
crack... control joints help the slab to crack in
the joint.
Since concrete expands due to climate changes, expansion
joints are used when concrete is placed between immovable
objects (a driveway between a sidewalk and the garage
floor), or when new concrete is placed against old
concrete. Expansions joints are used in these situations
and generally every 30-40'.
Reinforcement in walkways, and driveways helps to
hold the concrete together. 6" x 6" wire
mesh and rebar are commonly used to reinforce concrete
slabs. If the base is properly prepared reinforcement
is not crucial to the longevity of a concrete slab.
It is interesting to note that steel reinforcement
is rarely required in city sidewalks.
Concrete should never be poured on frozen ground.
Concrete can be poured in freezing weather as long
as it is protected from freezing. Hydration will help
to protect the concrete from freezing for the first
couple of nights after pouring. Concrete blankets
or a combination of plastic and straw are often used
to keep the concrete warm.
Rain Storms can ruin a concrete finish. If the forecast
call for rain, it is a good idea to reschedule the
pour.
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Chemical reaction between concrete and water gives
off heat as the concrete cures.
For Concrete to cure properly, it needs to be kept
relatively moist for the first 3-4 weeks after pouring.
If concrete dries out to quickly it will loose strength.
Most concrete contractors will pour the concrete and
finish it, but it is up to the homeowner, builder
or contractor to protect it from the elements and
make sure that it is properly cured. After the concrete
has hardened for a day or two place a sprinkler and
moisten surface periodically for the next few weeks.
Concrete poured in the cooler part of the year or
in humid climates will need less watering. Ask your
concrete contractor for recommendations.
Some contractors like to pour the exterior concrete
as soon as the foundation is complete. This may sound
like a good idea... to "get it out of the way
as soon as possible". However, we believe that
unless weather will be a problem later on, it is usually
best to wait.
Waiting allows the ground a chance to settle where
it has been excavated. Laterals still need to be put
in, and it takes more preparation to plan that out.
Subcontractors with oil leaking vehicles will park
on the driveway if it is there. No matter when the
driveway is poured it must be protected from traffic
for a minimum of 7-10 days (longer when the weather
is cold). Place barriers around freshly poured concrete
or someone will walk or drive on it.
