Q. What
makes good quality concrete?
A. There
are several basic requirements for good quality concrete:
- Good materials-cement, aggregate
admixtures and water
- Proper proportioning of these
materials
- Thorough mixing
- Skillful placing and finishing
- Proper curing
All of these requirements are
important, and failure to satisfy any one of them will often result
in a reduction in quality. The materials used by Central Valley
Concrete. meet quality requirements. The specifications relate to
important properties such as amounts of nondurable substances
permitted in aggregates and how fast the cement will set and gain
strength. Multiple tests are conducted to verify that, the materials
we use meet and exceed applicable specifications.
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Another key to
having excellent concrete is proportioning. Proportioning
is simply deciding how much of each ingredient will be put
in the concrete. A temptation to use too much water
generally results from an effort to make placing
operations easier for the contractor. Overly wet concrete
flows into the forms more readily, is easier to compact
and to strike off. |
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Everything about overly
wet concrete is easier-except the maintenance problems that will
begin to appear on a surface that could just as easily have been
trouble-free for many years.
Q.
Why is too much mixing water dangerous?
A.
Simply because excessive water makes a more dilute cement paste. Not
all of the water in any concrete reacts with cement and when there
is an excessive amount of water it leaves small voids in the
hardened paste. This makes the concrete weaker, more porous and less
durable.
Q.
What are typical concrete requirements for around-the-house
flatwork-patios, walks, driveways?
A.
The contractor should provide the ready
mix producer with the following basic information:
- Maximum size of coarse
aggregate.
- Slump of the concrete. Slump
is a measure of how readily a concrete flows. Slump values are
determined by test. Increasing the water content increases
slump, and a slump that is too high indicates that too much
water may have been added to the concrete. A 4 to 5-inch slump
will give a good, workable mix. A stiffer mix (lower slump) will
usually be more difficult to place and a wetter, soupy mix won't
be as strong and durable.
- Strength required. In
severe climates where concrete will be exposed to freezing and
thawing, a compresive strength of about 4000 pounds per square
inch (psi) is a common specification. Strength is measured on
standard test cylinders cured under standard conditions. A
strength of 3000 to 3500 psi at 28 days is adequate for nearly
all concrete not exposed to numerous freezing and thawing cycles
or to deicing salts.
- Air-entrainment. Entrained
air is air that is purposely incorporated into a concrete mix,
primarily to make the concrete resistant to deterioration caused
by freezing and thawing and the use of deicing salts. To be
effective for this purpose the air content should be about 6
percent. The air bubbles also make the concrete easier to work
because they act as a lubricant. For this reason some builders
prefer to use air-entrained concrete for most flatwork around
the home, whether or not the flatwork will be exposed to
freezing and thawing.
Q.
What steps can be taken during construction to ensure good
performance?
A.
The homeowner will want to be
sure that, for flatwork, the underlying soil (subgrade) is well
compacted by the contractor and not extremely wet or muddy when
concrete is placed. Workers should also check the elevation of the
subgrade by measuring down from a straightedge laid on the forms.
High spots should be removed;
otherwise the slab will be too thin in spots. Low spots should be
filled in and compacted; otherwise the buyer may pay for more
concrete than is needed. Flatwork for driveways, sidewalks and
patios is typically 3 1/2 to 4 inches thick.
Side forms should be set so that the finished concrete surface is
sloped slightly for drainage. A slope of 1/4 inch per foot of width
is recommended for driveway slabs.
Joints are needed in most concrete slabs. They control cracking by
dividing the slab into square sections for driveways and patios, the
joints should usually be spaced 10 feet apart or less. For sidewalks
they are typically spaced only 4 or 5 feet apart. Joints must be cut
to a depth of at least one inch. If the joints aren't cut deep
enough or if no joints are provided, the slab will probably crack at
random locations.
Where flatwork abuts another structure, it should be separated from
the structure by an expansion joint. Usually a premolded strip of
compressible material is inserted at locations such as the
intersection of a driveway and garage slab or a patio and the
foundation wall of a house.
Q.
What is meant by curing?
A.
A chemical reaction between
cement and water makes concrete strong and durable. The purpose of
curing is to make sure that little or no water is lost from the
concrete during the early stages of this chemical reaction.
Q.
How is curing accomplished?
A.
The newly placed concrete should be covered or
sprayed with a material that will retain the moisture within the
mix. Membrane curing compounds sprayed on the surface are the
materials most commonly used. They are very convenient but they must
be applied immediately after the concrete is given its final finish
and care must be taken apply them thoroughly and uniformly.
Coverings used to cure concrete include wet burlap, plastic sheets
and Kraft paper. If burlap is used it must be kept continuously wet.
The curing process should be continued for at least 72 hours and
twice as long if the temperature is below 50 degrees F.
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It is up to the
contractor to cover the concrete and start the
curing process. But it is usually up to the homeowner to
follow through from there and make sure that curing
proceeds uninterrupted for the required time period. |
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Q.
How do you find a good contractor?
A. First
of all, don't pick him entirely on the basis of cost. In concrete
work, as in any business, you get what you pay for. Cost should be a
consideration, but it should not be the only consideration.
Knowledge, competence, integrity and experience are the qualities to
seek in a contractor. To obtain the names of qualified contractors,
talk to the dispatchers or friends and family. Then take a look at a
few of the contractors' completed jobs, particularly some that were
done several years ago. If you like what you see, you can feel
easier about seeking bids from them, knowing that they do good
quality work.
Q.
What performance should you expect from concrete placed around your
home?
A. Good
quality concrete-properly proportioned, mixed, placed, finished and
cured-will give maintenance-free service for many years. Concrete
work, properly done, will usually last much longer than the need for
which it was originally installed.
Q.
How do I know how much concrete to order?
A.
When you have measured the length and distance of
the area you are interested in pouring, call our dispatch center and
they will provide an exact measurement for your project.
Q.
What kind of reinforcement should I use in my concrete?
A.
Much of that depends on what the concrete is being used for.
In most cases, Fibermesh is an excellent no-hassle solution to
providing additional strength. We also suggest using rebar or
wire depending on the use.
