- Is there really a cement shortage?
- What is concrete?
- What is cement?
- What is ready mix?
- What is the difference between cement and concrete?
- What is flyash?
- What do you do when concrete hardens in the truck?
- How much does concrete weigh?
- How much does a
concrete truck weigh?
- What is finishing?
- When is it too cold to
place concrete?
- When is it too hot to
place concrete?
- Where do your raw
materials come from?
- Why does my concrete
job cost so much?
- When can I drive on my
new driveway?
- Does concrete hurt the
environment?
- What are those orange spots
on my sidewalk?
- Can I put a topping on my
concrete floor?
- "Suppliers Can be Central to
Your Business Success" from the ConcreteSherpa. Adobe Reader required.
Concrete is a material made from cement, aggregates (rock and sand), water, and
admixtures (chemicals that enhance or modify the properties of concrete).
Concrete is one of the most widely used and versatile building products known to
man.
Cement, or Portland cement, is a dry powder made from limestone and other
materials. It is burned in a kiln and then ground finely. Cement reacts
chemically with water to cause concrete to harden. There are several
different types of Portland cement, designated as Type I, Type II, etc.
Ready mix, also known as ready mixed concrete, is a type of concrete that is
delivered in trucks that agitate and/or mix the concrete on the way to the job
or at the job site. The concrete is delivered in a plastic, unhardened
state. The truck-mounted, revolving drum mixer was
invented by Stephen Stepanian. Stepanian filed the first truck mixer
patent application in 1916.
Cement is an ingredient in concrete. It is the
ingredient that forms a paste with the water, sand, and admixtures and fills the
space between the coarse aggregates (rock) and binds the rocks together.
The term "cement" is commonly misused to refer to concrete.
Flyash is a byproduct of coal-burning power plants. When used properly
in concrete, it improves the quality of the mix and saves money. Concrete
with flyash will typically have a higher ultimate strength, although early
strength will be lower than with straight cement. Sometimes users will
complain about "too much flyash in the mix." That is
rarely the problem. More often the problem is not enough cement in
the mix.
First of all, you do everything you can to prevent that from happening.
If it happens anyway, then you get inside the mixer drum with an air hammer and
break it out. It is a difficult job and nobody likes to do it.
Normal weight concrete weighs about 4000 lb. per cubic yard.
Lightweight concrete weighs about 3000 lb. per cubic yard.
If a truck is carrying 10 cubic yards, then the weight of the concrete is
40,000 lb. The truck will weigh approximately 26,000 lb. for a total of
66,000 lb.
Finishing is the process used to create the surface texture of the completed
concrete pour. Finishing involves several different steps. Depending
on the type of surface desired, it can involve striking off, floating, edging,
jointing, troweling, texturing, and curing.
It depends. Concrete will not set when the concrete temperature is
below about 35F. Many times specifications will say something like
"Concrete may not be poured when the temperature is 37F and
falling." With heated water and aggregates, accelerating admixtures,
and other methods, jobs can be poured below freezing, but it is more
expensive. In most southern states there are so few freezing days that it
is not worth it to try to pour when the temperature is below freezing.
It depends. High temperatures (90F and above) cause concrete to set
faster. High temperatures also reduce the strength of concrete.
Strong winds and low humidity can also cause problems with plastic shrinkage and
drying shrinkage cracks, even at moderate temperatures. To avoid these
problems, planning, timing of the finishing operations, proper use of retarding
admixtures, and proper curing are necessary.
More...
Supplies of cement, the key ingredient in concrete, are sometimes very tight. Several
factors are involved.
First, strong construction markets cause increased demand. If winter weather is
good, there may be little opportunity to prepare a large
inventory for spring construction activity.
Another factor is the availability of imported cement. The booming Asian
economies are straining worldwide cement capacity and shipping availability.
These conditions make imports of cement more expensive and difficult to acquire.
Regulations and a “build it anywhere not anywhere near anything” attitude have
restricted increases in domestic production. Domestic plants have not kept up
with the increased demand. In Texas, demand is much greater
than the amount of cement produced in Texas.
The cement comes from plants in Midlothian, Texas. The natural river sand we use
comes from the Trinity River bottoms southeast of Dallas. Crushed limestone, the
primary coarse aggregate we use, comes from quarries near Bridgeport, Texas.
Concrete is the most versatile and economical building material in the world,
but there are costs involved in doing the job properly. The cost of the concrete
itself is a relatively small percentage of the overall job. Insurance, labor,
benefits, materials, excavation, hauling, disposal, supervision, taxes, tools,
and equipment are some of the expenses a contractor faces. If you receive a
“cheap quote” it is probably because the contractor is not including all of
these things. For example, they may not have insurance, which can be more
expensive than the cost of the concrete. However, if they don’t have insurance,
the job could end up costing you many times the original quote.
It depends on many variables, two of which are mix design and temperature. If
you need your driveway quickly, ask your supplier to provide a high early
strength mix. Normal concrete will reach 1/3 of its ultimate strength in 3 days,
2/3 in 7 days. Ideally, you should wait 7 days, however that is not always
possible. Some high early strength concrete
mixes can be driven on in 24 hours or less.
Actually, concrete construction is environmentally friendly. Concrete is energy
efficient. Concrete contains recycled materials. Concrete reduces urban heat
islands. Concrete is manufactured locally. Concrete structures are durable.
Concrete pavements can be recycled. The raw materials used in
concrete come from the most abundant minerals on earth.
It may be fertilizer. Some fertilizer includes
iron, and when the granules get wet on the concrete they leave rust spots.
Fortunately, the effect is temporary.
It depends. Keep in mind that any cracks in the underlying
surface will probably be reflected in the new surface. For thin overlays
up to 2 inches, polymer-based overlay cement products
should be used. One product is good for applications in the 0- to
1/8-inch category, another for up to 1/2 inch, and another for 2-inch-thick
applications. Polymer cement
products have good flexural and tensile strength ratings and are much
more crack-resistant. They also have much higher compressive strengths than
regular concrete.
For toppings of 2 or more inches, a very rough
profile on the existing floor is needed. Shotblasting or
chipping will give the new floor a surface to bond to. The day
before you place concrete, wet down the surface and keep it wet until you place
concrete but don’t have any standing water on the slab during placement.
For bonding, use a 50% mix of portland cement and sand
slurry. You can use a 50% mix of water and latex bonding agent (like
Acryl 60) to make the slurry. Squeegee or sweep with a stiff broom the slurry
into the existing floor just ahead of concrete placement. The slurry must not
dry before concrete is placed over it. Consult your ready mix
supplier for the proper concrete mix to use for topping.
Source: Concrete Construction, February 2003
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