Concrete 101 Basics
Concrete has four main ingredients: Portland cement, large
aggregate, small aggregate, and water. The large aggregate can be gravel or
crushed stone. The small aggregate is almost always sand. Portland cement
comes in five types, each formulated for specific characteristics. By choosing
the right Portland cement type, the right size and type of aggregates, and
adjusting the ratio of ingredients, you can create the best concrete for each
The job of aggregate is to increase the strength of the
concrete while reducing the cost. The individual stones that make up the
aggregate interlock with each other while the sand fills in the voids. The
cement paste created from the cement and water is the glue that holds the rock
and sand together. With less water comes more strength.
Type I General Purpose - This is the most widely used type,
especially for residential projects.
Type II - This generates less heat than type I, which is
important in large volume jobs because it sets up more slowly to give crews more
time to work during hot weather.
Type III High Early- This sets and cures rapidly. It’s used
for jobs that must bear weight shortly after being poured. It is also used
during very cold weather to reduce the time in which to concrete is prone to
Type IV - This produces very little heat as it cures and is
used for huge-volume projects such as dams.
Type V - This is very sulfate-resistant and is used in areas
in which the soil or groundwater is very alkaline.
Accelerators speed up the hydration
(hardening) of the concrete.
Retarders slow the hydration of concrete,
and are used in large or difficult pours where partial setting before the
pour is complete is undesirable.
Air-entrainers add and distribute tiny
air bubbles in the concrete, which will reduce damage during freeze-thaw
cycles thereby increasing the concrete's durability. However, entrained air
is a trade-off with strength, as each 1% of air may result in 5% decrease in
Plasticizers (water-reducing admixtures)
increase the workability of plastic or "fresh" concrete, allowing it be
placed more easily, with less consolidating effort. Superplasticizers
(high-range water-reducing admixtures) are a class of plasticizers which
have fewer deleterious effects when used to significantly increase
workability. Alternatively, plasticizers can be used to reduce the water
content of a concrete (and have been called water reducers due to this
application) while maintaining workability. This improves its strength and
Pigments can be used to change the color
of concrete, for aesthetics.
Corrosion inhibitors are used to minimize
the corrosion of steel and steel bars in concrete.
Bonding agents are used to create a bond
between old and new concrete.
Pumping aids improve pumpability, thicken
the paste, and reduce dewatering – the tendency for the water to separate
out of the paste.
Accelerants reduce the curing time substantially so you can
shorten the length of time needed to protect the concrete from freezing.
Retarders are added to increase the setting time, allowing more time to finish
the concrete especially in warm weather. Retarders also help to reduce the
initial stress cracking that happens when concrete sets and cures too rapidly.
Water reducers let the concrete become more plastic and
workable with less water. The benefit is a stronger concrete with less labor
needed for placing and working it.
Air Entrainment admixtures allow for the formation of millions
of microscopic air bubbles in the concrete mix. These bubbles give water in the
concrete room to expand as it freezes during the freezes during the freeze/thaw
cycles in cold climates. Air entrainment also acts as a plasticizer and allows
the concrete to be more workable with less water.
Color pigments are used as a way to introduce color to
concrete. The pigments must be insoluble in water, free from salts and acids,
and colorfast in sunlight. Both liquid and powder pigments are available. It is
also important to note that using white Portland cement instead of gray produces
cleaner, brighter, more vivid colors.
The cost of reinforcing concrete is minimal compared to the
added value it gives. Concrete excels in the area of compressive strength yet
leaves a lit to be desired in tensile strength. The addition of reinforcing can
significantly offset this shortcoming. The most popular method of reinforcing is
with welded wire. Other reinforcing methods include fiber mesh or wire mesh that
is mixed into the concrete at the plant so the fibers are distributed through
Three physical characteristics give reinforced concrete its
special properties. First, the coefficient of thermal expansion of concrete is
similar to that of steel, eliminating internal stresses due to differences in
thermal expansion or contraction. Second, when the cement paste within the
concrete hardens this conforms to the surface details of the steel, permitting
any stress to be transmitted efficiently between the different materials.
Usually steel bars are roughened or corrugated to further improve the bond or
cohesion between the concrete and steel. Third, the alkaline chemical
environment provided by calcium carbonate (lime) causes film to form on the
surface of the steel, making it much more resistant to corrosion than it would
be in neutral or acidic conditions.
The relative cross-sectional area of steel required for
typical reinforced concrete is usually quite small and varies from 1% for most
beams and slabs to 6% for some columns. Reinforcing bars are normally round in
cross-section and vary in diameter. In the United States, rebar comes in two
grades of carbon content, Grade 60 and Grade 40, which typically sell for the
same price. Grade 60 has a higher carbon content and, therefore, a higher
tensile strength, but its stiffness can make it difficult to bend and cut.
Galvanized, epoxy-coated, and stainless steel rebar are also available for use
in corrosive environments.
Polypropylene fibers are usually used in concrete to control
plastic shrinkage cracking and drying shrinkage cracking. They also lower the
permeability of concrete and thus reduce bleeding of water. Some types of fibers
produce greater impact, abrasion and shatter resistance in concrete.
Generally fibers do not increase the flexural strength of
concrete, so it can not replace moment resisting or structural steel
reinforcement. Some fibers reduce the strength of concrete. The amount of fibers
added to a concrete mix is measured as a percentage of the total volume of the
composite. The volume of fibers typically ranges from 0.1 to 3%.
The strength of concrete is typically measure in pounds per
square inch (psi). Most residential project requires at least 2,500 psi. In the
case of standard commercial and industrial applications the use of at least
3,000 psi concrete is required.
The chemical reaction between water and cement is called
hydration. In order to maintain proper hydration, and therefore proper curing,
the rate of evaporation and the temperature of the water in the concrete must be
controlled. If the water rapidly evaporates or freezes during the first seven
days of curing, the concrete will be substandard. This is why weather plays a
big role in concrete work.
Even thought the cement reaction with water is completed over
period of time (normally about around 240 minutes which is referred to as final
setting time), the hardening of concrete and gain of strength is over a period
of time 95% to 98% strength is achieved in 3 weeks or about 28 days. During this
period concrete needs to be maintained in ideal conditions by controlling
temperature and humidity. In practice this is achieved by spraying or ponding
water on the concrete surface there by protecting concrete mass from ill effects
of ambient conditions.
Moisture curing is generally the best way to cure concrete.
This can be accomplished in several ways such as ponding of water on the
concrete shortly after the initial set of surface. In today’s fast passed world
the use of plastic sheeting, burlap or straw is most prevalent with its ability
to allow trades to work on the recently placed slab.
In lieu of water and plastic covering there are many types of
liquid-membrane curing compounds available that one can easily spray on any type
of concrete project. Once a curing compound is applied it will seal moisture in
and the concrete should cure properly under all but freezing weather conditions.
It is important to us the correct compound for the
application. Depending on its base, which can be acrylic, asphalt, rubber, wax,
epoxy, or some other type, curing compounds have many side effects. For example
some will discolor the concrete, some leave a film that must be removed for the
proper adhesion for coatings, vinyl flooring or carpet. Worst of all some when
left in place will chemically bind with lift truck tires leaving long lasting
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