| Frequently Asked Questions
What is the difference between cement
Although the terms cement and concrete
often are used interchangeably, cement is actually an ingredient
of concrete. Concrete is basically a mixture of aggregates and paste.
The aggregates are sand and gravel or crushed stone; the paste is
water and portland cement. Concrete gets stronger as it gets older.
Portland cement is not a brand name, but the generic term for the
type of cement used in virtually all concrete, just as stainless
is a type of steel and sterling a type of silver. Cement comprises
from 10 to 15 percent of the concrete mix, by volume. Through a
process called hydration, the cement and water harden and bind the
aggregates into a rocklike mass. This hardening process continues
for years meaning that concrete gets stronger as it gets older.
So, there is no such thing as a cement
sidewalk, or a cement mixer; the proper terms are concrete sidewalk
and concrete mixer.
How is portland cement made?
Materials that contain appropriate
amounts of calcium compounds, silica, alumina and iron oxide are
crushed and screened and placed in a rotating cement kiln. Ingredients
used in this process are typically materials such as limestone,
marl, shale, iron ore, clay, and fly ash.
The kiln resembles a large horizontal
pipe with a diameter of 10 to 15 feet (3 to 4.1 meters) and a length
of 300 feet (90 meters) or more. One end is raised slightly. The
raw mix is placed in the high end and as the kiln rotates the materials
move slowly toward the lower end. Flame jets are at the lower end
and all the materials in the kiln are heated to high temperatures
that range between 2700 and 3000 Fahrenheit (1480 and 1650 Celsius).
This high heat drives off, or calcines, the chemically combined
water and carbon dioxide from the raw materials and forms new compounds
(tricalcium silicate, dicalcium silicate, tricalcium aluminate and
tetracalcium aluminoferrite). For each ton of material that goes
into the feed end of the kiln, two thirds of a ton the comes out
the discharge end, called clinker. This clinker is in the form of
marble sized pellets. The clinker is very finely ground to produce
portland cement. A small amount of gypsum is added during the grinding
process to control the cement's set or rate of hardening.
What does it mean to "cure"
Curing is one of the most important
steps in concrete construction, because proper curing greatly increases
concrete strength and durability. Concrete hardens as a result of
hydration: the chemical reaction between cement and water. However,
hydration occurs only if water is available and if the concrete's
temperature stays within a suitable range. During the curing period-from
five to seven days after placement for conventional concrete-the
concrete surface needs to be kept moist to permit the hydration
process. new concrete can be wet with soaking hoses, sprinklers
or covered with wet burlap, or can be coated with commercially available
curing compounds, which seal in moisture.
Can it be too hot or too cold to place
Temperature extremes make it difficult
to properly cure concrete. On hot days, too much water is lost by
evaporation from newly placed concrete. If the temperature drops
too close to freezing, hydration slows to nearly a standstill. Under
these conditions, concrete ceases to gain strength and other desirable
properties. In general, the temperature of new concrete should not
be allowed to fall below 50 Fahrenheit (10 Celsius) during the curing
What is air-entrained concrete?
Air-entrained concrete contains billions
of microscopic air cells per cubic foot. These air pockets relieve
internal pressure on the concrete by providing tiny chambers for
water to expand into when it freezes. Air-entrained concrete is
produced through the use of air-entraining portland cement, or by
the introduction of air-entraining agents, under careful engineering
supervision as the concrete is mixed on the job. The amount of entrained
air is usually between 4 percent and 7 percent of the volume of
the concrete, but may be varied as required by special conditions.
What are recommended mix proportions
for good concrete?
Good concrete can be obtained by using
a wide variety of mix proportions if proper mix design procedures
are used. A good general rule to use is the rule of 6's:
- A minimum cement content of 6 bags
per cubic yard of concrete,
- A maximum water content of 6 gallons
per bag of cement,
- A curing period (keeping concrete
moist) a minimum of 6 days, and
- An air content of 6 percent (if
concrete will be subject to freezing and thawing).
Why does concrete crack?
Concrete, like all other materials,
will slightly change in volume when it dries out. In typical concrete
this change amounts to about 500 millionths. Translated into dimensions-this
is about 1/16 of an inch in 10 feet (.4 cm in 3 meters). The reason
that contractors put joints in concrete pavements and floors is
to allow the concrete to crack in a neat, straight line at the joint
when the volume of the concrete changes due to shrinkage.
Why test concrete?
Concrete is tested to ensure that the
material that was specified and bought is the same material delivered
to the job site. There are a dozen different test methods for freshly
mixed concrete and at least another dozen tests for hardened concrete,
not including test methods unique to organizations like the Army
Corps of Engineers, the Federal Highway Administration, and state
departments of transportation.
What are the most common tests for
Slump, air content, unit weight and
compressive strength tests are the most common tests.
- Slump is a measure of consistency,
or relative ability of the concrete to flow. If the concrete can't
flow because the consistency or slump is too low, there are potential
problems with proper consolidation. If the concrete won't stop
flowing because the slump is too high, there are potential problems
with mortar loss through the formwork, excessive formwork pressures,
finishing delays and segregation.
- Air content measures the total air
content in a sample of fresh concrete, but does not indicate what
the final in-place air content will be, because a certain amount
of air is lost in transportation, consolidating, placement and
finishing. Three field tests are widely specified: the pressure
meter and volumetric method are ASTM standards and the Chace Indicator
is an AASHTO procedure.
- Unit weight measures the weight
of a known volume of fresh concrete.
- Compressive strength is tested by
pouring cylinders of fresh concrete and measuring the force needed
to break the concrete cylinders at proscribed intervals as they
harden. According to Building Code Requirements for Reinforced
Concrete (ACI 318), as long as no single test is more than 500
psi below the design strength and the average of three consecutive
tests equals or exceed the design strength then the concrete is
acceptable. If the strength tests don't meet these criteria, steps
must be taken to raise the average.
How can you tell if you're getting
the amount of concrete you're paying for?
The real indicator is the yield, or
the actual volume produced based on the actual batch quantities
of cement, water and aggregates. The unit weight test can be used
to determine the yield of a sample of the ready mixed concrete as
delivered. It's a simple calculation that requires the unit weight
of all materials batched. The total weight information may be shown
on the delivery ticket or it can be provided by the producer. Many
concrete producers actually over yield by about 1/2 percent to make
sure they aren't short-changing their customers. But other producers
may not even realize that a mix designed for one cubic yard might
only produce 26.5 cubic feet or 98 percent of what they designed.
Why do concrete surfaces flake and
Concrete surfaces can flake or spall
for one or more of the following reasons:
- In areas of the country that are
subjected to freezing and thawing the concrete should be air-entrained
to resist flaking and scaling of the surface. If air-entrained
concrete is not used, there will be subsequent damage to the surface.
- The water/cement ratio should be
as low as possible to improve durability of the surface. Too much
water in the mix will produce a weaker, less durable concrete
that will contribute to early flaking and spalling of the surface.
- The finishing operations should
not begin until the water sheen on the surface is gone and excess
bleed water on the surface has had a chance to evaporate. If this
excess water is worked into the concrete because the finishing
operations are begun too soon, the concrete on the surface will
have too high a water content and will be weaker and less durable.
Will concrete harden under water?
Portland cement is a hydraulic cement
which means that it sets and hardens due to a chemical reaction
with water. Consequently, it will harden under water.
What does 28 -day strength mean?
Concrete hardens and gains strength
as it hydrates. The hydration process continues over a long period
of time. It happens rapidly at first and slows down as time goes
by. To measure the ultimate strength of concrete would require a
wait of several years. This would be impractical, so a time period
of 28 days was selected by specification writing authorities as
the age that all concrete should be tested. At this age, a substantial
percentage of the hydration has taken place.
What is 3,000 pound concrete?
It is concrete that is strong enough
to carry a compressive stress of 3,000 psi (20.7 MPa) at 28 days.
Concrete may be specified at other strengths as well. Conventional
concrete has strengths of 7,000 psi or less; concrete with strengths
between 7,000 and 14,500 psi is considered high-strength concrete.
How do you control the strength of
The easiest way to add strength is
to add cement. The factor that most predominantly influences concrete
strength is the ratio of water to cement in the cement paste that
binds the aggregates together. The higher this ratio is, the weaker
the concrete will be and vice versa. Every desirable physical property
that you can measure will be adversely effected by adding more water.
How do you remove stains from concrete?
Stains can be removed from concrete
with dry or mechanical methods, or by wet methods using chemical
Common dry methods include sandblasting,
flame cleaning and shotblasting, grinding, scabbing, planing and
scouring. Steel-wire brushes should be used with care because they
can leave metal particles on the surface that later may rust and
stain the concrete.
Wet methods involve the application
of water or specific chemicals according to the nature of the stain.
The chemical treatment either dissolves the staining substance so
it can be blotted up from the surface of the concrete or bleaches
the staining substance so it will not show.
To remove blood stains, for example,
wet the stains with water and cover them with a layer of sodium
peroxide powder; let stand for a few minutes, rinse with water and
scrub vigorously. Follow with the application of a 5 percent solution
of vinegar to neutralize any remaining sodium peroxide.
What are the decorative finishes that
can be applied to concrete surfaces?
Color may be added to concrete by adding
pigments-before or after concrete is place-and using white cement
rather than conventional gray cement, by using chemical stains,
or by exposing colorful aggregates at the surface. Textured finishes
can vary from a smooth polish to the roughness of gravel. Geometric
patterns can be scored, stamped, rolled, or inlaid into the concrete
to resemble stone, brick or tile paving. Other interesting patterns
are obtained by using divider strips (commonly redwood) to form
panels of various sizes and shapes rectangular, square, circular
or diamond. Special techniques are available to make concrete slip-resistant
How do you protect a concrete surface
from aggressive materials like acids?
Many materials have no effect on concrete.
However, there are some aggressive materials, such as most acids,
that can have a deteriorating effect on concrete. The first line
of defense against chemical attack is to use quality concrete with
maximum chemical resistance, followed by the application of protective
treatments to keep corrosive substances from contacting the concrete.
Principles and practices that improve the chemical resistance of
concrete include using a low water-cement ratio, selecting a suitable
cement type (such as sulfate-resistant cement to prevent sulfate
attack), using suitable aggregates, water and air entrainment. A
large number of chemical formulations are available as sealers and
coatings to protect concrete from a variety of environments; detailed
recommendations should be requested from manufacturers, formulators
or material suppliers.
Is there a universal international
specification for portland cement?
Each country has its own standard for
portland cement, so there is no universal international standard.
The United States uses the specification prepared by the American
Society for Testing and Materials-ASTM C-150 Standard Specification
for Portland Cement. There are a few other countries that also have
adopted this as their standard, however, there are countless other
specifications. Unfortunately, they do not use the same criteria
for measuring properties and defining physical characteristics so
they are virtually "non-translatable." The European Cement
Association located in Brussels, Belgium, publishes a book titled
"Cement Standards of the World."
What is alkali-silica reactivity (ASR)?
Alkali-silica reactivity is an expansive
reaction between reactive forms of silica in aggregates and potassium
and sodium alkalis, mostly from cement, but also from aggregates,
pozzolans, admixtures and mixing water. External sources of alkali
from soil, deicers and industrial processes can also contribute
to reactivity. The reaction forms an alkali-silica gel that swells
as it draws water from the surrounding cement paste, thereby inducing
pressure, expansion and cracking of the aggregate and surrounding
paste. This often results in map-pattern cracks, sometimes referred
to as alligator pattern cracking. ASR can be avoided through 1)
proper aggregate selection, 2) use of blended cements, 3) use of
proper pozzolanic materials and 4) contaminant-free mixing water.
Are there different types of portland
Though all portland cement is basically
the same, eight types of cement are manufactured to meet different
physical and chemical requirements for specific applications:
- Type I is a general purpose portland
cement suitable for most uses.
- Type II is used for structures in
water or soil containing moderate amounts of sulfate, or when
heat build-up is a concern.
- Type III cement provides high strength
at an early state, usually in a week or less.
- Type IV moderates heat generated
by hydration that is used for massive concrete structures such
- Type V cement resists chemical attack
by soil and water high in sulfates.
- Types IA, IIA and IIIA are cements
used to make air-entrained concrete. They have the same properties
as types I, II, and III, except that they have small quantities
of air-entrained materials combined with them.
White portland cement is made from
raw materials containing little or no iron or manganese, the substances
that give conventional cement its gray color.
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