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Frequently Asked Questions
What
is the difference between cement and concrete?
How is portland cement made?
What does it mean to "cure"
concrete?
Can it be too hot or too
cold to place new concrete?
What is air-entrained concrete?
What are recommended mix
proportions for good concrete?
Why does concrete crack?
Why test concrete?
What are the most common
tests for fresh concrete?
How can you tell if you're
getting the amount of concrete you're paying for?
Why do concrete surfaces
flake and spall?
Will concrete harden under
water?
What does 28-day strength
mean?
What is 3,000 pound concrete?
How do you control the strength
of concrete?
How do you remove stains
from concrete?
What are the decorative
finishes that can be applied to concrete surfaces?
How do you protect a concrete
surface from aggressive materials like acids?
Is there a universal international
specification for portland cement?
What is alkali-silica reactivity
(ASR)?
Are there different types
of portland cement?
What is the difference
between cement and concrete?
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"
concrete?
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
new concrete?
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 period.
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
fresh concrete?
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
spall?
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
concrete?
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 or water.
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 and sparkling.
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 cement?
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 as dams.
- 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.
Additional
frequently asked questions
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