Is there a difference between
cement and concrete?
The terms cement and concrete are often used
in the same context but cement is actually an
ingredient of concrete. Concrete is actually
a mixture of aggregates (sand and gravel or
crushed stone) and paste (water and Portland
cement).
Portland cement is a generic term for the type
of cement used in concrete. By volume, the concrete
mix comprises 10 - 15% of cement. It's through
a process called hydration that the cement and
water harden and binds the aggregates into a
rock hard mass. As concrete gets older it actually
gets stronger. In reality, a cement sidewalk
or cement mixer does not exist; rather, it's
a concrete sidewalk and a concrete mixer.
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What does it mean to cure
concrete?
One of the most important steps in the construction
of concrete is the curing stage. Proper curing
will greatly increase the strength and durability
of the concrete. The chemical reaction between
cement and water (hydration) hardens the concrete
but hydration only occurs if water is present
and the temperature of the concrete remains
within a suitable range. In order to allow the
hydration process, the concrete surface must
be kept moist during the curing period; from
five to seven days after the placement for conventional
concrete. The new concrete can be kept moist
with sprinklers, soaking hoses, covered with
wet burlap or it can be coated with curing compounds
which will seal in the moisture.
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Is temperature a factor
in placing new concrete?
Temperature is an important factor in order
to properly cure concrete. Too much water can
be lost on newly placed concrete if performed
on a hot day. If the temperature is close to
the freezing mark, hydration is slow to nearly
a standstill. Under these conditions, the concrete
will cease to strengthen and affect other desirable
factors. During the curing period, the temperature
of new concrete should not be allowed to drop
below 10 Celsius (50 Fahrenheit).
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What is air-entrained concrete?
Air-entrained concrete has billions of microscopic
air cells per cubic foot, which provides tiny
chambers for water to expand into when it freezes.
Air-entrained concrete can be produced through
the use of air-entraining Portland cement or
by introducing air-entraining agents through
careful supervision as the concrete is mixed.
Usually between 4 and 7% of the volume of concrete
is entrained air but can be varied depending
on special conditions.
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Why do cracks appear in
concrete?
As concrete dries, it will slightly change
in volume by about 1/16 of an inch in 10 feet.
This is why contractors put joints in concrete
floors and pavements, allowing the concrete
to crack in a straight line at the joint when
shrinkage is caused by a change in the volume
of concrete.
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Why do some concrete surfaces
flake?
In areas where freezing and thawing occurs,
the concrete should be air-entrained to resist
flaking and scaling. To improve durability of
the surface, the water/cement ratio should be
as low as possible. Flaking and spalling of
the surface could occur when too much water
is applied to the mix. If the excess water is
worked into the concrete before the water sheen
on the surface is gone and excess water on the
surface has evaporated, the high water content
on the surface will make the concrete weaker
and less durable.
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What is the meaning of 28
day strength?
The hydration process occurs rapidly at first
and slows down as time goes by but hardens and
strengthens as it hydrates. It would take several
years to measure the ultimate strength of concrete,
so for practical purposes, a time period of
28 days was selected as the age that all concrete
should be tested. A substantial percentage of
the hydration process has taken place at this
age.
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What is Dura-Mix™ concrete?
DURA-MIX™ identifies, in a word, the type
of ready mixed concrete which is required to
produce durable exterior concrete flatwork.
The DURA-MIXTM name has been registered as a
trademark by ARMCA.
DURA-MIX™ is designed to provide superior
surface durability to deicing chemicals and
freezing conditions.
Concrete flatwork which is exposed to freezing
and thawing has always required superior materials
and workmanship. The increasing use of deicing
chemicals has created a need for further improvements
in standards of materials and workmanship. Concrete
techno-logists can describe the appropriate
materials to alleviate these problems. The language
used is increasingly complex and sometimes confusing.
From now on, anyone who wants to receive the
appropriate materials for outdoor concrete flatwork
needs only to order DURA-MIX™ from his
concrete supplier. Homebuilders and homeowners
will be confident that they will be getting
a durable product which will remain attractive.
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Can you actually control
the strength of concrete?
Yes. Adding cement is the easiest way to add
strength. The ratio of water to cement in the
cement paste that binds the aggregates together
is the most predominant influence on concrete
strength. The higher this ratio, the weaker
the concrete and vice versa. By adding more
water, every desirable physical property that
can be measured will be adversely effected.
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How can stains be removed
from concrete?
Stains can be removed by using wet methods
with chemicals or water or by dry or mechanical
methods. Wet methods consist of the application
of water or chemicals depending on the nature
of the stain. Treating the stain with specific
chemicals will dissolve the stain substance
so it can be blotted up or bleach the stain
so it will not show. For example to remove blood,
wet the stain with water and apply sodium peroxide
powder to cover the stain. After a few minutes,
rinse with water while vigorously scrubbing
the stain. Follow this procedure with a solution
of 5% vinegar to neutralize any sodium peroxide
that may remain.
Common dry methods include sandblasting, grinding,
flaming and shotblasting, scabbing, planing
and scouring. Be careful using steel brushes,
since they can leave metal particles on the
surface which may rust and stain the concrete
later.
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What type of finish can
be applied to a concrete surface?
There are ways to add colour to concrete. Pigments
can be added before or after the concrete is
placed and white cement can be used rather than
the usual grey cement, chemical stains can be
used, or by exposing colourful aggregates at
the surface. Finishes can be textured from a
smooth polish surface to the roughness of gravel.
The concrete can be made to resemble stone,
brick or tile paving by using geometric patterns
that are stamped, scored, rolled or inlaid into
the concrete. Divider strips (commonly redwood)
can be used to form panels of various shapes
and sizes; circular, square, diamond or rectangular
to create other interesting patterns.
There are other special techniques that can
be used to make concrete sparkle or slip resistant.
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How do you protect the
concrete surface from salt and acids?
Even though most material will have no effect
on concrete, there are some aggressive materials
like salt and most acids that can pose a deteriorating
effect on concrete. The use of quality concrete
with maximum chemical resistance and an application
of protective treatments to keep corrosive ingredients
from contacting the concrete will deter a chemical
attack. The chemical resistance of concrete
can be improved by implementing practices including
the use of a suitable cement type like sulfate-resistant
cement to prevent sulfite attack, using a low
water/cement ratio and using suitable aggregates,
water and air-entrainment. Sealers and coatings
are available to protect concrete from a variety
of environments.
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What is ASR (alkali-silica
reactivity)?
ASR is an expansive reaction between reactive
forms of silica in aggregates, potassium and
sodium alkalis, mostly from cement but also
from pozzolans, admixtures and mixing water.
Other sources of alkali from soil, industrial
processes and deicers can also contribute to
reactivity.
An alkali-silica gel is formed by the reaction
and as it draws water from the surrounding cement
paste, swells and induces pressure, expansion
and cracking of the aggregate and surrounding
paste. As a result, map-pattern type cracks,
referred to as, alligator patter cracking appears.
There are steps to avoid ASR; select proper
aggregate; use of blended cements; use of proper
pozzolanic materials and contaminant free mixing
water.
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Are there different types
of Portland cement?
All Portland cement is basically the same but
eight types of cement are manufactured in order
to meet the chemical and physical requirements
for specific applications.
Type 1 - a general purpose Portland cement
for most uses.
Type 2 - for structures in water or soil that
contains moderate amounts of sulfate or when
heat build-up is a factor.
Type 3 - provides an early state of high strength,
usually in a week or less.
Type 4 - moderates heat generated by hydration
which is used for substantial structures such
as dams.
Type 5 - to resist attack from chemicals from
soil and water high in sulfates.
Type 1a, 2a, 3a - these have the same properties
as types 1, 2 and 3 but they have small quantities
of air-entrained materials combined with them
to make air-entrained concrete.
White Portland cement is made from raw materials
that contain little or no manganese or iron
which are the substances that provide the grey
colour in conventional cement.
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If
you have a question that did not appear in this
section, please email
us and we will be happy to assist you with
any questions or concerns you may have.
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Is
there a difference between cement and concrete?