CONCRETE
It is a mixture of a cement, aggregates
(gravel and sand), gauged with water, which may contain admixtures
(i.e. plasticisers, accellerators, retarders and air-entraining agents)
and other pozzolanic materials.
Cncrete is normally poured into formwork, a mould (for pre-cast elements)
or a ready-dug hole to create a specific shape. Metal reinforcement
may be included in the structure to improve its tensile/flexural performance.
Concrete can also be divided into two categories by its intended use,
but the composition will be much the same. Mass concrete e.g. for
foundations does not necessarily contain reinforcement, whereas structural
concrete (e.g. for bridge spans) does.
Some times the reinforcing steel is likely to be tensioned. If this
is the case, it will either be pre- or post-tensioned. Pre-tensioned
reinforcement is tensioned before the concrete is poured, and when
the concrete sets, the tension is maintained by the bond between concrete
and reinforcement. Post-tensioned reinforcement is tensioned in a
duct running through the concrete after the concrete has achieved
sufficient strength. Both systems have their advantages and disadvantages.
The relatively few catastrophic failures of concrete in normal service
bear testament to the reliability of the technology.
CEMENT
Although the use of cements (both hydraulic and non-hydraulic)
goes back many thousands of years (to ancient Egyptian times at least),
the first occurrence of "portland cement" came about in
the 19th century. In 1824, Joseph Aspdin, a Leeds mason took out a
patent on a hydraulic cement that he coined "Portland" cement
(Mindess and Young, 1981). He named the cement because it produced
a concrete that resembled the color of the natural limestone quarried
on the Isle of Portland, a peninsula in the English Channel. Since
then, the name "portland cement" has stuck and is written
in all lower case because it is now recognized as a trade name for
a type of material and not a specific reference to Portland, England.
ASTM C 125 and the Portland Cement Association (PCA) provide the following
definitions of two basic types of cements:
a. HYDRAULIC CEMENT: An inorganic material or a mixture of inorganic
materials that sets and develops strength by chemical reaction with
water by formation of hydrates and is capable of doing so under water.
b. PORTLAND CEMENT: A hydraulic cement composed primarily of hydraulic
calcium silicates.
CEMENT MANUFACTURING
The chief chemical components of portland cement are calcium, silica,
alumina and iron. Calcium is derived from limestone, marl or chalk,
while silica, alumina and iron come from the sands, clays and iron
ore sources. Other raw materials may include shale, shells and industrial
byproducts such as mill scale. The basic manufacturing process heats
these materials in a kiln to about 1400 to 1600°C (2600 - 3000°F)
- the temperature range in which the two materials interact chemically
to form calcium silicates. This heated substance, called "clinker"
is usually in the form of small gray-black pellets about 12.5 mm (0.5
inches) in diameter. Clinker is then cooled and pulverized into a
fine powder that almost completely passes through a 0.075 mm (No.
200) sieve and fortified with a small amount of gypsum. The result
is portland cement.
The chemical properties of general purpose portland cement are given
below:
| Chemical
Name |
Chemical
Formula |
Shorthand
Notation |
Percent
by Weight |
| Tricalcium
Silicate |
3CaO×SiO2 |
C3S |
50 |
| Dicalcium
Silicate |
2CaO×SiO2 |
C2S |
25 |
| Tricalcium
Aluminate |
3CaO×Al2O3 |
C3A |
12 |
| Tetracalcium
Aluminoferrite |
4CaO×Al2O3×Fe2O3 |
C4AF |
8 |
| Gypsum |
CaSO4×H2O |
CSH2 |
3.5 |
TYPES OF CEMENT
AASHTO M 85 and ASTM C 150, Standard Specification for Portland Cement,
recognize eight basic types of portland cement concrete. There are
also many other types of blended and proprietary cements that are
not mentioned here.
| Type |
Name |
Purpose |
| I
|
Normal |
General-purpose
cement suitable for most purposes. |
| IA |
Normal-Air
Entraining |
An
air-entraining modification of Type I. |
| II |
Moderate
Sulfate Resistance |
Used
as a precaution against moderate sulfate attack. It
will usually generate less heat at a slower rate than Type
I cement. |
| IIA |
Moderate
Sulfate Resistance-
Air Entraining |
An
air-entraining modification of Type II. |
| III |
High
Early Strength |
Used
when high early strength is needed. It is has more C3S
than Type I cement and has been ground finer to provide a
higher surface-to-volume ratio, both of which speed hydration.
Strength gain is double that of Type I cement in the first
24 hours. |
| IIIA |
High
Early Strength-
Air Entraining |
An
air-entraining modification of Type III. |
| IV |
Low
Heat of Hydration |
Used
when hydration heat must be minimized in large volume applications
such as gravity dams. Contains about half the C3S
and C3A and double the C2S of Type I
cement. |
| V |
High
Sulfate Resistance |
Used
as a precaution against severe sulfate action - principally
where soils or groundwaters have a high sulfate content.
It gains strength at a slower rate than Type I cement.
High sulfate resistance is attributable to low C3A
content. |
AGGREGATES IN CONCRETE
"Aggregate"
is a collective term for the mineral materials such as sand, gravel
and crushed stone that are used with a binding medium (such as water,
bitumen, portland cement, lime, etc.) to form compound materials
(such as asphalt concrete and portland cement concrete or reinforced
cement concrete). By volume, aggregate generally accounts
for 92 to 96 percent of HMA (hot mix asphalt) and about 70 to 80
percent of portland cement concrete. Aggregate is also used for
base and subbase courses for both flexible and rigid pavements.
Aggregates can either be natural or manufactured. Natural
aggregates are generally extracted from larger rock formations through
an open excavation (quarry). Extracted rock is typically reduced
to usable sizes by mechanical crushing. Manufactured aggregate
is often the byproduct of other manufacturing industries.
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