HISTORY OF CONCRETE
Fig: Portland
cement
The time period during which
concrete was first invented depends on how one interprets the term
“concrete.” Ancient materials were crude
cements made by crushing and burning gypsum or limestone. Lime also refers to crushed,
burned limestone. When sand and water were added to these cements, they became
mortar, which was a plaster-like material used to adhere stones to each
other. Over thousands of years, these
materials were improved upon, combined with other materials and, ultimately,
morphed into modern concrete. Today’s concrete is made using Portland cement,
coarse and fine aggregates of stone and sand, and water. Admixtures are chemicals added to the
concrete mix to control its setting properties and are used primarily when
placing concrete during environmental extremes, such as high or low
temperatures, windy conditions, etc. The precursor to concrete was invented in
about 1300 BC when Middle Eastern builders found that when they coated the
outsides of their pounded-clay fortresses and home walls with a thin, damp
coating of burned limestone, it reacted chemically with gases in the air to
form a hard, protective surface. This wasn’t concrete, but it was the beginning
of the development of cement.
Fig: History of concrete
Early composite materials
typically included mortar-crushed, burned limestone, sand and water, which was
used for building with stone, as opposed to casting the material in a mold,
which is essentially how modern concrete is used, with the mold being the
concrete forms. As one of the key constituents of modern concrete, cement has
been around for a long time. About 12 million years ago in what is now Israel,
natural deposits were formed by reactions between limestone and oil shale that
were produced by spontaneous combustion. However, cement is not concrete.
Concrete is a composite building material and the ingredients, of which cement
is just one, have changed over time and are changing even now. The performance
characteristics can change according to the different forces that the concrete
will need to resist. These forces may be gradual or intense, they may come from
above (gravity), below (soil heaving), the sides (lateral loads), or they might
take the form of erosion, abrasion or chemical attack. The ingredients of
concrete and their proportions are called the design mix.
HISTORY OF STEEL FIBER
REINFORCED CONCRETE
A French gardener by name
Joseph Monier first invented the reinforced concrete in the year 1849. If not
for this reinforced concrete most of the modern buildings would not have been
standing today. Reinforced concrete can be used to produce frames, columns,
foundation, beams etc. Reinforcement material used should have excellent
bonding characteristic, high tensile strength and good thermal compatibility.
Reinforcement requires that there shall be smooth transmission of load from the
concrete to the interface between concrete and reinforcement material and then
on to reinforcement material. Thus the concrete and the material reinforced
shall have the same strain.
Fig: Steel Fiber reinforced concrete
The steel bars are reinforced
into the concrete. The bars have a rough, corrugated surface thus allowing
better bonding with steel rebar’s the concrete gets extra tensile strength. The
compression strength, bending also shown marked improvement thermal expansion
characteristic of steel rebar’s and concrete shall match. The rebar shall have
cross sectional are equal to 1% for slabs and beams, this can be 6% in case of
columns. The concrete has alkaline nature, this forms a passivating film around
the bars thereby protecting it from corrosion. This passivating film will not
form neutral or acidic condition. Carbonation of concrete takes place along
with chloride absorption resulting in failure of steel rebar. By comparing the
tension capacity of steel bars and concrete + steel reinforcements the
reinforced concrete can be called as under reinforced (tensile capacity of bars
in less than concrete + bar) it is over reinforced (tensile capacity of steel
is greater than concrete + steel tensile strength. The over reinforced fails
without giving prior warning and under reinforced fails but gives a deformation
warning before it fails. Therefore it is better to consider an under reinforced
concrete. The long process of inventing modern steel fiber reinforced concrete
started in 1874, when A. Bernard, in California, patented the idea of
strengthening concrete with the help of the addition of steel splinters (Maidl
1995). Another 36 years passed before Porter in 1910 mentioned the possibility
of applying short wire to concrete. This was supposed to improve homogeneity of
concrete reinforced by thick wire. In 1918, in France, H. Alfsen patented a
method of modifying concrete by long steel fibers, long wooden fibers, and
fibers made of other materials. According to him, the addition of such fibers
was to increase tensile strength of concrete (Maidl 1995). Alfsen was the first
to mention the influence of coarseness of the surface of fibers onto their
adhesiveness to matrix, and it was also he who paid special attention to the
problem of anchorage of fibers. After these first patents, there were numerous
others, but generally they concerned different shapes and probable applications
of readymade SFRC.
Fig: Concreting
For instance, the patent from
1927 worked out in California by G.C. Martin, regarded the production of SFRC
pipes. In 1938, N. Zitkewic patented a way to increase the strength and impact
resistance of concrete by adding cut pieces of steel wire (Jamrozy 1985). Steel
fibers, patented in 1943 by G. Constancinesco, were already very similar to the
ones used at present. The patent, apart from different shapes of fibers,
contained information about the kind and dispersion of cracks during loading of
SFRC elements and it made mentioned of the great amount of energy which is
absorbed by SFRC under impact. The largest number of patents concerning the use
of steel fibers to modify concrete have been submitted in the USA, France, and
Germany in the years following. Wide applications of fiber reinforced
composites in civil engineering were limited for a long time by lack of
reliable methods of examination and mainly by the sudden progress of
traditional rod reinforcement. Concrete
is most widely used construction material in the world due to its ability to
get cast in any form and shape. It also replaces old construction materials
such as brick and stone masonry. The strength and durability of concrete can be
changed by making appropriate changes in its ingredients like cemetitious
material, aggregate and water and by adding some special ingredients. Hence
concrete is very well suitable for a wide range of applications. However
concrete has some deficiencies as listed below:
1) Low tensile strength
2) Low post cracking capacity
3) Brittleness and low
ductility
4) Limited fatigue life
5) Incapable of accommodating
large deformations
6) Low impact strength
The presence of micro cracks in the
mortar-aggregate interface is responsible for the inherent weakness of plain
concrete. The weakness can be removed by inclusion of fibers in the mixture.
Different types of fibers, such as those used in traditional composite
materials can be introduced into the concrete mixture to increase its
toughness, or ability to resist crack growth. The fibers help to transfer loads
at the internal micro cracks. Such a concrete is called fiber-reinforced
concrete (FRC). The concept of using fibers in order to reinforce matrices weak
in tension is more than 4500 years old.
