Type of concrete design
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Reinforce concrete
Pre-cast concrete
In – Situ concrete
Pre stressed concrete
Post stressed concrete
Light weight concrete
Heavy weight concrete
Reinforce concrete
Reinforced concrete is one of the most widely used modern building materials. Concrete is an “artificial stone” obtained by mixing cement, sand, and aggregates with water. Fresh concrete can be molded into almost any shape, giving it an inherent advantage over other materials. It became very popular after the invention of Portland cement in the 19th century; however, its limited tension resistance initially prevented its wide use in building construction. To overcome poor tensile strength, steel bars are embedded in concrete to form a composite material called reinforced concrete. The use of RC construction in the modern world stems from the wide availability of its ingredients – reinforcing steel as well as concrete. Except for the production of steel and cement, the production of concrete does not require expensive manufacturing mills. But, construction with concrete does require a certain level of technology, expertise and workmanship, particularly in the field during construction. Despite this need for sophistication and professional inputs, a large number of single-family houses or low-rise residential buildings across the world have been and are being constructed using RC without any engineering assistance. Such buildings, in seismic areas, are potential death traps. This is the motivation behind developing this tutorial.
Reinforced concrete is one of the most widely used modern building materials. Concrete is an “artificial stone” obtained by mixing cement, sand, and aggregates with water. Fresh concrete can be molded into almost any shape, giving it an inherent advantage over other materials. It became very popular after the invention of Portland cement in the 19th century; however, its limited tension resistance initially prevented its wide use in building construction. To overcome poor tensile strength, steel bars are embedded in concrete to form a composite material called reinforced concrete. The use of RC construction in the modern world stems from the wide availability of its ingredients – reinforcing steel as well as concrete. Except for the production of steel and cement, the production of concrete does not require expensive manufacturing mills. But, construction with concrete does require a certain level of technology, expertise and workmanship, particularly in the field during construction. Despite this need for sophistication and professional inputs, a large number of single-family houses or low-rise residential buildings across the world have been and are being constructed using RC without any engineering assistance. Such buildings, in seismic areas, are potential death traps. This is the motivation behind developing this tutorial.
Pre - cast concrete
Precast
concrete is a
construction product produced by casting concrete in a reusable mold or "form"
which is then cured in a controlled environment, transported to the
construction site and lifted into place. In contrast, standard concrete is poured into site-specific forms and
cured on site.
In – Situ concrete
Cast in place. Concrete is
poured where at its permanent location. For example, a manhole cast in-situ
will be formed and poured at the pipe connection. This is the opposite of
pre-cast, meaning it is cast off-site and transported to its permanent
location.
Pre stressed concrete
Prestressed Concrete is an architectural and structural material
possessing great strength. The unique characteristics of prestressed concrete
allow predetermined, engineering stresses to be placed in members to counteract
stresses that occur when the unit is subjected to service loads. This is
accomplished by combining the the best properties of two quality materials:
high strength concrete for compression and high tensile strength steel strands
for tension.
Actually, prestressing is quite simple. High tensile strands are
stretched between abutments at each end of long casting beds. Concrete is then
poured into the forms encasing the strands. As the concrete sets, it bonds to
the tensioned steel. When the concrete reaches a specific strength, the strands
are released from the abutments. This compresses the concrete, arches the
member, and creates a built in resistance to service loads
Post stressed concrete
Post-tensioned concrete is a
term heard more and more in the construction industry today. This method of
reinforcing concrete enables a designer to take advantage of the considerable
benefits provided by prestressed concrete while retaining the flexibility
afforded by the cast-in-place method of building concrete structures.
Post-tensioning is simply a
method of producing prestressed concrete, masonry, and other structural
elements. The term prestressing is used to describe the process of introducing
internal forces (or stress) into a concrete or masonry element during the
construction process in order to counteract the external loads applied when the
structure is put into use (known as service loads). These internal forces are
applied by tensioning high-strength steel, which can be done either before or
after the concrete is placed. When the steel is tensioned before concrete
placement, the process is called pretensioning. When the steel is tensioned
after concrete placement, the process is called post-tensioning. Because
pretensioning requires specially designed casting beds, it is used generally in
the precast manufacturing process to make simple shapes that can be trucked to
a jobsite. Post-tensioning is done onsite by installing post-tensioning tendons
within the concrete form-work in a manner similar to installing rebar.
Light Weight concrete
Lightweight concrete mixes are commonly used in the
construction industry where weight savings is an important factor. One of the
most common uses for lightweight concrete is with floor, roof or bridge decks;
others include pavement systems, masonry blocks and offshore oil structures.
Lightweight concrete is made by replacing some or all of the normal weight
aggregate with lightweight aggregate. Often the coarse fraction is replaced
with lightweight aggregate and the fines are normal weight sand.
Structural lightweight aggregate concrete is defined as
concrete which:
is made with lightweight aggregates conforming to ASTM C
330,
has a compressive strength in excess of 2500 psi at 28 days
when tested in accordance with methods stated in ASTM C 330,
and has an air dry density of no more than 115 pounds per
cubic foot as determined by ASTM C 567.
High performance lightweight concretes are typically made
using expanded clay, shale or slate. These lightweight aggregates weigh less
than normal weight aggregates (crushed limestone, granite, quartz, etc.) due to
the porous cellular structure of the individual aggregate particles. The
cellular, or “foamed”, structure is created at temperatures of about 2000
degrees F or higher. At these high temperatures the parent material “puffs” and
expands to form foamed rock.
Heavy weight concrete
Heavyweight concrete
uses heavy natural aggregates such as barites or magnetite or manufactured
aggregates such as iron or lead shot. The main land-based application is for
radiation shielding (medical or nuclear). Offshore, heavyweight concrete is
used for ballasting for pipelines and similar structures.
The density achieved
will depend on the type of aggregate used. Typically using barites the density
will be in the region of 3,500kg/m3, which is 45% greater than that
of normal concrete, while with magnetite the density will be 3,900kg/m3,
or 60% greater than normal concrete. Very heavy concretes can be achieved with
iron or lead shot as aggregate, 5,900kg/m3 and 8,900kg/m3 respectively.
Cement contents and
water/cement ratios are similar to those for normal concretes, but the
aggregate/cement ratios will be significantly higher, because of the higher
density of the aggregates. Heavyweight concrete can be batched, transported and
placed using conventional equipment, though there are obviously certain
aspects, such as the amount that can be carried by a ready-mixed truck or
handled in a skip, that will be limited by the density. Because of the higher
density, formwork pressures will be increased. The rate of wear of mixers and
pumps will also be increased. Compaction will require more energy than normal
concrete and poker vibrators will have to be inserted at closer centres. There
may be a greater tendency for the mix to bleed.
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