Post-tensioned concrete looks
& acts just like other reinforced concrete. Post-tensioning is
simply a way
to reinforce in a more active way.
What is Post-Tensioning ?
Post tensioning
is a technique for reinforcing concrete. Post-tensioning tendons, which are
prestressing steel cables inside plastic ducts or sleeves, are positioned in
the forms before the concrete is placed. Afterwards, once the concrete has
gained strength but before the service loads are applied, the cables are pulled
tight, or tensioned, and anchored against the outer edges of the concrete.
Post-tensioning
is a form of prestressing. Prestressing simply means that the steel is stressed
(pulled or tensioned) before the concrete has to support the service loads.
Most precast, prestressed concrete is actually pre-tensioned-the steel is
pulled before the concrete is poured. Post-tensioned concrete means that the
concrete is poured and then the tension is applied-but it is still stressed
before the loads are applied so it is still prestressed.
ADVANTAGES & APPLICATIONS OF
POST-TENSIONING
Post-tensioning,
which is a form of prestressing, has several advantages over standard
reinforcing steel (rebars):
·It reduces or eliminates shrinkage cracking-therefore no joints,
or fewer joints, are needed
·Cracks that do form are held tightly together
·It allows slabs and other structural members to be thinner
·It allows us to build slabs on expansive or soft soils
·It lets us design longer spans in elevated members, like floors
or beams
Some of the more common applications are:
·Slabs on ground: Today, PT is used extensively for slabs on grade where
soils are likely to move (expansive soils)-especially in the American
southwest. Jim Rogers, editor and publisher of Post Tension Magazine,
says that until housing construction ground to a halt last year, about half of
all post-tensioning work was slabs-on-ground for homes.
·Another good application for PT slabs is producing crack-free tennis
courts.
·A recently developed application of PT is external post-tensioning for
strengthening of existing structures, especially as an upgrade to resist
seismic forces.
Bridge designers have used PT both for cast-in-place concrete and for
precast segmental construction. PT allows longer spans and keeps cracks tight.
·Concrete water tanks are often post-tensioned to reduce crack width and
leakage. The companies that make prestressed concrete tanks are Crom, DYK, Natgun,
and Preload.
·Masonry walls can be post-tensioned-this is usually done with a solid
steel bar fastened to the foundation and stressed with a nut at the wall's top.
A fire sprinkler or sprinkler head is
the component of a fire sprinkler system that discharges water when
the effects of a fire have been detected, such as when a predetermined
temperature has been exceeded. Fire sprinklers are extensively used worldwide,
with over 40 million sprinkler heads fitted each year. In buildings protected
by properly designed and maintained fire sprinklers, over 99% of fires were
controlled by fire sprinklers alone.
Contents
1History
2US
regulations
2.1Quick
Response Sprinklers
4Types
4.1ESFR
History
In 1812, British inventor Sir William Congreve patented
a manual sprinkler system using perforated pipes along the ceiling. When
someone noticed a fire, a valve outside the building could be opened to send
water through the pipes. It was not until a short time later that, as a
result of a large furniture factory that repeatedly burned down, Hiram
Stevens Maximwas consulted on how to prevent a recurrence and invented the
first automatic fire sprinkler. It would douse the areas that were on fire and
report the fire to the fire station. Maxim was unable to sell the idea
elsewhere, though when the patent expired, the idea was used.
Henry S. Parmalee of New Haven, Connecticut created
and installed the first automatic fire sprinkler system in 1874,
using solder that melted in a fire to unplug holes in the otherwise
sealed water pipes. He was the president of Mathusek Piano Works, and
invented his sprinkler system in response to exorbitantly high insurance rates.
Parmalee patented his idea and had great success with it in the U.S., calling
his invention the "automatic fire extinguisher". He then
traveled to Europe to demonstrate his method to stop a building fire before
total destruction.
Parmalee's invention did not get as much attention as he had
planned, as most people could not afford to install a sprinkler system. Once he
realized this, he turned his efforts to educating insurance companies about his
system. He explained that the sprinkler system would reduce the loss ratio, and
thus save money for the insurance companies. He knew that he could never
succeed in obtaining contracts from the business owners to install his system
unless he could ensure for them a reasonable return in the form of reduced
premiums.
In this connection, he was able to enlist the interest of
two men, who both had connections in the insurance industry. The first of was
Major Hesketh, a cotton spinner in a large business in Bolton who was
also Chairman of the Bolton Cotton Trades Mutual Insurance Company. The
Directors of this Company and its Secretary, Peter Kevan, took an interest in
Parmalee’s early experiments. Hesketh got Parmalee his first order for
sprinkler installations in the cotton spinning mills of John Stones &
Company, at Astley Bridge, Bolton. This was followed soon afterwards by an
order from the Alexandra Mills, owned by John Butler of the same town.
An 1897 Grinnell automatic sprinkler advertisement
Although Parmalee got two sales through its efforts, the
Bolton Cotton Trades Mutual Insurance Company was not a very big company
outside of its local area. Parmalee needed a wider influence. He found this
influence in James North Lane, the Manager of the Mutual Fire Insurance
Corporation of Manchester. This company was founded in 1870 by the Textile
Manufacturers' Associations of Lancashire and Yorkshire as
a protest against high insurance rates. They had a policy of encouraging risk
management and more particularly the use of the most up-to-date and scientific
apparatus for extinguishing fires. Even though he put tremendous effort and
time into educating the masses on his sprinkler system, by 1883 only about 10
factories were protected by the Parmalee sprinkler.
Back in the U.S., Frederick Grinnell, who was
manufacturing the Parmalee sprinkler, designed the more effective Grinnell
sprinkler. He increased sensitivity by removing the fusible joint from all
contact with the water, and, by seating a valve in the center of a
flexible diaphragm, he relieved the low-fusing soldered joint of the
strain of water pressure. By this means, the valve seat was forced against the
valve by the water pressure, producing a self-closing action. The greater the
water pressure, the tighter the valve. The flexible diaphragm had a further and
more important function. It caused the valve and its seat to move outwards
simultaneously until the solder joint was completely severed. Grinnell got a
patent for his version of the sprinkler system. He also took his invention
to Europe, where it was a much bigger success than the Parmalee version.
Eventually, the Parmalee system was withdrawn, opening the path for Grinnell
and his invention.
US regulations
Fire sprinkler application and installation guidelines, and
overall fire sprinkler system design guidelines are provided by the National
Fire Protection Association (NFPA) 13, (NFPA) 13D, and (NFPA) 13R.
California, Pennsylvania and Illinois require
sprinklers in at least some new residential construction.
Fire sprinklers can be automatic or open orifice. Automatic
fire sprinklers operate at a predetermined temperature, utilizing a fusible
element, a portion of which melts, or a frangible glass bulb containing liquid
which breaks, allowing the plug in the orifice to be pushed out of the orifice
by the water pressure in the fire sprinkler piping, resulting in water flow
from the orifice. The water stream impacts a deflector, which produces a
specific spray pattern designed in support of the goals of the sprinkler type
(i.e., control or suppression). Modern sprinkler heads are designed to direct
spray downwards. Spray nozzles are available to provide spray in various
directions and patterns. The majority of automatic fire sprinklers operate
individually in a fire. Contrary to motion picture representation, the entire
sprinkler system does not activate, unless the system is a special deluge type.
Open orifice sprinklers are only used in water spray systems
or deluge sprinklers systems. They are identical to the automatic sprinkler on
which they are based, with the heat-sensitive operating element removed.
Automatic fire sprinklers utilizing frangible bulbs follow a
standardized color-coding convention indicating their operating
temperature. Activation temperatures correspond to the type of hazard against
which the sprinkler system protects. Residential occupancies are provided with
a special type of fast response sprinkler with the unique goal of life safety.
Quick Response Sprinklers
The NFPA #13 standard was revised in 1996 to require Quick
Response Sprinklers in all buildings with light hazard occupancy
classification.
The 2002 edition of the NFPA #13 standard, section 3.6.1
defines quick response sprinklers as having a response time index (RTI) of 50
(meters-seconds)1/2 or less. RTI is a measure of how thermally
responsive the heat-responsive element of the sprinkler is, measured as: the
time it takes to comes up to 63% of the temperature of a hot air stream times
the square root of the velocity of the air stream.
The term quick response refers to the listing of the entire
sprinkler (including spacing, density and location) not just the fast
responding releasing element. Many standard response sprinklers, such as
extended coverage ordinary hazard (ECOH) sprinklers, have fast responding (low
thermal mass elements) in order to pass their fire tests. Quick response
sprinklers are available with standard spray deflectors, but they are also
available with extended coverage deflectors.
QUICK RESPONSE FIRE SPRINKLERS
Quick Response per NFPA 13 RTI < 50 (ms)1/2
Nominal Diameter in mm
Norbulb Model[
Operating Time in Seconds
Response Time Index (RTI) (ms)1/2
Yes
2.5
N2.5
9
25
Yes
3
N3
11.5
33
Yes
3.3
N3.3
13.5
38
No
5
NF5
23
65
No
5
N5
32
90
Operation
Standard spray sprinkler head with a blue bulb indicating a
high release temperature
Each closed-head sprinkler is held closed by either a
heat-sensitive glass bulb (see below) or a two-part metal link held together
with a fusible alloy such as Wood's metaland other
alloys with similar compositions. The glass bulb or link applies pressure
to a pipe cap which acts as a plug which prevents water from flowing until the
ambient temperature around the sprinkler reaches the design activation
temperature of the individual sprinkler. Because each sprinkler activates
independently when the predetermined heat level is reached, the number of
sprinklers that operate is limited to only those near the fire, thereby
maximizing the available water pressure over the point of fire origin
The bulb breaks as a result of the thermal expansion of
the liquid inside the bulb. The time it takes before a bulb breaks is
dependent on the temperature. Below the design temperature, it does not break,
and above the design temperature, it breaks, taking less time to break as
temperature increases above the design threshold. The response time is
expressed as a response time index (RTI), which typically has values between 35
and 250 m½s½, where a low value indicates a fast
response. Under standard testing procedures (135 °C air at a velocity
of 2.5 m/s), a 68 °C sprinkler bulb will break within 7 to 33
seconds, depending on the RTI. The RTI can also be specified in imperial
units, where 1 ft½s½ is equivalent to 0.55 m½s½.
The sensitivity of a sprinkler can be negatively affected if the thermal
element has been painted.
Maximum Ceiling Temperature
Temperature Rating
Temperature Classification
Color Code (with Fusible Link)
Liquid Alcohol in Glass Bulb Color
100 °F / 38 °C
135-170 °F / 57-77 °C
Ordinary
Uncolored or Black
Orange (135 °F / 57 °C) or Red (155 °F /
68 °C)
150 °F / 66 °C
175-225 °F / 79-107 °C
Intermediate
White
Yellow (175 °F / 79 °C) or Green (200 °F /
93 °C)
225 °F / 107 °C
250-300 °F / 121-149 °C
High
Blue
Blue
300 °F / 149 °C
325-375 °F / 163-191 °C
Extra High
Red
Purple
375 °F / 191 °C
400-475 °F / 204-246 °C
Very Extra High
Green
Black
475 °F / 246 °C
500-575 °F / 260-302 °C
Ultra High
Orange
Black
625 °F / 329 °C
650 °F / 343 °C
Ultra High
Orange
Black
From Table 6.2.5.1 NFPA13 2007 Edition indicates the
maximum ceiling temperature, nominal operating temperature of the sprinkler,
color of the bulb or link and the temperature classification.
Types
There are several types of sprinklers:
Quick
response
Standard
response
CMSA
(control mode specific application)
Residential
ESFR
(early suppression fast response)
ESFR
ESFR (early suppression fast response) refers to both a
concept and a type of sprinkler. "The concept is that fast response of
sprinklers can produce an advantage in a fire if the response is accompanied by
an effective discharge density — that is, a sprinkler spray capable of fighting
its way down through the fire plume in sufficient quantities to suppress the
burning fuel package." The sprinkler that was developed for this
concept was created for use in high rack storage.
ESFR sprinkler heads were developed in the 1980s to take
advantage of the latest fast-response fire sprinkler technology to provide fire
suppression of specific high-challenge fire hazards. Prior to the
introduction of these sprinklers, protection systems were designed to control
fires until the arrival of the fire department.
Almost two third of electricity requirement of the world is fulfilled
by thermal power plants (or thermal power stations).
In these power stations, steam is produced by burning some fossil fuel
(e.g. coal) and then used to run a steam turbine. Thus, a thermal power station
may sometimes called as a Steam Power Station. After the steam
passes through the steam turbine, it is condensed in a condenser and again fed
back into the boiler to become steam. This is known as ranking cycle.
This article explains how electricity is generated in thermal power plants.
As majority of thermal power plants use coal as their primary fuel, this
article is focused on a coal fired thermal power plant.
Typical Layout And Working Of A Thermal Power Plant
A simplified layout of a thermal power station is shown
below.
Coal: In a coal based
thermal power plant, coal is transported from coal mines to the generating
station. Generally, bituminous coal or brown coal is used as fuel. The coal is
stored in either 'dead storage' or in 'live storage'. Dead storage is generally
40 days backup coal storage which is used when coal supply is unavailable. Live
storage is a raw coal bunker in boiler house. The coal is cleaned in a magnetic
cleaner to filter out if any iron particles are present which may cause wear
and tear in the equipment. The coal from live storage is first crushed in small
particles and then taken into pulverizer to make it in powdered form. Fine
powdered coal undergoes complete combustion, and thus pulverized coal improves
efficiency of the boiler. The ash produced after the combustion of coal is
taken out of the boiler furnace and then properly disposed. Periodic removal of
ash from the boiler furnace is necessary for the proper combustion.
Boiler: The mixture of
pulverized coal and air (usually preheated air) is taken into boiler and then
burnt in the combustion zone. On ignition of fuel a large fireball is formed at
the center of the boiler and large amount of heat energy is radiated from it.
The heat energy is utilized to convert the water into steam at high temperature
and pressure. Steel tubes run along the boiler walls in which water is
converted in steam. The flue gases from the boiler make their way through
superheater, economizer, air preheater and finally get exhausted to the
atmosphere from the chimney.
Superheater: The superheater tubes are hanged at the hottest part of the
boiler. The saturated steam produced in the boiler tubes is
superheated to about 540 °C in the superheater. The superheated high
pressure steam is then fed to the steam turbine.
Economizer: An economizer is essentially a feed water heater which heats the
water before supplying to the boiler.
Air pre-heater: The primary air fan takes air from the atmosphere and it is then
warmed in the air pre-heater. Pre-heated air is injected with coal in the
boiler. The advantage of pre-heating the air is that it improves the coal
combustion.
Steam turbine: High pressure super-heated
steam is fed to the steam turbine which causes turbine blades to rotate. Energy
in the steam is converted into mechanical energy in the steam turbine which
acts as the prime mover. The pressure and temperature of the steam falls to a
lower value and it expands in volume as it passes through the turbine. The
expanded low pressure steam is exhausted in the condenser.
Condenser: The exhausted steam is
condensed in the condenser by means of cold water circulation. Here, the steam
loses it's pressure as well as temperature and it is converted back into water.
Condensing is essential because, compressing a fluid which is in gaseous state
requires a huge amount of energy with respect to the energy required in
compressing liquid. Thus, condensing increases efficiency of the cycle.
Alternator: The steam turbine is
coupled to an alternator. When the turbine rotates the alternator, electrical
energy is generated. This generated electrical voltage is then stepped up with
the help of a transformer and then transmitted where it is to be
utilized.
Feed water pump: The condensed water is
again fed to the boiler by a feed water pump. Some water may be lost during the
cycle, which is suitably supplied from an external water source.
This was the basic working principle of a thermal power station and
its typical components. A practical thermal plant possess more complicated
design and multiple stages of turbine such as High Pressure Turbine (HPT),
Intermediate Pressure Turbine (IPT) and Low Pressure Turbine (LPT).
Advantages And Disadvantages Of A Thermal Power Plant
Advantages:
Less initial cost as compared to other generating stations.
It requires less land as compared to hydro power plant.
The fuel (i.e. coal) is cheaper.
The cost of generation is lesser than that of diesel power plants.
Disadvantages:
It pollutes the atmosphere due to the production of large amount of
smoke. This is one of the causes of global warming.
The overall efficiency of a thermal power station is low (less than
30%).
Efficiency Of A Thermal Power Station
A huge amount of heat is lost in various stages of the plant. Major part
of heat is lost in the condenser. That is why the efficiency of thermal plants
is quite low.
Thermal Efficiency: The ratio of 'heat equivalent of mechanical energy transmitted to
the turbine shaft' to the 'heat of coal combustion' is called as thermal
efficiency.
Thermal efficiency of modern thermal power stations is about 30%. It
means, if 100 calories of heat are produced by coal combustion, the mechanical
energy equivalent of 30 calories will be available at the turbine shaft.
Overall Efficiency: The ratio of 'heat equivalent of electrical output' to the 'heat
of coal combustion' is called as overall efficiency.
The overall efficiency of a thermal plant is about 29% (slightly less than the
thermal efficiency).
There are various types of cement used in concrete construction. Each
type of cement has its own properties, uses and advantages based on composition
materials used during its manufacture.
Types
of Cement and their Uses
1.Ordinary Portland Cement (OPC)
2.Portland Pozzolana Cement (PPC)
3.Rapid Hardening Cement
4.Quick setting cement
5.Low Heat Cement
6. Sulfates resisting cement
7.Blast Furnace Slag Cement
8.High Alumina Cement
9.White Cement
10. Coloured cement
11.Air Entraining Cement
12.Expansive cement
13. Hydrographic cement
1. Ordinary Portland
Cement (OPC)
Ordinary Portland
cement is the most widely used type of cement which is suitable for all general
concrete construction. It is most widely produced and used type of cement
around the world with annual global production of around 3.8 million cubic
meters per year. This cement is suitable for all type of concrete
construction.
2. Portland Pozzolana
Cement (PPC)
Portland pozzolana
cement is prepared by grinding pozzolanic clinker with Portland cement. It is
also produced by adding pozzolana with the addition of gypsum or calcium sulfate
or by intimately and uniformly blending Portland cement and fine pozzolana.
This cement has high
resistance to various chemical attacks on concrete compared with ordinary
Portland cement and thus it is widely used. It is used in marine structures,
sewage works, sewage works and for laying concrete under water such as bridges,
piers, dams and mass concrete works etc.
3. Rapid Hardening Cement
Rapid
hardening cement attains high strength in early days it is used in concrete
where form works are removed at an early stage and is similar to ordinary
Portland cement (OPC). This cement has increased lime content and contains
higher c3s content and finer grinding which gives greater strength development
than OPC at an early stage.
The strength
of rapid hardening cement at the 3 days is similar to 7 days strength of OPC
with the same water-cement ratio. Thus, advantage of this cement is that form work can be removed earlier which increases the rate of construction and
decreases cost of construction by saving form work cost.
Rapid
hardening cement is used in prefabricated concrete construction, road works,
etc.
4. Quick setting
cement
The
difference between the quick setting cement and rapid hardening cement is that
quick setting cement sets earlier while rate of gain of strength is similar to
Ordinary Portland Cement, while rapid hardening cement gains strength quickly. Form works in both cases can be removed earlier.
Quick
setting cement is used where works is to be completed in very short period and
for concreting in static or running water.
5. Low Heat Cement
Low heat
cement is prepared by maintaining the percentage of tricalcium aluminate below
6% by increasing the proportion of C2S. This makes the concrete to produce low
heat of hydration and thus is used in mass concrete construction like gravity
dams, as the low heat of hydration prevents
the cracking of concrete due to heat.
This cement has
increased power against sulfates and is less reactive and initial setting time
is greater than OPC.
6. Sulfates Resisting Cement
Sulfate resisting
cement is used to reduce the risk of sulfate attack on concrete and thus is
used in construction of foundations where soil has high sulfate content. This
cement has reduced contents of C3A and C4AF.
Sulfate
resisting cement is used in construction exposed to severe sulfate action by
water and soil in places like canals linings, culverts, retaining walls,
siphons etc.
7. Blast Furnace Slag Cement
Blast
furnace slag cement is obtained by grinding the clinkers with about 60% slag
and resembles more or less in properties of Portland cement. It can be
used for works economic considerations is predominant.
8. High Alumina Cement
High alumina
cement is obtained by melting mixture of bauxite and lime and grinding with
the clinker. It is a rapid hardening cement with initial and final setting time
of about 3.5 and 5 hours respectively.
The compressive
strength of this cement is very high and more workable than ordinary Portland
cement and is used in works where concrete is subjected to high temperatures,
frost, and acidic action.
9. White Cement
It is
prepared from raw materials free from Iron oxide and is a type of ordinary
Portland cement which is white in color. It is costlier and is used for
architectural purposes such as precast curtain walls and facing panels,
terrazzo surface etc. and for interior and exterior decorative work like
external renderings of buildings, facing slabs, flooring, ornamental concrete
products, paths of gardens, swimming pools etc.
10. Coloured cement
It is
produced by mixing 5- 10% mineral pigments with ordinary cement. They
are widely used for decorative works in floors.
11. Air Entraining Cement
Air entraining cement
is produced by adding indigenous air entraining agents such as resins, glues,
sodium salts of sulfates etc. during the grinding of clinker.
This type of
cement is especially suited to improve the workability with smaller water
cement ratio and to improve frost resistance of concrete.
12. Expansive Cement
Expansive cement
expands slightly with time and does not shrink during and after the time of hardening.
This cement is mainly used for grouting anchor bolts and prestressed
concrete ducts.
13. Hydrographic cement
Hydrographic
cement is prepared by mixing water repelling chemicals and has high
workability and strength. It has the property of repelling water and is
unaffected during monsoon or rains. Hydrophobic cement is mainly used for the
construction of water structures such dams, water tanks, spillways, water
retaining structures etc.
Post-tensioning
is a form of prestressing. Prestressing simply means that the steel is stressed
(pulled or tensioned) before the concrete has to support the service loads.
Most precast, prestressed concrete is actually pre-tensioned-the steel is
pulled before the concrete is poured. Post-tensioned concrete means that the
concrete is poured and then the tension is applied-but it is still stressed
before the loads are applied so it is still prestressed.
