Prestressed Concrete Design – Concept
LOSS OF PRE-STRESS
A reduction in initial pre-stress resulting from the combined effect of creep, shrinkage or elastic shortening of the concrete, relaxation of the reinforcing steel, frictional losses resulting from the curvature of the draped tendons and slippage at the anchorage.
- The steel wires of a pre-stressed concrete member do not retain all the preliminary pre-stress .
- The initial pre-stress in concrete undergoes a gradual reduction with time from the stage of transfer due to various causes.A loss of pre-stress will affect the stress distribution on the section of the member. The loss of pre-stressed takes place due to many causes. In general these can be classified as:Loss of pre-stress during the tensioning process Loss of pre-stress at the anchoring stage. Losses occurring subsequently
- In addition there may be losses of pre-stress due to sudden changes in temperature, especially in steam curing of pre- tensioned units.
The rise in temperature causes a partial transfer of pre-stress (due to elongation of the tendons b/w adjacent units in the long line process) which may cause a large amount of creep if the concrete is not properly cured.
LOSS OF PRE-STRESS DURING THE TENSIONING PROCESS DUE TO FRICTION
- Friction in the jacking and anchoring system and on the walls of the duct where the wires fan out at the anchorage with the result, the actual stress in the tendons is less than what is indicated by the pressure gauge.
The losses due to friction in the jack and at the anchorage are different for different system of pre-stressing.
This loss due to friction may be classified into:
- Loss Due To Length Effect
The extent of friction met with in a straight tendon due to slight imperfection of the duct (the straight tendon).
- Hence the cable will touch the duct or concrete, wobbing effect, or wave effect
- Loss due to curvature effect
- In the case of curved ducts, the loss of pre-stress depends upon the radius of curvature of the duct and the coefficient of friction between the duct surface and the tendons.
LOSS OF PRESTRESS AT THE ANCHORING STAGE
- This loss is due to the fact that the anchorage fixtures themselves are subjected to a stretch.
- It is also possible that the friction wedges holding the wires the wires may slip a little
- The necessary additional elongation may be provided for at the time of tensioning to compensate for this loss.
LOSS OF PRESTRESS OCCURIING SUBSEQUENTLY
The loss which occur subsequently to pre-stress are:
- Loss Of Stress Due To Shrinkage Of Concrete:
Contraction of concrete due to chemical changes and drying. This depends only on the interval of time and the moisture conditions, but is independent of the stresses in the members due to loads
By minimizing the water cement ration and proportion of cement, the shrinkage can be reduced.
- Loss Of Stress Due To Creep Of Concrete
Creep of concrete means the deformation of concrete, which depends upon the interval of time to which the member is loaded
This additional deformation of the stressed member is remaining in a stressed state is called CREEP.
- Loss Of Stress Due To Elastic Shortening Of Concrete
(a) Pre-tensioned member
Due to the pre-stress transfer to the concrete, the concrete will shorten. This results in a corresponding shortening of steel
(b) Post tensioned member
Suppose only a single tendon has been provided in a member, the concrete gets shortened as the tendon is jacked against it.
Hence, after tightening, no more shortening of concrete can take place
· Loss Of Stress Due To Creep Of Steel(Stress Relaxation)
Total loss of pre-stress depends on many factors such as properties of concrete and steel, moisture and curing condition, the magnitude and system of pre-stress
- The first three types of losses take place due to reduction in the length of concrete resulting in reduction in reduction of the initial extension of the steel.
- The loss of stress due to elastic shortening of concrete is maximum in pre-tensioned members.
- In the case of post –tensioned members those losses occur only when a number of cables are progressively stressed one after another.
APPLICATIONS OF THE PRE-STRESSED CONCRETE:
MEGA FLOOR,the Prestressed slab
Slab: hollow slab, preslab or predalle, Prestressed ribs and blocks , lintels.
Beam: Prestressed rectangular beam and I-beam for bridges
Other prestressed components: Lintels , Wineyard stud.
Concrete is an all-round construction material. Almost every building contains some concrete, but its questionable application in certain buildings-for example in its use in the style of brutalism – has brought it into discredit. Its dull grey colour has contributed to the fact that the word concrete has become a synonym for ugly. In the field of bridges, concrete deserves a more favourable judgement.
- Not all concrete bridges have turned out to be beauties, but pleasing bridges can be built with concrete if one knows the art. Concrete is poured into forms as a stiff but workable mix, and it can be given any shape; this is an advantage and a danger. The construction of good durable concrete requires special know-how – which the bridge engineer is assumed to have.
- Pre-stressed concrete – if correctly designed – also has high fatigue strength under the heaviest traffic loads.
- Pre-stressed concrete bridges soon became much cheaper than steel bridges, and they need almost no maintenance – again assuming that they are well designed and constructed and not exposed to de-icing salt.
- In bridge building, concrete beams and arches predominate. The shaping of concrete is usually governed by the wish to use formwork, which is simple to make. Plain surfaces, parallel edges and constant thickness are preferred. This gives a stiff appearance to concrete bridges, and avoiding this is one task of good aesthetic design
- All types of structures can be built with reinforced and pre-stressed concrete: columns, piers, walls, slabs, beams, arches, frames, even suspended structures and of course shells and folded plates.
- Foundation panels
- Modular block retaining wall system
- Wall panels
- Concrete units
- Roofing and flooring
- Lintel and sunshade
- Columns girders
- STRAND Wrapped circular pre-stressed concrete tanks are long life liquid storage structure with virtually no maintenance.
- Concrete construction makes for a substantial, sturdy tank structure that easily contain the internal liquid pressure while comfortably resisting external forces such as earthquake, wind.
- These tanks are used in portable water treatment and distribution system, wastewater collection and treatment system and storm water management.
- They are also used in a variety of commercial applications including thermal energy storage, LNG containments, large industrial process tanks and large bulk storage tanks.
- Pre-stressed concrete is the most efficient material for water tanks and coupled with the circular shape, eliminates all stress conditions.
- By placing the steel of the pre-stressed strands in tension and the concrete in compression, both materials are in an ideal states and the loads are uniformly distributed around the tank circumference.
- Low maintenance can be enjoyed throughout the life as these are built with concrete, durable material that never corrodes and does not require coatings when in contact with water or the environment.
- Pre-stressing counteracts the differential temperature and dryness loads that a tank core wall experience. The tank walls are wet on the inside and dry on outside and the temperature varies between the two sides. If not properly accounted for, these moisture and temperature differential will cause a tank wall to bend and crack. Counteract these force in both the vertical and horizontal direction and diminish subsequently the cracking and leaking
- Tanks are very ductile, enabling to withstand seismic forces and varying water backfill.
- Tanks utilize material efficiently – steel in tension, concrete in compression
- Pre-cast tanks can store or treat anything from potable water to hazardous waste to solid storage bins.
- Storage capacities can range from 0.4 to 120 mega liters
- Diameters of the tank can vary up to 90 m
- Pre-cast concrete wall elements are usually pre tensioned vertically in the plant and post tensioned horizontally through ducts cast in the panels.
- Vertical pre stressing diminishes vertical bending in the wall and subsequent leakage
- Circumferential pre-stressing counteracts bursting loads from interior liquid.
- Joints closures are usually poured concrete on site. This method of sealing the joints allow the tank to [perform (after post – tensioning) as a monolithic structure to resist hydraulic, temperature and seismic forces.
SHAPE OF THE TANKS
- PRECAST CONCRETE foundation panels consist of steel reinforced concrete studs, reinforced tops and bottom beams and concrete facing.
- Insulation can be placed between the studs.
- A typical panellised foundation can be erected in four to five hours, according to a manufacturer, with no on site concrete work (the panels sit on gravel bed in lieu of footings.)
- Fast Track Easy Installation
- Low Cost Walling
- Durable Low Maintenance Walling
- All Weather Construction
- No Trades – Foundation Free Walling
- High Impact Resistance – Cladding Protection
- Added Security and Fire Stop Properties
- Steel Frame Compatible Walling System
- Fast Simple and Robust Angle Bracket Connection
- Tongue and Groove Joints
- Variable Height Walling
- Simple Top Lifter Installation
- Any Length
- Ideally suited for many Building Application such as Industrial, Light Industrial & Commercial, Heavy Duty Warehousing, Bulk Storage and Waste Transfer Stations
- Pre-stressed Concrete Panel Division are manufacturers and suppliers of pre-stressed concrete panels for use in both agricultural and construction industries.
- These panels are used in a variety of applications including fast track wall construction, retaining walls, bulk storage bunkers, grain stores and silage clamps.
- Manufactured in lengths of up to 7000mm, the pre-stressed panels are offered in a selection of thickness to suit various applications and a range of widths. Their simple clamp bracket connection and tongue/grooved profile make for a fast and efficient installation system.
- Low maintenance, competitive price and aesthetic appearance of the pre-cast concrete poles make them superior to steel or wood for use in utility, communication area lighting application
- The use of concrete poles preserve our forest requires no chemical treatment and utilizes environmentally safe materials in he production and placement.
- Some other benefits are corrosion resistance, long service life
- Because of mineral vibration and deflection, pre-cast concrete poles offer greater service life to ballast for light and this in turn means less down time and less costly equipment repair.
Pre-cast concrete poles can save erection time and money by eliminating the need for anchor based structure, which may take days or weeks to install.
PRECAST STAIRS AND LANDINGS
MODULAR BLOCK RETAINING WALL SYSTEM
- Modular block or segmental, retaining walls employ interlocking concrete units that tie back into the earth to efficiently resist loads.
- These pre- engineered modular systems are an attractive, economical and durable an alternatives to atone or poured concrete retaining walls.
The inherent design flexibility can accommodate a wide variety of site constraint, project sizes and aesthetic preferences.
- Controlled manufacturing conditions ensures a durable, damage resistant product.
- These systems allow for some flexibility, such as curved walls .
- Construction is generally faster than poured in place concrete or stonewalls
- Site conditions have a major impact on cost
APPLICATIONS (some examples):
- The following fig. Shows the typical pre-stressed concrete flat slab floor construction using the lift slab technique