Pre-Stressed Concrete

Introduction Lakshitha Fernando First Class in BSc.Eng(Hons) University of Moratuwa

What is Pre-Stressing?

External Compressive Force

Tensile force due to self weight of the books

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• “The act of applying forces to a structure other than the loads the structure is designed to carry, in order to enhance the structure's ability to carry those loads”

What is Pre-Stressing?

Principles in Pre-stressing

• There is a deficiency

• The opposite of the deficiency could be provided

• There is a method of applying the force

• There is a method of locking the force in the position

Significant amounts of energy are stored in the system

Concrete

• A building material which is strong in compression but relatively weak in tension

• Solutions

– Reinforcing- Steel carries the tension

– Pre-stressing – Application of pre-compression to counteract the tensile stresses

Pre-Stressed Concrete

• There is a deficiency- lack of tensile strength

• The opposite of the deficiency could be provided – compressive strength

• There is a method of applying the force – using a jack

• There is a method of locking the force in the position – anchorages/bonds

• Significant amounts of energy are stored in the system

History

• Considere- French Engineer (1880s)

• Sir William Glanville – USA (1886s)

• Wettstein (1921)

• Dill – USA (1925)

“The Father of Pre-Stressed concrete”

Basic Concepts

• Design based on stress control (French approach)

– The pre- stress is selected to ensure that stresses under working loads do not exceed permissible stresses

• Design based on strength (German approach)

– Concerned with the ultimate strength of the section

• Design based on deflections (American approach)

– Load balancing

Advantages of Pre-Stressed Concrete

• Crack free structure under service loads – Full section is utilized - higher stiffness – smaller deflection – improved serviceability – Reduction of steel corrosion-increase in durability

• Slender sections can be used – reduction in self weight – Economical sections – Larger spans are possible

• More adaptable to pre-casting

– Rapid construction – Repetitive construction

Disadvantages of Pre-stressed Concrete

• Costly

– Requires high strength material

– Requires larger capital

• Requires highly trained personnel

– Design

– Production – supervision and quality assurance

Concrete for Pre-stressed Concrete

• OPC based concrete with strength usually greater than 50N/mm2

• A high early strength is required to enable quicker application of pre stress

Steel for Pre-stressing

• Wires- (diameter:3mm-7mm)

• Strand- produced by spinning wires around a central core wire (diameter: 8mm-18mm)

• Bars – (diameter: 20mm-40mm)

Forms of Pre-stressing

• Pre-tensioning & Post-tensioning

• Bonded & Unbonded

• Partial & Full

Pre-tensioning & Post-tensioning

• Pre-tensioning

– Step1 : Steel tendons either pass through a single mould or a line of moulds for multiple members arranged end to end and are attached at one end to a fixed anchorage

– Step 2: The tendons are then tensioned from the fixed anchorage between an external independent anchorage to give the required tensile force in the tendon

– Step 3: The tendons are held in place while the concrete is poured

– Step 4 : When the concrete has hardened sufficiently the ends of the tendons are slowly released from the external anchorages. The tendons are restrained from regaining their original length by the development of bond stresses between the concrete and the tendon, and it is these bond stresses that transfer the compressive stress to the concrete

– Step 5 : The tendons are then trimmed

• Key Points

– Straight profiles – holding down for curved profiles

– Small diameter wires – for better bonding

– Long line method – for mass production

Post Tensioning

– Step 1: The tendon is placed in the correct position in the formwork with a dead-end anchorage and a ‘live-end’ anchorage, through which the tendon passes. The tendon may be placed in plastic or metallic sleeve or later inserted into a duct made out of thin steel tubes or rubber tubes

– Step 2: Concrete is poured and left to harden

– Step 3: When the concrete has gained sufficient strength a jack is attached to the ‘live-end’ anchorage and the tendon is stressed to the required force

– Step 4: To prevent the tendon from slipping back it is locked into the anchorage by means of a split wedge located in the barrel of the recessed anchorage. The tension force in the tendon is transferred to the concrete as a compressive force by the reaction at the anchorages

– Step 5: The jack is then removed

• Key points

– If needed, the tendon can be jacked from both ends – to reduce friction losses

– The ducts may or may not be grouted – grouting could provide additional security

• If grouted- bonded pre-stressing

• If not grouted- unbonded pre-stressing

Pre-Tensioning vs Post-Tensioning

Pre-Tensioning

• Suitable for short spans

• Generally straight tendon profiles

• Cheaper

Post-Tensioning

• Suitable for medium and long spans

• Can use curved tendons to get any desired profile

• Costlier

Bonded and Unbonded Pre-stressing

• Bonded Pre-stressing

– Pre- tensioned and grouted post-tensioned

– For grouted post- tensioned with tendons placed in metallic or plastic sleeves

• When concrete has gained sufficient strength the tendon is stressed and then the sleeve is filled with grout under pressure

• The grout bonds the tendons to the sleeve thus enabling the transfer of stress from the tendons to the concrete

• For grouted post- tensioned with tendons placed in ducts formed in the concrete during placement

– A rubber tube is placed in the required position before the concrete is poured

– When the concrete has hardened sufficiently the tube is removed and the tendon is passed through the duct

– Once the tendon is in the correct position, it is stressed and finally grouted to provide the required bond between the concrete and tendon

Unbonded Pre-stressing

• Post tensioned members in which sleeves or ducts are not grouted

Bonded Pre-stressed Unbonded Pre-stressed

Pre- tensioned Yes No

Post-tensioned with tendons placed in metallic

or plastic sleeves

If grouted If not grouted

Post- tensioned with tendons placed in ducts formed in the concrete

during placement

If grouted If not grouted

Full and Partial Pre-stressing

• Full pre-stressing

– When a member is designed so that no tensile stresses develop under service loads

• Partial pre-stressing

– When some tensile stresses are allowed and concrete is allowed to crack under maximum service load

Applications

• Railway sleepers

• Communication poles

• Pre-tensioned pre-cast Hollowcore Slabs

• Pre-tensioned pre-cast piles