CE7005 Prestressed Concrete Chapter-2 Materials For Prestressed Concrete

Outline High strength concrete High tensile steel

High strength concrete Higher strength is necessary for prestressed concrete for following reasons: Anchorages for prestressing steel always designed on the basis of high strength concrete. High strength concrete offers high resistance in tension and shear ,as well as bond and bearing.

High strength concrete High strength concrete is lees liable to the shrinkage cracks. It also has a higher modulus of elasticity and smaller creep strain ,resulting in smaller loss of prestress. By using high strength concrete the c/s area required for member will be reduced resulting less dead weight moment.

High strength steel Cold working: rolling the bars through a series of dyes. Stress relieving: heating the strand to about 350°C and cooling slowly. Improves plastic deformation of steel Strain tempering for low relaxation: heating the strand to about 350°C while it is under tension .Improves the stress- strain behaviour of steel . Also the relaxation is reduced.

Properties of prestressing steel High Strength Adequate Ductility Bendability High Bond Low Relaxation Minimum Corrosion

CE7005 Prestressed Concrete Chapter-3 Systems of prestressing

Out line Systems in pre-tensioning Systems in post-tensioning

Systems of prestressing As system of prestressing involves the process of tensioning the tendons and securing them firmly to the concrete. Pre- tensioning Hoyer long line system

Post-tensioning Freyssinet system Magnel balton system Gifford -Udall system P.S.C mono wire system Lee-mc-call system Electrical prestressing Chemical prestressing

Hoyer long line system The Hoyer system is usually adopted for the production of pre –tensioned members on a large scale. It consists of stretching wires between two abutments at a large distance apart. With this Hoyers process, several members can be produced along one line, by providing suitable shuttering.

Hoyer long line system After concrete has hardened ,the wires are released from abutment The prestress is transferred to the concrete through bond between the tendons and the concrete

Freyssinet system Freyssinet system was first to be introduced among the post –tensioning systems. Anchor element consisting in two parts, cylindrical piece with tapered hole (sleeve)and a cone(wedge). The cylindrical piece is a helically reinforced concrete unit cast with M60 concrete.

Freyssinet system The tapered hole in the cylindrical piece is lined with closely wound helix of 2.5mmǿ steel wires of strength of 200N/mm2 is that it should be stronger than the wires pulled through the cone. The cone is made with M100 concrete A small tube is inserted inside the cone to facilitate grouting.

Freyssinet system The surface of the cone is grooved with grooves depending on the number of wires to be stretched. As the wires are stretched and stressed ,released, cone slips in to the sleeves. The space between the wires will be filled with the grout. This provides additional restraint against the slipping of the tendons. Normally Freyssinet system is used for pulling 5mm wires 12 to18Nos.

Magnel Balton System The wires are arranged with four wires per layer. The wires in the same layer and the wires in adjacent layers are separated with a clearance of 4mm spacers. The wires are anchored by wedging two at a time in to sandwich plates. The sandwich plates are about 25mm thick and are provided with two wedge-shaped grooves on its two faces.

Magnel Balton System The wires are taken two in each groove and tightened. Then a steel wedge is driven between the tightened wires to anchor them against the plate. A complete anchorage unit may consists of one to eight sandwich plates. Each plate can anchor eight wires. Two wires are tensioned at a time

Magnel Balton System

Gifford Udall System (method-1) Gifford Udall system is another wedge system. Small wedge is slit in to 2 units and the cut pieces have teeth on the inner face. The wires are stressed and anchored one by one in a separate cylinder using small wedging grips. Theses wedge grips are called Udall grips.

Gifford Udall System(method-2) Tube anchorage consists of a bearing plate, anchor wedges and anchor grips. Anchor plate may be square or circular and have 8 or 12 tapered holes to accommodate the individual prestressing wires. These wires are locked into the tapered holes by means of anchor wedges.

Gifford Udall System In addition, grout entry hole is also provided in the bearing plate for grouting. Anchor wedges are split cone wedges carrying wires on its flat surface. There is a tube unit which is a fabricated steel component incorporating a thrust plate, a steel tube with a surrounding helix.

Lee mc call system This method is used to prestress steel bars. The diameter of the bar is between 12 and 28mm. bars provided with threads at the ends are inserted in the performed ducts. After stretching the bars to the required length, they are tightened using nuts against bearing plates provided at the end sections of the member

P.S.C Monowire system Wires are tensioned individually The anchorage consists of a single piece collect sleeve wedging in a conical hole A steel truncated guide leads each wire from the cable to the anchorage point along a gentle curvature In addition to the guide a central block is also provided to anchor the central wires.

Electrical Prestressing in this method, reinforcing bars is coated with thermoplastic material such as sulphur or low melting alloy and buried in the concrete. After the concrete is set, electric current of low voltage but high amperage is passed through the bar. Electric current heats the bar and the bar elongates. Bars provided with threads at the other end are tightened against heavy washers, after required elongation is obtained. When the bar cools, prestress develops and the bond is restored by resolidification of the coating.  

Chemical Prestressing Chemical prestressing is done using expanding cement. Prestressing can be applied embedding steel in concrete made of expanding cement. Steel is elongated by the expansion of the concrete and thus gets prestressed. Steel in turn produces compressive stress in concrete.