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Prestress Engineering

What are the main potential benefits in using the bridge form of precast prestressed beams supporting in-situ concrete top slab?

The potential benefits of using the bridge form of precast prestressed beams supporting in-situ concrete top slab are:

(i) For bridges built on top of rivers and carriageway, this bridge form provides the working platform by the precast beams so that erection of falsework is not required.

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For incremental launching method, the span depth ratio of bridges is normally low. Why?

Bridges constructed by incremental launching method are usually low in span depth ratio and typical values are 14 to17. With low span depth ratio, the bridge segments are stiff in bending and torsion which is essential to cater for the launching process.

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What is the optimum size of cable duct for prestressing?

The cross sectional area of duct is normally 2.5 times that of the area of prestressing steel. The size of ducts should be not designed to be too small because of the followings:

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Why type of prestressing is better, external prestressing or internal prestressing?

At several locations in the span (i.e. third or quarter points) the tendons are deviated to the correct tendon profile by concrete deviators in external prestressing. The advantages of external prestressing are listed below:

(i) Owing the absence of bond between the tendon and structure, external prestressing allows the removal and replacement of one or two tendon at one time so that the bridge could be retrofitted in the event of deterioration and their capacity could be increased easily. This is essential for bridges in urban areas where traffic disruption is undesirable.

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Central prestressing is normally required during construction in Incremental Launching method. Why?

The erection condition plays an important role to the structural design of bridges when incremental launching method is adopted.

Each section of superstructure is manufactured directly against the preceding one and after concrete hardens, the whole structure is moved forward by the length of one section. When the superstructure is launched at prefabrication area behind one of the abutments, it is continually subjected to alternating bending moments. Each section of superstructure (about 15m to 25m long) is pushed from a region of positive moment and then to a region of negative moment and this loading cycled is repeated. As such, tensile stresses occur alternately at the bottom and top portion of superstructure section. For steel, it is of equal strength in both compression and tension and it has no difficulty in handling such alternating stress during launching process. However concrete could only resist small tensile stresses and therefore, central prestressing is carried out to reduce the tensile stress to acceptable levels.

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Grouting of Post Tensioned Cables In I- Beam

The purpose of grouting is to provide permanent protection to the post-tensioned steel against corrosion and to develop bond between the Prestressing cables and the surrounding structural concrete.Grouting shall be carried out as early as possible, but generally not later than two weeks of stressing.Whenever this stipulation cannot be completed with for unavoidable reasons adequate temporary protection of the cables against corrosion by methods or products, which will not impair the ultimate adherence of the injected grout shall be ensured till grouting.

1. Water : Only Clean potable water free from impurities shall be used.

2. Cement : Ordinary port land cement 43Grade shall be used. It should be as fresh as possible and free from any lumps.

3. Admixture : Non-shrink powder compound. (Intra Plast N-200 of Sika Brand).

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