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:

(i) Potential blockage by grout
(ii) Excessive development of friction
(iii) Difficulty in threading prestressing tendon

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

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.

(ii) It usually allows easy access to anchorages and provides the ease of inspection.

(iii) It allows the adjustment and control of tendon forces.

(iv) It permits the designer more freedom in selecting the shape of cross section of bridges.

(v) Webs could be made thinner so that there is a reduction of dead load.

(vi) It enhances a reduction of friction loss because the unintentional angular change like wobble is eliminated. Moreover, the use of polyethylene sheathing with external prestressing has lower friction coefficient than corrugated metal ducts in internal prestressing.

(vii) Improvement of concrete placing in bridge webs owing to the absence of ducts.

The major distinction between internal prestressing and external prestressing lies in the variation in cable eccentricity. The deflected shape of external tendons is not exactly the same as beams because the
displacement of external tendons is controlled by deviators. This is a second order effect at working load and it is very important at ultimate load.

Based on past research, for small span with shallow cross section (i.e. less than 3m deep), the use of internal prestressing requires less steel reinforcement. However, for deeper bridge cross section, the employment
of external prestressing results in smaller amount of steel reinforcement.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

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.

Central prestressing means that the prestressing cables are arranged such that the resultant compressive stresses at all points in a given cross section are equal and it does not matter whether tensile stresses occur in upper or lower portion of superstructure during launching process.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

GENERAL
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.

Material
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).

Equipments
1. Grout Mixer Mechanical type
2. Grout Pump J-600
3. Grout Screen
4. Connection and air vents
5. Generator
6. Thermometer, Stopwatch etc.

Procedure of the Grout
1)After measuring the slip of 24hrs, the extended cables shall be cut off
50mm away from bearing plates.
2) Cement mortar of 1:1 ratio is applied over the Bearing Plates on both ends of girders to prevent the leakage of Grout. Grouting operation shall be commenced after two days of sealing the ends.
3) Water cement ratio should be as low as possible consistent with workability. This ratio is 0.42 (not more than 0.45 as per MOST,P-677) proportions of material shall be based on field trials made on the Grout before commencement of grouting. As per specifications, the temperature of the Grout maintained at 250C by adding ice into water if necessary.
4) Water shall be poured in to the mixer with Port land cement and admixture is added into it. Mixing shall be continued for duration to obtain uniform and thoroughly blended Grout. Grout shall be continuously agitated and then pour into another tank after passing through the screen.
5) Ducts shall be flushed with water for cleaning as well as for wetting the surface of the duct walls.
6) The water in the duct shall be blown out with compress air.

Injection Of Grout:-

1) After mixing of Grout, all connections from tank to pump and pump to inlet shall be checked.
2) The grout shall be allowed to flow freely from the other end until the consistency of the grout at this end is the same as that of the Grout at the injection end.
3) When the Grout flows at the other end, it shall be closed off and grouting is continued so that pressure commenced, full injection pressure at about 5 kg/cm2 shall be maintained for at least one minute before closing the injection pipes.
4) If there is leakage observed at any of ends the grouting operation shall be discontinued and the entire duct flushed with high-pressure water. Grout not used within 30minutes of mixing shall be rejected.
6) Check the Compressive strength of the cubes for the grout in 10 cm cubes for 7 days, which should not be less than 17 Mpa.
7) Grouting record for each cable shall be maintained as per Performa in MOST.

This information was submitted by Er. Neha Sood on 3d June 2008