Grouting of Post Tensioned Cables In I- Beam

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