The complete procedure of the grouting of pre/post-tensioned cable ducts, its materials, equipment and properties is explained in detail in the below article.
The 10 important precautions to be taken for effective grouting in tensioning ducts are explained below.
1. Optimum Temperature
The optimum temperature at which the grouting procedure can be carried out shall not exceed 25 degrees celsius. However, to increase the workability of grout, its temperature may be lowered by the use of chilled water or by putting ice outside the grout storage container.
2. Grouting in Cold Weather
In cold and frosty weather, the grout injection process shall be postponed. In extreme conditions, grouting can be carried out with special precautions. Prior to the commencement of grouting, care must be taken to ensure that the duct is completely free of frost/ice by flushing with warm water, but not with steam.
If frost is likely to occur within 48 hours after injection, heat must be applied to the member and maintained for at least 48 hours after injection so that the temperature of the grout does not fall below 5 degrees Celsius.
3. Threaded Cables
When the cables are threaded after concreting, the duct must be temporarily stiffened during concreting by inserting a bunch of strands, wires or reinforcement or a rigid PVC pipe or any other suitable method.
4. Care for Sheathing Damage
During concreting, care shall be taken to ensure that the sheathing is not damaged. Needle vibrators shall be used with extreme care by well-experienced personnel only, to ensure safety against such damage.
To avoid any blockages in the duct, it is a good practice to move the cables in both directions during the concreting operations. It is also advisable that after 3 to 4 hours of concreting, the cable should be moved both ways through a distance of about 20 cms.
With such movement, any leakage of mortar which has taken place in spite of all precautions, loses bond with the cables, thus reducing the chance of blockages.
6. Separation of Cables
The cables to be grouted should be separated by as much distance as possible to ensure effective grouting.
7. Stage-wise Stressing
In the case of stage prestressing, cables tensioned in the first stage should not remain ungrouted till all cables are stressed. After grouting the particular cable, the water in the other cables should be drained and removed with compressed air to prevent corrosion.
It is a good practice while grouting any duct in stage prestressing, to keep all the remaining ducts filled up with water containing 1% lime or by running water through such ducts till the grout has set.
8. Care at Joints
At the places where the 2 precast members are joined, special care should be taken to avoid leaks from one duct to another.
The end faces where anchorages are located are vulnerable points of entry of water. They have to be necessarily protected with an effective barrier. Recesses should be packed with mortar concrete and should preferably be painted with waterproof paint.
10. Protection of Vents
The grouting vents installed for grouting must be protected while concreting and after grouting is completed. The projecting portion of the vents should be cut off and the face of the vent must be protected to prevent corrosion.