2. We are the Main Contractor, our Employer intends to use Post-Tensioning Beams in our Multi-Storey Structure Construction, but we are unfamiliar with the construction sequence and other details.
Our Answer to you:
The Post-Tensioning Beams System to be used in your project would depend on the design of the Employer’s Consultant, it would thus be a UNIQUE layout to cater for your building requirements and details. Your Employer’s Consultant would be able to advice you further on the post-tensioning requirements before you start work.
In order to make things simpler, I would share with you my experience in my 1st Post-Tensioning Beams Project in 1994, when I was involved in the supervision and construction of the Inland Revenue Authority of Singapore Building at Thomson Road.
In this project, there is a 24 Storey Tower Block. More than 80% of the beams supporting the Tower Block slabs were designed to be post-tensioning beams. The maximum length of the post-tensioning beams were about 15m supporting a slab width of 3.5m.
Tendon Profile Checking & Stressing
After the profile of the tendon ducts were checked to be within the tolerance of ±5mm vertically and ±20mm horizontally, the post-tensioning beams were casted with the concrete slabs at that storey in one operation.
The Primary Post-Tensioning Beams were stressed first, after which, we checked that the concrete and formwork were able to resist the stressing force of the Secondary Beams before stressing the Secondary Beams.
The wires or strands were only cut or bent beyond 300mm of the anchorages until 7 days after the grouting of the tendon ducts. The tendons were cut back to give a minimum 30mm cover after concreting the recesses. The interior surfaces of anchorages were scabbled and coated with an approved epoxy resin before casting of the recesses.
For Post-tensioning of the beams, the stressing forces, extensions and sequence were planned so that the prestress was as uniformly as possible. In order to reduce the risk of early shrinkage cracking, we stressed the tendons in 2 stages. The 1st stage was 50% of the final prestress force and was carried out when the concrete had obtained adequate strength of 20 N/mm2 for the anchorage being used (your project requirements could be different).
The stressing of the tendons were performed smoothly, tendon ducts were cleaned by blowing oil-free compressed air through them and anchorage prior to commencement of tensioning. A force was initially applied to take up the slack of the tendon. After taking up the slack, the tendon would then be marked at the end and elongations or draw-in measured from these markings.
Elongation of Tendons after Stressing
After the post-tensioning work, if the duct were not grouted within 14 days of the commencement of stressing, the ducts were sealed to prevent tendons from corrosion. Before grouting of the tendon ducts, they were washed, blown off the excess water by oil-free compressed air. Allowance was made for any slippage or draw-in at the anchorage devices on the release of the jacks. The final forces in each tendon as measured by the dynamometer were within the accuracy of 3% of the value stipulated. In completion of tensioning, each tendon to the required force, the check measurement of elongation from its calculated value was within 5% tolerance.
(Your Project Requirements may differ from the above.)
Elongation of Tendons being Excessive?
During the discussion with the Post-Tensioning Specialist, I was informed that the following steps could be adopted when the elongation of the tendon was more than ±5% compared with the calculated value:
A. Re-calibration of the equipment;
B. Testing of Tendon Material to check secant modulus;
C. Tendon released and re-stressed (secant modulus applicable to 2nd stressing to be adopted);
D. Lubrication of Tendons to reduce friction losses, only water soluble oils should be used in ducted system and these should be washed out before grouting (for elongation of tendon more than –5% i.e. could be excessive friction loss due to seepage of grout into the duct);
E. Where only one jack was used previously, the tendon to be tensioned from both ends using two jacks;
F. Tendon released and re-stressed to 80% of the Ultimate Tensile Strength of the tendon (normally 75% in design consideration);
G. Other method to be proposed by the Post-Tensioning Specialist and agreed by the Consultants.
Possible Slippage of Tendon?
If one or more components of tendons of a group stressed together slips during stressing operation (but this did not happen in my project), you will permit a compensating increase in the elongation of the remaining tendons of the same group, provided the jacking force does not exceed 85% of the minimum ultimate tensile strength of the remaining tendons.
In the case of a tendon breaking or slippage after tensioning (but it did not happen in my project), the tendon will need to be released, replaced if necessary, and re-stressed. However, the jacking force is not to exceed the rated capacity of the jacking equipment used, or 85% of the specified minimum ultimate strength of the tendon, whichever is the lesser.
(Your Project Requirements may differ from the above.)
When should the Slabs behind the Stressing End to be casted?
The slabs behind the stressing end were casted together with the Post-tensioning beams with additional reinforcement to resist cracking of the slab during stressing (to be installed according to the Designer’s detail), so that the slabs would not have excessive cracking during the post-tensioning process. An opening was left at the stressing end locations to enable the stressing work to be done smoothly.
Some designers may prefer to cast the slabs behind the stressing end after the post-tensioning process, in this case, cold joints will form in the slab and it may not be desirable for the entire structure.
With the above mentioned, you would realize that to coordinate and construct the aforesaid system is never an easy task. The Post-tensioning Work in the aforesaid project was a success; as there was no excessive elongation of the tendons and no slippage occurred in any of the Post-tensioning beams. And after the construction, there was no major defects seen.
This could never be possible without the hard work of the entire team; as all of us had worked late or even overnight to ensure the quality of the materials used on site; thorough checking were also conducted before each casting operation and before each stressing operation.
Although so many years have past after we completed this project, and I have lost touch with many of our team members. I want to use this opportunity to thank them again:
Thanks for your commitment and hard work. I think you would feel proud like me whenever you drive pass or walk pass the Building, it’s the fruit of our hard work as we had left our sweat and blood there; especially in the midst of chaotic situations when mistakes were made by others and we had to rush to help to ratify them... Thankfully, with God’s help & blessings, He has crowned our efforts with success. It’s been a pleasure working with you all in the team & I look forward to other opportunities in the future, where we could still work together. Finally, I wish you all the best in all your endeavours.
Er. Goh Cheow Leng Maria
First drafted on 10 September 2008.