Grout encapsulation of post tensioning strand

[tassie]

We are in the process of completing a post tensioned bridge with a specified 100 year maintenance free design life. Part of the post tensioning force is provided by 6No. 12.7mm strand in a flat duct, 19mm x 85mm. Tendons are grouted normally - cement/water grout pumped under pressure until a consistent flow emerges from the next vent. Vents are capped under pressure.
My client has questioned how we can ensure complete encapsulation of tendons given the congestion within the duct. I don't see this problem as any different from the congestion you would normally get at the top of a draped duct where all tendons are forced to the top of the duct.
Does anyone have any knowledge of the long term affect on durability of strand in contact with metal duct and not fully coated with grout? Ducts are fully surrounded by concrete.

 

[flamby]

The space is really tight and even if you could ensure the encapsulation of the cables by grout, the grout between the cables is going to crush anyway making the whole excercise of grouting pointless. If you have some healthy corrosion allowance considered in the cables, leave the cables without grouting.

 

[RiBeneke]

I have not seen flat ducts is use. Is the duct profile straight or draped ?

If the situation is similar to the more common circular ducts, then I do not agree with flame's comments.
Grout that is correctly mixed and placed has a considerable strength, easily 25 MPa / 3500 psi, and will not crush unless it is subjected to some localised load. It serves the purpose of corrosion protection and also of bonding the tendon to the duct to give greater resistance to failure of the structure in certain circumstances.

To answer the original question, if the grout fill is sufficient to ensure that water and air cannot migrate through along the duct, then it probably does not matter if the strand is fully coated, as it will be in a corrosion-inhibiting alkaline environment.
The problem, however, is that the grout fill is seldom airtight, as the grout cannot easily get to the central wire of the strand. Then if there are any air pockets present in the grout, the air in them will expand/contract with temperature and barometric changes, and there will be a tendency to draw in oxygen from the ends of the strands.

Perhaps you should do tests by grouting short sample lengths of both circular and flat duct and then cut them open for inspection.

 

[Ingenuity]

tassie,

We have used flat galvanised duct extensively for grouted/bonded PT tendons of the size that you indicate, with great success over many years.

Grouting is a critical construction excercise that must be undertaken with an experienced crew using materials and equipment that are proven to work in the given application and environment.

For special structures (100 year life probably counts here) you should possibly consider the PTI "Specification for Grouting of PT Structures" dated Feb 2001. It has several classes of grout - with various tests for bleed, thixotropy, trials, batching etc.

I disagree with FLAME on leaving the tendons without grout - that WILL create corrosion problems. The annular space between strand and duct needs to be filled with some corrosion protection product - grease for unbonded tendons and grout for bonded tendons. All you need to initiate corrosion is oxygen and water - if the grout bleeds then that will provide the water, and corrosion soon after!

Given that this is near the end of the construction of this structure, you may wish to recommend to your client that they invest in a monitoring system that can determine strand and even individual wire breaks (in both gronted and ungrouted tendons). It will not stop the corrosion but it will continuously monitor the structure for such problems.

If your client is asking to you to verify that there are no voids that will be a challenge. Impact echo has been used in the past with some limited success, but I am guessing that these tendons are transverse in a bridge deck and hence the profile is probable very shallow and grouting should have been relatively straight forward with an experienced crew. For larger multistrand tendons with circular duct, the individual strand cross-over each other, and for large/deep profiles it does create some grouting challenges.

The wicking effect that RiBeneke discusses can be reduced with good grout pocket parging and ensuring full grout inside the tendon trumpet and anchorage.

For problematic tendons, vacuum assisted techniques are available, but specialised.

If you do find/know of voids, very low viscosity epoxy resin has been successfully injected into such voids for long-term durability, but QA/QC of thhis technique is a concern.

HTH

 

[austim]

Surely the grout could not possibly 'crush', since it would be injected after the tendons were tensioned, and all of the transverse loads on the tendons had already been taken up by direct contact tendon/tendon or tendon/duct?

I agree with the generally accepted practice of making sure that the ducts have been 'fully' grouted. As far as QA of the grouting is concerned, see if you can get hold of a copy of the Institution of Civil Engineers Proceedings for September 1964, Paper No 6799, Medway Bridge - Design by O.A. Kerensky and G. Little. (Downloadable from the ICE Virtual Library

http://www.iceknowledge.com/,

£5 to ICE members, £15 to others). That has some useful stuff regarding the gamma ray photography used to verify grouting in the precast viaduct beams.

 

[flamby]

All,
I am still feeling the grout will crush. The gap between the cables can as small as 1.5 mm, or even less even with the best workmanship. When the cables go through load changes, the change in diameter (due to poisson effect)will be sufficient to crack or crush the 'film' of 1.5 mm grout, assuming your grout had actually reached this gaps. I have nothing to say against greasing however, as RiBeneke has proposed. Nor I am against grouting in a circular duct, which I also do. My answer was in context of 6-12.7mm dia cable passing through 19x85 mm opening.
flame

 

[boo1]

In Florida, (US) the department of transporation has done extensive research on tendon corrosion: see

http://www11.myflorida.com/structures/posttensioning.htm

From what i read they recommended;
1. Epoxy seal the pre-cast tendon in place.
2. Fully grout (approved product) the internal tendons.
3. Insure proper cover.
4. Consider an elasomeric coating system to seal the surfaces.