Bonded Post-tensioning Cover 1

[slickdeals]

All:
In the Indian code, the reinforcing cover is measured from face of concrete to face of duct for post-tensioned work. However, in the case of pre-tensioned concrete (no duct), the cover is measured to the face of the strand.

If I understand the logic correctly, is it to allow for force transfer through bond between duct & concrete? Can cover to bonded post-tensioning be to face of strand in the duct?

What are the requirements in other codes?

 

[ishvaaag]

I quote,
"FEDERAL HIGHWAY ADMINISTRATION 5/26/2004
POST-TENSIONING TENDON INSTALLATION AND GROUTING MANUAL
Chapter 3 – Post-Tensioning Duct and Tendon Installation
3.4.1.1 Ducts for Internal Tendons: Ι-Girders and Cast-in-Place Construction

Recommendations for ducts in concrete in Ι-girders (Figure 3.13):
• Maximum allowable size of aggregate should be specified
• The distance between the outside of the duct and the side of the web should be dequate to accommodate the vertical reinforcing and specified cover and provide the minimum concrete section to satisfy design requirements."

So sidewise filling 40% of the duct with strained tendon cover really amounts to practically to duct cover.

Will continue looking flexurally-wise.

 

[ishvaaag]

ACI 318 08 2.2 Definitions

"Cover, specified concrete — The distance between the outermost surface of embedded reinforcement and the closest outer surface of the concrete indicated on design drawings or in project specifications."

Embedments...

"6.3.10 — Specified concrete cover for pipes, conduits,
and fittings shall not be less than 1-1/2 in. for concrete
exposed to earth or weather, nor less than 3/4 in. for
concrete not exposed to weather or in contact with
ground."

Tolerances at 7.5.2.1

"R7.6.7.2 — When ducts for prestressing steel in a beam are arranged closely together vertically, provision should be made to prevent the prestressing steel from breaking through the duct when tensioned. Horizontal disposition of ducts should allow proper placement of concrete. A clear spacing of one and one-third times the size of the coarse aggregate, but not less than 1 in., has proven satisfactory. Where concentration of tendons or ducts tends to create a weakened plane in the concrete cover, reinforcement should be provided to control cracking."

Finally

"R7.7 — Concrete protection for reinforcement
Concrete cover as protection of reinforcement against weather and other effects is measured from the concrete surface to the outermost surface of the steel to which the cover requirement applies. Where concrete cover is prescribed for a class of structural members, it is measured to the outer edge of stirrups, ties, or spirals if transverse reinforcement encloses main bars; to the outermost layer of bars if more than one layer is used without stirrups or ties; or to the metal end fitting or duct on post-tensioned prestressing steel; or to the outermost part of the head on headed bars."

Without proceeding further, ACI 318 also requires cover be measured to duct surface for post-tensioned members, flexurally-wise or in whatever circumnstance.

 

[ishvaaag]

By the way these checks within a pdf document are started with Ctrl-F, that opens the entry box for the word to be searched. It is quite useful to ascertain the incidences of some specific word or concept.

 

[hokie66]

For structural capacity, corrosion protection, and fire rating, the important thing is the distance to the strand, not the duct. This will always be greater than the cover to the duct, so I am not sure of the reason for your question.

 

[slickdeals]

@Hokie,

Precisely what I was getting at. The reason for my question is that the Indian code requires the cover to be specified to the duct. I lose precious drape in a 200mm slab when cover is to duct and not strand.

I want to be able to use the cover as the distance to the strand and not the duct and hence, the question about what is out there in practice and other codes.

 

[rapt]

Slickdeals,

All codes I hace seen, exposure is cover to the duct.

Also, minimum cover for bonded is 25 mm to the duct to avoid blowout under grouting perssure.