Concrete Column Capacity, Reduced Cover

[DtaylorStructeng]

Investigating the moment capacity of some 1200 reinforced concrete Piles.
Due to a bad pour the cover has been compromised and has reduced strength, say 40Mpa in for 50Mpa design strength.

There are inclusions and voids in the cover region of the piles due to issues with bentonite. Some reo was exposed when investigated. The internal diameter within the bars appears sound after investigation.

The problem: There will be a reduced section capacity, I am going to do an analysis with Moment interaction diagram neglecting any cover, (ag cover = 0) for the structural analysis.

However with the cover compromised, can you still develop stress in the steel without the full bond of the bars within the concrete matrix?

Any ideas?

 

[ishvaaag]

For limit loads quite likely yes, think for example in external prestress being still an able structural mechanism, or cable-post trusses. Mechanisms where the deformation in the steel becames the same between friction points.

However since quite likely a proper model of the lack of bond (geometrical 3D shape of the situation will be lacking) you may add a factor to reduce capacity on such intent. If after rational estimate you still have reasonable doubt of the piles having the necessary strength, adding more or microplile may be the solution.

 

[Zambo]

Are there 1,200 piles or the diameter is 1,200mm?

Is it just one pile affected or many? If just one has a check been made to see if the problem continues at a depth exceeding say -5m?

 

[csd72]

sounds like a major durability headache to me.

 

[InDepth]

The main issue will be durability and possible issues with lap splice length. Are your longitudinal bars confined by spiral ties? If so, you could possibly take advantage of concrete confinement to effectively increase your concrete strength. We do this in seismic design, and we also assume the cover spalls and is thus not typically included in our analysis. You may want to also look at making a spring model that takes the soil properties (bracing...)...which then can effectively shorten the column and increase capacity.

 

[InDepth]

In terms of the corrosion issue, you can either calculate a corrosion rate and compare it to the expected life time of the structure....you can combine this with comparing it to the demands. This is what we do with sheet pile calculations...take the upper limit of what is required for demand and what is required for corrosion serviceability. Your worst case is going to be at the top of a water table...and for corrosion to occur, water needs to be present...so...do you have water?

You can also solve the problem by adding sacrificial anodes "pukes", I believe Sika makes some, and i've specified them for concrete repairs. But...they have an effective distance. The other more complicated solution is to add active corrosion protection through an electric circuit.

 

[InDepth]

Galvashield

http://usa.sika.com/en/solutions_products/02/02a002/02a002sa14.html

...or if all else fails...core a micropile(s) through the center.