Concrete repair and repair of stirrups 2

[jjalex1]

I have to do a concrete repair detail for a pier column that has a spall measuring 8.5'high x 3' wide x 2.5" deep.There is exposed rebar and 7 of the 8 stirrups exposed have 100% loss as well as 2 main vertical reinforcement bars.I was wondering do I repair the vertical reinforcement bars by a lap splice (of 30 bar diameters)and repair the splices with a lap splice (of 30 bar diameters)

 

[rowingengineer]

I would consider repair by using a full strength mechanical splice if possible to an area of the vertical steel that is unaffected by corrosion. to replace the stirrups I would consider FRP wraps to the outside of the column.

How you stop this from happening in the future is also of interest, the new bar could be galv or similar, but how do you ensure the remaining steel is likey to remain serviceable for an extended period, that are of spalling is high. Maybe you could wrap the entire column to increase corrosion resistance.

A picture may help us suggest for ideas, and also we would be interested.

http://www.nceng.com.au/

"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected” Petroski 1992

 

[bridgebuster]

You can also weld a piece of bar.

 

[Teguci]

It is quite likely that your concrete is contaminated with chlorides. Chip out and finger each of the vertical rebars. As for lapping or mechanical splicing, that is up to you. For the ties I would consider a larger pier section with brand new ties space allowing.

 

[JAE]

One other idea for the tie replacement is to wrap the column with steel bands at similar spacing to the column ties. The steel can be cut to an exact length, heated and welded to cool tight around the column once the vertical bars are repaired and the concrete patched.

I would agree with the concerns above about contaminated concrete - there is a good chance that the rust (and further section loss) occurs beyond the spalled area. Some further investigatory demolition of remaining concrete cover beyond the spall might be warranted.

 

[jjalex1]

@ bridgebuster

I have a similar detail to the one you have and was wondering can I also use that for the repair of the stirrup because I assumed that could only be used for straight bars.The column is square.

 

[jjalex1]

My detail has the contractor to chip out concrete at least an inch past the spalled concrete.

@JAE do you have a detail of a column with steel bands?

@rowingengineer I was always curious about FRP but isn't it expensive in comparison to tradional concrete and shotcrete?I was going to use Sika Armatec 110

 

[SkiisAndBikes]

If dealing with an 8.5ft high x by 3ft wide spall, it is very likely, if not certain that the removal area will extend beyond those dimensions. Solid concrete is normally removed around the perimeter of the spalled area, until at least 6" of clean, rust free rebar is exposed. The removal may have to extend further to accommodate lap splices, etc...

I request that the concrete be removed to an even depth of at least 1" behind the reinforcement, not 1" past the spalled concrete. With columns and walls, I often find that one of the causes relates to lack of proper concrete cover, therefore, the face of the column or wall is built out with the repair material to provide proper cover. I do like using Armatec 110 as it has a long open time, but the contractor better have his ducks in a row to get the forms up and concrete replaced within the open time.

With 100% section loss there must be a significant reason for the deterioration ... i.e. source of chlorides, perhaps a drainpipe, etc... If the source cannot be rectified, a thin type waterproofing membrane might prolong the life of the repair.

Depending on the precentage of the column face you are removing, you might want to consider removing 100% of the cover concrete, replacing the stirrups and building out the entire column to provide adequate concrete cover to the new stirrups.

 

[jjalex1]

@SkiisandBikes

The spall is 2 1/2" with a 3" cover so I guess it would make sense to removing 100% of the cover concrete.How would you indicate that you are replacing all of the stirrups?Would you do it in a note or mention it in the procedure and detail drawing?

 

[bridgebuster]

@jjalex1 - you can repair the stirrup by welding.

 

[JAE]

jjalex1, I don't have a detail with the bands - but basically it was a 1/4" to 3/8" thick x 2" (or maybe 3") wide strap. The strap was designed to extend completely around the square columns in two separate "U" shapes. The dimension of the strap was initially designed just a hair short (so for a 24" concrete column the strap would be initially fabricated to be 23.9" or so. The strap was heated up (expanding) and placed on the column. Then the ends were full-pen welded. Upon cooling it was tight to the column.

Fairly simple and provided the necessary confinement - only external to the column. This was done on a parking garage where they discovered (after construction) that the contractor had left off the column ties in the lower sections of the columns where the vertical bars lapped just above the floors. Apparently with the laps the ties wouldn't fit so they left them off. Contractor logic I guess.

 

[SkiisAndBikes]

jjalex1, seems strange to me that the spall is only 2-1/2" deep when the concrete cover is 3". Normally the corrosion of the rebar at cover level or a bit deeper is where the delaminations start. Once you get into removals (unless the delam has already spalled), I am sure you will find areas deeper than 2-1/2".

Specifying the extent of removal is quite often an iterative process. For small areas of repair I would suggest:
1. Contractor/Consultant meet on site with can of paint and hammer. Sound area and agree on initial removal, keeping in mind geometry of removal patch.
2. Contractor completes initial concrete removal as per the details.
3. Consultant visits site, reviews exposed repair area and makes decision on whether or not further removal is required to chase any corroded rebar, further concrete removal for patch profile and/or overall patch geometry, further removal for lap lengths, bruised concrete, etc..... Stirrups would be evaluated at this point.
4. Contractor completes final removal and patch preparation.

Having a contractor experienced in concrete rehabilitation work can reduce the steps above.

The replacement of steel for a small job can be handled within the procedures/details. Rebar can be added to the bid form on a per weight basis for new bar supplied and installed.

 

[rowingengineer]

jjalex1,
yes FRP is expensive, but if this is a corrosive zone, probably the least likely to need high maintenance in the next few years.

http://www.nceng.com.au/

"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected” Petroski 1992

 

[TXStructural]

Longitudinal bars: lap splices will prove ineffective unless lapped inside a proper mass of concrete, and not lapped in areas of high stress. For this reason, use full strength mechanical splices.

Welding bars is unlikely to be satisfactory unless you can ascertain the carbon equivalent (CE) of the steel. The carbon content of common steel reinforcing bar makes it difficult to weld, plus you must avoid heating the bar below the cover. AWS D1.4 describes the required procedures.

External confinement and shear reinforcement can be tricky, but applying a strut and tie analysis/design will assure the basic forces are covered. External flat bar ties could be used but they should be be grouted on to assure 100% contact. There aren't any good guidelines for bend radii and capacity after bending for this application, so you are on your own. Hot bending may or may not change the properties significantly from the specification (ASTM A36, etc.) I found that by the time you grind the corners, fabricate and install the ties, and provide new cover for fire protection, it was generally better to remove the old cover and damaged bars, replace the bars, and reinstall cover. This also removes any chloride-laden concrete near the bars.

Installing new ties which will provide proper capacity is tricky. Do not have closed ties bent, and then straighten and re-bend - the cold work of the steel will damage the bar (possibly cracking or breaking it.) Overlapping U-bars can be used, but that depends on the stress level in the bars and size of the bars and columns. The use of 135 hooks on ties requires clearance to seat the tails, do not cut the tails shorter than the code-required minimums, remove more concrete as required.

On a beams, I built stirrups using the slab on top, flat bars on bottom, and ductile all-thread on the sides, taking all of the shear into the all-thread. I'm not sure about the equivalent on a column.

Replacement cover should surround all exposed old and new bars, and proper consolidation is very important. New concrete should be very low shrinkage or shrinkage-compensating to reduce the likelihood that the cover with crack away. ICRI describes the proper surface preparation, including roughness and moisture conditioning. Consider use of lightweight aggregates in the new concrete to help reduce shrinkage due to loss of moisture during curing.

 

[bgatto]

I would consider FRP too.

 

[jjalex1]

@bridgebuster

I like the detail you sent to me but where can I find the reference (which publication) that says I can use minimum lap length plus 25mm?

 

[TXStructural]

The detail shows the sister bar extending one lap length beyond the damaged area in the existing bar. The 25 mm refers to the use of welds, which do not require a regular lap length.
The use of a straight, lapped repair on a stirrup/tie is not recommended. The first thing to go in a deteriorating column or beam is the cover, and once the cover is gone, the lap will no longer function.

 

[bridgebuster]

jjalex1 - the detail came from a set of plans I have. It's made the rounds over the years on a variety of NYSDOT projects prepared by various consultants.