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Spalled Concrete 1

SE2607

Structural
Sep 24, 2010
272
I'm going to look at a situation where a concrete beam has spalled concrete and the owner would like to repair it. I don't have any issue with doing this. I would probably specify something like Sika Dur Hi Mod 32 for a bonding agent, but my concern is, unless I shore the beam to relieve the load, the new concrete won't help much except for any live load that might be applied after the repair. I've never specified shoring of a concrete beam before. Is that practical? Any other thoughts?
TIA
 
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Wouldn’t the spalled concrete be protective in nature, rather than contributing to capacity? The only real reason to be concerned is live load vibration. I don’t think shoring would do much regardless. I think a removing loose concrete, squaring up area, rapid setting repair mortar would be best.
 
Yes, assuming the repair material needs to be loaded. Consider the cause before slapping more concrete on there, lest the spall recur (or worse). The bonding agent is not nearly as important as the surface prep. Lastly, I’ll add that the repair strategy changes greatly with spall depth and cause.
 
You certainly would not want to specify that particular bonding agent. You have an issue with open working times where the repair has to be formed, and concrete placed, prior to it expiring. With the product you selected that looks to be about 2 hours at room temperature from the datasheet. That is far too short a time to do everything that the contractor needs to do. We don't always use a bonding agent on our repairs but when we do it's usually SikaTop Armatec 110 Epocem, which has an open time of about 12 hours at room temp. That gives the crew a day or so to form & place concrete, which is realistic for smaller beam / soffit / vertical repairs (slab repairs it doesnt matter since a worker goes ahead of the pour and coats as they go). If you use two coats it's also a corrosion inhibitor, which is a bonus.

In terms of shoring, we do that all the time. What you do will depend on the particulars. Usually we use a passive system (No.2 post shores or 10kip frames). You can use an active system (where you jack the beam up) but we usually do not do that except for columns under high axial load, such as a building column.

There is a pair of papers by John Cairns (one in 1996 in the Journal of Structural Engineering and one in 1993 in the Journal of Civ. Engrs Structs & Bldgs) that compare beams that were repaired under load to those that were actively relieved of load and repaired. The results more or less turned out to be about the same for strength indicating relieving the load prior to repair was not necessary (though ductility and other aspects were affected, as you'd suspect). That said, we generally just don't do it in practice because of the expense involved.

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Generally do what enable does, usually skip the bonding agent, the correct mortar and prep, and join geometry, is much more important.
 

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