Spalled Concrete 2

[SE2607]

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

 

[jhnblgr]

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.

 

[ANE91]

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.

 

[Enable]

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.

 

[canwesteng]

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

 

[JAE]

Agree but the need for temporary shoring highly depends on the extent, and position, of the spalling/repair on the structural members.

Don't just add the high cost of shoring if you don't need to.

 

[gte447f]

In terms of shoring, we do that all the time.

That said, we generally just don't do it in practice because of the expense involved.

@Enable Excellent info you provided. Thank you for sharing. Yet, the above two statements confused me. Do you generally shore concrete elements before repairing or not?

 

[BridgeSmith]

@Enable Excellent info you provided. Thank you for sharing. Yet, the above two statements confused me. Do you generally shore concrete elements before repairing or not?

It appears the first statement was in reference to shoring. The second was about lifting the beam, etc. to relieve the existing stress on it.

Blocking up a beam to keep it in place, is fairly simple by comparison to actually lifting the beam, the additional analysis to determine how much it needs to be lifted, and the verification involved in making sure it's done properly and doesn't cause more issues.

 

[Enable]

@Enable Excellent info you provided. Thank you for sharing. Yet, the above two statements confused me. Do you generally shore concrete elements before repairing or not?

I probably could have been clearer with those statements. But @BridgeSmith has is right. Passive Shoring and Active Shoring are the technical terms sometimes used to distinguish shoring that is merely placed to handle any loads that redistribute during the repair (Passive Shoring) and shoring that unloads the member prior to the repair (Active Shoring). Below is a rough, rough guide to when each is typically utilized.

No Shoring: Minor repairs that do not extend beyond the cover. These are cosmetic in nature and don’t go into the core of the element being repaired. You would patch these with SikaTop 123 or similar.

Passive Shoring: Flexural members (e.g. beams, slabs, foundation walls, etc) or very lightly loaded axial members (e.g. garage columns) where a modest increase in deflection post-repair is not a huge issue. Usually the repair extends into the core of the member and at least past the primary steel. This constitutes the bulk of shoring used on repairs in my experience.

Active Shoring: Axially loaded members that have a fair amount of load on them (e.g. building columns) where very sudden movements during / after repairs would be a big deal. The active nature of it also doubles as a load test of the shoring assembly, so even if movement is not a concern, it is the system of choice for axially loaded building elements.

Here are two projects that should get the idea across (for Passive vs Active).

Project 1 (Passive): Single story parking garage with base column repair. We used passive shoring here simply distributing over a large enough area that movements would be tolerable during load redistribution.



Project 2 (Active): Building column holding up an 18-story occupied building (~ 1million pounds factored) that needs to be demolished and replaced. Active shoring with hydraulic jacks used to transfer the load to the shoring prior to demolition. For obvious reasons we want to test the system before it is relied upon for safety of the structure! We’re actually just completing this guy and you can follow along online where we push the data to an online dash board we made (we should start jacking tomorrow and continue over a 3 day period): https://www.enable-inc.com/live-feed

 

[gte447f]

@Enable Great info. Thanks for sharing.