Spalling CMU Face Shell - Old Football Bleacher

[marinaman]

I have been asked to review an old football bleacher at a local school. While my official meeting is not for a few days, I was in the area, and dropped by today to "pre-view" what I've been asked to observe.

The existing football bleachers are comprised of precast planks vertically supported by CMU walls. The CMU walls are perpendicular to the field and perpendicular to the planks. The load bearing CMU walls are about 30' on center.

When I looked at the CMU, it appears to be 12" CMU that has its face shell spalling off. I first thought it was just paint, but upon closer inspection, it appears that the face of the block is spalling. The thickness of spall is about 3/8" thick, over the face of the block. This stadium was built 25 yrs ago or more and the CMU has been exposed to water due to rain for a long long time.

I'm meeting a testing service at the site in a day or two....but was wondering if anyone has seen this before. I dont believe the CMU is grouted nor reinforced.

I do not believe the CMU can be repaired, as there is no way to grout nor reinforce the block, as atop the block sits the concrete planks. Right now, preliminarily, I believe the planks will need to be taken down, the CMU demolished, and the walls rebuilt as grout filled CMU....or.....I will have to go on each side of each wall and re-support the planks with some sort of steel structure.

Have you guys dealt with this before?

 

[Ron]

Have you considered sandblasting the block, pinning a reinforcement grid to the block and applying shotcrete? You can design the shotcrete to handle the loads and the block will serve as a backform. With this technique, you won't have to dismantle anything and your construction remediation cost will go way down.

Good luck.

 

[marinaman]

Ron - Thanks for the reply.

I have considered shotcreting the wall, but right now, I dont know the condition of the wall.....having just visited the site once.....since I happened to be out that way.

I'm sure that the backside of the wall would have to be temporarily shored while the shotcrete was applied, as I would not want to "blow down" the wall.

Ron, do you have a good source, book, or publication illustrating such a repair?

 

[Ron]

Marinaman...I'm not sure you'll find a book that outlines that same type of repair, but you can get a good handle on the process from ACI 506.1-506.4. These are the definitive industry documents for shotcrete.

My one caution is that you write your specification similar to the ACI Guide Spec,requiring certification for the nozzleman, etc. Otherwise, you're going to get every swimming pool builder with a pickup truck and concrete pump.

I've been involved with shotcrete structural repairs and new shotcrete construction. They are not vastly different. Quality Assurance and Quality Control are absolute necessities with either. If you've ever been to Universal Studios-Florida, pretty much all of the lagoon and structures in the lagoon are constructed from shotcrete. That was done 20 years ago and still performs well.

 

[marinaman]

Ron - I'm going to find out in the coming days if the block is grouted and reinforced or not. IF it is grouted and reinforced, and I've simply lost 3/8" to 1/2" of the face shell on each side of the wall, I'm wondering if there is method of "building out" the face of block again?

My concerns about that are that I don't know the condition of the internal reinforcing (if it has any).....but I suspect the reinforcing would be heavily deteriorated, given its 25 yr exposure to moisture.

Lets say the block turns out to be reinforced and grouted....and lets say I find the reinforcing is ok........would shotcrete adhere to the face of the block as a means of "building back out" the faceshell? What if a bonding agent was added and then the face shotcreted?......or perhaps I'd be better off going with a bonding agent and a repair mortar.

Just trying to get options in my mind before I dive into this project.

 

[Ron]

Marinaman...if the cells are grouted and reinforced, I would not worry too much about the condition of the reinforcing. You have no indication that exfoliation is occurring, since you have no significant spalling other than the face shell.

If your only concern is the face shell spalling, then there are several ways to repair that, with shotcreting being one of them. A bonded polymer mortar repair could be done and a carbon fiber overlay could be done.

If you know anyone with an infrared thermographic scanner, you can determine pretty readily if the cells are grouted or not. Even tapping with a hammer will usually get you some indication (I've found that a plastic (not rubber) faced mallet works well for this)

 

[dcarr82775]

You could build masonry sister walls each side, or one side if it works.

The shotcrete will stick to the masonry just fine so long as it is cleaned first.

 

[JAE]

It looks to me like the vast majority of the distress is adjacent to a control joint - i.e. much more water getting in perhaps? Is there a freeze-thaw cycle in your area?

Bonding to block masonry with such significan water infiltration might be tough as the whole matrix of the block walls might be deteriorated. If you sand blast you might run the risk of simply disolving the wall.

On the left side of the joint the block appears to be in good condition and further right in the photo it looks better.

You might be able to simply shore up the precast elements that are supported by this segment of wall (near the joint) and then simply replace the affected blocks.

If there is reinforcing - I'd concur with Ron above. dcarr82775's idea also might have merit but would thicken the wall and might affect functionality of the spaces near the wall.

 

[vandede427]

i think it's probably a water issue. most stadiums are not water tight on purpose, so they can hose off and wash the top of the seating. the wall looks wet and i thought i saw some moss growing on it, but i wonder why the spalling is mostly occuring on the bottom half of each block.

i do not think you have to take down the precast seating to fix the block. that seating is designed for a simple span beam. shore the precast seating on each side near the block wall while the repair is taking place.

 

[hokie66]

I think the spalling is cause by freezing of water in the block. Water ran down the wall, wicked into the wall, the less pervious mortar delayed the gravity drainage, and freezing led to spalling.

I would open up some of the cores to check what is inside and decide on repair methods after that. If hollow, building another wall as suggested above, shotcrete or masonry, would solve the problem. If grouted, it probably only requires cleaning and plastering. Whichever, I don't think the seating planks needs to come down unless they have also deteriorated.