Concrete Patch Repair

[Gopher13]

I haven't really done any concrete repair work in my career until now, so I am looking to get an opinion on concrete patching details. In my specific case, I have 7 areas on a concrete pedestrian bridge that have spalled and need to be repaired. Each spall is approximately 14 inches by 14 inches and is about 4 inches deep, and all are located at the transverse edges of the bridge. The overall thickness of the bridge is 9 inches and it is lightly reinforced with #4 transverse bars at 12 inches o.c. top and bottom. There are no longitudinal bars where the patches will be. The bridge is located in Minnesota. The spalls all occur around existing guardrail posts that are embedded in the concrete. We will be removing and replacing the guardrail posts with surface mounted posts, and we will be moving the locations of the posts away from the patches.

I have included the typical patching detail that my previous employer used. Yesterday (1-19-18) someone posted a whole bunch of details from the New York City Transit Authority. Similar details were found in that set.

One of my questions is, why do some details include a series of anchors embedded into the existing concrete (identified as "D" in the attached detail) with a layer of welded wire fabric (identified as "C" in the attached detail) attached to it, and other details do not include any new reinforcing? I don't believe these bars are for strength because they aren't properly lapped with the existing bars nor would they be the same size. Are they for temperature and shrinkage? Do large patches contain this reinforcing and small patches don't?

Also, what type of patching material would be the best to use? I have read up on latex modified concrete, epoxy modified concrete, and concrete carefully designed to control shrinkage. I am leaning on going with carefully designed concrete because it can be placed in one lift and the fact that the other products requires a more skilled laborer to execute. Do you agree/disagree?

Any comments would be appreciated. Thank you!

 

[oldestguy]

First off, why not post the drawing right side up?

My experience has been great using a few simple steps.

Rattle off as much of the weakened concrete as possible with small jack hammers or similar. A riveting gun with chisel works. The surface left should be sound concrete,if possible and LEAVE IT DRY!!! Never add water. If weather is wet, use a flame or similar dryer to get rid of all moisture at AND WITHIN that surface.

Vacuum or blow off all dust.

Brush on a paste consistency of Portland cement and water. Work it into the dry concrete, the object being the dry concrete takes this on by capillary action. This is the glue, keep it thin.

Follow immediately with as stiff a concrete mix as possible.
Cure as you would regular concrete (moist).

If you wish, wait a few weeks and drill a core sample to see how you did. Then try to break it with a chisel at the bond.

 

[Gopher13]

OG: I apologize. The scanner rotated it 90 degrees, and I didn't take the time to figure out how to fix it. Next time I will make sure the drawing is uploaded in the correct orientation. You should be able to right click and rotate 90 degrees though? Thanks for your help! Any thoughts on adding welded wire fabric?

 

[JedClampett]

I'd call the experts. I know Sika has many products for concrete repair, maybe even some special products for bridges.
Besides that, their representatives are knowledgeable and helpful. If they don't know something, they call their technical experts in New jersey and get the answer.

 

[bridgebuster]

For the most part ditto what oldestguy said. Essentially, concrete repair details are standard, chip out the bad stuff, get behind the bars, patch, although agencies and individuals have their nuances, A trend I've seen lately on NYSDOT projects is to add mesh to vertical repairs. Does your state DOT have any standards? On a NYSDOT project, the typical repair material is Portland cement concrete; when the repair is less than 5 1/2"" deep we use a 3/8" aggregate (NYSDOT Class D concrete); repairs deeper than that we use concrete with larger aggregate (NYSDOT Class A). The specifications also allow for specialized products such as Sika. I attached some details.

One thing with deck repairs, it's possible that your partial depth repairs could end up going full depth.

 

[bridgebuster]

some more

 

[bridgebuster]

Another

 

[bridgebuster]

and another

 

[bridgebuster]

and last but not least (actually I have dozens more but they're all about the same).

One other thing to keep in mind, speaking from my field experiences: Concrete repair quantities have been known to overrun.The quantity that you will show in your plans is something that should be discussed with your client. If you feel good about your number a 25% contingency is not unreasonable. On one project I did a few years ago, the client told me to triple my quantity because of a bad experience they had on a recent project.

 

[Ron]

The method OG outlined is tried and true. It requires proper preparation as he noted. You will either need to supervise closely or hope that you have a contractor who knows how to do this properly.

If you want to use a different material such as a polymer modified concrete, consult with Sika as JedClampett noted. They still require good preparation in accordance with the manufacturer's instructions, but are a bit more forgiving.

 

[epoxybot]

In my experience the only time you would use mesh is in a greater than 1-1/2" thick vertical or overhead application and even then it would have to be applied using a low pressure pneumatic spay rig. This is how silica fume & fiber modified cementitious patching materials are laid up in single lifts. The anchors are only needed to secure mesh in the absence of local rebar. Imagine trying to trowel a patching material through mesh and the mesh throwing it back at you. I've seen really good cement masons flick material through mesh, off their mortar board but the consistency of the material needs to be just right.

In a horizontal application, mesh might be warranted if you are using a manufactured product with a fast strength development in a deep patch. The mesh would help prevent shrinkage cracking for hot mixes. This was a problem with latex modified patching materials in the 80's & early 90's but the manufacturers have gotten better at mix design. If you go the manufacturer packaged route be sure your patching material has a compatible modulus.

Cement slurry returned to popular use towards the end of the 90's and is in many cases the best way to bond a patch but timing is important, as you don't want to cover more area than you can place patching material before the slurry sets up or dries out too much. Not all latex bonding agents are the same. If they are designed to be applied & used in a wet condition, they will have good bonding & water resistance but if they are of the apply and let dry variety they may not hold up to water ponding (bird baths). Some materials have powdered latex mixed in with the patching product. Again not all of these products are good in the frequently wet conditions. Sika & Master Builders would be safe choices.

It is better to be a hovering presence when the work is done the first time, than to be back in two years correcting the patches where the cement mason, re-tempered the mix.

 

[Gopher13]

Thanks for all the responses and example details. Everything is very helpful.

I do have a couple questions about the cement slurry though: if the existing surface is properly prepared (roughened, cleaned, dried) won't the new concrete sufficiently bond to it without the slurry? Also, if I am requiring them to apply the slurry, what consistency (1 part portland cement, 1 part water?) and thickness is recommended?

 

[WARose]

I don't know if anyone's mentioned it yet but Five Star makes a lot of good repair epoxy concretes/grouts. I've used them before and they always bond real good.

 

[epoxybot]

See page 90 in the following link. The slurry should be applied with a Masonry or Kalsomine Brush. Link

 

[Gopher13]

Very helpful. Thanks epxoybot!!!

 

[oldestguy]

The reference says you can clean off the surface with water jet and then leave it "saturated surface dry". I don't agree. the best set of instructions easily can be done wrong if yu don't really know what happens, such as capillary action drawing cement and water into a porous surface. Also if you do not know that water/cement ratio affects strength. So the book says use water, and says surface dry. How dry? Leave it up to the recently hired laborer with his mind on quitting time being delayed by waiti9ng. Ya, surface dry. Humbug. The saying that "If anything can go wrong on construction, it will". So think about these possibilities with any new procedure.

edit: I'd be sure by not allowing a water hose on the job.

 

[Gopher13]

I don't know if I totally understood you the first time OG, but I think I get it now: The surface and underlying areas need to be dry so the existing concrete "sucks in" the new creating a stronger bond. Also any remaining moisture will increase the w/c ratio adversely affecting the patch. Is that right?

 

[oldestguy]

You got it.