Exposed Slab Bottom Bars in 50 year old building 1

[PhDE]

What is the best way of patching up slabs? Materials? Do you need to chip out concrete around each bar? There is some minor corrosion. Also, the building is not exposed to the elements.

 

[samdamon]

PhDe-

One could write a small book in response to this question. It sounds like the bottom bars have debonded from the surrounding concrete in certain locations. Is this typical for many of the bars? If its just local,occasional spalling then you MAY have less of a problem.

What caused the damage? Was the floor overloaded, or exposed to a corrosion-producing combination of oxygen and moisture at some time in the past?

Fyi there are a variety of concrete repair materials out there that might be used to repair this type of problem, ranging from shotcrete to trowel-applied vertical and overhead repair mortars. Some of the mortar makers are Master Builders, Sika, and Euclid, to name a few.

The v and o mortars are typically manufactured to be job site mixed in small batches, with light coarse and fine aggregates so that its less work to get them in place. These mortars are also designed to stick to overhead surfaces.

Problems can arise in their use when the manufacturer's directions are not followed. Masons who have been working with traditional masonry or concrete construction do not necessarily understand how to use these materials. Mistakes or omissions in substrate prep, priming (if required), mixing, lift thickness and curing measures can all ruin an otherwise fine repair attempt. (V and o repair materials are not cheap, to say nothing of the labor!)

Some repair contractors out there have a wealth of knowledge and experience, while others always seem to be hiring guys off the street who will thus be learning a new trade on your repair job. Its important that the repair contractor be experienced, or short of that, review the progress of the job yourself often. Paying attention during the repair work is even more important than in new construction, I think.

If you could describe what you are looking at a little more, I might be able to comment further. Is this a flat plate, concrete pan-joist system, or some other type of concrete framed slab? Have repairs been attempted in the past? How extensive is the area you are looking at?

ALso fyi there is an organization called ICRI (International Concrete Repair Institute) that has many useful publications. Try a web search.

Regards

 

[PhDE]

Thank you SamDamon! I apologize for framing the question so general but wanted as much info as possible. The rebar exposure from the underside is about 10% of the 10,000sf space in random areas (typically between spans). The bars look as if they are a triangular mesh draped over steel beams spanning 7-8 feet beam to beam. There may not be sufficient cover in the midspan. Any other advice would be greatly appreciated.

 

[samdamon]

PhDe-

1000 square feet of spalls over a 10,ooo sf area is quite a bit. Sounds like you have some sort of systemic problem. If I understand you right, the concrete floor is designed as a one way slab and is supported by steel beams at about 7' on center. (Is this an old industrial building?)

As a first step I would suggest measuring the as-built cover on the bottom bars, and looking at how much section was lost in the bars due to corrosion, and how much of the bar lengths are unbonded. Just visual review won't be enough, it might be best to systematically sound the bottom and maybe also the top with a hammer. The bottom bars are probably acting as positive bending reinforcement for the middle of the slab spans, so damage towards the middle of the slab span becomes important. (How does the top of the floor look? Are any bars showing there?)

In general you want to get a feel for how much the bending and shear capacity of the slab has degraded, and try to really pin down why the spalls occurred. If you are familiar with the basics of concrete design, you may be able to take your field information and solve backwards to get a rough idea what the floor's current capacity is.

If the client and budget can agree, suggest you open a small test area of the floor to see what the typical reinforcing is on both the top (over the beams, and at the bottom (near the center of the slab). Bear in mind that if its a very old building you may be dealing with grade 40 (or less!) reinforcing.

Its hard to comment further without seeing things firsthand, but good luck with your project.

 

[StephenEd]

Companies like SIKA offer great tech help. They can recommend the proper materials and techniques. There are construction companies that specialize in repairs who can test for things like slab alkilinity, which will enable you to choose a proper patch.

One of the keys is getting an experienced contractor who will use the proper methods (i.e. thorough surface prep).

 

[Rawand]

PhDE
I know a method of covering the concrete members using Kevlar texture, pasted on the concrete surface using Epoxy based grout.
So if the building is important Go for it, remove the bottom cover part by part and reiforce the bottom using this textile. otherwise try to cover the damage using Ultimax Cement, the manufactoror says that this cement reacts with the rust and adheres with the rust bars