Repair Garage Slab-on-grade Finish

[ajk1]

For a large repair garage slab-on-grade, that will service municipal equipment like snow ploughs etc. is it fairly standard practice to specify a shake hardener?

Note that the garage is heated; there are large doors but I expect they are not kept open long enough to cause freeze-thaw cycles of the slab.

I was planning on the following.

- 8" slab on grade, with sawcut control joints at 15' centres each way;
- no reinforcement in slab because it is a corrosion issue;
- CSA exposure class C-4, but waive the air entrainment requirement (as permitted in CSA A23.1) since the dry shake hardener requires a steel trowel finish and should not steel trowel air entrained concrete.

 

[hokie66]

For heavy equipment, I would prefer using steel fibres in the concrete. Steel fibre concrete is very good for toughness and abrasion resistance.

 

[TehMightyEngineer]

Plain concrete with large stress concentrations and stress reversals will likely have cracking issues. I would highly recommend against not using reinforcement. Welded wire in the middle third of the slab will have so much cover it will not corrode for longer than the building is standing. If corrosion is truly a concern then I'd suggest a small stainless wire mesh.

I agree with hokie that steel fibers will help out a lot here with improved abrasion, toughness, and impact resistance. Steel fibers also add a minor increase to strength. However, the concrete contractors will hate working with it and it makes finishing difficult. Synthetic macro fibers will be much more contractor friendly, no corrosion, and almost as good of properties as steel fibers.

I'd get rid of the shake hardener and put the air back. In the limited research I've done I'm not convinced that dry shake or liquid hardeners do much, unless they are a metallic dry shake, polymer-resin, or an epoxy topping.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[ajk1]

Be interesting to know what geographic area you are located in, as that may be impacting your answers. Where I am in Canada, road deicing chemicals must be designed for, by Code.

Also, interested if either of you have designed such a facility in your area, in accordance with your recommendations, and how you found that it has performed.

In my area, steel fibres will corrode and leave the surface pock marked with rust stains, unless stainless steel fibres are used, which I expect would be really expensive.

We do not use mesh in slabs because it cannot be chaired and ends up at the bottom of the slab where it does no good....is actually counter-productive because it renders the sawcut control joints less effective.

Our investigation of wire mesh a few hears ago, in a topping on precast Tees, showed that it does corrode. But the corrosion products are not generally large enough in volume to cause concrete spalling. One of the reasons for the myth that wire mesh does not corrode, is that it is almost always down lower in the concrete than it was specified to be, so it has lots of concrete cover.

We have checked the plain concrete slab for cracking under the wheel load, using the Westergaard method and it works. I know it will crack in practice, but so will steel bar reinforced slab. The thing that the bars will do is to limit the crack width. But with control joints at 15 feet centres,the crack widths should not be all that wide.

I don't believe that concrete road pavements are generally reinforced, and they seem to perform ok as far as I know. Why then would the repair garage slab have to be reinforced?

All answers are of course much appreciated, but if there is anyone out there who has done such a facility, it would be especially interesting to hear from you.

Thanks.

 

[TehMightyEngineer]

Central Maine (a.k.a. Southern Canada), we have a pretty heavy deicing salt usage and we see a lot of chloride related corrosion. I haven't had a performance report on the last few facilities I've worked on, most of my recommendations are based on what I've seen in older industrial facilities and seeing what worked and what didn't for them. The biggest thing I saw was that reinforcement corrosion was a concern for sure but slab cracking and breaking up was worse when there wasn't reinforcement. Abrasion was a huge concern though but this was under tracked skid loaders moving logs so I imagine it was WAY worse than your application.

I agree 100% about the mesh, it will corrode just like rebar but at deep covers will not present a concern for serviceability.

The problem with cracking of plain concrete is that, once cracked, it's much easier to crack it further. It also provides a sharp corner which increases wear from abrasion. I'd keep the slab designed as plain concrete but provide at least temperature and shrinkage reinforcement in the middle third to provide crack control.

I've seen both reinforced and unreinforced concrete roads used. In Maine we usually have heavy vehicle truck traffic and thus design everything for very large loads, thus reinforced concrete for concrete roads is typical around here.

The precast plant where I work at right now has a repair garage for our trucks and I'm almost positive it's reinforced. No issues so far but I don't know much details about it's construction. We have a section of concrete pavement outside the plant that recent abraded away down to the reinforcement. No corrosion on the reinforcement but the heavy dump trucks bringing our aggregate was just killing the concrete.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[ajk1]

To TehMightyEngineer (Structural)- ok, that is very interesting. Much appreciated.

 

[jike]

If you want to go with bar reinforcement, consider using epoxy coated bars. Hardeners can be expensive and I do not believe liquid hardeners are effective. Perhaps an aggregate hardener would be better compromise than a metallic shake-on. Make sure you have a well designed concrete mix with larger angular aggregate.

 

[ajk1]

We do not generally use epoxy coated bars. Neither does the Ministry of Transportation of Ontario. Also, they are not an acceptable corrosion protection system according to Table 1 of CSA S413 "Parking Structures". Also, if the concrete cracks, the crack widths are 20% to 30% wider than if reinforced with uncoated bars (see Mitchell et al). Yes they will buy you a few years of life, but I don't think they are the best choice.

The hardener would be dry shake Surflex by Euclid. I do not know about liquid hardeners, but dry shake hardeners I have looked at on site seem effective. The hardener is primarily to prevent damage to surface because of dropped tools and the like. It seems to be generally used around here for similar facilities.

I really do not want to go with bars, because eventually they are going to be one hell of a corrosion and concrete spalling problem, just like the parking garages were before CSA S413 came out and made membranes mandatory on reinforced concrete suspended slab garages. If the cover is made large for corrosion protection, then they are less effective in controlling surface crack widths, so have you really then accomplished much, for a lot of extra expense?

 

[TehMightyEngineer]

Epoxy coated is not worth the time IMO. Go stainless or fiberglass if you actually want corrosion protection. I'd take MMFX rebar over epoxy if I had to.

Try to find independent reviews of the Surflex hardener. I suspect you'll find that it's not as good as the manufacturer wants you to believe.

As for the rebar, placing it at mid-depth in a slab has proven almost as effective at closing crack widths at the surface when compared to lower cover reinforcement (at least from the research that I recall). Plus, the greatly increased flexural and shear strength means that eventual cracking will propagate much less.

A third option is very high doses of steel fibers with no reinforcement. This will REALLY make the contractor hate you as it's a horrible nightmare to pump and finish but at high dose rates it will control the cracking and add measurable amounts to the strength in flexure and shear. Potential corrosion issues here.

A fourth option would be designing in an epoxy, self-leveing topping. This would be the best system IMO, but probably somewhat pricey and requires two pours (one for the slab, one for the topping).

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

I was involved in a pursuit for a maintenance vehicle facility at an airport once. The client specifications were:

1) Minimum 6" SOG.
2) Class A institutional floor finish.
3) Control Joints at 4500 min.
4) Follow recommendations in this doc: Link

I doubt this is your answer but it may provide some useful insights and details. Obviously, ignore the airplane stuff.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

My suggestion to use steel fibre concrete ignored the corrosion issue, mostly because where I have used steel fibre concrete, freezing conditions never occur.

I was of the understanding that corrosion at the surface was only a staining issue, but I could be wrong.

The issue of contractors not liking to deal with high dosed steel fibre concrete is dissipating gradually as more concreters get familiar with the material.

My experience with similar floors to the OP has been in mining workshops, where repairs are undertaken to big stuff like drag line buckets, bulldozer blades, etc. Abrasion resistance and toughness are big issues there, and the use of armoured floors, embedded rails, etc. has largely been discontinued and replaced with steel fibre concrete. But corrosion...I don't know.

 

[ajk1]

To Kootk - that is interesting information you sent. So far I have only skimmed it, but I did not see a requirement to use reinforcement, but maybe I missed it.

To hokie66 - yes I believe that the fibre suppliers say it is a only a surface staining issue, but is hard to see why the fibre will not eventually corrode away, just like eventually rebar does in our environment where so much corrosive chloride deicing chemicals are used on the roads and tracked into the garage. In Australia I suppose that is not an issue.


To TehMightyEngineer (Structural)- I would be interested in the research that you note says "placing it at mid-depth in a slab has proven almost as effective at closing crack widths at the surface when compared to lower cover reinforcement". I am not saying that couldn't be true, but I would like to see that research paper.

I believe that the main reason for using reinforcement in the slab is so that when it cracks (as it will irrespective of whether there is reinforcement or not), we will be able to say that we did everything that was possible.

I could see the need for rebar if the soil was not good, and the Westergaard analysis gave an uneconomical required thickness. But otherwise, in a corrosive environment, it seems to me that the rebar is a negative. Whether a cracked slab performs better with rebar than without I don't know but perhaps it does. The rebar may be useful in reducing joint curling.

 

[TehMightyEngineer]

ajk1: I believe it was in ACI 360 Guide to Design of Slabs-on-Ground [URL unfurl="true"]https://www.concrete.org/store/productdetail.aspx?ItemID=36010[/url]
but as I no longer work at the office that had this reference I can't double check for you.

I did also come across this reference from the US army: [URL unfurl="true"]http://armypubs.army.mil/eng/DR_pubs/dr_a/pdf/tm5_809_12.pdf[/url]
see Chapter 5. That document references this document: [URL unfurl="true"]https://www.wbdg.org/ccb/DOD/UFC/ufc_3_260_02.pdf[/url] which has information on placement of rebar on page 13-5.

The general conclusion I've found after a bit of googling is place reinforcement in slabs-on-grade at between mid-depth to 1/4 thickness below the surface.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[Ron]

Steel fiber corrosion is only an issue where exposed to the surface and even then is only a minor staining issue. Since the fibers are discontinuous (discrete), the corrosion issue does not go subsurface except in the instances of advanced chloride permeability.

ajk1...since you are in an area that routinely deals with chloride permeability, I do not anticipate this would be an issue. An hokie66 notes, steel fibers will provide additional toughness to the concrete. I would not waste my time with synthetic fibers.

Steel fiber will provide some property enhancement to the concrete mix. Most of the steel fibers on the market are relatively long and stiff. At least one manufacturer still provides shorter, ductile fibers that would work well in your case. I do not work for nor do I have any interest in the following manufacturer; however, I was involved with some of their research in the early 80's for both concrete and asphalt, so that's my disclosure!

Fibercon International, Inc.

Compare these with Novocon, Helix Micro-rebar (the name always makes me chuckle!), Dramix and others.

 

[ajk1]

To Ron - The corrosive chlorides will penetrate the concrete unless a waterproofing membrane is installed as per CSA S413, so I am afraid I cannot agree with your statement "Since the fibers are discontinuous (discrete), the corrosion issue does not go subsurface except in the instances of advanced chloride permeability", unless by "advanced permeabilty" you mean what we see in places like Toronto after several years.

I am not aware of any standard that says you can use steel fibres to reduce the depth of chloride penetration and required cover. The fact that the fibres are discrete has nothing much if anything to do with how deeply the chlorides penetrate.

The depth of penetration of the chlorides into the concrete is governed mostly by the pore structure of the concrete, but if you are aware of any independent research that shows that the chlorides penetrate less deeply into a concrete mix with steel fibres than the same mix without the steel fibres, I would be interested in seeing it.

I expect that the steel fibres lose metal as they rust in the concrete, just the same as rebars do, so that after a certain number of years there will be little if any of the steel fibre area remaining in the top 1 to 2" of concrete, unless you have a concrete mix with an ultra low w/cm (or you are in a location where very little chloride is applied to the roads).

If you have any photos of what the surface of a steel fibre slab looks like after say 5 years of use in a chloride deicing chemical environment, I would be really interested in seeing them.

I am really interested in whether anyone experience with an unreinforced slab on grade in a maintenance facility and how it has performed?

Thanks.

 

[Ron]

ajk1....the fibers have nothing to do with chloride penetration....my only point is that the steel fibers would only be likely to corrode below the surface if the chlorides penetrate into the concrete. Yes, what I meant by advanced chloride penetration is the type that you see in the Toronto area; however, you are aware of the issues and can accommodate that in the mix design, placement, curing and maintenance of the concrete.

the dry shake hardener requires a steel trowel finish and should not steel trowel air entrained concrete.

A steel trowel finish can be put on air entrained concrete....we do it routinely here. The problems you see with scaling and blistering of air entrained concrete that has a steel troweled surface are usually due to 2 issues.....excessive bleeding of the concrete and getting on the concrete too quickly with the steel trowel. The finishing technique is critical to getting a proper floor surface.

 

[ajk1]

Ron - "accommodate" is perhaps a rather vague word. Yes, we can reduce the rate of chloride penetration into the concrete by specifying CSA A23.1 C-1 chloride exposure mix or C-XL exposure mix, but we cannot stop it. Eventually the chloride will reach the critical chloride corrosion threshold at which corrosion will begin in the presence of moisture and oxygen. This corrosion threshold is generally taken as between 0.025% and 0.035% of water-soluble chloride by weight of concrete. See the annexes in Canadian Standard Association (CSA) Standard S413 "Parking Structures" or any text or standard on corrosion of metals in concrete.

If steel fibres are used then the fibres within the top 1" are going to corrode away within a few years (and the fibres at greater depths will corrode later), unless there is some particular phenomenon that happens with steel fibres that does not happen with rebar.

Anyway, I think we hold different views on this, and I respect your convictions.

Seem to be straying from the thrust of the original question. Seems that there is no one here who has designed an unreinforced concrete slab on grade for a repair garage for municipal equipment...or is there?

 

[Brad805]

Bekaert offers different coating options for their fibers and some offer stainless steel options.

http://www.bekaert.com/en/products-and-applications/our-core-competences/coating-technologies

I talked to a tunnel engineer (TBM size tunnels) in England and he echoed Ron's comments. They spray a thin layer of shotcrete over the tunnels reinforced with SFRC to hide stains, and not for structural reasons.

The question of an un-reinforced slab will depend largely on your geotech report and the drainage characteristics. We work with expansive clays, so it is never an option. If the potential heave in your case is very small and a proper sub-base is used it could be feasible.

 

[ajk1]

A tunnel engineer in England? Really? I did not know they used a lot of corrosive deicing salts in England. Sorry I don't see any relevance whatever. I just would not use steel fibres in a chloride environment.

 

[Brad805]

I think a PHd research engineer working on projects of the scope of the Chunnel between England and France might have some experience in such a matter. A research paper is attached if you are interested. Anyway, good luck.

http://publications.lib.chalmers.se/records/fulltext/187376/local_187376.pdf