Slab On Grade - Seal or No Seal? 2

[justhumm]

I just got a small side-job thrown on my desk. It's for the design & detailing of 6 or 8 slabs on grade in a contractor's equipment facility.

It's in New England (lots of salt in the winter). The slabs are completely outdoor and exposed to weather. They'll mainly be used for truck parking and equipment/material storage (generators, transformers, cable rolls, etc.).

I don't have a lot of experience with slabs on grade, as far as watching them and seeing how they degrade over 25 years. And I really don't have any useful experience with selecting a sealing / coating compound

I'm sort'of of the opinion that you should NOT seal concrete surfaces, following the logic that concrete is a living, breathing thing that has air voids. And it will expand and contract with the temperature. If you seal it up...as far as I know...you're trapping the air in the voids and you're just going to end up with cracking, anyway...so what's the point in sealing it?

So does anyone have any input on why you should or should not seal an outdoor, industrial slab on grade?

Thanks!

 

[dik]

If For pavements, subject to chlorides, I'd seal it using a good silane or siloxane sealant. I've use the below often.

EXPOSED CONC SHALL BE SEALED WITH MEADOWS SEALTIGHT CS-309 OR EQ. 21 DAYS AFTER FINISHING COAT WITH 1 COAT OF BASE MASTERPROTECT H 400 OR EQ, WATER BASED SILANE SEALER. 7 DAYS AFTER APPLYING THE FIRST COAT APPLY A SECOND COAT

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[SlideRuleEra]

Dik's suggestion sounds good; I'll offer an unrelated suggestion for an outdoor industrial slab:

Check with your client, but subject to their approval, slope it in one direction. For slabs subjected to water (both indoor & outdoor) at our generating stations, I had our Consultants use a 2% slope (in spite of the Consultant's howls and objections). Our employees hated having to walk through "flat" slab, water-holding "birdbaths" but had no objection, and actually preferred working on "dry" (sloped) slabs. In New England, I expect having positive drainage would be a big advantage for freeze-thaw conditions, too.

 

[dik]

If outside it should be drained. Slope can be a problem if sufficient to provide drainage it may be unsuitable for purpose. I usually use 1-1/2%. Be careful of the finish... a smooth steel trowelled finish can be 'slippery as catsh*t on linoleum' when it has a little water on it in particular with a sealant.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ron]

This is a pavement, not just a "slab on grade". Design accordingly. Sealing the slab is a good idea. Siloxanes work well, but for a horizontal slab, pavement or otherwise, exposed to weather, count on recoating every 2 or 3 years.

 

[dik]

Interesting, I consider aircraft runways as a variation of a slab-on-grade/ground.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[STrctPono]

We do a decent amount of PCC pavement design. I honestly, do not like sealers for outdoor slab on grades and I would argue to the death anyone who actually thinks that you can get a better product with a sealer vs. proper slab detailing and proper concrete mix design specification. Not to mention the fact that if you design this as an unreinforced slab you don't actually need the sealer.

You say that the Contractor is going to be parking trucks on this. As Ron stated, this is not a slab on grade but more similar to PCC pavement due to the loads that you are likely to be seeing. A concrete slab (~10" thick) with a properly prepared thick aggregate basecourse/subgrade, properly sized panels and sawcut lines, and properly sized and oriented corrosion proof dowel and tie bars at the joints is really all you need.

EDIT: The only reason I would believe that a sealer should be used if for aesthetic reasons. If petroleum product stains are unsightly and undesirable then I could see why you may want to put a sealer down, but otherwise, it's not needed IMO.

 

[dik]

Since I consider sealers to be of great importance, I should clarify my comments above... for pavements (aka SOG) I generally use silane sealers. I use the products spec'd above and have used these or their predecessors for decades. As a disclaimer, I do not own shares in these companies and gain no benefit from recommending them.

Silanes have a much smaller molecule and penetrate the concrete deeper. They also rely on the high concrete pH as a catylist. Silanes generally penetrate deeper and can be used for surfaces that are subject to abrasion. Silanes are generally used for horizontal surfaces and siloxanes are used for vertical surfaces generally. Although clear, I think most of them discolour over time; check with the supplier.

For historic brickwork I still use Prosoco products... currently the king, I believe.

As a result of this thread, I've decided to upgrade my SOG paper... I'll post it here when it's done...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

Updated SOG... any comments or criticisms welcome...

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1630210458/tips/Slab_on_Grade_ufpe8r.pdf[/url]

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[STrctPono]

dik. I read your writeup. You don't mention anything about dowel bars at joints. Perhaps for interior slab-on-grades you just run the reinforcing continuously through and sawcut wherever without concern for the reinforcing steel but for exterior SOG's or pavements this would not be desirable.

I admit, I am a huge advocate for the use of lithium silicate. There isn't a job that I design that doesn't specify the use of them. However, it is not the sealing properties that I am after as I believe these to be water repellers at best. The densifier quality of the lithium silicates is also a benefit but is not my main end goal of what I am looking to achieve. To me, lithium silicates are curing compounds and I specify them because they help prevent slab curling and are so much better than your other options of water curing or the wax-based curing compounds. Also, for other projects, (not SOG's) where you may have a construction joint with a subsequent concrete pour, the lithium silicates do not need to be removed from the surface as they do not create a bond breaker like the wax based curing compounds do. So, in the end, I do like Lithium Silicates but it sounds like I specify them for a very different reason than you.

Since most highway pavement doesn't have reinforcing in it and all dowel bars at CJ locations are either GFRP or epoxy coated dowels, chloride issues are not really a concern and thus sealers offer no additional benefit. However, a very low w/c ratio (with a strict maximum cementitious content) and an added corrosion inhibitor, fibers, and SRA will outperform any "sealer" applied to plain vanilla concrete any day.

 

[dik]

found this:

http://www.specialtysealantsmi.com/wp-content/uploads/2015/02/CD_LS_PDS_072508_C.pdf

I understand Lithium Silicates can increase the surface hardness by 30 or 40%.

I've not done much traffic pavement design, except for a bunch of parking lots (no dowels) and parking garages (proper expansion joints-they can either be good, or cheap, but not both). I've done a bunch of runways and taxiways, where dowels are nearly always used... for interior SOG, I usually use 2" cover and 1-1/2" deep sawcuts and no dowels.


That I'm not sure of. But, I will check into it.

Will add dowels... I guess it's never been an issue... thanks. It's been a work in progress for the last few decades; it was originally prepared to give an owner/client some SOG background and has continually been upgraded. It started life as a 2 page document. Comments appreciated.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

ST... still using "epoxy coated dowels"?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

ST... forgot to ask... did you notice any errors, other than the omissions?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Envelope3]

I would focus less on sealers and more on specifying a higher quality concrete mix and requiring the slab to be installed in the spring or summer so that the slab has ample time to cure before being exposed to deicers. If it's poured in October/November expect problems.

 

[STrctPono]

dik, I did not notice any errors. I also noticed that you didn't have any information on finishing of the concrete slabs, whether it be floated, power trowel, or tining. In pavement design, slip formed concrete is common but covering all types of concrete SOG's is nearly impossible in an 8 page write-up. I noticed that you referenced pavement design and stated that it was a special sub-set of SOG design.

We do still use epoxy coated steel dowels for highway and airport PCC pavement design. Can't get enough capacity out of GFRP dowels for those kind of loads. We do use GFRP dowels for pretty much anything else that has lighter concentrated loads. We are in a highly corrosive environment but it is atmospheric corrosion and salt spray as you get closer to the ocean. We don't have any issues with salted roads and therefore our bridge decks and roadways don't have the same issues that you guys might. If you know of anything better than epoxy coated for dowel bars subjected to high concentrated loads, I am all ears.

 

[dik]

I'll add some finishing stuff, too...

HDG or 'plate' dowels... I'm not big on GFRP, either...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

Thanks for the review... as I noted, it's a work in progress, and it started out as a page and a half document several decades back...

We do have freeze-thaw issues, but not as bad as some places. Here it freezes in November and thaws in April. Southern Ontario is far worse.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[STrctPono]

Nice job on the writeup dik. I hope your clients appreciate that.

 

[dik]

It's a little frosting that keeps them coming back. In 50 years, I've only lost one client because he was unhappy, and it really wasn't my fault. I can be difficult, but I don't think unreasonable (everyone says that )

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik