Slag Cement in Slab Concrete Mix

[kennyb04]

We are EOR for a large shopping center and received a mix design for the slab containing up to 50% GGBF slag. (C595 Ty IS w/ max 50% C989 G120 GGBF slag). We specified Type II cement with max 20% flyash, that's just what is more commonly available as far as I can tell, but due to the location of the mix plant being close to a plant producing slag the mix designer said 90% of their mixes are slag and not fly ash.

That is fine with us, it seems like slag is good to use if it is available, we just aren't as familiar with it and has brought up a couple concerns.

We called out 3000 psi concrete but did not set a w/cm ratio. Typically, calling out 3000 psi concrete with max. 20% fly ash would give us w/cm ratio of 0.5. This mix using up to 50% slag has given us w/cm ratios of 0.6 and as high as 0.65. Does this seem right? Will a mix design with slag give a higher w/cm ratio than with fly ash because it can obtain a higher strength (or same strength with less cementatious material therefore higher w/cm ratio)?

Our concern is not with strength but with the high w/cm causing cracks throughout the building. The slab will be polished concrete slab in a shopping center so any cracks will be seen. We would like to consider limiting the w/cm ratio to 0.5 but since it is such a large store, any "small" change can be a big cost so we need to really be able to justify our answer.

Also, due to weather and issues the slab has been put off and the roofing and precast walls have been put up so the slab will be essentially placed in an enclosed building (still not completely sure how the cotractors are going to handle the grading and stuff with that). I understand that slag already will delay the early strength gain so does anyone have any comments about placing this in an essentially enclosed building? Should be be thinking about and accelerator? (Again, more cost we would need to justify)

There is also a water reducer in the mix in addition to the .6 w/cm ratio. Does that seem right? (4" slump +- 1")

Thanks for any comments.

 

[Ron]

kennyb04...you have a bit of a screwy mix design. Inherently, there's nothing wrong with using 50% GGBFS, although it is a bit on the high side. Expect your strength gain to be slow....your 7 day strengths will likely be in the 40 to 50% range, rather than the expected 70% range....as long as the 28 day strengths come up, not a big deal.

The 0.6 w-c ratio is not good. That compromises durability. If you lower the w-c ratio, your strengths will go up.

The mix design you received is more likely to be used on a residential slab than a commercial slab. I would bet that the slump is higher than 5", as you suspect as well.

Reject the mix design. Require a more reasonable water-cement ratio for durability purposes, not just strength. Consider that barrier floor coverings (vinyl tile, etc.) will likely be used on this slab and a high w-c ratio will impact the performance of such coverings.

 

[kennyb04]

Ron, thanks for your quick response.

The mix shows max 50% slag so I'm not sure of the actual content. Is that something I should push the mix designer for?

As far as I know, codes don't specify a w/cm for slabs. I agree that the 0.6 seems high for a commercial slab but we need to figure out the best way to justify our answer if it is going to increase the cost much (large shopping center = lots of concrete slab). Would the main concern be cracks in the concrete or would there be other durbablity concerns likes spalling?

The slab is currently going to be polished (no additional coverings) but that doesn't mean something won't be placed on it in the future.

We are also trying to determine whether to have the vapor barrier below slab (above granular fill) or below granular fill. We previously called out below slab, but now the building is enclosed (roofing and precast walls up) before the slab is getting placed so we are trying to determine if this will change the location of the vapor barrier.

 

[dik]

Ron has provided a quick summary...

With current concrete juju, it's possible to get 3000 psi concrete with W/C of 0.6... Your spec of Type II may give you a little wiggle room for negotiating a different mix. The proposed mix is considerably less expensive to produce (in our environs).

You might include stipulating a max W/C of 0.5 for future work and for resistance reasons, even a lower one. For commercial stuff of little exposure, I typically use approx 3600 psi... (25 MPa in our area) and a maximum slump of 100 mm (4", and not a spec'd slump, but a max one else you can get 5" or more slump) and sometimes 75 mm (3").

Also include for proper sawcutting and curing. These two items have a real impact on the final slab quality.

Dik

 

[Ron]

Cracks can be handled without regard to w-c ratio. Space the control joints at no more than 10 to 12 feet apart for a 4" thick slab and make sure the length-to-width ratio of the slab sections does not exceed 1.2. Make sure the sawcut control joints are done the same day as slab placement....don't let them wait until the next day, particularly with a higher w-c ratio...shrinkage will be higher and initial shrinkage will be significantly higher.

Since the slab will be polished, it will also have a sealer applied after polishing. If there is a lot of latent water in the concrete, you'll likely see some white spots in the concrete and in some cases, dark spots. Another reason to keep the w-c ratio low.

As I noted, the shrinkage will be higher with higher w-c ratio. This means that the sawcut joints will widen with time and it increases the potential for cracks between the control joints.

 

[dik]

Should add that unless you are using a special sofcut type saw, that sawcutting should occur within 6 or 8 hours after finishing... as soon as the concrete has enough strength to prevent ravelling of the aggregate when sawcut. You have o use a method of curing consistent with the use of the sealer after polishing (missed that, Ron, thanks)...

Dik

 

[Ron]

Dik...no, thank you...I missed the whole curing thing!!

 

[shobroco]

The fact that the building is closed in is good. No temperature extremes, sunlight, wind, etc. to accelerate drying & start cracking. slower is better, & I agree with Dik, 25MPa for a floor slab. The VB location or even existence won't be as critical without floor coverings & the WC ratio is mostly going to affect sawcut spacing (or the other way around). A lot of guys these days have saws with a shoe to hold down the aggregate while they saw a green floor, which is really effective in getting the cracks where you want them because they can saw sooner, but not likely acceptable for an exposed floor because you can have a lot of microcracks along the cuts where the aggregate pulls up against the shoe.

 

[dik]

shobroco... that's a sofcut type saw and the slab can be cut a couple of hours after finishing... or maybe sooner.

The 25 MPa is min... and for slabs exposed to light traffic, etc and no chloride or sulphate exposure issues... then it jumps to 35 MPa with a W/C <= 0.4... Same max slump... sometimes max 75 mm, depending on the mood... same with heavy industrial slabs... and sawcutting and curing is critical... if you wait until the next day or so, you may as well wait until the cracks have formed, route them out, and caulk... <G>

Dik

 

[jdgengineer]

I don't have a lot of experience with slag concrete but I recommend you read through ACI 233. That is ACIs specification on slag as a material in concrete. A couple imporant things I saw -- 1) water demand should be slightly lower when using slag. Therefore, I'm not sure why they are proposing such a high w/c ratio. 2) they recommend using class 100 or 120 slag.

 

[jdgengineer]

The document also mentioned that up to 70% slag has been used as a substitution, but they recommend using a 50% substitution as a maximum threshold.

Also if you ave all the ACI manuals I'd check out ACI 302.1 which is the guide for concrete floor construction and ACI 201.2 which is the guide to Durable Concrete. I believe ACI 302.1recommends a w/c ratio between 0.47-0.55 for slabs. I think Mike's recommendation of 0.50 or lower seems appropriate.

 

[Lutfi]

I have been using slag in my concrete mix for the last few years. I like it very much. It will increase the compressive strength by the 28the day. Ask the concrete provider to give you a historical test results based on various WC rations. I think w/c ratio of 0.6 high. I like it to be less in the mid 0.5 or lower.
We used 60% mix for slip forming two silos and worked like a charm. Slag will improve strength over time, produce higher resistance to chlorides and will yield nice white concrete rather than the green concrete we are used to seeing. If you want a nice exposed concrete, use slag.


Regards,
Lutfi