Slag in the mix design 4

[UcfSE]

I have had a few mix designs come across my desk that are using GGBF slag, around 40% of the cementious material if I remember correct (at home right now). What are the consequences of substituting slag for Portland? I have access to the MCP 2002, and I can read that all day long and may never read what some of you who have used this or permitted its use will know. What are your opinions and experiences with slag in the mix?

 

[SlideRuleEra]

UcfSE - I have never used slag (just lots and lots of fly ash), but here are some links that may help:
Federal Highway Administration

http://www.fhwa.dot.gov/legsregs/directives/policy/ggbfsmem.htm

EPA

http://www.epa.gov/cpg/products/cement.htm

Environmental Council of Concrete Organizations

http://www.ecco.org/pdfs/EV18.pdf

http://www.SlideRuleEra.net

 

[patswfc]

What type of structures are the mix designs going to be used on?

In the UK cement replacement products are often used, especially in water retaining structures. PFA and GGBS take longer to hydrate compared to OPC and thus the peak heat of hydration for concrete with some cement replacement material is lower then a pure OPC concrete. This can lead to substantial savings in rebar to limit early age thermal cracking.

One drawback is that as the blended cement takes longer to hydrate the rate of strength gain of the concrete can be slower which may increase formwork stripping times (this could have serious effects if the mix is used on large multistorey framed buildings), however with 40% GGBS replacement the rate of strength gain should not be affected that much.

This link is for a ggbs manufacturer and gives some guidance on the effects of using GGBS.

http://www.civilandmarine.com/pages.en/products/ggbs/ggbsfaqs.html

I've got a few useful documents in pdf format that I could send to you. Whats the easiest way to do this? Do you know of any file sharing websites.

 

[Ussuri]

The link below is to a report published by the concrete society, you may be able to get a copy from a library.

As patswfc it is often used (along with pfa) as a method of reducing the overall temperatures generated during hydration as it replaces the OPC in the mix. It also leads to a denser more durable concrete. From memory you can go to about 30% maximum cement replacement with PFA and up to about 60-70% replacement with GGBS. I'm not sure of the chemistry behind it though.

A word of warning, while it may be benficial from a design point of view I have been caught out by its use before. Some contractors dont like it as it can extend formwork stripping times. This can cost more in terms of schedule can often be greater than any material savings reinforcement, particularly if its a small job.

http://www.concrete.org.uk/bookshop/detail.aspx?ID=633

 

[Ussuri]

Also, CIRIA Report 91, Early Age Thermal Crack Control In Concrete might provide some useful information.

 

[UcfSE]

Excellent, thanks for your insight. Some of that I have read about and some was new to me. That's just what I was looking for.

 

[Ron]

UcfSE....GGBFS is fairly common in Florida mixes and is becoming moreso. The most significant effect we see is delayed strength gain. It is even more pronounced than with fly ash. I commonly see 55 to 60 percent of the 28-day strength at 7 days (where we normally see 70 percent with Type I portland cement) when slag is used. While ultimate strengths are relatively unaffected, the slow gain can be significant for vertical construction.

One submittal item we generally request from the supplier is a strength gain curve. They are not always willing to supply that (mostly because they don't have them for some of their mix designs, and because that presents another potential liability for them if the mix doesn't follow the curve reasonably closely), but it doesn't hurt to push for it.

At one time, Rinker was the leader in your area for backup info. They would readily supply info for my requests. Since I haven't been involved day-to-day in your area in about 10 years, I'm not sure how much info is available from the suppliers.

If you're active in the Central Florida Chapter of ACI, you might bring that up as a topic for one of your meetings.

 

[UcfSE]

Thanks, Ron. I'll keep that in mind. Do you plan on visiting us in here anytime?

 

[Ron]

UcfSE....any time you'd like! I maintain several clients in your area and was very active in your local ACI Chapter for about 8 or 9 years. I'm only about 2 hours away.

 

[concrete206]

Ireland we have successfully used up to 40% replacement, and have achieved a significent increase in 28 day strength. I would strongly recommend a mid range admixture, reducing w/c is essential. One of the main problems with all blended cements is formwork stripping times. We are currently testing a New Wireless Temperature Matched Curing Tank, to show temperature vs strength, this new TMC tank looks extremly promising, and will finally give industry a product that can be used everyday, and take the quess work ou of blended cements.

 

[Admixengineering]

Slag cement is a wonderful material to work with, from 4,000 psi concrete to 15,000 psi concrete. I've used 50% in 13,000 psi concrete and it works beautifully. Don't be afraid to use 30% or 40%, you won't be sorry. Unfortunity, slag is not available in all areas, especially hard to find in the West. The East coast has quite a bit and your chances are much better in the East than any where else. There are advantages and disadvantages, but they can easily modified. Advantages: you can increase the percentages in the warm/hot climatic conditions, slag will retard or slow the set of the cement paste, but if the concrete temps are in the 80's, you will have not problem with the set-times. The contractors will love it. Slag works great with HRWR's, both Mid Range and High Range. On the other side of that, you will need an accelerator in cool/cold climatic conditions and you will need to drop your replacement percentages down most likely. Slag is expense, about the same cost of portland cement, but worth it. The strength gains between slag and flyash are quite different. Slag is a "hydrate" than flyash and you get the bang for buck, in 16-20 hours strengths, especially in hot weather. Slag isn't cheap, just about the same cost as cement, but in my book, its worth every penny you spend. Your air contents will remain stable, not always in flyash, depending upon the LOI (loss of ignition) in the ash. Go for it!

 

[Kivi]

in the mining game, we use slag cements for mine backfill. the standards for cement replacements are not as strict regarding astm, but we do testwork to prove. we get over 90% slag replacement of cement. the slag cements in this case take longer to set initially but surpass 100% opc in the long term. for surface type work follow the astm regs (or CSA) with respect to slag chemistry and grind fineness. Slag used to be a waste product and free, now cement suppliers are tieing up the sources (pig iron plants). and now a preblend for slag cement for our use is more expensive than cement, because it has built in retardant and sulphate resistance. you can make your own slag cement by grinding and mixing your slag with opc.
regards,

 

[concrete206]

I have alot of experience with 40% slag replacement, there is no difficulty in replacing the portland cement, provided you use a mid range plasticer to reduce your overall water content. A mid range plasticer from Grace Chemicals or Sika, will give you a reduction of 25/30 litres or kg of water, the water/cement ratio is critical. Most concree plants the plasticer starts as soon as the water begins, with a mid range plasticer change the settings on the plant so that 80% of the water has gone in before the plasticer goes in, then you will see the power of the plasticer.

 

[mporisky]

Any concrete sent out without at least one SCM is in my opinion "stone age" concrete and belongs in a museum. Slag is a great SCM that we especially use in the summer to mantain set times. Reinterating with a previous comment the finishers will love it interms of its ability to improve the workability, and finishability however it can reduce bleeding depending on the fineness. Using it at dosages above 35% also gives you the added benefit of mitigating Alkali Silica reactivity and Sulfate Attack, although watch your alumina content as greater than 12% Al2O3 will decrease sulfate attack resistance. We usually see a modest water reduction ~2%.

 

[jprusinski]

The Slag Cement Association maintains a website that has practical information about the use of slag cement in concrete. Information sheets in the "About Slag Cement" area are downloadable. Also, there are case studies in the "News and Resources" area.

http://www.slagcement.org

 

[WAstruc10]

I agree with most of the above - I've used 40-45% replacement on many jobs with good success, only been scared at some very low 7-day breaks but it seems to go off the charts after that. Hard to sell it on multi-story flatwork but for everything else the delayed set and slow strength gain seems to be no big deal to most contractors, unless it's real cold out. Supply is typically no problem in western WA.

And no one has mentioned the environmental benefits. Everytime I get the question of "why do we need this stuff in the mix" from the contractor or redi-mix supplier, I describe the durability and engineering benefits, plus I don't feel guilty adding in "because it's the right thing to do". Our industry ignores it (mostly) but cement is a big part of a big problem - and requiring GGBFS is one relatively painless way engineers can make a significant difference. Most contractors, architects, and owners will agree that it's the right choice once they know that 1. it doesn't really cost any more, 2. it makes a better final product (by the way, architects like the whiter color that results), and 3. it is an environmentally consciencious choice.