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Hardened cement in concrete

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Stan in Oly, WA
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PostPosted: Fri Aug 30, 2013 7:15 pm    Post subject: Hardened cement in concrete Reply to specific post Reply with quote

I bought three 94 lb. bags of cement at Lowe's two days ago. I wasn't glad to see that I had to take them from the last few on the pallet, but it was that or go somewhere else, and I'd already hit my limit on shopping. When I was mixing the concrete, one of the bags had caked cement clumps in it. Not real bad, I could easily pulverize them with the shovel, but it made me wonder what would happen if they didn't break up or dissolve---what would the concrete be like?. Does anyone know?

Stan
 
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dpendzic
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PostPosted: Fri Aug 30, 2013 7:23 pm    Post subject: Re: Hardened cement in concrete Reply to specific post Reply with quote

those clumps would get encapsulated by the concrete and would be weak pockets in the mix
 
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Tom vertiz
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PostPosted: Fri Aug 30, 2013 7:27 pm    Post subject: Re: Hardened cement in concrete Reply to specific post Reply with quote

Yeah you would have a spot in the concrete with dry cement in it. Your concrete would end up like Swiss cheese with lots of holes in it. With the bags on the bottom that is not uncommon. The pressure compacts the power together.
 
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da.bees
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PostPosted: Fri Aug 30, 2013 7:43 pm    Post subject: Re: Hardened cement in concrete Reply to specific post Reply with quote

If you are wondering how the portland would compare in strenth if it absorbed enough water (which is unlikly) to wet it then dry along with the rest of material. It would be much weaker. It is somtimes thought that sand and rock is added as a filler to reduce amount of portland required but that is far from true. Portland only binds the things with strength togeather.
 
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Leroy
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PostPosted: Sat Aug 31, 2013 3:59 am    Post subject: Re: Hardened cement in concrete Reply to specific post Reply with quote

Don't know how it does for strength but caked stuf like that I have used a sledge to but things to smaller pieces then put them dry in the mixer and after a while it looks like fresh.
 
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Jim in Ma.
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PostPosted: Sat Aug 31, 2013 6:24 am    Post subject: Re: Hardened cement in concrete Reply to specific post Reply with quote

I had some with hard lumps in it. I put the whole bag full in the mixer with a half dozen rocks (base ball size) and let it run dry for a while. Worked great.
 
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Stan in Oly, WA
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PostPosted: Sat Aug 31, 2013 7:23 am    Post subject: Re: Hardened cement in concrete Reply to specific post Reply with quote

I've put hard caked pieces in a cardboard box on concrete and pulverized them with the end of a 4X4. The cement seems to work normally, but I don't trust it enough to use it as the primary cement. I'll use it a scoop at a time to enrich concrete I mix using fresh cement. Technically, I don't think it makes a whole lot of sense; I mean, you mix your concrete to be strong enough for the application. I wouldn't use reconstituted cement to reach that strength, but I think I'd have trouble explaining the benefit of making the concrete stronger than just strong enough. But then, putting any amount of effort into saving a quarter bag of cement or less doesn't make much sense, either. That's $2.50 worth of cement.

Stan
 
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Stan in Oly, WA
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PostPosted: Sat Aug 31, 2013 7:49 am    Post subject: Re: Hardened cement in concrete Reply to specific post Reply with quote

Frankly, it's the "Worked great" part I'm most uncertain about. Concrete is more complicated than most of us imagine. Years ago, I poured a small pad between gate posts on a cold November day. I didn't make any effort to protect it from freezing other than to hope the temperature wouldn't get that low that night. The concrete seemed fine for a few years, so I thought I'd gotten away with it, but then the surface started to deteriorate, so I knew I hadn't.

Another time I poured a short garden wall (in warm weather) using fresh cement, a good mix ratio for the application, doing everything carefully, I thought. Recently I used a wire brush to clean some lichen off the top of the wall, and was startled to see that it was also removing some of the surface of the concrete. So I'm not entirely comfortable doing something unconventional with my concrete mix when the results might not become apparent for years, and the consequences of it failing might be a serious problem.

Stan
 
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Billy NY
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PostPosted: Sat Aug 31, 2013 8:52 am    Post subject: Re: Hardened cement in concrete Reply to specific post Reply with quote

Technically, its faulty material, should be discarded by the supplier, though I've bought torn bags at a huge discount, any that have absorbed moisture and have started the hydration process, would be of no interest or value, well except to pack dry around a post or similar.

Like was said, it would create voids. If a windsor probe were to hit one of these voids, you may easily see the strength reduction, say as opposed to probing an area that is comprised of good material.
http://www.youtube.com/watch?v GhlMhwBdAN8

 
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Billy NY
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PostPosted: Sat Aug 31, 2013 8:57 am    Post subject: Re: Hardened cement in concrete Reply to specific post Reply with quote

You may find this testing method of interest, I had to use it in NYC when the developer/owners consultant pushed to commence work before I had a controlled inspection company hired and on site. They certified that the concrete underpinning,( support of existing and occupied buildings that had footings above the new high rise) was of the strength specified on the structural drawings, project specifications, 03300 etc.
More WIndsor Probe Info

 
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Dick L
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PostPosted: Sat Aug 31, 2013 9:44 am    Post subject: Re: Hardened cement in concrete Reply to specific post Reply with quote

Take one of the clumps and pulverize it then mix the proper amount of sand and water with it for a little batch and let is set to see what happens. If the clumps will crumble into a powder fairly easy I would doubt that it had been wet enough activate the catalyst to the point you would be able to tell a difference. It does not take much of the calcium sulfate to cause it to set. It might take a little bit longer to set than normal.
 
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Stan in Oly, WA
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PostPosted: Sat Aug 31, 2013 10:20 am    Post subject: Re: Hardened cement in concrete Reply to specific post Reply with quote

Hi Billy,

It's easy to picture the situation of using gravel sized clumps of hydrated cement, and how they would form actual voids or, at best, pockets of a substance with less strength even than sand. What I can't get a clear idea of is what it would do to the concrete to contain a proportion of pulverized---to the point of appearing as fine as fresh cement---fully hydrated cement, not as the only cement in the mix but, let's say, as an addition to the required amount for the desired mix, or as a small (< 20%) portion of the total cement. Would it create concrete that was microscopically honeycombed with either voids or pockets of no strength material? In the past I've treated the addition of a small amount of reconstituted cement (to a mix that would be adequate without it) as an action that would either improve the strength of the concrete, or at least not hurt it. I would feel dismay, chagrin even, to discover that my efforts had not only been pointless, but had actually made the situation worse.

Stan
 
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Billy NY
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PostPosted: Sat Aug 31, 2013 11:49 am    Post subject: Re: Hardened cement in concrete Reply to specific post Reply with quote

For what you are doing, its probably marginal, however we know that this material, subject to any deviation from a designed and tested mix, can and will produce adverse results. Technically speaking, it needs to be done within a certain tolerance ie; additional water for lower slump, admixtures, pump design mixes and so on.

If in fact that amount of portland has begun the hydration process, (clumps) you have "technically" changed the water/cement ratio for the measured quantity in the bag. How much, well that depends on how much of the mix absorbed enough moisture, and I suppose, chemically, you could determine how much of the portland has reacted or how far along its been hydrated. Thats putting things under a microscope, and I think all who participate in the discussions you have posted (which I find highly interesting, and enjoy participating in) realize that most things you are doing, it just may not make too much difference. Its still an unknown to be honest, only way to really know is testing.

If in fact a certain portion of the portland in the bag, be it clumps or not, was compromised, its hard to say for that particular measured quantity, how much tolerance there is for something like that and what the result would be for compressive strength. This would be an interesting test, like (remember Puddles ?) did with the welding processes and the bending/breaking etc. You could create test cylinders per ASTM criteria, from the compromised mix, say after the clumps were pulverized and dispersed evenly into the mix. Break tests would prove out its effect on compressive strength, and that would truly answer this question as to the effects on a small measured quantity of an already designed and tested mix. I think it would be more along the lines of less portland cement in the mix. I also look at it from another view, it could also then be inert, weakened, to the point where its just filler, or an increase of small fine aggregate, which if was done to a design mix, you would proportionally have to add more portland to bond more aggregate, very small fines in this case. The mix is a uniform and measured quantity of all materials, change things and its performance may easily change as well, requiring testing to determine the results. I think what was in the bag has the possibility to bind, just maybe not as strong as if it were new material.

In the construction industry, with a stuctural material such as concrete, anytime there is deviation, or any compromise of this material, in my mind, a red flag pops up, hence my views as posted here, its just something you do not fool with. At home or in non critical situations, it may be fine, but I did see you took high notice of results or performance with this material where it spalled or did not perform abrasively, given what you describe. That is precisely why we do not, in good project management practice, tolerate deviation with concrete unless the engineer of record approves and it proves out through ASTM test criteria.

I did see mention of a wire brush being used, this may leave iron deposits onto the abrasive material, and leave rust stains, often times with masonry you want to use another brush type, so as not to leave rust stains.
 
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tomtirediron
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PostPosted: Sat Aug 31, 2013 12:00 pm    Post subject: Re: Hardened cement in concrete Reply to specific post Reply with quote

Stan, I'll look it up Monday when I get to work, I have several concrete manuals at my desk that I've used over the years. I would think that if you assumed all the material that was in clumps was a certain percentage by weight of the total concrete in the mixture you could figure out the percent of compressive strength lost compared to total possible strength. That is assuming the clumps were broken enough to not create actual large physical voids. I have some lab people I can ask also if no one comes up with satisfactory answer this weekend. Tom B.
 
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Stan in Oly, WA
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PostPosted: Sat Aug 31, 2013 12:35 pm    Post subject: Re: Hardened cement in concrete Reply to specific post Reply with quote

Thanks Billy,

I'm always grateful for the complete explanations you provide in situations like this. And, while it's accurate to say that much of the concrete I'm mixing is to be used in applications where there's a lot more leeway than if it were a commercial job, I prefer to know how to do things right, even when I choose not to, rather than do things wrong because I don't know better. In any case, I'm always interested in understanding how things work.

Stan
 
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