concrete slab on grade 2

[wildehond]

I am investigating a proposal by a European supplier to use shrinkage compensating agents in a slab on-grade. Their claim is that by countering the shrinkage of the concrete together with the use of steel fibre-reinforcement, much thinner slabs can be used. While the savings all-round are very attractive, I am needing to be confident that their claims are real. Does anyone out there have any experience of using slabs like this?

 

[Lion06]

I haven't had an opportunity to use this yet, but I've seen a couple of presentations on the subject and have been eager to use it with a PT structure. That opportunity has not presented itself yet.

My takeaways, regarding SOG, from the presentations I've seen we're not that thinner slabs could be used or reinforcement taken out, but rather that control joints can be eliminated. There is cost savings there, both upfront for the cutting of the joints and long term with the maintenance of joints.

 

[Ron]

Each variable introduced into a concrete project, in this case steel fiber and shrinkage compensating cement, requires tighter controls during production and placement. Don't forget that. The concept is plausible, but not without specific controls and overview that most suppliers and placement crews are not familiar with.

 

[hokie66]

I think to gain confidence in these claims you would have to do your own extensive investigation, including visiting sites where the system has been used.

Jointless concrete slabs, particularly in industrial and storage facilities, are probably the way of the future. Joints in slabs are a major maintenance problem. But at this stage, there are not recognised standards or guidelines which allow the average engineer or owner to make a decision. While everything is proprietary, it is difficult to separate the real from the snake oil.

I am yet to be convinced that there is such a thing as "no shrink" concrete, but certainly the shrinkage can be reduced, at a cost. The usual approach to eliminating joints, up until now, has been to use enough reinforcement, about 0.006Ag, to control crack widths, not to eliminate cracks. I don't think any research indicates that steel fibre concrete can control crack widths with the same degree of confidence. Like Lion06, I don't think reducing slab thickness is appropriate.

Another approach to limiting the number of joints in slabs on grade is to use bonded post-tensioning, without edge restraint and with as near frictionless subgrade as possible.

 

[wildehond]

thanks very much for all your responses. Unfortunately, the only slab like this in South Africa was only cast in Nov last year. Not enough of a track record to give me comfort. I have been doing a lot of reading around the topic though:

The stock response to adding steel fibre into concrete seems to be that the steel fibre will, over and above avoiding macro-cracking (and turning it into micro-cracking), give a level of ductility. In a design process that seems to favour convenience over rigour, the product of the section modulus and a deemed tensile stress capacity gives the allowable moment. This is then combined with a yield line-style analyses to produce a "safe-design". With shrinkage compensation, the usual residual tension component in the concrete is ignored and I believe that this is how this particular european "slab-on-grade company" justifies their thinner slabs.

It looks seductive on paper, but the true test result will only be several years down the line. I may have to get out my passport and go and see one of the European slabs with sufficient age to act as a proper reference;-)

thanks again for your help. if i find out more, i'll post it.


PS. Does post tensioning have to be bonded? Can one consider unbonded PT?

 

[hokie66]

I would never consider unbonded PT for any reason, but realise that it is used in some countries. Many of the locations where it was used have now realised that it is not a particularly good idea.

For a slab on grade which cracks (and PT will not necessarily prevent cracking), there is nothing to prevent the crack opening quite widely, as the slab can slide along the unbonded tendons.

 

[wildehond]

thanks Hokie66, i appreciate your input.

 

[TXStructural]

This appears to be cross-posted from another spot on the forum, please don't do that. And see my reply on your other post for more specifics about shrinkage comp.

ACI's slabs on ground committee is working on a new guide which will address slab design and does include fiber reinforcement - the fiber industry is investing heavily in making their product visible and getting it accepted into the codes.
By whatever means you achieve it, the method to control crack width is to be sure that sufficient steel crosses the cracks as they try to form. Widely-spaced, larger diameter rebar may not initially control cracks as well as smaller, closely-spaced bars or wires (as in mesh). Fiber is attractive for the control of initial, visible crack development, but does not prevent cracking although the width of cracks is generally well-controlled. Over time, bars and wires may perform better than fibers, since they are not subject to the raveling that can make shorter fibers useless. In a properly designed, placed and cured cured slab, of sufficient thickness, over a well-prepared subgrade, most slabs do not need reinforcement for some period of use. But over time, we find that performance and appearance deteriorates in unreinforced slabs, particularly with the economic considerations required for very large slab areas (that is, it is important to save a little thickness/admix/curing/reinforcement in each square foot since you could have millions of sq ft.) It is impractical in most cases to construct the BEST slab possible, so we construct the best slab we can afford. There are always tradeoffs of performance for economy.

Almost all slab on ground PT we see here is unbonded.

 

[hokie66]

wildehond,
As you can see by these comments and also from other similar threads on the site, slab on ground design is probably the least understood, most contentious, and widely varying with geographical location of all our structural pursuits.

 

[wildehond]

TXStructural, Sorry about the cross-posting. It was a mistake rather something intentional. Thanks, too, for your input.

Hokie, thanks for your summation. It certainly is starting to dawn on me quite how many compromises have to be stuck.

 

[hokie66]

We will be interested to hear more about your project...especially if you go on that investigative trip in Europe.