Shrinkage Compensating Concrete Slab 1

[McQSE]

I am interested in some feedback from people who have experience using shrinkage compensating concrete for slab-on-grade applications. I am considering using this as an alternate to a post-tensioned slab. I need a monolithic slab/grade beam/footing foundation due to expansive soils so it makes crack control difficult. I have seen problems occur when slabs and footings are cast monolithically due to the restrained condition but I need a stiff foundation/slab.

Thanks for your input.

 

[hokie66]

1. "Shrinkage compensating concrete" does not exist. There are some things which can be done to limit the shrinkage, but all concrete shrinks.

2. Post-tensioning will not provide crack control in a slab with restrained edges unless the strands are bonded. Even if they are, for a strong degree of crack control, you will need some additional deformed bar reinforcement.

 

[rapt]

Agree with both of Hohie66's points.

Also, shrinkage is not the only source of shortening. Temperature change will be at least as significant and possibly more so depending on the temperature at time of construction and the general working temperature.

 

[McQSE]

Thanks for the input.

The use of shrinkage compensating concrete is well documented. You are correct that all concrete shrinks, but shrinkage compensating concrete expands (first seven days or so) before it starts to shrink. The concrete goes into compression as it expands and this offsets the tensile stresses that cause shrinkage cracks when it starts to shrink. I have never used it on a job and I am interested in some firsthand knowledge.

I am not sure about your second comment. Post-tensioning whether bonded or unbounded produces a compressive stress on the concrete. This compressive stress helps to counteract the tensile stresses caused by shrinkage. Why do you say that “Post-tensioning will not provide crack control in a slab with restrained edges unless the strands are bonded?”

 

[kslee1000]

McQSE:

I think the most important parameter to watch out for is the end restrains around the elements that using "shrinkage compensating concrete". As you pointed out correctly - it expands, thus create permenent stresses on the confining elements. You may avoid that effect by carefully planning concrete casting sequence, and locating construction joints. Do you rebas in the slab?

 

[hokie66]

My second comment about post-tensioning being ineffective in providing crack control is, as stated, based on the edges being restrained, as they are in stiffened slabs. Most of the force of the tendons is resisted by the edge beams bearing against the earth rather than by the slab. If the tendons are not bonded, any cracks which form in the slab are free to open.

 

[McQSE]

hokie66

Thanks for the clarification. That makes sense.

 

[concretedoc2]

Check ACI 223 "Standard Practice for the Use of Shrinkage-compensating Concrete". In addition, check out the web site for CTS cement which makes the expansive cement for shrinkage compensating concrete. They have FAQ's along with structural design recommendations. And you can always ask them for a list of projects near your area to view the performance for yourself.

 

[jheidt2543]

McQSE is correct. Some 25 years ago, when I worked for General Contractor, we built a warehouse for a local food plant. The Owner didn't want the normal floor joints and column boxouts, so a propritory shrinkage compensating cement was specified, Chem-Comp by the designer. The specifications called for significantly more slab reinforcing than normally used at the time, I want to say 25%-30% more. The added rebar "pretensioned" the floor, so as it tried to shrink back from it's expanded volumne (as McQSE described) it was restrained by the rebar.

The floor pours were made in 10,000 sf sections, no joints, no column boxouts and finished conventionally. That floor is still crack free even with all the forktruck traffic that it has had over the years.

Now, the reason this never really caught on is the added cost. The increased amount of reinforcing, the fact that at that time the ready-mix supplier had to order the Chem-Comp cement in bulk and empty his bins just for that job, and the large on-site construction crew to pour and finish the slab.

Today, you can get additives that are put in the ready-mix truck at the site, concrete pumps are readily available, as well as riding finishing machines all of which reduces the cost.