Cracks in a reinforced concrete SOG 2

[wdc]

We Pumped a SOG basement level 85' x 70' (slab was enclosed)Im sure during the set time it was hot in the basement and there was no air movement. the slab was a 6" slab #4 @12" EW 3"from top. control joints cut in 20'x20' panels at the column block outs 1" deep 1/4" wide cut in right after set up same day. curing compound used. The problem is that we have a number of cracks through out the floor more than can be expected. going in all directions. We are being told that are control joints are to far apart and the cuts are to small. I don't see that being the answer to the cracks. Is there any other reason or is that the reason for the cracks.

 

[dik]

The random cracking would indicate that it was sawcut at the wrong time, the sawcut depth was insufficient, or the concrete slump was too high.

If sawcutting started about 8 hours after finishing, it was likely sawcut at the right time. If you pumped it and didn't use a superplasticizer, the slump may have been too high. For slabs, I usually use a 3" slump or equivalent with a plasticizer. Also for a 6" slab, I would normally sawcut it 1-1/2" to 2" deep. The 20'x20' panels seem about right; I've done many slabs of this thickness and panel spacing.

Unless the subgrade was 'really' bad, it's likely too early for it to be a settlement problem.

 

[phuduhudu]

I would agree with dik that the depth of the sawcuts sounds quite small but it also sounds like the quality of the concrete mix might have something to do with it because the joint spacing sounds about right. There are a lot of things that can affect the amount of water a mix needs including sand quality and shape and aggregate shape. As dik said your mix may have been too wet. Carl Bauer

http://www.bauerconsultbotswana.com

 

[dik]

What is the approximate spacing of the random cracking? and what type of pattern?

 

[Ron]

The spacing is common but on the high side of acceptable. The sawcut depths are shallow...should be 20 to 25 percent of the thickness of the slab, more under adverse curing conditions.

As dik said, would be interested in the pattern of cracking. Closely spaced, shallow cracks or crazing at the surface indicate too much water in the mix or fast bleed rate. Short, discontinuous cracks that ultimately connected by shrinkage are indicative of plastic shrinkage that occurred early in the curing stage, so sawcuts would have no effect on them. They are caused when the evaporation rate of the surface moisture in the concrete exceeds the bleed rate of the mix water.

Give us a little more info as dik requested.

 

[dik]

Should add that under one of the other forums, response has included the absence of the PEVB layer allowing bonding of the concrete to the base material restraining the slab against shrinkage..

I usually use 1-1/2" to 2" conc cover to rebar (depends on sawcut depth) and for general developer use (inexpensive), caulk the sawcuts with a medium firm caulking material to support the edges of the slab at the cut.

The #4@12 are heavier than normal. The 12&quot; spacing might make it difficult to walk 'through'; any chance these are located at the bottom of the slab, on top of the base? It's easier to walk on them, then... <G>.