Two source of concrete for one suspend slab.

[hetgen]

Hi all,

Considering the size of parking deck slab two supply point were used and the 28 days cube strength for each supply point turn out to have a strength difference of 20MPa.

What are the long term implication of having 30Mpa and 50Mpa cube strength concrete on a suspended slab which only requires 30Mpa as per design? Stress concentration, variable shrinkage strain… ?

Currently after 51 days fine random crack are visible at bottom of the slab.

Thanks.

 

[hokie66]

I would seek an explanation for the variance, but the two different strengths should have no real affect.

 

[hetgen]

Thanks Hokiee66,

The explanation from the suppliers is different mix design; one supplier was clearly over conservative.

So, you don’t think having part of the slab with high strength or localized higher stiffness will cause different distribution of stress from what has been assumed for the designed?

Obviously it’s very difficult to identify where this 50Mpa concrete is within the slab and verify the above statement.

Thanks for your thought.

 

[concretemasonry]

I think that the approval of two different suppliers opened the flood gates.

Were the results of the two different samples tested in parallel or were they just historic results of previous testing of previously produced mixes?

It must have been in a project with limited production capability in the area and the resulting rivalry between suppliers. How many yards in the pour? Certainly, a contractor would like just one supplier for co-ordination and responsibility purposes.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[hetgen]

Hi Dick,

Yes, it was a capacity issue which led to this. The test was done in parallel, the values given are actual cubes tested results not historic. The slab area is about 300 square yards.

 

[dhengr]

Both suppliers meet the design spec., but the one will be working closer to its limit state. The working stresses and stress concentrations won’t change they are in your design and details. Some of the mechanical properties of the two mixes might be slightly different, but probably not enough to cause problems. Obviously, things like long term creep, shrinkage and cracking, etc. could be slightly different, but again probably not enough to cause problems. And, probably not much different (more?) than the std. variations from batch to batch due to allowable variations within a given mix design; i.e. high or low water content from batch to batch, max. allowable variations in any of the constituents, etc.

Hindsight is always better than foresight, but this could turn into a real can of worms if you do run into any problems with that slab. Without core sampling you don’t know who’s mix went where, and who to blame, if you do find distinct areas with problems. Hopefully they have different aggregate suppliers, or a comparison with known cubes from each supplier, would make this fairly straight forward.

 

[cvg]

300 square yards, assume 8 inch thick = 65 cubic yards, a fairly small job. I am wondering why this was considered so large as to require two plants to produce the concrete?

 

[concretemasonry]

If it was only 65 yards for two suppliers to handle it, I question the timing and mix design reports for being current. For a small supplier, it is easier and cheaper to add cement than to jump through hoops for a small job.

You only have 6 or 7 loads of concrete.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.