Two source of concrete for one suspend slab.
Two source of concrete for one suspend slab.
(OP)
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.
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.






RE: Two source of concrete for one suspend slab.
RE: Two source of concrete for one suspend slab.
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.
RE: Two source of concrete for one suspend slab.
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.
RE: Two source of concrete for one suspend slab.
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.
RE: Two source of concrete for one suspend slab.
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.
RE: Two source of concrete for one suspend slab.
RE: Two source of concrete for one suspend slab.
You only have 6 or 7 loads of concrete.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.