Highly varied concrete strength

[arawinda]

Hi Friends,

Recently I experienced a bit high variation of our strength results of C50 ready-mix concrete purchasing for slabs and columns casting. We expect 37.5 N/mm2 as 7 days strength for C50 (i.e. 75% of design strength as per Sri Lanka Standards). Can anybody help me on following concerns.
1. Does this high variation (0.8% to 20%) affect integrity or durability of element?
2. Are there any thresholds for this variation specified by any standard?

Please see attached excel spread sheet for data of recent slab/column cast.

Thanks in advance.

 

[MIStructE_IRE]

In my experience this is generally (but not always!) down to poorly made/cured cube tests which may not be reflective of the concrete as placed.

Perhaps take some cores from elements for crushing first.

 

[arawinda]

Dear MIStructE IRE,

I personally observing process of cube making and curing. We follow standards without fail.

 

[r13]

I wouldn't worry, as all test results are higher than the estimated/anticipated strength. The variation will be smoothed out with time, and proper curing.

 

[JLNJ]

Are you testing cubes or cylinders?

Generally I wouldn't worry about high strength samples, except, when the variation is that great, you should be concerned about the experience and skill of the ready-mix facility producing the concrete. Maybe they have poor quality control and try to cover it up by using extra cement in the mix.