Low strength test results

[sapperDAK]

I know there are many reasons why concrete compressive strength test specimens come up below the specified strength. Generally speaking, has anyone experienced or can anyone give me a basic explanation for low strength when the cylinders actually seem to crumble? Upon close examination, the top of the cylinder fails by "crumbling" 360 degrees around the circumference of the specimen. Internally there are no fractured pieces of aggregate - they aggregate simply "pulls away" from the paste. Am I wrong in presuming that this is an issue of the cement paste "bonding" to the aggregate? Additionally, there is a lot of "powder" (which can be brushed away from the interior of the specimen by finger) which I interpret to be unhydrated pozzolan/cement. Are there any common reasons for this to occur other than water/cement ratio? What about "dirty" coarse aggregate or excessively dry aggregate?

 

[Ron]

Dry aggregate does not cause this issue. It is usually related to a very high water-cement ratio. Even contaminated (dusty) aggregate will not "debond" uniformly.

Sounds like you have a real issue with the concrete. Get a local expert to put eyes on it. We can do a lot here in the forums, but not a substitute for actually seeing the issue.

Smell the broken concrete. If it has a pungent, chlorine-like odor, the cement has not hydrated appropriately.

Was an air entraining admixture used? If so, is there a possibility of overdosing?