Cocrete Compression Test Break types

[BobSmith54]

Greetings All

I am the Manager of a testing firm in OH and I am trying to exsplain the types of breaks that are desired as a result of testing a concrete 6x12 cyl to failure. There is a discussion as to what is best a conical break or sheer and or splintering concrete (col break) Keep in mind all that this is 3000 - 4000 psi concrete

Please provide references for the type if failure

Robert

 

[concretedoc]

The ASTM Manual of Aggregate and Concrete Testing is a helpful reference on this subject. It's included at the end of Vol. 04.02 of the Annual Book of ASTM Standards. For 3000-4000 psi concretes, conical fractures are typical and expected. Variation from a conical fracture may indicate a nonstandard testing procedure. You've probably seen the sketches of types of fracture in ASTM C 39. Unfortunately, that document tells you to report the type of fracture but has no information about what the fracture type indicates. I'll have to look up some other references to give you more information. I recall that highly sanded mixes may exhibit shear failures and columnar failures for 3000-4000 psi concretes may indicate a nonplane bearing surface or capping problems.

 

[Ron]

BobSmith54...I see that you are listed in "Geotechnical" so let's use a geotechnical analogy. The compressive strength test is an unconfined compression test (confining pressure=0). With uniform specimen fabrication and generally homogenous conditions, a "double cone" failure pattern would be expected for a triaxial stress state. To receive any other failure mode implies a deviation from a consistent stress state inside the sample. That deviation can come from a variety of sources as there are many variables in the process. As an example, lets assume that the specimen has a high water-cement ratio and a high entrained air content. Voids will result from these two conditions and will manifest in different locations depending upon the rodding process while fabricating the specimen. Variations in the size, quantity, and grouping of these voids will create a "discontinuity" in the homogeneity assumption. Internally, that will create stress raisers and variations in the stress distribution resulting in the various failure descriptions promulgated in the ASTM standards.

Most of these variations (while still achieving some variant on the cone shape) are minor and the end result is an insignificant effect on the macro-strength. As the failure mode deviates more from the cone, the significance becomes greater, leading to an appearance of erratic strength. These can be caused by internal physical specimen variations (voids, aggregate distribution, microcracking, etc.), variations in moisture content through the specimen during curing, variation in load application speed, variations in load distribution at the top and bottom of the specimen and others.

Achieving the "double cone" failure shape is a good indication of procedure consistency in making the specimen, curing, and testing. That leaves only variations in the actual strength of the concrete to explain differences in the observed strength values.