Date: January 1, 2010

News - Testing Compressive Strength

 

The compressive strength of concrete is the most common performance measure used by the engineer in designing buildings and other structures. The compressive strength is measured by breaking cylindrical concrete specimens in a compression-testing machine. 

Compressive strength is determined to determine that the concrete mixture as delivered meets the requirements in the job specification.

A test result is the average of at least tow standard-cured strength specimens made from the same concrete sample and tested at the same age. In most cases strength requirements for concrete are at an age of 28 days.
 
Cylinders are made and tested according to ASTM C 39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens.
 
To comply with the strength requirements of a job specification, the following criteria should apply:
 
  • The average of 3 consecutive tests should equal or exceed the specified strength.
  • No single strength test should fall below the specified strength by more than 500 psi or by more than 10 percent of the specified strength when the specified strength exceeds more than 5000 psi.
When strength test results indicate that the concrete fails to meet the requirement of the specification, it is important to recognize that the failure may be in the testing, not the concrete. This is especially true if the fabrication, handling, curing and testing of the cylinders are not conducted in accordance with standard procedures [link to cylinder tip]
 
How to Test the Strength of Concrete
 
Cylinders should by 6” by 12”in size or 4” by 8” when specified. The diameter of the cylinder used should be at least 3 times the maximum size of the coarse aggregate used in the mix.
 
Recording the mass of the specimen before capping provides useful information in case of disputes.


Testing Compressive Strength









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