For the past few Friday’s we’ve focused on concrete (read more: Why Concrete Testing is Essential for New Construction Projects, How Cold Weather Impacts the Structural Integrity of Concrete, How to Determine the Temperature of Freshly Mixed Hydraulic Cement Concrete and Why It Is Important). Last week, we learned about making and curing concrete test specimens. That brings us to the end of the road for our concrete test specimens: compressive strength testing.
Don’t be crushed to read this – it’s the specimen’s purpose in life!
After our concrete cylinders have been properly cured, they are ready to meet the compression testing machine. After donning the appropriate safety gear, the technician will place the sample in the machine. There is a right way to do this – the procedure and specifications outlined in the appropriate ASTM Standard Test Method must be followed in order to get accurate results and keep the technician safe.
The testing machine has two thick, flat plates. The sample is placed between the top and bottom plates, and pressure applied hydraulically or electromechanically from the top and bottom. The amount of pressure needed to crack the sample indicates how much load the concrete will be able to bear in field, as a foundation, support, or whatever it is used for.
A window in the machine allows the technician to see what’s happening while keeping him or her safe from flying debris while load is applied gradually to the specimen. The test cylinder will begin to crack and splinter – this is what we’ve been waiting for! The technician records the load at which specimen cracks, and notes any unusual features or actions in the sample. The Compressive Strength Formula is then applied to calculate the compressive force required to compress the cylinder to its breaking point (wait, no one said there would be math!).
Concrete fragments are removed from the machine, and the procedure is repeated with two more samples from the same group. If the strength of any of these specimens varies by more than 15% of the average strength, its result should be rejected. The average of the three (or more) samples should be used as the final strength measurement of the concrete (more math!).
If you need concrete sampled and tested, call Encorus’s Civil Laboratory Supervisor Jeremy Lake at (716) 592-3980, ext. 133, or email him at [email protected].