Comparison of UCS crushing machine to CBR crushing machine

[defg]

Dear Sir,
Currently I'm working as Materials Engineer at road site laboratory.Usually we are doing crushing the stabilised subbase specimen on our new compression testing machine(digital models).The obtained test results were found good and satisfy the specification requirements by the use of 3%cement we got an average of 1.8MPa and Sometimes 2.1MPa.The norminal specified was 1MPa.As part of confirmatory test,we took samples from the same borrowpits and test them at a reputable University laboratory.The test results obtained from them especially the Unconfined Compressive Strength were found too low compared to our site laboratory,which was about 0.9MPa to 1MPa.Under these circumstance I have tried to crush some specimen by using the CBR machine because it is well calibrated.On top of the specimen we put a thick metal plate to enable plunger to transmit the penetration force evenly over the whole specimen.When it starts crushing the dial reading are recorded and the load(KN) are read from the CBR calibration chart.Nthi case we manage to obtain 1.7MPa with this CBR machine instead of 2.1Mpa by using our compression machine.Is it a correct way or there is another way of checking the test results Please advise!

 

[jdonville]

See ASTM D 2166 for discussion of the method and calculations.

The cross-section area of the UCS specimen at failure is calculated as A*=A/(1-e), where A is the original cross-section area of the specimen prior to test, and e = deltaL/L = absolute value of strain of sample at failure.

Naturally, UCS = load at failure/A*

UCS is calculated at failure or 15% strain, whichever is reached first.

Not correcting for the change in cross-section can result in UCS results which are too high.

Jeff

 

[emmgjld]

The speed of the load application is very important. A CBR machine is quite rapid, which would significantly increase the apparent strength. Look at the machine rates of load application and read the appropriate test specifications. If you are comparing with another lab. be sure that everyone is recording the rates of load application.It would be ideal if everyone could use the same application rate but, limitation of equipment and procedures make this nearly impossible.