For this purpose, either concrete cube or cylinder specimens are tested in the laboratory.
3 concrete specimens are generally made and cured for 28 days at site and then sent to laboratory for testing.
In this post, I’ll describe step by step procedures of concrete compression test at laboratory.
You’ll need following things for testing concrete compressive strength -
* Measuring tape
* Safety goggles
* Testing machine, and ofcourse,
* Concrete specimens (Cylinder/Cube)
Procedure of Concrete Compression Test
Step1 – Preparation: Check all the things you need are ready. Check concrete compression machine is in working order.
Step2 – Safety: Wear hand gloves and safety goggles.
Step3 – Taking measurement: Take the measurement of concrete specimens (which are sent to laboratory for testing). Calculate the cross sectional area (unit should be on mm2) and put down on paper. Do the same for each specimen.
Step4 – Start machine: Turn on the machine. Place one concrete specimen in the centre of loading area.
Step5 – Lowering piston: Lower the piston against the top of concrete specimen by pushing the lever. Don’t apply load just now. Just place the piston on top of concrete specimen so that it’s touching that.
Step6 – Applying load: Now the piston is on top of specimen. It is the time to apply load. Pull the lever into holding position. Start the compression test by Pressing the zero button on the display board.
Step7 – Increasing pressure: By turning pressure increasing valve counter-clockwise, adjust the pressure on piston so that it matches concrete compression strength value. Apply the load gradually without shock.
Step8 – Test is complete: Observe the concrete specimen. When it begins to break stop applying load.
Step9 – recording: Record the ultimate load on paper displaying on machine’s display screen.
Step10 – Clean the machine: When the piston is back into its position, clean the creaked concrete from the machine.
Step11 – Turning off machine: Match your record once again with the result on display screen. The result should still be on display screen. And then turn off the machine.
Step12 – Calculate concrete compressive strength: The result we got from testing machine is the ultimate load to break the concrete specimen. The load unit is generally in lb. We have to convert it in newton (N). Our purpose is, to know the concrete compressive strength.
We know, compressive strength is equal to ultimate load divided by cross sectional area of concrete specimen. We took the concrete specimen’s measurement before starting the testing and calculated cross sectional area.
Now we got the ultimate load. So we can now calculate the concrete compressive strength.
Compressive strength = Ultimate load (N) / cross sectional area (mm2).
The unit of compressive strength will be N/mm2.
Normally 3 sample of concrete specimens are tested and average result is taken into consideration. If any of the specimen compressive strength result varies by more than 15% of average result, that result is rejected.