After a compressive strength test of three concrete cubes, how can I compare and

Question

After a compressive strength test of three concrete cubes, how can I compare and discuss the results with standard characteristic compressive strength(at 28 days (fc) in the range of 20 MPa to 65 MPa ), identify the exposure category satisfied by the concrete produced based on the appropriate Australian standard .

Maximum Load (failure load) 259460.55 N Compressive Strength: Cube 1 Compression 259460.55 0.12 25 946 055 Pa 25.95+ 26.37+29.32 mean 3 = 25.95 MPa mean = 27.21Mpa Maximum Load (failure load) 263721.16 N Compressive Strength: Cube 2 Compression a (25.95-27.21)2 -1.5876 b (26.37-27.21)2 0.7056 c-29.32-27.21)2-4.4521 263721.16 0.12 26 372 116 Pa a+b+c 6.7453 26.37 MPa 2 Standard Deviation = 3.37 Mean = 27.21Mpa Cube 3 Compression Maximum Load (failure load): 293196.56 N Compressive Strength: 293196.56 0.12 Characteristic Strength Mean (SD*X) where X= 1.64 for 95% Ch·Strength 29 319 656 Pa 29.32 MPa 27.21- (3.37 1.64) = 21.68 So this mixture has a 95% Characteristic Strength of 21.68Mpa

Explanation / Answer

Here three cubes are made according to the Australian standards of the characteristic compressive strength of 20 MPa. Characteristic compressive strength is the strength of the cube after 28 days of curing in which 95% percentage of the tested specimens satisfies or falls above the required strength and 5% specimens may fail. So, the three tested cubes gave 25.95, 26.37, 29.32 MPa of strength and their mean is calculated as 27.21 MPa and the standard deviation as 3.37. For the 95% confidence level the value of x is 1.64 and then the equation for characteristic strength is shown above and calculated and finally, the obtained value is 21.68 MPa at 95 % confidence level which is more than 20 MPa which is according to the Australian codal provisions. So, it comes under no risk or corrosion attack exposure category.

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