I conducted a compressive uniaxial stress-strain test for normal concrete, it achieved a peak stress of 35 MPa but when i calculated the young's modulus from the stress-strain curve in excel, it showed a young's modulus of only 6900MPa. It is quite a contrast to normal C35 concrete which has a 30 000+ MPa young's modulus. How can it achieve the same strength but different stiffness?
I think you have over estimated the strain which is often the case as accurate measurement strain is quite challenging. It would be helpful to know whether you are using strain gauges or LVDT? If you share more details and the actual stress-strain data, one can help in more detail.
Dhanada Kanta Mishra I'm using LVDT (50mm). Attached is the stress-strain graph in compressive. The strength is 35MPa but the elastic moduli is a bit low.
If you tested a normal concrete, then there was an error certainly in the recorded strain.
Did you use calibrated LVDT ?
How many samples that you tested ?
Sheikh Mohd Iqbal S. Zainal Abidin - thanks for sharing the data. This seems like result from one sample or is it an average of multiple samples (minimum 3 is recommended). Also it will be helpful to look at individual LVDT data assuming you used a set up as shown in attached photograph.
I would take a guess that the LVDTs were not mounted on the sample itself bit on the loading platen or frame. The correct way is shown in the attached photograph. If you followed this method and used reliable (calibrated) LVDTs, then it could be error in gauge length, strain calculation and any some other possibilities. My experience is measurement of strain using bonded electrical strain gauges gives most accurate results followed by LVDTs which would be generally an over estimation leading to underestimation of modulus. But generally it would be less than 10%. Hope this is of help.
It is not realistic even if it is lightweight concrete. If you review the elastic modulus equation in some codes of practice, e.g.; ACI, BS, EU, AS, CAN., ...ETC you will see that it is not logical. The method of testing could affect the results if it is not according to the recommneded test procedures.
Dhanada Kanta Mishra I think it may have been the strain calculation. At the moment i'm calculating strain using (deflection/ LVDT gauge length). However i did get a young's modulus of 44GPa which is more realistic if i calculate strain using (deflection/ cylinder sample length). Which is more appropriate?
Thanks for the help.
Its the first approach which is correct. However, 44 GPa is too high for 35 MPa compressive strength any way. You should calculate the theoretical value based on ACI formula ( E = 47,000 (fc’)^0.5 for normal density concrete) for elastic modulus as a function of strength and validate. Also the strain at peak load should be around 0.003 which is another way to know your data is reliable.
Based on your attached data sheet, I calculated the young's modulus from the stress-strain curve in excel, it showed a young's modulus of 70342 MPa. Therefore, you were mistaken and I think you dropped one zero to be 69000 MPa instead of 6900 MPa as you mentioned above. So, would you please check your data as well as your calculations especially for strains.
Mohamed I. Abukhashaba can you describe how you calculate the young's modulus? 70342 MPa is unrealistic as the compressive strength is after all only 35MPa.
Firstly, I have to apologize because I did a big mistake since I have calculated E from the curve you plotted (I touched the 2 followed points at the straight line by mouth and I determined their coordinates both x and y and I used the small calculator to get the resulted E). However, after your notice, I went back to your data for accurately recalculation and I found your previous calculation is correct. Any way, I think the mistake may be in the strain gauges readings or may be you used the wrong gauge factor.
Would you send me that excel sheet to observe if there are any mistake, please
This is possibly because of the way you observed the deformation. Did you use extensometer or any other setup. I have such kind of experience once. I have used seperate LVDT and extensiometer. The data acquired through LVDT , extensometer and from machine stroke are different with each other. So please explain the procedure you adopted in measuring the deformation.
R.Siva Chidambaram So what did you do? I used LVDT instead of strain gages because i followed the set up from an article in a Q1 published journal. I followed it strictly, and got the curve almost similar to the published results but the youngs modulus is off the chart. Very different from the ACI or Eurocode 2.
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