Supposing that we have splitting tensile strength determined by using a concrete cylinder 150mm x 300 mm. Based on this result we now have to calculate the splitting tensile strength corresponding to concrete cylinder 50 mm x 100 mm and concrete cylinder 500 mm x 1000 mm. What conversion factors (values) should be used in this case? Thanks.
I believe there's no universal factor to determine this as set up for split tensile testing varies.
However, I'll suggest to carry out split tensile tests on a particular mixture for all the 3 sample sizes to obtain a conversion factor for your set up. Then this factor can be used for subsequent calculations
@ Adeyemi Adesina: For small samples we can do the test relative easy. What should we do when we can not carry out the test for the large sample?
Ngoc Linh Tran: I believe maximum aggregate size has a great effect on the size dependency of tensile strength of concrete specimen. If the ratio diameter of the specimen/max aggregate size is above a certain limit (I'd say 5 or more) the tensile strength result values should not be influenced by the specimen size. What you may encounter is a reduction of standard deviation as the size increases.
Ngoc Linh Tran, In that case, you can assume they are the same since the height to diameter ratio of all your samples are the same. However, it will be worthy to mention in your article/paper that those assumptions were made and justify the assumption with a reason such as the constant height to diameter ratio
Adeyemi Adesina, the height-to-diameter ratio has little influence. Here the crack area (height x diameter) is more important. Existing experimental results support this, e.g. results of Malárics (2010) in the following link:
https://d-nb.info/1013134796/34
A further question is: why cylinder 150 mm x 300 mm is chosen for determining the reference strengths of concrete (tensile strength, compressive strength, elastic modulus), which are directly used in the calculation. Why not other size?
Ngoc Linh Tran I was unable to go through the file as its not in English
The rational behind the choice of specimen size used by standards is based on the maximum aggregate size that will be present in your mixture.
However, apart from special types of concrete such as preplaced concrete (ACI 304) where large aggregates are used, other standards uses specimen sizes less than that of a cylinder with 150mm and 300 mm height as dimensions are not explicitly mentioned in the standard. But ASTM C 192, section 5.1 stated that "If dimensions are not stipulated in a specification, test method, or practice, the specimen selected shall have a length that is twice the diameter "
For example;
ASTM C39 / C39M - 18
Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens does not explicitly state the dimensions of the cylinders to be used. That is why most researchers use either 75mm by 150mm or 100mm by 200mm. However, once your nominal aggregate size exceed 20mm, 150mm by 300mm is used i.e you don't want to use a mold size of 75mm by 150mm when you have a larger aggregate size.
The same also applies to the tests below
ASTM C496 / C496M - 17
Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete Specimens
ASTM C469 / C469M - 14
Standard Test Method for Static Modulus of Elasticity and Poisson’s Ratio of Concrete in Compression. For this test, most studies also use the 75mm by 150mm or 100mm by 200mm cylinders because of the availability of the MOE test apparatus.
So basically, depending on your size of aggregate, you have to make a deliberate choice of the most suitable specimen size that can give a significant representation of the mix. It is also worth to mention that according to ASTM C 192; specimens with 75mm diameter should be filled in two layers and rodded. Specimens with 100mm and 150mm diameter should be filled in 3 layers and rodded. While specimens with diameter 225mm should be filled in 4 layers and rodded. Any specimen with diameter greater than 225mm should be filled and two layers and vibrated.
The size of specimens will affect the apparent strength not only because of the size of the aggregate, but also because of the distribution of critical defects. Smaller specimens will result in a larger apparent strength. As a thought experiment: suppose I test one large specimen. It will fail due to its critical defect. If I were to split that specimen into four and test each one separately, one of the four would have the same strength as the original. The other three would be stronger than the original, because their respective critical defects are stronger (we know this because the "original" failure did not pass through them). So, on average, the material would appear stronger (though, of course, it's the same material and is not actually stronger).
I have never seen conversion factors for tensile strength, though I've seen them for compressive strength. I suppose my counter-question to you would be: what is the value of having such a conversion factor? What are the comparisons that you are trying to make?
- Badges
- Science topic