Why is there QA
Because this issue is often controversial among the parties: design consultants, inspectors, and appraisal agencies. Above all, this issue is related to the investor’s pocket, because increasing materials means increasing money💶.
First, let’s talk about the Standard. TCVN 10304:2014 section 7.1.6 All calculations of piles, pile foundations and foundations must use the calculation characteristics of materials and ground. This leads to a very reasonable point of view: the calculated strength of bored pile concrete must be multiplied by the working condition coefficient of 0.85.0.7, including pouring concrete in bentonite solution, … Leading to the material bearing capacity being unable to reach the test load twice the mass pile load if the concrete grade is kept the same. Many projects have even been required by the appraisal agency to increase materials, for example, test piles of grade 600, mass piles of grade 400 (💶).
In fact, the opposite is true: For many generations before TCVN 10304 was born, test piles were still made with the same mass pile grade and compression tests were almost never broken. Except for a few rare cases due to defective concrete, …
Is it necessary to increase the material: use higher grade concrete, more reinforcement?
To explain this issue clearly, it is necessary to
Disseminate types of strength:
When determining the strength of concrete, people compress the test samples to the point of destruction (breakage of the sample). The test samples are cast from a concrete truck poured directly, each batch casts at least 3 samples. The common shape of the test sample in Vietnam is a cube with side size a=150mm. The common test sample in the West is a cylindrical sample, as shown in the drawing.
Concrete compressive strength test sample
Strength is equal to the compressive force when the sample is broken divided by the compressed area (square or circle), the unit is force / area. According to TCVN 5574:2018 on concrete structures, there are several types of strength as follows:
💎 Average strength Rm, equal to the average strength of the cubic test samples. This is the popular M concrete grade, which is the average strength in the unit of kg/cm2.
💎 Characteristic strength: The standard comes from statistical work, to make sure to eliminate the probability of a damaged sample. So in addition to taking the average, a type of strength is born that is determined according to the guaranteed probability of 95%, called characteristic strength. This value in the unit of MPa (N/mm2) is the compressive strength level B of concrete. The probability relationship leads to the formula:
B = Rm (1 – 1.64ν) with ν=0.135
💎 Standard strength: Experiments show that the strength obtained when compressing a cube sample is greater than the strength of a cylinder sample. The problem is that the cylinder sample describes the concrete’s behavior in the structure more accurately. Therefore, it is necessary to convert the compression in the structure to the standard strength Rbn. The formula of the standard:
Rbn = (0.77-0.001B)B ≥ 0.72B
This means that the strength of the concrete in the structure is about 0.8 times less than the strength of the cube sample.
In conclusion, when the concrete in the structure is damaged, it reaches the standard strength.
💎 The calculated strength Rb is the standard strength divided by the safety factor (the reliability factor γb=1.3 according to the standard). The meaning of Rb is for the first limit state (ttgh1), ensuring that the structure is not damaged.
So, in terms of the working nature of the structure:
– Mass piles: large quantity, not allowed to be damaged, according to ttgh1, it is correct to use the calculated strength. Since it is the calculated strength, it is necessary to multiply the working condition coefficients
– Experimental piles: small quantity, because it is not possible to test all piles. The working condition is compression to failure, according to both the ground (the pile is oversettled) and the material: the concrete is compressed and broken. At that time, the concrete reaches the standard strength, not the calculated strength. Because the calculated strength is not used, there are no further reduced working condition coefficients. The same reasoning applies to the steel.
Conclusion
The SCT of the experimental pile material is calculated according to the Standard Strength. Therefore, just keep the same material as the mass pile, in terms of concrete grade, steel reinforcement, to withstand 2 times the experimental load. If you don’t believe it, try it in reality to see if it is compressed and broken.
I hope you can protect your investment 💶 in case of conflict of views like this.
