Assertion A: For determining uniaxial compressive strength of concrete, cube is a better test specimen compared to cylinder.
Reason R: Stress distribution is more uniform over the cross section of a cylinder as compared to cube.

Explanation:
To determine the uniaxial compressive strength of concrete, two common test specimens are used - cubes and cylinders. The choice between these two specimens depends on various factors such as the purpose of testing, available resources, and standards followed.

Understanding the Assertion:
The assertion states that a cube is a better test specimen for determining the uniaxial compressive strength of concrete compared to a cylinder. However, this assertion is false.

Understanding the Reason:
The reason provided for the assertion is that stress distribution is more uniform over the cross section of a cylinder as compared to a cube. This reason is true.

Analysis:

1. Uniaxial Compressive Strength:
- Uniaxial compressive strength is the maximum compressive stress that a material can sustain under uniaxial loading conditions.
- It is an essential property of concrete that helps in design and analysis of structures.

2. Test Specimens:
- Cube Specimens:
- Cubes are widely used as test specimens for determining the compressive strength of concrete.
- The most common cube size used is 150mm x 150mm x 150mm.
- Cube specimens are easy to prepare and test.
- They are suitable for quality control purposes and provide a reasonable estimation of the compressive strength of concrete.

- Cylinder Specimens:
- Cylinders are also commonly used as test specimens for determining the compressive strength of concrete.
- The most common cylinder size used is 150mm diameter and 300mm height.
- Cylinder specimens are usually used for research purposes and validation of design assumptions.
- They provide a closer representation of the in-situ stress distribution in structural members.

3. Stress Distribution:
- Cube Specimens:
- In cube specimens, stress distribution is non-uniform due to the presence of stress concentrations at the corners.
- The corners of the cube experience higher stresses compared to the central portion.
- This non-uniform stress distribution can lead to premature failure and reduced accuracy in determining the compressive strength.

- Cylinder Specimens:
- In cylinder specimens, stress distribution is relatively more uniform compared to cubes.
- The absence of sharp corners reduces stress concentrations.
- The stress distribution is more representative of the actual stress distribution in structural members.

4. Choice of Specimen:
- The choice of test specimen depends on various factors such as the purpose of testing, available resources, and standards followed.
- For routine quality control purposes, cube specimens are more practical and provide satisfactory results.
- For research purposes and validation of design assumptions, cylinder specimens may be preferred due to their closer representation of in-situ stress distribution.

Conclusion:
In conclusion, the assertion that a cube is a better test specimen for determining the uniaxial compressive strength of concrete compared to a cylinder is false. However, the reason provided, i.e., stress distribution being more uniform over the cross section of a cylinder compared to a cube, is true. The choice of test specimen depends on the specific requirements of the testing and the desired