The correct answer is A. 5%.
When hand mixing concrete, it is important to add a little extra cement to the mix. This is because hand mixing is not as effective as machine mixing, and the extra cement helps to ensure that the concrete is strong and durable.
The amount of extra cement that needs to be added will vary depending on the type of concrete being mixed, the size of the batch, and the experience of the person mixing the concrete. However, a good rule of thumb is to add 5% extra cement for hand mixing.
It is important to note that adding too much extra cement can make the concrete difficult to work with. It is also important to make sure that the extra cement is evenly distributed throughout the mix.
Here is a brief explanation of each option:
- Option A: 5%. This is the recommended amount of extra cement to add when hand mixing concrete.
- Option B: 10%. This is a higher amount of extra cement that may be necessary for some types of concrete or for large batches.
- Option C: 15%. This is a very high amount of extra cement that should only be used in special circumstances.
- Option D: 20%. This is an excessive amount of extra cement that should never be used.