I have tested and demonstrated that the defects of reinforced concrete floors can be improved in 15 years. Can I apply for a patent?

As long as your technology is not well-known to the society, that is to say, you have done confidential work, and then no one else has applied for a patent similarly, then the possibility of your successful patent application is still very high.

The patent law stipulates that patent applications need novelty, so if it is a brand-new technology or an innovation of existing technology, it is generally possible.