Concrete does not actually have a melting point, but it decomposes into various components due to the makeup of concrete, which is mostly sand and gravel with Portland cement added. A temperature of thousands of degrees Fahrenheit is needed to decompose concrete.
As heat is applied to concrete, it breaks down. The water in the mix evaporates into water vapor, while the sand and gravel become molten lava. The different stones contained in the gravel melt at different temperatures, as does the sand. Other metallic components remain unchanged. When the mix cools off and reforms, it is no longer concrete.