Yea 15% was the level most the plants I went to were comfortable using and it was certainly not used to improve roads but more so maintain the standards back then by using RAP. By using RAP plants would effectively lower the amount of new aggregate being used thus saving themselves money on grinding. And it was also a requirement.

I am curious to know how even higher levels are achievable and engineered, it was certainly an interesting job but I'm glad my engineering degree landed me in a more cushy and less smelly and lung destroying environment lol