When trails turn muddy this winter, the Appalachian Mountain Bike Club will reach for a new kind of gravel — recycled concrete. "We would normally use a crushed aggregate anyway, and instead of quarrying it out of the earth, we think it’s a much better idea to use something that’s already around," said Matthew Kellogg, president of the AMBC, which maintains public trails in Knox County. "We can essentially get a similar product and have it be recycled." That product comes from Harrison Construction, which, for three years, has been collecting discarded concrete and asphalt from razed buildings and their own road projects and recycling it.