A basic problem in recycling concrete is its contamination with plaster, masonry and other materials, since many have very different densities and service lifetimes, and they may also affect the aggregate size and shape. Separating materials during deconstruction (and even in the building process) is crucial for materials efficiency. But even the design of concrete mixes (particularly the size of aggregates) can influence the eventual recyclability of concrete. Three principles can facilitate more comprehensive recycling: (1) reduce the variety of constituent materials; (2) avoid composites that are difficult to dissolve in construction, and (3) separate units and materials with different service lives & recycling techniques. Because the composition of concrete mixes can vary considerably, maintaining a careful record of material constituentsas a part of a general passport for buildingscan be invaluable when it comes time for replacing the building (Mller, 1999). These principles and strategies become even more important when we acknowledge that in a green economy, there will be many fewer roads built, and that more and more material loops will be geared to recyclingas opposed to downcycling. I will explore these relationships more thoroughly as they apply to the recycling of all building materials in the next chapter.