One possibility is that the Dressel 20 amphora, the principal type found at Monte Testaccio, may have been unusually difficult to recycle. Many types of amphora could be re-used to carry the same type of product or modified to serve a different purpose—for instance, as drain pipes or flower pots. Fragmentary amphorae could be pounded into chips to use in "opus signinum", a type of concrete widely used as a building material, or could simply be used as landfill. The Dressel 20 amphora, however, broke into large curved fragments that could not readily be reduced to small shards. It is likely that the difficulty of reusing or repurposing the Dressel 20s meant that it was more economical to discard them.