Fibers are added to asphalt to prevent drain-down, add strength and stability, or both. Drain-down occurs when liquid binder becomes separated from the stone during either production, storage, or transport. This is more problematic in SMA and open graded mixes due to the higher liquid asphalt amounts. Some fibers are effective at alleviating drain down due to their high surface area. Fibers such as Polyester fibers are added to some mixes to provide strength and stability and acheive a high performing mix.