Used in permeable paving, contains a network of holes or voids, to allow air or water to move through the concrete. This allows water to drain naturally through it, and can both remove the normal surface-water drainage infrastructure, and allow replenishment of groundwater when conventional concrete does not. It is formed by leaving out some or all of the fine aggregate (fines). The remaining large aggregate then is bound by a relatively small amount of Portland cement. When set, typically between 15% and 25% of the concrete volume is voids, allowing water to drain at around 5 gal/ft²/ min (70 L/m²/min) through the concrete.