 Monolithic structures consist of a single continuous porous medium such as porous concrete or porous pavement (asphalt) while modular structures include porous pavers individual paving blocks that are constructed so that there is a gap in between each paver.Commercial products that are available are for example, pavements made from special asphalt or concrete containing minimal materials, concrete grid pavements, and concrete ceramic or plastic modular pavements.Porous pavements are usually laid on a very porous material (sand or gravel), underlain by a layer of geotextile material.