Porous concrete block paving, like its non-permeable counterpart, substantially reduces runoff surface water by allowing it to drain through the base and into the sub-base. A key factor affecting the water absorption rate is the amount of void space within the block, which is determined by its porosity. This in turn is affected by the type of aggregate used, as well as how it's been compacted during manufacture. The level of compaction required will depend on the intended use of the block.