PERVIOUS CONCRETE DESIGN

Two  factors  determine  the  design  thickness  of  pervious  pavements:  the  hydraulic  properties, such as permeability and volume of voids, and the mechanical properties,  such as strength and stiffness. ACI  522R-10  states  that  pervious  concrete  used  in  pavement  systems  must  be  designed  to  support  the  intended  traffic  load  and  contribute  positively  to  the  site- specific  storm-water  management  strategy.  The  designer  selects  the  appropriate  material  properties,  the  appropriate  pavement  thickness,  and  other  characteristics  needed  to  meet  the  hydrological  requirements  and  anticipated  traffic  loads  simultaneously. Separate analyses are required for both the hydraulic and the structural  requirements, and the larger of the two values for pavement thickness will determine  the final  design  thickness.  Numerous  applications  have  used  a  5-  to  6-in.  (12-  to  15-cm)-thick pervious concrete over an aggregate base generally of the same dimension. Field performance of these projects has shown that they are adequate to handle  the  traffic  loads  expected  in  parking  lot  type  applications  (passenger  cars),  where  the  heaviest  loads  are  generally  from  garbage  trucks.  If  heavier  loads  and  higher  traffic are expected, then a thicker pavement (8–12 in. [20 –30 cm]) is used. Another  approach would be to try to use the structural design techniques outlined in the ACI  522R report, which could help optimize the pavement thickness.

Initial  recommendations  had  been  that  pervious  concrete  should  be  used  only  in  sandy  soils  with  an  infiltration  rate  greater  than  0.5  in./h.  However,  a  detailed  hydrologic analysis for a specific example with soils with infiltration rate of 1, 0.5,  0.1, and 0.01 in./h has shown that the postconstruction runoff was lower in all four  soils when compared to the preconstruction runoff. The draw-down time in all cases  was acceptable, except for the soil with the lowest infiltration rate, and that, too, only  when an aggregate base was used. The authors concluded that pervious concrete can  be used in silty soils with a soil infiltration of only 0.1 in./h and that there is no need  to limit its use arbitrarily only to sands. In soils with infiltration rates considerably  less than 0.1 in./h, one way to reduce the draw-down time could be to use buried  perforated pipes that can transfer the collected water elsewhere. If that is not feasible,  the  pervious  concrete  system  could  be  placed  without  an  aggregate  base  and  the  resulting excess runoff over the pervious concrete (but still lower than if an impervious system had been used) could be handled using additional detention devices.