CONSTRUCTION PERVIOUS CONCRETE

The  success  of  pervious  concrete  pavements  depends  on  the  experience  of  the  installer. As with any concrete pavement, proper subgrade preparation is important.  The subgrade should be properly compacted to provide a uniform and stable surface.

When pervious pavement is placed directly on sandy or gravelly soils, it is recommended to compact the subgrade to 92%–95% of the maximum density (ASTM D  1557). With silty or clayey soils, the level of compaction will depend on the specifics  of the pavement design, and a layer of open graded stone may have to be placed over  the soil. Engineering fabrics are often used to separate  ne-grained soils from the  stone layer. Care must be taken not to overcompact soil with swelling potential. The  subgrade  should  be  moistened  prior  to  concrete  placement  to  prevent  the  pervious  concrete  from  setting  and  drying  too  quickly.  Also,  wheel  ruts  from  construction  traffic should be raked and recompacted.

The pervious concrete is sensitive to changes in water content, so  eld adjustment  of the fresh . mixture is usually necessary. The correct quantity of water in the concrete  is critical. Too much water will cause segregation and too little water will lead to ball- ing in the mixer and very slow mixer unloading. Water content that is too low can also  hinder adequate curing of the concrete and lead to a premature raveling surface failure.  Pervious concrete has little excess water in the mixture. Anytime the fresh material is  allowed to sit exposed to the elements is time that it is losing water needed for curing.  Drying of the cement paste can lead to a raveling failure of the pavement surface.

All  placement  operations  and  equipment  should  be  designed  and  selected  with  this in mind and scheduled for rapid placement and immediate curing of the pavement. A pervious concrete pavement may be placed with either fixed forms or slip- form pavers. The most common approach to placing pervious concrete is in forms on  grade that have a riser strip on the top of each form such that the strike-off device is  3/8–1/2 in. (9–12 mm) above final pavement elevation. Strike-off may be by vibratory or manual screeds. After striking off the concrete, the riser strips are removed  and  the  concrete  compacted  by  a  manually  operated  roller  that  bridges  the  forms.  Rolling consolidates the fresh concrete to provide a strong bond between the paste  and aggregate, creating a smoother riding surface. Excessive pressure when rolling  should be avoided because it may cause the voids to collapse. Rolling should be per- formed immediately after strike-off. Since floating and troweling tend to close up the  top surface of the voids, they are not carried out.

Jointing pervious concrete pavement follows the same rules as for concrete slabs  on grade, with a few exceptions. With significantly less water in the fresh concrete,  shrinkage of the hardened material is reduced significantly; thus, joint spacing may  be wider. The rules of jointing geometry, however, remain the same. Joints in pervious concrete are tooled with a rolling jointing tool. This allows joints to be cut in  a short time and allows curing to continue uninterrupted. Saw-cutting joints also is  possible, but is not preferred because slurry from sawing operations may block some  of the voids, and excessive raveling of the joints often results. Removing covers to  allow sawing can reduce the effectiveness of curing, and it is recommended that the  surfaces be rewet before the covering is replaced. Some pervious concrete pavements  are not jointed because random cracking is not viewed as a significant de cit in the  aesthetics of the pavement (considering its texture) and has no significant effect on  the structural integrity of the pavement.

Proper curing is essential to the structural integrity of a pervious concrete pavement. The open structure and relatively rough surface of pervious concrete expose more  surface area of the cement paste to evaporation, making curing even more essential  than  in  conventional  concreting.  Curing  ensures  sufficient  hydration  of  the  cement  paste to provide the necessary strength in the pavement section to prevent raveling.  Curing should begin within 20 minutes of final consolidation and continue through 7  days. Plastic sheeting is typically used to cure pervious concrete pavements. Realizing  the  importance  of  the  installer,  the  National  Ready  Mixed  Concrete  Association (NRMCA) developed and administers a pervious concrete contractor cerfitication. The goal of the certification program is to ensure that knowledgeable contractors are selected to place the product and thereby minimize the chance for failure.