Pervious Specification – Forward
Since we pioneered efforts to create and publish a Pervious Concrete Specification on the internet in the 1990s, the use of Pervious Concrete Pavement, (also known as “no fines concrete” “zero slump concrete” and “porous concrete pavement”) has continued to gain acceptance. It is now well recognized as a very ecologically friendly alternative to asphalt, impervious concrete, pervious asphalt and porous paving blocks. It is used to treat, filter, control and partially contain storm water runoff. It will continue to demonstrate growth as a sustainable, unique building product for the long term future.
Earliest attempts at creating specifications for pervious concrete were based upon trying to adapt test procedures from impervious concrete pavements. Many of such procedures have proven irrelevant. Because of the void structure and the absence of sand, such tests have been trying to force what should be a square porous sample into a round hole designed to test solid cylinders. Thus actual performance, possible by following good guidelines and specifications have now resulted in thousands of successful, well functioning installations. Sadly there are many others that failed due to poor base preparation, lack of installation skill, poor ready mix preparation, lack of proper understanding of the pervious materials, concrete equipment and correct site preparation procedures. It is hoped that this updated specification will help overcome some of these problems.
Though its largest demand is for parking and paving areas, it can also be used as a sound absorption pavement and as an underlayment to relieve hydrostatic pressure under asphalt or impervious concrete paving. It may also be used for light aircraft taxiways or tarmacs to make use of its ability to prevent puddling and to enhance the grip of rubber tires on pavements. It is also being used as a base for playgrounds where it is covered with shredded rubber tire mulch or other pervious, soft, materials.
It can reduce or eliminate the need for retention and detention ponds, especially when used in conjunction with a sub-base designed with void containing soils and crushed aggregates. A ten-inch sub-base of crushed limestone or recycled concrete can store about three inches of rain water until it permeates through the ground below. This sub-base in combination with soils of variable permeability above the seasonal high water table will determine the water storage capacity of the total drainage-absorption system. Generally, soils that will support septic systems will also support Pervious Concrete.
Engineers and officials who are reluctant to work with Pervious Concrete, have usually developed that reluctance due to, 1. Failures due to installation by untrained installers, 2. Lack of sufficient appreciation or education about the positive environmental impact that Pervious Concrete is making internationally, or, 3. Inertia. (It took a while for computers to be appreciated and not fought!)
An old EPA publication dated September 1999 listed among possible disadvantages for porous pavement, the following:
Many pavement engineers and contractors lack expertise with this technology.
Porous pavement has a tendency to become clogged if improperly installed or maintained.
Porous pavement has a high rate of failure.
Over the last two decades through education and progressive experience Pervious Concrete is now a proven green product that is reducing the environmental impact of outdoor pavements and helping reduce heat islands and support struggling aquifers while reducing storm water pollution!
The proliferation of recently published Pervious Concrete specifications has had a mixed impact on the construction industry. With a few well thought-through exceptions, many are having a negative impact on the acceptance of Pervious Concrete. Why? Because some are too simple or general, failing to address major serious concerns while others are too complicated. Some of these have since been withdrawn!
Not fully understanding the unique properties of Pervious Concrete, many try to simply reference test criterion which may not be designed for its physical or chemical properties. Others fail to convey full understanding of the true qualities of well designed, installed and maintained Pervious Concrete.
Some who publish the newer specifications make the statement: “... moderate amounts of surface raveling are normal.” This is not normal. It is primarily due to lack of experience. This specification outlines how to greatly reduce raveling and prevent further degradation from turning these jobs into examples of the “so called” failure of Pervious Concrete. We have seen numerous jobs that start with that premise, being either covered with asphalt or torn out and replaced with some other product.
To elaborate: traditional Portland cement pavement testing procedures that are based upon strength, air content and slump control are not applicable to this type of pavement material. Whatever test procedures are to be used, or not used, including, but not limited to those specified herein, must be agreed to by the owner, engineer/architect, material suppliers and anyone else with a vested interest, before work commences. This specification is based upon Charger Enterprises' more than 30 years experience with installing pervious concrete.
Slab thickness is best tested, first, by verification of the depth of forms, and then may be verified by random thickness measurements taken during or after the placement process. If core samples are to be taken after the pour, we recommend using 1' square samples for compression testing. The testing organization will be responsible for repairing the sample cavities with Pervious Concrete under Charger supervision.
As continued testing of this product yields additional test methods that are relevant, cost effective and reproducible in the field, these specifications will continue to be modified. The entire layered pervious concrete system is critically dependant upon the quality of the pervious concrete surface layer. If the mix is designed properly and then it is placed expeditiously and correctly, the system will perform exceptionally well in retaining and filtering storm water. However, many contractors familiar with other materials may not be skilled or sufficiently educated to properly apply this product in compliance with its unique application requirements. Furthermore they may not have the correct tools and equipment required by this material to work within the placement time constraints and its finishing and curing specifications. Steel forms and power screeds are a necessity in Pervious Concrete Placement. Proper weight steel rollers must be used immediately behind screeding.
The system is also critically dependent on what is beneath it. Important factors include the preparation of the sub-grade, the percolation ability of the base soils and the fluctuating level of the water table below. Also important is whether the water below is constricted or flowing. In almost all cases the flow of storm water through the pervious concrete will closely equal the ability of the system below to receive it.
This specification is provided on the basis of our many years of pervious concrete work experience. It reflects the state of the art of pervious concrete design and placement as seen through the eyes of a successful, pioneer, Pervious Contractor, Charger Enterprises, Inc. It is not intended to be applied to conditions unforeseen at the time of contractual agreement, and it may be altered with notice if significant changes in material sources, new equipment, or new techniques become available. Responsibility for its application and its wording is limited to contractual agreements between Charger Enterprises, Inc. and its customers and suppliers.