Frequently Asked Questions (FAQ)
Details & Specifications
Pervious Concrete Pavements, and especially the
Dual-Use (Parking in the Pond) systems,
are unique designs. Because
of increased interest in the systems, guidelines and specifications for
their
construction and use
have been developed, and are available on the Specifications page. These Specifications pertain directly to the
Dual-Use system and should be used by Specifiers.
The
design and construction details for the Dual-Use system have been derived
from the NIBS -
Federal Green Construction Guide for Specifiers.
(See the EPA web page on
Porous Pavement.)
This page also contains Sections on: Plans and Specifications, Testing, Criteria for Acceptance, Dual-Use Designs, Pavement Thickness, Pervious Concrete Mix-Designs, Pavement Reinforcement and Pavement as a Staging-Area. (Click your "back-button" to return to this list.)
The Project Specifier should be aware of the properties, and allowable field applications, for pervious concrete systems. Being familiar with the contents of this site will give the Specifier a basic knowledge and understanding of both, pervious concrete, and the dual-use system.
Magruder Construction is turning gray concrete into green parking lots.
Plans and Specifications should be an integral part of every project that involves the construction of pervious concrete pavements. If possible, the Developer, Project Architect & Engineer, Ready-Mix Producer and Concrete Contractor should work together as a team to discuss the design and construction of the project. Because everyone's knowledge of pervious concrete systems, Dual-Use systems and Parking-in-the-Pond systems will not be the same, it is important that all team members be aware of what can, or cannot be done, to create a successful project. It is critical, that at least one person on the team have a thorough knowledge of both the material properties, and the standard construction methods, of pervious concrete pavements. (See the next section below on testing.)
The Industry Standards for testing and Quality Control (QC) of Pervious Concrete are currently being developed by the ASTM, ACI and the NRMCA. However, at this time, no nationally recognized, standardized tests, have been developed by any of these agencies. As a consequence, the Pervious Concrete Industry, in conjunction with each of the above mentioned agencies, have developed recommended guidelines for the design, construction and testing of pervious concrete pavements. (See also the guidelines promoted by the National Institute of Building Sciences (NBIS) in their Federal Green Construction Guide for Specifiers.)
Acceptance Tests for Pervious Concrete, at the jobsite, are generally completed in two phases. The first phase of testing is done for Quality Control and acceptance of the material when it arrives on the jobsite. These tests are generally completed by a qualified representative of the Concrete Contractor. (See the section below on the Criteria for Acceptance.) The second phase of testing is done by the Owner, or Owner's Representative, for acceptance of the finished pavement. The major elements generally required of finished pavements are permeability and surface durability.
Industry Guidelines for Quality Control and Testing of Pervious Concrete are not the same as those used for conventional concrete.
Tests that are mandatory for conventional concrete, such as "slump", air-content, or taking "cylinders" for strength tests, are all procedures that do not have a statistically-verified field relationship with Pervious Concrete pavements. (Concrete Testing procedures are typically supported by recommendations by ASTM or ACI that have been statistically validated.) Neither of these agencies recognize any "Nationally-accepted" testing procedures for Pervious Concrete.
- Slump & Water/Cement Ratio: There is typically only "one" mix-design used for a Pervious Concrete mix at any specific Ready-Mix Plant. It is generally left to the Concrete Contractor to determine from a "visual" inspection of the material, at the jobsite, the correct properties of the mix. Pervious Concrete is a very "stiff", dry material, and essentially has no slump. If there is enough water in the mix to create a measurable slump, the mix is probably already too wet. Note: The visual-testing performed by the Concrete Contractor is generally the most important test of all acceptance criteria. Therefore, the Concrete Contractor must be qualified to make the test, and have the field-based experience to be able to make recommendations on any required field-adjustments to the mix. Only NRMCA Certified Craftsmen have the Industry-recognized, verifiable, knowledge and experience to make the decisions. See the section below on Criteria for Acceptance.
- Test for Air: Because of the "void-space" in Pervious Concrete, an air-content test would give invalid readings, and is therefore not recommended as a test of the properties of the material. Addition of "air" in the mix design may be recommended in areas of freeze-thaw. However, its value has yet to be accepted as a National Standard.
- Strength Tests: Pervious Concrete is compressible, and different strengths can be achieved with the same mix-design depending on the compression used in the place & finish of the pavement. There is also an inverse relationship between the compression of Pervious Concrete and the resulting permeability. Therefore, a balance must be achieved between strength-by-compression, and the resulting permeability. In addition, the design-strength requirements for the durability and design of Pervious Concrete pavements, under parking-lot traffic loadings, are not known. Pavement thicknesses, that would typically be based on material-strengths, are based primarily on performance of installed pavements. The Industry Standard for pavement thicknesses, for automobile and light-truck loadings, in a parking lot, are generally five to six-inches. Truck-delivery lanes are typically eight-inches thick. Because the strength requirements for the design of parking-lot pavement thicknesses are unknown, "cylinders" or "beams" typically created at the jobsite for laboratory strength-tests are not recommended. In addition, there are no standard practices on the procedures for making beams or cylinders using Pervious Concrete.
- Unit Weight: Unit weight tests are often recommended as the only "valid" test for the properties of Pervious Concrete. They are used to determine the void-space within the volume of Pervious Concrete. However, all of the void spaces, within any sample of Pervious Concrete, will not be interconnected. Therefore void-space is not the same as "permeability", or storage-capacity, as would be in an equal volume of gravel. As a consequence, tests to determine the unit-weight, or void-content, of Pervious Concrete are of little value.
The two items that are most meaningful to any completed Pervious Concrete pavement, are the permeability (perk-rate), and the resistance to wear from automobile traffic. Nationally accepted testing-procedures of either of these two items, as it pertains to pervious concrete, have not been adopted as yet.
Criteria for Acceptance: A mix-design (see below) should be submitted, and accepted by the Concrete Contractor and Project Engineer, before the project starts. In many cases, the Concrete Contractor must also have the knowledge and experience to work with suppliers that may not have an equal knowledge of the material. Because not everyone is familiar with the properties of Pervious Concrete, the Concrete Contractor may be placed in the position that they must make suggestions on how to prepare, mix and transport the Pervious Concrete. These issues must be resolved before the first delivery is made. Many Suppliers will not even deliver Pervious Concrete to a Concrete Contractor unless they have verifiable experience with the material.
Note: A vitally important aspect of fresh Pervious Concrete, that is often overlooked, is the paste to aggregate ratio (par). The par is defined as the quantity of paste (still in the plastic/fluid state) that is available in the mix to glue the aggregate together. In many cases, during transit, the aggregates become coated by successive layers of the available portland-cement paste until very little free-paste is left in the mix. The reduction in par can be caused by many factors to include: aggregate types, water content and ambient weather. It is essential for the Concrete Contractor to have the knowledge, and experience with the material, to be able to identify, and correct, any potential problems with the par. In some cases, the addition of a small quantity water to the mix at the jobsite will change the par, but most often this is not the case. Experience has shown that this one factor has caused most of the problems with pervious concrete pavements. This is especially true with inexperienced crews, or concrete suppliers.
It is recognized in the Industry, that only NRMCA Certified "Craftsmen" have the verifiable, industry-recognized, knowledge and experience to make decisions relating to the par of pervious concrete. (See the Contractor's Page to learn how to become NRMCA Certified.)
It may be in the best interest of the Owner/Developer to use a Sole Source Contractor to help in the design, as well as the construction of the project. (See the Sole Source Justification page.)
In many cases a Concrete Contractor may not have the knowledge and experience to meet the "primary qualifications" for construction of a pervious concrete system. (See the Specifications page.) If the selected Contractor does not meet the primary qualifications, it is suggested they retain a NRMCA Certified Craftsman to act as their field-representative. Another option is to have the Concrete Contractor construct a test-section of the system which will be evaluated (by someone that has enough knowledge and experience) as the criteria for acceptance and payment. Once the test section has been accepted by the Owner's Representative, it becomes the "standard" for all the rest of the pavement. No other material testing should be required. It becomes the responsibility of the Concrete Contractor to construct the pavement in accordance with the accepted test section, and the Industry accepted guidelines. The Concrete Contractor must also have the ability to make the correct decisions on field-adjustments to the material if changes are noted from one load to the next.
Important: Because of the lack of standardized test for quality control and acceptance of Pervious Concrete, it is critical that whoever is making the decisions for acceptance of both the fresh concrete, and the finished pavement, that they are qualified to do so. The decision-maker must have adequate knowledge and experience to recognize the essential properties of Pervious Concrete, and know how to make field-adjustments. (Note: Both the NRMCA, and the National Institute of Building Sciences, recognize that only NRMCA Certified "Craftsmen" have the 1500-3000 hours of experience necessary as a basis of qualification.)
Contact Don Wade for additional information on the par of Pervious Concrete, and Industry accepted guidelines and recommendations for the testing of Pervious Concrete.
It is also suggested site permeability tests be taken in several areas where the Dual-Use system will be constructed. These tests should be taken in undisturbed areas before the selected areas are graded and compacted, as per the specifications. Site permeability tests (perk-tests), or drain-field tests, and soil- types testing, should be used by the Project Engineer to design the stormwater BMP to meet the NPDES permitting requirements. (See the specifications of the NIBS - Federal Green Construction Guide.)
The soil within the "foot-print" of the infiltration-basin area, (that is designated as the on-site stormwater storage/treatment area), should have very little compaction. However, most soils of a construction-site are going to receive a lot of compaction due to the construction traffic. In the selected areas, the soil should be brought to grade, and proof-rolled, such that there is a "uniform" compaction. These areas may have to be roto-tilled, re-graded, and re-compacted if they have received too much compaction during construction. Too much compaction will restrict the permeability of the soil. Water-compaction may be the best method of compaction for infiltration systems. Conventional lawn-sprinklers can be used in some areas to achieve the desired soil compaction. In all cases the soil compaction should not exceed 95%.
Dual-Use Designs for infiltration-systems starts from the ground up. Stormwater that passes through the pervious concrete pavement must go somewhere else. Either the water is held on-site for a specific period of time, or it is allowed to perk into the native soil below the pavement. System designs, incorporating pervious concrete, must take into consideration the water that will exist "below" the pavement (not on top of the pavement). It is suggested the system-bottom be designed flat (no slope) to allow for maximum percolation, or the settlement of pollutants. The permeability, or "perk-rate", of the native soils, or imported-fill, is important considerations when calculating discharge of the stormwater. If a site has a low perk rate, then out-flow pipes may be designed into the system to discharge the over-flow, or emergency-discharge, of the stormwater.
Pavement Thickness design is also performance related. Experience has shown that six-inches of pervious concrete will support auto and light-truck traffic. Eight-inches is recommended for heavier truck traffic such as delivery lanes or warehouse parking lots frequently used by large trucks. A six-inch thick, clean-gravel layer, below the pavement will give support to the pavement, and act as the detention layer for the on-site stormwater storage. Additional thicknesses of gravel may be necessary in colder climates, or where additional "storage" volume is required.
Pervious Concrete Mix-Designs are typically dependent on the availability of local materials. Generally, local Ready-Mix plants store a single "brand" of chemical additives, and maybe several sizes of locally available aggregates, along with some form of dry portland cement. These items make up the key ingredients of conventional concrete, and also of pervious concrete. The Concrete Contractor must have the multiple-job experience necessary to recognize the properties of the different mix designs. If the Contractor is not familiar with the selected supplier's mix design, then both parties should work together with "trial-batches" until an agreement has been made on a final mix. The final-mix may need to be adjusted during the project depending on a variety of factors such as local weather, types of aggregate or jobsite conditions. The Concrete Contractor must be able to recognize when adjustments need to be made.
A "general" mix-design for one cubic yard of Pervious Concrete starts with one cubic yard of 3/8-inch gravel (single-grade) - approximately 2,700-pounds. The weight will vary depending on the specific gravity of the local supply. (Specific-Gravity of the aggregate will also affect the unit-weight of the final mix.) No sand (fine aggregate) should be used. Next, add about 600-pounds of type-I, or Type-II, portland cement. (Other types of cement may also be used, including blended cements.) The third key ingredient is water. Use about 18-gallons (150-pounds) per cubic yard. It should be noted that for every new mix design created, trial-batches should be made, and visually inspected to insure the right mix proportions. Properties of local materials, weather, travel distance to jobsite, and admixture quantities can make a lot of difference on how a particular mix will perform.
It is suggested that Concrete Ready-Mix Facilities monitor their aggregate supplies during the time pervious concrete is being manufactured. Hot, dry aggregate, especially during the summer months, can have an adverse affect on the properties of pervious concrete. In transit, the available cement-paste may coat the aggregate, (and inside the mixer-drum), in successive layers, but not leave enough paste to adequately form the paste-bridge between aggregates. This can result in a weak bond between aggregates after placement, and can lead to severe raveling of the finished pavement. (See also the discussion on "par" of fresh pervious concrete on the General Information page.) Note: This one factor has been determined to be the major cause of failure of pervious concrete pavements, especially when inexperienced crews or suppliers are involved. It is extremely important that only qualified contractors are allowed to place and finish pervious concrete pavements, as they must be able to identify, and correct, potential problems of the mix based on their experience. (See the Contractor's Page to see how Contractors can become Certified.)
Grace Chemicals, and Degussa Admixtures, have both developed chemical additives that can be used to enhance the properties of pervious concrete. Contact their local representatives, or check for contact information on the Contacts and References page.
Pavement Reinforcement in pervious concrete pavements is normally not required. Steel reinforcement, or stay-in-place steel keyways, should not be used in pervious concrete pavements. Fibers are recommended in some areas, but should not be approved unless the use of the fibers can be proven beneficial to the appearance, workability or endurance of the pavement.
Magruder Construction is experienced in the design, and installation, of pervious concrete pavements and should be contacted to give assistance to design professionals or specifiers throughout the U.S. Professional Designers should contact Don Wade, Program Manager, and NRMCA Certified Craftsman.
Pavement as a "staging" area for other subcontractors is an element that should be closely observed throughout the entire construction process. It should not be allowed. Construction traffic generally creates a lot of dust and debris that should not be allowed on, or within, the pavement, or the clean-gravel base. It is suggested that the pavement/on-site stormwater detention/treatment area be constructed toward the tail-end of the project. If the pavement is constructed early in the project it should be barricaded and protected against construction traffic and debris, and not be allowed as a staging area.