Pervious Pavement Design
Structural Design Considerations
This section provides guidelines for the structural design of pervious concrete pavements. Procedures described provide a rational basis for analysis of known data and offer methods to determine the structural thickness of pervious concrete pavements.
Pervious concrete is a unique material that has a matrix and behavior characteristics unlike conventional portland cement concrete or other pavement materials. Although these characteristics differ from conventional concretes, they are predictable and measurable. Projects with good to excellent performance over service lives of 20 to 30 years provide a great deal of empirical evidence related to material properties, acceptable subgrades, and construction procedures. Laboratory research in these areas has only recently begun.
Pavement Structural Design
Pervious concrete pavements can be designed using either a standard pavement procedure (AASHTO, WinPAS, PCAPAV, ACI 325.9R, or ACI 330R) or using structural numbers derived from a flexible pavement design procedure. Regardless of the procedure used, guidelines for roadbed (subgrade) soil properties, pervious concrete materials characteristics, and traffic loads should be considered.
Sub-base and Subgrade Soils
The design of a pervious pavement base should normally provide a 6-to-12-inch (150-300 mm) layer of permeable sub-base. Unless precautions are taken as described in “Clays and Highly Expansive Soils” (see below), the permeable sub-base can be either 1-inch (25 mm) maximum-size aggregate, or a natural subgrade soil that is predominantly sandy with moderate amounts of silt, clay, and poorly-graded soil. Either type of material offers good support values as defined in terms of the Westergaard modulus of subgrade reaction (k). It is suggested that k not exceed 200 lb/in.³ (54 MPa/m), and values of 150-175 lb/in.³ (40-48 MPa/m) are generally suitable for design purposes. Table 4 (below) lists soil characteristics and their approximate k values.
The composite modulus of subgrade reaction is defined using a theoretical relationship between k values from plate-bearing tests (ASTM D 1196 & AASHTO T 222), or estimated from the elastic modulus of subgrade soil (MR, AASHTO T 292), as:
(Eq. 1) k (pci) = MR/19.4, (MR in units of psi), or
(Eq. 1a) k (MPa/m) = 2.03 MR, (MR in units of MPa)
where MR is the roadbed soil resilient modulus (psi). Depending on local practices, the California Bearing Ratio (CBR), R-Value and other tests may be used to determine the support provided by the subgrade. Empirical correlations between k and other tests, CBR (ASTM D 1883 and AASHTO T 193), or R-Value test (ASTM D 2844 and AASHTO T 190) are shown in Table 4.
Determining the subgrade’s in-situ modulus in its intended saturated service condition can increase the design reliability. If the subgrade is not saturated when the in-situ test is performed, laboratory tests can develop a saturation correction factor. Two samples (one in the “as field test moisture condition” and another in a saturated condition) are subjected to a short-term, 10 psi consolidation test. The saturated modulus of subgrade reaction is the ratio of the “field test moisture” to the saturated sample multiplied by the original in-situ modulus.
Clays and Highly Expansive Soils
Special design provisions should be considered in the design of pervious concrete pavement for areas with roadbed soils containing significant amounts of clay, silts of high compressibility, muck, and expansive soils. It is recommended that highly organic materials be excavated and replaced with soils containing high amounts of coarser fill material. Also, the design may include filter reservoirs of sand, open-graded stone, and gravel to provide adequate containment and increase the support values. Another design alternative is a sand subbase material placed over a pavement drainage fabric to contain fine particles. In lieu of the sandy soil, a pervious pavement of larger open-graded coarse aggregate (1½ in. or 38 mm) may provide a sub-base for a surface course of a pervious mixture containing ⅜-in. (9.5-mm) aggregate. Figure 9 shows several options as examples.
The anticipated traffic carried by the pervious pavement can be characterized as equivalent 18,000-lb single-axle loads (ESALs), average daily traffic (ADT), or average daily truck traffic (ADTT). Since truck traffic impacts pavements to a greater extent than cars, the estimate of trucks using the pervious pavement is critical to designing a long-life pavement.
Other Design Factors
Depending on the pavement design program used, design factors other than traffic and concrete strength may be incorporated. For example, if the AASHTO design procedure is used, items such as terminal serviceability, load transfer at joints, and edge support are important considerations. The terminal serviceability factor for pervious concrete is consistent with conventional paving. At joints, designers should take credit for load transfer by aggregate interlock. If curbs, sidewalks, and concrete aprons are used at the pavement edges, using the factors for pavement having edge support is recommended.
Pervious concrete should be jointed unless cracking is acceptable. Since the pervious concrete has a minimal amount of water, the cracking potential is decreased and owners generally do not object to the surface cracks.
Materials Properties Related to Pavement Design
The flexural strength of concrete in a rigid pavement is very important to its design. Rigid pavement design is based on the strength of the pavement, which distributes loads uniformly to the subgrade. However, testing to determine the flexural strength of pervious concrete may be subject to high variability; therefore, it is common to measure compressive strengths and to use an empirical relationship to estimate flexural strengths for use in design. Since the strength determines the performance level of the pavement as well as its service life, the properties of the pervious concrete should be evaluated carefully.
A mix design for a pervious pavement application will yield a wide range of strengths and permeability values, depending on the degree of compaction. Pre-construction testing should determine the relationship between compressive or splitting tensile and flexural strength, as well as the unit weight and/or voids content for the materials proposed for use. The strength so determined can be used in standard pavement design programs such as AASHTO, WinPAS, PCAPAV, ACI 325.9R, or ACI 330R, to name a few.