As stated in the previous sections, permeable pavement is a complete system designed using various individual materials including paving units, aggregates, and geotextiles. Each of these materials have specific technical properties that designers should be aware of and incorporate into a design and specification document to ensure optimal, long lasting designs are achieved.
The figure below shows a typical cross section with a breakup of materials.
The aim of Part 3: Specification of Materials Used in Permeable Pavements is to explain in greater detail the technical properties of these materials, and which relevant standards and industry documents designers should refer to.
The paving units are the surfacing element of the permeable paving system. The surface needs to remain durable throughout the pavements design life and allow the constant infiltration of water into the substructure. Depending on the application, the technical properties of the paving unit will differ due to contrasting loading conditions. The technical properties of the paving units for both pedestrian and trafficked applications taken from AS/NZS 4455.2:2010 - Masonry units, pavers, flags and segmental retaining wall units, are shown below:
Breaking load: The maximum force the paving unit can withstand when subjected to a load applied through the centre of the top face.
1. Pedestrian: >3kN as per AS/NZS 4456.5:2003 - Determining the breaking load of segmental pavers and flags
2. Trafficked: >6kN as per AS/NZS 4456.5:2003
- Abrasion Resistance: The surface mass loss of each paving unit when subjected to the constant impact of 600 steel balls over the duration of 1 hour. A reduction in the abrasion resistance value reflects a paving unit that is more resistant to effects of abrasion and wearing.
1. Both Pedestrian and Trafficked: <7 as per AS/NZS 4456.9:2003 - Determining abrasion resistance
- Slip Resistance: The paving unit should provide an adequate amount of frictional resistance to enable a person to traverse the surface without an unreasonable risk of slipping.
1. Both Pedestrian and Trafficked: >P4 as per AS4586:2013 - Slip Resistance classification of new pedestrian surface materials
Durability: The paving units should provide adequate durability to perform their required function when exposed to external elements throughout the design life of the pavement. Depending on the region this may include exposure to saline coastal environments, exposure to de-icing salts in heavy snow environments, or exposure to various chemicals in industrial applications.
1. Both Pedestrian and Trafficked: "Exposure Grade" as per AS/NZS 4456.10:2003 - Determining resistance to salt attack
Surface Infiltration Rates: The surface infiltration rates will vary depending on the type of paving units, joint width and opening. Immediately after installation the paving units can have a surface infiltration rate of up to 2.5 litres/sec/m2, however designers typically adopt a 6-10 year surface infiltration rate which takes into account fine material clogging the voids. This value can be as little as 10% of the initial value but can still satisfy the rainfall experienced during intense storm activities.
More details can be found in AS/NZS 4455.2:2010 and the Concrete Masonry Association of Australia (CMAA) PA03 Concrete Segmental Pavements - Specifying Guide.
The substructure of permeable pavement is not only the main structural element of the pavement system, it is used to store water within its voids for harvesting, re-use or discharging. When selecting a base material, the designer needs to consider cost, availability, structural adequacy (when dry and saturated) and water storage ability. The importance of each will differ in various permeable pavements depending on the application. Trafficked permeable pavements will need to take careful consideration of the structural performance of the substructure whilst pedestrian pavements will not. Crushed rock roadbases, cement and asphalt bound roadbases, and no fines concrete have all been used successfully as substructure materials. Below are the main technical properties of the substructure designers should consider.
Grading: The grading of the substructure in a permeable pavement is important. A densely graded material will perform better structurally however will provide less voids to store the water. Alternately, a single sized material will have a greater water storage capacity but will perform poorly under load. Below are some various grading profiles for different unbound materials.
Modulus: The vertical modulus is used to determine the structural adequacy of the substructure when performing a mechanistic design. Typical modulus values of various roadbase materials can be found in Austroads Guide to Pavement Technology Part 2: Pavement Structural Design (2017).
However, research has shown that the modulus values of the substructure in a saturated condition can be less than half of the value when in the dry condition. The designer should adopt an appropriate modulus value depending on the anticipated long-term moisture condition of the substructure. Assuming saturated condition 100% of the time would be a conservative approach.
Void Ratio: The void ratio of the substructure is the most important technical property when performing a hydraulic design. This is the collective volume of voids available within the substructure available to store water. The void ratio of materials will vary depending on grading, particle size and particle shape. Tests can be performed on a sample to determine the void ratio however in the absence of test data, the void ratio can be determined by the relationship between bulk density and particle density.
Void Ratio = 1 - (Bulk Density / Particle Density)
Typical void ratios of materials:
Single Sized (Uniform) Granular Base = 40%
Graded Permeable Road Base = 15%
Cement Bound Permeable Base = 20%
No Fines Concrete Base = 15%
Long term partial clogging of voids may occur by fine material entering the substructure, therefore designers should adopt a lower void ratio to compensate.
The subgrade is the natural material on which pavements are constructed upon. The subgrade may be left exposed to the substructure to allow water to pass through it into the natural water table (Full Infiltration) or it may be tanked and offer no exchange of water between the substructure and water table (No Infiltration). The technical properties of the subgrade are important to perform both a structural and hydraulic design of a permeable pavement. Below are the technical properties of the subgrade the designer needs to consider:
- Californian Bearing Ration (CBR): The CBR value represents the strength of the subgrade. This value is used to perform the structural design of a trafficked pavement. In the absence of data, the CBR can be used to estimate the Hydraulic Conductivity of the subgrade to perform an infiltration hydraulic design. As the CBR increases, so does the ability for water to permeate through it.
- Hydraulic Conductivity: The subgrade hydraulic conductivity represents the ease with which water can move through the pore spaces. As mentioned above, inorganic silts and clay with low CBR values will also have low hydraulic conductivity, meaning they will not easily allow the movement of water through the subgrade. Alternatively, sands and gravel subgrades will have a higher CBR value and as a result, a higher hydraulic conductivity value. Typical hydraulic conductivity values are below.
Inorganic silt and clay: 1 x 10-7 to 1 x 10-11 m/s
Sands and Gravels: 1 x 10-3 to 1 x 10-6 m/s
The quicker the permeable paving system can remove water from the substructure through the subgrade, the less water is required to be externally discharged. In a tanked (No Infiltration) system, there is no movement of water between the substructure and subgrade so hydraulic conductivity value is assumed to be 0.
The correct use of geotextiles within a permeable pavement is important. Various geotextiles are used including filter fabrics, impermeable liners, and high tensile subgrade stabilisation geogrids. The technical properties the geotextiles vary and are listed below.
- Filter Fabric:A filter fabric is inserted into the permeable paving system between the bedding course and the substructure to minimise fine material entering the substructre and clogging the voids. The designer should ensure the infiltration rate of the filter fabric is greater than the expected infiltration rate of the surface.
- Impermeable Liner: An impermeable liner is used in a tanked system to prevent the movement of water between the substructure and subgrade. The designer should ensure the liner is resistant to puncturing from angular subgrade or substructure particles during installation, compaction, and throughout the intended design life. Typical tear and tensile strength values are below:
Tear Strength: >200N
Tensile Strength: >1kN
- High Tensile Subgrade Stabilisation: The structural performance of the permeable pavement can be improved by introducing a high tensile grid at the subgrade, or within the substructure to mechanically stabilise the layer.
A greater understanding of the technical properties of all materials allows designers to select the most appropriate materials to create an effective, functional, long lasting, and durable permeable pavement system.
This Permeable Paving Guide will explore the common aspects of permeable pavements by covering the following topics:
Part 3: Material Specifications
Part 4: Applications
Part 5: Design
Part 6: Maintenance
Stay tuned for Part 4: Applications to be published. This article will showcase the benefits of permeable paving by outlining the problem, exploring the various design options, and how permeable paving was applied to provide the ideal solution.
Reference: Concrete Masonry Association of Australia (CMAA) PE01 Concrete Permeable Interlocking Concrete Pavements - Design and Construction Guide