A few California developers and local agencies are already using PICP. Caltrans development of specs and a design guide should support increased PICP use.
The California Department of Transportation (Caltrans) is the first state transportation agency to develop a specification and design guide for permeable interlocking concrete pavement (PICP). Typically, state governments rely on their stormwater departments to develop such information. Stormwater departments reside within agencies that have “environment,” “water” or “conservation” in their titles. Examples include permeable pavement guidelines from the Virginia Department of Conservation and Recreation, North Carolina Department of
Environment and Natural Resources, and the Rhode Island Department of Environmental Management, to name a few.
Caltrans has developed a draft specification for PICP and pervious concrete that will be released in the coming months. This was done with support by the Concrete Masonry Association of California and Nevada (CMACN), the California Nevada Cement Association (CNCA) and the Interlocking Concrete Pavement Institute. All permeable pavement specifications are written in the Caltrans “plain language” style and initially will be “non-standard special provision specs” used for non-highway facilities. This specification status allows for greater flexibility in modifying specs during construction. In addition, this status enables Caltrans to initiate permeable pavement projects and then return to the specifications a year or two later with revisions drawn from construction experience. Based on successful experience and further
technical review within Caltrans, the specifications eventually should be a permanent entry in the Caltrans book, Standard Specifications, periodically updated by the agency.
The draft PICP specification uses the existing structure of Standard Specifications, placing PICP in Division V Surfacing and Pavements, Section 40 Concrete Pavements. PICP is situated in a subsection labeled 40-9 Permeable Interlocking Concrete Pavement. This section covers the concrete pavers, which must conform to ASTM C936, Standard Specification for Solid Concrete Interlocking Paving Units, as well as the permeable jointing and bedding stone. Jointing stone sizes conform to ASTM No. 8, 89 or 9 stone, and selection is based on the maximum joint width between pavers. Bedding stone is consistently No. 8 stone. Other changes to Caltrans specs include Section 26 Aggregate Bases, which introduces a new Class 4 base with gradation, cleanness and durability requirements per Caltrans California Test methods, as well as a minimum 30 percent void space. Changes were also made to Section 19 Earthwork to allow for lower compacted soil densities than that required for conventional pavement.
One requirement of the Caltrans PICP specification is that the job foreman holds a certificate of completion in the Interlocking Concrete Pavement Institute PICP Technician Course. This requirement includes qualified construction personnel on job sites. Also, Caltrans acceptance of the completed PICP includes surface infiltration testing using ASTM C1701, Standard Test Method for Infiltration Rate of In Place Pervious Concrete. The minimum construction acceptance infiltration rate is 100 in./hr (250 cm/hr). This infiltration rate can be consistently attained regardless of the percentage of paver open surface area or the jointing materials.
All permeable pavement projects are subject to review and approval by the Caltrans Office of Stormwater Management-Design, and structural designs are reviewed and approved by Caltrans Pavements Program. The hydrologic design criteria require capturing 85 percent of all storms, also known as the water quality volume (WQV) capture depth. The emphasis is on treatment in managing stormwater across most of California rather than on volume reduction due to low rainfall. This WQV calculation method is well established within Caltrans and by the California Stormwater Quality Association. The most effective means of treatment—by reducing volumes—is accomplished with permeable pavement.
Storms with higher depths can be managed by the permeable pavement if required. The maximum infiltration time is 48 hours and the minimum acceptable soil infiltration rate is 0.01 in./hr (0.25 mm/hr). This minimum rate enables permeable pavements to be used in compacted soils, including some clay soils, resulting in about ½ in. (13 mm) of infiltrated water over 48 hours.
Caltrans structural design is essentially for cars and limited truck use on low-speed areas such as parking lots. The design approach is conservative and does not account for experience in other states with heavier loads/repetitions. The CMACN, CNCA and the ICPI Foundation for Education and Research are preparing to conduct PICP structural testing at the University of California Pavement Research Center in Davis, which will provide design charts to Caltrans for PICP base thicknesses. This study is slated to start before the end of 2012 and has wider implications on structural design for state and local agencies, as well as designers and industry outside California.
As with many states, numerous California city and county agencies reference state highway specifications for pavement materials and construction practices in their development standards. The inclusion of permeable pavements in Caltrans’ provisional specification sends a credible message to city and county engineering departments and further supports permeable pavement application beyond limited use expected by Caltrans.