California is in a desperate situation. Overpopulation, severe drought, widespread inefficient use of available water, and brazen groundwater pumping have regulators and engineering and environmental professionals grasping for sustainable water sources.
Up until this century, stormwater infrastructure was designed to concentrate and release runoff as quickly as possible. Today, due to state-mandated water use reductions, record groundwater and precipitation lows, as well as state and local regulations, the need for new water sources has been a high priority. As a result, development and redevelopment engineering has shifted to implement droughttolerant or low-water-need plantings, and stormwater capture for onsite reuse or groundwater aquifer replenishment.
Various state and local regulations have surfaced during the last two decades that establish the basic structure for regulating discharges into waters of the United States and associated quality standards for those surface waters. The U.S. Environmental Protection Agency (EPA) and California State Water Resources Control Board develop total maximum daily loads (TMDLs) based on identified impairments, which establish metrics for the capacity of receiving water to absorb a pollutant. These TMDLs become enforceable with civil and/or criminal penalties once incorporated into municipal separate storm sewer system (MS4) permits, which are renewed approximately every five years.
The MS4 permits include pollution control requirements for construction-related activities, illicit discharges and connections, and business and commercial stormwater inspections. Of particular interest to engineers and designers are structural stormwater quality control measures required by the TMDLs and MS4 permits. Most MS4 permits in California require onsite retention of at least the volume resulting from a 0.75-inch rain event. This generally equates to about 85 percent of all rain events in Southern California. Therefore, only storms that produce greater than 0.75 inches will result in site discharge.
A key component of this regulation is that no runoff will leave the site from the first part of any storm, often referred to as the “first flush.” The first flush contains higher concentrations of pollutants than later in a storm due to the initial wash-off of pollutants.
Pervious (or porous) concrete has been around for a few decades but is not a material many are familiar with or comfortable designing or constructing. When permeable surfaces are desired, most landscapers or designers lean toward pervious pavers, which allow runoff to flow through small gravel-filled areas between each paver. They can be effective but are not as efficient as pervious concrete, since the entire surface of pervious concrete is permeable as opposed to only the small area between adjacent pavers.
In its simplest form, concrete incorporates cement, gravel or crushed stone (coarse aggregate), sand (fine aggregate), and water. Pervious concrete incorporates the same materials but excludes all sand in order to produce voids between the coarse aggregates. These voids encourage stormwater flow through the material to the soils below.
Pervious concrete should have a void ratio of 18 to 25 percent and a water-to-cement ratio of less than 0.3. Standard concrete has zero voids and typically has a water-to-cement ratio between 0.4 and 0.8.
Standard concrete acts like a liquid, filling in voids, while pervious concrete is a zero-slump material (keeping its form to maintain voids). Without sand, pervious concrete is naturally weaker than standard concrete. But with close attention to density and placement, pervious concrete can be incorporated effectively within most vehicle-traveled areas.
The Ventura County Government Center Parking Lot Green Streets Urban Retrofit and the El Rio Retrofit for Groundwater Recharge are two Ventura County Public Works Agency (VCPWA) projects that incorporate unique designs utilizing pervious concrete.
Government Center Project
The Ventura County Government Center, built in 1978, has 39 acres of impervious parking and roadway surfaces. The storm drain network consists of 40 catch basins throughout 11 parking lots and a campus perimeter access road with five different locations where 18- inch or wider storm drain pipes leave the property.
Site-specific infiltration testing completed prior to design revealed high clay content and poorly draining soils to depths of about 13 feet below ground surface. Below the clay cap, soils infiltration potential improved dramatically (2 to 32 inches/hour) and seasonal high groundwater was analyzed to be almost 28 feet below ground surface. The final design section is a combination of three Best Management Practices (BMPs):
- Eight-inch-thick pervious concrete gutters (18 inches wide) overtop 3.5-foot-deep aggregate-filled infiltration trenches (for storage purposes, although minor infiltration was anticipated through poorly draining shallow soils), supplemented with aggregate-filled 15-footdeep, 1-foot-diameter dry wells spaced about 10 feet on center along the entire length of the pervious gutter improvements (see Figure 1).
- Pervious concrete strips up to 15 feet wide were constructed around sump condition catch basins to ensure no bypass of flows, except in large events. The improvements were sized to store the volume of the first flush; any additional volume reduction from infiltration was a bonus.
- Trenching depths were kept to around 4 feet to ensure that Occupational Safety & Health Administration (OSHA) regulations and costs associated with shoring were not required. By minimizing the width of the pervious concrete and storing water vertically in the larger void ratio aggregates of the infiltration trenches and dry wells, pervious concrete material costs were lower, traffic control costs were minimized since traffic could be maintained adjacent to work areas, and there was less pervious concrete to clean at the surface.
In total, about 22,000 square feet of pervious concrete improvements were constructed for $1.2 million. During the 2013/2014 wet season, about 60 percent of all rainfall that hit the site was infiltrated. Pre- versus post-construction flow monitoring showed an average infiltrated volume of more than 200,000 gallons per event with a maximum of 700,000 gallons for a rainfall event extended over a three day period. This project was partially funded by California’s Proposition 84 Storm Water Grant Program and recognized by several professional associations for its innovative concept.
El Rio Panel Project
El Rio is a residential community within unincorporated Ventura County with old, nonexistent, or undersized storm drain infrastructure. The community has a history of local flooding even after small rain events due to infrastructure inadequacies, very large drainage areas, and exceptionally flat slopes. Building on its success with pervious concrete at the Government Center, VCPWA decided to implement a pervious gutter system in El Rio. VCPWA engineers came across Stormcrete precast pervious concrete panels by North Yarmouth, Maine-based Porous Technologies (http://storm-crete.com). The precast pervious concrete panels had been successfully implemented in New York, Maine, and other East Coast states: The main appeal of the panels is that pervious concrete is a “touchy” material with very little room for error, and the allure of pouring, rolling, curing, and testing the materials in a controlled environment could avoid headaches related to quality control and material consistency.
Additionally, the panels are manufactured offsite and brought onsite when needed, minimizing time of construction, traffic delays, or access to driveways. Lastly, the panels have embedded threaded lifting points that allow for anchor bolts to be connected to chains for easy installation, removal, and replacement.
VCPWA worked on design utilizing the panels as Porous Technologies searched for a facility in Southern California for manufacturing the panels. A partnership with Precon Products was established and VCPWA worked with them for about six months on various iterations of design mixes and testing to get panel density and infiltration rates into desired ranges.
Construction was completed in approximately half the time of the Government Center project as there were no materials rejected onsite, placement of the precast panels was faster than the pouredin- place option, and the pervious concrete panels could be driven on immediately, without the minimum seven days of curing prior to vehicle traffic needed for poured-in-place pervious concrete.
The El Rio project involved about one mile of pervious concrete gutters (2.5-foot-wide by 5-foot-long by 5-inch-thick panels) for 14,500 square feet of precast pervious concrete panels at a cost of $900,000. The project infiltrates stormwater runoff from 46 acres of the urban residential neighborhood with anticipated annual infiltration volumes of 49 acre-feet.
To our knowledge, this is the first application of precast pervious concrete panels in California. Like the Government Center project, El Rio was funded by Proposition 84 Integrated Regional Water Management Drought Relief Grant Program.
Pervious concrete testing
Material acceptance testing of standard concrete includes slump testing at the time of pour and breaking of collected concrete cylinders or cores throughout curing to assess compressive strengths. The primary objective of pervious concrete is not compressive strength but to maintain desired void ratios.
As a result, different acceptance methods are employed that allow the engineer to verify desired wet density (ASTM 1688) at the time of pour (+/- 5 pounds/cubic foot of design mix) and minimum infiltration rates (ASTM 1701) after curing (no less than 200 inches/hour but prefer more than 400 inches/hour), and determine that materials were not overworked during placement, causing surface and void sealing. Excellent design references for pervious concrete are the latest editions of American Concrete Institute (ACI; http://www.concrete.org) publications ACI 522R and ACI 522.1. Additionally, the National Ready Mixed Concrete Association (http://www.nrmca.org) provides pervious concrete design and specification-focused webinars.
The main concern most engineers raise is whether pervious concrete clogs, and the answer is, yes it clogs. However, just like with any other type of post-construction BMP to mitigate water quality issues, maintenance is expected, and should be part of planning, design, and budget. Maintenance of pervious concrete includes removing vegetative debris, sediment, or trash through frequent (biweekly is recommended) dry vacuuming with a regenerative air sweeper or manually using a blower and broom for clean-up of fines. Street sweeper brushes should not be used as they degrade the surface and can add to clogging.
At least twice a year the pervious concrete surface needs to be pressure washed and vacuumed simultaneously to loosen and remove more packed in fines and debris. The most effective pressure washing and vacuuming equipment VCPWA has tested is the B.I.R.D. vacuum head manufactured by Bunyan Industries (http://www.bunyanusa.com). It is an attachment to a sewer vacuum truck that effectively revitalizes heavily clogged pervious concrete surfaces.
Annual pervious concrete maintenance costs described above are about $2 per square foot. Lastly, periodic infiltration testing (ASTM 1701) of the pervious concrete is recommended to calculate estimated clogging rates over time, which will help guide maintenance practices and frequency.
DAVID KIRBY, P.E., QSD/P, GISP, is water quality engineer for the Ventura County Watershed Protection District where he is the lead engineer on design, construction, and implementation of water quality and low-impact development projects.