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Designing Plaza Hardscapes: Considerations from insulation and waterproofing to structural support

All images courtesy Raths, Raths & Johnson

All images courtesy Raths, Raths & Johnson

By Kurt R. Hoigard, PE, SECB, FASTM, and Brian T. Lammert, SE, PE, CDT

Outdoor plazas provide open spaces that break up the massing of neighboring buildings and provide a respite from busy schedules. In urban environments, they are frequently constructed over underlying occupied spaces used for parking, storage, conference rooms, and classrooms. Landscape treatments typically include planters, trees, and paving of various types for pedestrian and/or vehicular traffic. The resulting construction is a complex sandwich of materials ranging from the landscape and hardscape components visible at the surface to the structural elements keeping the plaza from falling into the occupied space below.

The materials and components used in plaza construction over occupied space are needed to provide thermal resistance, water management, structural capacity, a durable wearing surface, and an attractive finished appearance. Designers select product characteristics to fulfill these functions, typically relying on technical data and recommendations from product manufacturers.

An example of split-slab construction including components to distribute surface loads, provide thermal insulation, manage water, and protect interior space from water ingress.

An example of split-slab construction including components to distribute surface loads, provide thermal insulation, manage water, and protect interior space from water ingress.

The authors have found the available published manufacturer technical data is inadequate for designing plazas to which heavy loads will be applied. When not properly accounted for in the plaza design, the interaction of the various materials and components can result in unplanned movement and damage to the exposed hardscape materials. Plaza damage investigated by the authors attributed to hardscape/substrate stiffness interaction include concrete topping slab cracking, paver cracking, paver edge raveling, and paver joint deterioration.

Supporting actors in compression
Figure 1 depicts some of the products frequently used in plaza construction. These include:

  • extruded polystyrene (XPS) foam board insulation for thermal resistance or adjusting finished elevations;
  • waterproofing membrane and protection course applied to an underlying concrete structural slab to keep water out of the occupied space below;
  • composite drainage and air layers providing flow channels for water drainage; and
  • hardscape paving materials such as concrete topping slabs and unitized pavers of brick, concrete, or stone.

There is a common misconception that plaza hardscape materials, whether monolithic (e.g. cast-in-place concrete) or modular (e.g. concrete, stone, or brick pavers), simply ‘sit’ on the underlying materials and do not ‘move.’ However, in reality, most supporting materials can compress somewhat. Loads applied at the hardscape surface are transmitted into the supporting materials, creating compressive stresses that in turn cause the material to locally shorten like a spring.

Anyone who has ever stood on a bed has encountered this phenomenon—feet sink down as the mattress springs compress, as shown in Figure 2. The heavier the person, the deeper his or her feet sink into the mattress since the springs compress more. In plaza hardscape construction, compressible materials that can behave like the springs in the mattress analogy include the XPS, waterproofing membrane, drainage mat, and protection course. Each of these materials can be defined by, among other things, a unique ‘springiness,’ more technically known as compressive stiffness.

The downward displacement of a mattress resulting from a person’s weight is related to the mattress springiness—more technically known as compressive stiffness. This weight is distributed to only a few springs because the mattress does not include a stiff surface element to laterally distribute the weight.

The downward displacement of a mattress resulting from a person’s weight is related to the mattress springiness—more technically known as compressive stiffness. This weight is distributed to only a few springs because the mattress does not include a stiff surface element to laterally distribute the weight.

The addition of a 6.4-mm (¼-in.) thick plywood sheet on the top surface of a mattress laterally distributes the person’s weight. The plywood sheet and mattress are analogous to a plaza hardscape material such as a concrete topping slab and the underlying support.

The addition of a 6.4-mm (¼-in.) thick plywood sheet on the top surface of a mattress laterally distributes the person’s weight. The plywood sheet and mattress are analogous to a plaza hardscape material such as a concrete topping slab and the underlying support.

 

 

 

 

 

 

 

 

 

 

Returning to the ‘standing on the bed’ analogy, the surface of the mattress that sinks down the most is directly under the feet, since those springs directly beneath are the ones carrying the person’s weight. Most of the mattress surface was unchanged, though, because the fabric enclosing the top of the mattress is flexible and cannot help spread the weight out onto other adjacent springs.

If this experiment was modified by first placing a sheet of 6.4-mm (1/4-in.) plywood on the bed, and then the subject stood on the plywood, the behavior would be changed. The mattress springs directly under the person’s feet would still be compressed the most, but some of the adjacent springs would also be compressed and the plywood would bend as it spread the load to the adjacent springs (Figure 3).

Depending on the weight of the person standing on the bed, the plywood might or might not crack due to the induced bending stresses. Cracking would depend on whether the bending stress exceeded the plywood’s flexural strength. In this case, the plywood is analogous to plaza hardscape materials such as concrete topping slabs and mortar setting beds for unitized pavers. The ability of a material to spread load out to supporting materials not directly under the load application point is related to flexural stiffness, which is, in turn, related to the material configuration, including thickness and the position, type, and size of any reinforcement present.

Multiple materials and manufacturers
The various components used below the hardscape surface of a plaza are in many cases manufactured by multiple entities, each with published product-specific properties. For example, XPS insulation types are described in ASTM C578, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation, which includes product requirements such as minimum compressive strength and density.

Graph showing results of compression testing performed on a dimple-type drainage composite, 50-mm (2-in.) thick extruded polystyrene (XPS) insulation, and assembly incorporating both components. The slope of each graph line is equal to the compressive stiffness. The assembly test (red line) compressive stiffness is significantly less than the dimple-type drainage composite (blue line) or XPS insulation (green line) individual component compressive stiffness.

Graph showing results of compression testing performed on a dimple-type drainage composite, 50-mm (2-in.) thick extruded polystyrene (XPS) insulation, and assembly incorporating both components. The slope of each graph line is equal to the compressive stiffness. The assembly test (red line) compressive stiffness is significantly less than the dimple-type drainage composite (blue line) or XPS insulation (green line) individual component compressive stiffness.

XPS insulation used in plaza construction includes, but is not limited to, Types VI, VII, and V, with minimum compressive strengths of 275, 415, and 690 kPa (40, 60, and 100 psi), respectively. The compressive strength is based on material uniformly loaded according to the procedure in ASTM D1621, Standard Test Method for Compressive Properties of Rigid Cellular Plastics. Similar to the XPS insulation, drainage composites will have a compressive strength value included on a manufacturer data sheet based upon uniform load testing according to the procedure in ASTM D1621. These compressive strength values represent individual material properties.

Compressive stiffness values for XPS insulation are also typically available through the manufacturer and, similar to the compressive strength, are based on uniform loading. Stiffness values are typically larger for higher compressive strength material (i.e. Type V XPS is stiffer than Type VI XPS) and decrease as the material thickness is increased (i.e. 25-mm [1-in.] thick XPS is stiffer than 50-mm [2-in.] thick material). Limited data is available on the stiffness of the other components previously described, including drainage/air composites, waterproofing membranes, and protection courses.

Designing for durability
The success or failure of a plaza hardscape installation hinges on many things, one of which is the durability of the finish materials. Cracking, spalling, and joint deterioration mar the plaza appearance and are generally considered unacceptable outcomes.

The authors have found hardscape/substrate interactions governed by compressive and flexural stiffness are often at the root of these types of problems. Avoiding them requires understanding the behavior of not only the individual component materials, but also the resultant assembly. The authors have found the industry literature lacking in this regard. To fill in some of the data gaps, the authors undertook a laboratory test program to evaluate representative plaza materials and assemblies for compressive strength and stiffness.

The authors tested hot-applied rubberized asphalt waterproofing membrane applied to concrete slabs, the membrane manufacturer’s approved protection course, weave-type and dimple-type drainage composites, and Type VII 50-mm (2-in.) XPS insulation board. Each material was subjected to compression testing, with load and deflections recorded using a computerized data acquisition system. Assemblies constructed from the tested materials were then subjected to the same tests, allowing material versus assembly behavior comparisons. Some of the resulting comparisons were quite surprising, with the measured strength and stiffness of the assemblies much lower than would be predicted by the individual material tests.

An example of compressive stiffness and strength reduction due to component interaction is demonstrated by tests performed on dimple-type drainage composite and Type VII 50-mm thick XPS insulation board. The following compressive stiffness values were determined for these components when loaded individually:

  • 676 kPa/mm (2490 pounds per cubic inch [pci]) for the drainage layer; and
  • 638 kPa/mm (2350 pci) for the XPS board.

These stiffness values represent a ratio of the uniform surface load in kPa to the vertical compression displacement in millimeters. Stacking the insulation and drainage layer, as would be expected in a plaza system, is similar to placing two springs end-to-end, assuming the load is distributed uniformly between components. The stiffness of these two components when stacked together as an assembly can be theoretically calculated to be 328 kPa/mm (1210 pci). However, testing performed on drainage composite and insulation assemblies resulted, on average, in a compressive stiffness of only 128 kPa/mm (470 pci)—a 61 percent reduction from the theoretical value.

The relationship between load and displacement for this test series is shown in Figure 4, where the slope of each line on the graph is equal to the compressive stiffness. The stiffness reduction between theoretical and tested is caused by partial contact between the insulation and drainage composite, resulting in increased localized stresses in the insulation. Testing of other assemblies confirmed this same concept applied at the interface between other plaza system components, such as drainage composites and waterproofing membrane with or without a protection course.

XPS board insulation after compression testing of an assembly incorporating dimple-type (left) and weave-type (right) drainage composite. The insulation surface indentations demonstrate the uneven distribution of load at this interface.

XPS board insulation after compression testing of an assembly incorporating dimple-type (left) and weave-type (right) drainage composite. The insulation surface indentations demonstrate the uneven distribution of load at this interface.

The tests also revealed localized crushing failure of the XPS board at load levels well below the advertised material compressive strength. The reduced contact area at the high points of the drainage mat change the behavior of the assembly from the uniform bearing condition assumed by ASTM D1621 to a series of smaller load points with open space in between—at least initially.

With increased load, the drainage mat high points pushed into the XPS until it conformed to the drainage mat surface undulations, as shown in Figure 5. Beyond the obvious effect on assembly compressive stiffness, crushing of insulation can result in a reduction of drainage/air composite capacity due to intrusion into the flow channels.

Assessing configurations
All the foregoing discussion about plaza component and assembly behavior under compression loading provides the background on which realistic design-phase assessment of proposed configurations can be performed. This is particularly important when heavy loads will be present either early (i.e. during construction and landscaping activities) or later (i.e. from planned vehicular traffic, maintenance equipment, or emergency response vehicles).

Overestimating the strength and stiffness of a plaza assembly can have serious consequences. Insufficient supporting material compressive stiffness can cause concrete topping slabs, pavers, and paver setting beds to crack due to larger than anticipated flexural stresses, as shown in Figure 6, and paver edge raveling and mortar joint crushing, as shown in Figure 7. These problems can be avoided by evaluating the entire plaza assembly, and not just focusing on the properties of individual components.

Flexural stress levels in concrete topping slabs, pavers, and paver setting beds that may be subjected to heavy loads should be evaluated taking into consideration not only the strength and stiffness characteristics of the hardscape materials to which the loads will be directly applied, but also those of the supporting materials. Without accurate component and assembly strength and load/deflection performance data available to incorporate into plaza design calculations, project-specific testing should be considered.

Designers can use the test results to evaluate the anticipated behavior of a proposed plaza design using analytical techniques such as finite element modeling to quantify anticipated displacements and associated stress levels. If unacceptable levels of cracking are analytically predicted, the design should be revised.

Exaggerated deflection of topping slab subjected to vehicular loading with flexible plaza assembly represented by blue layer with springs. If not properly evaluated, the interaction between the concrete topping slab and supporting assembly can result in unsightly topping slab flexural cracks, shown in red.

Exaggerated deflection of topping slab subjected to vehicular loading with flexible plaza assembly represented by blue layer with springs. If not properly evaluated, the interaction between the concrete topping slab and supporting assembly can result in unsightly topping slab flexural cracks, shown in red.

Exaggerated deflection of pavers subjected to vehicular loading with plaza flexible assembly represented by blue layer with springs. If not correctly evaluated, the interaction between each paver and supporting assembly can result in paver edge raveling (shown above), paver flexural cracking, or mortar crushing.

Exaggerated deflection of pavers subjected to vehicular loading with plaza flexible assembly represented by blue layer with springs. If not correctly evaluated, the interaction between each paver and supporting assembly can result in paver edge raveling (shown above), paver flexural cracking, or mortar crushing.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Possible design modifications include:

  1. Increase topping slab, mortar bed, and paver thickness when project constraints allow, thereby spreading surface loads over a wider area and increasing the flexural resistance. A trade-off exists with this approach in that the underlying structure, which may be a parking garage or the basement level of a building, may not be capable of supporting added dead load from thicker materials.
  2. Add reinforcement to topping slabs and mortar beds to control cracking. Care must be taken with this approach since reinforcement can control, but not eliminate, cracking.
  3. Modify component selections below the hardscape materials to increase the support stiffness, such as selecting a different insulation board material with a higher compressive stiffness.
  4. Consider adding bollards to preclude vehicular access to particularly problematic areas.

Conclusion
As is true with any design exercise, the key to avoiding plaza hardscape/substrate interaction problems is to be able to answer four key questions:

  1. What are the loads that are likely to be applied to the plaza?
  2. What path will the loads follow from the point of application to the underlying structural support?
  3. What materials will be incorporated into the design?
  4. What are the strength and load/deformation behaviors of the proposed hardscape and underlying support, drainage, and waterproofing materials?

Answering these questions will go a long way toward developing a successful plaza hardscape design.

Kurt R. Hoigard, PE, SECB, FASTM, specializes in evaluation and repair of distressed buildings and structures. Since joining Raths, Raths & Johnson in 1985, his experience has encompassed preconstruction consulting, implementation of construction-phase quality assurance programs, investigation of water leakage, deterioration and complete collapse, and repair design. Hoigard has received numerous awards from the American Institute of Steel Construction (AISC), International Masonry Institute (IMI), and ASTM International. His memberships include the American Architectural Manufacturers Association (AAMA), American Concrete Institute (ACI), International Code Council (ICC), and Structural Engineering Institute (SEI). Hoigard can be reached at krhoigard@rrj.com.

Brian T. Lammert, SE, PE, CDT, specializes in field investigation, lab and field testing, structural analysis, collapse investigation, peer review of new construction, and repair design and implementation. Since joining Raths, Raths & Johnson in 2005, he has directed load tests to evaluate distressed structures, component suitability for new construction, and failure causation. Lammert’s structural analysis experience includes development of computer models used for failure analysis, structural design peer review, existing structure evaluation, and repair design. He can be contacted via e-mail at btlammert@rrj.com.

 

 

Controlling Stormwater at the Source: Exploring best management practices

Photo courtesy Katie McKain

Photo courtesy Katie McKain

by Katie McKain, ASLA, MLA, MUD

Conventional stormwater systems treat precipitation as a waste product, directing it into storm drains and pipes and pouring it into receiving waters. These traditional development systems also cause undesirable effects to the landscape, such as reducing the water table and its overall quality, as well as leading to erosion, sedimentation, and flooding issues.1

As the impervious surfaces characterizing urban sprawl—roads, parking lots, driveways, and roofs—replace meadows and forests, rain can no longer seep into the ground to replenish aquifers. This reduces the groundwater recharge serving as a natural hydrologic process where surface water infiltrates downward into groundwater to maintain the water table level.

The infiltration process naturally filters runoff through vegetation and soils. Not only do conventional systems prevent groundwater recharge, but they also cause significant stress to waterways and affect water quality. When the natural process does not happen, runoff spreads over impervious surfaces and gathers pollutants that wash into lakes, rivers, and streams, contaminating them.

There is also a negative financial connotation as building impervious surfaces and concrete curb and gutter systems are expensive. Curbs and gutters, and the associated underground storm sewers, frequently cost as much as $36 per 0.3 m (1 ft), which is roughly twice the cost of a grass swale. When curbs and gutters can be eliminated, the cost savings and positive effects on the environment can be considerable.

Destination of water after rainfall.  Data courtesy EPA, Water Quality Facts, www.epa.gov/owow/waterqualityfacts.html.

Destination of water after rainfall. Data courtesy EPA, Water Quality Facts, www.epa.gov/owow/waterqualityfacts.html.

What is low-impact development?
As shown in Figure 1, the ultimate destination of water after rainfall is divided into three categories:

  • absorbed into the ground;
  • evapotranspiration; and
  • runoff.

There is a dramatic difference between water movement on natural areas versus urban impervious environments.

The negative effects associated with unnaturally high runoff volumes from conventional practices have initiated the emergence of low-impact development (LID). The Low Impact Development Center is a non-profit organization in Beltsville, Maryland, dedicated to the promotion of this strategy, which it defines as:

a new, comprehensive land planning and engineering design approach with a goal of maintaining and enhancing the pre-development hydrologic regime of urban and developing watersheds.2

A detention pond where the maintenance access road uses void structured concrete. Photos courtesy Katie McKain

A detention pond where the maintenance access road uses void structured concrete. Photos courtesy Katie McKain

LID promotes integration of stormwater management into site and building designs, controlling stormwater at the source before it collects and deposits harmful pollutants. Another crucial component involves minimizing impervious areas and ensuring buffer zones between them. This allows infiltration and daylighting of runoff to the surface, controlling stormwater at the source. (In this context, ‘daylighting’ is used to describe an underground pipe conveying water that ends at the surface, so water rushes out of the pipe onto gravel or grass and the pipe is no longer underground.)

Advantages to using low-impact development
Some of the numerous benefits to designing with the LID model are discussed below.

Improved water quality
Many best management practices (BMPs) involve bioretention—a process using the chemical, biological, and physical properties of plants, microbes, and soils to improve water quality. Hyperaccumulators are unique plants with natural abilities to degrade, bioaccumulate, or render harmless contaminants in soil, water, and air.

There are many species of hyperaccumulators, including:

  • barley (i.e. hordeum vulgare);
  • water lettuce (i.e. pistia stratiotes); and
  • Indian mustard (i.e. brassica juncea).

These common types respectively counter aluminum, mercury, and lead. Bioretention techniques include adsorption, absorption, volatilization, decomposition, phytoremediation, and bioremediation.

Increased groundwater recharge
General water infiltration is important for groundwater recharge (i.e. replenishing the water table). Unsatisfactory groundwater recharge is becoming a serious concern as cities continue to develop land with impervious surfaces (Figure 2).

As the statistics are directly proportional, it is not surprising Atlanta earned the ‘top’ ranking for both loss of potential groundwater recharge and acres of new development. These extremely high numbers should also take the population increase into account, but Seattle managed much lower numbers across the board despite having a relatively high population increase. (While this is perhaps due to the West Coast city’s advances in stormwater management, it should also be noted Atlanta sees an average of 304 mm [12 in.] more annual rain than Seattle.)

When water is sent to a treatment facility instead of infiltrating to the groundwater, it is often taken far from its place of origin, depleting waterways of their natural processes. The sewer system not only diminishes groundwater supplies, but also causes significant stress to the waterways and affecting water quality.

When contaminated water runs off into rivers and lakes, it poisons the water and aquatic life; further, most of it evaporates without making it into the groundwater recharge cycle. Some runoff actually leaks into sewage systems of fading infrastructure. When there is not ample groundwater recharge, the water table is lowered and negatively affects all facets of nature, including the drinking water supply. BMPs aim to promote infiltration to satisfy the necessary groundwater recharge.

Development aids the loss of potential groundwater recharge. Data courtesy Paving Our Way to Water Shortages: How Sprawl Aggravates the Effects of Drought www.smartgrowthamerica.org/DroughtSprawlReport09.pdf.

Development aids the loss of potential groundwater recharge. Data courtesy Paving Our Way to Water Shortages: How Sprawl Aggravates the Effects of Drought www.smartgrowthamerica.org/DroughtSprawlReport09.pdf.

Reduced erosion, flooding, sedimentation, and water temperature
LID practices reduce stormwater rate, volume, and temperature. By lowering volume and rates of runoff, phenomenon occurrences (e.g. erosion, flooding, and sedimentation) also decrease. Pollutants increase the faster and farther runoff travels on impervious surfaces, with more speed causing runoff to warm up before depositing into lakes and streams and adversely affecting aquatic life.

Disconnecting impervious surfaces and adding permeable surfaces are the best ways to decrease flow rate and volume of runoff. Aesthetically, providing green space and visual attractions in a usually less appealing area, such as a parking lot, is always a benefit to consider.

Designing with LID principles and incorporating BMPs into site designs are responsible and affordable ways of incorporating the land and its natural processes into development. The U.S Environmental Protection Agency (EPA) defines a BMP as a:

technique, measure, or structural control that is used for a given set of conditions to manage the quantity and improve the quality of stormwater runoff in the most cost-effective manner.

Common types of stormwater BMPs
There are many types of stormwater BMPs to consider for a design. A site analysis should be performed to note the area’s size and the amount of water that must be accommodated. Each BMP has unique pros and cons and is site-dependent. In many cases, BMPs are cheaper alternatives to curb and gutter systems.

One way water may enter the bioswale from the parking lot.

One way water may enter the bioswale from the parking lot.

Bioretention swales, bioswales, vegetated swales
Bioretention swales are long, narrow landscaped channels that cleanse runoff using bioretention techniques, as well as infiltrate water and act as a conveyance system.

Phytoremediation is the process enabling plants to naturally remove toxic metals from the water. Typically, this process is not immediate, so the swale should be designed to hold water for more than 48 hours. A gentle slope is used within a swale to move water through it slowly enough for the plants to respond. Vegetation in the swale must be flood-tolerant, erosion-resistant, close-growing, and efficient at removing pollution. This is the case for plants able to extract high concentrations of metal from the soil into their roots, also called hyperaccumulators.

Swales can be wet, riparian areas, or they can be dry areas only wet during large storms. Typically, irrigating a swale is not a good practice except for the establishment of vegetation—a period that typically takes two to three years. Grassy swales, similar to vegetated ones in their design and activity, are landscaped solely with a mixture of grasses. The major difference is maintenance and form, as the grasses can be left to grow tall, be mowed constantly (e.g. buffer strips adjacent to streets), or mowed occasionally, depending on aesthetic and stormwater-filtering requirements.

Swales are inexpensive compared to traditional curb and gutter techniques. Although maintenance is an increased concern, a swale is still less costly and provides more benefits.

Rain gardens, infiltration basins, planter boxes
Rain gardens are meant to be a short-term bioretention area that fluctuates between wet and dry conditions depending on the space. Collectors of water runoff, these rain gardens typically consist of grasses adaptable to wet or dry conditions, but can also contain flowering plants and hyperaccumulator plants. The main function of a rain garden is to allow the stormwater to infiltrate into the ground and recharge the stormwater reserve, but they also allow for plant- and soil-filtering functions to improve water quality. These gardens are situated close to the runoff source; unlike swales, rain gardens do not convey the water to a specific place—they promote infiltration in a smaller contained area.

A small infiltration basin clearly delineating the entrance of the stormwater as it runs off the parking lot.

A small infiltration basin clearly delineating the entrance of the stormwater as it runs off the parking lot.

Rain gardens must be positioned close to the source, and the water table must be at least 1.5 m (5 ft) below the basin at its peak. This is because if runoff into a rain garden travels a long way and picks up excess pollutants and sediments, infiltration may not cleanse it enough. As a result, the ground water could be contaminated or the system clogged. Additionally, the type of soil needed to accommodate proper infiltration of 12.7 to 76.2 mm (1/2 to 3 in.) per hour is an extremely important design pre-treatment. The soil should have no greater than 20 percent clay content, and less than 40 percent silt/clay content.

Although vegetation within infiltration basins is encouraged for optimal filtering, basins can also have layers of sand and rocks in a type of soakage trench, without vegetation. Infiltration basins are cost-effective practices because little infrastructure is needed when constructing them.

Another way to collect stormwater is to use a rainbarrel or cistern to harvest rain water, commonly from a roof downspout. By collecting water, and repurposing it, runoff is prevented. Water collected in such barrels is then commonly re-used to irrigate lawns or gardens and can be used in other non-potable uses such as flushing toilets. However, it should be noted, due to water laws and restrictions, it is illegal to harvest rainwater in some areas of the United States.

Constructed wetlands: Detention ponds/retention ponds
Constructed ponds and wetlands are designed by engineers to prevent runoff by holding stormwater. Detention ponds are larger, less-particular versions of infiltration basins. They typically have a fore bay to allow particles and pollutants to settle and be treated; this prevents them from clogging the entire pond. Two important pieces of designing ponds include the removal of sediment buildup over time, as well as the simultaneous growth of hyperaccumulator plants that naturally filter water.

Typically, fore bays are a concrete surface, which allows for maintenance and removal of sediment buildup, but not vegetation growth. To plan around this problem a structured surface able to grow vegetation, such as a void-structured concrete, should be used. This material is similar to a concrete slab in structure, but has spaces large enough for hyperaccumulator vegetation to grow through. The vegetation’s roots are protected within the concrete cavity allowing for maintenance and removal of sediment as needed without distrubing the vegetation significantly.

Generally, detention basins can be used with almost all soils. While detention ponds can remain wet or sometimes dry up, retention ponds are typically deeper and always wet as well as retain stormwater for longer periods.

An interactive bioswale at a big box store with a pedestrian bridge connecting the paths.

An interactive bioswale at a big box store with a pedestrian bridge connecting the paths.

Sometimes people need to be reminded not to dump chemicals in the storm drains.

Sometimes people need to be reminded not to dump chemicals in the storm drains.

Green roofs
A green or vegetated roof consists of a waterproofing assembly, lightweight soil, and plants adapted to survive the area’s climate. An efficient BMP and prevention technique, vegetated roofs intercept rainwater directly at the source, preventing most of the water from becoming runoff. Since the rain is used by the vegetation, a major advantage to a green roof is its ability to decrease the volume of runoff, thus mitigating flow rates, flooding, erosion, and sedimentation.

Green roofs promote infiltration for the advantage of the vegetation on the roof, but not the water table. Additionally, green roofs provide wildlife habitat and attract birds. These assemblies also provide energy-saving benefits to the building, including increased insulation on the roof, mitigating building and roof temperatures, and possibly doubling the roof’s lifespan since it is protected from harsh weather.

There are two types of green roofs: intensive and extensive. The first type promotes human interaction where people are encouraged to use and interact with plant life amongst paths and gathering areas, these roofs can usually carry heavier loads and deeper soil. Conversely, extensive green roofs contain only vegetation over the entire roof with little human interaction. A green roof is a relatively high-cost BMP initially, but has energy-saving returns that are worthwhile in the long run.

Tree plantings
One of the most underused BMPs, tree plantings are effective at mitigating stormwater in urban settings due to their ability to absorb large amounts of water while needing little surface area. Trees provide shade and habitat, which helps reduce the urban heat island effect.

Trees are also an important factor in cleansing and filtering the air. The branches and leaves of trees help to soften rainfall speed, reducing both stormwater flow rates and erosion. Trees also help aid the view shed, breakup the impervious landscape, provide small but essential green spaces linking walkways and trails, and reduce the visual dominance of cars.

The success of tree plants, especially in urban settings, is largely determined by the species chosen and the size of the planting area, in addition to meeting its watering requirements. Species selection should be based on location, but an appropriately sized tree-well with plenty of soil volume makes the difference in how well it survives. Although there is a need to pave over tree roots in urban settings, leaving as much exposed to natural water as possible is the best plan; if paving is necessary, use permeable paving so water and oxygen can still get to the tree roots. Pouring concrete around a street tree of any variety guarantees stunted growth and little canopy.

A close up of the vegetation within the void structured concrete of this detention pond showing how roots are protected from maintenance scrapers.

A close up of the vegetation within the void structured concrete of this detention pond showing how roots are protected from maintenance scrapers.

Detention pond fore bays designed with a base made of void structured concrete offer designers the opportunity to have naturally cleansing and appealing vegetation in the pond and still offer the ability to perform scraping maintenance to remove sediment without destroying the vegetation.

Detention pond fore bays designed with a base made of void structured concrete offer designers the opportunity to have naturally cleansing and appealing vegetation in the pond and still offer the ability to perform scraping maintenance to remove sediment without destroying the vegetation.

Permeable paving
Arguably the most efficient BMP due to its practicality, permeable paving has been revolutionizing the green industry for years. Paved surfaces are a necessity in the built environment, and replacing traditional impervious concrete and asphalt with permeable materials is one of the easiest, effective, and cost-efficient methods of preventing runoff.

Permeable paving allows for water to percolate through cracks in the pavement and infiltrate directly to the soil, preventing runoff from ever occurring. Infiltration not only prevents runoff, but also replenishes the water table and allows for natural soil filtration to improve water quality. Incorporating stormwater treatment into parking areas and landscaped zones reduces required detention volume onsite. This allows for an increase in building area and the potential for further profitability for an LID-educated developer.

Permeable pavements also provide a reduction in the heat island effect, which is especially valuable in urban areas typically paved in dark colors and absorb light. Since permeable pavement allows for water infiltration, it also helps increase water quality using the soil as a filtering medium. There are many types of permeable pavements available with advantages and disadvantages, depending on the desired use and location.

Types of permeable pavement
Different types of permeable pavement can be used for various installations. Some examples are included below.

Void-structured concrete: grass- or stone-filled, or concealed with vegetation
Designed to combine the strength of traditional concrete with aesthetically pleasing vegetation, void-structured concrete is cast-in-place and contains a grid series of spaces that allow the system to be pervious in nature.

These voids can be filled with various porous materials such as vegetation or no-fines stone, or the entire system can be seeded over to completely conceal it, making it ideal for emergency access applications seeking to hide the eyesore of a road. Typically, void-structured concrete is used for fire and emergency access, military applications, parking lots, detention pond fore bays, and general stormwater management with heavy load requirements.

The typical lifecycle of the material is more than 15 years. Its benefits include:

  • high load-bearing capacity (suitable for heavy traffic);
  • high infiltration rates;
  • low maintenance costs;
  • freeze-thaw cycle resistance;
  • ability to be effective with saturated sub-base;
  • green space addition; and
  • moderate installation costs.

Some challenges associated with void structured concrete are:

  • not Americans with Disabilities Act (ADA)-compliant;
  • in pedestrian zones there is a need to incorporate bands of traditional concrete for ease of movability with all footwear; and
  • surrounding grass needs to be properly maintained.

Permeable interlocking concrete pavement
Available in various shapes, sizes, and colors, permeable interlocking concrete pavers (PICPs) are designed to either let water infiltrate within voids in the unit itself, the space around them, or both. PICP is ideal for light traffic where intricate and aesthetically pleasing designs are desired. Often these systems are used for pedestrian walkways and garden paths.

The longevity of PICP is moderate, spanning seven to 15 years. PICP benefits include:

  • high infiltration rates;
  • various patterns and colors;
  • can be ADA-compliant; and
  • ease of maintenance if integrity of paver fails or if utilities underneath need to be reached.

Some challenging aspects of PICP are:

  • system has a low load-bearing capacity (not suitable for heavy traffic applications);
  • pavers are susceptible to movement and damage in freeze-thaw climates;
  • high installation costs as a deep sub-base for optimal performance is required; and
  • high maintenance costs.
A study site pictured after a rainfall showing water pooling on top of the asphalt on the right, and water all infiltrated in the pervious concrete to the left.

A study site pictured after a rainfall showing water pooling on top of the asphalt on the right, and water all infiltrated in the pervious concrete to the left.

A test site in a parking lot comparing pervious concrete to traditional asphalt.

A test site in a parking lot comparing pervious concrete to traditional asphalt.

 

 

 

 

 

 

 

 

 

Plastic grid systems reinforced with grass or gravel
Reinforced systems are most commonly seen as plastic ring systems. They are designed to provide vehicle stability for grassed or gravel surfaces without the visual eyesore of the plastic rings showing. This system is best used for short-term pedestrian and light vehicular traffic and is often seen on trails and for emergency access. Its lifecycle is typically less than seven years.

Benefits include:

  • high infiltration rates;
  • adds green space; and
  • lightweight plastic is easy and cost-effective to install.

Challenges include:

  • low load-bearing capacity (not suitable for heavy traffic applications);
  • system commonly fails in saturated soils;
  • susceptible to movement and damage in freeze-thaw climates;
  • high maintenance costs as the system settles fast and plastic rings are commonly visible;
  • not ADA-compliant; and
  • grass needs to be properly maintained.
A close up image of stone filling the voids of a void structured concrete installation.

A close up image of stone filling the voids of a void structured concrete installation.

Gravel and crusher fines
Crushed rock type gravel and crusher fines, typically last less than seven years. Crusher fines can use a glue to help hold them together, reducing the infiltration rates. Due to their natural properties, they are commonly employed in park and trail settings without heavy slopes. Similar to some of the other materials discussed, they typically have high infiltration rates and low installation costs, along with the ability to withstand freeze-thaw cycles.

The system is not suitable for heavy traffic applications due to its low load-bearing capacity. It also commonly fails in saturated soils because the system is not stabilized well. Further, gravel and crusher fines are not appropriate for use on a slope because they develop ruts in heavy storms, resulting in high maintenance costs.

Pervious concrete and pervious asphalt
An alternative to their traditional counterparts, the pervious versions of concrete and asphalt employ larger aggregates in the mix. The result creates voids in the pavement that allow water to pass through to enter a temporary detention area and ultimately infiltrate to the ground. These products are different, but their advantages and disadvantages are similar.

Location is key to the performance of these systems. Typically, pervious concrete and asphalt perform the best in areas where available sediment is low, traffic volume is low, and maintenance can be regular and intensive. The lifecycle is typically seven to 15 years.

Benefits include:

  • high load-bearing capacity (suitable for heavy traffic);
  • moderate infiltration rates;
  • aesthetic design options;
  • ADA-compliant; and
  • moderate installation cost.

Some challenges associated with pervious concrete and asphalt include:

  • reduction in pavement surface (surface raveling is common where aggregate is dislodged or damaged);
  • high maintenance costs as the system needs the quarterly use of an intensive vacuum sweeper to pull out sediment, and special winter maintenance for snow and ice conditions;
  • reduction in porosity (clogging is the largest concern—even when regularly vacuumed, a clogged system cannot function and water ponds on the surface); and
  • it can be susceptible to damage in freeze-thaw climates if system freezes with water in it.

With the cost of permeable pavements being fairly similar to conventional methods, it seems permeable pavement could replace nearly all impervious surfaces, but there are certainly exceptions to this idea. It is not recommended to use permeable pavements in areas where heavy pollution, leaks, or chemical spills could occur. While small oil drips from parked cars is considered acceptable for the soil to handle, a large chemical spill or heavy agricultural use would put soil and the water table in danger of being directly polluted.

A conventional way of handling stormwater is to direct it into storm drains at fast speeds with water collecting harmful pollutants on the way.

A conventional way of handling stormwater is to direct it into storm drains at fast speeds with water collecting harmful pollutants on the way.

It is also not recommended to use permeable pavement in areas where heavy sedimentation can occur—this could clog the system faster than it is designed to handle and could result in heavy maintenance costs.

Additionally, care should be taken to design for ADA-compliant access as wheelchairs and those who use walkers as an aid typically prefer the steady, smooth surface of traditional impervious surfaces. Some tracked vehicles may also be better handled on traditional surfaces to limit wear and tear.

Conclusion
Now commonly used for residential driveways, military applications, offices buildings, government buildings, and even grocery stores, permeable paving is emerging everywhere. LID stormwater management methods, with a focus on handling stormwater at the source, will be important to incorporate for the environmental, social, and economic stability of the world’s future. As the shift from impervious to pervious surfaces continues to occur, it will be interesting to see the speed at which this concept permeates—how much use and what technological advances will we see out of the next generation to come?

Notes
1 An earlier version of this article appeared in the June 2010 issue of ROOT. (back to top)
2 For more, see www.lowimpactdevelopment.org. (back to top)

Katie McKain, ASLA, MLA, MUD is a proponent for sustainable design and green initiatives who promotes stormwater management principles as a representative for Sustainable Paving Systems, LLC. She earned her Master of Landscape Architecture and Master of Urban Design at the University of Colorado Denver in Denver, CO, and her Bachelor of Science from Purdue University in West Lafayette, IN. McKain also received her certification from GREENCO for Best Management Practices for the Conservation and Protection of Water Resources in Colorado. She can be contacted by e-mail at kdmckain@hotmail.com.