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Environmental
NRMCA Pervious Concrete Contractor Certification
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Hydrologic Design of Pervious Concrete Part 1 w Part 2 w Part 3 w Part 4 w Part 5
Examples and Discussion
In this section, application of the design methods presented in Part 4 are examined and the implications of the analysis for several common situations are discussed. Broad conclusions are drawn regarding pervious concrete pavement system design needs in many situations. Some of the limitations of the Rational Method with this type of system are also examined.
Tabular results of the analysis are provided and discussed in Section 5.5. Values of both Equivalent CN and total runoff are provided. The total runoff values provide important hydrologic information and may be needed for permit applications. The total runoff value is also used to assess the often significant benefits of impoundment. Sensitivity of the solution to the values of initial estimates used in the design, which may not be known with great precision, is assessed by examining changes in the total runoff with changes in the values of initial estimates. The draw-down times of pervious concrete pavement systems in various conditions are also examined to ensure appropriate longer term performance. Comments on the findings of the analysis are also provided.
5.1 Example Proposed Development
5.1.1 Development Plan
A single development is analyzed and discussed in subsequent sections. Various site conditions are considered so that comparisons of the effects of various elements are more evident. The proposed development consists of 300,000 ft2 (about 28,000 m2 or 6.9 acres) of pervious parking, onto which the runoff from 150,000 ft2 (about 14,000 m2 or 3.5 acres) of impervious roof and impervious pavement structures drain. The impervious pavement would be used in heavy traffic-load lanes, such as delivery areas, and in turning lanes. Vegetated islands, side slopes, and contiguous undeveloped land (some of which may not belong to the owner) occupying 200,000 ft2 (about 19,000 m2, or 4.6 acres) will also drain onto the pervious concrete pavement system. The islands, slopes, and undeveloped land will be landscaped with grass and some bushes. The CN of these pervious areas and the pre-development CN will depend on the soil (HSG) used in the different examples. Modifications to this adjacent, impervious area are considered later. The site use prior to development is pasture in very good condition with continuous forage available, and minimal bare areas or trails.
Pavement depth is normally controlled by anticipated traffic loading so a minimum thickness is selected prior to the hydrologic design and analysis. Pavement depth should be specified in increments of 1 in. (25 mm). Typical pervious concrete pavement characteristics are provided in Appendix D. In the example in this chapter, a pervious concrete pavement depth of 6 in. (150 mm) will be used. The design porosity of the pervious concrete in these examples is 15%. The pervious concrete pavement system is assumed to be level.
5.1.2 Site Conditions and Constraints
Hydrologic site performance of the approximately 650,000 ft2 (about 60,000 m2 or 15 acres) development is examined for a variety of site conditions or constraints, including four different soil types and the presence or absence of a base course composed of clean stone. Performance is examined in a 2-year, 24-hr storm and 10-year, 24-hour storm.
The four different soils used in the example analysis of the site are:
1. A sandy, well draining soil classified as HSG A;
2. A loamy sand with some silt, with an intermediate infiltration rate, still classified as HSG A;
3. Another silty soil with an intermediate infiltration rate, classified as HSG B; and
4. A poorly draining silty clay, classified as HSG D.
The effects of base course are examined by using 8 in. (200 mm) of clean stone. A compacted aggregate base consisting of size #57 or #67 stone has a porosity of 40%. The levels of precipitation used in this study are relatively conservative: the precipitation in the 2-year storm is given as 4 in. (100 mm) and the precipitation in the 10-year storm is given as 6 in. (150 mm).
5.2 Pre-Development Runoff and Post-Development Runoff Without Pervious Concrete Based on the soil classifications provided for pasturage in good condition at the time of analysis, estimates of the pre-development runoff can be determined. Runoff is estimated from the Curve Number and given in inches for 4 in. (100 mm) and 6 in. (150 mm) of precipitation (see Table 4).
Several comments are in order. First, although the values of runoff are reported in Table 4 to the nearest 0.01 in., such reporting precision is inappropriate given the variability in the input estimates and the uncertainty in the model itself. Second, since both Case 1 and Case 2 are in HSG A soils, the estimate of runoff is the same for both; in reality, one would expect a difference, but it is important to recognize the inherent simplifications made in this, as in all, hydrologic models. Third, the effect of the type of soil on the volume of runoff is significant and the designer should be careful to compare “apples to apples” when assessing the value of various alternatives in different areas.
Table 5A shows the pre-development CN’s and runoff values with more appropriate significant figures. Table 5B shows the post-development CN’s and runoff values anticipated from the site in the 2-year and 10-year storm without the benefit of a pervious concrete pavement system, (assuming all of the parking area has an impervious surface). Table 5B is based on composite CN’s (the area-weighted, average CN).
Table 4. Estimates of Pre-Development Runoff for Example Case
Table 5A. Pre-Development CN’s and Runoff
*Note: Metric conversions are not exact equivalents due to rounding in Table 4.
Table 5B. Post-Development CN’s & Runoff Without a Pervious Concrete Pavement
The increase in runoff associated with development is significant in the absence of some type of mitigation. Again, the effect of the underlying soil is also significant, even in the pre-development stage. As shown below, a pervious concrete pavement system appreciably improves the hydrologic performance of the site.
5.3 Preliminary Estimates for Use in the CN Method and Discussion
5.3.1 Initial Estimates of Infiltration Rate
Based on the soil classification and descriptions provided, estimates of the infiltration rates for the different soils were made based on TR-55 (SCS 1986) (see Tables 1A and 1B).
It is important to recall that these values must be estimated prior to construction and that the construction process itself will change in situ characteristics. This level of accuracy in the initial estimates of infiltration rate will provide a sufficiently robust model in most practical situations. It is best to modify marginally acceptable designs rather than depend on more accurate estimates of infiltration rates when modifying or finalizing a design, especially considering the variability inherent in the other design elements. Preliminary designs can determine feasibility and identify necessary design modifications as early as possible. Sensitivity analysis is discussed further in section 5.6.
5.3.2 Initial Estimates of the CN of Adjacent Areas
Based on the HSG of each case and the description of cover anticipated after development, the CN of the vegetated and landscaped areas on the site that will drain into the pervious concrete pavement can be estimated. Values are similar but slightly higher (more conservative) than the pre-development case to account for foot traffic, irregular watering, slope, and other reductions in the quality of the ground cover. Estimates are drawn from TR-55 (SCS 1986).
5.4 Results and Discussion of Site Analysis Including Pervious Concrete
5.4.1 Runoff and Equivalent Curve Numbers The runoff and equivalent CN’s for the proposed development, as affected by the soils considered in this study, are given in Tables 8A and 8B. Improvements in runoff (in inches) between pre- and post-development, where the site includes a pervious concrete paving system, are provided in Table 9.
Table 6. Initial Estimates of Infiltration Rate
Table 8A. Post-Development Runoff, Including Pervious Concrete
Table 8B. Equivalent Curve Number, Post-Development, Including Pervious Concrete
*Note: Calculated values of the equivalent Curve Number should not be given below about 36.
Table 9. Improvement from Pre- to Post-Development Runoff, Including Pervious Concrete
Note: A positive value indicates a positive (beneficial) improvement.
5.4.2 Discussion of Findings of Site Analysis
The pervious concrete pavement system significantly reduced post-development runoff and, in all cases where a clean stone base was used, the total runoff was actually lower than pre-development levels. This is a finding for a specific, but realistic, situation and not a generalization. This finding does, however, demonstrate the significant potential benefits of a pervious concrete pavement system.
5.4.2.1 Infiltration Effects
The analysis highlights a number of other findings. One important conclusion is that the infiltration rate of the subgrade is extremely important in terms of accurately modeling and assessing the hydrologic performance of a pervious concrete pavement system, and subgrade infiltration effects should clearly be included in the design methodology. Several other important conclusions are related to this observation.
5.4.2.1.1 System Recovery Time
The system recovery time (or “draw-down” time) of the pervious concrete pavement system at the end of the 24-h storm is acceptable for all cases except Case 4, the poorly draining soil. The draw-down time for the system in well draining soils (0.5 in./h to 1 in./h or 1.3 cm/h to 2.5 cm/h) is negligible; the draw-down time for the system in the moderately draining soil (a sandy silt) is less than 2 days. The draw-down time estimated for pervious concrete pavement alone over the poorly draining soil was almost 4 days, but was in excess of two weeks when an aggregate base was included. The extra volume of runoff captured and stored in the base required an unacceptably long time (greater than 5 days) to infiltrate and recover the capacity of the system. For this reason, the use of pervious pavements is likely to be limited in active mitigation of sites containing soils with very low infiltration rates, generally those considerably less than 0.1 in./h (0.3 cm/h). A pervious concrete pavement system could be used as a passive application for certain sites with these type of soils, however. In addition, an experienced designer might consider using a pervious pavement system in an area with infiltration rates considerably less than 0.1 in./h (0.3 cm/h) with the intention of controlling runoff rate using additional detention devices. This analysis requires detailed knowledge of storm water design and is beyond the scope of this publication.
5.4.2.1.2 Comments of Performance with Silty Soils
Some policy recommendations suggest that pervious concrete pavement systems be limited to areas with sandy soils with high infiltration rates. These policies were often adopted as interim measures prior to availability of in-depth studies. The analysis above clearly indicates that beneficial results can be attained using pervious concrete pavements in less than “ideal” soils. Although well drained sandy areas are optimal in many ways for pervious concrete applications, pervious concrete pavement systems can be used successfully in many other types of soils, including some silty soils. The analysis of the site with a soil infiltration of only 0.1 in./h (0.3 cm/h) indicates not only successful performance but shows how a developed site can, when using a properly designed pervious concrete pavement system, reduce the post-development runoff to less than that prior to development. The validity of this conclusion can be demonstrated analytically in many situations, and the analysis has been confirmed in practice (Knight 2003). There is no need to arbitrarily limit the use of pervious concrete pavement systems to sands. Another important observation is that runoff could still occur if pervious concrete were used without a clean stone base for additional storage, even in high infiltration rate sandy soils.
Infiltration characteristics of the subgrade are important for both passive and active systems. However, estimating the infiltration rate for design purposes is imprecise and the actual process of soil infiltration is complex. A simple model is acceptable for these applications and initial estimates for preliminary designs can be made with satisfactory accuracy, using conservative estimates for infiltration rates.
5.4.2.2 Equivalent Curve Number
The use of equivalent CN’s provides a way to describe the benefits of pervious concrete pavement systems in more qualitative, verbal terms, which can be useful in conveying the results of the analysis to decision makers without a strong technical background in hydrology. For example, the equivalent, post-development CN was 61 in the 10-year storm for the site with a moderate infiltration rate soil (0.1 in./h or 0.3 cm/h). In an HSG B region, this is slightly better than the CN expected in a residential area limited to 2-acre (87,000 ft2, or 8100 m2) lots with about 12% impervious surface. Another comparison is that the post-development runoff characteristics for this site would be hydrologically similar to woods, in which some grazing occurs, which are not burned, and with some forest litter covering the soil. As noted previously, it is important to compare “apples to apples” with this type of analysis.
An equivalent CN derived in this manner for a specific site can also be used for additional analysis of a larger watershed or downstream elements. The equivalent CN can be used to help estimate the peak runoff (cfs or m3/s) using the methods described in TR-55. The equivalent CN must be used with caution in these applications and estimates should be checked using alternate methods.
5.4.3 Other Design Considerations
5.4.3.1 Comments on the Use of Stone Base
While incorporation of a clean stone base reduces runoff, the reduction may be small in deep, well draining sands where the water table does not hinder infiltration. Since the use of clean stone base is usually recommended for both hydrological benefits and load carrying capability of pervious concrete pavements, additional analysis and discussion will focus on the results derived by including the base in the analysis.
The designer must consider additional factors when faced with fine grained soils. Historically, pavements have provided a level of waterproofing for the subgrade. A pervious concrete pavement system will ensure that the subgrade is saturated for a considerable period of its service life. This will reduce the subgrade modulus and, in the presence of appreciable traffic loads, can promote migration of the soil into the base course, reducing storage capacity and possibly affecting the system response to traffic. In these cases the inclusion of a filter fabric or designed sand filter is strongly recommended. The reduced modulus may affect required pavement depth; this issue is discussed in another document (Tennis, Leming, and Akers 2004).
5.4.3.2 Alternate Performance Specifications
It is reasonable to consider the minimum size of the parking lot required for various desired performance attributes. In the previous examples, the parking lot size is near that required to maintain post-development runoff at pre-development levels. If other requirements were in place, and there was a desire to optimize the use of pervious concrete pavement system, spreadsheet tools such as those found in Pervious Concrete, Hydrological Design and Resources (PCA 2006) could be used with successive approximation to find a solution.
For example, if criteria for development in the moderate infiltration area (0.1 in./h or 0.3 cm/h) were given to the effect that “…post-development runoff shall not exceed pre-development runoff by more than 25% in the 10-year storm…,” the parking lot area could be built with both impervious surface and pervious concrete pavement. Table 5A shows the pre-development runoff in this situation was estimated to be 2 in. (50 mm). Permitting total post-development runoff to be 25% higher, or 2.5 in. (64 mm), the 300,000 ft2 (about 28,000 m2) parking area could be converted into additional out-parcels (approximately 153 pervious pavement with an 8 in. (200 mm) deep stone base under the pervious concrete and 253 roof or other impervious surface) or used to offset development in an adjacent area.
5.4.3.3 Additional Impervious Surfaces
The site description used in the analysis so far included 200,000 ft2 (18,600 m2) of vegetated area. This may not be reasonable in heavily urbanized areas. If the same area were primarily paved, or very steep, or both, more runoff would clearly be expected. Converting this area into 150,000 ft2 (about 14,000 m2) of conventional paved surface and 50,000 ft2 (about 4,650 m2) of vegetated area, results in a total impervious area of 300,000 ft2 (about 28,000 m2), the same area as that occupied by the pervious concrete pavement system. Letting all of this drain into the pervious concrete pavement with 8 in. (200 mm) of clean stone base, in an area with a soil infiltration of 0.1 in./h (0.3 cm/h), would result in total runoff values of 0.8 in. (20 mm) for 4 in. (100 mm) of precipitation and 2.8 in. (71 mm) for 6 in. (150 mm) of precipitation.
Table 10 shows the comparison between the different alternatives. With almost half the site (about 46%) now impervious surface, the post-development runoff is still essentially the same (0.8 in.; 20 mm) as pre-development runoff for a large, 2-year storm, although greater (2.0 in. to 2.8 in.; 51 mm to 71 mm) for a relatively large, 10-year storm, even for a site with less than optimal soil infiltration. The pervious concrete pavement system is still providing significant hydrologic benefit. The site with a pervious concrete pavement system is providing more than 40% reduction in what the post-development runoff would be without the pervious concrete pavement system, even in this demanding situation.
An additional reduction in runoff is possible by incorporating a deeper stone base, consistent with maintaining an acceptable draw-down time. For example, using 12 in. (300 mm) of base instead of 8 in. (200 mm) would reduce the runoff back to 2.0 in. (51 mm) with 6 in. (150 mm) of precipitation, essentially the pre-development runoff. The draw-down time would be less than 3 days in this case, which is acceptable.
Table 10. Comparison of Extent of Impervious Surface
5.4.3.4 Comments on the Use of Pervious Concrete Pavement Systems in Sandy Regions
Conducting analysis of a variety of different types of developments in sites with well drained, sandy soils, those with infiltration rates of 0.5 in./h to 1.0 in./h (1.3 cm/h to 2.5 cm/h) or greater, it is quickly apparent that the systems have excellent hydrological characteristics. In these regions, the use of more complex analysis may provide little additional benefit and it is possible to design an effective, robust, pervious concrete pavement system using little more than simple rules of thumb.
5.4.3.5 Calculations for Passive Mitigation Applications
The analysis to this point has focused on active mitigation applications of pervious concrete pavement systems. The procedure for passive applications is identical except that no runoff from adjacent surfaces is included directly in the analysis. When using the software on CD063 (PCA 2006), enter “0” for all areas except the pervious pavement.
5.4.3.6 Analysis with a High Water Table
The presence of a high water table can complicate the analysis. A simple, reasonably conservative technique used in analysis of sites overlaying sandy soils is to ignore infiltration but include the storage capacity of the sand layer (depth and effective porosity) between the water table and the bottom of the pervious concrete pavement system. More detailed analysis may require the services of a geotechnical engineer as well as a hydrologist.
5.5 Estimation of Peak Discharge
Estimates of peak discharge are needed for the design of outlet structures and may be required for permit applications. Restricting peak flow after development compared to pre-development estimates provides a relatively simple way for permit granting agencies to help assure satisfactory water quality and water quantity performance of the developed area. This approach may not fully capture all of the benefits of a pervious concrete pavement system, however. This document describes a method appropriate for hydrologic design of pervious concrete pavement systems as impoundment structures, specifically including the effects of infiltration, and is therefore based on volumetric analysis. Estimates of peak discharge, when required, may be determined with additional analysis using the results of the Curve Number approach described above.
Peak discharge may be estimated by several methods. The Rational method is discussed below in Section 5.8. TR-55 (SCS 1986) describes two methods of estimating peak discharge, the Graphical Peak Discharge Method and the Tabular Hydrograph Method. The Graphical Method can be used to estimate the peak discharge (cubic feet per second) based on time of concentration (hours), total runoff (inches), area (square miles), precipitation (inches), and CN.* The Tabular Hydrograph Method is a routing technique that requires similar input and can incorporate hydrologic behavior in multiple reaches or sub-areas. [*Note: TR-55 provides values only in U.S. Customary units. For conversion of the final peak discharge values, 1 ft 3 / s = 0.283 m3 / s].
The area and precipitation information needed as input is the same as that used as input in the Curve Number method. Runoff values obtained as output of the Curve Number method can be used as input for the Graphical Peak Discharge Method. The time of concentration can be estimated using TR-55 (SCS 1986) or other means and is typically 15 to 30 minutes for most small, urban watersheds. More extensive studies often include complete hydrograph formulation and basin routing analysis. The hourly estimates of excess runoff provided by the Curve Number Method can be used as input in more complex analyses.
5.6 Comments on Designing a Robust Solution
All hydrologic models involve uncertainty and variability. The model described in this report is based on a commonly used, but synthetic storm, and requires estimates of various parameters, some of which cannot be determined with great accuracy or precision. Areas, depths and slope can be determined reasonably accurately. The CN of various surfaces must be estimated from tabular data and adjusted based on experience. The infiltration rate in service is probably the most difficult to estimate accurately. It is informative to consider the effects of variation in different input parameters on the output of the Curve Number Method with a simple sensitivity analysis.
5.6.1 Sensitivity Analysis
The conditions in Case 3 (moderate infiltration in an HSG B soil) are demanding and this case is re-examined with a variety of different initial estimates. The effects of differences in porosity of the pervious concrete, the depth of the base course, the CN of the adjacent, vegetated area, and the rate of infiltration on total runoff were determined. The original analysis used a 6 in.- (150-mm) thick pervious concrete with 15% porosity, an 8 in. (200 mm) clean stone base course with 40% porosity, a CN of 69 for the adjacent areas, and an infiltration rate of 0.1 in./h (0.3 cm/h). A total runoff of 2.0 in. (51 mm) was estimated to occur with 6 in. (150 mm) of precipitation in a NRCS Type II, 24-hour storm. The sensitivity of runoff estimates to variations in selected parameter estimates were also examined for Case 2, a site on silty sand.
The storage capacity of the pervious concrete accounts for just over 20% of the storage capacity of the total pavement system and the base course accounts for almost 80%. Slight differences in the porosity of the pervious concrete should not have a significant effect on storage capacity, so the effects of 10% porosity and 20% porosity, relatively large differences, were examined. The clean stone base course porosity should be relatively constant, but construction tolerances and sedimentation could affect the useable depth for storage, so the effects of a 1 in. (25 mm) reduction in depth were examined.
The adjacent, vegetated areas accounted for about 30% of the total area draining into the pervious concrete pavement system and the post-development CN of these areas must necessarily be estimated with some degree of uncertainty. The difference in runoff due to a CN of 61 and a CN of 79 in the Case 3 situation were examined. These values were selected as the likely range of values for the conditions and soil type given.
The default value of 0.1 in/h (0.3 cm/h) suggested in Table 2 was used in the original analysis in Case 3. The effects of variations in rate of infiltration from 0.05 in/h to 0.3 in/h (0.1 to 0.7 cm/h) were examined. This provides a reasonable range of values of a parameter which can be difficult to estimate accurately in the design phase.
Table 11 shows the runoff for Case 3 due to changing one parameter at a time. The values in the “Runoff” column are the excess runoff from the storm in inches (mm). The values in the “Difference in Runoff” column are the differences in runoff from that estimated using the original Case 3 parameter values.
Table 11. Sensitivity of Estimated Runoff to Variation in Selected Design Parameters, Case 3 with 6 in. (152 mm) of Precipitation
Note: Conversions are not exact due to differences in rounding. Values are given to reasonable and similar significant figures.
5.6.2 Discussion of Sensitivity Analysis Results
The uncertainty of hydrologic models easily exceeds several tenths of an inch (in excess of 5 mm) of runoff, but comparing differences in runoff permits observations on the effects of routine variations in characteristics or properties of important elements on performance of the system in service. One of the important observations is that relatively large variations in porosity of the pervious concrete have only a small effect on storage capacity. This implies that reasonable variations in the bulk properties of the pervious concrete during construction are acceptable. This analysis does not include effects on permeability, however, and routine maintenance is required to ensure satisfactory operation.
Likewise, a slight reduction in base course thickness will result in a slight increase in runoff. The designer should carefully consider the effects of construction tolerances and sedimentation in highly sensitive applications.
Another observation is that accuracy of initial estimates of the CN of adjacent areas is not critical. Reasonable differences in initial estimates of the adjacent area CN will result in only slight differences in the estimated runoff in most practical situations.
Variation in the estimate of the infiltration rate has a much greater effect on the estimated runoff for this site than routine variations in the other factors examined. An increase in the soil infiltration rate from 0.1 in/h to 0.3 in/h (0.3 cm/h to 0.7 cm/h) results in over a 50% improvement in capturing runoff. A reduction in the soil infiltration rate from 0.1 in/h to 0.05 in/h (0.3 cm/h to 0.1 cm/h) results in only a 25% reduction in impoundment, under the conditions given.
Sensitivity analysis of Case 2 with a 0.5 in./h (1.3 cm/h) infiltration rate and without a clean stone base leads to similar conclusions — that reasonable variations in the pervious concrete porosity, slab depth (a loss of 152 in. [13 mm]), and CN of adjacent areas have only a very slight effect, and that estimates of the infiltration rate should be accurate, but conservative. In this case, an increase in the infiltration rate to 0.8 in./h (2 cm/h) would decrease the runoff by 0.3 in. (7mm); a reduction in the infiltration rate to 0.3 in./h (0.7 cm/h) would increase the runoff by 0.4 in. (10 mm). These differences in runoff due to uncertainty in the infiltration rate are important, but may have only a marginal impact on behavior in service in many practical situations. The difference in estimated runoff is less than 10% of the precipitation.
These observations support the conclusions that it is important to properly classify the soil, that the effects of routine uncertainty in the estimate should be examined, and that the infiltration rate used in the analysis needs to be estimated conservatively when accurate estimates of the infiltration rate in service are not available. This is rarely a problem in sandy areas, but can be an issue in soils with considerable silt content.
The estimates in Table 2 based on soil type are reasonably conservative and include the positive effect of AMC assumptions. Table 2 can be used to estimate infiltration rates in preliminary and feasibility studies, or when it is difficult to obtain accurate estimates of infiltration in service. When the results of studies using these values clearly indicate acceptable performance in service, additional testing to develop more accurate estimates may not be needed.
5.6.3 Recommendations
Recognizing the limits of accuracy in the design assumptions, the uncertainty in all hydrologic models, the uncertainty in estimates of input parameters, including precipitation, and the inherent variability of construction materials, soil, and construction methods, the observations of the sensitivity analysis result in three primary recommendations:
1. Sensitivity analysis should be conducted using a reasonable range of estimates of the depth of the base course, porosity of the pervious concrete, the Curve Numbers of adjacent areas, and the infiltration rate of the soil;
2. Estimates of the infiltration rate need to be conservative — values in Table 2 are reasonable values, particularly for preliminary or feasibility studies; and
3. If the results of the basic analysis or the sensitivity study indicate a design with marginally acceptable hydrologic performance, the designer should modify the design such that it is no longer “borderline.”
This approach is generally preferable to trying to improve the accuracy or precision of the estimates with additional, extensive testing. The amount of excess storage capacity, generally obtained by providing additional depth of clean stone base course, will depend on the sensitivity of the project, regulatory requirements, and degree of uncertainty with estimates of inputs, which can vary significantly by location.
5.7 Design Factors in Cold Climates
Several additional factors must be considered in the design of pervious concrete pavement systems in areas with prolonged freezing temperatures: frost durability of the material, frost heave of the subgrade, and frost durability of saturated pervious slabs. Frost durability of the material must be ensured as with all concrete mixtures exposed to freezing temperatures. Frost heave of a saturated subgrade may cause excessive movement during long periods of freezing weather and may result in significant loss of subgrade support during spring thaws. Durability of the pervious concrete slab may be compromised if it freezes while completely saturated. This issue is linked to the draw-down time, and therefore the infiltration rate of subgrade materials at or above the frost line.
Frost durability of the material requires frost durable aggregates combined with frost durable paste. Frost durability of the paste is provided by the low water cement ratio common to pervious concrete and by using an air entraining admixture. The use of sand also improves frost durability.
5.7.1 Frost Heave
Frost heave occurs as moisture in certain soils migrates to existing ice formations resulting in the growth of ice lenses. The lenses can grow over time causing the pavement to move upward, resulting in an uneven pavement surface. The biggest problem with frost heave, however, is that the ice lens melts in the spring thaw reducing the ability of the subgrade to support load.
Frost heave is associated with fine grained soils and requires sufficient water supply. Sands and aggregates such as clean stone base are non-frost susceptible. Clearly the water supply will be adequate with a pervious concrete and frost heave must be considered in areas with susceptible subgrade soils. The techniques for mitigating potential damage associated with frost heave in pervious concrete pavement systems have not been fully established.
The techniques used with conventional pavement are to reduce the thickness of frost susceptible soil under the pavement and to increase the pavement thickness to accommodate the extra load carried by the surface course during the spring thaw. A sufficiently deep base course keeps the layer of frost susceptible soil between the bottom of the non-susceptible base and the frost line (below which no ice forms) thin enough to minimize damage. One rule-of-thumb is that the pavement system should extend to at least half of the depth of the frost line. Others recommend a more conservative approach of extending the depth to two-thirds the depth of the frost line (NRMCA). This may require a base course depth in excess of that required for storage capacity alone. The effects of ice and frozen soil on infiltration rates and draw-down time must also be considered. In areas with very deep frost penetration, alternate methods of draining the system may be required.
There are several factors which may help minimize distress associated with frost heave with many pervious concrete pavement systems, however. Minor pavement movement should cause few problems in areas of slow, relatively light traffic such as in automobile parking lots. Since a pervious concrete pavement system is designed for a saturated subgrade, the serious issue of subgrade support loss in the spring thaw may not be as critical with pervious concrete pavements as with conventional pavements.
5.7.2 Storage Capacity in Cold Climates
Determination of the required storage capacity of a pervious concrete pavement system in cold climates must include the effects of several factors. Except along the coasts, precipitation volumes are generally lower in winter months in most of North America subjected to freezing weather, but significant runoff can occur during spring thaws. The infiltration rate of frozen soil is very low, but the ground will not be frozen all winter, especially at the design frost depth. The effect of the latent heat of runoff and snowmelt on infiltration is not fully established and the effects of these factors can vary significantly with location. The storage capacity must be established, in general, such that freezing of a completely saturated pervious concrete slab will not occur. In many locations only minor adjustments to the pervious concrete pavement system may be required. When long-term freezing exposures are anticipated, such that infiltration will be essentially zero, additional methods to remove accumulated runoff may be required.
5.8 Comments on the Rational Method
The Rational method is commonly used to estimate the peak discharge of an area. The observation that runoff would occur from the site described in 5.4 if pervious concrete were used without a clean stone base even in high infiltration rate sandy soils, points one of the concerns in using the Rational method for pervious concrete pavement systems, even in well draining soils. The concerns are both technical and non-technical.
Short, intense storms, 15- to 30-minutes in duration, are typically used with the Rational method for small, urban watershed analysis. All of the rainfall from a storm of this duration could be stored in the structure, especially one which includes a clean stone base. In many situations, analysis indicates that all of the precipitation in the 2-year storm could be held in a pervious concrete pavement system as well, particularly in sandy areas with a high infiltration rate. It is tempting therefore to use a value of 0 for the rational C coefficient in the analysis, indicating that no runoff should be expected.
Using this value could be problematic in some circumstances for several reasons, however. Ignoring the antecedent rainfall effects on storage capacity could lead to a model which does not adequately estimate the actual runoff expected. It is easily possible to have a site over well draining sands that will have some runoff, even in a moderate storm, and, although peak flow will be reduced in any practical situation, the use of the Rational method fails to capture critical hydrologic features of the site. In addition, the Designer should consider the potential problems in obtaining permits for other sites when runoff has been observed or reported for a site with a “C = 0.” If a site containing a pervious concrete pavement system is designed to just hold the runoff in a 2-year storm, runoff in a 10-year storm is very likely; runoff in a 20-year storm is a virtual certainty. While a technical explanation is possible, the perception of value by all parties will likely have been compromised and subsequent developments may be forced to use less economical and less technically advantageous BMP’s.
The Rational method must be used with caution in designing pervious concrete pavement systems or in assessing the performance of these systems for technical reasons as well. The Rational method provides an estimate of peak flow rather than total runoff. Peak flow values are used in the design of outlet structures such as culverts or storm sewer pipes to ensure they can handle the largest volume occurring at any one time during the design storm, specifically including the effect of excess surface runoff traveling overland.
Since the intent of the designer is frequently to store some portion of the runoff temporarily until it can infiltrate into the underlying soil, the pervious pavement structure itself is the “outlet” and the permeability of the surface is typically well in excess of any rainfall rate during the design storm. The pervious concrete pavement system design methodology should consider the capture and infiltration of the design rainfall event as it occurs. The design professional may select the Rational method in the design of an outlet structure such as a culvert some distance downstream from the site. In these cases the pervious concrete pavement system can be analyzed separately and the value of C for the site estimated from the output of the analysis described here in Part 5. Alternately, and preferably, peak flow can be estimated using the methods of TR-55.
Table 12. Estimates of Rational Method C for Preliminary Studies
Estimates of the Rational method C for pervious concrete pavement systems were obtained based on back calculation from results of the Curve Number method described above and from the Graphical Peak Discharge Method in TR-55 (SCS 1986), along with separate regression analysis based on areas in the mid-Atlantic region (Malcom 2003). The values in Table 12 may be used for preliminary studies of pervious concrete pavement systems at least 6 in. (150 mm) thick in 2-year and 10-year storms. A minimum value of 0.05 is recommended. Values at the lower end of the range may be used if the pervious concrete pavement system includes a clean stone base.
Reference: Leming, M.L., Malcom, H.R., and Tennis, P.D., Hydrologic Design of Pervious Concrete, EB303, Portland Cement Association, Skokie, Illinois, and National Ready Mixed Concrete Association, Silver Spring, Maryland, USA, 2007, 72 pages. |
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