Pavements

ARCHIVED: Chapter 18

Chapter 18. Structural Design Of Recycled Pavements

Introduction

Structural design of pavements takes into consideration those aspects of design which provide required strength or stiffness to the pavement structure. The method has evolved from the application of engineering judgement to predominantly mechanistic or semi-mechanistic procedures. Pavement materials can now be characterized by resilient modulus and fatigue characteristics, and pavement materials with different strength and structure can be denoted by appropriate "structural numbers." Recycled asphalt materials can provide pavements similar or even better than pavements constructed with conventional hot mix asphalt. However, the wide range of properties of recycled mixes, resulting from variation in material and construction methods must be considered during structural design of recycled pavements.⁽¹⁾ On an average, the coefficients for both recycled surface and recycled base courses are found to be greater than the coefficients for respective conventional mixes determined in the AASHTO Road Test. The AASHTO guide indicates that in essence there is no difference between hot recycled and virgin HMA material, and recommends the structural rehabilitation analysis method (for conventional mix) for design of recycled pavements as well.⁽¹⁾ However, it also cautions that since long-term performance data is not available for recycled mixes, engineering judgement should always be applied for design of such mixes. In this chapter design guidelines recommended by AASHTO and the Asphalt Institute are discussed.

Structural Design Of Recycled Hot Mix Asphalt Pavement

AASHTO Method

The AASHTO guide⁽¹⁾ presents a method of overlay design based primarily on structural number, thickness of underlying layers, and drainage coefficients. Design of recycled pavements can be based on the same methodology.⁽¹⁾ Basically, a nomograph is used to calculate a combined total structural number for the whole pavement section, based on performance period, traffic, and change in Present Serviceability Index (PSI). The structural number can be represented by a combination of product of depth, structural number, and drainage coefficients for each of the pavement layers. The structural number of the recycled layer required is calculated by subtracting the effective structural number of the existing pavement from the structural number required by the "new pavement," which includes the recycled layer. The effective structural number of the existing pavement is modified by a remaining life factor for the existing pavement. The equation is as follows:

SN_OL = SN_Y - (F_RL X SN_xeff)

where:

Structural number (SN) is defined as follows:

SN = a₁D₁ + a₂D₂m₂ + a₃D₃m₃

where:

One important feature of the design method is the inclusion of reliability factor in traffic and performance prediction. The reliability design factor considers chance variations in both traffic predictions (the actual number of load applications during the analysis period in terms of equivalent 18-kip single-axle loads, ESAL) and performance prediction (the number of ESALs that will result in the pavement reaching a specified terminal serviceability level). A reliability level, R, and an overall standard deviation, So, are the required input parameters for calculating reliability. A higher value of R means a greater assurance of pavement serviceability for the design period, and hence a greater thickness and a higher cost. Values of So are based on pavement types and are available for flexible and rigid pavements.⁽¹⁾ The details of the design method are presented in the AASHTO guide.⁽¹⁾ A simplified flow chart is shown in figure 18-1.

Figure 18-1. Flow chart for AASHTO design method.

Asphalt Institute Method

For hot-mix recycling, the Asphalt Institute⁽²⁾ recommends the same design procedure as for conventional mixes. It has been recommended to use the method outlined in the Asphalt Institute's publication "Thickness Design - Asphalt Pavements For Highways and Streets."^(3,4) The parameters required for designing the pavement thickness include the following:

1. Equivalent 18-kip single-axle load (ESAL) applications. A simple traffic estimation procedure⁽⁴⁾ is presented in table 18-1. This can be utilized on the basis of roadway type. A detailed analysis can also be performed based on type of vehicle, truck factor for each vehicle, and single or multiple growth factors. The design ESAL is calculated by the summation of the products of number of vehicles and the corresponding truck and growth factors.

   Table 18-1. Traffic analysis.⁽⁴⁾

   Whenever possible the traffic analysis and design procedures given in The Asphalt Institute manual, Thickness Design - Asphalt Pavements for Highways and Streets (MS-1) should be used. However, in many cases it is necessary to estimate traffic using only limited information. In such cases, the following table may be used. Definitions The following definitions apply to the traffic analysis procedure: ESAL is defined as equivalent 80 kN (18,000 lb) single-axle load applications. It is the effect on pavement performance of any combination of axle loads of varying magnitude equated to the number of 80 kN (18,000 lb) single-axle loads required to produce an equivalent effect. Heavy trucks are described as two-axle, six-tire trucks or larger. Pickup, panel and light four-tire trucks are not included. Trucks with heavy-duty, wide-base tires are included. Traffic Classifications
   Traffic Class	ESAL	Type of Street or Highway	Approximate Range Number of Heavy Trucks Expected During Design Period
   I	5 x 103	Parking lots, driveways Light traffic residential streets Light traffic farm roads	5,000-7,000
   II	104	Residential streets Rural farm and residential roads	7,000-15,000
   III	105	Urban minor collector streets Rural minor collector roads	70,000-150,000
   IV	105	Urban minor arterial and light industrial streets Rural major collector and minor arterial highways	700,000-1,500,000
   V	3 x 106	Urban freeways, expressways and other principal arterial highways Rural Interstate and other principal arterial highways	2,000,000-4,500,000
   VI	107	Urban Interstate highways Some industrial roads	7,000,000-15,000,000

2. Resilient Modulus, MR of the subgrade. This can be determined by testing or through correlations with CBR or R-value, as presented in table 18-2.

   Table 18-2. Subgrade soil.⁽⁴⁾

   It is desirable to use laboratory tests to evaluate the load-supporting characteristics of subgrade soils. However, if laboratory test equipment is not available, designs may be made on the basis of a careful field evaluation by an engineer who can assign the subgrade soils to one of the following categories: Poor Subgrade Soils These soils become quite soft and plastic when wet. Included are those soils having appreciable amounts of clay and fine silt. The coarser silts and sandy loams may also exhibit poor bearing properties in areas where frost penetration into the subgrade is a factor. Typical properties: Resilient modulus = 30 Mpa (4,500 psi) CBR = 3, R-value = 20. Good to Excellent Subgrade Soils Good subgrade soils retain a substantial amount of their load-support capacity when wet. Included are the clean sands and sand-gravels and soils free of detrimental amounts of plastic materials. Excellent subgrade soils are unaffected by moisture or frost. They include clean and sharp sands and gravels, particularly those that are well graded. Typical properties: Resilient modulus - 170 Mpa (25,000 psi), CBR = 17, R-value = 43. The Asphalt Institute's Soils Manual (MS-10) describes in detail the commonly used soil evaluation systems and test procedures listed below. Field evaluation of the soil involves visual inspection and simple field tests. Resilient Modulus (Mr) A test used for evaluating the stress-strain properties of materials for pavement thickness design. California Bearing Ratio (CBR) A test used for evaluating bases, subbases, and subgrades for pavement thickness design. Resistance Value (R-value) A test used for evaluating bases, subbases, and subgrades for pavement thickness design.

3. Type of surface and base. The total required pavement thickness can be calculated by entering the design traffic and MR values in the design charts. In the comparison of properties of recycled materials to those of new materials, the recycled materials are to be considered equivalent to conventional mix in the design procedure.

The overlay design procedure, as outlined in the Asphalt Institute Manual, Asphalt Overlays for Highways and Street Rehabilitation,⁽⁵⁾ can also be used for thickness design. The overlay thickness is calculated as the difference between the thickness required by a new pavement to the design traffic ESAL and the effective thickness of the existing pavement. The effective thickness of the existing pavement can be determined by either of two methods. In one method a condition rating, the Present Serviceability Index, and equivalency factors for converting various pavement materials to equivalent thicknesses of asphalt concrete (figure 18-2 and table 18-3) are used. The second method uses the conversion factors for each pavement layer (based on the condition of each layer prior to overlay) to directly convert each layer to an equivalent thickness of asphalt concrete (table 18-4). Figure 18-3 shows the recommended chart for determining the thickness of a full depth HMA pavement for new construction. The effective thickness of the existing pavement should be subtracted from the thickness of the recycled layer. A simplified flow chart for the Asphalt Institute Design method is shown in figure 18-4. Three examples for determining the thickness of recycled layers are shown in figure 18-5.⁽⁵⁾

Other Design Methods

The National Stone Association method can also be used for design of hot recycled mix. The method is based on the Corps of Engineers method and mechanistic design procedures. The mechanistic design process assumes that the pavement can be modeled as a multilayered elastic or viscoelastic structure on an elastic or viscoelastic foundation, and stress, strain and deformations are calculated accordingly. A number of computer programs are available which can determine pavement responses (stress, strain) at different locations with the help of wheel load data, material properties, such as Elastic Modulus and the Poisson's ratio and the thickness of layers. Such computer programs include CHEV5L (Chevron Research Co.), BISTRO and BISAR (Shell Oil Co.), ELSYM5 (University of California at Berkeley), PDMAP (NCHRP 1-10B), and DAMA (The Asphalt Institute). The VESYS program developed by the Federal Highway Administration uses the viscoelastic approach for calculation of pavement responses.

Different state DOTs have also developed their own pavement design methods of which very few employ the direct use of the mechanistic design procedures.

Structural Design Of Recycled Cold-Mix Asphalt Pavements

Two main types of design methods are available for design of cold-mix recycled layers. One method uses the pavement layer coefficients and the other involves the characterization of pavement as a multi-layered elastic system. The AASHTO method, which uses the layer coefficient method, and the Asphalt Institute method, which is an example of multi-layered elastic structure approach, are discussed below.

AASHTO Method

The 1986 AASHTO Design Guide⁽¹⁾ presents the method of using a structural number, SN, which is a combination of layer coefficients and layer thicknesses for the various layers in the pavement. The required SN for a particular reliability level, R, and overall standard deviation, S_o, the estimated traffic level (ESAL) for the design period, the effective resilient modulus of the roadbed soil or the subgrade and the serviceability loss in terms of the Present Serviceability Index (PSI) can be determined from nomographs. A factor for including the effect of drainage conditions is also included for each of the unbound layers. The SN equation is as follows:

SN = a₁D₁ + a₂D₂m₂ + a₃D₃m₃

where:

Figure 18-2. Conversion factors.⁽⁵⁾

Notes:

• * Values and ranges of Conversion Factors are multiplying factors for conversion of thickness of existing structural layers to equivalent thickness of asphalt concrete.
• ** Originally meeting minimum strengths and compaction requirements specified by most state highway departments.

Figure 18-3. Design chart for full-depth asphalt concrete.⁽⁵⁾

Figure 18-4. Flow chart for Asphalt Institute design method.

Figure 18-5. Design examples for hot recycled mix.

If the recycled layer is treated as an overlay (above part of the original pavement) the equation for the structural number of the overlay is as follows:

SN_OL = SN_Y - ( F_RL X SN_xeff )

where:

The resulting overlay thickness would include the thickness of the cold recycled layer plus the thickness of the asphalt concrete surface layer, if used. Table 18-5 shows the typical AASHTO structural layer coefficients obtained from a variety of recycled test sections using several types of recycled material (a refers to layer coefficient). These values were derived from the results of AASHTO Road Test and layered elastic programs. Layer coefficients for cold-recycled mixes can be derived from these values. Coefficients of foamed-asphalt recycled layers were found to range from 0.20 to 0.42 with a midpoint value of 0.31 according to a study reported in 1984.⁽⁶⁾ The range for emulsion recycled layer ranged from 0.17 to 0.41 with a midpoint value of 0.29. A value between 0.30 and 0.35 can be considered appropriate for cold recycled mixes, as compared to a value of 0.44 for hot mix asphalt concrete.⁽³⁾ However, the structural coefficient of cold recycled mixes is dependent on several other factors such as cure rate, and must be evaluated on the basis of sound engineering judgement.

Note:

• a_i - Layer Coefficient.

Asphalt Institute Method

The thickness design method presented in The Asphalt Institute Manual for Cold-Mix Recycling⁽⁷⁾ is based on the use of emulsified asphalt mixes but is considered applicable for cold-recycled mixtures made with other types of asphalt binders such as asphalt cement. The required input parameters include estimated design traffic level and subgrade strength. Design charts, shown in figures 18-6 and 18-7⁽¹⁾ can be used to determine the thickness of the recycled layers.

Figure 18.6. Design charts (metric units) for recycled cold mixed Type A.⁽⁷⁾

Figure 18.7. Design charts (metric units) for recycled cold mixed Type B.(7)

Traffic is classified by ESAL, type of street or highway, or by volume of heavy trucks (table 18-2, mentioned before). The subgrade support is classified by type of subgrade or obtained from Resilient Modulus, CBR, or R-value test data (table 18-3, mentioned before). The mix can be classified into two types - Type A and B. Type A is the mix which consists of semi processed, crusher run, pit ran or bank run aggregates, mixed in central plants or by travel plants (figure 18-6). Type B includes mixes which use sands or silty sands, mixed in central plants, or by travel plants, rotary mixers or motor graders. This type of mix also includes Type A aggregate (as explained in table 18-6) when mixed by rotary mixer or motor grader (figure 18-7). The output from the design chart gives the combined thickness of a recycled cold-mix base and an asphalt surface course. Table 18-7⁽⁷⁾ shows the recommended thicknesses of asphalt surfaces over cold-mix recycled bases. A surface course of asphalt concrete or emulsified asphalt mix Type I (plant mixed, laboratory designed, emulsified asphalt mixes made with dense graded aggregate and having properties similar to asphalt concrete) may be substituted for a portion of the thickness of emulsified asphalt Type A or B mix obtained from the design chart. When Type I emulsified asphalt mix is used, a single or double surface treatment should be used as a wearing course, but this should not be substituted for any of the thickness obtained from a design chart. For light traffic conditions, ESAL less than 104, a surface treatment may be placed directly on, but should not be substituted for. any portion of the thickness of Type A or B emulsified asphalt mix obtained from a design chart. Two design examples are shown in figure 18-8⁽¹⁾

Notes:

1. Equivalent 80 kN (18,000 lb) single-axle load applications.
2. Single or double surface treatment.
3. Asphalt concrete or Type 1 emulsified asphalt mix with a surface treatment.

Structural Design For Asphalt Surface Recycling

The load carrying capacity of an existing pavement cannot be improved by asphalt surface recycling, since this method is used for only 50 mm (2 in) or less depth of the pavement. The improvement can be effected only by improving the existing HMA mix. Surface distress can be removed but structural or subgrade problems which cause the distresses cannot be eliminated by this method.⁽³⁾ The thickness of the overlay will depend on the purpose of recycling. If the objective is to rejuvenate the upper layer of the existing material and improve the ride quality of a structurally adequate pavement, then a minimum thickness should be considered on the basis of the maximum size of the aggregate used for the overlay mix. In general, the thickness of the overlay should not be less than 1½ times the maximum particle size in the new mix.⁽³⁾ On the other hand, if the primary purpose is to increase the load carrying capacity of the mix, then the overlay should be designed according to conventional methods to yield the required strength. Depending on the specific need of the overlay, the thickness can range from 25 mm to 100 mm (1 to 4 in).⁽⁸⁾

The thickness of the overlay will also depend on the construction method, since extra structural capacity can be added

1. through an overlay after heating, scarifying, rejuvenating, and compacting the recycled mixture,
2. by blending virgin mix with scarified old mix prior to compaction (re-mixer process), or
3. by overlaying the loose, scarified, and rejuvenated old mix with loose, virgin mix and compacting both at the same time (re-paving process).

Either of the three methods can produce an acceptable surface. When the two mixes are not mixed together, some extra structural advantage can be obtained, but only the new mix is considered to comprise the overlay. An allowance can be made for the rejuvenating effect of the recycling process in determining the effective thickness or remaining life of the existing pavement. The material that has been recycled with a modifier will provide a soft layer that can act as a stress-relieving layer. This layer acts as a barrier in crack propagation through the new surface, especially when thin overlays are used. It is possible that a 25-mm (1-in) overlay over 25 mm (1 in) of recycled material can provide better performance than 65 mm (2½ in) of new overlay over the original surface.⁽⁹⁾ If the recycled material is mixed with new aggregate or asphalt concrete mix, then the resulting additional thickness is considered as an overlay. Both mix design and structural design would be the same as for hot mix recycling.

Summary

The method of structural design, which provides the required strength to a pavement structure, has evolved from an empirical to a semi-mechanistic procedure. Since hot recycled asphalt material can provide similar or even superior performance compared to conventional hot mix asphalt, the AASHTO design guide indicates that there is essentially no difference between the recycled and virgin materials, and recommends the structural rehabilitation analysis method for conventional mix for design of recycled pavements as well.

The AASHTO method for design of hot mix recycled asphalt is based on the derivation of the structural number required for the pavement with the help of design traffic, reliability level of prediction of traffic and performance, performance period, and the pavement condition rating. The structural number can be expressed as the sum of the product of the depth, layer coefficient, and drainage coefficient of each of the layers. The structural number for the recycled layer, which can be treated as an overlay, can be calculated as the difference between the structural number required by the finished pavement and the structural number of the existing pavement. Values of layer coefficients are also presented in the AASHTO design guide. The Asphalt Institute method uses the traffic level, the subgrade resilient modulus, and the type of surface and base to calculate the design thickness. In this method also, the hot recycled material can be considered to be similar in performance to conventional hot mix. In another Asphalt Institute procedure, the recycled layer can be considered as an overlay and its thickness can be calculated as the difference between the total thickness required by the pavement and the thickness of the existing pavement. The total thickness required can be determined on the basis of condition rating of the pavement and a method of converting and expressing each type of material or pavement layer as equivalent thickness of asphalt concrete layer. Other methods include design procedures based on load-deformation response calculation by computer methods with the help of loading and material properties of the pavement layers. In such methods the pavement is assumed to behave as an elastic or viscoelastic laver on an elastic or viscoelastic layer.

The AASHTO design method for cold recycled mixes is similar to the design method for the hot-mix asphalt. However, layer coefficients for cold-recycled mixes are dependent on construction methods, and should be determined on the basis of engineering judgement. The Asphalt Institute method assumes the pavement as a multilayered elastic structure, and determines the required thickness on the basis of design traffic and subgrade strength. The combined thickness of cold-recycled base and surface course is obtained from charts. The thickness of cold-recycled bases can be obtained by taking into consideration the recommended thickness of hot mix asphalt overlay on the cold-recycled base.

Since asphalt surface recycling does not normally improve the structural capacity of an existing pavement, there is no method for thickness design of surface recycling. However, the thickness of any overlay should be based on conventional overlay design method. If the overlay is meant to improve the ride qualities only, then the minimum thickness should be based on the maximum aggregate size used in the mix.

References

1. American Association of State Highway and Transportation Officials (AASHTO). AASHTO Guide for Design of Pavement Structures, Washington, DC, 1986.
2. Asphalt Institute. Asphalt Hot-Mix Recycling, Manual Series No. 20 (MS-20), College Park, MD, 1986
3. Pavement Recycling Guidelines for Local Governments - Reference Manual, Report No. FHWA-TS-87-230, FHA, U.S. Department of Transportation, Washington, DC, 1987.
4. Asphalt Institute. Thickness Design: Asphalt Pavements For Highways and Streets, Manual Series No. I (MS-1), College Park, MD, September, 1981.
5. Asphalt Institute. Asphalt Overlays for Highways and Street Rehabilitation, Manual Series No. 17 (MS-17), College Park, MD, June, 1983.
6. A.J. Van Wijk. Structural Comparison of Two Cold Recycled Pavement Layers, Transportation Research Record 954, TRB, National Research Council, Washington, DC, 1984.
7. Asphalt Institute. Asphalt Cold-Mix Recycling, Manual Series No. 21 (MS-2 1), College Park, MD, 1986.
8. G.F. Whitney. Urban Surface Recycling, Transportation Research Record 780, TRB, National Research Council, Washington, DC, 1980.
9. FHWA. Techniques for Pavement Rehabilitation, Participants Manual for Training Course, National Highway Institute, Washington, DC, 1982.