Crumb Rubber Modifier in Asphalt Pavement

Chapter 2

Thickness Design Considerations

There is significant variation in the use of CRM-HMA in the United States. As pointed out earlier, the three States selected for inclusion in this early technology transfer document have substantial experience with CRM materials, particularly the wet process. The bulk of this experience was gained through the use of CRM mixes in functional and structural overlays of existing HMA and, to a lesser degree, portland cement concrete (PCC) pavements. CRM-HMA has been used in new pavement construction in these States mainly as a wearing course. With these limitations in mind, the design procedures described here necessarily reflect the experience of these States, and may not be appropriate for designs of pavements utilizing dry process CRM mixes and for other environmental regions in the United States.

Typical Applications

Arizona, California and Florida typically use CRM mixes in functional and some structural overlays of existing pavements. Florida uses mixes containing CRM as the surface friction course in new construction as well as rehabilitation. For purposes of this document, functional overlays are distinguished from structural overlays based on thickness and purpose.

Functional surfaces or resurfacings are generally less than 40 mm (1.6 in) thick and have as their main purpose the improvement of ride quality, skid resistance or prevention of pavement deterioration. All overlays placed in California are expected to satisfy ride quality, structural adequacy, and reflection crack control for a 10-year design period. Thirty-mm (1.2-in) overlays are placed under the Capital Preventative Maintenance program and must satisfy a 5-year design. When ride quality is poor, California Department of Transportation (CALTRANS) uses a two-pass operation consisting of a 30-mm (1.2-in) cold plane followed by a 30-mm (1.2-in) inlay.

In contrast, structural overlays are generally greater than 40 mm (1.6 in) thick and increase the load-carrying capacity of the roadway, normally in anticipation of increasing future traffic or to extend the life of the pavement. Only California makes use of CRM-HMA as a structural overlay.

New Construction

CRM materials have been used in new construction to a limited degree in all three States. Florida and Arizona use the American Association of State Highway and Transportation Officials (AASHTO) methodology (1972 and 1986, respectively) for their new pavement designs. If CRM materials are to be used in the new design, the mixes are assigned the same layer equivalency as a comparable nonmodified mix except as noted below. Typically, the CRM materials are used only in the wearing course.

CALTRANS has drafted a suggested modification to their new pavement design procedure that allows a slight reduction in the overall structural thickness when CRM mixes are used as the surface course. This modification is based on preliminary performance experience and results from early experimental lab tests. Long-term performance tests have not been considered. This preliminary design procedure is described below.

This modification is based on preliminary performance experience and results from early experimental lab tests. Long-term performance tests have not been considered. This preliminary design procedure is described below.

Functional and Structural Overlays

Florida has several years' experience with CRM as the final wearing courses. Florida DOT uses crumb rubber in three products: membrane interlayers (not discussed further), open-graded, and dense-graded friction courses. Open-graded friction courses (OGFC's) are required on all multilane facilities with design speeds equal to or greater than 73 km/h (50 mi/h). NQ structural value is typically assigned to the 6GFC. Dense-graded friction courses (DGFC's) are used where an OGFC is not required, such as in urban areas with average daily traffic (ADT) in excess of 3,000.

Of the agencies included in this study, agencies in Arizona have the longest history with CRM materials, dating back to the mid-i 960's. Much of the early experience was with stress-absorbing membranes (SAM's) and stress-absorbing membrane interlayers (SAMI's) when from 1974 to 1989, about 1,125 km (700 mi) of State highways were constructed using these materials. Asphalt rubber continues to be used in relatively thin, functional overlays of existing AC pavements. For example, the city of Phoenix places a gap-graded, wet process mix in thicknesses of approximately 30 mm (1.2 in) as part of a routine preventive maintenance program. The thickness design is based on experience and probably cannot be transferred to other agencies, due in part to the unique environmental conditions in Phoenix.

Arizona DOT uses asphalt-rubber mixes to reduce reflection cracking, improve surface course durability, and reduce noise in urban areas. Typical lift thicknesses for open-graded mixes are 12 to 25 mm (1/2 to 1 in) and 25 to 50 mm (1 to 2 in) for gap-graded mixes. The OGFC mixes are considered a functional overlay used to improve safety, aesthetics, and ride. The gap-graded mixes are used on low volume, badly cracked pavements; however, they have also been used on the interstate highways.

Design Methodologies

All three agencies base functional overlay thicknesses on agency experience. Two of the three agencies (Arizona and Florida) studied use older versions of the AASHTO overlay design procedures^(1,2) to determine the thickness of structural overlays. California has modified their structural design approaches for overlays and new construction to accommodate CRM mixes. Following are details of each State's procedure:

Arizona

The city of Phoenix uses CRM materials in 30-mm (1.2-in) functional overlays without a documented design procedure. Arizona DOT does not have a documented procedure for thickness substitution of gap-graded CRM materials for nonmodified dense-graded mixes. Reduction in thickness is based primarily on engineering judgment. A typical substitution would be 37 mm (1.5 in) gap-graded CRM material for 64 mm (2.5 in) of nonmodified dense-graded mix. The selection of asphalt rubber over conventional mixes is based on pavement management system (PMS)-derived performance information. Specifically, CRM mixes have shown better crack reflection resistance and durability. CRM-HMA sections have been monitored through the Arizona PMS for nearly 10 years.

California

California is the only State among the three included in this report that has made significant modifications to their structural design procedure to accommodate CRM materials. The modifications were based on careful observations of a wide variety of CRM mixes (wet, dry-process with dense-, open-, and gap-graded aggregates) placed throughout the State beginning in 1978.^(3,4) The implementation of reduced thickness overlays has been limited to selected applications. Investigations of CRM projects continue, and additional modifications to California Department of Transportation (CALTRAN S) design procedures may be warranted, based on field performance. Although the procedure described below for new construction has not been formally implemented by CALTRANS, both rehabilitation and new pavements are considered. CALTRANS is currently conducting extensive laboratory and field testing. The early field-testing program makes use of accelerated pavement testing using the South African Heavy Vehicle Simulator (HVS).

California is working on a design procedure based on their standard methodology that would allow partial substitution of CRM mixes for conventional HMA as the wearing course in new construction. A draft of this procedure is shown graphically in figure 1. The draft procedure would allow substitution of up to 60 mm (2.4 in) of CRM material in place of a conventional dense-graded asphalt concrete (DACE).

As an example, consider a pavement with a Traffic Index (TI) of 11. The design that does not include CRM would require about 182 mm (7.2 in) of DACE over class 2 aggregate base. As an alternative, 60 mm (2.4 in) of CRM mix could be placed over 75 mm (3 in) of DACE over the same aggregate base, saving approximately 45 mm (1.8 in) of mix. (It must be noted that this methodology is subject to change and does not constitute an accepted CALTRANS design standard.)

The structural design of overlays is based on several assumptions as well as field observations. These assumptions were first described in a memo prepared in March 1992 and reviewed in October 1993. The assumptions follow in figure 1 on page 2-4 50 that other agencies may judge the appropriateness of the CALTRANS procedure to their circumstances. They include the following:

The most promising form of CRM is asphalt-rubber hot mix-gap graded (ARHM-GG). Additional work with dense-graded asphalt-rubber mixes and dry process mixes is being undertaken to confirm this assumption.
ARHM-GG is recyclable. CALTRANS successfully recycled CRM-HMA in 1994 and is considering recycling additional jobs in 1995.
Cracking in overlays is caused by a combination of traffic loads and movement of the underlying pavement.
Even thinner layers of asphalt rubber hot mix-gap graded (ARHM-GG) may be appropriate but, to limit risk, higher equivalencies should be considered experimental. Furthermore, construction considerations, such as heat retention and surface irregularities in the surface being covered, limit the minimum thickness of ARHM-GG to 30 mm (1.2 in).
ARHM-GG may be little or no better than conventional dense-graded AC in preventing cold-weather-induced transverse cracking.
The degree of stiffening provided by a specific thickness of ARHM-GG is less than the amount of stiffening provided by the same thickness of DACE. Thus, the after-overlay ratio of tolerable deflection actual deflection for ARHM-GG may become less favorable when the ARHM-GG thickness is greater than 60 mm (2.4 in).
ARHM-GG can withstand considerably higher deflection than the same thickness of DACE without cracking.
The mild climate structural equivalency of ARHM-GG for nonexperimental work is less than or equal to twice that of DACE. Some recent performance observations may allow the elimination of the "mild climate" from this assumption.
The mild climate structural equivalency of a SAMI is <=15 mm (0.6 in) of ARHM-GG (i.e., <=30 mm [1.2 in] of DACE; assuming that the SAMI reduces that portion of the total overlay stress caused by reflection of underlying cracks/joints) Again, recent performance observations may allow the elimination of the "mild climate" from this assumption.
The reflection crack retardation equivalency of ARHM-GG is considerably greater than that of DACE.
The reflection crack retardation equivalency of a SAMI is 30 mm (1.2 in) of ARHM-GG when used in conjunction with ARHM-GG, and equivalent to 30 mm (1.2 in) DACE when used with a DACE over an untreated aggregate base.
There may be stability problems if ARHM-GG is placed thicker than 60 mm (2.4 in), even if multiple lifts are used.

Table 1
California Structural Equivalencies (in mm)
(from CALTRANS memorandum dated February 28, 1992)

Dense-Graded Asphalt Concrete Asphalt-Rubber Hot Mix: Gap Graded^1,5 Asphalt Rubber Hot Mix: Gap Graded on a Stress-Absorbing Membrane Layer

45 (1.8 in) 30² (1.2 in) -

60 (2.4 in) 30 (1.2 in) -

75 (3.0 in) 45 (1.8 in) 30 (1.0 in)

90 (3.6 in) 45 (1.8 in) 30 (1.2 in)

105 (4.2 in) 60 (2.4 in) 45 (1.8 in)

120 (4.8 in) 60 (2.4 in) 45 (1.8 in)

135 (5.4 in) 45³ (1.8 in) 60 (2.4 in)

150 (6.0 in) 45⁴ (1.8 in) 60 (2.4 in)

165 (6.6 in) 60³ (2.4 in) 45³ (1.8 in)

180 (7.2 in) 60⁴ (2.4 in) 45⁴ (1.8 in)

Notes:

The maximum allowable nonexperimental equivalency for ARHM-GG is 2:1.
The minimum allowable ARHM-GG lift thickness is 30 mm (1.2 in).
Place 45 mm (1.8 in) of new DACE first.
Place 60 mm (2.4 in) of new DACE first.
ARHM-GG may not prevent cold-weather-induced transverse cracks. ARHM-DG is used in lieu of ARHMGG in areas subject to snow chain wear.

California's overlay design procedure consists of the following basic steps:

Determine the thickness of conventional DACE required for the structural needs of the existing pavement (based on deflections and structural section stiffening using current CALTRANS procedures).
Determine the thickness of conventional DACE required to improve ride quality and retard reflection cracking (using current CALTRANS procedure).
Select a DACE overlay thickness that satisfies the requirements of 1 and 2 above. 2-4 Chapter 2: Thickness Design Considerations
Use either table 1 or table 2 to determine the ARHM-GG equivalent sections, with and/or without SAMI's. Use table 1 if structural needs control and table 2 if reflection crack retardation controls.
If the ride score of the pavement to be rehabilitated is greater than or equal to the allowable ride score of 45 and there is no structural need per 1 above, use the following strategy, if reflective cracking per 2 above is satisfied:
- Place one 60-mm-thick (2.4-in) lift of ARHM-GG; or
- Cold plane to a depth of 30 mm (1.2 in), then place ARHM-GG as determined in steps 1 through 4 to satisfy a two-pass operation.