Design Methods for Hot-Mixed Asphalt Rubber Concrete Paving Materials INTRODUCTION

Transcription

1 Design Methods for Hot-Mixed Asphalt Rubber Concrete Paving Materials James G. Chehovits INTRODUCTION The properties and use of asphalt-rubber materials for various paving and maintenance activities have been well documented in the literature. The majority of this literature reports on properties of asphalt-rubber materials (1-12), and use of asphalt-rubber in stress absorbing membranes (SAM) and interlayers (SAMI) (13-25). Research studies have shown that asphalt-rubber materials have significantly modified physical properties when compared to asphalt cement. These modifications include increased high temperature modulus, viscosity, and toughness; increased elasticity, reduced temperature susceptibility, and less age hardening. In several states and countries abroad, use of asphalt-rubber in SAM and SAMI applications has become routine construction practice for rehabilitating deteriorated pavements and extending overlay life. In the United States, asphalt-rubber materials have been used on a limited basis as the binder for hot-mixed asphalt concrete pavements. A recent survey by the Asphalt-Rubber Producers Group (ARPG) indicated that at least 35 projects which used asphalt-rubber binder were placed between 1975 and 1987 in 12 states. Several research studies have been completed which investigated use of asphalt-rubber as an asphalt concrete paving binder (26, 27). Additionally, there have been several well documented field test projects placed by several states (28, 29, 30, 31). The literature indicates that several other countries have used asphalt-rubber binders for hot-mixed paving (32, 33, 34). Limited information is currently available on methods to be used for designing hot-mixed asphalt concrete mixtures when using asphalt-rubber binders. Vallerga (35) has suggested several specification changes which should be made when using asphalt-rubber. Hoyt and Lytton (27) reported on a mixture design procedure for asphalt-rubber paving mixtures which was used in a lab research program that studied the feasibility of using asphalt-rubber binder in dense graded airfield pavements. The purpose of this paper is to describe design procedures which have been developed since 1984 as the result of approximately 30 hot-mixed paving projects which used asphalt-rubber binder. Procedures for selecting the asphalt-rubber material proportions and resulting desired properties, mixture aggregates, and binder contents for dense, open, and gap graded mixture types are presented along with suggested construction guidelines and specifications. 1

2 FACTORS WHICH INF~UENCE ASPHALT-RUBBER PROPERTIES The interaction which occurs between asphalt and recycled rubber has been shown to be dependent on a variety of factors: Asphalt physical and chemical properties Rubber physical and chemical properties Time Temperature Mixing conditions (high shear or low shear) Use of ad~itives Each of these factors needs to be considered when developing an asphalt-rubber formulation for a specific use. Asphalt Cement Chemical and physical properties of the asphalt influence several properties of the asphalt-rubber. Stiffness (asphalt grade) and temperature susceptibility will influence high temperature and low temperature performance of the asphalt-rubber. Chemical make-up of the asphalt will influence the degree of interaction which occurs between the asphalt and the rubber. Asphalts which have higher degrees of aromaticity tend to dissolve and interact with rubber to a greater degree than asphalts with lower aromatic contents. Rubber Several characteristics of the rubber influence properties of the asphalt-rubber blend. Physical rubber characteristics including particle size (gradation), shape (angular or elongated), surface texture (as influenced by grinding method), and contaminant presence (fibers, etc.) influence properties of the asphalt-rubber. Chemical compositional characteristics also influence blend properties. These characteristics include rubber hydrocarbon content, specific type of rubber polymer or blends (amount of SBR and natural), plasticizer content, and reinforcement type and content (carbon black and other materials). Time and Temperature Various research projects have shown that the time exposure and temperature of the asphalt-rubber blend influence physical properties. Increased time results in greater interaction. Increased temperature results in quicker interaction. When physical properties of asphalt-rubber are monitored, the material will thicken (increase in viscosity) as the rubber particles swell in the asphalt. After a period of time, depending on temperature and materials properties, the rubber will begin to break down (devu1canize and melt), and viscosity will reduce. Rate of devu1canization will also be influenced by the mixing conditions. Because of these influences, it is important that asphalt-rubber blends be checked for appropriate properties at a variety of time periods which can occur during actual use. 2

3 Mixing Conditions The amount of shear, or intensity of mixing, will influence the properties of are designed to' the asphalt-rubber. Production mixing insure uniform wetting and suspension systems of the rubber particles in the asphalt. It is important that lab mixing procedures do not subject the asphalt-rubber to excessive amounts of shear which could quicken the rubber devulcanization process. Additives Extending oils can be used to soften the material for improved low temperature performance and for improving the degree of interaction between the asphalt and rubber. Adhesion agents commonly used in asphalt paving (heat stable anti-stripping agents) can be used to improve film stripping resistance. Diluents which are used in asphalt-rubber chip seal application must not be used in hot-mixed applications. TEST METHODS FOR ASPHALT RUBBER The physical properties of asphalt-rubber have been shown in a variety of studies to be substantially different than for unmodified asphalt cement (1-12). Many of these studies have used common asphalt test procedures as well as non-standard procedures to attempt to quantify the modified physical properties of asphalt-rubber. The non-standard procedures include Schweyer Rheometer (2, 3), sliding plate viscometer (1,3), force ductility (2.3.8,10). torque fork viscosity ( ). mechanical spectrograph (I). and several others. Physical attributes of asphalt-rubber which should considered for hot-mixed applications include: Viscosity at high temperatures for appropriate mixing and compaction characteristics. Consistency at high pavement experienced during the summer. Consistency at moderate temperatures. surface temperatures e Elasticity Low temperature characteristics Testing methods which can be used to evaluate these attributes are as follows: Viscosity The Viscosity of asphalt-rubber materials temperatures ( o F) can easily be monitored using type viscometers such as a Haake hand held portable (10) or a Brookfield viscometer (ASTM 03236)(36). at high rotational viscometer 3

4 High Temperature Consistency The consistency of- asphalt-rubber at high pavement surface temperatures can be indicated by several procedures including Ring and Ball Softening Point (ASTM 036), Modified Flow (ASTM 03407), or a cone penetration at F (ASTM and 05)(37). Moderate Temperature Consistency Moderate temperature (77 0 F) consistency can be evaluated easily by using the ASTM Cone Penetration test at 77 0 F (37). Elasticity The elasticity of asphalt rubber can simply be evaluated using the ASTM 0~407 resilience test which indicates the amount of rebound under a 75 gram load at 77 0 F (37). Low Temperature Characteristics Several test method types can be used to provide an indication of low temperature properties. These include Cone Penetration (ASTM 03407) at 32 or F; Ductility at F (ASTM 0113) and Low Temperature Flexibility (Modified ASTM C 711, section 7.2 using a 90 0 bend in 10 seconds at lower and lower temperatures until fracture occurs) (37,38). SELECTION OF ASPHALT RUBBER FORMULA The physical properties of asphalt-rubber depend on the ingredients and interaction conditions. Therefore, to obtain the desired properties, appropriate ingredients and interaction conditions must be chosen. These choices which must be made are: e Selecting the asphalt cement source and type. Selecting the rubber source and type. Selecting the rubber content. Selecting the interaction conditions. Additionally, it is important that the asphalt-rubber material have appropriate stability of properties, since properties vary with interaction time and time of interaction can vary during actual use. Therefore, testing of an asphalt-rubber blend of ingredients during project formulation studies should be performed not just at a single interaction period, but at a variety of interaction periods to evaluate stability and retention of properties. A procedure for interacting the asphalt-rubber for 24 hours during formulation studies is contained in Appendix 1. Physical properties can be evaluated from samples poured at 30, 60, 90, and 120 minutes of interaction to identify property retention during normal usage periods after completion of field mixing. Tests at 6 hours can identify properties of the blend if 4

5 a job delay occurs and the asphalt-rubber is used the same day. Tests at 24 hours (with e~posure from 6 to 22 hours at a lower temperature to simulate overnight unheated storage) can indicate stability of properties if the asphalt-rubber is to be used the next day. Suggested physical property limits for asphalt-rubber materials for hot-mixed asphalt-concrete applications are listed in Table 1 for hot, moderate, and cold climates. Asphalt-Cement The grade of asphalt cement used in asphalt-rubber, is a major influence on blend properties over the entire temperature range. Asphalt cement for asphalt-rubber should meet appropriate specifications for paving use such as ASTM or Typical grades used range from AC-2.5 to AC-20, or penetration to penetration. It is important that the specific asphalt cement be compatible, or capable of interacting with the specific rubber being used. This is indicated by appropriate increases in viscosity during the interaction period. Since the interaction with rubber results is an increase in high temperature modulus, asphalt cements used are typically somewhat softer than usual unmodified asphalts used for similar applications. Figures 1,2, and, 3, show results of constant load creep tests performed on asphalt-rubber materials which contained a variety of asphalt-cement grades each with 17% by total weight of a minus 20 mesh tire rubber. Testing was performed using a procedure reported by Cotzee and Monismith (24). Data shown in these figures indicates that addition of rubber produces a stiffening effect at moderate temperatures (74 0 F) which is approximately equivalent to using an asphalt which is 2 to 3 grades harder. At 100oF, the effect is even greater. At low temperature (39 0 F) however, the effects of stiffening are much less. For the asphalts which were approaching the brittle point (the AC-20 and AC-7.5) at 39 0 F, creep with rubber was very similar to creep of the unmodified asphalt. This data therefore indicates that asphalt-rubber materials and unmodified asphalt creep at similar rates at low temperatures near the base asphalt brittle point. However, as temperature increases, the asphalt cement reduces in stiffness to a greater degree than the asphalt-rubber as indicated by reduced creep of the asphalt-rubber. The effect becomes greater as temperatures increase. The data suggests that it is possible to use asphalt cements in asphalt-rubber which are softer than the normal unmodified paving grade used for the specific application to provide reduced stiffness (higher degrees of creep) at low temperatures and increased stiffness (reduced creep) at high temperatures. 5

6 Base asphalt grades which have been found to produce asphalt-rubber materiala meeting the hot, moderate, and cold climate property limits of Table 1 are as follows: Climate Type Base Asphalt for Asphalt-Rubber AR Grade AC Grade Penetration Grade Hot AR-4000,AR-2000 AC-20,AC , Moderate AR-1000 AC-5,AC , Cold AR-1000 with 3% to 6% extender AC 2.5 with 0-3% extender with 0-3% extender Various extending oils can be used in asphalt-rubber materials to increase the interaction between the asphalt and rubber, and to improve low temperature performance by decreasing the stiffness of the asphalt-rubber when softer asphalt grades are not available. Extender types used are generally napthenic or aromatic petroleum oils which have a minimum flash point of F. For most common oils with viscosities between 500 and 3000 SUS at F, approximately 3% by asphalt weight is required to soften an asphalt an equivalent of 1 grade. Typically, each percent of extender oil lowers ring and ball softening point (ASTM 036) results by 1.5 to 2.0 degrees Farenheit. Rubber Rubber used for asphalt-rubber should be primarily made from recycled pneumatic tires. The rubber should be ground on appropriate systems, and should be free from contaminants including mineral matter, fiber and metal. The rubber should be sufficiently dry to prevent foaming when added to hot asphalt. Generally this means a moisture content of less than 0.75%. Mineral contaminants should not exceed 0.25%. The rubber may be produced from buffings, whole tire, or stampings. The rubber hydrocarbon content should be between 40 and 50% and should be relatively uniform throughout the rubber shipment. If low degrees of interaction occur as indicated by insufficient viscosity increase or low elongation, use of rubber with a smaller particle size, rougher surface texture or higher rubber hydrocarbon or natural rubber content can increase the degree of interaction. The gradation of the rubber is very important when using asphalt-rubber in hot-mixed paving mixtures. If the rubber particle size is too large for the void spaces within the aggregate, compaction difficulties can occur and mixes can act spongy after compaction. Since the voids in the aggregate depend on the specific mix type, different rubber gradations are suited for different mix types. For open-graded mixtures, large rubber particles can be used without problems. Dense-graded mixtures, however; require use of finer rubber to produce mixes which compact appropriately. Suggested gradations are as follows: 6

7 Sieve Size Percent Passing Open-graded mix Dense-graded mix No. 10 No. 16 No. 30 No. 80 No. 200 Rubber Content The maximum amount of rubber which can be used in asphalt-rubber for hot-mixed paving applications is limited by the requirement that the asphalt-rubber must be capable of being pumped, and mixed appropriately with the aggregate. Asphalt-rubber materials with viscosities of up to 4000cp at F (as indicated by a Haake Rotational viscometer) have been found to be acceptable for use in dense-graded mixtures. If viscosities significantly exceed 4000cp at 350 0, aggregate coating problems during mixing can result. Higher viscosities (up to 6000cp) have been used with open-graded mixtures without aggregate coating problems. Generally, with most asphalts and typical rubber types appropriate for hot-mixed paving, maximum rubber contents based on viscosity are approximately 18 to 20% by total weight of the asphalt-rubber mixture. The minimum rubber content required is based on producing appropriate consistency at high service temperatures (softening point) and elasticity (resilience). Increasing the rubber content provides both incr.eased elasticity and increased high temperature reinforcement of the asphalt-rubber. Generally, rubber contents of at least 15% by total asphalt-rubber mixture weight are required to meet requirements of Table 1. Table 2 shows properties of blends of asphalt-rubber at several rubber percentages. Figure 4 is a plot of the test data versus rubber content. Note that for these combinations, hot climate properties are met at rubber contents of between approximately 16 and 19% rubber. Also note that the mixture viscosity increases rather linearly up to a rubber content of 15% and then begins to more rapidly increase with increasing rubber contents. Figure 4 indicates that a 1500cp viscosity is achieved with 15% rubber, and that 4000cp is achieved at 19% rubber, a 4% range. An appropriate selection for rubber content for this mixture would be 17% to provide a mixture in the center of the viscosity range while meeting other properties for a hot climate asphalt-rubber. Testing would then be required during the additional heating periods as previously discussed. 7

8 Interaction Conditions The time and temperature conditions for the interaction between the asphalt and the rubber need to be specified because of the influence on resulting properties. As previously discussed, an important consideration for an asphalt-rubber material is. stability of properties over time periods experienced during construction. Appropriate temperatures for asphalt-rubber interaction are 350 +/ F. In this temperature range, the interaction generally proceeds quick enough to reach desired properties within 30 minutes to 1 hour after blending the rubber with the asphalt while providing adequate property retention during extended heating. At temperatures lower than F, interaction periods which are generally greater than 1 hour are required to achieve desired properties. This can cause low production rates.. Temperatures in the range of 3750 to 4250 F quickly produce desired properties, but may lack in property stability during extended heating periods. Table 3 shows test data during a 24 hour laboratory. interaction period for a typical asphalt-rubber blend. These data indicate a uniform viscosity from 30 minutes of interaction to 24 hours, and adequate stability of physical properties to meet moderate climate property limits of Table 1 throughout the 24 hour interaction period. DESIGN OF DENSE-GRADED ASPHALT-RUBBER CONCRETE Both Marshall and Hveem methods (39) with slight modifications can be used for design of dense-graded asphalt-rubber concretes. Both procedures essentially consist of' selecting aggregates and binder, compacting mixes at varying binder contents, analyzing compacted specimen voids, mechanical testing, and then selecting the binder content based on data obtained. The follouring discussions when using asphalt-rubber, can be applied to both Marshall and Hveem procedures. Aggregate Dense-graded asphalt-rubber concrete pavements are composed of typical dense-graded type aggregates and appropriate asphalt-rubber binder. Aggregate should meet the same quality requirements as for conv~ntional asphalt concrete which would be used in similar applications. Due to the presence of the rubber particles in the asphalt-rubber binder, the aggregate gradation for dense-graded mixtures should be maintained on the coarse side of the gradation band. Gradations which plot between the maximum density line and the upper limit of the band should be avoided (Figure 5). Maintaining the gradation on the middle to coarse side of typical dense-graded bands is important to provide sufficient void spaces in the aggregate for the rubber particles. If the gradation is too fine, or the rubber particles are too large, compaction problems resulting from rubber interference between aggregate particles can result. This effect is indicated by two 8

9 observations during the mixture design procedure. First, immediately after compaction and while hot, the mixture will appear to have a somewhat unstable and spongy characteristic if coarse aggregate particies are pressed into the mix. Second~ a relatively level trend in mixture air voids data will be noticed with increasing, asphalt-rubber contents, instead of the typical decrease in air voids. Both of these effects can generally be reduced and eliminated by coarsening the gradation or by reducing rubber particle size used. Suggested gradation limits for 3/8 inch, 1/2 inch, and 3/4 inch maximum sized dense-graded mixtures for use with asphalt-rubber binder are listed in Table 4. Asphalt-Rubber The asphalt-rubber for use in dense-graded paving mixtures should be composed of rubber meeting the previously stated gradation limits' for use in dense-graded mixes, and the appropriate asphalt cement or blend with extenders to meet desired physical parameters such as those listed in Table 1. Trial Asphalt-Rubber Contents Due to the replacement of a portion of the asphalt by rubber in the asphalt-rubber (15 to 20%), generally, asphalt-rubber contents to be investigated during dense-graded mixture designs are 15 to 25% higher than asphalt cement contents which would be used for the same aggregate type. During the design procedure, the rubber in the asphalt should be considered as an integral part of the overall binder. During specimen evaluation and analysis, the rubber is accounted for by measuring the asphalt-rubber specific gravity or by calculating the combined specific gravity of the asphalt and rubber by proportion. With typical asphalt cements (specific gravity of 1.00 to 1.02) and granulated tire rubber (specific gravity of ), the combined specific gravity of the asphalt-rubber is between 1.02 and 1.05 at 60 0 F. Specimen Mixing Prior to mixing, it is recommended that the asphalt-rubber be heated to F, and the aggregate to F. The 350 F temperature for the asphalt-rubber is recommended for each asphalt-rubber grade in Table I, regardless of base asphalt grade due to the specified viscosity of between 1500 and 4000 cpo The asphalt-rubber should be heated in an oven using the procedure contained in Appendix 2 and should be stirred to assure uniformity (approximately 15 seconds) immediately before adding to the aggregate. Mixing of the asphalt-rubber with the aggregate should be performed using standard types of mechanical mixers using whips or paddles. Mixing should be performed. immediately after addition of the asphalt-rubber to the aggregate. Mixing should continue for at least 30 seconds beyond the time required to obtain complete aggregate coating. Total mix time should not exceed 2 minutes. If complete aggregate coating is not achieved in 2 minutes (which may be due to very fine or dusty mixes) either the asphalt-rubber content should be increased or a liquid anti-stripping agent 9

10 6hould be added to the asphalt-rubber to assist aggregate coating.' Following completion of mixing, the mixture should be split into appropriate portions (approx to 1200 gms) for compaction of specimens. If Hveem compaction will be used, the specimens should be subjected to the standard F curing procedure for 15 hours. Specimen Compaction When using Marshall compaction, the individual specimens should be placed in a forced draft oven maintained at 280 +/- 5 0 F for between 1 and 2 hours prior to compaction. Insure that the mixture has reached the compaction temperature by checking the actual mix temperature with a thermometer. Specimen compaction consists of removing the specimen from the oven, placing into heated Marshall molds, spading for 15 times, and compacting using standard Marshall procedures. Compaction level can be 35, 50, or 75 blows per side as dictated for the anticipated traffic level. Compaction should be completed within 3 minutes following removal of specimens from the oven. When using the Hveem procedure, the mix should be heated to a compaction temperature of 280 +/- 5 0 F after the curing period. Compaction then procedes using the standard Hveem kneading procedure. Some agencies use a compaction temperature of F. Immediately following completion of compaction, the specimens can be evaluated for instability and spongyness as previously discussed. For both procedures, specimens should be allowed to cool off for a minimum of 4 hours prior to removing from the molds. The reason for this is that if specimens are removed while still warm, deformation due to rebound from the rubber particles may occur, which could distort results. Specimen Testing For both Marshall and Hveem procedures, following removal from the molds, specimens are tested using standard procedures to evaluate stability, flow, stabilometer value, density, voids, etc. Marshall Procedure: Test results should be reported using standard procedures and methods. The design asphalt-rubber binder content should be selected to provide a mixture with an appropriate level of air voids while providing appropriate stability flow, and V.M.A. as indicated for conventional mixtures in the MS-2 manual (40). Two modifications in design criteria should be used for asphalt-rubber dense-graded concrete. First, due to the increased viscosity, elasticity, and softening point of the asphalt-rubber, asphalt-rubber concrete mixtures tend to experience less compaction and densification from traffic after construction. Therefore, for dense-graded mixtures containing asphalt-rubber binder, the design air void level can be set at the lower end of the 3 to 5% range. The target therefore for air void level should be 3 to 4%. The second modification in analysis of results for determining design binder content is that maximum flow values can be raised to 24 for light traffic, 22 for medium traffic, and 20 for heavy traffic due to the higher binder contents which are typically required. An example of a mix design which shows data and results for a dense-graded hot mix using asphalt-rubber is 10

11 contained in Appendix 3. Typical design asphalt-rubber binder contents for dense-graded mixtures range between 6.5 and 7.5% by total mixture weight, (7.0 to 8.1% by aggregate weight). Hveem Procedure: As with the Marshall procedure, test results should be reported using standard procedures and methods. Aspahlt-rubber mixtures generally yield stabilometer values which are significantly lower than those obtained for conventia1 asphalt concrete (31). This may be due to the more elastic behavior of the compacted mixtures. Typical stabilometer results with asphalt-rubber dense-graded mixes are 20 to 30 when using aggregate which produces 35 to 40 stabilities with asphalt cement. For specification purposes, it is suggested that the aggregate to be used be verified to be capable of providing a minimum Hveem stability which meets standard specifications when using asphalt cement (35 or 37 minimum). The suggested value for stability when using the same aggregate and asphalt-rubber is 20 minimum. During specimen evaluation, as with the Marshall procedure, it is suggested that air voids for the design be targeted at 3 to 4 percent instead of the 4% minimum. As with the Marshall procedure, typical binder contents are 6.5 to 7.5% by total mix weight..moisture Resistance: After the asphalt-rubber binder content of the mix has been determined, the moisture resistance of the mix should be checked. Conventional procedures such as Immersion Compression (ASTM D1075)(37), Lottman (40), or Tunnic1iff-Root (41) can be used. Additives which are used to improve moisture resistance (liquid additives, hydrated lime, or cement), of conventional asphalt concretes can be used for' asphalt-rubber mixtures. Acceptance criteria should be the same as for conventional asphalt concrete. 11

12 DESIGN OF OPEN~GRADED ASPHALT-RUBBER CONCRETE The modified physical properties of asphalt-rubber binder permit its use, in a variety of manners with open-graded aggregates. Due to the higher viscosity of the asphalt-rubber, very high binder contents (up to 10 or 11%) can be used effectively without experiencing excessive drain off which occurs with asphalt cement. Use of a higher binder content results in mixes with thicker binder films, improved aging resistance and better durability. When using asphalt-rubber binder, high mix temperatures can be used, again without the drain off problem, to permit construction in cooler temperatures or at longer haul distances than with conventional open-graded mixtures. High binder contents produce mixtures which have crack reflection reduction characteristics. similar to spray applied and chipped stress-absorbing-membranes (SAM's) (42). The design procedures which follow generally use methods outlined in the Federal Highway Administration Report No. FHWA-RD 74-2 titled 'Design of Open Graded Asphalt Friction Courses' (43) with several modifications to account for the unique properties of asphalt-rubber materials. The procedures describe methods for determining the asphalt-rubber content for three different types of open-graded mixture applications. These applications are: Aggregate Normal free draining friction courses at low binder content Durable friction courses at a medium binder content Plant mix seals at a high binder content Aggregate used for open-graded asphalt-rubber concrete should meet the same quality requirements as for conventional asphalt concrete which would be used in similar applications. Recommended aggregate gradations are listed in Table 5. These gradations are typical of many 3/8 and 1/2 inch open-graded mixtures used throughout the United States. For the 3/8 inch gradation, overly thickness should not exceed 1 inch. For the 1/2 inch gradation, maximum thickness should be 1 1/2 inches. Asphalt-Rubber The asphalt-rubber for use in open-graded paving mixtures should be composed of rubber meeting the previously stated gradation limits for use in open-graded mixtures and the appropriate asphalt cement or blend with extenders to meet desired physical parameters such as those listed in Table 1. Due to the large void spaces which exist between aggregate particles in open-graded mixtures, larger rubber particles can be used in the asphalt-rubber than with dense-graded mixtures. Asphalt-Rubber Content The suggested method for determining content consists of three basic steps followed for mix type. the by asphalt-rubber an adjustment 12

13 Step 1. Determine the surface constant K c of the aggregate using the FHWA RD-74-2 procedure (oil soaking and drain off) (43). This procedure is also contained in the Asphalt Institute MS-2 manual. Step 2. Calculate the required asphalt cement content using the following formula: (43) Percent Asphalt (agg. wt.) K 2.0 K c Step 3. Determine the base asphalt-rubber content by dividing the percent asphalt from step 2 by the asphalt cement (and extender if used) content of the asphalt-rubber. This provides an asphalt-cement content in the asphaltrubber mix which is equivalent to that determined in Step 2. Open-Graded Asphalt-Rubber Concrete Types Open-graded mixes using asphalt-rubber can be classified into three basic types depending on the binder content used. Free Draining Friction Coarse: This type of mixture is constructed using the base asphalt-rubber content with no modifications. This provides a friction coarse which has skid resistance and draining characteristics similar to a conventional open-graded friction coarse constructed using asphalt-cement. Use of the asphalt-rubber binder provides improved durability and permits use of higher mix temperatures for cool climates. Typical asphalt-rubber binder contents are between 6.5 and 8.0% by aggregate weight. This mix type generally has between 15 and 18% air voids when compacted using 50 blows per side with a Marshall Hammer at F. An example design is shown in Appendix 4. Durable Friction Course: The binder content for the durable friction course is determined by multiplying the previously determined base asphalt-rubber by a factor of 1.2. Typical asphalt-rubber binder contents are 8.0 to 9.5 percent by aggregate weight. This mix type has somewhat thicker binder film thickness which results in increased durability, but with a somewhat lessened drainage capacity. This mix type generally has 12 to 15% air voids when compacted using 50 blows per side with a Marshall Hammer at F. Plant Mix Seal: The binder content for the plant mix seal type of open-graded asphalt-rubber concrete is determined by multiplying the previously determined base asphalt-rubber content by a factor of 1.4. Typical asphalt-rubber binder contents for this mix type are 9 to 11% by aggregate weight. When compacted at F using 50 blows of the Marshall Hammer per side, this mix type generally has 8 to 12% air voids. The high binder content produces a mix with improved aging resistance, durability and resistance to reflective cracking. When this mix type is placed to 13

14 a thickness of 3/4 inch, there will be between 0.65 and 0.8 gallons of asphalt-rubber per square yard on the pavement which is typical of a stress absorbing membrane type application. Specimen Mixing, Following determining the asphalt-rubber content for the application, mixtures of the open-graded asphalt-rubber concrete are made. The asphalt-rubber should be heated in an oven to F +/- 100F and be stirred immediately proir to addition to aggregate in order to insure that the mixture is homogeneous and rubber particles are not segregated. Proportioned aggregate should be heated to 300 o Fprior to mixing with the heated asphalt-rubber. Mixing of the asphalt-rubber with the aggregate should be performed using appropriate types of mechanical mixers using whips or paddles. Mixing should be performed immediately after addition of the asphalt-rubber to the aggregate. Mixing should continue for at least 30 seconds beyond the time required to obtain complete aggregate coating. Total mix time should not exceed 2 minutes. Following completion of the mixing, the mix should be split into 1000gr. portions for drainage testing and appropriate sized specimens for moisture reisistance testing. Mixture Production Temperature Determination Testing should be performed in accordance with the FHWA RD-74-2 drainage procedure (Section 6.1). A temperature of 290 F is recommended for starting the drainage evaluation. If drainage at F after both 15 and 60 minutes is acceptable, a mix production temperature of 2900F +/- 100F can be used. If excessive drainage occurs, lower temperatures should be investigated until. appropriate drainage levels are obtained. The appropriate drainage level is defined as no more than a slight puddle (less than 1/4 inch diameter at points of contact between aggregate and the glass plate. Moisture Resistance Testing Moisture resistance of the mixture should be determined in accordance with standard testing methods used for open-graded mixtures such as Immersion Compression or 24 hour Marshall Immersion. Immersion Compression: Testing using ASTM D 1075 procedures (37). Mix should be the previously determined mix pressure should be 2000 psi instead of 3000 should be performed compaction temperature temperature, and molding psi (43). Marshall Immersion: For Marshall Immersion testing, specimens should be compacted after conditioning at the mixing temperature for 1 to 2 hours. Specimen compaction should be 50 blows per side of the Marshall hammer and compaction should be completed within 3 minutes after removal from the oven. 14

15 DESIGN OF GAP-GRADED ASPHALT-RUBBER CONCRETE Gap-graded asphalt-rubber concrete mixtures are a variation of dense-graded 'mixtures in which the aggregate gradation is coarsened to provide a greater amount of mixture voids. The inceased voids permit use of an increased asphalt-rubber content to provide increased mixture durability. Suggested aggregnte grading limits are shown in Table 6. Aggregate should meet normal other quality requirements for asphalt concrete aggregates. Asphalt-rubber should be of the appropriate type listed in Table 1 and should use the dense-graded type of rubber. The Marshall design procedure for dense-graded mixtures which was discussed previously can be used for the design of gap-graded asphalt-rubber concretes. During the design, it is suggested that air void levels of 3 to 5 percent be achieved. Additional criteria listed in the dense-graded design procedure should be met except that flows can be raised to 26, 24, 22 for light, medium and heavy traffic.. Gap-graded mixtures which have a more open gradation which approaches' an open-graded mix can also be used. As the gradation is opened, greater amounts of asphalt-rubber binder are required to produce 3 to 5 percent air voids, and the mix will take on characteristics closer to an open-graded mixture. Typical binder contents for gap-graded mixture gradations listed in Table 6 are between 7.0 and 8.5% by total mix weight. Suggested thickness limits for 3/8 inch gap-graded asphalt-rubber mixtures are 3/4 to 1 1/2 inches, for 1/2 inch mixtures, are 1 to 2 inches, and for 3/4 inch mixtures, 1 3/4 to 3 inches. If gap-graded mixtures with more open gradations are used, maximum thickness should be reduced. Equipment ASPHALT-RUBBER CONCRETE MIX PRODUCTION Asphalt-rubber concrete can be mixed in either batch or drum. type production plants. It is suggested that in order to prevent contamination of the storage tanks, and to prevent segregation of the asphalt-rubber. that a seperate asphalt-rubber storage tank with appropriate agitation be used. Additionally, it is suggested that a seperate asphalt-rubber supply system equiped with a pump and metering system capable of adding binder to the aggregate at the correct percentage tbe used. Asphalt-rubber binder content should be maintained within plus or minus 0.5 percent of the' design value for ~ingle test values. Mixture Production Temperatures Suggested asphalt-rubber temperatures when being added to the aggregate for all mix types are between 325 and F. For dense and gap-graded mixtures it is recommended that the aggregate temperature be 290 to F. For open-graded mixtures, aggregate temperatures should be appropriate to result in the lab determined mix temperature. 15

16 CONSTRUCTION TECHNIQUES AND GUIDELINES Asphalt-rubber mixtures are hauled, placed, and compacted using conventional equipment and slightly modified techniques. When hauling asphalt-rubber mixtures, truck beds should be sprayed with a water-soap solution or dilute silicone emulsion instead of kerosene or diesel fuel. Kerosene or diesel fuel should not be used because of an affinity for absorption into the rubber particles which can result in mix tenderness. Mixture laydown temperatures should not be below F for open-graded mixtures or F for dense-graded or gap-graded mixtures. Asphalt-rubber mixtures should be compacted using steel-wheeled rollers. Pneumatic rollers should not be used due to an increased adhesiveness of the asphalt-rubber binder, which can stick to the rubber tires. Compaction should proceed quickly as soon as the mixture is capable of supporting the rollers without excessive shoving. Delays should be avoided because as asphalt-rubber mixtures cool, they become more difficult to compact due to the reinforcement provided by the rubber. Figure 6 shows lab density data obtained for dense-graded mixtures made with 120 and 60 penetration asphalt-cement, and asphalt-rubber made from the 120 penetration asphalt and 18% minus 20 mesh rubber. Note the reduced densities of the asphalt-rubber mixture in comparrison to the asphalt cement mixtures as temperature drops from F to 200 o F. Open-graded asphalt-rubber mixtures should be compacted using a minimum of 3 full roller coverages. Dense and gap-graded mixtures should br compacted to provide a minimum of 95% of the lab compacted density. Vibratory rollers can be used with dense and gap-graded mixtures, but should not be used for open-graded mixtures. With some asphalt-rubber the compacted mix may exhibit construction. If this occurs, application (approximately 4 lbs. be use to alleviate the problem. concretes at high binder contents, excessive stickiness just after it is recommended that a light per square yard) of blotter sand SUMMARY This paper covers design methods which can be used for hot-mixed asphalt-rubber concrete pavements. Properties of. asphalt-rubber binders appropriate for use in hot- mixed paving mixtures are discussed along with factors which influence asphalt-rubber properties. Criteria for selecting the specific asphalt-rubber formula and specifications for use in hot, moderate, and cold climates are presented. Mixture design methods for asphalt-rubber dense, open, and gap-graded mixtures are discussed. Each method follows conventional Marshall, Hveem, or FHWA procedures with suggested modifications to ipcorportate asphalt-rubber binder. The methods presented can easily be performed by most laboratories proficient at asphalt-concrete mix designs with aquisition of minor pieces of additional equipment. Evaluation criteria and suggested property limits for both the asphalt~rubber and asphalt-rubber concrete mixes could possibly be used as a basis for establishment of uniform construction specifications. 16

17 Table 1 Suggested Physical Property Limits for Asphalt-Rubber Materials for Use in Hot-Mixed Asphalt Concrete Applications Property Property Limitslll Hot(2) Moderate ---C-o-l-d-- Climate Climate Climate Viscosity, Haake, 3500F Softening Point, (ASTM D36) Cone Penetration, 770F (ASTM D3407) Resilience, 770F (ASTM D3407) Ductility, 770F (ASTM D113) Low Temperature Flexibility3 (ASTM C711 modified) F min % min 15 ern/min 35 max cp 120 F min % min 15 ern/min cp 110 F min % min 15 ern/min 150 max Notes: 1). Property limits should be stipulated at a specific interaction period such as 60, 90, and 120 minutes. 2). Make climate selction based on the following temperature ranges from the U.S. Department of Commerce Enviromental Data Service. Hot climate - average July max F-; average Jan. low 30 o F+. Moderate Climate average July max F-; average Jan. low F. Cold Climate - average July max F-; average Jan. low = 15 0 F-. Make the selection based on January low, then check July temperatures. If July temperatures exceed those of the grade selected based on January temperatures, use the next stiffer grade. 3). As an alternate, Cone Penetration at 39.20F, 200g, 5 sec. can be used. Limits would be 10 min. for hot climate, 25 min. for moderate climate, and 40 min. for cold climate. 17

18 Table 2 Physical Properties of Asphalt Rubber Blends with Differing Rubber Contents Percent Rubber (Mix Basis) Property ~...l1l Viscosity, 350 F,.cp Cone Penetration,77oF Resilience, 77 F Softening Point, F ~ Notes: 1). Asphalt is AC-20, rubber is minus No. 16 mesh. 2). Interaction period is 90 minutes at 350 F. 18

19 Table 3 Asphalt Rubber Test Data During a 24 hr Interaction Period Interaction Period TEST PERFORMED min min min min 6 hrs 24 hrs Viscosity. 350 F in centipoise Penetration. 77 F in 1/10 mm F in % rebound Ductility. 77 F 5 cm/min;cm Softening Point in of Fracture Temperature of Lowest Passing of Fracture Notes: 1). Asphalt cement is AC-5. Penetration (D5) = 198. Softening Point = 1100F. 2). Asphalt Rubber Blend is 83% AC-5; 17% 20 mesh rubber. 3). Interaction temperature is 3500F. 4). 6 hour to 22 hour holdover temperature is 3000F. 19

20 Table 4 Suggested Gradation Specifications for Dense-Graded Asphalt Rubber Concrete (Percent Passing) Mix Designation Sieve Size 3/S- 1/2 3/4 1 1 (25.0 nm) /4 (19.0 nm) /2 11 (12.5 nm) /S- (9.5 nm) S0 #4 (4.75 nm) 60-S #S (2.36 om) #30 (600-urn) ls #50 (300-urn) S-IS #200 (75-urn) 2-S 2-S

21 , Table 5 Suggested Gradations for Open-Graded Asphalt-Rubber Concrete Mixtures (Percent Passing) Mix Designation Sieve Size 3/8-1/2-3/4- (19.0 mm) /2- (12.5 1lJIl) /8- (9.5 om) #4 (4.75 mm) #8 (2.36 mm) #30 (600 urn) #200 (75 urn)

22 Table 6 Suggested Gradation Specifications for Gap-Graded Asphalt-Rubber Concrete (Percent Passing) Mixture Designation Sieve Size 3/8 1/2 3/4 1 (25.0 mm) /4 (19.0 om) /2 (12.5 rom) /8 1 (9.5 nm) #4 (4.75 rom) #8 (2.36 mm) #30 (600 urn) #50 (300 urn) #200 (75 urn)

23 z 0... to< < l:> :z; 0 &:l ~ 20 Iol U ~ Iol ~~~.. AC-7.5 with 14% Extender and 17% Rubber AC-7.5 with7: Extender and 17% Rubber AC 7.5 AC Rubber TIME, MINUTES AC-20 + Rubber AC-20 FIGURE 1. Constant Load Creep Plots For Asphalt Cement And Asphalt-Rubber At 39F With A 500 g. Load 45 AC7.5 with 7% Extender and 17% Rubber z to< < l:> Z 0 25 &:l 20 Iol U ~ Iol AC Rubber AC-40 AC-20 + Rubber TIME,MINUTES FIGURE 2. Constant Load Creep Plots For Asphalt Cement And Asphalt-Rubber At 74F With A 25 g. Load 23

24 AC-20 AC z l-< < to' Z 0,..l 20 l&l Ii: 15 \&l u ti Il J AC-7.5 with 14% Extender and 17% Rubber AC7.5 with 7% Extender and 17% Rubber ~~:_;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;~AC-7 AC-20+Rubber.5+Rubber 0 TIKE. MINUTES FIGURE 3. Constant Load Creep Plots For Asphalt Cement And Asphalt-Rubber At loaf With A 1 g. Load c. lj 0..., ~ to-! "" 150 z l-<,...,... <... l-< 0 \0 0 -:r.., l>:: ~ ~ l&l 3 ~ ~ ~ ~ :r: SOFTENING POINT ~ Ii: rae; 130 "" ,.,., to' Z l-< l&l... < Z lo.l ~ ~... t: ,..l... 0 til til 0 til 110 u til... > 0.., VISCOSITY "" l>:: Rubber Content of Asphalt-Rubber (Total Weight) -10 FIGURE 4. Properties Of Asphalt-Rubber Blends At Various Rubber Contents (Data From Table 2) 24

25 C> Z II) II) :..e o f:=ti=t= ~ ~ - -L CRADATlON AREA - -. TO AVOID ~ ~::L=. -, -. '-~ ~ f-. ~ ~ f _- :" '--- ~ fl..~ ~ ' =- ASTH 1/2" LIMITS ~..- MAXIMUM DENSITY -f- P.~ I. -- > ~" I#-._ ~'I..,- /: I-. =-. - I=L' 1/" - _ lot &0 ) /8 liz ~." I.e 1.& SIEVE SIZE O.4~ POWER o to 20 ) z $0 ;!... & 60 i! FIGURE 5. Illustration Of Gradation Area To Avoid With A Typical Dense-Graded Gradation When Using Asphalt-Rubber Binder 14S AC-7.S (120 pen.) u ~.... r:: E 135 c:l Compact1ve EffoTt 15/15 Blow MaTshal COMPACTION TDiP RAnJRl. r Figure 6. Variation Of Density Of A Dense-Graded 1/2" Mix With Asphalt Cement And Asphalt-Rubber When Compacted At Temperatures Ranging From From 200F To 300F 27S

26 APPENDIX 1 Procedure For Lab Interaction Of Asphalt-Rubber Materials The recommended procedure for preparing asphalt-rubber materials in the laboratory consists of subjecting the asphalt-rubber to temperatures and times which will occur in actual use. The mixture is then tested at several specific points in the interaction period to evaluate the properties of the mixture during normal application periods. The procedure for preparing asphalt-rubber. is as tolloo's: 1). Selection of asphalt cement, rubber, and additives for the mixture. 2). Selection of the proportions of each material to be tested. 3). The asphalt cement,extender (if used) and adhesion agent (if used) a~e placed in a standard 1 gallon open top metal can. Approximately 2,000 grams of the blended materials should be used. The materials are then heated using any convenient method to 50 +/- 10F above the desired temperature to be used during the interaction period. During heating, the materials should be stirred to insure uniformity.. 4). All of the granulated rubber to be used in the mix is then added to the heated asphalt cement and stirred in using an appropriate hand stirring device (spatula) for approximately 30 seconds or until all of the added rubber is wetted into the asphalt. 5). The mixed material is then placed in a forced draft oven maintained at an appropriate temperature to maintain the desired interaction temperature (typically approximately 25F above the interaction temperature). The container should be loosely covered. 6). After 15 minutes has elapsed, after the rubber has been added, the container is removed from the oven, then stirred by hand for 15 seconds, and then replaced in the oven. 7). The sample is then stirred for 15 seconds. after an additional 15 minutes, and then is stirred at 30 minutes intervals until 2 hours has elapsed. During this time period, while stirring, the temperature should be checked and recorded so that adjustments in oven temperature can be made, if required to keep the sample temperature within plus or minus 10F from the desired interaction temperature. 8). From 2 hours to 6 hours of interaction the sample is stirred at 1 hour intervals. 9). After 6 hours of interaction, the oven temperature is reduced to 300F and the asphalt rubber sample exposed to the 300F temperature until 22 +/- 1 hours have elapsed since the rubber was added to the asphalt. The asphalt rubber is then stirred by hand for 15 seconds, replaced in the oven, and then the oven temperature is raised to raise the temperature of the asphalt-rubber to the interaction temperature within 2 hours. This completes the interaction period. 26

27 APPENDIX 2 Recommended Procedure for Preparing Sampl~E of Asphalt Bubb~r for Testing Introduction: This procedure recommends methods which should be used for preparing mixed samples of asphalt rubber obtained from jobs or suppliers for laboratory use in asphalt concrete mix design, chipseal evaluations, or other testing procedures. Asphalt Rubber Preparation for Testing: Heating: The sample of asphalt rubber should be placed in an appropriate metal container, no larger than 1 gallon (a standard one gallon round open top paint can is suitable). The can is then placed in an oven. (preferrably forced draft) maintained at 25 F above the temperature the asphalt rubber is to be 'heated to. For example if the asphalt rubber is to be heated to 350 F, the oven temperature should be 375 F. Higher temperatures should not be used due to the possibility of over reacting the rubber components. The can containing the asphalt rubber should be covered while heating in'the oven. After the material has been in the oven approximately 1 hour, the can should be removed and the sample stirred for 15 seconds. At this time, the sample will not be totally melted. The can is then replaced in the oven, covered, heated for an additional 30 minutes, removed and stirred for 15 seconds again. Temperature is then checked with an appropriate thermometer. This procedure is then repeated until the sample is uniformly melted and has reached the mixing or application temperature. For a 1 gallon sample it will take approximately 2 1/2 hours to reach 350 F. 27