6.2 HMA Mix Design
Mix design is performed in a laboratory using one of the procedures outlined in
, “Design of Bituminous Mixtures,” where the applicable procedure varies according to mixture categories outlined above. In addition, material quality, aggregate gradations, and other mixture requirements are given in each of the specific mix standard or special specifications.
6.2.1 Performance Concerns
Mix design seeks to address a number of performance concerns in the finished HMA mat (
Hot Mix Asphalt Materials, Mixture Design, and Construction,
Roberts, et al., 1996). These include:- Resistance to Permanent Deformation.The mix should not distort or displace under traffic loading. The true test will come during high summer temperatures under slow or standing truck traffic that soften the binder and, as a result, the loads will be predominantly carried by the aggregate structure.Resistance to permanent deformation is controlled through improved aggregate properties (crushed faces), proper gradation, and proper asphalt grade and content.
- Resistance to Fatigue and Reflective Cracking.Fatigue and reflective cracking resistance is inversely related to the stiffness of the mix but proportional to asphalt film thickness. While stiffer mixes are desirable for rut resistance, design for rut resistance alone may be detrimental to the overall performance of the HMA mat if fatiguing or reflective cracking occurs. Stiff mixtures perform well when used in thick HMA pavements and can perform well when used as a thin overlay on a continuously reinforced concrete pavement (CRCP).Thin HMA mats placed on an unbound base or on surfaces prone to reflective cracking (e.g., jointed rigid pavements, bound bases subject to shrinkage cracking, etc.) should use a mix that strikes a better balance between rut and crack resistance. Fatigue and reflective crack resistance is primarily controlled by the proper selection of the asphalt binder. Application of a specialty designed crack-resistant interlayer is another option for mitigating cracking.
- Resistance to Low Temperature (Thermal) Cracking.Cooler regions of Texas are particularly confronted with thermal cracking concerns. Thermal cracking is mitigated by the selection of an asphalt binder with the proper low temperature properties.
- Durability.The mix must contain sufficient asphalt cement to ensure an adequate film thickness around the aggregate particles. This helps to minimize the hardening and aging of the asphalt binder during both production and while in service. Sufficient asphalt binder content will also help ensure adequate compaction in the field, keeping air voids within a range that minimizes permeability and aging.
- Resistance to Moisture Damage (Stripping). Loss of adhesion between the aggregate surface and the asphalt binder is often related to properties of the aggregates. The assumption on the part of the mix designer should be that moisture will eventually find its way into the pavement structure; therefore, mixtures used at any level within the pavement structure should be designed to resist stripping by using anti-stripping agents.
- Workability.Mixes that can be adequately compacted under laboratory conditions may not be easily compacted in the field. Adjustments may need to be made to the mix design to ensure the mix can be properly placed in the field without sacrificing performance.
- Skid Resistance.This is a concern for surface mixtures that must have sufficient resistance to skidding, particularly under wet weather conditions. Aggregate properties such as texture, shape, size, and resistance to polish are all factors related to skid resistance. Under the department’s 1 , aggregates are classified into three categories (A, B, or C) based on a combination of frictional and durability properties. A friction demand assessment is made by the engineer. The proper aggregate or blend (using categories A and B only) to achieve the assessed rating is then selected.
Design is facilitated by the use of a series of in Excel format. Mix designs can be generated in accordance with Tex-204-F by either department personnel or by a consultant/contractor who is certified by the department-approved hot-mix asphalt certification program. Plant mix or raw materials must be furnished by the contractor to the department project engineer to allow verification of the mix design.
6.2.2 Texas Gyratory Compactor (TGC)
For dense-graded hot-mix asphalt (Types A, B, C, D, and F of Items 340 and 341), the Texas gyratory compactor (TGC) is used to compact sample mixtures in accordance with
“Compacting Specimens Using the Texas Gyratory Compactor [TGC].” Item 347, Thin Overlay Mixture [TOM], often associated with pavement preservation operations, can also be compacted using the TGC. The TGC uses a 4.0-in. diameter mold, with a target specimen height of 2.0 in.
Compactive effort is achieved by a combination of gyratory compactions governed by achieving a low pressure threshold, followed by uniform axial compaction achieving a high pressure threshold. Optimum asphalt binder content is derived by molding specimens at various binder contents, plotting the asphalt vs. density curve and selecting the binder content that corresponds to the specified target laboratory molded density.
6.2.3 Superpave Gyratory Compactor (SGC)
Superpave mixtures (Item 344), Permeable Friction Course (Item 342), Stone-Matrix Asphalt (Item 346), Thin Bonded Friction Courses (Item 348), and mixture designs used in Hot In-Place Recycling of Asphalt Concrete Surfaces (Item 358) must be compacted using the SGC in accordance with Tex-241-F, “Superpave Gyratory Compacting of Test Specimens of Bituminous Mixtures.” Dense-graded mixtures (Items 340 and 341) may be compacted using the SGC with a density requirement of 96.0%.
The SGC uses a 6.0-in. diameter mold with a target specimen height of 4.5 in. The larger diameter mold allows retention of material 3/4-in. or larger in the compacted sample whereas Parts I and II of Tex-204-F require removal of this material because of the smaller TGC mold size. Samples are prepared at various asphalt binder contents around the estimated optimum content. Plots are generated within the design software to evaluate optimum binder content at the specified target laboratory molded density and to ensure other key parameters are met.
Mixture designs using the SGC are also controlled by the number of gyrations (N) required to achieve proper density. Depending upon the mix type, an N design (Ndes) related to minimum asphalt content and design air voids is established in each mixture specification. Ndes can be adjusted to ensure sufficient asphalt cement content and mix workability.
Traditionally, lab-molded specimens have been produced at an AC content that will yield a target density of 96% of the theoretical maximum density. Some variation is allowed to ensure mixes are workable under field compaction conditions, thus mitigating tendencies toward very dry mixes and improving field achieved densities.
6.2.4 Voids in the Mineral Aggregate
Another mix design parameter that has significant impact on mix workability and durability is the voids in the mineral aggregate (VMA). Conceptually, this is the volume of space within a mix that is available for asphalt binder to occupy; as a result, this mixture design parameter has a direct impact on the binder film thickness. For this reason, minimum values are placed on this parameter, specific to the mix type and gradation. Related to VMA is the voids filled with asphalt cement (VFA), the percent of the volume of VMA that is filled with asphalt cement. A range of acceptable VFA is a further control placed on Superpave mixtures.
6.2.5 Evaluating Mix Stability
Historically, mix stability for the traditional dense-graded mixes was evaluated using the Hveem stabilometer. The lab-compacted samples were subjected to axial compression and shearing resistance of the mixture was evaluated.
A more comprehensive evaluation of all hot-mix asphalt mixtures for problems related to stability and moisture susceptibility (with the exception of permeable friction course and mixtures using asphalt-rubber modified binders) is now accomplished using the Hamburg Wheel Tracking Device, or simply Hamburg (see
, “Hamburg Wheel-tracking Test").
For the case of a dense-graded mixture designed using the Texas Gyratory Compactor (TGC), once optimum asphalt cement is determined, new samples are molded to 93% theoretical maximum density in the Superpave Gyratory Compactor (SGC) using the optimum asphalt content (a requirement for all lab-prepared Hamburg test specimens).
The Hamburg test uses a pair of abutting, trimmed SGC samples placed in a 122°F (50ºC) water bath. A weighted steel wheel passes back and forth across the surface; rut depth is evaluated per number of passes. A minimum threshold of passes resulting in a rut depth no greater than 1/2-in. is established based on the PG binder grade.
6.2.6 Tools to Improve HMA Mixes
Research project 0-5123 developed a methodology to design a balanced HMA mixture, considering both rutting (Hamburg) and fatigue (Overlay Tester) properties.
The Overlay Tester has been implemented for select mixtures using test method
Part | Type Mix | Compactor Used | Must Meet | Mix Evaluation | Comment |
|---|---|---|---|---|---|
I | Dense-graded Types A, B, C, D, F; Thin Overlay Mix (TOM) | TGC | Density: 96.5-97.5% Min. VMA by mix type | Indirect tensile strength (
Hamburg (
, both at optimum AC content at 93 ±1% density; OT (Tex-248-F) for TOM only. | Mix designed by weight of constituent materials |
SGC | N des 50 gyrations; Density: 96%Min. VMA by mix type. | ||||
II | As above | TGC | As above | As above | Mix designed by volume of constituent materials when aggregate stockpile specific gravities vary by 0.300 or more. Volumes converted to weights. |
III | Refer to Part I | Previously Part III used for DG TY A/B mixes designed using the SGC | |||
IV | Superpave SP-A, SP-B, SP-C, SP-D | SGC | N des 50 gyrations; Density: 96%Min. VMA by mix type. Design VFA for SP mixes. By plan note, designate stone on stone contact. | Indirect tensile strength (Tex-226-F), Hamburg (Tex-242-F), both at optimum AC content at 93 ±1% density | Mix designed by weight of constituent materials |
V | Permeable Friction Course (PFC); Thin Bonded Permeable Friction Course | SGC | N des 50 gyrations; Min. optimum asphalt content of 6.0% [7.0% for AR mixture].Lab molded density 78% [PFC-F]–82% [all others]. Max. allowable draindown < 0.1%. No visible stripping by
| Cantabro Loss (
) at optimum AC content at 78-82% density. Hamburg (Tex 242-F) and OT (Tex-248-F) for PFC-F only. | Mix designed by weight of constituent materials |
VI | Stone-Matrix Asphalt (SMA) | SGC | N des 50 gyrations; Density: 96%Min. VMA Min. AC content 6% Must ensure stone- on-stone contact. Max. allowable draindown < 0.1%. No visible stripping by Tex-530-C. | Hamburg (Tex-242-F), OT (Tex 248-F) at optimum AC content at 93 ±1% density. | Mix designed by weight of constituent materials |
VII | Stone-Matrix Asphalt Rubber (SMAR) | SGC | N des 50 gyrations; Density: 96%Min. VMA higher for A-R binder Min. crumb rubber modifier content Other requirements as above for SMA | As above | Mix designed by weight of constituent materials |
VIII | Thin Bonded Wearing Course | SGC | N des 50 gyrations; Density: 92%Min. VMA Min. AC film thickness 9µm AC content % range: TY A: 5.0-5.8 TY B/C: 4.8-5.6 Max. allowable draindown < 0.1%. No visible stripping by Tex-530-C. | Cantabro Loss (
) at optimum AC content 92% density. |
The result of the mix design process is a job-mix formula (JMF), a starting point for the contractor in producing HMA for the project. The engineer and contractor generally verify the JMF based on plant-produced mixture from a trial batch. The engineer may accept an existing mixture design previously used by the department and may waive the trial batch to verify the JMF. It is recommended that if the trial batch is waived, the mix design should have been developed and verified within the past 12 months.
If the JMF fails the verification check using the trial batch, the JMF is adjusted or the mix may be redesigned. Additional plant-produced trial batches are run until the JMF is verified. During the course of the project, the JMF may be modified without developing a new mix design to achieve specified requirements as long as adjustments do not exceed tolerances established within the applicable mix specification.
1.
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