Section 5: Hot Mix Asphalt Concrete Pavement Mixtures

General

Hot mix asphalt (HMA) is a generic term that includes many different types of mixtures of aggregate and asphalt cement (binder) produced at elevated temperatures (generally between 300-350F) in an asphalt plant. Typically, HMA mixtures are divided into three mixture categories: dense-graded; open-graded; and gap-graded as a function of the aggregate gradation used in the mix.

A variation on tradtional hot-mix asphalt is warm-mix asphalt (WMA). WMA technologies are processes or additives to HMA that allow mixture production and placement to occur at temperatures (30-100F) lower than conventional HMA without sacrificing performance. Technology currently used to make the WMA process possible are chemical binder additives, chemical mixture additives, foaming admixtures, and plant modifications.

Additives and processes pre-qualified for use on department projects can be found on the approved WMA list .

Significant benefits derived from using WMA include:

reduced fuel requirements for mixture production

extended time available for compaction/workability, particularly for mixtures containing polymer-modified asphalt binders and thin lift/cool weather applications

ease of incorporating reclaimed asphalt pavement (RAP)

potential lower oxidation/improved fatigue life

The following information addresses HMA mixtures, but generally do not differ appreciably from WMA. Typically, the same parent test procedures or specification item applies to both types of asphalt.

Dense-graded mixes are produced with well or continuously graded aggregate (gradation curve does not have any abrupt slope change) and are specified under the current Items 340 and 341 (QC/QA). Typically, larger aggregates float in a matrix of mastic composed of asphalt cement and screenings/fines (see Figure3-3).

Open-graded mixes are produced with relatively uniform-sized aggregate typified by an absence of intermediate-sized particles (gradation curve has a nearly vertical drop in intermediate size range). Mixes typical of this structure are the permeable friction course (Item 342) and asphalt-treated permeable bases. Because of their open structure, precautions are taken to minimize asphalt drain-down by using modified binders (A-R) asphalt rubber, or by use of fibers. Stone-on-stone contact with a heavy asphalt cement particle coating typifies these mixes (see Figure3-4).

Gap-graded mixes use an aggregate gradation with particles ranging from coarse to fine with some intermediate sizes missing or present in small amounts. The gradation curve may have a flat region denoting the absence of a particle size or a steep slope denoting small quantities of these intermediate aggregate sizes (see Figure3-5). These mixes are also typified by stone-on-stone contact and can be more permeable than dense-graded mixes (Item 344, Performance-Designed Mixtures), or highly impermeable (Item 346, Stone-Matrix Asphalt).

Stone-matrix asphalt (SMA) will be missing most intermediate sizes but have a relatively high proportion of fines. Fibers or modified binders (A-R) are combined with these fines to build a rich mastic coating around and between large aggregate particles. Compacting and hand-working these mixes are usually more difficult than with either dense-graded or open-graded mixes.

Figure 3-3. Dense-graded mix cross-section and typical gradation curve for a dense-graded mix.

Figure 3-4. Open-graded (Permeable Friction Course) mix cross-section and typical gradation curve for open-graded mix.

Figure 3-5. Gap-graded (Performance-Designed or SMA) mix and typical gradation curve for a gap-graded mix.

HMA MIX DESIGN

Mix design is performed in a laboratory using one of the procedures outlined in Tex-204-F, Design of Bituminous Mixtures," when the applicable procedure varies roughly 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.

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 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. 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 departments Wet Weather Accident Reduction Program (WWARP)*, aggregates are classified into four categories (A, B, C, or D) based on a combination of frictional and durability properties. A friction demand assessment is made by the responsible engineer. The proper aggregate or blend (using categories A and B only) to achieve the assessed rating is then selected.

*Available through the TxDOT intranet only.

Design is facilitated by the use of a series of automated mix design programs in Excel format. Mix designs can be generated by either department personnel or by a consultant/contractor who is Level II (HMA mix design specialist) certified by the Texas Asphalt Pavement Association. Raw materials must be furnished by the contractor to the responsible department engineer to allow verification of the mix design.

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 under Parts I or II of the procedure (see Tex-206-F, Compacting Specimens Using the Texas Gyratory Compactor [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 a lab-molded density of 96%.

Superpave Gyratory Compactor (SGC). Alternatively, larger stone dense-graded mixtures (Types A and B) can be designed using the Superpave Gyratory Compactor (SGC) under Part III of the procedure (see "Tex-241-F, Superpave Gyratory Compacting of Test Specimens of Bituminous Mixtures").

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 require removal of this material because of the smaller mold size. Samples of all remaining mixture types are compacted using the SGC. Samples are again prepared at various asphalt binder contents about the estimated optimum content. Plots are generated within the design software to evaluate optimum binder content at 96.0% 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 and are related to the expected traffic loading. Depending upon the mix type, a N design (Ndes) related to design air voids will be established (see " Tex-241-F" and "Tex-204-F," Part IV). 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.

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 pavement design 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.

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 "Tex-242-F, 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 in the Superpave Gyratory Compactor (SGC) using the optimum asphalt content to 93% optimum density (a requirement for all lab-prepared Hamburg test specimens).

The Hamburg test uses a pair of abutting, trimmed samples placed in a 50C 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.

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.

Recently, a new test device (Overlay Tester) was implemented at the CST-M&P laboratory to test mixes for cracking potential found in Tex-248-F.

Table 3-6: Tex-204-F Mix Design Options

PartType MixCompactor UsedMust meetMix EvaluationComment

IDense-graded Types A, B, C, D, FTGCDensity - 96.0%1 (Rich Bottom Layer [RBL] 98%)

Min. VMA by mix typeIndirect 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

IIAs aboveTGCAs aboveAs aboveMix designed by volume of constituent materials when aggregate stockpile specific gravities vary by 0.300 or more.

Volumes converted to weights.

IIILarge stone (Types A, B)SGCDensity - 97% Min. VMA by mix typeIndirect tensile strength (Tex-226-F) at each asphalt content, Hamburg (Tex-242-F) at optimum AC content at 93 1% density. Mix designed by weight of constituent materials.

IVPerformance-designed (Superpave, Coarse Matrix High Binder) SP-A, B, C, D, CMBH-C, F SGCDensity - 96%1 (Rich Bottom Layer [RBL] 98%)

Min. VMA by mix type.

Design VFA for SP mixes.

By plan note, designate stone on stone contact for SP and CMHB mixes.

Density at Nini, Ndes, Nmax must all fall in allowable range SP mixes (summary worksheet of mix design program) 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

VPermeable Friction Course (PFC)SGCMin. optimum asphalt content of 6.0%.

Lab molded density 7882%.

Max. allowable draindown < 0.3%.

No visible stripping by Tex-530-C. Cantabro Loss ( Tex-245-F) at optimum AC content at 78-82% density. Mix designed by weight of constituent materials

VIStone-matrix Asphalt (SMA)SGCDensity - 96%

Min. VMA

Min. AC content 6% for aggregate bulk spec. grav. 2.75.

Must ensure stone- on-stone contact.

Max. allowable draindown < 0.2%.

No visible stripping by Tex-530-C.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

VIIStone-Matrix Asphalt Rubber (SMAR)SGCDensity - 97%

Min. VMA higher for A-R binder

Min. crumb rubber modifier content otherwise as aboveAs aboveMix designed by weight of constituent materials

1. Consideration should be given to increasing the lab molded density to 96.5 or 97.0% where achieving field compacted density has been problematic.

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 responsible engineer and contractor generally verify the JMF based on plant-produced mixture from a trial batch. The responsible engineer may accept an existing mixture design previously used by the department and may waive the trial batch to verify the JMF.

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.

Guidelines for Selecting HMA Mixtures

Selection of a HMA mix or combination of mixes to use in a project should be a conscious decision made by the responsible engineer based on mix attributes (suitability as part of the overall pavement design, existing pavement conditions, lift thickness, traffic loading characteristics), environment, past performance, local contractor experience, and economics.

Guidelines have been established to assist in the decision-making process in the form of a Mixture Selection Guide. The Guide provides general descriptions of the various HMA mixes used in the State (typical use, advantages, disadvantages); table ratings (subjective) of mixture characteristics for each of the mixture types; table of typical lift thicknesses; location within a pavement structure for each mixture type; and recommended choices for surface mixtures.

Use of Perpetual Pavements

Perpetual pavements take into account the increased structural demands due to heavy truck traffic, where cumulative one-direction traffic loading of more than 30 million ESALs over a 20-30 year design life is projected. By limiting the strain level at critical locations, these pavements are designed to have a virtually infinite fatigue and full-depth rut life, requiring only periodic surface renewal.

Stone Matrix Asphalt (SMA), Item 346, and Performance-Designed Mixtures, Item 344, should be used in lieu of conventional QC/QA dense graded specifications to ensure rut resistance within HMA layers. In addition, the use of a Rich Bottom Layer (RBL) should be considered when:

new or bottom up reconstruction is anticipated

full-depth HMA structures have a composite HMA thickness between 8-12 inches or

there are concerns for bottom up moisture intrusion into less rich (higher air void) bottom HMA layer

Along with establishing an engineered foundation, using RBL is designed to help mitigate conventional bottom up fatigue cracking (see Figure2-2).

Exceptions to the use of these improved performance mixes for high truck traffic pavements must be granted by the Director of CST-M&P or designated representative. If a district is interested in using these improved performance mixes on routes with less than 30 million ESALs, the district must obtain approval from CST-M&P.

Selecting Surface Aggregates to Comply With the Wet Weather Accident Reduction Program (WWARP)

The department is required to establish and maintain a program to ensure that pavements with good skid resistant characteristics are used. This program is commonly referred to as WWARP Program.

The department is charged with developing and implementing methodologies for the detection and improvement of locations with a significant incident of wet weather accidents using accident record systems and countermeasures to address those locations.

The department is also directed to utilize methods for the analysis of the skid resistant characteristics of selected roadway sections to:

1. ensure that pavements are being constructed that provide adequate skid resistance

2. develop an overview of the skid resistant properties of the highway system

3. provide information for use in developing safety improvement projects and the implementation of cost effective treatments at appropriate locations.

The WWARP allows the department to take advantage of the increased knowledge gained through our research efforts and to more effectively and efficiently address the various regional demands of Texas pavements. WWARP addresses three separate but interrelated phases of pavement friction safety. The three phases are accident analysis, aggregate selection, and skid testing.

Accident analysis is the first phase and it consists of the identification, evaluation and improvement (as needed) for all wet weather accident locations. This information is contained in the Crash Records Information System. The Traffic Operations Division will furnish reports through the Construction Division (CST) to the districts on an annual basis.

The second part of the program is aggregate selection. Each bituminous coarse aggregate source is classified into categories based on a combination of the frictional and durability properties of the aggregate. The classifications will be listed in the Bituminous Rated Quality Source Catalog provided (every 6 mos.) by the Geotechnical, Soils & Aggregates Branch of the Materials & Pavements Section of the Construction Division (CST-M&P).

The third part of the program will consist of skid analysis and will include a mandatory collection of skid data that will become part of the Pavement Management Information System (PMIS).

Although CST-M&P has been delegated responsibility for administering the WWARP program, it is the districts responsibility to manage frictional properties for their pavements through sound engineering judgment and application of the program.