Chapter 9: Thin Functional Hot Mix Asphalt Overlay Projects

1.0 Introduction

For the purposes of this advisory, functional overlays are defined as thin treatments using a hot mix system.  A thin treatment for the purposes of this chapter is a non-structural layer and is applied as a preservation or maintenance treatment, either corrective or preventive.  Nationally, thin treatments are less than 37.5 mm (1.5 inches) in thickness.

There are three thin functional hot mix asphalt overlay types, which may be used either alone or in combination with other treatments such as Stress Absorbing Membrane Inter-layer (SAMI) include:

• Dense Graded Mixtures 
• Gap Graded Mixtures
• Open Graded Mixtures

The different mixes are defined based on their aggregate grading, binder content, and voids content.  Figure 1 illustrates, in general, the differences in aggregate structure for these mix types.

This chapter describes each of these mix types in further detail and provides an overview of the design and construction of these mixtures.

1.1 Mix Design Methods(19)

1.1.1 Fundamentals

Hot Mix Asphalt (HMA) consists of two basic ingredients: aggregate and asphalt binder. HMA mix design consists of choosing an aggregate, an asphalt binder, and the optimum combination of these two ingredients. Several alternative methods are available, of which the Marshall, Hveem and Superpave methods are the most common. The mix design fundamentals discussed here are applicable to all mix design methods.

1.1.1.1 Variables

HMA is a complex material upon which many different, and sometimes conflicting, performance demands are placed. It must resist deformation and cracking, be durable over time, resist water damage, provide a good tractive surface, and yet be inexpensive, readily made and easily placed. In order to meet these demands, the mix designer can manipulate three variables:

1. Aggregate: Items such as type (source), gradation and size, toughness and abrasion resistance, durability and soundness, shape and texture as well as cleanliness can be measured, judged and altered to some degree.
2. Asphalt binder: Items such as type, durability, rheology, purity as well as additional modifying agents can be measured, judged and altered to some degree.
3. The ratio of asphalt binder to aggregate: Usually expressed in terms of percent asphalt binder by total weight of HMA, this ratio has a profound effect on HMA pavement performance. Because of the wide differences in aggregate specific gravity, the proportion of asphalt binder expressed as a percentage of total weight can vary widely even though the volume of asphalt binder as a percentage of total volume remains constant.

1.1.1.2 Objectives

By manipulating the variables of aggregate, asphalt binder and the ratio between the two, mix design seeks to achieve the following qualities in the final HMA product:

• Deformation resistance: HMA should not distort (rut) or deform (shove) under traffic loading. HMA deformation is related to aggregate surface and abrasion characteristics, aggregate gradation, asphalt binder content and asphalt binder viscosity at high temperatures.
• Fatigue resistance: HMA should not crack when subjected to repeated loads over time. HMA fatigue cracking is related to asphalt binder content and stiffness.
• Low temperature cracking resistance: HMA should not crack when subjected to low ambient temperatures. Low temperature cracking is primarily a function of the asphalt binder low temperature stiffness.
• Durability: HMA should not age excessively during production and service life. HMA durability is related to air voids as well as the asphalt binder film thickness around each aggregate particle.
• Moisture damage resistance: HMA should not degrade substantially from moisture penetration into the mix. Moisture damage resistance is related to air voids as well as aggregate mineral and chemical properties.
• Skid resistance: HMA placed as a surface course should provide sufficient friction when in contact with a vehicle's tire. Low skid resistance is generally related to aggregate characteristics or high asphalt binder content.
• Workability: HMA must be capable of being placed and compacted with reasonable effort. Workability is generally related to aggregate texture/shape/size/gradation, asphalt binder content and asphalt binder viscosity at mixing and laydown temperatures.

1.1.1.3 Basic Procedure

Regardless of the specific method used, the basic mix design procedure remains basically the same. (See Figure 2). All mix design processes involve three basic steps:

1. Aggregate selection: Different agencies / owners specify different methods of aggregate acceptance. Typically, a battery of aggregate physical tests is run periodically on each particular aggregate source. Then, for each mix design, gradation and size requirements are checked. Normally, aggregate from more than one source is required to meet gradation requirements.
2. Asphalt binder selection: Different authorities can and do specify different methods of asphalt binder evaluation. Many States have made the transition from the Hveem or Marshall Method to the Superpave PG system.
3. Optimum asphalt binder content determination: Mix design methods are generally distinguished by the way in which they determine the optimum asphalt binder content. This process can be sub-divided into the following steps:
   • Make several trial mixes with different asphalt binder contents.
   • Compact these trial mixes in the laboratory. This compaction is meant to be a rough simulation of actual field conditions.
   • Run laboratory tests to determine key sample characteristics.
   • Pick the asphalt binder content that best satisfies the mix design objectives.

1.1.1.4 Result: The Job Mix Formula (JMF)

A successful mix design results in a recommended mixture of aggregate and asphalt binder which includes aggregate gradation and asphalt binder type is often referred to as the job mix formula (JMF). Although the JMF may subsequently be altered based on field performance, the mix design provides the initial JMF. For HMA manufacturing, target values of gradation and asphalt binder content are specified based on the JMF along with allowable specification bands to allow for inherent material and production variability (see Table 1 and Figure 3). These target values and specification bands are based on the JMF and not any general HMA gradation requirements. Thus, the mix designer is allowed substantial freedom in choosing a particular gradation for the JMF and then the manufacturer is expected to adhere quite closely to this JMF gradation during production.

1.1.2 Hveem Method

The basic concepts of the Hveem mix design method were originally developed by Francis Hveem when he was a Resident Engineer for the California Division of Highways in the late 1920s and 1930s. The Method is still used by several western states.  Like the Marshall and Superpave methods, the Hveem method produces quality HMA from which long-lasting pavements can be constructed.  This section briefly discusses the Hveem mix design method.

The Hveem mix design method consists of three basic steps:

1. Aggregate selection:  Different agencies / owners specify different methods of aggregate acceptance.  Typically, a battery of aggregate physical tests is run periodically on each particular aggregate source.  Then, for each mix design, gradation and size requirements are checked.  Normally, aggregate from more than one source is required to meet gradation requirements.
2. Asphalt binder selection:  States which use the Superpave PG system for asphalt binder specification typically use it to also specify the asphalt binder in the Hveem method.  Older documents may refer to the now-replaced aged residue (AR) system.  Commonly used grades in the AR system were AR-4000W and AR-2000W. 
3. Optimum asphalt binder content determination:  In the Hveem method, this step can be broken into five sub-steps:
   • Prepare six initial samples, each at different asphalt binder contents.  For instance, one sample each might be made at 4.5, 5.0, 5.5, 6.0, 6.5 and 7 percent asphalt by dry weight for a total of six samples.
   • Compact these trial mixes in the California Kneading Compactor (see Figure 4).  This compactor is specific to the Hveem mix design method.    
   • Test the samples for stability and cohesion using the Hveem stabilometer (see Figures 5 and 6) and cohesiometer.  These tests are specific to the Hveem mix design method.  Passing values of stability and cohesion depend upon the mix class being evaluated.  Typically, all samples pass the cohesion test and three or four pass the stability test. 
   • Determine the density and other volumetric properties of the samples.
   • Select the optimum asphalt binder content.  The asphalt binder content corresponding to 4 percent air voids is selected as long as this binder content passes stability and cohesion requirements.

Typical Hveem mix design specifications are shown in Table 2 below.

1.1.3 Marshall Method

Although many states do not use the Marshall method, many private laboratories do because it is a proven method and requires relatively light, portable and inexpensive equipment. Like the Hveem and Superpave methods, the Marshall method produces quality HMA from which long-lasting pavements can be constructed. This section briefly discusses the Marshall mix design method.

The basic concepts of the Marshall mix design method were originally developed by Bruce Marshall of the Mississippi Highway Department around 1939 and then refined by the U.S. Army.

Typically, the Marshall mix design method consists of three basic steps:

1. Aggregate selection: Different agencies / owners specify different methods of aggregate acceptance. Private labs may or may not run periodic aggregate physical tests on a particular aggregate source. For each mix design, gradation and size requirements are checked. Often, aggregate from more than one source is required to meet gradation requirements.
2. Asphalt binder selection: Many States use the Superpave PG system for asphalt binder specification which is becoming more accepted in private industry. Older specifications may refer to the now-replaced viscosity (AC) system. Commonly used grades in the AC system were AC10, AC20 and AC-30.
3. Optimum asphalt binder content determination: In the Marshall method, this step can be broken into 5 sub-steps:
   • Prepare a series of initial samples, each at different asphalt binder contents. For instance, two to three samples each might be made at 4.5, 5.0, 5.5, 6.0 and 6.5 percent asphalt by dry weight for a total of 10 to 15 samples. There should be at least two samples above and two below the estimated optimum asphalt content.
   • Compact these trial mixes using the Marshall drop hammer, an instrument specific to the Marshall mix design method.
   • Test the samples in the Marshall testing machine for stability and flow. This testing machine is specific to the Marshall mix design method. Passing values of stability and flow depend upon the mix class being evaluated.
   • Determine the density and other volumetric properties of the samples.
   • Select the optimum asphalt binder content. The asphalt binder content corresponding to 4 percent air voids is selected as long as this binder content passes stability and flow requirements.

Basic Marshall mix design specifications from the Asphalt Institute are shown in Table 3 below.

1.1.4 Superpave Method

Many states are transitioning from the Marshall or Hveem mix design method to the Superpave mix design method. Like the Marshall and Hveem, the Superpave method produces quality HMA from which long-lasting pavements can be constructed. This section briefly discusses the Superpave mix design method.

The Superpave mix design method consists of three basic steps:

1. Aggregate selection:  Aggregate is specified in three ways.  First, restrictions on aggregate gradation are specified by using gradation specifications.  Second, there are requirements on aggregate angularity, flat and elongated particles, and clay content.  Third, aggregate criteria, which the Asphalt Institute (2001) calls "source properties" (because they are considered to be source specific), such as durability and soundness are specified.
2. Asphalt binder selection:  The Superpave PG system is typically used for the asphalt binder specification. Superpave PG asphalt binders are selected based on the expected pavement temperature extremes in the area of their intended use.  These extremes can be calculated using software (such as LTPPBind) or, more commonly, determined based on standard PG binders specified by many States.  Sometimes, these standard binder grades can be adjusted based on the anticipated traffic level, type or speed.
3. Optimum asphalt binder content determination:  In the Superpave method, this step can be broken into 4 sub-steps:
   • Prepare 6 initial samples, two at the proposed design asphalt content, two at 0.5 percent below the design asphalt content and two at 0.5 percent above the design asphalt content.
   • Compact these trial mixes in the Superpave Gyratory Compactor (see Figures 7, 8 and 9).  This compactor is specific to the Superpave mix design method.
   • Determine the density and other volumetric properties of the samples.
   • Select the optimum asphalt binder content.  The asphalt binder content corresponding to 4 percent air voids.

In the Superpave mix design process there are no accepted standard performance tests so nothing analogous to the Marshall or Hveem stability and Hveem cohesion tests is used.  Research into creating a standard performance test is ongoing.