Chapter 5: Chip Seals

3.0 Design and Specification

3.1 Material Specifications

3.1.1 Binders

Binders are selected for their ability to provide good adhesion (stickiness). Polymer-modified emulsion binders usually contain latex additives, although other elastomeric polymers are often used. The purpose of the polymer is to improve stone retention during the early life of the treatment and to increase the softening point of the binder after cure (i.e., the temperature at which the binder changes phase from being primarily solid to being primarily fluid). The general-purpose base binder is an 85/100-penetration grade asphalt cement. This base binder mostly controls low temperature properties. For cold climates, a softer base asphalt (e.g. an 120/150 penetration grade) may be warranted. For hot climates, a harder base binder (e.g., a 40/50 penetration grade) might be considered.

As their names imply, performance-based asphalts may contain a range of materials to enhance certain performance characteristics. PBA-6 and PBA-6a usually contain elastomeric polymers, which increase the binder’s softening point and improves its crack resistance. Asphalt rubber binders CMCRA contain high levels of crumbed tire rubber and high natural rubber materials, which increase the softening point of the binder, improve stone retention, and produce good resistance to reflection cracking. In general, selection of the base binder largely determines the low temperature properties; softer bases should be used in lower temperature areas. Selection of a particular binder type should also take into consideration climatic conditions, traffic levels, and types of loads associated with the project (e.g., consideration of snow plow use, AADT, and percent of trucks using the roadway).

Emulsion specifications are discussed in Section 94 of the Standard Specifications (2) and related SSPs as discussed in Chapter 2 of this guide.

3.1.2 Aggregates

For best performance of chip seals, the aggregate should have the following characteristics:

• Single-sized, if possible
• Clean
• Free of clay
• Cubical, with limited flat particles (low Flakiness Index )
• Crushed faces (75% of aggregate particles have at least two fractured faces)
• Compatible with the selected binder type (ASTM D1664 assists in determining compatibility)
• Aggregates must be surface damp for use with emulsions, but must be dry for use with hot binders

The specifications for aggregates used in chip seals are included in Section 37-1.02 of the Standard Specifications, Supplemental Specifications and Special Provisions.

3.2 Chip Seal Design

Properly designed chip seals are cost-effective in sealing pavements and providing a new riding surface with enhanced friction.  Internationally, many countries have developed rational chip seal design methods and, as a result, have used chip seals on major highways.  Caltrans does not currently employ a formal design process for Chip Seals.  The methods currently used are based on experience and do not address adjustments for the factors identified below.  This section is included for information purposes only and to provide a foundation for an improved design process.  The evolution of the North American practice of chip seal design is based largely on experience and observation, whereas in overseas practice, chip seals are designed based on engineering principles.

The basics of chip seal design are straightforward.  Binder application rate and aggregate application rate are important variables to consider.  Calculating these application rates, however, requires an understanding of the materials and the surface on which they are to be applied.  Additional factors to consider include traffic type and volume, climate, and existing surface condition.  Determining the proper binder and aggregate application rates is covered in greater detail in the following two sections.  The design of multiple seal coats is also briefly described.  However, sand seals and sandwich seals are designed strictly from experience and are not included in this discussion of design procedures.

3.2.1 Binder Application

In chip seal design, the residual asphalt application rate is the most important factor affecting seal performance.  There must be enough binder to hold the aggregate in place, but not so much that the binder fills, or is forced by traffic action to cover the aggregate (commonly referred to as bleeding or flushing).  The proper amount of binder helps create the desired rough surface texture.  Binder application rates are determined based on the average least dimension of the largest aggregate, as well as other aggregate properties such as shape, density, absorption and grading.  Optimal binder content also depends on how much binder flows into existing voids in the pavement.

The McLeod method is most commonly used to design chip seals (6) (6); however it is not used by Caltrans. This method assumes that 70% of the voids in the aggregate must be filled (i.e., 70% embedment).  In some states, this is adequate and has been adopted as the standard; however, modifications can be made for varying project conditions.  See Figures 2 through 5.

A more detailed discussion on this design method can be found in A General Method of Design for Seal Coats and Surface Treatments by N.W. McLeod.  The McLeod method also assumes the use of a cubical, single-sized aggregate.  This may not always be the case (e.g. California specifications specify graded aggregates) since many agencies specify graded aggregates. The main modification for graded aggregates is determining a median aggregate size (50% passing).  The aggregate shape must also be examined; this is done by measuring the flakiness index (3).  The average least dimension (ALD) can then be determined using the following equation (3):

ASTM C29 is used to measure the loose unit weight.  This approximates the voids in the loose aggregate when it is dropped onto the pavement.  The voids in this state are 50% for cubical, single-size aggregate and lower for graded aggregate.  It is assumed that, once rolled, a cubical aggregate will reduce its unit weight to a point where the voids content is 30% and finally reduces to 20% once trafficked.  These assumptions are adjusted when using graded aggregates.  Figures 2 through 4 illustrate the average least dimension (ALD) concept, along with the effects of flakiness and changes in voids based on compaction.

Figure 2: Illustration of ALD (4)

Figure 3a: Illustration of Flakiness of Aggregates (4)

Figure 3b: Illustration of Flakiness of Aggregates (4)

Figure 4: Effects of Compaction on Voids in Cubical Aggregate (4)

The voids in loose aggregate may be calculated using the following equation (3):

Most design methods calculate the specific requirements for each job by considering the required corrections in addition to the basic application rate, i.e., the rate designed to result in 70% embedment.  One method for estimating the binder content is as follows (3):

B = [0.40(H) ( T ( V + S + A + P] / R    (4.3)

For projects in areas maintained by snowplows, the binder content is calculated using both the median particle size and the ALD.  The average of these two results is the starting application rate in these areas.

Corrections to the basic application rate for the aggregate must address certain variables that affect the level of embedment in the binder.  The corrections are ultimately applied to the calculation of the binder application rate.  These variables include:

• Aggregate Characteristics: Important aggregate characteristics include absorption and shape. Corrections for absorption are based on experience and the characteristics of the local aggregates. Chip shape effects are variable: rounded chips leave greater voids and do not interlock and require additional binder. This type of chip is not recommended. Non-uniform sized aggregates produce uneven surfaces. Figure 5 illustrates both rounded and non-uniform chip applications

Figure 5: Aggregate Shape Characteristics (5)

• Traffic Volume: This factor accounts for the role of traffic volume in achieving the desired embedment. The traffic factor is lower for higher traffic volumes and higher for lower traffic volumes. Table 3 lists the application rate correction factors associated with varying traffic levels.

• Loss of Aggregate Due to Traffic (Traffic Whip-Off): A traffic whip-off correction accounts for the effects of traffic operations on removing aggregates from newly chip sealed roads. Reasonable values for losses are 5% for low-volume roads and residential streets and 10% for high-speed roads and highways. Table 4 lists road types and associated whip-off correction factors.

• Existing Pavement Condition: Existing pavement conditions play an important role in determining the optimum binder content. A smooth surface will require less binder than a rough or porous surface. Table 5 details the correction factors associated with various existing pavement conditions.

• Embedment: Aggregates may be punched or embedded into soft pavement surfaces by roller compaction and traffic.  Table 6 provides corrections based on surface hardness and related traffic volume using a Ball Penetrometer test (7).

3.2.2 Aggregate Application

Calculating the design aggregate application rate is based on the amount of aggregate needed to create an even, single coat of chips on the pavement surface.  The amount of cover aggregate required can be determined using the following equation (3):

Metric  C = ( 1 - 0.4V) H G E

English C = 46.8 ( 1 - 0.4V) H G E     (4.4)

Equation 4.1 calculates H (average least dimension) and Equation 4.2 calculates V (voids in loose aggregate).  The bulk-specific gravity of coarse and fine aggregates, G, can be determined using ASTM C127 & C128 CT 206 and CT 208, test methods, respectively.  The waste factor (E) compensates for whip-off and handling; normally the designer estimates E based on experience with local conditions.  While other design methods are available, Equation 4.4 provides a good starting point and covers most situations.  It requires that the user consider both the attributes of the surface being sealed and its intended use.

The design of multiple coat seals is based on the same concepts as the single chip seal.  First, a design is performed for each layer as if it were the only layer in the system.  Next, the following three rules are applied:

1. Maximum nominal top size of each succeeding layer of cover aggregate should be no more than half the size of the previous layer’s aggregate.
2. No allowance is made for waste.
3. Except for the first application, no correction is made for the underlying surface texture.  The amount of binder determined for each layer of aggregate are added together to calculate the total binder requirement.  For two-layer chip seals, 40% of the total binder requirement is applied to embed the first layer of aggregate; the remaining 60% is applied to embed the second layer of aggregate.

3.2.3 Application Rate for Polymer Modified and Chemically Modified Crumb Rubber Asphalt (CMCRA) Asphalt Rubber Modified Seals

For CMCRA asphalt rubber (e.g., SAMI), typical binder application rates of 2.2 to 2.5 l/m² (0.49 to 0.55 gal/yd²) are used.  For asphalt rubber seals, the binder application rate is significantly higher compared with the base application level calculated for unmodified binder.  The higher binder rates are possible due to the higher viscosity of these binders.  Application of cover aggregate should be the same in a SAM or SAMI to avoid damage to the membrane due to pick-up by the construction equipment or when the membrane is opened to traffic.

Caltrans practices for these materials are summarized in their standard specifications, Section 37-1.05.