Chapter 5: Chip Seals

1.0 Introduction

Chip sealing is the application of a bituminous binder immediately followed by the application of an aggregate.  The aggregate is then embedded into the binder using pneumatic-tired rollers.  (This is commonly referred to as “rolling,” but is not intended to imply that the aggregate is rolled by the rollers.)  Multiple layers may be placed and various binder and aggregate types can be used to address specific distress modes or traffic situations.

1.1 Types of Chip Seals

There are many different types of chip seals used by agencies, but only treatments currently being used by Caltrans are discussed in detail in this manual. However, additional treatments not currently in use are also described to promote a broader understanding of other methods. Types of chip seal treatments include:

There are many different types of chip seals used by agencies.  Several treatments are discussed in detail in this manual to promote a broader understanding of a variety of methods.  Types of chip seal treatments include:

• Single Chip Seal: A single chip seal is an application of binder followed by an aggregate.  This is used as a pavement preservation treatment and provides a new skid-resistant wearing surface, arrests raveling, and seals minor cracks.  Figure 1 illustrates a single chip seal application.
  

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Figure 1: Single Chip Seal

1.1.2 Multiple Chip Seal:

A multiple chip seal (or armor coat) is a built-up seal coat consisting of multiple applications of binder and aggregate.  For example, installing a double chip seal consists of the following steps:

1. Spraying an application of binder.
2. Spreading a layer of aggregate.
3. Rolling the aggregate for embedment.
4. Removing excess aggregate.
5. Applying a second application of binder.
6. Spreading another layer of aggregate (approximately half the average least dimension of the base coat aggregate).
7. Rolling the second layer of aggregate.
8. Removing excess aggregate. 

This process may be repeated as necessary to build up a pavement’s edges.  Multiple chip seals are used where a harder wearing and longer lasting surface treatment is needed.  Caltrans does not use multiple chip seals at this time.  Figure 2 illustrates a multiple chip seal application.

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Figure 2: Multiple Chip Seal

1.1.3 Stress Absorbing Membrane (SAM) Seal

SAM is a single chip seal in which a modified binder (normally asphalt rubber), often asphalt rubber, is applied, followed by a layer of aggregate. The aggregate is then rolled into the binder. Binder application rates are much higher than those used for conventional chip seals.

1.1.4 Stress Absorbing Membrane Inter-layer (SAMI)

SAMI is a membrane seal that retards the rate of reflection cracking in new overlays. It consists of an application of modified binder followed by a layer of aggregate which is spread and rolled. An overlay is then placed over the membrane. If necessary, traffic may be allowed on the SAMI prior to construction of the overlay.

1.2 Binder Types

Binder type varies according to the type of chip seal being used.  Binder types include:

• Asphalt Emulsion: The type of asphalt emulsion must be compatible with the proposed aggregate. Polymer-modified emulsions will influence performance.Polymer-modified emulsions (PME), such as PMCRS-2h, as included in the Standard Specifications, Section 94 (1).
  
• Performance Grade Binders Performance-Based Asphalt (PBA) Cements: Hot applied modified binders that can be placed at cooler temperatures than emulsion binders and can be placed at night. Examples include PBA 6 and PBA 6a binders (2).
  
• Chemically Modified Crumb Rubber Asphalt Asphalt Rubber Binder: Binders modified with high levels of crumbed tire rubber and a high natural rubber content material. These binders are sprayed hot and require hot chips pre-coated with asphalt. Hot applied CMCRA binders can be placed at cooler temperatures than emulsion binders and can be placed at night.
  
• Rejuvenating Emulsion: These are emulsions modified with rejuvenating oils (and sometimes polymers) that are used to penetrate and soften existing asphalt pavements.

Table 1 lists common binder types and their suitable applications.

Table 1: Binder Type and Suitable Applications

Binder Type	Application
	Chip Seal Single Course	Chip Seal Multiple Course	Sand Seal	SAM & SAMI
Asphalt Emulsions	Yes	Yes	Yes	No
Performance Graded Binders PBA	Yes	Yes	Yes	No
Chemically Modified Crumb Rubber Asphalt Asphalt Rubber	Yes	Yes	Yes	Yes
Rejuvenating Emulsions	Yes	Yes	Yes	No