Reference Manual
Module 3-8. HMA Pavement Recycling and Reconstruction
MODULE 3-8. HMA PAVEMENT
RECYCLING AND RECONSTRUCTION
1.
INSTRUCTIONAL OBJECTIVES
This module presents information on recycling and reconstruction of HMA pavements. Asphalt
pavement recycling techniques include hot in-place recycling, cold in-place recycling and hot central
plant recycling. Upon completion of this module, the participant should be able to accomplish the
following:
1. Identify options when a pavement is near the end of its service life.
2. Describe feasibility and approach for cold in-place, hot in-place, and hot central plant recycling.
3. Identify considerations associated with reconstruction.
2.
INTRODUCTION
The need to reuse or recycle existing pavement materials for the reconstruction and rehabilitation of
HMA and PCC pavements is of increasing importance. Recycling can help to optimize the use of
available materials and energy supplies, and to decrease the cost of maintaining highways, roads, and
streets in the United States.
Rehabilitation and maintenance of the highway and street transportation system in the United States is
costly, time-consuming, material-intensive, and an ever-increasing burden on public agencies. The
recycling of existing pavement materials for rehabilitation and maintenance purposes offers several
advantages over the use of conventional materials and techniques in easing this burden. As the
techniques for recycling improve and specification-writing agencies and contractors become familiar with
the various processes available, the use of recycling has demonstrated cost savings over the use of new
materials for major maintenance and rehabilitation of pavements. The Federal Highway Administration
(FHWA) estimates the pavement industry generated $105.5 million in savings using recycled materials in
1985 and that 34 States had accepted some form of asphalt recycling in their specifications by 1985
(Roads and Bridges 1986). By the year 2000, all 50 States have recycled HMA pavements using various
methods and to varying degrees (FHWA 2000). Other major benefits of recycling are conservation of
aggregates, binders, and energy, as well as preservation of the environment and existing highway
geometrics. In 1999, industry estimated that a billion dollars had been saved in landfill tipping fees thru
the use of recycling. An industry survey estimated 34 million tons of HMA were recycled and not placed
into landfills (ARRA 2000).
Recycling or reuse of existing pavement materials for pavement rehabilitation, reconstruction, and
maintenance is not a new concept; literature indicates that pavement recycling existed as early as 1915
(NAPA 1977a). However, the quantity of pavement materials recycled from 1915 to 1975 is small in
comparison to the amount of recycling that has taken place since 1975.
The engineering community’s interest in recycling starting in 1975 was largely based on economics, with
some interest in energy conservation. During the mid and late 1970s in the United States there were
problems related to a) reduced funding for transportation facilities, b) materials supply, c) equipment
availability, d) trained manpower availability, and e) energy awareness and availability. Recycling of
existing pavement materials for construction, rehabilitation, and maintenance purposes offered a partial
solution to these problems. Specifically, recycling offered the following major potential benefits
compared with conventional techniques:
HMA Pavement Evaluation and Rehabilitation
3-8.1
Module 3-8. HMA Pavement Recycling and Reconstruction

Reduced costs.

Preservation of existing pavement geometrics.

Conservation of aggregates and binders.

Preservation of the environment.

Energy conservation.
Reference Manual
Because recycling appeared promising from a wide variety of viewpoints, a number of agencies,
including the National Cooperative Highway Research Program (NCHRP), FHWA, Corps of Engineers
(for the Air Force), and United States Navy sponsored recycling research and implementation studies
(Epps 1978; Epps et al. 1980; Beckett 1977; Brown 1977; FHWA 1975, 1977, 1978a, 1978b; Anderson et
al. 1978; Lawing 1976; Brownie and Hironaka 1978; Sullivan 1996b; McDaniel and Anderson 2000b).
Associations and institutes also contributed to the development of recycling in the United States. These
groups include The Asphalt Institute (1983a, 1983b, AI 1986), National Asphalt Pavement Association
(NAPA) (NAPA 1977b; 1978), Portland Cement Association (PCA) (PCA 1976), Pacific Coast userProducer Group on Asphalt Specifications (PCUPSC 1978), American Society for Testing and Materials
(ASTM 1978), American Concrete Pavement Association (ACPA), Asphalt Emulsion Manufacturers
Association (AEMA), and the Asphalt Recycling and Reclaiming Association (ARRA).
Early research, development, and implementation efforts led to the categorization of four types of
pavement recycling:

Surface recycling.

Cold recycling.

Hot recycling.

Portland cement concrete pavement recycling.
These forms of recycling are addressed in a comprehensive manner in several publications. (See
references by TRB [1980], Epps et al. [1980], FHWA [1978b], Departments of the Army and the Air
Force [1984], Newcomb and Epps [1981], and ARE [1987a, 1987b, 1987c].)
In the past 20 years there has been increasing emphasis on the need to reduce pavement rehabilitation
costs and to conserve energy. Because of these twin emphases, many public agencies have reexamined
and recognized the great value of recycling techniques. Recycling is considered capable of not only
producing cost and energy savings, but also reducing the demand for asphalt during supply interruptions.
Common practice for many years has been to waste the old pavements removed before pavement
reconstruction. In recent years, the use of pavement grinding to restore pavement ride quality, remove
corrugations, smooth faulted joints, and so on, has produced additional quantities of waste asphalt and
concrete pavement materials. A comparatively recent development has been to treat salvaged pavements
as materials having economic value.
Contractors and plant operators in some parts of the country have routinely blended small quantities
(approximately 5 percent) of waste asphalt pavements into mixes produced for commercial and private
work. In some States, contractors bidding on recycling work are required to purchase or submit price
reductions for waste asphalt pavements removed during construction.
The significance of these developments is that most public agencies no longer consider recycling an
experimental process. Many public agencies routinely permit recycling alternates in their standard
3-8.2
HMA Pavement Evaluation and Rehabilitation
Reference Manual
Module 3-8. HMA Pavement Recycling and Reconstruction
specifications, as well as in special provisions. Production equipment is now available to permit the
effective use of recycling and cost benefits of 10 percent or more have been reported. FHWA estimates
that 80 percent of asphalt pavement that is removed each year during widening and resurfacing projects is
reused as part of new roads, road beds, shoulders, and embankments (Morris 2000).
3.
DEFINITIONS
The two types of paving material recycled can classify pavement recycling operations: asphalt and
portland cement concrete. These categories can be broken down further according to the particular
procedure used. Figure 3-8.1 illustrates the framework in which the types of pavement recycling are
defined. The different categories themselves are described further below.
Cold
Surface
Recycling
Asphalt
Pavement
Recycling
Cold-Mix
Recycling
Hot
(Hot In-Place
Recycling)
In-Place
(Cold In-Place
Recycling)
Central Plant
Pavement
Recycling
Portland
Cement
Concrete
Recycling
Hot-Mix
Recycling
Central Plant
Hot Central
Plant Recycling
Figure 3-8.1. Categorization of pavement recycling.
Pavement Recycling. The reuse of material from in-place pavements that are processed to provide quality
paving materials suitable for use in new construction or in the rehabilitation of pavements. Pavement
recycling is applicable to asphalt concrete pavements, portland cement concrete pavements (other
pavement surfaces, such as chip seals and slurry seals) and to roads and streets that are unpaved or that
have asphalt, portland cement or pozzolanic base courses. Definitions of the types of recycling follow
(AASHTO 1993; AI 1986).
Asphalt Pavement Recycling. The reuse of an existing asphalt pavement by employing one of three
recycling procedures: surface recycling, cold-mix recycling, or hot-mix recycling. The existing asphalt
surface may or may not be an existing HMA pavement, and that in turn determines the type of recycling
that is used.
Asphalt Pavement Surface Recycling. The reworking in-place of the surface of an asphalt pavement to a
depth of less than about 50 mm (2 in) by any of the suitable machinery available. This operation is a
single or multi-step process that may involve the use of added materials, including aggregate, modifiers,
or asphalt mixtures (virgin or recycled).
HMA Pavement Evaluation and Rehabilitation
3-8.3
Module 3-8. HMA Pavement Recycling and Reconstruction
Reference Manual
Cold-Mix Asphalt Pavement Recycling. The reuse of treated or untreated base materials and/or asphalt
concrete pavement that is either processed in-place or at a central plant with the addition of asphalt
emulsions, cutbacks, portland cement, lime and/or other materials as required to achieve a desired mix
quality. After the material is processed, it is placed and compacted.
Hot-Mix Asphalt Pavement Recycling. The removal of more than the top 25 mm (1 in) of a HMA
pavement with or without removal of underlying pavement layers (e.g., untreated base materials) that is
processed by sizing, heating and mixing in a central plant with additional components such as aggregate,
bitumen, or recycling agents; then laid and compacted according to standard specifications for
conventional hot mixtures (e.g., asphalt concrete base, binder, and asphalt concrete leveling or surface
course).
As pavement recycling technology continues to develop, these definitions continue to be refined. For
HMA pavements, the three most common types of recycling are hot in-place recycling, cold in-place
recycling, and hot central plant recycling. Definitions for these three forms of pavement recycling are
given below:
Hot In-Place Recycling (HIR) (Button, Little, and Estakhri 1994). A process of correcting asphalt
pavement surface distress by softening the existing surface with heat, mechanically loosening the
pavement surface, mixing as necessary with recycling agent, aggregate, or hot-mix asphalt, and replacing
the loosened material on the pavement without removing the recycled material from the original
pavement site. HIR may be performed as either a single-pass operation that recombines the restored
pavement with virgin material, or as a two-pass operation, wherein the restored material is recompacted
and the application of a new wearing surface then follows a prescribed interim period.
Cold In-Place Recycling (CIR) (Epps 1990). A process of correcting asphalt pavement distress by
processing, without heat and in-place, the existing pavement material(s) and combining as necessary with
a stabilizing agent, recycling agent, and/or aggregate. After processing and mixing, the material is placed
and compacted. A surface or wearing course is typically applied to the surface.
Hot Central Plant Recycling (HCPR). The removal of a portion of a HMA pavement, with or without
removal of underlying pavement layers (e.g., untreated base materials), that is processed by sizing,
heating, and mixing in a central plant with additional components such as aggregate, asphalt, or recycling
agents, and then relaid and compacted.
Additional definitions associated with asphalt pavement recycling are given below:
Reclaimed Asphalt Pavement (RAP). Removed and/or processed pavement materials containing asphalt
cement and aggregate.
Reclaimed Aggregate Material (RAM). Removed and/or processed pavement materials containing no
asphalt cement.
Recycled HMA. The final mixture of RAP, new asphalt cement, recycling agent and, if necessary, RAP
or new aggregates, produced at a hot-mix plant.
Asphalt Recycling Agent. A petroleum product additive with a combination of chemical and physical
properties designed to restore aged asphalt to desired specifications. The recycling agent should conform
to the specifications described in ASTM D 4552: “Standard Practice for Classifying Hot-Mix Recycling
Agents.”
3-8.4
HMA Pavement Evaluation and Rehabilitation
Reference Manual
Module 3-8. HMA Pavement Recycling and Reconstruction
Asphalt Modifier. A generic term describing any compound or material that is used as an admixture to
alter or improve the properties of the asphalt binder in the recycled asphalt mixture. Included are asphalt
cements, cutback asphalts, emulsified asphalts, recycling agents, and polymers.
Heater-Scarification. Heating, scarifying, rejuvenating, leveling, reprofiling, compacting.
Repaving. Heating, scarifying, rejuvenating, leveling, laying new hot-mix, reprofiling, compacting.
Remixing. Heating, scarifying, rejuvenating, mixing (and/or adding new hot-mix), leveling, reprofiling,
compacting.
4.
SELECTION OF RECYCLING AS A REHABILITATION ALTERNATIVE
The reasons for selecting a form of recycling as a rehabilitation alternative include initial cost, life-cycle
cost, time required for the rehabilitation alternative, reliability, chance of success of the alternative, and so
on. Table 3-8.1 is provided to generally define some of the advantages and disadvantages of the major
types of asphalt recycling alternatives.
More detailed information on recycling can also be found in NHI Course No. 131050, Asphalt Pavement
Recycling for State and Local Governments.
5.
COLD IN-PLACE RECYCLING
As shown on figure 3-8.2, cold recycling may be performed in-place or at a central plant. Cold in-place
recycling is used more often than cold central plant recycling. Cold in-place recycled materials have been
used for subbases, bases, and surfaces. The most common use to date has been for base courses.
Although stabilization with bituminous materials is the most popular process, literature indicates that
lime, portland cement, flyash, and calcium chloride have also been used (Epps 1990; Cross 1999).
Two forms of cold in-place recycling with bituminous binders have evolved in the United States: fulldepth and partial-depth. Full-depth (reclamation/stabilization) cold in-place recycling is a rehabilitation
technique in which the full flexible pavement structure and predetermined portions of the base material
are uniformly crushed, pulverized, and mixed with a bituminous binder, resulting in a stabilized base
course. Additional aggregate may be transported to the site and incorporated in the processing. This
process is normally performed to a depth of 100 to 300 mm (4 to 12 in) (Epps 1990).
Partial-depth cold in-place recycling is a rehabilitation technique that reuses a portion of the existing
asphalt-bound materials. Normal recycling depths are 50 to 100 mm (2 to 4 in). The resulting
bituminous-bound recycled material is often used as a base course, but can be used as a surface course on
low to medium traffic volume highways. When this form of cold in-place recycling is performed on an
old uniform pavement, a higher-quality end product is expected (Epps 1990).
Background
The use of full-depth cold in-place recycling with bituminous binders probably dates to the 1910s. States
with extensive experience with full-depth, cold in-place recycling include California, Indiana, Kansas,
Michigan, Nevada, and New Mexico. A number of other States have completed numerous projects
identified later (Epps 1990).
Partial-depth cold in-place recycling dates to 1980 with contract-size projects. The States of California,
Kansas, Maine, Nevada, New Mexico, Oregon, and Pennsylvania have experience with this form of cold
in-place recycling. Oregon, in particular, has placed numerous projects (Epps 1990).
HMA Pavement Evaluation and Rehabilitation
3-8.5
Recycling Techniques
Advantages
Disadvantages
• Reduces frequency of reflection cracking.
• Promotes bond between old pavement and thin overlay.
• Provides a transition between new overlay and existing
gutter, bridge, and/or pavement that is resistant to
raveling (eliminates feathering).
• Reduces localized roughness due to compaction.
• Treats a variety of types of pavement distress (raveling,
flushing, corrugations, rutting, oxidized pavement,
faulting) at a reasonable initial cost.
• Improves skid resistance.
• Provides limited structural improvement.
• Heater-scarification and heater-planing have limited
effectiveness on rough pavement without multiple
passes of equipment.
• Limited repair of severely flushed or unstable
pavements.
• Some air quality problems.
• Vegetation close to roadway may be damaged.
• Mixtures with maximum size aggregates greater than
25 mm (1in) cannot be treated with some equipment.
• Limited disruption to traffic.
In-place
•
•
•
•
•
•
•
•
•
•
•
Central
•
•
•
•
•
•
•
Provides significant structural improvements.
Treats all types and degrees of pavement distress.
Can eliminate reflection cracking.
May improve frost susceptibility.
Improves ride quality.
Improves skid resistance.
Minimizes hauling.
Provides significant structural improvements.
Treats all types and degrees of pavement distress.
Can eliminate reflection cracking.
Improves skid resistance.
May improve frost susceptibility.
Enables geometrics to be more easily altered.
Improves quality control if additional binder and/or
aggregates must be used.
• Improves ride quality.
Quality control not as good as central plant.
Traffic disruption.
PCC pavements cannot be recycled in-place.
Curing often required for strength gain.
• Potential air quality problems at plant site.
• Traffic disruption.
Reference Manual
HMA Pavement Evaluation and Rehabilitation
Surface
Module 3-8. HMA Pavement Recycling and Reconstruction
3-8.6
Table 3-8.1. Major advantages and disadvantages of asphalt pavement recycling techniques (Epps et al. 1980).
Reference Manual
Module 3-8. HMA Pavement Recycling and Reconstruction
Pulverize
pavement and
base material with
multiple and two
step sequences
Fine grade
and
compact
Add and
mix stabilizing
agent
Fine grade
and
compact
Windrow
material
Prepare
Construction
Area
Rip and break up
asphalt concrete
surfaces
Pulverize
pavement and
base material
Fine grade
and
compact
Pulverize, add and
mix stabilizing
agent, place on
grade with
equipment train
Add and
mix stabilizing
agents
Fine grade
and
compact
Compact
Pulverize, add and
mix stabilizing
agent, place on
grade with single
machine
Compact
Prime and
place
surface
course as
required
Tack and
place
surface
course as
required
Prime and
place
surface
course as
required
Tack and
place
surface
course as
required
Figure 3-8.2. Full-depth cold in-place recycling.
A nationwide survey of cold in-place recycling was conducted in early 1987 for the ARRA (ARRA
1988). This survey did not differentiate between full-depth and partial-depth cold in-place recycling.
ARRA received responses from all state highway agencies, as well as numerous counties, cities, and
private contractors. Twenty-four states indicated use of cold in-place recycling, five states indicated that
they have placed only experimental test sections, and the remaining 21 states did not use cold recycling.
Several states, including California, Kansas, New Mexico, Oregon, and Pennsylvania, indicated that they
have constructed numerous projects. Based on the ARRA survey, county roads and secondary highways
comprised equal proportions of cold in-place recycling projects (31 percent of responses each) (ARRA
1988). City street projects accounted for 19 percent and primary and interstate highways comprised 12
and 7 percent shares, respectively (ARRA 1988). A similar survey conducted in 1994 found that at least
32 States have used or are using cold recycling (Collins and Ciesielski 1994).
The literature indicates that cold in-place recycling has been used for all types of roads and structural
section components. However, some agencies restrict its use. Twenty percent of the agencies reporting
to ARRA restrict cold in-place recycling to rural areas; an additional 20 percent limit use to road with low
traffic volumes. Most agencies limit the use of cold in-place recycling to base courses (95 percent). Of
these base course projects, 12 percent placed fog, sand, or slurry seals as surfaces; 33 percent of the
projects were surfaced with aggregate chip seals; and 50 percent were surfaced with HMA. Three States
use cold in-place recycling for shoulder reconstruction on interstate highways (ARRA 1988).
A listing of several comprehensive references on cold in-place recycling is given below.

NCHRP Synthesis 54 (Epps 1978).

NCHRP Recycling Guidelines (Epps et al. 1980).

TRB National Seminar (TRB 1980).
HMA Pavement Evaluation and Rehabilitation
3-8.7
Module 3-8. HMA Pavement Recycling and Reconstruction

Chevron Cold Mix Recycling Manual (Chevron USA 1982).

The Asphalt Institute (Asphalt Institute 1983a).

Scherocman 1983.

FHWA (ARE 1987b).

Wood, White, and Nelson 1988.

NCHRP Synthesis 160 (Epps 1990).
Reference Manual
References dealing with partial-depth cold in-place recycling are primarily those based on Oregon
research and field experience. (See references by Cooper, Dline, and Allen [1987], Highway and Heavy
Construction [1988], Allen [1985], Allen et al. [1986], and the Pacific Coast User-Producer Conference
[1989]). NCHRP Synthesis 160 was used as the base for the background for this module (Epps 1990).
Recycling Methods and Equipment
Figures 3-8.2 and 3-8.3 describe construction operations associated with full-depth and partial-depth cold
in-place recycling. The type of equipment and the sequence of operations are largely dictated by the
specifications, the contractor’s experience, and the type of cold in-place recycling (full-depth or partialdepth).
Cold in-place recycling consists of nine identifiable operations:

Pavement sizing.

Addition of new aggregate.

Addition of new asphalt/recycling agent.

Mixing.

Lay down.

Aeration.

Compaction.

Curing.

Application of wearing surface.
A wide variety of equipment has been used. Many of these operations are combined with a single
machine or operation, whereas others, such as “addition of new aggregate,” may not be necessary on
some projects. For convenience of discussion, several of these operations have been combined.
Sizing and Mixing Operations
The methods acceptable for in-place sizing and mixing for cold-recycling operations can be conveniently
separated into four techniques:

Multiple-step sequence.

Two-step sequence.

Single machine.

Single-pass equipment train.
3-8.8
HMA Pavement Evaluation and Rehabilitation
Reference Manual
Module 3-8. HMA Pavement Recycling and Reconstruction
Prepare
construction
area
Equipment train:
pulverize, crush, add
and mix stabilizing
agent, place on roadway
Single machine:
pulverize, add and mix
stabilizing agent,
place on roadway
Compact
Tack and place surface
course as required
Figure 3-8.3. Partial-depth cold in-place recycling.
All of these methods are used for full-depth, cold in-place recycling; only the single machine and
equipment train are used for partial-depth cold in-place recycling. These methods are briefly discussed
below.
Two-Step Sequence
This method combines the breaking and pulverizing or sizing steps as described above into a single
operation using a cold-milling machine or large pulverizing machine. The stabilizer is then added and
mixed in the second step. Common methods of adding stabilizers in this cold-recycling approach include
the use of soil stabilization mixing equipment and traveling mixers.
Cold-milling machines have a rotating drum lined with a variable number (depending on width) of
replaceable, tungsten-carbide-tipped cutting teeth to grind the old pavement. The advantages of coldmilling machines for breaking and pulverization include:

Accurate control of depth and profile.

Ability to pulverize and size in a single pass, resulting in less interference with traffic.

Handling of conventional curb-reveal and other cold-planing work (i.e., use is not restricted to
recycling).

Use for mixing when fitted with pump and metering system.

High productivity in almost any weather.
The disadvantages of cold-milling machines include the need for trained personnel to operate them and
their relatively high cost of operation, which can make them uneconomical for use on seal-coat or thin
plant-mixed asphalt roads. Care must be taken to ensure that the entire pavement is reduced to the proper
size and that the mix design takes into account the increase in the amount of fines created by this
equipment.
HMA Pavement Evaluation and Rehabilitation
3-8.9
Module 3-8. HMA Pavement Recycling and Reconstruction
Reference Manual
The drum of the cold-milling machine may be set to operate in an upcutting mode, in which the teeth cut
from the bottom of the pavement layers upward as the machine moves forward, or in a downcutting
mode, in which the teeth strike the top of the pavement surface in a downward direction as the machine
travels ahead. For partial-depth cuts, the upcutting mode generally offers the most accurate cutting depth,
with lower cost, greater speed, less tooth wear, less power to operate, and less damage to the underlying
surface. However, upcutting can result in the production of significant amounts of oversize material.
With downcutting, the reclaimed materials are pinched against the underlying layers, resulting in proper
sizing. The productivity of a milling machine is a function of the resistance of the pavement material to
the penetration of the cutting teeth. Three of the most important factors affecting this resistance are
material quality, aggregate characteristics, and depth of cut.
Single Machine
Single-pass equipment, capable of breaking, pulverizing, and adding stabilizers, is used for both fulldepth and partial-depth cold in-place recycling. These operations have the same advantages and
disadvantages as cold-milling machines used for pavement removal only.
Single-Pass Equipment Train
Several contractors have developed a single-pass equipment train capable of full-depth and partial-depth
cold in-place recycling. Large quantities of pavement can be recycled daily. The equipment train usually
consists of a cold-milling machine, portable crusher, travel-plant mixer, and laydown machine. The
small, portable screen and crusher unit sizes the oversize material from the milling operation. The coldmilling machine’s conveyor discharges the RAP into the crusher unit, which passes it over a screen with
large sieve sizes (e.g., 38 mm [1.5 in]). The particular sieve size will depend on the job specifications.
The material retained on the screen is rerouted to the roll unit for crushing and then back to the screen.
Eventually, 100 percent of the RAP will pass through the screen and onto another conveyor where it can
be weighed before being deposited into the pugmill or a paver. The screen and crusher unit can also be
fitted with a pugmill and asphalt feeder system for mixing. The recycled mix can then be windrowed
directly behind or to either side of the mixer or, in some cases, directly into the hopper of a self-propelled
asphalt laydown machine.
Comparison of Sizing and Mixing Operations
A partial list of advantages and disadvantages associated with each category of breaking, sizing, and
mixing operation is given in table 3-8.2.
Mixing Operations
Asphalt products used as modifiers in cold recycling include emulsified asphalts (usually either slowsetting or medium-setting), cutback asphalts, high-penetration asphalt cements (heated to a minimum
temperature of 166 ºC [331 ºF] for in-place recycling), and emulsified versions of commercial recycling
agents. In addition, water may be added initially to help in the dispersion of the asphalt modifier during
the mixing process.
Foamed asphalt is a method where a small amount of water is injected along with the heated asphalt
cement into the mixing chamber. The water immediately boils and creates a foaming action in the asphalt
cement, helping it to disperse. Foamed asphalt also differs from conventional methods in that the asphalt
cement does not really coat the particles but rather is used as a mortar-like matrix to cement the particles
together.
3-8.10
HMA Pavement Evaluation and Rehabilitation
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Module 3-8. HMA Pavement Recycling and Reconstruction
Table 3-8.2. Comparison of sizing and mixing operations.
Sizing and Mixing Operation
Advantages



Multi-step sequence
Two-step sequence
Single machine
Single-pass equipment train
Readily available equipment can
be used.
 Partial-depth removal of
asphalt concrete possible.
 High production capacities.
 Partial-depth removal of
asphalt concrete possible.
 High production capacities.
 Partial-depth removal of
asphalt concrete possible.
 Aggregate gradation control.
 High production capacities.











Disadvantages
Depth-control problems.
Removal of entire asphalt
concrete layer is necessary.
Mixing of asphalt concrete and
base.
Limited width operations.
Slow production rates.
Traffic control problems.
Construction coordination.
Aggregate oversize.
Depth limitations.
Aggregate oversize.
Specialized equipment.
Depth limitations.
Aggregate oversize.
Specialized equipment.
 Depth limitations.
 Specialized equipment.
A small percentage of portland cement, flyash, or lime may also be added with emulsified asphalts to help
stabilize the recycled mix and reduce curing time. The percentages of any added modifiers should be
established in a laboratory mix design as discussed in a later section.
As with pavement removal and size reduction, there are several alternatives for mixing. There are four
general types of soil-stabilization construction equipment that can be used for in-place cold recycling:

Blade type.

Flat type.

Windrow type.

Hopper type.
All of these equipment types are used for full-depth cold in-place recycling; however, the hopper-type
mixer is most often used for partial-depth cold in-place recycling.
Blade Mixing
Blade mixing is the simplest method, but it usually is slow and inefficient. The basic sequence involves:

Using a motor grader to windrow the pulverized reclaimed material.

Adding the prescribed amount of water (if required) to the windrow, preferably using a
pressurized water truck rather than gravity flow for reasons of accuracy of application.

Blading the windrow across the road with a rolling action to blend in the water.
HMA Pavement Evaluation and Rehabilitation
3-8.11
Module 3-8. HMA Pavement Recycling and Reconstruction
Reference Manual

Reshaping into a windrow and adding the prescribed amount of asphalt modifier, normally in two
or three passes, using an asphalt distributor.

Using the grader to fold the material around the applied asphalt modifier, followed by working
the mixture back and forth across the roadway surface until the modifier is uniformly distributed
and proper fluids content is achieved.
If new aggregate is to be added, it should be windrowed next to the existing pulverized material and
mixed in with the motor grader before water or modifier is added.
Flat Type
Mixing operations are often performed with single and multiple transverse-shaft rotary (flat type) mixers.
An asphalt distributor can either apply the asphalt modifier to the windrowed material before mixing, or it
can be added directly by the mixer by means of a spray-bar in the cutting chamber fed by an asphalt
supply tanker. With the spray-bar system, mixing can be combined with pulverization in a single-pass
operation provided the recycled pavement is sufficiently reduced in size with one pass of the mixer.
However, several passes of the machine are normally required to add the proper amount of asphalt and to
achieve uniform mix quality. Typically, pulverizing and mixing are completed in separate passes. This is
the method used most often with foamed asphalt applications. Initial passes are made to pulverize the
material and reach the proper grade, and a final mixing pass is made to add the asphalt cement.
Windrow Type
Windrow mixers can pick up the material from the grade and mix with parallel shafts. These types of
mixers are not commonly available today. Windrow, transverse-shaft mixers that do not elevate the
material above grade are available. Improved quality control is normally obtained from those mixers that
elevate the material above grade.
Hopper Type
Hopper type or travel-plant mixers are pugmill plants that can mix recycled pavement with liquid
modifier, applied at a controlled rate as they move along the road. There are several options when using
these mixer-pavers for in-place recycling. One is to have a windrow pickup attachment for loading the
pulverized, recycled pavement directly from the roadway surface into the pugmill. The windrow type of
equipment can use either parallel or transverse shafts. Another option is to feed the recycled pavement,
new aggregate, or both, into the plant’s aggregate receiving hopper. This requires an intermediate step of
loading the recycled pavement into trucks by conveyor or other means. A third option when using coldmilling machines, is to load the receiving hopper directly by means of a truck-loading conveyor set at the
proper angle.
If water is required in addition to the asphalt modifier, a separate water-delivery system is required.
Difficulties may arise if the recycled mix requires variable water content.
Laydown, Aeration, Compaction, Curing, and Surfacing
Construction techniques for in-place cold recycling laydown, compaction, and surfacing are the same as
those used for conventional stabilization operations. Proper curing prior to the application of the
surfacing is important to the success of a project.
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Module 3-8. HMA Pavement Recycling and Reconstruction
Recycled Mixture Design
The ARRA questionnaire addressed the mix design process for cold in-place recycling, although it did not
differentiate between full-depth and partial-depth processes (ARRA 1988). Experience reported was that
public agencies obtained block, core, and loose-milled samples for mix design purposes. Sixteen percent
of the agencies obtained block samples, 42 percent core samples, and 42 percent obtained samples from
the milling operation. Sample location and frequency is based more on judgment than statistical
procedures.
New Aggregate
The addition of new aggregate to the recycled pavement appears to be a widespread practice.
Approximately two-thirds of the reporting agencies allowed for the addition of aggregate. The reasons
cited for adding aggregate include providing additional thickness, correcting gradation, and allowing the
use of additional binder. The new aggregate can be added in front of the pulverizing or milling machine
or after partial pulverization; or the existing base course can be used with the pulverized asphalt concrete.
The amount of new aggregate ranges from 15 to 50 percent and the amount of salvaged base ranges from
33 to 50 percent (ARRA 1988). Many agencies have also added precoated aggregates to assure an
adequate coverage of the particles when partial depth recycling is being used.
Binder
Slow-setting and medium-setting asphalt emulsions are most often used for cold in-place recycling, with
one-third of the respondents to the ARRA questionnaire using CMS-2 and CSS-1h. In general, the fulldepth cold in-place recycling operations use the slow-setting emulsions, whereas the partial-depth
operations have used medium-setting emulsions. High-float emulsions have also been used on several
projects, both full-depth and partial-depth. The New Mexico State Highway Department has used a highfloat emulsion with a polymer additive to reduce thermal cracking, resist rutting, and provide improved
early strength. The western United States uses emulsified recycling agents proposed by the Pacific Coast
User-Producer Group, among other types of binders (Pacific Cost User-Producer Conference 1989).
Cutbacks and soft asphalt cements are used by some agencies. The engineer should know the type and
amount of diluent before any of these liquid asphalts is used.
Amount of Binder
The amount of binder for cold in-place recycling generally ranges from 0.5 to 3 percent emulsion, with
0.5 to 1.8 percent suggested by Oregon, and 1.2 to 1.5 percent in Pennsylvania, as starting points for
mixture design. This equates to 0.3 to 2 percent residual asphalt for emulsions. States using full-depth
cold in-place recycling operations usually require binder contents at the upper end of this range, whereas
the partial-depth operations usually use less than 2 percent emulsions. One-third of the respondents to the
ARRA questionnaire use laboratory mix design procedures to determine binder and additive content.
Mix Design
The Marshall mix design procedure was used by 20 of 30 agencies using mix design procedures. The
Hveem resilient modulus and indirect tensile tests were used by the other agencies. One-fourth of
responding agencies reported relying on field workability or experience for determining binder content
(ARRA 1988).
Eighty percent of reporting agencies analyze the recycled pavement for asphalt content and aggregate
gradation. Sample preparation was performed by processing or crushing in the laboratory (47 percent),
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heating and breaking of bulk samples (22 percent), and use of samples from field pulverized or milling
operations (31 percent) (ARRA 1988).
Agencies have used Marshall compaction (50 and 75 blows), kneading, and gyratory method of
compaction. Curing after compaction varies among agencies and ranges from 1 hour to 7 days. Curing
temperatures among agencies range from 23 to 121ºC (73 to 250 ºF). Density, stability, and air voids are
frequently used to select binder contents (ARRA 1988).
States, agencies, and groups that appear to have the most developed mix design procedures for cold inplace recycling are shown in table 3-8.3.
Table 3-8.3. Mix design procedures.
Procedure
Full-depth
California
Chevron
Corps of Engineers
Nevada
New Mexico
Oregon
Pennsylvania
Purdue
Texas
Asphalt Institute
Application
Partial-depth
Partial-depth CIR
X
X
X
X
X
X
X
X
X
X
X
X
A standard national method is not available; however, certain basic steps are normally included in the mix
design process. These include:
1. Obtain representative field samples from the pavement or from stockpiles of reclaimed materials.
2. Process field samples for use in mix design.
3. Evaluate recycled pavement.
a.
b.
c.
d.
Asphalt content.
Asphalt physical properties (penetration, viscosity).
Aggregate gradation.
Recycled pavement gradation.
4. Select amount and gradation of new aggregate.
5. Estimate asphalt demand.
6. Select type and amount of recycling agent.
7. Mix, compact, and test trial mixture.
a. Initial cure properties.
b. Final cure properties.
c. Water sensitivity.
8. Establish job mix formula.
9. Make adjustment in field.
Methods proposed by California, Chevron, Oregon, Pennsylvania, and the Asphalt Institute are reviewed
by Epps (1990).
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Module 3-8. HMA Pavement Recycling and Reconstruction
These methods provide approaches for selecting the type and amount of binder and the amount of water.
Methods of compaction, curing, and testing differ. Most methods define mix property measurements
soon after compaction, at or near a final cure condition, and after exposure to water. These are desirable
properties from a rational mixture design approach.
A good, cold in-place recycling binder is recognized as one that (a) produces initial softening of the RAP
asphalt, (b) has good initial coating of RAP and new aggregate at low fluids contents, (c) allows for early
compaction and traffic, (d) is relatively insensitive to binder content, and (e) does not continue to soften
for several months to create rutting and bleeding problems. The grade of binder is chosen to soften the
RAP asphalt to a selected level. Depending on environmental conditions, complete mixing of the new
binder and the RAP binder may or may not occur in a timely manner. If complete mixing does not occur,
it is possible that the new binder or recycling agent will remain on the surface of the hard RAP aggregate
and create an unstable mixture. In selecting recycling agents, it is better to err on the hard side of the
residual asphalt (high viscosity or low penetration) in the emulsified and asphalt cements. Studies have
also shown that the addition of 20 to 25 percent virgin aggregate in the CIR results in less voids and
flushing and improved stability (Murphy and Emery 1995).
Guidelines for Use
Guidelines for the use of cold in-place recycling can be found in the report from Task Force 38 of the
Joint AASHTO-AGC-ARTBA Committee (AASHTO 1998). Cold in-place recycling is a very versatile
rehabilitation alternative for pavements with low to moderate traffic. Significant structural improvement
can be achieved with full-depth cold in-place recycling operations without adversely affecting the
horizontal and vertical geometry of the pavement. The full-depth option of cold in-place recycling has
the ability to treat all forms of distress as the entire asphalt-treated portion of the pavement is pulverized
and recycled. Thus full-depth recycling eliminates reflection cracking. Cold in-place recycling is among
the most economical forms of pavement recycling for a relatively large number of projects.
Cold in-place recycling operations depend upon a curing of the asphalt material or other stabilizing
material to gain strength. This strength gain is, therefore, dependent upon the environment and the depth
of recycling. Cold in-place recycling operations usually have difficulty obtaining density. A loss of
strength and perhaps permanent deformation in the recycled layer can cause performance problems.
Large variability in the quality of construction can cause performance problems. The placement of a
wearing surface is usually required to achieve the desired performance of the rehabilitated pavement.
CIR is a viable rehabilitation alternative. Partial-depth, CIR has a proven performance record in several
States, including New Mexico and Oregon. Full-depth, CIR is very economical and has a proven
performance record, provided project selection has been performed appropriately.
Poor performance on CIR projects can often be traced to one or more of the following causes:

Excessive recycling agent resulted in excessive softening.

Placing a tight seal or dense-wearing course too soon, resulting in trapping water and diluent,
followed by stripping and rutting. Surfacing should not be placed before the moisture content of
the recycled mixture is reduced to 1 to 1.5 percent. This usually takes one to two weeks of curing
in summer weather conditions and longer in cooler temperatures.

Depth of recycle stopped at a delaminated layer of old pavement, resulting in loss of bond.

Failure to provide some type of seal before freeze/thaw conditions.
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Pennsylvania has developed guidelines for its climate to define the use of cold in-place recycling, which
are summarized in table 3-8.4.
Table 3-8.4. Guidelines for use of cold recycling in Pennsylvania (Kandhal and Koehler 1987).
Average Daily Traffic
Wearing Surface
1,500 or less
1,500 to 3,000
More than 3,000
Surface treatment (double application) as a minimum
Hot-mix wearing course
Do not use cold recycling
Specifications
Specification and quality control procedures for cold in-place recycling operations have been developed
by a number of State highway agencies. Epps (1990) reports specifications developed by Michigan,
Pennsylvania, and the Asphalt Recycling and Reclaiming Association and used primarily for full-depth
cold in-place recycling operations. Specifications developed by Oregon and New Mexico are used
primarily for partial-depth cold in-place recycling operations. These specifications contain elements of
both method specifications and end-result specifications.
These types of specifications rely on the expertise of the public agency, contractors, material suppliers,
and equipment manufacturers to obtain the desired end product at a reasonable cost. A discussion of the
general elements of the specifications follows.
Most cold-asphalt recycling specifications tend to focus on the material properties of the reclaimed
material and of the recycled mix rather than on the exact methods of construction. Specifications are
often written to allow the use of a wide variety of equipment, provided the recycled mix meets the job
specification for depth, maximum particle size, and gradation. The typical specification for cold-mix
recycling will contain sections on some or all of the following topics:

Overall description of work.

Materials (RAP, new aggregate, asphalt binder, water).

Equipment.

Method of construction:

Scarification and pulverization.

Addition of asphalt modifier and mixing.

Aeration.

Spreading.

Compaction.

Approval of job-mix formula.

Inspecting, sampling, and testing.

Quantity and basis of payment for each material.

Wearing course (asphalt concrete, chip seal coat, slurry seal).

General (weather, traffic control, safety).
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Module 3-8. HMA Pavement Recycling and Reconstruction
Materials
The primary materials specifications deal with aggregate gradation, asphalt binder type, asphalt binder
content, water content (if applicable), and density requirements.
Maximum Size
Most specifications limit the top size of the RAP produced by the pulverization equipment. Some
agencies require 100 percent passing the 25 mm (1 in) sieve. This type of specification can be overly
restrictive (and can slow down the recycling process). The top size of milled or pulverized materials is a
function of all of the following:

Condition of existing pavement (if alligator cracking is present, oversized pieces will most likely
be produced).

Top size of original aggregate.

Speed of milling (higher speeds produce larger sizes).

Depth of cut (thicker cuts tend to produce chunks of greater size).
The preferred alternative is to allow some oversized material to be present in the recycled mix by
specifying a minimum percentage for the nominal maximum size instead of placing restrictions on top
size (e.g., 97 percent passing the 37.5 mm [1.5 in] sieve) with no chunks larger than 100 mm (4 in) or
with the remaining 3 percent, not so large as “to affect adversely the stability and structural integrity of
the mixture, nor to hamper the shaping operation” (Scherocman 1983).
Aggregate Gradation
It is not practical to have the aggregate gradation of the RAP specified for all sieve sizes because of the
variability associated with the pulverization process. However, consideration should be given to the
amount passing the 75 m (#200) sieve because milling tends to increase the filler content by two to three
percentage points (a maximum of 12 percent is reasonable). Because of the variability of the material
being cold-recycled, allowance must be made in the specification (from the viewpoints of both
engineering and economics) such that the gradation reflects what is present in the roadway and not what
the designer considered to be optimum values. In addition to meeting RAP gradation requirements, the
scarification and pulverization equipment should also be capable of reasonable accuracy in cutting the
existing pavement to a specified depth.
Asphalt Binder
The specified asphalt binder should conform to the appropriate standard specifications for emulsified
asphalt, cutback asphalt, emulsified recycling agents, or asphalt cement. AASHTO, ASTM, or State
Highway Agencies publishes standard specifications. A committee of the Pacific Coast User-Producer
Conference on Asphalt Specifications has developed specifications for emulsified recycling agents.
Binder Content
The equipment for adding the modifier should be capable of an accurate application rate such that the
total binder content of the recycled mix is equal to the job-mix formula amount within a specified
tolerance, typically ±0.5 percent. Provision should also be made for the accurate application of any
required pre-mix water as specified by the job-mix formula.
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Job-Mix Formula
The responsibility for establishing the job-mix formula and required sampling procedures, test methods,
and design criteria for the mix design needs to be clearly outlined in the job specifications. The
specifications for full-depth cold in-place recycling generally do not place limits on the amount of RAP in
the mix, unless additional aggregate materials are required to increase the thickness of the stabilized layer.
Extraction and recovery tests, which are part of the mix design process, can be used to determine if any
new aggregate is needed to improve the quality of the RAP.
New Aggregate
If new aggregate is to be incorporated into the recycled mix, the aggregate should be tested for
compliance with standard specifications for virgin aggregate.
Equipment
Equipment specifications for the various phases of construction can be either the method or end-result
type. The user agency’s choice of which to use will depend on factors such as contractor and equipment
availability, economics, and the desired quality of work.
Density
A major item in the job specifications is the required density of the compacted mix, which can be
specified in one of three ways:

Percentage of theoretical maximum density.

Percentage of laboratory density.

Percentage of field density.
Some agencies recommend the use of percentage of theoretical maximum density instead of percentage of
lab density. Agencies citing the problem with variation in the original pavement suggest that a target
density (i.e., an actual density in kg/m3 [lb/ft3] or other units) combined with a rolling pattern that can be
changed may be the most realistic type of density specification. This control-strip approach is used in
Nevada and Pennsylvania.
The extent of agency experience with cold recycling and environmental factors will probably determine
which type of density specification is appropriate. Typical specifications require air void contents in the
12 to 15 percent range.
Performance
Comprehensive nationwide information on performance of cold in-place recycling is not available. The
FHWA-sponsored research project to define performance of recycled pavements is limited in its
evaluation of cold in-place recycled pavements (ARE 1987b). Reports that define performance of cold
in-place recycled pavements are available in the literature; however, they do not use a common method of
defining performance nor do they provide an equal amount of project detail. A summary of information
from California, Indiana, Kansas, Maine, Nevada, New Mexico, Oregon and Pennsylvania is presented by
Epps (1990). These performance studies have identified advantages and problem areas associated with
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Module 3-8. HMA Pavement Recycling and Reconstruction
cold in-place recycling. Using Oregon as an example, of the 672 km (420 mi) of low-volume roads that
used CIR between 1984 and 1989, 75 percent of these projects were rated fair or better in 1991 (Scholz et
al. 1991).
The benefits most often cited by those using cold in-place recycling, regardless of the type (full-depth or
partial-depth), include (Epps 1990):

Significant pavement structural improvements may be achieved without changes in horizontal
and vertical geometry and without shoulder reconstruction.

All types and degrees of pavement distress can be treated.

Reflection cracking normally is eliminated if the depth of pulverization and reprocessing is
adequate.

Pavement ride quality can be improved.

Hauling costs can be minimized.

Old pavement profile, crown, and cross slope may be improved.

Production rate is high.

Only thin overlay or chip seal surfacing is required on most projects.

Engineering costs are low.

Aggregate and asphalt binder are conserved.

Energy is conserved.

Air quality problems resulting from dust, fumes, and smoke are minimized.

It is a cost-effective solution for a number of situations.

Frost susceptibility may be improved.

Pavement widening operations can be accommodated.

It is environmentally desirable, because disposal problems are eliminated.
Identified problems with cold in-place recycling include (Epps 1990):

Construction variation is larger for in-place versus central plant operations. (Partial-depth cold
in-place recycling can result in a uniform pavement layer.)

Curing is required for strength gain.

Strength gain and construction are susceptible to climatic conditions, including temperature and
moisture.

Traffic disruption can be greater relative to other rehabilitation alternatives. (The use of the
recycling train greatly reduces traffic disruption.)

Placement of a wearing surface is required.
Economics
Table 3-8.5 gives a summary of agency costs associated with cold in-place recycling operations. Detailed
performance histories are not available to allow the identification of the best time to do a CIR project.
Thus, detailed life-cycle cost information does not appear in the literature. However, preliminary
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Table 3-8.5. Full- and partial-depth cold in-place recycling cost differences (Epps 1990).
1
Agency
California (99)
Year
2
1979-83
California (40)
Cost Difference (%)
Typical
Range
Value
15-43
31
Cost Data
Cold In-Place Recycling ($)
Range
Typical Value
16.20-26.80/Mg
(14.70-24.30/T)
22.22/Mg
(20.15/T)
24.25/Mg
(22.00/T)
6.18/m2
(/sy)
4.55/m2
(3.80/sy)
Relative to
conventional mix
7.60/Mg
(6.90/T)
Relative to HMA
37
21
Comments
California (46)
1980
Illinois (56, 57)
1982
Indiana (64)
1976
Iowa (65)
1988
67
Kansas (70)
1977
53
Relative to
equivalent section
Kansas (172)
Missouri (181)
1988
1978
50
Relative to
equivalent section
Montana (86)
1978
21
New Mexico3
1984-86
13.17-24.25/Mg
(11.94-22.00/T)
23.80/Mg
(21.58/T)
1.67/m2
(1.40/sy)
4.77/m2
(4.00/sy)
4.14/m2
(3.46/sy)
2.40/m2
(2.00/sy)
1.25-2.40/m2
(1.05-2.05/sy)
N. Carolina
(180)4
1977
Oklahoma (92)
1979
Oregon (41)
1984
24
Pennsylvania
(98)
Vermont (107)
1983
16
1978
28
Vermont (108)
1982
31
Wisconsin
(111)
1978
FHWA (114)
Relative to
equivalent section
6
2.16-2.90/m2
(1.81-2.43/sy)
9.45/m2
(7.90/sy)
1.64/m2
(1.37/sy)
0.014/m2-mm
(0.25/sy-in)
5.65/m2
(4.73/sy)
Mean 66 mm (2.6
in) of recycling
Relative to
equivalent section
Relative to
equivalent section
Relative to
equivalent section
Relative to
equivalent section
Relative to
equivalent section
1
Relative to commonly used rehabilitation alternatives used by identified States.
References are identified in Epps (1990).
3
Personal communication with D. Hanson (1987).
4
Cost increase on one project.
2
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Module 3-8. HMA Pavement Recycling and Reconstruction
performance information obtained from State records indicates that significant life-cycle cost savings will
be obtained when comparisons are made between conventional overlay techniques and cold in-place
recycling operations. In some instances, the first cost of cold in-place recycling will be greater than
conventional overlays; however, improved performance and the use of stage construction techniques with
the cold in-place recycling option will lower life-cycle costs.
A review of the FHWA Demonstration Project 39 reports on pavement recycling, as well as other
information, indicates the following component costs for cold in-place recycling operations:

Materials  46.6 percent.

Equipment  29.7 percent.

Labor  23.7 percent.
The main economic advantage that recycling offers is in material cost savings. The majority of the
material costs are associated with new binder. Increases in new aggregate will increase recycling costs.
Typical square yard costs for cold in-place recycling operations are shown in table 3-8.5.
First costs of cold in-place recycling operations are project dependent. Generalizations should be used as
guides only. First-cost savings of 6 to 67 percent are reported in the literature.
6. HOT IN-PLACE RECYCLING
The use of hot in-place recycling operations dates to the 1930s with the development of heater-planer
equipment in California (Epps 1978; Epps et al. 1980). Since the 1930s, a wide variety of hot in-place
recycling equipment has been developed and improved. Heater-scarifying equipment was developed by
the 1960s and heater-remixing equipment was developed in the 1980s and 1990s. Heater-scarifiers were
developed to heat, scarify, and reprofile the pavement. Over the years, equipment has been developed
which allows for a greater depth of heating and scarification, as well as improved pavement smoothness
associated with the laydown operation. Typical heater-scarification operations heat and scarify to depths
of 10 to 25 mm (0.4 to 1 in). The use of hot millers in place of scarifiers and improved heaters has
increased depth and versatility of the equipment.
Hot in-place recycling repaving equipment was developed in the 1950s and 1960s. A layer of hot-mix
asphalt is applied on top of a heated and scarified layer. A single- or two-pass equipment operation can
be used in the process, and scarification depths of 10 to 25 mm (0.4 to 1 in) are typical.
Hot in-place remixing operations were developed in the 1980s and 1990s. This equipment heats,
scarifies, or hot mills the existing equipment, mixes new materials, and lays the combined recycled and
new mixtures. Removal depths from 10 to 50 mm (0.4 to 2 in) are typical.
Details of hot in-place recycling operations are described in more detail below. References by Pyrotech
Asphalt Equipment Manufacturing Company; Rogge, Hislop, and Dominick (1996); Terrel, Epps, and
Sorenson (1996); Nevada Transportation Technology Transfer Center (1995); and Button, Little, and
Estakhri (1994) are the primary recent references on hot in-place recycling and have been used as the
basis for this discussion.
Methods and Equipment
A chronological record of the evolution of HIR shows an increasing understanding of and improvement in
the concept. As indicated earlier, the modern era of recycling began in the mid 1970s, but trials and
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experiments began much earlier. The following is a summary of the evolution of HIR, based on the
terminology adopted by ARRA.
Heater Scarification
Sometimes called a reshaping process, a Utah contractor originally developed heater-scarification
sometime in the 1930s (NAPA 1977a; Whitney 1992), but common usage did not evolve until the 1960s.
By the 1970s, further evolution moved the technology into more complex systems. This relatively simple
process includes several steps, as follows:
1. Heating the old pavement surface.
2. Scarifying the softened surface with a bank of stationary teeth.
3. Adding a liquid recycling agent (when needed).
4. Mixing and leveling the recycled loose mixture with an auger and/or laydown machine.
5. Compacting with conventional rollers.
Figure 3-8.4 shows a typical equipment train for heater-scarification. The depth of scarification and
treatment usually was about 10 to 20 mm (0.4 to 0.8 in). Depending on the depth of scarification and
condition of the pavement, the resulting surface is not always smooth and uniform.
Figure 3-8.4. Heater-scarifier process (Button, Little, and Estakhri 1994).
Early attempts at heating used direct flame, but radiant infrared heaters fired by propane gas gradually
replaced this approach. Infrared (IR) heating helped reduce the overheating (and excessive hardening)
and smoking caused by direct flame. One or more heater units (see figure 3-8.4) are used to gradually
raise the pavement temperature sufficiently to allow the scarifying teeth to scrape through the surface.
Surface temperatures ranging from 110 to 150 ºC (230 to 302 ºF) are generally achieved when at least two
heaters are used in tandem. The scarifying teeth are normally spring-loaded tines that are able to override
obstacles such as manholes, but the use of tines for scarification may limit the depth of scarification and
cause aggregate breakage. The recycled layer contains relatively hard asphalt binder because of both the
normal aging of the surface and the heating required to soften it. Thus, recycling or rejuvenating oils are
commonly used to restore flexibility. The heater-scarified surface is usually overlaid using conventional
HMA.
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Module 3-8. HMA Pavement Recycling and Reconstruction
Repaving
When heater-scarification is simultaneously combined with an overlay of HMA, it is called repaving.
Often called the Cutler process (named after its inventor), repaving is a process that started in the 1950s
and was upgraded in the 1960s (Rathburn 1990). The repaving process has several steps as follows:
1. Heating (i.e., preheating).
2. Scarifying using teeth or a rotary mill.
3. Adding a recycling agent.
4. Mixing the recycling agent and loosened mixture.
5. Spreading and screeding the recycled mixture.
6. Placing a new HMA overlay.
Figure 3-8.5 and figure 3-8.6 show typical equipment trains for the repaving process. Current practice is
to heat the existing surface to approximately 190 ºC (374 ºF) using IR preheaters as well as heaters in the
recycling unit. The heat-softened pavement is then removed to a depth of about 10 to 20 mm (0.4 to 0.8
in), depending on how well the heaters have softened the asphalt. For those machines that use milling to
loosen the old pavement, some variations in the design and layout allow for adjustment of depth during
operation. On some machines, the cutter heads can be raised or manipulated to avoid obstacles such as
manholes.
Figure 3-8.5. Multiple pass repaving process used by Dustrol (Button, Little, and Estakhri 1994).
HMA Pavement Evaluation and Rehabilitation
3-8.23
Module 3-8. HMA Pavement Recycling and Reconstruction
Reference Manual
Figure 3-8.6. Single pass repaving process used by Cutler (Button, Little, and Estakhri 1994).
Rejuvenation of the loosened mixture is accomplished by spraying liquid additive onto the pavement or
mixing chamber or windrow at a rate determined by laboratory testing. This predetermined application
rate is then locked into the machine so that it is adjusted by the forward motion or progress of the train.
Auger mixers that also transfer the mixture into a windrow usually accomplish mixing of the loosened
mixture and recycling agent. Additional transverse augers then spread the recycled mixture in front of the
screed, and the screed levels and shapes the recycled material. Finally, a new HMA surface is added and
spread with another screed directly on top of the recycled layer. The recycled layer may still retain
temperatures well over 100 ºC (212 ºF) so that the new HMA is well bonded and integrated with the old
pavement surface. Depending on the equipment’s design, the lift thickness and/or cross shape may be
controlled manually or by using automatic controls. Repaving is a practical solution to restore and
improve a pavement surface in one pass of a recycling train.
Remixing
When additional materials are needed to recycle the pavement, such as mineral aggregate or virgin HMA,
the remixing process is used. This approach permits upgrading the existing pavement with additional
thickness and/or improving the old HMA by changing the aggregate gradation or adjusting the binder
properties. The process is somewhat similar to the repaving process; but usually more thorough heating
and mixing is accomplished.
Figures 3-8.7 to 3-8.12 show sketches of the equipment generally used in remixing. One or more
preheater units, usually infrared, are used to warm and soften the pavement ahead of the others. In
reasonably stable weather (not windy or cold pavement temperature), the preheaters can raise the
temperature of the pavement to about 85 to 105 ºC (185 to 221 ºF). The preheaters are large, a full lane
wide and up to 12 m (39 ft) long. Each unit in the paving train typically has a heater, including the
remixer, although it may be smaller than the preheaters.
A few manufacturers still utilize stationary tines or teeth to scarify the warm pavement, but most currently
use rotating milling heads. These are similar to those used for cold milling, but require less power
because the warm pavement is softer. Most systems can mill to a depth of 25 to 50 mm (1 to 2 in),
although a target (desired) may be 50 mm (2 in). Experience has been that the higher boundary of milling
depth is controlled by the temperature and is about 50 mm (2 in) with conventional IR heaters. When
greater depths are obtained, the lower temperatures cause aggregate breakage and the overall average
temperature of the loose RAP is lowered, making it more difficult to obtain high quality recycled
mixtures.
The remixer unit, as shown in figures 3-8.7 to 3-8.9, usually has a hopper to receive virgin HMA when
needed. Some equipment picks the virgin HMA from a windrow and blends it with the hot milled RAP.
3-8.24
HMA Pavement Evaluation and Rehabilitation
Reference Manual
Module 3-8. HMA Pavement Recycling and Reconstruction
Figure 3-8.7. Artec multistage remixer process (Terrel, Epps, and Sorenson 1996).
Figure 3-8.8. Single pass remix process, Taisei Rotec HIPR-5 (Button, Little, and Estakhri 1994).
HMA Pavement Evaluation and Rehabilitation
3-8.25
Module 3-8. HMA Pavement Recycling and Reconstruction
Reference Manual
Figure 3-8.9. Pyrotech Pyropaver 300E remixer process (Terrel, Epps, and Sorenson 1996).
Figure 3-8.10. Artec four-state remixer process (Terrel, Epps, and Sorenson 1996).
3-8.26
HMA Pavement Evaluation and Rehabilitation
Reference Manual
Module 3-8. HMA Pavement Recycling and Reconstruction
Figure 3-8.11. Wirtgen 4500 remixer (Button, Little, and Estakhri 1994).
Figure 3-8.12. Martec four-stage remixer with hot air/infrared heaters and
recirculating vacuum system to improve air quality (Terrel, Epps, and Sorenson 1996).
HMA Pavement Evaluation and Rehabilitation
3-8.27
Module 3-8. HMA Pavement Recycling and Reconstruction
Reference Manual
The remixer unit shown in figure 3-8.7 has a conveyor that lifts the virgin HMA over the heater and
milling head and then into the pugmill. Here, the RAP, recycling agent and virgin HMA or aggregate are
blended into the final mixture. Although some equipment designs have attempted to accomplish all the
mixing on the pavement surface, this procedure has been rather unsuccessful. Most specifying agencies
call for pugmills to be used.
Two general approaches to relaying the recycled mixture have been used. In one, a paving machine is
snugged up to the rear of the repaver and accepts the recycled mixture directly into its hopper, after
which, paving proceeds the same as for a new mixture. In the other, the recycled RAP is windrowed
behind the repaver and an on-board auger screed system spreads the mixture, ready for compaction.
Improvements in Remixing
Variations of the repave and remix processes were developed in the mid and late 1980s and early 1990s,
and this technology is being used in North America. The hot in-place systems described earlier are often
called multistage because of the progressive stages of preheating, hot milling, remixing, and paving. But,
they were also single step; meaning that a single milling operation and the full depth of milling (with one
pass of the cutting heads) followed the preheating. Several manufacturers, including Rorison-Wiley
Blacktop, Wirtgen, Taisei Rotec, and others, utilized these single-stage equipment styles.
The later variations, two-step or multistep, were developed because of the recycle depth limitations of
single step trains. It is difficult to heat pavement at a depth in a timely fashion using a single IR source.
Figure 3-8.13 shows typical heating depth rates for IR surface heating (Rogge, Hislop, and Dominick
1994). It is apparent that the surface is easily heated, but adequately high temperature for hot milling and
remixing is not feasible at 50 mm (2 in) depth, for example. Further, if the heater is slowed in an attempt
to heat at depth, the high surface temperature tends to oxidize the asphalt excessively.
Pyrotech, Inc. and Artec, Inc., both of British Columbia, Canada, developed two-step and four-step
processes. The idea was to take advantage of the ability to heat the top 12.5 mm (0.5 in) or 25 mm (1 in)
of the pavement effectively. In the two-step process (figure 3-8.13), two miller heads are used, one on
each heater unit following the preheater. The first heats the original pavement surface and then mills off
approximately 25 mm (1 in), about the limit of heating. The RAP removed is windrowed in the center to
expose most of the underlying surface. The second unit follows, picking up the windrow, milling off the
top 25 mm (1 in) under the windrow, heating the now-exposed surface, then milling a second 25 mm (1
in) depth. The heating procedure is more efficient and the recycling process is much faster, thereby
increasing productivity.
The Artec four-step process is similar to the two-step process, except that four heater-miller units are used
(see figure 3-8.10). Each unit heats the surface or exposed underlying surface and mills off about
12.5 mm (0.5 in) of pavement. Again, taking advantage of the rapid IR heating at the surface, it is easy
and efficient to heat 12.5 mm (0.5 in) for easy milling, so forward progress is increased to very acceptable
levels. Also, there is less aggregate fracture since the layer being milled is much warmer. The entire
quantity of loose RAP material is heated adequately since each 12.5 mm (0.5 in) layer is mixed with the
others.
Figure 3-8.14 shows how the two-step process is effective in heating to 50 mm (2 in) depth, even though
there is rapid cooling between heater-miller units (Rogge, Hislop, and Dominick 1994). The four-step
process does the same, even more effectively. Figure 3-8.14 shows how the higher temperature of each
underlying layer contributes to a more rapid rise in overall temperature. The ability to heat a warm
underlying surface is improved over a cold surface and the step-wise process takes advantage of this
factor.
3-8.28
HMA Pavement Evaluation and Rehabilitation
Module 3-8. HMA Pavement Recycling and Reconstruction
Temperature, ºC (ºF)
Reference Manual
149
(300)
Surface
93
(200)
25 mm (1 in) Depth
50 mm (2 in) Depth
38
(100)
5
10
Time (minutes)
Temperature (º C )
Figure 3-8.13. The heating process raises the surface temperature rapidly, while heating at depth takes
much longer (Pyrotech Asphalt Equipment Manufacturing Company).
149
(300)
50 mm (2 in) Depth
93
(200)
2 Step
Process
38
(100)
1 Step
Process
5
10
Time (minutes)
Figure 3-8.14. Temperature profile over time with one-step versus two-step heating process
(Pyrotech Asphalt Equipment Manufacturing Company).
The hot in-place recycling developed by Wirtgen (Germany) is similar to the systems previously
described. Figure 3-8.11 shows the remixer unit that includes a receiving hopper for admix, IR heaters,
and milling heads. A preheater (not shown) is similar to other IR preheaters. This unit has its own
leveling screed and does not rely on a paving machine. There is no special provision for air quality
control.
HMA Pavement Evaluation and Rehabilitation
3-8.29
Module 3-8. HMA Pavement Recycling and Reconstruction
Reference Manual
There have been incremental changes in the HIR equipment and processes with each succeeding year of
operation. For example, alternative heating methods, such as microwaves, have been attempted on an
experimental basis, but no commercial size units have been placed in service as yet (Terrel, Epps, and
Sorenson 1996). This idea is intriguing because the microwaves heat the pavement at depth (sometimes
too deeply) and without the smoking of other flame-type heaters. On-board electric power requirements
are high, but improvements in concentrating the microwaves where needed are making it more effective.
Through the early 1990s, the further development of HIR technology seemed to reach a plateau. There
were a number of HIR trains operating in the United States and Canada and they were using the various
technologies previously discussed. Because of the enormous potential of hot in-place recycling, several
user agencies began to seriously evaluate its effectiveness and conducted surveys of projects through key
people who were directly involved. These studies were based on projects constructed in 1992 and earlier.
(See references by Washington State University [1993], Haughton [1993], Gavin and McMillan [1993],
Kazmierowski, Bradbury, and Marks [1993], and Rogge, Hislop, and Dominick [1994].) For example,
questionnaires sent to various users addressed questions on 1) advantages, 2) deficiencies, and 3) cost.
The responses from these queries and other sources are summarized below:
Advantages

Conservation of energy and materials, including aggregates, asphalt, and fuel (less truck hauling).

Construction improvements realized through shorter duration projects; less traffic delay or
control; safer site conditions; easy mobilization; no milling disposal costs.

Improved pavements through correction of surface conditions and up to 50 mm (2 in) depths;
correction of mix deficiencies; elevation and curb lines are unaffected; improved ride and skid
resistance.

Environmental concerns addressed through improved air quality due to less trucking; smoking on
site reduced by using afterburners on recent upgrades of equipment.
Disadvantages


3-8.30
Concerns about the finished pavements.

Compaction problems resulting in segregated and open texture and low density.

Cracks that soon reappeared.

Smoothness deficiencies.

Inadequate depth of milling.

Insufficient mixing.

Excess aggregate fines caused by milling to a depth where the pavement was too cold, thus
causing aggregate fracture.
Equipment related problems.

Frequent breakdowns.

Too much smoke and steam.

Too long a paving train.

Remixed RAP was too cold at laydown and the HIR equipment did not do well in windy or
cool weather or when the pavement was damp.
HMA Pavement Evaluation and Rehabilitation
Reference Manual

Module 3-8. HMA Pavement Recycling and Reconstruction
Poor longitudinal joint matches and undersized paver may be due to factors other than the actual
HIR equipment. Remixer would not accept enough virgin HMA admix for effective remixing to
upgrade the pavement.
Costs

Cost savings over a 50 mm (2 in) HMA overlay ranged from 19 to 45 percent for 14 projects
reported by one study; equal to a 25 mm (1 in) overlay in cost.

Costs for HIR vary because of different project needs: detailed curb and driveway paving,
location, weather, and moisture in the pavement, all of which affect productivity.
Needed Improvements
Through 1993, HIR in North America evolved rapidly and several improvements resulted. The evolution
is continuing as a result of manufacturers’ innovations created by competition as well as
demands of the customers. Again, through the questionnaire process, a long list of needed improvements
has emerged. Included among these are at least the following:
Equipment

Improved air quality.

Increased mixture temperature.

Deeper recycling.

Adjustable width of milling.

Quieter operation.

Increased capability to add virgin mixture and cold aggregate.

Improved ability to climb steeper grades.

Better interlocking of additive and remixing speed.

Additional instrumentation for improved QC monitoring.
Procedures

Better project evaluation and core sampling.

More on-site testing to improve QC.

Development of better procedures and criteria for selecting potential HIR projects.
This last factor was emphasized by marginal results in the State of Oregon when three of four HIR
projects attempted in 1992 and 1993 were deemed inappropriate for HIR rehabilitation (Rogge, Hislop,
and Dominick 1996; Terrel 1994). For example, a typical problem was an attempt to recycle to 50 mm (2
in) depth in a layer that was about 70 mm (2.75 in) thick, and that was delaminated. The milling process
broke loose large chunks of underlying pavement that the machinery could not accommodate.
Equipment developers continue to address the identified problems and improve the hot in-place recycling
operation.
HMA Pavement Evaluation and Rehabilitation
3-8.31
Module 3-8. HMA Pavement Recycling and Reconstruction
Reference Manual
Recycled Mixture Design
The mixture design process for hot in-place recycling and hot central plant recycling is identical. The
procedure published by Epps et al. (1980) serve as an excellent reference. Further detail can also be
found in National Highway Institute Course No. 131050, Asphalt Pavement Recycling for State and Local
Governments. Typically, softer recycling agents and lower recycling agent contents are used for hot inplace recycling operations. Low percentages of new hot-mix asphalt and high percentages of RAP that
are typically associated with hot in-place recycling operations are responsible for the differences in the
type and amount of recycling agent.
Structural Design
A phone survey of fifty States reported in the reference by Button, Little, and Estakhri (1994) indicated
that only seventeen States considered the structural value or load carrying ability of hot in-place
recycling. Fourteen States considered the structural value of hot in-place recycling to be about the same
as that of new hot-mix asphalt. Three States indicated that they assigned a structural value that is slightly
less than new hot-mix asphalt. One research project indicated the same structural value for hot in-place
recycling and new hot-mix asphalt (Bandyopadhyay 1982).
Performance
The 1994 survey of state practices indicated that twenty-eight States used hot in-place recycling on an
experimental basis and an additional ten States used hot in-place recycling on a somewhat regular basis.
Thirteen States have reported using heater-scarification while fifteen States use repaving and sixteen
States remixing (Button, Little, and Estakhri 1994). Hot in-place recycling has been on both major and
secondary highways. Some States place a surface seal or hot-mix asphalt overlay depending on the
specific project conditions (Button, Little, and Estakhri 1994).
Projects completed in Canada have the best performance documentation, as most of the recent
developments in HIR development have been in Canada. Ontario recently conducted a follow-up review
of 31 HIR projects they have completed since 1987 (Kazmierowski, Marks, and Lee 1999). This study
found that the penetration of the asphalt binder was increased by an average of 18 penetration units.
Projects constructed in later years have been more successful in raising the penetration to acceptable
levels comparable to that achieved from conventionally recycled HMA. This same study found that about
half of the edge, transverse, and midlane cracks reflected through in the first year. This would be
comparable for thin overlays of the same pavement. One major difference was that longitudinal and
centerline cracks have not reflected through. The author hypothesized that this may be due to hot joint
construction during the paving process. Smoothness of the HIR pavements was comparable to new
construction. Conclusions from this study include:

HIR is an acceptable rehabilitation technique for pavement exhibiting moderate surficial
pavement distresses that are not associated with structural deficiencies.

Contractors can achieve, and are achieving, the recovered penetration values specified for the
HIR process provided adequate attention is paid to the existing pavement and any changes in the
material while determining the mix designs for use on the project.

Pavement with steel slag aggregates should not be HIR because of their porous and insulative
properties.

Excessive quantities of temporary maintenance treatment materials (sand seals and cold mix) and
rubberized asphalt sealant should be removed before HIR.
3-8.32
HMA Pavement Evaluation and Rehabilitation
Reference Manual
Module 3-8. HMA Pavement Recycling and Reconstruction
It is difficult to generalize about how well pavements are recycled, because each project is different, but it
is fair to say that the objectives were met on most projects. For example, the earlier hot in-place recycling
projects conducted in British Columbia by Pyrotech and Artec were usually rejuvenated with recycling
agent, so the binder was softened as measured by viscosity and penetration test results. The resilient
modulus of recycled mixtures generally is a good indicator of the recycling and is much more sensitive to
binder changes than is the Marshall Stability, for example. If an existing old pavement shows age
hardening, then the resilient modulus is relatively high and the values are significantly lowered after hot
in-place recycling using a recycling agent (Pyrotech Asphalt Equipment Manufacturing Company).
Alternatively, a too-soft, rut-prone mixture in an existing pavement may be stiffened by the HIR process
due to additional aging by the heating, as well as by addition of HMA with better aggregate and perhaps a
stiffer asphalt rather than a recycling agent.
The considerable experience with HIR in Canada has led to a Government-sponsored project to evaluate
the performance of HIR pavements constructed in Alberta since 1990 (AGRA Earth & Environmental
Limited 1996). The ten projects were recycled using single-pass, two-stage HIR trains at 50 mm (2 in)
depth. The project focused on assessing the binder rheology and the mixture volumetric properties before
and after construction. In addition, these same properties (binder and mixture) were compared at the time
of construction and again in 1996 to evaluate the effects of time and traffic. For example, the data in
figure 3-8.15 from the Alberta study indicate that adding a recycling agent increased the penetration about
30 percent and thus was effective in restoring the binder. For those projects where no recycling agents
were used, the HIR process reduced the penetration about 20 percent.
Recovered Penetrations @ 25 º C (77 ºF)
120
Preconstruction
1996
100
94
94
cyclogen
80
no cyclogen
78
77
78
71
62
62
62
60
51
46
50
51
41
40
20
0
A
J
G
C
Section
D
H
B
Figure 3-8.15. Effect of recycling agent on HIR pavements
(AGRA Earth & Environmental Limited 1996).
The early trials using the Martec HIR equipment in British Columbia (B.C.) have provided an opportunity
to assess the process. A good project for this evaluation of mixture properties was a section of Highway
1A in Abbotsford, British Columbia, recycled in October 1994. Because a recycling agent was not used,
the properties of the asphalt mixture and binder could be compared before and after hot in-place
recycling. The process was selected by B.C. Ministry of Transportation and Highways because the
pavement was rutting prematurely after only 1 year of service. Both the B.C. Ministry and a consultant
conducted tests.
HMA Pavement Evaluation and Rehabilitation
3-8.33
Module 3-8. HMA Pavement Recycling and Reconstruction
Reference Manual
Table 3-8.6 shows a summary of data from the Abbotsford project. The data from core samples taken at
four locations show that the air voids increased an average of 0.7 percent, intended to provide space for
future traffic compaction. The gradation of the aggregate after hot in-place recycling was finer, but the
dust (<0.075 mm [0.003 in]) increased only 0.4 percent. As would be expected, the asphalt content did
not change and the viscosity increased only slightly, from 395 to 318 mm2/sec (395 to 318 cSt), while the
penetration decreased only one point (dmm). This value can be compared to the 20 percent loss in
penetration as shown in figure 3-8.15. These data would indicate that the hot air/IR heating was relatively
gentle and that the age hardening of the binder was insignificant.
Table 3-8.6. Summary of test data from trial project at Abbotsford, B.C., Canada, October 1994.
Property
From core samples:
Thickness of top lift, mm (in)
Air voids, %
Bulk specific gravity, kg/m3 (pcf)
Compaction, % of Marshall
Before Recycling
After Recycling
54 (2.125)
3.3
2379.0 (148.5)
100.3
56 (2.205)
4.0
2387.0 (149.0)
98.5
Marshall samples:
Bulk specific gravity, kg/m3 (pcf)
Theoretical max specific gravity, kg/m3 (pcf)
Air voids, %
Stability, N (lbf)
Flow, 0.25 mm (0.01 in) units
2415.0 (150.7)
2506.0 (156.4)
3.6
10,072.0 (2264)
9.3
2432.0 (151.8)
2498.0(155.9)
2.6
10,105.0(2272)
10.2
Aggregate gradation:
Sieve size, mm (in)
19.00 (0.75)
12.50 (0.5)
9.50 (0.375)
4.750 (#4)
2.360 (#8)
1.180 (#16)
0.600 (#30)
0.300 (#50)
0.150 (#100)
0.075 (#200)
Percent passing
100.0
92.3
81.5
63.8
50.4
35.9
24.6
12.8
7.1
5.2
Percent passing
100.0
94.9
82.4
64.3
49.4
36.8
25.9
14.0
8.3
5.6
5.0
305 (305)
5.0
318 (318)
36
35
Asphalt binder:
Asphalt content, % total mix
Recovered (Abson) asphalt properties
Kinematic viscosity @ 135ºC, mm2/sec (cSt)
Penetration @ 25 ºC (dmm, 100g/5 sec)
Notes:
1
Average of four sets of core samples from four locations on the four-lane highway.
2
See the following references: Terrel, Epps, and Sorenson 1996; Nevada Transportation Technology Transfer
Center 1995; Button, Little, and Estakhri 1994; NAPA 1977; Whitney 1992; Rathburn 1990; Washington State
University 1993; Haughton 1993; Gavin and McMillan 1993; Kazmierowski, Bradbury, and Marks 1993;
Rogge, Hislop, and Dominick 1994; Terrel 1994; Bandyopadhyay 1982; AGRA Earth & Environmental
Limited 1996; and Fyvie 1994.
3-8.34
HMA Pavement Evaluation and Rehabilitation
Reference Manual
Module 3-8. HMA Pavement Recycling and Reconstruction
Limitations
HIR, like all rehabilitation methods, has its limitations. Button, Estakhri, and Little (1999) summarized
the limitations as:

Existing pavement needs to have adequate load carrying capacity.

Pavements with obvious base failures, irregular and frequent patching, and needing major
drainage improvements are not candidates for HIR.

HIR should be limited to the top 50 mm (2 in), although projects have been completed to a depth
of 75 mm (3 in).

Existing pavements should be at least 75 mm (3 in) thick.

Limited ability to affect large changes in grades. Rutting may be corrected but the user will not
be able to correct large undulations due to swelling or heaving soils.

Narrow roads are not good candidates for HIR due to the width of the heating and milling
equipment unless traffic can be rerouted.

Streets with numerous metal appurtenances, such as manholes and utility access covers, are not
good candidates for HIR.

Preferred weather is hot, calm days with no moisture in or on the existing pavement. Less than
ideal weather conditions will slow the operation due to increased heating time.

Surface treatments of the existing pavement negatively affect the ability of the equipment to heat
the pavement. It may be advisable to remove surface treatments (especially multiple chip seals)
before HIR is employed.

Surface courses with aggregate larger than 25 mm (1 in) in diameter may be unsuitable for
current HIR practices.

The pavement must be heated to the desired temperature without creating air quality concerns or
burning the asphalt cement.

Many of the operations of a HIR operation are not visible to observers. Quality control best
becomes the responsibility of the contractor and not the owner. Specifications should be endresult or performance related.
A pavement that exhibits stripping may not be improved by hot in-place recycling, even though the initial
coating may appear to be adequate. Test results show that there is often a loss in resistance to water
damage after hot in-place recycling probably because the binder is usually softened, thus resulting in a
more water susceptible mixture (Pyrotech Asphalt Equipment Manufacturing Company; ARE 1987b).
Therefore, it may be prudent to consider anti-stripping measures such as liquid agents or lime as
additives.
Economics
A limited amount of comparative cost information is available in references by the Pyrotech Asphalt
Equipment Manufacturing Company and Button, Little, and Estakhri (1994). Because of different
processes, equipment and pavement/project conditions, comparisons between hot in-place recycling and
conventional rehabilitation alternatives are difficult. Some typical first costs are given on table 3-8.7.
HMA Pavement Evaluation and Rehabilitation
3-8.35
Module 3-8. HMA Pavement Recycling and Reconstruction
Reference Manual
Table 3-8.7. Typical cost information (Pyrotech Asphalt Equipment Manufacturing Company;
Button, Little, and Estakhri 1994).
Hot-In Place Recycling Operation
Heater-scarification - (25 mm + recycling agent)
Heater-scarification + 25 mm overlay
Repaving - (recycle 25 mm + 25 mm hot-mix
asphalt mixed together)
Remixing - (recycle 25 mm + 10-20 percent
new hot-mix asphalt)
Remixing - (recycle 50 mm + 10-20 percent
new hot-mix asphalt)
Approximate Cost, Dollars/
Sq Meter (Sq. Yard)
1.20 (1.00)
3.17 (2.65)
3.50 (2.93)
2.75 (2.30)
3.25 (2.72)
First costs are important, but life-cycle costs must also be considered. The reference by Button, Little,
and Estakhri (1994) indicates that States have not developed life-cycle costs. A few life-cycle cost
examples have been prepared and are contained in the reference by the Pyrotech Asphalt Equipment
Manufacturing Company. Favorable first costs and life-cycle costs are possible, depending upon the
project particulars. First cost savings of from 5 to 50 percent have been reported (Button, Little, and
Estakhri 1994).
Guidelines for Use
Guidelines for selecting hot in-place recycling as a rehabilitation alternative can be found in references by
the Pyrotech Asphalt Equipment Manufacturing Company, Nevada Transportation Technology Transfer
Center (1995), Button, Little, and Estakhri (1994), and Emery, Gurowka, and Hiramine (1989).
The major considerations that must be taken into account when designing a mixture for a hot in-place
recycling project are as shown below:

Uniformity.

Depth of HMA.

Presence of chip seals.

Asphalt content (bleeding).

Aggregate gradation.

Asphalt properties.

Traffic.

Types of pavement distress.
The uniformity of the existing asphalt-bound materials within the project limits must be established. The
construction, rehabilitation, and maintenance records should be consulted to determine the uniformity
along the project and with depth. A single project may require several mix designs if the existing
materials are not uniform.
Uniformity with depth is an important consideration. The specified recycling depth is the depth of the
material to be used for mixture design. If nonuniform materials are found at depth, the mixture design
phase of the project may require that a change in the specifications be made.
3-8.36
HMA Pavement Evaluation and Rehabilitation
Reference Manual
Module 3-8. HMA Pavement Recycling and Reconstruction
The presence of chip seals or other types of seals within the depth of recycling may require special mix
design considerations. The uniform gradation of the chip aggregate and its relatively high asphalt content
must be taken into account.
Hot-mix asphalt pavements experiencing bleeding may require the addition of new hot-mix or aggregate
of a gradation that will create voids in the mixture. The gradation of the aggregate may have to be altered
to improve stability, increase air voids, provide skid resistance, or other factors. New aggregate is
typically precoated with a new asphalt binder.
The properties of the binder in the existing asphalt bound pavement must be determined to provide
information for the selection of the characteristics of the new binder and the amount of new binder to be
added. The physical properties of viscosity and penetration are normally determined. Binder physical
properties associated with PG graded binders will be used in the near future.
The traffic that will use the pavement should also be considered in the design process. High volumes of
heavy traffic will require mixtures with high stability and resistance to permanent deformation.
The type of pavement distress occurring in the existing pavement may indicate deficiencies in the hot-mix
asphalt to be recycled. A pavement with rutting and corrugations indicates that the mixture in its present
condition is unstable. A pavement with alligator cracking indicates a structural deficiency that most
probably should be corrected with additional thickness. Transverse cracking may indicate that the asphalt
binder is excessively stiff for the environment in which it has been placed. Altering the physical and
chemical properties of the binder during the recycling process may be required.
The type, extent, and severity of distress will therefore determine if hot in-place recycling should be
selected as a rehabilitation alternative. The performance of hot in-place recycling is also dependent upon
the type, extent, and severity of distress in the existing pavement. Guidelines for predicting performance
of hot in-place recycling based on these criteria in an existing pavement are not available in the literature.
The above factors affect the mixture design and structural design considerations. Other considerations
include the time required for rehabilitation, project and cross section geometry, thickness of asphalt
abound materials and presence of manholes, utility covers, vegetation, etc.
Specifications
Specifications for hot in-place recycling operations are available from several States, ARRA, and the
reference by Pyrotech Asphalt Equipment Manufacturing Company. The key sections of the hot in-place
recycling guide specifications include a description of the process, materials required, mixture design
information requirement, equipment, construction operation, quality control and quality assurance,
measurement, and payment. Many agencies are using end-result or performance related specifications.
Ontario has developed a warranty specification that they employ on HIR projects (Kazmierowski, Marks,
and Lee 1999). Key factors for some of the specifications sections are:

A general description of the recycling process to be utilized should be included in the
specifications.

The capability of the equipment to heat and to scarify/mill to a specific minimum depth.

A specification reference for the recycling agent (such as ASTM D 4552) and other new
materials, such as anti-strip agent and new hot-mix asphalt.

The party responsible for conducting the mixture design should be designated (contractor or
public agency).
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
The information required for the mixture design submittal should also be identified.

A description of the recycling equipment may be needed in the specifications.

Equipment capability for heating, removal; and distribution of recycling agent, anti-strip
agent and new hot-mix asphalt.

Spreading and leveling unit, and compaction equipment descriptions should also be
considered for inclusion.

This section of the specifications should not prevent equipment capable of performing the
work, but should be prepared to insure that a high degree of success can be expected if the
equipment specified is utilized.

Specify recycled mixture temperatures behind the recycling equipment, but prior to compaction.
Since heating should not adversely harden the asphalt, an extracted and recovered penetration,
viscosity or dynamic shear rheometer value could be specified. Air quality requirements should
be identified and required safety equipment defined.

The depth of pavement removal and the test method to determine this depth should be defined in
the specification.

Criteria for acceptance complete with point of sampling and test method should be specified.

A section on measurement and payment is appropriate.
Quality Control/Quality Assurance
Quality control/quality assurance requirements for hot in-place recycling should be similar to those for
conventional hot-mix asphalt. The quality of hot in-place recycling projects is very dependent upon the
uniformity of the existing pavement (along with pavement, across the pavement and with depth).
Therefore, the specification requirements and pay factors may have to be adjusted for this variability.
Key items that should be considered for inclusion in QC/QA specifications include asphalt binder content,
in-place density, laboratory molded density, smoothness, and depth of recycling. QC/QA specifications
with pay factors for hot in-place recycling are not widely used at this time. Project variability needs to be
defined, test methods developed, and so on. As discussed in previous sections HIR lends itself to quality
control by the contractor since many of the operations are not readily accessible for inspection.
7.
HOT CENTRAL PLANT RECYCLING
Hot central plant recycling is a process in which reclaimed asphalt pavement materials, reclaimed
aggregate materials or both, are combined with new aggregate, asphalt, or recycling agents, as necessary,
in a central plant blending and mixing operation to produce HMA paving mixtures. Hot central plant
recycled materials can be used for base courses, intermediate layers, or surface courses. The finished
product is generally required to meet standard materials specifications and construction requirements for
the type of HMA mixture being produced. With equipment now available, all hot-mix producers can
recycle using relatively inexpensive additions or modifications to their existing plants, or using plants
designed specifically for recycling, without violating air quality regulations. Small percentages of RAP
are routinely recycled in both conventional batch and drum mix plants. Inclusion of high percentages of
RAP requires special mixture designs and often requires specialized equipment.
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The uses and benefits of hot central plant recycling include the following (AI 1986):

Surface and base structural problems can be corrected.

Significant structural improvements can be obtained with little or no change in thickness. For
example, untreated granular bases can be recycled into hot-mix asphalt (HMA) concrete and then
placed back in the same thickness.

Existing mix deficiencies, such as aggregate gradation problems, can be corrected.
Hot central plant recycling is a relatively proven technology and is much less experimental in nature than
cold in-place and hot in-place recycling. Hot central plant surface and base recycling has been practiced
for a number of years. In 1915, Warren Brothers practiced the recycling of asphalt paving surface into
asphalt concrete using central plant operations, but very little experimentation was conducted from that
time until 1974 (Epps et al. 1980; Epps 1978). A widespread rebirth of central plant recycling occurred in
1974 because of the rapid increase in the price of asphalt cement, and increased costs of other
construction materials and equipment. With the increased use of the drum mixer and its adaptability for
recycling, the amount of hot central plant recycled material being produced has increased.
Comparing cold in-place recycling with hot in-place recycling, there are several advantages of hot central
plant operations. Improved quality control can be obtained in terms of modifier and total binder contents,
blending percentages of new and recycled aggregate, and mixture homogeneity. Processes involving the
use of heat generally produce mixtures that do not have to be cured before obtaining near maximum
strength. The process can be used to repair all types of pavement, including high-traffic volume facilities.
With proper scheduling, it is possible to remove a section of pavement and replace it the same day, using
recycled mixtures made with aggregate from the previous day’s removal operation.
The disadvantages of hot central plant recycling compared to in-place methods are that 100 percent RAP
generally cannot be used in the recycled mix, additional costs result from transporting the material to and
from the central plant, and traffic may be disrupted for longer periods of time.
Background
FHWA’s recent review of hot central plant recycling operations indicates that States remove about 50
million metric tons of HMA pavements annually (Sullivan 1996a). About 33 percent is reused in hot
central plant recycling operations. An additional 47 percent of the HMA pavement removed is used in
some type of highway application (unstabilized base, shoulder, erosion control, cold in-place recycling,
and hot in-place recycling) (Sullivan 1996a).
Recycled HMA pavement comprises only about 4 percent of all HMA produced in the United States of
American (450 million metric tons of HMA produced annually). From a survey of 17 States, 9 indicated
that HMA containing RAP (any percentage of RAP) was less than 20 percent of their total production of
HMA. Six States reported that between 20 and 50 percent of all HMA produced contained some
percentage of RAP. One State (Florida) indicated that more than 50 percent of all HMA produced
contained some RAP (AI 1986). A 1986 survey performed by NAPA indicates that about 23 percent of
all HMA produced contained RAP (Sullivan 1996a; The Futures Group, Inc. 1988). A comparison of the
production surveys conducted in 1986 and 1992 indicates that the amount of RAP utilized in the United
States has not increased during this period (Sullivan 1996a; The Futures Group, Inc. 1988).
A review of the literature suggests that while some States have significant hot central plant recycling
operations, other States perform little or no hot central plant recycling. Common reasons given for not
using hot central plant recycling include:
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
RAP variability.

Uncertainty of blending of new binder and aged RAP binder.

High variability of hot central plant recycled mixtures.

Unavailability of performance information on hot central plant mixtures.
Reference Manual
Information relative to these cited barriers are summarized below. Key references for hot central plant
recycling are:

NAPA Information Series 123. (Young 1996).

FHWA Report TA 92-76 (Sullivan 1996a).

FHWA Report SA-95-060 (Sullivan 1996b).

NHI Reference Manual (ARE 1987b).

NCHRP Synthesis 54 (Epps 1978).
Recycling Methods and Equipment
Four basic construction activities are required in the hot central plant recycling process:

Removing the existing pavement.

Preparing the RAP for hot-mix recycling, (stockpiling and crushing).

Processing the blend of old and new materials in a hot-mix plant.

Placing the materials on the roadway.
The recently completed NAPA report and other publications are summarized below and define the
equipment and processes used for hot central plant recycling (Young 1996).
Pavement Removal
Much of the same equipment that is used for pavement removal and size reduction in cold recycling
processes can also be used for hot recycling. The methods can be generalized as:

Ripping and Crushing. The pavement is removed and then reduced in size at another location by
crushing.

In-Place Removal and Sizing. The pavement is reduced in size as part of the removal process.
In the ripping and crushing operation, earthmoving equipment, scarifiers, grid rollers, or rippers are used
to break up the existing HMA pavement, which is then loaded into trucks and hauled to a crushing site.
The type of removal equipment that is used will depend on the maximum size of the RAP pieces that
available crushing equipment can handle. For example, relatively large pieces from a simple ripping
process may be suitable for a primary crusher, while the use of a grid, sheepsfoot, or similar type roller or
dozer after ripping may be needed to reduce the size for acceptance by secondary jaw or roll crushers.
On-grade pulverizers, such as a traveling hammermill, can also be used for preliminary crushing. Ripping
and central plant crushing is usually economical only when the full depth of the asphalt-treated layer is to
be removed. Its main advantage is that no investment in new or specialized equipment is required, but
special care must be taken to minimize contamination of the RAP by underlying untreated base or
subgrade materials. The disadvantages include increased time to complete the recycling process due to
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the need to crush the removed RAP, and greater potential for segregation of the material and congealing
after stockpiling.
The alternative to ripping and crushing is to remove and size the RAP in-place, using equipment normally
associated with in-place recycling such as single- or multiple-shaft rotary mixers and cold-milling
machines. This type of procedure is primarily used on projects that require only partial depth removal.
The major advantage of this method is that the RAP is reduced to a manageable size on the spot, and no
further crushing, or only a minimal amount of crushing, is required at the plant site. The advantages of
using cold milling include:

High productivity in almost any weather.

No heat, dust, or toxic fumes.

Usefulness for nonrecycling work (i.e., pavement profiling or texturing).
The disadvantages of this procedure include:

Special attention needed to ensure that entire pavement is reduced to proper size.

Increase in the amount of fines generated.
Crushing and Sizing
The degree to which reclaimed pavement materials must be processed after removal depends largely on
the removal method and the requirements of the mix design. HMA pavement removed by ripping will
have to be crushed and screened to reduce the maximum particle size to acceptable limits. The Asphalt
Institute recommends that at least 95 percent of the RAP pass the 50 mm (2 in) sieve, while another
guideline is that the RAP be processed such that 100 percent will pass the 37.5 mm (1.5 in) sieve and 90
percent will pass the 25 mm (1 in) sieve (AI 1986; Epps et al. 1980). The National Asphalt Pavement
Association (1993) recommends that if the RAP contains aggregate that is larger than the maximum size
permitted by the mix specifications, the material should be sized by screening or crushing to the
maximum size of the comparable virgin mix (i.e., 19 mm [0.75 in] maximum in mix specifications, crush
to 19 mm [0.75 in]), although larger sizes such as 37.5 mm (1.5 in) or 50 mm (2 in) are acceptable for
thick base course mixes produced in drum-mix plants. Obviously, the size specification will depend on
the characteristics of the particular job and economic considerations, since the larger the allowable
crushed RAP size, the less expensive the crushing process. The downside of this is that larger pieces of
RAP are harder to heat adequately.
Different types of RAP crushing and sizing are available. Typical equipment presently available to the
contractor is identified below (Young 1996).
Rap Breaker. This type of equipment is not designed for extensive crushing and downsizing of RAP.
Occasional large pieces of RAP may pass through the cold feed. The RAP breakers on “lump
breakers” are designed to handle these larger pieces. Scalping screens placed between the RAP cold feed
and transfer belt conveyors can be used to divert the large size material to the RAP breaker. Some RAP
breakers resemble a small cold milling machine head, while others resemble a small roll crusher.
Rap Crushers. Horizontal impact crushers, hammermill impact crushers, jaw/roll combination crushers,
and milling/grinding reduction units have been used to reduce the size of RAP.
Horizontal impact crushers have solid breaking bars fixed to a solid rotor. RAP is crushed as a result of
impact with the breaking bars and a striker plate. Horizontal impact crushers can be used as either a
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primary or secondary crusher. Jaw primary and horizontal impact crushers are sometimes used to
produce the desired RAP size reduction.
Hammermill impact crushers use breaker bars that pivot on a rotor, creating a swing-hammer type action.
The swing-hammer allows for “foreign material” to pass through the crusher unit without damage.
Hammermill impact crushers can be used as either a primary or secondary crusher. Jaw primary and
hammermill impact secondary crushers are often used to process RAP.
Combinations of jaw and roll crushers are effective in reducing the size of RAP. The jaw crusher is used
as a primary crusher to reduce large RAP “slabs.” Secondary roll crushers are then typically used to
produce RAP to minus 37.5 to 50 mm (1.5 to 2 in).
On hot days, both roll and jaw crushers can cause agglomeration or “pancaking” of RAP. The “pancake”
will reduce production and may have to be removed. Horizontal impact and hammermill impact crushers
typically have little agglomeration.
Crushing-sizing equipment that is infrequently utilized can be described as milling/grinding units. These
units are not designed to reduce the stone size in the RAP, but to break the asphalt-aggregate bond or the
asphalt binder.
Stockpile Operations
Literature of the 1970s and 1980s indicated that RAP stockpiles should be low in height, as RAP will
agglomerate under its own weight. Experience has shown that stockpiles of considerable height do not
agglomerate significantly. However, independent of stockpile height, a crust of RAP of 200 to 250 mm
(8 to 10 in) will form on the surface of the stockpile. The exterior crust on the stockpile is caused by heat
generated by the air and the sun. The crust may help “shed” water from entering the stockpile. A frontend loader usually can easily break through this crust and the RAP material inside the stockpile is easily
handled.
RAP stockpiles have a tendency to hold rainwater and not drain like conventional aggregate stockpiles.
Low height stockpiles (because of their relatively large surface area) tend to absorb larger percentages of
water than stockpiles of considerable height. Water contents of 7 to 8 percent are not unusual in low
height stockpiles. High moisture content RAP reduces production and limits the amount of RAP that can
be recycled in a mixture.
Covering RAP stockpiles will reduce the water content in the material and increase production. Tarps or
plastic covers should not be used as water will accumulate under the covers and increase the water
content in the stockpiles. The use of open-sided buildings with a paved surface for the stockpile is the
best method to use for controlling water content in stockpiles.
Material handling equipment should not drive directly on the stockpiles of RAP. The resulting
compaction from this traffic will create agglomerations of RAP and handling problems.
Management of stockpiles is an important part of a successful hot central plant recycling operation. RAP
stockpiles should be formed to ensure that the gradation of the aggregate, the type of aggregate, the
amount of asphalt binder, and the hardness of the asphalt binder are as uniform as possible. When
pavement is removed from large projects, single stockpiles of the RAP should be formed from single
projects.
If small quantities of RAP are received from several sources or projects, the crushing and processing
operation should be used to blend the materials and to ensure that uniform stockpiles of processed RAP
are created.
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Hot central plant recycling operations that use small percentages of RAP can tolerate more variability in
the stockpile than recycling operations that use large percentages of RAP. The management of
stockpiling operations to ensure material uniformity, low moisture contents, and a minimum amount of
agglomeration is important for successful hot central plant recycling operations.
Cold Feed Operations
Conventional cold feed systems can feed RAP materials, but the best feeder bins have steep sides.
Longer feeder belts and larger openings onto the feeder belts are utilized. Since RAP has a tendency to
agglomerate, the front-end loader operator must feed the cold feed bins slowly (an entire front-end loader
bucket should not be dropped into the hopper). RAP cold feed bins should not be filled to capacity.
RAP should not remain in the cold feed bin for an extended period or agglomeration may occur.
Pneumatic air “cannons” or “blasters” have been used to ensure the free flow of RAP from the cold feed
bin to the collection belts. Vibrators on the bins tend to agglomerate the RAP and should not be used.
Agglomeration problems are most severe on hot and humid days.
Mixing Equipment
In the 1970s and 1980s, several innovative approaches were developed to heat RAP in hot central plant
facilities. Two different types of heat transfer techniques are primarily used to heat RAP in central plant
facilities, conductive heat transfer and convective heat transfer. Conductive heat transfer occurs when
two materials of different temperatures are in contact with each other. Convective heat transfer occurs
when solid particles are exposed to a hot gas stream.
When RAP is used in batch plant types of operations and in most counter-flow drum mixers, conductive
heat transfer is responsible for the majority of the heat transfer between the new aggregate and the RAP.
Parallel flow drum mixers primarily use convective heat transfer.
A number of heating and mixing plants that use these principles of heat transfer are presently used to hot
central plant recycle. A brief summary of this equipment is provided below (Young 1996). Most of the
equipment developments have been focused on solving environmental problems (blue smoke) caused by
the use of larger percentages of RAP: by developing equipment that can recycle relatively large
percentages of RAP from a heat transfer point of view and by providing adequate mixing of the RAP,
new aggregate and recycling agent.
Batch Plant or Weight Bucket Method
The Minnesota or weight bucket method is a batch plant process in which the RAP (cold and wet) is
introduced into a weigh hopper and mixed with the “superheated” new aggregate in the pugmill.
Conductive heat transfer takes place between the RAP and the superheated new aggregate.
Since RAP stockpiles are often relatively high in water content, a significant amount of steam is
generated when the RAP comes into contact with the superheated new aggregate. Environmental control
systems on the plant must be capable of handling this large amount of steam. Additional air handling
equipment may have to be installed to handle the steam problem.
A typical batch plant can recycle up to about 50 percent RAP. Most batch plant operations limit the RAP
content to about 25 to 30 percent. Moisture content of the RAP, capability of the bag house to handle
high air exhaust temperatures, and the capacity of the emission control system associated with the pugmill
are often the controlling factors that limit the quantity of RAP.
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Batch Plant-Separate RAP Dryer
This process typically utilizes two conventional parallel flow dryers, one for heating the new aggregate
and the second for heating the RAP. The exhaust gas from the RAP dryer is introduced into the burner
end of the dryer for the new aggregate to reduce air quality problems and to reduce heating costs of the
new aggregate. The heated RAP is stored in a bin and is proportioned into the pugmill mixing chamber.
The capacity of this system to handle RAP is typically controlled by the ability of the new aggregate dryer
to handle the steam and “blue smoke” from the RAP dryer. The “blue smoke” is typically burned in the
combustion area of the new aggregate dryer.
Parallel Flow Drum Mixer-RAP Collar
Mid length entry systems on conventional parallel flow drum mixers have been used extensively for hot
central plant recycling operations. The mid length entry allows for the RAP to enter the drum when the
hot gases from the burner have been reduced in temperature. Depending on the characteristics of the
asphalt binder on the RAP, the type of recycling agent used and the temperatures of the gases in the drum,
air quality problems may arise. Some plants can operate with RAP percentages to 70 percent without
significant air quality problems. Percentages as low as 25 percent may be required with other plant-RAP
combinations.
Parallel Flow Drum Mixer-RAP Collar
Mid length entry systems on conventional parallel flow drum mixers have been used extensively for hot
central plant recycling operations. The mid length entry allows the RAP to enter the drum when the hot
gases from the burner have been reduced in temperature. Depending on the characteristics of the asphalt
binder on the RAP, the type of recycling agent used and the temperatures of the gases in the drum, air
quality problems may arise. Some plants can operate with RAP percentages to 70 percent without
significant air quality problems. Percentages as low as 25 percent may be required with other plant-RAP
combinations.
The parallel flow drum mixer is presently the most common type of plant utilized to produce hot central
plant recycled material. Because of air quality problems, coating problems and the desire to increase
production, a number of other approaches have been utilized and are summarized below.

Parallel flow drum mixer-RAP collar and continuous mixing device.

Parallel flow drum mixer-RAP collar and isolated mixing area.

Parallel flow drum mixer-counter flow RAP drying tube.

Parallel flow drum mixer-RAP introduced in continuous mixing device.

Counter flow triple drum dryer

Counter flow dryer-RAP introduced in continuous mixing device.

Counter flow dryer-RAP introduced in the aggregate dryer.

Counter flow drum mixer.

Counter flow dryer and continuous mixer.

Indirect heat transfer.

Microwave heat transfer methods.

Laydown and compaction.
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Module 3-8. HMA Pavement Recycling and Reconstruction
Recycled Mixture Design
Several methods are available for designing hot central plant mixtures (AASHTO 2000; McDaniel and
Anderson 2000b; AI 1986; Epps et al. 1980). The procedure published as appendix A in a reference by
Epps et al. (1980) serve as an excellent reference. Further detail can also be found in National Highway
Institute Course No. 131050, Asphalt Pavement Recycling for State and Local Governments.
The recycling of old bituminous-bound pavements often requires special consideration because the binder
is often hard and brittle. Asphalt recycling agents can be used to soften these old binders and produce
mixtures with properties similar to those of conventional asphalt-bound materials. General steps in a mix
design include:
1. Evaluation of salvaged materials.
2. Determination of the need for additional aggregates.
3. Selection of modifier type and amount.
4. Preparation and testing of mixtures.
5. Selection of optimum combinations of new aggregates and asphalt modifiers.
The overall philosophy of this approach is to use the recycled materials, new aggregate, and modifier to
produce a mixture with properties as nearly like a new HMA as possible. Standard test methods are used
where possible.
Superpave Procedures
The introduction of the Superior Performing Asphalt Pavement (Superpave) mix design method in the
1990’s complicated hot central plant recycling since no methodology was included in Superpave mix
design for consideration of recycled mixtures. The reason for this was that Superpave concentrated on
defining performance parameters for virgin asphalt cements and aggregates. Original guidance for use of
reclaimed HMA in Superpave mix design is summarized below (Bukowski 1999).
Procedures for the design of mixtures containing RAP are divided into the following three categories:

Tier 1: <15 percent RAP by weight of total mixture.

Tier 2: 16 to 25 percent RAP by weight of total mixture.

Tier 3: >25 percent RAP by weight of total mixture.
Mix Design
It is recognized that experience with RAP varies among States. Thus, these tiers are given as guidelines
that may need to be changed based on local experience. For mix designs containing RAP, the following
requirements are suggested:

General mix design requirements remain unchanged for Superpave mixtures containing RAP.
Requirements for aggregate properties, gradation, and volumetric properties should be met by the
blend of virgin and reclaimed materials.

The gradation of aggregate in the RAP should be used in calculation of the mix gradation. RAP
is treated like a stockpile of aggregate during this analysis. Aggregate consensus properties may
be run on the individual RAP aggregate stockpile at the agency's discretion. While fine aggregate
angularity, sand equivalency, and flat and elongated particles might not be measured on the
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individual RAP aggregate stockpile, some amount of RAP aggregate will need to be extracted,
combined with the total aggregate blend and tested for compliance with aggregate consensus
properties.

The percentage of asphalt binder in the RAP should be considered when determining the trial
asphalt content. Asphalt binder content of the total mixture for mix batching includes virgin and
reclaimed asphalt binder. The mixture trial asphalt content is calculated or estimated by
experience during the trial blend analysis. Thus the amount of asphalt binder in the RAP is
considered when determining how much virgin asphalt binder is required. The ability to obtain
satisfactory mix volumetric properties is a requirement for all tiers.

RAP should conform to the following requirements, as well as any other individual agency
requirements:

Maximum 2 percent deleterious materials, or as specified by the agency.

No particle in the mixture made with RAP should exceed the mixture maximum aggregate
size at the time of production and discharge into the transport vehicle.

For mixture design, the specific gravity of the virgin asphalt binder should be used as the specific
gravity of the asphalt binder in the RAP.

The effective specific gravity of the aggregate in the RAP should be determined and used as the
bulk specific gravity of the RAP aggregate for calculation purposes. When the RAP contains
highly absorptive materials, the amount of absorbed asphalt should be estimated based on
experience and used to backcalculate the bulk specific gravity of the aggregate.

During the laboratory mix design procedure the RAP is handled as a combined material, asphalt
and aggregate are not extracted and handled individually, but are left together.

Moisture content of the RAP should be initially determined to facilitate batching for mix design.
A representative sample of RAP should be "pre-dried" to a constant mass prior to the batching of
the mix specimens. This sample used for determination of the moisture content should not be
used for other mix testing because it will be overheated.

During batching of specimens, virgin aggregates and RAP should be heated to mixture
temperature. RAP once heated to mixing temperature should not be reheated.

Further hardening of the RAP asphalt binder during heating of the material to the mixing
temperature should be avoided. Therefore, heating time for RAP should be kept at a minimum.
The RAP should not be held at mixing temperature for more than one hour.

After laboratory mixing and prior to compaction, the mix of virgin and reclaimed materials
should be short-term oven aged. The same short-term oven aging procedure is used for the
specimens containing RAP as would be used for a mix with all virgin materials.

Use mixing and compaction temperatures for intended asphalt binder grade or as specified by the
agency. Mixing and compaction temperature for virgin asphalt binders may be based upon equiviscous temperatures measure with a rotational viscometer using AASHTO TP48 (ASTM D4402)
or as specified by the agency. For combinations of virgin and reclaimed asphalt binder, actual
measurement on a homogenous blend is not required. Mixing and compaction temperature can
be obtained from a typical virgin binder or as specified by the agency.
Binder Selection
Tier 1 (<15 percent RAP) - The asphalt binder grade for the mixture is selected for the environmental and
traffic conditions the same as required for a mixture with all virgin materials. No grade adjustment is
made to compensate for the stiffness of the asphalt binder in the RAP.
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Tier 2 (16 to 25 percent RAP) - The selected binder grade for the new asphalt binder is one grade lower
for both the high and low temperature than the grade required for a virgin asphalt binder. Thus, if the
specified binder grade for a virgin mixture were PG 64-22, the required grade for the binder in the
recycled mixture would be a PG 58-28. In moderate climate areas, the low temperature portion of the
binder grade may not need to be adjusted at this RAP tier. Thus, based on local climate and experience,
some areas may only need to adjust the high temperature portion of the binder grade to account for
inclusion of RAP. The asphalt binder grade can also be selected using an appropriate blending chart if the
designer chooses to adjust the binder selection to compensate for the stiffness of the reclaimed asphalt
binder.
Tier 3 (> 25 percent RAP) - The binder grade for the new asphalt binder is selected using an appropriate
blending chart for high and low temperatures.
NCHRP 9-12
One recurring question concerning Superpave and RAP is whether RAP acts like “black rock.” If RAP
acts like a black rock, the aged binder will not combine, to any appreciable extent, with the virgin binder
and will not change the binder properties. If this is the case, then the premise behind blending charts,
which combine the properties of the new and old binders, is void. This question was addressed through
National Cooperative Highway Research (NCHRP) Project 9-12, “Incorporation of Reclaimed Asphalt in
the Superpave System.” The objectives of this research were to investigate the effects of RAP on binder
grade and mixture properties and to develop guidelines for incorporating RAP in the Superpave system on
a scientific basis.
The findings of this research largely confirmed current practice (McDaniel and Anderson 2000a). The
concept behind the current blending charts was supported, as was the use of a tiered approach. The
advantage of this approach is that for relatively low levels of RAP extensive testing of the RAP binder is
not required. If higher RAP contents are used, conventional Superpave binder tests can be used to
determine how much RAP can be added and the proper virgin binder to blend with. A possible binder
selection guide is shown in table 3-8.8.
Table 3-8.8. Possible binder selection guidelines for RAP mixtures (NCHRP 2000b).
Recommended Virgin Asphalt Binder Grade
No change in binder selection
Select virgin binder one grade softer than normal (i.e.,
select a PG 58-28 if a PG 64-22 would normally be used)
Follow recommendations from blending charts
RAP Percentage
Recovered RAP Grade
PG xx-22
PG xx-10
or lower
or higher
PG xx-16
<20%
<15%
<10%
20 – 30%
>30%
15 – 25%
>25%
10 – 15%
>15%
Performance
FHWA has recently completed a literature review and visited 17 States to collect performance data on hot
central plant recycled projects (Sullivan 1996a). Only limited performance information was available,
and it is summarized below.
If proper mixture design is performed, asphalt binder properties of recycled mixtures should be typical of
those obtained from conventional HMA. A study for Georgia reports on material properties and
performance information, since none of the studied pavement sections had significant distress, no
HMA Pavement Evaluation and Rehabilitation
3-8.47
Module 3-8. HMA Pavement Recycling and Reconstruction
Reference Manual
difference in performance has been noted between recycled and conventional HMA (Kandhal, Rao, and
Young 1994). No statistical difference exists between properties of recycled HMA and conventional
HMA (table 3-8.9).
A Washington State study of hot central plant recycled pavements indicated comparable performance
between recycled and conventional HMA pavements as shown in table 3-8.10 (Sullivan 1996a).
A study conducted by Louisiana indicated that there is no significant difference between recycled and
conventional HMA when performance is measured by pavement condition rating indicators. A
significant difference was not noted on asphalt binder properties (viscosity, penetration, and ductility)
between recycled and conventional HMA. Some increase in cracking was noted on the recycled
pavements with higher recovered asphalt viscosity properties (Paul 1996).
FHWA concludes that hot central plant recycled material that was designed under established mixture
design guidelines and produced under appropriate quality control and acceptance measures will perform
comparably to conventional HMA (Sullivan 1996a). Note that a range of performance has resulted on hot
central plant recycled projects. Premature cracking and rutting have been observed on some projects.
Economics
Limited first cost comparison information is available in the literature. Detailed life-cycle cost studies
have not been conducted. Typical first cost savings of 5 percent to 20 percent are possible, depending
upon the following:

Amount of RAP in mixture.

Haul distance to central plant.

Availability of mixing plant.

Sequencing of construction operations.

Tipping fees for landfills that accept construction waste.

Adjustment of roadway features.

Raising bridges for vertical clearance.

Raising guardrail.

Retaining curb reveal.

Adjustment of drainage inlets and manholes.
The following first cost savings have been reported:

New York—$7.25 per metric ton ($6.57/T).

Wisconsin—$4.10 to $4.50 per metric ton ($3.72 to $4.08/T).

Georgia —$1.25 to $6.35 per metric ton ($1.13 to $5.76/T).

FHWA—$0.45 to $10.40 per metric ton ($0.41 to $9.43/T).
If performance period and maintenance requirements are nearly equal for both hot central plant and
conventional HMA, life-cycle costs will favor hot central plant recycling as compared to conventional
HMA.
3-8.48
HMA Pavement Evaluation and Rehabilitation
Construction Details
Project
%
RAP
Absolute
Viscosity
(Pa s)
18C
0
18R
Field Core Information
Age
298.8
6.0
9.0
1.5
15
298.8
5.7
9.3
1.5
22C
0
270.3
6.0
6.6
1.75
22R
10
191.2
5.7
6.9
1.75
23C
0
280.7
23R
25
199.0
5.4
6.5
1.5
25C
0
296.5
5.8
7.9
2.25
25R
20
205.5
5.7
7.4
2.25
28C
0
304.7
6.0
8.3
1.5
28R
20
304.6
5.8
7.8
1.5
10 Poise = 1 Pa s
1 kPa
64 ºC
-22 ºC = -8 ºF
= 147 ºF
1.5
= 0.145 psi
%
Voids
(Mat)
7.6
(0.45)
8.2
(0.81)
9.4
(0.70)
7.5
(0.89)
3.6
(0.80)
4.9
(0.52)
6.2
(1.07)
5.3
(0.70)
8.3
(1.34)
6.5
(0.99)
5581.0
20.8
2078
5537.0
21.9
2012
3309.2
12.1
781
3677.3
11.9
655
3467.7
12.3
1030
3300.2
10.3
721
10344.0
28.1
1789
5934.1
16.1
1341
4627.2
16.0
1102
4990.7
16.9
1712
1 MPa = 145 psi
25 ºC
= 77 ºF
G*/Sin(Delta) G*/Sin(Delta)
64ºC (kPa)
-22ºC (kPa)
Spec > 2.2kPa Spec < 5000kPa
Indirect
Tensile
25ºC
(kPa)
1766
(104)
1594
(145)
1035
(28)
980
(62)
1166
(124)
1111
(117)
1511
(166)
1346
(104)
1497
(69)
1428
(62)
MR
25ºC
(MPa)
7505
6572
4942
4210
4816
4728
8289
5732
7104
9519
Module 3-8. HMA Pavement Recycling and Reconstruction
3-8.49
% AC
%
Voids
(Mat)
Absolute
Viscosity
(Pa s)
Reference Manual
HMA Pavement Evaluation and Rehabilitation
Table 3-8.9. Comparison of recycled HMA and conventional HMA average test results (standard deviation) (Kandhal, Rao, and Young 1994).
80 kN
(18 kips)
ESAL
(millions)
1993
PSC
Predicted
Program
Rehab
Date
Reason
for
Rehab
Mileposts
RAP
Content
Year
Complete
I-90
I-90
I-90
121.92 to 126.14
102.61 to 106.34
239.11 to 255.29
72%
79%
65%
1977
1978
1982
4.3
7.3
58
86
80
4/93
93
9/92
Rut & PSC
Rut
Rut
SR-395
SR-2
I-90
I-90
I-90
I-90
183.69 to 190.61
240.77 to 245.40
126.14 to 137.20
164.25 to 175.62
191.89 to 200.35
244.90 to 254.31
70%
40%
75%
75%
65%
71%
1982
1982
1982
1982
1982
1982
1.1
0.4
3.3
2.6
2.7
73
60
76
51
81
80
9/95
8/92
9/95
2/93
3/98
9/92
PSC
PSC
PSC
PSC
PSC
Rut
SR-9
SR-97
I-90
SR-99
SR-5
SR-527
5.35 to 7.15
144.64 to 149.56
175.62 to 179.05
22.53 to 25.98
88.02 to 102.70
8.90 to 10.34
8%
9%
35%
33%
70%
35%
1982
1982
1983
1984
1984
1985
0.6
0.4
2.8
4.3
11.3
0.4
80
76
85
79
97
76
1/98
8/94
5/98
1/98
1/93
1/98
PSC
PSC
PSC
PSC
Rut
PSC
16 years
15 years
10 years
performance
13 years
10 years
13 years
11 years
16 years
10 years
performance
16 years
12 years
15 years
15 years
9 years
13 years
Predicted
Average
Convention
HMA
Service Life
15 years
15 years
9 years
9 years
13 years
12 years
13 years
9 years
13 years
16 years
18 years
21 years
Reference Manual
HMA Pavement Evaluation and Rehabilitation
Route
Predicted
Project
Service
Life
Module 3-8. HMA Pavement Recycling and Reconstruction
3-8.50
Table 3-8.10. Performance summary of Washington State DOT recycled HMA recycling projects (Sullivan 1996a).
Reference Manual
Module 3-8. HMA Pavement Recycling and Reconstruction
Guidelines for Use
Hot central plant recycling is a very versatile rehabilitation alternative for pavements at all levels of traffic
and types of distress. Partial depth and full depth recycling, using the hot central process, can treat all
types of distress, provided adequate mixture design and structural design is performed. Full depth
recycling removes all of the HMA and hence can eliminate reflection cracking. Since the pavement can
be removed and replaced in this recycling operation, vertical and geometric control problems can be
avoided.
Hot central plant recycling has the lowest construction variability of all forms of recycling and can
produce the highest quality paving material. Performance information suggests that a properly designed
hot central plant recycled mixture will perform the same as conventional HMA.
Specifications
The 1996 summary of State specifications presented in Roads and Bridges magazine indicates that some
States limit the amount of RAP in the various asphalt-bound pavement layers and that some States do not
allow RAP in surface courses. Higher RAP percentages are typically allowed in the lower pavement
layers. The reason for these limits is based primarily on State perceptions or performance information
indicating that poor performance can be obtained at high RAP percentages, that recycling agents do not
soften the aged asphalt binder on the RAP, and that air quality problems exist at the higher RAP
percentages.
Quality Control/Quality Assurance
Quality control/quality assurance practices used for hot in-place recycling are those typically used for
conventional HMA. Variability of the RAP should be considered when preparing QC/QA limits and pay
factors for individual projects.
8.
RECONSTRUCTION
Reconstruction of a roadway is another rehabilitation alternative that should always be considered. In
some instances the condition of the existing roadway does not lend itself to economic rehabilitation
treatments. Recycling of the existing pavement structure and/or base material may still be incorporated
into a reconstruction project as an economical and environmentally sound way of disposing of the
existing material. Reconstruction is defined as the removal and replacement of the existing pavement
structure. Some of the recycling methods discussed above could also be considered reconstruction if the
entire pavement structure is removed and replaced.
Although rehabilitation is the predominant form of pavement work in the United States today there are
still many miles of pavement that are reconstructed every year. There have been several mega-projects
undertaken in the last few years involving the reconstruction of major urban interstates, such as the
Central Artery project in Boston, Massachusetts and the reconstruction of I-15 in Salt Lake City, UT.
Before reconstruction is undertaken there are several factors that should be considered.
Condition of subgrade
Many existing pavements were built upon subgrades that are considered poor. Before reconstruction is
selected a thorough investigation of the existing subgrade condition should be undertaken. This
investigation should include the ability of the existing subgrade to support construction traffic and the
placement of the new pavement. If the subgrade is not adequate for these operations then the cost of
improving or replacing part of the subgrade must be added to the project.
HMA Pavement Evaluation and Rehabilitation
3-8.51
Module 3-8. HMA Pavement Recycling and Reconstruction
Reference Manual
Maintenance of Traffic
As a general rule, reconstruction projects have a longer timeline for construction than do most
rehabilitation alternatives. During the reconstruction, traffic must generally be severely restricted or
detoured. The counterpoint is that generally once a reconstruction project is completed the traveling
public should not be inconvenienced again by rehabilitation operations for 15 to 20 years. If user costs
are considered in the selection process this may very well be a dominant factor.
Utilities
Many urban roadways have had utilities placed under and along the pavement. If the pavement is simply
being replaced in-kind, the conflict with existing utilities will be minimal and generally limited to
adjusting manholes and utility boxes. Most reconstruction of urban pavements involves improvements to
the subgrade, widening, and geometric adjustments. Many utility companies also take this as an
opportunity to perform upgrades and maintenance on their facilities. On some major urban reconstruction
projects, utility relocation has been estimated at 20 percent of the total project costs. In addition to the
cost of the relocation of the utilities, the time involved should also be considered. Since much of that
construction cannot begin until the existing roadway is closed and the pavement structure removed, this
adds to the total construction time of the project.
Geometrics
Reconstruction provides the opportunity to correct geometric deficiencies, add additional capacity, and
provide a roadway built to the latest standards that will serve the public for many years without additional
work. Correcting existing deficiencies usually requires the acquisition of additional right-of-way and may
require more a more extensive environmental process. This may add to the overall project cost and time
to complete the project.
Safety
Reconstruction also provides the opportunity for safety enhancements to the roadway. Correction of
geometric deficiencies, updating or eliminating the need for safety appurtenances, and providing a
pavement surface with adequate drainage and friction characteristics, will all reduce the rate and severity
of crashes.
Performance Period
Another advantage of reconstruction is the longer performance period associated with this work. Many of
the treatments discussed in this block have performance lives from five to ten years. Reconstruction with
HMA pavement should provide 20 to 30 years of service or longer with proper preventive maintenance
treatments.
Project Budget
Cost of reconstruction is obviously considerably higher than the other rehabilitation treatments discussed
in this block. Reconstruction costs of the pavement itself can vary from $20/m2 to $50/m2 ($17/sy to
$42/sy) depending upon traffic, local construction costs, and design. In urban areas on high volume
freeways, consideration is given to bridges, utilities, signing, traffic control, and so on, the total cost can
run as high as $62 million per centerline kilometer ($100 million per centerline mile). With limited
3-8.52
HMA Pavement Evaluation and Rehabilitation
Reference Manual
Module 3-8. HMA Pavement Recycling and Reconstruction
budgets, agencies must weigh the benefits and costs associated with the rehabilitation and reconstruction
options and make the appropriate decision. A pavement management system is helpful in assisting with
this decision on a network basis.
9.
SUMMARY
CIR
Cold in-place recycling is a viable engineering and economical rehabilitation alternative for asphaltsurfaced pavements with moderate to low traffic volumes. Some States have successfully used cold inplace recycling on interstate highways. Cold in-place recycling is a rehabilitation alternative that can be
used to rehabilitate the pavement from the “bottom up” and thus can be used to strengthen a roadway with
minimal change in the vertical cross section. This technique also lends itself to stage construction.
Several States have collected general performance data on this technique. Overall performance has been
very good on a large percentage of the projects. Some problems with raveling, rutting, and cracking have
been noted. Adequate specification and quality control guidelines have been developed by both State
highway agencies and industry.
HIR
During the past 20 plus years, the concept of hot in-place recycling has grown steadily, although more
rapidly in the late 1980s and early 1990s. Early attempts were limited by equipment, but the development
of companion technologies such as cold milling and hot central plant recycling has added to the
knowledge base and spurred equipment and materials improvements that are useful to hot in-place
recycling.
Throughout the early 1990s, the techniques and equipment developed by the pioneers in organizations
like Artec, Pyrotech, Wirtgen, Cutler, Jackson, and Taisa have incrementally approached the goal of
being able to recycle at depth and put down an acceptable high quality asphalt pavement that can directly
compete with other HMA pavements.
Guidelines for selecting suitable hot in-place recycling projects are important so that the process is used
where most effective. Further education and training will be needed to help expand the industry. This is
being accomplished through the efforts of manufacturers, ARRA and other associations, highway
agencies, contractors, and others. An important group that will be working toward standardization is Task
Force 40 of the AASHTO-AGC-ARTBA Joint Committee.
Hot Central Plant Recycling
While almost all the States allow RAP in hot central plant operations, most States do place limits on the
amount. Additionally many States do not allow hot central plant recycled materials as surface courses.
Performance of hot central plant recycled materials that have been properly designed has been equivalent
to conventional HMA. First cost savings are possible when using hot central plant recycling in addition
to environmental savings of landfill space.
Reconstruction
Reconstruction is a viable alternative that should be considered during the selection process. Advantages
include a longer performance period and an opportunity to correct existing safety and operational
deficiencies. Disadvantages are a longer construction period requiring more disruption to traffic and
increased cost.
HMA Pavement Evaluation and Rehabilitation
3-8.53
Module 3-8. HMA Pavement Recycling and Reconstruction
Reference Manual
10. REFERENCES
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Anderson, D. I., D. E. Peterson, M. L. Wiley and W. B. Betenson. 1978. Evaluation of Selected
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Highway Administration, Washington, DC.
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(MS-21). Asphalt Institute, Lexington, KY.
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Reference Manual
Module 3-8. HMA Pavement Recycling and Reconstruction
Asphalt Recycling and Reclaiming Association (ARRA). 2000. “ARRA Member Firms Recycle 34
Million Tons.” ARRA Newsletter Fall 2000. Asphalt Recycling and Reclaiming Association, Annapolis,
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Bituminous Pavements.” Transportation Research Record 1684. Transportation Research Board,
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Button, J. W., D. N. Little and C. K. Estakhri. 1994. Hot In-Place Recycling of Asphalt Concrete.
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Reference Manual
Epps, J. A. 1990. Cold Recycled Bituminous Concrete Using Bituminous Materials. NCHRP Synthesis
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