Table of Contents

4.7.6 Reclaimed Asphalt Pavement (RAP)

Hot mix asphalt

The addition of up to 15% RAP has little impact on the properties of a dense graded asphalt mix with conventional bitumen binders. Little change is required to asphalt mix design procedures or production methods other than the preparation of separate mix designs for the required proportion of RAP and establishment of suitable protocols for handling, stockpiling and adding RAP.

Where the proportion of RAP exceeds around 15% of the total mix, the bitumen grade may need to be adjusted to compensate for the stiffness of the aged binder in the RAP.

Modified asphalt plants that provide for improved heat transfer, reduced emissions and effective mixing of recycled materials, are generally desirable for higher proportions of RAP addition. Higher proportions of RAP also require greater control over the uniformity of RAP materials, and crushing and screening into separate size fractions is generally recommended.

Guidelines for manufacture of asphalt containing RAP are provided in Section 5.

Specifications generally prescribe the maximum proportion of RAP permitted in particular mix types and applications as well as specific test requirements for mix design or quality assurance purposes.

Hot-in-place recycling

Hot-in-place recycling involves heating and scarifying or milling of asphalt surface material, mixing with fresh binder or rejuvenator, and re-laying, generally in one operation. Fresh asphalt may also be added during the hot recycling process to improve asphalt properties or supplement the layer thickness.

Mix design procedures for in situ recycling are not as well established as those for conventional recycled hot mix. Unlike conventional recycled hot mix where the RAP usually comprises no more than 40% of the mixture, in situ recycled materials may involve up to 100% recycled material.

In general the design steps for hot-in-place recycled asphalt are:

  1. Characterisation of the in situ asphalt
    – binder content
    – penetration, softening point and ductility or, alternatively, viscosity of recovered binder
    – grading and quality of the aggregate.
  2. Determination of the need for, and proportions of, additional mix.
  3. Selection of the type and quantity of rejuvenating agent.
  4. Mix preparation and testing for optimum combination of in situ materials and added aggregates, binder and rejuvenating agent, using standard mix design criteria.

Assessment of the material properties of the existing asphalt pavement, including binder content, binder viscosity and aggregate grading is the first part of the process. This information is used to determine any adjustments to aggregate grading to develop the required voids in mineral aggregate (VMA) and selection of the appropriate viscosity binder or rejuvenating agent.

The process of determining the correct type and quantity of rejuvenating agent or blended bitumen binder is largely iterative.

Table 4.5 provides a guide to typical target values of rejuvenated binder for design purposes. These values are intended to provide a binder of similar consistency to freshly mixed asphalt.

Consistency may be measured as viscosity, apparent viscosity using a sliding plate viscometer, or penetration.

The penetration of Class 170 bitumen is initially between 65 and 90 and drops to about 40 during the manufacture of new asphalt. The penetration values of recovered binder from old pavements are often as low as 10 or 20, and a target penetration of 35 to 40 is recommended for the recycled binder.

Table 4.5: Typical requirements for rejuvenated binder

PropertyRequirement
MinMax

Penetration at 25° C

Viscosity at 60° C

Viscosity at 45° C (heavy traffic)

Viscosity at 45° C (light traffic)

Softening point

35 dmm

350 Pa.s

52° C

900 Pa.s

4.5 log Pa.s

4.2 log Pa.s

56° C

The quantity of rejuvenating agent required, or grade of fresh added binder, may be calculated by various methods including nomographs for individual products or estimated using Equation 4.

   \[\mathrm{r  =  }\frac{\log\left( \mathrm{V  +  3} \right) - \mathrm{ log(T  +  3)}}{\log\left( \mathrm{V  +  3} \right) - \mathrm{ log(R  +  3)}}\]4
where    
 r=the mass fraction of the total binder in the mix that is rejuvenating agent or fresh bitumen 
 R, T and V=viscosity (log Pa.s) at a single temperature that is usually either 45 or 60 °C 
 R=viscosity of the rejuvenating agent 
 T=target viscosity of the final product 
 V=viscosity of the bitumen extracted from the RAP 

Rejuvenation is a physical-chemical phenomenon that takes time under field conditions, as follows:

  • fluxing of the existing binder takes place gradually by migration
  • fluxing is speeded up by compaction, traffic and higher temperatures
  • there is a maximum amount of rejuvenating agent that can be assimilated by a bitumen binder.

The rejuvenating process continues to occur after mixing, leading to further softening of the bitumen. The extent of this post-mixing softening depends on the materials used and the type and quantity of the rejuvenating agent. Full rejuvenation is usually achieved in about three to six months after the completion of the recycling.

The amount of rejuvenating agent that can be added in hot-in-place recycling is limited by the air voids of the existing asphalt. When the air voids content of the old mix is too low to accommodate sufficient recycling agent for proper rejuvenation or softening of the old asphalt without mix flushing, it may be necessary to add additional fine aggregate or improve the mixture with fresh hot mix to open up the mix or increase the air voids. The selection of the appropriate addition (either fine aggregate or add-mix) and the amount to be added are determined by conventional hot mix asphalt design methods. Similarly, criteria used for the testing and evaluation of hot recycled mix construction are generally the same as those used for comparable hot mix asphalt types.

Cold recycled asphalt

Cold recycling of asphalt may be undertaken in situ or as plant mixed materials.

Cold in situ recycling involves milling of in situ asphalt, mixing with fresh binder or rejuvenator, and re-laying. Filler or other additives may also be used to improve mix properties.

Plant mixing comprises crushed and screened RAP blended with fresh bitumen of rejuvenating agent. Fresh bitumen is usually in the form of bitumen emulsion of foamed bitumen. Additional crushed aggregate, natural sand or mineral filler, as well as moisture, may be added to the mixture to improve grading, strength, handling and curing properties of the mixture.

Typical design steps for cold recycled asphalt are:

  1. Characterisation of the in situ asphalt or RAP.
  2. Determination of the need and proportions of additional aggregates or filler.
  3. Estimation of the quantity and type of new binder (or rejuvenating agent).
  4. Mix preparation and testing for optimum combination of RAP, aggregates, filler and binder or rejuvenating agent.

Selection of optimum content of additional binder or rejuvenating agent is somewhat different to the procedure described for hot recycled mixes. Generally, it is not intended to restore the binder to the condition of fresh asphalt. Bitumen emulsion and foamed bitumen have minimal effect on the viscosity of the existing binder in the RAP. Optimum binder content is generally determined as a result of stiffness measurement such as the indirect tensile test.

The optimum amount of added binder or rejuvenator is selected as a combination of economics and minimum resilient modulus of cured materials. Typical proportions of design binder content are shown in Table 4.6.

A further difference in the design of cold recycled mixes is the use of water as an essential component to facilitate dispersion of binders and workability and compaction of the mix.

Mixtures only develop their full strength after a period of curing. The rate of curing and strength development depends on the type of binder or rejuvenator, moisture content and curing conditions. Accelerated curing for laboratory testing is typically undertaken in a fan-forced oven for three days at 40 °C. Development of full strength in the field may take several months, or even longer, depending on ambient conditions and ability of moisture to drain or evaporate from the mixture. The rate of cure of field materials can also be significantly influenced by the choice of added filler.

Table 4.6: Design binder content for cold recycled asphalt

Binder typeAdded binder content
(% by mass of total mix)
Foamed bitumen2.0 to 4.0
Bitumen emulsion1.5 to 3.5 (residual binder)

Further details of cold recycled materials are provided in Austroads (2009b).