Pavement Recyclability - Factors to be Considered

Karlsson and Isacsson (2006) stated that any road in need of reconstruction or new overlay is a candidate for recycling. This assertion is just too general, though they later opined that the re-use of asphalt means that the complexity of the maintenance operation increases, as the reclaimed material is more difficult to characterise compared to new material. However, on a more technical point, PIARC (2003) opined that, the criteria for ascertaining the feasibility of recycling a

34 Figure 2.6: Cold Recycled Mix Plant (Wirtgen, 2005)

Figure 2.7: Cold Recycled Mix Placement (Wirtgen, 2005)

35

Figure 2.8: Process involved in Cold Recycling Ex Situ (ARRA, 2001)

36 pavement is to identify whether its deterioration comes mainly from the poor quality of the pavement itself (insufficient thickness, granular layers contaminated with clay, de-bonded bituminous layers, etc) or from problems related to the subgrade. They further submitted that the recycling feasibility should be established from knowledge of the structure of the pavement and of the characteristics of the materials present in it.

Thus for this reason, it is necessary to examine the road, to determine the characteristics of the materials of the pavement and to collect data on the traffic and climate. ARRA (2001) while elaborating on this gave general guideline for the preliminary selection of roads that are candidates for recycling or reclamation methods for the rehabilitation of asphalt pavements. Table 2.8 details the guidelines. However, ARRA (2001) cautioned that all of the candidate rehabilitation techniques have disadvantages and advantages since not all rehabilitation techniques are equally suited to treat the various pavement distress types. Also, the ability of a rehabilitation technique to correct pavement distress is dependent on the type of pavement distress, as well as the extent of the distress and its severity. A suggested guide by ARRA is shown in Figure 2.9. The local aggregate quality, amount/type of traffic, and climatic conditions are also important factors which need to be considered.

ARRA (2001) further stated that, though economic analysis which includes life cycle costs must be done before a decision is made on the method to be adopted, the following factors must be generally considered:

 The type and severity of the existing distresses

 Age/condition of the existing pavement materials and their potential for recycling

 Expected design life and performance requirements of the rehabilitation

 Traffic growth

 Structural capacity of existing roadway

 Environmental conditions

 Acceptable future maintenance activities

 Geometric, drainage, underground and surface utilities

 Traffic accommodation and safety

 Construction limitations

 Project location and size

 Contractor availability and experience

 Impacts on adjacent businesses and public

 Available budget

 Good engineering judgement

37 Table 2.8: Guidelines for the Preliminary Selection of Candidate Recycling or Reclamation Methods for Rehabilitation of Asphalt Pavements (ARRA, 2001)

Pavement Distress Mode

Candidate Rehabilitation Techniques

Cold Planing Hot In-place Recycling Cold In-place Recycling Thin HMA Thick HMA Full Depth Recycling Combination Treatments Reconstruction

Raveling Potholes Bleeding

Skid Resistance Shoulder Drop Off Rutting

Corrugations Shoving

Fatigue Cracking Edge Cracking Slippage Cracking Block Cracking

Longitudinal Cracking Transverse Cracking Reflection Cracking Discontinuity Cracking Swells

Bumps Sags

Depressions Ride Quality Strength

Most Appropriate Least Appropriate

38 Figure 2.9: A Guide for Choosing Recycling Type to Employ in Addressing Pavement Problems (ARRA, 2001)

39 2.15 Recycling Agents

A number of materials with the purpose of altering the properties of old or aged binders during the recycling of asphalt pavements have been advanced. Karlsson and Iscasson (2006) stated that some of the materials go by names such as, reclaiming, recycling, modifying or softening agents (additives), recycling modifiers, rejuvenators but also fluxing, extender or aromatic oils. They further classified rejuvenators into two general groups:

 Softening agents- examples of such are, asphalt flux oil, lube stock, lubricating or crankcase oil, and slurry oils.

 Rejuvenating agents- examples of such are lube extracts and oil extenders.

They also reported that bitumen, bitumen emulsions, cut backs, foamed bitumen and a number of other proprietary additives have also been used as rejuvenators.

Rejuvenators generally are expected to restore the reclaimed binder characteristics to a consistency level appropriate for construction purposes and pavement performance, and, at the same time, optimise the chemical characteristics with regard to durability. A rejuvenator should also provide sufficient additional binder to coat any new aggregate that is added to the reclaimed mixture and satisfy mixture design requirements. Chen et al (2007) in buttressing this affirmed that the softening agent lowers viscosity of the aged bitumen while the rejuvenator restores the physical and chemical properties of the aged bitumen. FHWA (1997) stated that the types of recycling agents used include emulsified recycling agents, softer grade asphalt cements and cutback asphalts and that however, the most commonly used recycling agents for completely cold recycling processes are emulsified asphalt cements or emulsified recycling agents. This is because the emulsions are liquid at ambient temperatures and have the capacity for being dispersed throughout the mix and do not cause major air pollution problems. At ambient temperature, the softening effect of the recycling agent is a time and temperature dependent physico-chemical process. Schiavi et al (2003) reported that rejuvenators are solutions or emulsions of bituminous binders that are used to restore the properties of softness and low viscosity to hardened (aged) bitumen and that they do this by restoring the balance between maltenes, which are the volatile part of the bitumen lost with time, and the asphaltenes, which are produced and precipitated as a consequence of the ageing. Epps et al (1980) informed that rejuvenators should easily be dispersed in the old binder, and as well be uniform from batch to

40 batch, and practical to use, for example preventing flashing, smoking and health risks.

On the same note, PIARC (2003) identified cement, lime and cement, and cement and emulsion or foamed bitumen in addition to those materials earlier mentioned as materials which have been found very useful in quite a number of recycling projects.

In this research work bitumen emulsion will be used as the recycling agent. This has been chosen since cutback which is similar in operation to emulsion, is commonly used in Nigeria. Therefore switching to emulsions should not pose much difficulty compared to foamed bitumen and furthermore existing equipments for cutbacks can be easily adapted to suit emulsions. Hence further discussions will focus on bitumen emulsion.