Material that is recovered from aged asphalt pavements is known as RAP (Reclaimed Asphalt Pavement) (1, 2). The requirement for using RAP in new asphalt is becoming increasingly urgent (3) because of the economic and environmental benefits (3), reducing demand on finite resources, generation of waste materials and embodied energy (4).

Cold recycling of asphalt is a proven technique that reduces energy consumption (5–7). This reduction is largely achieved by avoiding aggregate drying (8) and mixing the material at ambient temperature instead of 170°C-185°C, as required for hot mix asphalt (HMA) (9). The use of bituminous stabilising agents produces a flexible layer with superior fatigue performance to those with purely cementitious binders (10). This study looks at foamed asphalt with cement, material which is classified in the UK as a cold recycled bound material (CRBM) (8, 11).

Foamed bitumen is produced by injecting air and water droplets under high pressure (e.g. 5 bar) into hot (160–180°C) liquid bitumen, resulting in the formation of foam (12). The volume of bitumen increases while viscosity considerably reduces (13). Typically foam bitumen is added to the mixture at between 3% and 5% by weight of aggregate; however, when the bitumen content of the recycled material is high, this can be reduced to 2–3% (2, 13, 14).

Early life CRBM mechanical properties change over time (15). This phenomenon, during which the cohesion between the binder and the aggregates increases as the mixture loses water, is known as curing (16, 17). No standard curing procedure has been established for CRBM; however, from previous research, it has been demonstrated that curing specimens fully wrapped at 20°C for 28 days is an appropriately conservative practice (16, 18); therefore, this curing procedure was chosen for this study.

Despite the increasingly common use of CRBMs in roads (11, 19), the specifications for the use of these materials in airfields are underdeveloped (8) and there is no guidance to ensure that pavement design with these materials is trustworthy (20–22). Design guides for airfield pavements such as FAArfield (23), BAA (24) or Design and Maintenance Guide 27 (DMG27) (25) do not readily allow the introduction of new material properties (20), making it difficult for authorities and practitioners to use these materials on airfield pavements. A new design approach is therefore required (22). In Figure 1 a harmonised approach for analytical pavement design of pavements using CRBM is proposed (22).

In this investigation Kenlayer, an empirical-mechanistic software package, was selected to undertake the multilayer-elastic analysis. This software allows analysis which can incorporate CRBM behaviour (26). It was decided to use a mechanistic design method because of the lack of the necessary performance data to undertake an empirical analysis (21; 22). Furthermore a mechanistic method provides a theoretically sound approach as it relates the stresses, strains and deflections within a pavement structure with the loads and material properties.

The structure of airfield pavements comprises surface, binder and base courses laid on a foundation as shown in Figure 2, for new build and full rehabilitation designs (24; 25). The surface and binder courses have typical thicknesses of 40 and 60 mm respectively; the base course thickness is designed as a function of traffic, subgrade conditions and desired design life (26).

According to the BAA approach the base thickness calculated is then divided into 1/3^rd asphalt and 2/3^rd dry lean concrete (24), while DMG27 requires a minimum of 120 mm of asphalt base to avoid reflective cracking for high traffic scenarios (25).

To carry out a pavement analysis with Kenlayer (or any other multi-layer linear elastic program), material mechanical properties need to be defined, such as stiffness, Poisson’s ratio and failure criteria; these parameters therefore have to be determined for CRBM.

The material stiffness can be obtained from conventional indirect tensile stiffness modulus (ITSM) tests (27). Poisson’s ratio has a relatively small effect on the pavement response (26); thus a typical value of 0.3 has been adopted here for CRBM (28). The failure criteria can be adjusted by modifying cracking and permanent deformation algorithms (26). Fatigue cracking is a common distress that affects pavement service life (29).

The failure criterion for permanent deformation is expressed by equation [1]:

where N_d is the allowable number of load repetitions to limit permanent deformation, ɛ_c is the compressive strain at the top of the subgrade, and f₁ and f₂ are coefficients determined from road tests or field performance (26). The compressive strain at the top of the subgrade is used as a failure criterion as the permanent deformation is considered to be caused by subgrade weakness rather than by the overlying layers (26). Taking this into account, for this research f₁ and f₂ were selected as 1.365x10^-9 and 4.477 respectively, these values being taken from the Asphalt Institute analytical design procedure (26;30).

The failure criterion for fatigue cracking is expressed by equation [2]:.

where N_f is the allowable number of load repetitions to prevent fatigue cracking, ɛ_t is the tensile strain at the bottom of the asphalt layer, E₁ is the elastic modulus of the asphalt layer and f₃, f₄ and f₅ are coefficients determined from laboratory fatigue tests, with f₃ modified to correlate with field performance observations (26).

A key aim of this paper was to evaluate at a laboratory level the parameters needed to perform a pavement analysis with Kenlayer incorporating CRBM with foamed bitumen layers, namely stiffness and fatigue coefficients, f₃, f₄ and f₅.

To obtain the inputs needed for Kenlayer mentioned in the introduction, a laboratory program was established. The materials used were specified fully in previous work (20) and are summarised in the next section.

In this paper, the fundamental CRBM input variables for undertaking a pavement design analysis with Kenlayer have been identified as stiffness and fatigue life.

Laboratory determination of these inputs showed significant difference in the performance of CRBM versus HMA. In terms of stiffness, calculated values for CRBM are within specifications; therefore this material is identified as potentially appropriate for airfield pavement design. Fatigue coefficients have been established for CRBM; however, the failure criterion used in this research was the conventional target of 50% reduction of stiffness value, as generally used for HMA, and it remains to be investigated whether this failure criterion is also valid for CRBM.

Fatigue is a determining factor for understanding CRBM behaviour under cyclic loading. For this reason, further investigation is needed in order to develop fuller understanding of how CRBM performs, and how pavement design should best be progressed.