Cold recycled bound materials (CRBMs) provide an economic and environmental advantage for pavements since they decrease energy and raw material consumption. However, design methods for airfield pavements do not include key CRBM properties. In this paper an empirical-mechanistic method is used to study airfield pavement design with CRBM in order to develop design guidance. The aim of the paper is to obtain the inputs related to material properties needed for use in this method. For this purpose, CRBM containing reclaimed asphalt, with fly ash, cement and foamed bitumen as stabilising agents, was characterised. The methodology included indirect tensile stiffness modulus (ITSM) and indirect tensile fatigue tests (ITFT) in strain control mode. The inputs needed for a pavement design analysis with CRBM were then obtained. The results showed the importance of further study on CRBM fatigue to understand the behaviour of these mixes under cyclic loading.
Material that is recovered from aged asphalt pavements is known as RAP (Reclaimed Asphalt Pavement) (
Cold recycling of asphalt is a proven technique that reduces energy consumption (
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 (
Early life CRBM mechanical properties change over time (
Despite the increasingly common use of CRBMs in roads (
Analytical design principle for pavements with cold recycled layer (
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 (
The structure of airfield pavements comprises surface, binder and base courses laid on a foundation as shown in
Airfield flexible pavement structure.
According to the BAA approach the base thickness calculated is then divided into 1/3rd asphalt and 2/3rd dry lean concrete (
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 (
The failure criterion for permanent deformation is expressed by equation [
where Nd is the allowable number of load repetitions to limit permanent deformation, ɛc is the compressive strain at the top of the subgrade, and f1 and f2 are coefficients determined from road tests or field performance (
The failure criterion for fatigue cracking is expressed by equation [
where Nf is the allowable number of load repetitions to prevent fatigue cracking, ɛt 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 (
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 (
For CRBM mixture manufacture, RAP, fly ash, cement and foamed bitumen were used with the mix design shown in
CRBM mix design
Ingredient | Proportion by mass (%) |
---|---|
|
43.5 |
|
39.1 |
|
6.3 |
|
1.6 |
|
3 |
|
6.5 |
The binder contents in the RAP and in the final mixture were calculated in accordance with BS 598-102 (
Bitumen characterisation
Bitumen | Binder content (%) | Penetration (25°,1/10 mm) | Softening point (°C) |
---|---|---|---|
|
NA | 107 | 44.2 |
|
7.2 | 30 | 58.6 |
|
4.4 | 32 | 55.0 |
|
7.5 | 46 | 52.4 |
A Wirtgen WLB 10 mobile foaming plant, with the settings established in
Foaming conditions
|
4 bar |
|
5 bar |
|
100/150 |
|
170°C |
|
1% |
The testing methodology comprised:
Determination of indirect tensile stiffness modulus (ITSM) to BS EN 12697-26:2004 Annex C (
Indirect Tensile Fatigue Tests (ITFT) in strain control mode to BS EN 12697-24:2012 Annex E (
Asphalt material stiffness relates to its load spreading ability and temperature susceptibility, parameters used to assess pavement structural condition. In a structural asphalt layer, high stiffness indicates good load-spreading ability.
In determining ITSM the rise-time, which is the time taken for the applied load to increase from the initial contact load to its maximum value, was selected as 124 ms. 10 conditioning pulses were applied to set the load needed to obtain a peak horizontal deformation of 5µm. To calculate the stiffness modulus 5 pulses were applied across two perpendicular diameters (
As stated before, the stiffness value is a material property required to undertake analysis with Kenlayer and it was measured on 37 specimens at 10, 20 and 30°C.
This test was developed at the University of Nottingham (
The strains selected for the ITFT were between 150 and 300 µɛ (
The stiffness results presented in
CRBM stiffness modulus.
Experimental data from strain control fatigue tests for CRBM
Nf | εt | E1 (MPa) |
---|---|---|
184279 | 0.00015 | 3426 |
202363 | 0.00015 | 3081 |
165563 | 0.00018 | 2219 |
78554 | 0.00018 | 2330 |
123526 | 0.0002 | 2343 |
64123 | 0.0002 | 1660 |
88723 | 0.0002 | 2895 |
65933 | 0.00022 | 1996 |
90173 | 0.00025 | 1711 |
36413 | 0.00025 | 2179 |
82223 | 0.00027 | 1390 |
59043 | 0.00027 | 1343 |
15183 | 0.0003 | 1432 |
21673 | 0.0003 | 1646 |
To obtain the coefficients, the difference between the Nf obtained in the laboratory and the Nf calculated using equation [
CRBM fatigue law calculation.
Maggiore’s data (
Experimental data from strain control fatigue tests for HMA
Nf | εt | E1 (MPa) |
---|---|---|
149243 | 0.000125 | 10900 |
126500 | 0.000135 | 10329 |
88923 | 0.000145 | 11081 |
42613 | 0.000155 | 10231 |
49383 | 0.000165 | 10220 |
22393 | 0.000175 | 10582 |
30963 | 0.000185 | 9828 |
18683 | 0.0002 | 9928 |
17773 | 0.00022 | 9245 |
Fatigue coefficients
f3 | f4 | f5 | |
---|---|---|---|
|
0,074 | 4.842 | 3.109 |
|
0.0685 | 5.671 | 2.363 |
|
0.0796 | 3.291 | 0.854 |
|
7.61·10-6 | 2.826 | 0.110 |
It is noted that the values calculated using Maggiore’s HMA data and the values proposed by Shell are comparable. Thus it seems likely that the ITFT in strain control mode is a suitable test for fatigue coefficient calculation.
Previous researchers report various values for these coefficients, with the typical range of values for f4 being between 3 and 6 (
With the new coefficients obtained in
The fatigue curves from Maggiore’s data and the CRBM mix are compared in
Fatigue laws comparison.
The fundamental material input variables for CRBM assessed with the Kenlayer model have been identified as stiffness and fatigue with permanent deformation being dependent on the subgrade. These variables are summarised in
Kenlayer inputs
Input | Value | |
---|---|---|
|
0.3 | |
|
20°C | 3500 MPa |
f3 | 7.61·10-6 | |
|
f4 | 2.826 |
f5 | 0.110 |
It is also interesting to study the material behaviour in terms of stress evolution during testing to analyse if the modes of failure of the two materials are comparable. In
a) HMA fatigue behaviour (
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.
The research presented in this paper was carried out as part of the Marie Curie Initial Training Network (ITN) action, FP7-PEOPLE-2013-ITN (