Field performance and fatigue characteristics of recycled pavement materials treated with foamed asphalt

doi:10.1016/j.conbuildmat.2013.07.056

Construction and Building Materials

Volume 48, November 2013, Pages 677-684

https://doi.org/10.1016/j.conbuildmat.2013.07.056 Get rights and content

Highlights

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Pavement material properties were estimated from field measurements.
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Laboratory fatigue characteristics of the layer mixes were used in the analysis.
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The contribution of the AC overlay in the CIR pavement condition was investigated.
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The curing of the foamed material is important for the damage of the AC overlay.

Abstract

The paper concentrates on the field performance and the fatigue characteristics of cold recycled materials treated using foamed asphalt (FA). For this purpose, field measurements were performed approximately one month and one year after the pavement was placed in service. Based on this data and laboratory analysis results, there is an indication that fatigue is a minor distress factor for both the cured FA material and the asphalt concrete (AC) overlay which covers the FA. The contribution of the AC overlay in the structural condition of the cold recycled pavement is vital during the early days of the pavement life.

Introduction

As the cost of hot mixed asphalt mixtures continuously increases and the availability of good materials is limited, Cold In-Place Recycling (CIR) offers an environmental friendly and attractive alternative to other pavement rehabilitation options. CIR is an advantageous rehabilitation technique that is eminently suited for the reworking of the upper layers of distressed pavements. Among several cold recycling systems, the foamed asphalt technique [1] has gained popularity over the last two decades for its efficient use of salvaged construction material. The use of the foamed asphalt technique in rehabilitating road pavements was given a boost in the 1990s through the incorporation of advanced cold recycling machine technology. Worldwide since 1991 many foaming systems were developed after the ending of the Mobil patent rights on the nozzle [2].

Information concerning foaming procedures in the laboratory, as well as related construction aspects, although not standardized, is satisfactorily documented in international literature, as for example in [3] and [4]. However, limited information is provided concerning the field performance and the fatigue characteristics of recycled pavement materials treated with foamed asphalt (FA) and when published it is usually focused on low and medium volume roads.

Foamed asphalt treated materials have strong cohesive bonds and a relative high resilient modulus (in comparison to untreated materials) as long as the cohesive bonds is retained. This intact condition is maintained for a certain period of traffic loading and is referred to in [5] as the “effective fatigue life phase”. Eventually, the cohesive bonds are destroyed through the repeated flexing of the material under traffic loading to the point where the effective resilient modulus of the material has decreased to a value comparable to that of an untreated granular material and the layer is referred to as being in an “equivalent granular state”.

Following the research done by [6] on the behavior of foamed asphalt treated materials and based on Heavy Vehicle Simulator (HVS) tests [7], the resilient modulus of the treated material initially starts at a relatively high value and then decreases under the action of traffic until a constant resilient modulus or stiffness state is reached. The latest (“constant stiffness state”) is a scientific interpretation replacing the term “equivalent granular state” [6]. The load repetition from the initial state to the constant stiffness state is more recently referred to in international literature as the “stiffness reduction phase” replacing the term “effective fatigue life phase” [6]. However, an area of concern was the lack of field performance data, as well as differences between the performance under accelerated testing versus long term field conditions [8]. More recent analysis results of Non Destructive Tests (NDT), on a CIR pavement treated with foamed asphalt [9] show no reduction of the modulus of the FA recycled material with time and traffic (see Fig. 1).

In order to gain more information about the field performance of foamed asphalt treated mixes, a field experiment was undertaken by the NTUA Laboratory of Pavement Engineering on sections of a heavily trafficked Greek highway pavement (approximately 1.2 × 10⁶ ESALs per year). In situ Non Destructive Tests (NDT) for approximately one year after the pavement was placed in service laboratory tests and a data analysis research study was performed. The deterioration of the materials during the design period was estimated by calculating the in situ critical strains during the early life of the pavement and using appropriate fatigue laws. The main findings of the data analysis concerning the conducted field experiment are presented and discussed in the present research work.

Section snippets

Experimental test sites and foamed asphalt mix design

The experiment was carried out on two test sections with different FA mix compositions and “soil-support” of the FA treated layer hereafter referred to as S1 and S2, respectively. The term “soil support” for the FA treated layer is defined as the remaining layer beneath the recycled layer, which “supports” the FA layer. The test section S1 provides a stiffer “soil support” for the FA layer, than test section S2. Prior to the CIR implementation, foamed asphalt mix designs were undertaken on

Field data collection

Data from measurements (NDT) performed approximately one and twelve months after the pavement was placed in service were used for further analysis. The data collected approximately one month after the pavement was placed in service was utilized for the evaluation of the performance during the first days of the life of the pavement. Following research concerning relevant CIR pavements and FA recycled mixes (see Fig. 1) the curing of the FA material is expected to be completed one year after

Backanalysis

A thorough field data analysis was performed including a detailed backanalysis using appropriate software Elmod [12]. Considering the level of the subgrade at the bottom of the remaining layer, the backanalysis model consisted of four layers. For backanalysis purposes, the thicknesses of all layers were estimated using the GPR analysis results. Fig. 3 shows the pavement modeling for the backanalysis, including the loading condition parameters (R, P) and the layers characterization through the

Strain response analysis

A strain response analysis was conducted to estimate the strains at critical against fatigue positions in the “body” of the recycled pavements. The strains were used as an input in the fatigue laws in order to identify fatigue characteristics of the CIR pavements. The horizontal tensile strains at the bottom of the FA layer (ε_FA), as well as at the top of the FA layer, i.e. bottom of the AC overlay (ε_AC) (see Fig. 3), were calculated using a multi-layer linear elastic analysis [14]. It should

Fatigue characteristics of the FA treated material

The fatigue characteristics of the FA treated material were determined in the laboratory using the indirect tensile fatigue test (ITFT) [15]. A fundamental issue concerning the used fatigue characteristics determined in the laboratory is that they do not exactly relate to the fatigue in the field, where different stress situation normally occurs. Shift factors need to be applied on laboratory fatigue relations in order to be able to use them for field fatigue performance prediction [16].

Fatigue characteristics of AC overlay

A laboratory fatigue test [14] was performed in the laboratory on characteristic dense AC mixes, similar to the one used in the AC overlay. The AC fatigue low is presented graphically in Fig. 11 (code “AC”, continuous line). In Fig. 11 the laboratory-estimated fatigue characteristics of the FA material are presented with code “FA” (dotted line). The fatigue law for the FA material derived from laboratory testing done by [17] is presented in Fig. 11 with code “FA_ref”. According to the diagrams

Conclusions

The present research study has attempted to contribute towards the field performance and the fatigue characteristics of recycled pavement materials treated with foamed asphalt (FA). The research is based on the investigation of two test sections of cold in place recycled (CIR) pavements. The main findings and discussion points are the following:

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Before the completion of the curing of the FA material the critical tensile strain and consequently the fatigue damage potential was located mainly in

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