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Highlights: [•] We model the stresses in the subgrade of a structure of High Speed Rails. [•] The levels of the stresses in the capping layer have been calculated. [•] They are compatible with mechanical fatigue of stabilized fine-grained soils. [•] A thickness for capping layers made with treated materials is proposed. [Copyright &y& Elsevier]

Abstract Ballast deterioration, under dynamic loads, remains an important issue on high-speed tracks that can lead to high maintenance costs. This ballast deterioration leads to settlements. Several studies have shown that these settlements were linked to high accelerations produced in the ballast by high-speed train (HST) passages. The solution with bituminous underlayment was used since 1980s in several countries like United States, Italy, Spain, especially on high-traffic and high-speed lines (HSL). In France, the interest in this technique is recent. Following the East European HSL satisfactory behavior, a layer of asphalt concrete was used under the ballast layer of the Bretagne-Pays de la Loire (BPL) HSL. It is intended, in addition to the schedule savings and the protection of the subgrade during the construction phase, to reduce acceleration amplitudes produced at the passage of HST, to ensure moisture stability in the subgrade and thereby to decrease the maintenance costs of the tracks. BPL HSL includes 105 km of innovative track with an asphalt concrete (GB) ballast sublayer, and 77 km with a granular layer under the ballast (UGM). Out of the instrumented sections of the BPL track, 3 sections are constructed with GB subballast layer and one with a layer of UGM as a subballast layer. A total of 127 sensors that includes accelerometers, anchored displacement sensors, temperature and humidity probes, and extensometers are used. Sensors are placed at various positions and depths in the track structures. Data were first acquired during a speed up test phase, under controlled conditions, with the same train passing at speeds ranging from 160 to 352 km/h. This paper presents the different sensors used for the instrumentation as well as the acquisition system installed to collect all measurements. Data treatment and processing is explained in details. Finally, results obtained for different speeds are presented, with a focus on accelerometer and anchored displacement sensor measurements, on two sections, allowing, among others, comparisons between the response of structures with and without asphalt concrete. The role of the GB, as a subballast layer, in damping the vertical displacement of the sub ballast structure and reducing the accelerations peaks in the ballast layer for ballasted tracks is demonstrated. [ABSTRACT FROM AUTHOR]

• Recycled concrete aggregate. • Self-cementing properties. • Unhydrated cement content. • Unbound granular material. • Monotonic triaxial test. Recently, the use of recycled crushed concrete aggregates (RCA) as a substitution of natural aggregates in pavement base and subbase layers has become more popular. Occasionally, a growth of stiffness and strength in unbound base and subbase layers built with RCA can be observed, which can be considered as self-cementing properties of RCA. In this study, the potential self-cementing properties and the long-term mechanical behaviour of two different RCA (NRCA and ORCA), with significantly different self-cementing properties, were studied by pH value, thermogravimetric analysis (TGA) and monotonic triaxial tests under varying confining pressures (20/40/70 kPa) and varying curing times (1/28/360 days). Results show that the long-term storage can largely reduce the self-cementing properties of RCA (ORCA), while the RCA crushed recently (NRCA) exhibits much stronger self-cementing properties. Besides, the long-term mechanical behaviours are influenced by self-cementing properties and confining pressure as well as curing time, so that the strength and stiffness of RCA specimen, with stronger self-cementing properties (NRCA), increase more under low confining pressure and long curing time. The experimental results also indicate that the long-term mechanical behaviours of NRCA are consistent with cement treated materials, whose ductile stress–strain response gradually turns to stiff brittle behaviour as curing time increases. [ABSTRACT FROM AUTHOR]

• Continuous behavior of HSL with bituminous sub-layer over 4-years was observed. • The bituminous sub-layer reduces acceleration levels under the ballast even after 4 years of service. • No fatigue damage was caused in the bituminous sub-layer under high-speed train traffic. • This study underlines the very good overall behavior of the track. • Detection of track damage using accelerometers appears possible. Tamping and ballast wear, due to dynamic stresses, lead to frequent and costly maintenance operations. To mitigate the impact, an innovative track structure combining a bituminous (asphalt concrete) layer below the ballast has been built on the Bretagne-Pays de la Loire High-Speed Line (BPL HSL). The track is intended to reduce the amplitude of the accelerations produced by passing High-Speed Trains (HST) which are the main reason for the settlement of ballast. BPL HSL includes 105 km of asphalt concrete sub-layer below the ballast and 77 km of granular sub-layer. In order to study the dynamic responses of these different structures, some track sections have been instrumented with anchored deflectometers, accelerometers, strain gauges and temperature probes. The aim of this paper is to present the results achieved after four years of commercial traffic. The data processing methods developed for this acquisition phase and the measurements of the substructure deflection, the vertical accelerations under ballast, and the vertical and horizontal strain of the various sections were presented. Deflection measurements, vertical strains and vertical accelerations peaks at the top of the sub-layer are compared between the granular and the bituminous track structures and shows that there is no evolution of the measurements during the four years of monitoring. The study clearly demonstrated that the presence of the bituminous layer in the structure reduces the acceleration levels under the ballast that cause its deterioration. Such conclusions suggest that a bituminous sub-layer might contribute to lower track maintenance needs over the service life of the line, decreasing operational costs and increasing capacity. [ABSTRACT FROM AUTHOR]

Highlights: [•] The stress states in the capping layer of several HSRs structures have been modeled. [•] Results are compared with stress paths induced by several laboratory fatigue tests. [•] A test is highlighted and a design dedicated to treated/stabilized soils is proposed. [Copyright &y& Elsevier]

• The fatigue process of aged asphalt mixtures was assessed under two-point bending fatigue tests. • Acoustic emission technique can characterize the fatigue damage and failure mechanisms. • AE-hits could reflect the onset of damage and failure stages. • New thresholds to identify damage initiation and fracture propagation were proposed. • Classical failure criteria were analyzed and compared to the acoustic emission thresholds. This paper presents a new method for analyzing experimental data stemming from the Two-Point Bending (2 PB) fatigue test conducted on French standardized trapezoidal specimens, in accordance with the European standard EN 12697-24. These tests were performed on an asphalt mix specimen sawn from an in-service road pavement, adopting a loading frequency of 25 Hz and a target temperature of 10 °C. The Acoustic Emission (AE) technique was implemented to monitor the damage and failure processes during fatigue. The fatigue life, defined as the critical number of loading cycles inducing specimen failure, is investigated herein according to several methods, including a normalized criterion, models proposed in the literature, experimental fitting, and Acoustic Emission activity analysis. First, based on the AE analysis, new damage and failure criteria are proposed (Damage Initiation and Failure Propagation Thresholds). In a second step, the mechanical behavior is fitted, in terms of strain, force, stiffness modulus, and phase angle, using four functions, namely: a polynomial function, a continuous Fourier series, a sum of sine functions, and a Gaussian function. After the fitting process, the first derivative is calculated and the singular points of the curves are detected and discussed. Lastly, a phenomenological analysis is carried out in order to understand the damage and failure process stages. [ABSTRACT FROM AUTHOR]