Repeated Load Research Articles

Study of the Influence of the Mastic Coating of Untreated Reclaimed Asphalt Pavement on the Permanent and Resilient Behavior

Abstract Reclaimed asphalt pavement (RAP) arises from the demolition of asphalt road layers. The Optimal Recycling of Reclaimed Asphalt Pavements Project concerns the cold recycling of 100 % RAP without binder addition in base and subbase layers of low-traffic roads. In this context, a test program was performed with Rhine region materials to evaluate the impact of the mastic coating of untreated RAP. A source of reclaimed asphalts with 4.4 % of bitumen is tested before (RAP) and after binder extraction (RAP-BE) and then is compared to an unbound granular material (UGM) with a similar particle-size distribution. Repeated load triaxial tests were conducted at several frequencies to study the permanent and resilient behaviors. The results show that the RAP before binder extraction and the UGM present similar permanent behaviors, while the aggregates without binder reveal low strains. Concerning the resilient phase, the RAP and the RAP-BE show similar levels of strains, but the resilient moduli of the UGM are significantly lower than those of the RAP. From the experimental results, the nonlinear elastic Boyce model was fitted and brings out a highly anisotropic mechanical behavior of the RAP. Finally, the RAP shows a frequency-sensitive viscous behavior because of the bitumen in the mastic, while the effect of the frequency is negligible for the RAP-BE.

LOW-CYCLE FATIGUE OF COLD-DRAWN TYPE 304 AUSTENITIC STAINLESS STEEL IN ANNEALING CONDITIONS

Type 304 austenitic stainless steel, or better known as SS304, generally has alloying elements: C < 0.1%, Cr 18 – 20%, Fe 66 – 74%, Mn <2%, Ni 8 – 10.5%, P <0.045%, Si <1%, and S <0.030%. In general, this material has good ductility, high tensile strength, and excellent corrosion resistance. In application, type SS304 will be subjected to repeated loading, and eventually, the material will undergo plastic deformation, which leads to structural failure in a short life. The failure of SS304 is generally due to the inability of the material to repeat loading, which results in large amounts of plastic deformation so that the SS304 will experience fatigue and then fracture. Based on the description above, this study aims to evaluate the LCF properties of 304 CDS stainless steel with annealing heat treatment. The parameter used was the strain amplitude 0.003 – 0.013 mm/mm. The results of this study revealed that the highest fatigue life in the LCF test was experienced by steel with heat treatment at an amplitude of 0.003 mm/mm with 48367 cycles. In contrast, at the amplitude condition of 0.013 mm/mm, the fatigue life of the steel decreased drastically with the resulting plastic strain being larger, namely 0.0094 mm/mm and elastic strain of 0.0035 mm/mm, with an average modulus of elasticity of 194 GPa. Annealing treatment conditions experienced decreased mechanical strength but tended to be ductile. Using Basquin-Coffin-Manson Equation, empirical equations to predict LCF of 304 stainless steels can be determined.

Calcium bentonite and sodium bentonite as stabilizers for roads unbound

Several technologies are currently available to stabilize the unbound layers of road pavements. The use of these solutions, often manufactured by means of chemical processes, is steadily increasing worldwide. As an alternative natural resource, the use of bentonite represents a valid option when it comes to road stabilization and its application in this context is still relatively unexplored. This study characterises the use of calcium based bentonite and sodium based bentonite to stabilize a typical road base layer. Considering two types of aggregates with different geological origin, the laboratory investigation is performed on dry specimens by means of repeated load triaxial tests, which assess the enhancement in stiffness and resistance to permanent deformation. The findings show that both the investigated types of bentonite are suitable for road stabilization. Even if negligible from a road engineering standpoint, the performance stemming from calcium bentonite was slightly better than the one pertaining to sodium bentonite when it came to the increase in resilient modulus. • Bentonite clay is a natural effective resource to stabilize unbound layers of roads. • Repeated load triaxial tests evaluate modifications in stiffness and deformation. • The performances related to calcium bentonite and sodium bentonite are compared.

Reclaimed Asphalt Pavement (RAP) as an Unbound Base Course Material: A Mechanistic Design Approach Based on Multi-stage Repeated Load Triaxial Tests

Reclaimed asphalt pavement (RAP) and blends of RAP with virgin aggregate (VA) were characterized in this study to be utilized as base course aggregate. Multi-stage and single repeated load triaxial (RLT) tests were conducted in the laboratory to characterize the materials. The results from these tests were used to develop a mechanistic design approach that is suited not only for virgin but also for recycled materials that are prone to excessive plastic deformations under repeated loads. The design approach presented in this study allows the users to select different pavement structure configurations in terms of base course and HMA thicknesses, where the selected configuration can withstand the repeated loads without undergoing significant permanent strains. Such design approach was achieved with the laboratory tests that were conducted with variety of stress ratios (σ1/σ3) that represented different pavement layer configurations and beyond. Principles of shakedown theory were utilized to define the boundaries of plastic shakedown, creep shakedown, and incremental collapse. This study demonstrated that the use of failure and plastic shakedown boundaries were crucial to the proposed mechanistic design approach.

Fan-shaped Shear Dampers Strengthen Mortise-tenon Joints in Chinese Traditional Timber Structures

ABSTRACT Looseness, deformation, and tenon pulled in the mortise-tenon joint constitute the primary causes of failure in traditional Chinese timber structure building. However, the introduction of the joint damper technology and installation of the fan-shaped shear damper at the mortise-tenon joints of traditional wooden structure solves these issues. Herein, we explored the strengthening effect of dovetail mortise-tenon joints with dampers. We designed six dovetail mortise-tenon joint models, three without dampers, and the other three were installed with dampers. Using the low cycle repeated loading test, we examined the hysteretic response, skeleton curve, stiffness degradation curve, and equivalent viscous damping coefficient curves of the six test models. Consequently, we established that the joint damper effectively controls the joint tenon pulled problem and enhances energy dissipation, strength, and rotational stiffness. Besides, the ultimate bearing capacity of the damper models was nearly four-fold that of models without dampers.

Cracking Diagnosis in Fiber-Reinforced Concrete with Synthetic Fibers Using Piezoelectric Transducers

The addition of short fibers in concrete mass offers a composite material with advanced properties, and fiber-reinforced concrete (FRC) is a promising alternative in civil engineering applications. Recently, structural health monitoring (SHM) and damage diagnosis of FRC has received increasing attention. In this work, the effectiveness of a wireless SHM system to detect damage due to cracking is addressed in FRC with synthetic fibers under compressive repeated load. In FRC structural members, cracking propagates in small and thin cracks due to the presence of the dispersed fibers and, therefore, the challenge of damage detection is increasing. An experimental investigation on standard 150 mm cubes made of FRC is applied at specific and loading levels where the cracks probably developed in the inner part of the specimens, whereas no visible cracks appeared on their surface. A network of small PZT patches, mounted to the surface of the FRC specimen, provides dual-sensing function. The remotely controlled monitoring system vibrates the PZT patches, acting as actuators by an amplified harmonic excitation voltage. Simultaneously, it monitors the signal of the same PZTs acting as sensors and, after processing the voltage frequency response of the PZTs, it transmits them wirelessly and in real time. FRC cracking due to repeated loading ad various compressive stress levels induces change in the mechanical impedance, causing a corresponding change on the signal of each PZT. The influence of the added synthetic fibers on the compressive behavior and the damage-detection procedure is examined and discussed. In addition, the effectiveness of the proposed damage-diagnosis approach for the prognosis of final cracking performance and failure is investigated. The objectives of the study also include the development of a reliable quantitative assessment of damage using the statistical index values at various points of PZT measurements.

Mechanical properties of roads unbound treated with synthetic fluid based on isoalkane and tall oil

The unbound layers are an essential component of both highly trafficked and low-volume roads as their mechanical properties are of major importance when it comes to ensuring a well-performing pavement. However, as the aggregates employed as road construction materials naturally display a wide variation in the mechanical properties according to their geological origins, the pavement engineer may be interested in utilizing stabilization technologies to deliberately modify and adjust the behaviour of the road courses. This study sheds light on an innovative synthetic fluid technology composed of isoalkane and tall oil pitch. Repeated load triaxial tests investigate the resilient modulus and the deformation behaviour of samples treated with the synthetic system according to different percentages. Furthermore, cyclic triaxial tests are also performed after exposing stabilized specimens to a series of freeze–thaw cycles. The principle of the rolling bottle test is adopted to assess the integrity with stripping loss on loose aggregates coated by the product. The laboratory results show that evident variations in the mechanical properties can be attained once the aggregates are mixed with the synthetic agent and that a strong coating resistant to water and external actions is formed.

Performance Studies on Stone Mastic Asphalt Mixes with Reclaimed Asphalt Pavement

Stone mastic asphalt (SMA) is a gap graded mix which is categorised by more quantity of coarse aggregate, high asphalt content and fibre. Due to stone on stone contact and presences of high filler content, it acts as a stiff matrix and reduces the rutting due to heavy traffic load. This research presents a study on fatigue performance RAP replaced SMA mixes using VG 30 as binder along with elastomer as a modifier and results were compared with conventional SMA mix. The specimens prepared were tested using several laboratory test procedures: Marshall mix design, indirect tensile strength, moisture susceptibility, drain down test and Repeated load fatigue test. Test results showed Marshall Properties of the RAP mix improved up to a RAP content of 30% without elastomer modifier and RAP content up to 60% with elastomeric modifier. From the moisture susceptibility test results, the elastomeric modified SMA mix showed high resistance to moisture damage when compared to conventional mix and 30% RAP replacement mix. Repeated load fatigue test was conducted for different stress load and temperature and results showed elastomeric modified SMA mix offered high resistance to deformation across all stress level and temperature when compared to conventional and optimum RAP mix. As a fatigue loading increased resulted in decrease of number of fatigue cycles and increased in the initial tensile strain of the mix. As the percentage of RAP addition increased the initial tensile strain decreased.

Transient delamination growth in GFRP laminates with fibre bridging under variable amplitude loading in G-control

To predict fatigue-driven delamination growth in real structures, it is inevitable to understand delamination growth under real load spectra, also known as in-service loads, which most commonly are variable amplitude load patterns. In this work, delamination growth in glass fibre-reinforced polymer laminates subjected to two-level block amplitude loading is investigated using a pure moment loaded DCB test configuration for G-controlled cyclic testing. The variable amplitude load patterns consist of single- and periodic repeated load amplitude changes. The method utilises a digital image-based technique that allows for precise tracking of delamination fronts in translucent materials. The work characterises transient crack growth phenomena following load amplitude changes with emphasis on the actual fatigue crack growth rate. The load amplitude changes may increase the crack growth rate by factors of 5–6 in comparison to the crack growth rate in constant amplitude base-line tests. Periodic repeated load amplitude changes particularly increase the fatigue crack growth. For example, after just 10 periodic repeated low- and high-load blocks, the delamination has extended more than a factor of 2 times the delamination extension predicted from a non-interaction model based on constant amplitude data. The effect of the frequency of load amplitude changes on the fatigue crack growth is also investigated. Frequent load amplitude changes are proven to increase the fatigue crack growth significantly because of the transient crack growth responses following the load amplitude changes.

Seismic Performance of Precast Columns Connected with Two Different Connection Modes

The precast reinforced concrete building is made of precast components, which are connected at the joints via reliable connection approaches. Therefore, the study on the stability and reliability of the connections should be conducted to improve the safety and integrity of the precast structure. In this paper, experiments on two connection modes, i.e. corrugated pipe confined with spiral stirrup connection and steel sleeve connection, were carried out. The experimental tests consisted of six specimens, including one cast-in-situ specimen, three precast specimens using steel sleeve connection and two precast specimens using corrugated pipe confined with the spiral stirrup. The influence of artificially unbonded length at the connection joint is also studied. All specimens were tested under low-frequency cyclic repeated loading to analyse their seismic performance under different connection modes. The experimental results showed that the precast columns using the two proposed connections have a similar or even better seismic performance compared with the cast-in-situ column. The installation of proper unbonded treatment on the longitudinal reinforcement in the connection is beneficial in improving the seismic performance of precast columns and the length of 5 times of the longitudinal reinforcement’s diameter is recommended.

Cracking Diagnosis in Fibre Reinforced Concrete Cubes and Cylinders with Synthetic Fibres using a PZT-based Health Monitoring System

This paper carried out experimental research of concrete specimens with synthetic fibres under repeated splitting tension and uniaxial compression to investigate the efficiency of a proposed health monitoring methodology that implements surface-bonded piezoelectric transducers. The developed technique uses small-sized custom-made devices for diagnosing damage due to concrete cracking caused by the imposed repeated loading at various load levels. A test program of standard 150/300 mm cylinders and 150 mm cubes made of plain and fibre reinforced concrete subjected to splitting tensile and compressive repeated loading is presented and discussed. The proposed method utilises the frequency response measurements of a network of small-sized piezoelectric patches mounted to the surface of the tested specimens in order to detect the onset of damage since cracking has probably been developed in the inner concrete, whereas no visible cracks appeared on their surface. The objectives of this study also include development of a quantitative assessment procedure of damage using the statistical index values distribution at various points of measurements. The influence of distance, type of material (plain or fibrous concrete) and damage level triggers changes in the measurements of the piezoelectric transducers and the adopted statistical index seems to be a reliable assessment tool for damage quantification. Further, for the first time, a wireless and portable structural health monitoring system display feasible detection of premature cracks for damage prognosis in fibre reinforced concrete structural members.

Evaluation of permanent deformation and resilient modulus of pavement base with different gradations and percentages of clay plasticiser

Characteristics of unbound granular pavement materials vary among transportation and road association around the world based on function, environmental conditions, and materials availability. Plasticity index (PI) is one of the most important characteristics of unbound granular pavement courses both in the construction stage and throughout its service life. It indicates the range of moisture contents over which the fines in the pavement material behave plastically. This study investigates the effect of a refined clay plasticiser called Claypro (CP) on the elastic and plastic behaviour of unbound granular base materials at different gradations. This paper aims to find the optimum clay content (CP) that achieves an acceptable PI and maintains high elastic performance and high plastic strain resistance under traffic loading. Elastic and plastic responses of 18 mixtures at three different gradations, representing two types of crushed rock materials namely; Hornfels and Basalt, were evaluated at three different CP contents. Repeated load triaxial (RLT) test was carried out with a high number of cycles to estimate the resilient modulus (MR) and the permanent deformation of these mixtures. The shakedown concept was also applied to assess the deformation behaviour of the tested engineered mixtures under multi-stage RLT test. Results showed that 1% Claypro (CP) content is considered the optimum for all gradations of both aggregate types considered herein.

Hydromechanical behavior of unsaturated expansive clay under repetitive loading

Compacted layers of expansive soils are used in different engineering projects, such as subgrades, engineered clay barriers, and buffers for radioactive waste disposal. These layers are exposed to a variety of stresses and wetting conditions during field serviceability. Coupling between hydraulic and mechanical repeated loading provides insight understanding to the induced progressive deformation of expansive clay. This study was conducted to investigate the hydromechanical behavior of unsaturated compacted expansive clay under repeated loading–unloading (RLU) conditions. Two series of one-dimensional (1D) oedometer tests were conducted under controlled matric suction up to 1500 kPa using the axis translation technique (Fredlund soil-water characteristic curve device, SWC-150). The first test series was carried out at different levels of controlled matric suction for non-repeated loading–unloading (NRLU) cycles. RLU cycles were applied in the second test series at different repetitive-stress levels and under different levels of matric suction. The results indicated increasing axial wetting strain εa(s), axial swell pressure σs(s), compression index Cc(s), and swell index Cs(s) with suction reduction. The estimated load–collapse (LC) curves obtained from NRLU series (LCN) and RLU series (LCR) indicated increasing yield stress σy(s) with increasing suction. This is attributed to the developed apparent cohesion between soil particles, which in turn rigidifies the material response. Applying repetitive loading induced a notable reduction of compression index Cc(s) at the same level of suction, whereas swell index Cs(s) seems to be independent of repetitive loading. Finally, repetitive loading exceeding initial yield stresses results in plastic hardening and, hence, enlargement of yield stress locus (i.e. LCR curve).

Critical Dynamic Stress and Accumulative Deformation Evolution of Embankment Silty Clay Subjected to Cyclic Freeze-Thaw

In cold regions, the permanent settlement of embankment is mainly caused by the repeated freeze-thaw process and long-term repeated train loads. Meanwhile, the critical dynamic stress (σdcr) is an important parameter index for determining embankment stability. Therefore, the accumulative permanent deformation evolution and critical dynamic stress of embankment soil subjected to cyclic freeze-thaw were studied using dynamic triaxial tests. Firstly, a numerical model for calculating critical dynamic stress considering the repeated freeze-thaw process was proposed, which shows that the critical dynamic stress of embankment soil rapidly decreases in the first two repeated freeze-thaw cycles, whereas it tends to be stable after the subsequent freeze-thaw process. Next, based on the normalization of the critical dynamic stress, an explicit model for predicting accumulative plastic strain (εp) of embankment soil was established. The above model considers freeze-thaw times, repeated dynamic stress amplitude (σd), and loading times, in which all material parameters of Qinghai-Tibet silty clay were presented. Thus, the critical dynamic stress and accumulative plastic strain models established in this paper can be applied to judge the embankment stability and predict the embankment settlement induced by train loads in cold regions.

Resilient modulus of coarse-grained subgrade soil for heavy-haul railway: An experimental study

Large-scale dynamic triaxial tests under train-induced repeated load were conducted to study the variation of resilient modulus of the coarse-grained soil under different moisture contents, dynamic stress amplitudes, number of loading cycles and confining pressures. The results show that the state of specimens under the dynamic load can be divided into three categories: stable, critical and failure in accordance with the development trends of the accumulated plastic strain. For the saturated stable specimen, the resilient modulus tends to be stable at about 1000 cycles, while for the unsaturated stable specimen, the resilient modulus shows an increase trend with the increasing number of loading cycles. Resilient modulus increases with the increase of dynamic stress amplitude and confining pressure, but decreases with the increase of moisture content. The larger the amplitude of confining pressure and dynamic stress is, the more the resilient modulus decreases with the increase of moisture content.

A Comparison between Static and Repeated Load Test to Predict Asphalt Concrete Rut Depth

Rutting has a significant impact on the pavements' performance. Rutting depth is often used as a parameter to assess the quality of pavements. The Asphalt Institute (AI) design method prescribes a maximum allowable rutting depth of 13mm, whereas the AASHTO design method stipulates a critical serviceability index of 2.5 which is equivalent to an average rutting depth of 15mm. In this research, static and repeated compression tests were performed to evaluate the permanent strain based on (1) the relationship between mix properties (asphalt content and type), and (2) testing temperature. The results indicated that the accumulated plastic strain was higher during the repeated load test than that during the static load tests. Notably, temperature played a major role. The power-law model was used to describe the relationship between the accumulated permanent strain and the number of load repetitions. Furthermore, graphical analysis was performed using VESYS 5W to predict the rut depth for the asphalt concrete layer. The α and µ parameters affected the predicted rut depth significantly. The results show a substantial difference between the two tests, indicating that the repeated load test is more adequate, useful, and accurate when compared with the static load test for the evaluation of the rut depth.

Shear Resistance Assessment of the Y-Type Perfobond Rib Shear Connector under Repeated Loadings

The steel–concrete composite structures consist of two different material parts, which are connected with reliable shear connectors to enable the combined action of the steel and concrete members. The shear connectors may experience either one-directional repeated cyclic loadings or fully reversed cyclic loadings depending on the structural functions and acting loadings. It is essential for structural engineers to estimate the residual shear strength of the shear connectors after action of repeated loads. The characteristics of deteriorating shear capacities of Y-type perfobond rib shear connectors under repeated loads were investigated to estimate the energy dissipating capacity as well as the residual shear strength after repeated loads. To perform the repeated load experiments four different intensities of repeated loads were selected based on the monotonic push-out tests which were performed with 15 specimens with five different design variables. The selected load levels range from 35% to 65% of the representative ultimate shear strength under the monotonic load. In total, 12 specimens were tested under five different repeated load types which were applied to observe the energy dissipating characteristics under various load intensities. It was found that the dissipated energy per cycle becomes stable and converges with the increasing number of cycles. A design formula to estimate the residual shear strength after the repeated loads was proposed, which is based on the residual shear strength factor and the nominal ultimate shear strength of the fresh Y-type perfobond rib shear connectors. The design residual shear strength was computed from the number of repeated loads and the energy dissipation amount per cycle. The reduction factor for the design residual shear strength was also proposed considering the target reliability level. The various reduction factors for the design residual shear strength were derived based on the probabilistic characteristics of the residual shear strength as well as the energy dissipation due to repeated loads.

Analysis of Reinforced Concrete Brackets Strengthened with Steel Fiber

An experimental investigation into the structural behavior of brackets with plain and fibrous concrete and in the presence of steel bars is addressed in this research. Both brackets have the same shape and dimensions. Testing of nine steel fibrous reinforcement concrete specimens and one reinforced concrete with compressive strength (28.6) MPa under vertical repeated load. The test results in terms of load versus deflection curves, stiffness, ductility and crack patterns show the effectiveness of using steel fibrous concrete to ensure a superior strength and deformation capacity in reinforced concrete bracket. Test results showed that, addition of steel fibers (1.5) % by volume of concrete and horizontal stirrups improve both shear strength and ductility of the tested brackets, and results in a more ductile failure mode.

Mechanical response of full-scale geosynthetic-reinforced asphalt overlays subjected to repeated loads

This study aims at evaluating the influence of geosynthetic reinforcements on the structural improvement of asphalt overlays placed on distressed pavement layers using repeated load tests. Full-scale instrumented pavement models were constructed in an indoor steel tank measuring 1000 mm in length, 1000 mm in width and 1000 mm in depth. Full-scale instrumented pavement models consisted of a 650-mm-thick weak subgrade, 250-mm-thick base, 90-mm-thick distressed asphalt layer, binder tack coat, geosynthetic reinforcement (except in control sections), and 50-mm-thick hot mix asphalt overlay. Sensors used in the instrumentation program included earth pressure cells and linear variable displacement transformers installed on the subgrade and surface layers, respectively. Four different geosynthetic types, including woven geo-jute mat (GJ), polypropylene geogrid (PP), polyester geogrid (PET), and fiberglass geogrid composite (FGC) were adopted as asphalt reinforcements. A servo-hydraulic actuator was used to replicate a live traffic wheel load by applying an equivalent single axle contact pressure of 550 kPa at a frequency of 1 Hz. Repeated load tests were terminated after 100,000 load cycles and the behaviour of geosynthetic-reinforced full-scale models was compared with that of unreinforced model. Performance indicators, including Traffic Benefit Ratio (TBR) and Rut Depth Reductions (RDR), were estimated and repeated load test results indicated an increase in the structural performance of geosynthetic-reinforced full-scale models in relation to that of unreinforced model. Among the geosynthetic-reinforced models considered in this study, the FGC-reinforced model showed a comparatively better performance with a maximum TBR of 20 at a permanent deflection of 5 mm and the highest RDR of 56% after 100,000 load cycles, respectively. Maximum reductions of 56% in surface deflection and of 30% in vertical pressure on the subgrade were also observed after 100,000 load cycles in the FGC-reinforced model.

YIELD STRESS UNDER REPEATED LOADING AFTER APPLYING DIFFERENT DEFORMATION PATH FOR LARGE TENSION AND SHEAR

This paper describes the yield phenomenon when a repeated loading is applied to the structure after giving a large pre-strain on it. In the series of our previous research, focusing on the fundamental deformations such as tension or shear, changes of yield stress as the number of repetition increases have been investigated experimentally by using test specimens of annealed pure copper. In the determination of yield stress value, the method by using slope of stress-strain curve at yield has been used instead of proof stress. As a consequence, it has been found that if the type of pre-deformation and the type of deformation in repeated load are the same, yield stress at the pre-deformation side has a declining tendency as compared with opposite side. Therefore, it is predicted that the yield stress under repeated loading after applying a large deformation is closely related to the loading history in large deformation previously applied. Thus, in this study, in order to clarify the influence of pre-deformation on the yield behavior under repeated loading, the experiments are performed by changing the order of tension and shear in pre-deformation. Consequentially, it is clarified that the declining tendency of yield stress under repeated loading is closely related to the yield surface anisotropy, which is formed during the second half of the pre-deformation.