Reduction In Initial Stiffness Research Articles

Analysis of viscoelastic behaviour in asphalt pavement through four-point beam bending tests

This study, conducted in accordance with ASTM T321-14 standards, offers crucial insights into the behaviour of asphalt materials subjected to cyclic loading. For proper maintenance and pavement design, it is essential to understand the material response under different loading conditions. This study focuses on the four-point beam bending test to investigate the viscoelastic behaviour of asphalt pavement. The four-point beam bending test is a useful method for determining the material's ability to withstand cyclic loading and deformation, which the material experiences during field traffic conditions. The experimental setup involves subjecting asphalt samples to cyclic loading using a four-point bending apparatus. The imposed load causes the specimen to experience bending strains, representing the actual loading conditions that pavements endure. The data gathered during testing include stress, strain, and deformation properties under various loading conditions. The stress-strain response demonstrates the material's resilience to fatigue, with a gradual decrease in stiffness beyond 10,300 cycles. Fatigue failure criteria include a 50% reduction in initial stiffness for strain-controlled fatigue tests and cracking in stress-controlled tests. The dynamic modulus in a compressive-type, repeated load test follows a three-phase pattern, highlighting the impact of temperature and binder characterization methods on sample performance. The results provide information about the material's resilience to rutting and fatigue cracking, the most significant distresses indicated in asphalt pavements. The findings from this study contribute to an in-depth understanding of the viscoelastic behaviour of asphalt pavement and can aid in the development of improved design guidelines and maintenance strategies characterizing the material response to cyclic loading. Engineers and researchers can make better decisions on the durability and performance of asphalt pavements, resulting in more cost-effective and sustainable road infrastructure.

Simplified methodology for fatigue analysis of reinforced asphalt systems

Fatigue analysis has an important role in evaluating the durability and performance of asphalt pavements, especially when novel or alternative materials are used. Numerous laboratory studies have investigated fatigue performance with the aim of estimating field behavior as accurately as feasible. Therefore, there are currently many different test methods and data analysis approaches that can be used. One of the most common laboratory test methods is the four-point bending beam (4PBB) test, which results are usually analyzed using the so-called traditional (where 50% reduction in initial stiffness is considered as failure criterion) or the energy ratio (ER) approach. However, outcomes from previous studies have shown that these approaches may not be appropriate and reliable if geogrids are used as a reinforcement. As a possible solution, this study proposes a new simplified flex point (SFP) approach that considers the flex point of the strain amplitude curve, measured during 4PBB fatigue tests, to calculate the number of cycles to failure. These three approaches were applied to four double-layered asphalt sets: one unreinforced and three reinforced with geogrids of different strength (50 x 50, 100 x 100 and 100 x 200 kN/m). The impact of reinforcement on the fatigue life was evaluated by comparing the critical strain values (ε6) of reinforced and unreinforced sets through the fatigue resistance improvement factor. The research findings showed that the use of geogrids improves fatigue life when the SFP and ER approaches are applied and that the traditional approach might not always be appropriate for assessing the fatigue resistance of reinforced asphalt mixtures.

Predicting corrosion in reinforced UHPC members through time-dependent multi-physics numerical simulation

Ultra-high-performance concrete (UHPC) has superior durability characteristics over traditional concrete. Improved durability behavior of UHPC has been attributed to the dense material property, which effectively constrains harmful material ingress and oxygen supply. However, the corrosion performance of reinforced UHPC under sustained mechanical loading, which may induce cracking, in addition to environmental conditioning is still not well characterized. This study investigates the electrochemical-mechanical coupling effects on deterioration mechanisms of reinforced UHPC beams subjected to service loading conditions and chloride attack through multi-physics simulation techniques. Finite element models were simulated considering the cracking induced by service loads and deformations caused by rust expansion on the cementitious matrix. Material transport properties such as chloride and oxygen penetration parameters were updated in each step of the time-dependent process based on cracking conditions. The simulation results show that there is substantial steel cross section and yielding load capacity loss, initial stiffness reduction, and damage area increase in reinforced concrete members while UHPC members showed relatively small cross section loss, negligible increase in UHPC damage, no observable reduction in initial stiffness, and minimal loss in yield load capacity. The effect of oxygen supply level on the corrosion performance is also explored showing that the corrosion rate of reinforced UHPC specimens were limited due to depleted oxygen at the steel-UHPC surface. Numerical simulation results were verified against experimental trends on corrosion performance of reinforced UHPC specimens. Further, the numerical framework herein provides an alternative way in predicting deterioration processes and evaluating service life performance of ductile reinforced concrete infrastructures in a computationally efficient manner.

Experimental cyclic performance of steel shear walls with single rectangular opening

SummaryThis paper presents the results of evaluating the effects of the location and size of rectangular opening on the cyclic performance of steel plate shear wall (SPSW) systems through an experimental study. A total of seven 1:6 scale SPSW specimens were tested under quasi‐static cyclic loads. Apart from a solid (without opening) specimen, six specimens with different opening locations and sizes were considered. The rectangular opening in the SPSW specimens with the relative opening‐to‐plate area of 4.00, 6.76% and 10.24% caused a reduction in maximum shear capacity by 28%, 33%, and 46% respectively. A similar trend was also noticed in the reduction of initial stiffness. The test results proved good performance of the stiffeners around the opening perimeter in preventing the zipping of the plate at these regions. However, from the drift ratio of 1.0%, the tearing of the plate began gradually and was followed by significant strength degradation. Because of the buckling and snap‐back of the plate, pinching in force‐displacement of all the specimens was observed. Also, a finite element simulation of all the specimens was performed, which led to acceptable results in terms of the characteristics of force‐displacement response.

Lateral load resisting behavior of steel moment frames with reduced web section (RWS) beams

Many studies have been conducted on the monotonic and cyclic behavior of beams with Reduced Web Section (RWS) moment connections. However, the use of these structural elements in Steel Moment Resisting Frames (SMRF) have not been much considered. This paper presents a numerical investigation on the lateral load carrying behavior of steel moment resisting frames with reduced web beam sections. For this purpose six multi-story multi-bay structures with different seismic design category, framing type and geometric configurations are designed and modeled numerically. A design procedure for calculating the proper size of opening in the beam web is presented. Then, nonlinear static analysis is conducted on the frames with reduced web beam sections and results are compared with the ones corresponding to the conventional moment resisting frames. Based on the results, the average reduction in initial stiffness and ultimate base shear of the models with perforated beams are 2.5–8.5% and 3.5–6%, respectively. This is while the investigation of yielded areas in these models showed that the plastic zone moved from beam-column connection zones to the openings which is consistent with “Strong column-weak beam” rule. Therefore, the proposed design algorithm can be considered as an effective way to select the proper perforation size which decreases demand for plastic deformation at beam-column connections without causing further decrease in stiffness and strength of the frame.

Quantification of mixed mode loading and bond line thickness on adhesive joint strength using novel test specimen geometry

This study quantifies the effect of mixed mode loading and bond line thickness on adhesive joint strength for automotive structural applications. This research is motivated by the need to address the complex loading that occurs during automotive crash events, as well as the variation in bond line thickness that may occur due to gap variability when joining mass-produced structural components. A newly developed specimen geometry for Mode II and Mixed Mode loading is presented, while a recently published test methodology was used to characterize the Mode I response. Three nominal bond line thicknesses (0.18, 0.30 and 0.64 mm), were investigated for a toughened structural adhesive and steel adherends. The traction-separation response, required for cohesive zone modeling (CZM) of adhesive joints, was determined for each combination of bond line thickness and mode of loading. Mode I loading resulted in higher peak traction and lower critical energy release rates compared to Mode II loading, with the Mixed Mode responses typically falling between Mode I and II, in relation to the loading angle tested. Increasing bond line thickness resulted in a reduction in initial stiffness and peak traction, as well as an increase in critical energy release rate for all modes of loading. Two existing CZM mixed mode failure criteria were assessed and demonstrated a good fit to the tested mixed mode responses, despite the limited ability of the CZM implementation to predict the end of the plateau region of the traction-separation response. The experimental approach described in this study was shown to provide repeatable results that could be directly used to fully define an adhesive CZM, ready for use in finite element modeling without the need for inverse modeling.

Fatigue Evaluation in Hot and Warm Mix Asphalts Based on Dissipated Energy

Warm mix asphalts (WMA), because of their low production and compact temperatures, may have different behaviors in long term. In the present work, the energy-based criteria along with the 50% reduction in initial stiffness (Nf50%) using four-point bending test under controlled-strain conditions of 1000 microstrain were applied to compare the prepared two warm mix and HMA samples. All these criteria illustrate properly the effect of mix asphalt properties (additive type) on its fatigue performance. A noteworthy point in this regard is the difference between Nf50% values of the studied samples with the real failure point. For HMA and zycotherm WMA (ZWMA), loading cyclic number at the failure moment occurs almost 80% higher than the fatigue life estimated using Nf50% while for Sasobit WMA (SWMA) this value is declined to 28%. The RDEC method, compared to other methods, indicated the maximum fatigue life and consistency with the failure point. Comparing the energy-based methods with Nf50% method revealed that ERR, ERR&B, and ERP have the maximum consistency with fatigue life in terms of 50% reduction in initial stiffness. For SWMA, the fatigue life at Nf50% was larger than that of various energy-based methods but almost equal to that of the RDEC method. However, for two WMA mixes prepared using ZWMA and HMA, all energy methods revealed a fatigue life longer than that of Nf50%.

Experimental evaluation of fatigue resistance of asphalt mixtures containing waste elastomeric polymers

This study concerning the asphalt mixture fatigue life, affected by the addition of waste elastomeric polymers obtained from polymer production process. Since the use of polymers is economically costly, and also there are some by-products in polymer production process, considering recycled polymers and waste of polymers is important in terms of environmental clean-up. Investigation of fatigue characteristics of HMA mixtures containing waste of elastomeric polymers and their comparison with unmodified mixtures are the main objectives of this research. In addition, different fatigue analysis approaches were evaluated in order to figure out the most precise results. Fatigue resistance of hot-mix asphalt (HMA) modified by elastomeric polymers such as SBR and PBR and the waste of their production process, was studied by performing four-point bending (4 PB) test and indirect tensile fatigue test (ITFT) based on conventional and energy approaches. As far as the technical aspects are concerned, the traditional method based on 50% reduction in initial stiffness is not suitable enough to be employed for calculating fatigue life, because this method underestimates the fatigue life of polymer modified mixtures and leads to high variations among the responses. Therefore, two alternative fatigue analyses were conducted: The Ratio of Dissipated Energy Change method and the Energy Ratio method. Both these methods provided a more detailed comparison between the fatigue life of these mixtures. As the results shows, fatigue life of mixes modified by the waste of polymers, is significantly higher than unmodified mixes and considerably close to mixes including ‌‌original polymers.

Effect of pore structure on fatigue of pervious concrete

Pervious concrete (PC) is a class of porous pavement whose properties are largely influenced by pore parameters. Under repeated vehicular loads, PC undergoes reduction in stiffness leading to failure. The purpose of this study was to understand the influence of porosity and pore sizes on fatigue of 13 PC mixtures encompassing 104 beams, and establish stiffness reduction model / damage evolution in respect of repeated loads for PC based on the existing model used for conventional concrete. Fatigue life and stiffness depend on porosity at the fractured face of specimens as investigated using image analyses, which was more influential than the volumetric porosity determined through ASTM C1754. The reduction in stiffness followed S-curve depicting three distinct stages of damage evolution. The damage model fitted the experimental data with very high accuracy (R2 > 0.95) and precision. The specimens shifted from secondary to tertiary stage when the reduction in initial stiffness was 65–85%, which occurred at 90–98% of the total life. This study is envisioned to assist in the development of fatigue test protocols as well as recommend PC pavements as part of the maintenance and rehabilitation programmes.

Cyclic behavior of in-situ exterior beam-column subassemblies with cold joint in column

Four sets of exterior-beam-column subassemblies with and without construction joint (cold joint) in column to simulate multistage casting at site were tested under cyclic loading. Flexural strength ratios of the specimens were 1.2, 1.4, 1.7 and 2.0. It was observed that the reduction in energy dissipation capacity of the specimens with cold joint were 24–49% compared to control specimens. It was also found that there was 22–36% decrease in ductility of the specimens with cold joint. Reduction in initial stiffness of the specimens with cold joint in column was found to be significant. Difference of peak loads between control and the corresponding specimens with cold joint were negligible. Four sets of reinforced concrete beam specimens with and without cold joint with different percentage of tension reinforcement were tested for flexural capacities under static load. Reduction in peak load of beam specimens with cold joint in comparison to control specimens was found to be insignificant.

Fatigue Behavior Analysis of Asphalt Mixes Containing Electric Arc Furnace (EAF) Steel Slag

This research was conducted in order to evaluate fatigue behavior of asphalt mixes containing Electric Arc Furnace (EAF) steel slag. After initial evaluation of the properties of EAF steel slag using X-ray Diffraction (XRD) and Scanning Electric Microscope (SEM), six sets of laboratory mixtures were prepared. Each set were treated replacing various portions of limestone aggregates of the mix with EAF steel slag. Four point bending beam fatigue tests were performed in both controlled strain and stress mode of loading at various strain and stress levels to characterize the fatigue behavior of asphalt mixes containing different percentages of EAF slag. Different approaches based on stiffness and dissipated energy were used to analyze the fatigue tests data. The results show that the inclusion of EAF in mixes improved the fatigue life considerably under both stress and strain control mode of loading. In the stress control mode, very good correlations were observed between responses and fatigue life of mixes. However, correlation coefficients in the strain control mode were relatively lower than those in the stress control mode (particularly in the tests that were based on 50% reduction of initial stiffness).

Comparison of Fatigue Resistance of HMA and WMA Mixtures Modified by SBS

This study aims to investigate the effect of warm mix asphalt (WMA) additives on fatigue life of unmodified and SBS- modified asphalt mixes. Two different WMA additives, including Aspha-min and Sasobit were used to fabricate WMA specimens. Fatigue test was performed on warm polymer modified asphalt (PMA) by indirect tensile test on two stress levels (300,450kPa) at 20°C and fatigue life was compared with PMA mixture. Also, indirect tensile strength (ITS) test was performed to determine the moisture susceptibility of warm PMA. In order to evaluate the fatigue performance of warm mix asphalt (WMA) mixtures in comparison with hot mixture asphalt (HMA), four-point flexural beam test was employed. Four- point flexural beam test was conducted at four strain levels (650, 800, 1000 and 1200μɛ) and frequency of 7.5Hz at 20°C. According to the test results, it was shown that warm PMA mixtures prepared by Aspha-min and Sasobit, have less fatigue resistance in comparison with PMA mixture. Also the fatigue resistance of warm PMA mixture made by Sasobit is greater than fatigue resistance of warm PMA mixture made by Aspha-min. Based on the ITS test results, warm PMA with Sasobit additive and PMA achieved the desirable tensile strength ratio (TSR) criterion (80%) but warm PMA Aspha-min had TSR lower than 80%. Also fatigue life determination based on 50% reduction in initial stiffness showed that the fatigue life of HMA mixture is less than WMA mixtures at lower strain levels whereas it was comparable with warm mix asphalt at higher strain levels.

Laboratory evaluation of strain controlled fatigue criteria in hot mix asphalt

The fatigue lives of hot mix asphalt are affected by the test method and the loading mode. In the strain controlled mode, the 50% reduction in initial stiffness is widely used by asphalt professionals. Also, different approaches based on dissipated energy concept have been used to define the fatigue life of hot mix asphalt mixtures. In this research study, the 50% reduction in initial stiffness, Energy Ratio and Rate of Dissipated Energy Change approaches are used to depict fatigue life and compared with the observed failure point in beam fatigue test. The beam fatigue tests were conducted in accordance with AASHTO T321 using 1000 microstrain and semi sinusoidal loading at 10Hz. The results show that the 50% reduction in initial stiffness underestimates fatigue life of hot mix asphalt mixtures. According to the tests results, the fatigue life corresponding to 65% reduction in initial stiffness is close to the observed failure point.

Indirect tensile versus two-point bending fatigue testing

This paper describes a comparison between an indirect tensile fatigue method and the currently preferred European standard two-point bending, trapezoidal fatigue test. A study was undertaken using both tests to determine the fatigue properties of a standard UK asphalt mixture at two temperatures of 10 and 30°C. The fatigue life, in addition to using the traditional 90% reduction in initial stiffness, was also computed using a fatigue failure point based on the transition between the quasi-stationary phase (associated with a uniform stiffness decrease and the development of micro-cracks) and the failure phase (associated with localised crack propagation). This fatigue failure point, determined by taking the peak of the product of loading cycles and stiffness versus loading cycles, was also converted into an alternative form used to determine the fatigue failure point for the indirect tensile fatigue test. Separate fatigue relationships were produced for the four combinations of test method and temperature based on the phenomenological initial tensile strain versus fatigue life relationship. Although the indirect tensile fatigue test produced shorter fatigue lives compared to the two-point bending fatigue test, it was also possible to combine all four fatigue relationships to produce one unique fatigue function for the 20 mm dense bitumen macadam asphalt mixture.

Fatigue damage analysis in asphalt concrete mixtures using the dissipated energy approach

An asphalt concrete damage–energy fatigue approach based on the concept of change in dissipated energy is presented in this paper. The damage–energy based fatigue approach is simple and based on a sound theoretical background. The central concept of the energy approach is the energy fatigue curve, which is based on two key elements, namely the plateau value (PV) and the number of load cycles to true failure (Ntf). The plateau value represents the constant value of the percentage of dissipated energy that produces damage to the material under cyclic loading. Failure is defined as the number of load cycles at which this percentage of dissipated energy begins to increase rapidly, indicating instability. Flexural fatigue testing was used to test hundreds of asphalt concrete beams, mainly under controlled-strain testing conditions. It was found that PV is highly dependent on the initial loading conditions, stress, strain, and dissipated energy. As a result, it can be used conveniently in pavement design. The number of load cycles to 50% reduction in initial stiffness was found to be highly correlated with the new failure point (Ntf). Using the dissipated energy concepts in fatigue analysis makes it possible to account for damage accumulation in a straightforward manner.Key words: fatigue of asphalt concrete, dissipated energy, damage, energy ratio.

Effects of Prior Earthquake Damage on Response of Simple Stiffness-Degrading Structures

A systematic study involving over 20,000 single-degree-of-freedom (SDOF) analyses was made to assess the effects of prior earthquake damage on the peak displacement response of simple oscillators. Principal variables were oscillator strength, period of vibration, degree of prior damage, and load-deformation relation. Base input was provided by 18 recorded ground motions, representing different frequency content, duration, and the presence or absence of near-field forward directivity effects. Prior damage was modeled as a reduction in initial stiffness under the assumption that residual displacements are negligible. The study is restricted to cases in which prior demands are less than those that would result if the structure were initially undamaged. For these conditions, prior damage usually has a minor effect on peak displacement response. This was observed for SDOF oscillators having the Takeda load-deformation relation with positive post-yield stiffness and for a modified Takeda relation incorporating pinched hysteresis and strength degradation. Takeda oscillators with negative post-yield stiffness were prone to collapse, whether or not they had experienced prior damage.