Asphalt Concrete Stiffness Research Articles

Mechanistic-empirical assessment of axle load legal limits on Brazilian roadways

This paper presents a comprehensive mechanistic-empirical assessment of axle load legal limits policy on Brazilian roadways, considering the intricate balance between enhanced freight transportation and potential pavement deterioration. Focusing on the prevailing flexible pavements, the study explores factors such as asphalt concrete stiffness, thickness, and axle-load increments, accounting for variations in tire-load and pressure. The analysis reveals complex relationships between these factors and their impact on pavement distress mechanisms, particularly fatigue cracking and rutting. The investigation highlights the substantial influence of axle load legal limits adjustments, even seemingly minor ones, on pavement damage, ride quality, safety, and operational efficiency. A calibrated model for assessing mechanistic-empirical load equivalency factors (LEFs) was developed, offering insights into the disparities between the model and existing empirical equations used in the Brazilian Pavement Design Guide (BPDG). This comparison underscores the potential underestimation of axle load legal limits adjustments in the current approach, emphasizing the importance of informed decision-making in transportation policy. In conclusion, this study provides critical insights for policymakers and engineers while considering axle load policy adjustments. The findings emphasize the significance of accurate engineering considerations to sustainably balance economic growth, road safety, and efficient freight transportation within Brazil’s evolving road network.

Optimal time-frequency equivalency factor for approximate interconversion between the dynamic and relaxation moduli

The viscoelastic stiffness of asphalt concrete is commonly represented using relaxation modulus and dynamic modulus, which are functions of loading time and loading frequency, respectively. They are typically measured via experimental testing whereby only one of the moduli is determined, and interconversion techniques can be used to obtain the other modulus if needed. Although exact approaches exist for pavement modulus interconversion, they can be difficult to implement in practice, and approximate conversion techniques have therefore been developed for conventional use. A popular approach is to approximate a direct relationship between the time and frequency domains via an equivalency factor, but there is no apparent consensus on its proper value. In this paper, a new numerical technique is applied to experimental data to ascertain the optimal value of the time-frequency equivalency factor. Approximate conversions from dynamic modulus to relaxation modulus are conducted using the optimal factor, and results are compared to popular alternative approaches. The optimal factor is determined to be with 95% confidence. Using the mean value of 0.0673 produced conversion errors of 1.41% on average among 30 samples of hot mix asphalt.

Influence of Additives on the Fluctuation of Stiffness of Asphalt Concrete under Dynamic Stresses

Influence of Additives on the Fluctuation of Stiffness of Asphalt Concrete under Dynamic Stresses

Influence of Testing Environment on Fluctuation of Initial Stiffness of Asphalt Concrete

Influence of Testing Environment on Fluctuation of Initial Stiffness of Asphalt Concrete

Research Of Asphalt Concrete Frost Resistance According To Fatigue Resistance And Stiffness Criteria

The formation of snow cover has been unsustainable in recent decades, which means that freezing and thawing are becoming more frequent in Latvia and that is one of the reasons why asphalt concrete frost resistance matters. Asphalt concrete beams were exposed to multiple freeze-thaw cycles and afterwards tested on a 4-point bending test for stiffness and fatigue resistance. Frost resistance testing for aggregates and concrete is a popular and widely described and used test method worldwide, but there is no such testing standard for asphalt concrete. This study is aimed to develop a methodology for determining the frost resistance of asphalt concrete by adapting the requirements of cement concrete standart AASHTO C666, as well as to experimentally evaluate the effect of frost resistance on the fatigue strenght and stiffness of asphalt concrete. The results show significant correlation between asphalt concrete void content and fatigue life after multiple freeze-thaw cycles and correlation between the amount of freeze-thaw cycles and asphalt concrete fatigue life.

Influence of Additives on the Fatigue Life and Stiffness of Asphalt Concrete

Implementation of additives to the asphalt binder can enhance the overall physical properties of the modified asphalt concrete. In the present assessment, an attempt has been made to use 2 % of silica fumes and 4 % of fly ash class F for modification of asphalt binder in wet process. Asphalt concrete wearing course mixtures have been prepared and compacted by roller in the laboratory. The beam specimens of 400 mm length and 50 mm height and 63 mm width were extracted from the slab samples. The specimens were subjected to the four-point repeated flexural bending beam test. The flexural stiffness was calculated under three constant micro strain levels of (250, 400, and 750). The fatigue life was monitored in terms the number of load repetitions to reach the required reduction in stiffness. It was concluded that the flexural stiffness increases by (11, and 15) %, (17.7, and 63.6) %, (57.2, and 65) % when 2% of silica fumes or 4 % of fly ash are implemented and the specimen’s practices 750, 400, and 250 microstrain levels respectively. However, the fatigue life of asphalt concrete beam specimens increases by (40, and 72.8) %, (115, and 220.6) %, (46, and 94.6) % when 2% of silica fumes or 4 % of fly ash are implemented and the specimen’s practices 750, 400, and 250 microstrain levels respectively. It is recommended to use modified binder with fly ash and silica fumes in asphalt concrete to enhance the fatigue life and stiffness.

Assessment of Fatigue Life and Stiffness of Asphalt Concrete After Implementation of Additives

Modifying asphalt binder with additives can enhance the overall physical properties of asphalt concrete. In the present investigation, an attempt has been made to use 2 % of silica fumes and 4 % of fly ash class F for modification of asphalt binder in wet process. Asphalt concrete wearing course slab samples have been prepared under roller compaction. The beam specimens of 400 mm length and 50 mm height and 63 mm width were extracted from the slab samples. The beam specimens were subjected to the four-point repeated flexural bending beam test. The flexural stiffness was calculated under three constant micro strain levels of (250, 400, and 750). The fatigue life was monitored in terms the number of load repetitions to reach the required reduction in stiffness of 50 %. It was concluded that the flexural stiffness increases by (11, and 15) %, (17.7, and 63.6) %, (57.2, and 65) % when 2% of silica fumes or 4 % of fly ash are implemented and the specimen’s practices 750, 400, and 250 micro strain levels respectively. However, the fatigue life increases by (40, and 72.8) %, (115, and 220.6) %, (46, and 94.6) % when 2% of silica fumes or 4 % of fly ash are implemented and the specimen’s practices 750, 400, and 250 micro strain levels respectively. It is recommended to use modified binder with silica fumes and fly ash in asphalt concrete to enhance the fatigue life and stiffness.

Rutting Resistance of Hot Mix Asphalt Containing Coarse Recycled Concrete Aggregates Coated with Waste Plastic Bottles

The use of recycled concrete aggregate (RCA) as a part of coarse aggregates in asphalt pavements confers economic and environmental benefits. Coarse RCA (CRCA) has inferior mechanical and physical properties compared to natural aggregates due to very porous and weakly adhered cement mortar. In this study, CRCA surfaces were coated with waste plastic bottles (WPB) and used at 15%, 30%, and 50% in the asphalt concrete. The Marshall, stiffness modulus, and dynamic creep tests were performed to determine the strength of hot mix asphalts against rutting. The results revealed that the use of untreated CRCA reduced the Marshall quotient and the rutting resistance of the asphalt concrete. The results of the stiffness modulus and dynamic creep tests indicated that CRCA incorporation increased permanent deformation in the tested specimens due to the reduction of asphalt concrete stiffness. However, the asphalt concrete containing treated CRCA had lower permanent deformation because WPB promotes CRCA stability by penetrating its void and reinforcing cement mortar. Furthermore, by raising the temperature, the strength of all asphalt concretes decreased against rutting, and the reduction rate was higher in the modified specimens.

Simulation of the asphalt concrete stiffness degradation using simplified viscoelastic continuum damage model

A procedure based on the simplified viscoelastic continuum damage model is developed to simulate asphalt concrete stiffness degradation caused by the application of cyclic loads. Several tests are performed to characterize the linear viscoelastic and fatigue damage properties of four asphalt mixtures. It was found that the piecewise approach proposed to simulate the stiffness evolution successfully describes the damage growth under loads applications. It was also found that the exponential model used to fit the characteristic curves best describes the stiffness evolution for the entire load history. Finally, stiffness evolution simulations showed higher fatigue resistance for polymer-modified mixtures.

Relationship between Backcalculated and Estimated Asphalt Concrete Dynamic Modulus with Respect to Falling Weight Deflectometer Load and Temperature

There are several methods for determining the stiffness of asphalt concrete in an existing pavement. The three primary methods are: dynamic modulus testing in the laboratory, predictive equations, and falling weight deflectometer (FWD) testing. Asphalt over asphalt (AC/AC) overlay design procedures allow the use of multiple methods to characterize fatigue damage in the existing asphalt concrete. Therefore, understanding the difference between these methods is critical for AC/AC overlay design. The differences between the methods for determining asphalt concrete stiffness and how these differences are related to FWD load magnitude and asphalt temperature are examined. Data from the Federal Highway Administration’s Long-Term Pavement Performance Program (LTPP) are used in this investigation. It is found that the stiffness determined through FWD testing and backcalculation is generally less than that estimated using the Witczak predictive equation and binder aging models. Furthermore, it is found that both FWD load magnitude and asphalt temperature have a significant effect on the difference between backcalculated and estimated stiffness of asphalt concrete. Backcalculated stiffness increases relative to estimated stiffness as FWD load and temperature increase. These effects must be considered when multiple methods of determining asphalt concrete stiffness are used interchangeably for overlay design.

Optimal Structure Combination for Inverted Asphalt Pavement Incorporating Cracks in Cement-Treated Subbase

The structure design and mechanistic calculation of inverted asphalt pavements are mainly based on linear layer elastic theory with the assumption that the cement-treated subbase (CTB) is complete without cracks. This study investigates the optimal structure combination for inverted pavements according to calculated critical responses considering cracks in the CTB layer. A three-dimensional finite element (3D FE) model of inverted pavement with a transverse crack through the CTB layer was developed. Four full-scale inverted pavement sections were built, and a crack 0.01 m wide and 0.05 m deep was sawn on top of each CTB layer after construction. The 3D FE model was validated by strain and deformation measured in falling weight deflectometer tests and used for a parametric study of dominating structure combination factors. Variance analysis results show that interactions with thickness or stiffness of the asphalt concrete (AC) layer presented the most significant effect on critical responses, while CTB stiffness (12588~7668 MPa) had the least impact. Structure variation effect analysis results illustrated that 0.1 m aggregate base (AB) thickness is enough to prevent the CTB crack propagating to the surface. Thin AC structures are highly sensitive to variations in AC and AB stiffness. A thin AC and AB combination (0.05 and 0.10 m) can provide low critical strains similar to a thick AC and thin AB (0.15 and 0.10 m) combination if the stiffness of AC and AB can be maintained at 7175 and 358 Mpa, respectively, or higher. AC thickness of 0.1 m and the combination of thin AC and thick AB are two unfavorable conditions for inverted pavements.

Modelling of Asphalt Concrete Stiffness in the Linear Viscoelastic Region

Stiffness modulus is a fundamental parameter used in the modelling of the viscoelastic behaviour of bituminous mixtures. On the basis of the master curve in the linear viscoelasticity range, the mechanical properties of asphalt concrete at different loading times and temperatures can be predicted. This paper discusses the construction of master curves under rheological mathematical models i.e. the sigmoidal function model (MEPDG), the fractional model, and Bahia and co-workers’ model in comparison to the results from mechanistic rheological models i.e. the generalized Huet-Sayegh model, the generalized Maxwell model and the Burgers model. For the purposes of this analysis, the reference asphalt concrete mix (denoted as AC16W) intended for the binder coarse layer and for traffic category KR3 (5×105 <ESAL100kN <2.5×106) was prepared. Measurement of the stiffness modulus of asphalt concrete under steady-state strain was performed using the simple axial compression-tensile test under controlled strain mode. The fixed strain level was set at 25με to guarantee that the stiffness modulus of the asphalt concrete would be tested in a linear viscoelasticity range. The master curve was formed using the time-temperature superposition principle (TTSP). The stiffness modulus of asphalt concrete was determined at temperatures 10°C, 20°C and 40°C and at loading times (frequency) of 0.1, 0.3, 1, 3, 10, 20 Hz. The model parameters were fitted to the rheological models using the original programs based on the nonlinear least squares sum method. All the rheological models under analysis were found to be capable of predicting changes in the stiffness modulus of the reference asphalt concrete to satisfactory accuracy. In the cases of the fractional model and the generalized Maxwell model, their accuracy depends on a number of elements in series. The best fit was registered for Bahia and co-workers model, generalized Maxwell model and fractional model. As for predicting the phase angle parameter, the largest discrepancies between experimental and modelled results were obtained using the fractional model. Except the Burgers model, the model matching quality was more than 0.985 (determination coefficient) at the root mean squared error of less than 10%. From the point of view of their practical application, it is best to apply the generalized Huet-Sayegh model and the sigmoidal model.

Correlation Between Asphalt Mixture Stiffness Determined Through Static and Dynamic Indirect Tensile Tests

At present, Italian technical specifications only provide stiffness measurements in few cases, whereas they require the execution of the static indirect tensile strength test, both in mix design and in quality control phases. Despite this test is typically carried out only to determine the material strength, it is, however, possible to obtain a measure of the specimen stiffness properties from the stress–strain curve. The present paper deals with the correlation between asphalt concrete stiffness measurements obtained through dynamic indirect tensile tests, according to EN 12697-26, and static indirect tensile strength tests, according to EN 12697-23. In particular, data from static tests have been processed to obtain stiffness measurements through the application of Hondros’ theory or graphically from the stress–strain curve. Although based on empirical derivation, this relationship would enable the laboratories that are not equipped with a proper machine for dynamic modulus tests to estimate the stiffness properties of the bituminous material, by exclusively performing simple static tests. The experimental program included static and dynamic indirect tensile tests at 10, 20 and 30 \(^\circ \)C on three asphalt concretes, different for binder type and compacted to two air voids contents. Results proved that good correlations (R\(^2\) values higher than 0.92) can be established between dynamic modulus and static moduli, independently from test conditions and mixture composition.

Influence of Aging, Temperature and Moisture Damage on the Stiffness of Asphalt Concrete through the Fatigue Process

The stiffness of asphalt concrete is considered as a key element in the fatigue process under load repetitions. In this study, an investigation was made in order to better understand the influence of long term aging, moisture damage, low testing temperature, asphalt content, and Microstrain level on the stiffness through the fatigue process of asphalt pavement. Repeated four-point flexure bending beam test in controlled strain mode has been implemented using Nottingham apparatus. Asphalt concrete mixtures were prepared using different percentages of asphalt cement, (optimum and 0.5% above and below the optimum). Asphalt concrete slab samples of (300x 400x 60) mm were prepared using roller compaction, beam specimens of (400x 50x 60) mm were cut from the slab samples. Beam specimens were divided into four groups, the first group was tested for fatigue life under the influence of three levels of micro strain (750, 400, and 250). The second group was tested at 30, 20, and 5 °C, while the third group was subjected to long-term aging, before testing for fatigue life. The fourth group was subjected to moisture damage impact, and then tested for fatigue life. The changes in stiffness per load cycles have been monitored. It was concluded that the stiffness is susceptible to moisture damage and aging, the increase in Microstrain level leads into a remarkable decrease in initial and failure stiffness's. The stiffness is susceptible to the testing temperature and asphalt content, lower testing temperature of 5o C exhibits higher stiffness value, while higher binder content has a negative impact on the stiffness.

Influence of asphalt binder on time-temperature shift function for asphalt concrete at large strains

This paper presents a study of the time-temperature superposition principle for asphalt concrete. The idea behind the timetemperature superposition principle is to allow the construction of master curves representing the material viscoelastic behaviour at a given reference temperature in a range of reduced loading times which is wider than that tested in the laboratory. In addition, the asphalt concrete stiffness is in fact assumed to be a function of temperature and time. A penetration number is an empirical parameter and it could be linked to the viscosity of asphalt. This research has been extended upon previous studies on the viscoelastic behavior of asphalt concrete made from different penetration gradation of asphalt binder. The total 96 experiments were run through the unconfined compression and indirect tensile tests under various temperatures and different strain rates. The results obtained from both unconfined compression and indirect tensile tests can be used to construct the stress-reduced time master curve and timetemperature shift function for each asphalt concrete. The analysis results show that the time-temperature shift function can be simplified as a single function for both compressive and tensile modes at large stains and it can also be employed for multiple penetration-graded asphalts.

Deformation Characteristics of Textile and Grid Reinforced Asphalt Concrete Pavement Model

In this paper, a laboratory investigation has been conducted to evaluate the deformation characteristics of reinforced Asphalt concrete pavement. Two types of reinforcements have been implemented; the first type is biaxial plastic geogrids of two different aperture sizes and rib thickness, while the second type was the nonwoven continuous fiber geotextile with two different thicknesses. The Asphalt concrete mix was designed as per Marshall Procedure using super pave requirements for gradation. To investigate the deformation characteristics, Circular slab specimens of different thickness have been constructed using static compaction and optimum asphalt content. Slab specimens were coupled and the reinforcements were introduced in between, then, tested in a model box of 50 x50 x70 cm filled with loose sand layer of 40 cm thickness representing the existing poorsubgrade condition. A total of 60 circular slab specimens have been constructed and tested in duplicate. The load - deformation data were recorded, plotted, and analyzed for the deformation characteristics. The effect of pavement thickness, Asphalt concrete stiffness, reinforcement type were also analyzed and discussed.

Numerical investigation into the stiffness anisotropy of asphalt concrete from a microstructural perspective

A micromechanical model was built in this paper to investigate the stiffness anisotropy of asphalt concrete (AC) using the discrete element method. Four three-dimensional cubic AC digital samples with different aggregate particle orientations were built using discrete element software PFC3D. The aggregate gradation and shape, air voids and mastic included in the digital samples were modeled using different contact models, with due consideration of the volumetric fractions of the different phases. Laboratory uniaxial complex modulus test and indirect tensile strength test were conducted to obtain material input parameters for numerical modeling. Simulation of the uniaxial cyclic compressive tests was performed on the four cubic samples loaded in three different directions. Dynamic stiffness in different directions was calculated from the compression stress–strain responses. Results show that the AC stiffness is significantly dependent on preferential orientation of aggregate particles. The AC stiffness in the long-axis direction of aggregate particles is shown to be up to 43% higher than the stiffness in the particle short-axis direction. The stiffness anisotropy of AC decreases as the mixture temperature drops.

Long-Term Oven-Aging Effects on Fatigue and Initial Stiffness of Asphalt Concrete

An investigation of the effects of long-term oven aging (LTOA) on initial stiffness and fatigue of asphalt concrete was made using two typical California asphalts, known to have different aging characteristics, in mixes with one aggregate. Asphalt content, air-voids content, and days of LTOA were varied independently. Stiffness and fatigue were evaluated using the controlled-strain flexural beam test developed by the Strategic Highway Research Program Project A-003A. The results indicated that both mixes exhibited an increase in initial stiffness with LTOA periods of up to six days. The sensitivity of beam fatigue life to LTOA depended on the asphalt. Beams containing Valley asphalt had virtually no change in fatigue life due to LTOA, whereas beams with Coastal asphalt showed some sensitivity to LTOA. For both asphalts, the average reduction in fatigue life from 6 days of LTOA was less than that caused by a 3 percent increase in air-void content or a 1 percent decrease in asphalt content. Simulations of thick and thin pavement structures were performed to reconcile the effects of LTOA, asphalt content, and air-void content on mix fatigue life and stiffness by evaluating their combined effects on predicted pavement fatigue life. The simulations indicated that aging, as induced by LTOA, increased fatigue life for all cases except one.

INTERFACIAL DEBONDING OF ICE-ASPHALT CONCRETE

In order to clarify the nature of the bonding mechanism between ice and asphalt concrete and to develope new techniques to reduce debonding resistance, a series of experimental investigation was conducted. Surface energy was varied by using various water repellents, and stiffness of asphalt concrete was varied by mixing crushed rubber particles with the asphalt. Correlations between these variables were investigated and the following conclusion was obtained. Reducing surface energy and/or increasing deformability of the asphalt concrete is an effective way to decrease interfacial bonding between ice and asphalt concrete. In this regard, use of water repellents or replacement of aggregate with rubber particles is a promising method to facilitate debonding of ice and asphalt concrete.