Effect Of Air Voids Research Articles

Effect of air voids on the fracture resistance of HMA in the indirect tensile cracking (IDEAL-CT) test

ABSTRACT Air voids (density) is a key volumetric property of asphalt mixes and play an unignorable role in anti-cracking. While lower voids were expected to result in more durable asphalt mixtures, contradictory trends were found in lab testing. In this study, several plant-produced mixtures were compacted to three levels of air voids (7%, 9%, and 11%) and tested to evaluate the sensitivity to the crack resistance. The results indicate that the higher the air voids, the better the cracking resistance as measured by the CTindex. To understand the cause, a stepwise regression analysis was conducted to model the IDEAL-CT parameters with volumetric and gradation parameters. The air void, percent passing the 0.6 mm (P0.6) and binder content (Pb) are found to have significant effects on CTindex and post-peak slope. The analysis of the CT change rate shows that the Pb, dust to binder ratio (D/B), and P0.6 can be used to predict the effect of the voids with a correlation factor (R2) of 0.96. For further understanding, the asphalt film thickness was calculated and found that compared with using film thickness as the single fitting parameter, combining the film thickness with D/B in modelling can dramatically increase the R2 of correlation from 0.67 to 0.92.

Influence of bitumen grade and air voids on low-temperature cracking of asphalt

Low-temperature cracking is one of the main causes of pavement distress in colder areas with below-freezing temperatures. Resistance of asphalt to this cracking depends on a number of factors. A complex knowledge about this phenomena is a key factor during the design process, the building and the use stage of roads. Among the leading drivers are the type of bitumen and air voids in the bituminous mixture. In this research project, both the bituminous binders and mixtures were tested to determine the influence of these components on the low-temperature performance of asphalt pavements. A wide range of bituminous binders were tested, including 20/30, 35/50, 50/70, 70/100 and 100/150 grades. This approach allowed for an in-depth evaluation of the impact of bitumen type and air voids on the low-thermal susceptibility of asphalt mixture. The basic tests for bitumen were performed such as the penetration value at 25 °C (EN 1426), the softening point with the Ring & Ball method (EN 1427) and the group composition with SARA analysis using the Thin Layer Chromatography with Flame Ionization Detector (TLC/FID) method. Both neat bitumen and bitumen after Rotating Thin Film Oven Test (RTFOT) short-term simulated ageing (EN 12607–1) were tested. Next, bituminous mixtures based on mentioned bitumen with 2.2%, 3.5% and 6.5% air voids were prepared and subjected to uniaxial tension in Thermal Stress Restrained Specimen Test (TSRST) to determine their low-temperature performance. The test results point to the binder type as the key determinant of the low-temperature performance of asphalt. Softer bitumens of a higher penetration have a better low-temperature performance and the group composition is the determinant of the TSRST failure temperature. The effect of air voids were of secondary importance in this respect.

Effect of air voids and asphalt content changes on laboratory and simulated long-term fatigue performance of asphalt concrete pavements

Effect of air voids and asphalt content changes on laboratory and simulated long-term fatigue performance of asphalt concrete pavements

Experimental and Statistical Analysis of Bitumen’s Field Ageing in Asphalt Pavements

Field ageing gradients of bitumen samples recovered from 14 asphalt road sections were investigated via rheological and chemical characterizations and statistical analysis. The effects of air voids and environmental factors on the ageing gradient were evaluated using the field ageing and climate data. The effectiveness of the pressure ageing vessel (PAV) test was assessed in accelerating the bitumen’s long-term ageing by comparing with the field ageing data. Critical factors for field ageing were identified using statistical methods. A statistical model for the Glover-Rowe (G-R) parameter was formulated and verified by incorporating the screened key factors. Results indicate that a threshold air void content (around 6%) exists in differentiating the field ageing gradient patterns in the asphalt pavements. An increasing tendency is observed between the ageing gradient and annual days below 0°C. The chemical indices, stiffness-related indices, and G-R parameter can quantify the field ageing gradient of asphalt pavements. The PAV test can condition the bottom slices’ bitumen to the same ageing level as that in a pavement depth of 0.5–2 cm after 8 years’ field service. Pavement service life, binder content, minimum temperature, days above 32°C, and days below 0°C are the critical material and environmental factors that significantly affect bitumen’s complex shear modulus, crossover frequency, G-R parameter, and Δ Tc. The statistical model is verified with an acceptable mean absolute error of 28.1% and a R2 value of 0.95.

Laboratory Evaluation of Electrically Conductive Asphalt Mixtures for Snow and Ice Removal Applications

The study evaluates the electrical conductivity and mechanical performance of graphite modified asphalt mixtures. The effects of air voids, carbon fiber, and binder performance grade (PG) on the electrical resistivity of graphite modified asphalt mixtures are also assessed. Three graphite grades, two asphalt binders (polymer-modified PG 76-22 and neat PG 64-22), one aggregate type, and one carbon fiber were used to produce graphite modified asphalt mixtures. The mixtures were produced without graphite (control mix, PG 76-22), with only graphite (three grades and PG 76-22), with both graphite and 1% carbon fiber (three grades and PG 76-22), and with graphite (all three grades) and PG 64-22. The electrical conductivity, resistance to rutting, resistance to cracking, and durability of these mixes were evaluated using electrical resistivity (using a multi-meter), asphalt pavement analyzer, Hamburg wheel tracking device, semi-circular bend, indirect tension cracking, and Cantabro loss tests. Test results showed that graphite improves the electrical conductivity of asphalt mixtures when added at dosages of 10% to 15% or higher by volume of binder. Graphite grades with larger particle sizes helped improve the conductivity of asphalt mixtures better than graphite grades with smaller particle sizes. Air voids (higher air voids increased resistivity), carbon fiber dosage (decreased resistivity), and binder performance grade (neat binders had lower resistivity) affected the electrical resistivity of graphite modified asphalt mixtures. Furthermore, graphite modified mixes had better rutting resistance but higher susceptibility to breakdown and cracking when compared with unmodified mixtures.

A Comprehensive Research of the Air-void Defect of Concrete-filled Steel Tube

Welded spiral steel tubes are adapted for use in a majority of the large-diameter concrete-filled steel tube (CFST) arch bridges due to technical as well as economic reasons. However, the welding temperature and other factors during the process of manufacturing of steel tube, initially result in a reduced spiral-welded seam (SWS) strength. Moreover, with the pumping of concrete into a steel tube, numerous deformities of the likes of laitance coating and air-voids occurred inevitably, particularly with the lack of air-entraining agent in China almost 20 years ago. This work makes use of ultrasonic scanning field experience to investigate the capacity of bearing within a repaired and reconditioned CFST arch bridge rib having defects such as air-void and reduced SWS strength under negligible compression of eccentric axial by carrying out its finite element model analysis of group. The outcome reveals that CFST bearing capacity is only minimally affected by the SWS strength as compared to concrete air-void, which can be ignored. The effect of air-voids and the nonlinear behavior of the constituents on the CFST behavior have also been probed into. The impact of the air-void on the rib capacity of bearing was investigated by conducting parametric studies. To conclude, we proposed a new index defining the ultimate strength of the rib for the defect, and presented a rather simple blueprint to determine the influence of air-void on the final strength of the arch bridge rib of CFST.

Effect of air voids on permeability and durability of hot mix asphalt

The Journal of the National Science Foundation of Sri Lanka publishes the results of research in all aspects of Science and Technology. The journal also has a website at http://www.nsf.gov.lk/. 2020 Impact Factor: 0.515The JNSF provides immediate open access to its content on the principle that making research freely available to the public supports a greater global exchange of knowledge.

A time-dependent vapor flow model of asphalt concrete

ABSTRACT It is straightforward to determine vapour flow for a non-absorbing medium using Darcy’s law. However, asphalt concrete (AC) sample absorbs moisture over time if it is exposed to a humid environment which causes swelling of the binder materials. This action reduces pore connectivity. As such, the flow properties of AC exposed to moisture can change with time due to clogging of capillary pores and reduced pore connectivity by swelling. To this end, this study develops a model by modifying Darcy’s law to quantify vapour permeability of an AC sample. The resulting model considers asphalt’s swelling using air voids and sample thickness (represents flow length) in addition to regular parameters of Darcy’s law. As a first step, the effects of air voids and thickness on vapour flow are evaluated. It is shown that vapour flow increases with an increase in air voids and with a decrease in thickness. Vapor flow shows a logarithmic correlation with time. As a second step, a permeability model is developed considering the time-dependent vapour flow rate in Darcy’s law. Next, the model is validated using laboratory tested data. The model is found to have good agreements with the tested data. The developed model is advantageous over simpler Darcy’s model for determining asphalt permeability considering binder swelling, pore connectivity and clogging.

Poisson’s Ratio of Asphalt Concrete Mixes Using Indirect Tensile Test

Abstract Increased interest in mechanistic evaluation of flexible pavement structures has brought a demand for accurate and practical methods, models, or both to estimate the mechanical properties of asphalt concrete mixtures. One of these properties is the Poisson’s ratio (ν) of asphalt concrete mixtures, which is often assumed to have a constant value of 0.35 in asphalt concrete evaluations. However, investigations have reported that mixture type, air void content, and temperature produce considerable variation in measured ν-values that could have a significant effect on evaluations of asphalt concrete mixes. The objective of this study was to evaluate the effect of air voids, binder type, and testing conditions on the measured ν-values. Indirect tensile (IDT) tests were conducted to measure ν-values. The study indicated that the Poisson’s ratio of the asphalt concrete mixes, on average, attained a maximum value at a particular level of air void content. Furthermore, when comparing the Poisson’s ratio values in relation to the dynamic modulus, calculated using the Mechanistic Empirical Pavement Design Guide (MEPDG) equation, higher ν-values were attained. This demonstrates the importance of measuring the Poisson’s ratio of a mix type.

Numerical investigation into the effect of air voids on the anisotropy of asphalt mixtures

The aim of this study is to investigate the asphalt mixture anisotropy of both the modulus and Poisson’s ratio due to air voids using a discrete element modeling simulation method. Three three-dimensional cubic digital samples of asphalt mixture with different shapes of single air void were built using discrete element software PFC 3D. The aggregate gradation, 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 three cubic samples loaded in three different directions. Dynamic modulus in three directions and Poisson’s ratio in six directions were calculated from the compression stress-strain responses. Results show that both the modulus and Poisson’s ratio are dependent on the preferential orientation of air voids. The anisotropy of the modulus and Poisson’s ratio increases as the pressure loading on the asphalt mixture increases. Compared to the modulus, Poisson’s ratio due to air voids has been shown to be more anisotropic. The maximum of Poisson’s ratio and modulus is shown to be up to 80% and 11% higher than the minimum, respectively.

Predicting the role of geotechnical parameters on the output from shallow buried explosives

Experiments have been conducted to quantify the effect the geotechnical conditions surrounding a buried charge have on the resulting output. From the results obtained the critical importance of moisture content in governing the magnitude of impulse delivered is highlighted. This has led to the development of a first-order predictive model for the impulse delivered from a buried charge, based on bulk density and moisture content, allowing rapid assessment of the effect of varying the geotechnical conditions. The work utilised a half-scale impulse measurement apparatus which incorporated a deformable target plate. Impulse, peak and residual target deflections were recorded for each test. No variations the charge geometry, mass of explosive, burial depth or stand-off were considered, with the focus solely being on the effect of the geotechnical conditions on the magnitude of loading and structural response. Five different types or grades of soils were used in the work, with both cohesive and cohesionless soils represented. The effect of air voids on the impulse generated was also investigated which showed that while strongly correlated, air voids alone is a poorer predictor of impulse than moisture content.

An innovative microwave cavity sensor for non-destructive characterisation of polymers

This paper investigates the feasibility of using an innovative microwave sensing technology to characterise and study various properties of polymer material such as difference between various polymer types, particle size and particle size distribution, contamination and pigmentation. A microwave sensor designed previously has been utilised to carry out this initial study to analyse the capability of microwave techniques to carry out the analysis. The curves obtained from material response to microwaves are distinguishable showing the shift to the lower frequency end with the insertion of polymer material. S11 measurements have shown distinctive peaks for each size and type of the sample tested. The results are quantifiable in terms of various polymer properties under consideration. In terms of S21 measurements, microwave sensor clearly distinguishes between coarse and fine polymer samples in terms of particle size. The effect of air voids in the sample and the particle size distribution has also been studied. The results are promising and justifies a thorough design and development of a dedicated microwave sensor unit for the characterisation of polymers. The sensor will have a significant industrial benefit in terms of costs associated with the industrial analysis, increase in the efficiency of manufacturing and production operation as well as material quality, control and validation.

Micromechanical modelling of typical Dense Bitumen Macadam (DBM) from laboratory testing and X-ray Computed Tomography (CT)

The application of finite element modelling for assumed mesh elements is limited for asphalt mixtures because of the inclusion of air voids within the structure. A multiphase finite element (FE) model was applied to consider the effect of air voids on the properties of asphalt mixtures. The virtual microstructure of an asphalt mixture was incorporated into the FE model from the X-ray CT images of asphalt specimens. Considering the asphalt mixture as viscoelastic, the mechanical properties determined from laboratory tests were applied to the FE model. Analysis was performed on a representative volume element using the concept of continuum damage mechanics. It was found that X-ray CT images can be converted into a finite element mesh of the real geometry of the asphalt sample. It was also found from the analysis that the mesh generated from the actual geometry account of the stress concentrations due to air void distribution compared with the assumed simplified geometry. The FE model was also analysed using CAPA 3D to give a validation of the results.

Improved method of considering air void and asphalt content changes on long-term performance of asphalt concrete pavements

Mixture properties (aggregate gradation and volumetric quantities), rate of loading and environmental conditions are the most important factors that affect the |E*| values. The main objective of this study was to develop a rational approach to investigate and model the effect of air voids and asphalt content on the |E*| master curves and consequently predict pavement performance. In this study, |E*| tests were conducted on three asphalt concrete mixtures with the same aggregate gradation, but different binder grades. For each of these mixtures, the air void and asphalt contents were varied at three levels. It is found that the developed method provides a more accurate estimate of the effects of volumetric changes in hot mix asphalt. The application of the proposed approach would be most beneficial for quality control/quality assurance purposes, performance-related specifications and for estimating contractors' incentives and penalties, where |E*| is utilised to predict the pavement performance.

Dynamic Properties to Evaluate Moisture Sensitivity of Asphalt Mixtures

An experimental measurement of dynamic properties, including dynamic modulus and phase angle, of asphalt mixtures with varying saturation levels of moisture has been carried out in the study. The specimens of asphalt mixture were compacted with three different air void contents and two different levels of moisture saturation. The relaxation modulus expressed by Prony series was estimated from the Prony series expression of dynamic modulus. The purpose of this study is to identify the effect of air voids on dynamic properties that affect and control moisture susceptibility of asphalt mixtures and then compare the stress relaxation behavior of asphalt mixtures between dry and moisture conditioning. The results show that dynamic properties are quite sensitive to temperature and loading frequency in dry and different saturation levels of moisture. Moreover, and the smaller the air voids and saturation level, the greater the increase in dynamic modulus and rate of stress relaxation, indicating better resistance to cracking and less moisture susceptibility.

Using Asphalt Mixture Beams in the Bending Beam Rheometer

ABSTRACT The paper is presented to validate the use of asphalt mixture beams in the Bending Beam Rheometer (BBR) test. Experimental work was performed to address the concerns regarding the sources of variability of using asphalt mixture beams for the BBR tests. The dimensional differences on the accuracy of deflections of a beam are evaluated using finite element method and statistical analyses. The effect of air voids on the overall stiffness of asphalt mixtures is found to be less significant at low temperatures through the volumetric measurements on three nominal maximum aggregate sizes NMAS (i.e., 12.5 mm, 9.5 mm, and 4.75 mm) mixture beams. The homoscedasticity of variances among the 12.5 mm, 9.5 mm, and 4.75 mm NMAS mixture groups is statistically validated using the Bartlett's test, so the measurements and comparisons among these NMAS mixture groups are valid for the BBR tests. A linear viscoelastic analysis (LVE) was used to evaluate the thermal properties of the three NMAS mixture beams. The LVE results show that the effect of the aggregate size on the low temperature properties of these three NMAS mixtures is not significant. A band of the confidence interval was established to show highway agencies how to run the Quality Control/Quality Assurance (QC/QA) operations using data collected from the BBR testing. Through the experimental and numerical work, the sources of variability during a BBR test are evaluated and the concerns are addressed. The use of asphalt mixture beams in the BBR tests to control the thermal-induced failure in asphalt pavements is experimentally and numerically validated and properly used for QC/QA.

Multiscale Prediction of Viscoelastic Properties of Asphalt Concrete

The low viscosity of asphalt concrete at T>135°C is necessary for the construction and compaction process of high-quality asphalt concrete layers. Whereas the continuous increase of viscosity with decreasing temperature is desirable for the reduction of permanent deformations during warm periods, so-called top-down cracking may occur in the course of temperature drops during cold periods. In order to explain the complex viscoelastic properties of asphalt concrete, a multiscale model is proposed. Hereby, the viscoelastic behavior of bitumen serves as input and the effect of air voids and aggregates is investigated. The viscous properties of bitumen are identified, using the bending-beam rheometer and the dynamic-shear rheometer, providing access to the viscoelastic material response for different temperature and loading regimes. With the rheological properties of bitumen at hand, the viscoelastic properties of mastic, mortar, and asphalt concrete are determined using continuum micromechanics, employing the...

Effect of air voids on salt scaling and internal freezing

By combining calorimetric measurements with dilatometry, it has been possible to calculate the contributions of thermal expansion, pore pressure, and crystallization pressure of ice to the strain observed in a mortar during freezing/thawing cycles. Air-entrained mortars contract upon freezing, while non-air-entrained mortars expand. The expansion of the latter is attributed primarily to hydraulic pressure, owing to the rapid growth of ice, which nucleates at low temperatures in laboratory samples. Poromechanical calculations account quantitatively for the contraction of samples with air entrainment, assuming that ice crystals form in the air voids. As originally proposed by Powers and Helmuth, those crystals create suction in the pore liquid that offsets the crystallization pressure of ice in the mesopores of the paste, resulting in a net contraction. Ice in the matrix also contributes significantly to the increase in the thermal expansion coefficient of the mortar. The magnitude of the contraction in air-entrained mortar is shown to account for a reduction of salt scaling damage. According to the glue-spall theory, the damage results from cracking of the ice on the surface of concrete, when the thermal expansion mismatch stress exceeds the strength of the ice. The contraction of the mortar caused by air entrainment offsets the thermal expansion mismatch sufficiently to prevent cracking. Based on observations of the nucleation temperature of ice in laboratory samples of various sizes, it is estimated that there is one site capable of nucleating ice at − 1 °C in a cube of mortar roughly 34 cm on an edge (or, one per square meter in a slab 3 cm thick). This suggests that ice nucleates in the field at high temperatures, compared to what is typically seen in the laboratory, and propagates slowly through the pores as the temperature drops. This mode of growth may lead to fatigue damage over many cycles, owing to local stresses from crystallization pressure, where the contribution of hydraulic pressure is insignificant.

METHODS FOR EVALUATING THE EFFECT OF LARGE VOIDS ON FOOD PACKAGE COOLING TIMES

Design of food package cooling operations requires accurate prediction of chilling or freezing time, temperature-time profile and heat release rate. Current algebraic and ordinary differential equation prediction methods assume an homogenous product and account for the effects of air voids through the use of effective thermal properties. Many packaged foods, however, contain voids large enough to allow natural convection to develop, or gaps that change the directions and lengths of heat conduction pathways. The effect of these phenomena may be significant enough to render current prediction methods unsuitable. The initial step in developing a simple method to predict the cooling times of these packages is to measure the effects of large voids on temperature-time profiles in the packages. In this paper, an experimental methodology for evaluating the effects of large voids was developed. The accuracy of the method is investigated and compared to numerically simulated results. Preliminary conclusions about the effect of various void sizes are made.

Prediction of Asphalt Cement Content of Compacted Hot-Mix Asphalt Specimens Using Nuclear Gauges

Determining asphalt cement (AC) content of hot-mix asphalt (HMA) for quality control/quality assurance using extraction methods is a lengthy, time-consuming, and hazardous process. A demand for a faster and safer method led to the development of different nuclear gauges capable of predicting the AC content of HMA samples. Measuring the AC content of compacted specimens by nuclear gauges is a new technique whose feasibility was evaluated. A total of 216 Marshall compacted specimens were cast and tested in an attempt to develop correction models. The study program investigated the effect of various HMA parameters on the measured AC content, including aggregate gradation and type, air void content, moisture content, AC content, and specimen weight. Specimens were prepared at two different mix design formulas using AC-30 and compacted at two different compaction efforts to investigate the effect of air voids. Specimens (1200 g each) used for calibration were prepared at 3 to 7 percent AC content, whereas spec...