Air Void Distribution Research Articles

Investigation of aggregate gradation on air voids distribution in porous asphalt concrete using X-ray CT scanning images

Porous asphalt concrete (PAC) is typically used as a paving material for porous asphalt pavement in rainy areas due to its abundant interconnected pores structure. Not all interconnected pores in PAC are valid for permeability, mainly influenced by the composition of aggregate gradation. The main purpose of this paper is to investigate the effect of aggregate gradation on pores structure distribution in PAC by X-ray computed tomography (CT) system and digital image processing (DIP) techniques. The distributions of porosity, number and average equivalent diameter of air voids in middle part are much more evenly compared to the both ends in PAC specimens, accounting for about 70 % of the total of specimen height. The average equivalent diameter of interconnected air voids mainly ranges from 0.5 mm to 5 mm in PAC, accounting for about 80 % of the total numbers. For PAC with similar porosity, the larger the normal maximum aggregate size (NMAS), the less the number and the larger the average equivalent diameter of air voids (interconnected air voids), the smaller the difference between the interconnected porosity and porosity. For PAC with same NMAS, as the porosity increase, the number and average equivalent diameter of air voids (interconnected air voids) are respectively decreased and increased gradually, the difference between the interconnected porosity and porosity is also showed a decreasing trend. And then, the correlated equation is established among porosity, interconnected porosity and the composition of aggregate gradation. The distribution of air voids (interconnected air voids) size could be well described by the two-parameter Weibull model, and the relationships are explored and developed between Weibull model parameters and the composition of aggregate gradation.

Development of insulating backfill materials produced from the ternary mixtures of red mud, fly ash and preformed foam

Many public utility lines to transport power, water, natural gas, water, sewer, and communication are placed beneath trafficable areas. However, insufficient or inadequate backfill induces sudden subsidence, damage, or pothole on the road. This study aims to develop lightweight controlled low-strength materials (CLSM) with low thermal conductivity using the ternary mixtures of red mud to replace aggregate sand, high carbon fly ash, and preformed foam. Changes in flow consistency, air void characteristics, bulk unit weight, unconfined compressive strength (UCS), and thermal conductivity (k) of the tested materials with varying red mud contents were investigated as a function of the foam volume ratio (FVR). The results demonstrate that the UCS and k of tested materials decreased with increasing FVR due the decrease in unit weight, and a greater UCS but smaller k was observed at a given FVR with increasing red mud content because the inclusion of red mud in the lightweight CLSM mix design helps improve the stability of air bubbles and achieve uniform distribution of air voids. In addition, the red mud can act as a NaOH supplier, leading to the developed material had additional strength gain from the alkali activation. Thus, the developed insulating backfill material showed 43 % decrease in k while maintaining UCS similar to non-foam CLSM without red mud.

Dynamic response and sound radiation of cracked asphalt pavements under moving loads and variable thermal profiles

ABSTRACT This study investigates the acoustic performance and dynamic response of cracked, viscoelastic porous asphalt pavements under moving loads and variable thermal conditions. Realistic pavement cracks are modelled using an advanced line spring model with fracture compliance coefficients. The novelty lies in the synergistic integration of a heterogeneous triple-porosity media model, a non-uniform depth-dependent temperature profile, and an advanced fracture mechanics-based line spring model with arbitrary crack orientation. Variations in temperature that are uniform, linear, and nonlinear with respect to the thickness layer axis are taken into account. The governing equations are derived by Hamilton’s principle and solved analytically via Fourier-Laplace transforms. The acoustic pressure is first-time predicted through Rayleigh integral analysis. Durbin’s numerical inversion validates the model’s accuracy versus sophisticated finite element simulations. Parametric analysis offers new insights into the effects of crack length, pore size distribution, loading frequency, and thermal gradients on noise pollution and fatigue cracking. The integrated chemo-thermo-mechanical modelling enables optimal structural design and accelerated pavement testing to limit acoustic radiation and premature failure in porous asphalt pavements. It allows for the development of noise-reducing porous asphalt mixtures by modifying aggregate gradation, binder content, and air void distribution.

Diagnostics of premature damage to surface-hardened industrial concrete floors

The article presents the diagnostic results on surface-hardened industrial concrete floors. Selected examples of floors showcased premature damage to surface layers, characterized by intense dusting, delamination, and local spalling, while the structural system remained unaffected. Quantitative petrographic analysis of concrete was applied to core specimens from the floors, involving the examination of digital images from a polarizing optical microscope and a scanning electron microscope. The hardening compound and powdered specimens of the cement matrix were characterized using differential thermal analysis and X-ray diffraction. A multiple microindentation method was employed to assess local variations in mechanical properties. Concrete cross-section analysis revealed areas with a non-uniform distribution of air voids, identified regions exhibiting increased porosity, highlighted areas of cracking in the concrete, indicated local variability in the phase composition of cement hydration products, and pointed out the presence of carbonated areas. The causes of the damage were discussed based on these findings,. The crucial role of quantitative petrographic analysis in diagnosing premature surface damage to industrial floors was demonstrated.

Porous asphalt mixture performance in cold regions: Case study of Chicago

Porous Asphalt Mixtures (PAMs) improve resilience of flexible pavements in the context of climate change. PAM application in coastal cities like Chicago is of interest; however severe winters compromise the durability of PAMs given their susceptibility to freeze-thaw. This work aimed to evaluate the to-date experience with PAMs in Chicago, exploring benefits and limitations to be considered for future projects in the Southern Lake Michigan region. Full-depth PAM cores were obtained from seven sections, constructed in different years. Tests were conducted to assess the evolution of surface texture, internal structure, and multi-scale material properties. Regarding surface texture, consistent change over time was observed, such as loss of the “channel” structure, and increase in mean profile parameters because of raveling. Also, binder content varies with time within different layer depths. An aging gradient in terms of stiffness from top to bottom within the layer does not necessarily occur. However, binder ductility shows drastic and consistent reduction throughout the layer as aging increases. Analysis of internal structure using a CT scan revealed that there is also a lack of homogeneity in the air void and particle size distributions. Finally, this study suggests that full-depth PAM viscoelastic behavior is sensitive to saturation level, and this is a relevant subject to be considered, since partial saturation may be a condition often observed in the field.

Dispersion and Spatial Distribution of Air Voids or Microspheres in Assessing Frost Resistance of Concrete

Abstract The standard spacing factor developed by Powers is typically used to evaluate the quality of the air void system in hardened concrete, but it does not always correlate with durability of the concrete. Several air void spacing equations, which are also applicable when polymeric microspheres are used in place of air entrainment, have been proposed because of the need for a more robust and comprehensive basis to evaluate the quality of the air void system. However, the spacing parameters provided by the various proposed equations, when used as sole measures in predicting the frost resistance of concrete, do not seem to do any better than the standard spacing factor. Dispersion and spatial distribution have been shown to be effective ways of describing air void or microsphere systems in hardened concrete because they have been quantified to establish criteria to assess the frost resistance of concrete. In this paper, dispersion and distribution factors are further elaborated upon to explain how they characterize zones that are protected by air voids or microspheres in the concrete. Criteria to assess the durability of concrete under rapid cycles of freezing and thawing based on the dispersion and distribution factors are linked to the exposure classes defined in the ACI 318 Code and in the recently proposed Unified Durability Guidance in ACI Committee Documents.

Proposed automated contrast-enhanced microscopical method for evaluating air void structure in compacted asphalt concrete mixtures

There is no direct, less expensive, and practical method to measure the interconnectivity and parameters of air voids in asphalt mixtures other than the total air void volume calculated traditionally using bulk and maximum theoretical specific gravities. Unlike the conventional method, this paper proposes a method that is suitable for advanced applications and studies, such as for the evaluation of air void distribution and air void gradient to model and simulate long-term aging, predict top-down fatigue cracking, evaluate the effectiveness of compaction of thick lifts, and satisfy the representative volume element (RVE) requirements (a homogenous and representative specimen) for performance tests. Developing the proposed compact asphalt evaluation method included modifying the sample preparation methods and analysis from the American Society of Testing & Materials (ASTM), C457 Method C, and recently applied in the analyses of two independent studies at the Federal Highway Administration (FHWA). The first study was focused on checking the air void distribution within the small specimen for dynamic modulus and uniaxial cyclic fatigue tests using AASHTO TP 132 and TP 133, respectively. Results of air void analyses conducted on the interior and exterior sides of several slices sawn from different compacted asphalt mixtures consistently showed that the air void contents on the external sides are higher than on the interior. The second study was to evaluate whether the thicker-than-typical lift thickness to nominal maximum aggregate (NMAS) ratio was causing any top-to-bottom air void gradient in the pavement. Results showed that air void sizes and contents are consistently higher along the top ends of all the cores, and the total air void contents measured using the new method on several specimens from the field were comparable with the indirectly calculated air void percent with up to ± 0.4 percent random error. Overall, the new method is practical to apply to larger-size samples and is a tool that both agencies and contractors can use for confirming in situ void uniformity. Its application can support innovations and improve performance in asphalt pavement construction.

DEM analysis of the effect of geocell on splitting tensile behavior of asphalt mixture based on multi-phase model

Herein, an investigation was conducted upon the reinforcement of geocell on the splitting tensile process of asphalt mixture using laboratory tests and the discrete element method (DEM). Micromechanical modeling of geocell-reinforced asphalt mixture and virtual splitting tensile test was built using PFC3D based on the vital material features and mechanical properties of different ingredients (e.g., the three-dimensional irregular shapes and elastic properties of coarse aggregates and geocell, the viscoelastic properties of asphalt mastic, and the random distribution of air voids in asphalt mastic). Meanwhile, the parameters of the DE model were determined by splitting tensile test, and the effects of geocell on the splitting tensile process of contact force, displacement and stress were evaluated through the virtual splitting test. The results indicated the efficiency of the built DE model and virtual test in effectively predicting the splitting process of geocell-reinforced asphalt mixture. Besides, geocell acted mainly in the falling stage of the load-displacement curve during the splitting process, where the contact force distribution of each phase component shifted from the lower to the higher range, and the effect between coarse aggregates was the greatest, followed between coarse aggregate and asphalt mastic, with the least within asphalt mastic. Further, the maximum displacement of asphalt mixture in the X and Y directions was reduced by the geocell, significantly constraining the coarse aggregate and mastic in geocell-reinforced layer. In addition, the stress in the splitting center of asphalt mixture was greater than that at the sides, and the stress within the geocell of the reinforced layer was significantly lower compared to the unreinforced layer, especially in the falling stage. Overall, the present results forge a theoretical basis for the tensile performance of geocell-reinforced asphalt mixture from the microscopical perspective.

Drainage Performance of Long Longitudinal Slope and High Safety Permeable Asphalt Pavement

Permeable asphalt pavement refers to an asphalt mixture layer with an air void content of more than 18% and internal water permeability and drainage capabilities that can quickly drain away water on the road surface, improve rainy day travel safety, and improve ride comfort. This paper aims to explore the optimal asphalt mixture design for long longitudinal slope pavement (referred to as the FAM mixture). By using CT scanning technology to analyze the air void content of different rotated and compacted asphalt mixture specimens and extensively testing and evaluating the performance of permeable pavement mixtures, the following conclusions are drawn: Based on the research philosophy of functional integration, a new asphalt mixture gradation suitable for long longitudinal slope roads is proposed, with the optimal key factor composition being: 0.075 mm passing rate of 7%, 2.36 mm passing rate of 20%, 9.5 mm passing rate of 55%, and an oil-stone ratio of 4.8%. The FAM mixture was divided into three parts for air void analysis, with the upper part having a slightly higher air void content than the lower part. The air void distribution diagram of the FAM mixture is concave, with higher air void rate curves on both sides and a lower middle curve. Through dynamic modulus testing, the strength requirement for the road asphalt mixture in the pavement structure design was evaluated. It was found that at high temperature conditions (50 °C), the minimum dynamic modulus value of the FAM mixture was 323 MPa, with a peak value of 22,746 MPa at a temperature of −10 °C and a frequency of 25 HZ. The dynamic modulus value at high temperature conditions is lower than at low temperature conditions, while the dynamic modulus value at high frequency conditions is higher than at low frequency conditions. This study provides useful information and experimental data for the design of new asphalt mixtures for long longitudinal slope roads and has conducted in-depth research on the air void distribution and performance of the mixture, providing strong support for related research fields and practical applications.

Uniaxial Compressive Process Simulation of Geocell-Reinforced Asphalt Mixture by Discrete Element

Based on laboratory tests and the discrete element method (DEM), this study investigated the reinforcement by geocells on the compressive process of asphalt mixture. By considering the vital material features and mechanical properties of different ingredients (the three-dimensional irregular shapes and elastic properties of coarse aggregates and geocells, the viscoelastic properties of asphalt mastic, and the random distribution of air voids within asphalt mastic), micromechanical modeling of geocell-reinforced asphalt mixture and virtual uniaxial compression test was built by using PFC3D. The effects of geocell reinforcement on the compression process of the asphalt mixture were evaluated through the virtual compression test. The results show that the built DEM and virtual test can effectively predict the compression process of geocell-reinforced asphalt mixture. The geocell reinforcement positively affected the compression process of asphalt mixture by affecting the distribution of micromechanical forces and displacements in the asphalt mixture. Moreover, the geocell reinforcement was more significant on asphalt mastic. This indicates that the geocell significantly improves the compressive performance of asphalt mixture from the microscopic perspective.

Laboratory investigation of gyratory-vibratory compaction method for better simulating asphalt mixture field compaction

To better simulate the effect of vibration and kneading for steel-wheel roller, a novel lab compaction method combining the effect of vibratory and gyratory compaction was proposed. X-ray tomography was used to compare the air void distribution of the in-situ core samples and the laboratory-compacted samples by different compaction methods, including vibratory, gyratory and the combinatory method. It was found that different compaction methods significantly affected the air void distribution of specimens. The combination of vibratory and gyratory compaction methods presented comparable air void distribution and compaction effect to the in-situ core samples.

Characterizing the Effect of Freeze-Thaw Cycling on Pore Structure of Asphalt Concrete Mixtures Using X-ray CT Scanning.

Freeze-thaw (F-T) cycling presents a challenge when building durable pavement structures in cold regions. Understanding the changes within the microstructure of asphalt concrete (AC) due to F-T conditions is crucial for developing a resilient pavement design. This study investigates the impact of F-T cycles on five AC mixtures using X-ray computed tomography (CT) scanning. Image analysis was completed to evaluate the changes in the microstructure of the AC samples before and after exposure to 30, 60, and 90 F-T cycles. The changes/degradation in the microstructure were evaluated based on analyzing the distribution and properties of air voids within the AC samples. The results showed that an X-ray CT scan can successfully capture the impact of F-T cycles on the structure of air voids in different AC mixtures. The findings of this research provide guidelines for understanding the mechanism of F-T damage within AC, which can assist in optimizing the performance of AC in cold regions.

Improvement of microporous structure and impermeability of cement mortars using fly ash and blast furnace slag under low curing pressures

Low air pressures in high plateau will degrade the impermeability and air void structure of cement mortars. In this study, the feasibility and mechanism of fly ash (FA) and ground granulated blast furnace slag (GGBFS) improving the microporous structure and impermeability of cement mortars under low curing pressure were investigated. Results showed that the slag significantly improved the impermeability and air voids distribution of cement mortars, while the fly ash did not benefit the impermeability, as well as the air void in mixed mortars. The heat releasing rate and hydration product composition of the slag mixed paste were superior to that of the FA. The reduction of Ca(OH)2, gentler hydration rate, and filling effect of fine slag particles are believed to contribute to the improvement of impermeability. The water vapor isotherm sorption test demonstrated that low curing pressure changed the surface characteristics of hydrates, and inhibited the adsorption and desorption process (including the capillary condensation process and ink-bottle effect) with the range of relative humidity (RH) > 40%. Meanwhile, the titting parameters variation certified according to Guggenheim-Anderson-deBoer (GAB) model certified that the addition of slag achieved the same result as increasing curing pressure. Moreover, futher analysis showed that if partial OPC was replaced by slag, it could befinit energy consumption, CO2 emission, and materials cost compared with pure OPC. This study provides an initial evidence for applications of slag in low air pressure conditions.

Mechanical responses of buoyant bio-inspired foamed concrete structures

This work proposes a new type of lightweight cellular foamed concrete (FC) inspired by natural cellular structures with controllable mechanical properties for different prefabricated engineering applications. The 3D printed formworks are inspired by the lattice structure and triply periodic minimal surface (TPMS) architectures, including gyroid and primitive prototypes. These 3D printed sacrificial thermoplastic Polylactic Acid (PLA) formworks were then infiltrated with two types of foamed concrete with different densities of 600 kg/m3 and 800 kg/m3 and subjected to uniaxial compressive loadings. Furthermore, the X-ray computed tomography (CT) scan was conducted on the prefabricated structures to further observe the PLA-foamed concrete contact region and to investigate the air void characteristics. Numerical simulation was employed for further investigation on the structural performance and failure mechanism of three bio-inspired structures (TPMS-Primitive, -Gyroid, and strut-based lattice). By both numerical and experimental investigations, the gyroid cellular structure yields the highest compressive capacity, followed by the primitive and lattice structures, respectively. Furthermore, based on the X-ray micro-CT scan, the bio-inspired architecture of the moulds showed a significant effect on the size and distribution of air voids, which highly influenced the compressive strength of the infiltrated foamed concrete.

Modelling the hydraulic performance of open graded asphalt using the discrete element method and computational fluid dynamics

Porous asphalt (PA) pavements are used as an alternative for non-permeable pavements to tackle hydroplaning issues and reduce splash and spray. The key aspect of designing a PA pavement is its hydraulic performance. This study introduces a novel numerical approach combining the discrete element method (DEM) and computational fluid dynamics (CFD) to model the hydraulic performance of PA. Different from the traditional methods of scanning the internal asphalt matrix using X-rays, this study uses a photogrammetry-analysis technique to convert the aggregate structure into a DEM. DEM modelling was then used to create a random aggregate structure with different sizes to simulate the asphalt samples for testing. The information about the sample structure was transferred to CFD for the hydraulic performance simulation. The effects of the air voids content, air voids distribution, and sample structure on the vertical and horizontal hydraulic performance were investigated. The results showed that the proposed approach is capable of producing virtual test samples resembling the laboratory PA sample structure for hydraulic performance tests. It was found that the outlet flow rate was mainly affected by the total air voids content of the sample. Higher content and evenly distributed air voids over the depth of the PA samples led to better hydraulic conductivity. Samples created and tested using DEM-CFD methodology could also effectively predict the flow rate at the outlet and demonstrate the flow movement between connected pores inside the sample structure.

Simplified Method of Estimating the A300 Micropore Content in Air-Entrained Concrete

It is well known that the most important parameters for predicting the frost resistance of pavement concrete are the air pore spacing factor, L, and the micropore content, A300. The A300 parameter requires complex calculations with the estimation of the air-void size in a 3D space. The procedure is based only on one-dimensional chord lengths. The air-void distribution is used only to determine the content of micropores and has no other practical application. Based on the results of the analysis, it was found that there is a simpler way to estimate the A300 parameter without the tedious calculations described in the EN 480-11 Standard. The presented approach is based on the existing linear correlation between the A300 parameter and the number of chords in 28 length classes. The developed function includes only a few coefficients (eight classes) because only chord lengths of 10-350 µm are statistically significant. This fact is important not only for the simplification of calculations but may also have consequences for the methodology of testing parameters characterizing the structure of air-entrained concrete using the 2D method. The presented function allows the estimation of A300 with a standard error not exceeding 0.02%, so it is useful for practical use.

Calculation and Characterization of Air Void in Mortar of the Hot Mix Asphalt (HMA) Based on CT Scanning and Image Analysis Methods

The air void content is one of the most important volumetric properties of asphalt matrixes, such as asphalt mixtures or mortars, because it can greatly affect the performance of the matrix. At the mixture level, there are standardized methods for measuring the air void content, which is an important design parameter of a mixture. However, at the mortar scale, no unified method has been proposed to determine the air void content corresponding to the asphalt mixture. Therefore, this research aims to, first, characterize the air void distribution within the mortar of an asphalt mixture and then develop an updated theoretical method for calculating the air void content of asphalt mortar. The internal structures of air voids from three commonly used asphalt mixtures with different gradations were captured using an industrial CT scanning technique and then morphologically characterized using the image analysis method. Three-dimensional models of the air voids were reconstructed, and the air void content calculated from the CT images was also verified by density tests. The scanning results show that the air void content and air void size fit the cumulative Weibull curve. The results show that the nominal maximum particle size (NMPS) of mortar and the mixture air void content were the main parameters that affected the mortar’s air voids. The mortar air void content had linear relationships with both the mixture air void content and the mixture asphalt aggregate ratio. The mortar air void content was more sensitive to the NMPS of the asphalt mixture compared with the air void content or asphalt aggregate ratio of the mixture. This research provides a quantitative method to calculate the air void content of asphalt mortar within a mixture, which may help in the matrix design of mortar.

Numerical simulation of pore structure and heat transfer behavior in aerated concrete

The pore structure of porous concrete is randomly formed, which brings thorny obstacle to accurately predict its thermal performance. In this study, a numerical approach was proposed for the simulation of pore structure and heat transfer behavior in aerated concrete. According to the porosity (ε) and log-normal pore size distribution parameters (μ, σ), an idea of “random distribution” was developed to reconstruct the pore structure via computer modeling, and also an overlap ratio (δ) was defined and adopted to generate a structure closer to reality. In parallel with this, a Finite Volume Model (FVM) was established to solve the energy control equation, and thus the effective thermal conductivities (ETCs) were numerically calculated. The calibrated and validated FVM model was also used to systematically study the effects of air-void content, air-void distribution and structural parameters, including pore size and porosity on the heat transfer behavior of aerated concrete. The numerical results show that the ETCs tend to slightly decrease with the increase of overlap ratio (δ), pore size (μ) and distribution (σ) due to an increased degree of consolidation between pores, which leads to the solid phase easily blocked by the connected pores so that the optimal heat transfer path through the solid phase conduction is blocked. Further, the comparisons of predictions of ETCs with the experimental data and other existing models indicate that the numerical predictions agree well with the experimental data, and the experimental and simulated values of the ETCs are intermediate between the Maxwell-Eucken 1 and Effective Medium Percolation Theory (EMPT) models.

Relationship among air void microstructural characteristics, stiffness, and fatigue of asphalt concrete mixtures

It is a common sense that the air void content significantly affects the field performance of asphalt mixtures. Thus, this parameter is a key mix design criterion and is also adopted as a quality assurance/control tool. Even though the air voids play a notorious role in the behaviour of asphalt mixtures, the knowledge about their microstructural characteristics is still limited. To overcome this limitation, advanced techniques such as the X-ray micro-computed tomography technology may be pursued as an alternative. This paper uses X-ray micro-computed tomography to identify microstructural parameters of air voids within asphalt mixtures, i.e. distribution, content, type, size, and degree of anisotropy. These microstructural characteristics are compared to the stiffness and the resistance to fatigue cracking of asphalt mixtures. The findings suggest that the air void distribution is strongly correlated with the mixture stiffness and fatigue behaviour, and that the air void heterogeneity should be avoided.

Predicting Spacing Factor in Hardened Concrete Using the Fresh Concrete Measurements

Damage induced by freeze–thaw action in Portland cement concrete pavements can be minimized by ensuring the presence of a well-entrained air-void distribution within its cementitious paste. Several methods exist for assessing the air-void system in fresh concrete, principally ASTM C231-17a, ASTM C173-16, and AASHTO T 395. These methods accurately measure the fresh air content, but this alone is insufficient to assess air-void distribution parameters. ASTM C457-16 enables such an assessment but can only be performed on hardened concrete and consequently does not provide real-time assessment. Hence it was highly desirable to develop a system to accurately predict the air-void distribution properties, principally the spacing factor, of a concrete mixture, in real time, to support quality control/-assurance activities. A set of 271 observations was used to develop statistical models for predicting the spacing factor obtained by the RapidAir-based ASTM C457-16 method. The models were based on paste content and fresh concrete test measurements (predictors), for which least absolute shrinkage and selection operator regression analysis were used to select/reject variable(s). The predicted spacing factors (PSFs) obtained using these models in fresh concrete were evaluated on their ability to differentiate between a well-entrained (i.e., spacing factor <0.2 mm) and coarsely entrained (i.e., spacing factor >0.2 mm) air-void system. The PSFs from the base and final models exhibited 88.5% and 81.1% accuracy, respectively—much higher than that of the SAM number alone (48.4%). A larger fraction of the data set consisted of well-entrained spacing factor observations, which limited validation for coarsely entrained air-void systems.