Journal of Materials in Civil Engineering

Multiscale modeling is becoming an increasingly useful method of evaluating the behaviors of asphalt concrete. Reasons for this increased interest include the fact that many of the critical behaviors of this material are affected by localized behaviors that cannot be completely captured using more traditional continuum approaches. Computational methods are popular for this type of evaluation because in principle they can directly account for many of the localized mechanisms. However, computational expense can be excessive, particularly if all of these localized mechanisms are accounted for rigorously. An alternative method of multiscale modeling relying on analytical models is developed and presented in this paper. The model is referred to as the microstructure association model because it accounts for the ways that the multiple scales within asphalt concrete associate together to yield the gross behaviors of the finished composite, asphalt concrete. This model is formed from the hypothesis that asphaltic composites at different length scales evolve along a continuum, which is related to the degree of internal structuralization that exists within the composite. In this way asphalt mixture may behave in a quantitatively similar way as asphalt mastic if the concentration of particles within the mastic produces the same degree of internal structuralization as that which exists in the asphalt mixture. Through this modeling effort two separate, but possibly related, mechanisms are identified: (1) interaction of particles separated by some distance, but still influenced by an adsorbed and nonadsorbed asphalt binding medium; and (2) close-proximity contacts of aggregate particles that lead to a significant increase in stiffness with relatively small changes in volumetric concentration. Each of these mechanisms is modeled separately within the microstructural association model and combined using a rational mechanical analog approach. The model is characterized using only experiments on asphalt mastic at different concentrations, but is verified with its ability to upscale to the modulus of asphalt concrete. In the final section of this paper the ability of the model to perform useful engineering tasks is shown when it is used to assess the impact of asphalt content and air void content changes on the modulus of two different materials.

An investigation of the existence of predominant frequency, $f_p$ (or frequencies), and the ability to predict critical pavement responses in asphalt layers using those predominant frequencies has been presented. The study used an extensive database of computed pavement response histories of four different asphalt pavement structures (thin and thick) at two temperatures (21 and 40°C) subjected to a tandem axle load at three speeds (16, 65, and $97\mathrm{km}/\mathrm{h}$) using the pavement response analysis program 3D-Move Analysis. The conversion of load duration to frequency and the validity of using one response component (${\sigma }_{zz}$) alone in the estimation of the pulse time ($t_p$) were explored. Instead of focusing on only ${\sigma }_{zz}$, attention is given to all pavement response components (stresses and strains). In addition, verification of whether the use of a consistent set of $f_p$ value(s) can in fact adequately capture all components of pavement response has also been undertaken. The viscoelastic properties that are representative for the predominant (or equivalent) frequencies from frequency sweep data were used. A major contribution of the study is that the critical responses and performance in a flexible pavement system can be successfully simulated with a static multilayer elastic analysis (within $\pm 10\%$) by assigning elastic modulus to hot-mix asphalt (HMA) based on appropriately selecting the corresponding predominant or equivalent frequency, $f_p$ (or frequencies). Generalized equations for estimating those $f_p$ values for all pavement structures, temperatures, and vehicle speeds under consideration have been developed. Good fitting parameters ($R^2$) were observed for all evaluated cases. The impact of loading frequency on predicted asphalt distresses was assessed for rutting and bottom-up fatigue cracking following the AASHTO mechanistic-empirical pavement design guide approach. The study showed that the proposed procedure for estimating $f_p$ values resulted in rut depths and bottom-up fatigue cracking that are within $\pm 10\%$ of those determined using the viscoelastic dynamic responses.

Thermal cracking of Portland cement concrete (PCC) decreases rideability and accelerates deterioration of concrete pavements. Coefficient of thermal expansion (CTE) is one of the most important parameters to evaluate the thermal sensitivity of PCC. The AASHTO mechanistic-empirical pavement design guide (MEPDG) requires CTE as a basic input for concrete pavement design, which has increased interest in studies related to concrete CTE in the United States. Several test methods have been developed and used to determine concrete CTE. Nevertheless, concrete CTE testing is time-consuming. Most of the currently available concrete CTE prediction models are empirical and do not reflect the microstructure of PCC. This paper developed a micromechanical model based on thermal mechanical analysis to predict concrete CTE. Concrete CTE data found in the literature validated the applicability of the developed model. Factors affecting concrete CTE were examined using the proposed model. The model has the potential to estimate concrete CTE for concrete pavement design and to help select appropriate raw materials for PCC to achieve low CTE.

The mechanical behavior of bituminous mixes is very complex. It is also characterized by great thermal sensitivity. In the small strain domain (for strain amplitudes below approximately ${10}^{-4}$), it can be considered to be linear, therefore the linear viscoelasticity theory can be applied. In addition, the time temperature superposition principle (TTSP) may be assumed, with a good approximation. This has been widely verified regarding the unidimensional behavior of bituminous mixes in the viscoelastic linear domain. This paper provides an overview of TTSP for bituminous materials, and three-dimensional (3D) behavior and nonlinear domain are also considered. The 3D behavior is characterized by measurements of the complex modulus and complex Poisson’s ratio. For both parameters, master curves are plotted to check the validity of TTSP in 3D. Some experiments in the nonlinear domain (considering strains up to some percentages and some crack propagation tests) are also conducted in equivalent conditions of time and temperature. The results show that the TTSP may be extended to the nonlinear domain, including the domain of crack propagation.

When an asphalt mixture is subjected to a destructive compressive load, it experiences a sequence of three deformation stages, as follows: the (1) primary, (2) secondary, and (3) tertiary stages. Most literature research focuses on plastic deformation in the primary and secondary stages, such as prediction of the flow number, which is in fact the initiation of the tertiary stage. However, little research effort has been reported on the mechanistic modeling of the damage that occurs in the tertiary stage. The main objective of this paper is to provide a mechanistic characterizing method for the damage modeling of asphalt mixtures in the tertiary stage. The preliminary study conducted by the writers illustrates that deformation during the tertiary flow of the asphalt mixtures is principally caused by the formation and propagation of cracks, which was signaled by the increase of the phase angle in the tertiary phase. The strain caused by the growth of cracks is the viscofracture strain, which can be obtained by conducting the strain decomposition of the measured total strain in the destructive compressive test. The viscofracture strain is employed in the research reported in this paper to mechanistically characterize the time-dependent fracture (viscofracture) of asphalt mixtures in compression. By using the dissipated pseudostrain energy-balance principle, the damage density and true stress are determined and both are demonstrated to increase with load cycles in the tertiary stage. The increased true stress yields extra viscoplastic strain, which is the reason why the permanent deformation is accelerated by the occurrence of cracks. To characterize the evolution of the viscofracture in the asphalt mixtures in compression, a pseudo J-integral Paris’ law in terms of damage density is proposed and the material constants in the Paris’ law are determined, which can be employed to predict the fracture of asphalt mixtures in compression.

Fatigue cracking is a common distress in asphalt pavements, but most existing models to predict its growth are generally empirical or phenomenological in nature. To fill this gap, this paper aims at developing an energy-based mechanistic approach to model the fatigue crack growth in asphalt mixtures. The core of this approach is establishing the energy balance equations between the apparent energy of the bulk specimen and the true energy of the intact material. A controlled-strain repeated direct tension (RDT) test is used to generate fatigue cracking damage in asphalt mixtures. The true stresses, true strains, and true pseudostrains are simulated through the force equilibrium and energy balance equations. The true stress in a damaged asphalt mixture is found to be the driving force for crack growth; the crack growth in turn aggravates the localization of the true stress/true strain. The ratio of the true stress and the apparent stress in a damaged asphalt mixture is used to calculate the damage density. The evolution of the damage density with repeated loading demonstrates the development of fatigue cracking in the asphalt mixture. In addition to modeling fatigue cracking in asphalt mixtures, the energy-based mechanistic approach developed in this paper can be used with a wide range of tests to predict crack growth of different types of materials because of its mechanical nature.

Several different modifiers are increasingly being used to improve the performance of asphalt binders (e.g., polymers) or to promote environmentally friendly technologies (e.g., warm-mix asphalt, ground tire rubber, and organic waste). In such cases, it is important to evaluate the inherent improvement in engineering properties and damage resistance of the asphalt binders while being subjected to a state of stress that is similar to full asphalt mixtures. The first part of the paper presents a review of the literature that investigates the state of stress of the asphalt binder in an asphalt mixture as well as the test methods that have been used to achieve this state of stress experimentally. The second part of the paper presents a test method by which the fatigue-cracking characteristics of four different asphalt binders with similar true temperature grades were evaluated. Composite test specimens were fabricated using asphalt binders and glass beads with a specified gradation. The viscoelastic and fatigue-cracking characteristics of the binders were measured using the glass bead-binder composite specimens in a dynamic shear rheometer at an intermediate temperature. Results were analyzed using viscoelastic continuum damage theory. Damage evolution in the glass bead-binder test specimens used in this study was qualitatively very similar to the damage evolution in full asphalt mixtures using the same binders.

Fatigue damage is one of the most common distresses observed in the asphalt-concrete pavements. Both the initiation and propagation of fatigue cracking are complicated and very difficult to detect because they develop generally within pavement structure. To understand the fatigue performance of asphalt concrete thoroughly, the behaviors of the major components of asphalt concrete under cyclic loading were investigated in this study. A new experimental method was developed to evaluate the performance of asphalt binder, mastic, and fine-aggregates mixture under cyclic tensile loading. The test results showed that fatigue performance of asphalt binder is closely related to the loading magnitude, temperature, and loading frequency. Mastic specimens with varied filler contents were tested. It was found that the mastic specimen with 30% filler content had better fatigue resistance and higher permanent strain. The differences between the test results of mastic and mixture were then compared and analyzed using X-ray tomography imaging. It was indicated that the fatigue resistance is closely related to the air-void content of the specimens. Three-dimensional digital specimens of asphalt binder, mastic, and mixture were developed and tested in the numerical simulation of fatigue tests based on the finite-element method. Both mastic and mixture were modeled as heterogeneous composite materials in which the asphalt binder, filler, and aggregate were described by different mechanical models. Fatigue damage of asphalt concrete was simplified by a damage model, which characterizes the degradation of the elastic modulus of the asphalt binder. It was found that with proper selection of damage parameters, the simulation results agree well with laboratory test results and can be used as a basis for future fatigue research. Finally, the reinforcement effect of basalt fiber was evaluated as an additive to improve the fatigue resistance of asphalt-binding materials. It was found that with proper content, the fatigue resistances of asphalt binder and mastic were significantly improved. The numerical simulation analysis also showed that fibers release the stress concentration in the interface area of two fillers and mitigate the fatigue damage caused by cyclic loading.

Low-temperature cracking of an asphalt pavement is caused by the deformation mismatch between the asphalt overlay layer and the underlying pavement layer in cold climates or where rapid temperature change exists. When longitudinal tensile stress in the overlay reaches a certain level, transverse cracks will initiate at the surface to release the energy stored in the asphalt material. When crack spacing reduces to a certain value, crack density becomes saturated and no new cracks will form. The fracture behavior significantly changes with the properties of the underlying layer and the interface. A recently developed theory of stress transfer is used to obtain a closed-form solution for the elastic field in an asphalt overlay fully bonded to the underlying layer, based on the periodic boundary-value problem. Using the correspondence principle, the formulation is extended to consider the viscoelastic material behavior of asphalt materials, so that the time-dependent material behavior of asphalt pavements under climate change can be analyzed. The modeling results compare well with finite-element results. The analytical solution can be used in the simulation of low-temperature cracking of asphalt pavements under extreme temperature conditions.

The early-stage cement hydration is critical to the strength and durability of cement-based materials. Due to the complex processes involved, holistic multiphysics and multiscale simulation is challenging. This paper introduces the concept of a water characteristic curve (WCC) used in unsaturated soils to cement-based materials. The WCC characterizes the internal structure of cement and essentially relates the pore morphology and physical chemistry of solid-liquid interface in microscale to the concepts of suction and water content in macroscale. A thermo-hydro-mechanical model was developed based on this multiscale technique. A modified Fourier’s heat equation for the thermal field, a modified Richard’s equation for the hydraulic field, and an extended Navier’s equation for the mechanical field were employed for developing the theoretical multiphysics framework. Auxiliary relationships, e.g., temperature-induced flow and chemical-reaction information of cement components, were adopted for the completeness of the formulation. This simulation framework was implemented with the finite-element method. Phenomena such as shrinkage, which is typical of early-stage cement hydration process, were reproduced. The methodology of this model provides a solid cornerstone for the simulation-based prediction of concrete performance.

This study investigates the internal-frost damage due to ice-crystallization pressure in the concrete pore system. The methodology integrates thermodynamic analysis and a microdamage model as well as a unique time-domain reflectometry (TDR) sensor. The crystallization pressure in the microscale pore system of concrete at subcooling temperatures was calculated based upon thermodynamic analysis. An extended finite-element method (XFEM) was applied to simulate the fracture development induced by internal frost, with the estimated internal crystallization pressure as the input. The XFEM fracture simulation was conducted on a digitized concrete sample obtained with imaging processing and ellipse-fitting techniques. The simulated crack development under the crystallization pressure was found to match the observed fracture patterns of the tested single-edge notched specimen. The XFEM simulation results were verified by the open-mode fracture behavior in both middle-notched single-edge notched beam bending test and freezing-damage tests. Furthermore, the crystallization-pressure analysis and freezing-damage simulation were conducted to demonstrate the freezing-damage process using cement samples with idealized pore structures. To provide direct estimation of the crystallization pressure, an innovative TDR tube sensor was developed to nondestructively monitor the extent of freezing in concrete specimens. The results show that this new sensor provides noninvasive measurement of freezing degree, which can be used to directly estimate the internal crystallization pressure for XFEM analyses. A volume-based damage criterion was also proposed based on the new TDR sensor. This work established a framework to integrate sensor and simulations to holistically predict the internal-frost damage process in concrete specimens.

Although abundant plastic waste contaminating the environment may be utilized as reinforcing materials, a potential pozzolanic material (rice husk ash blended with lime) possesses superior properties in stabilizing soils. Engineering behavior of the stabilized clayey/silty soil reinforced with randomly distributed discrete plastic waste fibers is investigated in this paper. The results indicate that the proposed method is very effective to improve the engineering properties of the clayey/silt soil in terms of compressive, tensile, and shear strength, which further enhanced the stability and durability of the soil. Based on the compressive strength, California bearing ratio (CBR), shear strength, and failure characteristics, the optimum amount of fiber mixed in soil/lime/rice husk ash mixtures ranges from 0.4–0.8% of the dry mass.

This paper documents experiments conducted to examine the time-dependent, one-dimensional swelling response of loaded granular packs composed of metal-free tire-derived aggregate (TDA) inundated with organic liquids. The results indicate that the magnitude and rate of sorption-induced swell are dependent on the fluid type and on the stress imposed at the time of inundation.

Tensile stress develops in concrete when shrinkage deformations and restraints (internal or external) exist. Utilization of appropriate creep or relaxation functions is crucial for assessing stress and the associated cracking potential in concrete, especially at early ages when chemical and physical properties change rapidly. The existing models, developed either from compressive creep tests or conventional tensile creep tests with constant loads applied at certain ages, were found to be unsuitable for such stress evaluation. This study was performed to investigate the restrained strain-stress development and tensile creep behavior of concrete subject to constant restraint starting at very early ages, which well represents field conditions of actual structures. Restrained stresses were measured using a specially designed frame. The concretes were cured and tested at two different temperatures (23°C versus 33°C) and ground granulated blast furnace slag contents (0% versus 30% of the total cementitious materials by mass). A modified tensile creep model is proposed to account for the high viscosity of concrete under constantly restrained conditions, allowing more accurate assessments of stress and cracking potential in structures such as slabs or pavements on the ground.

Tire-derived aggregate (TDA) has been successfully used for highway embankment applications in the past. Previous applications mainly used small and medium tire sizes as TDA sources. There are no published test results in the literature regarding the compression behavior of TDA made solely from off-the-road tires (OTR). In this study, large-scale, one-dimensional compression tests are carried out to study the compression behavior of TDA from OTR as well as from passenger and light-truck tires (PLTT). Samples for the tests are prepared by varying the initial unit weights. The results show that there is a general trend of decreasing compressibility with increasing initial unit weight for both TDA sources. The compression test results are also used to compare compression behavior between the two TDA sources. It is found that the compression behavior of TDA from OTR and PLTT is more or less similar. Moreover, one-dimensional stress-strain regression equations were developed for TDA from OTR.

This paper focuses on the feasibility of using acoustic emission (AE) techniques to evaluate stability of foams produced from paving-grade asphalt binders. The study compared asphalt foamability index [expansion ratio (ER) and half-life (H-L)] measurements with AE measurements for the same foamed asphalt produced using a Wirtgen Model WLB 10 S Laboratory Asphalt Foaming Device. Both ER and H-L are used to characterize the suitability of asphalt binders to produce good-quality foamed asphalt binders for pavement construction. However, current methods for determining foamability are considered as empirical, operator-dependent, and cannot be used for continuous monitoring of the foaming process. Two performance-graded asphalt binders, four foaming water contents, and two foaming temperatures were used. Spectral analyses using a test for white noise suggest AE signals resulting from asphalt foaming are not white noise ($p\text{-}\mathrm{value}<0.0001$) and therefore could be useful for evaluating foamed asphalt stability. The evolution with time of AE amplitude during foamed asphalt decay suggests two distinct stages related to foam stability. A clear amplitude dynamics characterized by shorter-duration lower-amplitude foam decay followed by longer-duration higher-amplitude foam collapse was found for all the asphalt binders tested. AE energy, a measure of elastic energy released by collapsing foam bubbles, was found to be sensitive to H-L. The results demonstrate that AE may be useful for characterizing foaming characteristics in a more fundamental manner than existing manual methods. The techniques and results summarized here provide the basis for further investigation into application of AE measurements to monitor and control foamed asphalt stability.

The mechanistic performance of hybrid asphalt-concrete mixture incorporating coarse recycled concrete aggregate (RCA) was investigated. The hybrid mixture (W3B-RCA-C37) and control mixture (W3B-granite) were evaluated by the Marshall mix design method to determine their optimum asphalt content. Both W3B-RCA-C37 and W3B-granite mixtures had similar optimum asphalt content. However, volumetric properties (voids in total mix, voids filled by asphalt, and voids of mineral aggregate) of W3B-RCA-C37 mixture at its optimum asphalt content were lower than that of W3B-granite mixture. The Marshall stability, resilient modulus, and creep resistance of W3B-RCA-C37 mixture were higher than that of W3B-granite mixture, but it was more susceptible to moisture damage. The substitution of coarse RCA affected the mixture’s performance in several ways. Equivalent mass substitution of the aggregate resulted in relatively larger RCA volume per unit mass of the mixtures. Friable RCA induced greater aggregate breakages; broken aggregates may easily reorientate and fill available space in the mixture, resulting in a lesser space for air voids and effective asphalt. More asphalt was absorbed into the aggregates in W3B-RCA-C37 mixture but lesser asphalt was needed to fill the voids between the more tightly packed aggregates. Incorporation of RCA had resulted in a mixture with denser aggregate structure and a thinner asphalt film, giving higher Marshall stability and better deformation resistance. However, thinner asphalt film and inadequately coated broken aggregates increase susceptibility to moisture-stripping damage to the mixture.

For the evaluation of damage to a structure after exposure to fire, the residual mechanical properties of structural materials need to be evaluated first. Many factors can affect the post-fire behavior of structural and reinforcing steels. In this paper, existing test data are collected from an extensive survey of the literature. A statistical analysis is conducted to analyze the effects of heat exposure on key parameters, such as the residual modulus of elasticity, residual yield strength, and residual ultimate strength, which control the full-range stress-strain curves of steel. A simplified stress-strain model is developed, which can be used for both structural and reinforcing steels after heating and cooling down to room temperature.

Based on the equilibrium, continuity, and energy principles of multiphase porous media, a thermal-hydro-mechanical coupling model was developed for concrete upon freezing. To solve the governing differential equations, a three-dimensional finite-element analysis is performed based on the software program Comsol Multiphysics. Finally, temperature distributions, frozen water contents, and strains as a function of freezing time are predicted. A comparison of the analytical results with available experimental data for concrete mortars illustrates the validity of the proposed model. This numerical model can be used to assess the effect of cooling rate, the role of pore-size distribution, the role of permeability, the effect of cross section size and shape, the effect of superimposed stresses attributable to applied loads, and other factors that cannot easily be evaluated by current tests.

Steel degradation due to corrosion in prestressed concrete bridges has been of major concern as it presents a threat to the integrity of structures adjacent to marine environments or where deicing salts are regularly used. To assess the potential for monitoring of the corrosion process, an accelerated corrosion testing program has been conducted. A series of specimens with dimensions $114\times 114\times 508\mathrm{mm}$ ($4.5\times 4.5\times 20\mathrm{in.}$) were subjected to constant potential application through the embedded steel strand while being continuously monitored with acoustic emission (AE). Depassivation of the strand was detected by monitoring the fluctuations in applied anodic current. Half-cell potential measurements assessed the probability of corrosion, and all results obtained were compared to acoustic emission data. The mass loss of the corroded strands was correlated to acoustic emission intensity analysis to quantify the degree of damage. Results show that acoustic emission is as sensitive as half-cell potential for detecting the presence of corrosion and may be used to effectively locate corroded areas. The intensity analysis proved useful for categorizing the level of damage, making it a strong candidate for structural prognosis.

The durability of reinforced concrete structures depends, in the main, on the performance of the cover-zone concrete, which protects the steel from the external environment. This paper focuses on the use of discretized electrical property measurements to study depth-related features during both the curing and postcuring period, thereby allowing an integrated assessment of the protective properties of the cover region. In the current work, use is made of a small, multielectrode array embedded within the surface 75 mm of concrete specimens. Concretes were manufactured with different European cements (CEM) and water/binder ratios representing mixes that satisfied the minimum requirements for a range of environmental exposure classes, including exposure to chlorides. Electrical resistance measurements were taken over a period in excess of 300 days which showed ongoing hydration, pozzolanic reaction, and pore-structure refinement; in addition, in the postcuring period, when exposed to a cyclic chloride ponding regime, measurements could be used to study the convective zone and ionic enrichment of the surface layer.

A laboratory investigation was carried out to study the effects of various compaction temperatures on moisture susceptibility and rutting resistance of warm-mix-asphalt (WMA) mixtures containing moist aggregates using a foaming technology. The experimental design included two aggregate moisture contents (0 and $\sim 0.5\%$ by weight of aggregate dry mass), one lime content (1% lime by weight of dry aggregate), two foaming water contents (2 and 3%) with control, and two aggregate sources. The major properties, such as the gyration number of various samples, indirect tensile strength (ITS), tensile strength ratio, rut depths of dry and conditioned specimens, and deformation (flow), were measured for all mixtures. The test results indicated that the aggregate source significantly affected the ITS and rutting resistance regardless of the foaming water content, compaction temperature, and aggregate moisture content. In addition, mixtures from moist aggregate had lower gyration numbers regardless of aggregate type, foaming technology, and compaction temperature. Moreover, the ITS values of foamed mixtures containing moist aggregate increased as the compaction temperature increased. Furthermore, the rut depths of all the dry and wet mixtures slightly increased as their compaction temperatures decreased regardless of the aggregate moisture content, foaming water contents, and aggregate type. A compaction temperature below 102°C (215°F) generally had no noticeable aging (2 h) effects on the mixture and did not significantly increase the rutting resistance of these WMA mixtures.

The infrastructure of concrete sewage systems is affected notably by biogenic sulfuric acid attack, which causes rapid deterioration of each of its components. To provide a better understanding of this mechanism of damage, this paper reports on research carried out to study the response of the cementitious concrete matrix and the reinforcement steel under the attack of sulfuric acid. Concrete industrial pipes were cast using ordinary portland cement and pulverized fly ash, and specimens obtained from the mixes were left for several cycles in a solution of sulfuric acid at different pH. Experiments were carried out to assess the loss of weight and the electrical corrosion potential. Results showed that sulfuric acid attack can be considered a surface phenomenon, in which the acid attack destroys the cover of concrete before it reaches the reinforcement.