Identification of Viscoelastic Functions for Hot-Mix Asphalt Mixtures Using a Modified Harmony Search Algorithm

This study proposes a modified harmony search (MHS) algorithm for determining the time-domain viscoelastic function of hot-mix asphalt (HMA) concrete materials. This MHS technique, employing a global optimization technique as well as a Wiechert model for the relaxation function, substantially enhances accuracy and consistency in the determination of viscoelastic functions of several HMA mixtures. In addition, this study shows how to determine a time-domain Prony series representation from the complex modulus in the frequency domain using the MHS algorithm. This can be efficiently used for numerical analysis with techniques such as the finite-element method. The results from lab frequency sweep tests of unmodified and lime-modified HMA at various asphalt contents were consistent with the functions obtained from the MHS algorithm.     

Discrete-Element Modeling: Impacts of Aggregate Sphericity, Orientation, and Angularity on Creep Stiffness of Idealized Asphalt Mixtures

Hot-mix asphalt (HMA) contains a significant amount of mineral aggregate, approximately 95% by weight and 85% by volume. The aggregate sphericity, orientation, and angularity are very important in determining HMA mechanical behaviors. The objective of this study is to investigate the isolated effects of the aggregate sphericity index, fractured faces, and orientation angles on the creep stiffness of HMA mixtures. The discrete-element method was employed to simulate creep compliance tests on idealized HMA mixtures. Two user-defined models were used to build 102 idealized asphalt-mix digital specimens. They were the R-model and the A-model, short for a user-defined rounded aggregate model and a user-defined angular aggregate model, respectively. Of the 102 digital specimens, 84 were prepared with the R-model to investigate the effects of aggregate sphericity and orientation, whereas the remaining 18 were built with the A-model to address the effect of aggregate angularity. A viscoelastic model was used to capture the interactions within the mix specimens. It was observed that (1) as the sphericity increased, the creep stiffness of the HMA mixture increased or decreased, depending on the angles of aggregate orientation; (2) as the angle of aggregate orientation increased, the creep stiffness of the HMA mixture increased, with the rate depending on the sphericity index values; and (3) compared with the sphericity index and orientation angles, the influence of aggregate fractured faces was insignificant.     

Modeling Viscoelastic Crack Growth in Hot-Mix Asphalt Concrete Mixtures Using a Disk-Shaped Compact Tension Test

The growth of viscoelastic cracks in hot-mix asphalt (HMA) concrete mixtures was simulated by using a disk-shaped compaction test. Modeling techniques, including the interconversion and continuous spectrum methods, the harmony search algorithm, and the state-variable approach, substantially enhanced the computational efficiency of the generalized J-integral. The generalized J-integral was used for determining the viscoelastic crack growth parameter on the basis of the fracture mechanics under cyclic loading conditions. This paper shows that the parameter can be used for simulating the viscoelastic crack growth in HMA concrete mixtures even under conditions of different loading magnitudes and frequencies.     

Prediction of Creep Stiffness of Asphalt Mixture with Micromechanical Finite-Element and Discrete-Element Models

This study presents micromechanical finite-element (FE) and discrete-element (DE) models for the prediction of viscoelastic creep stiffness of asphalt mixture. Asphalt mixture is composed of graded aggregates bound with mastic (asphalt mixed with fines and fine aggregates) and air voids. The two-dimensional (2D) microstructure of asphalt mixture was obtained by optically scanning the smoothly sawn surface of superpave gyratory compacted asphalt mixture specimens. For the FE method, the micromechanical model of asphalt mixture uses an equivalent lattice network structure whereby interparticle load transfer is simulated through an effective asphalt mastic zone. The ABAQUS FE model integrates a user material subroutine that combines continuum elements with viscoelastic properties for the effective asphalt mastic and rigid body elements for each aggregate. An incremental FE algorithm was employed in an ABAQUS user material model for the asphalt mastic to predict global viscoelastic behavior of asphalt mixture. In regard to the DE model, the outlines of aggregates were converted into polygons based on a 2D scanned mixture microstructure. The polygons were then mapped onto a sheet of uniformly sized disks, and the intrinsic and interface properties of the aggregates and mastic were assigned for the simulation. An experimental program was developed to measure the properties of sand mastic for simulation inputs. The laboratory measurements of the mixture creep stiffness were compared with FE and DE model predictions over a reduced time. The results indicated both methods were applicable for mixture creep stiffness prediction.     

Viscoelastic Model for Discrete Element Simulation of Asphalt Mixtures

This paper presents a viscoelastic model of asphalt mixtures with the discrete element method, where the viscoelastic behaviors of asphalt mastics (fine aggregates, fines, and asphalt binder) are represented by a Burger’s model. Aggregates are simulated with irregular shape particles consisting of balls bonded together by elastic contact models, and the interplaces between aggregates are filled with balls bonded with viscoelastic Burger’s model to represent asphalt mastic. Digital samples were prepared with the image analysis technique. The micromechanical model was developed with four constitutive laws to represent the interactions at contacts of discrete elements (balls) within an aggregate, within mastic, between an aggregate and mastic, and between two adjacent aggregates. Each of these constitutive laws consists of three parts: a stiffness model, a slip model, and a bonding model in order to provide a relationship between the contact force and relative displacement and also in order to describe slipping and tensile strength at a particular contact. The relationship between the microscale model input and macroscale material properties was derived, and an iterative procedure was developed to fit the dynamic modulus test data of asphalt mastic with Burger’s model. The favorable agreement between the discrete element prediction and the lab results on dynamic moduli and phase angles indicates that the viscoelastic discrete element model developed in this study is very capable of simulating constitutive behavior of asphalt mixtures.     

Continuum Microviscoelasticity Model for Aging Basic Creep of Early-Age Concrete

We propose a micromechanics model for aging basic creep of early-age concrete. Therefore, we formulate viscoelastic boundary value problems on two representative volume elements, one related to cement paste (composed of cement, water, hydrates, and air), and one related to concrete (composed of cement paste and aggregates). Homogenization of the “nonaging” elastic and viscoelastic properties of the material’s contituents involves the transformation of the aforementioned viscoelastic boundary value problems to the Laplace-Carson (LC) domain. There, formally elastic, classical self-consistent and Mori-Tanaka solutions are employed, leading to pointwisely defined LC-transformed tensorial creep and relaxation functions. Subsequently, the latter are back-transformed, by means of the Gaver-Wynn-Rho algorithm, into the time domain. Temporal derivatives of corresponding homogenized creep and relaxation tensors, evaluated for the current maturation state of the material (in terms of current volume fractions of cement, water, air, hydrates, and aggregates; being dependent on the hydration degree, as well as on the water-cement and aggregate-cement ratios) and for the current time period since loading of the hydrating composite material, allow for micromechanical prediction of the aging basic creep properties of early-age concrete.     

Determination of Shear and Bulk Moduli of Viscoelastic Solids from the Indirect Tension Creep Test

Because of its efficiency in analyzing complex viscoelastic problems, the finite-element (FE) analysis has been widely used to identify the time- and rate-dependent effects of viscoelastic materials on various structural conditions. When performing the FE analysis on a viscoelastic structure, most FE programs require fundamental material properties, shear and bulk moduli, of the given viscoelastic material as their input. However, the shear and bulk modulus tests are difficult to perform, so they have been commonly estimated from a single material test on the basis of the assumption that the Poisson’s ratio of viscoelastic materials is a time-independent constant. Such an assumption, however, might not be suitable because the Poisson’s ratio of the viscoelastic materials is also a function of time. Therefore, this study developed computation algorithms for determining the time-dependent Poisson’s ratio and shear and bulk moduli of asphalt mixtures, which have been well recognized as a viscoelastic material, by employing the indirect tension testing system. The shear and bulk moduli determined by the developed approach appear to be reasonable and realistic. Their applicability and reliability were also verified by comparing experimental data to the results of the FE analysis performed on the same circular specimen as that used in the indirect tension creep test.     

Computational Constitutive Model for Predicting Nonlinear Viscoelastic Damage and Fracture Failure of Asphalt Concrete Mixtures

A computational constitutive model was developed to predict damage and fracture failure of asphalt concrete mixtures. Complex heterogeneity and inelastic mechanical behavior are addressed by the model by using finite-element methods and elastic–viscoelastic constitutive relations. Damage evolution due to progressive cracking is represented by randomly oriented interface fracture, which is governed by a newly developed nonlinear viscoelastic cohesive zone model. Computational simulations demonstrate that damage evolution and failure of asphalt concrete mixtures is dependent on the mechanical properties of the mixture. This approach is suitable for the relative evaluation of asphalt concrete mixtures by simply employing material properties and fracture properties of mixture components rather than by performing expensive laboratory tests recursively, which are typically required for continuum damage mechanics modeling.     

Numerical Investigation of Creep Effects on FRP-Strengthened RC Beams

Numerical analysis using a finite-element model was performed to simulate and investigate the long-term behavior of two RC beams with similar steel reinforcement, cast from the same batch of concrete. One beam was a plain RC beam and the other beam was strengthened using carbon fiber-reinforced polymer (FRP) strips. The deflections of both beams have been monitored for 5 years after loading. The finite-element model included both creep of concrete and viscoelasticity of the epoxy adhesive at the concrete-carbon FRP (CFRP) interface. The results of the finite-element analysis are compared to experimental observations of the two beams. The finite-element analysis was found to be able to simulate the long-term behavior of the CFRP-strengthened beam and help us understand the complex changes in the stress state that occur over time.     

Cyclic Behavior of Asphalt Concrete Used as Impervious Core in Embankment Dams

Asphalt concrete is used as a water barrier (interior core or upstream facing) in embankment dams. This paper investigates the behavior of hydraulic asphalt specimens subjected to cyclic loading in a triaxial cell. The specimens were tested at various sustained static stress states and temperatures and at maximum cyclic shear stress levels corresponding to severe earthquake shaking of the dam. The cyclic modulus versus mean sustained static stress showed an approximately linear relationship in a logarithmic diagram, and an empirical expression was developed to determine the cyclic modulus. At a mean sustained stress of 1.0?MPa, the cyclic modulus at 20°C was about 900?MPa; at 9°C, it was 1900?MPa and at 3.5°C, about 2500?MPa. The damping ratio was found to be between 0.07–0.30, depending on stress state and temperature level. The number of load cycles (up to 6000) had no significant effect on the magnitude of cyclic strain, and the cyclic loading was documented to have little effect on the postcyclic monotonic stress-strain-strength behavior and permeability (watertightness) of the asphalt concrete.     

Simulation of Soil Behavior under Blast Loading

A viscoplastic cap model was previously developed to address the high strain rate effect on soil behaviors. Although the model is an improvement over the inviscid cap model, it does not update soil density and bulk modulus as the shock wave propagates through the soil. Further, soil should be modeled as a three-phase porous media to accommodate various degrees of water saturation. This is especially true for the soil mass surrounding the source of energy release because each of the three phases responds differently to shock loading. A revised cap model comprising a Gruneisen equation of state for each of the three phases has been developed. These equations of state for solid, water, and air have been integrated with the viscoplastic cap model to simulate behaviors of soil with different degrees of water saturation. Numerical results from this revised soil cap model compared closely with experimental data from explosive tests in both dry and saturated soil.     

Time-Dependent Reliability of PSC Box-Girder Bridge Considering Creep, Shrinkage, and Corrosion

Bridge performance undergoes time-varying changes when exposed to aggressive environments. While much work has been done on bridge reliability under corrosion, little is known about the effects of creep and shrinkage on the reliability of concrete bridges. In this paper, the CEB-FIP model for creep and shrinkage is applied by using finite-element (FE) analysis in conjunction with probabilistic considerations. Verification is made by comparing the analytical findings with existing test data. More specifically, a time-dependent reliability assessment is made for a composite prestressed concrete (PSC) box-girder bridge exposed to a chloride environment. This realized via an advanced probabilistic FE method. The postcracking behavior of the thin-walled box girder is described using composite degenerated shell elements, and importance sampling is used to improve the efficiency of the reliability analyses. It is shown that concrete creep and shrinkage dominate during the early stages of bridge structure deterioration. This is accompanied by a decrease in reliability owing to the combined action of creep, shrinkage, and corrosion. The reliability indexes for the serviceability and the tendon yielding limit state fall below the target levels prior to the expected service life. Therefore, early maintenance and/or repair measures are required.     

Conformable Tire Patch Loading for FRP Composite Bridge Deck

Fiber-reinforced polymer (FRP) composites are increasingly being used in bridge deck applications. However, there are currently only fledgling standards to design and characterize FRP deck systems. One area that should be addressed is the loading method for the FRP deck. It has been observed that the type of loading patch greatly influences the failure mode of a cellular FRP deck. The contact pressure distribution of a real truck loading is nonuniform with more concentration near the center of the contact area as a result of the conformable contact mechanics. Conversely, the conventional rectangular steel patch on a FRP deck act like a rigid flat punch and produces stress concentration near the edges. A proposed simulated tire patch has been examined for loading a cellular FRP deck with the load distribution characterized by a pressure sensitive film sensor and three-dimensional contact analysis using ANSYS. A loading profile is proposed as a design tool for analyzing FRP deck systems for strength and durability. Local top surface strains and displacements of the cellular FRP deck are found to be higher with proposed loading profile compared to those for the conventional uniformly distributed loading. Parametric studies on the deck geometry show that the global displacement criterion used for characterizing bridge deck is inadequate for a cellular FRP deck and that the local effects must be considered.     

Micromechanical Modeling of the Viscoelastic Behavior of Asphalt Mixtures Using the Discrete-Element Method

This paper presents a methodology for analyzing the viscoelastic response of asphalt mixtures using the discrete-element method (DEM). Two unmodified (neat) and seven modified binders were mixed with the same aggregate blend in order to prepare the nine hot mix asphalt (HMA) mixtures used in this study. The HMA microstructure was captured using images of vertically cut sections of specimens. The captured grayscale images were processed into black and white images representing the mastic and the aggregate phases, respectively. These microstructure images were used to represent the DEM model geometry. Rheological data for the nine binders were obtained using the dynamic shear rheometer. These data were used to estimate the parameters of the viscoelastic contact models that define the interaction among the mix constituents. The DEM models were subjected to sinusoidal loads similar to those applied in the simple performance test (SPT). The DEM model predictions compared favorably with the SPT measurements. However, the simulation results tended to overpredict the dynamic modulus, E*, for mixtures made with neat binders and underpredict E* for those that consisted of modified binders. The DEM models gave mix phase angles, ?mix, higher than the experimental measurements.     

Response of Foam- and Concrete-Filled Square Steel Tubes under Low-Velocity Impact Loading

This paper presents the results of experimental and numerical studies of the comparative behavior of square hollow section (SHS) tubes filled with rigid polyurethane foam (RPF) and concrete undergoing transverse impact loading. A series of instrumented drop hammer tests were performed on mild steel and stainless steel SHSs for both filled and unfilled constructions. The concrete-filled tubes had the highest impact resistance and energy absorption capacity, followed by the steel tubes filled with RPF, and then the hollow tubes. The results also show that RPFs can be used as an effective infill material in structural steel hollow columns when expedient enhancement of the energy absorption capacity is required, e.g., to increase blast and impact resistance of hollow structural elements. Nonlinear dynamic finite-element analyses were carried out to simulate drop hammer test conditions. The predicted impact forces, deformation histories, and failure modes were found to be in good agreement with the experimental results.     

Dynamic Response of Retrofitted Masonry Walls for Blast Loading

A full-scale blast test was conducted on eight masonry walls reinforced with two and four layers of carbon fibers and two types of polymer matrices. The walls were then subjected to a 0.45-kg pentolite booster suspended from the ceiling of a test structure. The pressure-time history caused by the blast and the resulting displacement response were measured during the test. This paper presents a summary of the test program and the corresponding results from a nonlinear single degree of freedom analysis. The results provide a basis for determining effective means of retrofitting existing masonry walls and designing new structures to withstand blast loads. The paper also outlines a fiber-reinforced polymer retrofit design procedure for walls subjected to blast loading.     

Flexural and Interfacial Behavior of FRP-Strengthened Reinforced Concrete Beams

Although there is a large amount of experimental data available on the fiber-reinforced polymer (FRP) strengthening of concrete structures, a full understanding of the various debonding phenomena is somewhat lacking. As a contribution to fill this need, two-dimensional and three-dimensional (3D) nonlinear displacement-controlled finite-element (FE) models are developed to investigate the flexural and FRP/concrete interfacial responses of FRP-strengthened reinforced concrete beams. Interface elements are used to simulate the FRP/concrete interfacial behavior before and after cracking. The analysis is carried out using two different relations for the interface; namely, nonlinear and bilinear bond–slip laws. The results predicted using these two laws are compared to those based on the full-bond assumption. The FE models are capable of simulating the various failure modes, including debonding of the FRP, either at the plate end or at intermediate cracks. The 3D model is created to accommodate cases of FRP-strengthened reinforced concrete beams utilizing FRP anchorage systems. In addition, the models successfully represent the actual interfacial behavior at the vicinities of cracks including the stress/slip concentrations and fluctuations. Results are presented in terms of the ultimate load carrying capacities, failure modes and deformational characteristics. Special emphasis is placed on the FRP/concrete interfacial behavior and cracking of the concrete. The numerical results are compared to available experimental data for 25 specimens categorized in six series, and they show a very good agreement.     

Effects of Loading Rate and Duration on Axial Behavior of Concrete Confined by Fiber-Reinforced Polymer Sheets

In this study, 18 concrete cylinder specimens were tested either under uniaxial compression at different loading rates or exposed to sustained axial stresses after being jacketed externally with carbon-fiber-reinforced polymer (CFRP) sheets. The specimens were cast using medium strength concrete. All the specimens had identical dimensions and level of confinement. Loading rate and applied sustained stress level were the main test parameters. Applied loading rate varied between 0.0002 and 0.04 strain/min. Four stress levels between 0.52 and 0.85fcc′ (0.90 and 1.46fco′) were used in short-term creep tests. Test results showed that the stress-strain behavior of CFRP confined concrete was influenced by the change in loading rate, and CFRP confinement provided considerable increase in the creep performance of concrete. The strength enhancement was more pronounced for specimens loaded at higher strain rates, while specimens loaded at slower strain rates exhibited better deformability. Results obtained from short-term monotonic loading tests were also compared with the results of two analytical approaches originally developed for plain concrete. None of the specimens failed during the short-term creep tests. However, the lifetime of the specimen, which was subjected to 0.85fcc′ (1.46fco′) sustained axial stress, was predicted as 20 days. Results of residual strength tests showed that specimens did not have any strength loss due to sustained loading.     

Block FEM for Time-Dependent Shear-Lag Behavior in Two I-Girder Composite Bridges

This paper presents a time-dependent finite-element analysis of a two I-girder composite bridge with a concrete slab. The creep and shrinkage of the concrete slab are considered as sources of time-dependent behavior. This analysis, unlike others, includes the shear-lag effect of the concrete slab on the time-dependent behavior of two I-girder bridges. An example calculation is given for a two-span continuous composite bridge with a cracking region in the concrete deck near the interior support. It is shown that the shear-lag effect becomes significant at the edge of the cracking region and at the bridge ends.     

Analysis of Dynamic Response of Architectural Glazing Subject to Blast Loading

The effect of blast loading on civilian structures has received much attention over the past several years. The behavior of architectural glazing is of particular interest owing to the disproportionate amount of damage often associated with the failure of this component in a blast situation. This paper presents the development of a simple yet accurate finite element-based tool for the analysis of architectural glazing subjected to blast loading. This has been achieved through the creation of a user-friendly computer program employing the explicit finite-element method to solve for the displacements and stresses in a pane of glass. Both monolithic and laminated panes have been considered, in single and insulated unit configurations, and employing several types of glass. In all cases, the pane of glass has been modeled as a plate supported by an array of boundary conditions that include spring supports, and two failure criteria are employed. Furthermore, the program is designed to predict the hazard level, given a particular glazing configuration and blast load.