<p> Recently, increased awareness of the significance of developing sustainable materials for construction has renewed the interest in exploring Alkali activated concrete (AAC), a concrete that contains no cement, but only industrial by-products such as fly ash and slag, as a low energy alternative to the conventional concrete. Although the feasibility of making alkali&ndash;activated concrete with acceptable strength and mechanical properties is well documented, the information regarding the long-term durability, including resistance to acid attack and alkali silica reaction (ASR), is far from comprehensive and there is a need to increase the understanding of these durability issues. In this dissertation, these durability issues are addressed, and improvements in this novel technology will increase acceptance in industry. This dissertation presents a comprehensive evaluation into the acid leaching resistance of Alkali-Activated Concrete (AAC) and Ordinary Portland Cement (OPC). The deterioration in AAC and OPC when exposed to different types of acid laden (organic and inorganic) environments are quantified by characterizing the strength degradation, mass change and visual appearances. The changes in microstructure development and chemical composition are examined and analyzed in order to determine the mechanism of deterioration. Additionally, the effect of the addition of nanoparticles on the mechanical properties and resistance to sulfuric leaching of Alkali Activated Slag concrete (AAS) are also explored in this study. </p><p> Furthermore, this dissertation summarizes the findings of an experimental evaluation of alkali silica reaction (ASR) in cement free alkali activated concrete (AAC). The susceptibility of AAC to deleterious ASR was evaluated in this study in accordance with relevant ASTM standards. This study also compares the resistance of AAC with ordinary portland cement concrete (OPC) while exposed to ASR under ASTM C 1293 and ASTM C1567 tests. In particular, the focus of this investigation is to assess the effectiveness of existing ASTM test methods in identifying the occurrence of ASR in alkali activated slag cement (AAS) concrete. In addition to that, influences of activator parameters including the effect of binder type, activator concentration, activator type and water content to the resistance of ASR in AAC were also evaluated. Finally, a scanning electron microscopic study coupled with EDX analyses was used to explain the mechanism of ASR occurrence in AAC and OPC.</p><p>

The micro-structural approach, which relates the mechanical behavior of a material to its micro-fabric and the properties of the constituent particles, is a more rational way of modeling the mechanics of granular materials. Within this approach is the numerical simulation method in the framework of the Discrete Element Method (DEM) of analysis. Instead of a continuum, DEM treats granular material as an assemblage of distinct particles, each governed by the laws of classical mechanics. Deformation analysis of inter-particle contacts does not imply continuity at particle boundaries. As this technique has evolved, it has been used in a wide variety of research applications in mechanics and Geotechnical engineering. However, there are some drawbacks to its use especially in the simulation and interpretation of real granular material behavior. Inadequate understanding of the micro-kinematics of particle rotation and contact rolling have rendered most DEM models ineffective in translating its usefulness to the overall study of the mechanics of granular assemblies. This study evaluated DEM analysis for the purpose of improving computer simulation models of granular materials in order to enhance the capability of predicting real granular behavior and its usefulness as an alternative to full-scale modeling. Implicit and explicit numerical integration algorithms are discussed on the basis of a generalized collocation formulation. In relation to DEM, it is shown that the explicit velocity Verlet method improves convergence, stability and accuracy. Using the concept of rolling friction, closed-form expressions were derived for contact rolling stiffness for both 2-D and 3-D problems. The developed DEM simulation model shows that the effects of rolling friction on the stress-strain behavior, shear strength and the development of shear bands are very significant. The study proves that simulation of granular media is greatly enhanced and the microstructure and micro-mechanisms are better revealed. Validation tests showed good agreement between DEM simulation results and available experimental tests on rod assemblies. Comparisons of heterogeneous deformation fields and the uniform strain fields indicate the need to incorporate a high gradient of strain theory in predicting the constitutive law of granular materials.

Metal foam is a relatively new and potentially revolutionary material that allows for components to be replaced with elements capable of large energy dissipation, or components to be stiffened with elements which will generate significant supplementary energy dissipation when buckling occurs. Metal foams provide a means to explore reconfiguring steel structures to mitigate cross-section buckling in many cases and dramatically increase energy dissipation in all cases. The microstructure of metal foams consists of solid and void phases. These voids have random shape and size. Therefore, randomness, which is introduced into metal foams during the manufacturing processes, creating more uncertainty in the behavior of metal foams compared to solid steel. Therefore, studying uncertainty in the performance metrics of structures which have metal foams is more crucial than for conventional structures. Therefore, in this study, structural application of metal foams considering material and geometrical uncertainty is presented. This study applies the Sobol’ decomposition of a function of many random variables to different problem in structural mechanics. First, the Sobol’ decomposition itself is reviewed and extended to cover the case in which the input random variables have Gaussian distribution. Then two examples are given for a polynomial function of 3 random variables and the collapse load of a two story frame. In the structural example, the Sobol’ decomposition is used to decompose the variance of the response, the collapse load, into contributions from the individual input variables. This decomposition reveals the relative importance of the individual member yield stresses in determining the collapse load of the frame. In applying the Sobol’ decomposition to this structural problem the following issues are addressed: calculation of the components of the Sobol’ decomposition by Monte Carlo simulation; the effect of input distribution on the Sobol’ decomposition; convergence of estimates of the Sobol’ decomposition with sample size using various sampling schemes; the possibility of model reduction guided by the results of the Sobol’ decomposition. For the rest of the study the different structural applications of metal foam is investigated. In the first application, it is shown that metal foams have the potential to serve as hysteric dampers in the braces of braced building frames. Using metal foams in the structural braces decreases different dynamic responses such as roof drift, base shear and maximum moment in the columns. Optimum metal foam strengths are different for different earthquakes. In order to use metal foam in the structural braces, metal foams need to have stable cyclic response which might be achievable for metal foams with high relative density. The second application is to improve strength and ductility of a steel tube by filling it with steel foam. Steel tube beams and columns are able to provide significant strength for structures. They have an efficient shape with large second moment of inertia which leads to light elements with high bending strength. Steel foams with high strength to weight ratio are used to fill the steel tube to improves its mechanical behavior. The linear eigenvalue and plastic collapse finite element (FE) analysis are performed on steel foam filled tube under pure compression and three point bending simulation. It is shown that foam improves the maximum strength and the ability of energy absorption of the steel tubes significantly. Different configurations with different volume of steel foam and composite behavior are investigated. It is demonstrated that there are some optimum configurations with more efficient behavior. If composite action between steel foam and steel increases, the strength of the element will improve due to the change of the failure mode from local buckling to yielding. Moreover, the Sobol’ decomposition is used to investigate uncertainty in the strength and ductility of the composite tube, including the sensitivity of the strength to input parameters such as the foam density, tube wall thickness, steel properties etc. Monte Carlo simulation is performed on aluminum foam filled tubes under three point bending conditions. The simulation method is nonlinear finite element analysis. Results show that the steel foam properties have a greater effect on ductility of the steel foam filled tube than its strength. Moreover, flexural strength is more sensitive to steel properties than to aluminum foam properties. Finally, the properties of hypothetical structural steel foam C-channels foamed are investigated via simulations. In thin-walled structural members, stability of the walls is the primary driver of structural limit states. Moreover, having a light weight is one of the main advantages of the thin-walled structural members. Therefore, thin-walled structural members made of steel foam exhibit improved strength while maintaining their low weight. Linear eigenvalue, finite strip method (FSM) and plastic collapse FE analysis is used to evaluate the strength and ductility of steel foam C-channels under uniform compression and bending. It is found that replacing steel walls of the C-channel with steel foam walls increases the local buckling resistance and decreases the global buckling resistance of the C-channel. By using the Sobol’ decomposition, an optimum configuration for the variable density steel foam C-channel can be found. For high relative density, replacing solid steel of the lips and flange elements with steel foam increases the buckling strength. On the other hand, for low relative density replacing solid steel of the lips and flange elements with steel foam deceases the buckling strength. Moreover, it is shown that buckling strength of the steel foam C-channel is sensitive to the second order Sobol’ indices. In summary, it is shown in this research that the metal foams have a great potential to improve different types of structural responses, and there are many promising application for metal foam in civil structures.

A comprehensive review was made of the design and construction of flexible metal and rigid reinforced concrete large-span culverts, past documented field experience of monitored culvert performance, and culvert failures. Full-scale field testing of a flexible metal and a reinforced concrete large-span culvert was conducted and the results compared with finite element computer analyses. Based on this work recommendations for improved design and construction of large-span culverts were developed. The review of metal culvert design and construction practice revealed numerous differences among current methods as well as deficiencies. Proposed design limit states were identified and discussed for improved practice. An improved earth load thrust prediction model was developed based on past analytical work considering the flexural rigidity of the structure relative to the surrounding soil, in addition to other factors. The design curves for arching factors were extended to cover a wider range of structural backfill width conditions and shallower burial. Also, a proposed construction procedure was outlined to control construction moments based on deflection limits as a function of the expected level of construction control. None of the existing methods explicitly deals with large-span reinforced concrete culvert design and construction practice. Therefore, a proposed design approach for these culverts was outlined. Construction practice was based on recommendations from the manufacturers. The review of failure cases showed that most failures of large-span metal culverts occurred as a result of poor backfill procedures and/or poor backfill material selection. Other causes were excessive construction loads and invert uplift. Excessive deformation was the most common limit state reached before or at failure. Furthermore, significant variations in structural response may occur over time after construction. Therefore, better construction provisions and control are needed, coupled with consideration of flexural stiffness and moment capacity in design. The field tests showed significant differences in structural behavior between backfilling and live load testing. The metal structure was successfully subjected to very heavy live loads at shallow cover conditions without the use of thrust beams or ribs (current practice). Finite element computer analyses of the two tests showed that earth load responses could be reasonably modeled; however, live load predictions were poor.

<p> This study develops a novel sandwich composite material using plant based materials for potential use in nonstructural building applications. The face sheets comprise woven hemp fabric and a sap based epoxy, while the core comprises castor oil based foam with waste rice hulls as reinforcement. Mechanical properties of the individual materials are tested in uniaxial compression and tension for the foam and hemp, respectively. The sandwich composite is tested in 3 point bending. Flexural results are compared to a finite element model developed in the commercial software Abaqus, and the validated model is then used to investigate alternate sandwich geometries. Sandwich model responses are compared to existing standards for nonstructural building panels, showing that the novel material is roughly half the strength of equally thick drywall. When space limitations are not an issue, a double thickness sandwich biocomposite is found to be a structurally acceptable replacement for standard gypsum drywall.</p>

<p> Externally bonded fiber-reinforced polymer (FRP) composites represent a simple and economical solution for many repair and strengthening applications in concrete structures. However, the potential occurrence of sudden and brittle debonding failure in such repairs becomes prominent when FRP-concrete bond undergoes environmental degradation induced by moisture. Ambient-cured low-viscosity Bisphenol A epoxy adhesives are most commonly utilized in the engineering practice to bond wet-layup FRP to the concrete substrate. This study aims to elucidate the effects of Bisphenol A-based epoxy modified with commercial surface-modified nanosilica (SMNS), core-shell rubber (CSR) nanoparticles and multi-walled carbon nanotubes (MWCNT) on the improvement of mechanical properties of the epoxy adhesives, and strength and durability of FRP-concrete adhesively bonded joints. Moisture ingress in epoxy, DSC, tensile test on epoxy and three-point bending beam bond tests were performed. To determine the effects of environmental degradation, all specimens were subjected to the following environments: control&mdash;23 &deg;C at RH 50 &plusmn; 10% for 18 weeks; and accelerated conditioning protocol (ACP)&mdash;water immersion at 45 &plusmn; 1 &deg;C for 18 weeks. Improvement in mechanical properties were observed in dogbone specimens modified with nanoparticles without any reduction in glass transition temperature (Tg). In control conditions, nanomodified epoxy groups exhibited enhanced mechanical properties compared to the neat epoxy. Following ACP, strength, elongation and modulus of elasticity of neat epoxy deteriorated significantly, while no significant deterioration was observed in the nanomodified group of adhesives. Among all the nanomodified adhesive groups CSR Type-1 showed most improvement in mechanical properties over neat epoxy group both in control condition and in ACP. CSR-modified adhesive joints experienced practically no degradation when subjected to ACP and showed the highest maximum bond strength retention of 100% among all the adhesive groups. The bond strength of neat epoxy adhesive joints degraded most dramatically (15%) following ACP.</p><p>

<p> Cement based brittle matrix composites that show deflection hardening called high performance fiber reinforced cementitious composites (HPFRCC) have the potential of offering high resiliency and environmentally sustainable benefits in numerous applications. However, more information is needed to fully understand, predict the behavior, and add functionality to HPFRCCs. This experimental research program aims to develop and characterize a new type of HPFRCC. This new HPFRCC is composed of polyvinyl alcohol (PVA) microfibers, carbon nanofibers (CNF), and a high volume of fly ash (HVA) to form a self-consolidating and self-sensing HPRFCC. The multi-functionality of the CNFs allow for increased mechanical properties and strain and damage sensing capabilities. The hybrid fiber reinforced cement composite developed is environmentally sound, due to the large amounts of recycled fly ash, with enhanced stiffness, and tensile strain capacity. This research considers the determination of fresh properties, hardened mechanical properties (elastic modulus, first cracking stress, ultimate stress, and maximum plastic strain), and electrical conductivity of the composite in response to strain, which is measured simultaneously through uniaxial tension tests. Digital Image Correlation (DIC) is used extensively to capture the tensile strain and provide a visualization of the behavior of the composite under increasing displacements. Results from this research program provide a preliminary understanding of the behavior of CNF HPFRCCs, which will aid in future research of similar composites. A standard mixing procedure is established that can be adopted in large scale processing of CNF HPFRCC. Increased mechanical properties and damage detection offers engineers with the ability to quantify structural health and optimize designs. The use of multi-functional self-sensing HPFRCCs is a step towards providing the public with a resilient and sustainable infrastructure for their communities.</p>

<p> This thesis consists of two topics: Thermal strain of concrete under low temperatures, and durability and processing techniques of concrete with CNTs. The low temperature test studies the anti-freezing action of concrete under cooling environment. Concrete mixes using different amounts of air-entraining agent and water/cement ratios were made and cured under different humidity environments. During the cooling process from 10 &ordm;C to -25 &ordm;C, the strain of concrete was measured every 3~5 &ordm;C. The strain-temperature curve of concrete under different mixing proportion was produced from these results. A numerical model was developed based on the theory of the self-consistent model. No knowledge of real pore shapes is needed to apply in the model. The only inputs for the model came from the (Brunauer-Emmett-Teller) BET test, which gave the pore size distribution of concrete sample. The validity of the numerical model was compared to the experimental results, and showed similarity in trend and peak strain. CNT is one of the most popular topics in engineering. CNT has an extremely high strength and Young's modulus. CNT is a nano-scale material, however, and tends to clump together, which makes it difficult to apply. Other research has successfully incorporated CNT into cement paste and polymer materials. This has not yet been done into concrete. This research mainly focuses on the important factors that must be solved to adopt CNT in concrete area. An ultrasonicator was used to aid the dispersion and distribution of CNT in water, while several chemicals were also adopted for this purpose. Both strength and durability were tested for CNT concrete of different mix designs. It is suggested that ultrasonicator can improve the strength of pure CNT concrete (without chemicals) by around 100%. In addition, the sodium polyacrylate treated CNT concrete has showed best durability result and good strength result. </p>

<p> The objective of this research effort focused on the evaluation of asphalt mixtures with respect to thermal cracking. Preliminary investigations soon indicated that a fundamental evaluation of thermal cracking was highly dependent upon the more complicated understanding of asphalt binder oxidation. The oxidation of asphalt binders within an asphalt mixture were understood to potentially be influenced by the mixture characteristics (i.e. air void levels, binder content, etc.) and aggregate properties (i.e. aggregate absorption, gradation, etc.). Therefore, this study was conducted in order to investigate and quantify the effects different aggregate sources and mixture properties may have on the oxidation and thermal cracking performance of asphalt mixtures. </p><p> The investigation specifically focused on quantifying the oxidation of the asphalt binder alone and as part of the asphalt mixture when subjected to isothermal oven aging. The oxidation parameters of pan-aged asphalt binders were quantified, according to the standard of practice in the industry. These parameters were then compared to extracted and recovered mixture-aged asphalt binders to examine the influence of the main aggregate and mixture factors on the binder oxidation. The study observed differences between the pan-aged and mixture-aged asphalt binders in terms of oxidation kinetics, rheological measures, and the combined effect represented as the hardening susceptibility. </p><p> Further evaluation of the binder oxidation based upon the dynamic modulus measures indicated marked influences of the mixture characteristics, the individual component materials, and the interactions between the investigated factors. </p><p> Differentiation of the experimental factors was further identified by the newly developed low-temperature evaluation method, Uniaxial Thermal Stress and Strain Test (UTSST). The UTSST provides a fundamental approach to characterize the thermo-viscoelastic properties of asphalt mixtures permitting the pragmatic evaluation of changes in the stiffness and overall behavior of mixtures as a function of oxidative aging. Five distinct stages in the UTSST modulus were identified as thermo-viscoelastic properties, which are identified as a function of temperature: viscous softening, viscous-glassy transition, glassy hardening, crack initiation, and fracture stages. </p><p> Through consideration of the thermo-viscoelastic properties, marked differences in the binder oxidation were noted between the experimental factors. Typically, decreases in the viscous response of the mixtures as well as increases in both the stiffness and brittle behavior were observed with aging. The evaluation method provides definitive measures to monitor multiple aspects of the performance of asphalt mixtures subjected to thermal loading.</p>

Thesis (Ph.D.)--Lehigh University, 2007.