Theses and Dissertations

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Characterization and modeling of long-term behavior of frp-to-concrete interface in aggressive environments
(University of Alabama Libraries, 2015) Amidi, Shahrooz; Wang, Jialai; University of Alabama Tuscaloosa
Fiber reinforced plastics (FRP) composites have emerged as one the foremost structural materials in retrofit/rehabilitation of concrete structural members in last decades. The long-term durability of the FRP-to-concrete interface in aggressive environments places a critical role in the success of this technique. A comprehensive program using both the analytical and experimental approaches has been carried out in this study to examine the integrity and long-term durability of the FRP-to-concrete interface in presence of aggressive environments. Novel analytical solutions based on three-parameter elastic/viscoelastic foundation models have been developed for adhesively bonded joints first in this study to gain better understanding of stress transfer through FRP-to-concrete interface. These models overcome drawbacks in existing models by satisfying all boundary conditions and producing different peel stress distributions along two adhesive-adherend interfaces, making it possible to accurately predict the location of debonding initiation. These models have been verified with finite element analysis and experimental observations. Comprehensive experimental programs have been carried out to evaluate the deterioration of the FRP-to-concrete induced by moisture. In the first part, a novel environment-assisted subcritical debonding method using a wedge driving test has been proposed to examine the synergistic effect of the mechanical loads and environmental conditions on the deterioration of the FRP-to concrete interface. The deterioration of the interface induced by water has also been evaluated through measuring the residual fracture toughness of the FRP-to-concrete interface conditioned in water through two different ways. It has been found that conditioning method can have significant effect on the testing results. A novel wedge-split test has been proposed and carried out to directly measure the traction-separation law of the epoxy-concrete interfaces under mode I loading, which is not available in the literature. The potential of using silane coupling agent to improve the durability of the FRP-to-concrete interface has also been examined in the experimental program. Testing results confirm that the residual fracture toughness of the FRP-to-concrete interface attacked by moisture can be significantly increased by silane treatment.
Characterization of reinforced structural composites with carbon nanotubes grown directly on the fibers/fabrics using the Poptube Approach
(University of Alabama Libraries, 2017) Guin, William Edward; Wang, Jialai; University of Alabama Tuscaloosa
Carbon nanotubes (CNTs) are ideal candidates for the reinforcement of the matrix and interphase zone in polymer matrix composites (PMCs), due to their ability to more effectively bind the reinforcing fibers to the matrix material. This can lead to the enhancement of several critical composite properties – including interfacial shear strength and interlaminar fracture toughness – that are typically associated with a composite material’s resistance to delamination. Direct dispersion of CNTs into the matrix of the composites has been shown to be very difficult. A more effective way to reinforce PMCs using CNTs is to grow CNTs directly on the reinforcing fibers. To this end, a novel technique used to grow CNTs directly on carbon fibers has been developed at The University of Alabama and Auburn University. This method, referred to as the PopTube Approach, uses microwave irradiation to grow CNTs at room temperature in air, without the need for inert gas protection or additional feed stock gases. The simple nature of the PopTube Approach lends itself to large-scale, high-yield manufacturing that can be done in a cost effective manner. However, before this technique is developed beyond the laboratory scale, its effectiveness as a route to produce CNT-reinforced composites must be evaluated in a comprehensive manner. The objective of this work is to do just that – characterize the mechanical properties of CNT-reinforced composites produced via the PopTube Approach. A systematic experimental program is carried out to provide a comprehensive assessment of the effects of the PopTube Approach on a wide range of composite mechanical properties. Results show that the PopTube Approach provides for enhanced resistance to delamination with respect to several different loading events. Fractography studies are used to qualitatively understand the mechanisms responsible for these improvements in delamination resistance on the micro-scale. Results also suggest that improvements in delamination resistance via CNT reinforcement may come at the expense of the tensile properties of PMCs – which gives rise to the conclusion that in practice, the degree and manner of CNT reinforcement in PMCs should be carefully considered on an application-by-application basis. Together, the collection of studies performed herein provides a wide-ranging quantitative and qualitative assessment of the effects of the PopTube Approach CNT reinforcement scheme on the mechanical properties and behavior of polymer matrix composites.
Characterization of Ultra-High Performance Concrete Tension Behavior Using Different Test Methods
(University of Alabama Libraries, 2024) Mendes Rodrigues Farias Mello, Eunice; Aaleti, Sriram
Ultra-high performance concrete (UHPC) is a relatively new type of cementitious construction material known for its superior mechanical and durability properties, which has contributed to its increasing popularity and usage in the construction industry. Unlike conventional concrete, UHPC possesses a sustained post-cracking tensile strength resulting from the presence of discrete steel fibers in its composition. Furthermore, it exhibits exceptional compressive strength and durability. These remarkable properties are a result of the optimized gradation of cementitious materials, chemical admixtures, and steel fibers that compose UHPC. However, the standard test methodologies for accurately describing the tensile behavior of UHPC are still in the process of being developed and standardized. Direct tension (DT) and four-point bending (4PB) test methods are two commonly used methods in research and practice to assess the tension behavior of fiber-reinforced concrete. This thesis work provides a comprehensive analysis of the experimental investigation, test methodologies, calculations, and the correlation between the tension behavior computed from both test methods. For this, a total of 50 UHPC specimens were constructed, including 10-2 in. square prisms for direct-tension tests and 40 four-point bending beam specimens with different cross-sections. The experimentally obtained moment vs. curvature responses from four-point bending tests were used to calculate the tensile stress-strain response of UHPC. These results were then compared to the ones obtained experimentally from direct tension tests. Additionally, to investigate the impact of beam size on the measured tension behavior in the four-point bending method, four different sizes of specimens were fabricated for testing. It was observed that the UHPC tension stress-strain behavior obtained from the inverse analysis of 4PB tests and direct tension test methods were similar. The average tension behavior obtained from 2 by 4 in. bending specimens especially matched closely with results from direct tension tests. Ultimately, all 4PB and DT beams were cut into evenly spaced cuboids to examine differences in the number of fibers in the cross-section, dispersion, and orientation throughout the length of the beam. To also analyze the fibers present in the cross-section of the failure region, the specimens were cut around the location of critical crack propagation. The observed variability in the measured tensile strength can then be explained by the fiber count and orientation factor.
Design and analysis of a carbon composite flap for the Cirrus SF50 jet aircraft
(University of Alabama Libraries, 2017) Haefner, Andrew; Mulani, Sameer B.; University of Alabama Tuscaloosa
A new carbon fiber composite flap was designed and analyzed for the Cirrus SF50. This new flap will replace the existing aluminum flap and has the potential to save 5.30 lbs per aircraft. The new flap has the same OML profile as the existing flap and the same hinge locations. This allows the new flap to be either an upgrade option for customers or a supplemental type certificate (STC) option for aircraft in the field. The flap was designed with the same spar location and similar rib locations which allow existing tooling to be used for assembly. The design was analyzed to the static and damage tolerance requirements specified in 14 CFR Part 23. The loads that were utilized for the analysis were calculated using the method in 14 CFR Part 23 Appendix A. The loads are conservative since they consider a load factor of 3.6 instead of 2.2, this was done to make the design and analysis future proof. Since a significant portion of the structure uses minimum gauge layups (2 core 3, 4 ply solid), the weight increase from using the significantly higher load factor is minimal. The flap design and analysis are considered future proof because the loads used will be greater than the required loads if the SF50 were to have either a gross weight increase, a deployment speed increase, a deployment angle increase, or all in combination.
Development and Experimental Testing of Ultrahigh-Performance Concrete Pile Foundation Elements
(University of Alabama Libraries, 2023) Willis, Gabrielle; Aaleti, Sriram
Many aging bridges in the United States, built in the 1960s with a 50-year design lifespan, are deteriorating. Recent research seeks to improve construction materials and methods in order to extend bridge lifespans, reduce repair requirements, and retrofit structurally deficient elements. Driven pile foundations are critical to a structure's long-term stability and safety but can be damaged during installation or deteriorate over time due to harsh environmental conditions. Repairing or replacing these systems can be difficult and expensive.Ultra-High-Performance Concrete (UHPC) is an advanced cementitious material with superior mechanical and durability properties when compared to standard concrete. The problems associated with driven pile foundations can be alleviated by designing a pile foundation system using UHPC. This study aims to create a viable UHPC foundation system as an alternative to traditional steel and concrete driven piles.As an alternative to conventional HP steel piles, two H-shaped 12-in. and 14-in. deep UHPC pile sections and a 16-in. H-shaped section as an alternative to 16-in. square prestressed pile were designed. Full-scale piles were cast and tested under various flexural and shear loading conditions. The results show that these piles are viable alternatives to the standard piles that are currently used, and that current first principles methods adequately capture their behavior.To ensure that these piles can be used as field applications as part of a bridge structural system, two other aspects of foundation elements were investigated: interface shear behavior and splice design. The shear behavior along UHPC-UHPC interfaces is crucial for ensuring that UHPC piles embedded in UHPC pile caps or abutments will perform adequately. Over 100 small-scale specimens with varying low-amplitude texture morphologies were constructed and evaluated for this purpose. It was determined that current code specifications should account for texture spacing.A construction-friendly splice design similar to the current standard practice for steel HP piles was developed for field implementation of UHPC piles. Full-scale splice design, construction, and testing under flexure, shear, and tension conditions. According to the findings, the splice can provide at least 60% of the strong-axis flexural capacity. To achieve full moment capacity, the splice must be lengthened.
Efficient algorithms for uncertainty quantification using polynomial chaos expansion and its applications to composite structures
(University of Alabama Libraries, 2019) Thapa, Mishal; Mulani, Sameer B.; University of Alabama Tuscaloosa
Uncertainty Quantification (UQ) deals with the study of variation in the response due to the presence of uncertainties in input parameters and governing models. Among the prevalent probabilistic techniques for UQ, non-intrusive Polynomial Chaos Expansion (PCE) has become more popular recently due to its mean square convergence property and ability to integrate deterministic codes as black-box. However, the number of basis terms in the expansion increases exponentially with the number of random inputs - ‘curse of dimensionality,’ and demands a huge number of function evaluations. Hence, this dissertation has attempted to extensively explore new robust algorithms for PCE while maintaining a proper balance between accuracy and computational efficiency. At first, a new non-intrusive method for PCE called Polynomial Chaos Decomposition with Differentiation (PCDD) is developed. The PCDD utilizes higher-order sensitivities of the responses and requires samples equal to the number of basis terms only. Secondly, the PCDD is utilized to develop a stochastic multi-scale modeling framework for composite structures since the response of composites is hugely influenced by the uncertainties existing at different scales such as micro-scale and macro-scale. Another framework for stochastic progressive failure analysis (PFA) of composites is also developed that allows performing global sensitivity analysis (GSA) to identify the relative importance of random inputs as a post-processing step. To further reduce the number of samples and make the stochastic problem more tractable, an adaptive L2-minimization algorithm that allows basis adaptivity along with sequential adaptive sampling is developed. Finally, an adaptive algorithm to obtain sparse PCE models with L1-minimization and sequential sampling is also proposed for high-dimensional problems. The L1-minimization is capable of solving the under-determined system when the number of samples is minuscule. It is also advantageous in terms of computational storage and memory because of its ability to provide a sparse solution. In general, the overarching goal of obtaining high-fidelity stochastic response models while maintaining a balance between accuracy and computational cost was successfully achieved by the novel algorithms developed in this dissertation. Furthermore, the invaluable information obtained with PCE for composite structures highlighted the benefits of its implementation in engineering problems.
End zone design for Alabama deep prestressed girders
(University of Alabama Libraries, 2016) Burkhalter, David; Aaleti, Sriram; Song, Wei; University of Alabama Tuscaloosa
Deep prestressed concrete bridge girders are becoming increasingly popular due to their ability to span longer distances and reduce the total cost of bridge projects. However, these girders have frequently been subject to end zone cracking during the transfer of prestress forces despite being designed to current AASHTO specifications. Previously, the Alabama Department of Transportation (ALDOT) has designed deep prestressed girders which can span up to 165 ft. During the fabrication of these girders, crack formations in the end zone were typically noticed. To address this concern, longitudinal reinforcement was added to the end zones. This solution controlled cracking to some extent but could not completely eliminate cracking. An experimental study was conducted to find a practical engineering solution to the problem of end zone cracking, as well as to develop a 78 in. deep prestressed bulb-tee girder design to reach a span length of 180 ft. 3D finite element modeling was used to find three practical alternative end zone modifications to the standard design. The modified designs included a lowered draping angle, partial debonding of the strands, and a combination of the two. Four 54 ft. long specimens, including three with end zone modifications, were fabricated at Hanson Pipe & Precast in Pelham, Alabama, and monitored during the detensioning process. The end zones were instrumented with steel and concrete strain gauges to better understand the complex behavior of girder end zones. External DEMEC instrumentation was also included at the girder ends to measure the transfer length of the strands in each specimen. The specimens were then load tested at the UA Large Scale Structures Laboratory (LSSL) to determine the effects of the modified end zone details on the girder capacity. Based on the study, modified girder end zone details are recommended to ALDOT for implementation.
Experimental and FEA Study of Structural Behavior of CLT Shear Walls for Wood Building Applications
(University of Alabama Libraries, 2021) Chowdhury, Farhan A.; Dao, Thang N.; Aaleti, Sriram; University of Alabama Tuscaloosa
Cross Laminated Timber (CLT) is emerging as a sustainable alternative to traditional building construction materials for tall buildings in high seismic regions. However, before they can be incorporated into the building codes, different categories of CLT shear wall systems need to be fully characterized and novel solutions must be implemented to overcome the limitations of existing timber systems. This dissertation addresses analytical modelling of unbonded post-tensioned (PT) CLT rocking wall (CLTRW) system for its use in both rectangular and nonrectangular configurations. A set of laboratory experimental tests and nonlinear finite element analysis (FEA) were compared to investigate and characterize the flexural and lateral behavior of CLTRW system. The flexural performance was studied using a full-scale, 5-layer rectangular CLT panel testing in three-point loading configuration. A three-dimensional (3D) finite element (FE) model with nonlinear geometry and contact properties was developed in ABAQUS using measured material properties. The FE model was validated using the experimental data from the CLT panel bending test. Based on the validated FE model, additional FE models of previously tested post-tensioned CLTRW panels with different wall aspect ratios and tendon configurations were also developed. A close agreement between the measured and FEA models was found in the stiffness behavior, which suggested that the proposed nonlinear FE modelling approach can be employed to predict the performance of other CLTRW systems. This research further analytically investigates shear behavior of high stiffness web-to-flange connections for nonrectangular CLTRW system. The proposed shear connections use spatially arranged self-tapping screws (STS) together with ultra-high-performance concrete (UHPC) shear-keys. 3-D nonlinear FE models for connections with and without UHPC were validated using the available experimental data, which showed that the inclusion of UHPC would significantly improve the stiffness of the connections. The FEM technique for connections was subsequently used to evaluate the lateral load performance of a 36-ft tall, T-shape CLTRW wall with multiple connections. The results from this model were compared to a T-wall without shear-key connections. It was found that the wall with high-stiffness shear-key connections showed better lateral load performance compared to traditional connection wall.
Experimental investigation of long term and lateral load behavior of CLT shear walls for mid-rise wood buildings
(University of Alabama Libraries, 2019) Hossain, Md Kobir; Aaleti, Sriram; University of Alabama Tuscaloosa
There is a recognized need for tall building (8-20 story) construction in the United States due to growing population in the urban areas. In addition, there is a significant emphasis placed by the communities on sustainability. Wood is a sustainable construction material with a negative carbon footprint in comparison to steel and concrete, which are the traditional building materials used predominantly in tall buildings. Traditional Light Wood Frame Shear Wall system used in residential building construction in the U.S. cannot be used to construction tall building as it fails to provide necessary strength and stiffness. Mass timber panel such as Cross-Laminated Timber has been recognized as a promising building construction material in recent times and has been used in hundreds of building mostly in low seismic regions. However, to realize an all wood tall building in high seismic areas, lateral load resisting system (LLRS) using CLT needs to be developed and characterized before its implementation into building construction. This research focusses on addressing some of the issues in developing a robust LLRS using unbonded post-tensioned rocking CLT wall system for high seismic application. The parameters that affect the behavior of this system such as the compression, moisture diffusion and creep behavior of CLT material were studied by conducting laboratory testing. The performance of CLT rocking wall system was investigated through laboratory testing of four full scale specimens using different wall dimensions and detailing to gain a thorough understanding of this system behavior under reverse cyclic loading. The results show that this system can provide full recentering up to 3% drift with limited sustained damage at the rocking toes and limited energy dissipation capability. The rocking wall system can be designed as a robust LLRS but can be further improved by incorporating external energy dissipating elements into the system. To improve the system performance by including damping in the rocking wall system, o-connector and LiFS are used to connect two rocking walls. The coupled walls still provide the recentering while reduces the seismic displacement demand resulting from higher damping. Tests and finite element analysis of o-connectors were carried to understand its force-displacement behavior and energy dissipating capacity under reverse cyclic load. Design equations based on the test and FEA results are proposed. Laboratory tests were conducted on two CLT-LiFS hybrid walls in addition to component level tests on LiFS and CLT-LiFS connection. A load transfer mechanism in CLT-LiFS hybrid wall is proposed and used with a simplified calculation procedure to predict the force-displacement behavior of hybrid wall. The study shows that the analysis procedure predicts force capacity within 20% of the test results and can be conservatively used for practice. The test results show that the hybrid wall system has improved energy dissipation capacity while providing almost full recentering at 4% drift. The CLT building performance can be improved and the cost associated with LLRS may be reduced by taking into account the beneficial effect of non-rectangular shear walls (such as T-wall, I-wall). Non-rectangular walls can be achieved by connecting them at web-to-flange interface with high stiffness connections. Also, a high stiffness wall-to-foundation connection which can transfer the high base shear to foundation needs to be developed. Two connections, one for web-to-flange interface and another for wall-to-foundation interface, is developed by using grouted shear key incorporating ultra-high performance concrete and self-tapping screw. Laboratory tests on these two connection show that the connections have very high stiffness (4 times) compared to traditional bracket type ones and they have high strength as high as 3 to 4 times.
Experimental Investigation of Ultra-High-Performance Concrete Panels Under Tornado Impact Loads
(University of Alabama Libraries, 2021) Kniffin, Hannah Rose; Aaleti, Sriram; University of Alabama Tuscaloosa
Tornado events pose a threat to millions of people living in the tornadic-prone areas of the United States. Although many tornado shelters and safe rooms are commercially available that satisfy the extreme loading conditions required by the International Code Council and National Storm Shelter Association, there is a need for a simple yet safe design which can be easily assembled and used for multiple purposes. New engineering materials, such as ultra-highperformance concrete (UHPC), have the potential to improve tornado shelter options and save lives. This study experimentally investigates the performance of thin UHPC panels subjected to impact of standard wood 2x4 projectiles, following the requirements of ICC/NSSA 500, the leading standard on storm shelter design. 1.25-inch-thick and 1.625-inch-thick UHPC panels were cast and impacted with 15-lb wood projectiles at speeds ranging from 50 mph to 100 mph to maintain a similar impact-energy-to-panel-mass ratio. The failure response of each panel was characterized by excessive flexural deflection or punching shear. In the case of excessive deflection, a single-degree-of-freedom dynamic displacement model describes the motion of the panel during impact and the profile of the maximum deflection. In the case of punching shear, a modified equation from ACI 318 predicts the capacity of the panel. The results of the impact testing show UHPC is a promising material for future tornado shelters: UHPC panels with half the thickness of a traditional concrete shelter can be built for a similar or lower price, creatively integrated into homes, and increase accessibility of the tornado shelter for residents.
Hybrid structures using ultra high performance concrete and normal concrete for bridge applications
(University of Alabama Libraries, 2018) Ronanki, Vidya Sagar; Aaleti, Sriram; University of Alabama Tuscaloosa
The aging transportation infrastructure problem coupled with rapidly increasing traffic volumes and tightening budgets necessitates the need for cost effective and durable bridge components which can be easily implemented using current construction techniques. These solutions must also be suitable for accelerated construction in order to ensure minimum disruption to existing traffic. In this regard, Ultra High Performance Concrete (UHPC), a highly engineered cementitious material with enhanced mechanical properties lends its self as an ideal material. UHPC cost is nearly 30 times the traditional concrete, making full UHPC structures uneconomical. Through this multipart research, the emerging UHPC material and the traditional normal concrete are optimally combined to exploit both their beneficial features and yield new economical hybrid bridge components. The rebar development length in UHPC was experimentally investigated using pull out and beam specimens with lap splices. The results from these tests add significant new data on the bond stress distribution for rebar embedded in UHPC and a simplified design equation is proposed. An embedment length of 8 db (db -diameter of rebar) in UHPC with 3db clear cover was found to be sufficient to yield a Grade 60 mild steel reinforcement. A hybrid prestressed girder concept utilizing UHPC in the end zones of the girder with normal concrete in the remainder of the girder was proposed for a long-span girder with existing shapes . The endzone and shear behavior of a deep prestressed girder was investigated experimentally and analytically using four 78 in. deep, normal concrete bulb-tee (BT-78) girders. A detailed finite element (FE) model was developed in ABAQUS and calibrated using the experimental data. UHPC-NC interface behavior under direct shear and flexural loading was also experimentally investigated using direct shear testing of small-scale interface samples and flexural testing of UHPC-NC beams. A detailed finite element model for the interface was developed in ATENA and calibrated using the experimental results. Further, a detailed 3D FE model of a 205ft. long UHPC-NC hybrid girder was developed in ATENA and used to evaluate the feasibility of the hybrid girder concept. It was found that the hybrid girder concept is not only feasible but also reduces significantly the amount of end-zone reinforcement and end-zone cracking. A hybrid bridge pier system using a precast UHPC shell as permanent formwork for traditional bridge piers or as a retrofit option for existing columns was proposed. Experimental tests were conducted on 24in. long UHPC-NC columns to quantify the effectiveness of the UHPC shell in providing the confinement to normal concrete. Results obtained from the tests indicate that UHPC-shell-confined specimens exhibit a 15 to 30 % increase in peak load carrying capacity along with a 26 to 46% increase in failure strain values.
In-situ production of calcium carbonate nanoparticles in fresh concrete using pre-carbonation method
(University of Alabama Libraries, 2017) Qian, Xin; Wang, Jialai; University of Alabama Tuscaloosa
To reduce the carbon footprint of ordinary Portland cement (OPC)-based concrete, a novel technique, pre-carbonation process, has been developed to produce CaCO3 nanoparticles in fresh concrete. In this technique, gaseous CO2 is first absorbed into a slurry of calcium-rich minerals which is then blended with other ingredients to produce mortar/concrete. The objective of this work is to obtain an in-depth understanding of the underlying scientific mechanisms associated with the enhancement of strength and durability of the concrete induced by the new method. A comprehensive research plan has been carried out to study the carbonated slaked lime slurry and the effect of carbonated slaked lime slurry on the performance of OPC-based concrete, and to evaluate the potentials of the pre-carbonation method. Experimental studies show that carbonating the calcium-rich mineral slurry with CO2 can produce CaCO3 nanoparticles and Ca(HCO3)2 in the slurry, and these carbonation products were dictated by four parameters of the pre-carbonation method: the duration and temperature of the carbonation, the concentration of the calcium source slurry, and the stirring method of the calcium source slurry during the carbonation. The mechanical properties and durability of the mortar/concrete made with the carbonated slurry were significantly improved, which can be attributed to major mechanisms induced by the pre-carbonation method: promoted hydration of the cement and denser microstructure of the mortar/concrete. Calorimetry testing showed that the hydration of OPC was greatly improved by the pre-carbonation because of the extra heterogenous nucleation sites provided by the CaCO3 nanoparticles. XRD and TGA results revealed that more ettringite was produced in the mortar/concrete with pre-carbonated slaked lime slurry. The overall volume of the hydration products of the cement was increased by the pre-carbonation, leading to denser microstructure of the mortar/concrete. It has been found that the pre-carbonation can be used to the OPC-supplementary cementitious materials (SCMs) blended cement mortar/concrete, as evidenced by the improved mechanical properties achieved by these mortars produced by using the pre-carbonation method. A preliminary study was also conducted to examine whether other calcium-rich minerals, such as Class C fly ash and limestone, can be used as calcium source in the pre-carbonation method.
An investigation of the structural capacity of the Alabama Department of Transportation’s standard prestressed precast concrete piles
(University of Alabama Libraries, 2019) Gould, Emily; Aaleti, Sriram; University of Alabama Tuscaloosa
Prestressed precast concrete piles (PPCPs) are commonly used in bridge foundations throughout the southeastern United States. The Alabama Department of Transportation (ALDOT) particularly uses them in the coastal regions in non-cohesive soils and where corrosion would be expected from salt or brackish water. ALDOT’s currently listed pile design capacities in their structural design manual (SDM) were found to be lower than those for similar PPCP pile sizes used in surrounding states. This prompted the question as to why these values were lower, and whether they could safely be increased. A joint research project involving researchers from the University of Alabama and the University of South Alabama set out to answer this question, considering the structural and geotechnical implications respectively. This thesis particularly investigates the structural capacity of ALDOTs PPCPs through a series of tasks. First, an in-depth review of the practices of surrounding DOTs was conducted, evaluating how ALDOT’s practices are similar and where they diverge, to arrive at plausible explanations for ALDOT’s current standard capacities. Next, a simple analysis program was developed using Microsoft Excel, to calculate the combined axial and moment capacities of the standard ALDOT piles. Following this, to evaluate the expected demand on pile bents using the current bridge loading (HL-93), three prototype bridges were modeled and analyzed using standard structural analysis software. Using the analysis, the demand on pile bents under different load combinations were estimated and compared with the previously determined capacities. Based on this analysis, the listed structural capacities of ALDOT’s standard PPCPs could be increased.
Low-Cost Cenosphere Microencapsulation Technology for Phase Change Materials: a Sustainable Approach to Improving the Thermal and Mechanical Performance of Construction Materials for Operational Carbon Reduction
(University of Alabama Libraries, 2024) Ismail, Abdulmalik Bamidele; Wang, Jialai
Operational carbon emissions from heating, ventilating, cooling, and air conditioning in buildings account for 28% of global greenhouse gas emissions. These emissions can be reduced by improving the energy efficiency of buildings. One way to do this involves incorporating phase change materials (PCM) into cementitious composites to optimize thermal energy storage and regulation. The challenge lies in preventing PCM leakage and maintaining heat transfer efficiency, as existing PCM microcapsules (MPCMs) are costly and compromise the strength of cementitious composites due to their inherent low mechanical properties.To address these limitations, a novel microencapsulation technique was proposed. This involved utilizing cenosphere, a low-cost, high-strength, hollow microsphere derived from coal-burning power plants, as the protective shell for the new MPCM. The PCM in liquid phase can be loaded into the cenosphere after removing a thin silica film through chemical etching, resulting in a cenosphere-PCM microcapsule (CPCM). To seal perforations induced during production, three novel and cost-effective coatings (silica, ethyl cellulose, and bio-inspired silica) were applied to the CPCM.An extensive experimental plan was executed to optimize the etching process, characterize the coated CPCMs, evaluate their energy density, and assess the impact of each coating on the strengths of resulting cementitious composites. Additionally, the thermal performance of the cementitious composites with incorporated microcapsules was thoroughly examined. The results showed that the concentration of the etching agent significantly affects the etching duration, morphology, and size of the perforations on the cenosphere. Furthermore, the results showed that all the microcapsules exhibited superior thermal and mechanical properties compared to existing microcapsules due to their high crushing strength. Generally, the coatings improved the thermal stability of the CPCM accompanied by some reduction in latent heat. This reduction is largely attributed to the thickness of the coating layer. Bio-inspired silica-coated CPCM exhibited the best thermal performance, delayed the thermal decomposition of the PCM by about 50°C, and showed the best thermal performance profile. Cenosphere provides huge potential for the integration of PCM into building materials for thermal energy storage while still maintaining the structural integrity of the cementitious composites.
Low-cost, ubiquitous biomolecule as next generation, sustainable admixture to enhance the performance of ordinary portland cement-based concretes
(University of Alabama Libraries, 2021) Fang, Yi; Wang, Jialai; University of Alabama Tuscaloosa
The production of ordinary Portland cement (OPC) is highly energy-intensive and responsible for approximately 6% of anthropogenic greenhouse gas emissions. To reduce the carbon footprint of OPC based concrete, this research proposes to use a low-cost, ubiquitous, naturally occurring compound, tannic acid (TA) as a small-dose additive to significantly enhance the strength of OPC based concrete.This study is inspired by biosystems’ protein-based materials, which generally exhibit superior strength and toughness owing to their hierarchical structures via hydrogen-bonding assembly. With abundant reactive terminal phenolic hydroxyl groups, TA has an ability to complex or cross-link macromolecules sites through multiple interactions. Thus, TA can be used to complex or cross-link hydration products of cement at multi-binding sites so that the strength and durability of concrete can be significantly improved. A comprehensive research plan has been carried out to evaluate the potential of TA on performance enhancement of OPC-based concrete, understand how TA modifies the hydration of cement, mitigate the retardation of TA on cement’s hydration, and evaluate application potentials in concretes with SCMs. Experimental studies show that TA can strongly retard the hydration of cement and alite due to its ability to bind to various particles and chelate with calcium ions, causing less calcium hydroxide produced by the hydration. The strong interaction between the TA and hydration products leads to morphology change of the hydration products and generates nanoparticles at early age. Furthermore, addition of TA can significantly densify the nanostructure of cement pastes. Particularly, capillary pores smaller than 70nm are drastically reduced by TA. This finding is not only explaining why TA can enhance the micromechanical properties of concrete, but also opening a new approach to tune the nanoscale pores in concrete. Besides, a pre-hydration method is proposed and verified to mitigate the retarding effect of TA for widely adopted in practical application. Significant strength improvement at late age can be achieved by pre-hydration with TA without losing of strength at early age. TA is also successfully used in mortars with silica fume to achieve over 30% strength improvement, suggesting its huge potential to reduce the carbon footprint of concrete.
Modeling of CLT creep behavior and real-time hybrid simulation of a CLT-LiFS building
(University of Alabama Libraries, 2017) Nguyen, Tu T.; Dao, Thang N.; Kenneth, Fridley J.; University of Alabama Tuscaloosa
CLT-LiFS is an innovative hybrid structural system. This type of structure has emerged as a promising structural system for mid-rise to tall wood buildings in the seismic areas. CLT-LiFS is made by integrating post-tensioned Cross Laminated Timber (CLT) panels with Light Frame Wood Systems (LiFS). The post-tensioned CLT panels can provide excellent load bearing and self-centering capacity. And the LiFS can dissipate a large amount of energy through the slip of fasteners when it deforms. The behaviors of CLT-LiFS have been studied through a series of experimental tests under cyclic loading protocols and earthquake motions at different hazard levels using real-time hybrid simulations. Results from the experimental tests showed that the CLT-LiFS performed well under MCE (Maximum Considered Earthquake) hazard level with the maximum drift less than 1%. To obtain a good self-centering performance, the post-tensioned tendon force in CLT panels needs to be maintained at a desired level in long-term until the earthquake hits. Under the creep behavior, the post-tensioned force in the tendon will reduce as a function of time and moisture content in CLT panels. In this dissertation, a moisture content diffusion model was introduced by applying Fick’s law to estimate the moisture content migration in CLT panels under the variation of environmental relative humidity. A numerical model combined with data from a series of moisture content experiments were used to obtain the moisture content diffusion coefficients for CLT material. The four-element creep model was also proposed. This creep model could predict the creep deformation of CLT panels versus time under variations of ambient environmental conditions. A 3D finite element model (FEM) was developed with an integration of the creep and moisture content diffusion model to predict the tendon force of post-tensioned CLT panels versus time. This FEM model was used to predict the loss of tendon force in CLT panels. This tendon force loss did not include the instant loss at the beginning due to anchor slip or other factors.
Optimal discrete-time compensation design for real-time hybrid simulation
(University of Alabama Libraries, 2017) Hayati, Saeid; Song, Wei; University of Alabama Tuscaloosa
Real-Time Hybrid Simulation (RTHS) is a powerful and cost-effective dynamic experimental technique. In civil engineering, RTHS has the advantage of investigating the dynamic behavior of full-scale and complex structures by testing only the critical components. To implement a stable and accurate RTHS, the time delay in the experiment loop needs to be compensated. This delay is mostly introduced by servo-hydraulic actuator dynamics and can be reduced by applying appropriate compensators. Several existing compensators have demonstrated effective performance in reducing the actuator time delay. But most of them have been applied only in cases where the structure under investigation is subjected to inputs with relatively low-frequency content such as earthquake motion. To make RTHS an attractive technique for engineering applications with broader excitation frequency, a discrete-time feedforward compensator is developed via various optimization techniques to enhance the performance of RTHS. The effectiveness of the proposed compensator is demonstrated through both numerical and experimental studies. The proposed compensators are successfully applied to RTHS tests to study the seismic behavior of a linear-elastic reinforced concrete building equipped with a new type of tuned mass damper, known as the Disruptive Tuned Mass (DTM) damper designed by the National Aeronautics and Space Administration (NASA). The obtained results show that the proposed compensator reduces the time delay adequately and leads to a successful RTHS test. Results also suggest that the DTM damper can successfully reduce the response of the building subjected to the seismic loads. In addition, the dynamic properties of the DTM damper are fully investigated and a mathematical model is suggested for it.
Optimization and uncertainty quantification of multi-dimensional functionally graded plates
(University of Alabama Libraries, 2018) Hussein, Omar Shokry Ahmed; Mulani, Sameer B.; University of Alabama Tuscaloosa
Functionally graded structures (FGS) are structures that have varying properties in one or more directions that yield better performance over homogenous structures. The grading is usually considered through the thickness of beams, plates, or shells with different grading profiles. In this work, the design and analysis of multi-dimensional functionally graded nanocomposite structures are of interest with a focus on the material grading in the in-plane directions of plates, and the effect of the uncertainties in the elastic properties on the mechanical performance. The dissertation consists of six chapters; chapter one provides a literature review of the recent developments in the area of functionally graded structures, a brief overview of the properties and modeling of nanocomposites, and the uncertainty quantification of nanocomposites. The second chapter proposes a method for the design of multi-dimensional functionally graded structures based on the polynomial expansion of the volume fraction of the reinforcement. The third chapter extends the proposed method to design complex non-rectangular domains via coordinates transformations, and study the effects of the boundary conditions, loading type, and grading direction. The fourth chapter studies the reliability of in-plane FG plates by considering multiple sources of uncertainties (e.g. reinforcement size, volume fraction, and distribution). The fifth chapter studies the nonlinear dynamic and static responses of the FG plates by considering the post-flutter and the post-buckling behaviors. The sixth and last chapter provides a summary of the work done and the proposed future work. Throughout the dissertation work, the in-plane grading is optimized such that the minimum amount of reinforcement is used to satisfy certain mechanical performance constraints. The in-plane FG clamped plates showed a 45% average saving in the reinforcement amount compared to homogenous plates, while for simply supported plates the saving strongly depends on the problem nature and varies from 4% to 45%. For stiffened plates, the in-plane grading of the stiffeners led to a saving that can reach up to 200%. The reliability analysis showed that both homogenous and FG plates have the same level of uncertainty in the global responses. Also, the non-linear analysis indicated that both plates will in general behave similarly
Plastic Deformation Behavior of a Compositionally Complex Alloy Fe_50Mn_30Co_10Cr_10 (at.%) At Subzero Temperatures
(University of Alabama Libraries, 2022) Little, Jack Charles; Kumar, Nilesh; University of Alabama Tuscaloosa
Compositionally complex alloys (CCAs) are emerging class of alloys have shown promise for a wide range of applications. Although a range of properties including mechanical behavior of CCAs have been investigated at and above room-temperature, the properties investigated at subzero temperatures are limited. This paper focused on the effects of subzero temperature on the plastic deformation behavior of a dual-phase hot-rolled Fe50Mn30Co10Cr10 with transformation induced plasticity (TRIP) characteristics. The alloy was subjected to uniaxial tensile test using a subsize dog-bone shaped flat tensile test coupon at an initial strain rate of 10-3 s-1 (constant cross-head velocity) at room temperature (RT), 263 K (-10 ˚C), 243 K (-30 ˚C), 223 K (-50 ˚C,) and 77 K (-196 ˚C). The fractured samples were analyzed using a scanning electron microscope and electron backscattered diffraction (EBSD). The EBSD analysis revealed the alloy studied predominantly consisted of ε-martensite hexagonal close packed (HCP) crystal structure. The γ-austenite face-centered cubic (FCC) crystal structure was present as minor phase (<10% area fraction). Although the yield strength (YS) changed from 289 MPa at RT to 657 MPa at 77 K, the YS showed athermal characteristics from 263 K to 243 K. The ultimate tensile strength of the alloy changed from 877 MPa to 1481 MPa in this temperature range. Due to high work hardening rate, the ductility of the alloy remained high at all temperatures. The increase in ductility with an increase in strength from 243 K to 77 K is remarkable and contradicts the strength–ductility paradox. Moreover, the alloy exhibited a true tensile strength of 2.1 GPa with a true plastic strain of 38%. The stress – strain curves showed the presence of serrations at subzero temperatures which could be due to the twinning phenomena at subzero temperatures. However, the EBSD results obtained from the deformed samples and fractography were not sufficient to confirm the effect of twinning on serrated flow of the alloy. Although ε-phase is typically avoided in high-Mn steel for cryogenic applications as it introduces brittleness, the present research shows the alloy consisting primarily of ε-martensite shows a remarkable combination of strength and ductility.
Seismic Retrofit of Reinforced Concrete Shear Wall by Integrating Selective Weakening and Self Centering
(University of Alabama Libraries, 2022) Sharma, Sumedh; Aaleti, Sriram; University of Alabama Tuscaloosa
Many reinforced concrete (RC) buildings built prior to implementation of seismic design provisions in the 1970s are non-ductile and are at risk of excessive damage or even collapse during a major earthquake. In this dissertation study, a low damage retrofit scheme for non-code performing RC shear walls was investigated. In the retrofit scheme, traditional monolithic RC shear walls were converted into rocking walls by introducing a cold joint at the wall foundation interface and by adding external post-tensioning. Two retrofitted rectangular RC shear walls were tested in laboratory to investigate the proposed retrofit scheme. The retrofitted shear walls showed minimized damage, improved self-centering but lower energy dissipation capacity in comparison to the benchmark shear wall. A novel scheme using Ultra-High-Performance Concrete (UHPC) was proposed and investigated using laboratory testing and finite element (FE) simulation to anchor external PT elements to existing foundation in the retrofitted shear walls. Four anchorage specimens, that were designed to anchor 2/5th of the maximum PT force expected in the retrofitted shear walls, were subjected to laboratory testing. 3D FE models were calibrated based on the measured response and were used to investigate the proposed anchorage scheme at full-scale. The current code provisions to limit residual drift and predict critical concrete strains in precast rocking shear walls were examined based on published results. The provisions on residual drift were found to be satisfactory for hybrid rocking walls. Two code-conforming rectangular hybrid rocking shear walls were tested in the laboratory to provide improve alternative for predicting plastic hinge length in rocking walls, which is critical in estimating critical concrete strain.Laboratory testing of precast rocking shear walls has been limited to rectangular cross section. This dissertation addresses the gap in literature by testing a T-shaped precast rocking shear wall. The test specimen was designed in accordance with current design guidelines, at one-third scale of a prototype wall and tested under multi-directional loading up to 1.50% drift. Test observations showed damage to be limited at the rocking corners. The measured residual drift was lower than 0.25% and the measured energy dissipation ratio exceeded the prescribed limit of 12.5%.

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