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Early detection of damage in concrete infrastructure is critical to prolonging structural service life through timely maintenance, ensuring safety and preventing failure. Current management practices rely on regular visual inspections, which can be subjective and limited to accessible locations. In the field of structural health monitoring (SHM), there remain key limitations: (1) Indirect damage sensing, which requires complex physics-based models and algorithms to correlate structural response measurements to damage state; (2) Point-based sensing, which cannot accurately identify spatially distributed damage such as cracking and corrosion. Researchers have begun to explore distributed sensing methods such as ultrasonic guided waves and acoustic emission. These methods require expensive instrumentation, intricate network of sensors, and suffer from data contamination due to background noise and secondary sources. Most importantly, they are significantly more difficult to apply to concrete structures than steel structures. Distributed sensing that can provide the spatial resolution necessary to localize and quantify the severity of concrete infrastructure deterioration and damage is direly needed.In lieu of reliance on point-based sensors, this dissertation develops a distributed, direct damage and strain sensing approach based on novel multifunctional strain-hardening and self-sensing cementitious materials (MSCs). In this work, multifunctional cementitious materials are encoded with damage tolerance and spatial self-sensing capacity. The sequential formation of steady-state microcracks, rather than localized cracking, enables a prolonged and intrinsically controlled damage process, while allowing detection of microcracking damage level in the material long before failure occurs. The beauty of multifunctional concrete materials is two-fold: First, it serves as a major structural material component for infrastructure systems with greatly improved resistance to deterioration under service loading conditions, and to fracture failure under extreme events. Second, it offers capacity for distributed and direct sensing of cracking and straining wherever the material is located, while eliminating the need for sensor installation and maintenance. The dissertation research is driven by two central hypotheses: (I) Cementitious materials exhibit an AC frequency-dependent electrical response, which would depend on its heterogeneous microstructure. Mechanical straining and damage process would lead to a change in the microstructure, thus affecting the frequency-dependent electrical response. A strong, high signal-to-noise coupling between cementitious material electrical response and mechanical behavior at different length scales would enable a self-sensing functionality during elastic and post-cracking stages; (II) Using electrical stimulation and advanced tomography methods, spatial mapping offering a visual depiction of concrete damage and deterioration can be gained. Spatial sensing of damage location and level inside a structural element can be achieved through electrical probing only from the boundaries. This dissertation generates new understandings on how the composition and microstructure affect the frequency-dependent electrical response of cementitious materials. 4-point AC impedance spectroscopy is integrated with equivalent circuit modeling analysis to reveal the electrical microstructures and properties of cementitious materials, and their interfaces with electrodes. The work also elucidates whether and how cracking and healing (as a reversed damage process) lead to changes in material electrical response. An idealized model circuit is formulated to predict the frequency-dependent electrical behavior of cementitious composite materials at various cracking and healing levels. The model is tested and validated through a series of experimental measurements. Analyzing the changes of model parameters due to material composition, mechanical strain, and damage processes reveal the mechanisms that contribute to the overall electromechanical response of cementitious materials. This dissertation develops novel multifunctional cementitious composite materials that integrate self-sensing functionality with a pseudo-strain-hardening behavior accompanied by multiple steady-state microcracking process. The macroscopic electro-mechanical properties, which are strongly coupled through high signal-to-noise ratios, are achieved by tailoring the material electrical microstructure as well as micromechanical parameters. Through experimental investigation, the dissertation reveals the material strain sensing behavior at elastic, pseudo-strain-hardening, and tension softening stages under various loading scenarios and environmental conditions. In addition, a modeling framework is established to link length scales from single fiber/matrix interfacial electromechanical behavior, to a single crack bridged by numerous fibers with statistically random embedment lengths and orientation, and to multiple cracking process and final localized failure. The analytical model couples micromechanics theory with equivalent circuit model, to bring mechanistic insights into the material electromechanical response as the basis for self-sensing. Furthermore, this dissertation realizes spatial sensing and visual depiction of damage through combining with advanced electrical impedance tomography (EIT) methods. It establishes two EIT methods that are suitable for solid heterogeneous concrete elements: time-difference EIT and frequency-difference EIT. It evaluates different variables, e.g., material type, real or imaginary parts of impedance, probing frequency, and finite element algorithm, on the effectiveness of EIT methods and image reconstruction. Impedance reconstruction is an ill-posed inverse problem. Finite element models that describe the forward problem are implemented, and the inverse solution is solved using regularized least square analysis (e.g., Tikhonov regularization). This work makes it possible to spatially visualize material impedance and damage upon voltage measurements collected from probe locations at element boundaries. It demonstrates the effectiveness of the EIT methods using 2D (plate) and 3D (beam) specimens with different levels of cracking damage. Finally, the EIT methods are further extended for corrosion sensing in reinforced concrete specimens.

In contrast with the brittleness of concrete, strain-hardening cementitious composites (SHCC) have high ductility and toughness. However, SHCC mixtures generally contain a high cement content, which increases environmental and economic costs. The objective of this study is to use fly ash to replace a large portion of cement in SHCC and further improve their rheological and mechanical properties. The thesis approaches this objective from three aspects. First, composites reinforced with different types of fibres in high strength matrices containing high volume fly ash were investigated. Their mechanical properties were examined by compressive, direct tensile and flexural testings. The results showed that straight steel and micro-PVA fibres had the best reinforcing effect in the matrix under question. It was indicated that the dimension and material nature of the fibre are crucial in developing self-compacting high-strength SHCC with high-volume fly ash. Second, the mass ratios of fly ash to cement (FA/C) ranging from 1 to 4 were investigated. The results showed that high strength SHCC with a compressive strength of more than 80 MPa at 28 days were produced at the FA/C ratio of 1. With the further increase in FA/C ratio up to 4, the composites have a medium compressive strength value of more than 30 MPa at 28 days. The increase in the fly ash content resulted in reduction in the first-crack strength, which is favourable for achieving strain-hardening. The usage of high-volume fly ash also contributed to other benefits such as self-compactness and lightweightness. Third, the effect of high-volume fly ash on the bond between PVA fibre and the matrix was investigated. It was found that the majority of non-oil-coated PVA fibres pulled out without rupture in the matrices containing high-volume fly ash. The results indicated that the surface oil-coating on PVA fibres decreased the strength and ductility of high-volume fly ash SHCC. This implied that the high-volume fly ash effectively reduced the bonding of PVA fibre with the matrix and that the surface oil-coating on PVA fibres ought to be avoided. Furthermore, the theoretical analysis showed that the crack width largely depends upon the adhesional and frictional shear strengths of the interface. The crack width and the ultimate strain of SHCC can be controlled by adjusting the two strengths. This can be achieved by suitably designed usage of fly ash.

Cells can sense, signal and organize via mechanical forces. The ability of cells to mechanically sense and respond to the presence of other cells over relatively long distances across extracellular matrix (ECM) has been attributed to the strain-hardening behavior of the ECM. In this study, we explore an alternative hypothesis that the fibrous nature of the ECM makes long-range stress transmission possible, which could provide a mechanism for long-range cell-cell mechanical signaling. To test this hypothesis, we built 2-D and 3-D finite element models of stress transmission within cell-seeded collagen gels. To examine the role of collagen fibers in lateral stress transmission, confocal reflectance microscopy was used to develop 2-D image-based finite-element models. Models that account for the gel’s fibrous nature were compared with homogenous linear-elastic and strain-hardening models to investigate the mechanisms of stress propagation. To examine the role of collagen fibers in cell thickness sensing, 3-D finite element models with idealized fiber networks were built, and the stress transmissions in fibrous and homogeneous ECM were compared. Finite-element analysis revealed that stresses generated by cell contraction are concentrated in the relatively stiff ECM fibers and are propagated farther in a fibrous matrix as compared to linear elastic or strain-hardening homogenous materials. These results support the hypothesis that ECM fibers, especially aligned ones, play an important role in long-range stress transmission. Further, fluid-structure interaction models were built to investigate the interplay between collagen fibers and interstitial fluid. The results suggest that in cell culture, cell movement is the key factor in defining fluid-flow development at the microscopic ‘cellular’ level, and the cross-links are the key factor that determines the micro-mechanical environment.

Recent particle-tracking velocimetry (PTV) has revealed inhomogeneous elastic breakdown of entangled polymer networks during shear. Such cohesive failures indicate a many-bodied network picture for entangled polymers. The Doi-Edwards tube model appears to be inadequate to capture the crucial physics of this behavior.Observations of shear deformation have motivated the study of the extensional rheology of a series of SBR melts and blends. During step and startup extension, localized failures similar to those found in simple shear have been observed. After a large step extension, entangled polymers exhibit elastic breakdown, when high retraction forces overcome cohesion imparted by chain entanglement. In startup extension, engineering stress maxima indicate yielding just as stress overshoots in the start-up of simple shear mark the onset of yielding. The present work clarifies confusion regarding strain-hardening and shows that the basic physics of shear and extensional flow are similar.The yield-to-rupture transition in melts at high rates of uniaxial extension has also been explored. Here the failure mechanism changes from chain disentanglement (yielding) to chain scission. Birefringence of a monodisperse melt shows that elastic rupture occurs only when chains between entanglements are near full extension, and there is no sign of glassy dynamics.

Since corrosion of steel reinforcements in concrete is a major factor in the deterioration of bridge decks, this research proposes a new bridge deck system in which the top reinforcement is totally eliminated and instead, a strain-hardening fiber reinforced cement composite (or HPFRCC) is used. With the HPFRC matrix, the restraining resistance provided by the fibers, acts isotropically, thus temperature and shrinkage reinforcements are not needed. Also, the fibers help reduce crack widths, therefore minimizing ingress of corrosive agents. The bottom reinforcement is kept continuous and placed far away from the surface of the deck to prevent contact with penetrating corrosive agents, if any. This research deals with verifying the possibility of implementing this new system in real bridges. It comprises both experimental and analytical investigations. The first phase of the experimental work consists of testing the mechanical properties of HPFRC composites in order to better understand their behavior. The results obtained are used for modeling the proposed bridge deck system. The second phase of the experimental program includes the testing of thirteen two-span continuous beams simulating slab slices of bridge decks to evaluate their behavior. Parameters investigated included three types of fibers (Spectra, Torex, PVA), three types of reinforcements (steel bars, prestressing strands, CFRP), and two design concepts (reinforced concrete, prestressed concrete). The analytical work focused mainly on predicting the overall response of the proposed deck system with the combined use of nonlinear section analysis and nonlinear member analysis. Based on the findings, the proposed deck system works as anticipated. Indeed, with strain-hardening matrices, a plastic hinge mechanism formed under increasing load, leading to a response better than that of conventional reinforced concrete decks, even though the top reinforcement was eliminated, also, the proposed deck system showed higher inelastic stiffness, higher energy absorption, and higher ductility. Fibers in the HPFRC matrices help improve deck serviceability; under maximum service load, smaller permanent deflection and rotation as well as smaller crack width were observed in comparison to conventional reinforced concrete deck. Design recommendations are provided and conclusions are finally drawn.