Damage Assessment in Concrete Structures using PZT patches

Piezoelectric based PZT smart sensors offer significant potential for continuously monitoring the development and progression of internal damage in concrete structures. PZT-based damage sensors consisting of piezo-electric patches, which are bonded to the surface of a concrete structure can be developed for assessing the damage progression of concrete members. The primary challenge in developing a PZT-based sensor lies in developing a methodology to infer about the level of damage in the material from measurement. Changes in the resonant behavior in the measured electrical conductance obtained from electro-mechanical (EM) response of a PZT bonded to a concrete substrate is investigated for increasing levels of damage. The sensitivity of EM impedance- based measurements to level of damage in concrete is reported. Incipient damage in the form of microcracks in the concrete substrate produces a change in the electrical conductance signature associated with the resonant peaks. Changes in the conductance resonant signature from EM conductance measurement are detected before visible signs of cracking. The root mean square deviation of the conductance signature at resonant peaks is shown to accurately reflect the level of damage in the substrate. The findings presented here provide a basis for developing a sensing methodology using PZT patches for continuous monitoring of concrete structures

PZT Sensor Arrays for Integrated Damage Monitoring in Concrete Structures

The broad objective of the work reported here is to provide a fundamental basis for the use of Lead Zirconate Titanate (PZT) patches in damage detection of concrete structures. Damage initiation in concrete structures starts with distributed microcracks, which eventually localize to form cracks. By the time surface manifestation in the form of visible cracking appears there may be significant degradation of the capacity of the structure. Early detection of damage, before visible signs appear on the surface of the structure is essential to initiate early intervention, which can effectively increase the service life of structures. Development of monitoring methodologies involves understanding the underlying phenomena and providing a physical basis for interpreting the observed changes in the parameters which are sensed. PZT is a piezoelectric material, which has a coupled constitutive relationship. In the case of the PZT patches bonded to a concrete structure, any sensing strategy requires developing an understanding of the coupled electromechanical (EM) response of the PZT-concrete system. The challenges associated with the use of PZT patches for damage monitoring in a concrete substrate include providing the following: a clear understanding of the fundamental response of the PZT patch when bonded to a concrete substrate; interpretation of the coupled response of the PZT patch under load induced damage; and development of an efficient, continuous monitoring methodology to sense a large area of the concrete substrate. Due to a lack of a fundamental basis, the use of PZT patches in concrete structures often involves inferring the measured response using model-based procedures. The work outlined in this thesis addresses the key issue of developing the theoretical basis and providing an experimental validation for PZT-based damage monitoring methodology for concrete structures. A fundamental understanding of response of the PZT patch when bonded to concrete substrate is developed. The outcome of the work is an integrated local and distributed sensing methodology for concrete structures by combining the electromechanical impedance and stress wave propagation methods using an array of bonded PZT patches. The work presented in this thesis is focused on using PZT patches bonded to a concrete substrate. A fundamental understanding of the coupled electromechanical behaviour of a PZT patch under an applied electrical excitation in an electrical impedance (EI) measurement, is developed. The influence of the substrate size and its material properties on the frequency dependent EI response of a PZT patch is investigated using concrete substrates of different sizes. The dynamic response of a PZT patch is shown to consist of resonance modes of the PZT patch with superimposed structural response. The resonance behaviour of the PZT patch is shown to be influenced by the material properties of the substrate. The size dependence in the EI response of a PZT patch bonded to a concrete substrate is produced by the dynamic behaviour of the structure. The size of the local zone of the concrete material substrate in the vicinity of the bonded PZT patch, which influences the frequency dependent EI response of the PZT patch is identified. For each resonant mode, a local zone of influence, which is free from the influence of boundary is identified. The dynamic response of the PZT resonant mode is influenced by the elastic material properties and damping within the zone of influence. The structural effects of the concrete substrate produced by the finite size of the specimen are separated from the material effects produced by the material properties and the material damping in the coupled EM response of the bonded PZT patch. The influence of size of the concrete substrate on the coupled impedance response of the PZT is identified with peaks of structural resonance, which are superimposed on the resonant peaks of the bonded PZT patch The EI response of the PZT patch when bonded to concrete for detecting load-induced damage from distributed microcrack to localized cracks within the zone of influence of the PZT patch is investigated. Using an approach which combines an understanding of the coupled EM constitutive behaviour of PZT with experimental validation, a methodology is developed to decouple the effects of stress and damage in the substrate on the coupled EM response of a PZT patch. The features in the EI signature of a bonded PZT patch associated with stress and damage are identified. An increasing level of distributed damage in the concrete substrate produces a decrease in the magnitude and the frequency of the resonant peak of the bonded PZT patch. The substrate stress produces a counter acting effect in the EI spectrum of the bonded PZT patch. A measurement procedure for the use of bonded PZT patches for continuous monitoring of stress-induced damage in the form of distributed microcracks in a structure under loading is developed. An integrated methodology for damage monitoring in concrete structures is developed by combining the EI method for local sensing and the stress wave propagation-based method in a distributed sensing mode. An array of surface mounted PZT sensors are deployed on a concrete beam. The EI measurements from individual PZT sensors are used for detecting damage within the local zone of influence. PZT sensor pairs are used as actuators and sensors for distributed monitoring using stress wave propagation. A stress-induced crack is introduced in a controlled manner. It is detected very accurately from the full-field displacement measurement obtained using digital image correlation. The crack opening profile in concrete produced by the fracture is established from the surface displacement measurements. From the measurements of bonded PZTs, the localized crack is detected in the zone of influence by EI. The change in compliance of the material medium due to a localized crack is small and it is reflected in the smaller change in the measured EI when compared to distributed damage. Stress wave based measurements sensitively detect crack openings on the order of 10m. The material discontinuity produced by a closed crack, after removal of the stress is also detected. A damage matrix is developed for stress wave based method which is independent of transmission path to assess the severity of damage produced by the crack in a concrete structure

Evaluation of Crack Propagation and Post-cracking Hinge-type Behavior in the Flexural Response of Steel Fiber Reinforced Concrete

An experimental evaluation of crack propagation and post-cracking behavior in steel fiber reinforced concrete (SFRC) beams, using full-field displacements obtained from the digital image correlation technique is presented. Surface displacements and strains during the fracture test of notched SFRC beams with volume fractions (Vf) of steel fibers equal to 0.5 and 0.75% are analyzed. An analysis procedure for determining the crack opening width over the depth of the beam during crack propagation in the flexure test is presented. The crack opening width is established as a function of the crack tip opening displacement and the residual flexural strength of SFRC beams. The softening in the post-peak load response is associated with the rapid surface crack propagation for small increases in crack tip opening displacement. The load recovery in the flexural response of SFRC is associated with a hinge-type behavior in the beam. For the stress gradient produced by flexure, the hinge is established before load recovery is initiated. The resistance provided by the fibers to the opening of the hinge produces the load recovery in the flexural response

Damage Assessment in Concrete Structures using PZT patches

Piezoelectric based PZT smart sensors offer significant \ud potential for continuously monitoring the development and \ud progression of internal damage in concrete structures. \ud PZT-based damage sensors consisting of piezo-electric \ud patches, which are bonded to the surface of a concrete \ud structure can be developed for assessing the damage \ud progression of concrete members. The primary challenge \ud in developing a PZT-based sensor lies in developing a \ud methodology to infer about the level of damage in the \ud material from measurement. Changes in the resonant \ud behavior in the measured electrical conductance obtained \ud from electro-mechanical (EM) response of a PZT bonded \ud to a concrete substrate is investigated for increasing \ud levels of damage. The sensitivity of EM impedance- \ud based measurements to level of damage in concrete is \ud reported. Incipient damage in the form of microcracks in \ud the concrete substrate produces a change in the electrical \ud conductance signature associated with the resonant \ud peaks. Changes in the conductance resonant signature \ud from EM conductance measurement are detected before \ud visible signs of cracking. The root mean square deviation \ud of the conductance signature at resonant peaks is shown \ud to accurately reflect the level of damage in the substrate. \ud The findings presented here provide a basis for developing \ud a sensing methodology using PZT patches for continuous \ud monitoring of concrete structures

Concrete using Siliceous Fly ash at very High Levels of Cement Replacement: Influence of Lime Content and Temperature

Potential for producing viable binders at very high levels of cement replacement (60% and above) with fly ash is explored. The role of lime content and temperature on the efficiency of fly ash in contributing to strength gain is investigated using quantitative X-ray diffraction (XRD) analysis. Results of fly ash characterization are presented using quantitative X-ray diffraction (XRD) to identify its reactive potential associated with the amorphous silica content. A new method for quantitative phase analysis of the amorphous phase contributions in the XRD spectrum is presented. A strength-based efficiency factor which provides a measure of the contriubtion of fly ash is introduced. Temperature is shown to increase the efficiency of fly ash by accelerating the dissolution of the reactive amorphous content. Efficiency of fly ash in the binary high volume fly ash-cement blend is limited by the availability of lime. Increasing the lime content in the system provides significant enhancement in strength, but it does not influence the dissolution of fly ash With the availability of lime, the efficiency is limited by the rate of contribution of reactive Silica from fly ash, which is influenced strongly by temperature. Concrete strengths of 30 MPa and higher were achieved with 65% replacement of cement with fly ash and total cement content of 100 kg/m3. The strength gain in concrete is shown to be related to the depletion of lime in the system, formation of amorphous hydration products and the depletion of Si/Al content from fly ash. An investigation of the underlying mechanisms reveals the potential for further strength enhancement by effectively engaging all the reactive components of fly ash

Experimental Investigation of Blast-Pressure Attenuation by Cellular Concrete

Results from an experimental investigation of the dynamic response of cellular concrete subjected to blast-pressure loading are presented. The cellular concrete has large entrained porosity in the form of uniformly distributed air cells in a matrix of hardened cement. Under quasi-static loading, once the applied stress exceeds the crushing strength of the cellular concrete, crushing and densification of material results in an upward concave stress-strain response. The shock-tube experimental test setup used for generating blast-pressure loading in a controlled manner is described. Experimental results from the cellular concrete subjected to blast-pressure loading with pressure amplitude greater than its crushing strength indicate that a compression stress wave, which produces compaction of the material due to collapse of the cellular structure, is produced in the material. As the compaction front propagates in the material, there is a continuous decrease in its amplitude. The impulse of the blast pressure wave is conserved. When a sufficient length of the cellular concrete is present, the applied blast pressure wave is completely attenuated to a rectangular stress pulse. The transmitted stress to a substrate from cellular concrete when an applied blast pressure wave is completely attenuated resembles a rectangular stress pulse of amplitude slightly higher than the crushing strength of the material with a duration predicted by the applied blast impulse

Stress-Crack Separation Relationship for Macrosynthetic, Steel and Hybrid Fiber Reinforced Concrete

An experimental evaluation of the crack propaga tion and post-cracking response of macro fiber reinforced concrete in flexure is c onducted. Two types of structur al fibers, hooked end steel fibers and continuousl y embossed macro-synthetic fibers are used in this study. A fiber blend of the two fibers is evaluated for spec ific improvements in the post peak residual load carrying response. At 0.5% volume fraction, both steel and macrosynthetic fiber reinforced concrete exhibits load recovery at large crack opening. The blend of 0.2% macrosynthetic fibers and 0.3% steel fibers shows a significa nt improvement in the immediate post peak load response with a significantly smaller load drop and a constant residual load carrying capacity equal to 80% of the peak load. An analytical formulation to predict fle xure load-displacement behaviour considering a multi-linear stress- crack separation (σ -w) relationship is developed. An inverse analysis is developed for obtaining the multi- linear σ -w relation, from the experimental response. The � -w curves of the steel and macrosynthetic fiber reinforced concrete exhibit a stress recovery after a significant drop with increa sing crack opening. Significant residual load carrying capacity is attained only at large crack separation. The fiber blend exhibits a constant residual stress with increasing crack sepa ration following an initial decrease. The constant residual stress is attained at a small crack separation

Ultrasonic monitoring of capillary porosity evolution and strength gain in hydrating cement pastes

An ultrasonic test procedure for determining the capillary porosity in hydrating cement paste is presented. The response of hydrating cement paste through setting is monitored using horizontally polarized shear waves (SH). Changes in the ultrasonic signal through setting are related with changes in the porosity and stiffness of an equivalent water- filled poroelastic material, which provides identical acoustic impedance. The porosity obtained from the ultrasonic measurements, is identical to capillary porosity obtained from the conventional themo-gravimetric analysis. A unique relationship between capillary porosity and compressive strength is established for hydrating cement pastes

Evaluation of seismic displacement demand for unreinforced masonry shear walls

Unreinforced, non-engineered low-strength brick masonry structures comprise a large percentage of buildings in the Himalayan region and have been extensively damaged in recent earthquakes. Due to the high seismic hazard of the region and the inherent vulnerability of non-engineered masonry structures, a seismic assessment of masonry construction in this region is imperative. In this study, a suite of strong ground motions is developed using data from major Himalayan earthquakes. Using a mechanistic-based procedure for predicting the monotonic load envelope which identifies limit states of cracking, strength, and collapse using stress-based criteria, a hysteretic model was calibrated to experimental data of unreinforced masonry shear walls. Nonlinear time history analyses are performed on the validated single degree of freedom models of two unreinforced masonry walls. The analytical results correlate well with observed damage to masonry structures in Himalayan earthquakes. Peak ground acceleration of ground motion is observed to be the key parameter influencing displacement of walls. A linearly increasing trend is observed between the PGA and the observed displacement up to a PGA value of 0.1g. A weak correlation is observed between displacement and ground motion frequency parameters

Impedance-based Damage Measurement in Concrete using Bonded PZT patches

Structural Health Monitoring (SHM) is a process of assessing the structural integrity of the constituent parts and the level of damage level in the structure during its life period. SHM relies on non-destructive evaluation (NDE) procedures and continuous monitoring of structural parameters to determine the intensity and location of the damage. Coupling the structure to the PZT changes the mechanical impedance of the PZT, which produces a change in its vibration characteristics. The change in the electrical impedance of the PZT due to the elastic restraint by the surrounding medium provides the basis for impedance-based measurements. Thesis evaluating the existing analytical formulations and develop a numerical framework for interpreting the electro-mechanical impedance-based measurements of a PZT patch bonded to a concrete substrate. Assessment of incipient damage and the evolution of damage are studied using electromechanical impedance based technique. Root mean square deviation (RMSD) is used to quantify the effect of stress and damage in the concrete