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In this study, the buckling-resistant antispring-clad reinforcing bar (BRASR) for improving the performance of the longitudinal reinforcing bars embedded in concrete columns is presented. B...

The 2001 \(M_{\text{w}}\) 7.6 earthquake which struck the Kutch region in the western part of India was one of the largest intraplate earthquakes to have struck the Indian subcontinent. There was extensive destruction of unreinforced brick masonry (URM) buildings in the region. Assessing the vulnerability of the masonry structures of this region is complicated due to a lack of accelerograms and realistic models for the cyclic response of masonry. In this paper, synthetic accelerograms for the region are generated using the modified stochastic finite-fault method. The monotonic backbone curve is developed using a mechanics-based approach. Stress-based criteria are used to identify the displacement limit states. Experimental data of two URM shear walls are used to calibrate hysteretic models for performing nonlinear time-history analyses. Damage predictions show good comparison with the earthquake reconnaissance reports on URM damage. Seismic fragility curves indicate the high vulnerability of URM structures under the expected range of seismic intensities for the region.

Experimental investigation on the cohesive–frictional shear behavior of soft-brick masonry under various levels of applied compression is reported. The progressive failure in the bed joint ...

An experimental evaluation of cracking and post-cracking behavior in the steel fiber reinforced concrete (SFRC) beams using the displacement field obtained from digital image correlation (DIC) is presented. The physical basis of the hinge-type behavior in flexure is established from an analysis of the displacement discontinuity across the crack. In SFRC beams, the load recovery following the localization of strain to a single crack is shown to be associated with opening of the hinge. An analytical framework for implementing a multi-linear stress-crack separation (σ-w) relationship within the cracked hinge model is presented. Multi-linear σ-w relations are obtained for SFRC with different fiber volume fractions (V f ) by an inversion procedure. The σ-w relationship for SFRC exhibits an initial softening to values lower than the tensile strength which is followed by a stress recovery with increasing crack separation. In SFRC, the stress attains a constant value with increasing crack separation, larger than 1 mm. For V f equal to 0.75%, application of cracked hinge model predicts a constant stress of magnitude less than the tensile strength with increasing crack separation in the part of the load response associated with multiple cracking.

Steel fiber reinforced concrete beams with fiber volume fractions equal to 0.5% and 0.75% are tested with a shear span to depth ratio equal to 1.8. The cracking in the beams is evaluated using the full-field surface displacements obtained from the digital image correlation (DIC) technique. Analysis of images shows that a full depth shear crack is established before the peak load. The displacements measured from across the shear crack indicate a continuous increase in the crack opening displacement associated with increasing slip between the two crack faces. From crack opening and sliding measurements across the shear crack, the dilatant behavior is identical in beams with and without steel fiber reinforcement. Failure in control beams is brittle and results in a large opening of the shear crack. In the SFRC beams with 0.5% volume fraction, there is a continuous decrease in the residual load carrying capacity after the peak load which is associated with an increase in the crack opening displacement. In SFRC beams with 0.75% fiber volume fraction, the increased resistance to crack opening provided by the fibers results in a significantly smaller crack opening and a large increase in the peak load. The crack opening due to dilatancy is arrested, resulting in shear failure by the formation of a secondary shear crack or by flexural failure. The crack opening displacement across the shear crack at the peak load in the load response of the control and the SFRC beams are nominally identical. Failure in shear occurs when the crack opening control provided by the flexural reinforcement and steel fibers is inadequate to sustain the aggregate interlock.

APLAS/ APS is an auto antibody mediated acquired thrombophilia with recurrent arterial / venous thrombosis and pregnancy morbidity . It primarily affects females . 5 cases per 1,00,000 population Diagnosis of APLAS should be considered in cases of thrombosis, CVA in individuals less than 55yrs of age . We report a case of a 21 yr old female who had history of jaundice ( on and off episodes) excessive menstrual bleeding with past history of 3 miscarriages. She was screened and evaluated for Autoimmune diseases and found to have ANTIPHOSPHOLIPID ANTIBODY SYNDROME . She was kept on anticoagulants and aspirin . And patient was doing well on follow up. Any miscarriages/ anaemia in young females needs active investigation so that appropriate treatment can be started to halt the disease process.

An experimental investigation of compressive failure in masonry made of soft clay bricks is presented. The research attempts to address the concern of strong/stiff mortars available in the market today in combination with poor quality burnt clay bricks available in India. Masonry assemblages are tested to evaluate the influence of the relative strength of mortar on the observed damage evolution and compressive failure. Damage evolution associated with the formation and propagation of vertical splitting cracks during the compressive load response of masonry assemblages in the stack bonded arrangement are studied using a full-field optical technique based on digital image correlation. The stress state in the composite material produces tension in the mortar and confined compression in brick. Using image correlation clear evidence of the crack forming in the mortar and propagating into the brick is established. Failure is produced by vertical splitting and the number of cracks depend upon the tensile strength of the mortar relative to the brick and the number of head joints. Head joints act as stress risers, leading to high tensile stress.

An experimental investigation on the fracture test response of notched concrete beams with two types of discrete macrosynthetic fibres is presented. The influence of high-modulus polypropylene macro fibres on crack propagation and opening was evaluated using the digital image correlation technique. The surface displacements measured close to the tip of the notch were analysed to evaluate the crack opening profile in concrete in relation to the observed load response. A method for precisely estimating the displacement discontinuity across the crack from measured surface displacements was developed. Post-peak softening in the flexural load response was found to be associated with crack advance in the cementitious matrix. The measured crack opening profiles in the post-cracking response indicated a hinge-type behaviour in the beam. The physical hinge in the post-cracking flexural response of the beam was directly determined from the surface measurements. It was found that the addition of macrosynthetic fibres up to 8 kg/m3 (0·9% by volume) did not significantly influence crack propagation but provided resistance to opening of the hinge, which resulted in an increase in toughness and significant load recovery in the post-peak flexural load response.

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.

An analytical formulation for flexure behaviour of concrete considering a multi-linear stress-crack separation (σ-w) relationship is developed using the cracked hinge model. An inversion procedure for obtaining the multi-linear cohesive stress response from the flexural load response of a beam is presented. The procedure is applied to obtain the σ-w relationship for macro-synthetic fiber reinforced concrete. An experimental investigation of the crack propagation in flexural response of macro-synthetic fiber reinforced concrete is presented using the digital image correlation technique. The post-cracking response of macro-synthetic fiber reinforced concrete during the initial softening and the subsequent load recovery is experimentally shown to be associated with a hinge-type behaviour and is produced by crack closing stresses contributed by fibers. From the optical measurements the hinge length is identified with a zone of length equal to twice the aggregate size. Using the measured hinge length, the multi-linear σ-w relationship for macro-synthetic fiber reinforced concrete obtained by matching the experimental and the analytical load responses exhibits a stress recovery following initial softening. The cohesive stress subsequently decreases following the recovery at large crack separation. The crack closing stresses contributed by the pullout of fibers produce stress recovery in the σ-w relationship and are primarily active after the formation of the hinge resulting in significant contribution to fracture energy at large crack openings. There is a good correlation in the fracture energy obtained from load response and the σ-w relationship at different values of crack opening displacements.

The debonding mode of failure, which is observed in concrete beams strengthened using externally attached CFRP composite sheets, is investigated using the direct shear test. The Mode II, cohesive stress-crack relative slip relationship is established using full-field displacements obtained from digital image correlation. The interface crack is associated with a cohesive stress-transfer zone of fixed length. The load capacity of the CFRP composite bonded to concrete is attained when the cohesive crack is fully established. The acoustic emission monitored during the interface fracture initiation and propagation indicates that microcracking events accumulate at a constant rate up to failure. The variations in the local fracture parameters are quantified and are adequately represented using the normal probability distribution. A numerical analysis of the direct-shear debonding response of CFRP composite attached to a concrete substrate is performed to study the influence of the variability of the loca...

An experimental investigation into the shear transfer across the brick mortar interface at different levels of applied pre-compression is reported. Direct shear tests are performed in the triplet test configuration on masonry made with a soft brick. The failure across the brick–mortar interface is progressive and is produced by a crack in the brick–mortar interface. The interface crack propagates in a thin layer where the mortar penetrates the brick. A cohesive-frictional interface relationship which combines progressive cohesive failure with increasing frictional resistance is developed. The maximum shear resistance obtained from the cohesive-frictional relationship follows a Mohr–Coulomb type relationship. The residual frictional resistance increases linearly with an increase in the applied compression.

Honey bee sting induced Cerebral infarction is a rare entity . We report a case of 55year old male presented with anaphylactic shock following honey bee sting along with weakness of left side of the body and the face . He was managed with anti histaminics, adrenaline injections, Intravenous fluids , vasopressors , anti platelets and anticoagulants .

The cohesive stress transfer during the sub-critical crack growth associated with the debonding of FRP from concrete under fatigue loading is experimentally investigated using the direct shear test set-up. The study focused on high-amplitude/low-cycle fatigue. The fatigue sub-critical crack growth occurs at a load that is smaller than the static bond capacity of the interface, obtained from monotonic quasi-static loading, and is also associated with a smaller value of the interfacial fracture energy. The strain distribution during debonding is obtained using digital image correlation. The results indicate that the strain distribution along the FRP during fatigue is similar to the strain distribution during debonding under monotonic quasi-static loading. The cohesive crack model and the shape of the strain distribution adopted for quasi-static monotonic loading is indirectly proven to be adequate to describe the stress transfer during fatigue loading. The length of the stress transfer zone during fatigue is observed to be smaller than the cohesive zone of the interfacial crack under quasi-static monotonic loading. The strain distribution across the width of the FRP sheet is not altered during and by fatigue loading. A new formulation to predict the debonding crack growth during fatigue is proposed.

Interface cohesive stress transfer between FRP and concrete during debonding is typically obtained using measured surface strains on the FRP, along the direction of the fibers. The cohesive material law is derived under a set of assumptions which include: (a) the bending stiffness of the FRP laminate is insignificant with respect to that of the concrete test block; (b) the strains in the bulk concrete produced by debonding are negligible, thus concrete substrate can be considered rigid; (c) there is stress transfer between FRP and concrete through the FRP–concrete interface which is of zero thickness; and (d) the axial strain in the FRP composite is uniform across its thickness. In this paper, a test procedure for directly obtaining the through-thickness strains in the FRP and the concrete substrate during cohesive stress transfer associated with debonding is presented. The displacement and strain fields are measured on the side of a direct-shear specimen with the FRP strip attached on the edge. Based on the experimental results, the influence of the assumptions which have been introduced to determine the cohesive law is discussed. Within the stress transfer zone there is a sharp gradient in the shear strain. The location of the interface crack within the stress transfer zone and the cohesive stress transfer during the propagation of the interface crack are determined.

Reinforced concrete beams with discrete hooked-end steel fibers at 0.5% volume fraction are tested with a shear span to depth (a/d) ratio equal to 1.8. Digital image correlation (DIC) technique was used to obtain the full-filed displacements from the beam. The formation and propagation of a shear crack which directly influences the load response and the peak load in the load response of the beam is monitored using the displacement information available from DIC. There is a continuous increase in slip across the crack faces with increasing load, which produces an increase in the crack opening. The dilatant behavior indicated by the proportion of crack opening to slip displacement obtained from the control and the SFRC beams is identical. Failure in control beams is brittle and was produced by the opening of dominant shear crack within the shear span. At the peak load, the shear crack pattern in fiber reinforced concrete is identical to the crack pattern in the control beam. The fiber reinforced concrete beams exhibit post peak load carrying capacity with continued slip of the dominant shear crack. The crack bridging stress provided by the fibers results in a significant increase shear transfer across the crack which provides significant post-peak load carrying capacity with increasing slip of the shear crack.

The masonry-FRP interface fracture is a topic of considerable research interest due to the heterogeneity of the masonry subgrade. The size of the material inhomgeneity associated with the presence of mortar joints is significant compared with the length scale of fracture process. In this paper, the results of an experimental investigation into the shear debonding of FRP sheets from brick, mortar and masonry blocks are reported. The test procedures (Ali-Ahmad et al. 2006,2007) developed previously for obtaining the FRP-concrete cohesive fracture response are applied to study interface debonding from the three substrates. During each test, spatially continuous measurements of the surface strains on the FRP and masonry are obtained using an optical technique known as digital image correlation. The interface cohesive fracture response of FRP-brick and the FRP-mortar interfaces are obtained from the results of the strain analysis. The interface fracture energy associated with the FRP-mortar interface is shown to be significantly smaller in magnitude than that of the FRP-brick interface. The contributions of the mortar and the brick to the overall load response of the masonry are analyzed using the cohesive material response of the FRP-brick and the FRP-mortar interfaces. It is shown that complete debonding is achieved at the FRP-mortar joint while the FRP is still attached to the bricks on either side of the joint. The cohesive crack front stretches across the fully debonded mortar joint before the cohesive crack completely crosses the joint. The local debonding at the mortar joint produces stresses higher than those associated with the main cohesive crack front in the brick-FRP interface close to the joint, thereby accelerating the crack advance.

Cracking in single-span concrete bridge decks was investigated. An assessment of the cracking as it relates to in situ concrete strength was performed. In cracked decks, concrete was of uniform quality and the compressive strength obtained from cores was in excess of the design strength. Evaluation of cores using an optical technique revealed that the cracks in the bridge decks formed at an early age, when the strength of the cementitious matrix is less than the strength of the coarse aggregate. Tests on mature concrete obtained from bridge decks indicate that the cementitious matrix is stronger than the coarse aggregate, resulting in aggregate fracture under tensile loading. Findings indicate that transverse cracking in the midspan portion of bridges with elastomeric bearings and longitudinal cracking at the supports in bridges with integral abutments result from restraint of early-age thermal movement of the bridge deck. The available evidence suggests that cracking in the bridge decks occurs in...

The overarching goal of this work is to provide a fundamental understanding of the behavior of solid brick masonry columns confined with fiber reinforced cementitious matrix (FRCM) composites. FRCM is a newly-developed type of composite material comprised of a cementitious inorganic matrix (binder) and embedded fibers that are usually bundled to improve the bond between the matrix and the fibers. Compression tests were carried out to investigate the influence of the FRCM confinement and the brick patterns on the load-carrying capacity of the confined columns. Compression tests were conducted on brick masonry columns with different brick configurations. Digital image correlation measurements on the surface of the composite and on the surface of the brick for the control specimens were attempted in order to understand the role of the mortar joints and the arch effect across the section of the columns due to the confinement. The experimental results indicate that FRCM composites can effectively increase the load-carrying capacity of brick masonry columns and the failure mode could be different from the one observed for masonry columns confined with fiber-reinforced polymer (FRP) composites.

An active sensing approach is employed to analyze the damage behavior of concrete cylinders under compression using smart aggregates. The smart aggregate sensor used in this study is developed by embedding a soldered water-proof piezo-ceramic patch inside stiff cement mortar. These smart aggregates are embedded at the desired locations in the concrete cylinders. A pair of smart aggregates is used with one acting as an actuator and other as a sensor. Wavelet packet analysis is used to analyze the recorded signals. Variation of cracking pattern observed at different levels of loading modifies the signal characteristics. Experimental investigations are carried out by testing eight concrete cylinders with and without steel fibers. The experimental result shows that the proposed method of using smart aggregates and evaluating damage index based on wavelet analysis is effective in monitoring the health of the concrete cylinders. Compression tests on steel fiber reinforced concrete cylinders indicate that the addition of fibers improves the ductility and post peak behavior under compression. Damage index variation shows that the steel fibers resist the crack propagation and results in improved performance.

An experimental investigation of compressive failure in masonry made of soft clay bricks is presented. Masonry assemblages are tested to evaluate the influence of the relative strength of mortar on the observed damage evolution and compressive failure. Damage evolution associated with the formation and propagation of vertical splitting cracks during the compressive load response of masonry assemblages in the stack bonded arrangement are studied using a full-field optical technique based on digital image correlation. Using image correlation, clear evidence of the crack forming in the mortar and propagating into the brick is established. Damage is associated with cracking in the mortar due to lateral tension produced by the confined expansion of brick. Failure depends upon the tensile strength of the mortar. In mortar with lower strength than the brick unit, failure is produced by spalling associated with multiple cracks, which results in a loss of load bearing area. For mortar with a higher strength than the brick, cracking occurs when the level of compression is a significant proportion of the compressive strength of the brick material. Localized crushing resulting from localization of strain in a small region at the brick–mortar interface is produced at axial stress close to the unconfined compressive strength of the brick material. Failure in the case of high strength mortar is a result of the localized crushing and the resulting global instability.

The scaling of the ultimate load in fibre-reinforced polymer (FRP)–concrete debonding with the relative width of the FRP is experimentally investigated in this paper. Shear debonding tests are performed to evaluate the cohesive stress transfer between the adherents during the interface crack growth which produces debonding. Concrete specimens with two different widths and different widths of FRP are used in the experimental programme. The nominal stress at debonding increases with the FRP-to-concrete width ratio. For a given width of FRP composite sheet, lower debonding stress is obtained from concrete specimens with a larger width. The strain distribution on the FRP and concrete free surface at different stages of debonding was determined using a full-field optical technique known as digital image correlation. The contribution of the two factors, the boundary effect and the restraint from the surrounding concrete, was studied from the measured strain distribution. The strain distributions across the FRP composite sheet and the concrete within the cohesive stress transfer zone associated with the interface crack are shown to be very inhomogeneous. A region of constant width associated with high shear strains is found at the edge of the FRP sheet during the entire debonding process. The increase in the ultimate nominal stress at debonding is shown to be due to the decrease in the proportion of the total width of the FRP occupied by the edge region. It is shown that the boundary region within the FRP is of a fixed width. The width of concrete close to the edge of the FRP involved in stress transfer, however, increases with the width of FRP. It is established that when the FRP-to-concrete width ratio is smaller than 0.5, the level of restraint from concrete increases with the FRP width.

The primary focus of this paper is to develop an understanding of temperature changes introduced by hydration heat release in the first few hours after casting in the thermal movements and stresses of the concrete deck and girders. Temperature and strain measurements from a simply supported, single-span, steel girder bridge with a composite concrete deck are presented. It is shown that setting occurs during the temperature rise and partial strain compatibility between steel girder and concrete deck is initiated at the end of the concrete temperature rise. Full strain compatibility between concrete deck and steel girder is achieved at the end of the cooling period following the initial temperature rise. The stresses in the steel girder associated with temperature changes are interpreted using an analytical model. It is shown that the concrete deck gains sufficient stiffness at the end of the temperature rise to restrain the movement of the top flange. Concrete deck movement in the period associated with cooling following the initial temperature rise is restrained, which could potentially produce tensile stress in concrete. The magnitude of tensile stress at the end of the cooling period depends upon the difference in the temperatures of the concrete deck and top flange and on the temperature gradient in the steel girder at the end of the heating period.

Numerical simulations and laboratory measurements have been used to illuminate the interaction of a moving shock wave impacting on metallic grids at various shock strengths and grid solidities. The experimental work was carried out in a large scale shock tube facility while computational work simulated the flow field with a time-dependent inviscid and a time-dependent viscous model. The pressure drop measured across the grids is a result of two phenomena which are associated with the impact of the shock on the metallic grids. First are the reflection and refraction of the incoming shock on the grid itself. This appears to be the main inviscid mechanism associated with the reduction of the strength of the transmitted shock. Second, viscous phenomena are present during the reflection and refraction of the wave as well as during the passage of the induced flow of the air through the grid. The experimental data of pressure drop across the grid obtained in the present investigation are compared with those obtained from computations. The numerical results slightly overpredict the experimental data of relative pressure drop which increases substantially with grid solidity at fixed flow Mach numbers. The processes of shock reflection and refraction are continuous and they can be extended in duration by using thicker grids that will result in lower compression rates of the structural loading and increase the viscous losses associated with these phenomena which will further attenuate the impacting shock. Preliminary theoretical analysis suggests that the use of a graded porosity/solidity material will result in higher pressure drop than a constant porosity/solidity material and thus provide effective blast mitigation.

The increasing use of carbon fiber reinforced polymer (FRP) sheets for strengthening existing reinforced concrete beams has generated considerable research interest in understanding the debonding mechanism of failure in such systems. The influence of the width of the FRP on the load-carrying capacity is investigated in this paper. The interfacial crack propagation and strain distribution during shear debonding are studied using a full-field optical technique known as digital image correlation. The results indicate the development of high stress/strain gradients at the interface as a consequence of the relative slip between the FRP and the concrete. The interface stress transfer between the FRP and concrete produces axial strain gradients in the FRP along its length. In the vicinity of the edges along the width of the FRP, edge regions comprising of both FRP and concrete are established. The edge region is characterized by high strain gradients in a direction perpendicular to the length and is of fixed width throughout the debonding process. The size of the edge regions is also found to be quite independent of the width of the FRP. Mode-II fracture condition exists in the interface directly below the FRP away from the edge regions. The interfacial crack is shown to be associated with a cohesive stress transfer zone of fixed length. During debonding, the stress transfer zone is shown to propagate in a self-similar manner at a fixed load. The interface fracture properties obtained from the portion of FRP away from the edge regions are shown to be independent of the FRP width. It is shown that when the width of concrete is larger than that required for establishing the edge regions, the nominal stress at debonding increases with an increase in the width of FRP. The scaling in the load carrying capacity during shear debonding is shown to be the result of the edge regions which do not scale with the width of the FRP.

The debonding mode of failure, which is observed in girders strengthened using externally attached fiber-reinforced polymer (FRP) sheets, is studied in this paper. A numerical analysis of the direct-shear response of FRP attached to concrete substrate is performed to study the initiation, formation, and propagation of an interfacial crack between the two adherents. The material response of the bimaterial interface, which includes postpeak softening, is incorporated into the numerical model. The load response obtained numerically is shown to be in close agreement with that determined experimentally from direct shear tests on concrete blocks strengthened with FRP sheets. An instability in the load response is predicted close to failure and the arc-length method is used to obtain the entire load response past the displacement-limit point. The instability in the load response is shown to be a result of snapback, where both the load and the displacement decrease simultaneously. The effect of the bonded length on the stress transfer between the FRP and concrete and on the ultimate failure is also analyzed. It is shown that there is a scaling in the load capacity when the bonded length does not allow for the establishment of the full stress-transfer zone associated with interface crack growth. From the results of the numerical analysis, a fundamental understanding of interfacial crack propagation and instability at failure in concrete members strengthened using externally bonded FRP is developed. Using a simple energy based formulation; it is shown that in strengthened girders, the instability at complete debonding of FRP from concrete translates into an explosive failure associated with a sudden release of energy.

The last few years have witnessed a wide use of externally bonded fiber reinforced polymer (FRP) sheets for strengthening existing reinforced and prestressed concrete structures. The success of this strengthening method relies on the effectiveness of the load-transfer between the concrete and the FRP. Understanding the stress transfer and the failure of the concrete–FRP interface is essential for assessing the structural performance of strengthened beams and for evaluating the strength gain. This paper describes an experimental investigation of the interfacial bond behavior between concrete and FRP. The strain distributions in concrete and FRP are determined using an optical technique known as digital image correlation. The results confirm that the debonding process can be described in terms of crack propagation through the interface between concrete and FRP. The data obtained from the analysis of digital images was used to determine the interfacial material behavior for the concrete–FRP interface (stress...

The ultrasonic wave reflection method, as applied to characterize early age cement-based materials, is reviewed. The topic is first introduced with a historical review of method development. The theoretical basis of the reflection technique within the context of wave mechanics is then considered, followed by a description of experimental techniques and associated requirements of testing setups. Then fundamental background information about the method is described, including effects of ultrasonic wave mode (compression and shear waves) and buffer material (metal, ceramic and polymer) on the results and required experimental apparatus. Several test application tasks, across a range of cementitious material ages (hydration states), are summarized, including determination of global constitutive properties, estimation of setting time (initial and final sets), evolution of strength, and assessment of microstructure (e.g. flocculation state in setting material and porosity in solid material). The analyses of various techniques are presented in a unified manner for elastic, viscoelastic and poroelastic idealizations of the cementitious material.

Cementitious materials are transformed from a fluid to a solid state due to a chemical reaction known as hydration. These cementitious materials exhibit a continuous change in the mechanical properties with time; there is a steady increase in the stiffness after setting. An ultrasonic test setup and the data analysis procedure, which provide for continuous monitoring of the hydrating cementitious materials from a very early age, have recently been developed. The test procedure for obtaining the ultrasonic test data from cementitious material at different stages of hydration and the theoretical analysis, which allows interpreting the ultrasonic response in terms of the changes in the acoustic shear impedance of the hydrating cementitious material, are presented in this paper. Experimental test results obtained from mortar mixtures of known composition are presented. It is shown that the initial and final setting times correspond approximately with the occurrence of distinctive features in the ultrasonic response.

Fiber-reinforced polymers (FRP) composites are used as supplementary reinforcement to increase the in-plane shear capacity or to provide out-of-plane load-carrying capability of masonry walls and to modify the collapse mechanism in arches and vaults. In these applications, the efficiency of load transfer is limited by the debonding of FRP from the masonry substrate. In this paper, the debonding mechanism of FRP-masonry is experimentally studied. Experimental procedures and test specimens are designed to investigate the progressive debonding of FRP from brick and mortar substrates and relate it to the response obtained from the FRP-masonry interface. Surface displacements during debonding are obtained using digital image correlation. A one-dimensional numerical model is developed for predicting the fracture behavior along the FRP-masonry interface using the cohesive fracture parameters from the brick and mortar interfaces obtained from the computed strains.

During the last two decades, externally bonded uni-directional fiber-reinforced polymer (FRP) composites have been widely used for strengthening, repairing, and rehabilitation of reinforced concrete (RC) structural members. The bond characteristics contribute to the effectiveness of the stress transfer achieved between the FRP composite and the concrete substrate. Debonding of the FRP composite reinforcement is the most critical concern in this type of application. Under monotonic and fatigue loading conditions, FRP-concrete shear debonding has been idealized as a Mode-II fracture problem along the bi-material interface. A cohesive material law is used to describe the interfacial stress transfer at the macroscopic level. The area under the entire curve represents the fracture energy and is related to the load-carrying capacity of the interface. In this paper, previous experimental results and literature are discussed to show how the fracture energy can be considered a true fracture parameter. In addition, a simplistic one dimensional numerical analysis of the direct shear test is presented with the intent of pointing out the effect of the fracture parameters related to the cohesive material law on the load carrying capacity. The results are instrumental to discuss the strain limits provided in the ACI 440.2R-08 document.

In this paper, the behavior of concrete under static and fatigue compressive load is studied. Cylindrical specimens were subjected to static cyclic and constant amplitude fatigue loading. The static cyclic tests were performed by unloading and reloading the specimen at three different points in the postpeak period of the static loading response. Low cycle, high amplitude fatigue tests were performed to failure using three load amplitudes. It is found that under the term of structural compliance the static compressive response of concrete can be used as an envelope for the fatigue failure compressive response. The change rate of stiffness or compliance under fatigue loading follows a two-stage process: a deceleration stage followed by an acceleration stage up to failure. The failure mechanisms for both static and fatigue loading are explained by the band damage zone model and found to be consistent in the term of an inelastic displacement. The results agree well with the previous research work.

Concrete structures such as rigid airport pavements are subjected to repeated high-amplitude loads resulting from passing aircraft. The resulting stress-state in concrete is a biaxial combination of compression and tension. It is of interest to understand the response of plain concrete to such loading conditions, which will enable development of realistic material models for implementation in mechanistic pavement design procedures. The objective of this work is to characterize the quasi-static and low-cycle fatigue response of concrete subjected to biaxial stresses in the biaxial tension region, where the principal tensile stress is larger than or equal in magnitude when compared with the principal compressive stress. An experimental investigation of material behavior in the biaxial tension region is conducted. The experimental setup consists of the following test configurations: (a) notched concrete beams tested in three-point bend configuration, and (b) hollow concrete cylinders subjected to torsion. Failure of concrete in the biaxial tension region is shown to be a local phenomenon under quasi-static and fatigue loading, wherein the specimen fails owing to a single crack. The crack propagation is studied using the equivalent elastic crack concept. It is observed that the crack growth rate in constant amplitude fatigue loading exhibits a two-phase process: a deceleration phase followed by an acceleration stage. The crack growth in the acceleration stage is shown to follow Paris law. The model parameters obtained from uniaxial fatigue tests are shown to be sufficient for predicting the considered biaxial fatigue response.

Subway tunnel condition assessment presents significant challenges for engineers and managers and is becoming increasingly important as the systems continue to age. Tunnels are in constant heavy use in an aggressive environment. Tunnel systems are vast, dark, and noisy. The national investment in subway tunnels is enormous, and careful maintenance and management are necessary to protect this investment. Technologies that can rapidly and accurately access the condition of subway tunnels without interfering with the normal operation of the system were studied. First, issues and problems in subway tunnel maintenance were reviewed through the literature and by interviewing transit agency managers and engineers. Next, different nondestructive evaluation (NDE) methods including spectral analysis of surface waves, impact echo, ground-penetrating radar, and impulse response were evaluated to determine the advantages and limitations of these methods on different problems like water leakage, corrosion, and cracks in subway tunnel systems. Issues of data and infrastructure management were also considered. NDE technologies have considerable potential for improving the maintenance and management of transit infrastructure. However, to fully realize that potential, further development is needed. It is necessary to distinguish between methods that require interruption of subway traffic from those that do not. Rapid screening NDE methods must be researched to develop clear signals of delamination, moisture-related damage, and other issues of concern. It is also necessary to develop automated procedures to process the vast amounts of data generated during extensive NDE testing. Case studies and demonstration projects must be developed and documented to convince managers of the utility of this approach.

In this paper cyclic quasi-static and constant amplitude fatigue responses of concrete subjected tensile compression-tension (\IC-T\N) biaxial stress are presented. In the tensile \IC-T\N region within the biaxial stress space, magnitude of the principal tensile stress is larger than or equal to that of the principal compressive stress. An experimental program consisted of subjecting hollow, cylindrical concrete specimens to torsional loading. Failure in both quasi-static and fatigue is due to crack propagation. It is shown that the crack propagation resulting from the biaxial loading can be predicted using Mode I fracture parameters. The fatigue crack growth is observed to be a two-phase process: an acceleration stage that follows a deceleration stage. The crack length where the rate of crack growth changes from deceleration to acceleration is shown to be equal to the crack length at the quasi-static peak load. Analytical expressions for crack growth in the deceleration and acceleration stages are developed in terms of the mechanisms that influence quasi-static crack growth. The model parameters obtained from uniaxial fatigue tests are shown to be sufficient for predicting the biaxial fatigue response. Finally, a fracture-based fatigue-failure criterion is proposed, wherein the fatigue failure can be predicted using the critical Mode I stress intensity factor.

The NSF-sponsored Center for Advanced Cement-Based Materials is actively involved in research aimed at the development of technologies for health monitoring and nondestructive evaluation of the concrete infrastructure. This paper summarizes pertinent research performed at the center. Basic findings from several new laboratory-based nondestructive evaluation techniques for concrete are reported. The described techniques are based on measurements of mechanical waves that propagate in the concrete. First, ultrasonic longitudinal wave (also called the L-wave or P-wave) signal transmission (attenuation) measurements are shown to be sensitive to the presence of damage in the form of distributed cracking in concrete. Next, experimental procedures that enable practical one-sided wave signal transmission measurements to be performed on concrete structures are described. The utility of the signal transmission measurement is demonstrated by two experimental test series; the depths of surface-opening cracks in concrete slabs are estimated and the extent and nature of autogenous healing in concrete disks are studied. Finally, an approach by which fatigue-induced damage in concrete structures is nondestructively monitored is described. Vibration frequencies are shown to be sensitive to the presence of fatigue-induced cracking in concrete specimens; changes in the vibration frequency of a concrete specimen during fatigue tests are related to the remaining fatigue life of the test specimens.

Torsion tests have been used to evaluate the shear response of concrete. The complete load-deformation response of unnotched concrete specimens subjected to torsion is difficult to obtain because of test instabilities. In this paper torsional tests with a closed-loop test system are performed on hollow concrete cylinders. The test instability is shown to be a result of snapback in the postpeak part of the torque-twist response of these specimens. The failure is a result of the propagation of a single major crack. A test signal that is a combination of torque and gauge rotation is introduced and is demonstrated to give improved control by eliminating the test instability. The response of concrete specimens subjected to both controlled and uncontrolled torsion tests, using specially designed test equipment, are presented and analyzed in terms of propagation of a single major crack.

The influence of stress and induced damage in concrete on the electro-mechanical (EM) impedance response of bonded PZT patches is evaluated for applied compressive loading. Full field displacements obtained from digital image correlation are used to evaluate the level of stress-induced damage in concrete. Stress in the substrate produces an imposed strain on the PZT. A change in the imposed strain produces a rightward frequency shift and an increase in the amplitude of the resonant peak in the EM conductance spectrum of the PZT. An increase in the substrate compliance produces a decrease in the resonant frequency and an increase in the amplitude of the resonant peak. Changes in the resonant peak in the conductance spectrum induced by increasing substrate stress are of a significant magnitude when compared with the changes induced by damage. In the early stages of damage associated with distributed microcracking, the counteracting influences of increasing level of damage and increasing stress on the resonant peak result in no shift in frequency for measurements under applied load. There is however an increase in the amplitude of the resonance peak. When the applied stress is removed, there is a net decrease in frequency resulting from damage in the form of distributed microcracks. Measures of changes in the resonant peak based on root mean square deviation (RMSD), do not show any observable change when measurements are performed under applied loading. There is a consistent increase in RMSD values and frequency shift with increasing damage when the applied stress is removed. The centroidal measure of the normalized frequency spectrum reflects changes in substrate stress. At higher applied stress levels, there is a nonlinear increase in damage, leading to localization and cracking. The influence of damage is dominant in this region and significant changes are obtained in the RMSD values in both loaded and unloaded conditions.

The main purpose of this study is to investigate experimentally granite masonry specimens as well as its constituent components, granite stone units and cement mortar, separately under compression so that the results can be used for numerical modeling and analysis of granite masonry structures. The compressive behaviour of granite stone units and cement mortar are characterized and their effects on the strength and deformation characteristics of the masonry structure are examined. The experimental results indicate that granite stone specimens exhibit high peak strength values and very fragile behavior when subjected to compressive loading. The stress-strain relation of the mortar specimens show a strain hardening behavior with more ductility than the granite stone. The strength of granite masonry is much higher than the mortar but less than the average strength of the granite stone units. By using an in-situ optical technique called Digital Image Correlation, quantitative measurements of strain localization in granite masonry are achieved in both deformation and failure modes. It was found that cracks initiated at the vertical mortar joint interface and formed multiple shear bands. Further deformation occurs by extending the shear bands into the mortar joint layers, and finally the granite unit split vertically due to the laterally biaxial tensile force induced by the confinement effect of the mortar joints.

Developing rational design guidelines for application of FRP strengthening techniques to concrete beams requires a fundamental understanding of the shear debonding phenomenon and the associated failure mechanism. Several finite element analyses have been performed to study the shear stress transfer across the FRP-concrete interface and several analytical formulations based on the classical section analysis have been proposed for predicting the ultimate load of a strengthened beam These previous analyses however did not explicitly consider the material law of the interfacial bond between concrete and the FRP. Recently, researchers have developed various constitutive laws that adequately model the FRP-concrete bond behavior during debonding (Ali-Ahmad et al. 2005, Leung 2004, Savoia et al. 2003). Fracture-based analytical and numerical formulations have also been proposed to predict the initiation of shear debonding of FRP from concrete surface (Taljsten 1996, Wu and Niu 2000). However, the full understanding of the uncontrolled failure in the strengthened beams and influence of size on the observed failure produced by shear debonding, is still emerging.

The polarization response of a steel bar with an established macrocell is presented. A simple circuit-based model, which allows for predicting the polarization response of the macrocell is described. The results indicate that there is a spatial variation in the potential relative to the crack. It is shown that the applied current is primarily confined to the active steel located near the crack. Corrosion of steel reinforcement is one of the main causes of damage in concrete structures. Thus, the detection and prevention of corrosion of steel in concrete is impor- tant for the condition assessment and rehabilitation of reinforced concrete structures. Often the corrosion process of steel embedded in concrete is influenced by factors such as cracks in the concrete. Cracks in the concrete are often produced due to action of loads, restrained shrinkage or thermal gradients. Once a crack is formed, it provides an easy access for ingress of water, chloride ions and oxygen to the steel surface. Further- more, a crack introduces a physical discontinuity in the material medium and also pro- duces spatial variation in the ionic concentration of ions, oxygen and moisture throughout the length of steel bar. Relatively little work has been done to understand the mechanism of corrosion of steel embedded in cracked concrete 1-5 . This paper presents the results of an experimental program which aims to investigate the influence of a crack on the corrosion mechanism and the polarization response of steel in reinforced concrete. It is shown that steel embedded in cracked concrete forms a macro-cell, which results in a spatial variation in the potential of the steel bar. The results from external polarization of steel embedded inside cracked concrete are also presented. Finally, it is shown that due to the low polarization resistance of the active area close to the crack, the applied current is confined to the active area.

The increasing use of carbon fiber reinforced polymer (CFRP) sheets for strengthening existing reinforced concrete beams has generated considerable research interest in understanding the debonding mechanism of failure in such systems. Less attention has been paid on the application of the FRP technology to masonry structures and specifically in the case of restoration of historical monuments and buildings. Since this technique is, in many cases, not invasive, easy to apply and free from corrosion, its application has been extended to preserve historical buildings.

In this paper, the applicability of the wave reflection factor (WRF) technique for developing in-place procedures for assessing the rate of strength gain in concrete structures is determined. The research placed particular emphasis on highway pavement structures. Through a carefully planned experimental study involving laboratory-sized slab specimens of typical concrete mixture compositions used in highway/bridge construction, it is shown that the WRF technique accurately predicts the trends in strength development in such structures. The results appear promising in developing a predictive approach for strength gain based on the observed WRF trends.

A nondestructive evaluation technique for monitoring damage due to crack growth in concrete beams is presented in this paper. The technique is based on monitoring the resonant frequencies of vibration. An introduction to the concepts of vibrational modes in beams is detailed first followed by a description of the experimental procedure for resonance frequency measurement. The result of a finite element (FE) analysis that is performed to identify the different modes of vibration of the beam specimen, are then presented. The resonance frequencies determined by the FE analysis are shown to match closely with the experimental values. A notch is introduced in the specimen and the effect of notch length on the resonant frequencies is studied by varying the notch depth. Experimental results are compared with the result of FE simulation of the beam with different notch lengths. The influence of a real crack on the frequencies of the vibrational modes is also studied by loading a specimen in a three-point bending configuration and propagating a crack in a controlled manner using a closed-loop testing machine. Analysis of the obtained data is performed to evaluate the response of the different vibrational modes of the concrete beam specimen to varying crack and notch lengths. Frequencies of the vibrational modes decrease consistently with increasing crack and notch lengths. There is a larger decrease associated with increasing notch length. Resonance frequencies are shown to be sensitive to crack and notch growth in concrete beams and can be used to effectively monitor the decrease in structural stiffness due to crack progress progression.

The fatigue behavior of concrete subjected to combined stresses in the compression-tension region of the biaxial stress space is studied. Hollow cylindrical concrete specimens are subjected to combined stresses through torsional loading. The load-deflection responses of specimens subjected to cyclic and constant amplitude fatigue loading are presented. Damage imparted to the specimens during cyclic and fatigue loading processes was monitored using mechanical measurements and a nondestructive evaluation technique based on the measurement of structural resonance frequencies of vibration. The complete load response of the specimen subjected to cyclic loading was obtained by unloading the specimen at different points in the postpeak part (descending branch) of the quasistatic response. Changes in the resonant frequencies during the loading procedure were monitored. Fatigue tests were performed to failure with three different torsional load ranges. The decrease in rotational stiffness during fatigue tests was obtained from mechanical measurements, and the resonance frequencies are presented. It was observed that the decrease in rotational stiffness at failure for the constant amplitude fatigue loading was comparable to the corresponding load in the postpeak part of the quasistatic response. The number of cycles to failure is closely related to the rate of the reduction of stiffness of the specimen as well as the resonant frequencies in linear portion of the fatigue response. This relationship is independent of the applied load range. The fatigue failure of concrete subjected to torsional loading is a local phenomenon similar to failure for quasistatic loading; the damage is seen to localize to a crack in the first few cycles, and the subsequent fatigue behavior is governed by the propagation of that crack. An approach for predicting the fatigue life and the stiffness of a pavement structure is presented using the results.

A simple mathematical model using polynomial functions is presented for uniaxial cyclic behavior of clay brick masonry under compressive loading. The proposed model predicts the reloading and unloading stress-strain curves at different values of plastic (residual) strains, for both cases of loading, i.e., loading perpendicular to the bed joint and loading parallel to the bed joint. The model transfers the reloading and unloading curves to a normalized stress-strain coordinate system on which the reloading and unloading curves plot within a narrow range. A parent polynomial is then developed to fit the curves on this coordinate system. The individual reloading and unloading curves can then be produced by transferring the parent equation to the sress-strain coordinate system. Comparison between the experimenlal and computed stress-strain curves shows good correspondence

Subway tunnel condition assessment presents significant challenges for engineers, and is becoming increasingly important as the systems continue to age. Technologies are needed that can rapidly and accurately assess the condition of subway tunnels, and identify distress such as delamination, moisture-related damage, without interfering with the normal operation of the system. Towards this goal, different nondestructive evaluation methods including SASW, Impact Echo, GPR, and Impulse Response were evaluated to determine their advantages and limitations for tunnel evaluation. Since tunnels are in constant heavy use in an aggressive environment, it is necessary to distinguish between methods that can be used for high speed screening, and those that require interruption of subway traffic. It is also necessary to develop automated procedures to process the vast amounts of data generated during extensive NDE testing. Introduction and Problem Statement There is an increasing need for better methods to evaluate the integrity of transit infrastructure. Some transit infrastructure, particularly rail transit tunnels and underground structures, is over 100 years old. While the risk of collapse may be remote, there is a possibility of injury to the public and/or significant service interruption caused by falling concrete, loose reinforcing bars, and leakage. Traditional destructive techniques, such as coring, and currently used non-destructive techniques, such as hammer sounding, are time consuming, cover limited area, and have limited effectiveness in identifying possible sites of deterioration. 1 Department of Civil and Environmental Engineering, The University of Alabama at Birmingham, 1075 13 ~ Street South, Suite 120, Birmingham, Alabama 35294-4440 2 Department of Civil Engineering, School of Engineering, The City College of New York NYC 100M 3 Civil Engineering Department, Cleveland State University, Cleveland, Ohio 44115-2440 4 412 S. Peoria, Suite 340, Urban Transportation Center (MC 357), University of Illinois at Chicago 5 Department of Civil & Environmental Engineering, Rhodes 730, PO Box 210071, Cincinnati, OH 45221-0071

Rigid airport pavement structures suffer damage from multi-axial high magnitude cyclic stresses resulting from passing heavy aircraft. It is of interest to model the response of plain portland cement concrete to such loading conditions; thus, the sensitive detection and characterization of such damage during the loading process is important. Low strain vibrational resonance frequency measurement offers direct information concerning the global, apparent elastic moduli of the material and preliminary results have shown such measurements are sensitive to the presence of damage in concrete.The work reported here includes the theoretical foundation and experimental results of a non-destructive technique, based on vibrational resonance measurement. The tests are applied to monitor damage imparted to end-mounted hollow concrete cylinders subjected to monotonic and cyclic torsional (bi-axial) loads. An introduction to the concepts of vibration testing and details of mechanical and vibrational test procedures employed are given first. The most significant vibrational modes, of all of the possible modes setup within the specimen, are identified. The frequency value of these significant modes in the concrete specimen are experimentally obtained throughout a controlled cyclic testing procedure to failure. The behavior of these modes is then monitored during a controlled cyclic testing procedure to failure. Distinctions between the frequency values of the various excited resonance modes are noted. Moreover, effects of the two damage types (monotonic and cyclic) on the frequency values of the modes are studied. Finally, conclusions concerning the applicability of the vibrational resonance techniques for monitoring imparted damage in these concrete specimens are drawn.