Delamination of the Dehumidified Repair Mortars: A New Experimental and Numerical Analisys
An innovative laboratory procedure, developed at the Non Destructive Testing Laboratory of the Po... more An innovative laboratory procedure, developed at the Non Destructive Testing Laboratory of the Politecnico di Torino, as a preliminary design stage for the pre-qualification of repair mortars applied to historical masonry buildings is described. The tested repair mortars are suitable for the new dehumidified plaster in order to stop the rising damp effects by capillary action on historical masonry walls. Long-term plaster delamination occurs frequently as a consequence of not compatible mechanical characteristics of mortar. Preventing this phenomenon is the main way to increase the durability of repair work. The use of materials similar to that historical in terms of mechanical, chemical and thermo-hygrometric performances is better. The procedure consists in the application of static loads to mixed stone block-mortar specimens having peculiar characteristics in terms of geometry and adhesion at the interface, with continuous monitoring of the longitudinal and transverse displacements. A numerical simulation based on the cohesive crack model was used to follow the experimental data, so as to describe the evolutionary phenomenon of de-bonding as a function of a small number of parameters. The methodology is in progress of carrying out at the Special Natural Reserve of the Sacro Monte di Varallo (UNESCO heritage site) in Piedmont (Italy)
When the long-time behaviour of a concrete dam is analysed, the International Commission of Large... more When the long-time behaviour of a concrete dam is analysed, the International Commission of Large Dams recommends to neglect the tensile strength of the dam-foundation joint and to include the uplift pressure due to the water penetrating into the crack. In this context a non-linear problem of contact with friction occurs in the vicinity of the point which separates the damaged part of the joint from the undamaged one. In these conditions the solution depends on the stress path followed during the quasi-static incremental process. Therefore the classical Newton-Raphson fails to converge and has to be replaced by a Large Time Increment method. In this way it was possible to obtain realistic solutions for three different mechanical regimes.
Time—dependent fracture of concrete dam models with fuzzy parameters
Nucleation and Atmospheric Aerosols, 2001
In order to apply the mechanical properties (measured on material specimens or laboratory-sized m... more In order to apply the mechanical properties (measured on material specimens or laboratory-sized models) to large structures (such as concrete dams), a nonlinear theory able to predict the size-scale effect has to be used. One of these theories was first proposed by Hillerborg and co-workers (fictitious crack model) and is based on the previous works of Barenblatt and Dugdale for metals (cohesive crack model). It is based on the existence of a Fracture Process Zone (shortened FPZ), where the material undergoes strain softening. The behavior of the material outside the FPZ is linear elastic. A large number of short time laboratory tests were executed, by varying the load, under CMOD control. Since concrete exhibits a time–dependent behavior, an interaction between creep and micro-crack growth occurs in the FPZ. Therefore a different testing condition can be applied: rupture can be achieved by keeping the load constant before peak value (pre-peak tests), or after peak value and after an unloading and reloadi...
The non-linear behaviour of concrete-like materials in tension is characterised by strain-softeni... more The non-linear behaviour of concrete-like materials in tension is characterised by strain-softening. One of the models most widely used to study this kind of behaviour is that of the cohesive crack. When applying this evolutive model to mixed-mode problems it is possible to meet with a number of singular points: this paper examines possible ways to cope with two of them: the singularity of the stiffness matrix and the loss in directional stability of the crack trajectory. In this manner, it is possible to obtain mechanical information of interest for application purposes, even in the presence of such singular points.
Failure Lifetime Prediction Of Cracked Concrete Structures
MRS Proceedings, 1997
ABSTRACTThis paper presents the results of creep tests on three notched dam models made with thre... more ABSTRACTThis paper presents the results of creep tests on three notched dam models made with three type of concrete. In order to characterise the time-dependent behaviour of the process zone, a series of tensile tests was performed for each kind of concrete. On the basis of the law obtained in this way, crack propagation was simulated numerically through the Cohesive Crack model.
The mechanical behaviour of joints plays a key role in concrete dam engineering since the joint i... more The mechanical behaviour of joints plays a key role in concrete dam engineering since the joint is the weakest point in the structure and therefore the evolutionary crack process occurring along this line determines the global load bearing capacity. The reference volume involved in the above mentioned process is so large that it cannot be tested in a laboratory: a numerical model is needed. The use of the asymptotic expansions proposed by Karihaloo and Xiao 2008 at the tip of a crack with normal cohesion and Coulomb friction can overcome the numerical difficulties that appear in large scale problems when the Newton-Raphson procedure is applied to a set of equilibrium equations based on ordinary shape functions (Standard Finite Element Method). In this way it is possible to analyse problems with friction and crack propagation under the constant load induced by hydromechanical coupling. For each position of the fictitious crack tip, the condition K 1 = K 2 = 0 allows us to obtain the external load level and the tangential stress at the tip. If the joint strength is larger than the value obtained, the solution is acceptable, because the tensile strength is assumed negligible and the condition K 1 = 0 is sufficient to cause the crack growth. Otherwise the load level obtained can be considered as an overestimation of the critical value and a special form of contact problem has to be solved along the fictitious process zone. For the boundary condition analysed (ICOLD benchmark on gravity dam model), after an initial increasing phase, the water lag remains almost constant and the maximum value of load carrying capacity is achieved when the water lag reaches its constant value.
The interaction between strain-softening and time-dependent behaviour is analysed in the case of ... more The interaction between strain-softening and time-dependent behaviour is analysed in the case of quasi-static fracture. A fractional order rate law is coupled with a micromecanical model for the fracture process zone. In this way, a whole range of dissipative mechanisms is included in a single viscous element. This problem is analised through the finite element method and the cohesive (or fictitious) crack model. The comparison with creep tests executed on prenotched beams shows a good agreement.
The safety of cracked concrete dams is fundamentally affected by their mechanical behaviour under... more The safety of cracked concrete dams is fundamentally affected by their mechanical behaviour under seismic excitation. Such a load is far from being harmonic and is characterized by intermittent spikes. Therefore the sequence effect is analysed. Two widely accepted non-linear methods were used: the Cohesive Crack Model to analyse the evolution of the process zone and the Continuous Function Model (CFM) to analyse the local hysteresis loop. In its original formulation, CFM predicts that a higher preloading arrests the fatigue crack growth at a subsequent lower load level. This unrealistic and unconservative behaviour is due to the fact that the above mentioned model neglects the damage occurring during the so-called inner-loops. In other words the CFM assumes that inner loops are mere loading loops and not fatigue loops. This assumption causes an incorrect prediction of the sequence effect. For the same reason the CFM predicts an endurance limit which is higher than attested by experimental evidence. In order to obtain more realistic results, in the present paper the CFM was enhanced, introducing a damage mechanism for the inner loops too. In the new model proposed, as well as in the original CFM, the endurance limit is seen to be almost constant relative to structural size.
The Foziling multi-arch dam, one of the few multi-arch dams in the world, was built on the bedroc... more The Foziling multi-arch dam, one of the few multi-arch dams in the world, was built on the bedrock with complicated geological conditions. It has undergone several reinforcements since it was put into service in the 1950s. In this study, the dam safety is evaluated by analyzing the measured displacements and simulating stresses in the concrete. Firstly, the multiple linear stepwise regression (MLSR) is used to train and test the relationships between the loads and displacement based on the hydrostatic-temperature-time (HTT) model. Subsequently, the contributions of water level, temperature, and time to displacements are determined, and the influence characteristics of water level and temperature on displacements are interpreted. Finally, the dam stress state is evaluated by establishing a dam finite element model and simulating the stress distribution in various operating conditions. The results indicate that (1) the dam is currently in an elastic state after the last reinforcement; (2) temperature contributes the most to the displacement, and the drastic fluctuation of temperature is the disadvantage factor for multi-arch dam safety; (3) the stresses generally can meet the requirements of code; and (4) the ideas and methods of the study can provide references for the safety evaluation of other concrete dams. Dams play an important role in water supply, flood control, irrigation, navigation, sedimentation control, and hydropower generation. Dam safety is closely related to property safety, economic development, and the ecological environment in the area around the dam. Dam safety monitoring by instruments is the most important project of dam operation management 1,2. The data on water level, temperature, stress-strain, and seepage is important monitoring variables in the safety evaluation of concrete dams. The actual response of the structure, such as displacement, is often compared with the model prediction so that the abnormal behavior of the dam can be detected in time and corrective measures can be taken 3-6. Displacement observation, which is intuitive and reliable, and can explain the dam behavior itself, has been widely used in practice 1,7,8. Studies have shown that the displacement is closely related to such factors as water level, temperature, and time. Therefore, establishing an accurate relationship between these factors and displacement can explain and predict dam behavior effectively. At present, there are many models used to describe and predict dam behavior, mainly including statistical models, deterministic models, and mixed hybrid models 9,10. Statistical models, dating back to the 1950s, are exclusively based on the past monitoring data for explaining the relationship between loads and dam behavior. The statistical models based on multiple linear regression (MLR) and stepwise regression (SR) are widely used for predicting dam behavior due to their simple formulation and fast calculation speed 11,12. Deterministic models typically explain the behavior of dam on the concept of mechanics based on the finite element method (FEM). However, due to the uncertainty of dam body and foundation materials, it is very difficult to accurately predict dam behavior by FEM. With the development of artificial intelligence technology, machine learning algorithms offer new ways to establish dam behavior prediction models. For instance, it is provided that the models based on the artificial neural network are more accurate in the prediction of the dam displacement than the MLR model 13-15. The predictive models based on the radial basis function (RBF) network are proved to have the ability to approximate any nonlinear function 16. Support vector machine shows high accuracy in predicting dam displacement 17,18. A hybrid model based on a swarm optimized neural fuzzy inference system performed better than multilayer perceptron neural networks and
Numerical Modelling of Mixed Mode Cohesive Crack Propagation
Springer eBooks, 1988
The influence of shear on the process of cracking in concrete structures is of great practical im... more The influence of shear on the process of cracking in concrete structures is of great practical importance. In the terminology of fracture mechanics, this is called Mode II or Mixed Mode loading. According to the ‘Fictitious Crack Model’,the non-linear crack behavior can be described by means of cohesive forces in the damage zone representing the aggregate interlocking and bridging. Using Finite Element Method (FEM) and inter-element crack propagation, continuous modification of the mesh and nodal renumbering is required. This paper presents a review of some computational results obtained by a program developed by the authors and performing the above mentioned procedure. Quadratic displacement isoparametric elements are used. Strain-softening at the crack surface is included.
International Journal of Architectural Heritage, Nov 21, 2013
An innovative laboratory procedure for the pre-qualification of repair mortars is described. The ... more An innovative laboratory procedure for the pre-qualification of repair mortars is described. The tested mortars are suitable for use with new dehumidified plasters applied to historical masonry walls. Long-term plaster detachment frequently occurs because of the mechanical incompatibility of mortar. The procedure consists of the application of static loads to mixed stone block-mortar specimens with particular characteristics, in terms of geometry and adhesion at the interface. A numerical simulation based on the cohesive crack model was used to follow the experimental data, in order to describe the evolutionary phenomenon of detachment as a function of a small number of parameters. The methodology is currently being used at Sacro Monte di Varallo Special Natural Reserve (UNESCO heritage site) in Piedmont (Italy).
Creep Induced Cohesive Crack Propagation in Mixed Mode
Westergaard (1939) gave the first solution to the problem of a Mode I crack in an elastic medium.... more Westergaard (1939) gave the first solution to the problem of a Mode I crack in an elastic medium. He admits infinite stresses (stress singularity) at the crack tip, introducing the concept of stress intensity factors as a measure of stress field severity. This theory was called Linear Elastic Fracture Mechanics (L.E.F.M.).
Delamination of the Dehumidified Repair Mortars: A New Experimental and Numerical Analisys
An innovative laboratory procedure, developed at the Non Destructive Testing Laboratory of the Po... more An innovative laboratory procedure, developed at the Non Destructive Testing Laboratory of the Politecnico di Torino, as a preliminary design stage for the pre-qualification of repair mortars applied to historical masonry buildings is described. The tested repair mortars are suitable for the new dehumidified plaster in order to stop the rising damp effects by capillary action on historical masonry walls. Long-term plaster delamination occurs frequently as a consequence of not compatible mechanical characteristics of mortar. Preventing this phenomenon is the main way to increase the durability of repair work. The use of materials similar to that historical in terms of mechanical, chemical and thermo-hygrometric performances is better. The procedure consists in the application of static loads to mixed stone block-mortar specimens having peculiar characteristics in terms of geometry and adhesion at the interface, with continuous monitoring of the longitudinal and transverse displacements. A numerical simulation based on the cohesive crack model was used to follow the experimental data, so as to describe the evolutionary phenomenon of de-bonding as a function of a small number of parameters. The methodology is in progress of carrying out at the Special Natural Reserve of the Sacro Monte di Varallo (UNESCO heritage site) in Piedmont (Italy)
When the long-time behaviour of a concrete dam is analysed, the International Commission of Large... more When the long-time behaviour of a concrete dam is analysed, the International Commission of Large Dams recommends to neglect the tensile strength of the dam-foundation joint and to include the uplift pressure due to the water penetrating into the crack. In this context a non-linear problem of contact with friction occurs in the vicinity of the point which separates the damaged part of the joint from the undamaged one. In these conditions the solution depends on the stress path followed during the quasi-static incremental process. Therefore the classical Newton-Raphson fails to converge and has to be replaced by a Large Time Increment method. In this way it was possible to obtain realistic solutions for three different mechanical regimes.
Time—dependent fracture of concrete dam models with fuzzy parameters
Nucleation and Atmospheric Aerosols, 2001
In order to apply the mechanical properties (measured on material specimens or laboratory-sized m... more In order to apply the mechanical properties (measured on material specimens or laboratory-sized models) to large structures (such as concrete dams), a nonlinear theory able to predict the size-scale effect has to be used. One of these theories was first proposed by Hillerborg and co-workers (fictitious crack model) and is based on the previous works of Barenblatt and Dugdale for metals (cohesive crack model). It is based on the existence of a Fracture Process Zone (shortened FPZ), where the material undergoes strain softening. The behavior of the material outside the FPZ is linear elastic. A large number of short time laboratory tests were executed, by varying the load, under CMOD control. Since concrete exhibits a time–dependent behavior, an interaction between creep and micro-crack growth occurs in the FPZ. Therefore a different testing condition can be applied: rupture can be achieved by keeping the load constant before peak value (pre-peak tests), or after peak value and after an unloading and reloadi...
The non-linear behaviour of concrete-like materials in tension is characterised by strain-softeni... more The non-linear behaviour of concrete-like materials in tension is characterised by strain-softening. One of the models most widely used to study this kind of behaviour is that of the cohesive crack. When applying this evolutive model to mixed-mode problems it is possible to meet with a number of singular points: this paper examines possible ways to cope with two of them: the singularity of the stiffness matrix and the loss in directional stability of the crack trajectory. In this manner, it is possible to obtain mechanical information of interest for application purposes, even in the presence of such singular points.
Failure Lifetime Prediction Of Cracked Concrete Structures
MRS Proceedings, 1997
ABSTRACTThis paper presents the results of creep tests on three notched dam models made with thre... more ABSTRACTThis paper presents the results of creep tests on three notched dam models made with three type of concrete. In order to characterise the time-dependent behaviour of the process zone, a series of tensile tests was performed for each kind of concrete. On the basis of the law obtained in this way, crack propagation was simulated numerically through the Cohesive Crack model.
The mechanical behaviour of joints plays a key role in concrete dam engineering since the joint i... more The mechanical behaviour of joints plays a key role in concrete dam engineering since the joint is the weakest point in the structure and therefore the evolutionary crack process occurring along this line determines the global load bearing capacity. The reference volume involved in the above mentioned process is so large that it cannot be tested in a laboratory: a numerical model is needed. The use of the asymptotic expansions proposed by Karihaloo and Xiao 2008 at the tip of a crack with normal cohesion and Coulomb friction can overcome the numerical difficulties that appear in large scale problems when the Newton-Raphson procedure is applied to a set of equilibrium equations based on ordinary shape functions (Standard Finite Element Method). In this way it is possible to analyse problems with friction and crack propagation under the constant load induced by hydromechanical coupling. For each position of the fictitious crack tip, the condition K 1 = K 2 = 0 allows us to obtain the external load level and the tangential stress at the tip. If the joint strength is larger than the value obtained, the solution is acceptable, because the tensile strength is assumed negligible and the condition K 1 = 0 is sufficient to cause the crack growth. Otherwise the load level obtained can be considered as an overestimation of the critical value and a special form of contact problem has to be solved along the fictitious process zone. For the boundary condition analysed (ICOLD benchmark on gravity dam model), after an initial increasing phase, the water lag remains almost constant and the maximum value of load carrying capacity is achieved when the water lag reaches its constant value.
The interaction between strain-softening and time-dependent behaviour is analysed in the case of ... more The interaction between strain-softening and time-dependent behaviour is analysed in the case of quasi-static fracture. A fractional order rate law is coupled with a micromecanical model for the fracture process zone. In this way, a whole range of dissipative mechanisms is included in a single viscous element. This problem is analised through the finite element method and the cohesive (or fictitious) crack model. The comparison with creep tests executed on prenotched beams shows a good agreement.
The safety of cracked concrete dams is fundamentally affected by their mechanical behaviour under... more The safety of cracked concrete dams is fundamentally affected by their mechanical behaviour under seismic excitation. Such a load is far from being harmonic and is characterized by intermittent spikes. Therefore the sequence effect is analysed. Two widely accepted non-linear methods were used: the Cohesive Crack Model to analyse the evolution of the process zone and the Continuous Function Model (CFM) to analyse the local hysteresis loop. In its original formulation, CFM predicts that a higher preloading arrests the fatigue crack growth at a subsequent lower load level. This unrealistic and unconservative behaviour is due to the fact that the above mentioned model neglects the damage occurring during the so-called inner-loops. In other words the CFM assumes that inner loops are mere loading loops and not fatigue loops. This assumption causes an incorrect prediction of the sequence effect. For the same reason the CFM predicts an endurance limit which is higher than attested by experimental evidence. In order to obtain more realistic results, in the present paper the CFM was enhanced, introducing a damage mechanism for the inner loops too. In the new model proposed, as well as in the original CFM, the endurance limit is seen to be almost constant relative to structural size.
The Foziling multi-arch dam, one of the few multi-arch dams in the world, was built on the bedroc... more The Foziling multi-arch dam, one of the few multi-arch dams in the world, was built on the bedrock with complicated geological conditions. It has undergone several reinforcements since it was put into service in the 1950s. In this study, the dam safety is evaluated by analyzing the measured displacements and simulating stresses in the concrete. Firstly, the multiple linear stepwise regression (MLSR) is used to train and test the relationships between the loads and displacement based on the hydrostatic-temperature-time (HTT) model. Subsequently, the contributions of water level, temperature, and time to displacements are determined, and the influence characteristics of water level and temperature on displacements are interpreted. Finally, the dam stress state is evaluated by establishing a dam finite element model and simulating the stress distribution in various operating conditions. The results indicate that (1) the dam is currently in an elastic state after the last reinforcement; (2) temperature contributes the most to the displacement, and the drastic fluctuation of temperature is the disadvantage factor for multi-arch dam safety; (3) the stresses generally can meet the requirements of code; and (4) the ideas and methods of the study can provide references for the safety evaluation of other concrete dams. Dams play an important role in water supply, flood control, irrigation, navigation, sedimentation control, and hydropower generation. Dam safety is closely related to property safety, economic development, and the ecological environment in the area around the dam. Dam safety monitoring by instruments is the most important project of dam operation management 1,2. The data on water level, temperature, stress-strain, and seepage is important monitoring variables in the safety evaluation of concrete dams. The actual response of the structure, such as displacement, is often compared with the model prediction so that the abnormal behavior of the dam can be detected in time and corrective measures can be taken 3-6. Displacement observation, which is intuitive and reliable, and can explain the dam behavior itself, has been widely used in practice 1,7,8. Studies have shown that the displacement is closely related to such factors as water level, temperature, and time. Therefore, establishing an accurate relationship between these factors and displacement can explain and predict dam behavior effectively. At present, there are many models used to describe and predict dam behavior, mainly including statistical models, deterministic models, and mixed hybrid models 9,10. Statistical models, dating back to the 1950s, are exclusively based on the past monitoring data for explaining the relationship between loads and dam behavior. The statistical models based on multiple linear regression (MLR) and stepwise regression (SR) are widely used for predicting dam behavior due to their simple formulation and fast calculation speed 11,12. Deterministic models typically explain the behavior of dam on the concept of mechanics based on the finite element method (FEM). However, due to the uncertainty of dam body and foundation materials, it is very difficult to accurately predict dam behavior by FEM. With the development of artificial intelligence technology, machine learning algorithms offer new ways to establish dam behavior prediction models. For instance, it is provided that the models based on the artificial neural network are more accurate in the prediction of the dam displacement than the MLR model 13-15. The predictive models based on the radial basis function (RBF) network are proved to have the ability to approximate any nonlinear function 16. Support vector machine shows high accuracy in predicting dam displacement 17,18. A hybrid model based on a swarm optimized neural fuzzy inference system performed better than multilayer perceptron neural networks and
Numerical Modelling of Mixed Mode Cohesive Crack Propagation
Springer eBooks, 1988
The influence of shear on the process of cracking in concrete structures is of great practical im... more The influence of shear on the process of cracking in concrete structures is of great practical importance. In the terminology of fracture mechanics, this is called Mode II or Mixed Mode loading. According to the ‘Fictitious Crack Model’,the non-linear crack behavior can be described by means of cohesive forces in the damage zone representing the aggregate interlocking and bridging. Using Finite Element Method (FEM) and inter-element crack propagation, continuous modification of the mesh and nodal renumbering is required. This paper presents a review of some computational results obtained by a program developed by the authors and performing the above mentioned procedure. Quadratic displacement isoparametric elements are used. Strain-softening at the crack surface is included.
International Journal of Architectural Heritage, Nov 21, 2013
An innovative laboratory procedure for the pre-qualification of repair mortars is described. The ... more An innovative laboratory procedure for the pre-qualification of repair mortars is described. The tested mortars are suitable for use with new dehumidified plasters applied to historical masonry walls. Long-term plaster detachment frequently occurs because of the mechanical incompatibility of mortar. The procedure consists of the application of static loads to mixed stone block-mortar specimens with particular characteristics, in terms of geometry and adhesion at the interface. A numerical simulation based on the cohesive crack model was used to follow the experimental data, in order to describe the evolutionary phenomenon of detachment as a function of a small number of parameters. The methodology is currently being used at Sacro Monte di Varallo Special Natural Reserve (UNESCO heritage site) in Piedmont (Italy).
Creep Induced Cohesive Crack Propagation in Mixed Mode
Westergaard (1939) gave the first solution to the problem of a Mode I crack in an elastic medium.... more Westergaard (1939) gave the first solution to the problem of a Mode I crack in an elastic medium. He admits infinite stresses (stress singularity) at the crack tip, introducing the concept of stress intensity factors as a measure of stress field severity. This theory was called Linear Elastic Fracture Mechanics (L.E.F.M.).
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Papers by Silvio Valente