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Experimental and numerical investigation of the fracture behaviour and fatigue resistance of self-compacting concrete

Sara Korte (UGent)
(2014)
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(UGent) , (UGent) and (UGent)
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Abstract
Unlike static loads, repeated loading actions on concrete structures can cause fatigue failure. Especially civil engineering structures, such as roads, (railway) bridges, beam cranes, marine and off-shore structures, are subjected to a large number of load cycles (millions or more), caused by traffic, waves or tidal currents. With each cycle, microscopic cracks are induced in the cement matrix, which gradually propagate during the further fatigue loading process until finally failure occurs. The more cycles a concrete structure has to sustain, the more the material irreversibly gets damaged and the less strength and stiffness remains. Hence, structural collapse may take place at a stress or strain level much lower than in case of a single static load. This well-known phenomenon is documented in literature for vibrated concrete, while this is not the case for self-compacting concrete. Because of its important advantages, this latter type of concrete is already used worldwide in many types of structures, including cyclically loaded ones, and its popularity is still growing. However, up to today, it is unsure whether self-compacting concrete performs better, worse, or equally under fatigue loading conditions, compared to conventional, vibrated concrete. The altered composition, affecting the whole microstructure, causes changes in the material characteristics, amongst which the crack resistance. Given the fact that fatigue damage is strongly related to crack propagation, a different response of self-compacting concrete to cyclic loading is not unrealistic. This study aims for a better understanding of the fatigue performance and failure mechanisms of self-compacting concrete, in comparison to vibrated concrete, in order to attain a correct and reliable application of the material. Both experimental and numerical research is performed, considering different concrete types. First, the fracture behaviour is investigated, and in a second part, the mechanical properties under diverse fatigue loading rates are examined. For studying some important fracture mechanics aspects, numerous notched specimens, made from normal, vibrated concrete, self-compacting concrete with similar compressive strength, and self-compacting concrete with equal water-to-cement ratio, have been subjected to three-point bending tests and wedge-splitting tests. Continuously registering the applied load and the according crack mouth opening displacement at the notch, allows to determine softening curves through inverse analysis and to calculate several fracture parameters (such as the fracture energy, the critical stress intensity factor, and the characteristic length) for evaluating the toughness/brittleness of the applied materials. The results point out that the vibrated mixture is tougher than the self-compacting concrete types. Furthermore, different crack resistance mechanisms are observed. In the vibrated concrete, the larger amount of coarse aggregates, causing interlock, affects the crack path by deflecting it around the aggregate particles. The longer and more complex crack propagation path thus enlarges the crack surface and the required fracture energy. On the other hand, the higher content of fine particles (limestone filler) and superplasticizer in the self-compacting concretes, as well as the higher water-to-cement ratio of the self-compacting concrete with similar strength, opposed to the vibrated type, contribute to a more brittle behaviour with cracking through the aggregates. Based on this more brittle nature of self-compacting concrete, a shorter fatigue life could be expected for this concrete type, opposed to vibrated concrete. Because during the fatigue failure mechanism of concrete a gradual strength and stiffness decrease takes place due to progressive growth of microcracks, the cracking resistance of concrete may be decisive for its fatigue performance. The fracture mechanical behaviour of the studied concrete types is also modelled in ABAQUS by using a simple, linear elastic material approach in combination with a description of the post-peak damage evolution law, based on the experimentally determined tensile strength and fracture energy. A 2D finite element analysis is conducted, for it suffices to fully capture the behaviour of the specimens, both in the wedge-splitting and the three-point bending test setup. Regarding the obtained crack patterns, a good agreement with the experimental observations is found, while the numerically generated load-displacement curves strongly deviate from the ones measured during the tests. However, it is proven that this is assigned to the fact that the constitutive model uses a linear softening diagram (instead of a bilinear one, as it is derived from the test results), thereby neglecting the long tail of the post-peak behaviour. The same concrete mixtures as applied in the fracture mechanics part of this work, have also been used to produce large, reinforced concrete beams for testing in a four-point bending rig. By designing an inversed T-shaped cross-section and by overdimensioning the reinforcement steel, there is aimed for failure due to crushing of the concrete in the compression zone. Based on the examination of several mechanical properties, including failure mechanism, deflection, strain, and crack width evolution, under cyclic loading conditions with different stress ranges, a comparison is made between the vibrated and the self-compacting concrete types. As intended, the most common failure mechanism during the cyclic tests is compressive fatigue failure, even though also rebar fracture occurs in some cases. The probability of these failure modes is shown to depend on the imposed loading interval (which is related to the amount of cycles). When upper load limits of 70% of the static bending strength or higher are applied, crushing of the concrete at midspan takes place, whereas less severe loading conditions rather cause rebar fatigue failure. When comparing the self-compacting concretes to the vibrated concrete type, no unique relationship, covering the full loading scope, can be found. Again, the loading range is crucial. Vibrated concrete demonstrates the largest fatigue resistance in case of lower loading levels (up to 70% of the static ultimate load), while both self-compacting concrete types perform best in the higher loading ranges. These findings only partly agree with the expectations, based on the outcome of the fracture mechanics experiments. Possibly, the cracking resistance of plain concrete specimens is not the only determinative factor for the fatigue performance in a reinforced structure. From the results of the cyclic four-point bending tests, it appears that the concrete-rebar interaction is equally (or even more) important. Regarding the observed mechanical properties, some important differences are noticed. The vertical displacement and the concrete strain increase during the experiments with higher cyclic loading levels is larger for both self-compacting concrete types, with respect to the vibrated one. Furthermore, self-compacting concrete (especially the one with equal water-to-cement ratio, compared to the vibrated concrete) generated, on average, a larger amount of cracks with a smaller crack spacing, and the fatigue crack propagation took place at an accelerated level. Practically, the results of the experimental program indicate that fatigue of concrete is crucial in low-cycle fatigue situations, such as earthquakes and storms, characterized by high loading amplitudes and a small number of cycles. For airport and highway pavements, bridges, and wind power plants, which are subjected to high-cycle fatigue loading, there is a greater possibility of rebar fatigue. Moreover, in cases with greater risk for crushing of the concrete, caution is required when applying self-compacting concrete, given the faster deterioration process, opposed to vibrated concrete. An attempt is made to develop a 3D numerical model (using the software ABAQUS) for assessing the fatigue life of the reinforced concrete beams in the four-point bending test setup. However, it needs to be mentioned that it is still in a preliminary stage, as it is only able to simulate a very small amount of load cycles. Nevertheless, for high loading intervals (e.g. between 10% and 85% of the static ultimate load), this restricted capability suffices to estimate the total number of cycles to failure by extrapolating the results of the initial period of rapidly increasing deformation. A fairly accurate agreement with the experimental data proves the correctness of this method. In case of lower load ranges, on the other hand, this technique cannot be applied yet, because the slower degradation process and the smaller deformation increments cause an unfeasibly large computation time with stability problems and convergence issues.
Keywords
Fracture Mechanics, Self-Compacting Concrete, Fatigue

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Citation

Please use this url to cite or link to this publication:

Chicago
Korte, Sara. 2014. “Experimental and Numerical Investigation of the Fracture Behaviour and Fatigue Resistance of Self-compacting Concrete”. Ghent, Belgium: Ghent University. Faculty of Engineering and Architecture.
APA
Korte, S. (2014). Experimental and numerical investigation of the fracture behaviour and fatigue resistance of self-compacting concrete. Ghent University. Faculty of Engineering and Architecture, Ghent, Belgium.
Vancouver
1.
Korte S. Experimental and numerical investigation of the fracture behaviour and fatigue resistance of self-compacting concrete. [Ghent, Belgium]: Ghent University. Faculty of Engineering and Architecture; 2014.
MLA
Korte, Sara. “Experimental and Numerical Investigation of the Fracture Behaviour and Fatigue Resistance of Self-compacting Concrete.” 2014 : n. pag. Print.
@phdthesis{5912710,
  abstract     = {Unlike static loads, repeated loading actions on concrete structures can cause fatigue failure. Especially civil engineering structures, such as roads, (railway) bridges, beam cranes, marine and off-shore structures, are subjected to a large number of load cycles (millions or more), caused by traffic, waves or tidal currents. With each cycle, microscopic cracks are induced in the cement matrix, which gradually propagate during the further fatigue loading process until finally failure occurs. The more cycles a concrete structure has to sustain, the more the material irreversibly gets damaged and the less strength and stiffness remains. Hence, structural collapse may take place at a stress or strain level much lower than in case of a single static load.
This well-known phenomenon is documented in literature for vibrated concrete, while this is not the case for self-compacting concrete. Because of its important advantages, this latter type of concrete is already used worldwide in many types of structures, including cyclically loaded ones, and its popularity is still growing. However, up to today, it is unsure whether self-compacting concrete performs better, worse, or equally under fatigue loading conditions, compared to conventional, vibrated concrete. The altered composition, affecting the whole microstructure, causes changes in the material characteristics, amongst which the crack resistance. Given the fact that fatigue damage is strongly related to crack propagation, a different response of self-compacting concrete to cyclic loading is not unrealistic.
This study aims for a better understanding of the fatigue performance and failure mechanisms of self-compacting concrete, in comparison to vibrated concrete, in order to attain a correct and reliable application of the material. Both experimental and numerical research is performed, considering different concrete types. First, the fracture behaviour is investigated, and in a second part, the mechanical properties under diverse fatigue loading rates are examined.
For studying some important fracture mechanics aspects, numerous notched specimens, made from normal, vibrated concrete, self-compacting concrete with similar compressive strength, and self-compacting concrete with equal water-to-cement ratio, have been subjected to three-point bending tests and wedge-splitting tests. Continuously registering the applied load and the according crack mouth opening displacement at the notch, allows to determine softening curves through inverse analysis and to calculate several fracture parameters (such as the fracture energy, the critical stress intensity factor, and the characteristic length) for evaluating the toughness/brittleness of the applied materials.
The results point out that the vibrated mixture is tougher than the self-compacting concrete types. Furthermore, different crack resistance mechanisms are observed. In the vibrated concrete, the larger amount of coarse aggregates, causing interlock, affects the crack path by deflecting it around the aggregate particles. The longer and more complex crack propagation path thus enlarges the crack surface and the required fracture energy. On the other hand, the higher content of fine particles (limestone filler) and superplasticizer in the self-compacting concretes, as well as the higher water-to-cement ratio of the self-compacting concrete with similar strength, opposed to the vibrated type, contribute to a more brittle behaviour with cracking through the aggregates.
Based on this more brittle nature of self-compacting concrete, a shorter fatigue life could be expected for this concrete type, opposed to vibrated concrete. Because during the fatigue failure mechanism of concrete a gradual strength and stiffness decrease takes place due to progressive growth of microcracks, the cracking resistance of concrete may be decisive for its fatigue performance. 
The fracture mechanical behaviour of the studied concrete types is also modelled in ABAQUS by using a simple, linear elastic material approach in combination with a description of the post-peak damage evolution law, based on the experimentally determined tensile strength and fracture energy. A 2D finite element analysis is conducted, for it suffices to fully capture the behaviour of the specimens, both in the wedge-splitting and the three-point bending test setup. Regarding the obtained crack patterns, a good agreement with the experimental observations is found, while the numerically generated load-displacement curves strongly deviate from the ones measured during the tests. However, it is proven that this is assigned to the fact that the constitutive model uses a linear softening diagram (instead of a bilinear one, as it is derived from the test results), thereby neglecting the long tail of the post-peak behaviour.
The same concrete mixtures as applied in the fracture mechanics part of this work, have also been used to produce large, reinforced concrete beams for testing in a four-point bending rig. By designing an inversed T-shaped cross-section and by overdimensioning the reinforcement steel, there is aimed for failure due to crushing of the concrete in the compression zone. Based on the examination of several mechanical properties, including failure mechanism, deflection, strain, and crack width evolution, under cyclic loading conditions with different stress ranges, a comparison is made between the vibrated and the self-compacting concrete types.
As intended, the most common failure mechanism during the cyclic tests is compressive fatigue failure, even though also rebar fracture occurs in some cases. The probability of these failure modes is shown to depend on the imposed loading interval (which is related to the amount of cycles). When upper load limits of 70% of the static bending strength or higher are applied, crushing of the concrete at midspan takes place, whereas less severe loading conditions rather cause rebar fatigue failure. 
When comparing the self-compacting concretes to the vibrated concrete type, no unique relationship, covering the full loading scope, can be found. Again, the loading range is crucial. Vibrated concrete demonstrates the largest fatigue resistance in case of lower loading levels (up to 70% of the static ultimate load), while both self-compacting concrete types perform best in the higher loading ranges. These findings only partly agree with the expectations, based on the outcome of the fracture mechanics experiments. Possibly, the cracking resistance of plain concrete specimens is not the only determinative factor for the fatigue performance in a reinforced structure. From the results of the cyclic four-point bending tests, it appears that the concrete-rebar interaction is equally (or even more) important.
Regarding the observed mechanical properties, some important differences are noticed. The vertical displacement and the concrete strain increase during the experiments with higher cyclic loading levels is larger for both self-compacting concrete types, with respect to the vibrated one. Furthermore, self-compacting concrete (especially the one with equal water-to-cement ratio, compared to the vibrated concrete) generated, on average, a larger amount of cracks with a smaller crack spacing, and the fatigue crack propagation took place at an accelerated level.
Practically, the results of the experimental program indicate that fatigue of concrete is crucial in low-cycle fatigue situations, such as earthquakes and storms, characterized by high loading amplitudes and a small number of cycles. For airport and highway pavements, bridges, and wind power plants, which are subjected to high-cycle fatigue loading, there is a greater possibility of rebar fatigue. Moreover, in cases with greater risk for crushing of the concrete, caution is required when applying self-compacting concrete, given the faster deterioration process, opposed to vibrated concrete.
An attempt is made to develop a 3D numerical model (using the software ABAQUS) for assessing the fatigue life of the reinforced concrete beams in the four-point bending test setup. However, it needs to be mentioned that it is still in a preliminary stage, as it is only able to simulate a very small amount of load cycles. Nevertheless, for high loading intervals (e.g. between 10% and 85% of the static ultimate load), this restricted capability suffices to estimate the total number of cycles to failure by extrapolating the results of the initial period of rapidly increasing deformation. A fairly accurate agreement with the experimental data proves the correctness of this method. In case of lower load ranges, on the other hand, this technique cannot be applied yet, because the slower degradation process and the smaller deformation increments cause an unfeasibly large computation time with stability problems and convergence issues.},
  author       = {Korte, Sara},
  isbn         = {9789085787419},
  keywords     = {Fracture Mechanics,Self-Compacting Concrete,Fatigue},
  language     = {dut},
  pages        = {XXIII, 388},
  publisher    = {Ghent University. Faculty of Engineering and Architecture},
  school       = {Ghent University},
  title        = {Experimental and numerical investigation of the fracture behaviour and fatigue resistance of self-compacting concrete},
  year         = {2014},
}