Mechanics of Structural Concrete, Civil, IISc



Mechanics of Structural Concrete                                                                                                                     Back to Top









Studies on fiber reinforced concrete

 An experimental research program focusing on the behavior of high strength concrete prestressed beams, which were expected to have dominant failure modes in (a) flexure or (b) shear, in the absence of fibers, was undertaken. The experimental program was supplemented by an analytical study, based on the development of empirical expressions to predict response quantities and a finite element simulation of the tests. An increased toughness and a more ductile stress strain response were observed with an increase in fiber content, when the fibers were distributed over either the full or partial depth of un-reinforced concrete beam cross-sections. Based on the test results, a robust analytical procedure has been proposed to establish the required partial depth of fibers in order to obtain the complete flexural capacity of full depth fiber reinforced concrete beams having the same fiber content. Behavior of fully or partially prestressed members designed to fail in diagonal shear in the absence of fibers were also studied experimentally to establish the influence of fiber content and fiber placement on the overall response of the member. A rigorous analytical model to predict the shear strength of prestressed high strength concrete beams containing steel fibers has been proposed. A finite element prediction of the test results from this study obtained using the ANSYS commercial software was also carried out. The presence of fibers in a concrete matrix was considered by suitably modifying the failure surface of concrete available in ANSYS to account for the presence of fibers and through the modeling of discrete fibers in the primary direction of the beams which were statistically quantified. The finite element solution was found to capture the salient features in the tests and accurately quantify the contribution of the fiber in sustenance of the load


Studies on strain rate of loading effects on concrete

A program of study that has been underway has focused on the development of a constitutive model for concrete including strain rate of loading effects. While it has been well known that concrete has a substantially enhanced strength when subjected to high strain rates of loading, traditional design methods of reinforced concrete structures have predominantly considered the effect of dynamic loads (earthquakes, wind gusts, blasts, etc.) through equivalent static loads. This has sometimes led to inconsistencies, and inadequacies in the designs of these structures leading to catastrophic failures. A constitutive model based on elasto-viscoplasticity for concrete and the reinforcement has been formulated and implemented into a special-purpose, 2-dimensional plane stress finite element program. The model accounts for strain rate sensitivity in the flow rule, hardening law and the initial yield and failure surface. The model was found to capture the rate effects to a fair extent over a strain rate range from 10-6 /s (psuedo-static loading) to 5 /s (low intensity blast loads).


Studies on the role of fiber reinforced plastic (FRP) reinforcement in concrete

Corrosion of steel reinforcement and high life cycle costs associated with the maintenance of reinforced and prestressed concrete construction has motivated researchers in recent years to seek alternative materials to serve as reinforcement in concrete in place of steel. Studies on the behavior of fiber reinforced plastic (FRP) rebars (Glass or carbon) in concrete, as a reinforcement for replacing steel rebars, has just been initiated. Some preliminary experimental work has been undertaken. The preliminary experimental program was designed to assess the performance of the indigenously available FRP rebars (Glass fiber) as reinforcement in lieu of the traditional steel reinforcement. Properties, such as the brittleness of the FRP rebar, the bond strength of these rebars with concrete and the ductility of the FRP reinforced beams in contrast to the benchmark steel reinforced beam have also been investigated. The use of latex additives and fibers in selective zones of the beam has been examined as part of this study, which is underway.


Studies on repair of structural elements with adhesively bonded patches

The cost of rehabilitation of concrete structures has led to a number of developments in the use of new repair materials. These materials in addition to their strength contribute through their flexibility in taking any shape to offering a quick and simple repair procedure in structural concrete applications in damaged regions that are often inaccessible. Studies on the use of FRP wraps wound around damaged areas of structural concrete sections and then bonded using thermosetting adhesives have been undertaken and found to be very attractive as a repair material. The use of adhesively bonded steel plates to damaged reinforced concrete sections has also been studied previously for its suitability as a repair material.


Fracture Mechanics of Concrete 

1.     Shear deformation beam theories on the fracture toughness of composites has been studied. It is observed that higher than 2nd order term in the contribution by shear deformation would significantly influence the results and those results are in better agreement with the experimental ones. Different types of specimens have been modeled and results obtained. A Ph.D. thesis resulted out of the above investigation.

2.      Influence of crack in a node of a space frame on its compliance has also been modeled and it is observed that plasticity contained in the material of the node contributes significantly in the delay of crack propagation. It is found that there is enough ductility in the nodes used in practice. A chapter in a thesis on the joint compliance on the behaviour of space structures contains the influence of crack.

3.      Fractal and lattice models for uniaxial tensile behaviour of concrete as a heterogeneous and quasi-brittle material have been successfully done. Fictitious crack model has been successfully tried to obtain the cohesiveness of crack.

4.   Ductility of beam column joint under slow cycle loading has been experimentally determined.

5.      Fracture toughness of high strength concrete has also been obtained experimentally. It was also a part of the BRNS Project for DAE. It also resulted in a Ph.D. thesis.


Fracture Mechanics and its applications in safety assessment of dams

For   over   five thousand years,  as  evidenced  in  the  cradles  of civilization, dams have played a vital role in regulating water flows to cater  to  the  needs  of irrigation. More recently, dams have also been extensively  used  for  the production of electricity using hydro power. Dams  require  huge capital outlay to serve various needs which warrants application   of  sound  design  principles  besides  good  construction practices.  Any  failure  of  dam leads to considerable loss of life and property. Thus the safety of dams is of paramount importance to mankind. Dam safety programs are of utmost importance to the society which call for  combined  use  of  multi-disciplinary  efforts. The concept of safety should  apply  not  only  to  the  pre-planning  stages  of  design  and construction  but  also  to the post operational and maintenance stages. Due  to  this, the recent years have witnessed a major research interest from the academic community of fracture mechanics of concrete and a high concern  from  the  engineering  community  and  power utility companies owning  dams,  in  issues related dam  safety. 

 From  recent  researches, it  is  well  understood that the concepts of fracture mechanics could be usefully applied for the failure analysis of concrete  dams.  In  a  concrete  dam,  the  interface  between concrete superstructure  and  rock  foundation  is  one of the potential sites of crack  formation  and subsequent failure. Not only do they contribute in weakening the mechanical strength, but they also constitute conduits for water  to  seep  through  and exert uplift pressure. Nevertheless, their response to seismic excitation is not well understood and it is commonly accepted that those constitute the weakest link in the safety of a dam during  and  after  an  earthquake.  Hence, it is important that proper mechanical behavior of this  interface  is understood in the light of realistic  loading  conditions.

 As a step towards understanding an interface crack lying in between concrete superstructure and rock foundation in dams, a finite element program INFAME (Interface Fracture Mechanics) incorporating contour integral method based on the reciprocal work theorem, catering the effects of body forces and surface tractions has been developed to compute the bi-material stress intensity factors on a linear elastic fracture mechanics dominance. Discrete crack approach is used to simulate cracks typically encountered in dams.

 Stress intensity factor forms a major parameter in linear elastic fracture mechanics, in terms of which, the fracture criteria for structures can be formulated. Hence, considerable effort is put in the computation of stress intensity factors using computationally simple and efficient methods. A method is formulated to obtain the mode I and mode II stress intensity factors for a crack lying between an interface of dissimilar materials using Weight Functions. The Weight Function method gives a boundary integral representation for the stress intensity factors. Since the weight functions are universal functions for a body specified by a particular crack, material combination and boundary conditions, once determined, the same weight functions can be used for determining stress intensity factors for any loading using the integral representation. This reduces the computational effort, which otherwise has to be put in analyzing each time for a newly applied load. This is an important feature of the method and advantageous when the stress intensity factors are required to be determined for a number of unforeseen loads on the body.

 An interface crack may propagate along the interface or kink into the adjoining material or may branch out, depending on the stress field at the crack tip. This phenomenon of crack propagation, kinking and branching are studied, stress and energy based criteria are proposed. Closed form solutions for determining the angle of crack kinking are developed.

 In real life situation, because of the combined compression and shear loading at the interface between concrete and rock in dams,  the crack faces come in contact so that sizeable contact zone emerge near the crack tip. Frictional contact of the crack surfaces cannot be neglected if the contact zones are finite. The frictional contact alters the stress singularity to become either weaker or stronger than the inverse square root stress singularity as observed in homogeneous crack problems. Consequently, the strain energy release rate as conventionally defined, either vanishes or becomes unbounded and thus cannot be used as a fracture parameter. An attempt has been made to include the effect of friction associated with the sliding of crack surfaces and compute the energy dissipated during crack propagation. Finite element analysis has been performed on an existing dam, to highlight the effect of friction on the nature of crack propagation along the rock-concrete interface.  Some of the main conclusions of this study are:

  • In the absence of friction, the total energy release rate remains constant as long as the crack lies within the interface. 

  • The total energy release rate increases with the crack length for a frictionless case and decreases with the crack length for the frictional case. The frictional energy dissipation contributes to the decrease in the total energy. As the value of friction reduces the total energy release rate decreases considerably. Thus, friction reduces the energy release rate for increasing crack length. This is not applicable for large water elevations since the crack has a tendency to open and the friction plays no role for an open crack.

In most literature pertaining to bi-material crack propagation, the effect of varying moduli ration�s (E1/E2), has been addressed but the combined effect of E1/E2 and n1/n2 (ni is the Poisson's ration of material i) on the crack propagation scenario was unknown. A parametric study has been carried out to understand the effects of different parameter like the moduli ratio�s (E1/E2), Poisson's ratios (n1/n2) and the load angles on the stress intensity factor�s and the crack kinking angles. Some of the main conclusions from this study are:

  • The difference in Poisson's ratio renders the material on either side of the interface as dissimilar even when their modulus of elasticity is the same. Thus, mixed mode conditions exists at the crack tip for materials having E1/E2 equal to one and different Poisson's ratio even for pure mode I loading. As difference in the Poisson's ratio increases the mode II stress intensity factor increases.

  • The effect of difference in Poisson's ratio between the two material on the stress intensity factors are negligible for all E1/E2 ratios for the case of pure mode II loading.

  • The absolute value of crack kinking angle increases as the shear load component increases, indicating that the presence of shear makes the crack to kink rather than to propagate in a self similar manner.

The response of an interface to seismic excitation is another area that constitute a weak link in the safety of dams during and after an earthquake. One of the major concerns in the seismic behavior of interfaces and joints is the mechanism of opening and closing without impact, although some localized impact may occur following joint separation. An attempt is made to solve this problem by developing a numerical model within the framework of finite element method using the fictitious force concept and special interface element.