Fracture Properties of Concrete-Concrete Interfaces Using Digital Image Correlation

The mode I and mode II fracture toughness and the critical strain energy release rate for different concrete-concrete jointed interfaces are experimentally determined using the Digital Image Correlation technique. Concrete beams having different compressive strength materials on either side of a centrally placed vertical interface are prepared and tested under three-point bending in a closed loop servo-controlled testing machine under crack mouth opening displacement control. Digital images are captured before loading (undeformed state) and at different instances of loading. These images are analyzed using correlation techniques to compute the surface displacements, strain components, crack opening and sliding displacements, load-point displacement, crack length and crack tip location. It is seen that the CMOD and vertical load-point displacement computed using DIC analysis matches well with those measured experimentally.

An experimental study on acoustic emission energy as a quantitative measure of size independent specific fracture energy of concrete beams

Notched three point bend specimens (TPB) were tested under crack mouth opening displacement (CMOD) control at a rate of 0.0004 mm/s and during the fracture process acoustic emissions (AE) were simultaneously monitored. It was observed that AE energy could be related to fracture energy. An experimental study was done to understand the behavior of AE energy with parameters of concrete like its strength and size. In this study, AE energy was used as a quantitative measure of size independent specific fracture energy of concrete beams and the concepts of boundary effect and local fracture energy were used to obtain size independent AE energy from which size independent fracture energy was obtained. (C) 2010 Elsevier Ltd. All rights reserved.

Proportioning of fly-ash cement concrete mixes

The accumulation of fly ash throughout the world is several million tons per day. The main problem with the usage of fly ash is the slow rate of strength gain, primarily due to slow pozzolanic reactions. Existing methods of proportioning fly ash concrete are lacking. These methods are involved and do not directly take into account the properties of the constituent materials. The Generalized Approach for Mix Proportioning developed at the Indian Institute of Science, Bangalore, is the basis for the development of the proposed method, which takes into account the characteristics of cement, fly ash, and aggregates. Based on the basic trial mix data obtained by using the American Concrete Institute (ACI 211.1-81) method, the proportions of fly-ash concrete mixes were arrived at using the Generalized Approach for Mix Proportioning. The method proposed was applied to and found applicable for fly-ash concretes using fly ashes from two different sources.

Fatigue crack propagation model and size effect in concrete using dimensional analysis

It is well known that fatigue in concrete causes excessive deformations and cracking leading to structural failures. Due to quasi-brittle nature of concrete and formation of a fracture process zone, the rate of fatigue crack growth depends on a number of parameters, such as, the tensile strength, fracture toughness, loading ratio and most importantly the structural size. In this work, an analytical model is proposed for estimating the fatigue crack growth in concrete by using the concepts of dimensional analysis and including the above parameters. Knowing the governed and the governing parameters of the physical problem and by using the concepts of self-similarity, a relationship is obtained between different parameters involved. It is shown that the proposed fatigue law is able to capture the size effect in plain concrete and agrees well with different experimental results. Through a sensitivity analysis, it is shown that the structural size plays a dominant role followed by loading ratio and the initial crack length in fatigue crack propagation. (C) 2010 Elsevier Ltd. All rights reserved.

Fracture Analysis of Concrete Structural Components Accounting for Tension Softening Effect

This paper presents methodologies for fracture analysis of concrete structural components with and without considering tension softening effect. Stress intensity factor (SIF) is computed by using analytical approach and finite element analysis. In the analytical approach, SW accounting for tension softening effect has been obtained as the difference of SIP obtained using linear elastic fracture mechanics (LEFM) principles and SIP due to closing pressure. Superposition principle has been used by accounting for non-linearity in incremental form. SW due to crack closing force applied on the effective crack face inside the process zone has been computed using Green's function approach. In finite element analysis, the domain integral method has been used for computation of SIR The domain integral method is used to calculate the strain energy release rate and SIF when a crack grows. Numerical studies have been conducted on notched 3-point bending concrete specimen with and without considering the cohesive stresses. It is observed from the studies that SW obtained from the finite element analysis with and without considering the cohesive stresses is in good agreement with the corresponding analytical value. The effect of cohesive stress on SW decreases with increase of crack length. Further, studies have been conducted on geometrically similar structures and observed that (i) the effect of cohesive stress on SW is significant with increase of load for a particular crack length and (iii) SW values decreases with increase of tensile strength for a particular crack length and load.

Synergetic roles of concrete ingredients in strength development

Concrete is basically a heterogeneous material made up of ingredients with distinct physical and mechanical properties. As a result, the presence of interphases is inevitable. In the processing of concrete, fresh and hardened states are the two distinct stages. In the fresh state, the presence of inert constituents in the cement mortar matrix only dilutes the overall potential of concrete to flow. In the hardened state the synergetics play a dominant role in strength development. When the strength of coarse aggregate is far higher than the strength levels for which the matrix or concrete is processed, interphase bonding plays a dominant role on the strength. When the matrix strength is comparable to that of the aggregate strength, in contrast, the concrete strength is affected by the aggregate strength. Besides these aspects, the effects of the size and the surface texture of coarse aggregates have also been analysed. Copyright (C) 1996 Elsevier Science Ltd.

Graphical method to determine the parameters of the two-parameter fracture model for concrete

To evaluate the parameters in the two-parameter fracture model, i.e. the critical stress intensity factor and critical crack tip opening displacement for the fracture of plain concrete in Mode 1 for the given test configuration and geometry, considerable computational effort is necessary. A simple graphical method has been proposed using normalized fracture parameters for the three-point bend (3PB) notched specimen and the double-edged notched (DEN) specimen. A similar graphical method is proposed to compute the maximum load carrying capacity of a specimen, using the critical fracture parameters both for 3PB and DEN configurations.

Punching shear strength of flat slab corner column connections .1. Reinforced concrete connections

This paper presents the details of an experimental study on punching shear strength and behaviour of reinforced concrete corner column connections in flat slabs; a quasi-empirical method is proposed for computing the punching shear strength. The method has also been extended for punching shear strength prediction at interior and edge column connections. The test results compare better with the strengths predicted by the proposed method than those by Ingvarson, Zaglool and Pollet available in the literature. Further, the experimental strengths of interior, edge and corner column connections have been compared with the strengths predicted by the proposed method and the two codes of practice, viz. ACI and BS code, to demonstrate the usefulness of the method.

Punching shear strength of flat slab corner column connections .2. Fibre-reinforced concrete connections

This paper gives the details of the studies undertaken to examine the strength and behaviour of fibre-reinforced concrete corner column connections in flat slabs. Tests have been conducted on 16 specimens with varying reinforcement ratio, moment/shear ratio (load eccentricity) and volume fraction of fibres. A quasi-empirical method has been proposed for computing the punching shear strength. The method has also been extended to fibre-reinforced concrete interior column connections, tests on which are available in the literature. The test results have been compared with the strength predicted by the proposed method for corner column as well as interior column connections and a satisfactory agreement noticed.

Influence of interface properties on fracture behaviour of concrete

Hardened concrete is a three-phase composite consisting of cement paste, aggregate and interface between cement paste and aggregate. The interface in concrete plays a key role on the overall performance of concrete. The interface properties such as deformation, strength, fracture energy, stress intensity and its influence on stiffness and ductility of concrete have been investigated. The effect of composition of cement, surface characteristics of aggregate and type of loading have been studied. The load-deflection response is linear showing that the linear elastic fracture mechanics (LEFM) is applicable to characterize interface. The crack deformation increases with large rough aggregate surfaces. The strength of interface increases with the richness of concrete mix. The interface fracture energy increases as the roughness of the aggregate surface increases. The interface energy under mode II loading increases with the orientation of aggregate surface with the direction of loading. The chemical reaction between smooth aggregate surface and the cement paste seems to improve the interface energy. The ductility of concrete decreases as the surface area of the strong interface increases. The fracture toughness (stress intensity factor) of the interface seems to be very low, compared with hardened cement paste, mortar and concrete.

A finite element assessment of flexural strength of prestressed concrete beams with fiber reinforcement

This paper presents an assessment of the flexural behavior of 15 fully/partially prestressed high strength concrete beams containing steel fibers investigated using three-dimensional nonlinear finite elemental analysis. The experimental results consisted of eight fully and seven partially prestressed beams, which were designed to be flexure dominant in the absence of fibers. The main parameters varied in the tests were: the levels of prestressing force (i.e, in partially prestressed beams 50% of the prestress was reduced with the introduction of two high strength deformed bars instead), fiber volume fractions (0%, 0.5%, 1.0% and 1.5%), fiber location (full depth and partial depth over full length and half the depth over the shear span only). A three-dimensional nonlinear finite element analysis was conducted using ANSYS 5.5 [Theory Reference Manual. In: Kohnke P, editor. Elements Reference Manual. 8th ed. September 1998] general purpose finite element software to study the flexural behavior of both fully and partially prestressed fiber reinforced concrete beams. Influence of fibers on the concrete failure surface and stress-strain response of high strength concrete and the nonlinear stress-strain curves of prestressing wire and deformed bar were considered in the present analysis. In the finite element model. tension stiffening and bond slip between concrete and reinforcement (fibers., prestressing wire, and conventional reinforcing steel bar) have also been considered explicitly. The fraction of the entire volume of the fiber present along the longitudinal axis of the prestressed beams alone has been modeled explicitly as it is expected that these fibers would contribute to the mobilization of forces required to sustain the applied loads across the crack interfaces through their bridging action. A comparison of results from both tests and analysis on all 15 specimens confirm that, inclusion of fibers over a partial depth in the tensile side of the prestressed flexural structural members was economical and led to considerable cost saving without sacrificing on the desired performance. However. beams having fibers over half the depth in only the shear span, did not show any increase in the ultimate load or deformational characteristics when compared to plain concrete beams. (C) 2002 Published by Elsevier Science Ltd.

Comparative study on flexural response of full and partial depth fiber-reinforced high-strength concrete

This study presents the results of an experimental and analytical comparison of the flexural behavior of a high-strength concrete specimen (no conventional reinforcement) with an average plain concrete cube strength of nearly 65 MPa and containing trough shape steel fibers. Trough shape steel fibers with a volume fraction ranging from 0 to 1.5% and having a constant aspect ratio of 80 have been used in this study. Increased toughness and a more ductile stress-strain response were observed with an increase in fiber content, when the fibers were distributed over the full/partial depth of the beam cross section. Based on the tests, a robust analytical procedure has been proposed to establish the required partial depth to contain fiber-reinforced concrete (FRC) so as to obtain the flexural capacity of a member with FRC over the full depth. It is expected that this procedure will help designers in properly estimating the required partial depth of fibers in composite sections for specific structural applications. Empirical and mechanistic relations have also been proposed in this study to establish the load-deflection behavior of high-strength FRC.

Modeling of concrete cracking-A hybrid technique of using displacement discontinuity element method and direct boundary element method

This paper presents a new approach by making use of a hybrid method of using the displacement discontinuity element method and direct boundary element method to model concrete cracking by incorporating fictitious crack model. Fracture mechanics approach is followed using the Hillerborg's fictitious crack model. A boundary element based substructure method and a hybrid technique of using displacement discontinuity element method and direct boundary element method are compared in this paper. In order to represent the process zone ahead of the crack, closing forces are assumed to act in such a way that they obey a linear normal stress-crack opening displacement law. Plain concrete beams with and without initial crack under three-point loading were analyzed by both the methods. The numerical results obtained were shown to agree well with the results from existing finite element method. The model is capable of reproducing the whole range of load-deflection response including strain-softening and snap-back behavior as illustrated in the numerical examples. (C) 2011 Elsevier Ltd. All rights reserved.

Fracture energy and softening behavior of high-strength concrete

An experimental investigation on the fracture properties of high-strength concrete (HSC) is reported. Three-point bend beam specimens of size 100 x 100 x 500 mm were used as per RILEM-FMC 50 recommendations. The influence of maximum size of coarse aggregate on fracture energy, fracture toughness, and characteristic length of concrete has been studied. The compressive strength of concrete ranged between 40 and 75 MPa. Relatively brittle fracture behavior was observed with the increase in compressive strength. The load-CMOD relationship is linear in the ascending portion and gradually drops off after the peak value in the descending portion. The length of the tail end portion of the softening curve increases as the size of coarse aggregate increases. The fracture energy increases as the maximum size of coarse aggregate and compressive strength of concrete increase. The characteristic length of concrete increases with the maximum size of coarse aggregate and decreases as the compressive strength increases, (C) 2002 Elsevier Science Ltd. All rights reserved.