Ravindra Gettu

Fracture properties and brittleness of high-strength concrete

The size effect method for determining material fracture characteristics, as previously proposed by Bazant is applied to typical high-strength concrete. Geometrically similar 3-point bending specimens are tested and the measured peak load values are used to obtain the fracture energy, the fracture toughness, the effective length of the fracture process zone, and the effective critical crack tip opening displacement. The brittleness of the material is shown to be objectively quantified through the size-effect method. Comparing the material fracture properties obtained with those of normal strength concrete shows that an increase of 16 % in compressive srength causes: (1) increase of fracture toughness; (2) decrease of effective fracture process zone length; (3) more than doubling of the brittleness number. The brittleness number, however, is still not high enough to permit the use of linear elastic fracture mechanics. The R-curves are demonstrated to derive according to the size effect law exclusively from the maximum loads of specimens of various sizes and yield remarkably good predictions of the load-deflection curves.

On the characterization of flexural toughness in fiber reinforced concretes

Abstract The article comprises two parts. The first part presents a summary of the available methods of characterizing the flexural toughness of fiber reinforced concretes (FRC), with a review of most of the toughness standards and guidelines from standards institutions and other professional agencies in North America, Europe and Japan. Also reviewed are other significant proposals available in the published literature. The second part of the article includes a discussion of the merits and drawbacks of these measures. Other related issues discussed include: the fundamental significance, problems with regard to experimental measurements and the potential for practical design implementation of a toughness measure.

IDENTIFICATION OF NONLINEAR FRACTURE PROPERTIES FROM SIZE EFFECT TESTS AND STRUCTURAL ANALYSIS BASED ON GEOMETRY-DEPENDENT R-CURVES

Abstract The size effect method, previously developed for concrete and mortar, is demonstrated for rock. Geometrically similar fracture specimens of limestone are tested and the measured maximum load values are used to obtain fracture energy, fracture toughness and effective size of the fracture process zone. Further, it is shown how to determine, from these results, other nonlinear fracture parameters including the critical effective crack-tip opening displacement. R-curves, dependent on specimen geometry, are calculated and used to predict load-deflection curves, which are found to agree very well with measurements. A modification of the R-curves for post-peak response is proposed.

Uniaxial tension test for steel fibre reinforced concrete: a parametric study

Abstract A RILEM Draft Recommendation was proposed in 2001 for obtaining the stress versus crack opening (σ–w) response of steel fibre reinforced concrete through a uniaxial tension test. The present study analyses the robustness of the recommended test through a parametric study. Furthermore, the methodology is extrapolated to cores extracted from cast elements. Also, the effect of the coring direction with respect to the preferential fibre orientation caused by the compaction procedure is examined. The study demonstrates that the test is robust and representative of the material response, and could be used for determining the σ–w relation of the material that may be needed for comparing the performance of different fibres or for providing input for finite element analysis. No significant influence of the characteristics of the specimen or problems of instability due to the loss of control were encountered. There is some relative rotation between the crack faces but its influence on the σ–w response is expected to be negligible. The parameters obtained from the tests exhibit coefficients of variation of up to 30%, which is mainly attributed to the randomness of the number of fibres bridging the crack, considering the relatively small cross-section of the specimen.

Toughness measurement — the need to think again

Abstract The first part of the paper reviews a number of concurrent developments which have taken place in recent years in the concrete research area. The developments include the commercial development of FRC materials, the proposals for evaluating the enhanced performance of FRC materials, the development of high performance (high strength/brittle) concretes, the application of fracture mechanics to concrete together with the revolution in testing machines with the introduction of closed-loop testing. The limitations of current standards and guidelines for evaluating the toughness of FRC materials are also reviewed and proposals for overcoming these limitations are advanced. It is proposed that notched beams, subjected to three-point loading, rather than unnotched beams subjected to four-point loading should be used for toughness measurement and that the deformation of the beams should be measured directly from the specimen rather than through the testing machine.

Study of the influence of superplasticizers on the hydration of cement paste using nuclear magnetic resonance and X-ray diffraction techniques

Melamine and naphthalene-based superplasticizers have been used, over the past few decades, in order to improve the workability of concrete. Recently, more efficient copolymer formulations have been introduced for the same purpose. However, the influence of these chemical admixtures on the microstructure of the hardened concrete and, consequently, on its properties still needs to be extensively evaluated. Accordingly, the present work analyzes the hydration characteristics of cement pastes with naphthalene, melamine and copolymer-based superplasticizers, using the techniques of X-ray diffraction (XRD) and nuclear magnetic resonance (NMR), up to the age of 28 days. The results indicate a significant influence of the superplasticizer on the growth rates of the hydrates and on the state of polymerization of the silicates.

Estimation of the modulus of elasticity for dam concrete

Abstract The modulus of elasticity of dam concrete is difficult to determine directly from tests, due to the necessity for large specimens and testing machines. In order to study the applicability of simple elastic models for predicting the modulus from standard size specimens, tests were conducted on prisms of 45 × 45 × 90 cm fabricated with dam concrete (maximum aggregate of 120 mm). The tests on standard 15 × 30 cm cylinders were made with the mortar and wet-screened components of this concrete. It is seen that the use of the data from these components together with estimated values of the modulus of the aggregates gives reasonable predictions of the moduli of the dam concrete. This has been verified for a range of ages, from 7 to 180 days.

Shear Failure of Steel Fiber-Reinforced Concrete Based on Push-Off Tests

Steel fiber-reinforced concrete (SFRC) uses fibers as shear reinforcement, a successful approach to coping with the brittle nature of shear failure. This article reports on a study undertaken to characterize the failure and toughness of SFRC subjected to direct shear loading at the material level. The study used a push-off test on a double-notched prism to quantify the shear stress-displacement behavior of SFRC. The shear stress-slip response (obtained experimentally) can be used to calculate toughness-based parameters, which can be employed in structural design. The authors note that the test can be performed in a stable manner for steel fiber-reinforced concrete, permitting the determination of the pre- and post-peak responses and, consequently, characterizing the shear stress that can be transferred across an open crack. The authors conclude that significant improvements in the ductility of concrete during shear failure and some increase in the shear strength are achieved through the incorporation of steel fibers in both normal- and high-strength concretes.

Utilization of impedance spectroscopy for studying the retarding effect of a superplasticizer on the setting of cement

The electrical resistivity and conductivity of cement pastes with different dosages of a superplasticizer have been determined using impedance spectroscopy. The evolution of these parameters during the first 24 h after mixing indicates that the incorporation of the superplasticizer leads to a retardation of the setting process, the significance of which increases with the dosage. It appears that the incorporation of the superplasticizer extends the dormant stage of hydration and delays the onset of the accelerating stage, without affecting its rate. Comparisons are made with penetration measurements with the Vicat apparatus.

Effect of Aging on the Fracture Characteristics and Brittleness of a High-Strength Concrete

Results of notched beam (fracture) tests on a 60-MPa silica fume concrete at the ages of 4, 10, 31, and 232 days are presented. Fracture parameters at the different ages were obtained using the size effect and cohesive crack models, which indicate that the fracture resistance (toughness and energy) decreases and the brittleness increases with the age of the concrete. This trend is attributed to the increase in the strength of the hardened cement paste and the interfaces that leads to less bond cracking and more aggregate rupture, and, consequently, to more brittle failure and lower toughening.