Nemkumar Banthia

Impact testing of concrete using a drop-weight impact machine

A detailed description of the instrumented dropweight impact machine is presented. The instrumentation, the calibration, the inertial loading correction, and the dynamic analysis of a concrete beam specimen undergoing three-point impact flexural loading are described. Some results, using such an impact testing machine, obtained from tests done on plain concrete, fiber-reinforced concrete, and conventionally reinforced concrete are presented. It is concluded that the use of such a testing machine may be successfully made in order to test cementitious materials under impact.

Crack growth resistance of hybrid fiber reinforced cement composites

Abstract Crack propagation in cement-based matrices carrying hybrid fiber reinforcement was studied using contoured double cantilever beam (CDCB) specimens. Influence of fiber type and combination was quantified using crack growth resistance curves. It was demonstrated that a hybrid combination of steel and polypropylene fibers enhances the resistance to both nucleation and growth of cracks, and that such fundamental fracture tests are very useful in developing high performance hybrid fiber composites. The influence of number of variables which would otherwise have remained obscured in normal tests for engineering properties become apparent in the fracture tests. The paper emphasizes the desired durability characteristics of these composites and discusses their current and future applications.

Fracture toughness of micro-fiber reinforced cement composites

Abstract Toughness and strength improvements in cementbased matrices due to micro-fiber reinforcement were investigated. Cement paste and cement mortar matrices were reinforced at 1, 2 and 3% by volume of carbon, steel and polypropylene micro-fibers, and these composites were then characterized in the hardened state under an applied flexural load. Both notched and unnotched specimens were tested in four-point bending. Considerable strengthening, toughening and stiffening of the host matrix due to micro-fiber reinforcement was observed. The test data from the notched specimens was used to construct crack growth resistance and crack opening resistance curves for these composites and to identify the conditions necessary for failure. This paper recognizes the potential of these composites in various applications and stresses the need for continued research.

Electrical resistivity of carbon and steel micro-fiber reinforced cements

Abstract Electrical Resistivity measurements were conducted on cement pastes reinforced with conductive micro-fibers of carbon and steel both in mono and hybrid (combination) forms. A high frequency alternating current was employed. Effects of the alternating current frequency and inter-electrode spacing were investigated. Composites with various fiber volume fractions were tested. Notable reductions in the electrical resistivity with fiber reinforcement were observed. Although carbon fibers themselves are are far less conductive than steel fibers, cement composites with carbon fibers were found to be better conductors than those with steel fibers. It was concluded that more than the conductivity of the fiber material itself, it is the size and distribution of the fiber in a composite which is of importance. For the same reason, some hybrid-fiber composites were found to have conductivities better than their equivalent mono-fiber systems.

CONCRETE REINFORCED WITH DEFORMED STEEL FIBERS, PART I: BOND-SLIP MECHANISMS

Bond-slip characteristics were investigated for three deformed steel fibers bonded in concrete matrixes with different strengths. Fibers were aligned at 0, 15, 30, 45, and 60 deg with respect to the loading direction, and complete load-versus-slip curves were obtained. It was found that the bond-slip characteristics of fibers aligned with respect to the loading direction were signifcantly superior to those for inclined fibers. Inclined fibers supported smaller peak pullout loads and absorbed less pullout energy than the aligned fibers. A high-strength matrix often caused brittle fiber and matrix failures, and led to reductions in the energy-absorption capability. The paper provides interpretations of the bond-slip curves based on various micromechanical processes in the matrix and fiber, and identifies the conditions that lead to a brittle response. The bond-slip information generated in this study for the various deformed fibers will be correlated to the actual behavior of fiber reinforced concrete in the second part of this paper.

Test Methods for Fexural Toughness Characterization of Fiber Reinforced Concrete: Some Concerns and a Proposition

The major advantage of fiber reinforced concrete over its unreinforced counterpart is in the improved energy-absorption capability, or toughness. The current methods of characterizing the toughness of fiber reinforced concrete, however, have proven to be largely inadequate and have caused a great deal of dissent and confusion. This paper discusses some of the major difficulties with these standard methods and demonstrates their susceptibility to human judgment errors. The paper also proposes an alternate technique that addresses some of these concerns and is capable of characterizing fiber reinforced concrete toughness in an objective manner.

Flexural Response of Hybrid Fiber-Reinforced Cementitious Composites

In fiber-reinforced concrete (FRC), fibers can be effective in arresting cracks at both macro and micro levels. Most of the FRC used today involves the use of a single fiber type, which implies that any fiber can provide reinforcement only at one level and within a limited range of strain or crack opening. For an optimal response, various types of fibers may be combined to produce hybrid fiber-reinforced concrete (HyFRC). The influence was quantified of various hybrid fiber combinations on fresh properties of concrete (that is, workability) and on hardened properties such as compressive strength. The objectives of the present study, however, were to investigate the flexural toughness properties of hybrid fiber-reinforced concrete and to identify synergistic effects between fibers, if present. The study found that some hybrid composites demonstrated some synergy between fibers.

IMPACT RESISTANCE OF STEEL FIBER REINFORCED CONCRETE

Concrete used in civil engineering structures, such as bridges in seismic areas and airport runways, must have adequate resistance to impact and impulsively applied loads. Unfortunately, the behavior of concrete to impact loads is not well understood and there is significant variability in the published literature. The main reason for this is the lack of a standardized technique of testing concrete under impact. This paper describes the construction of a simple impact machine capable of carrying out impact tests on concrete in uniaxial tension. Impact data for normal-strength, medium-strength, high-strength, and fiber reinforced concrete is presented. Researchers demonstrate that the machine can generate rational impact data for various concretes tested and that the data can be useful in design.

Predicting the flexural postcracking performance of steel fiber reinforced concrete from the pullout of single fibers

A model based on simple principles of mechanics is proposed to predict the flexural toughness of steel fiber reinforced concrete (SFRC), having the experimental pullout force-versus-slip relationships of single fibers at different inclination angles and the compressive strength of the matrix as the only two input parameters. The model is shown to be in good agreement with experimental results from tests on SFRC in terms of average response and kinematics of the failure mode. In addition, a reliability analysis has been incorporated in the model allowing for the prediction not only of the average expected response but also of its variability. This study of the stochastic nature of the process is also shown to agree well with the experiments and those reported in the literature in terms of the expected variability in the flexural toughness testing.

WATER PERMEABILITY OF CEMENT PASTE

Abstract The permeability of cement paste to water was determined by using a triaxial permeability cell. A new technique of specimen conditioning, based on cyclic flow reversal, was used for the early attainment of equilibrium flow conditions. Effects of cement type, age, and silica fume addition were investigated. With the specimen conditioning used, equilibrium flow conditions were achieved on the first day of the test itself. The coefficient of permeability was found to decrease with an increase in the degree of hydration. The use of silica fume was found to decrease the permeability. However, the permeability did not particularly depend upon the amount of silica fume added.