Surendra P. Shah

Permeability study of cracked concrete

Cracks in concrete generally interconnect flow paths and increase concrete permeability. The increase in concrete permeability due to the progression of cracks allows more water or aggressive chemical ions to penetrate into the concrete, facilitating deterioration. The present work studies the relationship between crack characteristics and concrete permeability. In this study, feedback controlled splitting tests are introduced to generate crack width-controlled concrete specimens. Sequential crack patterns with different crack widths are viewed under a microscope. The permeability of cracked concrete is evaluated by water permeability tests. The preliminary results indicate that crack openings generally accelerate water flow rate in concrete. When a specimen is loaded to have a crack opening displacement smaller than 50 microns prior to unloading, the crack opening has little effect on concrete permeability. When the crack opening displacement increases from 50 microns to about 200 microns, concrete permeability increases rapidly. After the crack opening displacement reaches 200 microns, the rate of water permeability increases steadily. The present research may provide insight into developing design criteria for a durable concrete and in predicting service life of a concrete structure.

SHRINKAGE CRACKING OF FIBER REINFORCED CONCRETE

Tests were conducted using a ring-type specimen to simulate restrained shrinkage cracking. The effects of two type of fibers (steel and polypropylene differing amounts of fibers (0.1 to 1.5% by volume); age of concrete (2 hr or 4 days); and length of drying (up to 6 weeks) under a given environment (40% relative humidity and 20 C (68 F)) were studied. The results were satisfactorily compared with a theoreticl model developed by the authors.

Measurement of deformations on concrete subjected to compression using image correlation

Because the nature of failure in concrete is complicated due to the material heterogeneity, a robust measuring method is essential to obtain reliable deformation data. A nondestructive displacement evaluation system using a digital image cross-correlation scheme, often called computer vision, is developed to make microscopic examinations of the fracture processes in concrete. This is a full-field measuring method that gives an accuracy within the micron range for a 100 mm × 75 mm viewing area. A feedback signal that combines the lateral and axial deformations provides a well-balanced imaging rate both before and after the peak load. Displacement vector diagrams or displacement contour maps of concrete reveal highly nonuniform deformations even in the elastic range. The processes of fracture in concrete are well defined at different deformation levels.

RILEM TC 162-TDF: Test and design methods for steel fibre reinforced concrete' - sigma-epsilon-design method - Final Recommendation

General information Publication status: Published Organisations: Section for Structural Engineering, Department of Civil Engineering Contributors: Vandewalle, L., Nemegeer, D., Balazs, L., Barr, B., Barros, J., Bartos, P., Banthia, N., Criswell, M., Denarie, E., Di Prisco, M., Falkner, H., Gettu, R., Gopalaratnam, V., Groth, P., Hausler, V., Kooiman, A., Kovler, K., Massicotte, B., Mindess, S., Reinhardt, H., Rossi, P., Schaerlaekens, S., Schumacher, P., Schnutgen, B., Shah, S., Skarendahl, A., Stang, H., Stroeven, P., Swamy, R., Tatnall, P., Teutsch, M., Walraven, J. Pages: 560-567 Publication date: 2003 Peer-reviewed: Yes

A Fracture toughness criterion for concrete

Abstract Many have attempted to calculate the fracture toughness of concrete in Mode I by testing notched beam specimens. It has been observed that when K I c is calculated from the measured values of the maximum load, the initial notch length and using the formulas developed from LEFM, its value is dependent on the dimensions of the beam. This size and geometry dependency can be attributed to slow crack growth and nonlinearity due to geometrical interlock effects. A method is suggested to calculate a size independent fracture toughness parameter. To calculate the value of K I c according to the proposed method, inelastic displacements are extracted from the total displacements and the slow crack growth is included by incorporating crack closing-pressure. Using this constant K I c criterion, the load vs crack mouth opening displacement and load vs deflection curves were accurately predicted for 40 beams of different sizes tested for this investigation as well as those reported by other investigators.

The influence of wall slip on yield stress and viscoelastic measurements of cement paste

The influence of wall slip on the shear yield stress and modulus of cement paste was investigated using a rotational rheometer with smooth-walled concentric cylinders and a vane. The results show that the concentric cylinders suffer from slip during yield stress measurements due to the formation of a water-rich layer at the walls of the cylinders. The use of a vane eliminates slip since shearing occurs within the material. Oscillatory tests were conducted to measure the viscoelastic properties of cement paste. The data for the vane and concentric cylinders are in excellent agreement at stresses below the yield point. It is difficult to determine the influence of slip for stress sweep measurements. Frequency sweep data for various materials show the general applicability of the vane method over several decades of modulus.

Failure of fibre-reinforced composites by pull-out fracture

A simple model is proposed to predict the ultimate tensile strength of fibre-reinforced composites when the failure is governed by fibre debonding. The theoretical analysis is based on the concept of fracture mechanics where the debonded zone is considered as an interfacial crack. The analysis is first applied to the classical pull-out test in order to determine the specific work of interfacial cracking. Using this value, the uniaxial tensile strength of the composites can be predicted from an approximate, closed-form equation proposed here. The theoretically predicted results seem to compare favourably with experimental values for fibrere-inforced cement based composite.

Image analysis techniques for characterization of pore structure of cement-based materials

Abstract This paper explores several image analysis techniques that provide insight into the nature of pore structure as observed in backscattered electron images of polished sections. Image analysis techniques used in this study include sizing, two-point correlation, and fractal analyses. The spatial parameters computed by these techniques have potential for relating microstructure to material properties. Specimens include plain cement paste, pastes with silica fume, and mortars. Tests include determination of total porosity, mercury intrusion porosimetry (MIP), compression and flexural testing. The two-parameter fracture model is used to characterize fracture properties. Pore size distribution from MIP is compared to pore size distribution derived from BSE images.

Interaction between Loading, Corrosion, and Serviceability of Reinforced Concrete

This research studies the mutual effects between mechanical loading and corrosion of reinforcing steel, as well as their combined effect on serviceability (flexural deflection and residual loading capacity) of reinforced concrete beams. Beam specimens 10-by-15-by-117 cm in size were subjected to four-point bending at various loading levels (0-75% of the ultimate load) with different loading histories (previous loading and sustained loading). Sodium chloride solution ponding was employed to accelerate the corrosion process. Half-cell potential and galvanized current measurements were taken to monitor time for corrosion initiation. After corrosion initiated, an external current was applied to some of the specimens to expedite corrosion propagation. Beam deflections were recorded throughout all of the tests. Residual flexural loading capacity of the beams was evaluated at the end of the experiment. The results indicate that loading history and loading level have significant effects on both corrosion initiation and the rate of corrosion propagation. The failure mode of the reinforced concrete beams appeared to shift from a shear failure of concrete to bond splitting as the degree of corrosion increased. The results suggest that for a rational service-life prediction of reinforced concrete structures, the influence of the service load on the structure performance should be considered in combination with environmental conditions and material proportions.

Strain-softening of concrete in uniaxial compression

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