Barzin Mobasher

MICROCRACKING IN FIBER REINFORCED CONCRETE

Abstract Micromechanisms of matrix fracture in Portland cement based fiber composites are studied by means of quantitative image analysis and acoustic emission technique. An experimental investigation has been conducted with different volume concentrations of polypropylene fibers. Uniaxial tensile specimens were loaded under constant strain rate and the acoustic emission response was monitored. Thin sectioned epoxydye impregnated samples were also prepared from specimens loaded to specified strain magnitudes. Fluorescence microscopy was used to quantitatively examine the thin sections for crack density, length, and spacing.

Pultruded Fabric-Cement Composites

The use of reinforcement in thin cement-based elements is essential to improve the tensile and flexural performance. The reinforcements can be either short fibers or continuous reinforcements, in a fabric form. Practical use of fabric-cement composites requires an industrial, cost-effective production process. The objective of this study was to develop the pultrusion technique as an industrial, cost-effective production method of prefabricated thin-sheet fabric-reinforced cement composites. Woven fabrics made from low-modulus polyethylene and glass meshes were used to produce the pultruded cement composites. The influence of fabric type, cell opening, application of pressure during casting, and cement-based matrix modification were examined. The tensile strength and ductility of the pultruded fabric-cement components were found to be relatively high, exhibiting strain hardening behavior even for fabrics with low modulus of elasticity. The best performance was achieved from glass fabric composites with a high content of fly ash. The mechanical properties were significantly affected by the matrix formulation, rheology of the matrix, and the intensity of the pressure applied after the pultrusion process. The promising combination of fabric reinforcement in cement composite products using the pultrusion process is expected to lead to a new class of high-performance fabric-cement composite materials.

The effect of copper slag on the hydration and mechanical properties of cementitious mixtures

The effect of copper slag on the hydration of cement-based materials is studied. Up to 15% by weight of copper slag was used as a portland cement replacement. Hydration reactions were studied through semiquantitative X-ray diffraction and TGA/DTA. Samples of copper slag and hydrated lime (ASTM type S) were used to test the pozzolanic properties of the slag. The porosity was examined using mercury intrusion porosimetry. A decrease in capillary porosity was observed while the gel porosity was increased. A significant increase in the compressive strength for up to 1 year is observed.

Effect of interfacial properties on the crack propagation in cementitious composites

Abstract The role of interfacial parameters on the fracture toughness of cement based composites are studied by means of a two-crack system. The first crack type represents the interfacial debonding of a fiber using a pullout model, while the second type simulates the crack growth in the matrix response subjected to the closing pressure generated by the fiber pullout force. A fracture mechanics approach is applied to the pullout of the short fiber. The interfacial zone is characterized as an elastic-perfectly plastic one-dimensional layer with a toughness lower than that of matrix or fiber. Stable debonding of fibers was modeled using R -curves so that the Interface toughness increases with an increase in debonded length. The closed-form solutions for strain energy release rate of a partially debonded frictional interface are solved for the critical debonding length. By obtaining the R -curve parameters, the fiber pullout load-slip response is simulated. The R -curve formulation is further applied to the crack growth in the composite. The toughening component is due to the closing pressure of fibers that depends on the matrix crack opening. The stress crack-width relationship is obtained as the crack growth in the stable region takes place. A parametric study of the effects of interfacial parameters on the crack growth in the composite is conducted. The present model is also compared with experimental data on the size effect in geometrically similar specimens.

EFFECT OF GROUND COPPER SLAG ON STRENGTH AND TOUGHNESS OF CEMENTITIOUS MIXES

The effect of ground copper slag (GCS) on the strength and fracture of cement-based materials is studied. Up to 15 percent by mass of GCS was used as a portland cement replacement. The strength and fracture toughness of concrete samples were studied using closed-loop controlled compression and three-point bending fracture tests. The compression test used a combination of the axial and transverse strains as a control parameter to develop a stable postpeak response. A cyclic loading-unloading test was conducted on three-point bending notched specimens under closed-loop crack mouth opening control. Test results were used to construct the resistance curve (R-curve) response of the specimens describing the dependence of fracture toughness on the stable crack length.

MECHANICAL PROPERTIES OF FIBER REINFORCED LIGHTWEIGHT CONCRETE COMPOSITES

Hybrid composites with variable strength/toughness properties can be manufactured using combinations of brittle or ductile mesh in addition to brittle and ductile matrix reinforcements. The bending and tensile properties of thin sheet fiber cement composites made from these mixtures were investigated. Composites consisted of a woven mesh of either polyvinyl chloride (PVC) coated E-glass or polypropylene (PP) fibers for the surface reinforcement. In addition, chopped polypropylene, acrylic, nylon, and alkali-resistant (AR) glass fibers were used for the core reinforcement. It is shown that by controlling fiber contents, types, and combinations, design objectives such as strength, stiffness and toughness, can be achieved. Superior post-cracking behavior was measured for composites reinforced both with glass mesh and PP mesh. Load carrying capacity of PP mesh composites can be increased with the use of 1% or higher chopped PP fibers. Glass mesh composites with short AR glass fibers as matrix reinforcement indicate an increased matrix cracking strength and modulus of rupture. Combinations of PP mesh/short AR glass did not show a substantial improvement in the matrix ultimate strength. An increased nylon fiber surface area resulted in improved post peak response.

Quantitative damage characterization in polypropylene fiber reinforced concrete

Abstract An automatic algorithm for microcrack characterization in cement-based materials is presented. The algorithm, test procedures, and various error sources are described and quantified. The algorithm is applied to crack images obtained from a polypropylene-FRC material and the relationship between specific crack surface and strain as well as the relationship between an orientation measure and strain are reported and discussed. The obtained damage evolution relationships are compared to the observed stress/strain behavior of the pp-FRC material.

Test Parameters for Evaluating Toughness of Glass Fiber Reinforced Concrete Panels

The paper describes the ability of different test methods to measure the toughness of glass-fiber reinforced composites (GFRC). The three loading configurations used were: uniaxial tensile, third-point flexural, and instrumented impact. Fracture was observed to be dependent on the specimen geometry; gage length used; strain rate; and the extent of accelerated aging. Assuming that the local tensile stress-strain response can be expressed in the form of a bilinear ascending portion and an exponentially descending portion, both gage-length effects and loading geometry effects can be predicted. Using the analytical tensile stresss-strain responses, the experimentally observed flexural load-deflection and toughness were predicted. Possible methods to formulate aging mechanisms in the composites are also discussed. It is concluded that gage length, nonuniform strain distribution, geometry, and rate of loading must be carefully considered to characterize the toughness of GFRC.

Refractive change induced by the LASIK flap in a biomechanical finite element model

PURPOSE To study the effect of varying four parameters on the refractive change induced by the LASIK flap. METHODS Using a variety of patient-specific data such as topography, pachymetry, and axial length, a finite element model is built. The model is used in a non-linear finite element analysis to determine the response and change in optical power of the cornea as a function of a material property of the cornea (corneal elasticity), flap diameter and thickness, and intraocular pressure. RESULTS The central flattening or hyperopic shift occurred atop the flap in all four of the simulated eyes tested with the creation of the LASIK flap. Of the four parameters tested, modulus of elasticity (Youngs modulus) had the most profound effect on the results of hyperopic shift, varying from <0.5 diopters (D) in the least elastic (stiffest) cornea to >2.0 D of hyperopic shift in the most elastic cornea. The depth of the lenticular cut was the second-most significant parameter tested varying from 0.24 D at 100 microns to 1.25 D at 275 microns of depth. Varying intraocular pressure demonstrated less difference, and varying corneal flap diameter demonstrated the least difference in induced refractive change on the model. The hyperopic shift was noted to be greater in hyperopic than in myopic eyes (simulated) tested. CONCLUSIONS Three-dimensional finite element analysis modeling of actual patient data could lead to a better understanding of the biomechanical response of corneal tissue to the lenticular flap creation and potentially for ablation patterns produced by the excimer laser. Understanding these biomechanical responses may lead to greater predictability and improvement of visual outcomes.

Testing of Concrete Under Closed-Loop Control

Abstract Closed-loop testing systems provide the ability to directly control the deformation of the loaded specimen. This considerably enhances the precision, stability, and scope of the experiments. Closed-loop machines can be used to determine the stable response of test specimen or structure by monitoring and controlling the physical quantities that are sensitive to its behavior. The importance of the various components of the closed-loop controlled system and the test configuration is reviewed in the paper. The most critical aspect of designing the test is the choice of the controlled variable. With appropriate controlled variables and good system performance, several interesting and intricate testing techniques can be developed, as seen in the examples presented here.