Development of high performance fiber -reinforced cement composites using twisted polygonal steel fibers.

Abstract

The ultimate goal of this research is to develop high performance fiber reinforced cement composites (HPFRCCs) using a new type of deformed steel fibers with an optimized geometry, identified as twisted polygonal steel fibers or Torex fibers. The research is divided into three parts: evaluating the components of bond in Torex fibers, optimizing the composite response of cement composites reinforced with Torex fibers, and modeling the pullout load versus slip response of Torex fibers. The parameters investigated in the first part include the fiber geometry, embedded length, fiber strength, matrix strength, interfacial properties, and fiber inclination. When the proper combination of these parameters is used, Torex fibers exhibit a pseudo-plastic pullout load versus slip behavior, that is a high pullout load is maintained almost constant up to complete pullout. In the second part, cement composites reinforced with Torex fibers are investigated experimentally in tension and bending. Test parameters include fiber volume fraction, L/d_e ratio and matrix compressive strength. The use of Torex fibers led to quasi-strain hardening and multiple cracking behavior, and induced a significantly higher strength and ductility in tension and bending than that of commercial steel fibers. The concept of hybridization with micro and macro fibers was also explored in this part of the study to further improve composite performance. Hybrid fiber reinforced composites presented improvement not only in tension and bending but in higher ductility and energy absorption capacity. A micromechanical model is developed in the third part to predict the pullout load versus slip response of Torex fibers embedded in a cement matrix. The total pullout load comprises two main components of bond: frictional bond (adhesion if any), and mechanical bond. However, both components of bond are modeled using the Coulomb's friction law, because mechanical bond is based on the concept of untwisting (thus sliding) torque distribution along the fiber developed during pullout. An extensive parametric analysis is carried out to better understand and optimize the components of bond in Torex fibers. Model predictions showed good agreement with the experimental data. In summary, this research contributes to the development of HPFRCCs with improved performance.