ارزیابی خواص مکانیکی و دوام کامپوزیت های سیمانی مهندسی (ECC)

One major weakness of concrete is the brittle fracture behaviour in tension, with low tensile strength and ductility. This brittleness has been recognized as a bottleneck hindering structural performances in terms of safety, durability and sustainability. The lack of structural ductility is due to brittle nature of concrete in tension which may lead to loss of structural integrity. Many infrastructure deterioration problems and failures can be traced back to the cracking and brittle nature of concrete. Many attempts have been made in the recent years to overcome these problems. To effectively solve these severe problems, a new type of composite, called as Engineered Cementitious Composites (ECC), reducing the brittle behaviour of concrete has been developed in recent decades. ECC with its flexible processing has emerged from laboratory testing to field applications leading to speedy construction, reduced maintenance and a longer life span for the Structures. Micromechanical design allows optimization of ECC for high performance, resulting in extreme tensile strain capacity while minimizing the amount of reinforcing fibers, typically less than 2% by volume. Tensile strain capacity exceeding 5% has been demonstrated on ECC reinforced with polyethylene and polyvinyl alcohol (PVA) fibers. Unlike ordinary cement-based materials, ECC strain hardens after first cracking, similar to a ductile metal, and demonstrates a strain capacity 350 to 550 times greater than normal concrete. Even at large imposed deformation, crack widths of ECC remain small, less than 80 μm. With intrinsically tight crack width and high tensile ductility, ECC represents a new generation of high performance concrete (HPC) material that offers significant potential to naturally resolving the durability problem of reinforced concrete structures. In the past few decades, substitution of mineral admixtures, such as fly ash (FA) and Ground Blast-Furnace Slag (GBFS), has been of great interest and gradually applied to practical applications of ECC. It has been found that incorporating high amount of FA can reduce the matrix toughness and improve the robustness of ECC in terms of tensile ductility. Additionally, unhydrated FA particles with small particle size and smooth spherical shape serve as filler particles resulting in higher compactness of the fiber/matrix interface transition zone that leads to a higher frictional bonding. This aids in reducing the steady-state crack width beneficial for long-term durability of the structure. In this study, the workability, mechanical properties and durability of ECC different mixtures contains two mineral materials (slag / fly ash) as to replace part of the cement weight and two types aggregate (Silica/ River sand) were evaluated. The results showed that mixtures containing fly ash despite lower mechanical strength to compared with mixtures containing slag, significantly have higher performance in strain- hardening behavior at post- cracking portion. ECC mixtures performance against the durability testing (Rapid chloride penetration, Electrical Specific Resistivity, Drying Shrinkage and Accelerated Reinforcement Corrosion) were appropriate and quantitatively was to form of slag> fly ash. In this study, in order to calculate the direct tensile strength of ECC mixtures, a new model (different geometry) compared to other models (used by prior researchers) proposed and tested. The its results showed that the tensile strength measured by the new model compared to the previous models, was higher 10% to 17%.