اثرات افزودن الیاف کربن بر خواص مکانیکی بتن مقاومت بالا (HSC)

ABSTRACT The Effect of Addition of Carbon Fibers on Mechanical Properties of High Strength Concrete

High Strength Concrete (HSC) is a special type of concrete that has a specified compressive strength of 60 N/mm2 or greater. High strength concrete is a most economic concrete, realized when it is used in the columns of High-Rise Buildings, Parking garages, Bridge decks, and other installations requiring improved compressive strength and density. For HSC usually selected pozzolanic and chemical admixtures are employed, and attainment of a low water to cementitious material ratio is considered essential. Many trial mixtures are often required to generate the data necessary to identify optimum mixture proportions Reported by ACI Committee 211.4R-93 [1].

High strength concrete is considered as a relatively brittle material as the concrete is strong under compression and weak under tension or flexure. Carbon fibers are inert, medically safe, chemically stable, low in density, and their strength-to-density ratio is one of the highest among all fiber types. Carbon fiber has a very high tensile strength (2110 to 2815 N/mm2)and young’s modulus. Pataudi,and Rathod, have concluded that, cement composites made with carbon fiber, as reinforcement will have very high modulus of elasticity and flexural strength[2]. Aziz and Tahahave found improved structural properties when flexural behavior of high strength concrete beams reinforced with carbon fiber polymer rebars with and without chopped carbon fiber, [3]. Addition of discontinuous fibers to concrete makes it a homogeneous and isotropic materials and converts brittleness into a ductile behaviour.

Aziz and Taha Carried out experimental work on mechanical properties of carbon fiber reinforced High Strength Concrete. The study reports that, when concrete cracks, the randomly oriented fibers start arresting both the randomly oriented micro-cracking and its propagation and thus improving strength and ductility[4]. These carbon fibers are manufactured by carbonizing polyacrylonitrile yarn at high temperatures and then aligning the resultant graphite crystallites by a process called hot stretching. Polyacrylonitrile based carbon fiber were not commonly used in FRC because of their high cost.In the early 1980s, the less expensive short pitch based carbon fiber was developed, and used as fiber reinforcing material in precast concrete applications[5]. The technique of dispersing short pitch based carbon fibers randomly in the concrete mix is carried out by two methods in the carbon fiber reinforced concrete technology. One is to mix the fibers with cement and aggregate in dry state known as “Dry Mix” method. Second is to first disperse the fibers in water with hydroxyl Ethyl cellulose (0.06% by weight of cement) and then pour the dispersion into the slurry with cement and aggregate known as “Wet Mix” method. The second method is much more practical for dispersion of short pitch based carbon fibers in the carbon fiber reinforced concrete technology, specially chosen so as to have appropriate strength. Wet Mix is an effective method of dispersion of carbon fibers only if dispersant chemicals are used with water[6].

In this study, cement content in the HSC mix is replaced with fixed percentages of fly ash (10%) and carbon fiber are added in volume fraction (0 to 0.60%), also the carbon fiber reinforced polymer (CFRP) strip are placed in different layer (single, double and triple layer) with varying width of CFRP strip (0 to 80 mm). The hardened properties of HSC were studied and the experimental results were validated by regression analysis.