Investigation of mechanical and durability properties of hybrid fiber concretes and performance optimization

Concrete is the most preferred building material and needs to be developed. While concrete shows excellent performance under compressive loads, it remains weak against tensile loads. These cracks are tried to be prevented with reinforcements or fibers that transfer loads and prevent sudden breakage of concrete under loading. By changing the dimensions, geometries and quantities of the fibers, it is possible to produce concrete with different features. However, each fiber type is used for a particular problem. Within the scope of this thesis, thirty different Hybrid Fiber Concrete (HLB) mixtures were produced with the seven-factor experimental design created by the Central Composite Design method to use the unique feature of each fiber type. A completely hybrid system was created by replacing the natural crushed stone aggregate with electric arc furnace slag aggregate and fly ash with cement at rates ranging up to 100% and 40% by weight, respectively. It has been determined that the most important variable affecting the slump value of HLBs and splitting tensile strength is the glass fiber ratio. With the hybridization of different fibers in small amounts instead of using a single type of fiber, the flexural strength and energy absorption capacity of concrete have increased. It has been determined that with the production of HLB whose total fiber volume does not exceed 1%, the pull-out capacity increases. It has been observed that the methods based on wave transmission can be misleading in the quality measurements of concrete containing fiber. Weight loss was observed in samples exposed to acidic environment and weight gain was observed in samples exposed to sulphate environment. However, the weight changes are similar by about 15% in both environments and synthetic fibers were insufficient in terms of the durability of HLBs. As the binder dosage and steel fiber content of HLBs increased, their rapid chloride permeability decreased. Regression models, which reveal the relationship between independent variables and the experimental results, have been created by using a commercial software programme. Variance analysis was conducted on the experimental results. By using Response Surface Method, the effect weights of independent variables and their interactions and statistically significance for each response were determined. Optimum production conditions that optimize mechanical, permeability, durability and all properties, with the highest desirability function were determined.