Effect of hybridization of fibers on the properties of high volume fly ash concrete and its numerical simulation

Abstract

High volume fly ash (HVFA) is used as cement substitute in many cementitious materials. In recent years, hybrid fibers such as steel and polypropylene have been added to improve the mechanical characteristics. The usage of three fibers combinations is relatively a new concept in HVFA concrete. In this thesis, natural basalt fiber has been combined with steel and polypropylene due to its availability and low cost. Due to the concern about chemical resistance, chemical durability and performance of silane coated modified and non-modified basalt fibers are studied. The fibers were immersed into twelve solutions for 62 days considering the concrete medium. The failure pattern and damage features of the fibers were sorted with the observation of surface by scanning electron microscope (SEM) and their compositions were identified using energy dispersive X-ray spectroscopy (EDX). Long term mass retention capacity was also summarized. The result revealed that the modified fiber exhibits superior properties compared to the non-modified fibers based on morphological and chemical analysis of basalt fibers. Post this analysis; varied hybrid fiber combinations (both two types and three types) were tested on cement mortar specimens to identify the optimal fiber percentages. Based on the experimental results, equations and artificial neural network were developed to predict the compressive and flexural strength. Two optimum hybrid fiber percentages were found out from the overall analysis of compressive and flexural properties i.e., 1.5% steel, 0.5% polypropylene (1.5S0.5P) and 1.0% steel, 0.5% polypropylene, 0.5% basalt (1.S0.5P0.5B). Optimal percentages were then used to test concrete specimens. Pre and post cracking mechanical tests of the concrete were carried out. The result of the compressive strengths of three types of hybrid fiber concrete showed appreciable increase by 5.44% and the splitting tensile strength increase was 6.77% in comparison with two type hybrid. Detailed durability properties of optimum hybrid fiber HVFA concrete were conducted along with advanced computed tomography X-ray scans to investigate the durability characteristics of HVFA hybrid fiber concrete. These techniques together confirmed the reduction in porosity and hence the durability of concrete directly suggesting the benefit of the added silane coated basalt fiber. Numerical models were validated using experimental results for the use of three different fibers in concrete. The model was prepared considering the separate geometry and random orientation of individual fibers. Nonlinear analysis was performed under static load to investigate their failure modes in terms of ultimate load. This model will provide a guideline for the optimum use of three different fibers