Investigating the bending behavior of light concrete beams reinforced with FRP rebars

Master thesis

in the field of structural civil engineering

Abstract

Steel reinforced concrete is a material that is widely used in civil engineering structures due to reasons such as good compressive strength, low production cost and availability of raw materials. But reinforced concrete prepared from cement paste, ordinary aggregates, and steel rebars has weaknesses such as heavy weight, steel corrosion, and shrinkage cracks, which limit its use in some cases. To compensate for these defects, special concretes have been invented in which light aggregates and separate and small pieces of fibers are used, and instead of steel rebars, fiber polymer composite rebars are used to solve the problem of corrosion in coastal areas. Since the use of special concretes is expanding due to their undeniable advantages, and their use in a country like Iran is justified and necessary in many cases, it is necessary to conduct sufficient studies on their various practical aspects.

In the present research, by preparing a laboratory program, the bending behavior of concrete beams made of light fiber concrete, armed with longitudinal bars made of glass polymer composite (GFRP) has been investigated. For this purpose, 9 concrete beams with a rectangular cross-section have been designed and built in three groups. The first group includes three light aggregate concrete beams, the second group includes three light aggregate concrete beams with steel fibers, and the third group includes three light aggregate concrete beams with polypropylene fibers. In each group, one hundred percent, two hundred percent, and three hundred percent composite rebars were embedded in each beam. The beams were subjected to increasing load step by step and this process continued until the final step, i.e. the destruction of the beams. In each step, the values ??of displacement, strain, and crack width were recorded in appropriate places on the beams, along with the applied force. By processing the obtained data, characteristics of the beams such as force-displacement behavior, bending capacity, how to create and spread cracks were studied. The results indicate that the force-displacement diagram of beams reinforced with composite rebars is almost linear until the final stage, and in all beams at a constant strain, a higher percentage of rebars can withstand a greater load than beams with a lower percentage of rebars. Also, the comparison of the experimental capacity of the beams with the regulatory relations shows that these relations give conservative results. Also, the beams with fibers started to crack at lower loads, but the load resistance and causing more deformations in them is more obvious.

Key words:

GFRP rebars, structural lightweight concrete, fiber concrete, bending behavior.

Chapter 1: General

1-1 Introduction

Concrete is the most widely used construction material, and in most countries of the world, the ratio of concrete to steel has exceeded 10 to 1. The only substance that humans consume in this amount is water. Concrete has advantages such as excellent resistance to water, ease of forming in various shapes, cheap and availability of primary materials. Also, compared to steel, it needs little maintenance, it has shown good resistance at high temperatures, and because it is under less local stress fields, fatigue is not considered an important problem for it [1]. Despite the advantages mentioned for concrete, due to the presence of different materials in concrete and the interaction of these materials, especially in the area between aggregates and cement paste, there are still many complications and unknowns in this material and the final product resulting from its construction. In some cases, concrete structures will not meet the operational needs. Among the defects of concrete structures, we can mention low tensile strength, corrosion of steel, ease of crack creation and propagation, and their heavy weight.

Efforts of researchers in the construction industry have always been to fix the defects of concrete structures and they have provided various methods for this purpose, some examples of which are mentioned below:

- FRP rebars to prevent corrosion and increase resistance and increase damping: the susceptibility of steel to corrosion against the alkaline environmental conditions that exist in concrete structures Reinforcement is used exposed to sea water, it has caused the use of FRP to be proposed as its alternative. The corrosion and tensile resistance of composite materials can be up to four times that of steel.These materials dampen absorbed energy due to their high damping coefficient, which is due to their inelastic properties.

- Use of fibers to increase tensile strength and reduce the width of cracks: Fibers are used in different sizes and shapes and made of steel, pulp, glass and natural materials. The use of fibers with different volumes and sizes in the mortar has increased the final tensile strength to some extent, but the tensile strain at the time of rupture in this type of concrete is much higher compared to normal types, which is due to preventing the opening of cracks and turning a large crack into several small cracks.

- The use of light concrete to reduce the overall weight of the structure: compared to steel, the ratio of strength to the weight of concrete is low, for the construction of towers and large spans of bridges and structures. Floating is considered as an economic problem. To increase the strength-to-weight ratio of concrete, a suitable solution is to use light aggregates such as Lika instead of ordinary aggregates, which have been successfully used in the construction of towers up to tens of stories in the world.

Obviously, new materials have shortcomings, including limited production and high cost, brittle failure, the need to hook polymer bars in the factory, and so on. . . which causes a decrease in their use in concrete structures at present. According to the growth of industry and technology, the increasing use of these materials in the near future will not be far from expected. 1-2 Research Objectives In this research, the main objective is to examine the advantages, disadvantages, and limitations of the combined use of two concretes, i.e. light structural aggregate concrete and fiber concrete in the construction of structural members under bending (beams), along with the use of different cross-sectional areas of rebars made of composite. It is a polymer armed with glass fibers to strengthen them longitudinally. In order to achieve this goal, for which a laboratory program was developed, the items that should be studied are:

- bending behavior (force-displacement) of beams,

- ultimate bending capacity and type of failure of beams,

- load such as the first crack and the way cracks are created and spread,

- tensile and compressive strains resulting in different stages of loading, and

- comparison of results with relations regulations and existing related researches.

It is necessary to explain that although the mentioned concretes and rebars have been considered in the researches related to concrete structures and there is a relatively comprehensive technical literature about them, but limited researches have been done for their combined use and with the objectives mentioned in the present work, and therefore, this research will have a contribution, however small, in making the existing scientific texts more complete in this field.

1- 3. Research method

In the present work, firstly, the research conducted on lightweight concrete, fiber concrete and FRP rebars was studied and then through experimental study or testing, the bending behavior of a number of beams made of the mentioned materials was evaluated. Laboratory samples of 9 beams, including 3 samples with ordinary lightweight concrete, 3 samples with lightweight concrete with metal fibers and 3 samples with lightweight concrete with propylene fibers were considered. In addition, for each beam, an appropriate number of cubic samples were prepared to estimate the compressive strength and volumetric mass of the concrete used in them. The test was carried out by the four-point bending method (simple supports at both ends and two concentrated loads at the opening of the beams) and in a step-by-step manner until the complete destruction of the beams, and by installing sensors in appropriate places on the beams, quantities such as displacement, strain, and crack width were measured, as well as the applied force in different stages of loading. In order to achieve the goals defined in the present work, the raw results obtained from the test are analyzed and processed, and the characteristics or desired parameters for the beams are extracted. Finally, by summarizing the results obtained, a comparison is made between them and the results of past researches as well as regulatory relationships.

1-4 Thesis Structure

In the first chapter, a brief explanation of concrete structures and the reason for their use is given. The accuracy and how to work with the equipment used in the laboratory is mentioned, and the stages of laboratory research, design, construction, how to conduct tests and the characteristics of materials and consumables have been discussed.