CHAPTER 1 INTRODUCTION 1.1
BACKGROUND OF STUDY Concrete is a composite material containing hydraulic cement, water, coarse aggregate
and fine aggregate. The resulting material is a stone-like structure which is formed by the chemical reaction of the cement and water. This stone like material is a brittle material which is strong in compression but very weak in tension. This weakness in the concrete makes it to crack under small loads, at the tensile end. These cracks gradually propagate to the compression end of the member and finally, the member breaks. The formation of cracks in the concrete may also occur due to the drying shrinkage. These cracks are basically micro cracks. These cracks increase in size and magnitude as the time elapses and they finally make the concrete to fail. (Thilagavathi et al., 2014) The formation of cracks is the main reason for the failure of the concrete. To increase the tensile strength of concrete many attempts have been made. One of the successful and most commonly used method is providing steel reinforcement. Steel bars, however, reinforce concrete against local tension only. Cracks in reinforced concrete members extend freely until they encounter steel bars. Thus need for multidirectional and closely spaced steel reinforcement arises. That cannot be practice call possible. Fibre reinforcement gives the solution to this problem So to increase the tensile strength of concrete a technique of introduction of fibres in concrete is being used. These fibres act as crack arrestors and prevent the propagation of the cracks (glass, carbon, steel, natural, sisal. Etc.).
These fibres are uniformly distributed and randomly arranged. This concrete is named as fibre reinforced concrete. The main reasons for adding fibres to concrete matrix is to improve the post cracking response of the concrete, i.e., to improve its energy absorption capacity and apparent ductility, and to provide crack resistance and crack control. Also, it helps to maintain structural integrity and cohesiveness in the material. The initial researches combined with the large volume of follow up research have led to the development of a wide variety of material formulations that fit the definition of Fibre Reinforced Concrete. (Rajalakshmi et al., 2014) The use of fibres to increase the structural properties of construction materials is not a new process. From ancient times, Fibres were being used in construction. In Egypt, horse hair was used to reinforce mortar. Egyptians used straw in mud bricks to provide additional strength. Asbestos was used in the concrete in the early 19th century, to protect it from formation of cracks. But in the late 19th century, due to increased structural importance, introduction of steel reinforcement in concrete was made, by which the concept of fibre reinforced concrete was over looked for 5-6 decades. Later in 1939, the introduction of steel replacing asbestos was made for the first time. But at that period it was not successful. From 1960, there was a tremendous development in the fibre reinforced concrete, mainly by the introduction of steel fibres. Since then use of different types of fibres in concrete was made. In the 1970s, principles were developed on the working of the fibre reinforced concrete. Later in the 1980s, a certified process was developed for the use of fibre reinforced concrete. In the last decade, codes regarding the fibre reinforced concrete have been undergoing development. (Ramya et al., 2014) 1.2
STATEMENT OF THE PROBLEM Reinforcing schemes are generally designed to resist tensile stresses in the tension zone
of reinforced concrete sections which causes unacceptable cracking. Cracking of the concrete
section is nearly impossible to prevent and can allow moisture to penetrate and corrode the reinforcement. This is a serviceability failure in limit state design. Cracking is normally the result of an inadequate quantity of rebar, or rebar spaced at too great a distance just as we currently have in most reinforced concrete elements, especially slabs and beams. The concrete cracks either under excess loading, or due to internal effects such as early thermal shrinkage while it cures. Limiting cracking in reinforced concrete elements has become a major center of focus in recent times in structural engineering studies. This problem has greatly necessitated this study in order to explore the option of using structural fibres (sisal and glass) to limit the occurrence of cracks in reinforced concrete beams. 1.3
AIM AND OBJECTIVES OF THE STUDY
The aims and objectives of this study is to develop concrete mixes reinforced with glass and sisal fibres and thus to investigate the engineering properties of the resulting glass fibre reinforced concrete (GRFC) and sisal fibre reinforced concrete (SFRC). Also, to make a comparison between the fibres with regards to their performance in the concrete matrix after undergoing the required tests. Specific Objectives: i. ii. iii.
To evaluate the compressive strength of concrete reinforced with glass fibre. To evaluate the compressive strength of concrete reinforced with sisal fibre. To evaluate the compressive strength of concrete not reinforced with fibre (mass
iv. v. vi. vii.
concrete). To evaluate the split tensile strength of concrete reinforced with glass fibre. To evaluate the split tensile strength of concrete reinforced with sisal fibre. To evaluate the split tensile strength of concrete not reinforced with fibre (mass concrete). To compare the compressive strength of concrete reinforced with glass fibre to that reinforced with sisal fibre.
viii.
To compare the tensile strength of concrete reinforced with glass fibre to that reinforced
ix.
with sisal fibre. To compare the compressive and tensile strengths of concrete not reinforced with glass or
x.
sisal fibre (that is, mass concrete) to that reinforced with the two fibres. To establish the physical properties of constituents (cement, fine aggregate, coarse aggregate and fibre).
1.4
SIGNIFICANCE OF THE STUDY 1. The essence of this study is to highlight the variations that would occur in the compressive strength and split tensile strength when the conventional steel bars in reinforced concrete are replaced with glass fibre and sisal fibre bars. 2. The study is meant to highlight the benefits / limitations of glass fibre and sisal fibre in concrete structures with regards to compressive and tensile strengths, thereby establishing the necessity or otherwise of incorporating fibre reinforced concrete in construction works. 3. Its conclusions and recommendations should give insight to structural engineers in order to know the suitability of fibre reinforced concrete for use in design of structures. 4. This research attempts to provide a long-lasting solution to the problem of occurrence of structural cracks in concrete structures by the use of environmentally friendly material such as glass and sisal fibres. 5. Finally, this work is expected to contribute significantly to existing literature in the subject under consideration.
1.5
SCOPE OF STUDY
This work is strictly limited to evaluating and studying the quality of the results obtained from the compressive and split tensile strength of fibre reinforced concrete and the mass concrete and it does not includes the evaluation of reinforced concrete. Also, the workability of the concrete was determined by carrying out some tests such as Slump Test and compacting factor test on the concrete,
1.6
LIMITATIONS OF STUDY
The major limitation to this study was financial. Other fibres such as carbon and kelvar fibres would have been used for the study but for the high cost of obtaining the fibres with the scare resources available for use.
