Introduction to Fibre Reinforced Concretes
Fibre Reinforced Concretes (FRC) have an untapped potential provision in distinctive structures because of its adaptability, quality, high vigor assimilation proficiency and comparatively basic development method. For more than 50 years, normal solid materials have been fortified with short haphazardly dispersed fibres (Mobasher, Test Parameters in Toughness Evaluation of Glass Fiber Reinforced Concrete Panels, 1989) (Mobasher, Mechanical Properties of Hybrid Cement Based Composites, 1996). In order to spout such potential, the existing grouping of learning on FRCs must be stretched to give a legitimate premise for authorities to add this technique for development to the procurements of the construction regulation. The point when plain cement is subjected to pressure, the launch and development of intrinsic micro-splits lessens the burden convey region and increment the anxiety focus at basic split tips, making the breaks spread further. Flopping of the cement happens, if these breaks are not adequately captured (Swaddiwudhipong, Lu, & Wee, 2003).
In spite of the fact that cement is a standout amongst the most flexible building materials, composite material grid is fragile. Including short steel fibres in this we can expand the material firmness, flexural, elasticity and affect safety, and additionally can acquire a bendable conduct for split material. The utilization of steel fibres in cement in place of customary support is valuable for most of the part because of the less complex shedding method. It might be fixed to any structural shape, paying little heed to the assembling complexities. It is promptly accessible in urban regions at generally minimal effort.
Numerous research studies have been performed on mechanical properties of fibre concrete and concrete structural members reinforced with fibres under various loading conditions. However, studies on behaviour of concrete slabs reinforced with fibres considering various parameters are limited. In the present investigation, influence of different types of fibres of strength and energy absorption of concrete slabs with a wide range of concrete strengths is studied. According to theoretical evaluation, design bases for application are presented.
Aims and objectives
The primary objectives of this research are:
- To develop a simple and generalized simulation model that can be used to analyse the slab on grades with or without fibre reinforced materials.
- To propose a developed method to define fibre reinforced materials such as SFRC’s regardless of the structure type sine existing methods are not adequate. The method includes the pre crack and post crack behaviour of fibre reinforced concrete which can be used in different situations.
The project was carried out from February 2013 until September 2013 to write a dissertation as a final report of the M.Sc. Advanced Engineering design at Brunel University London. The research was conducted by a giant chart in order to have a clear overview of the task and the milestones as well as progress presentation.
Effects of Steel Fibres
Three-dimensional plasticity theory is used to define the material equations. Deformation method and the incremental process are also considered. For a material-dependent reference curve for compression states, the usual key data for one-dimensional stress are applied. For the behaviour of tension states special approaches are used, which take softening into account given increasing crack width. In the next pages the material equations are presented based on the literature review and some calibrations to test results are described.
The general assumption believes that the addition of fibres to concrete will improve the mechanical and physical properties of the concrete such as ductility, toughness, strength, fatigue endurance and so on. New published papers, which are approved by ACI such as (Wang & Wang, 2013) and (Chen, 2004) allow to replace rebar steel by conventional fibres, which can save time, cost and materials. However, some disadvantages for SFRC exist
Modulus of Elasticity
Modulus of elasticity is a mathematical definition, which define the tendency of a mass to be deformed elastically while a force is applying to it. From engineering point of view, it is considered as the slope of pre-crack stress-strain curve. The modulus of elasticity of normal concrete is fairly constant at low stress levels but starts lessening at higher stress levels due to the crack propagation. Hence, steel fibres and their effects start in post cracking period should show a good improvement in the modulus of elasticity. However, fibre materials are considered as a material with a low Young’s Modulus. Although a numerical function is proposed as below to determine the modulus of elasticity for Fibre Reinforcement materials, the experimental results show the formula is not trustful.
Various literatures and results of the experiments (544.1R-96, 2002) (Akcay & Tasdemir, 2012) (Belletti B. , Cerioni, Meda, & Plizzari, 2008) (Gornale, Quadr, Quadri, Ali, & Hussaini, 2012) (Chandramouli, Rao, Pannirselvam, Sekhar, & Sravana, 2010) (Priti, Patel, Atul, Desai, Jatin, & Desai, 2012) indicates that the deviation in the Poisson’s ratio of the concrete due to the addition of steel, glass, polypropylene or carbon fibres is negligible. The quality of Poisson's ratio was considered to be fluctuating from 0.18 to 0.22 for different evaluations of concrete and did not represent critical change in Poisson's proportion of the concrete.
In general, concrete is measured as a fragile material with a low tensile strength and shear capacity. The shear failure of reinforced or plain concrete is a common concern for structural engineers. The study of the previous literatures show the shear strength of SFRCs are much more than the normal concretes (Adhikary & Mutsuyoshi, Prediction of shear strength of steel fiber RC beams, 2006), (Turmoa, Ramosb, & A.C.Apariciob, 2006), (Slater, Moni, & Alam, 2012), (Wang and Lee2007). However, it is difficult to predict the shear capacity of SFRCs.
The word “concrete” from the structural point of view assumed to be a non-tension material with some softening post-peak behaviour in compression due essentially to the limited transverse strain capability of the material. Then, normal plain concrete does not have considerable tensile strength and it is considered as a brittle material.
In this part, the typical strain-stress diagrams will be introduced, which are being gathered and comprised from several previous studies. It has tried to mention to the newest curves, which are correlated in the new publishes. These relationships are important to define a finite element model in software, especially in the non-linear stage analysis. Different techniques have been used to craw the caves include fracture energy and the laws of mixture.
Modelling Non-Linear Behaviour of fibre reinforced slab on grade
The importance of ability to have a clear prediction from the behaviour of fibre reinforced concrete regarding the usage tendency in several parts of industries has been increased during last decade. However, numerous types of fibre beside the complexity of concrete’s behaviour have led to make the analysis more complicated. This study is proposed to establish a simplified and generalised method for analysing fibre reinforced concretes. In addition, this model focuses on slabs on grades computer designation which is created by SFRC material.
As it mentioned in literature review, there is several approaches to define and model the fibres.
There are approaches for modelling the composite fibres in FE software:
• The first one is layer modelling where fibre, concrete and other contingency material are defined as a Sell for the software.
• The second approach is modelling based on equilibrium composite material where the volume fraction of fibre is calculated, and then it is defined as steel bars in two or three directions inside the concrete.
The strain and the displacement developed in the slab are assumed to be in the non-linear stage. For definition of the numerical modelling of the SFRC, the stress-strain behaviour of concrete in compression, and a post-cracking stress-strain relationship are defined. In addition, a crack representation model as well as failure criteria to simulate cracking and crushing fractures are used.
DISCUSSION AND RESULTS
Effect of Fibres on Plain Concrete
The effects of the steel fibre, glass fibre, carbon fibre and polypropylene fibre on physical properties, compressive strength, tensile strength, flexural strength, modulus of elasticity, Young’s modulus and shear strength have been reviewed in chapter 2 separately. In this chapter, the effects of the different fibres on the plain concrete, which are being gathered from several published resources, will be discussed to have a clear understanding from behaviour of fibre reinforced concretes. Then, in each section the data will be compared with the proposed formulas to find out whether they are reliable or not.
Steel Fibre Reinforced Concretes (SFRCs)
Compressive Strength:-Previous studies were being reviewed in part Above. Despite the general belief about increasing in compressive strength of SFRCs, the statistical analysis of the previous experiments indicates something else.
The review of previous studies reviewed in part Above. Both theoretical and experimental data indicate a comprehensive improvement in tensile behaviour of adding steel fibres on the plain concrete. However, there are some disagreements between different authors about the amount of improvement where some believes that it is less than 50% and some believe it is more than 100%.
A numerical model for the non-direct dissection of steel fibre strengthened concrete pieces backed on soil was created. Utilizing exploratory information, the principle characteristics of the fibre strengthening were presented in the cement constitutive laws, predominantly in the pressure and in the clamping post-top conduct. More tests ought to be maintained out so as to adjust the constitutive laws proposed. The trial exploration performed was confined to just two sorts of fibres. The conduct of the un-broke cement and cement between splits was recreated under the elastic-plasticity system. The splitting conduct was repeated utilizing a spread multi-settled break model. The soil or other base material, supporting the concrete slab was simulated by distributed springs orthogonal to the concrete slab middle surface. An elasto-plastic model was used to modulate the non-linear behaviour of the springs.
- 544.1R-96, A. (2002). Fiber Reinforced Concrete. Chicago: ACI Committee.
- A, A. S., S., H. G., & Wafa, F. F. (1992). Shear Behavior of High-Strength Fiber Reinforced Concrete Beams",. ACI Structural Journal , 89 (2), 176- 184.
- Abdullah, M., & Jallo, E. (2012). Mechanical Properties of Glass Fiber Reinforced Concrete. Al-Rafidain Engineering , 20 (5), 129-135.
- Adhikary, B. B., & Mutsuyoshi, H. (2006). Prediction of shear strength of steel fiber RC beams. Construction and Building Materials , 20, 801-811.