Seminar Report On STEEL FIBRE REINFORCED CONCRETE (Submitted in partial fulfillment for the award of the degree of Bachelor of Technology in Civil Engineering, Rajasthan Technical University Kota)
Dr. P.K. AGARWAL
SHOBHA MEENA CRN - 11/361 Enrolment No.11EUCCE106
DEPARTMENT OF CIVIL ENGINEERING
UNIVERSITY COLLEGE OF ENGINEERING RAJASTHAN TECHNICAL UNIVERSITY KOTA MARCH 2015
i
Department of Civil Engineering
University College of Engineering Rajasthan Technical University, Kota-324010 Dated:
CERTIFICATE This is to certify that
Ms. Shobha meena
College Roll No. 11/361 and
University Roll No. 11eucce106 has submitted the seminar report entitled “steel fibre reinforced concrete’’ in partial fulfillment for the award of the degree of Bachelor of Technology (Civil Engineering). The report has been prepared as per the prescribed format and is approved for submission and presentation.
Counter signature of Head
Signature of Guide
Dr. H.D. CHARAN Professor& Head Dept. of Civil Engg. UCE, RTU, Kota-324010
Dr. P.K.AGARWAL Associate professor Dept. of Civil Engg UCE, RTU, Kota-324010
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ACKNOWLEDGEMENT
I express my sincere thanks to my seminar guide Dr. P.K. AGARWAL (Associate Prof.) for guiding me right from the inception till the successful completion of the seminar. I sincerely acknowledge him for extending their valuable guidance, support for literature, critical reviews of seminar & the report & above all the mortal support. I would like to convey my thanks to seminar coordinators (Prof. B. P. Suneja, K.S. Grover and Mr. S.K. Nagar) and HOD (Prof. H.D. Charan ) giving me the opportunity to embark upon this topic. Finally, I am thankful to my parents & friends for their continued moral support & helping me finalize the presentation.
Date-
SHOBHA MEENA C.R. No. 11/361 Final B. Tech. (Civil)
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CONTENTS
ABSTRACT 1. INTRODUCTION
1-2
1.1. GENERAL
1
1.2. FIBRE REINFORCED CONCRETE
2
1.3. HISTORY
2
2. FIBRE REINFORCED MECHANISM & PROPERTIES
3-7
2.1. PROPERTIES OF FIBRE REINFORCED CONCRETE
3
2.2. FIBRE MECHANISM
5
2.2.1. Method1
6
2.2.2. Method2
6
2.3. WORKABILITY
7
3. STEEL FIBRE REINFORCED CONCRETE
8-11
3.1. STEEL FIBRE REINFORCED CONCRETE (SFRC)
8
3.2. COMPOSITION OF STEEL FIBRE REINFORCED CONCRETE
8
3.3. STEEL FIBRES
9
3.4. TECHNOLOGY FOR PRODUCING SFRC
10
4. BEHAVIOUR OF STEEL FIBRE REINFORCED CONCRETE UNDER
12-19
CONVENTIONAL LOADINGS 4.1. BEHAVIOUR OF STEEL FIBRE REINFORCED CONCRETE UNDER DIRECT COMPRESSION
12
4.1.1. FOR PLAIN CONCRETE
12
4.1.2. FOR STEEL FIBRE REINFORCED CONCRETE
14
4.2. BEHAVIOUR OF STEEL FIBRE REINFORCED CONCRETE UNDER FLEXURE 15 4.2.1. FACTORS AFFECTING THE FLEXURE BEHAVIOUR OF STEEL FIBRE
16
REINFORCED CONCRETE 4.1.2. FLEXURAL BEHAVIOUR OF STEEL FIBRE REINFORCED CONCRETE
iv
17
5. REVIEW OF LITERATURE
20-26
5.1. PREVIOUS PREPARATION, PROPERTIES AND MIX DESIGN OF FIBRE REINFORCED CONCRETE
20
5.2. FLEXURAL BEHAVIOUR OF SFRC AS PER PREVIOUS STUDIES
20
5.3. PREVIOUS INVESTIGATIONS ON FLEXURAL TOUGHNESS, ENERGY ABSORPTION, DUCTILITY AND LOAD DEFLECTION BEHAVIOUR
22
5.4. IMPACT RESISTANCE OF SFRC
23
5.5. FATIGUE
26
6. USES & APPLICATION OF SFRC
27-32
6.1. STRUCTURAL USE OF SFRC
27
6.2. APLLICATIONS OF SFRC
29
6.2.1. USE OF SFRC
29
6.3. BENEFITS OF SFRC
30
6.4. MAJOR STUCTURES OF SFRC ALL OVER THE WORLD
32
REFRENCE
33
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LIST OF FIGURES FIG. NO.
TITLE
PAGE
Fig. 1.1
STEEL FIBRE
2
Fig. 2.1
FIBRE MECHANISM
5
Fig. 2.2
PULL OUT MECHANISM
6
Fig. 3.1
COMPONENTS OF STEEL FIBRE REINFORCED CONCRETE
8
Fig. 3.2
DIFFERENT TYPES OF FIBRES
9
Fig. 3.3
DARMIX FIBRE
10
Fig. 4.1
STRESS & STRAIN CURVE
12
Fig. 4.2
SCHEMATIC REPRESENTATION OF BEHAVIOUR OF CONCRETE
13
Fig. 4.3
COMPRESSIVE STRENGTH OF SFRC
14
Fig. 4.4
.LOAD V/S DEFLECTION CURVE
16
Fig. 4.5
LOAD DEFLECTION CURVE OF SFRC SPECIMEN
17
Fig. 4.6
TYPICAL LOAD DEFLECTION CURVE OF SFRC BEAM WITH LOW VOLUME FRACTION OF FIBRES
18
Fig. 6.1
EXPERIMENTAL MOMENT V/S DEFLECTION CURVE OF SFRC
28
Fig. 6.2
KRCL-MSRC TUNNELS
30
Fig. 6.3
STEEL FIBRE IN CONCRETE
30
Fig. 6.4
MAJOR STRUCTURES
32-33
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ABSTRACT
It is now well established that one of the important properties of steel fibre reinforced concrete (SFRC) is its superior resistance to cracking and crack propagation. As a result of this ability to arrest cracks, fibre composites possess increased extensibility and tensile strength, both at first crack and at ultimate, particular under flexural loading; and the fibres are able to hold the matrix together even after extensive cracking. The net result of all these is to impart to the fibre composite pronounced post – cracking ductility which is unheard of in ordinary concrete. The transformation from a brittle to a ductile type of material would increase substantially the energy absorption characteristics of the fibre composite and its ability to withstand repeatedly applied, shock or impact loading. In this seminar, the mechanic properties, technologies, and applications of SFRC are discussed.
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CHAPTER-1 INTRODUCTION
1.1 GENERAL 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 the finally makes the concrete to fail. 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 encountering are bar. Thus need for multidirectional and closely spaced steel reinforcement arises. That cannot be practically possible. Fiber reinforcement gives the solution for this problem So to increase the tensile strength of concrete a technique of introduction of fibers in concrete is being used. These fibers act as crack arrestors and prevent the propagation of the cracks. These fibers are uniformly distributed and randomly arranged. This concrete is named as fiber reinforced concrete. The main reasons for adding fibers 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 Fiber Reinforced Concrete.
1
FIGURE 1.1: Steel Fibre 1.2 FIBRE REINFORCED CONCRETE Fiber reinforced concrete (FRC) is concrete containing fibrous material which increases its structural integrity. So we can define fiber reinforced concrete as a composite material of cement concrete or mortar and discontinuous discrete and uniformly dispersed fiber. Fiber is discrete material having some characteristic properties. The fiber material can be anything. But not all will be effective and economical. Some fibers that are most commonly used are:
• Steel • Glass • Carbon • Natural • NBD Steel fiber is one of the most commonly used fiber. Generally round fibers are used. The diameter may vary from 0.25 to 0.75mm.The steel fiber sometimes gets rusted and lose its strength. But investigations have proved that fibers get rusted only at surfaces. It has high modulus of elasticity. Use of steel fibers makes significant improvements in flexure, impact and fatigue strength of concrete. It has been used in various types of structures.
2
Glass fiber is a recently introduced fiber in making fiber concrete. It has very high tensile strength of 1020 to 4080Mpa. Glass fiber concretes are mainly used in exterior building façade panels and as architectural precast concrete. This material is very good in making shapes on the front of any building and it is less dense than steel. Use of carbon fiber is not a developed process. But it has considerable strength and young’s modulus. Also investigations have shown that use of carbon makes the concrete very durable. The study on the carbon fibers is limited. Mainly used for cladding purpose. Natural fibers are low cost and abundant. They are nonhazardous and renewable. Some of the natural fibers are bamboo, jute, coconut husk, elephant grass. They can be used in place of asbestos. It increases toughness and flexural strength. It also induces good durability in concrete. Disposal of non-biodegradable materials is a serious problem. It creates environmental problems. Reusing is the best option to reduce the waste. These NBD materials are noncorrosive, resistant to chemical attack, light in weight, easy to handle. NBD materials – fiber plastic, jute plastic, polythene, disposal glass, cement bags. Studies conducted so far, proved that the short and discrete, small fibers can improve the flexural load carrying capacities and impact resistance for non-ferrous fibers. 1.3 HISTORY The use of fibers to increase the structural properties of construction material is not a new process. From ancient times fibers were being used in construction. In BC, 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 fiber reinforced concrete was over looked for 5-6 decades. Later in 1939 the introduction 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 FRC, mainly by the introduction of steel fibers. Since then use of different types of fibers in concrete was made. In 1970’s principles were developed on the working of the fiber reinforced concrete. Later in 1980’s certified process was developed for the use of FRC. In the last decades, codes regarding the FRC are being developed.
3
CHAPTER-2 FIBRE REINFORCED MECHANISM & PROPERTIES
2.1 PROPERTIES OF FIBRE REINFORCED CONCRETE Properties of concrete is affected by many factors like properties of cement, fine aggregate, coarse aggregate. Other than this, the fiber reinforced concrete is affected by following factors: •
Type of fiber
•
Aspect ratio
•
Quantity of fiber
•
Orientation of fiber
Type of fibre: A good fiber is the one which possess the following qualities: •
Good adhesion within the matrix.
•
adaptable elasticity modulus (sometimes higher than that of the matrix)
•
compatibility with the binder, which should not be attacked or destroyed in the long term
•
an accessible price, taking into account the proportion within the mix
•
being sufficiently short, fine and flexible to permit mixing, transporting and placing
•
Being sufficiently strong, yet adequately robust to withstand the mixing process.
Aspect ratio: Aspect ratio is defined as the ratio of length to width of the fiber. The value of aspect ratio varies from 30 to 150. Generally the increase in aspect ratio increases the strength and toughness till the aspect ratio of 100. Above that the strength of concrete decreases, in view of decreased workability and reduced compaction. From investigations it can be found out that good results are obtained at an aspect ratio around 80 for steel fibers. Keeping that in view we have considered steel hooked end fibers with aspect ratio of 80 (Length 60 mm and Diameter 0.75 mm).
4
Fibre quantity Generally quantity of fibers is measured as percentage of cement content. As the volume of fibers increase, there should be increase in strength and toughness of concrete. Regarding our fiber, we hope that there will be an increase in strength, with increase in fiber content. We are going to test for percentages of 1.0, 2.0 and 3.0. Orientation of fibre The orientations of fibers play a key role in determining the capacity of concrete. In RCC the reinforcements are placed in desired direction. But in FRC, the fibers will be oriented in random direction. The FRC will have maximum resistance when fibers are oriented parallel to the load applied. 2.2 FIBRE MECHANISM Fiber work with concrete utilizing two mechanisms: the spacing mechanism and the crack bridging mechanism. The spacing mechanism requires a large number of fibers well distributed within the concrete matrix to arrest any existing micro crack that could potentially expand create a sound crack. For typical volume of fractions of fibers utilizing small diameter of fibers or micro fibers can ensure the required no of fibers for micro crack arrest. The second mechanism termed crack bridging requires larger straight fibers with adequate bond to concrete. Steel fibers are considered a prime example of this fiber type that is commonly referred as large diameter fibers or micro fibers.
FIGURE 2.1: Fibre Mechanism 5
2.2.1 Fibre Matrix Interaction: The tensile cracking strain of cement matrix is much lower than the yield or ultimate strain of fibers. As a result when a fiber reinforced composite is loaded the matrix will crack long before the fibers can be fractured. Once the matrix is cracked composite continues to carry increasing tensile stress. The peak stress and strain of the concrete composite are greater than those of the matrix alone during the inelastic range between first cracking and the peak. Multiple cracking of matrix occurs as indicated in fig 2.1 2.2.2 Bridging Action: Pullout resistance of fibers (dowel action) is important for efficiency. Pullout strength of fibers significantly improves the post-cracking tensile strength of concrete. As an FRC beam or other structural element is loaded, fibers bridge the cracks. Such bridging action provides the FRC specimen with greater ultimate tensile strength and, more importantly, larger toughness and better energy absorption. An important benefit of this fiber behavior is material damage tolerance. Bayasi and Kaiser (2001) performed a study where damage tolerance factor is defined as the ratio of flexural resistance at 2-mm maximum crack width to ultimate flexural capacity. At 2% steel fiber volume, damage tolerance factor according to Bayasi and Kaiser was determined as 93%. .
FIGURE 2.2: Pull-out Mechanism
6
2.3 WORKABILITY A shortcoming of using fibers in concrete is reduction in workability. Workability of FRC is affected by fiber aspect ratio and volume fraction as well the workability of plain concrete. As fiber content increases, workability decreases. Most researchers limit volume of fibers to 4.0% and aspect ratio to 100 to avoid unworkable mixes. In addition, some researchers have limited the fiber reinforcement index [volume of fibers as % ×aspect ratio] to 1.5 for the same reason. To overcome the workability problems associated with FRC, modification of concrete mix design is recommended. Such modifications can include the use of additives.
7
CHAPTER-3 STEEL FIBRE REINFORCED CONCRETE
3.1 STEEL FIBRE REINFORCED CONCRETE According to Exodus Egyptians used straw to reinforce mud bricks. There is evidence that asbestos fiber was used to reinforce clay posts about 5000 years ago. Prof. Alberto Fava of the University of La Plata in Argentina points out that the hornero is a tiny bird native to Argentina, Chile, Bolivia and other South American countries; the bird had been painstakingly building straw reinforced clay nests on tree tops since the advent of man. However, N.V.Bekaert is been regarded as the father of "Fiber Reinforced Concrete". 3.2 COMPOSITION OF STEEL FIBRE REINFORCED CONCRETE The components of Steel Fiber Reinforced Concrete (SFRC) can be explained with the help of
FIGURE 3.1: Components of Steel Fiber Reinforced Concrete
8
The Figure 3.1 Concrete containing hydraulic cement, water, fine aggregate, coarse aggregate and discontinuous discrete Steel fibers is called Steel Fiber Reinforced Concrete. It may also contain pozzolans and other admixtures commonly used with conventional concrete. Fibers of various shapes and sizes produced from steel, plastic, glass and natural materials are being used. However, for most structural and non-structural purposes, steel fiber is commonly used of all the fibers. 3.3 STEEL FIBRES This research focuses on steel fibers. Steel fiber length ranges from 1/4 to 3 inches (1.5 to 75 mm) and aspect ratio ranges from 30 to 100. Fiber shapes are illustrated below.
Straight
Crimped
Hooked End
Paddled
Deformed
Irregular FIGURE 3.2: Different shapes of Steel Fibres
The Steel Fibers that we are using in this project work are Hooked Fibers that are glued to each other and named `Dramix', manufactured by Bekaert Limited, Belgium. 9
FIGURE 3.3: Dramix fibre 3.4 TECHNOLOGY FOR PRODUCING SFRC SFRC can, in general, be produced using conventional concrete practice, though there are obviously some important differences. The basic problem is to introduce a sufficient volume of uniformly dispersed to achieve the desired improvements in mechanical behaviour, while retaining sufficient workability in the fresh mix to permit proper mixing, placing and finishing. The performance of the hardened concrete is enhanced more by fibres with a higher aspect ratio, since this improves the fibre-matrix bond. On the other hand, a high aspect ratio adversely affects the workability of the fresh mix. In general, the problems of both workability and uniform distribution increase with increasing fibre length and volume. One of the chief difficulties in obtaining a uniform fibre distribution is the tendency for steel fibres to ball or clump together. Clumping may be caused by a number of factors 1. The fibres may already be clumped together before they are added to the mix; normal mixing action will not break down these clumps. 2. 2. Fibres may be added too quickly to allow them to disperse in the mixer. 3. Too high a volume of fibres may be added. 4. The mixer itself may be too worn or inefficient to disperse the fibres. 5. Introducing the fibres to the mixer before the other concrete ingredients will cause them to clump together. In view of this, care must be taken in the mixing procedures. Most commonly, when using a transit mix truck or revolving drum mixer, the fibres should be added last to the wet concrete. The concrete alone, typically, should have a slump of 50-75 mm greater than the desired
10
slump of the SFRC. Of course, the fibres should be added free of clumps, usually by first passing them through an appropriate screen. Once the fibres are all in the mixer, about 30-40 revolutions at mixing speed should properly disperse the fibres. Alternatively, the fibres may be added to the fine aggregate on a conveyor belt during the addition of aggregate to the concrete mix. The use of collated fibres held together by a water-soluble sizing which dissolves during mixing largely eliminates the problem of clumping.
11
CHAPTER-4 BEHAVIOUR OF STEEL FIBRE REINFORCED CONCRETE UNDER CONVENTIONAL LOADINGS
4.1 BEHAVIOUR OF STEEL FIBRE REINFORCED CONCRETE UNDER DIRECT COMPRESSION Maximum stress a material can sustain under crush loading is known as Compressive strength. The compressive strength of a material that fails by shattering fracture can be defined within fairly narrow limits as an independent property. However, the compressive strength of materials that do not shatter in compression must be defined as the amount of stress required to distort the material an arbitrary amount. Compressive strength is calculated by dividing the maximum load by the original cross-sectional area of a specimen compression test. 4.1.1 For Plain Concrete The stress strain curve of concrete under uniaxial compression shows a linear behavior up to about 30% of the ultimate strength (Fu) because under short term loading the micro cracks in the transition zone remain undisturbed. For stresses above this point, the curve shows a gradual increase in curvature up to about 0.75 Fu to 0.9 Fu, then it bends sharply almost becoming flat at the top and finally descends until the specimen is fractured.
FIGURE 4.1: Relation between Concrete Performance and Extent of Cracking
12
From the shape of the stress strain curve it seems that, for a stress between 30 to 50% of Fu the micro-cracks in the transition zone show some extension due to stress concentration to the tips however, no cracking occurs in the mortar matrix. Until this point crack propagation is assumed to be stable in the sense that crack lengths rapidly reach their final values it the applied stress is held constant. For a stress between 50 to 75% of Fu increasingly the crack system tends to be unstable as the transition zone crack begins When the available internal energy exceeds the required crack release energy, the rate of crack propagation will increase and the system becomes above 75% of Fu when complete fracture of the test specimen can occur by bridging of mortar and transition zone cracks. Based on the described cracking stages, the behavior of concrete can be viewed at two levels: First, randomly distributed micro cracks are formed or enlarged under low level of stresses. When tile stress level reaches a specific value, these micro-cracks begin to localize (strain localization) and to coalesce into a macro-crack. This macro crack will agate until the stress reaches its critical stage. Steady state propagation of this macro-crack will result in the strain softening mechanism observed for concrete. This general view of cracking of concrete makes it clear that the first linear elastic portion of loading up to strain localization cannot be described by fracture mechanics but can be quantified using damage mechanics [Krajcinovic 1984].
FIGURE 4.2: Schematic Representation of Behavior of Concrete 13
4.1.2. FOR STEEL FIBRE REINFORCED CONCRETE Compressive strength is little influenced by steel fiber addition. High compressive this can be achieved using silica fume or fly ash. However, the use of steel fibers the mode of failure of high strength concrete from an explosive brittle one to a more ductile one, again showing the increased toughness of SFRC and its ability to absorb energy under dynamic loading.
FIGURE 4.3: Compressive Strength of SFRC The fiber type, volume fraction and aspect ratio play important roles in determining the compressive ductility and energy absorption capacity of fiber reinforced concrete. The material behavior is generally enhanced as the volume fraction and aspect ratio of fibers increase up to limits after which the problems with fresh mix workability and fiber dispersibility start to damage the hardened material properties. As the increases in both fiber volume fraction Vr and aspect ratio 1/d lead to improvement of the same nature in the compressive behavior of the material, their combined effect has been generally analyzed using the Fiber Reinforcing Index Vr1/d. in general. The higher the fiber reinforcing index, the higher is ductility and energy absorption capacity of fiber reinforced concrete. However, for high values of fiber reinforcing index, the problems with workability and fiber dispersibility of fresh mix tend to deteriorate the compressive behavior of the hardened material. Due to their material properties, steel fibers do not at all influence the strength parameters of concrete. Under compressive loading, when micro-cracking occurs because of transverse 14
tension forces, steel fibers cause crack-closing forces, on the one hand. This leads to an increase of compressive strength. On the other hand, porosity increases when steel fibers are mixed in with the fresh concrete. This effect decreases the compressive Strength of steel fiber reinforced concrete. Both effects in combination have the tendency to cancel each other out. The influence of fibers in improving the compressive strength of the matrix depends on whether mortar or concrete (having coarse aggregates) is used and on the magnitude of compressive strength. Otter and Naaman [1988] showed that use of steel fibers in lower strength concretes increases their compressive strength significantly compared to plain unreinforced matrices and is directly related to volume fraction of steel fiber used. Ezeldin and Balaguru [1992] conducted tests to obtain the complete stress-strain of steel fiberreinforced concrete with compressive strengths ranging from 35 MPa to 84 Mpa. The matrix consisted of concrete and three volume fibers fractions of 30 kg/m3, 45 kg/m3 and 60 kg/m3 and three aspect ratios of 60, 75 and 100 were investigated. It was reported that the addition of hooked-end steel fibers to concrete increased marginally the compressive strength and the strain corresponding to peak stress. 4.2 BEHAVIOUR OF STEEL FIBRE REINFORCED CONCRETE UNDER FLEXURE In numerous investigations, it has been displayed that the flexure, shear, torsion, punching, dynamic impact behaviors of structural elements improved by the use of Steel Fiber Reinforced Concrete. The positive effects of SFRC on the flexure behavior of the structural elements are given as follows by Craig (1984). •
Increases moment capacity and cracking moment,
•
Increase the ductility,
•
Increases crack control,
•
Increases rigidity,
•
Preserves the structural integrity after beam exceeds the ultimate load.
15
4.2.1 Factors Affecting the Flexure Behaviour of Steel Fibre Reinforced Concrete. 1. Influence of Steel Fibre Volume Fraction
The influence of fiber volume fraction is shown in Figure. For 90 and 120 kg/m3 fiber content, the post-crack increase in load is significant. This increase essentially provides the improvement in flexural strength and a stable post-crack behavior. The bending capacity increases as the fiber volume fraction increases. 2. Influence of Fibre Length
The influence of fiber length is very significant for straight fibers. However, it is an established fact that, longer fibers with higher aspect ratios provide better performance in both strength increase and energy absorption as long as they can be mixed, placed, compacted and finished properly. Since hooked-end fibers provide good anchorage, an increase in aspect ratio of hooked-end fibers has less influence compared with straight steel fibers. However, the difference between fiber lengths becomes even less significant at higher volume fractions. 3. Influence of Fibre Geometry
Three different fiber geometry, namely hooked-end fibers, corrugated fibers and deformed-end fibers with equal length are studied on the flexural behavior of Steel Fiber Reinforced Concrete by Gopalaratnam et al. (1991). According to test results, concrete with hooked-end fibers have higher tensile strength and post-crack response than the other two types. The drop after the first peak is much more pronounced for corrugated and deformed-end fibers.
FIGURE 4.4: Comparison of effects of steel fibre shapes on load-deflection curves
16
There are a number of factors that influence the behavior and strength of SFRC in flexure. They are fiber orientation and fiber shape, fiber bond characteristics (fiber deformation). Also, factors that influence the workability of SFRC such as water cement ratio, density, air content and the like could also influence its strength. The ultimate strength in flexure could vary considerably depending upon the volume fraction of fibers, length and bond characteristics of the fibers and the ultimate strength of the fibers. Depending upon the contribution of these influencing factors, the ultimate strength of SFRC could be either smaller or larger than its first cracking strength. 4.2.2
Flexural Behaviour of Steel Fibre Reinforced Concrete
Generally, there are three stages of the load-deflection response of SFRC specimens tested in flexure. The three stages are 1. A more or less linear response up to point A. The strengthening mechanism in this portion of the behavior involves a transfer of stress from the matrix to the fibers by interfacial shear. The imposed stress is shared between the matrix and fibers until the matrix cracks at what is termed as "first cracking strength" or "proportional limit". 2. A transition nonlinear portion between point A and the maximum load capacity at point B (assuming the load at B is larger than the load at A). In this portion, and after cracking, the stress in the matrix is progressively transferred to the fibers. With increasing load, the fibers tend to gradually pull out from the matrix leading to a nonlinear load-deflection response until the ultimate flexural load capacity at point B is reached. This point is termed as "peak" strength. 3. A post peak descending portion following the peak strength until complete failure of the composite. The load-deflection response in this portion of behavior and the degree at which loss in strength is encountered with increasing deformation is an important indication of the ability of the fiber composite to absorb large amounts of energy before failure and is a characteristic that distinguishes fiber-reinforced concrete from plain concrete. This characteristic is referred to as toughness.
FIGURE4.5 Load Deflection Curve of Steel Fiber Reinforced Concrete Specimens 17
The nonlinear portion between A and B exists, only if a sufficient volume fraction of fibers is present. For low volume fraction of fibers (Vf < 0.5%), the ultimate flexural strength coincides with the first cracking strength and the load deflection curve descends immediately after the cracking load.
FIGURE4.6: Typical Load Deflection Curves of SFRC Beams with low volume fraction of fibers Two concepts are proposed in the literature for explaining the factors that affect the magnitude of the "first cracking strength or proportional limit". One concept relates the "first cracking strength" to the spacing of the fibers in the composite [Romualdi and Batson 1963; Romualdi and Mandel 1964]. The other concept is based on the mechanics of the composite materials and relates the "proportional limit" to the volume fraction of the fiber, aspect ratio and fiber orientation. In the Fiber spacing concept, it is stipulated that the volume fraction of fibers and fiber aspect ratio must be such that there is a fiber overlap; however, except for this, the fiber aspect ratio L/df which has a significant effect on the flexural strength of SFRC is not a parameter in the fiber spacing approach. Experimental results by some investigators [Edington et al. (1974);
18
Swamy and Mangat (1974)] tend to show that the fiber spacing concept does not accurately predict the first cracking strength of fiber-reinforced concrete. The law of composite materials is believed to be simple and is proven experimentally [shah and Rangan 1971] to be more accurate for the prediction of first cracking strength comparison with the fiber spacing concept. The composite materials approach is based on the assumptions in that the fibers are aligned in the direction of the load, the fibers are bonded to the matrix, and the Poisson's ratio of the matrix is zero. In the law of composite materials the effect of fibers on the cracking behavior of SFRC composites can be viewed similarly to conventional reinforcing steel in concrete members. However, because the fibers are randomly distributed, an efficiency factor is commonly multiplied by the volume fraction of fibers to account for their random distribution.
19
CHAPTER-5 REVIEW OF LITERATURE
5.1 PREVIOUS PREPARATION, PROPERTIES AND MIX DESIGN OF FIBRE REINFORCED CONCRETE The influence of fibers in improving the compressive strength of the matrix depends on whether mortar or concrete (having coarse aggregates) is used and on the magnitude of compressive strength. Studies prior to 1988 including those of Williamson [1974], Naaman et al. [1974] showed that with the addition of fibers there is an almost negligible increase in strength for mortar mixes; however for concrete mixes, strength increases by as much as 23%. Furthermore, Otter and Naaman [1988] showed that use of steel fibers in lower strength concretes increases their compressive strength significantly compared to plain unreinforced matrices and is directly related to volume fraction of steel fiber used. This increase is more for hooked fibers in comparison with straight steel fibers, glass or polypropylene fibers. Ezeldin and Balaguru [1992] conducted tests to obtain the complete stress-strain curves of steel fiber-reinforced concrete with compressive strengths ranging from 35 MPa to 84 MPa (5,000 to 12,000 psi). The matrix consisted of concrete rather than mortar. Three volume fibers fractions of 50 psi, 75 psi and 100 psi (30 kg/m3, 45 kg/ m3 and 60 kg/m3) and three aspect ratios of 60, 75 and 100 were investigated. It was reported that the addition of hooked-end steel fibers to concrete, with or without silica fume, increased marginally the compressive strength and the strain corresponding to peak stress. 5.2 FLEXURAL BEHAVIOUR OF SFRC AS PER PREVIOUS STUDIES Shah and Rangan [1971] proposed the following general equation for predicting the ultimate flexural strength of the fiber composite 𝐹𝐹𝐹𝐹𝐹𝐹 = 𝐴𝐴𝐴𝐴𝐴𝐴(1 − 𝑉𝑉𝑉𝑉)𝐵𝐵(𝑉𝑉𝑉𝑉𝑉𝑉/𝑑𝑑)
Where FCC, is the ultimate strength of the fiber composite, G is the maximum strength of the plain matrix (mortar or concrete), A and B are constants which can be determined
20
experimentally. For plain concrete, A = 1 and B = 0. The constant B accounts for the bond strength of the fibers and randomness of fiber distribution. Swamy et al. [1974a] established values for the constants A and B as 0.97 and 4.94 for the ultimate flexural strength of steel fiber-reinforced concrete and 0.843 and 4.25 for its first cracking strength. A comparative evaluation of the static flexural strength for concretes with and without different types of fibers: hooked-end steel, straight steel, corrugated steel, a polypropylene fibers was conducted by Ramakrishnan et al. [1989]. The fibers were tested at 0.5, 1.0, 1.5 and 2.0% by volume. It was reported that maximum quantity of hooked-end fibers that could be added without causing balling was limited to 1.0 percent by volume. Compared to plain concrete, the addition of fibres increased the first cracking strength (15 to 90 percent) and static flexural strength (15 to 129 percent). Compared on equal basis of 1.0 percent by volume, the hookedend steel fiber contributed to the highest increase, and the straight fibers provided the least appreciable increase in the above mentioned properties. Ezeidin and Lowe [1991] studied the flexural strength properties of rapid-set materials reinforced with steel fibers. The primary variables were (a) rapid-set cementing materials, (b) Fiber type, and (c) fiber content. Four fiber types made of low-carbon steel were incorporated in this study. Two were hooked and one was crimped at the ends, and one was crimped throughout the length. Steel fibers were added in the quantities of 50, 75 and 100 lbs/yd3 (30, 45 and 6 kg/m). An increase in the flexural strength was observed. The fiber efficiency in enhancing the flexural strength is controlled by the fiber surface Deformation, aspect ratio, and fiber content. The results further indicate that steel fibers are very effective in improving the flexural toughness of rapid-set materials. Toughness indexes as high as 4 for I5 and 9 for I10 can be achieved with fiber contents of 75 lbs. /yd3
(45 kg/m3).
Johnston and Zemp [1991] investigated the flexural performance under static loads for nine mixtures, using sets of 15 specimens for each mixture. Each set of 102 x 102 x 356 mm (4 x 4 x 14 in.) specimens were prepared from five nominally identical batches and tested under third point loading over a 305 mm (12 in.) span. First crack strengths defined in ASTM C. 108 as the point on the load-deflection curve at which the form of the curve first becomes nonlinear, and ultimate strength based on the maximum flexural load (ASTM C 78) were established for the eight fibrous concretes, with only ultimate strength for the plain concrete control.
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5.3 PREVIOUS INVESTIGATIONS ON FLEXURAL TOUGHNESS, ENERGY ABSORPTION, DUCTILITY AND LOAD DEFLECTION BEHAVIOUR ASTM C 1018 provides Method A for evaluating the toughness of fiber reinforced composites through the use of a toughness index. The toughness index is calculated as the area under the load-deflection curve up to the prescribed service deflection divided by the area under the loaddeflection curve up to the first cracking deflection. Three indexes are described in ASTM C 1018: Is, I1o and I20 corresponding respectively to deflections of 3, 5.5 and 10.5 times the deflection at first cracking. These indices provide indications of the shape of the load-deflection response (post cracking) and available ductility. It should be pointed out that the value of I5, I10 and in is unity for elastic, perfectly brittle material behavior and is equal to 5, 10 and 20 respectively for elastic, perfectly plastic material behavior. Tests conducted on steel fiber-reinforced concrete by Otter and Naaman [1988] showed that the increase in toughness was directly related to the volume fraction of fibers with values of toughness index in compression of up to about 4 for specimens reinforced with 2.1 % volume fraction of hooked steel fibers. For concrete reinforced with straight, smooth fires, increasing the aspect ratio of fibers from 45 to 80 resulted only in a mineral increase in compressive toughness. The use of glass and polypropylene fibers led to low toughness values compared to steel fibers. Otter and Naaman [1988] attributed in part the low performance of glass and polypropylene fibers to the high aspect ratios of the fibers used which led to difficult mixing and possibly higher porosity. Johnston and Career [1989] established that fiber-matrix combinations produce on the loaddeflection curve with the maximum load at deflection much higher than the first crack deflection. The results of their work indicated that increasing the fiber content from 0.5 to 1.5% had a significant beneficial effect on the first crack (and ultimate) strengths despite the negative influence of increasing w/c and w/(c+f). The Increase in first crack strength of 31%, unadjusted for the differences in w/c and w/(c+f) is quite large, since it is widely believed that increasing fiber content has only a minor effect on first crack strength for many of the types of fiber in current use. The post cracking toughness (measured by means of a toughness index) of high strength concretes with steel and polypropylene fibers increases with increase in volume fraction Fibers was determined by Benaiche and Baar [1989]. The fracture toughness of cement-based composites reinforced with relatively high fiber volume (up to 15 percent) was studied by Mobasher et al. [1990]. Crack propagation and damage distribution were examined by laser holographic interferometry. Based on fracture mechanisms observed during experimental studies, an R-curve approach was proposed to predict the toughening of matrices due to fiber reinforcement. The theoretical predictions show a good agreement with the experimental results for both the steel fiber composites and glass fiber composites. Ezeldin and Lowe [1991] investigated the flexural toughness for rapid set materials. The primary variables were (a) rapid-set cementing materials, (b) fiber type, and (c) fiber content. 22
Three commercially available rapid-set materials were investigated. Four fiber types made of low-carbon steel were incorporated in the study. Two were hooked at the ends, one was crimped at the ends, and one was crimped throughout the length. Steel fibers were added in quantities of 50, 75 and 100 psi (30, 45 and 60 kg/m3). The results indicated that steel fibers are very effective in improving the flexural toughness of rapid- set materials. Toughness indexes as high as 4 for 5 and 9 for 10 can be achieved with fiber contents of 75 psi (45 kg/m3). Balaguru et al. [1992] conducted flexural tests on deformed steel fibers reinforced concrete beams. The variables investigated were fiber type, length and volume fraction, and matrix composition. The results indicate that fiber content in the range of 30 to 60 kg/m (50 to 100 lbs/yd3) provide excellent ductility for normal strength concrete. The fiber content has to be increase to about 90 kg/m3 (150 lbs/yd3) for high strength concrete. The load-deflection curves for normal strength concrete beams and high strength concrete beams with 30 mm long hooked-end fibers. It has been reported by Ashour [1993] that steel fibers also enhance the strength and ductility of high strength concrete beams. The toughness indexes were calculated based on the ASTM C 1018 procedure by Stevens et al. [1995]. Based on these results the following observations were made 1. Increase in fiber content results in consistent increase in ductility and energy-absorption capacity. The post peak load—deflection responses are flatter and the toughness index are higher. 2. Toughness Indexes 5 and 10 computed using the ASTM procedure are not sensitive enough to show the variations that are present in the load—deflection responses. If deflections are measured accurately, values 50 and 100 can be computed for all fiber types and fiber contents greater than or equal to 50 pcy (30 kg/m3). 3. Higher fiber contents result in much higher load-retaining capacity at large deflections. In almost all cases, there was a considerable difference in 100 between fiber contents. 4. The magnitude of toughness indexes are quite different from those reported in the literature. G. Appa Rao and B.K. Raghu Prasad [2005] conducted an experimental study on the Fracture energy and Tension Softening behavior of Steel Fiber Reinforced High Strength Concrete. They established that with a small fraction of steel fibers 0.62% by volume, the mode of failure changes from catastrophic to gradual. Also, with the addition of steel fibers, the energy absorption capacity of concrete increases very significantly. 5.4 IMPACT RESISTANCE OF SFRC M. Arockiasamy, A.S.J. Swammidas and K. Munaswamy [1987] published a paper regarding the experimental and analytical investigation on the impact behavior of fiber reinforced concrete cylindrical panels. It considered the impact forces and structural responses of the modelled concrete shell panels. The impact resistance modelling took into account the mass, velocity and shape of impacting surface and the impact interface stiffness as the parameters. 23
Ramakrishnan et al. [1989] used four types of steel fibers to evaluate the impact resistance. The impact specimens were tested at 28 days by the drop weight test method [ACI Committee 544 1990]. This method is simple, inexpensive and can also be conducted in the field. The impact tests were conducted for hooked – end steel (Type A), straight steel (type B), corrugated steel (Type C) and polypropylene (Type D) fibers. Type A had an apparent aspect ratio of 100, while type B had an aspect ratio of only 40. The maximum increase in impact resistance results from the use of Type A fiber but Type C fiber also contributes a higher impact resistance at higher fiber contents. The impact strength at first crack increased considerably with the increase in fiber content. Compared with plain concrete, the increase in impact strengths at full failure were 640%, 847%, 1,824%, 2,806% respectively for concretes with 0.5, 1.0, 1.5 and 2.0% (volume) Type C fiber content. The results of the Study prove that fiber concretes incorporating hooked – end and corrugated steel fibers (Type A and C) have an excellent impact resistance. The experimental results conducted by Hackman et al. [1992]; Krstulovic – Opara et al. [1994] suggested that there is no need for stirrups in flexural members with SIFCON matrix. The promising new development uses steel fiber mats to reinforce concrete matrix. This new approach, called SIMCON (Slurry Infiltrated Mat Concrete), produces concrete components with extremely high flexural strength. The advantage of steel fiber mats over a large volume of discrete fibers is that the mat configuration provides inherent strength and can utilize fibers with much higher aspect ratios. The fiber volume is less than required for SIFCON (Slurry Infiltrated Fiber Concrete), while achieving similar flexural strength and energy absorption capacity i.e..,. Improved toughness. J. Premalatha, D. Tensing and T.M. Murali [2004] carried out experimental investigation on the Impact strength of High strength Steel Fiber Reinforced silica flumes concrete. Impact specimens were cast for M60 grade of concrete with steel fibers of aspect ratio 50 in different proportions with partial replacement of cement by silica fumes. The casting and testing under impact loads were as per ACI 541 recommendations. The impact strength of SFRC without silica fumes was found to increase with increase in volume fraction of steel fibers. Same was observed in the case of silica fume for 1.5% volume but the impact strength was found to decrease for 2.0% volume. 24
5.5 FATIGUE No standard test (specimen size, type of loading, loading rate, fatigue failure criteria) is currently available to evaluate the flexural fatigue performance of fiber reinforced concrete. However, several earlier experimental fatigue studies were conducted on steel fiber – reinforced concrete and mortar in bending by Batson et al.[1972]; Zollo [1972] using a testing procedure, specimen sizes and loading conditions similar to those employed for static flexural test of FRC or tests for conventional concrete with reserved and non-reserved fatigue loading. In evaluating available fatigue data of steel FRC, Anderson [1978] indicated that past investigations could have probably underestimated the fatigue resistance of steel FRC. This is because those investigations used the firs cracking strength of fiber composite as the reference strength. Since fiber addition modifies the cracking strength of plain concrete, Anderson pointed out that proper reference strength for fatigue evaluation of FRC beams should be taken as unreinforced plain matrix beam strength. Using the proposed method of fatigue evaluation, Anderson showed that the fatigue resistance based on published fatigue data was much higher than reported. Fatigue tests conducted on steel fiber – reinforced concrete by Ramakrishnan et al. [1987] showed that the addition of collated hooked – end steel fibers results in a considerable increase in the flexural fatigue strength of concrete. The flexural fatigue strength was increased by 200% to 250%, and endurance limit (to achieve two million cycles) was increased 90% to 95%, when compared to plain concrete. From tests of similar beam specimens either dimensions 6 x 6 x 21 in. (150 x 150 x 525 mm) in flexural fatigue under 20Hz non reversed loading with different types of fibers (hooked, straight, corrugated steel fibers and polypropylene fibers) and different volume fractions of fibers (0.5% and 1.0%), Ramakrishnan et al. [1989] observed that fatigue strength and endurance limit (to achieve two million cycles) increased with the addition of fibers and increasing volume fraction of fibers. The endurance limit of FRC in flexural bending is defined as the maximum flexural stress at which the beam could withstand a prescribed number of loading cycles (usually two million cycles), expressed as percentage of either: (1). its virgin static flexural strength (first cracking strength or modulus of rupture), or (2). The maximum static flexural strength of similar plain unreinforced matrix (control). The flexural fatigue strength of steel FRC was reported to be 25
about 80 – 90% of its static flexural strength at two million cycles when non reserved loading is applied and about 70% of its static flexural strength when full reversed loading is used as proposed by ACI Committee – 544 [1990]. Johnston and Zemp [1991] investigated flexural fatigue behavior of steel fiber reinforced concrete involving nine different mixtures, including a control concrete without fibers. The fiber parameters were varied such that the effects of fiber content, fiber aspect ratio and fiber type could be studied independently, while other fiber variables were held constant. The fiber content was varied between 0.5 – 1.5 percent (volume). A total of 194 fatigue tests and 135 tests of static flexural strength were conducted. Comparison of S – N relationships based on stress as a percentage of first crack strength under static loading shows essentially the same trends as S – N relationships based on stress as a percentage of ultimate strength under static loading.
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CHAPTER-6 USES & APPLICATION OF SFRC
6.1 STRUCTURAL USE OF SFRC As recommended by ACI Committee 544, ‘when used in structural applications, steel fiber reinforced concrete should only be used in a supplementary role to inhibit cracking, to improve resistance to impact or dynamic loading, and to resist material disintegration. In structural members where flexural or tensile loads will occur. The reinforcing steel must be capable of supporting the total tensile load’. Thus, while there are a number of techniques for predicting the strength of beams reinforced only with steel fibers, there are no predictive equations for large SFRC beams, since these would be expected to contain conventional reinforcing bars as well. An extensive guide to design considerations for SFRC has recently been published by the American Concrete Institute. In this section, the use of SFRC will be discussed primarily in structural members which also contain conventional reinforcement. For beams containing both fibers and continuous reinforcing bars, the situation is complex, since the fibers act in two ways 1. They permit the tensile strength of the SFRC to be used in design, because the matrix will no longer lose its load-carrying capacity at first crack; and 2. They improve the bond between the matrix and the reinforcing bars by inhibiting the growth of cracks emanating from the deformations (lugs) on the bars. However, it is the improved tensile strength of SFRC that is mostly considered in the beam analysis, since the improvements in bond strength are much more difficult to quantify. Steel fibers have been shown to increase the ultimate moment and ultimate deflection of conventionally reinforced beams; the higher the tensile stress due to the fibers, the higher the ultimate moment.
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FIGURE 6.1: Experimental Moment versus Deflection Curves for SFRC
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6.2 APPLICATION OF SFRC Most common applications are pavements, tunnel linings, pavements and slabs, shotcrete and now shotCrete also containing silica fume, airport pavements, bridge deck slab repairs, and so on. There has also been some recent experimental work on roller-compacted concrete (RCC) reinforced with steel fibers. The list is endless, apparently limited only by the ingenuity of the engineers involved. The fibers themselves are, unfortunately, relatively expensive; a 1% steel fiber addition will The uses of SFRC over the past thirty years have been so varied and so widespread, that it is difficult to categorize them. The approximately double the material costs of the concrete, and this has tended to limit the use of SFRC to special applications. 6.2.1 Uses of SFRC in Indian Project
KRCL-MSRDC Tunnels. NafthaJakarihydro electric project. KOL hydroelectric project. Baglihar hydroelectric project. Chamera hydroelectric project. Uri dam. Sirsisilam project. Tehri dam project. Salalhydro electric project. Ranganadihydro electric project.
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FIGURE 6.2: KRCL-MSRDC Tunnels 6.3 BENEFITS OF USING STEEL FIBRES IN CONCRETE
FIGURE 6.3: Steel Fibre in Concrete The use of steel fibre in concrete can improve its many properties. The benefits of using steel fibres in concrete are as follows 1. Steel Fibres are generally distributed throughout a given cross section whereas reinforcing bars or wires are placed only where required 2.
Steel fibres are relatively short and closely spaced as compared with continuous reinforcing bars of wires. 30
3.
It is generally not possible to achieve the same area of reinforcement to area of concrete using steel fibres as compared to using a network of reinforcing bars of wires.
4. Steel Fibers are typically added to concrete in low volume dosages (often less than 1%), and have been shown to be effective in reducing plastic shrinkage cracking. 5. Steel Fibers typically do not significantly alter free shrinkage of concrete, however at high enough dosages they can increase the resistance to cracking and decrease crack width (Shah, Weiss, and Yang 1998). 6.3.1 Steel Fibres in Concrete can improve •
Crack, Impact and Fatigue Resistance
•
Shrinkage Reduction
•
Toughness- by preventing/delaying crack propagation from micro-cracks to macro-cracks.
6.3.2Benifits of Steel Fiber Reinforced Concrete •
SFRC distributes localized stresses.
•
Reduction in maintenance and repair cost.
•
Provides tough and durable surfaces.
•
Reduces surface permeability, dusting and wear.
•
Cost saving.
•
They act as crack arrestor.
•
Increases tensile strength and toughness.
•
Resistance to impact.
•
Resistance to freezing and thawing
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6.4 MAJOR STUCTURES OF SFRC ALL OVER THE WORLD
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REFERENCE 1. IS 456:2000 2. Concrete Technology: Theory and Practice by M.S.Shetty, 3. Perumalsamy N. Balaguru, Sarendra P. Shah, ‘‘Fiber Reinforced Cement Composites’’, Mc
Graw Hill International Editions 1992. WEBSITES www.wikipedia.com www.civilengineering.com
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