CODEN (USA): IJCRGG, ISSN: 0974-4290, ISSN(Online):2455-9555 Vol.10 No.8, pp 48-59, 2017
Abstract : Concrete is most widely used construction material. Because of its specialty of being cast in any desirable shape, it has replaced stone and brick masonry3,4. In spite of all this, it has some serious deficiencies such as lack of tensile strength, ductility etc. To improve the deficiencies steel fibers are added to the concrete, known as fiber reinforced concrete which is an emerging technology used in the construction industry. Fiber reinforced concrete is a concrete containing fibrous material which increases its structural integrity. It contains short discrete fibers that are uniformly distributed and randomly oriented. The addition of steel fibers to cement concrete leads to improvement in several properties of concrete. In this project the behaviour of HPFRC are studied and compared with conventional beams. Totally four numbers of beams were cast and tested for its cyclic behaviour. The specimen is incorporated with Hooked End and Crimpled fibers in the mix proportion of 70%-30% by volume at a total volume fraction of 1.5%. Silica fume and super plasticizers are added to modify the properties of concrete. The beams were subjected to single point cyclic loading by means of screw jack and the deflection is measured by using dial gauge. The load deflection behavior for all the beams were drawn and the important parameters like load carrying capacity, ductility, energy absorption, stiffness, first crack load and ultimate load has been studied and compared with other specimens. Index Terms : Fibers, FRC, High strength concrete, Hooked end, crimpled bars etc. (key words).
Due to enhanced mechanical properties such as strength, stiffness, toughness, ductility and durability, high performance concrete has gained wider acceptance in the construction of tall buildings, long span bridges, high rise building, off shore structures and other mega structures.
For past few decades, HPC has undergone many developments based on the influence of cement type, type and proportions of mineral admixtures, types of super plasticizer and the composition of coarse and fine aggregate. It is commonly accepted that the properties of an aggregate used in HPC have great influence on structural properties and on durability.
Fiber reinforced concrete is a concrete mix that contains short distance fibers that are uniformly distributed and randomly oriented2,5 . As a result of these different formulations, four categories of fiber reinforcing have been created; these include steel fiber, glass fibers, synthetic fibers and natural fibers. Within
these different fibers that character of fiber reinforced concrete change with varying concrete’s, fiber materials,
geometries distribution, orientation and densities.
The amount of fibers added to a concrete mix is measured as a percentage of total volume of the composite termed volume fraction (Vf). Vf typically ranges from 0.1 to 3%. Aspect ratio (1/d) is calculated by diving fiber length (l) by its diameter (d).
i. Applications of fiber reinforced concrete
FRC is used as crack control and shrinkage for water retaining and reservoir structure to reduce the permeability and freeze-thawing conditions. Increase of toughness in fiber reinforced concrete is ideal for building and pavements subjected to shatter-impact abrasion and shear. FRC has also been used in beam-column junction and penetration and impact resistance are found to be very high for FRC when compared to other materials. FRC replaces the temperature steel in sanitary sewer tunnels which prevents corrosion and improves ductility. FRC is used in repairs and rehabilitation of marine structures such as concrete pilling and caissons.
ii. High Performance Fiber Reinforced Concrete
High performance concrete (HPC) is a recent development in concrete technology. It is designed to give optimized performance characteristics for the given set of materials, usages and exposure conditions with requirements of with the requirements of cost, service life and durability5. Development of HPC is directly related to a number of recent technological developments, in particular the discovery of the extraordinary dispersing action of super plasticizers, use of micro fillers like silica fume increasing availability of fibers of different types and properties etc. The use of chemical admixtures in HPFRC reduces the water content, thereby reducing the porosity within the hydrated cement paste. It has been proved that steel fibers can be used to control cracking and deflection in concrete structural members. Addition of steel fibers to HPC makes it highly ductile and improves the energy absorption capacity1
iii. Properties Of Hpfrc
Significantly improved fatigue resistance Increased load bearing capacity and less palling damage Durability can significantly improve It increases the load carrying capacity and provides uniform multi-directional reinforcement in concrete It reduces the plastic shrinkage and thickness of concrete slab It improves the impact resistance and shear strength It resists abrasion and toughness Excellent crack control, the fibers control and settles cracks It improves ductility and controls the sudden deformation
iv. Scope of Investigation
The objective of the present investigation is to compare the behaviour of hooked end fiber reinforced concrete beams, crimpled fiber reinforced concrete beams, hybrid fiber reinforced concrete beams with conventional high performance reinforced concrete beams.
The properties such as energy absorption, stiffness, ductility factor, first crack load, ultimate load are also studied in this investigation.
The HPFRC material consists of steel fibers, silica fume, super plasticizer, reinforcing steel and cement materials. The behaviour of HPFRC material is studied through an experimental programme. Four beam such as hooked end fiber reinforced concrete beam, Crimpled fiber reinforced concrete beam, Hybrid fiber reinforced concrete beam and Conventional high performance reinforced concrete beams were cast and tested. All the beams were simply supported at both ends with concentrated point loading system and the beams are subjected to cyclic loading.
a) Coarse aggregate
The coarse aggregate used in the mixes were 12mm and 10mm are mixed in the proportions of 60% 40%. The specific gravity of coarse aggregate was determined and found to be 2.8.
b) Steel
The main reinforcement used for the beams were high yield strength deformed steel bars of 8mm and 6mm diameter deformed bars were used for shear reinforcement.
c) Super plasticizers
Cera hyperplast XR-W40 is an acrylic polymer based new range water reducing admixture. In this project we are using 0.8% of Super plasticizers.
d) Silica fume
In this project we are using 10% of silica fume by replacing the cement. It is extremely fine with particle size less than 1 micron. The silica fume increases the bond strength between cement paste and aggregate and it will fills the micro pores present in aggregate.
e) Steel fibers
In this specimen is in incorporated with hook end fiber and crimpled fiber are used separately and mixed in the mix proportion of 30% -70% by volume at total volume fraction of 1.45%
f) Mix Design
M60 grade of concrete has been designed as per IS code and the mix proportions is given in the table
Water -0.34 Cement -1 Fine aggregate -1.23 Coarse aggregate – 2.24
g) Reinforcement details
4 numbers of 12 mm diameter rods was used as main reinforcement, 2 numbers at top and 2 numbers at bottom. 8 mm diameter stirrups spaced at 250mm centre to centre were used as shear reinforcement. The reinforcement details are shown in the figure
Fig.1. Reinforcement Details
a) Casting of specimen
The materials were weighted accurately using digital weighing instrument. For HPC cement, fine aggregates, coarse aggregates, water and superplasticzers were added to mixture manually and mixed thoroughly in a concrete mixture and for HPFRC Hooked end fiber & crimpled fiber are used separately and also mixed in the mix proportion of 30% to 70%. The concrete is mixed in three layers in the mould & compacted manually.
The beams were kept in mould for 24 hours. After that they were marked for future identifications. Then the side plates of the beam mould were removed and the test specimen was transported to the curing pond. The specimens were taken out of the water after 28 days and dried out before testing
The beams were simply supported at both ends and were tested for central concentrated cyclic loading. The load was applied using screw jack. Small thin glass plates were fixed to the bottom surface of the beam to provide smooth surfaces where deflections are to be measured. The deflectometer was fixed at the bottom of the beam for measuring the deflection in the beam. A proving ring was fixed below the hydraulic jack to measure the applied load on the beam. The complete experimental setup is represented in figure
1. Behaviour of Beam under Loading
The beams are subjected to cyclic loading by using screw jack. No of cycles of loading were imposed on the beam till it fails. The deflections are measured at the centre of beam by using deflectometer.
Fig.2.Experimental Setup
i) Load Deflection Behaviour
The beams are subjected to cyclic loading by using screw jack. No of cycles of loading were imposed on the beam till it fails. The deflections are measured at the centre of beam by using deflectometer.
ii) First Crack Load
In general, the beams subjected to loading will develop crack gradually with increase in load. The initial load at which the crack formed is known as first crack load.
iii) Ultimate Load
Ultimate load is defined as the maximum load at which the beam can withstand its position without any failure. When compared to conventional concrete beams, the fiber reinforced concrete will have maximum load carrying capacity.
iv) Stiffness
Stiffness is defined as the load required causing unit deflection of beam. A tangent was drawn for each cycle of the hysteresis curves at a load of P = 0.75 Pu.
v) Ductility Factor
Ductility is one of the most important parameter to be considered in the design of structures subjected to various loading conditions. It is defined as the ability of a member undergoes inelastic deformations beyond the yield deformations without significant loss in its load carrying capacity. The ductility of a flexural member can be obtained from its load-deflection curve. The ratio of maximum deflection at each cycle to the deflection at first yield is known ductility factor.
vi) Relative Energy Absorption Capacity
When the frame is subjected to cyclic loading such as those witnessed during wind or earth quake loads some energy is absorbed. It is equal to the work done in straining and deforming the structure to the limit of deflection. The relative energy absorption capacities during various load cycles were calculated as the area under the hysteresis loops from the load versus deflection diagram. The cumulative energy absorption capacity of the frame was obtained by adding the energy absorption capacity of the frame during each cycle considered.
3.2 Results and Discussion
Fig.3.Behaviour of conventional concrete beam Fig.4.Variation of Ductility Factor with Load Cycle
Fig.5.Variation of cumulative ductility Fig.6. Variation of Cumulative Energy factor with load cycle Absorption with Load Cycle
Fig.7.Loop Diagram for HPC Beam Behaviour Fig.8.Variation of Ductility Factor of Hooked end Fiber RC beam with Load Cycle
Fig.9.Variation of Cumulative Ductility Factor with Load Fig.9. Variation of Stiffness with Load Cycle ii) Loop Diagram for Hooked End Fiber Beam
Fig.10.Behaviour of Crimpled fiber RC beam Fig.11.Variation of Ductility Factor with Load Cycle
Fig.12.Variation of Cumulative Ductility Fig.13.Variation of Stiffness with Load Cycle factor with Load
Fig.14.Variation of Energy Absorption with Load Cycle
Loop Diagram for Crimpled Beam Table1-Behaviour of Hybrid Fiber RC Beam
S No | Parameter | HPC | H | C | H+C |
---|---|---|---|---|---|
1 | First Crack Load (kN) | 12 | 15 | 13.5 | 18 |
2 | Ultimate Crack Load (kN) | 22.5 | 27 | 30.25 | 32.25 |
3 | Stiffness (kN/mm) | 3.74 | 4.19 | 4.24 | 4.89 |
4 | Ductility Factor | 14.68 | 36.42 | 16.48 | 76.11 |
5 | Energy Absorption (kN mm) | 550 | 1213 | 844.5 | 1312.5 |
Fig.14.Variation of Ductility Factor with Load Cycle Fig.15.Load Sequence Diagram
Fig.16.Loop Diagram for Hybrid Beam
In this project, the beams are compared with various parameters such as first crack load, ultimate load, cumulative ductility factor and energy absorption. The different parameters of conventional RC beam, hooked end, crimpled and hybrid fiber reinforced concrete beam were shown in Table
i) Comparison of First Crack Load
The first crack load for the conventional RC beam, hooked end, crimpled and hybrid fiber reinforced concrete beam were 12kN, 13.5kN, 15kN and 18kN respectively. The first crack load for the hybrid fiber reinforced concrete beam was 1.5 times greater than conventional RC beam. The graphical representation for the comparison of first crack load for different beam is shown in fig.
Fig.17.Comparison of First Crack Load
ii) Comparison of Ultimate Load
The ultimate load carrying capacity of the conventional RC beam, hooked end, crimpled and hybrid fiber reinforced concrete beam were 22.5kN, 30.75kN, 27kN, 32.25kN respectively. The ultimate load for the hybrid fiber reinforced concrete beam was 1.45 times greater than conventional RC beam. The graphical representation for the comparison of ultimate load for different beam is shown in fig
Fig.18.Comparison of Ultimate Load
ii) Comparison of Cumulative Ductility factor
Ductility is defined as the ability of a member undergoes inelastic deformations beyond the yield deformations without significant loss in its load carrying capacity. The ratio of ultimate deflection to the deflection at first yield is known was ductility factor. In this experiment, hybrid reinforced concrete beam will give maximum ductility when compare to conventional concrete beam. The comparison of cumulative ductility factor for different type of beam is shown in fig
Fig.18.Comparison of Cumulative Ductility factor
iii) Comparison of Energy Absorption
The cumulative energy absorption capacity of the frame was obtained by adding the energy absorption capacity of the frame during each cycle considered in this experiment, the hybrid fiber reinforced concrete beam will give maximum energy absorption when compare to other conventional beam. The comparison of energy absorption for various beams are shown in fig
Fig.19.Comparison of Energy Absorption
iii) Mode of Failure
As the load increases, the number of cracks or crack width increases for each beam. The presence of fiber inside the beam will resist the crack development by forming a bridging across the crack.ie the fibers act as crack arresting material.
i) General
The experimental investigation is carried out to study the behaviour of High Performance Fiber Reinforced Concrete Beam, such as Hooked end, Crimpled, Hybrid fiber reinforced concrete beam and Conventional high performance reinforced concrete beam, subjected to cyclic loading. The test results are compared with that of the Conventional high performance reinforced concrete beam.
It based on study parameters such as first crack load, ultimate load, cumulative ductility factor and energy absorption, we compare all the beams with that of conventional concrete beam. The following observation has been inferred from the experimental programme.
The ultimate load for the hybrid fiber reinforced concrete beam was 1.45 times greater than that of conventional RC beam The first crack load for the hybrid fiber reinforced concrete beam was 1.5 times greater than conventional RC beam The ultimate load for hybrid, crimpled and hooked end beams are about 43.33%, 36.6% and 20% respectively more than that of conventional RC beam The first crack load for hybrid, crimpled and hooked end beams are about 50%, 12.5% and 25% respectively more than that of conventional beam. The ductility value of hybrid fiber RC beam is about 5.18 times than that of conventional RC beam and
2.48 times than that of hooked end RC beams. The energy absorption of hybrid fiber RC beam is about 2.38 times than that of conventional RC beam and
1.53 times than that of crimpled RC beams. In general the presence of steel fibers increases both ductility and energy absorption capacity which is required for earthquake poof zones.
Moreover the presence of hybrid fiber results in higher load carrying capacity apart from enhanced ductility and energy absorption.
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