10

CHAPTER 2

REVIEW OF LITERATURE

2.1 GENERAL

The idea of adding a fibre or fibres into concrete in order to increase

strength and fracture energy goes back to a patent dated 1918 by H. Alfsen and this fact

is reported by Katzer [18]. Alfsen described a process to improve the tensile strength by

adding longitudinal bodies (fibres) of different materials into the concrete. Afterwards

several patents of different fibres and fibre geometries were proclaimed. In this chapter

studies on the High Performance Concrete, Fibre reinforced Concrete, Hybrid Fibre

Reinforced concrete and Ultra High Peformance Fibre Reinforced Concrete is reviewed.

2.2 STUDIES ON HIGH PERFORMANCE CONCRETE

High performance concrete (HPC) is a specialized series of concrete

designed to provide several benefits in the construction of concrete structures that

cannot always be achieved routinely using conventional ingredients, normal mixing and

curing practices. In other words a high performance concrete is a concrete in which

certain characteristics are developed for a particular application and environment, so

that it will give excellent performance in the structure in which it will be placed, in the

environment to which it will be exposed, and with the loads to which it will be

subjected during its design life.

It includes concrete that provides either substantially improved resistance to

environmental influences (durability in service) or substantially increased structural

capacity while maintaining adequate durability. It may also include concrete, which

11

significantly reduces construction time without compromising long-term serviceability.

While high strength concrete, aims at enhancing strength and consequent advantages

owing to improved strength, the term high-performance concrete (HPC) is used to refer

to concrete of required performance for the majority of construction applications.

The American Concrete Institute Committee 1993 [19] on HPC includes the

following six criteria for material selections, mixing, placing, and curing procedures for

concrete. They are (1) Ease of placement, (2) Long term mechanical properties,

(3) Early-age strength, (4) Toughness, (5) Life in severe environments and

(6) Volumetric stability

The above-mentioned performance requirements can be grouped under the

following three general categories.

(a) Attributes that benefit the construction process

(b) Attributes that lead to enhanced mechanical properties

(c) Attributes that enhance durability and long-term performance.

The performance requirements of concrete cannot be the same for different

applications. Hence the specific definition of HPC required for each industrial

application is likely to vary. The Strategic Highway Research Programme (SHRP) [20]

has defined HPC for highway application on the following strength, durability, and

water-cement ratio criteria.

1. a maximum water-cementitious ratio (W/C) of 0.35;

2. a minimum durability factor of 80% after 300 cycles of freezing and

thawing, and

3. a minimum strength criteria of either

(a) 21 MPa within 4 hours after placement (Very Early Strength, VES),

(b) 34 MPa within 24 hours (High Early Strength, HES), or

(c) 69 MPa within 28 days (Very High Strength, VHS).

12

In general, a ―High performance Concrete‖ can be defined as that concrete

which has the highest durability for any given strength class, and comparison between

the concretes of different strength classes is not appropriate. This means that, with the

available knowledge, one can always strive to achieve a better (most durable) concrete

required for a particular application

The use of high performance concrete results in many advantages, such as

reduction in beam and column sizes and increase in the building height with many

stories. In pre-stressed concrete construction, a greater span-depth ratio for beams may

be achieved with the use of high performance concrete. In marine structures, the low

permeability characteristics of high performance concrete reduce the risk of corrosion of

steel reinforcement and improve the durability of concrete structures. In addition, high

performance concrete can perform much better in extreme and adverse climatic

conditions, and can reduce maintenance and repair costs [21, 22, 23]. Hence, HPC

concretes are used for numerous applications and the studies related to HPC are

discussed.

Yogendran et al. (1987), have studied extensively High Strength Concrete

using Silica Fume with no air entrainment to achieve compressive strength in the range

of 50 to 70MPa in which cement was replaced by silica fume (0 to 30 percent by

weight) [24]. The efficiency of silica fume in improving the properties of concrete was

compared at medium and very low water cementitious ratios. Concretes with water

cementitious ratios of 0.34 and 0.28 with and without a superplasticizer, at a constant

slump (50 mm) and with varying slump were also investigated. It has been concluded

that for concretes with 5 to 30 percent replacement of cement by silica fume and a

constant slump of 50 mm there was no increase in water demand up to 5 percent

replacement. The water required to maintain a constant slump, however, increased

linearly as the percentage of silica fume replacement increased from 10 to 30 percent.

A 5 percent replacement produced the highest compressive strength at 7 and 28 days.

Replacement of 10 percent and 15 percent produced strength equal to the control mix at

7 and 28 days. Replacement levels of 25 percent and 30 percent produced lower

strengths at all test ages.

13

Ganesh Babu et al. (1993), has evaluated the efficiency of flyash in concrete

over a wide range of percentage replacements (15-75%) [25]. It was concluded that at

the replacement percentage of flyash and the water cement ratios increase the strength

of concrete decreases. It has been reported that while concretes above 70 MPa can be

produced with replacements up to 25% fly ash, replacements of about 75% can still lead

to concretes of 40 MPa through suitable adjustments in water/cement ratio and other

concrete constituents. It has been predicted that the strength of concretes varying from

20 to 100 MPa with Ground Granulated Blast Furnace Slag (GGBS) levels varying from

10% to 80% was found to result in a regression coefficient of 0.94, which was also the

same for normal concretes.

Khatri et al. (1995), has investigated the effect of different supplementary

cementitious materials on mechanical properties of high performance concrete [26]. The

investigation focussed on to compare the mechanical properties as well as fresh concrete

properties of concretes containing silica fume, ground granulated blast furnace slag,

fly ash and General Purpose (GP) Portland cement. Concrete mixes were prepared with

GP Portland cement, high slag cement and slag cement, and also mixes were prepared

with the addition of silica fume and fly ash The work focussed on concrete mixes

having a fixed water/binder ratio of 0.35 and a constant total binder content of

430 kg/m3. Apart from measuring fresh concrete properties, the mechanical properties

evaluated were development of compressive strength, flexural strength, elastic modulus,

and strain due to creep and drying shrinkage. The compressive strength of concrete

containing GP cement, fly ash and silica fume is higher (46%) compared to the

compressive strength of concrete containing only GP cement at the age of 28 days. It

was also observed that the addition of silica fume significantly increases the flexural

strength (45%) of GP concrete. The addition of silica fume also improved the elastic

modulus of GP concrete.

Duval et al. (1998), has studied the influence of silica fume on the

workability and the compressive strength of high-performance concretes [27]. The

workability and the compressive strength of silica fume concretes were investigated at

low water-cementitious materials ratios with a naphthalene sulphonate superplasticizer.

14

The results show that partial cement replacement up to 10% silica fume does not reduce

the concrete workability. Moreover, the superplasticizer dosage depends on the cement

characteristics (C3A and alkali sulfates content). At low water-cementitious materials

ratios, slump loss with time was observed and increases with high replacement levels.

Silica fume at replacement contents up to 20% produce higher compressive strengths

than control concretes.

Shannag (2000), has studied the mechanical properties of High strength

concrete containing natural pozzolan and silica fume [28]. Various combinations of a

local natural pozzolan and silica fume were used to produce workable high to very high

strength mortars and concretes with a compressive strength in the range of

69 – 110 MPa. The mixtures were tested for workability, density, compressive strength,

splitting tensile strength, and modulus of elasticity. The results of this study suggest that

certain natural pozzolan - silica fume combinations can improve the compressive and

splitting tensile strengths, workability, and elastic modulus of concretes, more than

natural pozzolan and silica fume alone. Furthermore, the use of silica fume at 15% of

the weight of cement was able to produce relatively the highest strength increase in the

presence of about 15% pozzolan than without pozzolan. The results show that the

natural pozzolan and silica fume combinations produced high to very high strength

concretes in the range of 69 - 85 MPa at 28-day compressive strength, with medium

workability, using total cementitious contents between 400 and 460 kg/m3. These

concretes also exhibited a 28-day splitting tensile strength of the order of 6.5% of their

compressive strength and showed relatively high values of modulus of elasticity.

Nihal Artoglu et al. (2006), have studied the relationship between the ratio

of splitting tensile strength to compressive strength of concrete (ftsp/fc) and the cylinder

compressive strength of concrete (fc), which is applicable to concrete at early ages

(12 hours and longer) as well as high strength concrete (up to 120 MPa) [29]. The

reliability of the equation is assessed on the basis of integral absolute error

(IAE percentage). The ratio of the two strengths (ftsp/fc) is strongly affected by the level

of the compressive strength fc. This ratio decreases with increase in compressive

strength. In fact the increase in the splitting strength (ftsp) occurs at a much smaller rate

15

compared to the increase of compressive strength. It is also evident, in comparison with

normal strength concrete (NSC), at higher strengths (80 to 120MPa very High Strength

Concrete) there is a significant decrease in the ratio. The ratio of (ftsp/fc) varies between

0.15 and 0.10 for the NSC, while the same ratio is between 0.08 and 0.06 for very high

strength concrete. At low compressive strength, the splitting tensile strength is as high

as 10 percent of the cylinder compressive strength but at extremely higher compressive

strengths, the ratio reduces to approximately 5 percent.

Elahi et al. (2010), has carried out an experimental investigation to evaluate

the mechanical and durability properties of high performance concretes containing

supplementary cementitious materials [30]. The mechanical properties were assessed

from the compressive strength, whilst the durability characteristics were investigated in

terms of chloride diffusion, electrical resistivity, air permeability and water absorption.

The test variables included the type and the amount of supplementary cementitious

materials (silica fume, fly ash and ground granulated blast-furnace slag). Portland

cement was replaced with fly ash up to 40%, silica fume up to 15% and GGBS up to a

level of 70%. It has been concluded that when the water-binder ratio kept constant at

0.3, the compressive strength was detrimentally affected by the replacement of Portland

Cement with both FlyAsh and GGBS at all ages up to 91 days. However, the

compressive strength increased at all ages due to the use of SF at 7.5% replacement

levels.

2.3 STUDIES ON FIBRE REINFORCED CONCRETE

The term fibre reinforced concrete (FRC) is defined by ACI Committee 544

[31] as a concrete made of hydraulic cements containing fine or fine and coarse

aggregates and discontinuous discrete fibres. Inherently concrete is brittle under tensile

loading. The mechanical properties of concrete can be improved by reinforcement with

randomly oriented short discrete fibres, which prevent and control initiation,

propagation, or coalescence of cracks [32].

16

Plain and unreinforced cementitious materials are characterized by low

tensile strengths and low strain capacities; that is, they are brittle materials. They thus

require reinforcement before they can be used extensively as construction materials.

Historically, this reinforcement has been in the form of continuous reinforcing bars,

which could be placed in the structure at the appropriate locations to withstand the

imposed tensile and shear stresses. Fibres, on the other hand, are discontinuous, and are

most commonly randomly distributed throughout the cementitious matrix. They are

therefore not as efficient in withstanding the tensile stresses. However, because they

tend to be more closely spaced than conventional reinforcing bars, they are better at

controlling cracking. Thus conventional reinforcing bars are used to increase the load

bearing capacity of concrete; fibres are more effective for crack control. Because of

these differences, there are certain applications in which fibre reinforcement is better

than conventional reinforcing bars. These include thin sheet components, in which

conventional reinforcing bars cannot be used and in which the fibres therefore constitute

the primary reinforcement. It also includes components which must withstand locally

high loads or deformations, such as tunnel linings, blast resistant structures, or precast

piles which must be hammered into the ground. The components includes in which

fibres are added primarily to control cracking induced by humidity or temperature

variations, as in slabs and pavements. In these applications, fibres are often referred to

as secondary reinforcement [3].

The applications of FRC are as varied as the types of fibres that have been

used. The fibres that can be used in concrete can be classified into two categories viz.,

natural fibres and artificial fibres. Natural fibres used in Portland cement composites

include akwara, bamboo, coconut, flax, jute, sisal, sugarcane bagasse, wood etc.,

Artificial fibres that have been tried with cement matrices include acrylic, aramid,

nylon, carbon, polyester, polyethylene, polypropylene, steel etc.,

Asbestos fibres have been used in pipes and in corrugated or flat roofing

sheets. Glass fibres are used primarily in precast panels (nonstructural). Steel fibres

have been used in pavements in shotcrete, in dams and a variety of structures.

Polyproylene fibres are used as secondary reinforcement, to control plastic shrinkage

17

cracking, and a newer generation of ―structural polymer fibres‘ may be applied for crack

control in the hardened concrete. Vegetable fibres have been used in low-cost building

materials [3].

Fibre reinforced cement and concrete composites have been used in

numerous applications, either as stand-alone or in combination with reinforcing bars and

prestressing tendons; they have also been used as support materials in repair and

rehabilitation work. Brief applications of FRC are shown in Figure 2.1. [33]

Hence, FRC concretes are used for numerous applications which resist

tension, flexure, shear, fatigue, impact etc., Hence, the studies related to FRC subjected

to impact, flexure, shear, tension and compression are discussed.

Figure 2.1 Brief Applications of FRC [33]

APPLICATIONS OF

FIBRE REINFORCED CEMENT COMPOSITES

STAND-ALONE

In Light

Structural

Elements

(e.g. Cement

boards, Sheets,

pipes, slabs on

grade, pavements,

shells, piles,

poles, light

beams, prefab

elements .…)

HYBRID

In Combination with

RC, PC, or Steel

Structures

(e.g., seismic and

blast resistant

structures, super

high- rise structures,

offshore structures,

space-craft launching

platforms, very long

bridges, encased steel

trusses, fire

protection ...)

HYBRID

In Selected Zones

of Structures

where Enhanced

properties are

needed

(e.g., beam-

column joints in

seismic frames

coupling beams,

anchorage zones

in PSC beams,

punching shear

zone in RC slabs.

…)

REPAIR AND

REHABILITATION

(e.g. tunnel lining,

jacketing around

columns, fire

protection. …)

18

Wimal Suaris et al. (1983), have identified the requirement for the rational

design of concrete structures subjected to impact and impulsive loading, the constituent

properties of concrete over a wide range of strain rates [34]. For this objective, concrete

and fibre reinforced concrete beams were tested in a drop-weight, instrumented impact-

testing machine. The influence of different types of fibres such as steel, polypropylene

and glass on impact strength was studied. The energy absorbed by fibre reinforced

concrete beams was as much as 100 times that for unreinforced beams. The energy

absorbed by the long and short steel fibre reinforced concrete, polypropylene fibre-

reinforced concrete and glass fibre reinforced concrete subjected to impact loading was

approximately 100, 41, 31 and 7 times, respectively that for unreinforced specimens.

Gopalaratnam et al. (1986), have discussed the effect of strain rate on the

flexural behaviour of unreinforced matrix and three different fibre reinforced concrete

(FRC) mixes [35]. The objective was to study the properties of Steel Fibre Reinforced

Concrete subjected to Impact Loading. A Conventional Charpy Impact Machine was

modified and instrumented to facilitate tests on FRC Specimens at different impact

velocities. Smooth brass coated steel fibres of length 25.4 mm and diameter 0.41mm

was used. Three different volume fractions of fibres 0.5, 1.0 and 1.5 were used. The test

results have shown that the inclusion of fibres in the matrix enhances the compressive

strength and the corresponding strains. Plain matrix had a compressive strength of

30.44 MPa while 1.5 percent FRC had strength of 40.98 MPa. The value of strain at

peak stress for the 1.5 percent FRC specimens was 3750 micro strains compared to

2700 micro strains for the unreinforced matrix.

Narayanan et al. (1987), have done investigations on the behaviour of steel

fibre reinforced concrete beams subjected to pre-eminent shear [36]. The report

establishes that the inclusion of steel fibres in reinforced concrete beams results in a

substantial increase in shear strengths. Beams when reinforced with 1 percent of volume

fraction, showed no increase in ultimate shear strength, while those reinforced with

stirrups showed increase in shear strength as the volume of stirrups is increased. The

pattern of cracks developed in fibre reinforced concrete beams subjected to shear was

found to be generally similar to those observed in the corresponding reinforced concrete

19

beams with conventional stirrups. When the cube strength was increased from 42 MPa

to 62 MPa (about 50 percent), this was reflected by an increase in shear strength from

1.97 MPa to 3.23 MPa (about 64 percent). The effectiveness of dowel resistance

increased with an increase in the fibre factor. This has indicated that the presence of

fibres improves the tensile strength of the concrete in the splitting plane along the

reinforcement.

Barr (1987), has studied the shear performance of fibre-reinforced concrete

(FRC) materials. The performance of FRC materials has been studied and characterized

by two-fracture parameters - fracture toughness (resistance to cracking) and toughness

index (quantifies the post first crack toughness) [37]. Three types of fibres (steel,

polypropylene and glass fibre) have been used to study the shear performance of fibre-

reinforced concrete specimens using double-notched specimens with conventional steel

stirrups. It has been found that the shear strength of steel FRC increased by the addition

of fibres (by weight). The test results have shown that the inclusion of fibres in the

matrix enhances the shear strength of the concrete. The matrix had shear strength of

8 MPa for 1 percent addition while 10 MPa for 4 percent addition of fibres.

Swamy et al. (1987), have investigated the influence of short steel fibres,

alone or in conjunction with conventional steel stirrups on shear transfer in concrete

[38]. The main variables investigated in the research include fibre content by volume,

amount of stirrups and the type of concrete. Only one type of fibre 0.50 mm diameter

and 50 mm length round, fully crimped fibres and two types of stirrup reinforcement,

mild steel and high tensile ribbed steel was used. The study was done using fibre

volume of 0 percent to 1.2 percent and the type of concrete was normal weight concrete

and lightweight concrete. The results show that the crack width varied from 0.15 mm to

0.53 mm with about 75 percent of the values in the range of 0.15 mm to 0.30 mm.

Fibres increased the residual shear transfer strength of cracked specimens and this

increased with increasing fibre volume.

Gopalaratnam et al. (1991), have studied the fracture toughness of fibre

reinforced concrete [39]. The research included the parameters, such as size, fibre

20

volume content, fibre type and the effect of notch. Two types of steel fibres crimped

steel and hooked end steel and fibrillated polypropylene were used as reinforcement.

Two types of volume fractions were used for each of the mixes (0.5 and 1.0 percent for

the steel fibre mixes and 0.1 and 0.5 percent for polypropylene mixes). The concrete

mixes were designed for a compressive strength of 34.48 to 41.37 MPa. It was found

that first crack deflection can vary as much as an order of magnitude depending upon

the method used to measure deflection. It was observed that the stress at first crack is

relatively independent of the fibre volume fraction. The ultimate strength was

influenced significantly by fibre volume fraction. Small increases in the initial elastic

modulus were observed with increases in steel fibre content. The elastic modulus was

unaffected by the polypropylene fibre content.

Mariano Valle et al. (1993), have studied shear strength and ductility

properties of fibre reinforced high strength concrete under direct shear [40]. Both

experimental and modelling studies were performed. The direct shear transfer behaviour

of fibre reinforced high strength concrete was investigated on rectangular notched

specimens. Two types of fibre, polypropylene and steel fibres, in conjunction with or

without conventional steel stirrups, were used. The concrete strength was identified as

Normal Strength Concrete (NC) 31 MPa, Steel fibre reinforced normal strength concrete

(SNC) 29 MPa, Polypropylene fibre reinforced normal strength concrete (PNC)

28 MPa, High Strength Concrete (HC) 62MPa, Steel fibre reinforced high strength

concrete (SHC) 80MPa and Polypropylene fibre reinforced high strength concrete

(PHC) 62MPa. It has been reported that higher shear strength values are obtained with

the high strength concrete specimens (M60 – M80) reinforced with steel fibres alone

compared to the Normal Strength concrete specimens (M25 – M35). Greater shear

strength increases were found with fibre reinforced high strength concrete specimens

(60 percent with steel and 17 percent with polypropylene fibres) than with fibre

reinforced normal strength concrete specimens (36 percent with steel fibres and no

increase with polypropylene), compared to the strength of their respective unreinforced

plain concrete specimens. In all cases fibres improved the shear deformation and

ductility characteristics of concrete.

21

Lianrong Chen et al. (1994), have studied the first crack strength and

flexural toughness of steel fibre reinforced concrete specimens with different

dimensions using ASTM C1018 and JSCE-SF4 method [41]. It was found that the

variations in specimen size influenced not only the stress and deflection at first crack,

and the ultimate strength, but also all the toughness parameters. It was reported that the

ASTM toughness indices and the JSCE toughness factor were both dependent on

specimen dimensions unless the specimens were geometrically similar. It was reported

that the toughness parameters decreased with an increase in the span-to-depth ratio of

the specimens. The toughness parameters were significantly affected by the width of the

specimen, even when both depth and span were unchanged; the toughness increased

with an increase in width.

Banthia et al. (1995), have studied the toughness characterization of steel

fibre reinforced concrete using deformed steel fibres [42]. The fibres investigated

include crimped fibres with circular and flat sections and end deformed fibres with

hooked and conical ends. Three matrices with compressive strengths of 42, 52 and 85

MPa were reinforced with fibres at a dosage rate of 40 kg/m3 and properties such as

compressive strength, elastic modulus etc., were determined. Toughness improvements

were characterized using ASTM C1018 and JSCE Standard SF-4 techniques. A strong

influence of both fibre geometry and matrix strength on toughness characteristics of

fibre reinforced concrete was observed. The flexural load deflection plots were analyzed

in an appropriate manner to derive quantities suitable for specification, design and a

general comparative assessment of the different fibres. It was found that at a low fibre

dosage of 40 kg/m3, no significant improvements in strengths or moduli were possible.

Deformed fibres, in general, bring about significant improvements in toughness or

energy absorption capabilities of concrete. Based on the four fibre geometries

investigated, fibres with deformations at the ends appear to be more effective than those

with deformations over the entire length. The matrix strength has a significant influence

on toughness characteristics. At high matrix strengths, there is usually a steeper and

sudden drop in load carrying capacity after the first crack. The influence of matrix

strength is dependent on the fibre geometry.

22

Ali Khaloo et al. (1997), have done critical comparison of strength and

ductility properties of steel fibre reinforced low to high strength concrete under direct

shear [43]. The concrete strength is identified as Low Strength Concrete (LC) 28MPa,

Normal Strength Concrete (NC) 42MPa, medium strength concrete (MC) 56MPa and

High Strength Concrete (HC) 70 MPa. It has been reported that concrete of higher

strength reinforced with fibre provided greater shear strength increase than those of

lower strength specimens compared to the strength of their respective unreinforced plain

concrete specimens. For specimens with one percent fibre volume and aspect ratio of

58, the percentage increase was 39, 47, 59 and 86 corresponding to LC, NC, MC and

HC specimens, respectively. For plain specimens, failure occurred in a very brittle

manner with limited warning before collapse, and the SFRC specimens manifested a

relatively ductile type of failure.

Mirsayah Amir et al. (2002), have studied the shear behaviour of fibre

reinforced concrete using direct shear tests [44]. Two types of fibres were used; one

with flattened ends and a circular cross section (FE) and the other with a crimped

geometry and a crescent cross section (CR). Shear tests were conducted using JSCE-

SF6 standard test method. Among the two fibres, the FE fibres were seen to be more

effective than the CR fibre. The comparison of shear toughness as function of shear

strain for FE fibres was found to be 1620 Nm where as for CE fibres it was found to be

1350 Nm for 1 percent fibre volume fraction.

Senthil Kumaran (2012), has studied the development of a ―New Generation

Rubberised Concrete (NGRC)‖ and evaluation of its basic engineering properties [45].

The research focuses on determining the correct proportioning of the tyre fibres that has

to be added to the concrete mix design to get optimum properties of concrete and fixing

up the exact dimensions of the tyre fibre that is most suitable for the concrete mix

because the length of the fibre, the diameter of the holes and the number of holes in

them play an important role in determining the properties of the concrete like toughness,

durability and deformation. The mechanical properties such as split tension, flexure and

direct shear have been studied on NGRC and Control Specimen (CS). The standard

cylindrical moulds 150mm diameter and 300mm height were made with two slits, each

23

of thickness 3mm to study the shear strength of concrete. The distance between the slits

were 30mm, 60mm, 90mm, 120mm and 150mm. The results show that, in both cases of

CS and NGRC, the shear stress reduces gradually with increase in the shear slit

distance. It was also observed that from 30mm slit to 60mm slit the reduction in shear

stress was around 68% for NGRC and CS. For 30mm to 90mm slit there is a major

variation in the shear stress value when compared to CS and NGRC. But for 120mm to

150mm slit the shear stress values of CS and NGRC are almost equal.

2.4 STUDIES ON HYBRID FIBRE REINFORCED CONCRETE

Most fibre reinforced concrete used in practice contains only one type of

fibre. It is known that failure in concrete is a gradual, multi-scale process. The pre-

existing cracks in concrete are of the order of microns. Under an applied load, these

cracks grow and eventually join together to form macrocracks. A macrocrack

propagates at a stable rate until it attains conditions of unstable propagation and a rapid

fracture is precipitated. The gradual and mutli scale nature of fracture in concrete

implies that a given fibre can provide reinforcement only at one level and within a

limited range of strains. For an optimal response, therefore, different types of fibres may

be combined. In hybrids, one type of fibre is stronger and stiffer and provides adequate

first crack strength and ultimate strength, while the second type of fibre is relatively

flexible and leads to improved toughness and strain capacity in the post crack zone.

Another combination has one type of fibre that is smaller, so that it bridges microcracks

and therefore controls their growth, which leads to a higher tensile strength of the

composite and the second fibre is larger and is intended to arrest the propagation of

macrocracks and therefore results in a substantial improvement in the toughness of

concrete [46]. The studies on Hybrid Fibre Reinforced Concrete (HyFRC) subjected to

compression, flexure and tension are discussed.

Parvez Soroushian et al. (1993), has reported the optimization of the

combined use of two different types of fibre in cementitious matrixes [47]. The two

types of fibre were a high modulus polyethylene fibre and a fibrillated polyethylene

pulp. The effects of different fibre volume fractions of the two fibres and their

24

interaction on the impact resistance, flexural strength and toughness and compressive

strength of cementitious materials manufactured with a high-performance mixer were

investigated. It has been reported that combined use of two fibres leads to higher impact

test results, where the interaction of the two fibres actually pronounces each other‘s

effectiveness in increasing the impact resistance. The flexural performance is also

improved while combining these two fibres; however, excessive amounts of the fibres

have negative effects on flexural performance. While fibres generally have negative

effects on the compressive strength of cement-based matrixes, the presence of one fibre

type was observed to reduce or eliminate the negative effects of the other one on

compressive strength.

Takashi Horiguchi et al. (1997), have studied the fracture toughness of fibre

reinforced concrete in compression as well as in flexure and have used four different

types of steel fibre with two types of polyvinyl alcohol (PVA) [48]. The different types

of steel fibre used were hooked fibre (Aspect ratio 50, 37.5 and 75) and straight fibre

(Aspect ratio 37.5). The two types of PVA fibres used were straight with aspect ratios

42.9 and 71.4. The study focused on the hybrid effects of fracture toughness in

compression as well as in flexure by mixing the steel and PVA fibre. The different types

of concrete studied were steel fibre reinforced concrete (SFRC), PVA fibre reinforced

concrete (VFRC) and hybrid fibre reinforced concrete (HFRC). It has been reported that

the compressive toughness of SFRC, VFRC and HFRC increased in proportion to the

volume fraction of fibre. The toughness of HFRC showed the highest value by adding

1.5 percent fibres, while it showed a mean value between that of SFRC and VFRC at

1.0 percent fibre content. This has indicated that the hybrid effect on compressive

toughness due to improvement of brittle property when two different fibres mixing

together is in good composition.

Ramanalingam et al. (2001), have investigated the development of strain

hardening composite using hybrid fibres and high volume fly ash [49]. In order to

achieve this property, different types of fibres may be suitably combined to exploit their

unique properties. In addition, partial replacement of cement by fly ash which has much

finer particles than ordinary cement is likely to modify the properties of the composite

25

by reducing the matrix fracture toughness and enhancing the interfacial bond strength

between the fibre and the matrix, thus promoting the strain-hardening response of the

composite material. Three types of fibres – Polyvinyl alcohol (PVA) micro fibres, PVA

fibres and steel fibres – were used in different combinations as reinforcement for the

mortar matrix. It has been concluded that the cement mortar reinforced with 1.5 percent

steel (6mm in length) and 0.5 percent PVA fibres (12mm in length) showed a flexural

strain hardening behaviour accompanied by multiple cracking when a large amount of

cement in the composite (about 50 percent) was replaced by fly ash. The result was a

composite with high flexural strength, high flexural toughness and high ductility.

Wu Yao et al. (2003), have studied the mechanical properties of hybrid fibre

reinforced concrete at low fibre volume fraction [50]. Concretes were produced with

different types of fibres at the same volume fraction (0.5 percent) and compared in

terms of compressive, splitting tensile and flexural properties. Three types of hybrid

composites were constructed using fibre combinations of polypropylene and carbon,

carbon and steel and steel and polypropylene fibres. Among the three hybrids, the

carbon and steel hybrid was the most beneficial for the improvement of strength and

flexural toughness. It has been shown that improvement of 31.4 percent in compressive

strength, 36.5 percent in splitting tensile strength, 32.9 percent in modulus of rupture

and 33.9 percent - 199.5 percent in toughness indices were obtained for carbon-steel

hybrid composite compared to unreinforced concrete. It is suggested that concrete can

be produced with enhanced strength and improved toughness from hybrid fibres at low

fibre volume fraction.

Banthia et al. (2004), have shown that two or more types of fibre in a

suitable combination may potentially not only improve the overall properties of the

concrete, but may also result in performance synergy [2]. The investigation was been

done by combining the fibres (often called hybridization) by achieving a very high

strength matrix (85 MPa). Control, single, two-fibre and three fibre hybrid composites

were cast using different fibre types such as macro and micro-fibres of steel,

polypropylene and carbon. A minimum change in the compressive strength was

observed due to the addition of various fibres, ranging between 70 MPa to 102.4 MPa.

26

The results show that at 5.9 percent air content, the strength was about 102 MPa and at

8.8 percent air content, the strength decreased about 86 MPa. The moment of resistance

values also decreased with an increase in the air content.

Banthia et al. (2005), have done tests to justify that the fibres can be

effective in arresting cracks at both micro and macro levels [46]. Most of the fibre

reinforced concretes used involve the use of a single fibre type. This implies that a given

fibre can provide reinforcement only at one level and within a limited range strain or

crack opening. For an optimal response, different types of fibres may be combined to

produce hybrid fibre reinforced concrete (HyFRC). The main objective of this research

was to investigate the flexural toughness of HyFRC and to identify the synergistic

effects between fibres. The different types of fibres used were steel fibre, carbon fibre,

micro polypropylene and macro polypropylene. Compressive strength of various

mixtures of fibres varies from 40.6 MPa to 64.3 MPa. It was reported that the

hybridization of micro polypropylene fibre with micro carbon fibre produced one of the

best responses with enhanced synergy. The micro carbon fibre demonstrated better

compatibility with the crimped steel macro fibre than with the flat ended steel macro

fibre.

Singh et al. (2005), have done a critical study on the fatigue strength of steel

fibre reinforced concrete (SFRC) containing mixed fibres[51]. The specimen

incorporated steel fibres of 1.0, 1.5, 2.0 percent volume fractions of corrugated mixed

steel fibres of rectangular cross section of size 0.6 x 2.0 x 25 mm and 0.6 x 2.0 x 50 mm

in different proportions. The fatigue strength of SFRC specimens subjected to flexural

loading is of great concern in the design of bridges and pavement slabs because the

flexural stresses in these structures are critical. It has been shown that the variability in

the distribution of fatigue life of SFRC is more as compared with that of plain concrete.

Further, it has been reported that the shape parameter decreases with an increase in the

fibre content in concrete, indicating higher variability in the distribution of fatigue life

with an increase in the number of fibres in concrete.

27

Chen et al. (2005), have investigated the contribution of hybrid fibres to the

workability, mechanical and shrinkage properties of lightweight concrete (LWC) with

high strength and workability [52]. Three types of fibres were used; steel fibre (S),

carbon fibre (C) and polypropylene fibre (PP). The investigation showed that when one

single type of fibres were used, the increase of the compressive strength of the LWC

with carbon fibres was up to 10 percent compared with the reference concrete; PP fibres

caused a decrease of compressive strength. For those concretes with hybrid fibres, the

strength increased at different degrees and the order of increase was: Carbon – steel

fibres (27.6 percent) > C – PP – S (24.3 percent) > S – PP (19.7 percent). For the split

tensile strength, the concretes with carbon or steel fibres also had great improvement

and that with steel fibres showed the highest split tensile strength. For those concretes

with hybrid fibres, similar to the compressive strength, the split tensile strength

increased and the order of increase was: Carbon – steel fibres (38.3 percent) > C – PP –

S (27.2 percent) > S – PP (11.3 percent). The positive effects of hybrid fibres on the

compressive and split tensile strengths may be due to the fact that hybrid fibres with

different sizes and types offer differing restraint. The results show that the ―holding‖

effect of different fibres in LWC reduced the surface bleeding of concrete and the

sedimentation of the aggregates and improved the uniformity of the mixture. However,

the slump of the mixture reduced somewhat as well. All combinations of different types

of fibres resulted in an increase in strength, among which C–S fibres combination

provides the best effects, i.e., a 27.6 percent increase in the compressive strength and a

38.3 percent of increase in the split tensile strength.

Ahmed et al. (2007), have studied the strain hardening and multiple

cracking behaviour of hybrid fibre reinforced cement composites containing different

hybrid combinations of steel and polyethylene (PE) fibres under four point loading [53].

The total volume fraction of fibres was kept constant at 2.5 percent to maintain a

workable mix. Effect of increase in flyash content as partial replacement of cement

beyond 50 percent, such as 60 percent and 70 percent on the flexural response of hybrid

steel – PVA (polyvinyl alcohol) and steel – PE fibre composites were also evaluated.

Hybrid Steel – PE fibre composites showed lower ultimate strength but higher

deflection capacity at the peak load than that of hybrid steel – PVA fibre composites.

28

Among composites with different volume ratios of steel and PE fibres, the composite

with 1.0 percent steel and 1.5 percent PE was found to show the highest flexural

strength and that with 0.5 percent steel and 2.0 percent PE exhibited highest deflection

and highest flexural toughness. It was reported that the replacement of cement by flyash

(50 percent) was found to be optimum in hybrid fibre composites.

Banthia et al. (2007), have studied the toughness enhancement in steel fibre

reinforced concrete through fibre hybridization [54]. The investigation was carried out

to enhance the toughness of FRC with larger diameter crimped fibres by hybridization

with smaller diameter crimped fibres to maintain the workability, fibre dispersability

and low cost. The results showed that hybridization is indeed a promising concept and

replacing a portion of the larger diameter crimped fibres with smaller diameter crimped

fibres can significantly improve the toughness. It was reported that all mixes were

workable and proper fibre dispersion was achieved. It has been concluded that the

flexural toughness of fibre reinforced concrete has been enhanced with large diameter

crimped steel fibres hybridizing with smaller diameter crimped steel fibres of the same

length. Although hybridization appears to be a promising concept, hybrid FRCs with a

combination of large and small diameter crimped fibres failed to reach the toughness

levels as demonstrated by FRCs with smaller diameter fibres alone. It was also critically

commented that there is no universal rule for hybrid composites involving small and

large diameter fibres.

Sivakumar et al. (2007), have studied the mechanical properties of high

strength concrete reinforced with hybrid fibres such as hooked steel and a non metallic

fibre up to a volume fraction of 0.5 percent [55]. The mechanical properties were

studied for concrete prepared using different hybrid fibre combinations – steel-

polypropylene, steel-polyester and steel-glass. The flexural properties were studied

using four point bending tests on beam specimens as per Japanese Concrete Institute

(JCI) recommendations. Due to fibre addition it is seen that the enhancement of pre-

peak as well as post-peak region of the load deflection curve causes an increase in

29

flexural strength and toughness, respectively. Among the various hybrid composites the

steel – polypropylene fibre concrete there was an enhancement over the individual steel

fibre concrete possibly as a result of the contribution by polypropylene fibres at small

crack widths. It was also reported that with increase in dosages of polypropylene fibres,

there is a decrease in toughness of the steel – polypropylene concrete, since there are not

enough steel fibres in the system for bridging the wider cracks.

2.5 STUDIES ON ULTRA HIGH PERFORMANCE FIBRE REINFORCED

CONCRETE

The brittle behavior of Ultra High Performance Concrete (UHPC) has been

one of main obstacles for its practical application, UHPC has demonstrated superior

mechanical and material properties, e.g., ultra high compressive strength

(150–200 MPa) and dense matrix structure [56 – 59]. As a result, various approaches

have been investigated to remedy its brittle failure. [60 - 66]. Therefore blending of

micro and macro fibres in a UHPC matrix will enhance both the post cracking strength

(tensile strength) and strain capacity (ductility) of Ultra High Performance Fibre

Reinforced Concrete (UHPFRC) by using a small amount of fibres without reduction in

workability. In blending micro and macro fibres, it is expected that macro fibres are

more effective in increasing ductility while micro fibres effective in enhancing tensile

strength [67].

Kang et al. (2011), have investigated the effect of fibre orientation and

distribution on the tensile behaviour of Ultra High Performance Fibre Reinforced

Cementitious Composites (UHPFRCC), considering the pull out behaviour of each fibre

in the composites and the actual fibre distribution obtained from experimental

specimens [68]. The concrete does not contain coarse aggregates and develops

compressive strength exceeding 150 MPa with improved toughness through mixing of

two percent of volume of steel fibres that are 13mm long and 0.2 mm diameter. Two

types of fibre orientation distribution were induced by adopting different placing

directions to investigate this effect on the tensile behaviour. Two cases were considered

for the placing directions; placement parallel to the tensile direction of the specimen and

30

placement transversely to the tensile direction. The pre-cracking tensile behaviour is

expressed using the mechanism of elastic shear transfer between the matrix and the fiber

in the composites. The analysis revealed that improvement of the first cracking strength

by the fiber orientation was limited to 10 percent for the entire range of the fiber

orientation coefficient. Experimental results showed that a higher fiber orientation

coefficient increases the first cracking stress slightly. The results show that the post-

cracking tensile behaviour of the composites was defined as the combined behaviour of

the resistance by the fibers and the matrix. Tension softening behaviour of the mortar in

UHPFRCC was found to be different from the typical softening curve of concrete. In

this research, the apparent matrix softening behaviour in UHPFRCC was studied under

the assumption that the resistant behaviour by mortar could be thought equal to the

actual post- cracking curve minus the fiber bridging curve, and a simplified bilinear

softening curve that was applicable to the matrix in UHPFRCC was suggested.

Seung Hun Park et al. (2012), have investigated the effects of blending

fibres on the tensile behaviour of Ultra High Performance Hybrid Fibre Reinforced

Concrete (UHP-HFRC) [67]. Four types of steel macro-fibres (of differing length or

geometry) and one type of steel micro-fibre were considered. In producing the

specimens, the volume content of the macro-fibre was held at 1.0 percent, whereas the

volume content of the micro-fibre varied from 0.0 percent to 1.5 percent. The

compressive strength of the concrete used was 200 MPa. The results show that the

overall shape of tensile stress–strain curves of UHP-HFRC is primarily dependent upon

the type of macro-fibre, although the addition of micro-fibres favorably affects the

strain hardening and multiple cracking behaviours. UHP-HFRC produced from macro-

fibres with twisted geometry provided the best performance with respect to post

cracking strength, strain capacity and multiple micro-cracking behaviour, whereas

UHP-HFRC produced with long, smooth macro-fibres exhibits the worst performance.

2.6 SUMMARY

A review of the works published on various aspects of Hybrid Fibre

Reinforced concrete (HyFRC) is presented in Table 2.1.

31

Table 2.1 Summary of Review of Literature on FRC and HyFRC

Authors Fibres

Investigated

Major Findings Observation

Studies on FRC

Wimal

Suaris et

al. (1983)

S, PP, G FRC structures subjected to

impact and impulsive loading

is studied. Energy absorbed by

Steel FRC (100%) is higher

compared to unreinforced

specimens.

The energy absorbed by

steel FRC is more

compared to PP FRC

and Glass FRC.

Gopalarat

nam et al.

(1986)

S Steel FRC subjected to impact

load is studied. The

compressive strength of FRC

is higher (34%) compared to

conventional concrete.

Composites made with

weaker matrices, higher

fibre content and larger

fibre aspect ratios are

more rate-sensitive than

those made with

stronger matrices, lower

fibre contents and

smaller fibre aspect

ratios.

Narayanan

et al.

(1987)

S When the cube strength was

increased from 42 MPa to 62

MPa (about 50 percent), an

increase in shear strength from

1.97 MPa to 3.23 MPa (about

64 percent) was observed.

In the beams with

conventional stirrups,

some spalling of

concrete occurred at the

ultimate stages; the

inclusion of steel fibres

eliminated the spalling

of concrete. The beam

was reinforced with

conventional stirrups

and fibres in concrete.

Barr

(1987)

S, PP, G The influence of shear

performance of FRC materials

is studied. The matrix had

shear strength of 8 MPa for 1

percent addition while 10 MPa

for 4 percent addition of fibres

The inclusion of fibres

in the matrix increased

the shear performance

of concrete.

Swamy et

al. (1987)

S The results show that the crack

width varied from 0.15 mm to

0.53 mm with about 75 percent

of the values in the range of

0.15 mm to 0.30 mm.

Fibres in conjunction

with steel stirrups are

studied.

Gopalarat

nam et al.

(1991)

S, PP The behavior of fracture

toughness of fibre reinforced

concrete is studied. The

concrete mixes were designed

for a compressive strength of

34.48 to 41.37 MPa.

It was observed that the

stress at first crack is

relatively independent

of the fibre volume

fraction.

32

Table 2.1 (Continued)

Authors Fibres

Investigated

Major Findings Observation

Mariano

Valle et

al. (1993)

S, PP The shear strength and

ductility properties of Fibre

Reinforced High Strength

Concrete under direct shear are

studied. Greater shear strength

increases were found with

fibre reinforced high strength

concrete specimens (60 percent

with steel and 17 percent with

polypropylene fibres)

compared to the strength of

their respective unreinforced

plain concrete specimens.

Steel fibres and PP

fibres were used to

study the direct shear

properties with

conventional stirrups.

Lianrong

Chen et al.

(1994)

S Flexural toughness of SFRC

with different dimensions

using ASTM and JSCE

method is studied. It was

reported that the toughness

parameters decreased with an

increase in the span-to-depth

ratio of the specimens.

The ASTM toughness

indices and the JSCE

toughness factor were

both dependent on

specimen dimensions

unless the specimens

were geometrically

similar.

Banthia et

al. (1995)

S The toughness characterization

of steel fibre reinforced

concrete using deformed steel

fibres is studied. . It was found

that at a low fibre dosage of 40

kg/m3, no significant

improvements in strengths or

moduli were possible.

Deformed fibres, in general,

brought significant

improvements in toughness of

concrete.

A strong influence of

both fibre geometry and

matrix strength on

toughness

characteristics of fibre

reinforced concrete was

observed.

Ali R

Khaloo et

al. (1997)

S The direct shear behavior of

SFRC from low to high

strength concrete is studied.

For plain specimens, failure

occurred in a very brittle

manner with limited warning

before collapse, and the SFRC

specimens manifested a

relatively ductile type of

failure.

Steel fibres were used to

study the direct shear

properties with

conventional stirrups.

33

Table 2.1 (Continued)

Authors Fibres

Investigated

Major Findings Observation

Mirsayah

A.Amir et

al. (2002)

S (various

geometry)

The shear behaviour of fibre

reinforced concrete is studied

using direct shear tests.

Shear tests were

conducted using

JSCE-SF6 standard

test method.

Senthil

Kumaran

(2012)

waste tyre

fibre

The development of a ―New

Generation Rubberised Concrete

(NGRC)‖ is studied. The shear

behavior of NGRC is studied using

direct shear with slits. The

distance between the slits were

30mm, 60mm, 90mm, 120mm and

150mm.

It was observed that

from 30mm slit to

60mm slit the

reduction in shear

stress was around

68% for NGRC and

Control Specimen.

Studies on HyFRC

Banthia

and Gupta

[2004]

S, CMP,

PP

A minimum change in the

compressive strength was

observed due to the addition of

various fibres, ranging between 70

MPa to 102.4 MPa. The results

show that at 5.9 percent air

content, the strength was about

102 MPa and at 8.8 percent air

content, the strength decreased

about 86 MPa.

The moment of

resistance values

also decreased with

an increase in the air

content.

Chen and

Liu [2005]

S, C & PP Combinations of Carbon and steel

fibre provided the best effects with

27.6% increase in the compressive

strength and 38.3% increase in the

split tensile strength.

The introduction PP

fibre decreased the

compressive

strength.

Soroushian

et al.

[1993]

PE, FPEP Hybrids of PE and FPEP have

important effects on flexural

strength, toughness and impact

strength at 95 percent level of

confidence;

Excessive amounts

of fibres have

negative effects on

flexural

performance. The

interaction of two

fibres actually

pronounces each

other‘s effectiveness

in increasing the

impact resistance.

34

Table 2.1 (Continued)

Authors Fibres

Investigated

Major Findings Observation

Horiguchi

and

Sakai

[1997]

S, PVA The toughness of HyFRC

showed the highest value by

adding 1.5% fibres, while it

showed a mean value between

that of concrete with steel

fibres (1%) and concrete with

PVA (1.0%) fibre content.

This has indicated that the

hybrid effect on compressive

toughness due to improvement

of brittle property when two

different fibres mixing together

is in good composition.

HyFRC showed greater

first crack deflection

for the same flexural

toughness.

Ramanalin

gam et

al. [2001]

PVA (micro

and

macro), S

Concrete reinforced with steel

(1.5%) and PVA (0.5%)

showed a flexural strain –

hardening behavior

accompanied by multiple

cracking when a large amount

of cement in the composite

(50%) was replaced by flyash.

The resulting

composite had a high

flexural strength, high

flexural toughness and

high ductility.

Hybridization provided

significant increases to

both ultimate load and

post-peak ductility.

Wu Yao et

al. [2003]

C, S & PP Improvement of 31.5% in

compressive strength, 36.5% in

splitting tensile strength,

32.9% in Modulus of Rupture

and 33.9 – 199.5% in

toughness indices were

obtained for carbon-steel

hybrid composite compared to

unreinforced concrete.

The PP fibre has low

modulus, the hybrid

systems containing PP

appeared to be less

effective in controlling

matrix crack opening.

Banthia

and

Soleimani

[2005]

S, CMP,

CIP, PP

Hybridisation of micro

polypropylene fibre with CMP

produced one of the best

responses with enhanced

synergy. Similarly, hybridizing

low modulus CIP with high

modulus CMP also produces

compatibility comparing to its

control mixture containing

only carbon fibre.

The CMP fibre

demonstrates better

compatibility with the

crimped steel macro

fibre than with the flat

ended steel macro

fibre.

35

Table 2.1 (Continued)

Authors Fibres

Investigated

Major Findings Observation

Ahmed et

al. (2007)

S, PE, PVA Hybrid Steel (1.5%) and PVA

(1%) exhibited highest flexural

strength and Hybrid Steel

(0.5%) and Polyethylene

(2.0%) exhibited highest

deflection capacity and energy

absorption capacity compared

to conventional concrete.

PE fibre is good in

increasing the highest

deflection capacity.

Banthia

and

Sappakittip

akorn

[2007]

S (various

diameters)

Large diameter crimped steel

fibres were partially replaced

with smaller diameter crimped

steel fibres.

This hybrid resulted in

a significantly higher

flexural toughness.

Sivakumar

and Manu

Santhanam

[2007]

S, PES, G Hybridization of steel and

polypropylene fibre performed

better in toughness

characteristics.

Among all hybrid fibre

combinations only the

steel polypropylene

combination performed

better in all respects

compared to the mono

steel fibre concrete.

Seung Hun

Park et al.

[2012]

S (various

geometry)

The overall shape of the tensile

stress–strain curves of

UHPHyFRC is primarily

dependent upon the type of

macro-fibre rather than micro

fibre.

Hybridization of

twisted steel fibre

produced the best

performance in terms

of strain hardening

behavior.

(PP: Polypropylene; S: Steel; G: Glass; C: Carbon, PVA: Poly Vinyl Alcohol;

PE: Polyethylene; FPEP: Fibrillated Polyethylene Pulp; CMP: Carbon Mesophase

Pitch-based, CIP: Carbon Isotropic Pitch-based, G: Glass, PES: Polyester)

2.7 CRITICAL COMMENTS

The literature reviewed has indicated that cracking in concrete is a gradual,

multi-scale process, occurring at both the micro and the macro levels. For fibre

reinforced concrete, therefore, it is very limiting when only one type and dimension of

fibre is used as reinforcement. Such reinforcement clearly restricts crack growth at its

own scale and has little or no influence on fracture processes at other scales [2].

36

The enhancement of tensile strength by the introduction of different types of

fibres (in hybrid form) into the concrete has been established from the literature survey.

When single macro fibres are used it helped in the post cracking behavior of concrete.

When micro fibres are added it helped in pre cracking behavior. The combination of

these two types of fibres enhanced the behavior both in the pre cracking as well as post

cracking stages of concrete [2].

The flexural strength of concrete is studied using various fibre combinations

and synergy is also observed in most combinations. [49, 50, 52, 54]. The fibres used

were mico fibre and a macro fibre. The mico fibre was Polypropylene, PVA, polyester,

polyethylene or steel. The macro fibre was a steel fibre. Hence studies on different

micro fibres such as steel and polyethylene for controlling the micro crack and hooked

end steel fibres for controlling the macro cracks are limited. The direct tensile strength

of HyFRC is also limited. The studies used were investigated with steel fibres of various

geometries [67] and hence studies related to use of synthetic micro fibre, steel micro

fibre and steel macro fibre is also limited. Up to date studies on the development of

HyFRC with High Performance Concrete using micro steel, polyethylene and macro

steel only limited and only little work has been done so far.

The direct shear strength of concrete is studied using notched cylindrical

specimens with a single type of fibre [38, 40, 43]. The study includes fibres in

conjunction with steel stirrups as reinforcement. Hence combining various types of

fibres in hybrid form to study the direct shear is limited. The high performance concrete

played a key role with the above combinations. In structures, the tensile cracking is seen

in pure flexure members and combination of flexure and shear in members. Studies on

shear behavior of the High Performance concrete together with fibres are scanty and

limited and warrant further research in this direction.

Therefore, there is ample scope to introduce all these fibres in concrete and

it is needed to investigate further important properties for the development of High

Performance Concrete for different grades of concrete which can be used in actual

practice.