Effect of Steel Fibre Reinforcement on Early Strength of Concrete

Cement concrete is the most extensively used construction material in the world. The reason for its extensive use is that it provides good workability and can be moulded to any shape. Ordinary cement concrete possesses a very low tensile strength, limited ductility and little resistance to cracking, internal micro cracks, leading to brittle failure of concrete. In this modern age, civil engineering constructions have their own structural and durability requirements, every structure has its own intended purpose and hence to meet this purpose, modification in traditional cement concrete has become mandatory. It has been found that different type of fibers added in specific percentage to concrete improves the mechanical properties, durability and serviceability of the structure. It is now established that one of the important properties of Steel Fiber Reinforced Concrete (SFRC) is its superior resistance to cracking and crack propagation. In this study the effect of Steel fibers on early strength of concrete have been studied by varying the percentage of fibers in concrete. The Chemical admixture used in the concrete is CaCl2 (Accelerator).Fibre content were varied by 0%, 0.50%, 0.75%, 1%, 1.5% and 2% by volume of concrete. Cubes of size 150mmX150mmX150mm to check the compressive strength, Cylinders of size 100mmx200m to check the split tensile strength and beams of size 500mmX100mmX100mm for checking flexural strength were casted. All the specimens were cured for the period of 3, 7 and 28 days before crushing. The results of Steel fiber reinforced concrete on early strength for 3days, 7days and 28days curing with varied percentage of fiber were studied and it has been found that there is significant strength improvement in Early Strength concrete. The optimum fiber content while studying the compressive strength, Split tensile strength and flexural strength is found to be 1.25%. Also, it has been observed that with the increase in fiber content up to the optimum value, the concrete gains maximum compressive strength at 28 days curing while split tensile and flexural performance shows maximum percentage increase in strength in first 3 days of curing. Slump cone test was adopted to measure the workability of concrete. The Slump cone test results revealed that workability gets reduced with the increase in fiber content.

engineering criteria for behavior and mechanical properties of HESFRC 4. To develop some guidelines and practical recommendations for professional use of HESFRC according to intended use and suitability.
Although a number of investigations have been dealt with Fibre Reinforced and High performance concrete before however we don't have much literature on fibre reinforcement of high early strength concrete.
The main intent will be to achieve, besides minimum compressive strength, a post cracking strength in bending higher than the cracking strength so as to minimize crack widths and also ensure a higher resistance to repeated loads after cracking. Particular attention will be given to recording key parameters such as compressive strength, tensile strength, flexural strength and their variation with different percentages of fibre addition.

OBJECTIVES
The main objectives of this investigation will be 1. To establish a consistent and comprehensive database on the properties of high early strength fibre reinforced concrete (HESFRC) 2. To document and synthesize information on the properties of the fresh mix and the mechanical properties of the hardened composite, and 3. To develop some practical recommendations for use of HESFRC by the profession.
The experimental investigation included several parts dealing with the properties of HESFRC: the properties of the fresh mix (workability by the slump test and unit weight), and the compressive and tensile, properties of the hardened composite. Only HESFRC will be considered which has been already defined as achieving a minimum target compressive strength of 35 MPa in 24 hours In relation to the part of the experimental program dealing with the compression tests, following goals will be undertaken: 1. To measure the fresh properties of various HESFRC mixes, which include plastic unit weight, workability (using the cone test) 2. To determine the values of strength and modulus of elasticity of the composite and their variation with time between 1 day and 28 days.
As far as current application of FRC is concerned, a low %age of fibres (0.01-1.0 %) is being used. In this study, we will try higher %ages of fibres (up to 2%)

SCOPE
High early strength concrete, when reinforced with fibres, can be used for several highway related applications. They include repair-type applications for which early strength properties are needed such as for potholes, bridge decks, overlays, pavement joints, and runways; and applications in new structures particularly bridge decks, pavements, median barriers, taxiways, and runways. Tunnel lining, slope stabilisation, dams, hydraulic structures, and blast resistant structures also utilize high early strength fibre reinforced concrete for various purposes. These are applications in which the specific advantages of HESFRC compared with HES concrete without fibres are needed, such as its increased resistance to cracking, its increased toughness (i.e., energy absorption capacity against dynamic and impact loadings), its increased ductility, and its increased fatigue life.
Moreover, HESFRC can be used in reinforced and prestressed concrete structures to replace the plain concrete matrix in these structures. In such cases, its use is expected to lead to substantially improved structural ductility, better hysteretic response under cyclic load reversals, better bonding of the reinforcing bars and prestressing tendons, improved resistance of the concrete cover to spalling, smaller crack widths, and overall improved energy absorption capacity of the structure.

METHODOLOGY
The procedure involved in our project work is summarized as:

Collection and testing of materials
The materials were collected from the university construction sites and stored in the laboratory and the necessary tests were carried out.
Fineness test and initial setting time of OPC and OPC with admixture was carried out. Sieve analysis of fine aggregates was done. For coarse aggregates, flakiness index, elongation index, abrasion, AIV, aggregate crushing value was done. The steel fibres needed to be cut to required size. The chemical was also powdered before use.

➢ Composition of cement
Cement comprises of compounds of calcium, silicon, aluminum, iron, and oxygen. The main function of cement is to bind the aggregates and to fill the voids in between the aggregates. The most commonly used cement is the Ordinary Portland Cement (OPC). It is obtained by burning together, in a definite proportion, a mixture of naturally occurring argillaceous and calcareous material to a partial fusion at high temperature (1450 °C). Differences in various types of ordinary Portland cements arise due to variations in relative proportions of these compounds in cement.

B. Aggregates
Aggregates are divided into two categories on the basis of size 1. Coarse aggregates (size greater than 4.75mm). 2. Finer aggregates (less or equal to 4.75mm).
Aggregate consists of large chunks of material in a concrete mix. Almost all natural aggregate material originate from bed rocks. Size of aggregates varies perhaps 80mm is maximum size that could be conveniently used. The shape of aggregates may be round, irregular or partly round or angular, flaky. The shape of aggregates greatly affects the workability.

C. Water
Combining water with a cementations material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it, and makes it flow more freely. A lower water-to-cement ratio yields a stronger, more durable concrete, whereas more water gives a freer-flowing concrete with a higher slump. Impure water used to make concrete can cause problems when setting or in causing premature failure of the structure.
Hydration involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles and other components of the concrete to form a solid mass. properties of concrete in cold weather. While as retarders slow the hydration of concrete and are used in large or difficult pours where partial setting before the pour is complete is undesirable. Typical polyol retarders are sugar, sucrose, sodium gluconate, glucose, citric acid, and tartaric acid.

D. Admixture
Plasticizers increase the workability of plastic or "fresh" concrete, allowing it be placed more easily, with less consolidating effort. A typical plasticizer is lingo-sulfonate. Plasticizers can be used to reduce the water content of a concrete while maintaining workability and are sometimes called water-reducers due to this use. Such treatment improves its strength and durability characteristics

HIGH PERFORMANCE CONCRETE (HPC)
In recent years, improvements in properties of concrete have been achieved in the so called High performance concrete by improvements involving a combination of improved compaction, improved paste characteristics and aggregate-matrix bond and reduced porosity. Subsequent reduction in water cement ratio is achieved through use of plasticisers. Although HPCs are made from same materials as normal concrete but they are considered quite different materials due to their higher qualitative and quantitative performances. They offer different advantages such as enhanced durability, reduced permeability, higher strength, early setting etc. Based on characteristic strength, a concrete can be called as HPC if it produces a strength of 600-100 Mpa in 28 days (compared to 10-20 of ordinary concrete). Fibre reinforcing concretes are becoming quite a common today due to accessory advantages besides increase in strength and reduction in cost. High early strength is a common HPC used mainly in underground construction, repair works, construction on busy routes etc.
There are three methods which make adjustments to obtain high-early strength in concrete: 1. Adding 30% more cement by weight to the normal cement content (the fine aggregate is reduced) while the water and air contents remain unchanged. 2. Adding chemical admixtures to the standard mix. 3. A combination of 1 and 2.
The additional 30% cement or addition of a water reducer increases the cement-voids ratio of the mix and thereby strength is increased. Accelerating admixtures added to a standard mix, without changing the cement or water content, increase the rate of hydration thereby increasing the early strength but reducing the ultimate strength.
Increasing the cement content 30% produces highearly strength concrete. Do not increase the water content more than 5% over that used with the normal cement content. There is a tendency to increase the water content to the extent that the same slump is obtained. The addition of excess water will nullify the benefits of the increased cement content and produce a lower early and lower ultimate strength than anticipated. The actual slump value is less in a higher cement content mix due to the increased workability of the mixture that is a result of the high cement content. The lower slump concrete with the additional cement is just as workable as the normal concrete

FIBRE REINFORCED CONCRETE
Concrete is the most widely used man-made construction material in the world, and is second only to water as the most utilized substance on the planet. It is obtained by mixing cementing materials, water and aggregates, and sometimes admixtures, in required proportions. The mixture when placed in forms and allowed to cure hardens into a rock-like mass known as concrete. The strength, durability and other characteristics of concrete depend upon the properties of its ingredients, on the proportions of mix, the method of compaction and other controls during placing, compaction, and curing. Concrete is weak in tension and has a brittle character. The concept of using fibres to improve the characteristics of construction materials is very old. Early applications include addition of straw to mud bricks, horse hair to reinforce plaster and asbestos to reinforce pottery. Use of continuous reinforcement in concrete (reinforced concrete) increases strength and ductility, but requires careful placement and labour skill. Alternatively, introduction of fibres in discrete form in plain or reinforced concrete may provide a better solution. The modern development of fibre reinforced concrete (FRC) started in the early sixties. Addition of fibres to concrete makes it a homogeneous and isotropic material. When concrete cracks, the randomly oriented fibres start functioning, arrest crack formation and propagation, and thus improve strength and ductility. The failure modes of FRC are either bond failure between fibre and matrix or material failure. A convenient parameter describing a fibre is its aspect ratio (LID), defined as the fibre length divided by an equivalent fibre diameter. Typical aspect ratio ranges from about 30 to 150 for length of 6 to 75mm.
A shortcoming of using fibres in concrete is reduction in workability. Workability of FRC is affected by fibre aspect ratio and volume fraction as well the workability of plain concrete. As fibre content increases, workability decreases. Most researchers limit volume of fibres to 4.0% and aspect ratio to 100 to avoid unworkable mixes. In addition, some researchers have limited the fibre reinforcement index [volume of fibres 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.

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 fibres 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.

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 fibre reinforced concrete is affected by following factors: ➢ Type of fibre ➢ Aspect ratio ➢ Quantity of fibre ➢ Orientation of fibre Type of fibre: A good fibre 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 fibre. 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 fibres. Keeping that in view we have considered steel crimped fibres with aspect ratio of 50-80 (Length 40-50 mm and Diameter 0.75 mm).

Fibre quantity:
Generally quantity of fibres is measured as percentage of cement content or concrete. As the volume of fibres increase, there should be increase in strength and toughness of concrete. Regarding our fibre, we hope that there will be an increase in strength, with increase in fibre content. We are going to test for percentages of 0, 0.5, 0.75, 1.0, 1.25, 1.5, and 2.0 % volume of concrete

Orientation of fibre:
The orientations of fibres play a key role in determining the capacity of concrete. In RCC the reinforcements are placed in desired direction. But in FRC, the fibres will be oriented in random direction. The FRC will have maximum resistance when fibres are oriented parallel to the load applied.
The stress strain curve of concrete under uniaxial compression shows a linear behaviour 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.

FIG 1: STRESS-STRAIN CURVE OF ORDINARY AND FIBRE REINFORCED CONCRETE STEEL FIBRES
Steel fibres have been used in concrete since the early 1900s. The early fibres were round and smooth and the wire will be cut or chopped to the required lengths. The use of straight, smooth fibres has largely disappeared and modern fibres have either rough surfaces, hooked ends or are crimped or undulated through their length. Modern commercially available steel fibres are manufactured from drawn steel wire, from slit sheet steel or by the melt-extraction process which produces fibres that have a crescent-shaped cross section Typically steel fibres have equivalent diameters (based on cross sectional area) of from 0.15 mm to 2 mm and lengths from 7 to 75 mm. Aspect ratios generally range from 20 to 100. (Aspect ratio is defined as the ratio between fibre length and its equivalent diameter, which is the diameter of a circle with an area equal to the cross-sectional area of the fibre). Carbon steels are most commonly used to produce fibres but fibres made from corrosionresistant alloys are available. Stainless steel fibres have been used for high-temperature applications.
International Journal of Trend in Scientific Research and Development (IJTSRD) ISSN: 2456-6470 Page: 205 Steel fibres have been used in conventional concrete mixes, shotcrete and slurry-infiltrated fibre concrete. Typically, content of steel fibre ranges from 0.25% to 2.0% by volume. Fibre contents in excess of 2% by volume generally result in poor workability and fibre distribution, but can be used successfully where the paste content of the mix is increased and the size of coarse aggregate is not larger than about 10 mm. Some fibres are collated into bundles using watersoluble glue to facilitate handling and mixing. Steel fibres have high tensile strength (0.5 -2 GPa) and modulusof elasticity (200 GPa), a ductile/plastic stress-strain characteristic and low creep. Steel-fibrereinforced concrete containing up to 1.5% fibre by volume has been pumped successfully using pipelines of 125 to 150 mm diameter. Steel fibre contents up to 2% by volume have been used in shotcrete applications using both the wet and dry processes. Steel fibre contents of up to 25% by volume have been obtained in slurry-infiltrated fibre concrete. Concretes containing steel fibre have been shown to have substantially improved resistance to impact and greater ductility of failure in compression, flexure, and torsion. Similarly, it is reported that the elastic modulus in compression and modulus of rigidity in torsion are no different before cracking when compared with plain concrete tested under similar conditions. It has been reported that steel-fibrereinforced concrete, because of the improved ductility, could find applications where impact resistance is important. Fatigue resistance of the concrete is reported to be increased by up to 70%.It is thought that the inclusion of steel fibre as supplementary reinforcement in concrete could assist in the reduction of spalling due to thermal shock and thermal gradients. The lack of corrosion resistance of normal steel fibres could be a disadvantage in exposed concrete situations where spalling and surface staining are likely to occur.

PREVIOUS WORK
The influence of fibres 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 fibres 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 fibres in lower strength concretes increases their compressive strength significantly compared to plain un reinforced matrices and is directly related to volume fraction of steel fibre used. This increase is more for hooked fibres in comparison with straight steel fibres, glass or polypropylene fibres.
Ezeldin and Balaguru [1992] conducted tests to obtain the complete stress strain curves of steel fibrereinforced 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 fibres 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 fibres to concrete, with or without silica fume, increased marginally the compressive strength and the strain corresponding to peak stress.

MATERIALS USED Cement
Ordinary Portland cement is used in the project work, as it is readily available in local market. We used 53 grade cement Khyber cement for our project purpose. The reason for using high grade is that for high performance cements, higher strength is preferable

Coarse Aggregate
Crushed angular granite metal from a local source was used as coarse aggregate. The specific gravity was 2.71, Flakiness index of 4.58 % and elongation index of 3.96%. The coarse aggregate used in the project work are 20 mm down grade. The reason for selecting a relatively small-size aggregate is to improve the efficiency of fiber reinforcement.

Fine Aggregate
River white sand was used as fine aggregate. The specific gravity was 2.55. The fine aggregate used in the project work is 4.75 mm down grade.

Fibre
Stainless steel zinc coated woven crimped wire mesh with dia 0.7 mm and density 2.9 -3.0 kg/cm2 was used to obtain fibres. The length of fires was kept in the range 3 -5 cm. The stainless steel wire mesh was obtained from local market.  The function of water in concrete ➢ It acts as lubricant ➢ It acts as chemically with cement to form the binding paste for coarse aggregate and reinforcement. ➢ It enables the concrete mix to flow into formwork.

EXPERIMENTAL PROGRAM CONCRETE MIX DESIGN MIX DESIGN
Mix design is the process of selection of suitable ingredients of concrete and to determine their properties with object of producing concrete of certain maximum strength and durability, as economical as possible. The purpose of designing is to achieve the stipulated minimum strength, durability and to make the concrete in the most economical manner.

SPECIFIC GRAVITY TEST OF FINE AGGREGATE
Specific gravity has been calculated using density bottle method. Specific gravity is given by   1. Aggregate Crushing Value 3kg of aggregate passing 12.5mm and retained on 10mm Sieve in a dry Condition is filled into the standard cylindrical measure in 3 layers. The apparatus with the sample in position is loaded uniformly up to a load of 40t in 10 minutes time.
After releasing the load, whole of the material is removed and sieved on a 2.36mm IS Sieve. The fraction passing through it is weighed.
Aggregate Crushing value = Wt. of fraction passing 2.36mm sieve/wt.of sample = 720/3000 = 26% Result: Aggregate Crushing value is less than 45% which is prescribed limit for coarse aggregate used for concrete.

Aggregate impact value
400 gms of the sample passing through 12.5mm sieve and retained on 10mm sieve is filled into a cylinder steel cup. The sample is subjected to 15 blows of a 14kg hammer, raised to a height of 380mm.The crushed aggregate is removed on 2.36mm IS Sieve.
Aggregate Impact Value = Wt. of fraction passing 2.36mm IS Sieve / Wt. of sample = 59/400 = 14.7 % Result: Aggregate is strong and the value falls below the max. Prescribed limit of 30%

Aggregate abrasion value
This test is carried in Los Angeles abrasion testing machine. According to the test specifications 5000gm of the aggregate passing 20mm sieve and retained on 10 mm sieve, the aggregate falls in grade B. No. of spheres taken = 11 Wt. of aggregates = 5000 gm Wt. of aggregates retained on 1.70 mm IS Sieve after the test = 4243 gm Loss in weight due to wear = 5000-4243 = 757 gm Los Angeles abrasion value, % = 757/5000 = 15% The maximum Specified value of abrasion for cement concrete pavements surface course is 30 %. Hence test result is satisfactory 4. Shape Tests Shape tests Shape tests of coarse aggregates were carried out using metal thickness gauge for flakiness index and metal length for elongation index. The flakiness index of aggregate is the percentage by weight of particles in it whose least dimension (thickness) is less than three-fifth of their mean dimension. The elongation index on an aggregate is the percentage by weight of particles whose greatest dimension (length) is greater than 1.8 times their mean dimension. The tests are not applicable to sizes smaller than 6.3mm.
The flakiness test was conducted by using a metal thickness gauge. A sufficient quantity of aggregate was taken such that a minimum number of 200 pieces of any fraction can be tested. The flakiness index is taken as the total weight of the material passing the various thickness gauges expressed as percentage of the total weight of the sample taken The presence of elongated particles in excess of 10 to 15 % is generally considered undesirable but no recognized limits are laid down. The results of all these tests are categorically shown as 1. Slump test It is observed that the workability (Slump) of concrete decreases as fibres content increases. The initial slump of control concrete was 50 mm and it was reduced to 17 mm at 1.0 % fibres and at 2%, no workability was observed.       The variation in the compressive stress and split tensile stress with respect to changes in the fibre content can be observed. From the results obtained, it is clear that the compressive and split tensile strength of concrete is maximum when the fibre content is 1.25% of the concrete.  repair-type applications for which early strength properties are needed such as for potholes, bridge decks, overlays, pavement joints, and runways; and applications in new structures particularly bridge decks, pavements, median barriers, taxiways, and runways. Tunnel lining, slope stabilization, dams, hydraulic structures, and blast resistant structures also utilize high early strength fibre reinforced concrete for various purposes. After Careful and elaborate study of the effect of steel fibre reinforcement on Early Strength concrete, it can be concluded that 1. The compressive strength of concrete for the cubes with steel fibers 0.25%, 0.50%, 0.75%,1%,1.25%,1.5%,and 2% is more than that of cubes without steel fibers. This may be due to the fact that the steel fibers will effectively hold the micro cracks in concrete mass. The percentage increase in the compressive strength for the cubes with steel fibers 0.25%, 0.50%, 0.75%,1%,1.25%,1.5% and 2% compared to the cubes without steel fibers are 11.70%, 16.70%,20.0%,22.50%,16.7% &10% respectively. However maximum percentage increase in compressive strength can be obtained for the cubes with steel fibers 1.25% by volume of concrete (+22.50%). Thus it is recommended to use steel fibers 1.25% by volume of concrete to get the maximum benefit in improving compressive strength.

FIG 6: SLUMP TEST
2. The addition of steel fibers at 1% by volume causes a considerable improvement in early as well as long term split tensile strength of concrete. The maximum improvement in 28-days strength was observed to be 52.1%. Hence 1.25% fiber content is optimum fiber content for 70 aspect ratio fiber from split tensile strength point of view. Thus it is recommended to use steel fibers 1.25% by volume of concrete to get the maximum benefit in improving split tensile strength.
3. The percentage increase in the flexural strength for the beams with steel fibers 0.25%, 0.50%, 0.75%,1%,1.25%,1.5% and 2% compared to the beams without steel fibers are +26.4%, +45%,+65%,+55% +22.4%, and +11% respectively. It can be seen from the observations that the maximum percentage increase in flexural strength can be obtained for the beams with steel fibers 1.0% by volume of concrete (+65%). Thus it is recommended to use steel fibers 1.0% by volume of concrete to get the maximum benefit in improving flexural strength.
4. The workability of fresh concrete was found to decrease with an increase in the fiber content. There was also a decrease in the workability with the increase in the aspect ratio.
In nutshell it can be concluded that the use of steel fibers is an effective method to improve the flexural, Split Tensile & compressive strength of concrete. To get the maximum benefit it is recommended to use steel fibers 1.0% by volume of concrete. More percentage of steel fibers will have the workability problem & also air cavities are left in the system.