ENHANCEMENTOF MECHANICAL PROPERTIES OF LIGHTWEIGHT HYBRID FIBRE REINFORCED CONCRETE (LWHFRC) Eniyachandramouli Gunasekaran, Govindhan Shanmugam and Ranjithselvan Karuppusamy

Eniyachandramouli Gunasekaran, Govindhan Shanmugam and Ranjithselvan Karuppusamy B.EDepartment Of Civil Engineering Bannari Amman Institute Of Technology Sathyamangalam, Tamilnadu, India. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 05 July 2020 Final Accepted: 10 August 2020 Published: September 2020

Many research works has been carried out on concrete which had encouraged the development of material with replacement of aggregate in order to reduce their weight which resulted in lightweight concrete. For lightweight concrete, weight of the aggregate should be reduced so that the self weight of the structural members is reduced and helps in constructing of larger precast units where reduction of self weight is an important factor which may increases thermal insulation .Fibre reinforced concrete(FRC) is a concrete consists of fibrous material which increases the tensile strength of the concrete members with distributed and oriented fibres. In this investigation the strength and durability properties of lightweight concrete by replacement of light weight aggregates will be studied and also discussed 4 types of lightweight aggregates. The properties of conventional concrete are compared with properties of light weight FRC . An optimum control mix is designed based on light weight concrete using the light weight aggregates and mechanical behaviour of fibre reinforced lightweight concrete with composition of various fibres and nature of fibres.For this purpose hybrid of hooked steel fibres of length 50mm and chopped glass fibres used for polymer products. This study results in the optimisation of fibres and lightweight aggregates with comparison of the concrete specimen strength at 7and 28 days with improved specific properties of FRC compositesshowing the compressive strength, ,flexural strength,split tensile strengthof the specimen.

…………………………………………………………………………………………………….... Introduction:-
Generally, concretemember is strong in compression and weak in tension. Concrete is brittle and will crack with increasing tensile force. Once concrete cracks it cannot withstand longer tensile loads. In order to make concrete capable of carrying tension at strains greater than those at which cracking initiates, it is necessary to increase the tensile strength by adding fibres. Lightweight concrete mixture is made with a lightweight aggregate instead of normal aggregates. For the most part light weight concrete is not as strong as concrete made with lightweight aggregates.Because of high density of traditional concrete with ordinary aggregates, a few endeavours have been made in the past to decrease the density and self weight of concrete called light weight concrete which is lighter than the conventional concrete made with lightweight aggregates. 87 It is extremely hard to design steady light weight concrete. One specific issue is that numerous light weight aggregates have high water absorption property. It is unenviable to utilize aggregates in the immersed, surface-dry condition ensuring to the water substance of the aggregates should be minimized with a specific goal to keep the concrete density low. For the most part dry lightweight aggregates are utilized. Furthermore, the amount of water consumed by the lightweight aggregates in the time of middle of blending and setting of the concrete, which will differ generally indicated by conditions, must be determined.
The major benefit in endorsing the light weight concrete mixes is of reducing the dead load of the structure up to 15% or more. When there is reduced unit weight of aggregate then comparatively the unit weight of concrete will be reduced and it is done by replacing the normal weight aggregate to the light weight aggregates. More research work is being carried out because of its lower density, porous and high volume occupancy. It includes an enlarging agent which increases the volume of the mixture. It is having low-density by maintaining large voids and when placed on the wall it forms cement films.
For a hybrid concrete , more than two different fibres are mixed with a concrete matrix to produce a composite concrete. Use of both steel and glass fibre can give potential advantages in improving the performance and properties of concrete matrix. Hence, this experiment proves the feasibility of HFRC with required grade of the concrete matrix with a weight of cement fraction of 1-4% in construction fields in future There are two different methods to categorize fibres according to their elastic modulus or their origin. In the view of modulus of elasticity, fibres can be classified into two basic categories, one having a higher modulus of elasticity than concrete mix (called hard intrusion) and those with lower modulus of elasticity than the concrete mix (called soft intrusion).
Steel, carbon and glass have higher modulus of elasticity than cement mortar matrix, and polypropylene and vegetable fibres are classified as the low elastic modulus fibres with less tensile strength. High elastic modulus fibres simultaneously can improve tensile, flexural and impact resistance; whereas, low elastic modulus fibres can improve the impact resistance of concrete but do not contribute much to its tensile and flexural strength.
As more experience is gained with Steel FRC, more applications are accepted by the engineering community. ACI Committee 318 "Building Code Requirements for Reinforced Concrete" does not yet concede the enhancements that SFRC makes accessible to structural elements. Then more data will be available to contribute to the recognition of magnified SFRC properties in this and other codes. The most considerable properties of SFRC are the improved flexural toughness (such as the potential to absorb energy after cracking), impact resistance, and flexural fatigue endurance.

Cement:
Ordinary portland cementwas used in this experimentation conforming to IS. -12269-1987.

Perlite:
Perlite is an amorphous volcanic glass occurred due to volcanic activity that has relatively high water content. It occurs naturally and has the property of expanding when heated sufficiently. It is an industrial mineral and a commercial product useful for its light weight after processing the product expands 4-20 times its original volume.

PUMICE:
Pumice stone is a consistent material formed from rapidly cooling molten rock by trapping gas bubbles which led to the formation in a foamy whipped glass. It is even formed in deep undergrounds and when the magma erupts from a vent by forming the gases which leaves a foamy structure. The transformed magma is the or pumice. Pumice is found in various textures such as pyroclastic flows,accumulated drifts,piles and at the river banks by the action of windthe pure pumice is obtained in a floating mass or near the shore as it saturates by sinking and near the water bodies by the action of wind.

Poraver:
Poraver expanded glass is adaptable lightweight aggregate for high-quality building materials and industrial products. Poraver offers a mix of formidable characteristics which no other product in this spectrum has. Despite its low density, Poraver is impenetrable to pressure. It not only acts as thermal insulation but also absorbs sound, dimensionally stable over the long term, and resistant to moisture, fire and chemicals. Poraver composed of recycled glass and is 100 % mineral.

M-SAND:
Manufactured sand is a substitute for river sand for concrete construction . Manufactured sand is produced by crushing of hard granite stone . The crushed sand is of cubical shape with grounded edges, graded and washed to as a construction material. The size of manufactured sand is less than 4.75mm so it seized and used.

LECA :
The expansion of LECA is Light weight expandable clay aggregate. LECA is produced of clay with low lime content, which is dried and fired in rotary kilns. The stove shape the clay into pellets and at high temperature of 1.150 ºc gas is formed. The pellets expandedby the gasemerge in the property of high porosity and low weight properties. The porosity is caved in by a matrix of solid bricklike ceramic material with upright compressive strength.

Load Vs. Deflection Behaviour
The load -deflection response for various percentages of LECA 10%,20%,30% replacement to coarse aggregate were as follows: The experimental investigation is divided into two phases.
In the first phase, pumice stone and LECA were replaced to coarse aggregate of percentages 0%,10%,20% and 50% and the mechanical properties were studied. By replacing LECA, load vs displacement behaviour was studied. In this phase, the mechanical properties were lowered to some extent but whereas the dead weight of the concrete found to be reduced than conventional concrete.
In the second phase of work, the mineral admixture Silica Fume is replaced by 5-10% weight of cement and with same percentages of pumice stone were taken and the mechanical properties and also load -deflection behaviour were studied.

Superplasticizer:
Water-reducing admixtures are primarily used to reduce the water-cement ratiocontent of concrete, thus increasing strength. In some cases, they can be used to increase the workability or slump of the concrete providing for smoother placement.

Silica fume:
Silica fume increases the compressive strength of concrete significantly (6-57%). The increase depends upon the replacement level. The tensile and flexural strength of silica fume concrete is almost indistinguishable to the referral concrete. By considering this result 5-10 % silica fume is replaced in cement content. Fly ash induced on the properties of concrete with a view to improving the level of fly ash used for a given application. The optimum amount of fly ash differ not only with the application, but also with proportions and also composistion of all the materials in the concrete mixture (especially the fly ash), the conditions during placing (temperature), construction practices (finishing and curing) and the exposure conditions. Fly ash contents of up to 0-50% may be suitable for most elements provided the early-age strength requirements of the project can be met and provided that adequate moist-curing can be made certain. Thus fly ash content of 10% is replaced in cement content.   The compressive strength of cubes is obtained after curing of 7 and 28 days and 28 days compressive strength will increases in a long period of time and exceeds the compressive strength values of conventional concrete.

Split tensile strength:
The concrete is weak in tension due to its brittleness . Hence. It is not expected to resist the direct tension when tensile forces exceed its tensile strength concrete cracks. Therefore, it is required to determine tensile strength of concrete to determine the maximum load at which the concrete members induce crack.

Fibres mixing and preparation of lwhfrc:
From the above results the optimum mix is identified. LECA and pumice are the lightweight aggregates that replaced normal coarse and fine aggregates with improved performance in test on concrete.
Mixing of FRC is practised by many methods. The mix should have a uniform dispersion of the fibres in order to prevent bleeding, segregation or balling of the fibres during mixing. Most balling happens in the course of fibre addition process. By the Increase of aspect ratio, volume percentage of fibre, and quantity and size of coarse and fine aggregate will intensify the balling tendencies and reduce the workability. To coat the large surface area of the fibres with paste, experience indicated that a water cement ratio of 0.4 -0.6, and minimum cement content of 400 kg/m^3 are necessary.
Process of vibration is used to prevent fibre segregation. Tube floats,Metal trowels and rotating power floats can be used to finish the surface. Mechanical Properties of FRC by Addition of fibres to concrete cause its effect on mechanical properties which significantly depend on the type and percentage offibre added . Fibbers with end anchorage and Properties and Applications ofFibre Reinforced Concrete.High aspect ratio were found to have enhanced effectiveness.

Modulus of elasticity :
Modulus of elasticity of FRC increases a bit with an increase in the fibre content. It was found that for each increase in fibre content by 1% volume there is an increase of 3 % in the elastic modulus.

Compressive strength:
The presence of fibre alter the failure mode of cylinders, but the fibre effect will be result in slight improvement of compressive strength values (0 -15 %).

Flexure Strength :
The flexural strength was reported to be increased by 2.5 times using 4 % addition of fibres.

Split Tensile Strength :
The presence of 3 % fibre volume was reported to increase the splitting tensile strength of mortar about 2.5 times that of the unreinforced one.

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Steel Fibres: Some steel fibres with hooked ends to improve resistance to pullout from a cement matrix. These are Most commonly used fibre. Their shape will be Round of diameter 0.25-0.75mm. They Enhances impact, fatigue and flexural strength of concrete and Used for-overlays of roads, pavements, bridge decks.
Shapes of steel fibre astm a 820: 1.Cold-drawn wire fibres are the most commonly available, manufactured from drawn steel wire. 2.Cut sheet fibres are manufactured as the name by laterally shearing off steel sheets. 3. Melt-extracted fibres are manufactured with a relatively complicated technique where a rotating wheel is used to lift liquid metal from a molten metal surface by capillary action. The extracted molten metal is then quickly frozen into fibres and thrown off the wheel by centrifugal force. The resulting fibres look like crescent-shaped cross section. 4. Other fibres are manufactured for tolerances in length, diameter, and aspect ratio, as well as minimum tensile and flexural strength, and bending requirement. The amount of fibres added to a concrete mix is measured as a percentage of the total volume of the cement composite (concrete).

Glass Fibre:
Glass fibres are used because of their high ratio of surface area to weight. By trapping air within them, blocks of glass fibre are considered as good thermal insulation, with a thermal conductivity of order of 0.05. The freshest and thinnest fibres have more ductile property. The resulting tenacity is less with scratched surface. Because glass has an amorphous structure, its properties remain unchanged along the fibre and across the fibre. Humidity is an important factor in the tensile strength and Moisture is easily absorbed also it can worsen microscopic cracks and surface defects, and lessen tenacity. Glass fibres improve the strength of the material by increasing the force needed for deformation and enhance the toughness by increasing the energy required for crack propagation.

Properties Of Fibre Reinforced Concrete: Volume Of Fibres:
The optimum fibre content in FRC is obtained as 40-65kg/m 3 and varies according to the volume of fibre content added. In this experiment both steel and glass fibres are used in the volume of 1.5% and 2% respectively.
The optimum fibre content for steel fibres is 1-2% of total volume of cement and for glass fibres it is about 1.5-3% of fibres enhanced effect of properties of concrete.

Aspect Ratio Of Fibre:
Structural behaviour of steel fibre reinforced concrete depends on different parameters such as aspect ratio of fibre, the geometrical shape of fibre and orientation of fibre in concrete and the effect of aspect ratio of fibres on resistance characteristics in fibre reinforced concrete such as concrete workability, compressive strength, tensile strength and resistance against impact by considering that steel fibres length were 3.5, 6.5 and 8 cm at 1.5% volume of cement. Results indicate that the uses of steel fibres don't have a specific effect on compressive strength of concrete but it has a significant increase in its tensile strength and toughness. Also by increasing the aspect ratio of fibre slump will be decreased. Steel fibres will improve impact resistance, and reduces crack expansion. 99

Impaction Strength Test
In this test, 150×20 mm disc specimens are used which one of them having zero fibres and the other three specimens have reinforced with fibres with different aspect ratio. This test is done by dropping an 8.5 kg weight from 250mm perpendicular distance on the specimens that concrete strength against impact is determined by number of impacts till initiating a first crack and complete crack(rupture).Results of this test according to the bar graph indicate that fibres increase the first crack strength Also, rupture strength of fibre reinforced concrete towards plain concrete has recovered considerably and as the aspect ratio of fibres increasing rupture.

Workability OF FRC:
Concreteslump cone test is to determine the workability or consistency of concrete mix prepared at the laboratory during casting. Concrete slump test is carried out from batch to batch to check the uniform quality of concrete . In FRC the slump value differs from 70-100mm. 100

Casting Of Specimens :
The materials are weighed accurately using a digital weighing machine and mixed with the mixture machine and mixed thoroughly. Steel fibres were mechanically scattered inside the mixture machine after thorough mixing of the ingredients of concrete. For preparing the specimen for compressive, tensile, and flexure strength steel moulds of their specific purpose were used. -19:-Casting Of Specimen.

Curing Of Specimen:
The test specimens were stored in place free from vibration and kept at a room temperature of 27˚±2˚C for 24 hours ± ½ hour from the time of addition of water to the dry ingredients. After this period, the specimen is marked and removed from the moulds and immediately submerged in clean fresh water and kept there until taken out prior 2-3 hours to test. The specimens were allowed to become dry before testing. The panels were cured by dry curing method, i.e. Moist gunny bags were used to cover over the panels.

Modulus Of Elasticity:
Fibres are added to concrete with optimum fibre range of 30-50kg/m^3. Steel fibres are added with 1% and 1.5% and glass fibre content of 1.5% and 2%.