Description:4.DESCRIPTION
FIELD OF THE INVENTION
[0001] The present invention is related to fibre reinforced concrete particularly using hybrid fibre reinforced concrete (HFRC) and conventional concrete.

BACKGROUND
[0002] Concrete is a composite brittle material composed of aggregates bonded together with a fluid cement paste that is cured over time. Concrete has low tensile strength and strain capacities and is vulnerable to crack propagation. The fibres are distributed randomly to develop fibre reinforced concrete to overcome crack propagation. A hybrid fibre reinforced concrete (HFRC) is formed when two or more different types of fibres are used in the concrete. HFRC is widely applied in blast and seismic-resistant structures where high impact energy and toughness are key parameters. Steel fibres are the most commonly used man-made metallic fibres, generally carbon or stainless steel. Steel fibres improve the toughness and post-cracking behaviour of concrete. Polypropylene fibres are synthetic in nature with a low density, less diameter and low modulus of elasticity and provides resistance to non-loading cracks.

[0003] The existing invention discloses the usage of fibres to improve the mechanical properties of concrete that mainly contribute to the enhancement of the flexural strength of concrete. Beam is a structural member subjected to transverse loads only which generate maximum bending stresses at extreme faces and zero at neutral axis. The hybrid fibre reinforced concrete improves the flexural strength, toughness, impact and post- cracking strength.

[0004] The above-addressed invention studies hybrid fibre reinforced concrete beams. The bending stresses are linearly reduced towards the neutral axis, thereby, the hybridization of fibres is not required in that portion. Hence, it is aimed at to develop three-layered concrete containing conventional concrete sandwiched between hybrid fibre reinforced concrete in the present investigation.

SUMMARY
[0005] The hybrid fibre is a combination of steel and polypropylene fibres and the conventional concrete made with the hybrid fibre is termed as hybrid fibre reinforced concrete.

[0006] The steel fibre reinforced concrete is composed of steel fibres that are randomly distributed in the conventional concrete of grade M30 whereas the polypropylene fibre reinforced concrete consists of polypropylene fibres in the conventional concrete of grade M30.

[0007] The three-layered concrete containing conventional concrete sandwiched between hybrid fibre reinforced concrete (TLCCHFRC) has been proposed in the present invention which is divided into three equal layers of depth d/3 each i.e., top, middle and bottom layers.

[0008] The top and bottom layers were cast with HFRC and the middle layer with conventional concrete as the bending stresses were maximum at extreme faces and linearly reducing to minimum in that portion at which conventional concrete was considered instead of hybrid fibre reinforced concrete.

[0009] The three-layered concrete containing conventional concrete sandwiched between hybrid fibre reinforced concrete (TLCCHFRC) specimens were cast and placed in ambient curing at room temperature for 24 hours and then were demoulded and cured in water for 7 and 28 days. Before testing, the specimens were taken out from curing tank and air-dried for 2 hours at room temperature.

[0010] The compressive and flexural strengths of M30 grade concretes were determined at the ages of 7 and 28 days respectively.

DESCRIPTION OF THE DRAWINGS
[0011] Figure-1 illustrates the M30 grade conventional concrete(left) and hybrid fibre reinforced concrete (right).

[0012] Figure-2 illustrates the casting of M30 grade three-layered concrete containing conventional concrete sandwiched between hybrid fibre reinforced concrete (TLCCHFRC) cubes.

[0013] Figure-3 illustrates the casting of M30 grade three-layered concrete containing conventional concrete sandwiched between hybrid fibre reinforced concrete (TLCCHFRC) prisms.

[0014] Figure-4 illustrates the specimens in ambient curing.

[0015] Figure-5 illustrates the specimens in water curing.

[0016] Figure-6 illustrates the M30 grade three-layered concrete containing conventional concrete sandwiched between hybrid fibre reinforced concrete (TLCCHFRC) cube specimens.

[0017] Table-1 illustrates the compressive strength of the various concretes.

[0018] Figure-7 illustrates the variation of compressive strength of various types of M30 grade concrete. at the ages of 7 and 28 days.

[0019] Figure-8 illustrates the comparison between flexural strength test on three-layered concrete containing conventional concrete sandwiched between hybrid fibre reinforced concrete (TLCCHFRC) prism.

[0020] Table-2 illustrates the flexural strength of the various types of M30 grade concrete.

[0021] Figure-9 illustrates the comparison of values of flexural strength of different types of M30 grade concretes at the ages of 7 and 28 days.

DETAILED DESCRIPTION OF THE INVENTION
[0022] A method has been proposed in the present invention to form a three-layered concrete containing conventional concrete sandwiched between hybrid fibre reinforced concrete (TLCCHFRC) as shown in figure-2 and figure-3. The three-layered concrete containing conventional concrete sandwiched between hybrid fibre reinforced concrete (TLCCHFRC) consists of three equal layers with a depth of d/3 each. The three equal layers are a top layer, middle layer and bottom layer. The top and bottom layers acts as the skin whereas the middle layer acts as core. A conventional concrete was prepared using cement, river sand, crushed stone, water and admixture whereas hybrid fibre reinforced concrete was prepared using steel and polypropylene fibres in addition to the constituents of the conventional concrete as shown in Fig.1.

[0023] Double-hooked end steel fibres of aspect ratio 63.64 with a dosage of 1.5% by volume of concrete used in the conventional concrete to prepare steel fibre reinforced concrete (SFRC) whereas polypropylene fibre reinforced concrete (PFRC) containing fibres of length 12mm, diameter 0.04mm and a dosage of 0.2% by volume of concrete were used. The bottom layer was cast with hybrid fibre reinforced concrete followed by the middle layer with conventional concrete and the top layer with hybrid fibre reinforced concrete to form three-layered concrete containing conventional concrete sandwiched between hybrid fibre reinforced concrete (TLCCHFRC) and placed in ambient curing for 24 hours as shown in Fig.4. After 24 hours, specimens are demoulded and kept in water for curing at 7 and 28 days as shown in Fig.5.

[0024] Steel fibres improve the toughness and post-cracking behaviour of concrete. Polypropylene fibres are synthetic fibres with a low density, less diameter and low modulus of elasticity provide resistance to non-loading cracks. Hybrid fibres (steel and polypropylene) improve toughness and ductility under static and dynamic loads. The hybrid fibre reinforced concrete specimens are effective against earthquake and blast loads and other aggressive environmental conditions. The preparation of conventional concrete, SFRC, PFRC, HFRC and TLCCHFRC are discussed in the following sections.

[0025] The ingredients used in the conventional concrete of grade M30 were cement OPC53 of specific gravity 3.12 with quantity of 394 kg/m3, the river sand of type zone -III with quantity 714 kg/m3, coarse aggregate of sizes 20 mm and 10 mm obtained from crushed stone, in the ratio of 60% and 40% with quantities of 728 kg/m3and 485 kg/m3. The water-cement ratio is considered as 0.42. The potable water with quantity of 165.58lit and SP conplast 430 as admixture with quantity 3.94lit.

[0026]. Cubes and prisms were cast laying prepared concrete mix in three layers. Each layer was tamped 35 strokes for cubes and 25 strokes for prisms respectively using 16mm diameter tamping rod. The compressive strength and flexural strength of the conventional concrete were determined by conducting compression test on cube of size 150mmx150mmx150mm and flexural test on prism of size 100mmx100mmx500mm as shown in Fig.6 and Fig.8. The values of compressive strength obtained at 7 and 28 days of curing for M30 grade conventional concrete were 29.17MPa and 40.59MPa as shown in table-1 and Fig.7 whereas for flexural strength of conventional concrete the values were 3.96MPa and 5.21MPa at 7 and 28 days respectively as shown in table-2 and Fig.9.

[0027] The compressive strength and flexural strength of the steel fibre reinforced concrete (SFRC) were determined by conducting compression test on cube of size 150mmx150mmx150mm and flexural test on prism of size 100mmx100mmx500mm. Double-hooked end steel fibres of 1.5% by volume of concrete having aspect ratio of 63.64 were randomly distributed in the conventional concrete to obtain steel fibre reinforced concrete (SFRC). Specimens were cast for both compression and flexure tests by laying SFRC in three layers each approximately 1/3 in height of the mould respectively. Each layer was tamped 35 strokes for cubes and 25 strokes for prisms respectively using 16mm diameter tamping rod. After curing specimens at required ages, cube specimens were tested in 300 ton compression testing machine and prisms in 10 ton universal testing machine as shown in Fig.6 and Fig.8.

[0028] The compressive strength and flexural strength of the polypropylene fibre reinforced concrete (PFRC) was evaluated by conducting compression test on cube of size 150mmx150mmx150mm and flexural test on prism of size 100mmx100mmx500mm. Specimens were cast for both compression and flexure test by laying polypropylene fibre reinforced concrete in three layers each approximately 1/3 in height of the mould respectively. Each layer was tamped 35 strokes for cubes and 25 strokes for prisms respectively using 16mm diameter tamping rod. After curing specimens at required ages, cube specimens were tested in 300-ton compression testing machine and prisms in 10-ton universal testing machine.

[0029] The hybrid fibre reinforced concrete specimens were cast by laying steel fibres at 1.5% and polypropylene fibres at 0.2% by volume of concrete in mixed form along with conventional concrete in three layers.

[0030] Similar to casting of steel fibre reinforced concrete (SFRC), polypropylene fibre reinforced concrete (PFRC) and hybrid fibre reinforced concrete (HFRC), three layered concrete specimens
were cast by laying HFRC in bottom layer (skin), conventional concrete in the middle layer (core) and HFRC in the top layer (skin) respectively. After curing specimens at required ages cube specimens were tested in 300-ton compression testing machine and prisms in 10 universal testing machines.

[0031] In the case of prisms, the bending stresses were maximum at the extreme faces and gradually reduce towards the neutral axis. Cracks were generated at the soffit of the beam (bottom surface) where tensile bending stresses are maximum. This is the reason to strengthen the skin portion of the beam with HFRC to arrest micro cracks which are developed. The concrete was effective in the structural components that are used in defence industry, nuclear industry and tunnels.

[0032] The values of compressive strengths of M30 grade SFRC, PFRC, HFRC and TCCHFRC were increased by 23.89%,17.89%; 12.24%,8.01%; 27.97%,22.64%; 25.95% and 19.81% at 7 and 28 days respectively when compared to conventional concrete. Similarly, the increase in the values of flexural strengths were 120.71%,102.88%; 19.19%,15.93%; 142.93%,115.74%; 136.87% and 111.71% at 7 and 28 days respectively. Marginal decrease in values of compressive strength and flexural strength was observed in three-layered concrete containing conventional concrete sandwiched between hybrid fibre reinforced concrete (TLCCHFRC) when compared to hybrid fibre reinforced concrete. However three-layered concrete containing conventional concrete sandwiched between hybrid fibre reinforced concrete (TLCCHFRC) specimens have reached the target strength. Specimens prepared as layered concrete using different types of mixes showed good results and effective in resisting the higher loads.
, Claims:1. A method of preparing a fibre reinforced concrete composition, comprising:
a composite concrete;
the composite concrete includes three equal layers with a depth of d/3 each;
the three equal layers are a top layer, middle layer and bottom layer;
a conventional concrete made up of cement, river sand, potable water, crushed stone and admixture;
a steel fibre reinforced concrete (SFRC);
the steel fibre reinforced concrete (SFRC) is a double-hooked end steel fibres with aspect ratio of 63.64 mm and 1.5% by volume of concrete induced into the conventional concrete;
a polypropylene fibre reinforced concrete (PFRC);
the polypropylene fibres were added in the conventional concrete with a length of 12 mm, diameter 0.04 mm and 0.2% by volume of concrete:
the steel and polypropylene fibres combined with a percentage of 1.5 and 0.2 by volume of concrete were added to the conventional concrete;
whereby forms a hybrid fibres (steel and polypropylene) reinforced concrete (HFRC);
The bottom layer (skin) is cast with HFRC followed by middle layer (core) with conventional concrete and top layer (skin) with HFRC;
Whereby forms a concrete composition;
That is termed as three-layered concrete containing conventional concrete sandwiched between hybrid fibre reinforced concrete (TLCCHFRC);
All types of prepared concrete specimens are cured in water for 7 and 28 days respectively before testing.
2. The method as claimed in claim 1, wherein values of compressive strength and flexural strength were increased by 19.81% and 111.71% respectively at 28 days.