KRS Narayan and Rajiv Gauri talks about the importance of Polyester Fibers as secondary reinforcement in concrete improes strength, durability, toughness and fatigue resistance.
Fiber Reinforced Concrete" is relatively a new construction material developed through extensive research and development work during the last two decades. Fiber Reinforced Concrete (FRC) is defined as composite material which consists of conventional concrete reinforced by randomly dispersed short length fibers of specific geometry, made of steel, synthetic (polymeric) or natural fibers. Plain cement concrete has very low tensile strength and causes formation of micro cracks in stressed and unstressed states of concrete. Also, it has a low strain at fracture and brittleness with less ductility especially in case of High Performance Concrete. Fiber Reinforced Concrete is the answer to modify these properties of Plain Concrete.
Advantages of FRC
Various advantages of Fiber Reinforced Concrete are,
• Resistance to Micro-Cracking.
• Toughness and Post-Failure Ductility
• Impact & Abrasion Resistances
• Resistance to fatigue
• Improved strength in shear, tension, flexure and compression.
• Reduced permeability
The interaction between the fiber and concrete matrix is the fundamental property that affects the performance of a cement based fiber composite materials. An understanding of this interaction is needed for forecasting the fiber contribution and for predicting the behavior of such composites. The following are the major parameters affecting the fiber interaction with the matrix.
• Condition of the matrix-uncracked or cracked.
• Matrix composition
• Geometry of the fiber-triangular or circular
• Type of fiber-steel, polymeric, mineral or naturally occurring fiber
• Surface characteristics of the fiber
• Stiffness of the fiber in composition with matrix stiffness
• Orientation of the fibers-aligned versus random distribution
• Volume fraction of fibers
• Rate of loading
• Durability of fiber in the composite and the long term effect in the concrete matrix.
Experimental investigation
The behaviour and strength of conventional and fiber reinforced concrete are ascertained by testing the specimens in the laboratory. This paper deals with the mix design, preparation of the specimen, and casting, testing and test results of the specimens.
Materials
It is necessary to get the maximum performance out of all of the materials involved in producing a concrete. The materials involved in this project are as follows
• Portland cement,
• Coarse aggregate.
• Fine aggregate and super plasticizers.
• Water
• Super Plasticizer
• Mixed design
• The additional material involved in this project is triangular polystor fiber-synthetic fiber. Material properties are given in Table 1.
Cement
The cement used for this investigation was OPC-53 grade Birla cement. The specific gravity of the cement was found to be 3.11 and it is conforming to IS 269-1979.
Fine aggregate
The fine aggregate used for all the specimens was complying with IS 383- 1970. The specific gravity of fine aggregate was 2.52, sieve analyses were conducted and it was found that the sand used was conforming to zone II grading. The fineness modulus of fine aggregate was 2.074.
Coarse aggregate
The coarse aggregate used was hard broken stone drawn from an approved quarry. Mean size of 20mm was used. The specific gravity of coarse aggregate was 2.73. And it was confirming to IS 383 – 1970
Water
Portable water available in the laboratory was used for casting all the specimens in this investigation. The quality of water was found to .satisfy the requirements of IS 456- 2000.
Synthetic Fiber (triangular polystor fiber)
The fiber used is a 12mm long ‘Virgin triangular monofilament’ Polystor, with an aspect ratio of < 360. For a mean sized aggregate of 20mm, 12mm fiber length is adequate. (Fig. 1)
Super plasticizer
Commercially available super plasticizer having a specific gravity of 1.2 at 25 degree centigrade. Desired Slump was 75 mm + – 25 mm for better workability
Mix design
In this study, Indian standard recommended method (IS 10262-1982) has been adopted for the mix design. The mix proportion adopted for concrete is 1: 1.238:2.917 with wlc ratio of 0.4 for a desired slump of 75mm +1- 25mm. All the samples are prepared from the desired mix. The volume of fiber added is 0.25% of weight of cement. Details of mix design is given in Table 2.
Testing procedure
Split Tensile Strength Test
The test was conducted as per IS 5816-1970. The test was carried out by placing the cylindrical specimen of diameter 150 mm and height 300 mm, horizontally between the loading surface of a compressive testing machine and the load was applied until failure of the cylinder along the vertical diameter. The maximum load applied was noted down.
Flexural Test
The test was conducted as per IS 516-1959. Beams of size 100 x 100 x 500 mm were used for the determination of flexural strength. The test was conducted using the universal testing machine adopting two points loading. The specimen was positioned in the testing reaching and a steel I section beam for transferring the concentrated load as the two point load (1/3 each other) was kept over the concrete beam. The supporting length of the prisms was fixed at 400 mm and load was applied up to final failure of the specimen.(Fig.2)
The test was conducted using compressometer as per IS516 – 1959. The cylinder of standard size 300 mm height and 150 mm dia were used to find the modulus of elasticity(Fig. 3). Specimens were placed on UTM of 100 tons capacity without eccentricity and uniform load was applied till the target load failure of the cylinder. The target load and deflection were noted and modulus of elasticity was obtained. The original length of the compressometer is 150mm. The deflection readings are change in length, from that the strain was calculated. For finding young’s modulus of concrete, the deformation of various loads was observed and the results are plotted graphically against the stress. Using the stress strain curve tangent in drawn and modulus of elasticity is found.
Test results
Split Tensile Strength
The cylinder specimens are cast and tested for split tensile strength as per IS 5816-1970 using compression testing machine of capacity 300 tonne. Table 3 gives the test results. This test was conducted as per IS 516-1959 on prisms of standard size I00 x l00 x 500 mm. Tests were carried out in universal testing machine. The-supporting length of the prisms was fixed at 400mm with two points loading at l/3rd distance with each other. Two uniform point loads were applied and the maximum failure load was noted. The modulus of rupture was calculated. Results are given in Table 4. Modulus of Elasticity
Youngs Modulus of Concrete Cylinder
The test was onducted using compressometer as per IS5516-1959. The cylinder of standard size 300 mm height and 150 mm dia were used to find the modulus of electricity. Specimens were placed on UTM of 100 tonne capacity without eccentricity and uniform load was pplied till the target load failure of the cylinder. The target load and defelection were noted and modulus of elasticity was obtained. The original length of the compressometer is 150 m. the defelection readings are in lengt, from that the starain was calculated.
(or) Young’s Modulus Of Concrete
The cylinder specimen is casted and tested for young’s modulus, using UTM of capacity of 100 tons. Results are given in Table 5.
Comparison of Results and Discussions Test results of the specimens are compared and the discussion is made from test results. The fibers concrete is then compared to the conventional concrete.
Split Tensile Strength
The split tensile strength is increased by 30.3 per cent for triangular polystor fibre reinforced concrete over plain concrete. The flexural tensile strength is increased by 17.93 per cent for the triangular Polyester Fibre reinforced concrete over the plain one.
Young’s Modulus of cylinder specimen
The Young’s Modulus is increased by 4.38 per cent for the Triangular Polyester Fibre Reinforced Concrete over plain concrete.
Conclusions
The conclusions of the above metioned tests are given as follows:
• Addition of triangular polyester fiber in concrete increases the split tensile strength at 28 days by 30.3 per cent at the fibre dosage of 0.25 per cent by weight of cement.
• Due to addition of triangular polyester fibre, the flexural strength is increased by 17.86 per cent compared with conventional concrete.
• Stress- Strain Curve for cylinder specimens of Normal v/s Fiber Concrete is given in Fig 4 and 5.