Concrete is the most widely used construction material. Apart from its excellent properties concrete is very poor in tensile strength. To improve its tensile strength, fibers are added to concrete which is known as fiber added concrete. In our study we are focusing on structural behaviour of coir fiber added concrete. Coir is abundantly available at low cost in the tropical areas especially Kerala. In this study coir fiber of aspect ratio 50 is randomly dispersed in M20 concrete for the preparation of 48 specimens. Two different varieties of concrete with different fiber cement ratio (0.5% and 1.5%) were casted. The specimens were tested for its compressive strength, splitting tensile strength, flexural strength and modulus of elasticity. From the test results obtained it can be seen that compressive strength, splitting tensile strength, flexural strength are greater than those of PCC. The optimum percentage f/c ratio may be occurring very near to 0.5%. From the cost benefit analysis coir fiber is proved to be far economical than any other fibers for a strength comparable to that obtained for 0.5% fiber cement ratio coir fiber added concrete. This study also focuses on the comparative study among presently using fiber added concrete which shows their structural strength and cost of production.
Occurrence, Distribution and Type of Coir
Coir is a versatile hard fiber obtained from the husk of coconut. Coir fibers are categorized in two ways. One distinction is based on whether they are recovered from ripe or immature coconut husks. The husks of fully ripened coconuts yield brown coir which is strong and highly resistant to abrasion, its method of processing also protects it from the damaging ultraviolet component of sunlight. On the other hand, white coir comes from the husks of coconuts harvested shortly before they ripen. Actually light brown or white in color, this fiber is softer and less strong than brown coir. Both brown and white coir consist of fibers ranging in length from 4-12 in (10-30 cm). It is the only natural fiber resistant to salt water and it is highly resistant to abrasion. It is Strong and nearly impervious to the weather.
Coir fiber is in great demand for its toughness, strength, resilience to dampness, rot resistance, durability, natural resilience, porous, hygroscopic, biodegradable, renewable, recyclable etc. it can withstand huge amount of weight and rubbing and recovers as soon as the weight is removed from it.
The coir fiber is one of the hardest natural fibers because of its high content of lignin. Coir is much more advantageous in different applications for erosion control, reinforcement and stabilization of soil and is preferred to any other natural fibers. Of all natural fibers coir processes the greatest tearing strength, retained as such even in very wet conditions. The chemical constituents have found to be cellulose, lignin, hemi cellulose and pectin. Higher lignin content makes the fiber stiffer and tougher. The physical and chemical properties of coir are shown in table I and II. Coir fiber is much coarser than most of the other natural fibers. However, the extent of elongation of coir is not approached by any of the best fiber and this fact is of greater advantage in its utilization in the preparation of materials which are expected to withstand the stresses resulting from operation involving repeated tension, bending and relaxation.
Results and Discussion
Concrete mix of various fiber cement ratios that is 0.5% and 1.5% was made. Six specimens of concrete cubes for compressive strength test, six beams for flexural strength test, twelve cylinders for splitting tensile strength and six cylinders for modulus of elasticity test were casted, cured and tested for each fiber cement ratio varieties.
Splitting tensile strength of the ordinary concrete was increased while adding coir fiber to the concrete. The maximum tensile strength that can be obtained by coir fiber added concrete may be very near to 3.14 N/mm2 which is about 9% more than the ordinary concrete. This strength is obtained when the fiber cement ratio is 0.5% while the same strength can be obtained by adding 0.5 volume fraction of steel fibers into the ordinary concrete.
Compressive strength of the coir fiber added concrete also increases with increase in the fiber content to a maximum value and then decreases. Compressive strength is never decreased by the addition of the coir into the concrete. A 9.15 % increase in the compressive strength is obtained as maximum increase for the f/c ratio of 0.5%, Which is Comparable with 1.0% volume fraction of Steel fiber added concrete.
Flexural strength of the ordinary concrete increased up to 4.75 N/mm2 by the addition of coir fiber into the concrete with 0.5% f/c ratio which shows an increase of 46.15 % in flexural strength which is comparable to steel fiber added concrete whose increase percentage is 48%.
Modulus of elasticity of the coir fiber added concrete composite is higher than the ordinary concrete. Modulus of elasticity is increased up to 3.384x 104 N/mm2 for the f/c ratio of 0.5%. This increase in modulus of elasticity may be due to the growth of cement hydration products within the hollow cellulose fibers which lead to excessive fiber to matrix bonding. Increase in the modulus of elasticity may be due to the tension stiffening behaviour of fiber added concrete. As the modulus of elasticity increases the tension stiffening by fibers also increases.
Strength of the coir fiber added concrete increases with the fiber content at first and then it is reduced. Maximum value for the compressive strength, tensile strength, flexural strength was obtained for the f/c ratio of 0.5%. So it can be inferred from the study that optimum f/c ratio of coir fiber to the concrete may be 0.5%.
Workability of the fresh concrete is getting reduced with the increase in the fiber content. But at the most efficient f/c ratio that is f/c = 0.5% decrease in the slump value is very less.
Coir fiber added concrete is more cost effective than any other fiber added concrete. Cost for the production of coir fiber added concrete of strength equal to the strength obtained by 0.5% volume fraction of steel fiber added concrete, polymer fiber added concrete and glass fiber added concrete is negligible.
From the testing of the specimen it can be seen that even after breaking load the concrete did not break more over it took more and more load. This is due to the post cracking integrity of the fiber added concrete which is the property of any fiber added concrete. It can be seen that the post failure integrity or post cracking strength increases with increase in fiber content. But we can see that an increase in fiber content beyond a limit will cause a decrease in strength of concrete. So the post failure behaviour and strength of concrete need to be compromised for an optimum fiber cement ratio.
The increase in the strength may be due the bridging of fibers and redistribution of moments. The splitting tensile strength and flexural strength increases as the fibers incorporated in the concrete matrix absorb the deformation energy and make the fiber added concrete a pseudo ductile material to an extent.