4. Natural fiber

Information on mechanical properties of unprocessed natural fibers is available. In this section, a brief summary of the results of research to determine the mechanical properties of various types of unprocessed natural fibers is presented. The types of fibers for which the mechanical properties have been evaluated are given in the table below. A brief description for some of the more commonly found natural fibers is presented:

a. Coconut fiber: A mature coconut has an outer covering made of fibrous material. This part of the coconut, called the husk, consists of a hard skin and a large amount of fibers embedded in a soft material. The fibers can be extracted simply by soaking the husk in water to decompose the soft material surrounding the fibers. This process, called retting, is widely used in the less developed countries. Alternatively, a mechanical process can be used to separate the fibers. Coconut cultivation is restricted to the tropical regions of Africa, Asia, and Central America.

b. Sisal fiber: In Australia, sisal fibers have been successfully used for making gypsum plaster sheets. A considerable amount of research has been carried out in Sweden for developing good quality concrete products reinforced with sisal fibers. These fibers are stronger than most of the other natural fibers, as can be seen from the table below.

c. Sugar cane bagasse fiber. Sugar cane is cultivated in both tropical and sub-tropical regions. Sugar cane bagasse is the residue remaining after the extraction of the juice and contains about 50 percent fiber and 30 percent pith with moisture and soluble solids constituting the remaining 20 %. In order to obtain good quality fibers, the pith and other solids are removed from the fibers. The properties of bagasse fibers depend, to a very large extent, on the variety of the sugar cane, its maturity, and on the efficiency of the milling plant. The properties given in the table below are considered to be typical.

d. Bamboo fiber: Bamboo belongs to the grass family and can grow to a height of 15 m with diameters varying within the range of 25 to 100 mm. It grows naturally in tropical and sub-tropical regions. Dried bamboo stems are commonly used for building temporary structures such as scaffolding. They may also be fabricated to form a continuous reinforcing material for concrete. Bamboo fibers are strong in tension (table below) and can be used as a reinforcing material. However, they have a high water absorption capacity, low modulus of elasticity, and special equipment may be needed to extract them from the stems.

e. Jute fiber: Jute is grown mainly in India, Bangladesh, China, and Thailand. It is grown solely for its fiber, which is traditionally used for making ropes and bags to transport grains and other materials ranging from cement to sugar. Strong in tension (table below), jute fiber can also be used in a cement matrix. The process of obtaining jute fibers is very simple. Mature plants are cut and soaked in water for about four weeks, which completely decomposes the bark. The fibers thus exposed are then stripped from the stem, washed, and dried.

f. Flax: Flax is a slender and erect plant grown mainly for its fiber. Both the tensile strength and the modulus of elasticity of flax are extremely high compared to those of other natural fibers, as may be seen from the table below.

g. Other vegetable fibers: Of the various vegetable fibers, only a few have been found to be potentially suitable as reinforcing materials. The mechanical properties of the more promising fibers, namely elephant grass, water reed, plantain, and musamba, are listed. Investigations have also been carried out to explore the possibility of using other natural fibers such as palm fiber and akwara fiber as reinforcing materials for concrete. These fibers are usually removed manually from the stem of the plant.