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Use of Coconut Husk Fiber for Improved
Compressive and Flexural Strength of Con- crete
1Anthony Nkem Ede and 2Joshua Olaoluwa Agbede
Abstract— Rapid crack propagation, brittle mode of failure and increased overload are common in concrete structures due to the low tensile strength of concrete. Although conventional steel reinforced concrete is the most popular method developed to reduce such problems, it is rather becoming expensive in terms of its costs and sustainability issues. Because of the huge capital investment to run the steel industry, many manufacturers in the developing nations try to cut corners by reducing the quality of steel thereby reducing the strength. This has led to a lot of challenges including building collapse accompanied by devastating economic and human loss. For these drawbacks, the development of contemporary concrete technologies such as eco-friendly and affordable coconut fiber reinforced concrete needs more investigation. This research studies the effect of coconut fibers on the strength of concrete which includes the compressive and the flexural strength of normal concrete. The fibers were used in different percentages (0%, 0.25%, 0.5%, 0.75%, and 1.0%) of the weight of the fine aggregates. 16 short beams were used for flexural strength at 0%, 0.5% and 1.0% fiber content which were tested after curing for 7 and 28 days. Destructive and nondestructive compressive tests were conducted on 40 concrete cubes to doubly validate the test results. The correlation of the two tests results were very good. The results showed that the compressive strength of coconut fiber- reinforced concrete increased with curing age and with increasing percentage of coconut fiber up to 0.5% then gradually began to decrease from 0.75% to 1.0%. The percentage strength gained at 28 days for 0.25%, 0.5%, 0.75% and 1.0% fiber contents with respect to the control sample are 4.58%, 38.13%, 8.56% and -2.42% respectively. The results for the flexural strength of concrete showed that strength gained at 28 days for 0.25%, 0.5% and 1.0% of coconut husk fiber were 28.82%, 22.15% and 0.42% respectively.
Index Terms— Coconut Fiber, Compressive Strength, Flexural Strength, Fiber Reinforced Concrete, Nondestructive Test.
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Concrete have been in use since the era of the Roman Empire and the earliest form of concrete was made from quicklime, pozzolana and an aggregate of pumice. Concrete was a new and revolutionary material to the Romans. Recent tests show that the Roman concrete had as much compressive strength as modern day Portland-cement concrete but had tensile strength far lower due to absence of reinforcement. Its mode of applica- tion was also discovered to be different [1]. Modern concrete is made from the mixture of Portland cement, fine and coarse aggregates, admixtures and water. Concrete is the primarily material adopted by the construction industry around the globe because of its fresh plastic state that gives it the ability to be molded into different shapes and affordability.
The usage of concrete worldwide is twice as much as wood, steel, plastics and aluminum put together and is only exceed- ed in the modern world by the usage of naturally occurring water.
Constituent materials of concrete production are at the base of
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1PhD, Lecturer, Depart of Civil Engineering, Covenant University, Canaan
Land, Km 10, Idiroko Road, P.M.B. 1023 Ota, Ogun state, Nigeria.
anthony.ede@covenantuniverity.edu.ng
2Student, Depart of Civil Engineering, Covenant University,
large industries and commercial activities. The concrete pro- duction in the United States alone is over $30 billion per year industry. In Nigeria, it accounts for a very high percentage of large industry/commercial activities as the nation is very fast developing. It is the primary business of the Africa’s richest man, Aliko Dangote. His cement plants are the largest manu- facturer of cement (one of the constituent materials of con- crete) in sub-Saharan Africa [2], [3].
Concrete is known for high compressive strength and low ten- sile strength (about 10% of the compressive strength). For this, ordinary concrete members are unable to resist tensile stresses and therefore is very prone to crack formation. Over the years, concrete technology have evolved through the aid of chemical admixtures and mineral admixtures such as fly ash, slag, etc., leading to increases in the compressive strength while its ten- sile strength is still very low. As a consequence of low tensile stresses, cracks form already under small loads and widen and propagate very quickly. Moreover, cracks are present even before loading start because of shrinkage phenomena. For this reasons, reinforcing bars are usually adopted to obtain rein- forced concrete (RC) structures and this increases the total costs since steel material, labour and control/monitoring op-
erations are expensive.
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The need for this research emanates from the numerous dura- bility problems effecting the Nigerian construction industry [4], such as frequent building collapse [5] confusion on the design strength of reinforcement steel for normal concrete structures due to declining strength of steel bars used in Nige- ria [6]. The British Standard [7] stipulates reinforcement strength 460N/mm2, while lack of integrity is driving manu- facturers to produce steel reinforcement of very low strength which is forcing the design engineers to adopt values of
410N/mm2 and a times as low as 380N/mm2. This makes the
overall strength of Nigerian reinforced concrete structures to be quite low while elsewhere like in Europe, a minimum strength of 500N/mm2 is the norm. The use of fibers rein- forced concrete could help to cover up to a certain extent for the weakness in tensile strength, thereby limiting and imped- ing the number of building collapse in the country and also providing more stability to concrete structures. Therefore, this research carries out various forms of tests and analyses to de- termine the effect of using coconut husk fibers on the com- pressive and flexural strengths of concrete for different per- centage fiber volumes. The major aim of this research is to examine the composite effect of coconut husk fiber on the strength of concrete.
To realize this aim, laboratory tests were carried on concrete cube samples to obtain compressive strengths of concrete us- ing Portland cement, a mix ratio of 1:2:4, and 0%, 0.25%, 0.5%,
0.75% and 1% percentages of coconut husk fiber. Destructive and Non Destructive test with Schmidt Hammer Test were carried out on concrete cubes as to define a correlation be- tween its readings and destructive compressive strength. Flexural tests were carried out 150mm x 150mm x 350mm con- crete beams using same cement type and mix ratio, and 0.25,
0.5 and 1.0 percentages of coconut husk fiber. A correlation between the concrete strength and the percentage of coconut husk was established.
A Fiber is a fine hair like structure of an animal, vegetable, mineral, or synthetic origin. Fibers that are commercially available have diameters ranging from less than 0.004 mm (0.00015 in) to 0.2 mm (0.008 in) and they come in several dif- ferent forms: short fibers (chopped), continuous single fibers (monofilament), untwisted bundles of continuous filaments (tow), and twisted bundles of continuous filaments (yarn). Fibers can be categorized according to their origin or chemical structure. High-strength fibers are used as reinforcements in composites materials and these include steel fibers, glass fi- bers, synthetic fibers and natural fibers [8]. Each of them can lend varying properties to concrete since concrete is a compo- site material per excellence. Fibers have been used as rein-
forcement in concrete since ancient times. Horse hair, asbestos fibers [9] and other alternatives were used and the notion of composite materials have grown over the years [10] causing fiber-reinforced concrete to remain a noteworthy topic of re- search interest.
Synthetic fibers are specifically engineered for concrete and they are made from man-made materials that can withstand the long-term alkaline environment of concrete.
Natural Fibers are made from plant, animal and mineral sources and they come in the form of leaf, skin, fruit and stalk fibers. The increase in the use of synthetic fibers after World War II saw to the significant decrease of natural fibers. Now, with the increased environmental considerations and the need to reduce waste of world natural resources, the use of natural fibers have resurged within the building industries. The inter- est in natural fibers within the building industry is basically technical and economical: technical because of its good insula- tion and safety properties and economical because of its favor- able cost implications.
Coconut husk is the fibrous material found between the hard,
internal shell and the outer coat of coconut. It is fundamental-
ly used in the modern times for the production of floor mats,
doormats, brushes, mattresses etc. while in the past times it
was basically used as ropes and cordage. The known proper-
ties include improves resistance to explosive spalling in case
of fire, improve impact resistance, improve structural strength,
improve mix cohesion, reduction of steel reinforcement re-
quirements and improved ductility.
Coconut fiber is extracted from the outer shell of a coconut. Its scientific name and the plant family of the coconut fiber is “Cocos Nucifera” and “Arecaceae (Palm)” respectively, while it is commonly referred to as “Coir”. The cultivation of coco- nut is concentrated in the tropical belts of Asia and East Africa [11]. There are two types of coconut fibers, white fibers that are obtained from immature coconuts and the brown fibers obtained from matured coconuts. Coconut fibers have low thermal conductivity while being tough and stiff [12]. Accord- ing to [13], Coconut fibers can be easily gotten in Nigeria from local farmers in Badagry area of Lagos and in many Nigerian villages.
The stress-strain relationship for coconut fibers have been re- ported by some researchers [14]. Coconut fiber amongst all natural fibers is the most ductile. Coconut fibers have the ca- pacity of taking strain 4-6 times more than that of other natu- ral fibers as shown in Figs. 1a and 1b.
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Fig. 1. Correlation of mechanical properties for natural fibres, [14], [15].
There are two basic strengths looked out for in construction materials: compressive strength and tensile Strength. The strength of concrete in the hardened state is measured by the compressive strength through compressive test and is perfor- mance measure used by the engineer in structural design. The tensile strength is measured through the split tensile test or flexural tests. Concrete tensile strength is low (about 10% of the compressive strength) and is responsible to for the fre- quent cracks found in concrete structures. Growth of concrete technology have seen the continuous effort of the scientist to improve the tensile strength of concrete structures. In fact, the drive to improve the tensile strength of concrete have been responsible for the various categories of concrete in use worldwide such as mass concrete (no reinforcement), rein- forced concrete (with steel re-bars), pre-stressed concrete (pre- stressed/post-tensioned with tendons) and the contemporary concrete made of various additives (including fiber reinforced concrete) [16]. The quest for improved tensile strength of con- crete is the driving force behind the novelties and new fron- tiers of concrete research globally. The characteristic strength of normal concrete measured at 28 days of age should not be less than 27N/mm2 [7].
The strength of concrete can be considerably increased by the addition of steel reinforcement which has been the norm over
the years, but for concrete to have uniform tensile properties and improved micro cracking resisting behavior short fibers are beneficial. According to Visconti [7], it is possible to obtain a close to isotropic behavior for composite materials (such as concrete) with short fibers as compared to the anisotropic be- havior of uni-direction fibers (such as steel re-bars of rein- forced concrete.) The introduction of fibers in concrete have been increasing the tensile, flexural and compressive strength. Fibers help to stop the initiation, growth and propagation of micro-cracks in concrete thereby increasing the strength, duc- tility and toughness. Many researches have been conducted on the effects of fibers on concrete [18], [19], [20], [21], [22], [23], [24].
The uniform distribution of fibers throughout the concrete mix
provides a near isotropic property not common to convention- ally reinforced concrete. Fiber Reinforced Concrete (FRC) have been widely used in various structural applications such as tunnel lining and slope stabilization, runway, aircraft parking and pavements, blast resistant structures, thin shell, walls, pipes and manholes, dams and hydraulic structures, lighting poles and concrete repairs [25].
All tests were performed at the material testing laboratory of the Department of Civil Engineering, Covenant University Ota, Nigeria. The methodology involved the preparation of specimens of concrete materials and coconut husk fibers for the research. The test samples were prepared using 1:2:4 mix ratio and applying 0%, 0.25%, 0.5%, 0.75% and 1.0% fiber vol- ume. Analysis were carried out on the strength of concrete by performing destructive and nondestructive compressive strength test and a two-point load flexural strength test. The same type of cement was used with fine aggregate (sharp sand), coarse aggregate (granite) and coconut husk fibers. Two cubes each for the five samples of concrete cubes at varying percentage of coconut fiber were used for the compressive test at 7 days, 14 days, 21 days and 28 days of curing. Two beams each for the flexural test of the first, second, third and fifth samples of fiber content were tested at 14 days and 28 days of curing. The average strength were computed for each sample after testing.
The cement used in this experimental investigation is ordinary Portland cement of 42.5 grade (Dangote Cement). The material used as fine aggregate in this project is Sharp Sand. The coarse aggregate used in this research is gravel and they are of
20mm, 10mm and 6mm sizes, crushed angular in shape. Fig. 2 shows the sequence used in this research study.
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Fig. 2. Schematic Diagram Showing the Sequence Used in Facilitating this
Research Study.
A good quality water was used for the concrete mix. The fibers selected for this experimental investigation is the Coconut husk fiber, which could be gotten from any part of Lagos State. They have higher tensile strength as compared to many other natural fibers. The length of the coconut husk fiber was
60 mm and the diameter 0.75 mm meaning the l/d ratio i.e.
aspect ratio will be 80. A water/cement ratio of 0.58 (i.e.
14.5kg) was adopted. For the nondestructive test with Schmidt Rebound Hammer, an average of about 15 readings was taken. Fig. 3 show the Schmidt rebound number chart for compres- sive strength.
Fig. 3. Schmidt rebound number chart for compressive strength.
The destructive compressive test was conducted as per [26]. The cubes of standard size 150x150x150mm were used to find the compressive strength of concrete. Specimens were placed on the bearing surface of UTM, of capacity 100 tones without eccentricity and a uniform rate of loading of 550 kg/cm2 per minute was applied till the failure of the cube. The maximum load was noted and the compressive strength was calculated. A two-point load flexural test was conducted on the beam samples of 150x150x700mm sizes.
The results compares the control samples of concrete without coconut fiber reinforcement and the samples with various per- centages of fibers.
Each of the two opposite faces were impacted to get at least 15 readings to illustrate the sensitiveness of the test to the pres- ence of aggregates and voids immediately underneath the plunger at the vertical position. Average of rebound numbers and standard deviations were calculated and the compressive strength were read off from the chart Fig. 3.
Fig. 4. Bar Chart showing 7, 14, 21 and 28 days rebound no of each per- centage of coconut fibers’
Fig. 4 contains the bar chart showing 7, 14, 21 and 28 days strength read off from the rebound number for each percent- age of coconut fibers.
Fig. 5 shows the rebound number versus compressive strength for each percentage of coconut fibers for 7 to 28 days curing. From the above results, 0.5% has the highest rebound number for all the samples while 1% fiber recorded the lowest rebound
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number as even the control (un-reinforced sample) had a
higher rebound number than it.
Fig. 5. Line Chart showing the rebound no of each percentage content of coconut fibers over 28 days
Fig. 6 contains bar chart showing 7, 14, 21 and 28 days com- pressive strength for the samples. The compressive strength is the average of two cubes at each age of the various samples is shown in figure table 1.
Fig. 7 shows the trend of strength gained by each percentage
of coconut fibers content from 7 to 28 days of curing.
Fig. 8 shows the correlation curve of compressive strength for NDT and destructive test results at 28 days. The trends of the two curves are very compatible and attest to the improve- ments derived from the coconut fiber reinforcement of con- crete. It is observed that there is a higher increase in the value of the compressive strength obtained by destructive testing when compared to the nondestructive tests. The values of the NDT can be easily altered by variety of factors such as the smoothness of the cube surface, the presence of voids in the cubes and aggregate angularity within test areas. However the relationship between both the NDT and the destructive tests as expressed in Fig. 8 is good.
Fig. 9 shows the 14 and 28 days flexural strength for each per- centage of coconut fibers. 0.25% fiber content offered the high- est flexural strength and then followed closely by 0.5% fiber content.
TABLE 1
COMPRESSIVE STRENGTH OF CUBES OF THE VARIOUS SAMPLES
Fig. 6. Bar Chart showing 7, 14, 21 and 28 days compressive strength of each percentage of coconut fibers
Fig. 7. Line Chart showing the trend of strength gained by each percent- age content of coconut fibers over 28 days
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Fig. 8. Shows the correlation curve of compressive strength for NDT and destructive test results at 28 days.
Fig. 9. Bar Chart showing 14 and 28 days flexural strength of each per- centage of coconut fibers.
The differences in the compressive strength and flexural strength with respect to changes in the fiber content can be observed. From the results obtained, it is clear that the com- pressive and flexural strength of concrete is maximum when the fiber content is 0.5% of the fine aggregate of the concrete.
The coconut fiber reinforced concrete with 0.5% fiber in-
creased by 38.13% for compression and 22.15% under flexural stress compared to the normal concrete. The sample with
0.25% fiber content achieved the optimum flexural strength
but has a lower compressive strength gain when compared
0.5% fiber content. This makes 0.5% fiber content the best op- tion to use in construction of concrete structures if coconut fibers are to be considered for compressive and flexural strength enhancement. The increase of fiber content beyond
0.75% will decrease the workability and compaction and thereby reduce the strength of concrete.
The phenomenon of Balling and Lumping of fibers was en- countered during the sample preparation stage. The problem was overcame by sprinkling the fibers after the water, cement and sand had been poured into the concrete mixer before add- ing the coarse aggregates.
This research studied the compressive and flexural strength of coconut fiber reinforced concrete using destructive and non- destructive test methods. Conventional compression tests and Schmidt Hammer Rebounds on cube specimens and two point bending test on short beam specimens with different coconut fiber content were conducted. From the results and analysis of this research work it can be concluded that the addition of a
0.5% coconut husk fiber as a constitutive material of concrete affected the rheological properties of the fresh concrete, in- creased the compressive and flexural strength of concrete by
35.8% and 22.15% respectively. It was verified that coconut fiber content in the excess of 0.75% reduces the workability and drastically weakens the compressive and flexural strength. The presence of coconut fiber significantly improves the toughness and the ductility behavior of concrete. The test results have shown that coconut fiber at 0.5% content is opti- mal for enhancing the rheological and mechanical properties of concrete. This research proves that coconut fiber can be sus- tainably adopted for enhancing the properties of concrete es- pecially in the tropics where this fiber abound and are not economically being put to use in the spirit of waste to wealth. Based on the test results obtained, the integration of coconut fiber in making FRC composite would be one of the promising strategies to improve the performance of concrete. The greater improvement in flexural and toughness behavior of FRC is highly encouraging as the tensile strength enhancement have been one of the biggest challenges of concrete material behav- ior over the years. The improve toughness behavior will allow the coconut fiber FRC to absorb sufficient amount of energy and hence increase the ductility of structural members and give adequate warning before the ultimate capacity is attained and possibly save lives as fragile collapse is avoided. This will help greatly to reduce the incidence of building collapse in Nigeria. Further researches are recommended for fatigue, shear capacity, durability and field applications to confirm the promising test results obtained in this research.
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The authors wish to thank the Chancellor and the Manage- ment of Covenant University for the platform made available for this research
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