ABSTRACT
This paper presents an experimental investigation on Euphorbia fibres as concrete reinforcement. The possibility of improving the mechanical characteristics of concrete through the use of reinforcement plant fibres has provoked in recent years a special interest for this new construction material, especially in those areas where plant fibres can easily be found and consequently have a low price like Kenya. In this study, the influences of addition of euphorbia fibres on properties of fresh and hardened concrete were carefully investigated. It was found out addition of these fibres in concrete significantly improved the tensile strength and the flexural strength of the composite. The test results also revealed that the compressive strength of the concrete was slightly improved. It was illustrated that plain concrete possesses a very low tensile strength, limited ductility and little resistance to cracking. Conversely, the concrete with short randomly spread euphorbia fibres in it was found to have relatively high tensile strength, high ductility and more resistant to cracking. Finally, the results presented suggest that Euphorbia fibres can be used in concrete reinforcement
Keyword: Euphorbia fibres, reinforcements, concrete, cracking, compressive strength, tensile strength
1.0 INTRODUCTION
Concrete has been proved to be an important construction material for more than a century all over the world. However, concrete is relatively brittle, and its tensile strength is typically only about one tenth of its compressive strength (Brown et al, 2002). Regular concrete is therefore normally reinforced with steel reinforcing bars. For many applications, it is becoming increasingly popular to reinforce the concrete with small, randomly distributed fibres. Their main purpose is to increase the energy absorption capacity and toughness of the material, but also increase tensile and flexural strength of concrete (Nemati, 2012).
Nemati (2012) defines fibre reinforced concrete (FRC) as a concrete containing hydraulic cement, water, fine or fine and coarse aggregate and discontinuous discrete fibres. In FRC, thousands of small fibres are dispersed and distributed randomly in the concrete during mixing, to improve some concrete properties. Fibres help to improve the post peak ductility performance, pre-crack tensile strength, fatigue strength, impact strength and eliminate temperature and shrinkage cracks. Several different types of fibres, both manmade and natural, have been incorporated into concrete in recent researches and applications of fibre reinforced concrete.
Use of natural fibres in concrete precedes the advent of conventional reinforced concrete in historical context. Early evidence of use natural fibre in reinforcement of building material is found in the Holy Bible in the book of Exodus 5:7 where Egyptians used straw in making mud bricks. However, the technical aspects of FRC systems remained essentially undeveloped. Since the dawn of fibre reinforcing of concrete in the 1940's, a great deal of testing has been conducted on the various fibrous materials to determine the actual characteristics and advantages for each product (Brown et al, 2012). Several different types of fibres have been used to reinforce the cement-based matrices. The choice of fibres varies from synthetic organic materials such as polypropylene or carbon, synthetic inorganic such as steel or glass, natural organic such as cellulose or sisal to natural inorganic asbestos. Currently the commercial products are reinforced with steel, glass, polyester and polypropylene fibres. The selection of the type of fibres is guided by the properties of the fibres such as diameter, specific gravity, young’s modulus, tensile strength and the extent these fibres affect the properties of the cement matrix.
Euphorbia tirucalli is probably the best known and most widespread of all Euphorbia tree species (Mwine J., Damme P., 2011). In Kenya it is commonly used for hedging due to its low herbivore pressure. This tree has wide range of habitats. It is also able to grow in areas where other plants cannot thrive. The stem and the branches of this tree are usually succulent and fibrous.
This study will aim at investigating the suitability of the fibres obtained from E. tirucalli stem as a reinforcing material in FRC.
2.0 FINDINGS FROM PREVIOUS RESEARCH
Over the past two decades intensive research and development work has taken place in the field of fibre reinforced concrete (FRC). This has seen knowledge on fibre manufacturing, fibre handling, and fibre concrete production increase substantially and various aspects of their applications studied to great extent that now can be found in various written literature. Previously, asbestos fibre reinforced concrete has been extensively used due to its durability, strength and economy (Swamy, 1990). However, recent researches have shown asbestos to be carcinogenic. This has led to the decline in asbestos use and in its place natural fibres are being adopted. Use of natural fibres in FRC can save energy, conserve scarce resources and protect environment whilst alleviating the housing problem and enhancing the country’s infrastructure (Swamy, 1990).
Nemati (2012) defines fibre-reinforced concrete (FRC) as concrete containing hydraulic cement, water, fine or fine and coarse aggregate and discontinuous discrete fibres. It may also contain pozzolans and other admixtures commonly used in conventional concrete. Fibres of various shapes and sizes produced from steel, plastic, glass, and natural materials are being used today; however, for most structural and nonstructural purposes, steel fibre is the most commonly used of all the fibres (Nemati, 2012).
There is an increasing worldwide interest in utilizing fibre reinforced concrete structures for civil infrastructure applications. The holding of international symposium on Vegetables Plants and their Fibres as Building Materials in Salvador, Bahia, Brazil from 17-21 September, 1990 serves as an indicator of the interest natural fibres have generated as building materials in the recent past.
Reinforcing the concrete with fibres substantially alters the properties of the non-reinforced cement-based matrix which is brittle in nature, possesses little tensile strength compared to the inherent compressive strength. The principal reason for incorporating fibres into a cement matrix is to increase the toughness and tensile strength, and improve the cracking deformation characteristics of the resultant composite. In order for fibre reinforced concrete (FRC) to be a viable construction material, it must be able to compete economically with existing reinforcing systems. Only a few of the possible hundreds of fibre types have been found suitable for commercial applications (Brown et al, 2002).
There are several properties that a good reinforcing fibre must have to be effective: a) The fibre must be much stronger than the concrete matrix in tension, since the load bearing area is much less than the matrix. b) For ductility improvements, the fibre must be able to withstand strains much greater than the matrix. Fibres subject to creep have a reduced effectiveness. c) Higher elastic modulus than the matrix. If the elastic modulus of the fibres is less than that of the concrete matrix, the fibres will contribute relatively little to the concrete behavior until after cracking. In addition, the composite strain after cracking will be higher. d) Fibres must have a high aspect ratio, i.e. they must be long relative to their diameter.
This project deals specifically with the concrete reinforced with the Euphorbia fibres. The objective of this research is to explore the properties of Euphorbia fibres in specific environments to which the commercial FRCs are exposed.
3.0 RESEARCH METHODS AND RESULTS
Experimental studies were performed to determine the suitability of euphorbia fibre as concrete reinforcement. The project involved investigating the effects of these fibres on properties of both fresh and hardened concrete, and finding out the optimal euphorbia fibres volume in the concrete that would ensure improvement of these properties of concrete. The aggregates that were used in the research had a maximum size of 20mm. A mix design of class 25 was adopted and the laboratory test methods were considered in this study for the fresh and hardened concrete properties. Hardened concrete properties were tested at 7, and 28days. For each test, three samples were tested and the averaged results were considered. A control test was also done for each test.
3.1 Euphorbia fibre Preparation
The euphorbia fibres used were extracted from the bark of Euphorbia trees by mechanical decortications. A lot of care was taken to avoid eye contact with the plant sap which is considered poisonous. The fibres were then allowed to dry for 3 days under the shade before being used in the concrete mix.
3.2 Batching
A mix design of concrete class 25 was done with water cement ratio of 0.6. The batching was done by weight according to the mix ratios which was obtained from the mix design results. The proportion of euphorbia fibre added was varied by percentage volume of concrete batched and ranged from 0.20-1%. The various weights of each material were clearly specified in the mix design.
3.3 Concrete mixing
The cement and fine aggregates were placed first in the mixing tray and mixed dry until a uniform colour was obtained. Coarse aggregates were then added and mixed together to attain homogeneity. The euphorbia fibres were added and mixed thoroughly using a trowel .Water was then sprinkled and mixed to form a homogeneous and a workable composite.
3.4 Concrete Testing a) Slump test.
This test was done to determine the workability of the concrete. The slump of the concrete used was recorded for each mix. The slump test was performed with compliance to BS 1881-102:1983.
Slump results for each concrete mix were as follows,
Table 1: Slump test results Euphorbia fibre content, % | 0% | 0.20% | 0.40% | 0.60% | 0.80% | 1.00% | Slump, mm | 21.2 | 16.5 | 14 | 11.5 | 10 | 9.3 |
Figure 1: A graph of concrete slump against fibre content
The graph shown above (Fig 6) indicates that there was a reduction in slump with increase in the amount of fibre reinforcement. This was due to an increase in surface area and water absorption of the euphorbia fibres with the addition of fibres in concrete mix.
b) Concrete density
The results recorded indicated a slight increase in concrete density with increase in fibre content up to 0.8% as illustrated in the table and the graph below.
The increase in bulk density may be due to the good homogeny and high compaction between the fibres and the cement matrix resulting to the decrease in air void and low porosity. However, as the fibre content exceeds the value of 0.8%, the bulk density decreases due to balling of fibres which introduce voids in the concrete specimen.
c) Compressive strength
For every percentage of fibre added, 6 cube specimens of 100 x 100 x 100mm were cast. Three of them were tested at the age of 7 days, and at 28 days after curing. The control cubes had no fibre reinforcements. The compressive strength test was performed as stipulated in BS EN12390-3:2002.
Compressive test results were as follows,
Table 3: Compressive test results Euphorbia fibre content | Density(Kg/m3) | 7 day strength (N/mm2) | Density(Kg/m3) | 28 day strength (N/mm2) | 0% | 2388 | 16 | 2398 | 24.5 | 0.20% | 2391 | 16.3 | 2397 | 24.7 | 0.40% | 2399 | 16.5 | 2407 | 25 | 0.60% | 2402 | 16.8 | 2403 | 25.3 | 0.80% | 2418 | 16.8 | 2414 | 25.5 | 1.00% | 2397 | 16.7 | 2395 | 25 |
Figure 3: Graph of 7 and 28 day compressive strength
The test was carried out on cubes measuring 100mm by 100mm by 100mm.
The graph above (fig 7) shows that compressive strength increased gradually with increase in the fibre reinforcement up to 0.8% before it started to decline. The effect of euphorbia on compressive strength is not so much as you see a slight difference in the strength of control mix and 0.8% fibre content which recorded the highest strengths in the 7 and 28 day curing. This is because concrete is strong in compression thus effect of euphorbia in its compressive strength was not significant. The slight increase in compressive strength with addition of euphorbia fibres could be explained by the fact that addition of fibres in concrete in small quantities improved the mixing and bonding of concrete ingredients. The later decline in compressive strength explains that high fibre content led to formation of balls which entrapped air in the concrete introducing voids and zones of weakness in the cast specimen.
d) Flexural strength
Flexural strength is one of the measures of tensile strength of concrete. It is a measure of the concrete test beam to resist failure in bending. The test specimens were three beams prepared as per BS 1881-109:1983 (for particular fibre content). The flexural strength test was done as stipulated in BS EN 12390-5:2000 - Two points loading. The maximum load at fracture was read. All the specimens failed within the middle third of the beam. The flexural strength of the specimen was thus computed using the formula;
= PLbd2 ……………. Equation 2
Where:
R = modulus of rupture, kPa
P = maximum applied load indicated by the testing machine, N l = span length, mm b = average width of specimen (mm) d = average depth of specimen (mm)
Flexural test results were as follows;
Table 4: Modulus of rapture results Fiber content, % | 0% | 0.20% | 0.40% | 0.60% | 0.80% | 1.00% | Flexural strength, N/mm2 | 3.62 | 3.66 | 3.75 | 3.85 | 3.99 | 3.95 |
Figure 4: Graph of flexural strength against fibre content
A graph of fibre content against flexural strength of the beams tested at 28 days is shown above. The results show that the amount of fibres used influence the flexural strength performance of the beams. The strength increases with the increment of fibre used but slightly reduce when the fibre content exceeds 0.8% where balling problem was apparent and contributed to lack of bonding between the matrix and the fibres.
Table 5: Beam deflections results Fiber content | 0% | 0.20% | 0.40% | 0.60% | 0.80% | 1.00% | Deflection, mm | 0.3 | 0.49 | 0.69 | 0.85 | 0.96 | 0.89 |
Figure 5: Deflection against fibre content
Central deflection and ductility also improved for fibre reinforced specimens as compared to the control. Specimens with 0.8% fibre show the highest central deflection. The large central deflections carried by the composites compared to control specimens also imply high surface tensile strains.
Cracking behavior of euphorbia fibre reinforced concrete also improved with formation of a number of cracks at failure compared to a single crack for the control specimen. Majority of the specimens failed by fibre rapture. This indicates a good bonding between the fibres and the matrix. e) Tensile splitting strength
This test is of considerable importance in resisting cracking due to changes in moisture content or temperature. A split test was carried out on three concrete cylinders (for each fibre content) to determine the horizontal tensile strength. These concrete cylinders were prepared in accordance to BS 1881-110:1983. The tensile splitting strength testing of these cylinders was done as required in BS 1881-117:1983. The maximum load at failure was observed and recorded.
Tensile strength test was carried out on cylinders measuring 100mm diameter and 200mm length. The tensile strength was computed as follows;
Tensile strength, T=2Fπld …………… Equation 2 Where; F = load at failure in Newton l = length of the cylinder in mm d = diameter of the cylinder in mm
Tensile test results were as follows;
Table 6: Tensile splitting test results Euphorbia fibre content | Density(Kg/m3) | 7 day strength(N/mm2) | Density(Kg/m3) | 28 day strength(N/mm2) | 0% | 2336 | 1.22 | 2330 | 2.18 | 0.20% | 2310 | 1.32 | 2333 | 2.28 | 0.40% | 2309 | 1.32 | 2315 | 2.34 | 0.60% | 2327 | 1.38 | 2324 | 2.44 | 0.80% | 2340 | 1.48 | 2339 | 2.55 | 1.00% | 2342 | 1.47 | 2320 | 2.5 |
Figure 6: Graph of 7 and 28 day tensile strength
The chart above shows an increase in tensile strength of concrete with addition of fibres up to 0.8% fibre content. The increase in tensile strength is due the fact that fibres are stronger in tension than concrete and their addition improves the tensile strength of the composite. The decline in tensile strength afterwards can be attributed to the decreased workability of concrete due to clumping of fibres thus a strong matrix was not formed.
5.0 CONCLUSION AND RECOMMENDATIONS
The main objective of this research was to determine the suitability of Euphorbia fibre for use as concrete reinforcement. The use of euphorbia fibres to reinforce concrete looks promising especially in developing countries where natural fibres can be obtained in abundance and at a low cost. The study provides evidence that addition of these fibres to concrete improves its mechanical characteristics like ductility, tensile strength, flexural and compressive strength. However, addition of fibres in concrete also reduces workability of concrete making mixing, placing and finishing of concrete difficult. Euphorbia fibres of 0.8% content by volume depicted the most desirable properties of concrete mix design tested. Fibre content beyond this showed clumping behaviour and high water absorption. This lead to decline in concrete workability and honeycombing of cast concrete. The hardened concrete showed decline in compressive strength, tensile and flexural strength at fibre content of 1%.
It can therefore be concluded that euphorbia fibres can be used in concrete reinforcement to improve it strength performance at 0.8% by volume.
It is recommended that further studies to be carried out to determine the long term behaviour of euphorbia fibres reinforced concrete especially in alkaline environment and to determine their susceptibility to fungal and insect attack. A study should also be done to determine whether there exists variability of properties amongst the euphorbia fibres depending on the age of the fibre. This research investigated the effect of euphorbia fibre reinforcement in a class 25 concrete mix. It is recommended that other concrete classes should be tried out too. The researcher also encourages further investigation on different types of natural fibres that are locally available for their suitability as concrete reinforcements.
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[ 2 ]. Graduate civil engineer, Jomo Kenyatta University of Agriculture and Technology, Nairobi, Kenya.
Email: muriets@gmail.com
