Strength and Fracture of Earth-Based and Natural Fiber-Reinforced Composites
SCOPE OF WORK
This study examines the effect of processing, composition and natural fiber reinforcement on the strength and fracture toughness of natural fiber-reinforced earth-based composites. The study includes:
1.The processing of local (earth-based) materials with different compositions;
2.The material characterization of local materials and processed materials with:
- X-ray diffraction (XRD) analysis;
- Scanning electron microscopy (SEM);
- Energy Dispersion X-ray Spectroscopy (EDS).
3. The measurements of the strength of the samples produced. In this study, two (2) forms of strength will be considered. These are:
4. The determination of the fracture toughness of the different samples produced, and
5. A study of toughening mechanism using indentation crack growth.
Earth-based materials are naturally occurring materials found on the earth. They are vital resources (raw materials) that provide the basic component of life, agriculture and industry. These materials include minerals, rocks, soil and water. This study focus on soils which could be used as alternative building materials. The earth-based materials to be studied include laterite, clay and straw.
Laterites are soil type rich in iron and aluminum. They are formed in hot and wet tropical areas. Nearly all laterites are rusty-red because of the iron oxide contained in them. They develop by intensive long-lasting tropical weathering of the underlying parent rocks. Tropical weathering (laterization) is a prolonged process of mechanical and chemical weathering which produce a wide variety in the thickness, grade, chemistry and mineralogy of the resulting soils. Laterites cover about one-third of the earth’s continental land area, with the majority of that in the land areas between the tropics of Cancer and Capricorn.
Francis Buchanan Hamilton first described and named laterite formation in southern India in 1807. He named it laterite from the Latin word “Later”, which means a brick. Thick rock can be easily cut into brick-shaped blocks for building (Thurston, 1913).
Historically, laterite was cut into brick-like shapes and used in monument buildings.
When moist, laterites can be easily cut into regular-sized blocks. Upon exposure to air, it gradually hardens as the moisture between the particles evaporates and the larger iron salts lock into a rigid lattice structure and become resistant to atmospheric conditions (Tardy, 1997). The act of quarrying laterite material into masonry is suspected to have been introduced from the Indian subcontinent.
After 1000 CE, construction at Angkor Wat and other south-east Asian sites changed to rectangular temple enclosures made of laterite bricks and stones. Since the mid-1970s, trial sections of bituminous-surfaced low volume roads have used laterite in place of stones as base course (Grace, 1991). Thick laterite layers are porous and slightly permeable, so layers can function as aquifers in rural areas. Locally available laterites are used in an acid solution followed by precipitation to remove phosphorous and heavy metal at sewage treatment facilities (Wood et al, 1996).
MATERIALS AND METHODS
The earth-based materials used in this study were collected directly from their deposition sites in Ogun state, South-West Nigeria. These materials are laterite, clay and straw. The laterite and clay were obtained from their deposition sites and then processed before they were used in the sample preparation. The laterite was obtained from Obada as wet soil. It was sun dried for three days and then sieved without crushing. The clay used was obtained from Olurunda Ayetoro. It was also acquired as wet soil with particles bonded together forming different sizes of moulds. These large moulds were broken into smaller pieces and sun dried for two weeks. They were then crushed into fine particles and sieved. The sieve used in the soil preparation had a pore size of 6 mm by 6 mm.
The straw used was obtained from an abandoned farmland at Ibara Orile and was also sundried to remove leaves in it. In addition, a bag of Portland cement (produced by West African Portland Cement Company Plc, Lagos, Nigeria) was obtained from a cement store. Figure 3.00 shows the pictures of the earth materials used.
RESULTS AND DISCUSSION
STRUCTURE OF MATERIALS/CONSTITUENTS
The XRD and EDS patterns obtained are presented below in Figures 4.00 and 4.04. The laterite consisted predominantly of potassium aluminum silicate hydroxide KAl2(Si3Al)O10(OH2) –illite.
CONCLUSIONS AND FUTURE WORK
Composites consisting of earth-based materials reinforced with natural fiber (straw), and plain matrices were prepared. The mechanical properties of the various compositions (in both matrices and composites) were determined. The results in both cases were compared to verify the effects of reinforcement. The mechanical properties were determined with the aid of a universal testing machine. In addition, the results obtained were compared to that of fired clay. Scanning electron microscopy was used to characterize the surface morphology of the prepared samples. X-ray diffraction and energy dispersive X-ray spectroscopy were carried out to provide information on the structure and compositions of the samples.
Fiber-reinforcement resulted in an increase in compressive strength from 2.57 MPa to 2.91 MPa at maximum compressive strength. Interestingly, a plain laterite sample had a compressive strength of ~3.03 MPa. This value was the closest to that of fired clay (4.95 MPa). Samples reinforced with straw fibres had increased flexural strengths and fracture toughness. Composites with fiber volume percentage of ~20% had flexural strengths and fracture toughness values up to 34.4 ± 1.5 MPa and 1.41 ± 0.13 MPa 𝑚 respectively. These values exceed those obtained for plain matrices (25.9 ± 2.4 MPa and 1.08 ± 0.04 MPa 𝑚) at the same fibre composition.
The measured matrix strengths are consistent with reactions that are revealed by the XRD analyses. Similarly, the composite strengths are consistent with the good bonding between the straw fibers and the matrix materials.
SUGGESTIONS FOR FUTURE WORK
- There is a need to study the underlying toughening mechanism and resistance- curve behavior of the different materials using techniques that are similar to those used in prior work on natural fiber-reinforced cementitious matrix composites (Savasrano et at., 2003). There is also a need to model the resistance-curve behavior using crack-tip shielding
- There is the potential to extend the current work to materials produced from black cotton soil and bentonitic rocks. Other natural fibers could also be studied to provide a range of composite materials for potential applications in affordable buildings.
- There is the need to study the potential degradation in the mechanical properties as a function of environmental exposure and cyclic Loading.
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