**5. Application in reinforced crude bricks by natural fibers as an eco-friendly material**

The emission of greenhouse gases, global warming and environmental concerns associated with modern construction materials have forced us to look for environmentally friendly construction materials. Construction materials such as concrete, cement and fired bricks manufacturing consume a higher amount of energy and are responsible for a considerable amount of CO2 emission. Moreover, these materials have higher thermal conductivity. On other hand, adobe bricks are environmentfriendly building materials manufactured with sediments and waste natural fibers. Dredged sediments and natural fibers are renewable waste materials, easily available in most regions of the world. Disposal of waste fibers and dredged sediments have negative impacts on the environment as they are the source of air, water and land pollution. Dredged sediments and natural fibers valorisation in adobe bricks generates sustainable and green construction material as adobe bricks manufacturing does not consume energy. Adobe bricks are sun-dried and they have a low thermal conductivity which reduces the energy consumption for heating and cooling.

Adobe bricks are the cheap and oldest construction materials, used in historical buildings. Adobe bricks are manufactured with soil and natural fibers. Soil suitability for adobe bricks is observed with standards such as AFNOR and MOPT [40, 41]. Sediment's mineralogy, molding moisture content, compaction energy, clay and sand content have a significant influence on the characteristics of adobe bricks. The strength and durability limitations of adobe bricks are the obstacles to their largescale use. The addition of natural fibers and the use of stabilization techniques reduce these drawbacks to some extent.

Natural fibers are agricultural waste generated by food industries such as sugar cane, palm oil, etc. Natural fibers act as reinforcement in adobe bricks and increase their tensile strength. The low thermal conductivity and density of natural fibers make them suitable additives for adobe bricks and composite materials. Distribution, orientation, quantity and length of fibers are some important parameters that affect the quality and performance of adobe bricks. Fibers are randomly distributed in crude bricks, and it is difficult to control their orientation. The longitudinal distribution of fibers parallel to the bricks axis increases the tensile strength of bricks while their distribution perpendicular to the bricks cross-section does not contribute to the tensile strength of bricks. The quantity of fibers has also a significant impact on the performance of composite materials. The quantity of fibers used in adobe bricks usually varies from 1–5% by mass. The length of fibers is another factor that influences the tensile and compressive strength of bricks. Length of fibers varies with the choice of fibers and available fibers cutting mechanism. The distribution of fibers in bricks is homogenous with fibers of short length. The tensile strength of bricks increases with increasing fibers length however, longer fibers harm the compressive strength of bricks. The presence of knots and weak bond in long fibers affects their performance. Therefore, the use of appropriate length fibers is suggested. The common length range of natural fibers for crude bricks varies from 2 cm to 10 cm in literature studies.

The stabilization of bricks is achieved either by compaction or by using binding materials such as lime, cement and gypsum. As the use of binders involves the cost and contributes to global CO2 emissions. Therefore, bricks stabilization with compaction is a reasonable option. Compaction of bricks can be achieved by static loading, dynamic compaction, and tamping. The compaction of bricks removes the voids inside the bricks and improves their mechanical characteristics along with durability [42].

*Recycling of Tropical Natural Fibers in Building Materials DOI: http://dx.doi.org/10.5772/intechopen.102999*

However, compaction of bricks with dynamic loading and tamping causes the upward movement of fibers along with water which may perturb the distribution of fibers. The durability of adobe bricks is also a challenge as the performance of fibers decreases with time due to degradation. Treatment of fibers improves their characteristics but it has environmental concerns.

Adobe bricks are manufactured by mixing sediments, fibers and water. Molding moisture content varies with the type of sediments and natural fibers. It is important to respect the water absorption coefficient of natural fibers. As water absorption of natural fibers is not instantaneous, prior fibers saturation helps to make homogenous sediments and fibers mixture.

### **5.1 Manufacturing of crude bricks**

Dredged sediments from the Usumacinta River and palm oil flower fibers (POFL) from the Tabasco State of Mexico were investigated to use them in adobe bricks. Usumacinta River sediments (USU) are shown in **Figure 18a**. Sediment's characteristics such as granulometry, density, Atterberg limits, optimum moisture content, carbonate content, pH, XRF and organic matter were analyzed. Sediment characteristics are summarized in **Table 4**.

Palm oil flower fibers were used as reinforcement in adobe bricks. POFL fibers are obtained from palm oil empty fruit bunches which are waste material from the palm industry. Empty fruit bunches were cut with a knife mill by using a grid of 3 cm. POFL fibers are shown in **Figure 18a**. Due to the grinding of fibers in the knife mill, there is length variation for grid-3 cm long fibers. The average length of fibers is 11.54 mm.

Fiber's characteristics such as length, cross-section, tensile strength, water absorption, chemical composition and thermal conductivity are important for their use in crude bricks. Cellulose is the main component of POFL fibers which play a key role in the tensile strength of fibers. The tensile load behavior of POFL fibers is elastoplastic behavior. Characteristics of POFL fibers are shown in **Table 3**.

#### **Figure 18.**

*Adobe bricks manufacturing, (a) dry sediment and fibers, (b) miniature proctor for prismatic sample, and (c) molded crude bricks.*


#### **Table 4.**

*Usumacinta sediments characteristics.*

Crude bricks were manufactured with POFL fibers and Usumacinta River sediments. Manufacturing of fired bricks consists of mixing sediments and fibers, molding, compaction and drying. USU sediments were crushed, grinded and passed through a 2 mm sieve. Sediments were mixed with 0, 1, 2, 3, 4 and 5% saturated POFL fibers. 450 g sediments are recommended for prismatic bricks of size 4\*4\*16 cm3 . The quantity of fibers for each brick can be calculated by Eq. (3).

$$\mathbf{m}\_{\text{fibers}} = \left(\mathbf{m}\_{\text{sol}} \times \text{96 of fibers}\right) / \mathbf{100} \tag{3}$$

where msed is the mass of dry sediments, mfibers the mass of fibers and % of fibers, the percentage of fibers used. The optimum moisture content of sediments was found by the Proctor test and its value is 19.3%. It was used as molding moisture content. The quantity of water for the mixture can be calculated by Eq. (4).

$$\mathbf{m}\_{\text{water}} = \left(\mathbf{m}\_{\text{sed}} \times \text{@ of water}\right) / \mathbf{100} \tag{4}$$

where msed is the mass of dry sediments, mwater the mass of water and % of water, the optimum water content.

USU sediments and POFL fibers were poured into a mixing bowl followed by the addition of water. Sediments and fibers were mixed with an electric mixer for 10 minutes. Sediment mixing was followed by molding. Sediment's mixture was molded into prismatic bricks of size 4\*4\*16 cm3 which is commonly used for manufacturing composite materials at laboratory scale [11]. Bricks were compacted with dynamic compaction. Compaction energy used is similar to the Proctor test energy, i.e., 600 kN.m/m3 . The compaction of adobe bricks is shown in **Figure 18b**. After compaction bricks were unmolded and oven-dried at 40°C and sun-dried. Bricks were kept in the oven until their mass variation was below 1%. It was observed that sun-drying of bricks at room temperature (20°C ± 2°C) takes 2–3 weeks while oven drying can be achieved in 3–4 days. **Figure 18c** shows oven drying of adobe bricks.

#### **5.2 Characteristics of bricks**

Adobe bricks characteristics include tensile strength, fibers distribution, thermal conductivity, shrinkage and density, etc. Characteristics of bricks are strongly influenced by fiber content. Tensile strength and toughness of bricks increase with fiber content up to optimum moisture content. Fibers contribute to the tensile strength of bricks and transform the brittle failure into ductile failure by increasing the post-peak load-bearing capacity of bricks. The higher quantity of fibers produces fibers clusters in the matrix and reduces the bonding between fibers and sediments which decreases the tensile strength and toughness of bricks. The indirect tensile strength of bricks is determined with a three-point bending test according to ASTM standard [43]. The toughness index of bricks is also be determined with a tensile strength test. The tensile strength and toughness index of Usumacinta sediments bricks at different fiber content are shown in **Table 5**.

**Table 5** shows that the tensile strength and toughness of Usumacinta sediments bricks increases with fiber content up to the optimum fiber content. Tensile strength of fiber-reinforced crude bricks, i.e., adobe bricks, reported in the literature and different standards vary from 0.04 to 2.05 MPa [44].


#### **Table 5.**

*Mechanical characteristics of bricks.*


#### **Table 6.**

*Physical characteristics of adobe bricks.*

Physical characteristics of bricks include mass, density, shrinkage, ultrasonic pulse velocity (UPV) and thermal conductivity. The addition of fibers decreases the density of bricks and makes them lightweight construction material. On the other hand, Ultrasonic pulse velocity of bricks also decreases with increasing fibers content due to the presence of voids as saturated fibers swell initially and shrink after drying the bricks which produce small cracks. Shrinkage is another important property of bricks which increases with higher molding moisture content and higher clay content of sediments.

Physical characteristics of adobe bricks made with USU sediments at optimum fiber content are summarized in **Table 6**.

Uniform distribution of fibers inside the bricks is important. The distribution of fibers inside bricks can be observed with ImageJ software [5]. Crude bricks are divided into 4 parts with 6 cross-sections. A microscopic image of each brick crosssection is analyzed with ImageJ software to find the number of fibers, their area and orientation in bricks. Upward movement of fibers in composite materials with dynamic compaction can be observed with image analysis.

This section includes a detailed analysis of the physical and mechanical characteristics of natural fibers especially tropical fibers for their recycling in construction materials such as crude bricks. Crude bricks specimens were manufactured at different fibers content and their characteristics were analyzed. The Addition of natural fibers in construction material has a positive impact on the tensile strength, density and thermal properties of these materials.
