Study of the implementation of fibrous materials and earth in hollow body slab in reinforced concrete: case of laterite, Borassus aethiopum and Calamus deerratus woods

This work enrolls in the context of eco materials. It concerns the field of transformation of local shrub forest resources and lateritic earth with low market values into building materials and is developing a process for the valorization of lateritic nodules, Borassus aethiopum (palmyra) and Calamus deerratus (rattan) in the engineering of modern reinforced concrete structures. The objective of this process is to promote the rational use of these local materials in the realization of the floors of social housing. Experimental studies of physical and mechanical characterizations of the lateritic concrete formulated, of the palmyra and rattan woods were carried out. Through tests of tearing and sliding of the interface between normal concrete and laterite concrete (push-out tests), it has been highlighted the adhesion of palmyra wood to concrete and the contribution of rattan lianas to oppose the longitudinal sliding of the interface between two concretes of different nature. The case study of a hollow body slab in mixed concrete (normal and laterite concrete) reinforced with palmyra and rattan woods,


Introduction
Rural communities in developing countries have significant deposits of local building materials among which we have: the palmyra (Borassus aethiopum), rattan (Calamus deerratus) (Fig. 1) and laterite. The local materials obtained from these natural resources were used in West Africa in the traditional and semi-modern construction practice, respectively by the villagers and the colonizer to build the housings (Fig. 2). a b In the context of modernity and construction methods using conventional sand mortar and cement, reinforced concrete and corrugated sheets, these green building practices have disappeared in the cities and are gradually disappearing in the villages. Thus, we are increasingly observing the establishment of housing made of modern materials in rural villages/ towns, although since the 1970s, many researchers, with the aim of promoting the use of eco materials in buildings modern, examine the issue of substitution of sea sands, gravel/crushed and steel, respectively by laterite/lagoon sand, laterite nodule [1][2][3][4][5][6][7][8][9][10][11], rice husks [12][13][14][15] and certain woody forest products [16][17][18][19][20][21][22][23][24]. Other researchers have used waste materials to improve the mechanical properties of rammed earth structures [25][26][27][28][29].
It is to reverse this trend, that we propose in this work the study of a possibility of using mixed concrete (conventional aggregate concrete and concrete with sand and lateritic gravel) reinforced, with original reinforcements vegetable palmyra wood and rattan lianas) in a hollow body slab which is of the range of modern structures of housing in urban cities.
If we analyze the stressing modes of the components of a hollow-body slab, we observe, that the ribs and the compressive table are stressed in simple flexion and simple compression respectively, while the interjoists serve as filling.
Considering that: -the palmyra wood and rattan wood have a certain capacity of tensile / shear strengh and of concrete adhesion, -the laterite concrete associated with rattan wood has a good compressive strength, -the interjoists in rice husks [12][13][14][15] have a thermal insulation capacity and a low density, we can without hesitation design slabs ( Fig. 3) with these eco-materials with low market value, so we could get solid, insulating, lightweight slabs.
Indeed, lateritic soils are compatible with cement paste. Lateritic sand and nodules can partially or totally replace conventional sand and gravel / crushed respectively, when they are used in optimal proportions with optimal water to cement ratio. Consequently, the density and the compressive strength of the hardened concrete thus obtained can reach respectively a value between 22 and 26 kN / m 3 , and 12 and 35 MPa [6,9,10], while their workability of these concretes is between 2 and 10 cm (slump test).
The relatively recent studies on the substitution of steels by vegetable reinforcements in the flexed structures of buildings and which concern the highlighting of the contribution of rattan lianas and bamboo to the mechanical resistance of unreinforced concrete beams, revealed, that the problem of using fibrous materials is the influence of their hygroscopic property, the adhesion to concrete and their low tensile strength compared to that of steel. So, Obilade and Olutoge [17]

Obtaining longitudinal reinforcing bars
After felling the palmyra tree, the stipe is harvested. From the outer crown of this stipe, slats are taken. These slats have a section of 4 x 4 cm 2 or 4 x 6 cm 2 depending on the thickness of the outer crown. These lattess are machined from rods or cylindrical bars with diameters between 15 to 30 mm. The rods are then characterized by physical and mechanical tests.

Obtaining transverse reinforcing bars and grid bars
After harvesting the stipe of the rattan, it is scraped to remove the bark and take out the cane.
The cane is then straightened and dried to allow better conservation. Rods that can be used as reinforcing rods must have a diameter between 10 and 15 mm. Their use as reinforcement requires physical and mechanical characterization tests.

Physical and mechanical characterization of palmyra and rattan woods
The rattan and palmyra woods used for this work come from the forests of Benin (West Africa).
The tests carried out on these materials concern the humidity rate, the density, the absorption, the shrinkage, the swelling, the tensile and the compression.
The humidity rate H (%) is measured in accordance with Standard NF B 51-004 [30].
The density (g/cm 3 ) is measured according to the protocol described by standard NF B51-005 [31].
The water quantity, that the plant materials used, can absorb is measured according to the experimental approach of standard NF EN 1097-6 [32].
The volumic shrinkage and swelling of the test pieces having a humidity equal or greater, than the saturation point of the cell walls and reduced to the anhydrous state are measured according to standard NF B51-006 [33].
The tensile tests of the test pieces were performed on the whole in accordance with standards NF B51-017 [34], NF B51-018 [35]. The palmyra wood sample used had prismatic shape and its dimensions 4x20x200 mm 3 followed the orthotropic directions of the palmyra tree. For the rattan wood, sample were directly cut on natural shape of the species under diameter of 9 mm and length of 200 mm. Displacement of sample during the test were captured using a device including a camera, two digital comparators and two rigid plates sticked on the sample (and in contact with the comparator probe) at a relative distance L0 = 100 mm. Deformation were calculated by dividing the difference of displacements displayed on comparators by the relative distance L0. The cross head speed during the test were 4 mm.s -1 .

Formulation of laterite concrete
The lateritic earth used in this work is taken from a quarry located in the south of Benin (West  The type of cement used for the formulation is CEMII / B-LL 42.5. It is a quick setting cement.
Its minimum compressive strength at 28 days is evaluated at 42.5 MPa. Its absolute density is 3.04 g / cm 3 .
The method of formulation which we retained in this work is that of the absolute volumes. It is based on the total sum of the absolute volumes of each constituent plus the volume of entrained air and trapped air corresponding to one cubic meter of concrete.
For the formulation of concrete, we considered aggregates of granular classes: The physical characteristics and the particle size curve of the mixture nodules and the sand lateritic for N/S = 2.8 are shown in Table 1 and Fig. 5.  Laterite concrete is obtained on the basis of the following expression: The unknowns of (1) being the proportions of aggregates, the cement dosage, the quantity of water and the voids. If we take , the ratio E C , , the ratio N S and put them in (1), we obtain: Not being able to study all the parameters at once, we set certain constants as follows: -cement dosage C = 400 kg/m 3 , -= 2.2 ; 2.5 and 2.8, The mass of water E is corrected by taking into account the absorption of the aggregates. This quantity is corrected by: where, = absorption coefficient of the material, = water content of the material. The subsidence test is carried out according to standard NF EN 12350-2 [36]. It consists in measuring the consistency of fresh concrete compacted in a mold having the shape of a truncated cone.
The compressive and tensile strengths of hardened concrete are determined in accordance with standards NF EN 12390-3 [39] and NF EN 12390-6 [40].

Study of the use of palmyra wood and rattan wood in concrete.
The flexed elements of the reinforced concrete structures are stressed both in compression and in tension. To guarantee the resistance and the stability of these structure, it must have an association of concrete -palmyra / rattan wood. The essential condition of this association is adhesion. This adhesion will allow the transmission of forces and the rational functioning of the reinforced concrete elements.
The calculations leading to the implementation, require knowledge of the physical and mechanical characteristics of laterite concrete, palmyra wood and rattan wood. In addition, it is also necessary to analyze the capacity of palmyra wood and rattan to resist tensile force and shear respectively.

Highlighting of the concrete-palmyra wood adhesion and behaviors at the interface of two superimposed beams stressed in pure shear
It was a question here of cheching whether there is a tangential connection to the wood-toconcrete interface due to friction and bracing to the concrete conneting rods. This test consisted in exerting a pull-out force along the axis of the test pieces of the palmyra wood anchored in a concrete block 110 mm in diameter and 220 mm in length. The anchoring length of the bar is taken to be 100 mm.
The maximum pullout force is divided by the lateral surface of the rebar in contact with the concrete so as to obtain the maximum apparent adhesion stress. This stress is calculated by

Shear test
The bending of the elements of the reinforced concrete structures is accompanied by a sliding of the fibers in the longitudinal direction (effect of the tensile and compressive forces) and the transverse direction (effect of the shearing force). In order to avoid longitudinal sliding and cracks, frames and stirrups are necessary. In the present work, these frames and stirrups are made of rattan lianas. The objective of the shear test carried out here, is to observe the impact that the rattan lianas can have on the longitudinal sliding, when one plans to carry out a slab in mixed structure (rib in conventional sand concrete and laterite concrete compressive table) as shown in Fig. 3.
The study of rattan lianas shear resistance is carried out, according to the prescriptions of standard EN 26891 [41]. The tests were carried out on symmetrical test pieces (Fig. 6) in order to eliminate the bending effect at the interface of the test piece. Three types of test pieces were tested: a first type consists of test pieces without connection; a second consisting of test pieces with connection (one rattan liana of 12 cm in length and diameter varying from 11 mm to 12.5 mm); a third consisting of test pieces with connection but this time with two rattan lianas of 12 cm in length and diameter varing from 11 to 12.5 mm. Figure 6 shows the configuration of the test pieces.

Applicability of palmyra wood, rattan wood and laterite concrete materials in the production of hollow body slab
In order to prove the effectiveness and efficiency of the palmyra, rattan and laterite concrete reinforcements in the hollow-body slab of social housing, a dimensioning of the compressive table and the rib is made. In order to validate the reinforcement of slab, an analytical and numerical study of the deformation state of said slab are carried out. Fig. 7 shows the slab structure. Numerical modeling is based on Finite Element analysis. It was an implicit calculation code whose calculation algorithm is based on iterative calculations. The mesh used is of the very tight type. The calculated rib is simply supported on its two ends and stressed in bending by a uniform load. The analytical and numerical calculation of the slab is done by analyzing the behavior of its homogenized rib whose simplified model is presented in Fig. 8. The equivalent elastic characteteristics of the slab were calculated on the basis of the data in the Table 2    The deformation of the slab is calculated by the expression: With,

≤ ≤
Where, p is taken equal to the value of the action in the service limit state (N ser = G + Q, here,

Results and discussion
Absorption kinetics of palmyra and rattan woods Fig. 9 shows the ability of rattan and palmyra woods to allow water to enter and circulate through its cells. We observe that the palmyra wood absorbs water less quickly than the rattan wood. The average saturation point of rattan wood of about 172% is reached at around 6 days, while the palmyra wood has an average saturation point of 64%. It's reached at around 23 days.
So these two materials subjected to humidity conditions become hygroscopic. Thus in fresh concrete, not only could they modify the quantity of mixing water but also undergo a significant swelling followed by a shrinkage (Table 2) when the concrete hardens. This dimensional instability could cause the adhesion to break between them and the concrete. To overcome this, it is necessary to waterproof them before associating them with fresh concrete. Fig. 9 Kinematics of absorption of specimens from the palmyra and rattan woods

Behavior and mechanical caracteristics in traction of palmyra and rattan woods
The laws of tensile behavior of the palmyra wood and the rattan wood are represented on Fig.   10 and Fig. 11. These curves show that the rattan wood has elastoplastic behavior while the

Times (hours)
Palmyra Rattan palmyra wood has brittle behavior. However, for the two cases of stresses, we note that the palmyra wood presents a fragile behavior, while the rattan wood a ductile behavior.
Generally, hard and dense woods have brittle tensile behavior, unlike soft and soft woods with low density are ductile.
Generally, hard and dense woods have brittle tensile behavior, unlike soft and soft woods with low density are ductile. Therefore, the analysis of the two curves of mechanical behavior shows us this evidence, because the palm tree wood has a density included in 700 and 800 kg / m 3 therefore hard, and the rattan wood a density lower than 450 kg / m 3 therefore soft The physical and mechanical properties of the palm and rattan woods calculated are presented in Table 3. In this table we designate by: H -humidity rate, Mv -basal density, Gv -volumic swelling, Rv -volumic shrinkage, E -modulus of elasticity, Ϭr -rupture stress, fe -elastic limit.  Table 3 Physical and mechanical properties of the palmyra and rattan woods

Caracteristics formuled laterite concretes
The laterite concretes formulated with a cement 400 kg / m 3 have the following characteristics at 28 days (Table 4)

Adhesion stress of palmyra wood frames
Highlighting the association of palmyra wood-concrete made through concrete-palmyra wood adhesion tests shows that the palmyra wood adheres perfectly to concrete. The quality of this adhesion improves when the surface of the wood has crenels. In fact, the pull-out strenght of smooth and crenellated reinforcements are respectively 2.77 MPa and 3.68 MPa.

Influence of rattan lianas on the sliding of the interface of superimposed mixed concrete blocks (normal concrete and laterite concrete)
The study of the sliding of the normal concrete-lateritic concrete interface with and without rattan connection reinforcement by push-out tests ( Fig. 6) reveals, that the presence of rattan lianas opposes slippage of interface of the concrete blocks. For this purpose, we have observed that: -the rupture of the connections whose interfaces are not provides with reinforcements occurred by a premature separation of the concrete blocks. This rupture is rapid and abrupt. Any deformation is not observed on the concrete blocks; -the interface connections provided with a single reinforcement rupture by a gradual separation of the concrete blocks and very often accompanied by the plasticization of rattan lianas. The concrete blocks are always connected to the connector rod. Any deformation is not observed on the concrete blocks; -the longitudinal sliding of the interfaces provided with two reinforcements causes cracking of the concrete blocks in the vicinity of the connector reinforcements.
In view of all of the foregoing, the laterite concrete, the palmyra and rattan woods can be used in the production of slab structures as show in the Fig. 3. However, these slabs must be lightly loaded, in view of the fact that the elastic limit and the breaking stress of the palmyra wood are low than those of the steels recommendes for reinforced concrete.

Effectiveness of rattan and Palmyra woods as reinforcement of hollow body slabs
It follows from the calculations carried out in accordance with the calculation codes for reinforced concrete structures, the following:

Conclusion
The palmyra and the rattan trees are monocots, the wood of which is used in a disproportionate way in the realization of the structures of traditional constructions in lateritic earth. This process does not contribute to the rational use of the shrub plant cover.
With the aim of remedying this state of affairs, the present work is developing a process for valuing palmyra wood, rattan wood and laterite for the realization of the flexed structure of social housing.
Analytical and numerical analyzes of the state of flexural deformation of the rib designed and dimensioned in normal concrete and laterite concrete reinforced with palmyra and rattan woods reinforcements, demonstrate the structural efficiency of concrete, reinforcements in palmyra and rattan woods in modern low-load slabs of social housing in rural towns.
This process brings to the studies previously carried out in the field of building materials the following innovations: -the behaviors and mechanical characteristics of the frames in palmwood and rattan have been described. These results allowed the use of reinforced concrete calculation code, the modeling and numerical calculation of the ribs of hollow body slabs, -a method for formulating lateritic gravelly concrete based on the total sum of the absolute volumes of each component plus the volume of entrained air and trapped air corresponding to one cubic meter of concrete has been developed.
-a process for valuing Borassus aethiopum and Calamus deerratus woods and laterite was developed.