The water content was determined by the method of weighing wet weight, then dry weight after oven drying according to the standard (NF P 94 050). Samples taken in the dry state and in powder form were placed in a tare and weighed (Pmh). Then they were introduced into the oven for drying. Once dry, they were weighed again (Pms). The mass of water was deduced and the water content of each tare of this sample was determined by the following relationship (1):

w ( 1 % ) = 100 × Pmh − Pms Pms (1)

With: Pmh: weight of wet material; Pms: dry material weight; w (%): water content.

The density of the soil was measured in accordance with standard NF P 94-054 using a water pycnometer. The pycnometer and its cap were weighed (m₁), then the sample was introduced into the pycnometer and weighed with the cap (m₂), then the pycnometer was filled with water, up to the mark. The whole was shaken to obtain a homogeneous mixture and weighed, after letting out all the air bubbles (m₃). Finally, the pycnometer was emptied, cleaned, dried and filled with water up to the stopper mark and weighed (m₄). The density is obtained from the expression in Relation (2). The density of the solid particles is obtained by taking the arithmetic mean of eight measurements on a soil sample

P s = ρ w × m 2 − m 1 m 4 + m 2 − m 1 − m 3 (2)

This test was carried out according to the NF P 94-056 and 057 standard. The dry sample taken for the study was soaked for one hour in a plastic bucket and washed with a 0.08 mm sieve. The sample was then dried in the sun and in an oven. Finally, it was sieved with interlocking square mesh sieve sets (5; 2; 1; 0.5; and 0.08) in mm and the accumulated rejects weighed.

The tests defined by Casagrande are carried out on the floor mortar (d < 0.4 mm) according to the procedure of standard NF P 94 051.

➢ Liquiditylimit (W_L)

The liquidity limit of a soil (W_L) is determined using the Casagrande cup. The well homogenised mixture is placed in the cup and spread well with a spatula. The thickness in the centre of the material should be 15 to 25 mm. The end of the test is announced when the groove in the material closes to a length of approximately 12 to 13 mm. The number of strokes is recorded and samples are taken from the vicinity of the closure area to determine their water content. The test is repeated at least eight times while varying the water content in order to cover a range of 15 to 35 strokes. For this material, lip closure is obtained at 17, 19, 27 and 34 strokes.

➢ Plasticitylimit (W_p)

The sample is spread out on a wooden plate, to make it lose a certain quantity of water and then a roll is made. The plasticity limit is obtained when the breakage occurs during the preparation of the roll or during its lifting. The clay samples taken from these rolls are dried in an oven at 105˚C for 24 hours to determine their water content. The plasticity index, noted I_p, is expressed by the Relation (3):

I p = W L − W p . (3)

The test was carried out in accordance with standard NF P94-093. The principle consists of weighing the empty Proctor mould, compacting three layers of soil sample in this mould and each layer receives 25 strokes of a 2.5 kg tamper. Then the mould is scraped with a metal ruler to remove the extra soil and weighed again. Finally a sample is taken from the top, middle and bottom of the mould for determination of their water contents.

The density is determined indirectly in accordance with standard NF P 94-064. A sample of rice straw is dried in an oven at 105˚C for 24 hours and weighed. The sample is then placed in a container (tare) and impregnated with paraffin oil. The sample is then weighed in the open air when its temperature is closed to ambient temperature and then completed by hydrostatic weighing. The calculation of the density is determined as follows:

Mass of anhydrous sample (m_S)

m S = m ( s + t a ) − m t a (4)

With m_(s+ta): Anhydrous and Tare Weight of sample

Paraffin mass:

m p = m ( S + P ) − m P (5)

With m_(S+P): Anhydrous and Paraffin Weight of sample

Paraffin volum:

V P = m P ρ P (6)

Hydrostatic weighing:

m T = m ( S + P ) 1 − m h ( S + P ) 1 (7)

With mh_(S+P): Humid and Paraffin Weight of sample

Total Volum:

V T = m T ρ W (8)

Sample volume:

V S = V T − V p (9)

The density of the sample:

ρ s = m S V S (10)

Approximately 35 g of rice straw was oven-dried at 105˚C for 48 hours to obtain dry aggregate. The absorption rate of rice straw is calculated according to the formula (11):

A ( T ) = m T − m S m S (11)

The oven-dried particles were immersed in water at ambient temperature within times of 5 minutes to 48 hours at ambient temperature.

The soil is lightly crushed with a hammer and 5 mm sieves are used to prepare the specimens of dimensions 40 × 40 × 160 mm for three-point bending and 40 × 40 × 40 mm for compression and absorption by capillary action, which are made from local materials using several formulations. The rice straw is first immersed in water until saturation and then added to the soil in a dry state. Then the water is topped up. The dimensions of rice straw used for this work are: (length 1, 2, 3 and 4 cm and diameter 2 to 4 mm) obtained by manual cutting with scissors.

The samples are dried in the oven at 65˚C for 24 hours [12] (Hay. B, 2002). The rice straw is soaked in water for 1 hour and added to the soil. Then, the whole is mixed with water manually. The mass of the dry mixture to make a 40 × 40 × 160 mm test piece is 400 g and for 40 × 40 × 40 mm test piece is 100 g. The formulations F_j are obtained from the compositions: F j = B     X A     Y P t

B: mud brick; XA= percentage X of clay. The unit of X is the percentage (%).

YPt: Percentage Y of the straw of size t.

X = { 100 ; 99.5 ; 99 ; 98.5 ; 98 ; 97.5 ; 97 } ;

Y = { 0 ; 0.5 ; 1 ; 1.5 ; 2 ; 2.5 ; 3 } et t = { 0 ; 1 ; 2 ; 3 ; 4 } .

The prepared mixture was placed in a plastic bucket for about 1 hour for good impregnation. Then, the dough was placed by hand in a plastic mold and unmolded just after compacting by hand. After demoulding, the bricks were dried at ambient temperature.

The specimen is weighed after drying in an oven at a temperature of 105˚C to constant mass, M the volume of the specimen, V this volume expressed in m³. The average dry density of the test piece is determined by the formula (12) expressed in kg/m³ according to standard NBN B 24.206.

ρ = M V (12)

The specimen is placed in the characterisation device, which consists of two pistons. An upper piston with a 2.5 KN load cell and a displacement comparator attached. The lower piston rises with the specimen until it comes into contact with the upper piston. The force sensor records the forces exerted on the specimen. A camera films the operation and the video is stored directly in a PC for analysis. The measurement of the bending strength is given by the relation (13) according to the standard (XP 13-901)

R f = 3 F t m a x ⋅ L 2 ⋅ b ⋅ h 2 (13)

A brick sample of 40 × 40 × 40 mm according to standard XP P 13-901 is subjected to compression using the civil engineering press until it breaks. The principle consists of placing the brick sample in the press; applying the load in a continuous manner and at a regular speed until the sample breaks completely and recording the maximum load supported by the sample during the test. The machinery system made of camera and Computer records all the measurements of pressure and displacement of the blocks, it has a maximum capacity of 2.5 KN and a sensor. The compressive strength of the blocks is the arithmetic average of the strengths of eight tests carried out on samples from the same batch. It is given by the formula:

R c = F S (14)

R_c: is the compressive strength of the blocks in mega pascal (MPa); F: is the maximum load supported by the two bricks in kilo newtons (N); S: is the average area of the test faces in square centimetres (mm²).

Water absorption by capillary action was measured by the detailed test according to the experimental standard NF XP 13-901. The principle is to partially immerse the brick at a 5 mm depth. The water absorption coefficient C_b corresponds to the rate of absorption rate after 10 min. The water absorption coefficient C_b is expressed by the following formula:

C b = 100 × m h − m s S t (15)

m_h − m_s: is the mass of water, in grams, absorbed by the block during the test.

S: area of the submerged face, in square centimeters; t: is the immersion time of the block in minutes.

The average natural water content value of the clay is 8.39% compared to that obtained by ABAKAR [2] which is 10.90%, this clay is less moist.

The density values of solid particles whose size are not more than 2 mm are 2025.73 Kg/m³ less than the value 2450 Kg/m³ obtained by ABAKAR [2], mean this clay contain more void than theirs.

The analysis was done on a sample of Ms = 361.27 g. The results of these analyses are given in Figure 1. For the different grain diameters, the soil was analysed by sieve opening between 5.00 mm and 0.08 mm or from sieve 38 to sieve 20. For sieve 38 with a sieve opening of 5 mm, no seeds were retained in the sieve, the cumulative sieving is therefore 100%, whereas for sieve 20 with an opening of 0.08 mm, 18.7% of the dry mass or 114 g was retained, also called cumulative refusal, and 81% of the cumulative sieving is also defined by the part passed through the sieve. From sieve 38 to sieve 20, the rate of cumulative refusal increases little by little contrary to the rate of cumulative sieving which decreases with each sieve. Thus, 81% represents the finer grains in the soil and 18.7% were rejected by the sieve due to their slightly larger size.

➢ Liquiditylimit

For this material, lip closure is obtained at 17, 19, 27 and 34 strokes and the result is given in Figure 2. From this diagram, the liquidity limit is deduced. It is worth 47.66% at 25 strokes.

➢ Plasticitylimit

The average plasticity limit value is summarized in Table 1. The average plastic limit is 29.75%. From these measurements, it appears that this material is

an organic silt that is not very plastic. This rough classification is frequently used in the road sector (NF P11-300, 1992) and ensures that soils are not susceptible to significant dimensional variations in the presence of water. That value corresponds to the water content in the brick as it changes from plastic to semi-solid consistency and as it dries.

After analysis of the Proctor curves of Figure 3, we note that the percentage of water in the soil to be compacted was 13.54%. Thus, the maximum dry density of the soil sample is 1.21.

The bulk density of rice straw is 464 kg/m³. We note that the density lower than that of clay which is 2045 kg/m³. This is due to the high porosity.

The result of the water absorption of rice straw is summarised in Table 2. We find that the water absorption capacity of rice straw is 206% at 5 min. This is too high compared to most construction materials. After 48 hours of immersion, the two materials reach a relatively close absorption rate. In construction, the hydrophilic character is a major defect. Rice straw must be immersed in water until it is saturated before mixing it with the binder or soil, otherwise it will absorb all the mixing water, thus generating a product that is difficult to use.

Figure 4 gives the results of the apparent densities of the specimens. This table shows that for each size, the apparent density decreases as the percentage of fibres increases. These masses are between 1082.57 and 1420.05 Kg/m³.

The Figures 5-9 show that the bending strength drops as a function of the straw intake percentage. For adobe without straw, the maximum strength is 1.52 MPa. For straw of 1cm length (Figure 5), the resistance is 1.04 MPa at 1% straw, 0.97 MPa at 0.5% straw to finish at 0.63 MPa with 3% straw while passing through the other percentages (Figure 6). Strength drops from 0.90 MPa at 0.5% straw to 0.42 MPa with 3% straw with a slight increase to 2.5% straw. Figure 6 has a resistance which varies between 0.61 and 0.73 MPa and that of Figure 8 varies between 0.57 and 0.84 MPa. Figure 9 gives the maximum resistance of each size. Apart from the simple clay formulation, the maximum strength is 1.04 MPa corresponding to the B99%A-1%P1 formulation (adobe made up of 99% clay and 1% straw of 1 cm size). This resistance is beyond the minimum resistance underlined by Doat [13].

The compression results are synthetized in Figure 10-14. The compressive strength drops. The maximum compression strength of Simple clay adobes is

0.19 MPa, that made with 1cm straws long is 0.16 MPa (Figure 10). The compression strengths of the bricks using straw of size 2, 3 and 4 cm are respectively 0.14 MPa; 0.2 MPa and 0.14 MPa (Figures 11-13). These characteristics allow us to conclude that the brick can be used to build houses [12]. The brick B98.5A1.5P3 made with 1.5% of 3 cm straw has maximum compression strength (Figure 14). The adobe brick reinforced with rice straw absorbs a lot of water when the percentage of straw is high (Figure 15). The absorption coefficient is 4.875% at 0% straw and 20.57% at 3% straw. This can also be explained by the void left by the straw in the adobe (Figure 16). The pores existing within the matrix of the brick influence the absorption of water by capillarity action and

absorption rate increases with void rate.