Introduction To partially address concerns such as reducing energy consumption in buildings, reducing the emission of gases through the greenhouse effect due to large air conditioning facilities and extra protection of environment by recycling waste polystyrene, we conducted an experimental study on the behavior of baked bricks with addition of polystyrene [1-4]. This is an interesting study in the Laboratory of Applied Mechanics and Energetics (LEMA) of the thermomechanical properties of cooked and lightened bricks. According to our surveys, the addition of polystyrene in mortars and concretes help to achieve good thermal insulation of the envelope [5-7]. The main objective of this study is to experimentally characterize the mechanical behavior of fired bricks. One of the objectives is to find earth blocks of acceptable mechanic properties to erect walls or fences and buildings that will be easy to implement, as stipulated by the French and Belgian standards [8, 9]. A weight ratio clay / laterite constant equal to 4 was adopted for all compositions. Then it was playing on the amount of polystyrene (PSE) introduced. Some preliminary tests have indeed shown that the workability of the mortar, the removal and the density of blocks, cracks of the clay during drying and firing, are influenced by the introduction of polystyrene. It has also been shown in this study the influence of the percentage of polystyrene on the bulk density, on the compressive strength of the cubic form of test specimens and the tensile strength of the samples. Material and methods Sample preparation Materials used. All the materials used in this work are of local origin. For making blocks, the materials used are: clay of Porto-Novo, laterite or earth from Bakhita bar, water and polystyrene, the latter component is also the only variable parameter. The Laterite or earth bar. Laterite used for making the blocks is taken from a soil in Calavi (Bakhita region). After drying the ground, it was sieved with two sieves (02 mm) for making test pieces to be cooked by taking into account the finer particle size clay. The picture in Fig. 1 shows the appearance of the size used. Fig. 1. Different sizes (Ф ≤ 2 mm) of laterite used (diameter less than 2 mm) The clay. The clay used to make the blocks stabilized by cooking, is a gray clay of Porto Novo. Its average bulk density measured in the dry state was 1200 kg · m-3 . Figure 2 shows pictures of the clay used in the rough, and after sifting. Before use, the clay was collected, dried, crushed and sieved with a sieve of 2 mm. Fig. 2. Pictures of the clay used The polystyrene. The expanded polystyrene beads used are suitable for incorporation into the blocks. They have varying diameters (Ф ≤ 2 mm; 2 ≤ Ф ≤ 4 mm; Ф ≤ 4 mm). Its bulk density is about 17.5 kg/m3, which is at least 60 times smaller than the densities of the clay and the earth bar. Figure 3 shows photos of PSE waste and ground product obtained. Fig. 3. Photos of the polystyrene used In this study, three granular classes were determined. Pictures 1, 2 and 3 in Fig. 4 show the appearance of the different classes. Only the granular beads class whose diameters are less than or equal to 2 mm was used (photo 1). Fig. 4. Different granular classes PSE The water dampening. Dampening water is distributed by the drinking water supply network of the university of Abomey-Calavi in Benin. Hardware used. For making blocks, many current materials and supplies (scales, containers, trowels, molds, formwork oil) are used. Figure 5 shows the pictures of some materials used. Fig. 5. Photos of some materials used Modus operandi. All the samples were prepared manually; the mortar mixture is made by using a trowel and the blocks were molded by hand (hand molding). Standard prismatic test (NFP 18-400, NA 2600) of dimensions 4 × 4 × 16 cm has been used for the determination of flexural strengths 3 points. These same samples were used to determine the weight loss. Equivalent cubic specimens (4 × 4 × 4 cm) were obtained by crushing the resulting equivalent cubic test half-prisms. To avoid cracking problems and withdrawal, the samples were therefore dried in the sunlight at the surrounding environment of the laboratory. All cooking preheating the cooling takes about 48 hours ; the product remains up for 10 hours in the open fire. The curing kinetics is adopted in accordance with the firing curve of Fig. 6. Fig. 6. Curve showing the various cooking phases  All the tests were made with a temperature of 105°C and a firing temperature of about 1000°C. For each type of test, the number of specimens is three (03) and the test body is: 4 × 4 × 16 cm for mechanical testing and measurements of bulk densities. For mechanical testing, testing in three flexure (03) dots and those in compression on the half blocks from the Flexural strength were performed. Formulation. Dosage. The following percentages were studied: - polystyrene: 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100% of the soil volume measured in accordance with the dosages adopted by Shafi ; this, to properly highlight the influence of this by-product in low dosages; - clay: 80% and 20% laterite. Of course, for each characteristic of interest, the results were compared with a reference block (control) performed according to the same methods of implementation and assays land and water. The only changing parameter is the mixed assay (content) of the PSE. The studied characteristics are as follows: - density; - compressive strength and tensile strength by bending. Notation C designates blocks stabilized by baking, followed by a number which represents the percentage by volume of polystyrene added to 10 (Table 1). Table 1 Notations adopted for the different samples № Designation Notation 1 Witness block C 2 Block reinforced 10% polystyrene C1 3 Block reinforced 20% polystyrene C2 4 Block reinforced 30% polystyrene C3 5 Block reinforced 40% polystyrene C4 6 Block reinforced 50% polystyrene C5 7 Block reinforced 60% polystyrene C6 8 Block reinforced 70% polystyrene C7 9 Block reinforced 80% polystyrene C8 10 Block reinforced 90% polystyrene C9 11 Block reinforced 100% polystyrene C10 The Table 2 shows the values of different strengths for the manufacture of each type of sample. Table 2 Compositions of the lightened blocks with polystyrene beads Mixture, ml: clay (80%) + laterite (20%) 2000 Water, ml 750 PSE volume, ml 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Corresponding PSE weight, g 0 3.5 7 10.5 14 17.5 21 24.5 28 31.5 35 Designation C C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 Testing the weight loss. The withdrawal measures were accompanied by with the weighing of 4 × 4 × 16 samples in order to determine their densities. Measurements of weight variation were performed using an electromechanical balance at a precision of 0.1 g (Fig. 5, photo 4). Tensile bending. The flexural tensile strength was determined using a 3-point bending machine 10 kN, on prismatic samples 4 × 4 × 16 cm in accordance with the standard NFP 18-407 (NA 428). The specimens were placed in the testing machine as shown in Fig. 7. Fig. 7. Bending machine used After a perfect centering, the loading was carried out with a speed of constant scalability. The machine is provided with a bending device whose principle is shown schematically in Fig. 8. Fig. 8. Device tensile failure in bending  Denoting by the failure of the test of load in bending, the moment of rupture is and the corresponding flexural stress is determined by: Compression testing. The compression test is to break the test specimen between the two plates of a compression press. The press used is a compression machine with a capacity of 150 kN. The compression test on equivalent cubes 4 × 4 × 4 cm was made on the same compression machine. The half-prisms 4 × 4 × 16 cm specimens obtained after Flexural strength were broken in compression as shown in Fig. 9. Fig. 9. Compression breaking device By appointing the breaking load of the compression cylinder, the stress fracture at the corresponding compression is calculated by : By expressing newton and considering the section specimens (40 mm × 40 mm), this resistance in MPa is: Following the withdrawal of the clay after firing, the compression breaking stress, for cooked blocks, is calculated by the following formula: The results for each of the 06 half-prisms are rounded to 0.1 MPa and then the close average is calculated. If each one of the results differs from 06 ± 10% of this average, it is discarded and the average is recalculated from the remaining 05 results. If again one of the 05 results deviates ±10% of this new medium, the series of 06 measures are dismissed as prescribed by the standard. When the result is satisfactory, the average thus obtained is the resistance of the mortar to the age considered. Results and discussion This section presents the different results obtained in this study so as to show the influence of the percentage of PSE on the bulk density, the compressive strength on cubic specimens and the sample tensile strength. The bulk density ρ. The influence of the incorporation of the polystyrene beads on the density of the blocks was studied. The results are compiled in Table 3 and represented by the curve of Fig. 10. In this table, ρc indicates the density of the C. Table 3 Density polystyrene block of samples Designation C C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 ρ, kg/m3 1750 1706 1642 1586 1527 1479 1344 1342 1337 1254 1199 ρ/ρc, % 100 97 94 91 87 85 77 77 76 72 69 Reduction, % 0 3 6 9 13 15 23 23 24 28 31 Fig. 10. Density of the samples tested It is obvious in Fig. 10 that the density of baked bricks eased decreases with the increase of the percentage PSE. For example, the density of the fired brick reference (C) is 1750 kg/m3, whereas the reinforced brick 100% PSE (C10) is only 1199 kg/m3. This corresponds to a decrease of 31%. The Ultra lightweight polystyrene aggregate regarded as voids created within blocks, explains this reduction in density. The lightweight materials become very beneficial when the density reduction rate is at least equal to 15%. With stabilized earth blocks and lightweight, discounts up to 31% were obtained and it is possible to go even further. PSE influence on the mechanical performance. One of the important points to consider in this research is indeed the mechanical performance. The strengthened polystyrene blocks were compared to control blocks in order to determine their influences depending on the dosages. - prismatic specimens 4 × 4 × 16 cm: for the testing of resistance to three-point bending; - half broken into two pieces during the bending test without changing the mechanical characteristics: for testing the resistance to compression. The results of the companion measures are presented as follows. Compressive strength.The influence of the incorporation of the polystyrene beads in the resistance of the compression blocks was studied. The results obtained are summarized in the Table 4 and represented by the curve of Fig. 11. Figure 11 shows the evolution of compressive strength of the blocks according to the polystyrene rate. In this figure, a lower mechanical strength of the polystyrene blocks was observed compared to control blocks, when the polystyrene content increases. At contents ranging from 10 to 100%, the compressive strength is reduced from 6 to 68% relative to the control. These decreases of the mechanical strength were observed by several authors such as Shafi, Collins & Ravindrarajah [6, 13]. Indeed, polystyrene aggregates create areas of weakness within the blocks and reduce the area of the resistant section of the test pieces. Table 4 Results of the compressive strength on samples cubic 4 × 4 × 4 cm Designation C C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 Rc, MPa 9.51 8.95 8.04 5.71 5.20 4.80 3.44 3.26 3.25 3.17 3.08 Reduction, % 0 6 15 40 45 50 64 66 66 67 68 Fig. 11. Evolution of the compressive strength versus percent polystyrene beads Flexural tensile strength. All results obtained are summarized in the following Table 5. The curve Fig. 12 shows and confirms that the addition of polystyrene leads to a decrease in strength of the blocks. Either the three-point bending or compression, the trend is the same: "The mechanical performance of blocks with addition of PSE decreases with PSE content". By comparing Fig. 11 and 12, the compressive strength is less sensitive to the polystyrene content than the flexural strength. The polystyrene consumed during firing improves the quality of the cooking but the voids left inside the surface and sometimes are not very favorable to bending. The improved cooking is useful to compression. Fig. 12. Evolution of the flexural strength based on the percentage of balls polystyrene Table 5 Results of the bending strength on prismatic specimens 4 × 4 × 16 cm (hot) Designation C C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 Rf, MPa 1.22 1.16 0.80 0.50 0.69 0.59 0.50 0.47 0.45 0.36 0.32 Reduction, % 0 5 34 59 43 57 59 61 63 70 74 Synthesis. The effects of the introduction of the polystyrene in the stabilized earth blocks are highlighted: - a dropping resistor was expected since the polystyrene reduced resistant surface of a section. It is to manufacture blocks full of air pockets that have no resistance; - a very different behavior between compression and bending stresses; - by analogy with the resistance obtained during the compressive strength test, it is found that falls are identical to those of apparent densities, so there is a correlation between the two characteristics; - the fall of the resistance to compression block is explained by the decrease in density due to charging of the internal structure of the latter. Conclusion The exploitation of experimental results, has quantified most mechanical quantities, characteristics of developed samples. The experimental part of the study on the mechanical properties provides guidance for the choice of material for the thermal insulation of buildings, however, considering other criteria for the final choice as behavior these materials are in contact with water. A first contribution concerns the development of new local composite materials for the building envelope. After the experiment, the following conclusions were reached: the variation of these parameters is due to the nature of polystyrene and to the dosing percentages in the composition. Depending on the percentage, the increase in the polystyrene has given: - a decrease in compressive strength; - a decrease in the flexural resistance; - a decrease in the mass and density.