To carry out this work; the materials used for concrete are CEM II 32.5 R cement, rolled sand from the Congo River, two classes of crushed gravel and iroko wood chip. The iroko wood chip used is shown in Figure 1.

In accordance with standard NF EN 933-1, a particle size analysis test by sieving to identify sand, gravel and chip was carried out.

Figure 2 shows the carrying out particle size analysis test.

In accordance with standard NF EN 1097-6, an absolute density test of these materials was carried out. These densities are useful for concrete formulation.

The particle size curves obtained from the study materials are shown in Figure 3.

The granular classes and absolute densities of these materials are presented in Table 1.

The sand with a particle size of 0 - 0.5 mm, is very fine sand.

The sand and the two classes of gravel used are ordinary aggregates since their densities are in the spindle of ordinary aggregates: 2500 to 2600 kg/m³.

The chip water absorption test was performed. This chip was previously dried in a controlled oven at a temperature of 50˚C. The chip material was considered dry when the mass variation of two successive weighings within 24 hours was less than 0.1%.

In accordance with the recommendations of RILEM TC 236-BBM [28], the chip water absorption test was performed at 1 minute, 30 minutes, 4 hours and 48 hours. It was determined in Equation (1):

W = 100 ( m s − m d m d ) (1)

In that equation, W is water absorption from chip (%), m_s is mass of submerged chip (g) and m_d is mass of dry chip (g).

To formulate the concrete, the classical method of Dreux-Gorisse was used. Thanks to the particle size curves and absolute densities of the materials, the composition was determined. With the Dreux method, the composition of the concrete for a given strength and workability was determined by involving the water dosage and the maximum size of the aggregates.

For the composition of wood chip concrete, chip intrusion was made by substituting in volume the sand and gravel in percentage (0, 2.5, 5, 7.5, 10, 12.5, 15 and 20%). The total substituted volume is the volume of chip to be introduced into the concrete. The water/cement ratio for concrete under the assumption is 0.56, the same ratio is kept to evaluate the influence of chip on the workability of concrete.

The composition of concrete is presented in Table 2.

When pouring concrete, the workability was determined using the Abrams cone subsidence test (Slum-Test) in accordance with standard NF EN 12350-2 [29].

After pouring the concrete and casting, the specimens were unmolded after 48 hours and stored at laboratory room temperature (24˚C ± 1˚C) until the test dates.

In accordance with NBN EN 12390-3, the compressive strength of concrete was determined by means of a concrete press. The specimens to be tested are subjected to an increasing load until they break. The concrete press for compression has a maximum capacity of 3000 kN with a loading speed of 0.50 MPa/s. This press is connected to the computer which directly displays the value of the resistance in MPa [30].

The mechanical tensile strength by splitting was determined by means of a concrete press. The specimens to be tested are subjected to an increasing load until they break. The concrete press for splitting traction has a maximum capacity of 150 kN with a loading speed of 0.05% ± 1% MPa/s. This press is connected to the computer which displays the maximum breaking force. The mechanical tensile strength by splitting is calculated in Equation (1) [31].

f c t = 1.243 * 10 − 2 F max (2)

In that equation, f_ct is the mechanical tensile strength by splitting (MPa) and F_max is the maximum breaking force (kN).

Figure 4 shows a specimen submitted in compression and one in tension by splitting.

The water absorption results of wood chip obtained are shown in Figure 5.

Up to 4 hours of immersion, the water absorption of the chip is exponential. Beyond that, it continues to grow but very slowly because at 48 hours, it has grown only 18% of its absorption at 4 hours.

The results of concrete workability obtained are presented in Table 3.

Although the workability of concrete has been fixed, in practice there have been some small variations in subsidence. Despite this variation, concrete is indeed plastic as fixed in the hypothesis.

The results obtained from the density of concrete are shown in Figure 6.

Has 2.5% chip; the density of fresh concrete decreased by 6% and hardened by 7%; to 20%, it fell in the fresh state by about 30% and in the cured state by about 34%. The higher the chip content in concrete, the lower its density. The density of concrete therefore decreases with the increase in the chip rate. These results are consistent with those found in the literature [7] [14] [26].

The results of the compressive strength of the concrete obtained are shown in Figure 7. Each value reported is the mean of 4 trials.

A decrease in concrete strength of 36%, 55%, 59%, 65%, 71% and 89% respectively was recorded for chip levels of 2.5%, 5%, 7.5%, 10%, 15% and 20%. The higher the chip content, the lower the mechanical compressive strength.

The chip incorporated into the concrete, therefore, reduces its mechanical strength in compression. These results are consistent with those of [4] [6] [8] [12] [22] [23].

The results of the tensile strength of concrete are shown in Figure 8. Each value reported is the mean of 3 trials.

A decrease in resistance of 17%, 22%, 24%, 30%, 37% and 45% respectively was recorded for chip levels of 2.5%, 5%, 7.5%, 10%, 15% and 20%. The higher the chip content, the lower the tensile strength by splitting.

The chip incorporated into the concrete, therefore, reduces its tensile strength by splitting.