The cement used in this study is portland cement with strength class 42.5 N (CEM I 42.5 N according to standard EN 197-1) from the Forspak cement works in the Niari department of Congo Brazzaville.The superplasticizer (SP) Dynamon Easy 738 is a modified acrylic polymer-based admixture, which is specifically suited to the ready-mixed concrete sector. The superplasticizer is incorporated into the mix when the concrete is mixed with a standard mixer. All the aggregates come from the Djoué river quarry on the outskirts of Brazzaville. They are hard and non-reactive to alkali-reaction. The 0/4 mm sand has a density of 2652 kg/m³ and a water absorption coefficient of 0.76% (by mass). Aggregates (04/12 mm and 12/20 mm) with a density of 2673 kg/m³ and a water absorption coefficient of 0.72% (by mass). The polypropylene fibers used ( Figure 1 ) in this study are fibers manufactured from propylene. They are Sikafibre-force 54. It has a length of 54 mm, a diameter of 0.34 mm, a tensile strength of 689 MPa and a tensile modulus of elasticity of 5.75 GPa. Figure 2 shows sugarcane bagasse fiber sieved through a 4 mm sieve.

The untreated sugarcane bagasse was sieved with a 4 mm sieve to extract elements smaller than 4 mm, then immersed in water for two hours before being dried again in a Controlab oven at 40˚C to prevent the mixing water from being absorbed by the fibers when the concrete was made. The mass of the sample was taken as a function of time. We also used Origin Pro software to process our data.

Our objective is to obtain a structural concrete with a compressive strength class of C30/37 in accordance with standard NF EN 206-1. To achieve this, several types of concrete were formulated using the Dreux-Gorisse method, of which seven typical concretes were selected [14] . These were five concretes based on sugarcane bagasse fibers (SBC: Sugar Bagasse Concrete), namely SBC-0. 1, SBC-0.15, SBC-0.17, SBC-0.23, SBC-0.25, containing respectively 0.1, 0.15, 0.17, 0.23, and 0.25 % sugarcane bagasse fibers (by volume), a concrete based on Polypropylene fibers (PC: Polypropylene Concrete) incorporating 0.1 % by volume of polypropylene fibers and an ordinary concrete (OC: Ordinary Concrete), taken as the reference concrete. Using the dreux-gorisse method, we started from the principle of making 21 specimens of concrete using cylindrical steel moulds with a diameter of 16 × 32 mm. And for the dosage of admixture in the matrix, depending on the quantity of fibers introduced, we carried out different dosages at 0.6, 0.54 and 1.20 %.

For each concrete formulation, concrete specimens with cylindrical moulds (Ø: 16 cm, height: 32 cm) were made. To carry out the experiments on the concretes, we began by weighing the quantities according to the mix code using a 30 kg Kern precision balance. The materials were then mixed in a Controlab C0198/3 vertical shaft mixer with a mixing capacity of 300 liters and a tank volume of 600 liters. The coarse and fine aggregates were dry-mixed for one minute. Water was then added gradually while the mixer was in motion. The polypropylene fibers were also gradually introduced into the mixer. The same process was repeated when mixing concrete with sugarcane bagasse fibers. The remaining admixture and water were then added to the still-running mixer. We estimated the total mixing time to be around five minutes for both the ordinary concrete and the sugarcane bagasse fiber concrete. In the case of mixes reinforced with polypropylene fibers and bagasse fibers, the fibers were added progressively in the middle and at the end of the mixing period, by placing them slowly and manually into the mix. To prevent the fibers from ‘balling up’, the addition of the fibers added two minutes to the total mixing time. The Abrams cone slump test was carried out at the end of the mixing process. All the moulds were first oiled and then filled in two layers and vibrated using a Heberth-type portable electric concrete vibrator for approximately one minute. Cylindrical moulds measuring 16 × 32 mm were cast vertically. The compacted specimens were covered with a rubber sheet to limit water loss for the first 24 hours after casting. The following day, the specimens were demoulded and weighed on a 30 kg kern precision balance. The specimens were then immediately placed in water tank with a temperature of 20˚C ± 2˚C and a relative humidity of over 60˚C ± 5˚C to determine the tensile and compressive strengths at 7, 14, 28 and 112 days, i.e., an average of 3 specimens per age of concrete. Also, 9 specimens were withdrawn from the batch for drying shrinkage and loss of mass and porosity tests.

Consistency classes (slump classes) are used to characterise the workability of concrete and to classify it according to standard NF 18-451 using the Abrams cone [19] . The classes of concrete from firm to very fluid are given in Table 1 . We used control brand equipment to carry out the Abrams cone test. For the last class, the Abrams cone slump test is not accurate enough, so the Abrams cone spread test is used.

The following formula was used to obtain percentage values for drying shrinkage

$\text{Mr}=\left(\frac{Dd-Da}{Dd}\right)\times 100$(1)

With Mr: Measurement of the dimension, Dd = Starting dimension, corresponds to the dimension on the 1st day, i.e., the first measurement, Da = Ending dimension, corresponds to the dimension on the 2nd day, i.e. the following day.

The total porosity (N_tot) of a concrete sample is given by:

${\text{N}}_{\text{tot}}=100\times \left[\frac{\left(\text{Ww}-\text{Dw}\right)}{\text{Ww}}\right]$(2)

Ww: wet weight, Dw: dry weight.

The following formula was used to find the compressive strength value:

$\text{Fcj}=\frac{\text{P}}{S}$(3)

Where Fcj is the compressive strength of a cylindrical concrete specimen measuring (16 × 32) cm. P is the breaking load obtained on the compression press and S is the surface area of the specimen.

The values of the different classes of the Abrams cone slump test are presented in Table 2 below. Two classes of concrete can be distinguished.

Among S3 concretes, the SBC-0.10 and SBC-0.15 differ in that the workability of the SBC-0.15 decreases due to the increase in sugarcane bagasse fibers. Both types of concrete have practically the extreme values of class S3. For class S2 concretes, it can be seen that as the amount of sugarcane bagasse fibers increases, the workability and workability of the mix in the fresh state decreases. These results are in agreement with those found in the literature [20] - [22] . This is due to the high porosity of the bagasse fibers: despite being impregnated in the alkaline solution before mixing, the bio-fibers of bagasse still continue to absorb some of the mixing water during the making of the composite, which reduces the workability of the concrete, hence the use of the superplasticizer, which made it possible to maintain a level of workability similar to classes S2 and S3 and to obtain a structural concrete that is easy to place.

Fibers (sugarcane bagasse and polypropylene) are additive and do not substitute components for cement, sand, or aggregates. They are considered as sewing fibers and their contribution in mass is negligible compared to the percentages of the aggregates. In the case of bagasse and polypropylene fibers concretes (SBC and PC), the fibers are mixed with the cement and aggregates before the water is added to ensure a better distribution of the fibers throughout the mix and to prevent the fibers from agglomerating and sticking together. We can add that problems of agglomeration or the formation of “sea urchin” balls can occur when fibers are introduced into the mix. The fibers could then disrupt the granular arrangement and thus reduce the workability and compactness of the mix.

Compressive strength and splitting tensile strength were tested on cured cylindrical specimens stored in a damp room (100% RH at 20˚C) at 7, 14, 28 and 112 days. Each value is the average of three specimen values per concrete age. Compression and tensile tests were carried out in accordance with standard NF P15-471.

The simple compression tests were carried out on the 16 × 32 cm cylindrical specimens. The results obtained are shown in Figure 3 below.

Figure 4 above shows the compressive strength as a function of fibers content. This clearly shows that as fibers content increases, compressive strength decreases.

Whatever the number of days, the compressive strength decreases with the compressive strength and shows a minimum function of the percentage of fibers incorporation at 28 and 112 days. The highest compressive strength is that of OC. There is no significant difference between the compressive strengths at 28 and 112 days, but although those of SBC-0.15, SBC-0.17 and PC decrease by 20% compared to OC, they remain within the strength values of ordinary structural concrete, whose strength is limited to between 25 and 50 MPa as stipulated in standard EN 206-1. We also note that there is a strong correlation between sugarcane bagasse fibers content and compressive strength for both maturities. As the sugarcane bagasse fibers content increases, the compressive strength of the concrete decreases. This relationship is in line with several research studies carried out on plant fiber-reinforced concrete. The use of plant fibers does not improve the compressive strength of concrete, as they increase the volume of voids and reduce the compactness of the composite [20] - [24] . These results are in agreement with those obtained by other authors [25] - [29] . It should be noted here that for both maturities (at 28 days and 112 days), compressive strength decreases progressively with increasing fiber content.

Tensile splitting tests (Brazilian test) were carried out on samples of cylindrical concrete (16 × 32) cm, as shown in Figure 5 below. The results of the tensile splitting test are shown after 28 and 112 days in water at 20˚C ± 2˚C.

Figure 6 below shows the tensile strength as a function of fibers content.

At 28 days, the strengths of bagasse fibers SBC-0.10 (2.90 MPa), SBC-0.25 (2.81 MPa) and PC (2.9 MPa) are lower than those of the reference concrete OC (3.05 MPa). On the other hand, SBC-0.15 (3.08 MPa), SBC-0.17 (3.09 MPa) and SBC-0.23 (3.06 MPa) have higher strengths than OC. At 112 days, all the fiber-reinforced concretes still show very good results compared with ordinary OC concrete (2.88 MPa). SBC-0.25 concrete (2.75 MPa) is the exception, remaining lower than the OC reference concrete. The best values are those of concretes SBC-0.15 (3.08 MPa at 28 days and 3.04 MPa at 112 days) and SBC-0.17 (3.09 MPa), which increased by almost 7% at 28 days and up to 6% at 112 days (3.06 MPa). For both 28 and 112 days, we note that above 0.17% incorporation of sugarcane bagasse fibers, the tensile strengths of the concretes decrease. And the coefficient of variation (the ratio between the standard deviation and the mean) of the concretes subjected to tensile strength at 28 days and 112 days gives us a value close to 100%. This gives us a homogeneous distribution for the concretes at 28 days and 112 days, so in terms of mechanical tensile strength by splitting, the resistance of all the concretes underwent a homogeneous variation.

Sugarcane bagasse fibers improve the tensile strength of concrete and reduce crack propagation, particularly in the early stages. This phenomenon depends on the quantity of fibers incorporated into the concrete, up to a certain threshold where the effect is reversed, as is the case with other types of natural fibers [30] [31] . In our case, the threshold not to be exceeded is 0.17%. We can also say that the improvement in tensile strength is due to the stitching effect of the fibers in the matrix [32] [33] .

The ability of sugarcane bagasse fibers to stretch, given their remarkable tensile strength, delays the cracking of concrete, thus preventing its sudden ruin, as in the case of a low-magnitude earthquake [32] .

However, once the fiber percentage threshold is exceeded, the tensile strength of the concrete decreases, probably as a result of the superposition of two potential phenomena: the effect of non-uniformly dispersed fibers in the matrix and the weakening of the cementitious matrix caused by the reduction in cement volume [28] [29] [34] . Taking into account the compressive and tensile strengths, we can conclude that the concretes SBC-0.15 and SBC-0.17 are the two composites with the best sugarcane bagasse fibers content in terms of mechanical properties. These results are in agreement with those found in previous work by other authors [25] [26] - [29] .

We measured the porosity over 24 hours of immersion in water. The specimens were manufactured and demoulded after 24 hours. The dry weight (Ps) was measured using a 30 kg Kern precision balance. The test tubes were then immediately and completely immersed in the water tank. The results of the different samples are shown in Figure 7 below.

The varies between 3.2 and 7.2%. Ordinary concrete (OC) is the least porous and SBC-0.17 concrete is the most porous. It can be seen that the total water porosity increases when the sugarcane bagasse fibers content varies between 0.10 and 0.17% (from SBC-0.10 to SBC-0.17) and decreases when the sugarcane bagasse fibers content varies between 0.17 and 0.25% (from SBC-0.17 to SBC-0.25). For propylene fibers concrete (PC), the porosity is close to that of SBC-0.10. Previous research studies confirm our results regarding the increase in total porosity in fiber-reinforced concrete [28] [29] . It is the fibers dispersed in the concrete that retain some of the water.

With the exception of SBC-0.23 and SBC-0.25, we can state that the total porosity values are proportional to the percentage of sugarcane bagasse fibers added. This exception, which is inconsistent with the literature, may be due to the poor dispersion of the sugarcane bagasse fibers in the concrete when they are added at a rate greater than 0.17%. Above this value, balls of sugarcane bagasse form, causing heterogeneous parts in the concrete samples, preventing water from entering certain parts of the composite by making it less porous. The unexpected results for SBC-0.23 and SBC-0.25 are probably distorted by the concentration of sugarcane bagasse fibers in certain parts of the composite. We can therefore say that the mechanism of variation is as follows: porosity increases with the addition of fibers up to 0.17% or it tends to stabilise, and drops after 0.17%.