2.1 Material

For the preparation of test specimens, ordinary Portland cement (Type I) conforming to ^{KS L 5201(2021)}(Portland cement) was used. The physical and chemical properties of the cement utilized are summarized in Table 1. River sand with a fineness modulus of 2.45 was employed as the fine aggregate, while coarse aggregate of sizes less than 20 mm was selected. To evaluate the particle size distribution characteristics of both fine and coarse aggregates, sieve analysis was conducted according to ^{KS F 2502 (2019)}(Standard test method for sieve analysis of aggregates). The particle size distribution curves resulting from the sieve analysis are presented in Fig. 1. The uniformity coefficient and curvature coefficient for the coarse aggregate were determined to be 1.81 and 1.05, respectively, while those for the fine aggregate were 3.70 and 1.25, respectively. The coarse aggregate was immersed in water for more than 12 hours and then used in a surface-saturated dry condition for the mix. The PVA mono-fiber used as the reinforcing fiber had a length of 12 mm, and its physical properties provided by the manufacturer are summarized in Table 2.

Fig. 1. Particle Size Distribution Curves of Aggregates

Fig. 2. Equipment Used for Grinding and Drying, (a) Oven for Drying, (b) Vibration Mill for Grinding

Fig. 3. Image of Biochar in Different Forms, (a) Biochar, (b) Grinded Biochar

Fig. 4. SEM images of Biochar at ×5,000 and ×10,000 Magnifications, (a) SEM of Grinded Biochar (×5,000), (b) SEM of Grinded Biochar (×10,000)

The biochar used in this experiment was a product of Company S, produced from wood pellets at high temperatures ranging from 650 to 800°C. To obtain a particle size similar to that of cement, the biochar was dried in an oven at approximately 100°C for over 24 hours, as illustrated in Fig. 2(a). It was then ground using a vibration mill, as depicted in Fig. 2(b). Fig. 3 shows the biochar before and after grinding. SEM (Scanning Electron Microscopy) images of the biochar at magnifications of ×5,000 and ×10,000 revealed a porous structure, as illustrated in Fig. 4. The elemental composition of the biochar, analyzed using SEM-EDS (Energy Dispersive Spectroscopy), is summarized in Table 3 and illustrated in Fig. 4. According to ^{Spokas (2010)}, an oxygen-to-carbon (O/C) ratio of less than 0.2 indicates long-term chemical stability. The biochar used in this study had an average O/C ratio of 0.18, indicating it possesses chemical stability.

The specific surface area, pore volume, and pore size of the ground biochar were measured using BET (Brunauer- Emmett-Teller) analysis, with the results presented in Table 4. The particle size distribution of both the cement and the ground biochar was analyzed using PSA (Particle Size Analysis), and the results are summarized in Table 5 and illustrated in Fig. 5. Emphasis was placed on achieving a particle size distribution for the ground biochar that closely matched that of the cement. It was found that over 90 % of the ground biochar had a particle size of 17.12 µm, which is less than 20 µm. Similarly, more than 90 % of the cement particles had a size of 19.01 µm, also less than 20 µm.

Fig. 5. Particle Size Distribution Curves of Cement and Grinded Biochar, (a) Grinded Biochar, (b) Cement

2.2 Experimental Variables and Mixture

BC is a concrete in which part of the cement is replaced with biochar at a specific ratio, and PBC is BC reinforced by adding PVA fibers. For each type, the percentage replacement of biochar varies from 1 % to 5 %. The concrete mix proportions are provided in Table 6, with a water-to-binder (W/B) ratio of 50 % and a sand-to-aggregate (S/A) ratio of 45 %. The biochar replacement levels were set to increase incrementally from 0 % to a maximum of 5 % by cement mass ratio (wt %). The inclusion rate of PVA fibers was fixed at 0.07 % by volume (vol %) as recommended by the manufacturer.

Fig. 6. Concrete Mixing Process, (a) Coarse Aggregate, (b) Fine Aggregate, (c) Cement, (d) Biochar, (e) Water + SP + AE, (f) PVA, (g) Curing in Air, (h) Curing in Water

To analyze the mechanical properties of the concrete, specimens were prepared in accordance with ^{KS F 2403(2019)} (Standard test method for making concrete specimens). The prepared specimens underwent 24 hours of air curing, were demolded, and then subjected to 28 days of water curing at a constant temperature of 20 ± 2°C, as illustrated in Fig. 6(h).

2.3 Test Method and Equipment

To measure the workability of the fresh concrete, the slump test was conducted in accordance with ^{KS F 2402(2022)}(Test method for concrete slump). The air content of the fresh concrete was determined using the pressure method as specified in ^{KS F 2421(2016)}(Standard test method for air content of fresh concrete by the pressure method (Air receiver method)). To evaluate the mechanical properties of the concrete, tests were performed following ^{KS F 2405(2022)}(Test method for compressive strength of concrete), ^{KS F 2408(2016)}(Standard test method for flexural strength of concrete), and ^{KS F 2423(2021)}(Standard test method for tensile splitting strength of concrete), as depicted in Fig. 7. The compressive strength and tensile splitting strength were measured using cylindrical specimens (Φ100 × 200 mm), as shown in Fig. 8(a). The flexural strength was measured using prismatic specimens (100 × 100 × 400 mm), as depicted in Fig. 8(b). A universal testing machine (UTM) with a capacity of 200 tons was used for these experiments.

Fig. 7. Test Setup, (a) Compressive Test, (b) Splitting Tensile Test, (c) Flexural Test

Fig. 8. Schematic of Test Specimens, (a) Cylinder (Φ100 × 200 mm), (b) Prism (100 × 100 × 400 mm)

3.1 Results of HS FE-SEM

Fig. 9(a) and (b) present the SEM images of BC (biochar added concrete) and BPC (PVA fiber reinforced biochar concrete) taken at a magnification of ×10,000. The images reveal the interaction between biochar, PVA fibers, and cement compounds. In both cases, the SEM images show that the biochar and PVA fibers are integrated into the cement matrix, indicating a cohesive bond with the cementitious materials.

Fig. 9. SEM Images of the Concrete with Biochar and PVA Fiber, (a) SEM Image of BC, (b) SEM Image of BPC

3.2 Results of Slump and Air Content Test

The results of the slump and air content tests for the fresh concrete are presented in Table 7 and Fig. 10. The slump test results, which measure the workability of fresh concrete, indicated that the slump value for the BC0 was 225 mm. It was observed that the incorporation of biochar and PVA fibers into the concrete resulted in a reduction in slump. This trend can be attributed to the bridging effect of PVA fibers, which prevents the concrete from flowing easily ^{(Lee, 2010)}. Additionally, as the replacement rate of biochar increased, the slump of the concrete decreased, likely due to biochar's water retention capability ^{(Lehmann, 2007)}.

The generally low slump value for PBC 4 % in Fig. 10(b) reflects the overall decreasing trend in workability due to the combined effects of biochar and PVA fibers. As the biochar replacement rate increases, the water retention capacity of the biochar reduces the free water available in the mix, leading to a lower slump. Additionally, PVA fibers create a bridging effect that further restricts the flow of the concrete. These combined effects result in a significant reduction in workability. Furthermore, the inconsistency in the internal structure of the concrete and the non-uniform dispersion of PVA during specimen preparation also contribute to the notably low slump value ^{(Kim et al., 2023)}.

Fig. 10. Comparison of Slump and Air Content of Fresh Concrete, (a) BC, (b) PBC

The air content test results for the fresh concrete showed that the biochar and PVA fiber-reinforced concrete (BPC) maintained an air content of approximately 3 % across all specimens. This consistency is likely due to the presence of voids at the interface between the PVA fibers and the concrete matrix, which are caused by the fiber inclusion ^{(Won et al., 2005)}. For the BC specimens as shown in Fig. 10(a), it was observed that as the replacement rate of biochar increased, the air content tended to decrease. This reduction in air content is associated with the particle size of the biochar. In this study, most of the biochar particles were ground to below 20 µm. The porous structure of the biochar particles allowed the cement matrix to fill the voids more densely, thereby reducing the overall porosity ^{(Alice et al., 2022; Han and Choi, 2023)}.

3.3 Experimental Results and Analysis of Mechanical Properties of Concrete

3.3.1 Results and Analysis of Compressive Strength Test

The compressive strength results for concrete specimens, both PVA-reinforced (BPC) and non-reinforced (BC), with varying biochar replacement ratio (0 % to 5 %), are summarized in Table 8. As shown in Fig. 11 the incorporation of biochar as a partial cement replacement generally leads to a decrease in compressive strength for both non-reinforced and PVA-reinforced concrete. This trend is more pronounced at lower biochar replacement levels (1 % to 3 %), where the reduction in strength is significant. However, at higher levels of biochar (4 % and 5 %), the adverse effect on compressive strength appears to stabilize or slightly improve, particularly in the PVA-reinforced specimens.

The presence of PVA fibers positively influences the compressive strength development, especially over the longer term (28 days). The PVA-reinforced concrete specimens exhibit higher 28-day compressive strengths compared to their non-reinforced counterparts. This suggests that PVA fibers enhance matrix integrity. They also contribute to improved load distribution and crack bridging. This effect can be attributed to the formation of a fiber network within the concrete, which helps to arrest crack propagation and distribute loads more evenly throughout the material ^{(Xie et al., 2024)}. This network formation is critical for maintaining higher strengths despite the addition of biochar.

Fig. 11. Results of Compressive Strength Test, (a) Day 7, (b) Day 28

Table 8. Summary of Compressive Strength Test (Unit : MPa)

Specimen name		Compressive Strength
		7 day		28 day
		Average	Standard deviation	Average	Standard deviation
B C	BC0	23.86	0.21	28.01	0.77
	BC1	17.65	0.15	23.37	1.06
	BC2	22.27	0.38	26.09	1.40
	BC3	21.30	0.57	24.64	0.57
	BC4	22.99	0.53	26.50	0.89
	BC5	21.42	1.21	26.64	0.68
B P C	BPC0	23.59	0.05	31.66	0.22
	BPC1	17.20	0.58	22.89	0.12
	BPC2	21.32	0.21	25.65	0.51
	BPC3	20.04	0.28	25.65	0.36
	BPC4	23.64	0.47	30.81	0.09
	BPC5	22.15	0.32	27.52	0.26

3.3.2 Results and Analysis of Tensile Splitting Strength Test

Table 9 presents the results of the tensile splitting strength tests for both non-reinforced (BC) and PVA fiber-reinforced concrete (BPC) specimens with varying percentages of biochar replacement from 0 % to 5 %. As shown in Fig. 12(a), both BC and BPC specimens exhibit a general trend of decreased tensile splitting strength with increasing biochar replacement up to 3 %, after which there is some recovery in strength for both series. The PVA-reinforced concrete generally shows higher tensile splitting strength compared to non-reinforced concrete, indicating the beneficial effect of PVA fibers. Fig. 12(b) shows a similar trend as the 7-day results, but with overall increased tensile splitting strengths due to continued curing. The BC4 and BPC5 specimens demonstrate relatively higher tensile splitting strengths, indicating optimal biochar replacement levels for maintaining or enhancing tensile strength over time.

Fig. 12. Results of Tensile Splitting Strength Test, (a) Day 7, (b) Day 28

Table 9. Summary of Tensile Splitting Strength Test (Unit : MPa)

Specimen name		Tensile splitting strength
		7 day		28 day
		Average	Standard deviation	Average	Standard deviation
B C	BC0	2.33	0.08	2.85	0.31
	BC1	2.26	0.16	2.60	0.04
	BC2	2.45	0.08	2.92	0.06
	BC3	2.25	0.14	2.53	0.22
	BC4	2.58	0.20	2.72	0.12
	BC5	2.45	0.02	2.87	0.03
B P C	BPC0	2.52	0.38	3.06	0.31
	BPC1	2.10	0.14	2.54	0.09
	BPC2	2.17	0.15	2.58	0.23
	BPC3	2.11	0.06	2.17	0.21
	BPC4	2.46	0.12	2.86	0.03
	BPC5	2.10	0.04	2.98	0.26

3.3.3 Results and Analysis of Flexural Strength Test

Table 10 presents the results of flexural strength tests for both non-reinforced (BC) and PVA fiber-reinforced concrete (BPC) specimens with varying biochar replacement levels ranging from 0 % to 5 %. As shown in Fig. 13(a), general trend of decreased flexural strength with 1 % and 3 % biochar replacement for both BC and BPC specimens, followed by a recovery or slight improvement at higher replacement levels (4 % and 5 %). PVA-reinforced concrete generally shows higher or comparable flexural strength to non-reinforced concrete. Fig. 13(b) shows that flexural strengths for both BC and BPC specimens improve over time. Higher biochar replacement levels (4 % and 5 %) show significant strength recovery or improvement, particularly for non-reinforced concrete. PVA-reinforced concrete shows slightly lower values compared to non-reinforced concrete at 28 days, suggesting a complex interaction between PVA fibers and biochar over time.

Fig. 13. Results of Flexural Strength Test, (a) Day 7, (b) Day 28

Table 10. Summary of Flexural Strength Test (Unit : MPa)

Specimen name		Flexural Strength
		7 day		28 day
		Average	Standard deviation	Average	Standard deviation
B C	BC0	5.50	0.07	5.87	0.05
	BC1	4.73	0.15	5.59	0.09
	BC2	5.47	0.45	6.11	0.40
	BC3	5.17	0.12	5.90	0.07
	BC4	5.64	0.09	6.05	0.22
	BC5	5.44	0.17	6.61	0.11
B P C	BPC0	5.64	0.30	5.36	0.43
	BPC1	4.63	0.35	5.75	0.35
	BPC2	5.29	0.24	5.72	0.50
	BPC3	5.19	0.02	6.16	0.05
	BPC4	4.76	0.21	5.86	0.06
	BPC5	5.22	0.12	5.82	0.39

The inclusion of biochar initially reduces flexural strength, especially at lower replacement levels. However, higher replacement levels (4 % and 5 %) show significant recovery or improvement in strength, particularly for non-reinforced concrete. PVA fiber reinforcement generally helps maintain or slightly improve flexural strength at higher biochar replacement levels. The increased flexural strength observed at higher biochar replacement levels can be attributed to the complexity of crack paths within the concrete matrix due to biochar and PVA fiber addition. This complexity likely increases the fracture energy required for crack propagation, leading to improved flexural performance ^{(Sachini et al., 2023;} Ahmad et al., 2014).