Influence of the Addition of Carob Ash to Concrete Under High Water Pressure

Concrete is the most widely used material in modern and contemporary construction due to the high global demand for buildings and infrastructure. Concrete structures are expected to have high quality and durability properties to withstand adverse environmental conditions, such as cold, heat, and rain, throughout their design life [1]. Despite this belief, numerous studies have shown that microcracks and the permeability of concrete allow the penetration of environmental liquids and gases, which can degrade and deteriorate the material directly or indirectly, reducing its lifespan [2].

The existence of this problem is common, and in recent years, positive studies have been conducted on the incorporation of different products into the construction industry, such as carob ash, for concrete. Australia is one of the countries with the highest annual waste utilization rate, where carob ash is among its main wastes used to manufacture concrete [3]. It is well known that carob ash, as a cementitious aggregate, meets physicochemical requirements and can promote binding properties when combined with cement. This improves the performance of concrete by making it more workable, resistant, and durable. Furthermore, this results in economic gains and environmental benefits, according to their online article [3].

Over the years, the population and housing construction have increased, and these are often built informally. This is, for many, the only option left for building their own homes at a low price. Without professional advice, they generate cost overruns, making their housing unsafe. It was then identified that due to this need, there are leaks in some concrete structures. These leaks can be caused by heavy rains, rising water tables, since these constructions are often built near rivers. The most frequent phenomena that cause penetration are: capillarity, penetration of water under pressure or permeability [4].

The increasing concern over environmental problems caused by the production and destruction of aggregates, and the insufficient storage area for the generated waste, has prompted many countries to explore waste reuse and recycling. Numerous studies have been conducted on these topics [5]. This growing global concern about energy and material consumption in the construction industry has sparked a growing interest in the use of organic-based materials in recent years. Furthermore, organic-based building materials can provide several advantages, such as improved thermal insulation, significantly improved concrete mechanics, and reduced weight in conventional building structures. They are also environmentally friendly due to the reuse of waste [6]. The problem of waste accumulation is a global problem and has become a concern in today's society, causing enormous environmental damage. Thus, the production of cement, as a chemical compound, generates highly polluting CO₂, as well as the longevity of concrete [7]. Given this situation, there is a need to explore alternative materials that can reduce the environmental impact, improve the durability of concrete, and utilize currently underutilized waste. One way to reduce its environmental impact and also increase its durability is to reuse this waste in new materials such as carob ash [8].

In this context, the use of agro-industrial waste, such as carob ash (CA), appears to be a promising alternative. This ash, generated from the combustion of carob wood residues, has been shown to have pozzolanic properties that can improve the mechanical strength and impermeability of concrete, in addition to contributing to more sustainable construction [3].

The construction sector is among the largest consumers of natural resources, leading to precious resources depleted each year and the environmental damage caused by their extraction. The construction industry generates a large amount of solid waste, commonly referred to as construction waste, which is often disposed inadequately [9]. The agricultural sector produces a large amount of different waste that can be studied for application in the construction industry. One of these wastes, considered in this article, carob ash (CA), can serve as a filler material in concrete. The development of sustainable concrete using agro-industrial waste responds to this need, reducing the consumption of natural resources, mitigating pollution, and improving the energy efficiency of the construction sector [10].

In the Puno region, a notable problem is the low strength of concrete. This is due to multiple factors such as low temperatures, high water pressure, constant humidity, and technical deficiencies, among others. As a result, concrete suffers from cracking, detachment, and low compressive strength [11]. Therefore, the application of this type of modified concrete can represent a practical and economical solution. Currently, there are problems caused by excessive stormwater runoff, as well as the lack of drainage systems in many cities, flash floods in urban areas, groundwater contamination, water shortages, increased contaminated water, and environmental impacts [12].

International and national studies have reported significant increases in compressive strength, tensile strength, modulus of elasticity, and water penetration in concrete when using plant-based ash as a partial substitute for cement [3, 8, 13, 14]. In addition to the structural benefits, its use contributes to mitigating the environmental impact of conventional concrete by reusing agro-industrial waste that would otherwise be discarded.

According to the study conducted by Sarabia Orihuela et al. [13], the objective of their project in Brazil is to evaluate the performance of using carob ash as a percentage substitute for cement. The method used is quantitative experimental. After 28 days of sample curing, compressive strengths of 26.52 MPa at 0%, 30.59 MPa at 6%, 29.38 MPa at 10%, and 31.21 MPa at 14%, respectively, were found. Therefore, it was concluded that the ideal proportion to improve compressive strength was 6% ash. According to Bernaola Fuentes and Guardapuclla Espinoza [14], in their project in Cusco, the objective is to evaluate the influence of eucalyptus trunk ash on the physical and mechanical properties of concrete with a density of 210 kg/cm². For this purpose, he uses a practically experimental and applied methodology. His research found a sample of 0% standard concrete along with three samples for the ash percentages of 5%, 9% and 13% of the additive. When breaking the concrete during the 7 days of curing, it presented average compressive and flexural strengths of 148.13 kg/cm² and 3.83 MPa for 0%, 149.11 kg/cm² and 3.98 MPa for 5%, 149.96 kg/cm² and 4.21 MPa for 9%.

In the study [15], the behavior of concrete with a design strength of 210 kg/cm² was evaluated by incorporating carob ash calcined at 600℃ in proportions of 5%, 6%, and 7%. The results after 28 days of curing indicated that the 6% addition produced the best results: 336 kg/cm² in compressive strength, 25 kg/cm² in tensile strength, and 26.4 kg/cm² in flexural strength, significantly surpassing the standard concrete. However, increasing the proportion to 7% resulted in a reduction in mechanical properties, with a compressive strength of only 260 kg/cm². A 6% addition was found to improve compressive, tensile, and flexural strength compared to the standard concrete. However, it was observed that exceeding this percentage resulted in decreased strength, highlighting the importance of determining the optimal dosage to maximize benefits without compromising concrete quality [16]. This study evaluated the effect of incorporating dry carob ash into concrete paving stones, using percentages of 0.75%, 2%, 4%, 6% and 8%. The results showed that the mixture with 6% carob ash showed the greatest improvement in compressive strength, reaching a value of 413.34 kg/cm², standing out as the strongest mixture compared to the other additions. However, when the addition of ash was increased to 8%, the strength began to decrease, suggesting that there is an optimal limit of ash to obtain the best results without compromising the strength of the concrete.

This research evaluates the resistivity and resistance of conventional concrete of 210 kg/cm², incorporating the ash derived from the calcination of firewood obtained from carob trees destined for the production of Junín wood, located in the province of Manabí. The ash was used as a partial replacement for cement at 5%, 10% and 15%, which makes a comparison between aggregates from two quarries, one located in the province of Manabí (Quarry "A") and the other in the province of Santo Domingo de los Tsáchilas (Quarry "B"). To produce the ash, sugarcane bagasse was calcined at a controlled temperature of 700℃ for 2 hours. Cylindrical specimens measuring 10 cm×20 cm were prepared and subjected to measure surface resistivity and perform the respective compression tests for ages of 7, 14, 21 and 28 days of curing. The results indicated an improvement in concrete strength compared to the standard design at 5% and 10%, with a decline observed at 15% CA replacement in both quarries. In addition, the resistivity range remains moderate for the standard design, 5% and 10% replacements, while reaching a high range at 15% CA replacement in both quarries.

The study aims to investigate the use of murumuru bark ash (MBA), an agro-industrial waste generated specifically in the Amazon region, as a partial replacement for cement in structural concrete. It also evaluates the physical, chemical, and mineralogical characteristics of the ash as a filler in concrete, as well as its fresh and hardened properties [17]. To this end, the MBA was subjected to physical-mechanical characterization tests, such as specific mass, pozzolanic activity with Portland cement, pozzolanic activity with lime, and BET testing. Mineralogical and chemical analyses of the ash were also performed. The results demonstrated the technical feasibility of partially replacing 6% of MBA in cement using a plasticizing additive to improve workability, demonstrating an improvement in the physical-mechanical and durability properties of the concrete.

The Ilo 21 thermal power plant generates electricity from coal and whose residue is fly ash, an environmentally polluting material, which is used as an additive in cement for the manufacture of concrete for different civil works, in this sense the objective of this study was to determine the dosage of concrete mixtures with fly ash in such a way that it does not decrease the resistance and helps mitigate the environment [3]. The material and method used is normal concrete with additions of fly ash in proportions of 2.5%, 5.0%, 10.0% and 15.0% for breaks at 7, 14, 28 and 90 days. The results indicate that, at 28 days, the average strengths were 221 kg/cm² for normal concrete, 223 kg/cm² for concrete with 2.5% fly ash, 231 kg/cm² for 5.0%, 200 kg/cm² for 10.0% and 192 kg/cm² for 15.0% fly ash. In conclusion, it is recommended to use fly ash as a substitute for cement in proportions less than 10%. Beyond this value, the strength of the concrete decreases, which can be detrimental when performing quality controls.

The purpose of this research was to analyze the behavior of low-permeability concrete with compressive strengths of f'c = 175 kg/cm², 210 kg/cm², 245 kg/cm², and 280 kg/cm² under high hydraulic pressure conditions [18]. During the development of the research, 56 specimens were prepared at 28 days of age, primarily for compressive strength, indirect tensile strength, and water penetration depth tests at a pressure of 50 mca (meters of water column), according to the European standard UNE-EN-12390-08 [19]. The conclusion was that concrete with a design of 210 kg/cm² is the most suitable for hydraulic construction, as it favors economic savings and minimizes potential damage to infrastructure and society. In this context, this research aims to evaluate the effect of different percentages of cement replacement with carob ash on the performance of structural concrete with a design strength of f'c = 210 kg/cm². The objective is to determine whether this addition improves the concrete's compressive strength, diametrical compressive strength, and modulus of elasticity, as well as its behavior under high-pressure water penetration. It also aims to identify the optimal percentage of ash that maximizes these properties. Ultimately, this study is expected to provide technical evidence on the viability of using carob ash as an alternative for the production of stronger, more economical, and environmentally friendly concrete.

In this project, a mix design was carried out for a concrete of 210 kg/cm², in which different percentages of carob ash (3%, 6%, 9% and 12%) are added, carrying out different laboratory tests.

• Recognize the chemical composition of carob ash to be used in concrete f'c=210 kg/cm².

Table 5. Chemical composition of carob ash

The ash was fired at 800°C and sieved through a 50 mesh screen. Table 5 shows that the chemical analysis revealed 58.14% SiO₂, 15.12% Al₂O₃, and 6.19% Fe₂O₃, giving a combined total of 79.45%. Other components were also identified, such as 0.74% SO₃, 9.06% CaO, and 1.34% MgO. Table 6 compares this composition with the limits established by ASTM C618 for class F materials, showing that the material meets the requirements for use as a pozzolanic admixture in concrete mixtures.

• To determine how much the addition of 3%, 6%, 9% and 12% carob ash influences the compressive strength of concrete f'c=210 kg/cm² subjected to high water pressures, Lambayeque 2024.

Table 6. Comparison of the chemical properties of the CA, according to the ASTM C618 standard

Table 7. Compressive strength at the age of 7 days

The compressive strength results after 7 days of curing show a significantly influenced behavior by the incorporation of CA in the concrete mix. The control sample reached an average strength of 155.52 kg/cm², corresponding to 74% of the design strength, which is consistent with the typical progress in mechanical development at that age. The addition of 3% CA generated a moderate increase of 8.5%, reaching 168.77 kg/cm², while the 6% dosage presented the maximum value at 190.27 kg/cm², which represents an increase of 22.3% compared to the standard concrete (Table 7). This behavior suggests that, at this proportion, carob ash acts as an effective pozzolanic addition, reacting with the free lime produced by cement hydration, promoting the additional formation of cementitious compounds such as hydrated calcium silicate (C-S-H), responsible for the increase in mechanical strength. In contrast, the 9% dosage showed a strength of 189.46 kg/cm², similar to the 6%, which would indicate a possible saturation of the pozzolanic effect, where a higher content does not lead to a proportional improvement. Finally, with a 12% addition, the strength decreased to 162.28 kg/cm², which can be attributed to a dilution effect of the cement content, in addition to possible physical interference with matrix compaction, reducing the effectiveness of the hydration process. These results indicate that there is an optimal addition threshold, and that excessive CA. content can compromise the mechanical properties of concrete.

The ANOVA test showed statistically significant differences between groups (F = 2031.497, p < 0.001), indicating that the addition of CA influences concrete strength (Table 8). The highest strengths were obtained with the 6% and 9% CA dosages (190.27 and 189.46 kg/cm², respectively). The confidence intervals reveal that these groups with higher activated ash content are clearly differentiated from the standard concrete, confirming a notable improvement in compressive strength (Table 9). The overall trend shows that the percentage increase in activated ash improves strength up to a certain point, before beginning to decrease slightly at 12%.

The ANOVA test performed between the 6% and 9% carob ash dosages showed a value of F = 3.618 and p = 0.130 (Table 10), indicating no statistically significant difference between the two compressive strengths. This suggests that, statistically, both dosages offer similar performance.

Table 8. ANOVA test for 7-day compressive strength for all CA ratios

Table 9. Confidence intervals for compressive strength at 7 days of age

Table 10. ANOVA test on 7-day compressive strength for 6% and 9% CA

After 14 days of curing, the concrete reached an advanced stage of mechanical strength development, with the standard sample recording an average value of 197.78 kg/cm², equivalent to 94.2% of the design strength (f'c = 210 kg/cm²), which is consistent with the levels expected at that age. The incorporation of carob ash, an agro-industrial waste with pozzolanic potential, showed varying effects depending on its dosage. With 3%, an average strength of 214.21 kg/cm² was obtained, exceeding conventional concrete by 8.3%, demonstrating a functional improvement within the concept of sustainable concrete. The mixture with 6% reached maximum performance with 243.20 kg/cm², representing an increase of 22.9%, suggesting an optimal interaction between the cement compounds and the reactive characteristics of the ash. In contrast, when using 9%, the strength decreased slightly to 229.97 kg/cm², indicating decreasing efficiency from this point on. Finally, with 12% replacement, the strength decreased to 208.89 kg/cm², implying a 4.5% loss relative to the standard mix, possibly due to the supersaturation of non-reactive fine particles that interfere with the proper development of the cementitious matrix (Table 11).

Table 11. Compressive strength at 14 days of curing

Table 12. Confidence intervals for compressive strength at 14 days of age

The standard concrete has a margin of error of 0.19, giving a confidence interval between 197.59 and 197.97 kg/cm². In the case of 6%, it had a margin of error of 0.84, and a range between 242.36 and 244.04 kg/cm². These results reflect a significant improvement in the concrete's strength with the ash, while maintaining narrow and consistent confidence intervals (Table 12).

Analysis of variance (ANOVA) indicated statistically significant differences in 14-day compressive strength between the various carob ash dosages (p < 0.001) (Table 13). The 6% dosage presented the highest strength (243.2 kg/cm²), with a 23% increase compared to the standard concrete (197.78 kg/cm²), demonstrating a positive effect of the additive up to that point. From then on, with 9% and 12%, the strength decreased, although it remained above the standard value. To further compare the higher dosages, a second ANOVA was performed only between 6% and 9%, which also showed statistically significant differences (F = 715.683; p < 0.001), confirming that 6% is statistically superior to 9% in terms of strength (Table 14). The low standard deviation across all groups supports the consistency of the results and suggests that the optimal dosage for improving concrete strength with carob ash is 6%.

Table 13. ANOVA test on compressive strength at 14 days

Table 14. ANOVA test on compressive strength at 14 days for the 6% and 9% CA ratios

Table 15. Compressive strength at 28 days of curing

After 28 days of curing, the standard stage for evaluating the final mechanical strength of concrete, the standard mix reached an average strength of 217.51 kg/cm², exceeding the design value by 3.6% (f'c = 210 kg/cm²) and satisfactorily meeting the established structural requirements. The inclusion of carob ash, an agro-industrial waste with pozzolanic potential, produced differentiated effects depending on the percentage used. With 3%, a strength of 234.97 kg/cm² was obtained, representing increases of 8% compared to the standard mix and 12% compared to the design, indicating a moderate improvement attributed to the cementing contribution of the ash. With a 6% proportion, maximum performance was achieved at 269.40 kg/cm², equivalent to an increase of 23.9% compared to the standard and 28.3% compared to the design value. This result reaffirms that this dosage optimizes concrete matrix densification and favors the formation of additional hydrated products, consolidating its application in high-performance sustainable concrete. In contrast, the 9% strength registered a strength of 256.23 kg/cm², still high (17.8% higher than the standard), but without exceeding the response obtained with the 6%, suggesting a saturation threshold in the reactivity of the supplementary material. Finally, the use of 12% yielded a strength of 226.82 kg/cm², which, while slightly exceeding the standard (4.3%), reflects a significant reduction compared to optimal dosages (Table 15). This decrease can be attributed to the excess of non-reactive material, which interferes with cement hydration and reduces the compactness of the system. Together, the results reinforce the potential for using agro-industrial ashes in the formulation of sustainable concrete, provided they are used in appropriate proportions to maximize concrete performance without compromising its structural properties.

The standard concrete reached 217.51 kg/cm², with a margin of error of 0.50 (range 217.01-218.01). The 6% ash content reached 269.40 kg/cm², with a margin of error of 0.25 (range 269.15-269.65), showing the greatest improvement. The 3% content also improved, reaching 234.97 kg/cm² (range 233.86-236.08). These results highlight that carob ash, especially at 6%, significantly improves strength with small and consistent margins of error (Table 16).

Table 16. Confidence intervals for compressive strength at 28 days of age

Table 17. Result of the analysis of variance for compressive strength at 28 days

The ANOVA performed on the 28-day compressive strength results showed that the differences between the mixtures with the addition of carob ash are statistically significant (Table 17). When specifically comparing the two dosages of 6% and 9%, it was observed that the mixture with 6% carob ash showed superior performance, reaching a strength of 269.4 kg/cm², compared to 256.23 kg/cm² obtained with 9%. These results indicate that, although both mixtures improved strength compared to the standard concrete, the 6% dosage maximizes the benefit of the carob ash, providing a considerable increase in strength without reaching the negative effects observed when using higher doses. The F value obtained in the ANOVA (2956.139) and the p value < 0.001 reinforce the validity of this difference, highlighting the effectiveness of 6% as the most optimal dosage for this type of addition in improving the mechanical properties of concrete (Table 18).

• To evaluate how much the addition of 3%, 6%, 9% and 12% carob ash influences the tensile strength of concrete f'c=210 kg/cm² subjected to high water pressures, Lambayeque 2024.

Table 18. ANOVA test on compressive strength at 28 days for the 6% and 9% CA ratios

The results of diametrical compressive tensile strength after 7 days of curing show a significant improvement in the mechanical behavior of the concrete with the addition of carob ash, an agro-industrial waste with pozzolanic properties. The standard mix, with a design strength of 210 kg/cm², reached an average tensile strength of 18.49 kg/cm², used as a reference to evaluate the effect of the different formulations. The mix with 3% ash showed a strength of 20.06 kg/cm², equivalent to an 8.5% increase compared to the standard. With 6% ash, the maximum value of 24.35 kg/cm² was recorded, representing a 31.7% improvement, suggesting a notable strengthening of the matrix due to the formation of additional hydrated products and greater internal cohesion. The 9% mixture showed similar performance, reaching 24.19 kg/cm² (30.9% increase), although it did not exceed the 6% mixture, which would again indicate a saturation point in the efficiency of the supplementary material. Finally, with a 12% replacement, the strength was 21.51 kg/cm², with a 16.3% increase, although lower than the maximum levels obtained (Table 19). These results confirm that, in addition to improving compressive strength, carob ash also contributes to the development of tensile strength, especially when used in optimal proportions, reaffirming its viability in the design of high-performance sustainable concretes.

The standard concrete had a standard deviation of 0.03 and a margin of error of 0.04, with a confidence interval between 18.45 and 18.53. With the addition of ash, the strength increased significantly, reaching 24.35 kg/cm² with 6% ash (standard deviation of 0.05) and 24.19 kg/cm² with 9% (standard deviation of 0.03). The margin of error in these mixtures was 0.06 and 0.03, respectively, giving confidence intervals between 24.29 and 24.41 for 6% and between 24.16 and 24.22 for 9% (Table 20).

Table 19. Diametric compression tensile strength 7 days of curing

Table 20. Confidence intervals for tensile strength at 7 days of age

Table 21. Result of the analysis of variance for tensile strength at 7 days

Table 22. ANOVA test on tensile strength at 7 days for the 6% and 9% CA ratios

The ANOVA test for all addition percentages revealed a highly significant difference between groups (F = 12721.9, p = 0.000), indicating that the addition of ash significantly affects tensile strength (Table 21). Focusing solely on the 6% and 9% concentrations, the ANOVA test also showed a significant difference (F = 23.585, p = 0.008), confirming that, although both concentrations improved strength, there are statistical differences between them (Table 22). These results highlight the effectiveness of carob ash in improving tensile strength, especially at 6% concentrations, with small and consistent margins of error.

The diametrical compressive tensile strength of the concrete at 14 days showed variations that depend on the percentage of carob ash used as a supplementary cementing material. In the standard concrete, the average strength was 21.71 kg/cm² (Table 23). With the addition of 3% ash, the strength increased to 25.75 kg/cm², indicating a significant improvement. When using 6% ash, the strength reached 29.32 kg/cm², the highest value observed. However, when using 9% ash, the strength decreased slightly to 28.75 kg/cm², and with 12%, the strength decreased to 24.71 kg/cm². These results suggest that increasing the ash ratio improves strength up to 6%, but higher concentrations (9% and 12%) appear to negatively affect the cohesion and hydration process of the concrete, which may explain the reduction in strength. The standard concrete had a margin of error of 0.11 and a confidence interval between 21.60 and 21.82 kg/cm² (Table 24). The 6% and 9% dosages showed low levels of dispersion (standard deviations of 0.10 and 0.17, respectively) and small margins of error (0.12 and 0.21), reflecting good consistency in the data.

Table 23. Diametric compression tensile strength 14 days of curing

Table 24. Confidence intervals for tensile strength at 14 days of age

The general ANOVA test showed statistically significant differences between groups (F = 2280.811, p = 0.000) (Table 25), and when specifically comparing the 6% and 9% dosages, a significant difference was also found (F = 26.204, p = 0.007) (Table 26). These results confirm that the addition of ash, especially at 6%, consistently and significantly improves the tensile strength of concrete.

Table 25. Result of the analysis of variance for tensile strength at 14 days

Table 26. ANOVA test on tensile strength at 14 days for the 6% and 9% CA ratios

Table 27. Diametric compression tensile strength 28 days of curing

The strength results of concrete with different percentages of carob ash show that, for the standard concrete with a design f'c of 210 kg/cm², the average strength was approximately 27 kg/cm², which is lower than the expected value. However, when carob ash is incorporated, a progressive improvement in strength is observed, with 3% being the first significant increase with an average of 31.85 kg/cm², exceeding the standard concrete by 17.3%. Upon reaching 6% ash, the average strength increased to 36.24 kg/cm², standing out as the highest value in this series, exceeding the standard by 34.5% and 3% by 13.8%. This behavior suggests that at this dosage, a greater activation of the ash's pozzolanic properties is achieved, favoring the formation of hydrated products that reinforce the concrete matrix. However, increasing the dosage to 9% resulted in a decrease in strength to 34.90 kg/cm², which is still 29.2% higher than the standard, but lower than the strength obtained with 6%. At 12%, the strength dropped to 29.96 kg/cm², which is only 11.4% higher than the standard concrete, and is also lower than 9% and 6% (Table 27). This behavior indicates that the incorporation of carob ash improves strength up to an optimal limit (6%), beyond which higher concentrations (9% and 12%) can negatively alter concrete cohesion, possibly due to an oversaturation of non-reactive fine particles that interfere with cement hydration and affect the internal structure of the material.

The standard concrete had a standard deviation of 0.09, with a margin of error of 0.11 and a confidence interval between 26.89 and 27.11 kg/cm². The highest strength was achieved with the 6% dosage, followed by the 9% dosage, both with low margins of error (0.09 and 0.13, respectively) and consistent data (Table 28).

Table 28. Confidence intervals for tensile strength at 28 days of age

The results of the ANOVA test performed to compare the diametrical compressive tensile strength between the different carob ash dosages showed highly significant differences between the groups. In the overall analysis of all addition percentages (0%, 3%, 6%, 9%, 12%), the F value was 7180.200 with a p value = 0.000, indicating that at least one of the dosages differed significantly compared to the others (Table 29). This difference reflects the significant effect of adding carob ash on improving compressive tensile strength.

Table 29. Result of the analysis of variance for tensile strength at 28 days

When comparing the 6% and 9% additions, the ANOVA analysis also showed a significant difference, with an F value of 327.522 and p = 0.000 (Table 30). This suggests that, although both percentages show an increase in strength compared to the standard, the 6% percentage performs significantly better in terms of tensile strength.

Table 30. ANOVA test on tensile strength at 28 days for the 6% and 9% CA ratios

• Determine how much the addition of 6% carob ash influences the elasticity of concrete f’c=210 kg/cm² subjected to high water pressures, Lambayeque 2024.

Table 31. Elasticity modulus with 0% and 6% M. A addition

Table 32. Confidence intervals for tensile strength at 28 days of age

Table 33. ANOVA test of the modulus of elasticity at 28 days

The results of the modulus of elasticity (Ec) test show that the standard concrete had an average value of 235,799.65 kg/cm². By incorporating 6% carob ash, the modulus of elasticity increased significantly to 276,983.01 kg/cm², representing an increase of 17.4%. This increase in the modulus of elasticity suggests that the addition of carob ash in this proportion improves the concrete's stiffness, indicating greater resistance to deformation under load. The improvement in the mechanical properties of the concrete with 6% carob ash can be explained by the activation of the ash's pozzolanic properties, which favor the formation of additional hydration products, thus strengthening the material's internal structure and optimizing its structural behavior (Table 31, Table 32 and Table 33). The standard deviation of the mixture with 6% ash is notably higher (339,264.60) compared to the standard concrete (201.52), suggesting a greater dispersion in the results of the mixture with ash, possibly indicating variability in its behavior. The margin of error and confidence intervals also reinforce this difference: while the standard concrete has a relatively narrow confidence interval (235,476.67–236,122.63), the interval for the mixture with 6% ash is much wider, reflecting the high uncertainty associated with the measurement of this material (between -266,764.62 and 820,730.64).

The ANOVA test shows that the difference between groups is highly significant, with an F value of 243.96 and a p-value of 0.000, indicating that the incorporation of carob ash into the concrete mix has a significant impact on the modulus of elasticity. This supports the belief that the observed variations are not due to chance, but rather to the addition of carob ash. Despite the high dispersion in the mix with 6% ash, the results point to a substantial change in the concrete's properties, suggesting that ash, while increasing variability, can also improve some mechanical properties, although these should be considered with caution due to the high uncertainty in the results.

• Determine the depth of water penetration under pressure of concrete f'c=210 kg/cm² with the addition of carob ash, Lambayeque 2024.

In the water penetration test, a pressure of 500 kPa was applied for 12 hours at 28 days of curing. The results showed good uniformity between the A and B sides of the samples. For the standard concrete, without the addition of CA, the average maximum penetrations were 25.69 mm, 25.62 mm, and 25.70 mm (Table 34). When incorporating 3% carob ash, a decrease in the size of the water penetration was observed, with average values of 17.00 mm, 25.62 mm, and 25.7 mm, suggesting an improvement in the impermeability of the concrete. However, increasing the carob ash dosage to 6% recorded an increase in penetrations, with values of 22.58 mm, 22.43 mm, and 22.67 mm. For mixtures with 9% and 12% ash, the average maximum penetrations were 24.08 mm, 24.00 mm, 24.13 mm, 24.12 mm, 23.96 mm, and 24.17 mm, respectively. These results indicate that 3% carob ash improves concrete's resistance to water penetration, while higher concentrations do not improve this property and, in some cases, may even impair it.

The margin of error for the standard concrete is relatively small (4.44 to 4.61 mm), indicating greater certainty in the penetration values, while mixtures with higher carob ash content have wider margins of error (1.31 to 3.01 mm), especially at 9%, reflecting greater variability, as shown in Table 35. The confidence intervals for the penetrations in the standard concrete and the mixtures with carob ash are fairly consistent, noting that the 3% ash intervals (15.67 to 18.29 mm) are narrower, reflecting greater reliability in the penetration values for that dosage. The standard deviation varies between the different groups, with lower values in the mixtures with a higher percentage of ash, suggesting greater consistency in the results as the ash addition increases.

Table 34. Maximum penetrations with CA addition (0%, 3%, 6%, 9%, 12%) – 12 hours

Table 35. Confidence intervals for the water penetration test after 12 hours of exposure

Table 36. ANOVA test of the water penetration test at 12 hours of exposure

Table 37. ANOVA test in the water penetration test after 12 hours of exposure to the 3% and 6% CA ratios

The results of the ANOVA test for maximum water penetration at 12 hours reveal a statistically significant difference between the different percentages of carob ash, with an F value = 29.336 and a p value = 0.000, indicating that the differences observed between the groups are not due to random variability (Table 36).

The ANOVA test also shows a clear difference between the results for 3% and 6% carob ash (F = 107.486, p = 0.000), according to Table 37, reinforcing the idea that mixtures with carob ash have a considerable impact on water penetration. In summary, the percentage of ash has a significant impact on water penetration. However, this relationship is not beneficial for concrete waterproofing at higher ash concentrations.

In the water penetration test, a pressure of 500 kPa was applied for 24 hours after 28 days of curing. The results showed good uniformity between the A and B sides of the samples. For the standard concrete, without the addition of carob ash (CA), the average maximum penetrations were 38.84 mm, 38.93 mm, and 38.76 mm (Table 38). When 3% carob ash was added, a decrease in the water penetration depth was observed, reaching average values of 28.76 mm, 28.78 mm, and 28.83 mm. This behavior suggests an improvement in the impermeability of the concrete with a low additive dosage. With 6% carob ash, penetration increased slightly, with average values of 34.30 mm, 34.23 mm, and 33.72 mm, indicating that this percentage does not significantly improve water penetration resistance compared to 3%. When the dosage increased to 9%, penetrations increased further, with values of 43.62 mm, 43.76 mm, and 43.50 mm, indicating that higher percentages of carob ash do not positively contribute to impermeability. Finally, with 12% carob ash, the average penetrations were 39.63 mm, 40.23 mm, and 39.65 mm, showing similar behavior to the standard concrete. In summary, concrete with 3% carob ash showed the lowest water penetration values, indicating that this dosage optimizes the material's impermeability, while higher concentrations did not contribute favorably.

Regarding the consistency of the results, the lowest standard deviations were observed in the mixture with 3% ash, all below 1 mm (Table 38), which translates into smaller margins of error (1.12 to 1.22 mm) and narrower confidence intervals (Table 39), reflecting greater reliability in the data. In contrast, the 6% ash mixture showed greater variability, with standard deviations as high as 1.38 mm and larger margins of error.

Table 38. Maximum penetrations with CA addition (0%, 3%, 6%, 9%, 12%) – 24 hours

Table 39. Confidence intervals for the water penetration test after 24 hours of exposure

Table 40. Confidence intervals for the water penetration test at 24 hours of exposure

Table 41. ANOVA test in the water penetration test after 24 hours of exposure to the 3% and 6% CA ratios

The water penetration results at 24 hours show statistically significant differences between the various percentages of carob ash addition, as confirmed by the general ANOVA test (F = 149.240, p = 0.000), as shown in Table 40. This finding is reinforced when specifically comparing the 3% and 6% dosages, where the ANOVA yielded an F value of 159.464 with a significance level of 0.000, indicating a notable difference between the two treatments, confirmed in Table 41. Although both mixtures reduced penetration compared to the standard concrete, the 3% mixture achieved better results with less variability (standard deviations less than 1 mm and error margins close to 1.1 mm), suggesting more stable and effective performance. In contrast, the 6% mixture showed higher penetration values and greater dispersion, reflected in standard deviations of up to 1.38 mm. The confidence intervals also support this trend, as the 3% confidence interval maintains lower and narrower ranges compared to the 6% confidence interval, reinforcing its effectiveness in improving durability against water ingress. Overall, the statistical analyses show that a moderate addition of ash (3%) significantly improves concrete performance against water penetration.

In the water penetration test, a pressure of 500 kPa was applied for 72 hours at 28 days of curing. The results showed good uniformity between the A and B sides of the samples. For the standard concrete, without the addition of carob ash (CA), the average maximum penetrations were 44.64 mm, 44.78 mm, and 44.77 mm (Table 42). In the design with 3% CA additive, the average maximum penetrations decreased slightly, reaching values of 43.27 mm, 43.20 mm, and 43.39 mm, indicating a slight improvement in the concrete's impermeability. When the carob ash addition was increased to 6%, the penetration values remained stable, with penetrations of 43.26 mm, 43.23 mm, and 43.29 mm. This behavior suggests that 6% carob ash optimizes cohesion between concrete components, improving its resistance to water penetration without generating adverse effects. Increasing the amount of additive to 9% resulted in greater water penetration, with values of 46.58 mm, 46.64 mm, and 46.74 mm, indicating that concentrations above 6% can reduce the concrete's efficiency in terms of impermeability. Finally, in the design with 12% CA additive, the average maximum penetrations were 45.69 mm, 45.78 mm, and 45.68 mm (Table 43), reflecting a trend similar to that observed in the standard concrete. After 72 hours of testing, the concrete with 6% CA additive showed the lowest penetration depths, standing out as the most efficient dosage in terms of impermeability. This optimal performance can be attributed to the ideal proportion of carob ash, which improves the concrete's microstructure without compromising its cohesion. Higher concentrations (9% and 12%) do not appear to provide the same benefits, suggesting that 6% is the optimal level for maximizing resistance to water penetration. All averages obtained meet the requirements of UNE-EN 12390-8, as the maximum penetrations were less than 50 mm, ensuring that concrete with carob ash is suitable for resisting water penetration within the established limits.

Table 42. Maximum penetrations with CA addition (0%, 3%, 6%, 9%, 12%) – 72 hours

Table 43. Confidence intervals for the water penetration test after 72 hours of exposure

The water penetration results obtained at 72 hours show a statistically significant difference between the groups with different ash percentages, as indicated by the general ANOVA test (p = 0.000), with an F value of 16.328, (Table 44), suggesting a clear influence of ash on water resistance. Although the ANOVA test between 3% and 6% ash did not show a significant difference (p = 0.969), (Table 45), the descriptive data indicate a clear trend: the group with 6% carob ash presents greater resistance to water penetration compared to the other percentages. This behavior reinforces the idea that a higher ash concentration improves the waterproof properties of the material.

Table 44. Confidence intervals for the water penetration test after 72 hours of exposure

Table 45. ANOVA test in the water penetration test after 72 hours of exposure to the 3% and 6% CA ratios

The trend observed in the confidence intervals and the margin of error indicates that the addition of 6% ash provides a slight improvement in resistance to water penetration (Table 43). However, it is important to note that as the ash percentage increases above 6%, water penetration also tends to increase. This suggests that higher ash concentrations (such as 9% and 12%) do not contribute positively to the material's watertightness and, in fact, may impair the desired properties, indicating that there is a limit to the effectiveness of carob ash in improving water resistance.