Evaluation of Volumetric Properties of Cassava Peel Ash Modified Asphalt Mixtures

In continuance to providing a reliable and cost-efficient road construction material that would aid the development of sustainable pavements while also eradicating agricultural wastes to protect the environment, Cassava Peel Ash (CPA) modified asphalt mixture is seen to be one of the most viable options. This study aimed to determine the suitability of Cassava Peel Ash (CPA) in hot mix asphalt for improved pavement performance. Using response surface methodology, a central composite design was employed for the mix design parameters, namely coarse aggregate (CA), fine aggregate (FA), mineral filler (MF), bitumen content (BC)


Introduction
In addition to focusing on sustainability, the asphalt pavement sector is dedicated to innovation in pavement construction [1,2]. A well-known technique for producing durable pavements involves combining aggregates with asphalt binder and applying heat and pressure to the mixture [3,4]. The mineral filler is one of the main components of these asphaltic mixtures. They are small, naturally occurring particles that typically pass US Standard Sieve No. 200 that make up the mineral filler used in pavement mixtures [5]. The filler interacts with itself and with other constituent aggregates to assist in stiffening the mix and resisting shearing pressures on the pavement [6]. It adds to the aggregate skeleton's strength by providing additional contact points and forms a high consistency matrix that serves as a cement coarse aggregate together at the mortar level after being combined with a binder [7].
Engineers and specialists are working to improve the qualities of asphalt mixtures to extend the life of asphalt pavement. The use of additives such as fibers and polymers to improve the qualities of asphalt mixtures is standard practice [8,9]. Similarly, the proliferation of various industries, combined with population growth, has resulted in a huge increase in the production of waste materials around the world. Waste resources offer the advantages of being both cost-effective and ecologically friendly when used as secondary materials [10,11]. Recent research efforts focus on incorporating recycled materials to enhance the properties of the mix. The recycled materials come from a variety of sources, including building, agricultural, and industrial waste. These various types of waste include hospital waste, agricultural waste, plastic waste, and industrial waste. The majority of these waste materials are either non-degradable (waste plastics, synthetic polymers, scrap tires, tire tubes, etc.) or degradable (cassava peels from cassava processing waste, as used in this study). In traditional bituminous mixes, they are typically employed as partial or total substitutions for coarse, fine, and filler aggregates [2]. The viability of using several degradable materials as fillers in asphalt concrete is being explored. For example, materials like rice husk ash, date seed ash, periwinkle shells, cow-bone ash, and milled egg shells [12][13][14][15][16].
Cassava peel (CP), which is a by-product of cassava processing, accounts for 20-35 percent of the tuber's weight, especially when peeling by hand [17]. Cassava peels are discarded indiscriminately due to gross under-utilization and a lack of sufficient technologies to recycle them, posing a major challenge and resulting in an environmental problem. As a consequence, there is a need to look for new ways to recycle it. The utilization of waste materials in road construction is a sustainable practice that will encourage waste reduction, a cleaner environment, and cost savings in road construction works, as well as serving as an alternative material [18]. Recent research has explored use of cassava peel ash as stabilizing agents for lateritic soils for pavement foundations [19] and strength modifiers for concrete.
Raheem et al. [20] in their research study partially replaced cement in concrete wit h CPA at varying proportions (0% to 20% by cement weight at 5% interval) using a mixture ratio of 1:2:4 and cured for 7 to 28 days. The strength development and slump were observed and it was concluded that the concrete strength increases with the increased hydration period but decreases with the increased quantity of the CPA. Also, Ofuyatan et al. (2018) [21] investigated cassava peel ash, which is locally available and utilized as a supplementary cementation material, to partially replace conventional cement in concrete at a varying ratio from 5% to 25%. The strength properties of the CPA concrete cured for 7 days to 180 days are determined in terms of compressive strength, flexural strength, porosity, durability, slump, water absorption, and shrinkage. From their results summary, 10% to 15% produced the optimal results for the response parameters. Other research carried out on the use of Cassava peel ash in concrete is also outlined in [22][23][24][25][26][27]. Following these developments and the limited research in exploring cassava peel ash use in asphalt, this study aims to evaluate the effects of cassava peel ash as a mineral filler on the engineering characteristics of asphaltic concrete.

Materials and Method
This study is based on laboratory testing and analysis. All tests were conducted using the equipment and devices available in the Civil Engineering laboratories of Landmark University, Omu-Aran, Kwara State, Nigeria. The materials used for this study are the basic constituents of Hot asphalt mix namely: bitumen, coarse aggregate, fine aggregate, filler (stone dust), and cassava peel ash.

Mix Design, Production and Testing
The experimental design was done using response surface methodology (RSM). All independent variables were varied simultaneously. The independent variables are coarse aggregate (CA), fine aggregate (FA), mineral filler (MF), bitumen content (BC), and cassava peel ash (CPA). Table 3 presents the independent variables and the range of variation, as given by the central composite design. The experimental plan designed was used to produce the bituminous mixtures using the Marshall Method for designing hot mix asphalt mixtures accordance with ASTM D1559-89 [50]. According to the standard 75-blow Marshal design method designated as a number of 10 samples each of 1200g in weight were prepared using the mix proportions shown in Table 4. As shown in Figure 2, Marshall Properties of the asphalt mix such as stability, flow, density, air voids in total mix, and voids filled with bitumen percentage were obtained for various mix constituents. The flowchart of the research methodology used to achieve the study is illustrated in Figure 3.

Volumetric Properties Of CPA-Modified Asphalt Mixtures
The volumetric properties of CPA-modified asphalt mixtures are shown in Table 5. The Marshall stability, flow, density, volume of the void, and void in mineral aggregates of the asphalt mixtures varied 1.8037-8.045 kN, 2.7-8.22 mm, 2.0426-2.3909%, 1.094-7.966% and 55.5105-93.1393% respectively. These results are the effect of simultaneously varying factors such as coarse aggregate, fine aggregate, mineral filler, bitumen content, and cassava peel ash on the asphalt mixtures. From Table 5, the highest stability has 20% of CPA while the lowest stability has 10% CPA. However, in terms of the flow property of the CPA-modified mixtures, the highest flow was 8.22mm with 0% CPA, and the lowest of 2.7mm with a CPA of 13.7%. Also, the highest values of void in mineral aggregate (VMA) were observed at 10% CPA content, whereas the lowest VMA value was found at the highest CPA content. These results imply the role of CPA in the mixture in influencing the volumetric properties of the mixtures. Furthermore, it was observed that only run 16 and 15 (CA:46%, FA:13.8%, MF:17.95%, BC:4%, CPA:20% and CA:48.97%, FA:11.8%, MF:15%, BC:8%, CPA:0% respectively) with stability value of 8.045kN and 7.886kN are higher than the minimum stability value specified by ASTM A530/A530M-18 [51].

Statistical Analysis
The RSM was used to obtain the interactions between the Outputs and the Independent variables from the ANOVA table, which displays the layout for experimental design and amounts for all of the responses with these values. For the responses, quadratic polynomial equations were created. The responses in the model are Coarse Aggregate (A), Fine Aggregate (B), Filler (C), Bitumen Content (D), and Cassava Peels (E). Checking the model's adequacy is a critical element of the data analysis because the model functions will produce incorrect responses if the fit is not good enough [52]. As a result, ANOVA analysis was used to determine the significance and applicability of models, and the findings are presented in Table 6. This table covers the various statistical variables as well as the quadratic models for coded factors. In this table, p-values<0.0001 shows that the model and variables are significant. In these tables p-values, less than 0.0001 imply that the model and parameter are significant (model and termp-values which are less than 0.0001 imply that the model and parameter are significant for 95% confidence intervals). All of the models proposed in this study are statistically significant, with p-values less than 0.05. However, for each response, a few unimportant interactions (p-value>0.100) were identified and specified. As a result, these irrelevant terms can be removed from the model to improve the model and maximize the result [53]. To check the fitness of the model regression coefficients R² and adjusted R² were calculated. The R² and adjR² values indicate that there is a good agreement between predicted and actual values [54]. Adequacy can be attained from AP value when AP>4 for all responses presented in all ANOVA Table 6. This supports that the model can be used to navigate the design space defined by CCD.

Normality and 3-D Plot
The one factor at a time (OFAT) technique is a well-known strategy for optimizing multifactor tests. For a certain experimental design, OFAT comprises a single variable that may be changed while the other parameters remain constant. OFAT is unable to produce suitable results since the effects of interactions among all relevant components in the designs are not thoroughly studied, and it is incapable of achieving the genuine optimal value. As a result, the RSM technique was established for parameter optimization to reduce the number of tests and parameter interactions to a minimum [52,[54][55][56][57][58]. This depicts the normal probability plots of both residuals and the actual versus predicted for hot mix asphaltic concrete HMA containing CPA. The experimental data obtained with the mixing process was analyzed using the Design expert software [59]. The influence of all the independent factors on the stability, flow, Volume of voids (Vv), GM, and VMA of Asphalt concrete composite was studied. Knowing model satisfactoriness is critical, and understanding diagnostic charts such as projected versus actual values can help. Figures 4(a-e) show that the plotted point falls either on or close to the distributed line of the plot.  Figure 5. As it could be seen from the plot, FA had a linear effect on the response (stability) while CA had a quadratic effect on the response. On the individual effect of the independent variable on the response, it could be seen that an increase in FA at low CA resulted in a decrease in the stability of the asphaltic concrete. On the other hand, an increase in CA up to 51.21% resulted in a slight increase in the stability of the asphaltic concrete, however, increasing the percentage CA above 51.21% caused a significant decrease in the stability of the concrete. A combined effect of the independent variables revealed a n increase in both the percentage of FA and CA led to a decrease in the marshal stability of the concrete. For maximum marshal stability, the percentage should be held at a lower level (9.5%) while the CA is set at 49%. The effect of CA and MF on the marshal stability of the concrete while holding other independent variables (FA, CB, and CPA) at a constant level is shown in Figure 6. The percentage CA had a quadratic effect on the response while the MF had a linear effect on the response. An increase in the percentage MF resulted in a slight decrease in the marshal stability of the concrete. An increase in the percentage of CA on the other hand led to an initial increase in the MS, however, an increase in the percentage of CA above 51.21% resulted in a slight decrease in the MS of the concrete. When the percentage MF is set at the barest minimum and the percentage CA is set at 49.03%, the maximum marshal stability of the concrete is achieved.

Conclusions
Numerous techniques can be employed to lessen pavement discomfort and increase the lifespan of the surface. However, one of the most effective remedies for pavement deterioration is thought to be a modified asphalt mixture. This study aimed to evaluate the volumetric properties of CPA-modified asphalt mixtures. Response surface methodology was utilized in this study to find the interactions between selected parameters. Based on the results achieved in this study, the following conclusions are drawn:  Higher stability was obtained from mixtures having a high asphalt content not less than and CPA value not less than 10%;  When the percentage MF is set at the barest minimum and the percentage CA is set at 49.03%, the maximum marshal stability of the concrete is achieved;  Based on the results achieved from RSM analysis, amounts of 4.018% asphalt content, 20% cassava peel ash, 46% coarse aggregate, 10% fine aggregate, and 15% mineral filler give the optimal asphalt content;  Good agreement was found between predicted and actual values which indicated that suggested models could successfully predict mixture properties within the range of defined factors, hence, the use of RSM can be utilized in place of the Marshall design method;  CPA showed good material as filler for asphalt mixtures meeting all volumetric conditions, therefore it can be used as a conventional material in the production of asphaltic concrete/pavement design. Subsequent research should use RSM for optimization in different test conditions;  In order to fully understand the implications of the findings of this work, it will be critical to consider the other properties of CPA-modified asphalt mixtures, such as fatigue.

Data Availability Statement
The data presented in this study are available in the article.

Funding
The authors received no financial support for the research, authorship, and/or publication of this article.