1. Introduction
Approximately, 60,000 km covering each state shows that Malaysia has one of the finest network systems, and the major types are flexible pavement and a small part of pavement made from concrete (rigid pavement). Flexible pavement is a mixture of asphalt binder and aggregates which are heated and mixed, and then laid and compacted above a granular layer; meanwhile, rigid pavement is a mixture of cement concrete and is laid over a dry lean concrete. The rigid concrete can also be compacted above the layer of aggregates. In the past, a thousand years ago, roads were being used in our daily lives. Both types of pavements will not last forever, in which they must be monitored regularly. Thus, improvising and making these roads better and safer for all users is a must to lessen road destructions and indirectly reduce road fatalities.
Every year, governments invest a lot of money to maintain and repair road conditions, and this indirectly affects the economy of the country. The road conditions in Malaysia, which were constructed using the conventional method, have a short lifespan and needed to be maintained regularly due to road defects (potholes) leading to 11.2 percent of road accidents [
1]. Furthermore, the changes in weather and temperature affect the roads; for instance, Malaysia, with its tropical monsoon climate, makes the pavement repetitively experience hot and wet conditions. The increasing number of road users leads to an increase in load stress and traffic volume, thus making the road surface experience more impacted in terms of stripping, potholes, and permanent deformation of repetition load.
Next, Stone Mastic Asphalt (SMA) is designed specifically to reduce rutting and skid resistance, as all aggregates have a stone-to-stone contact. Stone Matrix Asphalt (SMA) is a gap-graded bituminous mixture that has a comparatively higher proportion of coarse aggregates and binder mastic with a bituminous binder and mineral filler. A major problem with SMA is the drain down of mastic content at different phases of construction. This situation occurs due to SMA’s high coarse aggregate content, which creates gaps in the structural matrix and causes those voids to be filled with high-viscosity bituminous mastic with any stabilizing material, such as bitumen, crushed sand, or filler. Usually, additional fiber additives or a stabilizing agent material are used to stabilize the mixture and prevent binder drainage in stone mastic asphalt mixing [
2,
3]. Fiber reinforced bituminous mixtures are commonly used in the industry. However, research on the attributes and performance of stone mastic asphalt (SMA) using steel fiber is rather under published and has not yet been conclusively shown.
Various types of additives and modifiers were used in SMA to modify asphalt pavement. Ali et al. [
4] used mineral fiber, cellulose fiber, and styrene–butadiene–styrene (SBS) to modify stone matrix asphalt (SMA) mixtures, and performance tests were performed for modified and unmodified mixtures. According to the findings, SBS was superior to fibers in terms of improving the performance of asphalt mixtures and the service life of the pavement system modified with mineral, cellulose, and SBS was 1.07, 1.081, and 1.243 times longer than that of the unmodified mix, respectively. Another piece of research by Goutham Sarang et al. [
5] prepared mixtures with four different shredded waste plastics’ (SWP) content, and another mixture without any stabilizers was also prepared using polymer modified bitumen (PMB). Tensile strength, moisture susceptibility, rutting resistance, and fatigue behavior were determined using the obtained optimum bitumen content (OBC). The research findings indicated that the optimum level of SWP in the SMA mixture was selected as eight percent by the weight of bitumen. The result revealed that although a mixture with PMB performed the best, SMA with eight percent of SWP produced equivalent results. Therefore, it can be concluded that waste plastic in SMA should be used instead of a stabilizing additive.
As the number of registered vehicles in Malaysia increases, the quality of pavement needs to be improved as well, in order to resist the amount of stress and strain on the road. The presence of a good modifier has proven to reduce the asphalt binder content, increase the stability in the road mixture, and also strengthen the bond between aggregates and the fillers [
6]. For the purpose of improving the performance of road pavement, a modified asphalt mixture is essential. Many researchers have worked with various modifiers and additives such as polypropylene, polyethylenes, acrylonitrile butadiene styrene, styrene-butadiene-styrene, polyvinyl chloride, and polyethylene terephthalate [
7]. Recently, the use of fibers in asphalt mixtures is one of the many alternative materials that has been demonstrated to increase the stability and resistance against moisture damage of asphalt mixtures in dealing with pavement distresses. Previous studies have found that using fiber has become a significantly attractive option in road construction. In addition, many studies have reported that the use of fibers is beneficial for enhancing the strength of hot mix asphalt mixtures. Fibers can be used, particularly in SMA mixtures, to prevent asphalt from deteriorating during transportation and mixture construction [
8,
9,
10]. The addition of fibers changes the mixture’s viscoelastic properties, improves its dynamic modulus, increases its sensitivity to humidity, offers resistance to rutting, and reduces the number of reflective cracks in asphalt pavement [
11,
12,
13,
14,
15,
16]. This finding is consistent with research by previous researchers which indicates the addition of fibers modifies the mixture’s viscoelasticity, increases its dynamic modulus, increases sensitivity to moisture, enhances flow coherence, and provides rutting resistance [
17,
18,
19,
20,
21,
22].
Synthetic fibers, glass fibers, natural fibers, and steel fibers are among the polymer fibers that are frequently added to asphalt mixtures. Steel fibers are classified as metallic fibers or mineral fibers. It is a mineral fiber that was used as a fiber-reinforced material to enhance the tensile strength. The use of this fiber encourages the transformation of asphalt qualities into high-performance asphalt. Currently, the use of steel fiber in cement and concrete pavements has recently been the subject of some investigation [
23,
24,
25,
26]. The effect of micro steel fiber percentage (0.1 percent, 0.2 percent, 0.3 percent, and 0.4 percent) by volume of the total mix on the different grades, volumetric properties, and mechanical properties were determined by the Marshall method at different temperatures (50, 60, 70) °C and different compaction blows (50, 75, 125). The research findings indicated that the stability of hot mix asphalt increased when steel fiber was added, and then decreased in a crest-like manner. This indicates that using steel fiber as a modifier additive has improved Marshall Stability compared to using a conventional mixture without fiber [
26].
Research conducted by Serin et al. [
18] specimens were produced using different fiber rates (0 percent, 0.25 percent, 0.50 percent, 0.75 percent, 1.0 percent, 1.5 percent, 2.0 percent, 2.5 percent) and tested under the Marshall Stability Test, and the optimum bitumen content value for the aggregates sample to be used was determined. Results showed that the optimal value for the fiber rate that results in the best stability value was determined to be 0.75 percent. Steel fiber additions can therefore be used in the binder coarse of the flexible pavement due to their positive stability impact. Another piece of research by Ahmed and Mahmood [
27] used different types of mineral fibers. This research examined the potential of using different mineral fibers to withstand pressures on the pavement’s surface layer. Steel, aluminum, copper, and tin were the four different types of mineral fibers used in this research, along with four different fiber rates (1 percent, 1.5 percent, 2 percent, 2.5 percent) by total weight of the mixture, varying mineral fiber lengths (0.5, 1.0, 1.5, and 2.0 cm), and four different thicknesses (0.2, 0.4, 0.7, and 0.9 mm). According to the findings, adding copper fiber with a total weight of 1.5 percent, and dimensions of 0.5 cm in length and 0.4 mm in thickness, increased the Marshall stability by 34 percent, as compared to a conventional mix.
The use of statistical modelling and optimization approaches is effective in handling a variety of constraints, objectives, and articulating the interactions between dependent variables that influence a certain response [
28]. In order to meet the demand for the optimization approach, Response Surface Methodology (RSM), a statistical technique using Design Expert Software, was used to identify the most optimal properties from the experimental data. RSM is the most popular design of experiment (DOE) for optimization, which improves a system’s operational parameters and conditions [
29]. Regrettably, there are not many expert practices in this sort of procedure since the researcher must have excellent statistical and numerical skills. RSM is increasingly being used in asphalt research since it not only saves time and money, but also can predict if a material will persist over time, which is beneficial economically in the long term. Regression modelling, the central composite design (CCD) approach, and analysis of variance (ANOVA), which take a number of various factors and responses into consideration, have all been employed. [
30]. RSM techniques have been successfully used in a variety of fields, including biomass and clay science, material and mechanical engineering, and concrete technology.
The addition of steel fiber made in the modification of an asphalt mixture can enhance the asphalt mixture properties and mechanical performances and reduce the distress problems of a pavement which provides a safer road. The properties of steel fiber that are strong can increase the properties of the road mixture which provide a positive stability impact. Steel fiber is one of the fibers that has good properties as a modifier. Additionally, steel fiber-modified asphalt mixtures can reduce the cracking of the pavement, and increase its performance in the resistance to damage not only caused by concentrated loads, but also impact loading [
18,
31].
Therefore, in order to test the effect of mixture properties on asphalt performance associated with sensitivity to moisture and dynamic creep, steel fiber in various proportions (0 per cent, 0.3 percent, 0.5 percent, and 0.7 percent) was studied as a modifier in this study. The proportion of steel fiber was selected according to research conducted by AL-Ridha et al. [
26]. From Serin et al. [
18] and Ahmed and Mahmood [
27], the average optimum proportion of steel fiber that significantly improved the properties and performance of the asphalt mixture is between 0.5 percent to 0.75 percent. RSM was used to optimize and develop models for predicting the optimum amount of the steel fiber in a modified asphalt mixture and characterize the performance of steel fiber in the SMA mixture. It is thought that the addition of steel fiber to the asphalt mixture might offer excellent potential for pavement performance. Additionally, it is supposed that the enhanced characteristics of the asphalt pavement using steel fiber in the SMA asphalt mixture are significant. In that case, it can increase the durability of the asphalt pavement while reducing the need for maintenance, thereby extending the pavement service life and eventually reducing maintenance costs in the long run.