As a basic course for highway and bridge engineering majors, there were three major issues in the previous experimental course titled “Road Engineering Materials.” The traditional teaching mode hindered students’ innovation, experimental content was single and lacked engineering background, and experimental teaching focused excessively on macroscopic observations, while neglecting microscopic mechanisms. Based on this, relying on the teacher’s research project designed “warm-mix additive asphalt preparation, microstructure, and performance analysis,” a comprehensive experiment that uses high-viscosity and high-elasticity modified asphalt as the object of the study, which is mixed with different warm-mixing agents to reduce its viscosity, is proposed to help realize the dual carbon goals. The experiment is closely related to engineering practice, aiming to improve students’ problem-solving ability, stimulate their research interests, and cultivate innovation ability.

The experimental design took reducing the mixing temperature of asphalt mixtures as its entry point, focusing on three main control indicators: Brinell viscosity, 60°C dynamic viscosity, and storage stability. High-viscosity and high-elasticity asphalt is a composite-modified asphalt prepared by adding SBS rubber powder and other modifiers to #70 virgin asphalt. Basalt was selected as the coarse aggregate, and limestone was chosen as the fine aggregate. Based on the results of preliminary research, the optimal dosages of RH warm-mix additive and Aspha-min warm-mix additive were 3% and 6%, respectively. The optimal dosage of the surfactant warm-mix additive DWMA-1 was unknown, so four dosages (0.6%, 0.8%, 1.0%, and 1.2%) were set up for experimentation to investigate the effects of these three different warm-mix additives on the properties of high-viscosity and high-elasticity asphalt and to obtain the optimal ratio. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to analyze the impact of adding warm-mix additives on the molecular structure of asphalt, exploring their modification mechanisms.

Three viscosity reduction mechanisms, resulting from the addition of the warm-mix additive RH, Aspha-min, and different dosages of DWMA-1 to high-viscosity and high-elasticity asphalt, were analyzed. When the dosages of DWMA-1 were 0.8% and 1.0%, the viscosity reduction efficiency comparison of the three warm-mix additives at different temperatures revealed that the viscosity reduction effect of Aspha-min was lower than those of RH and DWMA-1, indicating that Aspha-min had the worst viscosity reduction effect. The reason was that Aspha-min contained a small amount of crystalline water, which could be released at high temperatures, but as the temperature rose, the released crystalline water amount decreased, and the viscosity-reducing effect became less and less obvious. Comparing the effect of RH and DWMA-1, the viscosity reduction efficiency of RH was only higher than those of 0.6% and 0.8% DWMA-1, and when the dosage of DWMA-1 was more than 1%, the viscosity reduction effect was higher than that of RH. In summary, the viscosity-reducing effects of the three warm-mix additives were ranked as follows: 1% DWMA-1 > RH > Aapha-min. In addition, through SEM test, it was found that the addition of a warm-mix additive could better disperse the polymer phase in high-viscosity and high-elasticity asphalt, and the polymer phase in the bright part of asphalt was obviously reduced, which had a positive effect on the reduction of the viscosity of asphalt. Finally, using AFM, some special structures in asphalt could be observed, namely the “bee” structures, which were based on a colloidal model and referred to as asphaltene particles in the model. Using AFM to analyze the 2D and 3D morphology of various asphalts, it was found that there were no obvious “bee” structures in high-viscosity and high-elasticity asphalt and 1% DWMA-1 warm-mixing high-viscosity and high-elasticity asphalt, but a light and dark interlaced structure was observed. This structure, when combined with the 3D morphology, could be viewed as finer and smaller “bee” structures. This was because high-viscosity and high-elasticity asphalt contained a large number of modifiers, including rubber powder and SBS. The addition of these modifiers reduced the light components and wax content in asphalt. Wax is a key substance for forming “bee” structures, so it was difficult for high-viscosity and high-elasticity asphalt to form common “bee” structures. Warm-mix high-viscosity and high-elasticity asphalt with the 1.0% DWMA-1 additive still exhibited a similar alternating light and dark structure as the original high-viscosity and high-elasticity asphalt. This was because DWMA-1, as an oil-based surfactant, did not contain key light components necessary for forming “bee” structures among its main components.

This paper follows the main line of “problem orientation - material preparation - microscopic analysis - mechanism exploration,” with the goal of comprehensively enhancing students’ comprehensive and innovative abilities. Taking high-viscosity and high-elasticity modified asphalt used in actual ultrathin wear layer engineering applications as the object of this study, different types of warm-mix additives are incorporated into high-viscosity and high-elasticity asphalt for research. The aim is to reduce its viscosity while maintaining the performance of the original high-viscosity and high-elasticity asphalt, thereby lowering the mixing and compaction temperatures of asphalt mixtures and reducing energy consumption. This experiment covers a wide range of knowledge points, including material selection, performance testing, structural characterization, and chemical analysis, which can enhance students’ abilities to solve practical problems by comprehensively applying interdisciplinary knowledge.

The micro-structure evolution and macro-property development of cement-based composites are affected by the wall effect. For the cement paste, the formation of the interfacial transition zone (ITZ) is related to wall effect, i.e., the lower density of cement particles near aggregate induces the formation of ITZ. In addition, the aggregate density is low near the formwork during the concrete placement and molding, which results in a difference of properties between the wall effect zone and the material interior. Characterizing the influences of aggregate or cement particle shape and size-polydispersity on the thickness of wall effect layer and further on the volume fraction distribution of particles is important for controlling the performance of cement-based composite. Compared with theoretical and numerical simulation methods, the experimental method, such as concrete section cutting is time-consuming and labor-intensive. With the development of computer technology, it provided a technical basis for the study of wall effects in concrete through numerical simulation. In general, cement-based composite can be regarded as a particle packing system. However, most of the existing numerical and theoretical studies employs simple particle, such as circles, ellipses and spheres to model the aggregate or cement particles, which makes it difficult to accurately analyze the role of particle shape and size-polydispersity on the wall effect. It is thus of great significance to construct a complex particle packing system and develop a micro-structure characterization algorithm applicable to the particle packing system to investigate the influence of the particle shape and size-polydispersity on the wall effect.

This work used variable shaped ovoidal particles to construct aggregate and cement particles. A contact detection algorithm for ovoidal particles was proposed based on a geometric potential concept, and ovoidal particle packing systems were generated to simulate the packing state of aggregate and cement particles. Moreover, the cross-section analysis algorithm applicable to the ovoid particle packing system was proposed. Based on the cross-section analysis algorithm and the stereology unbiased estimation method, the influences of particle shape and size-polydispersity on the thickness of the wall effect layer were analyzed and a quantitative relationship between the thickness of the wall effect layer and particle shape and size-polydispersity was established. A prediction formula was proposed to determine the particle volume fraction distribution at different section planes considering the wall effect, and the accuracy of the prediction formula was verified via comparing it with the results of the existing studies. Finally, the modulus of elasticity of concrete at different locations was calculated by the quantitative model of wall effect layer thickness and particle volume fraction distribution based on the differential effective medium theory.

For monoshaped-monosized particle packing system, the relationship between volume fraction of particle (V_V) and distance from boundary plane to cross-section plane (h) is obtained. The results show that the V_V–h curves for different shaped particle packing systems are basically similar, indicating that the influence of particle shape on the thickness of wall effect layer T_P can be ignored.

For monoshaped-binarysized particle packing systems, the V_V–h curves move in the direction of increasing distance as the proportion of larger size particles increases. The results indicate that the thickness of wall effect layer T_P enlarges with increasing the proportion of larger size particles. For monoshaped-polysized particle packing system, the thickness of wall effect layer T_P enlarges with the increase of average equivalent diameter of particles.

Based on the obtained results, the prediction formulas are proposed to determine the wall effect layer thickness and particle volume fraction distribution, and the reliability of prediction formulas is verified via comparing the predicted data with the results from this work and other publications.

The relationship between elastic modulus E^MT and distance h is similar to that between volume fraction of particle (V_V) and distance h. The variation of E^MT–h curves can be divided into the increasing and stabilizing parts. The results indicate that the elastic modulus E^MT is small near the boundary plane, which is induced at a low particle volume fraction.

This work could provide a guidance for the optimization of the microfine structure of concrete and the regulation of macroscopic properties, and the following conclusions were summarized: 1) The influence of particle shape on the wall effect was not dominant; 2) The thickness of wall effect layer T_P increased with enlarging the average diameter of aggregate, and the thickness of wall effect layer T_P decreased with enlarging the aggregate volume fraction; 3) The relationship between elastic modulus E^MT and distance h was similar to that between volume fraction of particle (V_V) and distance h.