Experimental Study on Mechanical Properties of Waste Steel Fiber Polypropylene (EPP) Concrete

Abstract

Polypropylene (EPP) concrete offers advantages such as low density and good thermal insulation properties, but its relatively low strength limits its engineering applications. Waste steel fibers (WSFs) obtained during the sorting and processing of machining residues can be incorporated into EPP concrete (EC) to enhance its strength and toughness. Using the volume fractions of EPP and WSF as variables, specimens of EPP concrete (EC) and waste steel fiber-reinforced EPP concrete (WSFREC) were prepared and subjected to cube compressive strength tests, splitting tensile strength tests, and four-point flexural strength tests. The results indicate that EPP particles significantly improve the toughness of concrete but inevitably lead to a considerable reduction in strength. The incorporation of WSF substantially enhanced the splitting tensile strength and flexural strength of EC, with increases of at least 37.7% and 34.5%, respectively, while the improvement in cube compressive strength was relatively lower at only 23.6%. Scanning electron microscopy (SEM) observations of the interfacial transition zone (ITZ) and WSF surface morphology in WSFREC revealed that the addition of EPP particles introduces more defects in the concrete matrix. However, the inclusion of WSF promotes the formation of abundant hydration products on the fiber surface, mitigating matrix defects, improving the bond between WSF and the concrete matrix, effectively inhibiting crack propagation, and enhancing both the strength and toughness of the concrete.

1. Introduction

Concrete is one of the primary materials used in civil engineering. However, the shortcomings of ordinary concrete, such as high self-weight, poor thermal insulation performance, and low ductility, restrict its new demands in engineering applications [1]. Developing new green, low-carbon, lightweight, and high-strength concrete materials has become a new goal to adapt to non-structural components [2]. In addition, lightweight low-carbon concrete has broad application prospects in the filling of pipe trenches and the external walls of lightweight buildings.

Expanded polypropylene (EPP) particles are widely used in daily life due to their excellent impact resistance and energy absorption properties [3]. EPP also shows moderate tolerance to water, oil, and chemicals, making it widely applied in shock-proof and moisture-proof packaging materials for products [4]. However, the dismantling of EPP products causes severe environmental pollution. Chopped EPP foam can be used as a substitute for concrete aggregates to produce lightweight and thermal-insulated concrete [5,6], which is suitable for non-load-bearing components. Therefore, incorporating EPP particles into concrete to form lightweight EPP concrete (EC) not only endows it with better thermal insulation performance but also improves its toughness and ductility compared to ordinary concrete.

The application of EC in buffer structures demonstrates remarkable energy-dissipating effects in relieving compressive stress on concrete, and the higher the content of EPP particles, the better the buffering performance [7,8,9]. The “ash-coating” technology was employed to modify EPP particles, and EPP load-bearing thermal-insulated concrete was successfully prepared using a two-stage dispersion process [10]. The test results indicate that while the incorporation of EPP particles negatively effects the mechanical properties of concrete to a certain extent, it significantly enhances its durability performance [11,12].

EPP and EPS both belong to polymer foams and exhibit closed-cell elastic characteristics. Compared with EPS, the mechanical properties of EPP are less affected by the heterogeneity in the microstructure, and it does not produce toxic gases during combustion, making EPP an environmentally friendly material [13]. Since the current research on EC and fiber-modified EC is still in the initial stage, with limited research achievements, incomplete research content, and extremely scarce relevant literature, EPS concrete with partially similar properties was selected for analysis in domestic and foreign research, providing a reference for subsequent studies on the properties and applications of EC.

As wall materials and filling materials, EPS concrete has obvious application advantages. It can form efficient internal thermal insulation walls, significantly improving the thermal insulation effect of walls and thus playing a positive role in energy conservation and emission reduction [14,15]. In steel structures, EPS concrete exhibits good thermal and sound insulation properties. Working together with light steel to form light steel–EPS concrete shear walls, it can enhance such structures’ structural integrity, comfort, and fire resistance [16,17]. The cast-in-place EPS hollow floor system features simple construction, lightweight slabs, large spans, and good structural integrity, demonstrating considerable economic and social benefits [18,19]. Reconstructing roads above subways has significant impacts on subway structural safety. Using EPS concrete for subgrade replacement can reduce the load above the subway, effectively mitigating effects on the subway structure and ensuring structural safety [20,21].

However, due to the low strength and small elastic modulus of EPS itself, the addition of EPS causes a loss of mechanical properties in concrete [22,23]. Studies have shown that incorporating steel fibers into EPS concrete can enhance the material’s strength, toughness, and ductility [24,25]. When copper-plated steel fibers were added to EPS concrete of three strength grades, compressive strength, flexural strength, and slump tests showed that specimens of all grades exhibited certain ductility during failure [26,27,28,29,30]. Furthermore, adding steel fibers to EPS concrete significantly improved its drying shrinkage and compressive strength [31,32].

Steel fiber-reinforced expanded EPS concrete specimens were prepared by combining steel fibers with EPS concrete, and drop hammer tests were conducted to determine the relationship between the initial cracking and final failure impact. The results showed that the impact resistance and energy dissipation capacity of EPS concrete containing steel fibers were significantly enhanced [33,34,35]. Impact tests were performed on steel fiber–EPS panels to study their failure modes, impact force, deflection, and strain. The results indicated that steel fiber–EPS concrete panels exhibited excellent performance in energy absorption and deflection reduction [36,37,38].

The use of virgin steel fibers in EPS concrete can enhance the strength, toughness, and ductility of concrete, but it also significantly increases costs. Existing studies have shown that waste steel fibers made from mechanical processing scraps, when added to concrete, improve the mechanical properties and energy dissipation capacity of concrete similarly to virgin steel fibers [39,40,41,42]. WSF formed by mechanical processing exhibits a 3D spiral shape. Incorporating WSF into EC creates waste steel fiber-reinforced expanded polypropylene concrete (WSFREC). WSF not only compensates for the strength loss of EC but also explores a path for WSF recycling. Additionally, WSF costs only one-tenth of the cost of virgin steel fibers, significantly reducing material usage costs.

The WSF incorporated into EC can closely bond with the concrete matrix. As a new type of environmentally friendly building material, WSFREC can effectively alleviate environmental pollution caused by waste EPP and WSF through its large-scale engineering application. Therefore, studying the mechanical properties of WSFREC is of great practical significance for the widespread promotion of this new material.

The waste steel fiber (WSF) incorporated into engineering cementitious composites (EC) can bond tightly with the concrete matrix, forming a new type of concrete composite metamaterial: WSF-reinforced engineering cementitious composite (WSFREC). As a new eco-friendly and green building material, its large-scale engineering application can effectively alleviate the environmental pollution problems caused by waste expanded polypropylene (EPP) and WSF. Therefore, studying the mechanical properties of WSFREC is of great practical significance for the widespread promotion of this new material. In this paper, using the volume dosages of EPP and WSF as parameters, five types of WSFREC specimens with different dosages were designed and fabricated. Tests on the cubic compressive strength, splitting tensile strength, and four-point flexural strength of the corresponding specimens were conducted to obtain the variation law of the mechanical properties of this material. The scanning electron microscopy (SEM) technique was used to explore the microscopic composition of WSFREC, aiming to reveal the specific mechanism of strengthening and toughening of WSFREC. Through these studies, we expect to provide a solid theoretical basis and practical exploration for the application of WSFREC in the field of civil engineering.

2. Materials and Specimens

2.1. Raw Materials

The materials required for the experiment include river sand, coarse aggregate (gravel), water, superplasticizer, cement, hydroxypropyl methyl cellulose (HPMC), silica fume, EPP particles, and waste steel fibers (WSFs). Some physical materials are shown in Figure 1.

2.1.1. Cement

Cement is a very important cementitious material used to form concrete. The selection of cement type and the determination of cement weight play very important roles in the strength and durability of concrete. We referred to the existing research results [43,44,45] to determine the cement type and cement weight to use in our tests. Ordinary Portland cement (P.O 42.5-grade) was used in this study, and its detailed technical specifications are presented in

Table 1. Technical indicators of cement.

Alkali Content (%)	Fineness (%)	Specific Surface Area (m2/kg)	Setting Time (min)		Compressive Strength (MPa)		Flexural Strength (MPa)
			Initial	Final	3d	28d	3d	28d
0.46	0.5	377	236	294	25.3	50.8	5.5	8.3

.

2.1.2. Aggregates and Water

The role of coarse aggregate stones in concrete is mainly to bear external loads. Stones with high compressive strength can form concrete components with high bearing capacity. Fine aggregate sand plays the roles of loading and filling in concrete. Sand with excellent performance can improve the compactness and durability of concrete. Water is an essential material for cement hydration. Clean water can improve the durability of concrete. The selection of aggregate and water in this test was completed with reference to [46,47,48]. The coarse aggregate used in the experiment was crushed stone with a particle size range of 5–10 mm, featuring continuous gradation to ensure testing accuracy and reliability. The fine aggregate consisted of natural river sand classified as medium sand, with an apparent density of 2.64 g/cm³. Tap water was used throughout all experimental procedures.

2.1.3. Superplasticizer

Superplasticizer has a strong dispersion effect on cement, which can greatly improve the fluidity of cement mixtures and the slump of concrete, significantly reduce the water consumption during the concrete-making process, and significantly improve the workability of concrete. References [49,50,51] were used to determine the type and dosage of water reducer used in this test. A high-range water-reducing admixture was employed in the experiment, added at 1% by weight of cement. The detailed technical specifications of the superplasticizer are presented in

Table 2. Technical indicators of superplasticizer.

Moisture Content (%)	Water Reducing (%)	Alkali Content (%)	Na2SO4 (%)	Cl− (%)
0.88	27	1.01	1.64	0.01

.

2.1.4. Silica Fume

Adding silica fume into concrete can improve the fluidity of concrete mixture, enhance the impermeability and frost resistance of concrete, improve the strength of concrete, and reduce the alkali aggregate reaction of concrete. It is an indispensable additive in the modern concrete industry [52,53]. Silica fume was utilized as a mineral admixture in this study. Previous experimental results [54,55,56,57] indicate that an optimal dosage of approximately 15% by mass of cement is appropriate for silica fume incorporation. The detailed technical specifications of the silica fume are presented in

Table 3. Technical indicators of silica fume.

SiO2 (%)	Cl− (%)	Water Demand Ratio (%)	Specific Surface Area (m2/g)
98.1	0.01	112	21

.

2.1.5. Cellulose Ether

As organic materials, EPP particles exhibit hydrophobic surfaces with poor bonding to inorganic cementitious materials. Additionally, due to their low density (approximately 120–250 kg/m³), EPP particles tend to float during concrete mixing. To address these issues, surface chemical modification was applied to EPP particles prior to EC mixing to enhance interfacial bonding and mitigate floating behavior.

Hydroxypropyl methyl cellulose (HPMC) was employed as the modifying agent, with a product fineness of 80–100 mesh, added at 1% by weight of cement. The detailed parameters are presented in

Table 4. Technical indicators of cellulose ether.

Type	Methoxyl Group (%)	Hydroxypropyl (%)	Condensation Temperature (°C)	Transmittance (%)	Ash Content (%)	Water Content (%)	Viscosity (MPa.s)
Hydroxypropyl methylcellulose	28.5	9.4	62	92	3.5	5	197,000

.

2.1.6. EPP Particles

EPP (expanded polypropylene) is a closed-cell foam plastic that is commonly used as anti-vibration packaging material for products. It exhibits characteristics such as high energy absorption, excellent chemical stability, and flame resistance, as shown in Figure 1d. The EPP particles used in this experiment were obtained from sorted residual materials from packaging box processing, with a particle size range of 3–5 mm and a bulk density of 13 kg/m³.

2.1.7. Waste Steel Fibers (WSFs)

The WSFs used in this study were derived from lathe machining scraps, which were processed and sorted into recycled spiral-shaped steel fibers. These fibers exhibit a distinctive 3D helical geometry with clean, impurity-free surfaces. Detailed technical specifications and physical characteristics are presented in Figure 2 and

Table 5. WSF size and mechanical properties.

Density (kg/m3)	Length (mm)	Thickness (mm)	Elastic Modulus (MPa)	Tensile Strength (MPa)
7850	30–40	0.75	2.05 × 105	≥380

, respectively.

2.2. Mix Proportion Design of Specimens

The mix design of WSFREC comprehensively considered multiple key parameters, including the EPP replacement ratio, WSF dosage, water–cement ratio, and unit water content. A thorough analysis was conducted to evaluate the influence of these parameters on both fresh mixture properties and hardened concrete mechanical performance, ensuring that the developed WSFREC meets comprehensive practical application requirements. Referring to some existing research results [16,58,59], considering that the amount of EPP is too small, and in combination with some initial test results, the mix proportion of this test was finally determined.

Using C30-grade concrete as the reference strength, the mix proportion was designed with specific ratios for each test group as detailed in

Table 6. WSFREC mix proportion.

Group Name	Specimen Number	Quantity of Various Materials Used (kg/m3)
		Cement	Sand	Stone	Water	EPP		Silica Fume	Cellulose Ether	Superplasticizer	WSF
						%	kg
Without WSF	NC	339	653	1230	185	0	0	0	0	0	0
	E10	339	653	1107	185	10	1.04	35	3.39	3.5	0
	E20	339	653	984	185	20	2.08	35	3.39	3.5	0
	E30	339	653	861	185	30	3.25	35	3.39	3.5	0
	E40	339	653	738	185	40	4.29	35	3.39	3.5	0
E10 Group	E10W0.5	339	653	1107	185	10	1.04	35	3.39	3.5	39
	E10W1.0	339	653	1107	185	10	1.04	35	3.39	3.5	78
	E10W1.5	339	653	1107	185	10	1.04	35	3.39	3.5	117
	E10W2.0	339	653	1107	185	10	1.04	35	3.39	3.5	157
E20 Group	E20W0.5	339	653	984	185	20	2.08	35	3.39	3.5	39
	E20W1.0	339	653	984	185	20	2.08	35	3.39	3.5	78
	E20W1.5	339	653	984	185	20	2.08	35	3.39	3.5	117
	E20W2.0	339	653	984	185	20	2.08	35	3.39	3.5	157
E30 Group	E30W0.5	339	653	861	185	30	3.25	35	3.39	3.5	39
	E30W1.0	339	653	861	185	30	3.25	35	3.39	3.5	78
	E30W1.5	339	653	861	185	30	3.25	35	3.39	3.5	117
	E30W2.0	339	653	861	185	30	3.25	35	3.39	3.5	157
E40 Group	E40W0.5	339	653	738	185	40	4.29	35	3.39	3.5	39
	E40W1.0	339	653	738	185	40	4.29	35	3.39	3.5	78
	E40W1.5	339	653	738	185	40	4.29	35	3.39	3.5	117
	E40W2.0	339	653	738	185	40	4.29	35	3.39	3.5	157

In Table 6, NC denotes plain concrete. The notation ErWn represents WSFREC with an EPP particle volume replacement rate of r% and a WSF volume fraction of n%.

.

2.3. Specimen Preparation and Curing

The slump of mixtures with different mix proportions is shown in Figure 3. The slump of normal concrete (NC) is 95.7 mm. After adding EPP, the maximum and minimum slumps of EC are 96.3 mm and 95.8, respectively, which are greater than those of NC. The reason for this is that the smooth surface of EPP particles is more lubricated after encountering water, forming a lubricant in the concrete, which increases the fluidity of the concrete, and so the slump increases. From E10 to E40, the slump decreases from 96.3 mm to 95.8 mm, and the slump value shows a downward trend. This is because the strength and elastic modulus of EPP particles are smaller than those of aggregates. After the replacement rate increases, the EPP is compressed to form more pores, resulting in insufficient liquidity, and the slump decreases.

WSFREC was formed after adding WSFs into EC. The slump of WSFREC decreased with the increase in EPP and WSF. In addition to the increase in internal pores formed by the above EPP, the larger the volume content of WSF, the more cement mortar wrapped on the surface of RSF, resulting in the concrete having worse fluidity and a lower slump. In addition, due to the spiral shape of WSF, a three-dimensional grid spatial structure formed inside the concrete, which enhanced the bonding force between cement mortar and WSF, and prevented the concrete from flowing. Therefore, the slump of the mixture also decreased significantly.

2.4. Slump Test of Mixture

According to the mix proportions, the corresponding weights of cement, sand, coarse aggregate, and silica fume were measured and placed into the mixer. The dry materials were mixed for 1 min to ensure uniformity.

Next, the pre-weighed hydroxypropyl methyl cellulose (HPMC) was thoroughly mixed with water, followed by the addition of the superplasticizer. The resulting solution and EPP particles were then poured into the uniformly mixed dry materials and stirred for an additional 1 min. When incorporating WSF, the fibers were added in batches and mixed thoroughly to ensure homogeneous distribution throughout the concrete mixture.

The molds were cleaned and coated with a release agent on their inner surfaces. The concrete mixture was poured into the molds and compacted using a vibrating table until paste surfaced and no significant air bubbles escaped. After casting, the specimen surfaces were leveled with a trowel and covered with plastic film to prevent moisture evaporation.

After 24 h of casting, the specimens were demolded and labeled. Following demolding, all specimens were continuously cured for 28 days. The equipment used in the specimen preparation process is shown in Figure 4.

3. Experimental Procedure and Failure Phenomena

3.1. Cube Compressive Strength Test

3.1.1. Testing Procedure

The cubic compressive strength test was carried out using a WHY-1000 microcomputer-controlled pressure testing machine with a maximum load of 1000 kN. The testing machine and specimen installation are shown in Figure 5. After installing the 100 mm × 100 mm × 100 mm cubic specimen, the compressive strength test was performed. The loading rate was controlled within the range of 0.5 MPa/s to 0.8 MPa/s. After the specimen was completely damaged, the failure load data were accurately recorded with a precision of 0.01 MPa.

3.1.2. Test Piece Failure Mode

Figure 6, Figure 7 and Figure 8 show the cubic compressive failure phenomena of NC, EC, and WSFREC specimens, respectively. It can be observed from the figures that there are significant differences in the cubic compressive failure morphology of NC, EC, and WSFREC specimens.

Figure 6 displays the failure mode of NC specimens. Without EPP particles or WSF reinforcement, fine vertical microcracks initially appeared on the specimen surface during early loading stages. As loading increased, these microcracks progressively propagated and widened, developing both vertically and laterally toward both ends. When the load reaches the ultimate bearing capacity of the specimen, the specimen completely loses its bearing capacity and exhibits typical brittle failure characteristics.

Figure 7 shows the failure mode of the EC specimen. Compared with NC, there was no obvious fragmentation phenomenon after EC compression failure, and the number of surface microcracks increased significantly, indicating that EC has better toughness. The reason for this is that EPP improves the brittleness of concrete, thereby enhancing its deformation capacity. As the EPP substitution rate increases, the crack width gradually increases and is mainly distributed vertically. As the load increases, cracks continue to propagate and penetrate the upper and lower ends of the specimen, accompanied by a few fragments falling off. The characteristic of ductile failure of the specimen is evident.

Figure 8 shows the cubic compressive failure morphology of WSFREC with an EPP substitution rate of 30%. The failure mode of WSFREC test blocks with other EPP particle substitution rates is similar to that shown in Figure 7. At the initial stage of loading, the cracks in the specimen are not obvious. When the ultimate load is reached, a few subtle cracks can be observed, and the entire failure process is relatively slow. WSF plays the role of “micro steel reinforcement” and has good tensile properties. The bonding force between WSF and the concrete matrix suppresses the generation and development of cracks, improves the compressive failure mode of concrete, and further enhances the toughness of concrete. Compared with the EC and NC test blocks, the WSFREC test block has fewer cracks and higher integrity. The entire test block is “cracked but not scattered”, with only a small amount of damage at the edges and ends. This phenomenon becomes more pronounced with the increase in WSF content.

3.2. Splitting Tensile Test

3.2.1. Testing Procedure

The splitting tensile strength test adopts the why-1000 microcomputer-controlled pressure testing machine with a maximum range of 1000 kN. The specimen installation is shown in Figure 9. After the 100 mm × 100 mm × 100 mm cube specimen is installed, the splitting tensile strength test is carried out. The test loading rate is controlled within the range of 0.02 MPa/s to 0.05 mMP/s. After the specimen is completely destroyed, the failure load data are accurately recorded to 0.01 MPa.

3.2.2. Failure Modes of Specimens

Figure 10 shows the splitting tensile failure mode of NC, and the specimen does not show any failure in the initial loading stage. As the load continues to increase, a fine crack appears on the surface of the test piece. Shortly after the fine crack appears, it is accompanied by a clear sound. At this time, the test piece is split into two parts by a through crack located on the central axis of the splitting surface. The crack development direction is singular, and there are almost no secondary cracks around the main crack. The test piece is completely destroyed, and the load of the testing machine rapidly drops to zero after reaching the peak value. The whole process shows a significant brittle failure characteristics.

This is because after the specimen is subjected to load, the microcracks in the concrete encounter no obstruction. The crack propagation occurs very rapidly. Once there are microcracks on the failure cross section, they will quickly expand into macro-cracks and extend further. When the cracks extend to the concrete surface, the entire cross section loses its shear resistance capacity. Then, failure occurs as the shear stress exceeds the strength stress of the concrete.

Figure 11 shows the splitting failure mode of EC. When the substitution ratio of EPP particles is low, it has little effect on crack development, allowing microcracks to interconnect and form macroscopic cracks, leading to specimen failure. Thus, it exhibits brittle failure characteristics similar to NC. When the substitution ratio of EPP particles is high, the development of microcracks is restricted, and no obvious cracks appear in the early stage of loading. As the load continues to increase, a large number of microcracks emerge. After the load reaches the tensile strength, the specimen fails but remains relatively intact. During the splitting failure of concrete specimens, the incorporation of EPP particles reduces the effective area of the tension-loaded section, thereby causing a decrease in splitting tensile strength.

Figure 12 shows the splitting tensile failure mode of WSFREC specimens with 30% replacement rate of EPP. The splitting tensile failure mode of specimens with other replacement rates of EPP is similar to Figure 11, which is not fully shown in the paper. The test process showed that WSFREC had no obvious damage at the initial stage of loading, and continued to increase the load. The test block had a small crack that did not penetrate the top and bottom, accompanied by a number of irregular microcracks, showing the phenomenon of “crack without separation”, which had good integrity. With the increase in WSF content, this phenomenon is more obvious. When the specimen is damaged, it will make a small tearing sound due to tears in the fiber and aggregate. When the load reaches the tensile strength, there will be a stage in the loading process where the load remains unchanged but the crack continues to develop. When the specimen is unloaded immediately after the crack appears, the crack width will be reduced. Compared with NC and EC, the crack development speed is slower, indicating that WSFREC shows good ductility. Careful observation of the fibers on the fracture surface shows that some fibers are pulled apart, indicating that WSF can effectively prevent the development of internal cracks in concrete and change the shape and direction of crack development. The bonding force between WSF and concrete ensures that the specimen does not directly break into two parts during failure, showing good toughness, and the specimen exhibits ductile failure.

3.3. Flexural Test

3.3.1. Testing Procedure

A Ya-500 pressure testing machine is used for the prism bending strength test. The installation of the testing machine and test piece is shown in Figure 13. After the 100 mm × 100 mm × 400 mm rectangular specimen is installed, the flexural strength test is carried out. The loading method is shown in Figure 14. The loading rate is kept within the range of 0.05 MPa/s to 0.08 MPa/s, and the load is applied continuously and uniformly. The ultimate load when the specimen is damaged is recorded, and the data is accurate to 0.01 MPa.

3.3.2. Flexural Failure Modes

Figure 15 shows the bending failure mode of NC. At the beginning of loading, there was no obvious crack. As the load increased, the microcrack appeared. After reaching the flexural strength, the microcrack rapidly expanded into the main crack. With a brittle sound, the load dropped sharply, and the specimen was completely broken into two pieces. During the failure process, only one main crack appeared, and the failure position was located in the middle 1/3 area of the span, which was a vertical straight crack. After careful observation of the cross section of the specimen, it was found that the cross section was in an uneven state, and a large number of stones were exposed on the cross section. This shows that the coarse aggregate of NC did not suffer obvious damage during the failure process, and the failure mainly occurred at the contact interface between coarse aggregate and cement paste.

Figure 16 shows the flexural failure mode of EC specimens. When the substitution ratio of EPP particles was low, the failure exhibited brittle characteristics. As the substitution ratio of EPP particles increased, the flexural failure mode of EC gradually transitioned from brittle failure to plastic failure.

No obvious cracks were observed in the initial loading stage. As the load increased continuously, microcracks appeared. After reaching the flexural strength, the microcracks rapidly propagated into macroscopic cracks, accompanied by a dull sound, and the load dropped sharply, leading to specimen failure without completely fracturing into two pieces. Only one main crack formed during the failure process, with no secondary cracks observed. The failure position was located in the middle 1/3 span area, presenting a longitudinal oblique crack. The fracture surface of EPP concrete was relatively smooth, with failure primarily occurring at the interface between EPP particles and the cement paste. The crack propagation rate slowed down with the increase in the EPP particle substitution ratio.

Figure 17 shows the flexural failure mode of WSFREC specimens with an EPP replacement rate of 30%, and the flexural failure strength of WSFREC specimens with other EPP replacement rates is similar to that in Figure 16. With the incorporation of WSF, the failure mode exhibits distinct plastic characteristics. No obvious cracks were observed at the initial loading stage. As the load increased continuously, microcracks emerged and propagated, while the specimen still sustained the load. Upon further loading, the specimen cracked gradually, accompanied by a tearing sound of metal pull-out during failure, demonstrating excellent toughness.

The failure position was located in the middle 1/3 span area. WSFREC did not completely fracture into two halves; instead, the middle part after fracture was still connected by sparse WSF, presenting a “cracked but unbroken” morphology and maintaining good overall structural integrity. The main reason for this is that WSF acts as a “micro-reinforcement,” undertaking part of the tensile stress, hindering crack development, and ensuring the specimen remains integrally intact.

4. Analysis of Factors Affecting Strength

4.1. Cubic Compressive Strength

4.1.1. Effect of EPP Content

Figure 18 shows the variation trend of the cubic compressive strength (hereinafter referred to as compressive strength) of NC and EC specimens. It can be observed that the compressive strength of the NC specimen without EPP particles is 34.12 MPa. The addition of EPP particles reduces the 28-day compressive strength of concrete to varying degrees. When the substitution ratios of EPP particles are 10%, 20%, 30%, and 40%, the strength ratios are 0.8423, 0.7371, 0.5642, and 0.4994, respectively. Compared with NC, the compressive strength of EC decreases by 15.77%, 26.29%, 43.58%, and 50.06%, respectively. When the EPP substitution ratio is less than 30%, the decrease in the compressive strength is less than 50%. When the EPP substitution ratio reaches 40%, the decrease in compressive strength exceeds 50%, indicating that an excessively high EPP substitution ratio leads to greater loss of compressive strength.

For NC, the strong bond between coarse aggregate and cement paste—that is, the interfacial transition zone—has a certain strength, which will affect the compression failure mode of concrete. In EC, EPP particles are organic particles with a hydrophobic surface, while cement slurry is inorganic cementitious material with poor compatibility. Therefore, it can be inferred that the interface transition zone between EPP particles and cement matrix will form a weak structural plane, which is easy to crack during loading, greatly weakening the compressive strength of concrete. With the increase in the EPP particle replacement rate, the cementitious materials used to wrap EPP lightweight aggregate in EC increase, and the cementitious materials used to bond EPP aggregate will be relatively reduced, resulting in the weakening of the bonding force between EPP lightweight aggregate, and thus resulting in a gradual decrease in the compressive strength of EC with the increase in the EPP particle replacement rate. At the same time, the strength of EPP particles is low. In the EC, EPP particles can be regarded as holes, reducing the contact area between aggregates in the concrete, the bonding performance, the total cementation area, and the compactness of the concrete matrix, resulting in a reduction in the compressive strength of the concrete.

4.1.2. Effect of WSF Content

An analysis of the experimental data reveals that the addition of WSF enhances the cubic compressive strength of EC. The compressive strength initially increases and then decreases with the increase in WSF content, reaching a peak at a dosage of 1.0%. The compressive strength of E10W1.0 concrete exceeds that of the NC control group.

Figure 19 shows the variation trend of compressive strength for the E10 group. It can be observed that the compressive strength values of WSFREC range from 29.68 MPa to 34.59 MPa, all exceeding the 28.74 MPa of E10W0 concrete. When the WSF content is 1.0%, the compressive strength reaches a peak of 34.59 MPa, which is 1.01 times that of NC and 1.21 times that of E10W0 concrete.

Figure 20 shows the variation trend of compressive strength for the E20 group. It can be observed that WSF exhibits the most significant enhancement effect on the compressive strength of EC. The compressive strength values of WSFREC range from 26.33 MPa to 31.08 MPa, all exceeding the 25.15 MPa of E20W0 concrete. When the WSF content is 1.0%, the cubic compressive strength reaches a peak of 31.08 MPa, which is 0.91 times that of NC and 1.24 times that of E20W0 concrete.

Figure 21 shows the variation trend of compressive strength for the E30 group. It can be observed that the compressive strength values of WSFREC range from 21.96 MPa to 23.18 MPa, all exceeding the 19.25 MPa of E30W0 concrete. When the WSF content is 1.0%, the compressive strength reaches a peak of 23.18 MPa, which is 0.68 times that of NC and 1.20 times that of E30W0 concrete.

Figure 22 shows the variation trend of compressive strength for the E40 group. It can be observed that the compressive strength values of WSFREC range from 17.96 MPa to 19.18 MPa, all exceeding the 17.04 MPa of E40W0 concrete. When the WSF content is 1.0%, the compressive strength reaches a peak of 19.18 MPa, which is 0.56 times that of NC and 1.13 times that of E40W0 concrete.

The addition of WSF improves the cubic compressive strength. The overall variation trend of compressive strength of WSFREC in the E40 group is consistent with that in the E10, E20, and E30 groups, but the increase amplitude is smaller than that in the E10, E20, and E30 groups, indicating that the enhancement effect of WSF on the compressive strength of EPP concrete with a high substitution ratio is limited.

An analysis of compressive strength variation trends and numerical changes in WSFREC specimens reveals that the main reason WSF can work together with concrete is the 3D spiral structure of WSF, which tightly connects and interlocks with the concrete matrix, exhibiting strong biting force and frictional force. This fully utilizes the mechanical properties of the materials.

The tensile strength of WSF itself is higher than that of concrete. Inside the concrete, the smaller the distance between WSF, the stronger the ability to restrict crack initiation and development, and the better the enhancement effect on the compressive strength of concrete. In other words, the improvement in compressive strength of WSFREC specimens mainly relies on the interface bonding force between WSF and concrete. When the specimen is subjected to external loads, this interface transfers the applied load to WSF, which bears the tensile force formed on both sides of the crack. After concrete cracking, WSF acts as a “micro-reinforcement” between cracks, allowing the specimen to continue bearing loads after cracking until WSF is pulled out or broken, at which point the concrete specimen fails. This also confirms the good crack resistance of WSF in concrete.

However, when the dosage exceeds a certain level, mutual overlapping and winding of WSF occurs, leading to a decrease in the amount of cement paste in the matrix. This weakens the strengthening effect of WSF, causing the compressive strength curve of the specimen to show a trend of first increasing and then decreasing.

4.2. Splitting Tensile Strength

4.2.1. Effect of EPP Content

Figure 23 shows the variation trend of splitting tensile strength (hereinafter referred to as tensile strength) for NC and EC. The addition of EPP particles reduces the splitting tensile strength of concrete. The splitting tensile strength of the NC specimen is 2.78 MPa, and the tensile strengths of EC with EPP particle substitution ratios of 10%, 20%, 30%, and 40% are 2.63 MPa, 2.52 MPa, 2.35 MPa, and 1.98 MPa, respectively. The tensile strength ratios relative to NC are 0.9460, 0.9065, 0.8453, and 0.7122, respectively. Compared with NC, the tensile strength of EC decreases by 5.40%, 9.35%, 15.47%, and 28.78%, respectively.

The main reason for the decrease in tensile strength is that EPP particles exhibit hydrophobic properties, preventing them from bonding well with the cement matrix. This reduces the effective tension area and creates numerous microcracks between EPP particles and the cement paste. When the EPP particle substitution ratio is relatively low, these microcracks primarily cause tensile failure during loading. When the EPP particle substitution ratio is relatively high, these microcracks interconnect to form critical failure cracks during loading, leading to tensile failure.

4.2.2. Effect of WSF Content

The addition of WSF significantly improves the tensile strength of EC. For EC with EPP particle substitution ratios of 10%, 20%, 30%, and 40%, the splitting tensile strength of each group of specimens exhibits a trend of first increasing and then decreasing with the gradual increase in WSF content. It is noteworthy that when the WSF content reaches 2.0%, the tensile strength begins to decline, indicating that this is an optimal proportion of WSF content.

Figure 24 shows the variation trend of tensile strength for the E10 group. It can be observed that WSF has a significant enhancing effect on the tensile strength of EC, though the increase amplitude is smaller than that of the E20 group. The splitting tensile strength of E10 WSFREC ranges from 2.85 MPa to 3.51 MPa, all exceeding the 2.63 MPa of E10W0 concrete. When the WSF content is 1.5%, the tensile strength reaches a peak, which is 1.27 times that of NC and 1.33 times that of E10W0 concrete. With the gradual increase in WSF content, the tensile strength ratios of WSFREC relative to EC are 1.025, 1.216, 1.263, and 1.227, respectively.

Figure 25 shows the variation in tensile strength for E20 group specimens, indicating a more pronounced enhancement effect of WSF on tensile strength. The splitting tensile strength of E20 WSFREC ranges from 2.91 MPa to 3.47 MPa, all exceeding the 2.52 MPa of E20W0 concrete. When the WSF content is 1.5%, the tensile strength reaches a peak, which is 1.25 times that of NC and 1.38 times that of E20W0 concrete. With the gradual increase in WSF content, the strength ratios of E20 WSFREC relative to NC are 0.964, 1.025, 1.094, and 1.022, respectively.

Figure 26 shows the variation trend of tensile strength for E30 group specimens, indicating a remarkably significant enhancement effect of WSF on splitting tensile strength. The tensile strength of E30 WSFREC ranges from 2.68 MPa to 3.04 MPa, all exceeding the 2.35 MPa of E30W0 concrete. When the WSF content is 1.5%, the cubic tensile strength reaches a peak, which is 1.09 times that of NC and 1.29 times that of E30W0 concrete. With the gradual increase in WSF content, the strength ratios of E30 WSFREC relative to NC are 0.964, 1.025, 1.094, and 1.022, respectively.

Figure 27 shows the variation trend in tensile strength for the E40 group, clearly indicating that the addition of WSF enhances the tensile strength. The splitting tensile strength of E40 WSFREC ranges from 2.15 MPa to 2.39 MPa, consistently exceeding the 1.98 MPa of E40W0 concrete. When the WSF content is 1.5%, the tensile strength reaches the maximum, which is 0.86 times that of NC and 1.21 times that of E40W0 concrete.

The overall variation trend of tensile strength in the E40 group is consistent with that in the E10, E20, and E30 groups, but the increase amplitude is smaller than that in the E10, E20, and E30 groups, indicating that the enhancement effect of WSF on EC with a high substitution ratio is limited. With the gradual increase in WSF content, the strength ratios of the E40 group relative to NC are 0.773, 0.817, 0.860, and 0.835, respectively.

The results of the data analysis show that WSF significantly enhances the tensile property of EC, primarily due to the high tensile strength of WSF itself. Additionally, the 3D spiral structure of WSF enables it to tightly bond with aggregates within concrete. During tension loading, this bonding force counteracts part of the tensile stress. Moreover, the strong mechanical frictional force formed by the close interlocking of WSF and concrete allows them to collectively bear external loads, effectively inhibiting the initiation and development of cracks, thereby enhancing the tensile property of EPP concrete.

As the WSF content increases, the tensile strength continues to grow. However, when the content reaches a certain threshold, the tensile strength decreases. The main reason for this is that excessive WSF content leads to agglomeration, winding, and uneven distribution, forming large fiber skeleton overlaps. This reduces the effective bonding area between the cement matrix and WSF, increases internal weak points in the material, and weakens the effect of WSF in preventing crack propagation.

4.2.3. Analysis of Tensile–Compressive Ratio

The tensile–compressive ratio is defined as the ratio of splitting tensile strength to cubic compressive strength, which directly reflects the toughness of concrete in evaluating its ductility. A larger tensile–compressive ratio indicates better toughness and ductility when concrete bears external loads. The tensile–compressive ratios of each group of concrete specimens in this test are shown in

Table 7. Tension compression ratio of WSFREC.

Group Name	Specimen Number	Cube Compressive Strength (MPa)	Splitting Compressive Strength (MPa)	Tensile–Compressive Ratio
Without WSF	NC	34.12	2.78	0.081
	E10W0	28.74	2.63	0.092
	E20W0	25.15	2.52	0.100
	E30W0	19.25	2.35	0.122
	E40W0	17.04	1.98	0.116
E10 Group	E10W0.5	29.68	2.85	0.096
	E10W1.0	34.59	3.38	0.098
	E10W1.5	31.46	3.51	0.112
	E10W2.0	31.02	3.41	0.110
E20 Group	E20W0.5	26.33	2.91	0.111
	E20W1.0	31.08	3.17	0.102
	E20W1.5	28.34	3.47	0.122
	E20W2.0	27.97	3.22	0.115
E30 Group	E30W0.5	21.96	2.68	0.122
	E30W1.0	23.18	2.85	0.123
	E30W1.5	23.01	3.04	0.132
	E30W2.0	22.46	2.84	0.126
E40 Group	E40W0.5	17.96	2.15	0.120
	E40W1.0	19.18	2.27	0.118
	E40W1.5	18.32	2.39	0.130
	E40W2.0	18.13	2.32	0.128

.

Figure 28 displays the variation trend of the tensile–compressive ratio for each type of concrete.

According to Table 7 and Figure 28, for EC, the tensile–compressive ratio generally exhibits a trend of first increasing and then decreasing with the increase in the EPP particle substitution ratio, indicating that EPP particles have a certain toughening effect on concrete. When the EPP particle substitution ratio is 30%, the tensile–compressive ratio is the highest overall, suggesting that EC at this dosage has the best toughness. When the substitution ratio reaches 40%, the tensile–compressive ratio is lower than that of NC, indicating that the toughness of concrete decreases at this dosage.

With a constant EPP particle substitution ratio, as the WSF content gradually increases, the tensile–compressive ratios of each group of concrete specimens generally show a pattern of first increasing and then decreasing, but remain higher than those of NC and EC. WSF still exhibits a certain toughening effect at higher dosages, with an overall excellent toughening effect. This reflects the remarkable toughening effect of WSF in improving the toughness of EC.

4.3. Flexural Strength

4.3.1. Effect of EPP Content

Figure 29 shows the variation in four-point flexural strength (hereinafter referred to as flexural strength) for NC and EC specimens, indicating that flexural strength decreases with the increase in the EPP particle substitution ratio. The flexural strength of NC is 4.26 MPa, and the flexural strengths of EC specimens range from 2.66 MPa to 3.85 MPa. Compared with NC, the flexural strengths of E10W0, E20W0, E30W0, and E40W0 decrease by 9.62%, 16.90%, 27.46%, and 37.56%, respectively.

The reasons for this can be attributed to two factors. On the one hand, the low strength of EPP particles themselves and the weak bonding force at the interface transition zone between EPP particles and cementitious materials contribute to the strength reduction. On the other hand, the introduction of EPP particles significantly affects the effective flexural section height of concrete specimens. Due to the dispersed distribution of EPP particles in concrete, they occupy the space originally belonging to the concrete matrix, leading to a reduction in the effective flexural section height. Furthermore, the test found a negative correlation between the content of EPP particles and the effective compressive section height, meaning that the higher the content of EPP particles, the smaller the effective bearing section of the specimen during bending.

4.3.2. Effect of WSF Content

Figure 30 shows the variation in flexural strength for the E10 group. It can be observed that the flexural strength of WSFREC reaches a maximum of 4.97 MPa at a WSF content of 1.5%, which is 1.17 times the flexural strength of NC and 1.29 times that of E10W0. The flexural strength of E10 WSFREC ranges from 4.29 MPa to 4.97 MPa, all exceeding the 3.85 MPa of E10W0 concrete. Compared with the flexural strength of NC, the strength ratios of the E10 group increase with the increase in WSF content, reaching 1.007, 1.106, 1.167, and 1.131, respectively. The flexural strength begins to decline when the WSF content reaches 2.0%.

Figure 31 shows the variation in flexural strength for the E20 group. It can be observed that the flexural strength of WSFREC reaches a peak of 4.76 MPa at a WSF content of 1.5%, which is 1.12 times that of NC and 1.34 times that of E20W0. The flexural strength of E20 WSFREC ranges from 3.83 MPa to 4.76 MPa, consistently exceeding the 3.54 MPa of E20W0 concrete. With the increase in WSF content, the strength ratios of the E20 group relative to NC’s flexural strength are 0.899, 0.977, 1.117, and 1.012, respectively. The flexural strength starts to decrease when the WSF content reaches 2.0%.

Figure 32 shows the variation in flexural strength for the E30 group. It can be observed that the flexural strength of WSFREC reaches a peak of 3.98 MPa at a WSF content of 1.5%, which is 0.93 times that of NC and 1.29 times that of E30W0. The flexural strength of E30 WSFREC ranges from 3.55 MPa to 3.98 MPa, consistently exceeding the 3.09 MPa of E30W0 concrete. With the increase in WSF content, the strength ratios of the E30 group relative to NC’s flexural strength are 0.833, 0.887, 0.934, and 0.897, respectively. The flexural strength begins to decline when the WSF content reaches 2.0%.

Figure 33 shows the variation in flexural strength for the E40 group. It can be observed that the flexural strength of WSFREC reaches a peak of 3.12 MPa at a WSF content of 1.5%, which is 0.73 times that of NC and 1.17 times that of E40W0. The flexural strength of E40 WSFREC ranges from 2.87 MPa to 3.12 MPa, consistently exceeding the 2.66 MPa of E40W0 concrete. With the increase in WSF content, the strength ratios of the E40 group relative to NC’s flexural strength are 0.674, 0.707, 0.732, and 0.718, respectively. The flexural strength starts to decrease when the WSF content reaches 2.0%.

The flexural strength of WSFREC exhibits a trend of first increasing and then decreasing with the addition of WSF, and it is consistently higher than that of the same-group EC, indicating a significant modifying effect of WSF on EC. This is mainly attributed to the 3D spiral structure of WSF itself, which can effectively bond with aggregates to form a 3D spatial skeleton structure, exhibiting excellent multi-directional connection effects.

In the specimen, the upper part of the failure section is in compression and the lower part in tension. When cracks appear in the tensile zone, WSF plays a role in crack resistance, delaying the initiation and development of cracks, thereby significantly improving the flexural strength of WSFREC. Meanwhile, the failure process is correspondingly prolonged. With the increase in WSF content, the flexural strength of EPP concrete shows a trend of first rising and then falling, reaching the maximum at a dosage of 1.0%. This is because at higher dosages, WSF particles agglomerate and overlap with each other, preventing the cement paste from completely filling the space between WSF. This reduces the bonding between the cement matrix and WSF, thereby weakening the strengthening effect of WSF.

4.3.3. Analysis of Flexural–Compressive Ratio

The flexural to compressive ratio is the ratio of the flexural strength to the compressive strength of concrete, and its magnitude reflects the toughness of concrete. The larger the compression ratio, the better the toughness of concrete when it is subjected to bending and compression. In order to investigate the effects of EPP particles and WSF on the toughness of concrete, the flexural compression ratio was introduced as an important indicator in this experimental analysis, and detailed analysis and comparison were conducted on the experimental data of each group.

Figure 34 shows the flexural–compressive ratio relationship of the test specimens, which allows intuitive observation of the changes in concrete toughness under different test conditions. In addition, the calculation results of the flexural–compressive ratio are tabulated in

Table 8. Flexural compression ratio of WSFREC.

Group Name	Specimen Number	Cube Compressive Strength (MPa)	Splitting Compressive Strength (MPa)	Tensile–Compressive Ratio
Without WSF	NC	34.12	4.26	0.1249
	E10W0	28.74	3.85	0.1340
	E20W0	25.15	3.54	0.1408
	E30W0	19.25	3.09	0.1605
	E40W0	17.04	2.66	0.1561
E10 Group	E10W0.5	29.68	4.29	0.1445
	E10W1.0	34.59	4.71	0.1362
	E10W1.5	31.46	4.97	0.1580
	E10W2.0	31.02	4.82	0.1554
E20 Group	E20W0.5	26.33	3.83	0.1455
	E20W1.0	31.08	4.16	0.1338
	E20W1.5	28.34	4.76	0.1680
	E20W2.0	27.97	4.31	0.1541
E30 Group	E30W0.5	21.96	3.55	0.1617
	E30W1.0	23.18	3.78	0.1631
	E30W1.5	23.01	3.98	0.1730
	E30W2.0	22.46	3.82	0.1701
E40 Group	E40W0.5	17.96	2.87	0.1598
	E40W1.0	19.18	3.01	0.1569
	E40W1.5	18.32	3.12	0.1703
	E40W2.0	18.13	3.06	0.1688

to facilitate further data analysis and comparative study.

According to Table 8 and Figure 34, without adding WSF, the flexural–compressive ratio of EC increases to varying degrees compared with plain concrete as the EPP particle substitution ratio increases, generally showing a trend of first increasing and then decreasing. The flexural–compressive ratio reaches the highest point overall when the EPP particle substitution ratio is 30%, while it decreases when the substitution ratio reaches 40%, indicating that the toughening effect of EPP particles on concrete is limited.

With a fixed EPP particle substitution ratio, the flexural–compressive ratio of each group of EC generally exhibits a trend of first increasing and then decreasing with the increase in WSF content, reaching a peak at a dosage of 1.5% and starting to decline at 2.0%, but still remains higher than that of NC and EC. WSF still has a certain toughening effect at higher dosages, with an overall excellent toughening performance. The flexural strength of WSFREC is significantly higher than that of EC, and the flexural–compressive ratios of WSFREC in E10, E20, E30, and E40 groups all increase to varying degrees compared with those of the same-group EC.

5. Analysis of WSFREC’s Enhancement Mechanism

Microstructural Study of WSFREC

The observation tool for the microstructural test was a Prisma E scanning electron microscope (SEM), as shown in Figure 35. Specimens numbered NC, E10, and E20W1.5 were selected as the objects for microstructural testing. The specimens were cut into approximately 10 mm × 10 mm × 10 mm cubes, and interfaces with good bonding were selected from the macroscale, including the aggregate–cement interface, EPP particle–cement paste interface, and WSF–cement paste interface. The specimen surfaces were cleaned and stored in a dry environment with unique numbering.

Before the test, samples were cleaned with alcohol pads and air-dried, followed by gold sputtering for surface coating, as shown in Figure 36. The prepared samples were placed in the SEM to observe the micro-morphological characteristics of each interface.

Through the scanning electron microscope test conducted on the sample, the micrographs of the interface transition zone (ITZ) after the compression failure of concrete in Figure 37, Figure 38, Figure 39 and Figure 40 were obtained at the same magnification after being magnified 500 times.

Figure 37 shows the SEM interface morphology of NC. The stone surface is uneven, and the cement slurry can be embedded on the stone surface to form meshing, which increases the stress and deformation capacity of ITZ. The ITZ produced is narrow, the microcracks are short and unobvious, the porosity is small, and the interface structure is relatively dense.

Figure 38 shows the microstructural image of the EPP particle–cement paste interface transition zone (ITZ) in E20 concrete. Part of the coarse aggregate is substituted by EPP particles, which exhibit a smooth spherical structure and cannot effectively bond with the cement matrix to form a homogeneous whole. The ITZ is wider with more obvious microcracks, and the porosity is higher than that of NC, indicating a loose interface structure. This phenomenon suggests that the internal structure damage of EC is primarily attributed to the insufficient bonding strength between EPP particles and the cement interface, as well as the impact of equal-volume substitution of coarse aggregate by EPP particles. Such substitution leads to the formation of pore structures and microcracks, thereby weakening the overall structural strength. This phenomenon is observable in EC with different substitution ratios, and the number of pore structures and microcracks increases rapidly with the increase in the substitution ratio.

Figure 40 shows the micro-morphological image of the EPP particle–cement paste interface transition zone (ITZ) in E20W1.5 concrete. Due to the introduction of WSF, the ITZ of E20W1.5 becomes narrower compared with E20, with a significant reduction in the number of cracks and a decrease in porosity. The interface structure is relatively dense, indicating that WSF can improve the mechanical properties of the ITZ in EPP concrete.

The reason for this is that when WSF is mixed into EC at an optimal dosage according to specification requirements, it can inhibit the development of harmful pore structures, reduce the looseness of EC, and optimize the mechanical properties of EC. When the ultimate load is reached, the 3D spiral morphology of WSF enables it to continuously bear tensile force, exerting a “bridging” function, thereby significantly improving the ductility of EPP concrete.

Figure 41 shows the micro-morphology of the WSF–cement matrix interface transition zone (ITZ) in E20W1.5. Due to the 3D spiral structure of WSF, the ITZ between WSF and cement matrix is indistinct. The cement matrix tightly connects with WSF and wraps the aggregates, with only a few indistinct microcracks occurring, leading to reduced porosity and a denser interface structure. This phenomenon becomes more pronounced with the increase in WSF dosage.

However, when the dosage reaches 2%, WSF particles are prone to mutual winding and agglomeration, which reduces the number of bonding points between the cement matrix and WSF. This increases internal weak points in the concrete, introduces loose pore defects into the matrix, and ultimately decreases its strength.

In addition to the overall microstructural features observed in the SEM analysis, the interfacial transition zone (ITZ) between the matrix and the reinforcement plays a crucial role in determining the mechanical properties of the composites. The ITZ is a region with distinct microstructural characteristics compared to the bulk matrix, and its properties can significantly influence the load-transfer mechanism and the overall performance of the material.

One of the key characteristics of the ITZ is bond strength. A strong bond at the ITZ ensures efficient load transfer from the matrix to the reinforcement. A higher bond strength at the ITZ means that the reinforcement can better resist being pulled out under a load, thereby improving the tensile and flexural strength of the composite.

Porosity is another important characteristic of the ITZ. High porosity in the ITZ can act as a stress concentrator and facilitate crack propagation. We quantified the porosity in the ITZ using image analysis of SEM micrographs. As the porosity in the ITZ increases, the compressive strength of the composite decreases significantly. This is because the voids in the ITZ reduce the effective cross-sectional area for load transfer and provide easy paths for cracks to initiate and grow.

6. Conclusions

Through the strength test and microstructure test and analysis of WSFREC with different mix proportions, the following conclusions were obtained:

(1)

The cube compressive strength, splitting tensile strength, and four-point flexural strength of EC decreased with the increase in EPP particle substitution rate, and the weakening of cube compressive strength was very significant. However, the addition of EPP particles also improved the toughness of concrete to varying degrees. When the EPP particle substitution rate was 30%, the toughness of concrete was the best.

(2)

The improvement effect of WSF on the splitting tensile and flexural strength of EC was significant, but its effect on enhancing the compressive strength of concrete cubes was limited. When the EPP substitution rate was low, the strength of WSFREC was similar to that of plain concrete. The improvement effect of WSF on high-substitution-rate EC was relatively limited, and it had the best compatibility with E20 concrete. In the E20 group, when the content of WSF was 1.5%, the splitting tensile strength, flexural strength, and cubic compressive strength of WSFREC increased by 37.7%, 34.46%, 23.58%, and 20.50%, respectively, compared to the EC of the same group. The experimental results fully demonstrate that the addition of WSF effectively compensates for the strength loss caused by EPP particles on concrete, thereby improving the overall performance of concrete.

(3)

After adding WSF to concrete, the specimens exhibited obvious ductility characteristics in compression failure. Through analysis of the tensile compression ratio and flexural compression ratio, it was found that WSF has a significant toughening effect on EC. The experimental results show that the optimal dosage of WSF is 1.5%. If the dosage of WSF in concrete is too high, it will entangle and form clusters with uneven distribution, which will reduce the mechanical properties of the concrete.

(4)

Through scanning electron microscopy testing of the WSFREC matrix, the morphology of the transition zone and WSF at the concrete interface was observed. It was found that the addition of EPP particles reduced the compactness of the concrete to a certain extent, leading to an increase in harmful structures such as microcracks and pores, but a decrease in the number of macroscopic cracks. However, when WSF was added, the concrete matrix became denser, resulting in a significant reduction in the number of EC harmful structures, and the mechanical and ductility properties of WSFREC significantly improved.

(5)

The current research primarily focuses on the short-term mechanical properties of WSFREC. However, in practical engineering applications, concrete structures are exposed to various environmental factors, such as freeze–thaw cycles, chemical corrosion, and carbonation, over an extended period. Therefore, it is necessary to conduct long-term durability tests to evaluate the performance degradation of WSFREC under different environmental conditions. This will provide crucial information for predicting the service life of concrete structures constructed using this material.