Effect of High-Tenacity Polypropylene Fibers on the Carbonation Resistance of Expanded Polystyrene Concrete

Abstract

Expanded polystyrene concrete (EPSC) is increasingly utilized in buildings as a green building material. To investigate the effect of high-tenacity polypropylene (HTPP) fibers on the carbonation resistance (CR) of EPSC, five groups of EPSC specimens with HTPP fiber volume fractions of 0%, 0.6%, 0.9%, 1.2%, and 1.5% were prepared. Rapid carbonation tests were conducted to measure the carbonation depth (CD) and uniaxial compression strength (UCS) of the specimens at different carbonation ages (3, 7, 14, and 28 days). The CD and UCS of the specimens were calculated and analyzed. The results indicated that the HTPP fibers dramatically improved the CR of EPSC, with a decrease in the CD of up to 29.5% at 28 days. A model for predicting the CD of EPSC was developed. The model for the strength after carbonation also showed good agreement with the experimental results. Scanning electron microscopy (SEM) was used to examine the microstructure of the HTPP-reinforced EPSC, while the mechanism of HTPP fibers to enhance the CR of EPSC was elucidated. The findings of this study provide valuable insights for the application of EPSC as a structural material.

1. Introduction

Expanded polystyrene concrete (EPSC) is a low-carbon and green building material known for its light weight, excellent thermal properties, and sound insulation performance [1,2,3,4,5,6]. In recent years, it has been extensively used in green buildings as a functional material and has begun to be utilized as a structural material for walls and floors in low-rise buildings [7,8,9,10,11,12]. However, the lower compressive strength of EPSC compared to ordinary concrete (OC) limits its application in high-pressure or heavy-load structural components. This limitation may pose safety concerns in multi-story buildings or load-bearing walls and columns [13].

To enable EPSC to be used as a structural material, it is essential to enhance both its mechanical and durability properties. Durability and load-bearing capacity are crucial for structural materials, with carbonation resistance being a key factor affecting concrete durability. Due to its porous and lightweight nature, the durability of EPSC is far inferior to that of OC. Expanding the use of EPSC as a structural material requires not only strength improvement but also enhanced durability. In recent years, significant advancements have been made in improving the mechanical properties of EPSC, which lays the foundation for its use as a structural material [14,15,16,17,18]. This progress has led to the application of EPSC as prefabricated components for modular construction. EPSC used as precast components needs to have sufficient strength, good toughness, and durability. Current research mainly focuses on improving the strength of EPSC, while studies on the durability of EPSC are relatively rare. The durability of EPSC warrants further in-depth investigation. Carbonation resistance (CR) is one of the most important indicators of the durability of EPSC. The key to improving the carbonation resistance of concrete is to enhance its crack resistance. Currently, many scholars have conducted research on the crack resistance of concrete, and most of them have used the method of adding fibers to enhance the crack resistance of concrete.

Liang et al. [19] showed that basalt fibers have a significant inhibitory effect on the initiation and propagation of microcracks in concrete, while polypropylene fibers have a more pronounced inhibitory effect on the propagation of macro-cracks. Li et al. [20] showed that the crack resistance and shrinkage resistance of high-ductility geopolymer concrete mixed with polyvinyl alcohol fiber (PVA fiber) were significantly improved. They also discovered that the optimal crack resistance and shrinkage resistance were achieved when the PVA fiber volume fraction was 2%. Zhao et al. [21] showed that the bridging effect of polypropylene fibers can constrain local cracks in concrete and delay the appearance of initial cracks. Liang et al. [22] also reached similar conclusions, as it was found that the addition of polypropylene fibers to concrete significantly reduced the likelihood of early-age cracking in the concrete matrix. Li et al. [23] effectively suppressed the development of internal cracks in lightweight aggregate concrete using bamboo fibers, which played a role in crack resistance and enhancing the toughness. Meng et al. [24] showed that incorporating polyacrylonitrile fibers into microbial self-healing concrete can effectively improve its crack resistance and enhance the self-healing effect of cracks.

These scholars found that fibers can effectively improve the crack resistance of concrete, inhibiting the initiation and development of cracks. This is undoubtedly beneficial for preventing gas penetration, indicating that fibers will be very effective in enhancing the CR of concrete. However, these fibers have limited effects on increasing the strength of concrete. When using EPSC as a structural material, it is necessary to comprehensively consider various performance aspects, such as the strength, toughness, and durability. If a fiber can be found that not only increases the strength and toughness of EPSC but also enhances its durability, it would greatly expand the application range of EPSC.

Relevant studies have found that high-tenacity polypropylene (HTPP) fibers can enhance both the strength and toughness of lightweight concrete [25]. If HTPP fibers can also improve the durability of EPSC, applying this material to EPSC would make it safer and more durable as a structural material. However, no researchers have studied the effect of this material on EPSC. This study attempts to add HTPP fibers to EPSC and investigate their effect on the carbonation resistance of EPSC. It explores the impact of different HTPP fiber contents on the CR of EPSC, and the research findings can serve as valuable references for practical engineering applications.

2. Experimental Program

2.1. Raw Materials

The P.O 42.5 cement produced by Conch Cement Company (Wuhu, Anhui Province, China) was employed for the experiments. The mixing water was Taizhou tap water. The Grade II fly ash from Taizhou Tianda Environmental Protection Building Materials Company (Taizhou, China) was adopted. The water reducer was a polycarboxylate superplasticizer produced by Jiangsu Sobute New Materials Company (Nanjing, China), with a solid content of 20%. The particle size of EPS beads was 2–3 mm, and the bulk density was 17.6 kg/m³. The morphology of the EPS particles is shown in Figure 1.

2.2. HTPP Fibers

HTPP fibers are high-performance synthetic fibers known for their smooth surface, abrasion resistance, high tensile strength, and good elasticity [26]. These fibers maintain excellent performance even when exposed to UV light, high temperatures, or humid environments, making them suitable for various engineering applications. When HTPP fibers are incorporated into concrete, they can form a network structure in the concrete matrix to prevent the carbonate aggregate surfaces from being in contact with carbon dioxide from the air [27].

In addition, HTPP fibers can be tightly bonded to the concrete, which enhances its durability, reduces its aging in harsh environments, and improves its load-bearing capacity and seismic performance [26]. The HTPP fibers have a tensile strength of 750 MPa, a breaking elongation of 14%, a length of 12 mm, and a diameter of 0.15–0.2 mm. Their tensile strength and elastic modulus are 800–1200 MPa and 8–10 GPa, respectively. The morphology of the HTPP fibers is shown in Figure 2.

2.3. Specimen Preparation

Five groups of EPSC specimens were prepared for the experiment. Group EP00 served as the control group without HTPP fibers. Groups EP06, EP09, EP12, and EP15 contained 0.6%, 0.9%, 1.2%, and 1.5% HTPP fibers by volume, respectively. All other materials were consistent across groups. The mixing ratios for the EPSC specimens are listed in

Table 1. Mixing ratios of EPSC specimens (per 1 m³).

Specimen ID	Cement (kg/m3)	Fly Ash (kg/m3)	EPS Particles (L)	Water (kg/m3)	Water Reducer (kg/m3)	HTPP Fiber Volume Content
EP00	370	370	580	272	2	0%
EP06	370	370	580	272	2	0.60%
EP09	370	370	580	272	2	0.90%
EP12	370	370	580	272	2	1.20%
EP15	370	370	580	272	2	1.50%

.

Preparation of EPSC specimens was in accordance with the “Standard for Test Methods of Concrete Physical and Mechanical Properties” (GB/T50081-2019) [28]. Sufficient cubic specimens of 100 mm side length were prepared for each group. At least 12 specimens from each group were tested for carbonation depth at 3, 7, 14, and 28 days, and the strength of another 12 specimens was tested at the same carbonation ages. In addition, three specimens were tested for 28-day strength, while the other three specimens were used for scanning electron microscopy (SEM) analysis. The molded specimens were naturally cured for 1 day, then they were demolded and moved to a standard curing room with the temperature at around 20 °C and the humidity at around 95%. One example of preparation of the samples is illustrated in Figure 3. The dry densities of the five groups of specimens ranged from 1000 to 1100 kg/m³.

2.4. Testing Methods

2.4.1. Carbonation Testing

Based on the “Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete” (GB/T50082-2009) [29], the specimens were first cured in a standard curing room for 26 days, and then they were baked in a constant temperature and humidity chamber for 2 days. After curing and drying, two opposite sides of each specimen were left waterless, while the other surfaces were sealed with heated paraffin. Parallel lines were drawn on the side surfaces with a pencil at 10 mm intervals to mark the measurement points for the CD. The treated specimens were then placed on iron racks in a carbonation chamber, and the surface distance between the specimens was ensured to be not less than 50 mm. Three specimens were taken from each group at 3, 7, 14, and 28 days of carbonation age, and the residual powder on the fracture surface was scraped off. A 1% phenolphthalein alcohol solution was sprayed on the surface of the specimens, and the CD was measured using a CD gauge with an accuracy of 0.1 mm.

2.4.2. Microstructure Analysis

Microstructure analysis was conducted using SEM. Representative concrete specimens after 28 days of curing were selected and crushed into samples of suitable sizes for SEM observation. The samples were gold-coated, and SEM images were taken to observe the concrete matrix, internal microcracks, and bonding between fibers and matrix. In addition, the morphology, orientation, and distribution of fibers within the concrete were examined.

3. Results and Discussion

3.1. Analysis of CD

Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8 illustrate the carbonation of the EPSC specimens at 3, 7, 14, and 28 days, while the CDs at these carbonation ages are displayed in

Table 2. CDs of EPSC specimens at various carbonation ages.

Specimen ID	CD at Various Carbonation Ages (mm)
	3 Days	7 Days	14 Days	28 Days
EP00	6.5	7.6	11.3	19.0
EP06	6.8	9.3	10.5	18.5
EP09	3.9	6.6	10.6	17.0
EP12	4.6	6.5	10.9	15.6
EP15	4.3	5.2	11.1	13.4

and Figure 9. Compared with the 7-day carbonation depth, the 14-day CD increased rapidly, with growth rates of 48.7%, 12.9%, 60.6%, 67.7%, and 113.5% for groups EP00, EP06, EP09, EP12, and EP15, respectively. As the carbonation age increased, the CD of the specimens also increased. The 28-day CD of control group EP00 reached 19 mm, which was obviously larger than that of OC.

Compared to group EP00, the 28-day CDs of groups EP06, EP09, EP12, and EP15 decreased by 2.6%, 10.5%, 17.9%, and 29.5%, respectively, which indicated that the HTPP fibers dramatically improved the CR of EPSC. This improvement was due to the fact that the HTPP fibers alleviated cracking and shrinkage in EPSC, thereby reducing its overall internal porosity. A lower internal porosity decreased the diffusion capacity of CO₂ within the HTPP-reinforced EPS concrete, thus enhancing its carbonation resistance.

3.2. Analysis of UCS after Carbonation

Figure 10 shows the 28-day UCS of the EPSC specimens. From Figure 10, it can be found that the strength of the EPC specimens with added HTPP fibers significantly increased, except for specimen EP12. The strengthening effect of the HTPP fibers on EPSC was particularly notable. The strength of specimen EP12 did not show a significant increase, mainly due to errors in specimen preparation.

Figure 11 confirmed that the strength of EPSC increased with the increasing carbonation age. This increase was due to the reaction between calcium hydroxide (Ca(OH)₂) and carbon dioxide (CO₂) in the concrete that generates calcium carbonate (CaCO₃) [30]. Calcium carbonate has a larger volume and density than calcium hydroxide, and the crystals formed can fill the pores in the concrete, thereby increasing its density and strength. The interface between the EPS particles and cement matrix may undergo changes during carbonation. The formation of calcium carbonate can create a transition zone between the EPS particles and cement matrix, improving interfacial bonding and increasing the overall strength of the concrete.

By comparing the results in Reference [31], it can be found that the strength of the carbonated EPS concrete with added HTPP fibers is higher than that of the carbonated EPS concrete with the partial replacement of cement by microspheres. This is because HTPP fibers form a complex network structure within the cement matrix, improving the interfacial bonding strength. Additionally, the calcium carbonate formed creates strong bonds between EPS particles and the cement matrix, and the calcium carbonate generated during carbonation may further enhance the bond between fibers and the matrix, thereby increasing the strength of EPSC.

Table 3. Relative strength of EPSC specimens at various carbonation ages.

Specimen ID	UCS (MPa)					Relative Strength
	0 Days	3 Days	7 Days	14 Days	28 Days	0 Days	3 Days	7 Days	14 Days	28 Days
EP00	8.80	7.27	6.40	11.50	13.60	1.00	0.83	0.73	1.31	1.55
EP06	9.37	8.30	10.50	13.33	12.57	1.00	0.89	1.12	1.42	1.34
EP09	11.07	9.57	15.70	16.27	16.90	1.00	0.86	1.42	1.47	1.53
EP12	8.73	8.97	12.23	13.63	14.67	1.00	1.03	1.40	1.56	1.68
EP15	10.17	9.77	13.23	15.77	15.07	1.00	0.96	1.30	1.55	1.48

shows the relative strength of the EPSC subjected to carbonation, and Figure 12 presents the variation in the relative strength of the EPSC specimens under various carbonation ages. From Table 3, it can be observed that the strength of most specimens increased after carbonation. This is because the CaCO3 produced by carbonation contributes to the increase in the concrete strength. However, in Table 3, the strength of some specimens did not increase but decreased after 3 and 7 days of carbonation. This is mainly due to errors in specimen preparation. In the early stages of the carbonation of the EPS concrete, the carbonation depth is relatively small, resulting in a minor increase in the strength due to carbonation. However, the fluctuations in the EPSC specimen strength caused by specimen preparation errors exceeded the strength increase due to carbonation. Therefore, the strength of a small number of EPC specimens decreased during the early stages of carbonation. To obtain accurate curve equations, a binary function was used to fit the relative strength curves of the EPSC specimens subjected to carbonation, as illustrated in Figure 13. The fitted curves matched well with the experimental curves, and the function model is expressed by Equation (1) [32]:

$$k=\alpha t^2+\beta t+\gamma$$

(1)

where k represents the relative strength; t represents the carbonation time; and α, β, and γ are the undetermined coefficients. β and α characterize the initial rate and acceleration of change in the relative strength, respectively, and γ represents the initial value of the relative strength. The model parameters derived from the fitted curves are shown in

Table 4. Model parameters for EPSC.

Specimen ID	α	β	γ	R2
EP00	5.84308 × 10−4	0.0092	0.86401	0.49272
EP06	−0.00109	0.04724	0.88969	0.61092
EP09	−0.00144	0.06255	0.9042	0.5745
EP12	−0.00132	0.0628	0.95417	0.90454
EP15	−0.00157	0.0647	0.91251	0.81647

.

Table 4 demonstrated that the β values of groups EP06, EP09, EP12, and EP15 were remarkably larger than those of group EP00, suggesting that the initial change in the UCS with the addition of HTPP fibers was greater. This was due to the fact that the UCS increased rapidly with the incorporation of HTPP fibers, leading to a faster change in the relative compressive strength, which was consistent with the observed patterns under carbonation.

3.3. Prediction of CD of EPSC

The CD of concrete is generally related to the carbonation time, and it is also influenced by factors such as the CO₂ concentration, humidity, and temperature [33,34,35,36]. Since the carbonation environment was the same for all five groups of specimens in this study, the relationship between the CD and carbonation time can be expressed by Equation (2) [37,38]:

$$d=k\sqrt{t}$$

(2)

where d denotes the CD, with a unit of mm; t denotes the carbonation time in days; and k denotes the carbonation rate coefficient. The carbonation curves of the EPSC specimens were fitted using Equation (2), and the resulting curves are shown in Figure 14. The carbonation rate coefficients obtained from the fitted curves are presented in

Table 5. Parameters of the CD fitting model.

Fitting Parameter	EP00	EP06	EP09	EP12	EP15
K	3.34973	3.33777	2.95835	2.8557	2.57008
R2	0.97311	0.96684	0.97035	0.98936	0.95881

.

Figure 14 revealed that the fitted carbonation curves matched well with the experimental curves. Table 5 demonstrated that the fitting coefficients for all groups of specimens were above 0.95, indicating that the fitted model can validly predict the CD of EPSC. A comparison between the actual and predicted values of the CD was performed for further assessing the reliability of the model. The comparison results in

Table 6. Comparison of calculated and measured CDs at various carbonation ages.

ID	3 Days			7 Days			14 Days			28 Days
	Measured Depth (mm)	Calculated Depth (mm)	Error (%)	Measured Depth (mm)	Calculated Depth (mm)	Error (%)	Measured Depth (mm)	Calculated Depth (mm)	Error (%)	Measured Depth (mm)	Calculated Depth (mm)	Error (%)
EP00	6.5	5.8	10.77%	7.6	8.8	−15.79%	11.3	12.5	−10.62%	19.0	17.7	6.84%
EP06	6.8	5.7	16.18%	9.3	8.9	4.30%	10.5	12.4	−18.10%	18.5	17.7	4.32%
EP09	3.9	5.1	−30.77%	6.6	7.8	−18.18%	10.6	11.1	−4.72%	17.0	15.6	8.24%
EP12	4.6	4.9	−6.52%	6.5	7.6	−16.92%	10.9	10.7	1.83%	15.6	15.1	3.21%
EP15	4.3	4.5	−4.65%	5.2	6.8	−30.77%	11.1	9.7	12.61%	13.4	13.6	−1.49%

Note: Positive error values indicate that the measured values are greater than the calculated values, while the opposite is true for negative error values.

indicated that the predicted values were close to the measured values with minimal error, which verified the applicability of the model in predicting the carbonation depth in EPSC.

It was observed from Table 5 that the carbonation rate was highest in group EP00, while groups EP06, EP09, EP12, and EP15 showed progressively lower carbonation rates. Compared with group EP00, the carbonation rates for groups EP06, EP09, EP12, and EP15 decreased by 11.68%, 14.75%, and 23.28%, respectively. This demonstrated that the HTPP fibers effectively enhanced the CR of EPSC, and that the CR improved with the increasing HTPP fiber content. Furthermore, Table 5 indicated that when the HTPP fiber content exceeded 0.9%, there was a significant reduction in the carbonation rate, suggesting that the fiber volume content should be greater than 0.9% to achieve noticeable improvement.

3.4. Micromechanism Analysis

Figure 15 shows the SEM images of the internal structure of the EPSC specimens. Figure 15a,b depict the EPSC without HTPP fibers, while Figure 15c–f present the EPSC with HTPP fibers. A comparison of Figure 15a,d revealed that the cracks in the EPSC with HTPP fibers were smaller, whereas the cracks in the EPSC without HTPP fibers were wider. This indicated that HTPP fibers reduced the formation and propagation of cracks. It was observed from Figure 15b that the interfacial transition zone of the EPSC without HTPP fibers was relatively loose. In contrast, Figure 15c showed that the EPS particles in the HTPP fiber-reinforced EPSC were compressed, resulting in a denser interfacial transition zone, which increased the density and reduced the porosity of the concrete, thereby improving the CR. The high elastic modulus of HTPP fibers allows them to dramatically improve the tensile and flexural strengths of EPSC under external loads, indirectly improving its CR. Figure 15e revealed that the fibers were present in short bundles or dispersed in the concrete. When the concrete is subjected to external stress, the fibers can absorb some of the tensile stress to prevent the formation and propagation of cracks, thus reducing the risk of carbonation. Figure 15f demonstrated numerous scratches on the surface of the HTPP fibers, indicating a strong bond between the fibers and the cement matrix, which is able to absorb more energy under stress and reduce the formation and propagation of microcracks.

3.5. Prediction of Carbonation Service Life

Carbonation can significantly compromise the durability of reinforced concrete by penetrating the concrete protective layer and causing the passive film on the steel reinforcement to break down, which eventually leads to corrosion. This corrosion leads to surface cracking, exacerbates the corrosion cycle, and gradually reduces the structural load-bearing capacity. Therefore, the diffusion of CO₂ in concrete structures is crucial to their durability.

When CO₂ reaches the steel surface, the concrete protective layer is completely damaged, and the steel begins to rust. The time required for complete carbonation of the protective layer is defined as the carbonation service life of the concrete. Researchers have derived empirical relationships from laboratory carbonation parameters to predict the natural environmental service life of concrete. The empirical relationship is expressed by Equation (3) [39]:

$${\displaystyle \frac{D_2}{D_1}}=\sqrt{{\displaystyle \frac{T_2·c_2}{T_1·c_1}}}$$

(3)

where D₁ denotes the laboratory CD; D₂ denotes the CD in a typical atmospheric environment (the concrete cover thickness is calculated as 25 mm); T₁ denotes the laboratory carbonation time (standard 28 days); T₂ is the carbonation service life in a typical atmospheric environment; c₁ is the CO₂ concentration in the accelerated laboratory test (20% in this study); and c₂ is the CO₂ concentration in a typical atmospheric environment (0.03% in this study). Based on Equation (3), the carbonation service life of EPSC components in a natural environment is approximately 88.5 years.

Figure 16 illustrates the predicted carbonation service life of HTPP fiber-reinforced EPSC components under accelerated carbonation test conditions, with the fiber volume content increasing from 0% to 1.5%. Furthermore, the carbonation service life of EPSC containing HTPP fibers increased significantly, and the EPSC containing 1.5% HTPP fibers showed the highest increase of about 101.1% in the carbonation service life. This demonstrated that adding HTPP fibers to EPSC can better protect steel reinforcement, significantly extending its carbonation service life and enhancing its durability.

4. Conclusions

This study investigated the enhancement of the CR of EPSC by incorporating HTPP fibers. The following conclusions are drawn:

• HTPP fibers effectively improved the CR of EPSC and reduced the 28-day CD by up to 29.5%.
• The UCS of EPSC after carbonation increased by 30−60%. The established relative strength model and CD model for EPSC matched well with the experimental curves, which offers a valuable reference for the performance evaluation of EPSC after carbonization.
• From a microstructural perspective, the enhancement mechanism of the CR of EPSC by HTPP fibers was mainly attributed to three factors: increased concrete density, reduced formation and propagation of cracks, and improved overall structural integrity.
• The carbonation service life of EPSC after introducing HTPP fibers was dramatically increased by up to 101.1%. This demonstrated that incorporating HTPP fibers can substantially improve the carbonation service life and enhance the durability of EPSC.