Durability and Flexural Response of RC Beams to Freeze–Thaw Cycles: Influence of Air Content

Abstract

Research on the long-term durability of concrete has continued due to its widespread application in construction. Freeze–thaw cycles significantly impact concrete durability, particularly in regions with harsh climates. While most studies focus on the material properties of concrete, limited research has addressed the performance degradation of reinforced concrete structures. This study investigates the freeze–thaw resistance of RC beams made with 35 MPa concrete, with particular emphasis on the influence of air content on flexural performance. RC beams were exposed to freeze–thaw cycles using the air freeze–thaw method and the ASTM C666/C666M-15 water freeze–thaw method. Their flexural behavior was evaluated through four-point bending tests. The results showed that low-air-content RC beams exhibited notable reductions in yield load and energy absorption capacity after freeze–thaw cycles, indicating decreased strength and ductility. Conversely, RC beams with appropriate or high air content exhibited minimal reductions, demonstrating superior freeze–thaw resistance. These findings underscore the importance of optimizing air content to enhance the durability of RC structures in harsh environmental conditions.

1. Introduction

With recent advances in construction technology, concrete with relatively high design compressive strength has been increasingly used in various reinforced concrete structures [1,2]. In particular, 35 MPa-grade concrete is widely applied in general structural members because it provides adequate mechanical performance, durability, and structural stability [3]. According to ASTM C94/C94M-23 (2023) [4], the recommended air content may be reduced by up to 1% for concrete with a compressive strength of 5000 psi, approximately 35 MPa, or higher. Although concrete in this strength range generally has a relatively dense matrix and improved resistance to water penetration, its freeze–thaw resistance may still be influenced by air content under repeated freezing and thawing conditions [5,6,7]. Therefore, it is necessary to evaluate the effect of air content on the freeze–thaw durability and flexural performance of 35 MPa-grade reinforced concrete members.

According to ASTM C94/C94M-23 (2023) [4], it is permissible to reduce the recommended air content by up to 1% for concrete with a compressive strength of 5000 psi (35 MPa) or higher. The general consensus is that high-strength concrete is relatively unaffected by freeze–thaw cycles. This resistance is attributed to its high density and low absorption, which make it difficult for water to penetrate the concrete, thereby reducing the likelihood of internal cracking or failure due to freeze–thaw cycles [8,9,10]. However, high-strength concrete can still be susceptible to freeze–thaw damage under certain conditions, underscoring the need for further research in this area. Structures such as dams, bridges, ports, and underground facilities are all vulnerable to freeze–thaw damage due to their constant exposure to water, making them more susceptible than inland concrete structures [11]. These structures are likely to weaken their internal structure due to repeated freeze–thaw cycles caused by contact with water. Additionally, the degree of damage to concrete structures subjected to freeze–thaw cycles in the air may differ from those subjected to freeze–thaw cycles in water [12,13,14,15,16,17,18,19,20,21,22,23].

Recent studies on freeze–thaw cycles have revealed a gradual increase in the strength of the concrete used in experiments over time [24,25,26,27,28,29]. Given this context, there is a need for experimental research to evaluate the freeze–thaw resistance of high-strength concrete with varying air contents. Such research is crucial for expanding the application of high-strength concrete, accurately predicting and managing its performance in various environments, and ensuring the safety and durability of structures. Moreover, it is important to conduct experimental research not only on the material properties of concrete under freeze–thaw conditions but also on the performance of reinforced concrete (RC) members. Research at the member level provides a more realistic assessment of the performance of actual structures.

However, existing rapid freeze–thaw testing standards (ASTM C666/C666M-15 (2015)) [30] primarily focus on material-level evaluations and are not well-suited for large-scale freeze–thaw experiments on RC structures. To address this limitation, this study applied the freezing-thawing in air method, recently proposed by the authors and used by several researchers [31,32]. This innovative approach replaces conventional water-based thawing with air-based thawing, overcoming the limitations of traditional standards and addressing the unique challenges of large-scale structural applications.

In this study, the freeze–thaw resistance of concrete was evaluated under three conditions: reduced air content (up to 1%), optimal air content, and significantly increased air content. Additionally, freeze–thaw and flexural tests were conducted on flexural members made from these concrete types. The results of the freeze–thaw tests were analyzed in terms of relative dynamic modulus of elasticity, durability index, mass loss rate, and compressive strength, while the results of the flexural tests on RC members were compared and analyzed based on failure patterns, load–deflection curves, and energy absorption capacity.

2. Experimental Program

2.1. Test Materials Design

A 35-MPa-grade concrete was designed, and the mix proportions are shown in

Table 1. Mixture design for concrete (unit: kg/m³).

Mixture ID	W	C	W/C	FG	CG	AE	SP
C35-LA	185	308	0.60	806	986	0	0.9
C35-MA	185	370	0.50	728	890	0.56	1.1
C35-HA	185	411	0.45	683	835	3.3	2.1

W = water; C = cement; W/C = water/cement; FG = fine aggregate; CG = coarse aggregate; AE = air entrainment; SP = superplasticizer.

. To evaluate the effects of freeze–thaw cycles on concrete with varying air content, an air-entraining (AE) agent was used to adjust the air content. The mix was designed to maintain consistent compressive strength across different air contents by increasing the coarse and fine aggregate contents while reducing the water–cement ratio. In general, an increase in air content can reduce the compressive strength of concrete; therefore, the water–cement ratio was adjusted to achieve comparable compressive strengths across the mixtures. The superplasticizer dosage was also adjusted to secure adequate workability as the water–cement ratio decreased. However, because changes in the water–cement ratio and superplasticizer dosage may also affect the pore and capillary structures of concrete, the results of this study should be interpreted as the combined effect of mixture adjustment and air content under a comparable compressive strength level.

The air content of the concrete was designed to evaluate its freeze–thaw resistance under three conditions: minimum, moderate, and over-severe air content. In accordance with ASTM C94/C94M-23 (2023) [4], the minimum air content was set at 1.0%, the moderate air content at 4.5%, and the severe air content at 6.0%. Using the ASTM C231 (2022) [33] pressure method, a Washington-type concrete air meter was used to determine the air content of the concrete. The measured air contents during concrete mixing were 1.2%, 5.3%, and 9.3%, with corresponding slump values of 130 mm, 180 mm, and 220 mm. The compressive strength results were 38.63 MPa, 38.71 MPa, and 38.51 MPa, respectively, with an average of approximately 38 MPa, satisfying the target compressive strength of 35 MPa. The mechanical properties of the mixed concrete are summarized in

Table 2. Mechanical properties of concrete.

Mixture ID	Air Content of Freshly Mixed Concrete (%)	Slump (mm)	Compressive Strength (MPa)
C35-LA	1.2	130	38.63
C35-MA	5.3	180	38.71
C35-HA	9.3	220	38.51

.

2.2. Reinforced Concrete (RC) Beam Specimen

To assess the flexural behavior of RC beams with varying concrete air content under freeze–thaw cycles, 12 specimens were fabricated. Figure 1 illustrates the dimensions of the RC beams, as well as the locations where strain gauges were attached. Each RC beam had an overall length of 1800 mm and a height of 200 mm. The transverse flexural reinforcement was positioned 35 mm from the bottom of the beam. To mitigate the risk of shear failure, stirrups were spaced at 85 mm intervals over a 680 mm length, extending from the loading points to the support regions. The flexural and shear reinforcements employed in this study consisted of D10 bars made of SD400 grade steel. The theoretical yield load of the RC beam was calculated as approximately 18.3 kN under the four-point bending condition. As presented in

Table 3. Design parameters of RC beam specimens.

RC Beam ID	Mixture ID	Freezing and Thawing Cycle (EA)	Air Content of Freshly Mixed Concrete (%)	Number of RC Beams
RCB-LA-N	C35-LA	0	1.2	2
RCB-LA-D	C35-LA	300	1.2	2
RCB-MA-N	C35-MA	0	5.3	2
RCB-MA-D	C35-MA	300	5.3	2
RCB-HA-N	C35-HA	0	9.3	2
RCB-HA-D	C35-HA	300	9.3	2

, the primary variables investigated were the number of freeze–thaw cycles and the air content in the concrete. The freeze–thaw cycles were conducted up to 300 cycles, and the concrete air content was classified into three levels: low air content (LA: A < 3%), moderate air content (MA: 3% < A < 6%), and high air content (HA: A > 6%).

2.3. Preparation of RC Beam Specimen

Two types of freeze–thaw tests were conducted to simulate conditions of freeze–thaw cycles under continuous year-round water exposure and in dry environments: a rapid freeze–thaw test in water (FTW test) and a novel approach, a rapid freeze–thaw test in air (FTA test). The FTW test was performed in accordance with ASTM C666/C666M-15 [30], while the FTA test differed in that thawing was conducted in air rather than water. The details of each testing method are summarized in

Table 4. Comparison of the freeze–thaw test in water (FTW test) and the freeze–thaw test in air (FTA test) methods used in this study.

	FTW Test	FTA Test
Standard	ASTM C666/C666M-15 [30] KS F 2456 (Republic of Korea)	N/A
Experimental condition	Rapid Freeze-Thaw in Water	Rapid Freeze-Thaw in Air
Freezing and thawing temperatures	4(±2) °C ↔ −18(±2) °C	4(±2) °C ↔ −18(±2) °C
Time required for one cycle	2–4 h	9–10 h
Test specimen	Concrete (material)	Concrete (material), RC beam (structure)
Damage	Relative dynamic modulus of elasticity, durability factor, mass loss rate	Material: relative dynamic modulus of elasticity, durability factor, mass loss rate, compressive strength/ Structure: failure mode, load–deflection curve, energy absorption capacity

.

The FTA test is an experimental method proposed by the authors and subsequently utilized by other researchers [31,32]. Unlike the conventional freeze–thaw procedures specified in ASTM C666/C666M-15 (2015) [30], which rely on water for the thawing process (Procedure A: freezing and thawing in water; Procedure B: freezing in air and thawing in water), the FTA test conducts both freezing and thawing in air. This modification was motivated by two factors: (1) the impracticality of using large volumes of water for freeze–thaw testing of full-scale structural members such as RC beams, and (2) the need to simulate dry winter conditions, as observed in regions like Seoul, where the freeze–thaw period coincides with very low humidity [31]. The freeze–thaw process in the FTW test was carried out using a chamber compliant with ASTM C666/C666M-15 (2015) [30], as shown in Figure 2a, whereas the FTA test was conducted using the chamber depicted in Figure 2b. The upper and lower temperature limits were set identically for both the FTW and FTA tests at 4 °C and −18 °C, respectively. To analyze the relationship between the FTW test and FTA, cylindrical specimens with dimensions of Φ100 × 200 mm were produced from the same concrete batch used for the RC beams. These specimens were subjected to both the FTW and FTA tests. This comparative testing was performed to verify the reliability of the FTA test by establishing a correlation with the standardized FTW test in terms of material degradation characteristics. After the freeze–thaw cycles, the relative dynamic modulus of elasticity and the concrete mass were measured and compared. Due to the RC beams’ larger size, which exceeded the capacity of the FTW test chamber, only the FTA test was conducted on them.

2.4. Flexural Test Setup

As illustrated in Figure 3, the flexural tests on the RC beams were conducted using the four-point bending method. The distance between the two supports was 1400 mm, and the distance between the two loading points was 400 mm. A hydraulic jack was used to apply the load, with a sensor placed between the jack and the RC beam to record the load values. Additionally, a linear variable differential transformer (LVDT) was installed at the midpoint of the RC beam to measure vertical displacement. The tests were conducted under displacement control, with the load applied at a rate of 1 mm/min. The experiments were terminated once significant damage occurred following the yielding of the RC beams.

3. Materials Test Results

3.1. Comparison of Relative Dynamic Modulus and Durability Index

The relative dynamic modulus of elasticity was measured using a Digital Type Dynamic Young’s Modulus Meter (HJ-5350, Heungjin, Gimpo, Republic of Korea), as shown in Figure 4. The measured resonance frequency was used to calculate the relative dynamic modulus using Equation (1).

$$P_c=\left({\displaystyle \frac{n_c^2}{n_o^2}}\right)\times 100,$$

(1)

Here, $P_c$ presents the relative dynamic modulus of elasticity (%) after $c$ cycles of freeze–thaw, $n_0$ denotes the fundamental resonance frequency of the deformation vibration at 0 cycles of freeze–thaw (Hz), and $n_c$ indicates the fundamental resonance frequency of the deformation vibration at c cycles of freeze–thaw (Hz).

Figure 4. Dynamic modulus measuring equipment.

Figure 4. Dynamic modulus measuring equipment.

The results of the relative dynamic modulus of elasticity for concrete subjected to different freeze–thaw methods are presented in Figure 5. For the FTW test, the relative dynamic modulus of elasticity was measured every 30 cycles, while for the FTA test, it was measured every 100 cycles. The relative dynamic modulus values reported in this study were obtained as the average of five repeated measurements for each specimen. After 300 freeze–thaw cycles, the relative dynamic modulus of elasticity for C35-LA, C35-MA, and C35-HA in the FTW test was measured as 80.82%, 89.91%, and 85.55%, respectively. In the FTA test, the relative dynamic modulus of elasticity for C35-LA, C35-MA, and C35-HA after 300 freeze–thaw cycles was recorded as 85.2%, 92.1%, and 89.9%, respectively.

As shown in Figure 5, both the FTW and FTA tests indicated a reduction in the relative dynamic modulus of elasticity with increasing freeze–thaw cycles, attributable to cumulative damage in the concrete. However, the degree of reduction was more pronounced in the FTW test compared to the FTA test. Specifically, the differences in relative dynamic modulus between the two tests for C35-LA and C35-HA after 300 freeze–thaw cycles were 4.38% and 4.35%, respectively, while for C35-MA, the difference was relatively smaller at 2.19%.

The durability index (D.I.) was calculated using Equation (2), where R represents the relative dynamic modulus of elasticity, and FTC denotes the number of freeze–thaw cycles.

$$D.I.={\displaystyle \frac{R\times \mathrm{F}\mathrm{T}\mathrm{C}}{300}},$$

(2)

The durability index (D.I.) is a composite parameter that reflects the residual stiffness of concrete after repeated freeze–thaw cycles. Since the relative dynamic modulus of elasticity represents internal damage such as microcrack propagation and pore-structure degradation, a higher D.I. indicates that the concrete retains a greater proportion of its original elastic properties under freeze–thaw exposure. Therefore, the D.I. can be used to evaluate the overall freeze–thaw resistance of concrete, with lower values indicating more severe stiffness degradation and internal deterioration.

After 300 freeze–thaw cycles in the FTW test, the durability indices for C35-LA, C35-MA, and C35-HA were measured as 82.71%, 89.22%, and 85.95%, respectively. In the FTA test, the durability indices for C35-LA, C35-MA, and C35-HA were 85.2%, 92.1%, and 89.9%, respectively. The differences in durability index between the FTW and FTA tests were 2.49% for C35-LA, 2.88% for C35-MA, and 3.95% for C35-HA. Figure 6 shows the density index corresponding to the freeze–thaw cycles.

The freeze–thaw resistance, as measured by air content, was consistent across both the FTW and FTA tests, with the order of performance being C35-MA, C35-HA, and C35-LA. This trend aligns with the findings of previous studies.

3.2. Comparison of Mass Loss

The mass loss rates of concrete subjected to different freeze–thaw methods are presented in Figure 7. The mass loss rate was calculated using Equation (3), where ΔW_n is the mass loss rate after n freeze–thaw cycles (%), W₀ is the initial mass of the specimen before freeze–thaw cycles, and W_n is the mass of the specimen after n freeze–thaw cycles.

$$\Delta {\mathrm{W}}_n={\displaystyle \frac{W_0-{ W}_n}{W_0}}\times 100\%,$$

(3)

After 300 freeze–thaw cycles in the FTW test, the mass loss rates for C35-LA, C35-MA, and C35-HA were measured as 1.66%, 0.47%, and 0.92%, respectively. In the FTA test, the mass loss rates for C35-LA, C35-MA, and C35-HA after 300 freeze–thaw cycles were 0.55%, 0.53%, and 0.48%, respectively. Both the FTW and FTA tests revealed that concrete mass decreased due to the freeze–thaw cycles. However, the degree of mass loss was significantly greater in the FTW test compared to the FTA test. As shown in Figure 7a, the greater mass loss in the FTW test can be attributed to the fact that water, the heat transfer medium in the FTW test, causes more significant surface spalling and cumulative damage with increasing cycles, leading to continuous mass reduction. In contrast, the FTA test, in which air serves as the heat transfer medium, results in less surface spalling and slower freeze–thaw progression, leading to less mass loss.

Figure 7b indicates that in the FTA test, the concrete mass decreases slightly until about 90 cycles, then shows a temporary small decrease between 90 and 120 cycles, after which the mass loss continues to decrease slightly regardless of the concrete’s air content. On the other hand, in the FTW test, as the number of freezing and thawing cycles increases, the mass loss rate varies with the air content in the concrete, with C35-MA showing the greatest resistance to mass loss, followed by C35-HA and C35-LA.

In the FTW test, differences in mass loss rate with air content were clear, and among the three mixtures tested, C35-MA showed the lowest mass loss rate, indicating a tendency toward improved resistance to mass loss. In contrast, in the FTA test, the differences in mass loss rate among the mixtures were relatively small because the external supply of moisture was limited under air freeze–thaw conditions.

3.3. Compressive Strength Change Due to a New Type of Freezing and Thawing Test

To evaluate the changes in the compressive strength of concrete in RC beams subjected to rapid freeze–thaw cycles in the air (the FTA test), cylindrical concrete specimens were fabricated from the same batch as the RC beams. The compressive strength of these specimens was measured in accordance with ASTM C39/C39M-21 standards [34], and the results are presented in Figure 8. After 300 cycles of the FTW test, the compressive strengths of C35-LA, C35-MA, and C35-HA were found to be 31.69 MPa, 33.95 MPa, and 32.32 MPa, respectively, representing reductions of approximately 18%, 8%, and 12% for C35-LA, C35-MA, and C35-HA, respectively. These findings indicate that C35-MA exhibits the highest resistance to freeze–thaw damage, whereas C35-LA shows the greatest reduction in strength. The results of the compressive strength tests are summarized in

Table 5. Compressive strength test results.

Mixture ID	Freezing and Thawing Cycle (EA)	Compressive Strength of Concrete (MPa)	Decrease Rate (%)
C35-LA	0	38.63	0
	300	31.69	17.95
C35-MA	0	38.71	0
	300	35.52	8.23
C35-HA	0	38.51	0
	300	33.95	11.83

.

4. Flexural Test Results of RC Beams

4.1. Failure Modes

Figure 9 shows the final failure modes of each RC beam specimen. As shown in Figure 9, all RC beam specimens failed in flexure, with tensile cracks forming in the tension zone and steel yielding as expected. Vertical cracks started in the tension zone and gradually spread as the load increased. With additional displacement, the flexural cracks further developed and expanded.

4.2. Load–Deflection Curves

Figure 10 shows the load–deflection curves for the tested RC beams. The yield point of the RC beams was defined as the strain in the rebar reaching 2000 × 10⁻⁶. The corresponding yield load and yield displacement are summarized in

Table 6. Yield load and yield deflection of the tested RC beams.

RC Beam ID	Yield Load (kN)		Yield Deflection (mm)
	Each	Mean ± SD	Each	Mean ± SD
RCB-LA-N-1	23.10	22.92 ± 0.26	5.78	5.99 ± 0.30
RCB-LA-N-2	22.74		6.20
RCB-LA-D-1	21.91	21.00 ± 1.29	5.41	5.31 ± 0.14
RCB-LA-D-2	20.09		5.21
RCB-MA-N-1	24.84	24.27 ± 0.81	4.43	4.54 ± 0.16
RCB-MA-N-2	23.69		4.65
RCB-MA-D-1	23.49	23.35 ± 0.21	4.71	4.69 ± 0.03
RCB-MA-D-2	23.20		4.67
RCB-HA-N-1	24.12	24.54 ± 0.59	5.25	5.43 ± 0.26
RCB-HA-N-2	24.96		5.61
RCB-HA-D-1	23.45	23.21 ± 0.35	5.55	5.39 ± 0.23
RCB-HA-D-2	22.95		5.23

Note: SD denotes standard deviation.

. The yield load and deflection characteristics of each RC beam were compared before and after the freeze–thaw cycles.

The load–deflection curves of all RC beam specimens demonstrated characteristic flexural behavior. During the initial phase, the curves exhibited linear elastic behavior up to the onset of cracking in the tensile zone, followed by a significant reduction in stiffness as the tensile reinforcement reached yield. Post-yielding, the applied load continued to increase with further increments in displacement.

Firstly, as shown in Figure 10a and Figure 10b, the load–deflection curves for RCB-LA-N and RCB-LA-D differ due to the freeze–thaw cycles. Before the freeze–thaw cycles, RCB-LA-N demonstrated an average yield load of 22.92 kN and deflection of 5.99 mm. However, after the freeze–thaw cycles, RCB-LA-D exhibited an average yield load of 21.00 kN and deflection of 5.31 mm. These results indicate that the freeze–thaw process reduced the strength and ductility of the concrete beam.

Next, the load–deflection curves for RCB-MA-N and RCB-MA-D are shown in Figure 10c and Figure 10d, respectively. The curves show minimal differences due to the freeze–thaw cycles. RCB-MA-N had an average yield load of 24.27 kN and a deflection of 4.54 mm, while RCB-MA-D, after undergoing freeze–thaw cycles, maintained an average yield load of 23.35 kN and a deflection of 4.69 mm. These results suggest that an optimal air content effectively preserves resistance to freeze–thaw cycles, preventing significant deterioration in the flexural performance of the RC beams.

Lastly, the load–deflection relationships for RCB-HA-N and RCB-HA-D are shown in Figure 10e and Figure 10f, respectively. RCB-HA-N exhibited an average yield load of 24.54 kN and a deflection of 5.43 mm, while RCB-HA-D, after the freeze–thaw cycles, showed an average yield load of 23.21 kN and a deflection of 5.39 mm. These results indicate that although the RC beam with higher air content experienced some reduction in flexural performance due to the freeze–thaw cycles, it still maintained a considerable level of resistance.

In conclusion, air content plays a significant role in the freeze–thaw resistance of beams. Beams with low air content showed a pronounced reduction in performance due to freeze–thaw cycles, while those with optimal air content exhibited relatively minimal deterioration. Beams with high air content exhibited some resistance to freeze–thaw cycles. However, Table 5 demonstrates that freeze–thaw cycles significantly reduce the compressive strength of concrete, whereas Table 6 indicates minimal impact on the yield load of RC beams. This is primarily because the yield load is dominated by the tensile capacity of the tension reinforcement, whereas the concrete’s compressive strength becomes more critical in post-yield behavior, especially in determining the ultimate load capacity.

4.3. Energy Absorption Capacity

The energy absorption capacity of RC beams is defined as the area under the load- deflection curve up to the yield point. This parameter indicates the amount of energy a beam can absorb under external loading and is closely related to its ductility and failure resistance.

Table 7. Energy absorption capacity of tested RC beams.

RC Beam ID		Energy Absorption Capacity (kN·mm)		Decrease Rate (%)
		Each	Avg
RCB-LA-N	1	67.19	70.26	0
	2	73.32
RCB-LA-D	1	59.35	56.35	20
	2	53.35
RCB-MA-N	1	55.80	57.83	0
	2	59.85
RCB-MA-D	1	53.88	54.04	7
	2	54.20
RCB-HA-N	1	63.55	66.57	0
	2	69.59
RCB-HA-D	1	64.46	61.72	7
	2	58.97

summarizes the energy-absorption capacities for each tested beam.

Firstly, the energy absorption capacity of RCB-LA-N was measured at an average of 70.26 kN·mm. However, following freeze–thaw cycles, the energy absorption capacity of RCB-LA-D decreased to 56.35 kN·mm, representing a reduction of approximately 20%. These results indicate that freeze–thaw cycles significantly affect the energy absorption capacity of concrete beams, particularly in low-air-content beams, where the effect is more pronounced.

In contrast, the energy absorption capacity of RCB-MA-N was measured at 57.83 kN·mm, and after freeze–thaw cycles, RCB-MA-D exhibited an energy absorption capacity of 54.04 kN·mm. This corresponds to a decrease of about 7%, demonstrating that beams with optimal air content show relatively good resistance to freeze–thaw cycles. The smaller reduction in energy absorption capacity suggests that the optimal air content had a positive effect on maintaining the beam’s ductility and failure resistance.

Lastly, the energy absorption capacity of RCB-HA-N was 66.57 kN·mm, decreasing to 61.72 kN·mm after freeze–thaw cycles for RCB-HA-D. This represents a reduction of approximately 7%. Although a decrease in energy absorption capacity was observed in beams with high air content due to freeze–thaw cycles, the reduction was relatively moderate. This suggests that high air content positively influences the ductility of the RC beams.

5. Discussion

The relative dynamic modulus of elasticity of the concrete, measured by both FTW and FTA tests, decreased with increasing number of freezing and thawing cycles. In the FTW test, the relative dynamic modulus of elasticity for C35-LA, C35-MA, and C35-HA was 80.82%, 89.91%, and 85.55%, respectively, while in the FTA test, the values were 85.2%, 92.1%, and 89.9%, respectively. The reduction in the relative dynamic modulus of elasticity was more pronounced in the FTW test. The durability index also followed this trend, with values of 82.71%, 89.22%, and 85.95% for C35-LA, C35-MA, and C35-HA in the FTW test, and 85.2%, 92.1%, and 89.9% in the FTA test. In both tests, the freeze–thaw resistance based on air content was ranked in the order of C35-MA, C35-HA, and C35-LA.

The mass loss rate of the concrete was also measured, showing that in the FTW test, the mass loss rates for C35-LA, C35-MA, and C35-HA were 1.66%, 0.47%, and 0.92%, respectively, while in the FTA test, the values were 0.55%, 0.53%, and 0.48%, respectively. Although the concrete mass decreased due to freeze–thaw cycles in both tests, the mass loss was greater in the FTW test. This is attributed to the heat-transfer medium in the FTW test being water, which led to more severe surface spalling of the concrete. C35-MA, with its optimal air content, exhibited the highest resistance to mass loss, while C35-LA, with the lowest air content, showed the greatest mass loss rate. This is because the optimal air content facilitates the formation of air pockets that can absorb water’s expansion, thereby alleviating the increase in internal pressure caused by freeze–thaw cycles.

The compressive strength of the concrete subjected to rapid freeze–thaw in air (FTA) was measured at 31.69 MPa, 33.95 MPa, and 32.32 MPa for C35-LA, C35-MA, and C35-HA, respectively, representing reductions of approximately 18%, 8%, and 12%, respectively. These results indicate that C35-MA exhibited the highest resistance to freeze–thaw cycles, whereas C35-LA showed the greatest reduction in compressive strength.

In beams with low air content (RCB-LA-N and RCB-LA-D), the strain was primarily concentrated in the mid-span region. Cracks initiated at the bottom of the beam, where tensile stress was greatest, and propagated upwards, resulting in distinct crack patterns and a brittle failure mode. In contrast, beams with optimal air content (RCB-MA-N and RCB-MA-D) showed similar crack concentrations in the mid-span region but exhibited greater ductility. Beams with high air content (RCB-HA-N and RCB-HA-D) exhibited a more uniform strain distribution over the entire length, with less severe cracking, indicating improved flexural failure resistance.

Before undergoing freeze–thaw cycles, RCB-LA-N exhibited an average yield load of 22.92 kN and a deflection of 5.99 mm. After freeze–thaw cycles, RCB-LA-D showed a reduced average yield load of 21.00 kN and deflection of 5.31 mm. This reduction demonstrates that the freeze–thaw process reduced both the strength and ductility of the concrete beam. In contrast, RCB-MA-N and RCB-MA-D showed little change in their load–deflection relationships before and after freeze–thaw cycles. RCB-MA-N had an average yield load of 24.27 kN and a deflection of 4.54 mm, while RCB-MA-D maintained an average yield load of 23.35 kN and a deflection of 4.69 mm after freeze–thaw cycles. These findings suggest that optimal air content effectively preserves resistance to freeze–thaw cycles, preventing significant degradation of the RC beam’s flexural performance. Lastly, RCB-HA-N and RCB-HA-D showed average yield loads of 24.54 kN and deflections of 5.43 mm before freeze–thaw cycles, and 23.21 kN and 5.39 mm after freeze–thaw cycles, respectively. Although freeze–thaw cycles negatively impacted the strength and ductility of the RC beams, beams with optimal and high air content experienced minimal degradation, maintaining effective flexural performance and resistance.

After undergoing freeze–thaw cycles, the energy absorption capacity of RCB-LA decreased by approximately 20%, from 56.35 kN · mm to 44.85 kN·mm, indicating that freeze–thaw cycles significantly affect the energy absorption capacity of concrete beams. In contrast, RCB-MA, with optimal air content, showed only a 7% reduction in energy absorption capacity, demonstrating good resistance to freeze–thaw cycles, while RCB-HA, with high air content, also exhibited a 7% reduction, indicating relatively strong resistance to freeze–thaw cycles.

6. Conclusions

This study investigated the freeze–thaw resistance and flexural behavior of 35 MPa-grade concrete and reinforced concrete beams with different air contents. Based on the freeze–thaw tests on concrete specimens and the flexural tests on RC beams, the following conclusions can be drawn:

• Among the three air content levels tested in this study, the concrete with moderate air content, C35-MA, exhibited the highest freeze–thaw resistance. This was confirmed by the relative dynamic modulus of elasticity, durability index, mass loss rate, and compressive strength results. In contrast, the low-air-content concrete, C35-LA, showed the most significant deterioration after freeze–thaw cycles.
• The FTW and FTA test results showed similar trends in freeze–thaw resistance according to air content. However, the degree of deterioration was more pronounced in the FTW test than in the FTA test, indicating that water-based freeze–thaw exposure caused more severe material degradation than air-based freeze–thaw exposure.
• In the flexural tests of RC beams, the low-air-content specimens showed reductions in yield load, yield deflection, and energy absorption capacity after freeze–thaw cycles. This indicates that insufficient air content can negatively affect the flexural performance and ductility of RC beams exposed to freeze–thaw conditions.
• The RC beams with moderate and high air contents exhibited relatively small reductions in flexural performance after freeze–thaw cycles. These results suggest that an appropriate level of air content contributes to maintaining the structural performance of RC beams under freeze–thaw exposure.
• Although freeze–thaw cycles caused noticeable degradation in the material properties of concrete, their influence on the flexural performance of RC beams was relatively limited. This is because the flexural behavior of RC beams is strongly affected by the tensile reinforcement and structural configuration. Therefore, the results should be interpreted as experimental trends, and further studies with a larger number of specimens are required to improve statistical reliability.