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Review

Microscopic Deterioration Mechanism and Different Reinforcement Methods of Concrete Under Freeze–Thaw Environment: A Review

by
Wenlong Niu
,
Tiesheng Dou
*,
Meng Li
and
Shifa Xia
State Key Laboratory of Water Cycle and Water Security, China Institute of Water Resources and Hydropower Research, Beijing 100038, China
*
Author to whom correspondence should be addressed.
Processes 2025, 13(12), 4064; https://doi.org/10.3390/pr13124064
Submission received: 3 November 2025 / Revised: 2 December 2025 / Accepted: 8 December 2025 / Published: 16 December 2025
(This article belongs to the Section Materials Processes)
Browse Figures
Versions Notes

Abstract

In cold regions, concrete is inevitably subjected to freeze–thaw (F–T) damage, where repeated water–ice phase transitions progressively erode its microstructure and shorten its service life. Compared with the abundant research focusing on macroscopic performance degradation, systematic summaries addressing the microstructural evolution of pores, cracks, and the interfacial transition zone (ITZ), as well as corresponding prevention measures, remain limited. This paper reviews studies from 2013 to 2025, outlining key deterioration mechanisms under F–T action, including pore coarsening, ITZ weakening, and microcrack propagation. Four frost resistance enhancement strategies are compared: introducing stable microbubbles, refining the pore structure with pozzolanic or nano admixtures, bridging cracks with fibers, and applying hydrophobic treatments to block water ingress. The findings indicate that combining multiple measures yields superior frost resistance. By integrating microstructural observations with engineering improvement approaches, this review provides a holistic perspective for the design of durable concrete in cold regions and highlights the need for further research on multi-factor coupling mechanisms, optimization of composite admixture systems, and the functional mechanisms of novel nanomaterials.

1. Introduction

Concrete is a construction material consisting of a specific combination of binder materials, water, aggregates, and admixtures that have better properties [1,2,3]. Concrete’s durability is defined as its capacity to fend off the impacts of numerous damaging forces and to hold onto its strength and aesthetic integrity for an extended period of time [4]. Extending the service life of civil infrastructure and social facilities is a current issue under consideration [5,6].
F–T damage is a type of permanent damage. Concrete is affected by F–T due to its exposure to cold conditions. For instance, as water easily enters the concrete, it can also reduce its structural durability [7], and it breaks down, leading to buildings often requiring repair [8,9,10]. Two types of F–T damage occur in concrete: (1) pure frost heave in which internal damage occurs without surface damage, and (2) surface scaling and external damage caused by the enlargement of an existing defect or the formation of microcracks from the surface ice to the concrete unit [11,12]. The concrete’s macroscopic properties under F–T cycling have been extensively studied. An increase in the number of F–T cycles leads to a steady decrease in relative elasticity as well as a gradual rise in mass loss that occurs over time [13,14]. When the mass loss rate is positive, the matrix has a good internal structure, and F–T failure starts from the surface. When the mass loss rate is negative, it means that the F–T failure is caused by internal water expansion, and the concrete has a poor internal structure [15,16,17].
In recent years, numerous experimental studies have examined the microscopic deterioration of concrete under F–T action and proposed various strategies to enhance its durability at the microscale. Nevertheless, systematic reviews in this field remain limited. Existing summaries, as listed in Table 1, mainly address macroscopic changes in mechanical properties during F–T cycles [4,7,18,19,20,21,22,23,24,25], without providing an in-depth discussion of the underlying microscopic damage mechanisms or evaluating how different reinforcement measures influence the microstructure. Meanwhile, advances in techniques such as scanning electron microscopy (SEM) and other high-resolution characterization tools have enabled direct observation of microstructural evolution during F–T cycling. To gain a more comprehensive understanding of concrete’s long-term performance, it is therefore essential to update the assessment of frost resistance by integrating recent findings on both microscopic deterioration patterns and microlevel reinforcement mechanisms.
In response to the shortcomings of previous studies, this study aims to provide a comprehensive review of existing reports on microscopic F–T damage patterns and enhancement mechanisms in concrete between 2013 and 2025. Figure 1 shows a general schematic of the review. In this paper, the microstructural changes in concrete after the occurrence of F–T cycling are summarized, and the patterns of change in porosity, pore distribution, and ITZ width are charted and analyzed comparatively. To enhance frost resistance, various methods can be employed, including mineral admixtures like silica fume (which densifies the matrix), fiber reinforcement (to control cracking), and nanomaterials (for pore refinement). However, the most effective approach combines multiple strategies, such as using silica fumes with polypropylene fibers and air-entraining agents, as this simultaneously improves pore structure, provides crack resistance, and accommodates ice expansion. Microscopic analysis confirms that such integrated solutions offer superior F–T durability compared to single-method approaches, providing critical insights for concrete design in cold climates [31].

2. Deteriorations in the Microstructure of Concrete After F–T

As a porous and brittle material, concrete is highly sensitive to the water–ice phase transitions caused by F–T action, making its microstructure prone to irreversible physical and structural deterioration [32,33], including pore coarsening and increased connectivity, initiation and propagation of microcracks, weakening of the interface transition zone (ITZ), and degradation and leaching of certain hydration products [34]; these microstructural changes collectively undermine the overall compactness and mechanical properties of the material.

2.1. Pore Structure

The pore structure of concrete plays a crucial role in determining its frost resistance and long-term durability [35,36]. Common measurement methods include MIP [37], nitrogen adsorption [38], NMR [39], and CT [40]. Figure 2 shows the variation in concrete porosity under different F–T cycles [41,42,43,44,45]. As the number of F–T cycles rises, concrete exhibits an increase in porosity [46]. Numerous studies have shown that the water-to-cement ratio (w/c) has a significant influence on the evolution of concrete pore structure during F–T cycles. Zhu et al. [47] reported that F–T cycles lead to an increase in porosity for concretes with different w/c ratios, with higher w/c values resulting in more pronounced growth. Specifically, concrete with a w/c of 0.55 exhibited a 9.8% increase in porosity after 100 cycles, showing a clear linear relationship with the number of cycles. Gonen et al. [43] confirmed this trend, reporting that concrete with a w/c of 0.5 showed a 56.4% increase in porosity after 100 cycles, whereas concrete with a w/c of 0.36 experienced only a 4.9% increase even after 300 cycles [41]. These findings indicate that increasing the w/c ratio significantly exacerbates pore structure degradation under F–T cycles. According to Bai et al. [48], concrete porosity tended to rise more quickly between 50 and 75 F–T cycles than it did between 0 and 50 F–T cycles. The explanation is the rise in fatigue stress and accelerated expansion of porosity when the number of F–T cycles gradually rises.
Concrete pores are mainly divided into gel pores (≤0.01 μm), transition pores (0.01–0.1 μm), capillary pores (0.1–1 μm), and large pores (≥1 μm); they can also be divided into harmless pores (≤50 nm) and harmful pores (≥50 nm), according to the influence of the pores on the frost resistance [49,50,51,52,53]. Figure 3 shows the percentage change in pores after different F–T cycles [54,55,56,57,58,59]. Sum total fraction of large as well as medium pores of concrete increased with the frequency of F–T cycles and after 100 F–T cycles increased by 10% [54,56,57]. While mesopore change is negligible, macropore change rate is very high, leading to the enlargement of gel pores into larger ones, showing that the sum of the ratios of transition pores and gel pores after 300 F–T cycles is still more than 60% [56,57]. Indicating that certain gel holes and transition pores were broken down and transformed into macropores during the F–T cycle and the pore structure towards laxity, the proportion of large pores increased dramatically [53]. After 60 F–T cycles, Qiu et al. [54] found a decrease of 15.73% in the percentage of gel pores, while the percentage of macropores increased by 8.43% in the samples. Guo et al. [60] demonstrated that the pore structure of concrete evolves from refinement to coarsening under F–T action. The volume fraction of gel pores decreased from 25.21% to 9.23%, while capillary pores and macropores increased from 9.11% and 3.47% to 17.89% and 10.52%, respectively. Transition pores remained dominant throughout the process, reaching up to 70.90%, indicating that F–T cycles significantly impair the microstructural compactness of concrete.
Unlike traditional techniques, which are limited in resolution and penetration depth and thus struggle to capture microscopic details clearly, their two-dimensional destructive imaging methods also fail to accurately reconstruct complex three-dimensional structures. By contrast, computed tomography (CT) offers high-precision, non-destructive 3D imaging of pore structure degradation processes, significantly enhancing the interpretability of microscopic evolution mechanisms [61,62,63]. Shen et al. [64] conducted a visual analysis of the pore structure in steam-cured rubber concrete, finding that porosity increased with the number of cycles. At the initial state, the slice porosity showed large fluctuations, with a maximum of up to 2.5%. After multiple cycles, it stabilized around 0.9%. Figure 4 shows the 3D air void images of the sample after 0–50 freeze–thaw cycles. Chen et al. [65] analyzed the three-dimensional pore structure characteristics of concrete under salt freeze–thaw cycles (see Figure 5) and pointed out that the degree of pore connectivity is a key indicator for characterizing the evolution behavior of pore structures.
Overall, the increase in F–T cycles leads to the transition of originally dispersed microscale pores toward interconnected structures, forming a continuous pore network and resulting in a continuous rise in total porosity. The pore size distribution tends to shift toward larger pores, with average pore diameter, pore volume, and pore surface area all increasing significantly with F–T cycles, indicating evident pore coarsening [63,65,66]. Therefore, it is necessary to study the evolution of pore structure through micro characterization technology to guide the optimal design and performance improvement of durable materials. For example, CT, SEM, optical microscope, mercury intrusion porosimetry, nuclear magnetic resonance, ultrasonic testing, etc.

2.2. Interfacial Transition Zone

The ITZ of concrete can be determined by nanoindentation technology and Vickers hardness. Under F–T cycles, micropores and microcracks first appear and gradually accumulate within the ITZ, which thickens over time and becomes the dominant region of microstructural deterioration in concrete [67,68]. Figure 6 shows the variation in concrete ITZ thickness under different F–T cycles [69,70,71,72]. The thickness of the ITZ exhibits a continuous increase with the number of F–T cycles. According to the literature [69], the ITZ thickness increased from an initial 60 μm to 80 μm after only 20 F–T cycles, representing a 33% increase. Another study [70] reported a growth from 41 μm to 57 μm after 120 cycles, an increase of 39%. When the initial ITZ was relatively dense (20–22 μm), its expansion potential was even more pronounced: after 300 and 1500 cycles, the thickness expanded to 50 μm and 60 μm, corresponding to increases of 150% and 172.7%, respectively [71,72]. Research results indicate that F–T cycles significantly deteriorate the microstructural integrity of the ITZ.
As an important indicator for evaluating the local mechanical properties of materials, microhardness characterizes the mechanical properties of localized regions by measuring a material’s resistance to indenter penetration at the microscale. This approach reflects both the mechanical properties and structural integrity of the ITZ [73,74]. Jia et al. [69] found that after 400 F–T cycles, the ITZ microhardness of recycled aggregate concrete decreased by 19.8%, while the corresponding mortar microhardness decreased by 14.7%. Li et al. Ref. [75] studied the degradation process of the ITZ before and after freeze–thaw cycles. The SEM image of concrete is shown in Figure 7, and the main cracks appear in the mortar and ITZ (see Figure 7b). The ice expansion force and the shrinkage of the mortar matrix cause a few new fractures and pores to form in the ITZ. Due to its initially high porosity and loose structure, the ITZ is the primary region for ice-crystal-induced stress concentration and crack initiation during F–T cycles [76]. Repeated expansion pressure from water–ice phase transitions drives the growth of preexisting microcracks along their length, increasing ITZ thickness [77]. Meanwhile, the mismatch between cement paste shrinkage and aggregate thermal deformation further accelerates ITZ degradation [78]. F–T cycles lead to a 10–20% reduction in microhardness, reflecting typical mechanical deterioration [75,79].
An increasing number of F–T cycles leads to an increase in the number of microcracks within the ITZ. Figure 8 demonstrates SEM photographs of ITZ microcrack structures after F–T cycles [49,80]. The majority of the fractures hang near the aggregate. As F–T cycling increases, microcracks form in the ITZ and progress into the cement matrix, finally converging in one location from many directions. Therefore, ITZ is most probably damaged in concrete when it is subjected to F–T cycles. Lu et al. [59] indicated that the ITZ crack width increased from 5 μm to 12 μm after 95 F–T cycles. Li et al. [81] evaluated ITZ crack widths after 98 F–T cycles using numerical image processing techniques found that ITZ cracks accounted for 70% of the total cracks in terms of length and percentage of area.

2.3. Microcrack Development

Crack development is the main factor affecting concrete durability [82]. Microcracks spread and connect to each other by gathering frozen and expanded water, ultimately destroying the microstructure of concrete. Figure 9 describes SEM images of microcracks in the concrete matrix during different F–T cycles [46,83]. Concrete in its initial state has a comparatively tight internal structure; the amount of micropores and microcracks increased with the increase in the amount of F–T cycles. During F–T cycling, water in micropores and microcracks becomes ice and the pressure causes the pores and cracks to expand, ultimately leading to concrete crushing. From the sample’s edge to its interior, the deterioration will gradually progress [23]. Liu et al. [84] found that the cement matrix is a weak link in concrete because of the presence of some initial cracks inside the matrix. The formation of these cracks is accelerated under the F–T cycle, leading to concrete failure. Medina et al. [85] found that after 56 F–T cycles, the average width for cracks within the cement matrix was 1.57 ± 0.329 μm with a maximum width of 1.88 μm.
Nondestructive detection of microcracks enables in situ monitoring of internal damage without compromising specimen integrity, facilitating the capture of early-stage crack initiation and propagation [86,87]. Ji et al. [88] employed micro-CT to conduct 3D characterization of mortar microcrack evolution after 0–300 F–T cycles. They found that when the crack volume ratio exceeded 0.25%, compressive strength loss surpassed 40%. The results indicate that the strength degradation of concrete is primarily governed by the connectivity of the crack, rather than being determined simply by the total volume or average width of the cracks [82]. Sehyuk et al. [89] introduced nonlinear ultrasonic subharmonic pulse compression (SPC) technology to detect early-stage microcracks in concrete after 25 F–T cycles. Compared to conventional harmonic ultrasound, SPC improved crack identification accuracy by approximately 35% at the crack initiation stage [90]. Jierula et al. [91] used rebound value test to test the strength of concrete. As shown in Figure 10, after freeze–thaw cycles, the rebound value of the concrete specimen decreases. The propagation speed of ultrasound in a medium is closely related to the strength of the material. Ding et al. [92] established a model for the relationship between ultrasonic velocity and strength damage caused by freeze–thaw cycles. As shown in Figure 11, the lowest strength loss of the A2 specimen is 35.2%. Morozova et al. [93] found that each 0.01 mm−1 increase in crack density led to a 75 m/s decrease in ultrasonic pulse velocity, confirming a quantitative relationship between acoustic parameters and crack damage. In summary, multisource integrated nondestructive testing technologies provide robust technical support for the early detection and precise quantification of microcracks in concrete.

3. Measures to Enhance F–T Durability

The microscopic damage to the concrete after F–T cycling included an increase in porosity, elevated harmful pore content, increased cracking, and a weakening of the bond between aggregate and concrete. Four improvement methods have been proposed from these aspects. (1) By incorporating an air-entraining agent (AEA), a substantial volume of uniformly distributed, stable, and discrete microscopic air voids is introduced into the concrete. These voids provide “buffer zones” to accommodate the expansion of freezing water. Concurrently, the freezing water migrates to the periphery of the nearest air void, thereby reducing the air-void spacing factor [33,36]. (2) The addition of mineral admixtures such as fly ash (FA), silica fume (SF), ground granulated blast furnace slag (GGBS), rice husk ash (RHA), metakaolin (MK), silica, alumina, carbon nanotubes (CNT), and graphite oxide (GO) can refine the pores and reduce water infiltration into concrete [94,95]. (3) By using the method of adding fibers to make them bear part of the tensile stress generated by the concrete, the occurrence of cracks in the concrete can be effectively avoided, or the cracks can be controlled (Figure 12) [7,96,97]. (4) By applying a waterproof coating on the concrete surface effectively reduces the capillary water absorption coefficient, blocks water penetration, mitigates F–T induced stress damage, and thus enhances the frost resistance of concrete [98].

3.1. Adding an Air-Entraining Agent

By adding an appropriate amount of air-entraining agent, the air content of fresh concrete can be maintained within the range of 4–6%, effectively enhancing its F–T resistance [36]. Concrete with a porosity of less than 0.2 mm is considered F–T resistant, and as the micropores and air content increase, the porosity decreases [99]. Figure 13 shows the relationship between porosity and AEA content before and after F–T [94,100]. After F–T cycling, the porosity of concrete without AEA increased by 291.8%, while the porosity of concrete with 7.92% AEA increased by only 4.3%, indicating that the addition of AEA has an inhibitory effect on the increase in porosity after F–T cycling.

3.2. Refining Concrete Pores by Using Pozzolanic Materials and Nanomaterials

The traditional method of incorporating volcanic ash materials can also achieve excellent frost resistance because the active SiO2 and Al2O3 in these materials react with Ca(OH)2 (a cement hydration product) to form C–S–H gel, which fills capillary pores [101]. This shifts the pore size distribution toward harmless pores and reduces the proportion of harmful pores [102]. Because pozzolanic materials react with cement and water, they can be utilized to regulate vulnerable locations. The pozzolanic reaction has the ability to consume less stable hydration products. Subsequently, gels composed of secondary hydration products (C–S–H) fill the voids that are created as a result of cement hydration. Since the majority of mineral admixtures feature extensive surface areas, this characteristic both enlarges the pores and improves the density of the concrete [7]. When used in combination with air-entraining agents, volcanic ash materials stabilize the bubble structure and increase the bubble spacing coefficient, thereby improving the frost resistance of concrete [103,104].

3.2.1. Fly Ash

FA is a component of the fine slag left over from burning pulverized coal. This compound can enhance concrete performance and improve freeze–thaw resistance [105]. In the same F–T cycle, Figure 14 depicts the link between porosity and FA content [94,100,106,107,108]. As the FA content rises, percentage of non-harmful pores increases and percentage of harmful pores decreases. FA introduces tiny air voids that provide good void spacing to protect the internal concrete from F–T damage. Li et al. [105] discovered that adding 10.5 wt% FA with 10.5 wt% thermally activated paper clay to concrete reduced porosity while increasing resistance to F–T damage. Due to the slow-reacting nature of the pozzolanic process in Fly Ash (FA), a high level of durability is expected to become evident during the later phases of the F–T cycle [44]. FA also contributes the addition of air content to concrete, and its conjunction with AEA will aid improved frost resistance.
The addition of an appropriate amount of FA reduces cracking, quality loss, relative dynamic modulus loss, strength loss, and surface spalling damage in concrete specimens after the F–T cycle. Microstructural changes in concrete after FA addition are shown in Figure 15. Yuan et al. [94] found that when the fly ash content is 30%, the frost resistance of steel fiber reinforced concrete is optimal. Wang et al. [39] found that the addition of FA reduced the number of cracks in concrete specimens after F–T cycling and resulted in a denser microstructure.

3.2.2. Silica Fume

As an industrial waste and auxiliary cementitious material, SF contributes significantly to concrete durability. Figure 16 depicts the connection between porosity and SF content when F–T cycles occur [109,110,111]; porosity decreases with increasing SF content. The high content of amorphous silica in SF can directly interact with Ca(OH)2 to produce C–S–H [112,113]. Appropriately increasing the proportion of SF can simultaneously reduce the macropore porosity and pore spacing, thereby significantly improving the F–T durability of concrete [114]. Oyunbileg et al. [115] reported that at W/C = 0.23 in SF modified concrete, the compressive strength reached 92.85 MPa, the water absorption was 0.7%, the material withstood 420 F–T cycles, and the water impermeability grade was W20.
SF also reduced the number of cracks in the concrete after the F–T cycle. Researchers used SEM to observe the effect of silica fume on the microstructure of concrete after freeze–thaw cycles (Figure 17) [116]. Significant microcracks appeared at the interface between PVA fibers and the matrix. The silica fume particles before freeze–thaw cycles mainly exhibit spherical or nearly spherical shapes, with smooth and flat surfaces and uniform particle size distribution. After 400 freeze–thaw cycles, a large number of gels were closely connected to form a tightly packed matrix structure. This tightly packed hydration matrix may contribute to the development of concrete strength and resistance to freeze–thaw fatigue loads [117].

3.2.3. Ground Granulated Blast Furnace Slag

GGBS comprises highly reactive calcium aluminosilicate, which improves the durability of concrete [118]. Figure 18 depicts the connection between porosity and GGBS content after F–T cycles occur [119,120,121,122]; porosity decreases with increasing GGBS level. GGBS can also convert Ca(OH)2 to C–S–H by replacing aggregates or cement for improved concrete durability [19,123].
The durability of concrete is enhanced by GGBS via the filling effect. Figure 19a shows that the surface of the specimen without added GGBFS contains unburned coal particles and calcite crystals, which are dispersed and distributed. At this stage, the degree of hydration is low and the interfacial adhesion is poor. Figure 19b shows that the hydration products of the specimen with added GGBFS are enriched and the structure is densified. A large number of C-S-H phases were filled in the particle gap in the form of gel, which significantly improved the interfacial adhesion between particles.

3.2.4. Other Pozzolanic Materials

In addition to the common pozzolanic materials mentioned above, MK and RHA can also improve the durability of concrete. MK primarily functions as a filler to encourage early cement hydration or to undergo a pozzolanic reaction later [124,125]. According to Sarıdemir et al. [112], MK particles can create connections between cement mortar and reduce concrete porosity, preventing seepage pressure brought on by the movement of supercooled water and enhancing concrete’s resilience to frost. The inclusion of RHA causes the pores in the concrete to become more refined, reducing the permeability of the concrete and increasing its resistance to frost [94,126]. Serag Faried et al. [127] added nano-rice husk ash to concrete, which resulted in an increase in microcracks and pores compared to the control group, but its structure was denser and uniform, with more calcium silicate hydrate filling the pores, achieving high gel density.

3.2.5. Nanomaterials

In terms of concrete’s freeze resistance, many scholars have also studied nanomaterials. Since nanoparticles are much smaller than cement particles, they can fill the smallest pores in cement paste, significantly reducing the proportion of harmful pores and blocking water penetration paths, thus achieving good results [128,129,130]. Figure 20 depicts the effect of different nanomaterial contents on the porosity in concrete specimens after F–T cycling has occurred [95,131,132,133,134]; the porosity decreases with increasing nanomaterial content. Zhang et al. [131] found that incorporating 1% to 3% of nanomaterials into concrete decreases the quantity of harmful pores and increases the volume of harmless pores, thereby enhancing the concrete’s frost resistance. Li et al. [135] added 1 wt%, 3 wt%, and 5 wt% of nanomaterials to concrete and observed that the concrete porosity exhibited a normal distribution after the F–T cycle. With the increase in the concentration of nanomaterials, the average porosity of concrete specimens after 100 F–T cycles was 13.35%, 7.3%, and 3.2%, respectively.
The addition of nanomaterials to concrete restrains crack development. Figure 21 shows the microstructure of the surfaces of three types of micro- and nano-rough structured coatings. When using Crn, SiC, and DIA as micron structures, the surface roughness of the coating is increased, resulting in good hydrophobicity of the coating [136]. The mechanics and durability of the combination have also been enhanced due to the nanomaterials, which also increased the pore structure of cement mortar through filler action [137]. Liu et al. [138] demonstrated that nano-SiO2 materials effectively enhance the frost resistance of concrete. The frost resistance life was increased by at least 43%, with a maximum improvement of up to 71%. Furthermore, the nanomaterials can restrain changes in the internal pore structure of the composite modified concrete and densify the ITZ.

3.3. Containing Cracks Using Fibers

Fibers help reduce porosity of concrete after F–T cycles. Figure 22 depicts the change in different fiber contents on the porosity of concrete after the F–T cycle has occurred [95,96,139,140]. With increased fiber content, porosity decreases. Fibers are capable of bearing certain tensile stresses within concrete, which restricts the occurrence of microcracks and the expansion of concrete. This action aids in preserving the compact microstructure of concrete and lessens the damage caused by F–T cycling [12,141]. Ren et al. [57] examined the pore structure of concrete containing 0.2% polypropylene fibers and concrete without polypropylene fibers after 300 F–T cycles using the MIP method. The porosity of concrete containing 0.2% PP fibers was 17.4% lower than that of concrete without PP fibers.
Fibers form a three-dimensional, randomly oriented micro-reinforcement network within the cement matrix, bridging crack surfaces and restricting the crack propagation speed, thereby preventing them from coalescing into continuous cracks [142]. Simultaneously, the interfacial bond between the fiber surface and the cement paste enhances the bond strength between aggregate and paste, reducing interface debonding and aggregate spalling caused by freezing–expansion stresses. Furthermore, an appropriate amount of fiber can optimize the pore structure, reduce free water content, and lower the risk of F–T damage [143,144]. The microstructure of concrete containing fibers after freeze–thaw cycles was observed through SEM testing, as shown in Figure 23 [145]. Fibers can transfer the loads they bear to the adjacent cement matrix, thereby avoiding stress concentration. Fibers exert a bridging effect in the microstructure, which can prevent crack propagation and stress concentration. The bonding state between fibers and the cement matrix is crucial for maintaining the freeze–thaw resistance of concrete. Fibers dispersed throughout the concrete matrix significantly slow down the initiation and spread of cracks; when cracks encounter randomly dispersed fibers in the matrix, their expansion is halted [146]. Compared to control concrete, fiber-added concrete samples exhibit fewer cracks. Previous studies have shown that adding fibers to concrete can improve its mechanical properties during repeated F–T cycles. The specific mechanisms include optimizing the microstructure, slowing down the decline in relative dynamic elastic modulus, inhibiting crack propagation, and enhancing freeze resistance [147,148].

3.4. Addition of Hydrophobic Materials

3.4.1. Hydrophobic Coating

The application of coating materials effectively enhances the durability of concrete structures by constructing a continuous and dense physical barrier layer on the concrete surface, thereby blocking the transport pathways of aggressive media [149,150]. Acting as a continuous film covering the concrete surface, the coating significantly reduces water absorption and air permeability. This minimizes the ingress of environmental moisture into the concrete interior, consequently reducing the amount of freezable water available during F–T cycles and mitigating frost heave stress [98,151]. Figure 24 presents SEM images of coated concrete after F–T exposure. After 200 freeze–thaw cycles, the total porosity of non-coating and modified epoxy composite coating (MECC) was 4.60% and 3.00%, respectively, indicating that the internal freeze–thaw damage of MECC was smaller. This demonstrates that the coating not only prevents water penetration and mitigates the impact of temperature fluctuations but also hinders the detachment of internally spalled cementitious material, effectively improving the frost resistance of the concrete and preserving a more intact internal structure [152]. Figure 25a–c show the surface morphology of the composite coating under SEM. The CS3-E coating exhibits more pronounced micro- and nano-scale structures, with a higher degree of aggregation. The number of functionalized nanoparticles can affect the hydrophobicity and mechanical properties of composite coatings. Figure 25d–f (obtained via CLSM) show that the CS5-E coating exhibits a higher density of protrusions, resulting in an increase in surface roughness to approximately 135 μm. By adjusting the ratio of functionalized nanoparticles to epoxy resin, the surface morphology and roughness of the coating can be customized, thereby affecting the performance of the coating. Guo and Weng [153] analyzed the F–T performance of concrete treated with silane, modified polyurea, and epoxy resin. Their findings revealed that silane provided the most effective protection, followed by modified polyurea, while epoxy resin exhibited the lowest efficacy. The protective mechanism of silane involves a condensation reaction, generating a hydrophobic network structure. This structure adheres firmly to the concrete surface, pores, and microcracks, enhancing resistance to water penetration and F–T damage. Wang et al. [23] reported that silane coatings can react with calcium hydroxide in concrete to form additional C–S–H gel, filling surface micropores and cracks, thereby reducing water absorption and enhancing frost resistance. Zhu et al. [154] investigated the effect of adding polypropylene fibers to inorganic coatings sprayed onto concrete. Under identical F–T cycling conditions, this modification significantly improved the frost resistance of the concrete.

3.4.2. Addition of Hydrophobic Agents

The incorporation of hydrophobic agents into concrete substantially reduces water ingress, significantly decreasing the water content within internal pores. This restricts ice crystal nucleation primarily to the surface layer, forming discontinuous ice sheets that reduce expansive pressure [155,156]. Hydrophobic materials form coatings on pore walls, increasing the water contact angle to >90° and thereby inhibiting water penetration into pores. The hydrophobicity of pores reduces water permeation through the matrix pore network (Figure 26) [97]. Hydrophobic or superhydrophobic additives are integrated during mixing to achieve bulk waterproofing. This process overcomes limitations of conventional surface protection methods, lowers water absorption, and enhances F–T resistance. With this approach, a thin hydrophobic layer forms both within pores and on the concrete surface, effectively blocking water intrusion into the concrete structure [7,157]. Zhang et al. [158] found that asphalt emulsions form protective films on aggregate surfaces upon water evaporation, establishing an effective sealing mechanism within the asphalt cement system. Consequently, water permeability decreases while F–T resistance improves. Pang et al. [159] developed a novel superhydrophobic additive that refines the pore structure within mortar, suppresses crack formation, increases harmless pores, decreases harmful pores, and enhances mortar’s F–T resistance.
Hydrophobic agents can reduce the formation of interconnected cracks in concrete subjected to freeze–thaw (F-T) cycles. Figure 27 shows the microstructure images of concrete with two hydrophobic agents (A1, A2) added [160]. It can be observed that the hydrophobic coating is well distributed inside the concrete structure, and no discontinuity, cracks, or dense aggregation of the hydrophobic coating is observed. The addition of hydrophobic agents ensures the frost resistance of concrete [161]. Researchers frequently combine hydrophobic materials with other admixtures. Zhang et al. [131] added a superhydrophobic admixture composed of 3.5% polymethyl hydro siloxane and 25% polyvinyl alcohol surfactant into the cement matrix and produced small pores with a diameter of 10–100 nm in the cement paste, which limited water entry and increased the concrete’s ability to resist freezing.

3.5. Discussion

The above methods for improving the durability of concrete have their own characteristics. Table 2 summarizes the corresponding admixtures, principles, advantages, and disadvantages of these methods. For example, AEA can increase the volume content of harmless pores by introducing small, enclosed bubbles, but it will weaken concrete’s ability to resist compression. PP fibers can reduce the formation of microcracks by absorbing part of the tensile stress generated by the concrete but are easily tangled and agglomerated. Based on the literature reviews and after weighing the benefits and drawbacks of each addition, a conceptual design was required. Depending on the size of concrete’s porous structure and microcracks, combined with the particle size of different additives, comprehensive improvement measures suitable for structures of different sizes have been proposed (Figure 28). The composite use of additives may increase construction costs. However, the potential of it is enormous because of its higher durability compared with ordinary portland concrete [162]. Therefore, it has certain engineering significance.

4. Conclusions

In this paper, based on the study of microstructural deterioration of concrete after F–T cycle and its mechanism, the techniques to improve the frost resistance of concrete are summarized. The conclusions that could be made are as follows:
(1)
Pore structure, cracks, and ITZ thickness are the primary manifestations of microstructural defects in concrete after F–T cycling. Porosity, crack width, crack density, and harmful pores all increase with F–T cycling. Cracks in concrete typically begin at the ITZ after F–T cycling. Additionally, as crack width increases, micro-cracks also become more numerous, and the thickness of the ITZ structure also increases.
(2)
AEAs function by introducing closed pores that provide a small expansion space for water during freezing. This alleviates volume expansion while blocking water flow to reduce internal pressure on the concrete, thereby enhancing its durability and freeze resistance. They are suitable for low-to-medium strength concrete.
(3)
Due to the filling characteristics of volcanic ash and nanoparticles, volcanic ash materials and nanomaterials can reduce the porosity in the microstructure, promote the development of concrete strength and resistance to load, and then improve the frost resistance of concrete but cannot effectively prevent concrete from water absorption.
(4)
Fibers can assume part of the tensile stress generated by concrete mainly by preventing or controlling the tensile cracking in concrete, hence limiting the development and growth of microcracks in F–T cycling and helping to maintain a dense microstructure.
(5)
Enhancing concrete freeze resistance primarily relies on two mechanisms: first, forming a continuous, dense physical barrier layer on the surface directly reduces the amount of water available for freezing during freeze–thaw cycles, thereby lowering frost heave stress. Second, utilizing hydrophobic materials—either by forming a surface film or incorporating them into the matrix—enhances pore hydrophobicity, fundamentally reducing water ingress. Both approaches effectively improve concrete’s freeze–thaw durability.
(6)
The comprehensive improvement measures suitable for the initial multiscale structure are provided based on the size of concrete’s porous structure and microcracks of concrete, and the benefits and drawbacks of various additives.

5. Future Perspectives

A growing interest exists in the study of microstructural degradation of concrete under F–T cycles in cold climates. Although there are some measures to enhance the durability of concrete at the microscopic level, further research is still required. The following are some possible directions for further investigation of concrete microstructure deterioration in F–T environments:
(1)
There are fewer studies about the effect of F–T cycle on concrete microhardness. These types of studies need to be intensified to investigate the pattern of change in concrete microhardness after the start of the F–T cycle.
(2)
Previous research has been directed towards single-factor F–T cycle theory. The F–T cycle and other chemical interactions may occur simultaneously and synergistically in real-world engineering. It is necessary to look at theories explaining the synergistic interactions between the many mechanisms that cause concrete to deteriorate microstructurally.
(3)
It has been demonstrated that the pozzolanic action decreases the quantity of dangerous pores in concrete and increases its toughness. Further research should be performed to determine how different forms of FA’s pozzolanic effects affect the outcomes of the F–T test. More consideration should also be given to the usage of FA as a secondary supplemental material and how it affects the durability of concrete when combined with AEA.
(4)
The application of nanomaterials in concrete varies significantly with their developmental stage. SF is a relatively well-established material. It possesses a high specific surface area and finer particles compared to cement. However, these characteristics lead to poor processability, necessitating the use of water-reducing agents for effective utilization.
(5)
Some nanomaterials, like graphene and nano clay, are still in the developmental stages. The methods by which graphene and nano clay increase the durability of concrete are not yet known. As a result, additional research into the impact of related materials is required.
(6)
The inclusion of fibers enhanced the microstructure of concrete. However, too many fibers can cause tangling and overlapping. As a result, more research should be performed on fiber orientation, distribution, and tangling and agglomeration prevention.
(7)
A new and improved technique involves the use of hydrophobic materials and coatings to improve F–T resistance. However, the mechanism of action of hydrophobic materials in the F–T cycle, the effect of hydrophobic materials on the ITZ of concrete after the F–T cycle, and the weak bonding performance of hydrophobic coatings with concrete have not received sufficient attention.

Author Contributions

W.N.: Conceptualization, methodology, writing—review and editing, validation. T.D.: Formal analysis, data curation, writing—original draft preparation, visualization, investigation. M.L.: Resources, supervision, project administration, funding acquisition. S.X.: Resources, supervision, validation, project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This study was sponsored by the National Natural Science Foundation of China (Grant No. 50979116).

Data Availability Statement

No new data were created or analyzed in this study.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Flowchart of the review.
Figure 1. Flowchart of the review.
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Figure 2. Porosity of concrete after the occurrence of different F–T cycles [41,42,43,44,45].
Figure 2. Porosity of concrete after the occurrence of different F–T cycles [41,42,43,44,45].
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Figure 3. Percentage of different pores of concrete after different F–T cycles [54,55,56,57,58,59].
Figure 3. Percentage of different pores of concrete after different F–T cycles [54,55,56,57,58,59].
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Figure 4. Comparison of air void in 3D of specimen during freeze–thaw test (from left to right, the 3D air void images of sample are shown for 0, 5, 15, 25, and 35 freeze–thaw cycles in order) [64].
Figure 4. Comparison of air void in 3D of specimen during freeze–thaw test (from left to right, the 3D air void images of sample are shown for 0, 5, 15, 25, and 35 freeze–thaw cycles in order) [64].
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Figure 5. Pore structures in concrete: (a) 0; (b) 100; (c) 200; and (d) 300 salt freezing–thawing cycles [65].
Figure 5. Pore structures in concrete: (a) 0; (b) 100; (c) 200; and (d) 300 salt freezing–thawing cycles [65].
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Figure 6. ITZ thickness of concrete after the occurrence of different F–T cycles [69,70,71,72].
Figure 6. ITZ thickness of concrete after the occurrence of different F–T cycles [69,70,71,72].
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Figure 7. SEM images of concrete ITZ before and after freeze–thaw cycles (a) Before freeze–thaw; (b) after freeze–thaw [75].
Figure 7. SEM images of concrete ITZ before and after freeze–thaw cycles (a) Before freeze–thaw; (b) after freeze–thaw [75].
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Figure 8. SEM images of concrete ITZ from various F–T cycles: (a) before F–T cycles [49]; (b) after 25 F–T cycles [49]; (c) after 50 F–T cycles [49]; (d) after 0 F–T cycles [80]; (e) after 50 F–T cycles [80]; (f) after 100 F–T cycles [80].
Figure 8. SEM images of concrete ITZ from various F–T cycles: (a) before F–T cycles [49]; (b) after 25 F–T cycles [49]; (c) after 50 F–T cycles [49]; (d) after 0 F–T cycles [80]; (e) after 50 F–T cycles [80]; (f) after 100 F–T cycles [80].
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Figure 9. SEM image of the microcracks in the cement matrix after different F–T cycles: (a) before F–T cycles [46]; (b) after 100 F–T cycles [46]; (c) before F–T cycles [83]; (d) after 300 F–T cycles [83].
Figure 9. SEM image of the microcracks in the cement matrix after different F–T cycles: (a) before F–T cycles [46]; (b) after 100 F–T cycles [46]; (c) before F–T cycles [83]; (d) after 300 F–T cycles [83].
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Figure 10. The rebound nondestructive testing of concrete [91]. (a) Test schematic diagram; (b) rebound values under freeze–thaw cycles.
Figure 10. The rebound nondestructive testing of concrete [91]. (a) Test schematic diagram; (b) rebound values under freeze–thaw cycles.
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Figure 11. Relationship between unconfined compressive strength and freeze–thaw cycle: (a) A1, (b) A2, (c) A3, (d) A4, (e) A5 [92].
Figure 11. Relationship between unconfined compressive strength and freeze–thaw cycle: (a) A1, (b) A2, (c) A3, (d) A4, (e) A5 [92].
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Figure 12. Principle of the fiber-reinforced interface properties [7].
Figure 12. Principle of the fiber-reinforced interface properties [7].
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Figure 13. Correlation between porosity and content of AEA before and after F–T cycles [94,100].
Figure 13. Correlation between porosity and content of AEA before and after F–T cycles [94,100].
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Figure 14. Relationship between porosity and FA content before and after F-T cycles [94,96,106,107,108].
Figure 14. Relationship between porosity and FA content before and after F-T cycles [94,96,106,107,108].
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Figure 15. Three-dimensional pore distribution of concrete with different dosages of FA based on CT scanning [94]. (a) 15%; (b) 30%; (c) 45%.
Figure 15. Three-dimensional pore distribution of concrete with different dosages of FA based on CT scanning [94]. (a) 15%; (b) 30%; (c) 45%.
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Figure 16. Correlation between porosity and content of SF after F–T cycles [41,109,110,111].
Figure 16. Correlation between porosity and content of SF after F–T cycles [41,109,110,111].
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Figure 17. Microscopic image of 20% SF and 0.5% PVA of the sample under different freeze–thaw cycles [116]. (a) 20% SF (0 F–T); (b) 20% SF (400 F–T); (c) 0.5% PVA (0 F–T); (d) 0.5% PVA (400 F–T).
Figure 17. Microscopic image of 20% SF and 0.5% PVA of the sample under different freeze–thaw cycles [116]. (a) 20% SF (0 F–T); (b) 20% SF (400 F–T); (c) 0.5% PVA (0 F–T); (d) 0.5% PVA (400 F–T).
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Figure 18. Correlation between porosity and content of GGBS after F–T cycles [119,120,121,122].
Figure 18. Correlation between porosity and content of GGBS after F–T cycles [119,120,121,122].
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Figure 19. SEM images presenting the microstructures of the (a) no GGBFS, (b) added GGBFS [116].
Figure 19. SEM images presenting the microstructures of the (a) no GGBFS, (b) added GGBFS [116].
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Figure 20. Correlation between porosity and content of nanomaterials after F–T cycles [95,131,132,133,134].
Figure 20. Correlation between porosity and content of nanomaterials after F–T cycles [95,131,132,133,134].
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Figure 21. The microscopic morphology of surfaces with three types of micro- and nano-rough structure coatings [136].
Figure 21. The microscopic morphology of surfaces with three types of micro- and nano-rough structure coatings [136].
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Figure 22. The effect of different fiber contents on the porosity of concrete after freeze-thaw cycles [95,96,139,140].
Figure 22. The effect of different fiber contents on the porosity of concrete after freeze-thaw cycles [95,96,139,140].
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Figure 23. Concrete SEM test results. (a,b) Fiber bridging effect; (c,d) fiber-bonded cement matrix; (e,f) fiber agglomeration [145].
Figure 23. Concrete SEM test results. (a,b) Fiber bridging effect; (c,d) fiber-bonded cement matrix; (e,f) fiber agglomeration [145].
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Figure 24. SEM images of coated concrete after freeze–thaw cycles [152].
Figure 24. SEM images of coated concrete after freeze–thaw cycles [152].
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Figure 25. SEM images of (a) CS1-E (100 mg nanoparticles and 98% resin), (b) CS3-E (200 mg nanoparticles and 94% resin), and (c) CS5-E (500 mg nanoparticles and 90% resin); CLSM images of (d) CS1-E, (e) CS3-E, and (f) CS5-E. [30].
Figure 25. SEM images of (a) CS1-E (100 mg nanoparticles and 98% resin), (b) CS3-E (200 mg nanoparticles and 94% resin), and (c) CS5-E (500 mg nanoparticles and 90% resin); CLSM images of (d) CS1-E, (e) CS3-E, and (f) CS5-E. [30].
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Figure 26. Photos of water droplets on concrete surfaces after using different hydrophobic agents. (a) Untreated, (b) BS4004 silane emulsion, and (c) C3033 organosilicone emulsion [97].
Figure 26. Photos of water droplets on concrete surfaces after using different hydrophobic agents. (a) Untreated, (b) BS4004 silane emulsion, and (c) C3033 organosilicone emulsion [97].
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Processes 13 04064 g027
Figure 27. SEM microstructure of the coatings [160].
Figure 27. SEM microstructure of the coatings [160].
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Processes 13 04064 g028
Figure 28. Suggestions for combined methods to improve the durability of concrete.
Figure 28. Suggestions for combined methods to improve the durability of concrete.
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Table 1. Previous reviews of the F–T cycle of concrete.
Table 1. Previous reviews of the F–T cycle of concrete.
LiteratureTest StandardMain ContentYears
[4]/Analyze the factors affecting the frost resistance of concrete and compare the frost resistance of new recycled concrete and ordinary silicate concrete.2013
[1]JIS A 1148 (Japanese Industrial Standards) [26]Frost resistance of fiber-reinforced cement-based composite materials under freeze–thaw cycles.2016
[18]Chinese standard GB/T 50082-2009 [27]The effect of additives on the frost resistance of concrete was qualitatively assessed but not quantitatively analyzed.2016
[11]ASTM C666-97 [28] and JTJ 270 [29]The influence of surface modification on concrete properties under freeze–thaw action.2017
[19]Chinese standard GB/T 50082-2009 [27]Analyze the factors affecting the frost resistance of concrete and predict the service life of concrete under F–T conditions.2019
[21]Chinese standard GB/T 50082-2009 [27]The effect of different factors on the freezing resistance of ultra-high performance concrete concretes was investigated.2021
[22]Chinese standard GB/T 50082-2009 [27]Compressive strength, mass loss and relative dynamic modulus of elasticity under the coupled effect of F–T and other factors.2022
[23]/Investigation of the progression of concrete damage due to F–T cycles, and the influence of additives on macroscopic parameters during F–T cycles.2022
[24]/Analyze the mechanical properties of concrete under low-temperature freeze–thaw cycles and summarize the existing mathematical models2022
[25]Chinese standard GB/T 50082-2009 [27]The variation law of dynamic mechanical properties of concrete with initial freeze–thaw damage in the low temperature range was investigated.2023
[30]/A comprehensive review of concrete durability in freeze–thaw conditions: Mechanisms, prevention, and mitigation strategies2025
Table 2. Micro-enhancement methods contributing to frost resistance.
Table 2. Micro-enhancement methods contributing to frost resistance.
Enhancement MethodsAdmixturesPrinciplesAdvantagesDisadvantagesReferences
Introduce tiny air voids using AEAAEAIntroduce tiny, closed bubbles.
Provide space for ice expansion and relieve expansion pressure.		Increase the volume content of harmless pores.Reduce the compressive strength of concrete.[7,94,99,100,163,164,165,166,167,168]
Improve pore structure and microcracks using pozzolanic materials and nanomaterialsFA, SF, GGBS, RHA, MK, CNT, GOUsed as a filler to fill pores or as a bridge to connect cracks.Improve ITZ.
Refine pore structure.
Reduce cracks.		Low early strength, high maintenance requirements.[19,39,94,95,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,169]
Using fibers to bridge cracksSteel fiber,
PP fiber, polyvinyl alcohol fiber, nanofiber		Bear part of the tensile stress generated by concrete.
Reducing the formation and propagation of microcracks.		Reduce cracks.
Reduce porosity.
Increase the volume content of harmless pores.		PVA fiber is hydrophilic.
PP fiber has poor dispersion.		[12,51,88,89,141,142,143,144,145,146,147,148,149,150]
Use hydrophobic materials and hydrophobic coatings.Silane,
modified polyurea,
epoxy resin,
AH material		Forming a water-repellent lining on the pore wall.
Build a continuous, dense hydrophobic coating on the concrete surface to block the transmission path of corrosive media.		Form membranes that encapsulate cement particles and aggregate.
Reduce the amount of water that can freeze during F–T cycles.		Does not directly improve the frost resistance.[7,23,30,97,98,131,149,150,151,152,153,154,155,156,157,158,159,160,161]
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Niu, W.; Dou, T.; Li, M.; Xia, S. Microscopic Deterioration Mechanism and Different Reinforcement Methods of Concrete Under Freeze–Thaw Environment: A Review. Processes 2025, 13, 4064. https://doi.org/10.3390/pr13124064

AMA Style

Niu W, Dou T, Li M, Xia S. Microscopic Deterioration Mechanism and Different Reinforcement Methods of Concrete Under Freeze–Thaw Environment: A Review. Processes. 2025; 13(12):4064. https://doi.org/10.3390/pr13124064

Chicago/Turabian Style

Niu, Wenlong, Tiesheng Dou, Meng Li, and Shifa Xia. 2025. "Microscopic Deterioration Mechanism and Different Reinforcement Methods of Concrete Under Freeze–Thaw Environment: A Review" Processes 13, no. 12: 4064. https://doi.org/10.3390/pr13124064

APA Style

Niu, W., Dou, T., Li, M., & Xia, S. (2025). Microscopic Deterioration Mechanism and Different Reinforcement Methods of Concrete Under Freeze–Thaw Environment: A Review. Processes, 13(12), 4064. https://doi.org/10.3390/pr13124064

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