Study on the freezing resistance and repair technology of prestressed high-strength concrete pipe piles

This article takes a prestressed high-strength concrete tube pile (PHC tube pile) in Tianjin Port as the research object, considers the influence of the type of admixture, forming process, and maintenance method in the process of concrete fabrication, and carries out a rapid freeze-thaw test of PHC tube pile concrete. The changes in the mass loss rate, relative compressive strength, and relative dynamic modulus of elasticity with the number of freeze-thaw cycles, as well as the influence characteristics and mechanism of the admixture type, forming process, and curing method, were obtained. Based on this study, the repair and reinforcement of the base concrete were carried out using ultrahigh toughness cementitious composites and cementitious permeable crystalline composites, and a rapid freeze-thaw test was conducted on the repaired concrete specimens to verify the repair and reinforcement effects of the two materials. The study showed that the mass loss rate, relative strength, and relative dynamic elastic modulus can better characterize the freezing resistance of PHC piles, and it is safer to use the relative dynamic elastic modulus as the evaluation index of the freezing resistance of PHC pile concrete than the mass loss rate. The concrete admixture, forming process, and maintenance method have significant effects on the freezing resistance of the PHC pipe pile mix. The splitting damage surface of both ultrahigh-toughness cementitious composites (UHTTC) and cementitious capillary crystalline waterproofing (CCCW) repaired concrete is a reinforced surface, and both repair materials can improve the freezing resistance of PHC pipe piles through the crack arresting effect and reduction in initial porosity.


Introduction
With the development of the port transportation industry, the application of concrete in water transportation engineering has rapidly developed, and the durability of concrete structures has become increasingly prominent, receiving great attention from the engineering and scientific communities.At present, China's port infrastructure inventory ranks first in the world, with most of the facilities being concrete structures.Freeze-thaw damage to hydraulic concrete buildings in ports in northern China is very common.Taking Tianjin Port as an example, its monthly average temperature in winter is below zero, and the pile caps and prestressed piles in the tidal and wave splash areas are subject to freezethaw cycles.The survey found that the concrete members of many terminals in Tianjin Port have freeze-thaw damage, and after repairing and reinforcing the concrete structure after freeze-thaw damage, some of the reinforced positions are broken again in a relatively short period, showing the phenomenon of poor bonding between repair materials and old concrete and cracking of repair materials.There are three main reasons for this phenomenon.(1) Due to improper treatment of the repair interface, the combination of old and new concrete is not solid.(2) The construction of the lower part of the pier needs to catch the tide operation, and the construction environment is harsh and covers difficulties, resulting in the new concrete not reaching the maintenance conditions, and the performance cannot give full play.(3) The repair material itself has a low cracking strength.Once dry shrinkage occurs after pouring, cracking occurs.With the internal pore moisture freeze-thaw alternation, concrete freeze-thaw damage develops, and further cracks appear inside the concrete.A prestressed high-strength concrete tube pile (PHC tube pile for short) is an equal cross-sectional hollow ring-shaped precast concrete member made by multiple processes, such as pretensioning, centrifugal forming, steam curing, and steam high-pressure curing.It is widely used in port waterworks construction because of its strong bending resistance, good sealing, corrosion resistance, and convenient construction.Therefore, it is of great theoretical significance and engineering application value to study the freezing resistance of PHC pipe piles and their repair and reinforcement technology, which can provide a reference for the prediction, evaluation, prevention, and reinforcement of freeze-thaw disasters in port infrastructure.
Freezing resistance testing is a safety guarantee for hydraulic concrete in its normal service life cycle, and the main parameters for evaluating the freezing resistance of concrete at home and abroad are the mass loss rate [1], strength loss rate [2], and relative dynamic elastic modulus [3].The mass loss rate can reflect the spalling of concrete after freeze-thaw damage, but it cannot quantitatively evaluate the internal damage of concrete [1].The strength loss rate can reflect the internal damage of concrete, but it is highly related to the quality of concrete specimens formed as a freezing resistance evaluation index [2].The mass loss rate and strength loss rate are commonly used in the evaluation of the freezing resistance of concrete in the laboratory, and there are limitations in the evaluation of the freezing resistance of bulk concrete structures in port projects.The dynamic elastic modulus is closely related to the inherent properties of the material.When there is a change in the internal structure of concrete, the dynamic elastic modulus also undergoes corresponding variations [3], which can reflect not only the overall modulus of elasticity of the material but also the local breakage of the material and is a common nondestructive test index used in the hydraulic industry to evaluate frost damage of concrete.
Concrete repair and reinforcement technology has developed very rapidly in recent years and has become an indispensable part of the construction industry.Scholars have studied and analyzed the reinforcement theory of concrete structures and summarized different types of concrete structure reinforcement methods according to specific engineering practices, mainly the following categories: increased section reinforcement method [4], externally clad steel reinforcement method [5], prestressing reinforcement method, replacement concrete reinforcement method [6], and highperformance cement mortar reinforcement mesh reinforcement method.The use of new highperformance composite cementitious materials as reinforcement materials is a new reinforcement method that has emerged only in recent years.Many scholars have studied the durability [7], toughness [8], flexural performance [9], and bond strength [10] of ultrahigh toughness cement-based composites in general or freeze-thaw environments from different perspectives, which significantly improves the mechanical properties of the structure, has many advantages over other traditional reinforcement methods, and can be coordinated with the original concrete structure in terms of common stresses and deformations.
This article considers the effects of additives, forming processes, and concrete curing methods during the production process and conducts experimental research on the freezing resistance and influencing factors of PHC pipe piles in Tianjin Port.Ultrah-high-toughness cementitious composites (UHTCC) and cementitious capillary crystalline waterproofing (CCCW) are used to repair and strengthen freeze-thaw concrete.The repair and reinforcement effect is evaluated by a fast freezethawing test. of PHC pipe piles are the type of admixture, molding process, and maintenance method.In this paper, the above three factors were comprehensively considered to design the antifreeze test scheme, as shown in Table 1.C-100 100×100×100

Specimen preparation and rapid freeze-thaw test
In Table 1, the test block of Group A is made by the PHC pile factory when making PHC pipe piles.Under the same conditions, a section of unreinforced PHC pipe pile with a length of 1 m, a wall thickness of 150 mm, and a diameter of 1000 mm is made, as shown in Figure 1 (a).After centrifugal molding and steam curing, the pipe piles were cut to make concrete test blocks of 100 mm×100 mm×0.4 m and 100 mm×100 mm×0.1 m, as shown in Figure 1 (a) and 1(c).The test blocks of group B and group C are poured into the test mold with concrete under the same conditions when making pipe piles in the PHC pile factory.After vibration, they are maintained following the curing mode designed in Table 1, among which the standard curing process is carried out in the concrete curing box, as shown in Figure 1(d).The concrete samples were cured until they reached 28 days of age.Subsequently, these specimens underwent a rapid freeze-thaw test using the HDK-9 type rapid freeze-thaw testing machine, under GB-T50082-2009.The testing procedure was commenced by immersing the specimen in water (ensuring that the water level remained below the top surface of the specimen) for 48 hours to facilitate complete water treatment.Following this immersion, the surface water was carefully removed, and the initial weight of the specimen was recorded.The testing cycle began with a freezethaw (FT) cycle, adhering to the subsequent requirements: (1) The freezing temperature at the center of the specimen reached -15°C, with a permissible deviation of -2°C.(2) Each cycle's cooling phase lasted between 1.5 and 2.5 hours.(3) The highest temperature during melting at the center of the specimen was 8±2°C.(4) The temperature rise phase within each cycle persisted for 1.0 to 1.5 hours.(5) A single freeze-thaw cycle was extended for a duration of 2.5 to 4.0 hours.(6) The temperature contrast between the specimen's center and its surface remained under 28°C.(7) Throughout the testing process, the experiment was paused every 50 freeze-thaw cycles.During these intervals, three specimens measuring 100 mm×100 mm×100 mm were extracted from each group to undergo compressive strength testing.Additionally, the mass and dynamic elastic modulus of specimens measuring 100 mm×100 mm×400 mm were measured.This freeze-thaw testing regimen continued for a total of 500 cycles.

PHC characterization of the freezing resistance parameters of pipe piles (1) Mass loss rate
A JA30k-1 electronic precision balance (range 30 kg, accuracy 0.1 g, as shown in Figure 2 (a)) was used to weigh the concrete specimens before and after experiencing FT cycles, and the mass loss rate was calculated according to Formula (1).The mass loss rate variation rule of concrete is shown in Figure 2 where  is the test specimen mass loss rate after freeze-thaw cycles (%) and  is the saturation weight of the concrete specimen before freeze-thaw cycles (g). is the saturated weight of the concrete specimen after n freeze-thaw cycles (g). and  are taken as the average of the test results of three specimens.illustrates the enhancement of concrete's FT resistance through the incorporation of silica fume.Analyzing the  alterations among specimens A, B1, B2, and B3 reveals that concrete's freezing resistance is least effective under standard curing conditions.Autoclave curing and steam curing, conversely, exhibit the capacity to bolster concrete's freezing resistance, although the difference in mass loss is not substantial.
(2) Relative strength Compressive strength tests were conducted on concrete specimens subjected to varying numbers of FT cycles, utilizing a 200 t class pressure testing machine (as illustrated in Figure 3(a)).The relative strength is defined as the ratio of the compressive strength of the unfrozen cube to that of the cube after undergoing n FT cycles.As an instance, considering the concrete specimens in group A, Figure 3(a) illustrates the variation in concrete relative strength with the number of FT cycles, as derived from the compressive strength test., it can be found that specimen C, without silica fume admixture and under autoclave and steam curing conditions, has a very large strength loss, although the  is not significant.Specimen B2 has a significant mass loss rate and strength loss under the standard condition with silica fume admixture.The admixture incorporation and curing conditions have a significant effect on the freezing resistance of PHC pipe pile concrete.
Literature studies have shown that the relationship between the compressive strength of ordinary concrete and the number of FT cycles satisfies an exponential relationship [11].Based on the test results and through parameter inversion analysis, the empirical formula of the attenuation of the compressive strength of PHC pipe pile concrete with the number of FT cycles is given in this paper, as shown in Formula (2).
Formula ( 2) is used to predict the compressive strength of Group A concrete specimens, and the calculation results are shown in Table 2.As seen from Table 2, the ratios between the empirical formula for the attenuation of the compressive strength of PHC pipe piles with the number of cycles and the measured test values are 0.996, 0.983, and 0.022.The formula is in good agreement with the experimental values and has good accuracy.
(3) Relative dynamic elastic modulus A DT-16 dynamic elastic modulus tester (Figure 4 (a)) was used to test the dynamic elastic modulus of concrete specimens with different FT cycles, and Formula (3) was used to calculate the relative dynamic elastic modulus.Under different working conditions, the mass loss rate of concrete specimens varies with the number of FT cycles, as shown in Figure 4 (b).
where  is the relative dynamic elastic modulus of the ith concrete specimen after a different number (n) of freeze-thaw cycles (%). is the initial value of the transverse fundamental frequency of the ith specimen before the freeze-thaw cycle test (Hz). is the transverse fundamental frequency of the ith concrete specimen after n freeze-thaw cycles (Hz). and  are the average values of the test results of the three specimens.As depicted in Figure 4(b), the P of PHC pipe pile concrete exhibits a gradual decline with the progressive increase in the number of FT cycles.According to the stipulations outlined in GB-T50082-2009, the testing procedure should conclude when the loss in the dynamic elastic modulus of the concrete reaches 25%.After undergoing 350 tests, it was observed that the dynamic modulus loss rate surpassed 25% for three sets of specimens.
In contrast, when considering S n as the criterion for assessing freezing resistance, only one set of specimens showed a mass loss exceeding 5%, as evident in Figure 2(b).This discrepancy highlights that employing P as the evaluation parameter for the freezing resistance of PHC pipe pile concrete is a more cautious approach.
In addition, Figure 4(b) shows that specimen B2 has the smallest S n and the fastest attenuation when silica ash is added and standard curing conditions are met.Specimens B3 and C, under the conditions of autoclaved + steamed curing, have a consistent variation law of P with the number of FT cycles, and both decay quickly.Under the conditions of silica ash and steam curing, specimens A and B1 were centrifugally formed and conventionally formed, respectively.Their P values were both high, and their attenuation was slow with the number of FT cycles.

Influence of concrete admixture
To further quantitatively study the influence of concrete admixtures on the freezing resistance of PHC pipe pile concrete, the compressive strengths of the B3 specimen with silica fume and the C specimen without silica fume are compared and analyzed, as shown in Figure 5.As seen from Figure 5, the initial strength and freezing resistance of concrete are improved after the addition of silica fume, and the curve of the strength-freeze-thawing cycles of specimens with silica fume is fuller.After 350 cycles, the strength decreases rapidly, indicating that the deterioration of its freezing resistance is relatively slow.The strength of the specimen without silica fume decreases rapidly after 200 FT cycles.The incorporation of silica fume can improve the freeze-thaw resistance of concrete from two aspects.First, the fineness of silica fume is lower than that of cement particles and mineral powder, which plays a filling effect after mixing, improving the concrete compactness and reducing the concrete porosity.Second, silica fume improves the pore structure of concrete through secondary hydration and reduces the connected pores of concrete.These two effects reduce and make it more difficult for water to flow through the concrete, which helps to reduce the void pressure inside the concrete when it freezes and the osmotic pressure when it melts, ultimately improving the concrete's freezing resistance.

Influence of the molding process
Under different forming process conditions, the variation rule of the PHC pipe pile concrete cube compressive strength with the number of FT cycles is shown in Figure 6.As seen in Figure 6, the specimens in Group A and Group B1 differed only in the forming process.The strength of centrifugally formed specimens reached 85.0 MPa, and after 500 FT cycles, their cubic compressive strength was 49.2 MPa, decaying by 41%.The compressive strength of ordinary formed concrete specimens was 78.0 MPa, and after 500 cycles, their cubic compressive strength was 40.8 MPa, decaying by 48%.The strength and freezing resistance of the centrifugally formed specimens are substantially better than those of the ordinarily formed specimens.This is because the raw material of the PHC pipe pile is concrete with a water-cement ratio of 0.27.Ordinary vibration cannot discharge the air bubbles in the concrete.Centrifugal molding, through a high-speed rotating pipe pile, makes the concrete material with a larger proportion move to the prefabricated pipe pile template, and the air bubbles move to the center, so the concrete porosity is very low.The strength and freeze-thaw resistance of concrete are inversely proportional to the porosity of concrete, so the strength and freezeresistance of concrete can be effectively improved by centrifugal molding.

Influence of curing methods
Under different curing methods, the variation rule of the PHC pipe pile concrete cube compressive strength with the number of FT cycles is shown in Figure 7. Figure 7 shows that steam curing, especially secondary autoclaved curing, can significantly improve the strength of concrete by 51.8%-70.2%.Steam and autoclaved curing can improve the freezing damage of concrete.After 300 and 500 cycles, the strength loss rates of steam-cured concrete specimens are 13% and 48%, respectively; the strength loss rates of secondary autoclaved concrete specimens are 23% and 64%, respectively; and the strength loss rates of standard cured concrete specimens are 59% and 77%, respectively.The initial strength of the secondary steam-forming specimen is 112.2% of that of the steam-curing specimen.After 300 cycles, the two strengths are the same; after 500 cycles, the initial strength of the secondary steam-forming specimen is 76.7% of that of the steam-curing specimen.It can be seen that the concrete using the secondary steam forming process can obtain higher initial strength.The strength decays quickly, and its freezing resistance is inferior to that of concrete formed by the steam curing process.

Repair materials (1) Ultra High-Toughness Cementitious Composites (UHTCC)
UHTCC is a new type of high-performance fiber cementitious composite.It can be fabricated by a conventional stirring process using short fiber reinforcement with a volume ratio of no more than 2.5%.After hardening, the composite material has significant strain hardening characteristics, the ultimate tensile strain capacity is more than 3% and can effectively limit the ultimate crack width within 100 μm, and some can even be limited within 5 μm, which can be called "seamless concrete" in a sense.The UHTCC material has excellent tensile properties, flexural properties, shear properties, freezethawing properties, and controllable crack width.To ensure the durability of materials and structures, these properties are exactly the needs of concrete freeze-thawing repair.UHTCC materials also have disadvantages, such as high cost and relatively complex construction.
(2) Cementitious Capillary Crystalline Waterproofing (CCCW) Cement-based permeable crystalline composite (CCCW) is a powdery rigid waterproof material made of special cement, quartz sand, which is infiltrated into a variety of active chemicals.After the action of water, the active chemical substances contained in the material permeate through the carrier water to the inside of the concrete, forming water-insoluble crystals in the concrete and blocking the capillary channel to make the concrete compact and waterproof.The concrete interface is coated with two layers of the material and can withstand more than 1.2 MPa water pressure.The physicochemical reaction produced by brushing the material on the concrete interface penetrates the concrete, and the penetration depth can reach more than 100 mm.In the concrete interface coating, the material forms insoluble tendering crystals, the gap will be dense, blocking the infiltration of waterways, and less than 0.4 mm of concrete cracks can be filled and self-repair.The construction method of the material is simple and labor-saving, and the concrete interface does not need to have a screed layer, and there is no need to have a protective layer after brushing.
(3) Interface agent In this paper, the cement-based permeable crystalline material is selected as the interface agent.Its working principle is that the permeable crystalline material will penetrate the old concrete after being painted on the interface and generate dendriform crystals in the gap of the old concrete, making the new and old concrete not only through the bonding force of the contact layer but also through the dendriform crystals between the new and old concrete to strengthen its bonding force.

Repair and reinforcement test
Table 4 displays the composition of the foundational concrete mix.The cement employed is PO42.5Rcement from the Hebei Jidong Cement Plant.The mineral powder was finely ground S95 grade mineral powder.The fine aggregate is sourced from medium river sand with an apparent density of 2650 kg/m 3 , a bulk density of 1480 kg/m 3 , a mud content of 1.0%, and a fineness modulus of 2.7.The coarse aggregate consists of limestone hammer-crushed stone, free from needle-like particles, with particle sizes ranging from 5 to 15 mm.Its apparent density is 2820 kg/m 3 , bulk density is 1435 kg/m 3 , mud content is 0.3%, and crushing index is 6%.These values conform to the standards stipulated for construction pebbles and gravel (GB/T14685-2001).
A naphthalene high-efficiency water-reducing agent from Yimeng Admixture Factory is employed as the water-reducing agent.Additionally, the air-entraining agent used is an SA-7 concrete airentraining agent, also from the Yimeng admixture factory.The essential characteristics are outlined in Table 4.The cement-based permeable crystalline material utilized is XYPEX Megamix II.
The specimen dimensions measure 100 mm×100 mm×100 mm.After formation, the specimens are cured with molds for 24 hours before being transferred to a constant temperature and humidity chamber for ongoing curing until reaching the targeted 28-day age.
The proportion of high-strength cement mortar is cement: sand: water: high-efficiency superplasticizer = 1:1:5:0.26:0.007.The PVA fiber content is 1.2 kg/m 3 , and the replacement amount of cement-based permeable crystal material is 20% of the cement content.(2) Preparation of test specimens When the benchmark test block reaches the age of 28 d, the square test block is precisely cut into 2 pieces by the cutting machine to obtain 50 mm×100 mm×100 mm test blocks.The cutting surface of the specimen was gouged and bristled.To obtain a better repair effect, the interface was treated with the grooving method.The grooving depth was 2~3 mm, and the interval was 1~2 cm.The interfacial agent is applied evenly to the concrete interface, as shown in Figure 8(a).The specimen was placed in the test mold, as shown in Figure 8(b).The premixed ultrahigh toughness cement-based mortar and the cement mortar mixed with crystallized material were poured into the test mold, smoothed after vibration, de-molded after wet curing for 48 h, and then moved to a constant temperature and humidity box for curing until the predetermined age of 28 d.The cured repaired and reinforced specimens were tested 125 times in an HDK-9 fast freeze-thaw testing machine according to GB-T50082-2009, and cubic compressive strength and split tensile strength tests were carried out on the repaired and reinforced specimens after freeze-thaw.The side of the repaired reinforced specimen is used as the bearing surface in the cube compressive strength test and the contact surface of the old and new concrete is used as the bearing surface in the split tensile strength test.

Evaluation of repair and reinforcement effect
(1) Characteristics of freeze-thaw deterioration and strength failure Under the influence of FT cycles, a transformation in the surface morphology of repaired and reinforced concrete specimens takes place.In the initial stages of the FT cycle, the concrete surface loses its smoothness and becomes marked, as depicted in Figure 9.As the count of FT cycles grows, the concrete surface experiences the detachment of its floating slurry.With an increased number of cycles, the hardness of the concrete surface diminishes, leading to a loose and porous texture.
After undergoing numerous FT cycles, concrete specimens that are combined with PVA fiber exhibit fewer surface pores compared to both the reference concrete test block and the specimens integrated with crystalline material.These PVA fiber-infused specimens feature fine fibers on the surface, with cement slurry adhering to these fibers.
The failure surfaces of the split tensile strength test are all reinforcement surfaces, as shown in Figure 10.  10, there are residual repair materials at the gouge of the fracture surface of the specimen, indicating that the protrusion formed by the repair materials at the gouge is subject to shear failure.Thus, the bond between the repair materials and the old concrete is not only the bond between the concrete but also the mechanical bite force, which can improve the bond performance of the new and old concrete to a certain extent.
(2) Mass loss rate and relative dynamic elastic modulus loss rate As the number of FT cycles increases, the S n and P of UHTTC-reinforced concrete specimens, CCCW-reinforced concrete specimens, and reference concrete specimens are shown in Figure 11 and Figure 12.As shown in Figure 11, when the number of FT cycles is less than 25, the quality of the three kinds of concrete specimens is improved to a certain extent, which may be because some active ingredients, such as cement, in the concrete, have not been fully hydrated.This component continues to react with water, resulting in improved concrete quality.When the number of FT cycles is greater than 25, the quality loss of the three kinds of concrete specimens gradually appears with the increase in the number of FT cycles.After 125 cycles, the quality loss of concrete mixed with PVA fiber is the least, and its curve tends to gradually become gentler.This is because the PVA fiber mixed into the concrete inside the formation of countless small bones, and the concrete parts together, reducing the loss of mass.
As shown in Figure 12, after multiple FT cycles, the P of the three concrete specimens decreases.The addition of PVA fiber and cement crystalline material can improve the P of concrete, but the amplitude is small.
(3) Tensile strength Regarding S n and P , the damage variable of the concrete rupture tensile strength is defined as where D is the damage variable of rupture tensile strength and  is the fracture tensile strength of the repaired and reinforced concrete specimen after n freeze-thaw cycles. is the initial rupture tensile strength of the repaired and reinforced concrete specimen.
As the number of FT cycles increases, the variation law of the split tensile strength of the UHTTCrepaired reinforced concrete specimen and CCCW-repaired reinforced concrete specimen is shown in Figure 13.From Figure 13, when the concrete specimens repaired by UHTTC and CCCW are subjected to FT cycles continuously, the splitting tensile strength decreases continuously, with maximum reduction rates of 24% and 29%, respectively.Among them, the strength of the UHTTC concrete reinforced specimens decreased gently and did not enter the sudden decline stage.Many studies show that after 75-100 FT cycles, the tensile strength loss rate of ordinary concrete can reach 50%~70%, while the failure tensile strength loss rate of concrete specimens repaired by UHTTC and CCCW is relatively low, both lower than 30%, which indicates that after several FT cycles, the residual adhesion between the repair material and the base concrete is high, and the repair and reinforcement effect is good.The reason for this is that both capillary holes and cementification holes are microstructure parts of concrete.Fine holes are usually formed due to insufficient reaction between water and cement particles in concrete, while adhesive pores are the spaces formed after the reaction between cement and water.When it is in a water-saturated state, at a certain freezing temperature, the water in capillary pores will freeze, while the water in the cementification hole is in a supercooled state.When the temperature drops below 0 ℃, the water in the pores begins to freeze into ice, and expansion occurs when the water freezes into ice.However, the water in the bonding hole is difficult to directly freeze due to its close binding with the cement mixture, resulting in supercooling.However, once the water in the bonding hole begins to freeze, it will also expand and cause pressure on the concrete.Therefore, when water in concrete is converted into ice, due to its volume expansion, the concrete will also expand accordingly.In addition, because the water molecules in the supercooled state in the cementitious pore permeate to the ice interface in the pressure pore, another osmotic pressure is generated in the capillary.When the concrete in the saturated state is frozen, its capillary wall is under two kinds of pressure at the same time: expansion and penetration.When the two kinds of pressure exceed the tensile strength of the concrete, the concrete will crack.After repeated FT cycles, as cracks develop, they will connect, and the strength of the concrete will gradually decrease until it is completely lost, causing damage to both the surface and interior of the concrete.After adding a certain amount of evenly distributed PVA fibers and cement-based crystalline materials into concrete, the fibers form countless tiny reinforcing bars inside the concrete, and the existence of fibers improves the pore structure of the original concrete, making the connectivity of the original pores change.On the one hand, when tiny cracks appear in the concrete, the tensile stress is shared by the fiber bonding effect.In this way, the weak tensile capacity of concrete is overcome to some extent, and the development of cracks is delayed.Finally, the freezing resistance of concrete is improved.On the other hand, the existence of pores in concrete is reduced, which makes the necessary condition of FT cycle failure of concrete-free water is reduced or becomes difficult to flow-thus improving the freeze-resistance of concrete.The mechanism of cement-based crystalline materials improving the freezing resistance of concrete is similar.

Conclusion
(1) The freezing-resistance test of PHC pipe pile concrete shows that the mass loss rate ( ), relative strength, and relative dynamic elastic modulus (P ) can better characterize the freezing resistance of PHC pipe piles, and P , as the evaluation index of the freezing resistance of PHC pipe pile concrete, is safer than the mass loss rate.The relationship between the compressive strength of PHC pipe pile concrete and the number of freeze-thaw cycles meets the subsection exponential.
(2) The concrete admixture, forming process, and curing method have a significant influence on the freezing resistance of PHC pipe pile mixtures.Silica fume can improve the freezing resistance of concrete through the filling effect and secondary hydration.Compared with ordinary formed concrete, the porosity of centrifugally formed concrete can be reduced to improve the freezing resistance of concrete.Steam and autoclaved curing can increase the strength of concrete by 51.8%~70.2%compared with standard curing.
(3) The splitting failure surface of UHTTC-and CCCW-repaired concrete is the reinforced surface.Both repair materials can reduce the  , P and tensile strength loss rate of concrete under freezingthawing cycles through the crack resistance effect and the reduction in initial porosity.
Due to the PHC pipe pile material and fabrication process, the main factors affecting the performance AMCE-2023 Journal of Physics: Conference Series 2713 (2024) 012041

Figure 2 .
Quality measurement and mass loss rate variation rule of concrete.(a) JA30k-1 electronic precision balance; (b) Change law of mass loss rate.As depicted in Figure 2(b), the  of PHC pipe pile concrete gradually increases as the number of FT cycles rises, albeit with overall insensitivity.According to GB-T50082-2009, we tested ceases when the concrete mass loss rate reaches 5%.Among the 350 tests conducted, only one set of specimens experienced a mass loss surpassing 5%.Moreover, Figure 2(b) indicates the substantial influence of the concrete admixture type and curing method on the freeze-thaw mass loss rate.A comparison between the  trends of specimens B3 and C

Figure 3 .
Concrete compressive strength test and relative strength variation rules.(a) Cube compressive strength test; (b) Relative strength change law.As seen in Figure 3(b), the strength of PHC pipe pile concrete has no attenuation up to 200 freezethaw cycles and begins to show significant attenuation after 250 cycles.Additionally, comparing Figure 2 b and Figure 3(b)

Figure 4 .
Determination of concrete compressive strength and the variation rule of relative dynamic elastic modulus.(a) DT-16 dynamic elastic modulus tester; (b) relative elastic modulus change law.

Figure 5 .
Figure 5. Influence rules of silica ash on the freezing resistance performance of concrete.

Figure 6 .
Figure 6.Influence laws of the molding process on the freezing resistance of concrete.

Figure 7 .
Figure 7. Influence laws of different curing methods on the freezing resistance performance of concrete.Figure7shows that steam curing, especially secondary autoclaved curing, can significantly improve the strength of concrete by 51.8%-70.2%.Steam and autoclaved curing can improve the freezing damage of concrete.After 300 and 500 cycles, the strength loss rates of steam-cured concrete specimens are 13% and 48%, respectively; the strength loss rates of secondary autoclaved concrete specimens are 23% and 64%, respectively; and the strength loss rates of standard cured concrete specimens are 59% and 77%, respectively.The initial strength of the secondary steam-forming specimen is 112.2% of that of the steam-curing specimen.After 300 cycles, the two strengths are the same; after 500 cycles, the initial strength of the secondary steam-forming specimen is 76.7% of that of the steam-curing specimen.It can be seen that the concrete using the secondary steam forming process can obtain higher initial strength.The strength decays quickly, and its freezing resistance is inferior to that of concrete formed by the steam curing process.

Figure 8
Preparation process of the repair and reinforcement specimens.Interface treatment of the repair and reinforcement specimens; (b) Forming process of the repair and reinforcement specimens.

Figure 9 Figure 10
Figure 9 Hemp surface of the concrete specimen

Figure 11 Figure 12
Figure 11 Variation law of the mass loss rate of different concrete specimens

Figure 13
Figure 13Variation in splitting tensile strength with the number of freeze-thaw cycles

( 4 )
It is recommended to optimize the design of concrete admixtures and mix proportions, strictly control the forming and curing processes, and obtain PHC pipe piles with excellent frost resistance.Silane impregnation or coating should be applied 10 meters below the pile head to further improve frost resistance.

Table 2 .
Calculation results of the empirical formula of compressive strength.

Table 3 .
Base concrete mix ratio.

Table 4 .
Basic properties of PVA fibers.