Low-temperature anti-freezing anti-cracking concrete and preparation method thereof

Abstract

The invention discloses low-temperature anti-freezing anti-cracking concrete and a preparation method thereof, and belongs to the technical field of concrete. The concrete is prepared from the following components: a cementing material, sand, gravel, temperature response modified bamboo aggregate, a water reducing agent, water, succinic anhydride, fucoidan and rock wool fiber, the preparation method of the temperature response modified bamboo aggregate comprises the following steps: S1, dissolving N-isopropylacrylamide and dodecyl acrylate in a solvent, adding an initiator I, carrying out polymerization reaction, and separating to obtain a copolymerized N-isopropylacrylamide monomer; s2, dissolving a copolymerized N-isopropylacrylamide monomer in a calcium citrate aqueous solution, and uniformly stirring to obtain a copolymerized N-isopropylacrylamide aqueous solution; and S3, mixing the bamboo aggregate, the copolymerized N-isopropylacrylamide aqueous solution, a cross-linking agent and an initiator II, and reacting at 40-50 DEG C for 4-5 hours, thereby obtaining the composite material. The succinic anhydride, fucoidan, the temperature response modified bamboo aggregate and the rock wool fiber synergistically improve the low-temperature crack resistance of the concrete.

Description

Technical Field

[0001] The present invention belongs to the technical field of concrete, and particularly relates to a low-temperature anti-freezing and crack-resistant concrete and a preparation method thereof. Background Art

[0002] China's JGJ / T 104-2011 "Code for Winter Construction of Building Engineering" has made specific regulations on the requirements for concrete winter construction. In this stage, each link of concrete from mix design to pouring, forming, curing, form removal, and use is different from that of concrete under normal working conditions. The main reason is that low-temperature working conditions will have a great impact on the formation of the concrete microstructure and the development of its performance. China has a vast territory with a large latitude difference between north and south. Among them, cold or severely cold regions account for about 60% of China's area. There is a large demand for winter engineering construction in cold regions. Due to the influence of its climate, the actual constructible date will be severely shortened. Many major projects even span 2 to 3 winter construction periods. Due to the significant reduction in the strength and durability of concrete structures caused by early frost damage, a large number of building damages and economic losses have occurred. Therefore, how to prevent the frost heaving and cracking damage of concrete at low temperatures has very important engineering application value.

[0003] After pouring concrete, the temperature difference between the concrete itself and the surrounding environment will cause heat exchange between the two. Especially when the concrete is in a negative-temperature environment, the stability of the fresh concrete is reduced through heat exchange. A large amount of free water remaining inside the concrete will generate a high frost heaving stress after freezing, resulting in concrete damage. Generally speaking, many studies have been carried out by scholars from various countries on the frost damage mechanism of concrete, and many theories have been proposed, such as the hydrostatic pressure theory, the osmotic pressure theory, the adsorbed water theory, and the theory of frost expansion of concrete aggregates. Among them, the hydrostatic pressure theory believes that the pressure caused by freezing expansion forces the capillary water inside the concrete and the external environmental moisture to migrate to the internal area with a smaller saturation. When the concrete permeability is large, a water pressure gradient will be formed, generating pressure on the pore wall. With the acceleration of the cooling rate, the increase in the water saturation in the capillary pores and the decrease in the pore size, the water pressure increases. When the water pressure exceeds the tensile ultimate strength of the concrete, the pore wall ruptures, and the concrete is frost heaving and cracked; the theory of frost expansion of concrete aggregates believes that the aggregates in the concrete absorb water and freeze under low-temperature conditions. The volume of ice is about 9% larger than that of water. The water inside the aggregates expands due to freezing, generating internal stress and volume change. Since the aggregates are relatively rigid, it will ultimately cause the aggregates to rupture or the interface between the aggregates and the cement paste to fall off, and the cracks expand from the inside, resulting in the surface cracking and structural damage of the concrete.

[0004] To improve the anti-freezing and anti-cracking properties of concrete, the existing technologies mainly focus on the following aspects: (1) Incorporating anti-freezing air-entraining agents to enhance the frost resistance of concrete, but it is likely to have an adverse impact on the strength and durability of concrete; (2) Improving the impermeability of concrete to reduce the inward migration of external moisture, but the effect of enhancing the compactness of concrete is limited. Moreover, during the mixing, forming and hydration processes of concrete, there is a large amount of free water that has not hydrated, and frost heaving still occurs under low-temperature conditions; (3) Introducing fiber anti-cracking and strengthening materials such as polypropylene fibers or polyvinyl alcohol fibers, which only have an anti-cracking effect and do not significantly improve the early hydration effect of concrete in a low-temperature environment.

[0005] Based on the above analysis, to improve the low-temperature anti-freezing and anti-cracking properties of concrete, it is necessary to systematically optimize in multiple aspects from the design of concrete materials. It is necessary to improve the compactness of the interior and surface of concrete, strengthen the early strength and impermeability, prevent external environmental moisture from seeping into the interior and freezing and cracking, and at the same time reduce the moisture in the capillary pores of concrete while ensuring the hydration effect of the cement-based concrete, and weaken the rigid expansion damage of the aggregate, so as to make the anti-freezing and anti-cracking effect of concrete reach the best state, improve the safety of the engineering structure during use and extend the service life. Summary of the Invention

[0006] Aiming at the deficiencies of the above existing technologies, one of the purposes of the present invention is to provide a low-temperature anti-freezing and anti-cracking concrete. Under the synergistic action of each component, it synergistically improves the low-temperature anti-freezing performance of concrete for winter construction from four aspects: enhancing cement activity, self-thermal insulation, temperature-responsive water absorption and toughening of concrete aggregates, and raising the freezing point of capillary water, so that the concrete has excellent low-temperature anti-cracking and structural durability, and solves the problems of low strength and easy frost heaving and cracking of traditional concrete in a low-temperature environment in winter.

[0007] To achieve the above purpose, the specific technical solutions of the present invention are as follows:

[0008] A low-temperature anti-freezing and anti-cracking concrete, the raw materials of which include the following components: cementitious materials, sand, gravel, temperature-responsive modified bamboo aggregates, water reducing agents, water, succinic anhydride, fucoidan, rock wool fibers;

[0009] The preparation method of the temperature-responsive modified bamboo aggregates includes the following steps:

[0010] S1. Dissolve N-isopropylacrylamide and dodecyl acrylate in a solvent, add initiator I, and carry out a polymerization reaction under an inert gas atmosphere. After precipitation, separation and drying, a copolymerized N-isopropylacrylamide monomer is obtained;

[0011] S2. Dissolve the copolymerized N-isopropylacrylamide monomer obtained in step S1 in an aqueous solution of calcium citrate, and stir evenly to obtain an aqueous solution of copolymerized N-isopropylacrylamide;

[0012] S3. Mix the bamboo aggregate, aqueous solution of copolymerized N-isopropylacrylamide, crosslinking agent and initiator II, and react at 40-50 °C for 4-5 h to obtain the temperature-responsive modified bamboo aggregate.

[0013] In the above technical solution, N-isopropylacrylamide is a reversible polymer with temperature responsiveness. Its macromolecular chain has both hydrophilic amide groups and hydrophobic isopropyl groups, and can undergo a phase change when the temperature changes. When the temperature is lower than the critical solution temperature, there is a strong hydrogen bond force between the amide groups in N-isopropylacrylamide and the surrounding water molecules, and the polymer shows hydrophilicity, and the molecular chain shows an extended state; when the temperature is higher than the critical solution temperature, the hydrophobic interaction between the isopropyl groups of N-isopropylacrylamide is enhanced, and the polymer shows hydrophobicity, and the molecular chain structure is a closely arranged granular structure.

[0014] The critical solution temperature of conventional N-isopropylacrylamide is generally between 30 and 35 °C. Dodecyl acrylate is a hydrophobic monomer with a long-chain alkyl group, and compared with the shorter alkyl chain of butyl acrylate, the dodecyl acrylate with a long-chain alkyl group helps the polymer to form a more hydrophobic network. In the present invention, by copolymerization, dodecyl acrylate with a long alkyl chain is introduced into the polymer to reduce the interaction with water and provide more hydrophobic parts. When the temperature rises, the molecular motion of the polymer is intensified. Since there are more hydrophobic parts in the copolymerized N-isopropylacrylamide, the hydrogen bond between it and water molecules is more likely to break earlier (that is, compared with N-isopropylacrylamide, the hydrogen bond in the copolymerized N-isopropylacrylamide requires lower thermal energy to break). Therefore, the critical solution temperature is reduced. At the same time, in the present invention, calcium citrate is introduced into the aqueous solution of copolymerized N-isopropylacrylamide. The calcium ions in calcium citrate can interact with water molecules through the ion shielding effect, weakening the hydrogen bond structure between water molecules. The citrate ion itself can compete with the hydrogen bond between water molecules, thereby further reducing the critical solution temperature of the copolymerized N-isopropylacrylamide. Through the above two aspects, the hydrophilic-hydrophobic transition temperature of the aqueous solution of copolymerized N-isopropylacrylamide is adjusted to 5-10 °C.

[0015] The copolymerized N-isopropylacrylamide can undergo physical state changes such as swelling and gelation with temperature changes, but its structure is relatively loose and unstable, and it is easily affected by environmental factors. In the present invention, by introducing a crosslinking agent and an initiator, the temperature-responsive modified bamboo aggregate is prepared by copolymerization method. On the one hand, it can effectively enhance the structural stability of the temperature-responsive monomer gel and strengthen the physical properties. On the other hand, it can make the temperature-responsive monomer gel form a good crosslinked structure with the natural fibers on the surface of the bamboo aggregate, improve the toughness of the bamboo aggregate, strengthen the adhesion between the gel and the surface of the bamboo aggregate, and enhance the frost resistance of the bamboo aggregate.

[0016] Bamboo has the characteristics of light weight, hygroscopicity, high toughness, etc. Adding bamboo aggregates to concrete can effectively improve the toughness and impact resistance of concrete and enhance the crack resistance performance. The surface of temperature-responsive modified bamboo aggregates is coated with copolymerized N-isopropylacrylamide. After it is added to concrete, in the early stage, due to the hydration of cement, the internal temperature of the concrete remains relatively high. The modified bamboo aggregates show hydrophobicity, do not absorb the water in the concrete and do not affect the normal hydration of the concrete, and can avoid the influence of the organic matter in the bamboo aggregates on the hydration process of the cement paste. When the external environmental temperature continues to decrease and the weakening of cement hydration leads to a continuous decline in the internal temperature, the copolymerized N-isopropylacrylamide begins to gradually become hydrophilic, and the excess water in the capillary pores of the concrete is gradually absorbed by the bamboo aggregates. The water that enters the bamboo aggregates freezes due to the low temperature and its volume increases. However, the bamboo aggregates have the characteristic of high toughness and will not cause a large rigid expansion to the cement matrix, which can effectively solve the problem that concrete is prone to cracking due to frost heaving and significantly improve the crack resistance performance of concrete at low temperatures. After the subsequent environmental temperature warms up and the temperature rises, the frozen water in the bamboo aggregates melts and can play the role of internal curing, further improving the later strength of the concrete.

[0017] Succinic anhydride contains easily reactive anhydride groups (-C=O) and is prone to hydrolysis reaction to form succinic acid in the presence of moisture or an alkaline environment. On the one hand, the succinic acid generated by hydrolysis has the effect of a surfactant, which can effectively reduce the surface tension on the surface of cement particles, promote better contact and dispersion between cement particles and water, accelerate the hydration reaction, and enhance the early strength of concrete. On the other hand, the carboxyl group (-COOH) in succinic acid can react with the free calcium ions generated by hydration and the calcium ions in calcium citrate (precipitated from the modified bamboo aggregates) to form calcium succinate, filling the interfaces and pore spaces of the cement matrix, promoting the hydration reaction of cement, improving the early strength and density of concrete, and enhancing the ability of concrete to resist the infiltration of external water.

[0018] Fucoidan has excellent water retention, gelation and polymer characteristics. The hydroxyl groups (-OH) and sulfate ester groups (-SO3 -It can form a stable hydrogen bond structure with water molecules, lower the freezing point of the aqueous solution in concrete, delay the growth rate of capillary water ice crystals inside the concrete, and enhance the frost resistance of the concrete; fucoidan will gelate when it meets water, but its gelation is reversible. When the pH value or calcium ion concentration is relatively low, the gel is in a dissolved state. During the mixing process of adding fucoidan to the concrete, the pH value and calcium ion concentration of the concrete are relatively low, and fucoidan can be effectively dispersed into the concrete without affecting the mixing and workability of the concrete. As the degree of hydration increases, the pH value and calcium ion concentration of the concrete increase significantly. Fucoidan forms a curved, flexible and dense three-dimensional network structure in the capillary pores of the concrete. On the one hand, it "fixes" the excess water in the concrete pores, reduces the free fluidity of the water, and weakens the migration of ice crystals after freezing to the weak interface of the cement matrix. On the other hand, it can endow the concrete pores with elasticity and toughness, disperse and absorb the stress caused by freezing expansion, and improve the density and structural stability of the concrete, enhancing the anti-permeability performance of the concrete.

[0019] By adopting the above technical solutions, the concrete of the present invention forms three anti-freezing and anti-cracking defense lines through the synergistic effect of each component, comprehensively enhancing the low-temperature frost resistance of the concrete. Firstly, by utilizing the cement-based activity strengthening effect of succinic anhydride and the heat preservation and toughening effects of rock wool fibers, the density, resistance to external water penetration and anti-cracking performance of the concrete are effectively improved, serving as the first anti-freezing and anti-cracking prevention and control means for the concrete; then, by using the temperature-responsive modified bamboo aggregate to absorb the excess free water in the capillary pores and taking advantage of the toughness of the bamboo aggregate to reduce rigid expansion, effectively solving the cracking of the concrete caused by the frost heave of the remaining capillary pore water and the water in the aggregate, serving as the second anti-freezing and anti-cracking prevention and control means for the concrete; finally, for the water remaining after being absorbed by the temperature-responsive modified bamboo aggregate in the capillary pores, using fucoidan to lower the freezing point of the remaining water, reduce the free flow of the water, and at the same time enhance the elasticity and toughness of the concrete capillary pores, effectively weakening the frost heave effect of the free water in the capillary pores (the water in the concrete itself and the water infiltrated from the outside into the inside), serving as the third anti-freezing and anti-cracking prevention and control means for the concrete.

[0020] Preferably, the molar ratio of N-isopropylacrylamide to dodecyl acrylate is (5~5.5):1.

[0021] Preferably, the temperature of the polymerization reaction is 70~80 °C and the time is 3~5 h.

[0022] Preferably, the mass ratio of N-isopropylacrylamide monomer, calcium citrate and water is 1:(0.1~0.2):5.

[0023] Preferably, the first initiator and the second initiator are ammonium persulfate or potassium persulfate. The mass of the first initiator is 0.8% - 1.0% of the mass of N-isopropylacrylamide. The mass of the second initiator is 1.0% - 1.5% of the mass of the copolymerized N-isopropylacrylamide monomer.

[0024] Preferably, the crosslinking agent is N,N'-methylenebisacrylamide. The mass of the crosslinking agent is 0.2% - 0.4% of the mass of the copolymerized N-isopropylacrylamide monomer.

[0025] Preferably, the preparation method of the bamboo aggregate is as follows: mature bamboo is crushed into bamboo particles, and after screening, bamboo aggregate raw materials with a size of 5 - 15 mm in length, 5 - 10 mm in width, and 2 - 3 mm in thickness are obtained, and the bamboo aggregate is obtained after drying.

[0026] Preferably, the density of the rock wool fiber is 20 - 40 kg / m 3 , the thermal conductivity is 0.030 - 0.045 W / (m•K), and the tensile strength is ≥100 kPa.

[0027] Preferably, before use, the rock wool fiber is soaked in styrene-acrylic copolymer emulsion mixed with expanded perlite, stirred evenly, and then taken out and dried.

[0028] Rock wool fiber can play a bridging role and disperse stress in concrete, improve the toughness of concrete, reduce cracks caused by temperature changes or load changes, and improve the low-temperature crack resistance of concrete. Moreover, rock wool fiber has excellent thermal insulation performance, can effectively improve the heat preservation performance of concrete, reduce heat conduction, and greatly slow down the decrease of the internal temperature of concrete affected by the low-temperature environment, maintaining the temperature required for normal hydration of cement. However, rock wool fiber has the disadvantages of large brittleness and strong water absorption, is easy to break and damage at low temperature, is easy to agglomerate or settle in concrete, and absorbs too much water, affecting cement hydration and its own heat preservation performance. Coating rock wool fiber with styrene-acrylic copolymer emulsion can form a flexible film on the fiber surface, enhance elasticity and toughness and reduce water absorption, and optimize the heat preservation performance of rock wool fiber. Expanded perlite is in granular form, with an irregular and rough surface, which can improve the roughness of the surface of the rock wool fiber coated with the copolymer emulsion, enhance the adhesion between the modified rock wool fiber and the cement matrix, optimize the interfacial transition zone, and at the same time, it has a very low thermal conductivity itself, which can further strengthen the heat preservation performance of rock wool fiber and reduce heat transfer.

[0029] More preferably, the solid content of the styrene-acrylic copolymer emulsion is 40% - 50%, the viscosity is 500 - 1000 mPa•s; the particle size of the expanded perlite is 60 - 70 mesh, and the thermal conductivity (25°C) ≤0.070 W / (m•K).

[0030] Preferably, by weight parts, the concrete comprises the following components: 413 - 486 parts of cementitious material, 710 - 750 parts of sand, 900 - 930 parts of crushed stone, 130 - 160 parts of temperature-responsive modified bamboo aggregate, 5 - 8 parts of water reducer, 145 - 155 parts of water, 5 - 8 parts of succinic anhydride, 1 - 2 parts of fucoidan, and 2 - 3 parts of rock wool fiber.

[0031] More preferably, the cementitious material comprises cement, fly ash, granulated blast furnace slag, and silica fume with a mass ratio of (300 - 340) : (60 - 80) : (50 - 60) : (3 - 6).

[0032] Another object of the present invention is to provide a method for preparing the low-temperature anti-freezing and crack-resistant concrete, which is characterized by comprising the following steps:

[0033] Mix the sand, crushed stone, and cementitious material evenly, add 2 / 4 of the water reducer and 2 / 4 of the water, and after mixing evenly, add the temperature-responsive modified bamboo aggregate, rock wool fiber, succinic anhydride, 1 / 4 of the water reducer, and 1 / 4 of the water, mix evenly, then add fucoidan, 1 / 4 of the water reducer, and 1 / 4 of the water, and mix evenly to obtain a concrete slurry.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] (1) The present invention forms three lines of defense for the anti-freezing and crack-resistant properties of concrete at low temperatures through succinic anhydride, rock wool fiber, temperature-responsive modified bamboo aggregate, and fucoidan, and synergistically improves the low-temperature anti-freezing performance of concrete for winter construction from four aspects: enhancing cement activity and self-insulation, improving the temperature-responsive water absorption and toughening of modified bamboo aggregate, and raising the freezing point of capillary water, thus solving the problems of low strength and easy frost heaving and cracking of traditional concrete in the low-temperature environment in winter.

[0036] (2) The present invention adds temperature-responsive modified bamboo aggregate to the concrete. The temperature-responsive modified bamboo aggregate does not absorb the water in the concrete during the early stage of hydration, does not affect the normal hydration and workability of the concrete. When the external environmental temperature continues to decrease and the cement hydration weakens, resulting in a continuous drop in the internal temperature of the concrete, it can absorb the excess water in the capillary pores of the concrete, reducing the influence of the ice expansion stress of the capillary pore water. And due to the high toughness of the bamboo aggregate, when the water that enters the bamboo aggregate freezes at low temperature and its volume increases, it will not cause a large rigid expansion of the cement matrix, solving the problem of easy cracking of concrete caused by aggregate frost heaving, significantly improving the crack-resistant performance of the concrete at low temperatures. Moreover, when the subsequent environmental temperature warms up and the temperature rises, the bamboo aggregate can also play an internal curing effect.

[0037] (3) The present invention modifies rock wool fibers with styrene - acrylic copolymer emulsion and vitrified microspheres, effectively improving the elasticity and toughness of rock wool fibers, solving the problems of their large brittleness and strong water absorption, enhancing the thermal insulation performance of rock wool fibers, and strengthening the adhesion between the modified rock wool fibers and the cement matrix. Utilizing the good fiber toughness of rock wool fibers, the cracks caused by temperature changes or load changes are reduced, and the low - temperature crack resistance of concrete is improved. More importantly, the thermal insulation performance of concrete is effectively enhanced, heat conduction is reduced, and the decrease in the internal temperature of concrete caused by the low - temperature environment is greatly slowed down, maintaining the temperature required for normal cement hydration.

[0038] (4) The present invention uses succinic anhydride to promote better contact and dispersion between cement particles and water, accelerating the hydration reaction. At the same time, calcium succinate is formed with calcium ions, filling the cement matrix interface and pore voids, improving the early strength and density of concrete. Using fucoidan to lower the freezing point of the aqueous solution in concrete, without affecting the mixing and workability of concrete, and a three - dimensional network structure can be formed in the capillary pores of concrete to "fix" the excess water and disperse and absorb the stress caused by freeze - thaw expansion. Specific embodiments

[0039] The technical solutions of the present invention will be described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0040] The low - temperature anti - freezing and crack - resistant concrete of the present invention comprises the following components in parts by weight: 413 - 486 parts of cementitious materials, 710 - 750 parts of sand, 900 - 930 parts of gravel, 130 - 160 parts of temperature - responsive modified bamboo aggregates, 5 - 8 parts of water - reducing agent, 145 - 155 parts of water, 5 - 8 parts of succinic anhydride, 1 - 2 parts of fucoidan, and 2 - 3 parts of rock wool fibers.

[0041] The cementitious materials include cement, fly ash, granulated blast - furnace slag, and silica fume with a mass ratio of (300 - 340):(60 - 80):(50 - 60):(3 - 6).

[0042] In the following examples and comparative examples, the cement is ordinary Portland cement P•O 52.5; the sand can be natural river sand and / or manufactured sand. When it is a mixture of natural river sand and manufactured sand, the mass ratio of manufactured sand to natural river sand is (2 - 3):(7 - 8), the fineness modulus of natural sand is 2.8 - 3.2, the fineness modulus of manufactured sand is 2.7 - 3.0, and the stone powder content is less than 10%; the gravel is limestone gravel aggregate with a particle size of 10 - 25 mm and an apparent density of 2600 - 2700 kg / m 3, the crushing value ≤ 20%; the water reducing agent is a polycarboxylate water reducing agent, and the water reducing rate is 22 - 28%. The density of rock wool fiber is 20 - 40 kg / m 3 , the thermal conductivity is 0.030 - 0.045 W / (m•K), and the tensile strength ≥ 100 kPa; the solid content of styrene - acrylic copolymer emulsion is 40% - 50%, and the viscosity is 500 - 1000 mPa•s; the particle size of vitrified microspheres is 60 - 70 mesh, and the thermal conductivity (25℃) ≤ 0.070 W / (m•K).

[0043] N - isopropylacrylamide, chemically pure, Shanghai Aladdin Biochemical Technology Co., Ltd.; ammonium persulfate (APS), analytically pure, Shanghai Aladdin Biochemical Technology Co., Ltd.

[0044] The preparation method of bamboo aggregate is as follows: crush mature bamboo into bamboo particles, and then screen out bamboo aggregate raw materials with particle size of 5 - 15 mm in length, 5 - 10 mm in width, and 2 - 3 mm in thickness through a vibrating screen, and then dry the bamboo aggregate raw materials at 65 - 85℃ under ventilation to obtain bamboo aggregate.

[0045] Example 1

[0046] This example provides a low - temperature anti - freezing and crack - resistant concrete, which includes the following components in parts by weight: 300 parts of cement, 60 parts of fly ash, 60 parts of granulated blast - furnace slag, 6 parts of silica fume, 750 parts of sand, 900 parts of crushed stone, 160 parts of temperature - responsive modified bamboo aggregate, 5 parts of water - reducing agent, 155 parts of water, 8 parts of succinic anhydride, 2 parts of fucoidan, and 3 parts of modified rock wool fiber;

[0047] Among them, the preparation method of temperature - responsive modified bamboo aggregate is as follows:

[0048] S1. Dissolve analytically pure N - isopropylacrylamide and dodecyl acrylate in dimethyl sulfoxide solvent according to a molar ratio of 5:1 to obtain a mixed solution. The concentration of N - isopropylacrylamide in the mixed solution is 1.2 mol / L. Then add 1% of ammonium persulfate based on the mass of N - isopropylacrylamide, mix and stir, maintain a temperature of 80℃, and carry out a polymerization reaction for 3 h under nitrogen protection until the monomers completely react. Pour the solution into a large amount of cold ethanol to precipitate for 4 h, filter and wash with ethanol to remove unreacted monomers, and dry to obtain copolymerized N - isopropylacrylamide monomer;

[0049] S2. Mix the copolymerized N - isopropylacrylamide monomer, calcium citrate and water according to a mass ratio of 1:0.2:5, and stir evenly at 15℃ to obtain a copolymerized N - isopropylacrylamide aqueous solution;

[0050] S3. Add bamboo aggregates (the mass of bamboo aggregates is 12% of the mass of the aqueous solution of copolymerized N-isopropylacrylamide) to the aqueous solution of copolymerized N-isopropylacrylamide, then add N,N'-methylenebisacrylamide at 0.4% of the mass of the copolymerized N-isopropylacrylamide monomer and ammonium persulfate at 1.5% of the mass of the copolymerized N-isopropylacrylamide monomer, stir evenly, and stir and react at 50 °C for 4 h to obtain temperature-responsive modified bamboo aggregates;

[0051] The preparation method of the modified rock wool fiber is as follows: Rinse the rock wool fiber with deionized water, and after drying, soak the fiber in a styrene-acrylic copolymer emulsion doped with vitrified microspheres. The doping amount of vitrified microspheres is 8%. Stir at a speed of 5200 rpm for 8 min, take out and dry at 100 °C for 1 h to ensure that the emulsion cures and adheres to the surface of the rock wool fiber, and then it is obtained.

[0052] The preparation method of the low-temperature anti-freezing and crack-resistant concrete in this example is as follows:

[0053] M1. Weigh each component by weight parts;

[0054] M2. Mix sand, gravel, cement, fly ash, granulated blast furnace slag, and silica fume evenly, add 2 / 4 water reducer and 2 / 4 water, and after mixing evenly, add temperature-responsive modified bamboo aggregates, modified rock wool fiber, succinic anhydride, 1 / 4 water reducer and 1 / 4 water, and after mixing evenly, add fucoidan, the remaining 1 / 4 water reducer and 1 / 4 water, and mix evenly to obtain a concrete slurry.

[0055] Example 2

[0056] This example provides a low-temperature anti-freezing and crack-resistant concrete, including the following components in weight parts: 340 parts of cement, 80 parts of fly ash, 50 parts of granulated blast furnace slag, 3 parts of silica fume, 710 parts of sand, 930 parts of gravel, 130 parts of temperature-responsive modified bamboo aggregates, 8 parts of water reducer, 145 parts of water, 5 parts of succinic anhydride, 1 part of fucoidan, and 2 parts of modified rock wool fiber;

[0057] Among them, the preparation method of the temperature-responsive modified bamboo aggregates is as follows:

[0058] S1. Dissolve analytical pure N-isopropylacrylamide and dodecyl acrylate in a dimethyl sulfoxide solvent at a molar ratio of 5.5:1 to obtain a mixed solution. The concentration of N-isopropylacrylamide in the mixed solution is 1.4 mol / L. Then add ammonium persulfate at 0.8% of the mass of N-isopropylacrylamide, mix and stir, keep the temperature at 70 °C, and carry out a polymerization reaction under nitrogen protection for 5 h until the monomers completely react. Pour the solution into a large amount of cold ethanol to precipitate for 2 h, filter and wash with ethanol to remove the unreacted monomers, and dry to obtain the copolymerized N-isopropylacrylamide monomer;

[0059] S2. Mix the copolymerized N-isopropylacrylamide monomer, calcium citrate, and water in a mass ratio of 1:0.1:5, stir evenly at 10 °C to obtain an aqueous solution of copolymerized N-isopropylacrylamide;

[0060] S3. Add bamboo aggregates (the mass of bamboo aggregates is 10% of the mass of the aqueous solution of copolymerized N-isopropylacrylamide) to the aqueous solution of copolymerized N-isopropylacrylamide, then add N,N'-methylenebisacrylamide at 0.2% of the mass of the copolymerized N-isopropylacrylamide monomer and ammonium persulfate at 1.0% of the mass of the copolymerized N-isopropylacrylamide monomer, stir evenly, and stir and react at 40 °C for 5 h to obtain temperature-responsive modified bamboo aggregates.

[0061] The preparation method of the modified rock wool fiber is as follows: Rinse the rock wool fiber with deionized water, soak the fiber in a styrene-acrylic copolymer emulsion containing expanded perlite after drying, the dosage of expanded perlite is 10%, stir at a speed of 4000 rpm for 15 min, take it out and dry it at 80 °C for 2 h to ensure that the emulsion solidifies and adheres to the surface of the rock wool fiber, then it is obtained.

[0062] The preparation method of the low-temperature anti-freezing and crack-resistant concrete in this example is the same as that in Example 1.

[0063] Example 3

[0064] The low-temperature anti-freezing and crack-resistant concrete in this example is basically the same as that in Example 1, the difference is only that unmodified rock wool fiber is used in this example.

[0065] The preparation method of the low-temperature anti-freezing and crack-resistant concrete in this example is as follows:

[0066] M1. Weigh each component by weight parts;

[0067] M2. Mix sand, gravel, cement, fly ash, granulated blast furnace slag, and silica fume evenly, add 2 / 4 water reducer and 2 / 4 water, after mixing evenly, add temperature-responsive modified bamboo aggregates, rock wool fiber, succinic anhydride, 1 / 4 water reducer and 1 / 4 water, mix evenly, then add fucoidan, the remaining 1 / 4 water reducer and 1 / 4 water, mix evenly to obtain a concrete slurry.

[0068] Comparative Example 1

[0069] The concrete in this comparative example is basically the same as that in Example 1, the difference is that bamboo aggregates are used in this comparative example to replace the temperature-responsive modified bamboo aggregates, and the dosage remains unchanged.

[0070] Comparative Example 2

[0071] The concrete in this comparative example is basically the same as that in Example 1, except that butyl acrylate is used to replace dodecyl acrylate when preparing the temperature-responsive modified bamboo aggregates in this comparative example.

[0072] Comparative Example 3

[0073] The concrete in this comparative example is basically the same as that in Example 1, except that in this comparative example when preparing the temperature-responsive modified bamboo aggregates, step S3 is as follows: The bamboo aggregates are immersed in the copolymerized N-isopropylacrylamide aqueous solution for 30 min.

[0074] Comparative Example 4

[0075] The concrete in this comparative example is basically the same as that in Example 1, except that in this comparative example when preparing the temperature-responsive modified bamboo aggregates, the bamboo aggregates are replaced with limestone gravel of the same weight fraction, and the physical and mechanical property indexes of the limestone gravel are the same as those of the limestone gravel in Example 1.

[0076] Comparative Example 5

[0077] The concrete in this comparative example is basically the same as that in Example 1, except that the concrete in this comparative example lacks fucoidan.

[0078] Comparative Example 6

[0079] The concrete in this comparative example is basically the same as that in Example 1, except that the concrete in this comparative example lacks succinic anhydride.

[0080] Comparative Example 7

[0081] The concrete in this comparative example is basically the same as that in Example 1, except that in this comparative example, polyvinyl alcohol fibers are used to replace the modified rock wool fibers.

[0082] Test Example

[0083] (1) Refer to GB / T14685-2022 "Crushed Stones and Cobbles for Construction" to test the water absorption rate of the aggregates at different water temperatures. The water temperatures are set to 33 °C, 20 °C, 8 °C, and 4 °C in sequence. The modified bamboo aggregates in Examples 1 to 2, the bamboo aggregates in Comparative Examples 1 to 3, and the limestone aggregates in Comparative Example 4 are tested respectively. The test results are shown in Table 1.

[0084] Table 1 Test Results of Aggregate Water Absorption Performance

[0085]

[0086] As can be seen from the test results in Table 1, the water absorption rates of the temperature-responsive modified bamboo aggregates in Example 1 and Example 2 are both 0 at water temperatures of 33°C and 20°C, and their aggregate surfaces exhibit an excellent hydrophobic state. When the water temperature is 8°C, the water absorption rate changes to around 35%, and at a water temperature of 4°C, the water absorption rates are 74% and 76% respectively. This indicates that when the temperature-responsive modified bamboo aggregates are around 5 - 10°C, the copolymerized N-isopropylacrylamide on their surfaces undergoes a hydrophilic-hydrophobic transition, and the modified bamboo aggregates start to absorb water at this temperature. For the ordinary bamboo aggregates in Comparative Example 1, their water absorption rates remain above 94% at different water temperatures, which will directly cause them to absorb a large amount of water in the concrete during the early mixing stage, and they are unable to absorb the water in the capillary pores of the hardened concrete at low temperatures in the later stage; for the modified bamboo aggregates in Comparative Example 2, the water absorption rate has reached 42% at a water temperature of 20°C. This shows that when butyl acrylate is used to replace dodecyl acrylate in the preparation of temperature-responsive modified bamboo aggregates, the phase transition temperature can only be adjusted to above 20°C, resulting in the bamboo aggregates starting to absorb the water required for hydration in the later stage of cement hydration when the internal temperature of the concrete drops during the hardening and forming process, and thus the concrete strength and density are reduced. Moreover, when the environmental temperature further drops, the bamboo aggregates basically do not have the function of absorbing the water in the capillary pores of the concrete; the test results in Comparative Example 3 prove that without using cross-linking agents and initiators for cross-linking and strengthening the polymerization of copolymerized N-isopropylacrylamide and bamboo aggregates, it will affect the structural stability and physical properties of the temperature-responsive monomer gel. The gel body is relatively loose, resulting in slight water absorption of the aggregates in the early stage of hydration and a decrease in the water absorption rate in the later stage; for the limestone crushed stone aggregates used in Comparative Example 4, their water absorption rates are basically the same at different temperatures. At the same time, freeze-thaw tests were carried out on the bamboo aggregates in Examples 1 - 2 and the limestone crushed stone aggregates in Comparative Example 4. After the aggregates were saturated with water and then cooled to -18°C and maintained for 36h, and then thawed in an environment of 15°C, after 20 cycles, it was found that there were no obvious changes on the surfaces of the bamboo aggregates in Examples 1 - 2, while expansion cracks appeared on the surface of the limestone aggregates in Comparative Example 4.

[0087] (2)Performance testing of concrete

[0088] Mechanical properties, impermeability, freeze-thaw resistance, and crack resistance tests were carried out on the concrete in the examples and comparative examples, and the test results are shown in Table 2.

[0089] The mechanical properties of concrete were tested in accordance with the Standard Test Method for Physical and Mechanical Properties of Ordinary Concrete (GB / T 50081-2019); the impermeability and early anti-cracking properties of concrete were tested in accordance with the Standard Test Method for Long-Term Performance and Durability of Ordinary Concrete (GB / T 50082-2009). Before the test, the specimens were subjected to a low-temperature environment. Specifically, after the specimens were formed, they were cured at a negative temperature (-10°C) for 7 days and then transferred to standard curing until 28 days. The "seepage height method" was used to measure the average seepage height of concrete under a constant water pressure to represent the water penetration resistance of concrete. The larger the average seepage height value of the specimen, the worse the water penetration resistance of the specimen. The early anti-cracking property of concrete was measured by the "anti-cracking flat plate test", and the test results were expressed by the total cracking area index per unit area calculated. The smaller the value, the better the anti-cracking property;

[0090] The specific test method for the frost resistance test of concrete is as follows: Make concrete standard specimens of 150 mm × 150 mm × 150 mm. Cure the specimens at a temperature of 2 ± 0.5°C for 7 days, then place the specimens in a rubber specimen box and fill it with water so that the specimens are soaked in water at a water temperature of 2 ± 0.5°C for 24 hours, and then cure them in a low-temperature environment of -15°C until 28 days. Test the compressive strength of the specimens and calculate the ratio of the low-temperature compressive strength of concrete P = (compressive strength after low-temperature curing / compressive strength after conventional curing) × 100%. The smaller the P value, the greater the strength loss rate and the worse the frost resistance of the concrete.

[0091] Table 2 Test Results of Concrete Properties

[0092]

[0093] The present invention forms three lines of defense for the frost resistance and crack prevention of concrete at low temperatures through succinic anhydride, rock wool fiber, temperature-responsive modified bamboo aggregates, and fucoidan. It synergistically improves the low-temperature frost resistance of concrete from four aspects: enhancing cement activity and self-insulation, improving the temperature-responsive water absorption and toughening of modified bamboo aggregates, and raising the freezing point of capillary water. From the test results in Table 2, it can be seen that the overall performance test results of Examples 1 to 3 are relatively good. The 28-day compressive strength is generally between 48.0 and 49.3 MPa, the seepage height is generally between 31.2 and 38.4 mm, and the total cracking area per unit area is generally between 79 and 94 mm 2 / m 2 , the anti-cracking grades are all Grade V, and the low-temperature compressive strength ratio is 90.3 to 94.1%. Among them, Example 3 uses unmodified rock wool fiber. Although each index is slightly worse than that of Example 1, the overall gap is small.

[0094] Comparative Example 1 uses bamboo aggregates instead of temperature-responsive modified bamboo aggregates. The bamboo aggregates are not treated with temperature-responsive modification. In the early stage of concrete mixing, they absorb more water, affecting the normal hydration of concrete. In the later stage, they cannot effectively absorb the excess water in the capillary pores of concrete, reducing the ice expansion stress caused by the freezing of capillary water. Therefore, compared with the performance test results of Example 1, there are obvious gaps in terms of density, crack resistance, and frost resistance. The low-temperature compressive strength ratio is only 82.1%, and the crack resistance grade is Grade IV.

[0095] In Comparative Example 2, butyl acrylate is used to replace dodecyl acrylate when preparing temperature-responsive modified bamboo aggregates. Its low-temperature compressive strength ratio is 5.7 percentage points lower than that of Example 1, and its frost resistance is relatively poor. This is mainly because compared with the shorter alkyl chain of butyl acrylate, the long-chain alkyl acrylate of dodecyl acrylate can adjust the critical temperature of the temperature-responsive monomer to a lower value, enabling the temperature-responsive aggregate to start the hydrophobic and water-absorbing conversion at a lower temperature. Then, without affecting the hydration of concrete, it can absorb the water in the capillary pores of concrete.

[0096] The frost resistance and crack resistance of Comparative Example 3 are significantly decreased compared with those of Example 1. This shows that only soaking the bamboo aggregates in the aqueous solution of copolymerized N-isopropylacrylamide for preparation, the structure of the copolymerized N-isopropylacrylamide is relatively loose and unstable, and no good cross-linked structure is formed with the natural fibers on the surface of the bamboo aggregates, so the modified bamboo aggregates fail to fully play an effective role.

[0097] The crushed stones in Comparative Example 4 are all conventional limestone crushed aggregates. Compared with Example 1, its compressive strength under normal curing is better, but the water seepage height of the specimens under low-temperature curing increases significantly, and the total cracking area per unit area also increases significantly. Moreover, the low-temperature compressive strength ratio is 76.2%. The anti-seepage performance, crack resistance, and frost resistance of the concrete at low temperature are all poor. This is because the concrete using rigid aggregates is prone to rigid expansion at low temperature, and the concrete is prone to cracking.

[0098] Compared with Example 1, Comparative Examples 5 to 7 are respectively the cases where fucoidan is missing, succinic anhydride is missing, and modified rock wool fibers are replaced with polyvinyl alcohol fibers in the concrete. The density, low-temperature frost resistance, and crack resistance of the concrete in the comparative examples all show varying degrees of decline compared with those of Example 1. This indicates that the use of fucoidan, succinic anhydride, and modified rock wool fiber materials in the present invention can synergistically improve the frost resistance and crack prevention performance of concrete at low temperature with the temperature-responsive modified bamboo aggregates.

[0099] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A low-temperature antifreeze and crack-resistant concrete, characterized in that: The raw materials include the following components: Cementitious material, sand, crushed stone, temperature-responsive modified bamboo aggregate, water reducing agent, water, succinic anhydride, fucoidan, rock wool fiber; The preparation method of the temperature-responsive modified bamboo aggregate comprises the following steps: S1. N-isopropyl acrylamide and dodecyl acrylate are dissolved in a solvent, an initiator is added, and a polymerization reaction is carried out under an inert gas atmosphere, and a copolymer N-isopropyl acrylamide monomer is obtained after precipitation, separation and drying; S2. The copolymerized N-isopropylacrylamide monomer obtained in step S1 is dissolved in an aqueous solution of calcium citrate, and stirred to obtain an aqueous solution of copolymerized N-isopropylacrylamide; S3. The bamboo aggregate, the copolymerized N-isopropylacrylamide aqueous solution, the crosslinking agent and the initiator are mixed, and reacted at 40-50° C. for 4-5 hours to obtain the temperature-responsive modified bamboo aggregate.

2. The low-temperature antifreeze and crack-resistant concrete according to claim 1, characterized in that: The molar ratio of N-isopropylacrylamide to dodecyl acrylate is (5-5.5):

1.

3. The low-temperature antifreeze and crack-resistant concrete according to claim 1, characterized in that: The polymerization reaction temperature is 70-80° C. and the reaction time is 3-5 hours.

4. The low-temperature antifreeze and crack-resistant concrete according to claim 1, characterized in that: The mass ratio of the N-isopropylacrylamide monomer, calcium citrate and water is 1:(0.1-0.2):

5.

5. The low-temperature antifreeze and crack-resistant concrete according to claim 1, characterized in that: The initiator 1 and the initiator 2 are ammonium persulfate or potassium persulfate, the mass of the initiator 1 is 0.8% to 1.0% of the mass of N-isopropylacrylamide; the mass of the initiator 2 is 1.0% to 1.5% of the mass of the copolymerized N-isopropylacrylamide monomer.

6. The low-temperature antifreeze and crack-resistant concrete according to claim 1, characterized in that: The cross-linking agent is N,N'-methylenebisacrylamide, and the mass of the cross-linking agent is 0.2% to 0.4% of the mass of the copolymerized N-isopropylacrylamide monomer.

7. The low-temperature antifreeze and crack-resistant concrete according to claim 1, characterized in that: The preparation method of the bamboo aggregate comprises the following steps: crushing mature bamboo into bamboo particles, screening to obtain bamboo aggregate raw materials with a length of 5-15 mm, a width of 5-10 mm, and a thickness of 2-3 mm, and drying to obtain the bamboo aggregate.

8. The low-temperature antifreeze and crack-resistant concrete according to claim 1, characterized in that: Before use, the rock wool fiber is soaked in styrene-acrylic copolymer emulsion mixed with glass microbeads, stirred evenly, taken out and dried.

9. The low-temperature antifreeze and crack-resistant concrete according to claim 1, characterized in that: The concrete includes the following components by weight: 413-486 parts of cementitious materials, 710-750 parts of sand, 900-930 parts of crushed stone, 130-160 parts of temperature-responsive modified bamboo aggregate, 5-8 parts of water reducing agent, 145-155 parts of water, 5-8 parts of succinic anhydride, 1-2 parts of fucoidan, and 2-3 parts of rock wool fiber.

10. The method for preparing low-temperature antifreeze and crack-resistant concrete according to any one of claims 1 to 9, characterized in that: The following steps are involved: The sand, gravel and cementitious material are mixed evenly, 2 / 4 of the water reducer and 2 / 4 of the water are added and mixed evenly, then the temperature-responsive modified bamboo aggregate, rock wool fiber, succinic anhydride, 1 / 4 of the water reducer and 1 / 4 of the water are added and mixed evenly, then fucoidan, 1 / 4 of the water reducer and 1 / 4 of the water are added and mixed evenly to obtain concrete slurry.

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