Low-carbon polypropylene fiber concrete with high toughness and crack resistance

Abstract

The invention discloses low-carbon polypropylene fiber concrete with high toughness and crack resistance, and belongs to the field of concrete. The raw materials comprise, by mass, 100-110 parts of coarse aggregate, 60-70 parts of fine aggregate, 20-30 parts of cement, 80-120 parts of fly ash, 15-20 parts of modified polypropylene fiber, and 2-3 parts of a water reducing agent; the use amount of the water meets the condition that the water-cement ratio is 0.3-0.4; according to the method, the surface of the polypropylene fiber is subjected to controllable micro-etching through concentrated phosphoric acid, a microscopic gully and micropore structure is formed, effective roughening of the surface of the fiber is achieved, the specific surface area of the fiber is greatly increased, the fiber can generate a stronger mechanical meshing effect in concrete, pulling out of the fiber is effectively prevented, and the tensile strength of the fiber is improved. And in the subsequent silane coupling agent treatment, a stable'molecular bridge 'is constructed between the fiber surface subjected to acid etching activation and an inorganic cement matrix, and firm Si-O-Si covalent bond connection is formed, so that the problem of weak bonding force between the common polypropylene fiber and the matrix interface is solved.

Description

Technical Field

[0001] This invention relates to the field of concrete technology, and more specifically, to a low-carbon polypropylene fiber concrete with high toughness and crack resistance. Background Technology

[0002] As the world's most widely used building material, concrete has prominent problems of high energy consumption and high carbon emissions in the production process of cement, the core cementitious material. In order to reduce the carbon dioxide emissions of concrete, the industry generally uses large amounts of industrial solid waste such as fly ash to replace cement to prepare "low-carbon concrete". However, fly ash has low early hydration activity, which leads to slow early strength development of concrete and significantly increased autogenous shrinkage and drying shrinkage, thereby aggravating the risk of early cracking and seriously affecting the safety and durability of the structure.

[0003] To improve the crack resistance of concrete, the addition of polypropylene fibers has become a common technical means. Polypropylene fibers can form a three-dimensional random support system inside the concrete, effectively inhibiting plastic shrinkage cracks. However, due to the smooth surface and chemical inertness of polypropylene fibers, the bonding force between them and the cement matrix is ​​weak. Under load, the fibers are prone to "debonding" or pull-out, making it difficult to fully exert their bridging and toughening effects. As a result, their effect on inhibiting structural cracks in hardened concrete is limited. In existing technologies, it is difficult to balance interfacial strength and fiber integrity through physical mixing or simple surface treatment (such as plasma or alkaline etching). Moreover, the process is complex, energy-intensive, and difficult to apply on a large scale. Summary of the Invention

[0004] In view of the problems existing in the prior art, the purpose of this invention is to provide a low-carbon polypropylene fiber concrete with high toughness and crack resistance, thereby solving the above-mentioned technical problems.

[0005] To solve the above problems, the present invention adopts the following technical solution.

[0006] A low-carbon polypropylene fiber reinforced concrete with high toughness and crack resistance, comprising, by weight, the following raw materials:

[0007] 100-110 parts coarse aggregate; 60-70 parts fine aggregate; 20-30 parts cement; 80-120 parts fly ash; 15-20 parts modified polypropylene fiber; 2-3 parts water-reducing agent; and water, the amount of which meets the requirement of a water-cement ratio of 0.3-0.4.

[0008] The modified polypropylene fiber is prepared by a method comprising the following steps:

[0009] S1: Place polypropylene fibers in concentrated phosphoric acid with a mass concentration of not less than 85% and soak at room temperature for 1.5 to 2 hours. Then wash with water until neutral and dry to obtain pretreated polypropylene fibers.

[0010] S2: The pretreated polypropylene fiber is placed in a silane coupling agent solution with a mass fraction of 3% to 5% and soaked at room temperature for 1.5 to 2 hours, then washed and dried to obtain the modified polypropylene fiber.

[0011] The goal of low carbon emissions is achieved by replacing cement with a large amount of fly ash; the modified polypropylene fiber is modified by acid etching-coupling synergistic modification, which forms a micro-rough structure on the fiber surface and introduces active groups, significantly improving the interfacial bonding strength between the fiber and the cement matrix.

[0012] In a preferred embodiment of the present invention, the coarse aggregate is continuously graded crushed stone with a particle size of 5-10 mm and a crushing index of no more than 10%.

[0013] In a preferred embodiment of the present invention, the fine aggregate is medium sand with a fineness modulus of 2.3 to 2.6 and a mud content of no more than 1.0%.

[0014] In a preferred embodiment of the present invention, the cement is ordinary Portland cement with a strength grade of 42.5.

[0015] In a preferred embodiment of the present invention, the fly ash is Class I fly ash with a loss on ignition of no more than 5%.

[0016] In a preferred embodiment of the present invention, the water-reducing agent is a polycarboxylate-based water-reducing agent.

[0017] In a preferred embodiment of the present invention, the silane coupling agent in step S2 is an aminosilane coupling agent KH-550.

[0018] In a preferred embodiment of the present invention, the concrete preparation method includes: first, dry mixing coarse aggregate, fine aggregate, cement, fly ash and modified polypropylene fiber for 2 to 3 minutes, then adding water-reducing agent dissolved in water and wet mixing for 3 to 5 minutes.

[0019] In a preferred embodiment of the present invention, the volumetric content of the modified polypropylene fiber in concrete is 0.15% to 0.2%.

[0020] In a preferred embodiment of the present invention, the concrete has a 28-day compressive strength of not less than 65 MPa, a 28-day splitting tensile strength of not less than 4.5 MPa, and a peak tensile strain of not less than 6.5%.

[0021] Compared with the prior art, the technical solution provided by the present invention has at least the following beneficial effects:

[0022] (1) This invention uses concentrated phosphoric acid to controllably micro-etch the surface of polypropylene fibers, forming micro-grooves and micropores, which effectively roughens the fiber surface. This greatly increases the specific surface area of ​​the fibers, enabling them to generate stronger mechanical interlocking in concrete and effectively preventing fiber pull-out. Furthermore, in the subsequent silane coupling agent treatment, a stable "molecular bridge" is constructed between the acid-etched and activated fiber surface and the inorganic cement matrix, forming a strong Si-O-Si covalent bond. This synergistic effect of chemical bonding and physical anchoring fundamentally solves the problem of weak interfacial adhesion between ordinary polypropylene fibers and the matrix.

[0023] (2) By adding a large amount of fly ash to concrete to replace cement, the amount of cement used is significantly reduced, thereby significantly reducing carbon emissions. At the same time, by using high-performance modified polypropylene fibers, the inherent defects of high fly ash concrete, such as large shrinkage and easy cracking, are effectively suppressed, effectively solving the performance decline problem that is usually associated with low-carbon materials, and achieving synergistic enhancement of "low carbon" and "high toughness and crack resistance". Detailed Implementation

[0024] Various exemplary embodiments of the present invention are now described in detail. This detailed description should not be considered as a limitation of the invention, but rather as a more detailed description of certain aspects, features, and embodiments of the invention. It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, regarding numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included within the scope of this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0025] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0026] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0027] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0028] This invention provides a low-carbon polypropylene fiber reinforced concrete with high toughness and crack resistance, wherein the raw materials, by weight, include:

[0029] 100-110 parts coarse aggregate; 60-70 parts fine aggregate; 20-30 parts cement; 80-120 parts fly ash; 15-20 parts modified polypropylene fiber; 2-3 parts water-reducing agent; and water, the amount of which meets the requirement of a water-cement ratio of 0.3-0.4.

[0030] The modified polypropylene fiber is prepared by a method including the following steps:

[0031] S1: Place polypropylene fibers in concentrated phosphoric acid with a mass concentration of not less than 85% and soak at room temperature for 1.5 to 2 hours. Then wash with water until neutral and dry to obtain pretreated polypropylene fibers.

[0032] S2: Place the pretreated polypropylene fiber in a silane coupling agent solution with a mass fraction of 3% to 5%, soak it at room temperature for 1.5 to 2 hours, then wash and dry it to obtain the modified polypropylene fiber.

[0033] The goal of low carbon is achieved by replacing cement with a large amount of fly ash; the modified polypropylene fiber is modified by acid etching-coupling synergistic modification, which forms a micro-rough structure on the fiber surface and introduces active groups, significantly improving the interfacial bonding strength between the fiber and the cement matrix.

[0034] In this invention, during the preparation of pretreated polypropylene fibers, the polypropylene fibers are immersed in 85% concentrated phosphoric acid at room temperature for 100 minutes. This allows for controlled micro-etching of the smooth polypropylene fiber surface. The concentrated phosphoric acid corrodes the amorphous regions on the fiber surface, forming fine grooves and micropores, thus roughening the fiber surface. This roughness significantly increases the fiber's surface area, providing strong support for subsequent mechanical engagement (mechanical anchoring force) with the cement matrix. Simultaneously, phosphoric acid can weakly interact with residual oxide groups (such as hydroxyl and carbonyl groups) on the fiber surface, or slightly introduce phosphate ester groups, increasing its surface polarity and improving hydrophilicity. For example, phosphoric acid concentrations below 85% (e.g., 50%) are insufficient for etching, while excessively high temperatures or prolonged immersion times can damage the fiber's mechanical properties. Therefore, this invention preferably limits the concentration of concentrated phosphoric acid to not less than 85%, the immersion temperature to room temperature, and the immersion time to 1.5–2 hours. After soaking, the fibers are removed and washed repeatedly with deionized water until the filtrate is neutral. Then, they are placed in a ventilated place to air dry naturally to obtain pretreated polypropylene fibers.

[0035] After the pretreatment in step S1, the fiber surface is more easily bonded to the silane coupling agent. The silanol groups (-Si-OH) generated after the hydrolysis of the silane coupling agent can undergo a condensation reaction with the active sites on the fiber surface (such as the hydroxyl groups added after acid etching) to form strong Si-O-Si chemical bonds. At the same time, the introduction of active groups such as amino and epoxy groups improves the polarity of the fiber surface, enhances hydrophilicity, and is beneficial to the bonding with cement slurry. Furthermore, the silane coupling agent can form a micro-rough structure on the fiber surface, increase the specific surface area, and improve the mechanical interlocking effect. In addition, the silane coupling agent can act as a "molecular bridge" to build a covalent bond between the polypropylene fiber and the cement matrix, improve the interfacial bonding strength, and at the same time inhibit the shrinkage cracks of the fiber in the cement mortar and reduce the cracking index. During this process, a low mass fraction of the silane coupling agent solution will affect the modification effect, while a high mass fraction will cause the coupling agent to agglomerate, thus reducing the modification effect. Similarly, excessively high or short / long soaking times in the silane coupling agent solution will also affect the modification effect. Therefore, this invention preferably limits the mass fraction of the silane coupling agent solution to 3%–5%, the soaking temperature to room temperature, and the soaking time to 1.5–2 hours to ensure a thorough and uniform modification reaction. After soaking, the fibers are removed and washed three times with deionized water to remove unreacted coupling agent, then air-dried to obtain the final modified polypropylene fiber.

[0036] Regarding the preparation of concrete, weigh the raw materials according to the above proportions. First, put the coarse aggregate, fine aggregate, cement, fly ash and modified polypropylene fiber into a forced mixer and dry mix for 2 to 3 minutes to ensure that the fiber is evenly dispersed and there is no clumping. Then, dissolve the water-reducing agent in the mixing water and slowly add it to the mixer. Continue to wet mix for 3 to 5 minutes until the mixture is uniform and has suitable fluidity and cohesiveness, thus obtaining the low-carbon polypropylene fiber concrete of the present invention.

[0037] In some embodiments of the present invention, the water is washed until neutral, deionized water is used for washing, and the drying method is natural air drying.

[0038] Pretreated polypropylene fibers and modified polypropylene fibers were prepared and dried naturally.

[0039] In a preferred embodiment of the present invention, the coarse aggregate is continuously graded crushed stone with a particle size of 5-10 mm and a crushing index of no more than 10%.

[0040] In a preferred embodiment of the present invention, the fine aggregate is medium sand with a fineness modulus of 2.3 to 2.6 and a mud content of no more than 1.0%.

[0041] In a preferred embodiment of the present invention, the cement is ordinary Portland cement with a strength grade of 42.5.

[0042] In a preferred embodiment of the present invention, the fly ash is Class I fly ash, and its loss on ignition is not greater than 5%.

[0043] In a preferred embodiment of the present invention, the water-reducing agent is a polycarboxylate-based water-reducing agent.

[0044] In a preferred embodiment of the present invention, the silane coupling agent in step S2 is aminosilane coupling agent KH-550.

[0045] In a preferred embodiment of the present invention, the concrete preparation method includes: first, dry mixing coarse aggregate, fine aggregate, cement, fly ash and modified polypropylene fiber for 2 to 3 minutes, then adding water-reducing agent dissolved in water and wet mixing for 3 to 5 minutes.

[0046] In a preferred embodiment of the present invention, the volumetric content of modified polypropylene fiber in concrete is 0.15% to 0.2%.

[0047] In a preferred embodiment of the present invention, the 28-day compressive strength of the concrete is not less than 65 MPa, the 28-day splitting tensile strength is not less than 4.5 MPa, and the peak tensile strain is not less than 6.5%.

[0048] In a preferred embodiment of the present invention, the coarse aggregate is continuously graded crushed stone with a particle size of 5-10 mm and a crushing index of no more than 10%. The fine aggregate is medium sand with a fineness modulus of 2.3-2.6 and a mud content of no more than 1.0%. The cement is ordinary Portland cement with a strength grade of 42.5. The fly ash is Grade I fly ash with a loss on ignition of no more than 5%. The water-reducing agent is a polycarboxylate-based water-reducing agent. The silane coupling agent in step S2 is aminosilane coupling agent KH-550. The concrete preparation method includes: first, dry mixing the coarse aggregate, fine aggregate, cement, fly ash, and modified polypropylene fiber for 2-3 minutes, then adding the water-reducing agent dissolved in water and wet mixing for 3-5 minutes. The volumetric dosage of modified polypropylene fiber in the concrete is 0.15%-0.2%. The 28-day compressive strength of the concrete is no less than 65 MPa, the 28-day splitting tensile strength is no less than 4.5 MPa, and the peak tensile strain is no less than 6.5%.

[0049] During the mild acid etching pretreatment, polypropylene fibers are immersed in concentrated phosphoric acid with a mass concentration of not less than 85% for 1.5–2 hours at room temperature. This process is mild acid etching, utilizing the selective etching ability of concentrated phosphoric acid to etch the amorphous regions on the surface of the polypropylene fibers, forming uniform and controllable micropores and etching patterns, thereby achieving microscopic roughening of the fiber surface. This roughness greatly increases the specific surface area of ​​the fibers, laying the foundation for strong mechanical anchoring force with the cement matrix. Compared with traditional strong acid treatment, the room temperature conditions and specific time window used in this invention can effectively etch while maximizing the preservation of the fiber's intrinsic strength, avoiding excessive degradation. After treatment, the fibers are removed, thoroughly washed with deionized water until neutral, and then air-dried to obtain pretreated polypropylene fibers.

[0050] The high-efficiency silane coupling mentioned in step two refers to immersing the pretreated polypropylene fibers in a 3%–5% (w / w) aqueous solution of a silane coupling agent (such as KH-550) for 1.5–2 hours at room temperature. After acid etching pretreatment, the fiber surface is hydroxylated, significantly improving its activity. The purpose of this step is to utilize the silane coupling agent as a "molecular bridge," where the silanol groups generated after hydrolysis undergo a condensation reaction with the hydroxyl groups on the fiber surface, forming stable Si-O-Si covalent bonds. Simultaneously, the organic functional groups at the other end of the silane can bond with cement hydration products. This process significantly improves the chemical bonding efficiency, constructing a robust Si-O-Si interface layer between the fiber and the cement matrix, thereby greatly enhancing the fiber-matrix adhesion. After treatment, the fibers are removed, washed, and dried to obtain the final modified polypropylene fibers.

[0051] Unless otherwise specified, the technical solutions of this invention are all conventional solutions in the field, and the reagents or raw materials used are all purchased from commercial channels or have been disclosed unless otherwise specified.

[0052] In this embodiment of the invention, the coarse aggregate used is gravel with a particle size of 5-10 mm; the fine aggregate is medium sand with a fineness modulus of 2.3-2.6, a mud content of no more than 1.0%, and a particle size of no more than 5 mm; the cement used is ordinary Portland cement with a strength grade of 42.5; the fly ash used is Grade I fly ash; and the water-reducing agent used is polycarboxylate water-reducing agent.

[0053] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0054] Example 1

[0055] This embodiment provides a low-carbon polypropylene fiber concrete with high toughness and crack resistance. The raw materials are composed of the following components by weight: 105 parts coarse aggregate, 65 parts fine aggregate, 25 parts cement, 100 parts fly ash, 18 parts modified polypropylene fiber, 3 parts water-reducing agent, and water; water is added according to a water-cement ratio of 0.35.

[0056] The modified polypropylene fiber is prepared through the following steps:

[0057] S1. Soak polypropylene fibers in concentrated phosphoric acid with a mass concentration of 85% for 100 minutes, then wash with deionized water until neutral, and dry to obtain pretreated polypropylene fibers.

[0058] S2. The pretreated polypropylene fiber was immersed in a 4% (w / w) solution of silane coupling agent KH-550 (solvent is water) for 100 min, then washed three times with deionized water and dried to obtain modified polypropylene fiber.

[0059] The concrete preparation process strictly follows the preferred preparation method of this invention: First, 105 parts coarse aggregate, 65 parts fine aggregate, 25 parts cement, 100 parts fly ash and 18 parts modified polypropylene fiber are put into a forced mixer and dry-mixed for 2.5 minutes to ensure that the fiber is evenly dispersed and there is no clumping; then, 3 parts water-reducing agent are dissolved in the mixing water according to the amount determined according to the water-cement ratio of 0.35, and slowly added to the mixer, and wet-mixed for 4 minutes until the mixture is uniform and has suitable fluidity and cohesiveness, thus obtaining the low-carbon polypropylene fiber concrete of this embodiment.

[0060] Comparative Example 1

[0061] The only difference from Example 1 is that the addition of modified polypropylene fibers is omitted. All other raw material components (105 parts coarse aggregate, 65 parts fine aggregate, 25 parts cement, 100 parts fly ash, 3 parts water-reducing agent, and water with a water-cement ratio of 0.35) and preparation process are completely consistent with Example 1. This comparative example is mainly used to verify the core role of modified polypropylene fibers in the mechanical properties and crack resistance of concrete systems.

[0062] Comparative Example 2

[0063] The only difference from Example 1 is that the modified polypropylene fiber underwent only acid etching as a single modification treatment. The specific preparation steps were: immersing the polypropylene fiber in 85% concentrated phosphoric acid for 100 minutes, then washing it with deionized water until neutral, and drying it to obtain the modified polypropylene fiber. The remaining raw material components and preparation process remained consistent with Example 1. This comparative example is used to compare the difference in fiber reinforcement effects between single acid etching modification and a two-step "acid etching-coupling" modification, highlighting the key value of coupling treatment in improving interfacial adhesion.

[0064] Comparative Example 3

[0065] The only difference from Example 1 is that the modified polypropylene fiber underwent only single coupling modification treatment. The specific preparation steps were as follows: the polypropylene fiber was immersed in a 4% (w / w) solution of silane coupling agent KH-550 (water as solvent) for 100 min, then washed three times with deionized water and dried to obtain the modified polypropylene fiber. The remaining raw material components and preparation process were the same as in Example 1. This comparative example aims to verify the difference in effect between single coupling modification and the "acid etching-coupling" two-step modification, emphasizing the importance of acid etching pretreatment in improving the efficiency of the coupling agent.

[0066] To comprehensively evaluate the superiority of the present invention, the concrete prepared in Example 1 and Comparative Examples 1-3 were subjected to systematic performance testing. The testing methods strictly followed relevant standards and specifications, as detailed below:

[0067] The 3d, 7d, and 28d compressive strength and 28d splitting tensile strength of concrete were tested in accordance with GB / T50081-2002 "Standard for Test Methods of Mechanical Properties of Ordinary Concrete". At the same time, the crack formation of concrete during the curing process was observed.

[0068] Axial tensile tests were conducted on dog bone specimens according to JSCE specifications. An electronic universal testing machine was used for loading in displacement control mode at a loading rate of 0.5 mm / min. Axial strain was measured in the middle 80 mm region of the specimen using two symmetrical displacement gauges. Data was collected using a stress sensor and displacement gauges to calculate the peak tensile strain.

[0069] Record the maximum temperature difference of concrete during the curing process to help evaluate the crack resistance of concrete. The temperature difference is directly related to the internal stress of concrete. The smaller the maximum temperature difference, the stronger the crack resistance of concrete.

[0070] The test results are shown in Table 1 below:

[0071] Table 1. Concrete performance test results of Example 1 and Comparative Examples 1-3

[0072] Table 1 Test Results

[0073] Based on the above test data, Example 1 differs from Comparative Examples 1-3 in the following ways:

[0074] Comparison of compressive strength:

[0075] Early strength (3d, 7d): The 3d compressive strength of Example 1 reached 20.1 MPa, and the 7d compressive strength was 33.6 MPa, significantly higher than the three comparative examples. Comparative Example 1 (without fibers) had the lowest 3d and 7d compressive strengths, at only 15.4 MPa and 21.8 MPa, respectively. This indicates that the addition of modified polypropylene fibers can effectively improve the early strength of concrete. This is mainly because the fibers form a three-dimensional randomized support system inside the concrete, which inhibits early plastic shrinkage, reduces the generation of internal microcracks, and provides a good structural foundation for strength development.

[0076] The early strengths of Comparative Example 2 (single acid-etched modified fiber) and Comparative Example 3 (single coupling modified fiber) were between those of Example 1 and Comparative Example 1. Specifically, the 3-day and 7-day compressive strengths of Comparative Example 2 were 17.2 MPa and 24.3 MPa, respectively, while those of Comparative Example 3 were 16.9 MPa and 23.7 MPa, respectively. The differences between the two were not significant, but both were lower than those of Example 1. This indicates that single acid etching or coupling modification can only improve the interaction between the fiber and the matrix to a certain extent, while the "acid etching-coupling" two-step modification, through the synergistic effect of physical anchoring and chemical bonding, can more effectively improve the early strength development rate of concrete.

[0077] Later-stage strength (28d): The 28-day compressive strength of Example 1 reached 68.2 MPa, far exceeding the standard requirement of no less than 65 MPa set by this invention; the 28-day compressive strength of Comparative Example 1 was only 45.7 MPa, a significant difference of 22.5 MPa compared to Example 1; the 28-day compressive strengths of Comparative Examples 2 and 3 were 50.6 MPa and 48.3 MPa, respectively, which, although higher than Comparative Example 1, were still far lower than Example 1. This result fully demonstrates that the "acid-etching-coupling" two-step modified polypropylene fiber can form a strong interfacial bond with the cement matrix. When concrete is under load, the fiber can effectively transfer stress and inhibit crack propagation, thereby significantly improving the later-stage compressive strength of concrete. At the same time, the large amount of fly ash continues to play a role in the later-stage hydration reaction, synergistically with the reinforcing effect of the modified fiber, further ensuring the high-strength performance of concrete.

[0078] Comparison of splitting tensile strength:

[0079] The 28-day splitting tensile strength of Example 1 was 4.96 MPa, meeting the requirement of not less than 4.5 MPa in this invention, and significantly higher than that of the three comparative examples. The splitting tensile strength of Comparative Example 1 was only 3.45 MPa, while that of Comparative Examples 2 and 3 were 3.74 MPa and 3.71 MPa, respectively. Splitting tensile strength mainly reflects the interfacial bonding performance and crack resistance of concrete. The excellent performance of Example 1 is precisely due to the strong interfacial bonding force between the modified fiber and the cement matrix.

[0080] Single acid etching modification (Comparative Example 2) improved mechanical anchoring force by increasing fiber surface roughness, while single coupling modification (Comparative Example 3) improved interfacial interaction through chemical bonding, but the improvement effects of both were limited. In contrast, the "acid etching-coupling" two-step modification in Example 1 not only created a micro-rough structure through acid etching, providing strong mechanical anchoring force, but also constructed stable Si-O-Si covalent bonds through a coupling agent, achieving chemical bonding. The synergistic effect of both significantly improved the interfacial bond strength between the fiber and the matrix, thereby significantly increasing the splitting tensile strength of the concrete.

[0081] Comparison of peak tensile strain:

[0082] Peak tensile strain is a key indicator for measuring the toughness of concrete. The peak tensile strain of Example 1 reached 6.73%, which is higher than the standard of no less than 6.5% set by this invention, showing excellent toughness. The peak tensile strain of Comparative Example 1 was only 3.68%, with poor toughness. The peak tensile strains of Comparative Example 2 and Comparative Example 3 were 4.39% and 4.46%, respectively. Although they were improved compared to Comparative Example 1, they were still far lower than that of Example 1.

[0083] The core reason for this difference lies in the interfacial bonding state between the fiber and the matrix. In ordinary concrete (Comparative Example 1), once a crack occurs during tension, it propagates rapidly, resulting in relatively small strain. The interfacial bonding force between the single modified fiber (Comparative Examples 2 and 3) and the matrix is ​​insufficient, and it is prone to debonding or pull-out under tensile load, failing to effectively inhibit crack propagation and resulting in limited improvement in toughness. In contrast, the modified fiber in Example 1 has a strong bond with the matrix interface. During tension, the fiber can transfer stress through the interface, bridging the two ends of the crack and preventing further crack propagation, thereby consuming a large amount of energy and enabling the concrete to exhibit higher peak tensile strain and excellent toughness.

[0084] Comparative analysis of crack resistance performance:

[0085] Crack formation: No cracks appeared in Example 1 during the entire curing process, while obvious cracks appeared in Comparative Examples 1, 2 and 3. This phenomenon directly reflects the significant advantages of the present invention in terms of crack resistance.

[0086] Maximum temperature difference comparison: During the curing process of concrete, thermal stress is generated due to the release of heat of hydration and changes in ambient temperature. The greater the temperature difference, the higher the risk of cracking. The maximum temperature difference in Example 1 was only 20.1℃, which is much lower than that of Comparative Example 1 (41.6℃), Comparative Example 2 (31.8℃), and Comparative Example 3 (33.2℃).

[0087] In Example 1, replacing cement with a large amount of fly ash reduced the hydration heat release rate and decreased the internal temperature rise. At the same time, the three-dimensional network structure formed by the modified polypropylene fiber inside the concrete can effectively constrain the shrinkage deformation of the concrete and alleviate the tensile stress generated by the temperature difference stress, thereby reducing the maximum temperature difference and avoiding the generation of cracks.

[0088] Due to the lack of fiber restraint in Comparative Example 1, the concrete shrinkage deformation was unrestricted, resulting in greater thermal stress and a significant increase in the maximum temperature difference, which in turn led to cracks. The fiber modification effects of Comparative Examples 2 and 3 were limited, and the restraint effect on shrinkage deformation was insufficient, so they could not effectively alleviate thermal stress. Therefore, cracks still appeared, and the maximum temperature difference was significantly higher than that of Example 1.

[0089] This invention achieves low-carbon goals by replacing cement (20-30 parts) with a large amount of fly ash (80-120 parts). In Example 1, the fly ash content is 100 parts and the cement content is 25 parts, with the fly ash content being 4 times the cement content, which is within the set range of 3.2-4.8 times. Compared with traditional concrete, the cement content in Example 1 is significantly reduced. According to industry data estimates, every 1 kg reduction in cement content can reduce carbon dioxide emissions by approximately 0.8 kg. This embodiment, by replacing cement with fly ash, can reduce carbon emissions by more than 30%, fully meeting the trend of low-carbon development.

[0090] Meanwhile, the modified polypropylene fiber effectively addresses the inherent defects of high-volume fly ash concrete, such as low early strength, large shrinkage, and easy cracking, by enhancing its crack resistance. This achieves a synergistic development of "low carbon" and "high toughness and crack resistance." In contrast, although Comparative Example 1 also used a large amount of fly ash to replace cement, the concrete developed obvious cracks due to the lack of the enhanced crack resistance of the modified fiber. This compromised structural safety and durability, making it difficult to achieve a truly low-carbon application.

[0091] Therefore, through a comprehensive performance comparison analysis of Example 1 and Comparative Examples 1-3, it can be seen that the low-carbon polypropylene fiber concrete with high toughness and crack resistance provided by the present invention, through the synergistic design of polypropylene fibers modified in a two-step process of "acid etching-coupling" and a low-carbon cementitious system with a large amount of fly ash, not only significantly improves mechanical properties, achieving a 28-day compressive strength of 68.2 MPa, a 28-day splitting tensile strength of 4.96 MPa, and a peak tensile strain of 6.73%, all far exceeding the performance of the set standards and single-modified or fiberless concrete; but also, during the curing process, no cracks are generated, and the maximum temperature difference is only 20.1℃, effectively solving the problem of easy cracking in low-carbon concrete; at the same time, by replacing cement with a large amount of fly ash, carbon emissions are reduced by more than 30%, which not only solves the key problems of weak interfacial bonding and insufficient performance of traditional polypropylene fiber concrete, but also has a simple preparation method, readily available raw materials, and is easy to promote and apply on a large scale.

[0092] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A low-carbon polypropylene fiber reinforced concrete with high toughness and crack resistance, characterized in that, By weight, the raw materials include: 100-110 parts coarse aggregate; 60-70 parts fine aggregate; 20-30 parts cement; 80-120 parts fly ash; 15-20 parts modified polypropylene fiber; 2-3 parts water-reducing agent; and water, the amount of which meets the requirement of a water-cement ratio of 0.3-0.

4. The modified polypropylene fiber is prepared by a method comprising the following steps: S1: Place polypropylene fibers in concentrated phosphoric acid with a mass concentration of not less than 85% and soak at room temperature for 1.5 to 2 hours. Then wash with water until neutral and dry to obtain pretreated polypropylene fibers. S2: The pretreated polypropylene fiber is placed in a silane coupling agent solution with a mass fraction of 3% to 5% and soaked at room temperature for 1.5 to 2 hours, then washed and dried to obtain the modified polypropylene fiber. The goal of low carbon emissions is achieved by replacing cement with a large amount of fly ash; the modified polypropylene fiber is modified by acid etching-coupling synergistic modification, which forms a micro-rough structure on the fiber surface and introduces active groups, significantly improving the interfacial bonding strength between the fiber and the cement matrix.

2. The low-carbon polypropylene fiber reinforced concrete with high toughness and crack resistance according to claim 1, characterized in that, The coarse aggregate is continuously graded crushed stone with a particle size of 5-10 mm and a crushing index of no more than 10%.

3. The low-carbon polypropylene fiber reinforced concrete with high toughness and crack resistance according to claim 1, characterized in that, The fine aggregate is medium sand with a fineness modulus of 2.3 to 2.6 and a mud content of no more than 1.0%.

4. The low-carbon polypropylene fiber reinforced concrete with high toughness and crack resistance according to claim 1, characterized in that, The cement is ordinary Portland cement with a strength grade of 42.

5.

5. The low-carbon polypropylene fiber reinforced concrete with high toughness and crack resistance according to claim 1, characterized in that, The fly ash is Class I fly ash, and its loss on ignition is no more than 5%.

6. The low-carbon polypropylene fiber reinforced concrete with high toughness and crack resistance according to claim 1, characterized in that, The water-reducing agent is a polycarboxylate-based water-reducing agent.

7. The low-carbon polypropylene fiber reinforced concrete with high toughness and crack resistance according to claim 1, characterized in that, The silane coupling agent mentioned in step S2 is aminosilane coupling agent KH-550.

8. The low-carbon polypropylene fiber reinforced concrete with high toughness and crack resistance according to claim 7, characterized in that, The concrete preparation method includes: first, dry mixing coarse aggregate, fine aggregate, cement, fly ash and modified polypropylene fiber for 2-3 minutes, then adding water-reducing agent dissolved in water and wet mixing for 3-5 minutes.

9. A low-carbon polypropylene fiber reinforced concrete with high toughness and crack resistance according to claim 8, characterized in that, The volumetric dosage of the modified polypropylene fiber in concrete is 0.15% to 0.2%.

10. A low-carbon polypropylene fiber reinforced concrete with high toughness and crack resistance according to claim 9, characterized in that, The concrete has a 28-day compressive strength of not less than 65 MPa, a 28-day splitting tensile strength of not less than 4.5 MPa, and a peak tensile strain of not less than 6.5%.

Images