POM fiber surface synergistic modification method and application of POM fiber surface synergistic modification method in toughened concrete

Abstract

The invention discloses a POM fiber surface synergistic modification method and application of the POM fiber surface synergistic modification method in toughened concrete. The modification method comprises the following steps: removing impurities on the surface of the POM fiber by using acetone, drying, adding the POM fiber and benzoyl peroxide into anhydrous DMSO, heating and stirring, and adding monomers (acrylic acid and KH570) for grafting reaction to obtain the modified POM fiber. The modified POM fiber can be used for toughening concrete, the interfacial acting force between the fiber and the concrete is enhanced through the synergistic effect of acrylic acid and KH570, and finally the fracture resistance and compressive strength of the concrete are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of concrete, in particular to a method for synergistically modifying the surface of POM fibers and its application in toughened concrete. BACKGROUND

[0002] Cement-based composites have become an indispensable part of modern civil engineering, and due to their ultra-high strength, durability and compressive strength after hardening, they have become one of the most widely used artificial composites in the fields of construction, tunnels, bridges and many others. Concrete is a typical cement-based composite material, which is a building material composed of cement, aggregate and water, i.e. the hydration reaction between water and cement forms a viscous paste that binds the aggregate together, resulting in a hard material. With the continuous development of the economy, higher performance concrete materials are needed to meet the rapid development requirements of high-rise buildings, earthquake-resistant buildings or civil structures. In recent years, the performance of concrete has been significantly improved. However, due to the poor toughness of concrete, it is brittle and has low tensile strength, and when subjected to tensile or bending loads, even a small force can easily cause cracks on the surface, leading to the destruction of the concrete.

[0003] In order to compensate for these defects in practical applications, various reinforcing agents have been added to concrete. Adding steel reinforcement to concrete can improve this situation to some extent. However, due to cracks and the penetration of aggressive ions, steel is easily corroded, which can lead to the destruction of the structure of the concrete. Fibers have characteristics such as flexibility, high aspect ratio, good corrosion resistance, different cross-sectional areas and molecular orientation, which can be used as reinforcing materials for cement-based composites. The uniform distribution of fibers in the concrete matrix not only reduces the formation of cracks, but also reduces shrinkage, controls crack propagation and improves the post-cracking performance of concrete, thereby improving the tensile and bending capacity of concrete and reducing maintenance costs. Therefore, natural fibers and synthetic fibers can be used in cement composites to improve the durability of concrete structures, which will increase the service life of the structure, reduce maintenance costs and improve the sustainability of the structure. These introduced composites are widely studied.

[0004] Natural fibers currently used to improve concrete include kenaf, jute, sisal, basalt, palm, coconut, flax, abaca, and bamboo to improve the mechanical and engineering properties of concrete. Synthetic fibers include polyethylene (PE), polypropylene (PP), acrylic (PAN), polyvinyl alcohol (PVA), polyamide (PA), and polyoxymethylene (POM) fibers. Among them, POM fiber reinforced concrete composites are the research focus of many scholars and researchers. POM fiber has the advantages of light weight, high strength and modulus, excellent wear resistance and corrosion resistance, good thermal stability, very stable in the alkaline environment of concrete, low cost, and easy to disperse in concrete with good anchoring capacity, which greatly improves the seismic resistance of composite concrete, making it the most widely used reinforcing material in concrete.

[0005] The interfacial adhesion between the fiber and the concrete matrix is a key factor for the excellent mechanical properties of fiber reinforced concrete. Only when there is sufficient interfacial adhesion between the fiber and the concrete, the mechanical properties of the composite concrete can be improved. Otherwise, when subjected to external force, a firm bond cannot be formed at the fiber-matrix interface, and slippage occurs between the fiber and the concrete matrix, resulting in no obvious effect of fiber reinforced cement. However, the low surface free energy and smooth surface of POM fiber result in poor bonding between the fiber and the concrete interface, which ultimately affects the performance of the concrete.

[0006] To solve this problem, several methods for improving the interfacial properties have been disclosed. The first method is to improve the strength of the cement matrix. For example, the incorporation of silica fume in concrete can densify the matrix and improve the bonding properties of the fiber and the cement matrix. However, the mechanical properties such as elastic modulus and hardness of the cement matrix are higher than those of the fiber / cement interface transition zone, indicating that further strengthening of the cement matrix may have limited effect on the interfacial properties. Therefore, another method is generally used, which is to modify the surface of the fiber to improve the adhesion between the fiber and the cement interface. In this regard, physical and chemical fiber surface modification techniques are very effective in enhancing the adhesion of the fiber and the concrete.

[0007] Generally, the physical surface modification of the fiber can be achieved by fiber, indentation, crimping or any other method that can change the topological structure of the fiber surface, that is, increase the surface roughness and surface polarity of the fiber, so as to improve the interfacial bonding strength between the fiber and the cement. Chemical modification is performed on the surface of the fiber, which can make the fiber surface have hydrophilic groups and have hydrophilicity. This can most actively help the chemical adhesion of the fiber to the concrete at the interface. The use of hydrophilic fibers improves the performance of the concrete. This is because strong hydrogen bonds are formed between the fiber and the water, enhancing the adhesion between the fiber and the concrete matrix. Compared with untreated fibers, the modification of the polymer surface chemistry and morphology increases the interfacial strength, the modified fiber is more firmly combined with the cement, and the effect of enhancing the mechanical properties of the cement is more obvious. However, the effect of simple physical / chemical surface modification on the interfacial force between the fiber and the concrete is limited, therefore, it is urgent to modify the surface of the fiber more effectively to improve the interfacial adhesion between the fiber and the concrete, so as to improve the mechanical properties of the concrete. SUMMARY

[0008] The primary object of the present application is to overcome the shortcomings and deficiencies of the prior art, and to provide a method for synergistically modifying the surface of POM fiber.

[0009] Another object of the present application is to provide modified POM fiber obtained by the method for synergistically modifying the surface of POM fiber.

[0010] Still another object of the present application is to provide the application of the modified POM fiber.

[0011] The objects of the present application are achieved by the following technical solutions: A method for synergistically modifying the surface of POM fiber, grafting acrylic acid (AA) and γ- (methacryloxy) propyl trimethoxysilane (MPS, hereinafter referred to as KH570) on the surface of POM fiber at the same time; specifically comprising the following steps: (1) Put the POM fiber into acetone, soak and ultrasonic vibration, to fully remove the impurities on the surface of the POM fiber, then dry it to remove the acetone remaining on the surface of the POM fiber, to obtain the pretreated POM fiber; (2) Put the pretreated POM fiber and benzoyl peroxide (BPO) into anhydrous dimethyl sulfoxide (DMSO), heat and stir at 70±1℃ for 30±1 min, then add monomers, continue to graft at 70±1℃ under stirring, after the reaction is completed, take out the grafted fiber, rinse with ethanol, and dry, to obtain the modified POM fiber; wherein the monomers are acrylic acid (AA) and γ- (methacryloxy) propyl trimethoxysilane (KH570).

[0012] The ultrasonic vibration in step (1) is under the following conditions: ultrasonic frequency 40 kHz, ultrasonic time 3-5 h; preferably: ultrasonic frequency 40 kHz, ultrasonic time 4 h.

[0013] In step (1), acetone removes impurities on the surface of POM fibers through physical-chemical combined action, providing a clean surface for subsequent grafting reaction and avoiding impurities hindering the adhesion of initiators or monomer grafting.

[0014] The drying in step (1) is under the following conditions: 60±1℃ drying for more than 24 h; the oven temperature is strictly controlled at 60±1℃ to avoid slight degradation of POM fibers caused by excessively high temperature. The drying time needs to be more than 24 h to ensure that the residual acetone is completely volatilized. After drying, it should be immediately transferred to a desiccator for cooling to avoid re-adsorption of moisture in the air, affecting the subsequent swelling effect.

[0015] In step (2), the strong polarity and swelling of DMSO are used to moderately swell the surface of POM fibers, increasing the surface porosity; at the same time, the initiator BPO is uniformly attached to the surface of the fibers, providing "active sites" for subsequent monomer grafting and improving the grafting efficiency; In this step, we need to pay attention to the fact that DMSO is easy to absorb water, so anhydrous DMSO (water can be removed by molecular sieve drying or reduced pressure distillation) should be used, otherwise the water may react with BPO (BPO is easily decomposed slowly when it meets water), reducing the activity of the initiator.

[0016] The amount of benzoyl peroxide (BPO) added in step (2) is 2% of the mass of the monomer; BPO needs to be weighed in a dry environment, otherwise it is easy to absorb moisture or be oxidized, and it needs to be accurately weighed according to the proportion of "2% of the mass of the subsequent monomer", a higher proportion will lead to an increase in monomer homopolymerization (self-polymerization), and a lower proportion will result in insufficient grafting rate; BPO needs to be added slowly and dispersed by stirring to avoid local high concentration leading to uneven subsequent reaction.

[0017] In step (2), heating and stirring at 70±1℃ for 30±1 min can produce soluble swelling on the surface of POM fibers and increase the probability of BPO attachment to the fiber surface to improve the monomer grafting rate; in terms of temperature, constant temperature heating is carried out at 70±1℃ using a constant temperature oil bath, because 70℃ is the best temperature for BPO initiation, and excessively high temperature will cause BPO to decompose prematurely, which will result in insufficient initiator for subsequent monomer grafting; excessively low temperature will result in poor swelling and insufficient BPO attachment; in terms of time, the stirring time of 30±1 min needs to be strictly controlled, as excessively short time will result in insufficient swelling and attachment, and excessively long time may cause a slight decomposition of BPO, leading to a decrease in activity.

[0018] The mass ratio of the pretreated POM fiber to the monomer in step (2) is 1:1-2.5; preferably 1:1.

[0019] The mass ratio of the acrylic acid (AA) to the γ-(methacryloyloxy)propyltrimethoxysilane (KH570) in step (2) is 1:1.

[0020] The time of the grafting reaction in step (2) is 5-8h; preferably 6h.

[0021] The rinsing with ethanol in step (2) is rinsing with ethanol for more than 3 times to remove the monomer and the homopolymer on the surface of the fiber as much as possible; it is noted that the ethanol needs to be excessive and rinsed for more than 3 times, and the unreacted monomer and the homopolymer are dissolved by the polarity of the ethanol; it is worth noting that the homopolymer is dissolved in ethanol, and the polymer grafted on the surface of the fiber is not easy to fall off because it is combined with the fiber.

[0022] The drying condition in step (2) is drying at 60±1℃ for more than 24h.

[0023] In step (2), the whole reaction process is carried out in a protective gas (such as nitrogen) atmosphere; nitrogen needs to be passed during the whole experimental process of the present application, and the purpose is to exclude oxygen, because oxygen can quench free radicals and inhibit polymerization; and nitrogen needs to be passed through a deoxidizing device in advance, and the air in the flask is exhausted for 30min before the reaction, and a slight positive pressure is maintained during the reaction to avoid air backflow, and the gas flow speed should not be too fast to prevent the monomer from volatilizing.

[0024] A modified POM fiber, which is modified by the method for surface synergistic modification of the POM fiber.

[0025] The apparent characteristics of the modified POM fiber are the same as those of the unmodified POM fiber, the strength and toughness of the fiber are not weakened after modification, and the surface performance is improved.

[0026] The method for surface synergistic modification of the POM fiber or the application of the modified POM fiber in toughening concrete.

[0027] The addition amount of the modified POM fiber in concrete is 5-15kg / m 3 ; further preferably 8-12 kg / m 3 ; and further preferably 10kg / m 3 .

[0028] A toughening concrete containing the modified POM fiber, which comprises the following components: the modified POM fiber 5-15kg / m 3 , cement 750-850 kg / m 3 , silica ash 100-140 kg / m3 microbead powder 70-90 kg / m 3 natural sand 1100-1200 kg / m 3 water 150-170 kg / m 3 water reducing agent 30-40 kg / m 3 ; further preferably comprising the following components: modified POM fiber 10 kg / m 3 cement 800 kg / m 3 silica fume 120 kg / m 3 microbead powder 80 kg / m 3 natural sand 1150 kg / m 3 water 160 kg / m 3 water reducing agent 35 kg / m 3 .

[0029] The present application has the following advantages and effects relative to the prior art: Since the interfacial adhesion between POM fiber and the concrete matrix is weak, the effect of POM fiber toughened concrete is poor, based on this, the present application provides a method for modifying the surface of POM fiber, specifically grafting acrylic acid (AA) and KH570 on the surface of POM fiber at the same time, enhancing the interfacial force between the fiber and the concrete through the synergistic effect of AA and KH570, and finally improving the flexural and compressive strength of the concrete. In addition, the present application also provides a toughened concrete containing the above modified POM fiber, which has good fluidity and mechanical properties. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 are scanning electron microscope (SEM) test and surface element distribution (EDS) test results before and after modification of POM fiber; wherein (a) is POM fiber before modification (comparative example 9); (b) is POM fiber after AA modification (comparative example 3); (c) is POM fiber after KH570 modification (comparative example 7); (d) is POM fiber after AA and KH570 synergistic modification (example 3). DETAILED DESCRIPTION

[0031] The present application will be further described in detail below in conjunction with examples, but the embodiments of the present application are not limited thereto. Unless otherwise specified, the reagents, methods and equipment used in the present application are conventional reagents, methods and equipment in the technical field. Unless otherwise specified, the test methods in the following examples are usually carried out according to conventional experimental conditions or according to the experimental conditions recommended by the manufacturer. Unless otherwise specified, the reagents and raw materials used in the present application are commercially available.

[0032] The fiber information involved in the examples and comparative examples of the present application is as follows: (1) POM fiber: purchased from Ganzhou Daye Metal Fiber Co., Ltd., performance parameters: specification 0.2*12 mm, product has high tensile strength; elongation at break < 30%; excellent alkali resistance, after soaking in 10% sodium hydroxide solution for 200 h, the strength retention rate is 100%.

[0033] (2) PP fiber: purchased from Ke Hui New Material Technology Co., Ltd., performance parameters: specific gravity about 0.91 g / cm³, tensile strength > 358 MPa, elastic modulus > 3.5 GPa, melting point > 165 °C, fiber diameter 18-48 µm, tensile limit > 15%, highly resistant to acid and alkali, non-toxic material, no water absorption.

[0034] (3) Steel fiber: purchased from Ganzhou Daye Metal Fiber Co., Ltd., performance parameters: diameter 0.55-0.9 mm, length 35-60 mm, tensile strength 1100 MPa, product meets the standards such as “Steel Fiber for Concrete — People’s Republic of China Black Metallurgy Industry Standard YB / T151-2017”.

[0035] Examples 1-4: First, 10 g of POM fiber was soaked in acetone and ultrasonically vibrated (ultrasonic frequency: 40 kHz) for 4 h to thoroughly remove impurities on the surface of the POM fiber. After treatment, it was dried in an oven at 60 °C for 24 h to remove the acetone remaining on the surface of the fiber. The above POM fiber and 0.2 g-0.5 g of benzoyl peroxide (BPO) (controlling the mass of BPO to be 2% of the mass of the monomers (KH570+AA) to be added later) were added to a three-necked flask containing 175 g-190 g of anhydrous dimethyl sulfoxide (DMSO), and heated and stirred at 70 °C for 30 min. A mixture of 10 g-25 g of γ-(methacryloyloxy)propyltrimethoxysilane (KH570) and acrylic acid (AA) was added to the flask, and heated and stirred at 70 °C for 6 h. The grafted fiber was taken out and rinsed with ethanol for 3 times, and the treated fiber was placed in an oven at 60 °C for 24 h to obtain modified POM fiber (POM-AA-KH570). The whole reaction was carried out under nitrogen atmosphere. The specific number and monomer ratio are shown in Table 1.

[0036] Table 1. Modified experimental drug mass ratio of Examples 1-4

[0037] Comparative Examples 1-4: First, 10 g of POM fiber was immersed in acetone and ultrasonically vibrated (ultrasonic frequency: 40 kHz) for 4 h to sufficiently remove other impurities on the surface of the POM fiber. After treatment, it was dried in an oven at 60°C for 24 h to remove the acetone remaining on the surface of the fiber. The above POM fiber and 0.2 g to 0.5 g of BPO (controlling the mass of BPO to be 2% of the mass of the monomer (AA) to be added subsequently) were added to a three-necked flask containing 175 g to 190 g of anhydrous DMSO, and heated and stirred at 70°C for 30 min. 10 g to 25 g of AA was added to the flask, and heated and stirred at 70°C for 6 h. After the grafting, the fiber was taken out and washed with ethanol for 3 times. The treated fiber was dried in an oven at 60°C for 24 h to obtain modified POM fiber (POM-AA). The whole reaction was carried out under nitrogen atmosphere. The specific number and monomer ratio are shown in Table 2.

[0038] Table 2 Modification experiment of comparative examples 1-4

[0039] Comparative examples 5-8: First, 10 g of POM fiber was immersed in acetone and ultrasonically vibrated (ultrasonic frequency: 40 kHz) for 4 h to sufficiently remove other impurities on the surface of the POM fiber. After treatment, it was dried in an oven at 60°C for 24 h to remove the acetone remaining on the surface of the fiber. The above POM fiber and 0.2 g to 0.5 g of BPO (controlling the mass of BPO to be 2% of the mass of the monomer (AA) to be added subsequently) were added to a three-necked flask containing 175 g to 190 g of anhydrous DMSO, and heated and stirred at 70°C for 30 min. 10 g to 25 g of AA was added to the flask, and heated and stirred at 70°C for 6 h. After the grafting, the fiber was taken out and washed with ethanol for 3 times. The treated fiber was dried in an oven at 60°C for 24 h to obtain modified POM fiber (POM-AA). The whole reaction was carried out under nitrogen atmosphere. The specific number and monomer ratio are shown in Table 2.

[0040] Table 3 Modification experiment of comparative examples 5-8

[0041] Comparative example 9: POM fiber without modification.

[0042] Comparative example 10: Steel fiber.

[0043] Effect example 1 The surface of the modified POM fiber prepared in comparative example 3, comparative example 3, comparative example 7 and comparative example 9 without modification was tested by scanning electron microscopy (SEM) and surface element distribution (EDS).

[0044] The results are shown in Figure 1 SEM morphology analysis and EDS element composition analysis show that the surface of the unmodified POM fiber is smooth and flat, without obvious attachments, showing the typical uniform morphology of POM fiber itself, indicating that the surface structure is single when not grafted, and EDS element composition analysis only detects C and O elements, with an atomic percentage close to 1:1, which is consistent with the element composition of POM (a) in FIG. 1). Figure 1 The surface of the POM fiber grafted with AA is slightly rougher than pure POM, and in addition, the atomic percentage of C and O is close to 3:2, which is consistent with the element composition of AA, indicating that AA is successfully grafted onto the fiber surface to form a grafted polymer layer, changing the surface morphology (b) in FIG. 1). Figure 1 The surface of the POM fiber grafted with KH570 is further roughened, and obvious granular or sheet-like attachments can be seen. After grafting KH570, the silicon-containing molecular chain on the fiber surface forms a grafted layer with a specific morphology, directly proving that KH570 is successfully grafted (c) in FIG. 1). Figure 1 The surface of the POM fiber grafted with AA and KH570 reaches the highest roughness, with more uniform and thicker coverage, combining the morphology characteristics after grafting AA and KH570, indicating that the grafting layer is more abundant and continuous when the two monomers are grafted together, and the modification effect is more significant. The Si content is further improved, indicating that the KH570 grafting amount increases when grafted together; at the same time, the change in C content reflects the contribution of AA grafting, proving that AA and KH570 are successfully grafted, and the synergistic grafting makes the element composition of the fiber surface change more significantly, and the chemical composition of the grafting layer is more complex (d) in FIG. 1). Figure 1

[0045] Effect Example 2 1. Concrete mixing and specimen forming experimental method: (1) Preparation of concrete mixture ​In the concrete mix proportion, the cementing material selects the cement with the strength grade of 42.5 (ordinary portland cement, produced by the cement manufacturers in the south, the specific surface area is 350 m2 / kg, and the average particle size is about 30 μm); the admixture adopts silica fume (new Bida silica fume produced by Wuhan Newruiqi New Material Co., Ltd., the average particle size is 0.1-0.15 μm, and the specific surface area is 15-27 m2 / g) and microbead powder (the microbead is extracted from fly ash, and the typical particle size range is 200 nanometers to 10 microns) to optimize the microstructure of the concrete; the fine aggregate is natural sand (medium sand is adopted, the clay content is 2.0%, the mud block content is 0.5%, the apparent density is 2650 kg / m3, the bulk density is 1450 kg / m3, the particle size distribution is good, and meets the requirements of “Sand for Building” (GB / T 14684-2011)); the mixing water is ordinary tap water; the water reducing agent (Jiangsu Subo New Material Co., Ltd., belongs to the acrylic acid series grafted polyether structure, the solid content is 20-40%, and the water reducing rate is ≥30%) is used to regulate the workability of the concrete mixture; the fiber is the modified (or unmodified) POM fiber prepared in examples 1-4 and comparative examples 1-8 to be studied, and the dosage is 10 kg / m 3 , and the specific amount of each component is: cement: 800 kg / m 3 ; silica fume: 120 kg / m 3 ; microbead powder: 80 kg / m 3 ; natural sand: 1150 kg / m 3 ; water: 160 kg / m 3 ; water reducing agent: 35 kg / m 3 . The specific preparation process is as follows: Firstly, 800 g of cement, 120 g of silica fume, 80 g of microbead powder and 1150 g of natural sand are sequentially put into a horizontal concrete mixer, the mixer is started to dry mix at a speed of 20-30 r / min for 120 s, to ensure that the cementing material and fine aggregate are macroscopically uniformly mixed, and to avoid local component enrichment in the subsequent wet mixing stage. 10 g of modified / unmodified POM fiber pretreated by the above method is added, and dry mixing is continued for 60 s. The fiber dispersion state is observed in real time during the mixing process, if fiber agglomeration occurs, stop immediately and use manual dispersion to break the agglomeration, and then re-dry mix for 30 s until the fiber is uniformly distributed. 50% of the designed water amount and all of the water reducing agent diluent are added to the mixer, wet mixing for 60 s, so that the water reducing agent is fully adsorbed on the surface of the solid particles, and the dispersion effect is initially played; the remaining 50% of the designed water amount is continuously added, and the stirring is continued for 180 s, until the mixture presents a uniform sticky state, and there is no visible particle separation or bleeding phenomenon. The slump of the mixture is determined according to “Standard Test Methods for Properties of Fresh Ordinary Concrete” (GB / T50080).

[0046] (2) Concrete specimen molding The preparation of the concrete specimen strictly followed the provisions of the Standard for Testing Methods of Mechanical Properties of Ordinary Concrete (GB / T 50081-2002), and the standard tamping method was used to cast the fiber reinforced concrete specimen to ensure the compactness of the mixture and the uniformity of the specimen. The specimen size was designed to be 40mm × 40mm × 160mm, which met the requirements of the specimen size for mechanical property tests such as flexural strength and compressive strength, effectively reflected the toughening effect of the fiber in the concrete matrix, and ensured the accuracy and comparability of the test results. The specific steps are as follows: A 40mm × 40mm × 160mm steel mold was used, and the flatness of the end face of the mold was checked before molding. The mold was then fixed on a horizontal operation table to ensure that there was no displacement or inclination during the molding process. Then the standard tamping method was used to mold the specimen, and the mixture was loaded into the mold in two layers, with the height of each layer controlled to be 1 / 2 of the total height of the mold. After loading each layer, a Φ16mm round steel tamper was used to tamping the mixture evenly along the inner wall of the mold for 25 times to ensure that the mixture was compact and had no internal voids. After casting, the excess mixture was removed by using a straight edge to scrape the top surface of the mold back and forth. Before the mixture was initially set, a second smoothing treatment was performed using an iron trowel to eliminate surface bubbles and small depressions and ensure the smoothness of the top surface of the specimen. The molded specimen was placed in a closed environment at room temperature without direct sunlight for 7 days. A special demolding device was used to slowly separate the mold from the specimen to avoid damage to the specimen caused by hard prying. After demolding, the appearance of the specimen was checked, and unqualified specimens with cracks, missing corners or surface honeycomb were removed. The qualified specimens were marked with the fiber number and the molding date for curing.

[0047] 2. Performance test (1) Flexural strength test: The specimen cured in the standard curing room for 7 days was taken out, and the surface moisture and impurities were wiped off with a dry cloth. The actual size of the specimen was measured. A four-point bending flexural strength testing machine was used, and the distance between the supports was adjusted according to the size of the specimen to ensure that the upper loading point and the lower support axis were coplanar and perpendicular to the length direction of the specimen. The equipment was started for empty load debugging, and the loading rate was confirmed to be stable at 0.05-0.10MPa / s.

[0048] The specimen was placed on the lower support of the testing machine, and the side surface during molding was used as the stress surface to avoid the influence of the deviation of the top surface flatness on the uniformity of the stress. The position of the specimen was adjusted to align the loading point with the center line of the width of the specimen. The testing machine was started, and the loading rate was uniform. The deformation process of the specimen was observed in real time until the specimen fractured along the stress surface, and the maximum breaking load was recorded. If the specimen did not fracture along the expected section during the loading process (such as edge cracking), the data of the specimen was marked as invalid, and additional testing was required. The average value of three measurements was taken.

[0049] (2) Compressive strength test: The test piece after the flexural strength test (a 40mmx40mmx40mm short column is formed after breaking), or a test piece of the same size is prepared separately (the curing conditions are consistent). The broken surface is leveled with gypsum to ensure that the flatness error of the loading surface is ≤0.05mm, and the surface impurities are wiped after drying. An electro-hydraulic servo compression testing machine is used, a ball hinge loading plate is installed, the loading rate is adjusted to 0.3-0.5MPa / s under no load, and the force value sensor is calibrated. The test piece is placed at the center of the loading plate of the testing machine, the axis of the test piece is coincided with the center line of the loading plate, and eccentric force is avoided. Start the device at a uniform speed, observe the change of the load-displacement curve, until the test piece is completely destroyed (the load drops suddenly or the deformation increases sharply), and record the maximum destruction load. If the test piece slips during loading, the flatness of the loading plate needs to be checked and retested. The average value of three measurements is taken.

[0050] (3) Fluidity: The determination of the fluidity of the concrete adopts the spread degree experiment, and the specific experimental steps are as follows: firstly, the slump cone is placed at the center of the bottom plate, the concrete mixture is loaded in layers (generally 3 layers, each layer is evenly inserted with a tamping rod for 25 times), and the cylinder mouth is scraped flat after being filled. Then, the slump cone is vertically and smoothly lifted, and the concrete mixture is allowed to slump freely. Finally, after the concrete mixture slumps and spreads stably, the maximum diameter and the minimum diameter are measured with a steel ruler or a caliper, and the average value of the two is taken as the spread degree (unit: mm). The average value of three measurements is taken.

[0051] 3. Test results (1) The fluidity, flexural strength and compressive strength of the PP fiber toughened concrete (the POM fiber is replaced with the PP fiber according to the same method, and the concrete test piece is prepared) and the POM fiber toughened concrete which are the most widely used in the market are compared. The fluidity can reflect the interface compatibility of the fiber and the concrete to a certain extent, which is an index of the fluidity of the concrete mixture, and directly reflects the interface interaction degree of the fiber and the concrete matrix, such as wettability and dispersibility. The better the interface compatibility, the smaller the hindering effect of the fiber on the fluidity of the concrete, and the higher the fluidity. The flexural strength depends on the interfacial adhesion between the fiber and the concrete matrix. When stressed, the stronger the interfacial adhesion, the more effectively the fiber can transfer stress and hinder crack propagation, and the better the flexural performance.

[0052] Table 4 Performance test before and after modification of POM fiber and PP fiber

[0053] The results are shown in Table 4: from Table 4, the fluidity of the unmodified POM fiber toughened concrete is higher than that of the unmodified PP fiber. This is because the POM molecule contains a polar group, which is more prone to physical / chemical interaction with the cement hydrate which is a polar substance; while PP is a typical non-polar polymer, the interfacial bonding force with the concrete matrix is weak, and the hindering of fluidity is more obvious, so the interfacial compatibility of POM with concrete is naturally better than that of PP. After modification, the fluidity of the POM fiber toughened concrete is still higher than that of the modified PP fiber, and the improvement range of POM fluidity is greater. This is because the surface grafting modification further optimizes the surface polarity and reactivity of the POM fiber, making it more easily infiltrated and dispersed by the concrete matrix; while the non-polar disadvantage of PP fiber leads to a weaker improvement of interfacial compatibility than POM, so the fluidity improvement is less.

[0054] In addition, the flexural strength of the unmodified POM fiber reinforced concrete is higher than that of the PP fiber group. This is because the interfacial compatibility of POM with concrete is better, the interfacial bonding force is stronger, and the fiber can more efficiently protect the matrix and delay fracture when the crack propagates. The flexural strength of the modified POM fiber reinforced concrete is not only higher than that of the modified PP group, but also has a greater strength improvement range. After modification of POM fiber, the interfacial bonding force is further strengthened; while after modification of PP fiber, the improvement degree of interfacial bonding is still weaker than that of POM, so the flexural strength improvement range is smaller. The compressive strength is closely related to the dispersion, interfacial compatibility and "skeleton support" effect of the fiber in the concrete. The more uniform the fiber dispersion, the stronger the interfacial bonding, the better the fiber can cooperate with the matrix under stress, and the more effectively the internal crack development of the concrete is limited, thereby improving the compressive strength.

[0055] Finally, the compressive strength of the modified POM fiber reinforced concrete is higher than that of the modified PP group, and the improvement range is more significant. High fluidity indicates that POM fibers are more uniformly dispersed in the concrete and are less likely to agglomerate, allowing them to play a more extensive "microskeleton" role. After modification, the interfacial bonding force between POM and the matrix is stronger, and the fiber and the matrix are more likely to deform cooperatively under pressure, effectively inhibiting the generation and propagation of microcracks. While PP fibers are limited by the improvement degree of dispersion and interfacial bonding, the improvement range of compressive strength is weaker than that of POM fibers.

[0056] (2) The performance of the concrete specimens prepared with the modified POM fibers in Examples 1-4 and Comparative Examples 1-8 above were tested (fluidity, flexural strength, compressive strength) with untreated POM fibers (Comparative Example 9) and steel fibers (Comparative Example 10) as controls. Among them, the fluidity reflects the flowability of the concrete slurry, and is directly related to the core and fiber dispersibility, fiber-concrete compatibility. The flexural strength reflects the ability of the fiber to bridge the concrete to resist cracking when the concrete is bent, and the core depends on the interfacial bonding strength of the fiber and the matrix and the uniformity of the fiber dispersion in the concrete. The compressive strength reflects the ability of "micro-crack inhibition" and "stress uniform transmission" when the concrete is under compression, which is directly related to the dispersibility and interfacial bonding strength of the fiber.

[0057] Table 5 Performance test data of Examples 1-4 and Comparative Examples 1-10

[0058] The results are shown in Table 5: from the data in Table 5, the flexural strength and compressive strength of the concrete toughened by the single-grafted AA fiber (Comparative Examples 1-4) have the smallest increase, but the fluidity has a large increase, indicating that after grafting AA, a large number of carboxyl groups (-COOH) are introduced onto the surface of the fiber, which not only improves the hydrophilicity of the fiber, but also reduces the ineffective consumption of the polycarboxylate superplasticizer due to the structural similarity between the carboxyl group and the polycarboxylate superplasticizer, thereby releasing more free water and greatly optimizing the fluidity; the higher the concentration, the more the modified fibers are effectively dispersed, and the more obvious the improvement in fluidity. The carboxyl groups of AA can strengthen the interfacial bonding but the improvement is limited, and the optimization of "microcrack inhibition" and "stress uniform transmission" is weaker than the double-grafted group. The flexural strength and compressive strength of the concrete toughened by the single-grafted KH570 fiber (Comparative Examples 5-8) steadily increase with the increase of the monomer concentration, but the fluidity has a small increase, and the mechanism is that the silane layer of KH570 is strongly combined with the cement matrix through Si-O bonds, improving the interfacial force transmission efficiency; but lacking the hydrophilic segment of AA, the fiber dispersion is poor, and the compatibility with the concrete is poor, resulting in a decrease in the number of "effective bridging fibers" and a weaker crack resistance than the double-grafted group. The flexural strength and compressive strength of the concrete toughened by the fiber simultaneously grafted with AA and KH570 (Examples 1-4) are significantly higher than those of the single-grafted group, and reach a peak when the concentration of the modified monomer is 10%. The carboxyl groups (-COOH) of AA improve the hydrophilicity and dispersion of the fiber, which is beneficial to reducing fiber agglomeration and allowing the fiber to more uniformly "block" crack propagation; the silanol (-SiOH) generated by the hydrolysis of the grafted KH570 reacts with the concrete, strengthening the "fiber-matrix" interfacial bonding through Si-O bonds, which is beneficial to more effectively transmitting and dispersing the bending stress through the fiber. The two work together to maximize the "bridging crack resistance" effect. In addition, the fiber can also inhibit the expansion of microcracks and improve the compressive performance. After modification under suitable conditions, the polyformaldehyde fiber can toughen the concrete to a level close to that of steel fiber modification, and the polyformaldehyde has lower density and better corrosion resistance, which shows that the POM fiber has the potential to replace steel fiber.

[0059] The above examples are the preferred embodiments of the present application, but the embodiments of the present application are not limited to the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application should be equivalent replacement methods and are included in the protection scope of the present application.

Claims

1. A method for synergistic surface modification of POM fibers, characterized in that, Specifically, the steps include the following: (1) POM fibers are immersed in acetone and subjected to ultrasonic vibration to fully remove impurities from the surface of POM fibers. Then they are dried to remove the acetone residue on the surface of POM fibers, and pretreated POM fibers are obtained. (2) The pretreated POM fiber and benzoyl peroxide were added to anhydrous dimethyl sulfoxide and heated and stirred at 70±1℃ for 30±1 min. Then the monomer was added and the grafting reaction was continued at 70±1℃ with stirring. After the reaction was completed, the grafted fiber was taken out, rinsed with ethanol, and dried to obtain modified POM fiber. The monomers were acrylic acid and γ-(methacryloyloxy)propyltrimethoxysilane.

2. The method for synergistic surface modification of POM fibers according to claim 1, characterized in that: The amount of benzoyl peroxide added in step (2) is 2% of the monomer mass.

3. The method for synergistic surface modification of POM fibers according to claim 1, characterized in that: The mass ratio of pretreated POM fibers to monomers in step (2) is 1:1 to 2.5; The mass ratio of acrylic acid and γ-(methacryloyloxy)propyltrimethoxysilane in step (2) is 1:

1.

4. The method for synergistic surface modification of POM fibers according to claim 3, characterized in that: The mass ratio of the pretreated POM fiber to the monomer in step (2) is 1:

1.

5. The method for synergistic surface modification of POM fibers according to claim 1, characterized in that: The grafting reaction time described in step (2) is 5 to 8 hours.

6. The method for synergistic surface modification of POM fibers according to claim 1, characterized in that: The conditions for ultrasonic vibration mentioned in step (1) are: ultrasonic frequency 40kHz, ultrasonic time 3-5h; The drying conditions described in step (1) are: drying at 60±1℃ for more than 24 hours; The ethanol rinsing mentioned in step (2) refers to rinsing with ethanol more than 3 times; The drying conditions described in step (2) are: drying at 60±1℃ for more than 24 hours; In step (2), the reaction is carried out under a protective gas atmosphere throughout.

7. A modified POM fiber, characterized in that: It is obtained by the method of synergistic surface modification of POM fibers as described in any one of claims 1 to 6.

8. The method for synergistic surface modification of POM fibers according to any one of claims 1 to 6, or the application of modified POM fibers according to claim 7 in toughened concrete.

9. The application according to claim 8, characterized in that: The modified POM fiber is added to concrete at a rate of 5–15 kg / m³. 3 .

10. A toughened concrete containing the above-mentioned modified POM fibers, characterized in that, It contains the following components: modified POM fiber 5-15 kg / m 3 Cement 750-850 kg / m³ 3 Silica fume 100-140 kg / m³ 3 Microbead powder 70-90 kg / m 3 Natural sand 1100~1200 kg / m 3 Water 150-170 kg / m 3 Water-reducing agent 30-40 kg / m 3 .

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