A method for evaluating the bond strength between reinforcing fibers and a young cement matrix

Abstract

This invention provides a method for evaluating the bond strength between reinforcing fibers and early-age cementitious matrix, relating to the field of cementitious material testing technology. The method includes: step S1, mixing raw materials; step S2, curing samples to a specified age; step S3, testing penetration pressure or penetration resistance values; step S4, calculating the average value, standard deviation, and mean deviation rate; and step S5, determining the degree of fiber dispersion. This method is simple to prepare samples and convenient to test. It can be used for screening reinforcing fibers in cementitious material research and preparation processes, and as a supplementary explanation for the improved tensile strength, toughness, and other properties of fiber-reinforced cementitious materials.

Description

Technical Field

[0001] This invention relates to the field of cement-based material testing technology, and in particular to a method for evaluating the bond strength between reinforcing fibers and early-age cement matrix. Background Technology

[0002] Cement-based materials used in transportation infrastructure in harsh environments such as freezing, salt spray, and extreme temperature differences, and subjected to repeated traffic loads, face increasingly stringent performance requirements. Introducing millimeter-centimeter length fibers for reinforcement can significantly improve the material's toughness, resisting brittle failure under repeated loading.

[0003] When reinforcing fibers are added to cement-based materials, a transitional layer structure similar to the aggregate-cement matrix exists between the fiber surface and the hardened cement. Currently commonly used reinforcing fibers, such as polyvinyl alcohol fibers, polyester fibers, and polypropylene fibers, are mainly smooth cylindrical shapes. Due to the presence of the relatively weak fiber-cement interface transition layer, the fibers can easily be pulled out of the cement matrix, greatly affecting the reinforcing effect of the fibers.

[0004] When evaluating the reinforcing effect of fibers, common methods include testing the flexural strength and tensile strength of cement-based materials, as well as characterizing the reinforcing effect through macroscopic mechanical properties such as flexural toughness. These methods have corresponding standards and specifications guiding sample preparation, curing, and testing procedures, and generally correspond to samples aged 3 days or longer. However, considering the screening of fibers in the laboratory stage and the influence of different fiber surface treatments, a simple and convenient method is needed to compare the reinforcing effects of fibers.

[0005] The Vicat apparatus for testing cement setting time uses the time it takes for the needle to penetrate the cement paste to a specific depth during the cement setting process as the initial and final setting times of the cement. The needle has two different circular flat-headed designs with different base areas. The penetration resistance meter for testing concrete setting time uses the penetration resistance meter to measure the initial and final setting times of the concrete when the needle penetrates the cement mortar (after removing coarse aggregate) to a depth of 25mm ± 2mm and the penetration resistance reaches 3.5MPa and 28MPa, respectively. The penetration needle also has a circular flat-headed design. Both the Vicat apparatus and the penetration resistance meter use circular flat-headed needles that penetrate only to a certain depth within the mixture sample, making them suitable for the cement setting and early hardening processes.

[0006] For cement paste samples containing added fibers, using a round, flat-tipped needle, such as a Vicat apparatus or penetration resistance meter, can easily cut off surrounding fibers during penetration rather than providing them with a pulling force. Therefore, flat-tipped needles are unsuitable for testing the bond strength between reinforcing fibers and the cement matrix. Furthermore, if the needle only penetrates to a certain depth within the sample, the penetration resistance only reflects the increase in internal bonding strength during the hydration and hardening process of the cement, and cannot effectively demonstrate the bond strength between the fibers and the cement matrix. Summary of the Invention

[0007] This invention provides a test and evaluation method for verifying the bonding strength between reinforcing fibers and early-age cement matrix. The method involves changing the indenter head to a hemispherical indenter and adjusting the sample to a thin plate shape (a flat fiber cement paste sample with a thickness of 2-4 mm). The bonding strength between the fiber and the cement matrix is ​​evaluated by the penetration resistance value obtained by the indenter penetrating the sample (i.e., the ratio of the penetration pressure obtained by the test to the area of ​​the hemispherical indenter head).

[0008] During the penetration of the sample by the penetrator, the surrounding cement matrix is ​​subjected to complex stresses such as compression, splitting, and tension. Due to their different distribution patterns, the fibers primarily bear pull-out forces from different directions, reaching their peak upon penetration. This method is suitable for early-stage fiber-reinforced cement paste samples (before 7 days of age), where the cement matrix strength is not yet fully developed and the bonding force between the reinforcing fibers and the cement matrix needs improvement. The test results better reflect the interaction and applicability of fiber surface properties with the cement paste, admixtures, or additives.

[0009] Specifically, the technical solution described in this invention is as follows:

[0010] A method for evaluating the bond strength between reinforcing fibers and early-age cementitious matrix includes:

[0011] Step S1: Mix the test fiber with cement and additives evenly in a mixing device to obtain fiber cement paste.

[0012] Step S2: Pour the fiber cement paste mixture into a mold, vibrate it, and smooth the surface. Cover the surface of the fiber cement paste sample with a plastic film, place it in a cement curing chamber (box), and demold it after curing for the specified age.

[0013] Step S3: After drawing a grid of lines at a certain interval on the flat fiber-reinforced cement paste sample, place it on a penetration resistance tester. A hemispherical penetrator, with a pressure head, penetrates the sample at the grid intersections at a certain rate. The penetration pressure value (i.e., the maximum pressure value, in N) is obtained from the pressure sensor. During the penetration process, the surrounding cement matrix is ​​subjected to complex stresses such as compression, splitting, and tension. Due to the different distribution patterns of the fibers, they mainly bear pull-out forces from different directions. This force reaches its peak when the sample is penetrated. The penetration resistance value, in MPa, is the ratio of the penetration pressure value to the area of ​​the hemispherical pressure head, calculated using the following formula:

[0014]

[0015] In the formula:

[0016] f p —Penetrating resistance value, in MPa, calculated to an accuracy of 0.01;

[0017] N pi — The penetrating pressure value at measuring point i (i=1, 2, 3…), in N;

[0018] A p — Surface area of ​​the needle indenter, mm 2 ;

[0019] Step S4: Based on different molds, obtain a certain number of penetration pressure values ​​or penetration resistance values, and calculate the average value, standard deviation, and mean deviation rate of the penetration pressure values ​​or penetration resistance values ​​using the following formula:

[0020]

[0021]

[0022] In the formula:

[0023] X i — The penetration pressure (N) or penetration resistance (MPa) at the i-th measuring point, calculated to an accuracy of 0.01.

[0024] N—Number of all measurement points in a single sample;

[0025] — The penetration pressure (N) or average penetration resistance (MPa) at each measuring point in a single sample, calculated to an accuracy of 0.01.

[0026] S—Standard deviation of test values ​​for a single sample;

[0027] D— Deviation rate of mean test value for a single sample;

[0028] Step S5: Determine the dispersion of the test values ​​based on the magnitude of the standard deviation, and calculate the proportion of test points with a deviation rate within 15% from the mean deviation rate. The larger the proportion, the better the fiber dispersion, and vice versa.

[0029] Furthermore, in step S1, the water-cement ratio is 0.35 to 0.55, the volume ratio of fiber to mixture is 0 to 5%, and the mass ratio of additive to cement is 0 to 30%.

[0030] Furthermore, in step S2, the mold dimensions are (100-200mm) × (100-200mm) at the bottom and 2mm-4mm in depth.

[0031] Furthermore, in step S3, the sample grid spacing is 20mm, the indenter head is a hemispherical shape with a diameter of Φ3mm~6mm, and the indentation rate of the indenter is 1mm / s.

[0032] Furthermore, in step S4, the number of penetration pressure values ​​or penetration resistance values ​​is 16-81.

[0033] Furthermore, the experimental fibers were organic fibers and / or inorganic non-metallic fibers with diameters in the micrometer range. Due to differences in fiber surface properties and the use of different additives, when fibers are mixed with cement, the hydration products adsorbed on the fiber surface and their bonding state in the cement paste differ, resulting in different pull-out forces on the fibers from the hardened cement paste. This can be used to identify the bonding effect between different fibers and cement paste. The additives used are those that enhance the bonding force between fibers and cement-based materials, such as nano-SiO2, nano-MgO, and nano-CaCO3. These additives can generate denser CSH gels on the fiber surface and at the fiber-cement-based material interface, increasing both fiber surface roughness and the tightness between the fiber and cement-based material, as well as improving fiber dispersibility.

[0034] Furthermore, regarding the magnitude of the standard deviation, a larger standard deviation indicates a higher degree of dispersion in the test values ​​and poorer fiber dispersion. Conversely, a smaller standard deviation indicates a lower degree of dispersion in the test values, closer to the average value, and better fiber dispersion. The fiber dispersion effect in the sample is evaluated by the mean deviation rate. The number of all test points with deviation rates within 15% is correlated with the total number of test points. If the calculated ratio is greater than 75%, it indicates a good fiber dispersion effect, resulting in small differences in penetration resistance values ​​at most test points. If this ratio is between 65% and 75%, it indicates good fiber dispersion, with penetration resistance values ​​being similar across most of the sample. If it is between 50% and 65%, it indicates a poor fiber dispersion effect. If it is less than 50%, it indicates poor fiber dispersion.

[0035] The beneficial effects of the technical solutions provided by the embodiments of the present invention include at least the following:

[0036] In this invention, the indenter head is intentionally modified to be hemispherical and the sample is adjusted to be a thin plate. The penetration resistance value obtained by the needle penetrating the sample is used to evaluate the bond strength between the fiber and the cement matrix. During the process of the needle penetrating the sample, the surrounding cement matrix is ​​subjected to complex stresses such as compression, splitting, and tension. Due to the different distribution patterns of the fibers, they mainly bear the pull-out forces from different directions. When penetrating the sample, this force reaches its peak.

[0037] This invention not only clearly distinguishes the reinforcing effects of different fiber types and addition amounts, but also simultaneously reveals the dispersion effect of fibers in cement paste. The method is simple to prepare samples and convenient to test, and can be used in the research and preparation of cement-based materials for screening organic and inorganic non-metallic reinforcing fibers, as well as as a supplementary explanation for the improvement in tensile strength, toughness, and other properties of fiber-reinforced cement-based materials.

[0038] The method of this invention is suitable for early fiber cement paste samples with an age of less than 7 days, where the strength of the cement matrix has not yet fully developed and the bonding force between the reinforcing fiber and the cement matrix still needs to be improved. The test results can better reflect the interaction and applicability between the fiber surface characteristics and the cement paste, admixtures or additives. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 This is a schematic diagram of the process of penetrating a sample using a hemispherical penetrating needle and a circular flat-headed penetrating needle as the indenter head of the present invention; wherein (a) is a schematic diagram of the hemispherical penetrating needle penetrating the sample, (b) is a schematic diagram of the circular flat-headed penetrating needle penetrating the sample, (c) is a schematic diagram of the hemispherical penetrating needle penetrating the sample, and (d) is a schematic diagram of the circular flat-headed penetrating needle penetrating the sample. Detailed Implementation

[0041] The technical solution of the present invention will now be described with reference to the accompanying drawings.

[0042] In embodiments of the present invention, words such as "exemplarily," "for example," etc., are used to indicate that something is an example, illustration, or description. Any embodiment or design described as "exemplary" in the present invention should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of the word "exemplary" is intended to present the concept in a concrete manner. Furthermore, in embodiments of the present invention, the meaning expressed by "and / or" can be both, or either one.

[0043] In this embodiment of the invention, the terms "image" and "picture" may sometimes be used interchangeably. It should be noted that, without emphasizing the difference, their intended meanings are consistent. Similarly, the terms "of," "corresponding," and "corresponding" may sometimes be used interchangeably. It should be noted that, without emphasizing the difference, their intended meanings are consistent.

[0044] In this embodiment of the invention, sometimes a subscript such as W1 may be mistakenly written as a non-subscript form such as W1. When the difference is not emphasized, the meaning they express is the same.

[0045] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0046] This invention provides a test and evaluation method for verifying the bonding strength between reinforcing fibers and early-age cementitious matrix, using fine organic and inorganic non-metallic fibers. By adding additives that promote the adhesion of the resulting material to the fiber surface or make the boundary between the fiber and cementitious material denser, such as nano-SiO2, nano-MgO, and nano-CaCO3, the bonding force between the fiber and cement paste is improved. Different fibers exhibit different surface adhesions under the influence of the cementitious material. The selected test fibers, cement, and additives are weighed and mixed in a mixer at a water-cement ratio of 0.35–0.55, a fiber-to-mixture volume ratio of 0–5%, and an additive-to-cement mass ratio of 0–30%.

[0047] The fiber-reinforced cement paste mixture is poured into a mold with a bottom surface of (100-200) mm × (100-200) mm and a depth of 2-4 mm. First, the cement paste mixture is tapped with a steel ruler and vibrated. Then, the mixture is filled until full, tapped again with a steel ruler, and the surface is smoothed. A layer of plastic film is then placed on the sample surface. The sample is placed in a cement curing chamber (box) and cured for 1 day before demolding. Samples requiring longer curing periods continue to be cured until the specified curing age.

[0048] When a Φ3-6mm indenter penetrates a thin sheet of fiber-reinforced cement paste, the rupture zone occurs within a circle with a radius of 10mm due to the effect of the fibers. A grid of 20mm intervals is drawn on a flat sample of fiber-reinforced cement paste. The sample is placed on a test platform pad of a penetration resistance meter. A Φ20mm hole is pre-drilled in the pad directly below the penetration needle, and the pad is 20mm thick. At the penetration test point, the corresponding test point is positioned within the Φ20mm hole in the pad. The penetration needle is manually or electrically driven to penetrate the sample at a rate of 1mm / s at the grid intersections. The maximum pressure value—the penetration pressure value—is obtained from the pressure sensor, in N. The penetration needle indenter is hemispherical with a diameter of Φ3mm-6mm.

[0049] Figure 1 This diagram illustrates the process of a penetrating needle with a hemispherical indenter penetrating a sample, and also shows a comparison of the process with a circular, flat-headed penetrating needle. When the penetrating needle is inserted into the sample, the sample below the indenter is compressed. For the hemispherical indenter, one end of the side fiber is bound by the hardened cement paste, while the other end below the indenter is stretched downwards. Figure 1 As shown in (a); for a circular pressure head, the side fibers are subjected to shearing action by the pressure head, and some fibers are cut off, as shown in (a). Figure 1 As shown in (b), when the indenter penetrates the sample to a certain depth, shear fracture begins to occur in the sample below the indenter until the sample is completely penetrated. For a hemispherical indenter, this process varies depending on the fiber distribution pattern, with the fibers primarily experiencing pull-out forces from different directions, such as... Figure 1 As shown in (c), this force reaches its peak when penetrating the sample; for a circular indenter, the side fibers are cut after being stretched, so the effect of the fibers cannot be demonstrated, as... Figure 1 As shown in (d).

[0050] The penetration resistance value is the ratio of the penetration pressure value to the area of ​​the hemispherical pressure head, in MPa. The calculation formula is shown in equation (1). According to the mold with different bottom areas, after planning the grid, 16 to 81 penetration pressure values ​​or penetration resistance values ​​can be obtained. The average value, standard deviation and mean deviation rate of the penetration pressure value or penetration resistance value are obtained according to formulas (2), (3) and (4).

[0051] Finally, the magnitude of the calculated standard deviation is used to determine the dispersion of the test values, and the proportion of test points with a deviation rate within 15% is calculated using the mean deviation rate. The standard deviation and mean deviation rate complement each other, illustrating the dispersion of the test values ​​relative to the average value. The difference in the magnitude of the test values ​​at each test point reflects the fiber distribution. In areas where the fiber distribution is more clustered, greater force is required for the indenter to penetrate; in areas where the fiber distribution is more sparse, the resistance of the fibers to the indenter is reduced, resulting in lower penetration pressure. Poor fiber dispersion leads to a greater difference in penetration resistance between test points; the larger the difference between the test value and the average value, and the more test points there are, the larger the corresponding standard deviation and mean deviation rate. Conversely, good fiber dispersion results in a smaller standard deviation and mean deviation rate. The number of all measuring points with a deviation rate within 15% is assigned to the proportion of all measuring points. If the calculated proportion is greater than 75%, it indicates good fiber dispersion, resulting in small differences in penetration resistance values ​​at most measuring points. If this proportion is between 65% and 75%, it indicates good fiber dispersion, with penetration resistance values ​​being similar across most of the sample. If it is between 50% and 65%, it indicates poor fiber dispersion. If it is less than 50%, it indicates poor fiber dispersion.

[0052] The reinforcing effect of fibers is commonly evaluated by testing the macroscopic mechanical properties of cement-based materials, such as flexural strength and tensile strength. Mortar is prepared according to the Cement Mortar Test (ISO method). Test fibers, cement, standard sand, and additives are mixed evenly in a mortar mixer at a water-to-binder ratio of 0.5, a binder-to-sand ratio of 1:3, a fiber-to-mixture volume ratio of 0–5%, and an additive-to-cement mass ratio of 0–30%. The fiber-reinforced mortar mixture is then poured into a 40mm × 40mm × 160mm mold, filled in two stages, and vibrated for a certain period. After the surface is smoothed, a plastic film is placed over the sample. The sample is then placed in a cement curing chamber (box) and cured for 1 day before demolding. Samples with longer early curing ages (3 days, 7 days, etc.) are cured to the specified age. The flexural and compressive strengths of the mortar are measured using a flexural and compressive strength testing machine at the corresponding curing age. Comparing the strength of the fiber-added group with the control group demonstrates the reinforcing effect of fiber incorporation.

[0053] The present invention provides a testing and evaluation method for assessing the bonding degree between reinforcing fibers and early-age cementitious matrices. This method not only clearly distinguishes the reinforcing effects of different fiber types and addition amounts, but also simultaneously determines the dispersion effect of the fibers in the cement paste. The method is simple to prepare samples and convenient to test. It can be used in the research and preparation of cement-based materials for screening organic and inorganic non-metallic reinforcing fibers, and as a supplementary explanation for the improved tensile strength, toughness, and other properties of fiber-reinforced cement-based materials.

[0054] The following description, in conjunction with specific embodiments, illustrates this point.

[0055] Example 1

[0056] Using PO 42.5 cement with a water-cement ratio of 0.5, 2% polyvinyl alcohol (PVA) fiber (12mm in length) was added to the mixture to prepare fiber cement paste. The weighed PVA fiber and cement were placed together in a mixing pot and stirred slowly first, then quickly to disperse the fibers, and finally water was added and stirred rapidly until homogeneous. The mixture was then filled into a 2mm thick 100mm × 180mm mold, vibrated after filling, and then filled again. The surface was leveled with a steel ruler, wrapped with plastic film, and placed in a curing chamber for 1 day. The mold was removed, and a 1-day sample was taken, gridded, and placed on a circular pad of a penetration resistance device. A Φ5mm hemispherical indenter was selected; 32 penetration pressure data points were obtained from this sample.

[0057] All penetration pressures were converted into penetration resistances according to formula (1), and the obtained data were processed according to formulas (2), (3) and (4). The standard deviation was calculated to be 0.17 MPa, and the final average penetration resistance was 1.79 MPa. The mean deviation rate was 86.36%, which shows that the measured data was concentrated near the average value, indicating good fiber dispersion. The 7-day flexural and compressive strengths of mortar blocks with the same water-cement ratio of 0.5 and 2% cement by weight of polyvinyl alcohol fiber were 7.67 MPa and 34.70 MPa, respectively (see Table 1), which were 23.1% and 39.9% higher than the blank group without fiber, respectively.

[0058] Example 2

[0059] Other conditions were the same as in Example 1, except that 2% of the 12mm polyvinyl alcohol fiber added to the mixture was replaced with 2% cellulose fiber, with the cellulose fiber length being 5mm. A certain amount of cellulose fiber was first weighed and soaked in a sealed bottle containing a measured amount of water for 24 hours. The cellulose fiber was then dispersed using a whisk at 22 rpm and subsequently added to cement, which was then mixed thoroughly using a cement paste mixer. The thoroughly mixed mixture was then filled into a 2mm thick mold measuring 120mm × 120mm. This sample yielded 25 penetration pressure readings.

[0060] All penetration pressures were converted into penetration resistances according to formula (1), and the data were processed according to formulas (2), (3), and (4) to obtain a standard deviation of 0.85, an average penetration resistance of 1.41 MPa, and a mean deviation rate of 70.6%, indicating good fiber dispersion. The 7-day flexural and compressive strengths of the mortar block with a cellulose fiber content of 2% under the same water-cement ratio were 7.19 MPa and 34.10 MPa, respectively, which were 15.4% and 37.5% higher than those of the control group.

[0061] Example 3

[0062] Other conditions were the same as in Example 1, except that 2% of the 12mm polyvinyl alcohol fiber added to the mixture was replaced with 2% of the 6mm long polyester fiber added to the mixture. The well-stirred mixture was filled into a 3mm thick 100mm×100mm mold, and 16 through-pressure data were obtained from this sample.

[0063] All penetration pressures were converted into penetration resistances according to formula (1), and the data were processed according to formulas (2), (3), and (4) to obtain a standard deviation of 0.28, an average penetration resistance of 3.45 MPa, and a mean deviation rate of 75%. The values ​​at each measuring point were concentrated, indicating good fiber dispersion. The 7-day flexural and compressive strengths of the mortar block with a polyester fiber content of 2% at the same water-cement ratio were 6.55 MPa and 31.42 MPa, respectively, which were 5.1% and 26.7% higher than those of the blank group.

[0064] Example 4

[0065] Compared with Example 1, the Φ5mm hemispherical pressure needle was replaced with a Φ4mm hemispherical pressure needle, which was filled into a 2mm thick mold of 120mm×140mm. This sample can collect 30 penetration pressure data.

[0066] All the penetration pressure was converted into penetration resistance according to formula (1), and the data were processed according to formulas (2), (3) and (4) to obtain a standard deviation of 0.79 MPa and an average penetration resistance of 2.97 MPa. The mean deviation rate was 60%, and the dispersion deteriorated after the amount of fiber added increased. From the results, it can be seen that the smaller the diameter of the indenter, the greater the dispersion of the measured penetration resistance value, and the lower the accuracy and responsiveness compared to the indenter with a diameter of 5 mm.

[0067] Example 5

[0068] Compared with Example 1, instead of filling the 2mm thick mold with the well-stirred mixture, the mixture was filled into a 3mm thick 140mm×140mm mold. The Φ5mm hemispherical pressure needle was changed to a Φ4mm hemispherical pressure needle. This sample can collect 35 penetration pressure data.

[0069] All penetration pressures were converted into penetration resistances according to formula (1), and the data were processed according to formulas (2), (3) and (4) to obtain a standard deviation of 0.86 MPa and an average penetration resistance of 5.87 MPa. The mean deviation rate was 68.18%, indicating a decrease in fiber dispersion. The decrease in fiber dispersion after the sample thickness increased may be due to the increased complexity of fiber dispersion with increased thickness. More fibers would agglomerate in some areas and the overlap between fibers would be more complex, while the fiber distribution would be less in other areas, resulting in greater dispersion of the measured penetration resistance. In this embodiment, the fiber length far exceeded the sample thickness, and the uneven fiber dispersion would easily increase the effect of fiber accumulation in the thickness direction. In addition, the smaller diameter of the indenter also amplified the deviation value.

[0070] Comparative Example 1

[0071] Other conditions were the same as in Example 1, except that no fibers were added to the prepared cement mixture. No data could be obtained for this sample; the entire sample broke when the indenter was inserted with a certain force, making data impossible to obtain and thus preventing testing. The unfiber-free specimen was brittle. The 7-day flexural and compressive strengths of the unfiber-free mortar block at the same water-cement ratio were 6.23 MPa and 24.80 MPa, respectively.

[0072] Comparative Example 2

[0073] The other conditions were the same as in Example 1, except that the hemispherical indenter was replaced with a round, flat-headed indenter. The sample was filled into a 120mm × 180mm mold, and 40 penetration pressure values ​​were measured.

[0074] According to formulas (2), (3) and (4), the standard deviation of the data is 0.97 MPa, the average penetration resistance is 3.85 MPa, and the mean deviation rate is 53.85%. The deviation rate and error of the values ​​measured by the flat-headed indenter are greater than those of the hemispherical indenter, and the measured data are less accurate. First, the flat-headed indenter is sharper around the edges than the hemispherical one. When penetrating the fiber cement paste sample, it is easier to cut the fibers and penetrate the sample in areas where the fibers are less dispersed. At this time, the fibers are destroyed without playing their role. In areas where the fibers are more distributed, it is difficult to cut the large number of fibers. Thus, using the flat-headed indenter will reduce the mean deviation rate and make it difficult to accurately represent the fiber dispersion effect. Second, when penetrating the sample, the flat-headed indenter uses the entire plane as the contact surface. It is difficult to penetrate when pressing into the measuring point with a large fiber content, which increases the penetration pressure. Penetrating in areas where the fibers are less distributed will lead to a larger cracking range. Therefore, it is even more difficult for the flat-headed indenter to test the bonding effect between the fibers and the cement matrix. The spherical indenter, on the other hand, slowly penetrates the sample through the contact point, which makes the penetration and damage range of the circular indenter much larger than that of the hemispherical one.

[0075] Comparative Example 3

[0076] Other conditions are the same as in Example 1, except that the 2% polyvinyl alcohol fiber content is changed to 3%, and the mixed mixture is filled into a 140mm×140mm mold with a thickness of 3mm. The Φ5mm hemispherical pressure needle is changed to a Φ4mm hemispherical pressure needle. This sample can be pressed to obtain 35 penetration pressure data.

[0077] All penetration pressures were converted into penetration resistances according to formula (1), and the data were processed according to formulas (2), (3) and (4). The standard deviation was calculated to be 0.95 MPa, and the final average penetration resistance was 6.19 MPa. The mean deviation rate was 58.82%, which shows that the measured data was concentrated near the average value, indicating poor fiber dispersion. In this experiment, the fibers were not properly dispersed, and a large number of long fibers were difficult to disperse in the cement paste, resulting in poor dispersion. Meanwhile, the 7-day flexural and compressive strengths of mortar blocks with the same water-cement ratio of 0.5 and 3% cement by weight of polyvinyl alcohol fiber were 6.83 MPa and 30.23 MPa, respectively (see Table 1), which were 9.6% and 27.8% higher than the blank group, respectively.

[0078] Table 1 Comparison of Embodiments and Comparative Examples of the Invention

[0079]

[0080] The reinforcing and dispersing effects of fibers are related to factors such as fiber surface properties, specifications, dosage, and mixing process of the fiber-reinforced cement paste. Under the same mixing process and test conditions, the reinforcing and dispersing effects of polyvinyl alcohol used in the experiment were superior to the other two fibers. For the same fibers and test conditions, by changing the mixing process, the method provided by this invention can also optimize the mixing process.

[0081] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for evaluating the bond strength between reinforcing fibers and early-age cementitious matrix, characterized in that, include: Step S1: Mix the test fiber with cement and additives evenly in a mixing device to obtain fiber cement paste. Step S2: Pour the fiber cement paste mixture into the mold to a depth of 2mm to 4mm; after vibration, surface smoothing, and covering with a film, cure to the specified age and then demold. Step S3: Draw grid lines on the flat fiber cement paste sample at a certain interval. On the penetration resistance meter, use a penetrating needle with a hemispherical indenter to penetrate the sample at the grid intersection at a certain speed and obtain the penetration resistance value. Evaluate the bonding strength based on the penetration resistance value. Step S4: Based on the obtained number of penetration resistance values, calculate the average value, standard deviation, and mean deviation rate of the penetration resistance values. Step S5: Determine the dispersion of the test values ​​based on the magnitude of the standard deviation, and calculate the proportion of test points with a deviation rate within 15% based on the mean deviation rate.

2. The method according to claim 1, characterized in that, In step S1, the water-cement ratio is 0.35 to 0.55, the volume ratio of fiber to mixture is 0 to 5%, and the mass ratio of additive to cement is 0 to 30%.

3. The method according to claim 1, characterized in that, In step S2, the mold dimensions are 100-200mm × 100-200mm for the bottom surface.

4. The method according to claim 1, characterized in that, In step S3, the sample grid spacing is 20mm, the indenter head is a hemispherical shape with a diameter of Φ3mm~6mm, and the indentation rate of the indenter is 1mm / s.

5. The method according to claim 1, characterized in that, In step S4, the number of penetration resistance values ​​is 16-81.

6. The method according to claim 1, characterized in that, The test fibers were organic fibers and / or inorganic non-metallic fibers with diameters in the micrometer range.

7. The method according to claim 1, characterized in that, In step S5, the smaller the standard deviation, the better the fiber dispersion; the higher the proportion of the number of measuring points with a deviation rate within 15% to the total number of measuring points, the better the fiber dispersion effect.

8. The method according to claim 1, characterized in that, The early stage is no more than 7 days.

Images