Cement-based material frost resistance evaluation index and evaluation method

Abstract

The application belongs to the technical field of civil engineering construction, and relates to a cement-based material frost resistance evaluation index and an evaluation method. The cement-based material frost resistance evaluation index and the evaluation method are based on a temperature stress hypothesis, propose a freeze-thaw fatigue characteristic parameter as the cement-based material frost resistance evaluation index, measure the fatigue resistance of the cement-based material to reflect the frost resistance of the cement-based material, and obtain the freeze-thaw fatigue characteristic parameter through a fracture energy test and related calculation formulas disclosed in the application. * The application does not need to perform a complex freeze-thaw cycle test, greatly shortens the test time, does not need to purchase or rent a high-value freeze-thaw test machine, greatly reduces the threshold of the freeze-thaw test, popularizes the freeze-thaw test research and test, and plays an important role in improving engineering quality.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of civil engineering construction, and relates to a cement-based material frost resistance evaluation index and an evaluation method. BACKGROUND

[0002] Frost-thaw cycle damage is an important issue of cement-based material structure durability, and is also the first cause of structure degradation in northern China. Therefore, the frost resistance of cement-based materials is very important. At present, the evaluation methods of the frost resistance of cement-based materials at home and abroad mainly depend on the frost resistance test of cement-based materials. The test methods mainly include the quick freezing method, the slow freezing method and the salt freezing method. The quick freezing method and the slow freezing method are mainly based on the hydrostatic pressure theory proposed by Powers, and ASTMC666 is taken as a representative. The RILEMTCl76-IDC-2002CF and CDF salt freezing methods are mainly based on the osmotic pressure theory. The evaluation of the frost resistance of cement-based materials in different environments should be selected according to the needs, and the weight loss rate, the relative dynamic elastic modulus retention and the strength loss rate are used to reflect the frost resistance.

[0003] Although the research on the frost resistance of cement-based materials has a long history and certain achievements have been made in many aspects, due to the complexity of the freeze-thaw problem, many contents still need to be further improved, which also includes the frost resistance mechanism. As the most basic content of the freeze-thaw damage problem, the evaluation of the frost resistance of materials still has many problems. The existing freeze-thaw test method is relatively complex, and the instruments used are mostly freeze-thaw testing machines with high price and high equipment requirements, which brings some disadvantages to the freeze-thaw research of cement-based materials. First, the total time of freeze-thaw cycle is long, and the workload is large. The minimum test time is about 20 days for every 100 freeze-thaw cycles, so it takes months to complete the entire freeze-thaw cycle process. Second, the cost of freeze-thaw cycle test is high. The special instruments used for freeze-thaw cycle test are mostly high in purchase cost, and it is also expensive to rent freeze-thaw testing machines from others. Finally, the freeze-thaw cycle test equipment has high requirements. The freeze-thaw testing machine has a relatively strict temperature control system, so the machine is generally equipped with a computer program for control. Although this can achieve more accurate test, it also puts higher requirements on the equipment. Therefore, the existing freeze-thaw test method has obvious problems such as long test period, large workload and high test equipment requirements, and also has other problems such as large result error.

[0004] With the development of high-strength / high-performance cement-based materials, high-strength cement-based materials have very high frost resistance due to their small water-binder ratio and high density. After more than 600 freeze-thaw tests, the dynamic elastic modulus is still more than 90%, the weight loss rate is negative, the surface does not peel off, and the mass even increases slightly. This leads to the fact that the traditional test method is very time-consuming and laborious, and it is not easy to evaluate the frost resistance of related cement-based materials.

[0005] With the development of research, the current anti-freezing evaluation index also includes new concepts such as critical expansion value and critical saturation, which provides a new idea for the evaluation of frost resistance. Therefore, it is necessary to further explore the evaluation method and evaluation index of the frost resistance of cement-based materials, and to design a simpler test method, which has important significance for the research and design of high frost resistance cement-based materials, and can better serve large-scale engineering practice and improve engineering quality. SUMMARY

[0006] Therefore, the purpose of the present application is to provide a cement-based material frost resistance evaluation index and evaluation method, mainly aiming at high-strength cement-based materials, which is simple in test process, small in workload, greatly reduces the test time, and does not need to perform complex freeze-thaw cycle test, thereby eliminating the need to purchase or rent expensive freeze-thaw test machines, so that the cement-based material frost resistance evaluation reaches the same level as the strength evaluation, and can be widely used in various fields of scientific research and actual engineering construction.

[0007] To achieve the above purpose, the present application provides the following technical solutions:

[0008] On the one hand, the present application provides a cement-based material frost resistance evaluation index, based on the hypothesis of temperature stress, a freeze-thaw fatigue characteristic parameter is proposed as the cement-based material frost resistance evaluation index, which reflects the cement-based material frost resistance by measuring the fatigue resistance of the cement-based material;

[0009] The freeze-thaw fatigue characteristic parameter is divided into a fatigue crack appearance stage and a fatigue crack propagation stage, so as to reflect the formation and development process of cracks under the action of freeze-thaw, the fatigue crack appearance stage is affected by the tensile strength and the elastic modulus, and the fatigue crack propagation stage can be measured by the resistance force in the microscopic propagation process of the crack tip, i.e. the microscopic fracture energy, so as to obtain the calculation formula of the freeze-thaw fatigue characteristic parameter as follows:

[0010]

[0011]

[0012] A * =d

[0013] In the above formula: D is the freeze-thaw fatigue characteristic parameter, f cf is the flexural strength of the cement-based material, E is the elastic modulus of the cement-based material, G * is the microscopic fracture energy; W is the area under the load-deflection curve, A * is the relative area of the fracture surface, A is the cross-sectional area of the fracture surface, and d is the fractal dimension of the fracture surface.

[0014] Further, the fracture energy test of the cement-based material is performed by the three-point bending test, so as to obtain the f cf , E, W, A and d.

[0015] In another aspect, the present application further provides a method for evaluating the frost resistance of a cement-based material, comprising the following steps:

[0016] Step 1: preparing a cement-based material test piece

[0017] Step 2: performing the fracture energy test of the cement-based material test piece, and recording the material flexural strength f cf , the material elastic modulus E, the area W under the load-deflection curve, and the cross-sectional area A of the fracture surface, and calculating the fractal dimension d of the fracture surface of the cement-based material test piece;

[0018] Step 4: calculating the frost resistance evaluation index, using the freeze-thaw fatigue characteristic parameter as the frost resistance evaluation index, and the calculation formula is as follows:

[0019]

[0020]

[0021] A * =d

[0022] wherein, * is the relative area of the fracture surface, G * is the micro-fracture energy, and D is the freeze-thaw fatigue characteristic parameter, and the higher the value of D represents the stronger the frost resistance.

[0023] Further, in step 2, the fracture energy of the cement-based material test piece is tested by the three-point bending test, and the loading method is three-point loading, and the displacement control loading method is used.

[0024] Further, the loading rate of the displacement control loading is ≤0.5 mm / min.

[0025] Further, the central cut of the cement-based material test piece in the three-point bending test is preformed when the cement-based material test piece is prepared or is cut after the cement-based material test piece is formed.

[0026] Further, the calculation of the fractal dimension d of the fracture surface of the cement-based material test piece comprises scanning the fracture surface of the cement-based material test piece by using a three-dimensional laser sensor, drawing the obtained three-dimensional coordinate data into a graph, and calculating by using the box-counting fractal dimension calculation method.

[0027] Further, when it is needed to judge whether the frost resistance of the cement-based material is qualified, the cement-based material meeting the frost resistance requirement is taken as a reference group, the cement-based material to be judged for the frost resistance is taken as a test group, the freeze-thaw fatigue characteristic parameters of the reference group and the test group are respectively determined, and the freeze-thaw fatigue characteristic parameters of the reference group and the test group are compared, and the freeze-thaw fatigue characteristic parameters of the test group is greater than or equal to the freeze-thaw fatigue characteristic parameters of the reference group, so that it can be judged that the frost resistance of the cement-based material is qualified.

[0028] The present application has the following advantages:

[0029] 1. The present application provides a cement-based material frost resistance evaluation index and evaluation method, based on the hypothesis of temperature stress, a freeze-thaw fatigue characteristic parameter is proposed as the cement-based material frost resistance evaluation index, the freeze-thaw fatigue characteristic parameter reflects the cement-based material frost resistance by measuring the fatigue resistance of the cement-based material, the freeze-thaw fatigue characteristic parameter can be obtained by the fracture energy test and the related calculation formula disclosed in the present application, without complex freeze-thaw cycle test, so that the cement-based material frost resistance evaluation index and evaluation method is not only suitable for large and medium-sized scientific research units and enterprises, but also can be used by small scientific research institutes and engineering project departments with weak funds and technical strength, greatly reducing the threshold of freeze-thaw test, making freeze-thaw test research and testing popular, and playing an important role in improving engineering quality.

[0030] And the test workload of the evaluation method is small, the test time is short, the evaluation of the frost resistance of high-strength and high-performance cement-based materials can obtain test results within one day after the cement mortar test piece is maintained, and the evaluation conclusion is clear. The cost required by the test is very low, and due to the small workload and short time, the cost of manpower, material resources and other aspects is significantly reduced, and the cost of freeze-thaw test of cement-based materials is greatly reduced. The steps of the evaluation method are clear, and the method is easy to master. The reliability of the test results can be easily guaranteed, the test precision can be improved by increasing the number of corresponding test pieces of the same age, and the test is simple and easy to operate, and there is no repeated test.

[0031] 2. By proposing the cement-based material frost resistance evaluation index, a freeze-thaw fatigue characteristic parameter value database of the national, regional and different cement-based materials is further established, the frost resistance of various mix proportion materials of different engineering sites can be easily judged, the corresponding conclusion whether the requirements are met is obtained, the related expenditure of the national and social levels is greatly reduced, and the foundation for widely and massively improving engineering quality is laid.

[0032] Additional advantages, objects, and features of the application will be apparent to those skilled in the art upon examination of the following specification. It is intended that the application not be limited by the disclosed implementation, but that it include all such variations and modifications to the full extent allowed by law. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to make the objectives, technical solutions and advantages of the present application clearer, the preferred embodiments of the present application will be described in detail below with reference to the drawings, in which:

[0034] Figure 1 A schematic diagram of a cement-based material test piece;

[0035] Figure 2 A schematic diagram of a three-point bending test of a cement-based material;

[0036] Figure 3 A schematic diagram of a three-dimensional laser sensor scanning the fracture surface of a cement-based material test piece;

[0037] Figure 4 A three-dimensional topography of the fracture surface of a cement-based material test piece. DETAILED DESCRIPTION

[0038] The embodiments of the present application will be described in detail below with reference to specific examples. Those skilled in the art can easily understand other advantages and effects of the present application from the content disclosed in the specification. The present application can also be implemented or applied in different specific embodiments, and the details in the specification can be modified or changed based on different views and applications without departing from the spirit of the present application. It should be noted that the diagrams provided in the following examples only illustrate the basic concept of the present application in a schematic manner, and the following examples and features in the examples can be combined with each other without conflict.

[0039] The drawings are only used for illustrative explanation, and the representation is only a schematic diagram, not a physical diagram, and should not be understood as a limitation of the present application. In order to better illustrate the embodiments of the present application, some components in the drawings may be omitted, enlarged or reduced, and do not represent the actual size of the product. It is understandable to those skilled in the art that some well-known structures and their descriptions in the drawings may be omitted.

[0040] The same or similar reference numerals in the drawings of the embodiments of the present application correspond to the same or similar components; in the description of the present application, it is understood that if the orientations or positional relationships indicated by the terms "upper", "lower", "left", "right", "front", "back" and the like are based on the orientations or positional relationships shown in the drawings, they are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, therefore the terms describing the positional relationships in the drawings are only used for exemplary illustration, and cannot be understood as a limitation on the present application, for those skilled in the art, the specific meanings of the above terms can be understood according to the specific circumstances.

[0041] A method for evaluating the frost resistance of high-strength / high-performance cement-based materials, comprising the following steps:

[0042] 1) Proposal of frost resistance evaluation index

[0043] An absolute value reflecting the characteristics of cement-based materials, i.e., a freeze-thaw fatigue characteristic parameter, is proposed to measure the fatigue resistance of cement-based materials and then reflect the freeze-thaw resistance of cement-based materials, the freeze-thaw fatigue characteristic parameter is divided into a fatigue crack appearance stage and a fatigue crack propagation stage, to reflect the formation and development process of cracks under freeze-thaw action; the fatigue crack appearance stage is affected by the tensile strength and the elastic modulus, and the fatigue crack propagation stage can be measured by the resistance in the microscopic propagation process of the crack tip, i.e., the microscopic fracture energy, the calculation formula of the freeze-thaw fatigue characteristic parameter is as follows:

[0044]

[0045]

[0046] A * =Ad (3)

[0047] In the above formula: D is the fatigue characteristic parameter; f cf is the bending strength of the material (reflecting the tensile strength); E is the elastic modulus of the material; G * is the microscopic fracture energy; W is the area under the load-deflection curve; A * is the relative area of the fracture surface; A is the cross-sectional area of the fracture surface; d is the fractal dimension of the fracture surface.

[0048] The freeze-thaw fatigue characteristic parameter is determined by a test method. When it is necessary to compare the frost resistance of cement-based materials, a cement-based material meeting the frost resistance requirement is taken as a reference group, a cement-based material to be judged for frost resistance is taken as a test group, the freeze-thaw fatigue characteristic parameters of the reference group and the test group are respectively determined, and the freeze-thaw fatigue characteristic parameters of the reference group and the test group are compared. The higher the value of the freeze-thaw fatigue characteristic parameter is, the stronger the frost resistance is. When it is necessary to determine whether the frost resistance of a cement-based material meets the requirement, the freeze-thaw fatigue characteristic parameter of the test group is greater than or equal to the freeze-thaw fatigue characteristic parameter of the reference group, and it is determined that the frost resistance of the cement-based material is qualified.

[0049] 2) Preparation of test pieces

[0050] For the cement-based material to be determined, the test group test piece is formed, and curing is performed. If necessary, the reference group test piece for comparison can also be formed and cured at the same time. The forming and curing test process is performed according to the related specifications in the prior art. The shape of the test piece is a three-point bending test piece of the cement-based material. The size of the test piece can be determined according to the actual situation. The central cut of the three-point bending test piece can be prefabricated or cut after forming.

[0051] 3) Perform fracture energy test of test piece

[0052] The fracture energy of the cement-based material is tested by referring to the three-point bending test of RILEM-Draft-Recommendation (50-FCM). The loading method is three-point loading. The displacement control loading mode is adopted, and the loading rate is ≤0.5 mm / min. Thus, the cement-based material modulus of rupture f cf , the material elastic modulus E, the area W under the load-deflection curve, and the area A of the fracture surface are obtained.

[0053] 4) Determination of fractal dimension of test piece fracture surface

[0054] The Gocator three-dimensional laser sensor is used to scan the fracture surface. The obtained three-dimensional coordinate data is plotted into a graph by using Matlab software. Then, the box-counting fractal dimension calculation method is used to calculate the fractal dimension. Thus, the fractal dimension d of the test piece fracture surface is obtained.

[0055] 5) Calculation of frost resistance evaluation index

[0056] According to the determined material modulus of rupture f cf , the material elastic modulus E, the area W under the load-deflection curve, the area A of the fracture surface, and the fractal dimension d of the fracture surface, the freeze-thaw fatigue characteristic parameters can be obtained by substituting the formula (1), (2), and (3).

[0057] When the anti-freezing ability of the comparative material is needed, the freeze-thaw fatigue characteristic parameter values of the reference group and the test group are calculated respectively, and the higher the value is, the stronger the anti-freezing ability is; when it is needed to determine whether the anti-freezing ability of the material meets the requirements, the freeze-thaw fatigue characteristic parameter value of the test group is compared with the freeze-thaw fatigue characteristic parameter value of the reference group that meets the anti-freezing requirement, and if it is greater than the value, it can be determined that the cement-based material of the test group is qualified.

[0058] The theoretical basis of the anti-freezing evaluation index of the high-strength / high-performance cement-based material and the evaluation method thereof lies in the temperature stress hypothesis proposed by Mehta in 1992, which is aimed at high-strength / high-performance cement-based materials, and believes that the destruction of the cement-based material is because the thermal expansion coefficient of the aggregate is different from that of the hydration product gel, and the deformation difference between the two during the temperature change is large, thereby generating temperature stress, which leads to the damage of the cement-based material under similar fatigue action; therefore, the anti-freezing and thawing ability of the high-strength cement-based material can be measured by the anti-fatigue ability of the cement-based material, according to the fatigue damage process, the fatigue damage is divided into the fatigue crack appearance stage and the fatigue crack propagation stage, the anti-fatigue damage ability of the material in the fatigue crack appearance stage is affected by the tensile strength and the elastic modulus, and the anti-fatigue damage ability of the material in the fatigue crack propagation stage can be measured by the resistance in the micro expansion process of the crack tip, i.e. micro fracture energy.

[0059] The anti-freezing evaluation index of the cement-based material and the evaluation method thereof can judge the anti-freezing ability of the cement-based material through one three-point bending fracture energy test and fracture surface fractal dimension test, greatly reduce the workload and test time, make the freeze-thaw test of the cement-based material be quickly completed, and reach the same level as the strength test of the cement-based material, so that the judgment of the anti-freezing ability of the cement-based material can be popularized.

[0060] Example 1

[0061] The freeze-thaw fatigue characteristic parameter value of the high-strength cement mortar is determined, and whether it meets the engineering needs and whether the related modification method is helpful to improve the anti-freezing performance of the material is determined.

[0062] Step 1: Proposing the anti-freezing evaluation index

[0063] List the calculation formula of the freeze-thaw fatigue characteristic parameter:

[0064]

[0065]

[0066] A * =Ad (3)

[0067] Wherein: D is the fatigue characteristic parameter; f cf is the bending strength of the material (reflecting the tensile strength); E is the elastic modulus of the material; G* is the micro-fracture energy; W is the area under the load-deflection curve; A * is the relative area of the fracture surface; A is the cross-sectional area of the fracture surface;

[0068] d is the fractal dimension of the fracture surface.

[0069] Step 2 Preparation of cement mortar test piece

[0070] As shown in Figure 1 , high-strength cement mortar test pieces were prepared according to the current cement mortar test piece molding method, with a test piece size of 330 mm * 70 mm * 35 mm. After molding, the test pieces were placed in a curing box and cured under the current specified standard curing conditions for 3 days. The test piece notch depth was 35 mm, which was formed by cutting after molding.

[0071] Step 3 Fracture energy test of cement-based material

[0072] As shown in Figure 2 , the fracture energy of the cement mortar test piece was tested by three-point bending test according to RILEM-Draft-Recommendation (50-FCM). The loading method was three-point loading, and the loading rate was taken as 0.5 mm / min in displacement control loading mode.

[0073] Thus, the material flexural strength f cf ; the material elastic modulus E; the area W under the load-deflection curve; and the cross-sectional area A of the fracture surface were obtained. The results are shown in Table 1.

[0074] Table 1 Test results of three-point bending test

[0075]

[0076] Step 4 Determination of fractal dimension of test piece fracture surface

[0077] As shown in Figure 3 and Figure 4 , the fracture surface was scanned using a Gocator three-dimensional laser sensor, and the obtained three-dimensional coordinate data was plotted using Matlab software. Then, the fractal dimension was calculated using the box-counting fractal dimension calculation method. Thus, the fractal dimension d value of the fracture surface of the cement mortar test piece was 2.63.

[0078] Step 5 Calculation of frost resistance evaluation index

[0079] According to the measured material flexural strength f cf ; the material elastic modulus E; the area W under the load-deflection curve; the cross-sectional area A of the fracture surface; and the fractal dimension d of the fracture surface, the freeze-thaw fatigue characteristic parameter D was calculated by substituting the values into formulas (1), (2), and (3). The result was 2.26 x 10-5 (N / mm), and the specific data are shown in Table 2.

[0080] Table 2 Freezing and thawing fatigue characteristic parameter values

[0081]

[0082] Since another cement-based material which is known to satisfy the engineering anti-freezing performance requirement has been obtained a freezing and thawing fatigue characteristic parameter D result of 1.47 x 10 -5 (N / mm), and the cement-based material is the cement-based material before modification of the cement-based material tested in the present example, it can be concluded that the high-strength cement mortar determined in the present test satisfies the engineering requirement, and since the freezing and thawing fatigue characteristic parameter D of the cement-based material before modification is 1.47 x 10 -5 (N / mm), and the freezing and thawing fatigue characteristic parameter D after modification is 2.26 x 10 -5 (N / mm), it is judged that the modification method used is helpful to the improvement of the anti-freezing performance of the cement-based material.

[0083] Finally, it is to be explained that the above examples are only used to illustrate the technical solutions of the present application and not to limit the present application, and although the present application has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present application can be modified or replaced equivalently without departing from the purpose and scope of the technical solutions, and they should all be covered in the scope of the claims of the present application.

Claims

1. A method for evaluating the frost resistance of cement-based materials, characterized in that: Based on the temperature stress hypothesis, a freeze-thaw fatigue characteristic parameter is proposed as an evaluation index for the freeze-thaw resistance of cement-based materials. This freeze-thaw fatigue characteristic parameter reflects the freeze-thaw resistance of cement-based materials by measuring their fatigue resistance. The freeze-thaw fatigue characteristic parameters are divided into the fatigue crack initiation stage and the fatigue crack propagation stage to reflect the formation and development process of cracks under freeze-thaw action. The fatigue crack initiation stage is affected by tensile strength and elastic modulus, while the fatigue crack propagation stage can be measured by the resistance force encountered during the microscopic propagation process of the crack tip—microscopic fracture energy. Thus, the calculation formula for the freeze-thaw fatigue characteristic parameters is as follows: In the above formula: These are characteristic parameters of freeze-thaw fatigue. It refers to the flexural strength of cement-based materials. It is the elastic modulus of cement-based materials. It is the microscopic fracture energy; It is the area under the load-deflection curve. It is the relative area of ​​the fracture surface. It is the cross-sectional area of ​​the fracture surface. It is the fractal dimension of the fracture surface; The evaluation method includes the following steps: Step 1: Prepare cement-based material specimens; Step 2: Conduct a fracture energy test on the cement-based material specimen and record the flexural strength of the material. Material elastic modulus Area under the load-deflection curve and the cross-sectional area of ​​the fracture surface And calculate the fractal dimension d of the fracture surface of the cement-based material specimen; Step 4: Calculate the freeze-thaw resistance evaluation index, using freeze-thaw fatigue characteristic parameters as the freeze-thaw resistance evaluation index. A higher value indicates stronger freeze resistance; In step 2, the fracture energy of the cement-based material specimen is tested by a three-point bending test. The loading method is three-point loading, and the displacement-controlled loading method is adopted. The loading rate of the displacement-controlled loading is ≤0.5mm / min.

2. The method for evaluating the frost resistance of cement-based materials according to claim 1, characterized in that: In the three-point bending test, the incision in the middle of the cement-based material specimen is either pre-formed during the preparation of the cement-based material specimen or cut after the cement-based material specimen has been formed.

3. The method for evaluating the frost resistance of cement-based materials according to claim 1, characterized in that: The calculation of the fractal dimension d of the fracture surface of the cement-based material specimen includes scanning the fracture surface of the cement-based material specimen with a three-dimensional laser sensor, plotting the obtained three-dimensional coordinate data into a graph, and calculating it using the box-shaped fractal dimension calculation method.

4. The method for evaluating the frost resistance of cement-based materials according to claim 1, characterized in that: When it is necessary to determine whether the frost resistance of cement-based materials is qualified, cement-based materials that meet the frost resistance requirements are used as the benchmark group, and cement-based materials whose frost resistance is to be determined are used as the test group. The freeze-thaw fatigue characteristic parameters of the benchmark group and the test group are measured respectively, and the freeze-thaw fatigue characteristic parameters of the benchmark group and the test group are compared. If the freeze-thaw fatigue characteristic parameter of the test group is greater than or equal to the freeze-thaw fatigue characteristic parameter of the benchmark group, the frost resistance of the cement-based material can be determined to be qualified.

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