Method for evaluating low-temperature performance of rubber asphalt based on fractional order viscoelastic model

Abstract

The invention discloses a method for evaluating the low-temperature performance of rubber asphalt based on a fractional order viscoelastic model, belongs to the technical field of road engineering, and solves the problem that an existing method for evaluating the low-temperature performance of the rubber asphalt cannot well represent the ductility of the rubber asphalt at low temperature. The method comprises the following steps: carrying out a rubber asphalt low-temperature performance test, making rubber asphalt into an asphalt beam, applying a constant load to the asphalt beam through a bending beam rheometer, and recording the test load of the asphalt beam and the change condition of midspan deflection along with time in the whole test process; carrying out a low-temperature bending beam rheological test on the asphalt beam under different test temperature conditions, and calculating the creep stiffness modulus and the creep rate of the rubber asphalt under different test temperatures; and calculating an improved creep compliance index. According to the method, the simple fractional order model is applied to the creep flexibility analysis result of the BBR test, the low-temperature evaluation index of the rubber asphalt is further researched, and the low-temperature performance of the rubber asphalt is evaluated more accurately.

Description

Technical Field

[0001] The invention belongs to the technical field of road engineering, and in particular relates to a method for evaluating the low-temperature performance of rubber asphalt based on a fractional-order viscoelastic model. Background Art

[0002] In the cold regions of northern my country, the temperature difference of the road surface can reach 70℃ in four seasons. The low temperature lasts for a long time in winter, the amount of rain and snow is large, and the frost heave and spring thaw are obvious. This unique cold climate with long-term repeated large temperature difference causes low-temperature cracks in asphalt pavements to become the most important structural damage form of the surface layer of highways in this area. Therefore, how to prevent and control the early damage of asphalt pavements, reduce or avoid the occurrence of low-temperature cracks, and improve the low-temperature durability of pavements has become a difficult problem that urgently needs to be solved in the construction, maintenance and maintenance of high-grade highways in cold regions. Improving the use quality of the mixture and extending the service life of the pavement by using high-quality modified asphalt has become one of the advanced technologies that are currently valued at home and abroad. Rubber asphalt, especially rubber asphalt produced by making rubber powder from waste tires, has excellent low-temperature crack resistance and water damage resistance. The natural rubber, synthetic rubber, sulfur and carbon black contained in waste tires are good modifiers for road petroleum asphalt. Adding rubber powder to the base asphalt can improve the flexibility of asphalt and improve the low-temperature performance of asphalt.

[0003] As a new road material, rubber asphalt has excellent performance and is increasingly widely used in cold northern regions. However, as a new material, there is no good low-temperature performance evaluation method and evaluation index, and the low-temperature evaluation system is not perfect, which greatly limits the application of rubber asphalt in cold regions. At present, my country's standards still use the 5℃ ductility index to evaluate the low-temperature performance of rubber asphalt. However, relevant studies have shown that the 5℃ ductility index cannot well characterize the ductility of rubber asphalt at low temperatures, and as a low-temperature evaluation index for modified asphalt, it needs further research and improvement. Based on the limitations of the current low-temperature evaluation index of rubber asphalt in my country, it is of great significance to study the low-temperature evaluation index of rubber asphalt. At present, the research on the low-temperature performance of rubber asphalt can be further developed and innovated on the basis of the research on the low-temperature performance of base asphalt. In terms of the research on the low-temperature performance of base asphalt, the US SHRP has made outstanding contributions in this regard, especially the bending beam rheology test (BBR) based on rheology has become one of the most commonly used, most practical and most effective means and methods for studying the low-temperature performance of asphalt materials. At present, more and more researchers have begun to use the BBR test to study the low-temperature performance and low-temperature evaluation index of polymer modified asphalt, and have achieved many results. However, the current research on BBR test mainly stays on the consideration of modulus or relaxation capacity index alone, and there are relatively few studies that comprehensively consider modulus and relaxation capacity index. The performance of rubber asphalt is complex, and the quality of its low-temperature performance is affected by factors such as deformation capacity and stress relaxation capacity at low temperatures. It is one-sided to evaluate its low-temperature crack resistance by considering S or m alone. There are many mechanical models that reflect the linear viscoelastic behavior of asphalt during the BBR test, such as Burgers model, generalized Maxwell model, generalized Kelvin-Voigt model and fractional order model, which can be divided into two categories: integer order model and real order model. The present invention adopts a fractional order viscoelastic model to evaluate the low-temperature performance of rubber asphalt. Compared with the integer order model, it has sufficient accuracy and fewer parameters, and is the simplest real order model. Summary of the invention

[0004] The purpose of the present invention is to provide a method for evaluating the low temperature performance of rubber asphalt based on a fractional viscoelastic model, so as to solve the problem that the current low temperature performance evaluation method of rubber asphalt cannot well characterize the ductility of rubber asphalt at low temperatures. The fractional viscoelastic model parameters fitted by BBR test data can better predict the creep rate of rubber asphalt, and a new index is proposed to further accurately evaluate the low temperature performance of rubber asphalt.

[0005] The technical solution of the present invention is: a method for evaluating the low-temperature performance of rubber asphalt based on a fractional-order viscoelastic model, comprising the following steps: A. Conduct low-temperature performance test of rubber asphalt, make rubber asphalt into asphalt beam, apply constant load to the asphalt beam through bending beam rheometer, and control and record the test load and mid-span deflection of the asphalt beam over time during the whole test process through computer acquisition system; B. Conduct low-temperature bending beam rheological tests on asphalt beams at different test temperatures, and calculate the creep stiffness modulus S(t) and creep rate m(t) of rubber asphalt at different test temperatures; C. Calculate the improved creep compliance index D′(t) by the following formula: D′(t)=αAt -α-1 (16) Where t is time in seconds. A and α are obtained by the following formula: log[S(t)]=log A-αlog(t) (5) The low temperature performance of rubber asphalt is evaluated by the improved creep compliance index D′(t).

[0006] Further, in step B, the test meets the following conditions: the creep stiffness modulus at 60s is not greater than 300 MPa, and the creep rate value is not less than 0.3.

[0007] The formula derivation process is as follows: Step 1: Conduct a low-temperature performance test on rubber asphalt. Make the rubber asphalt into an asphalt beam, apply a constant load to the asphalt beam through a bending beam rheometer, and control and record the test load and mid-span deflection of the asphalt beam over time during the entire test process through a computer acquisition system.

[0008] Step 2: Conduct low-temperature bending beam rheological tests on asphalt beams at different test temperatures, and calculate the creep stiffness modulus S(t) and creep rate m(t) of rubber asphalt at different test temperatures. The expressions of S(t) and m(t) are: log[S(t)]=A+B[log(t)]+C[log(t)] 2 (1) Where: A, B, C are regression coefficients, S(t) is the creep stiffness modulus, and m(t) is the creep rate.

[0009] Step 3: Assume that the power law equation for creep compliance is equivalent to Figure 1 ( Figure 1It is a viscoelastic model of rubber asphalt, which can simply represent the mechanical behavior of rubber asphalt in a certain temperature range. The mechanical behavior of rubber asphalt is described by a simple fractional element, and the creep compliance D(t) of rubber asphalt is described by the power-law equation, D(t)=At α 0 ≤ α ≤ 1.

[0010] Step 4: Determine the value of the creep rate m(t) using the simple fractional element. Fit the power-law by the value of m(t) determined by the test method in Step 2 and the value of m(t) determined using the concept of the simple fractional derivative to determine the parameters α and A. The value of the creep rate m(t) determined by the simple fractional element is related to the power of α. For the relationship between S(t) and m(t), it can be obtained that: where B = 1 / A. Then, by taking the logarithm of both sides of the equation and using the logarithmic property, it can be deduced that log[S(t)] = log A - αlog(t) (5) Using the definition of the value of m(t), it can be determined that

[0011] Step 5: Determine the relationship between S(t) and m(t) based on the simple fractional derivative. Obtain an improved creep compliance index D′(t) through the transformation formula. The process is as follows: By defining the function Substitute Equation (2) into (8), and it can be obtained that: From the relationship curve between log[S(t)] and the test temperature ( Figure 2 ), it can be clearly seen that B + 2Clog(t) < 0 (10) According to the property of the absolute function, it can be obtained that: B + 2Clog(t) = -m(t) (11) Finally, according to Equation (8) and Equation (10), it can be obtained that: As can be seen from Equation (3), S(t) is inversely proportional to D(t). Substitute Equations (2), (4), (5), (6), (8) into Equation (12) to obtain: D′(t) = αAt -α-1 (16).

[0012] The present invention evaluates the low-temperature performance of rubber asphalt by using a bending beam rheometer (BBR) test, obtains a fractional-order viscoelastic model describing the low-temperature performance of rubber asphalt using a power-law equation, and evaluates the low-temperature performance of rubber asphalt by calculating parameters A and α.

[0013] The beneficial effects of the present invention are as follows: the present invention introduces the theoretical basis of integer-order viscoelastic model and fractional-order viscoelastic model, utilizes the general power law, applies the simple fractional-order model to the creep compliance analysis result of the BBR test, and further studies the low-temperature evaluation index of rubber asphalt, so as to more accurately evaluate the low-temperature performance of rubber asphalt, and provides a certain reference for better studying the low-temperature performance of rubber asphalt and rationally using rubber asphalt. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is the viscoelastic model of rubber asphalt; Figure 2 It is the relationship curve between log[S(t)] and test temperature. DETAILED DESCRIPTION

[0015] The present invention is further described in detail below through specific implementation modes.

[0016] Embodiment 1, Rubber asphalt with rubber content of 10%, 15%, and 20% was prepared respectively, and the low temperature performance was evaluated at four temperatures of -12℃, -18℃, -24℃, and -30℃. The method is as follows: A. Conduct low-temperature performance test of rubber asphalt. Make rubber asphalt into asphalt beam. Apply constant load to the asphalt beam through bending beam rheometer. Control and record the test load and mid-span deflection of the asphalt beam over time during the whole test process through computer acquisition system.

[0017] B. Low-temperature bending beam rheological tests are carried out on asphalt beams under different test temperature conditions. The test meets the following conditions: the creep stiffness modulus at 60s is not greater than 300MPa, and the creep rate value is not less than 0.3; the creep stiffness modulus S(t) and creep rate m(t) of rubber asphalt at different test temperatures are calculated.

[0018] C. Calculate the parameters A and α by the following formula: log[S(t)]=log A-αlog(t) (5) Wherein, t is time, the unit is s; the calculation results of parameters A and α are shown in Table 1. Table 1

[0019] The improved creep compliance index D(t) is calculated by the following formula: D′(t)=αAt -α-1 (16) The calculation results of the improved creep compliance index D′(t) at different temperatures are shown in Table 2. The low-temperature performance of rubber asphalt is evaluated by the improved creep compliance index D′(t). Table 2 Asphalt Type -12℃ -18℃ -24℃ -30℃ CR10 3.82E-06 2.48E-06 1.17E-06 9.58E-07 CR15 4.41E-06 2.97E-06 1.55E-06 1.26E-06 CR20 7.69E-06 3.96E-06 2.49E-06 4.20E-06

[0020] Table 2 shows the relationship between D'(t) and test temperature of rubber asphalt at 60 seconds. As the test temperature increases, D'(t) also increases. Considering the coefficients mentioned in Table 1 and the concept of rheological behavior, it can be clearly seen that at higher temperatures (-12°C), the larger the α of rubber asphalt, the stronger the viscosity and the stronger the stress relaxation ability.

[0021] At this point, the performance evaluation of rubber asphalt at low temperatures has been completed.

Claims

1. A method for evaluating the low-temperature performance of rubber asphalt based on a fractional-order viscoelastic model, characterized in that The following steps are involved: A. Conduct low-temperature performance test of rubber asphalt, make rubber asphalt into asphalt beam, apply constant load to the asphalt beam through bending beam rheometer, and control and record the test load and mid-span deflection of the asphalt beam over time during the whole test process through computer acquisition system; B. Conduct low-temperature bending beam rheological tests on asphalt beams at different test temperatures, and calculate the creep stiffness modulus S(t) and creep rate m(t) of rubber asphalt at different test temperatures; C. Calculate the improved creep compliance index D′(t) by the following formula: D′(t)=αAt -α-1 (16) Where t is time in seconds. A and α are obtained by the following formula: log[S(t)]=logA-αlog(t) (5) The low temperature performance of rubber asphalt is evaluated by the improved creep compliance index D′(t).

2. The method for evaluating the low temperature performance of rubber asphalt based on the fractional-order viscoelastic model according to claim 1, characterized in that: In step B, the test meets the following conditions: the creep stiffness modulus at 60s is not greater than 300MPa, and the creep rate value is not less than 0.3.

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