An asphalt stress and cracking temperature measuring device and measuring method

Abstract

The application discloses a kind of asphalt stress and cracking temperature measuring device and measuring method, its device includes axial tension and compression loading frame, axial tension and compression loading frame is provided with programmable temperature control box, programmable temperature control box is provided with asphalt test piece;Programmable temperature control box bottom is provided with rigid base, the bottom of rigid base is fixedly connected with the bottom of axial tension and compression loading frame;The top of asphalt test piece is provided with upper clamp, the low end of asphalt test piece is provided with lower clamp, axial tension and compression loading frame is provided with displacement sensor for monitoring the displacement of upper clamp;Lower clamp bottom is fixedly connected with rigid base;Asphalt stress and cracking temperature measuring device in the application is directly measured and evaluates the thermal cracking of asphalt.No longer need to be indirectly and complexly calculated cracking stress and cracking temperature by creep test and viscoelastic modeling.By adjusting the cooling rate and geometric shape of asphalt test piece, the actual thermal shrinkage of field pavement asphalt can be better simulated.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of asphalt stress testing, in particular to an asphalt stress and cracking temperature determination device and determination method. BACKGROUND

[0002] Many studies consider that thermal contraction effect is the main reason for the cracks of asphalt pavement in the initial stage of use. The sudden drop in air temperature can cause the low-temperature contraction deformation of the pavement, but the friction between the asphalt pavement layer and the base layer will limit the free development of the contraction strain. In this case, the pavement tries to maintain its original length, but the low-temperature contraction stress (tensile stress) has been accumulated inside. When the accumulated low-temperature contraction stress is greater than the tensile strength of the pavement, a fracture will occur. The anisotropic response of the pavement structure to temperature drop leads to the fact that the cracks always develop in the width direction of the pavement (i.e. perpendicular to the traffic direction). However, most of the cracks will occur preferentially on the asphalt in the asphalt concrete, because the tensile strength of the asphalt is relatively low as the temperature decreases. Therefore, many studies show that the performance of asphalt has a decisive influence on the thermal cracking resistance of the pavement.

[0003] At present, there are many indoor test methods for evaluating the low-temperature performance of asphalt, such as low-temperature performance grading test (LTPG), low-temperature ductility test, double edge notched tensile test (DENT). However, these methods cannot directly measure the temperature stress development and low-temperature cracking temperature of asphalt, and their main function is to determine whether these asphalts have sufficient deformation ability at their lowest service temperature by measuring the stiffness or ductility. For a long period of time, these methods have successfully ensured that the pavements using base asphalt will not crack under normal service conditions. However, as more and more new and complex modified asphalts are applied to pavements, the results obtained by the traditional evaluation methods of asphalt performance by limiting the low-temperature stiffness and improving the low-temperature ductility of asphalt often do not agree with the actual situation of low-temperature cracking on site. This is mainly because the viscoelastic properties of base asphalt may change fundamentally after the addition of modifiers, and the continued use of the above traditional methods to evaluate the performance of modified asphalt may be inconsistent with the supporting theory or hypothesis behind these methods. In order to solve this problem, the American National Highway and Transportation Association has issued a new specification for determining the low-temperature performance grade evaluation of asphalt, which provides a method for evaluating the potential cracking risk of asphalt at low temperature by using two indicators of cracking temperature (critical temperature) and temperature stress. However, this method is also not a direct measurement of cracking temperature (critical temperature) and temperature stress, but a calculation of cracking temperature (critical temperature) and temperature stress through the results of specific asphalt viscoelastic constitutive model, bending beam rheological test and direct tensile test.

[0004] However, the calculation of temperature stress in the new specification of asphalt low-temperature performance grade is very complex, which has three key steps: the first step is to convert the creep compliance of asphalt binder to relaxation modulus by using the Hopkins and Hamming algorithm; the second step is to select a suitable rheological model, usually the CAM model, to generate the master curve of relaxation modulus; the last step is to estimate the temperature stress of asphalt according to the numerical value of the master curve of relaxation modulus, and the commonly used mathematical method is Gauss-Legendre numerical integration method. These three steps are too theoretical, abstract and complex, and it is difficult to popularize them to the practitioners in the pavement design and maintenance institutions. In addition, the problems that need to be optimized in the new specification of asphalt low-temperature performance grade include: the tensile strain rate used in the DTT test is too fast, resulting in too large dispersion of the results, and the asphalt rheological model used is too simple. Falchetto and Moon proposed an optimization scheme for this method, i.e. using Laplace transform method to directly convert creep compliance to temperature stress curve. The calculation results have high consistency with the original method, but it is still difficult for ordinary road design, construction and management and maintenance personnel to calculate.

[0005] Therefore, some researchers have developed special equipment and methods for directly measuring the development of asphalt temperature stress and critical temperature. Among them, the most recognized is the asphalt binder cracking device test (ABCD) proposed by Kim. ABCD uses the difference in thermal expansion coefficient between asphalt binder and aluminum, by adjusting the cooling rate and geometry of asphalt sample, it can simulate the actual asphalt low-temperature strain state under various field pavement conditions. ABCD test procedure is simple, measurement speed is fast, it can test unmodified and modified asphalt, and the theory is simple, these advantages make ABCD test become the standard test method of asphalt temperature stress recommended by AASHTO. In ABCD test, it is necessary to directly paste the electric strain gauge on the aluminum mold to monitor the strain change during the continuous cooling process, but the low-temperature shrinkage of the strain gauge itself may affect the test results.

[0006] Obviously, compared with the high temperature performance evaluation test method of asphalt, the test method for evaluating the low temperature performance of asphalt is less, especially the method for directly measuring the temperature stress and the critical temperature, which is mainly due to the unreliability of the loading device and the measuring device at low temperature. Specifically, at low temperature, compared with asphalt, the loading device itself can produce a large low temperature shrinkage, and the low temperature shrinkage of the device is difficult to accurately measure, which will result in that the shrinkage strain or stress provided to the asphalt cannot be controlled and predicted. On the other hand, it is difficult to measure the low temperature strain or stress of asphalt. Because the modulus of asphalt is low, if the strain gauge is directly embedded in the asphalt sample, the stress distribution in the sample can be changed. In addition, the low temperature shrinkage of the strain gauge itself will also affect the test results. SUMMARY

[0007] In view of the above problems in the prior art, the present application aims to provide an asphalt stress and cracking temperature measuring device and a measuring method, which can provide similar thermal average stress-strain conditions for asphalt as the field pavement in the laboratory, and solve the problems of the conventional indoor test device for evaluating the low temperature performance of asphalt, the difficulty in directly testing the low temperature strain of asphalt and the inaccuracy of the test results.

[0008] In order to achieve the above application purposes, the technical scheme adopted by the present application is as follows: an asphalt stress and cracking temperature measuring device is provided, which comprises an axial tension-compression loading frame, a programmable temperature control box arranged in the axial tension-compression loading frame, and an asphalt test piece arranged in the programmable temperature control box.

[0009] A rigid base is arranged at the bottom of the programmable temperature control box, and the bottom of the rigid base is fixedly connected with the inner bottom of the axial tension-compression loading frame.

[0010] The top end of the asphalt test piece is provided with an upper clamp, and the low end of the asphalt test piece is provided with a lower clamp. The top of the upper clamp is connected with the top of the axial tension-compression loading frame through a loading rod, and a displacement sensor for monitoring the displacement of the upper clamp is arranged on the axial tension-compression loading frame. The bottom of the lower clamp is fixedly connected with the rigid base.

[0011] The basic principle of the present application is that a continuous temperature field is applied to the asphalt test piece whose shrinkage deformation is limited, and the temperature stress generated in the test piece is calculated by measuring the force required to overcome the shrinkage strain caused by the temperature drop.

[0012] Further, aluminum end heads are arranged at the two ends of the asphalt test piece, and the contact surfaces of each aluminum end head and the asphalt test piece are roughened. The two ends of the asphalt test piece are fixedly connected with the upper clamp and the lower clamp through the aluminum end heads. The aluminum end heads can facilitate the fixed connection of the two ends of the asphalt test piece with the upper clamp and the lower clamp. At the same time, the contact surfaces of the aluminum end heads and the asphalt test piece are roughened to ensure the adhesion of the asphalt.

[0013] Further, the asphalt test piece is a rectangular block structure, and a semicircular structure notch is symmetrically arranged at the center of the length direction of the asphalt test piece. The main reason for the fracture position of the symmetric test piece not being at the geometric center is that the initial defect distribution inside the test piece is random at the initial stage of the test piece forming. During the loading process, stress tends to accumulate at the position where the initial defect is the most serious. In the test piece with equal cross section (uniform cross section), the maximum tensile stress of the test piece is symmetrically distributed in a wedge-shaped area about 1 / 4 of the full length of the test piece from the end; while the center of the test piece is a large area of uniformly distributed tensile stress, and the value is about 90% of the maximum stress. The stress distribution of the test piece with equal cross section shows that in most tensile or restrained thermal shrinkage tests using the test piece, the fracture position is often not at the center of the test piece, but close to the end face of the test piece; under this stress distribution, any congenital defect in the test piece, such as bubbles, cavities, impurities and micro-cracks, can cause stress concentration, change the failure temperature, the fracture position and the maximum stress.

[0014] For the test piece with a notch (non-uniform cross section), the maximum stress is concentrated in the central ligament, especially at the edge of the ligament. In the full cross section part of the test piece, the tensile stress is uniformly distributed, but the tensile stress is only 10% to 30% of the maximum tensile stress. The notch design induces temperature stress concentration to the pre-designed weak area in the center of the test piece to promote the occurrence of failure, which can effectively improve the repeatability and efficiency of the test, and also reduces the risk of congenital defects affecting the results.

[0015] More importantly, the asphalt test piece with non-uniform cross section (i.e. notch) used in the present application has another advantage, which can control the shrinkage strain of asphalt by adjusting the cross-sectional area of the notch section to match the actual shrinkage strain of the pavement asphalt. Therefore, a semicircular notch is designed in the asphalt test piece, which has the advantage of gradually changing the width of the test piece, thereby minimizing the stress concentration at the cross section transition.

[0016] Further, the minimum width of the two notch corresponding areas of the asphalt test piece is 5.6 mm; the radius of the two notches is 3.6 mm; the total length of the asphalt test piece is 45 mm, the effective length of the asphalt test piece is 25 mm, the effective length is the length of the asphalt test piece between the two aluminum end heads, the width of the asphalt test piece is 12.8 mm, and the height of the asphalt test piece is 4.8 mm. The asphalt test piece with the above dimensions can better simulate the low-temperature temperature stress of asphalt in the field pavement.

[0017] Further, the materials of the upper clamp and the lower clamp are both invar alloy, the thermal elastic coefficient of which is actually zero (i.e. the Young's modulus is constant) in a relatively wide temperature range, and the thermal expansion coefficient is also very low, which improves the test precision.

[0018] The present invention also provides a method for measuring asphalt stress and cracking temperature, which comprises the following steps:

[0019] Step 1: Prepare asphalt specimens according to preset dimensions;

[0020] Step 2: Install the asphalt specimen: Adjust the gap between the upper and lower fixtures to 25 mm. Securely connect the two ends of the asphalt specimen to the upper and lower fixtures with aluminum end caps. Align the long axis of the asphalt specimen with the center axis of the upper and lower fixtures.

[0021] Step 3: The programmable temperature-controlled chamber begins cooling at a set rate, gradually lowering the temperature of the asphalt specimen. The specimen shrinks, pulling the top of the upper fixture downward. The displacement sensor monitors the downward displacement of the upper fixture. The loading rod applies an upward force to the upper fixture, stretching the asphalt specimen and compensating for the downward displacement of the upper fixture, keeping the dynamic displacement of the top of the upper fixture at zero.

[0022] Step 4: Record the tension value of the loading rod and the temperature value of the programmable temperature control box in real time, and calculate the low-temperature stress inside the asphalt specimen based on the tension value and temperature value;

[0023] Step 5: When the tensile force is suddenly unloaded, the asphalt specimen breaks at the cut, and the current temperature of the programmable temperature control box is recorded as the low-temperature cracking temperature of the asphalt specimen.

[0024] Furthermore, in step 1, the method for making an asphalt specimen is as follows: a silicone mold is made in advance, and aluminum end caps are respectively provided at both ends of the silicone mold. The heated and flowing asphalt is poured into the silicone mold, and then the silicone mold is stored in ethanol at 0°C for 5 to 8 minutes. After cooling is completed, the asphalt specimen is taken out from the silicone mold together with the aluminum end caps.

[0025] Furthermore, in step 4, the formula for calculating the low-temperature stress inside the asphalt specimen is:

[0026]

[0027] Where σ(T) is the accumulated temperature stress in the asphalt specimen at temperature T; P(T) is the tensile force applied by the upper fixture to the asphalt specimen to overcome shrinkage deformation, which changes with temperature; A b is the cross-sectional area of ​​the asphalt specimen.

[0028] Furthermore, in step 1, the method for determining the two notch radii of the asphalt specimen is:

[0029] Step S1: Determine the width t(θ) of the area corresponding to the two cuts of the asphalt specimen:

[0030] t(θ)=w-2r·cosθ

[0031] wherein r is the radius of the notch; w is the width of the full cross section of the asphalt specimen; and θ is the angle corresponding to the position of the smallest cross section of the asphalt specimen;

[0032] Step S2: Calculate the shrinkage tensile strain ε at the variable cross section of the center notch of the asphalt specimen r and the shrinkage tensile strain ε of the complete cross section of the asphalt specimen (i.e. the position other than the notch) f :

[0033]

[0034]

[0035] wherein ε is the maximum shrinkage tensile strain inside the asphalt specimen; and h is the height of the asphalt specimen;

[0036] Step S3: Calculate the tensile force exerted by the upper clamp on the asphalt specimen to overcome the shrinkage deformation and cause the elastic elongation δ of the asphalt specimen according to ε r and ε f :

[0037]

[0038] Meanwhile, in order to satisfy the displacement balance requirement of the upper clamp, the elastic elongation δ is:

[0039] δ = [α b + (LF U + LF L ) α NSCT ] ΔT · l

[0040] wherein α b is the thermal expansion coefficient of asphalt, generally 1.7 x 10 -4 (℃) -1 ; α NSCT is the thermal expansion coefficient of the upper clamp and the lower clamp, i.e. the thermal expansion coefficient of Invar, 9.55 x 10 -7 (℃) -1 ; LF U and LF L are the length factors of the upper clamp and the lower clamp, i.e. the ratio of the length of the clamp to the length of the asphalt specimen, taking the values of 6.2 and 1.5; ΔT is the rate of temperature drop; and l is the total length of the asphalt specimen;

[0041] Step S4: Obtain the relationship between the maximum shrinkage tensile strain ε of the asphalt specimen and the radius r of the notch:

[0042]

[0043] wherein α b = 1.7 x 10 -4(°C) -1 ; a NSCT = 9.55 x 10 -7 (°C) -1 ; (LF U + LF L ) = 7.7; l = 0.025 m; w = 0.0128 m;

[0044] In the actual pavement, the asphalt contains aggregate, so the low-temperature shrinkage strain of the asphalt in the actual pavement is ε1= (α b + LF agg · a agg ) · ΔT; then

[0045]

[0046] In the formula, a agg is the thermal expansion coefficient of the aggregate in the actual pavement asphalt, 2-8 x 10 -7 (°C) -1 ; LF agg is the aggregate length factor, which is 18 for typical hot mix asphalt; is 2.6 x 10 -4 / °C, and the corresponding radius r of the notch of the asphalt test piece is 3.6 mm.

[0047] Further, in step 1, the method for determining the height of the asphalt test piece is:

[0048] The asphalt in the field pavement and the asphalt test piece in the test bear the same level of strain under the same temperature condition, which will produce similar temperature stress σ:

[0049] LF agg · ε agg = (LF U + LF F ) · ε NSCT-load

[0050]

[0051]

[0052] In the formula, A b-notch is the minimum cross-sectional area of the asphalt test piece, E NSCT is the elastic modulus of the upper clamp and the lower clamp, i.e., the elastic modulus of the non-rising steel, which is 112 GPa; A NSCT is the cross-sectional area of the upper clamp and the lower clamp, which is 1.15 x 10 -5 m 2 ; the height h of the asphalt test piece is 4.8 mm; and ε NSCT-load = P / (A NSCT · ENSCT ) is the tensile strain of the upper and lower clamps under the temperature load P.

[0053] The beneficial effects of the present application are: (1) The asphalt stress and cracking temperature measuring device in the present application directly measures and evaluates the thermal cracking resistance of asphalt. There is no longer a need for indirect and complex theoretical calculation of cracking stress and cracking temperature through creep test and viscoelastic modeling. By adjusting the cooling rate and geometry of the asphalt test piece, the actual thermal shrinkage of the field pavement asphalt can be better simulated.

[0054] (2) The test results of the asphalt stress and cracking temperature measuring device in the present application have better repeatability and low dispersion. The new device significantly improves the temperature and load control system, and uses asphalt test pieces with pre-cut, which greatly reduces the randomness of crack occurrence position, thereby reducing the variability of the results. The coefficient of variation of the asphalt temperature stress at the time of cracking measured by this method is at least 50% less than that of the existing measurement method.

[0055] (3) The asphalt stress and cracking temperature measuring device in the present application can be fully automatic and fast measurement. Using programmable temperature control box and displacement sensor, the temperature preservation, temperature loading and result recording of the asphalt test sample are fully automatic. The operator is relieved from the tedious experiment monitoring. BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1 is a structural schematic diagram of the asphalt stress and cracking temperature measuring device.

[0057] Figure 2 is a structural schematic diagram of the asphalt test piece.

[0058] Figure 3 is a comparison diagram of temperature stress distribution of asphalt test pieces with and without cut.

[0059] Figure 4 is a schematic diagram of the relationship between the cut radius and the maximum low-temperature shrinkage strain of the asphalt test piece.

[0060] 1, axial tensile and compressive loading frame; 2, programmable temperature control box; 3, asphalt test piece; 4, rigid base; 5, upper clamp; 6, lower clamp; 7, displacement sensor; 8, aluminum end. DETAILED DESCRIPTION

[0061] The specific embodiments of the present application are described below to facilitate the understanding of the present application for those skilled in the art, but it should be clear that the present application is not limited to the scope of the specific embodiments, and for those skilled in the art, it is obvious that various changes are within the spirit and scope of the present application defined and determined by the appended claims, and all the inventions utilizing the concept of the present application are within the scope of protection.

[0062] As shown in Figure 1 The present application provides a device for measuring the stress and cracking temperature of asphalt, which comprises an axial tension-compression loading frame 1, a programmable temperature control box 2 arranged in the axial tension-compression loading frame 1, and an asphalt test piece 3 arranged in the programmable temperature control box 2.

[0063] The bottom of the programmable temperature control box 2 is provided with a rigid base 4, and the bottom of the rigid base 4 is fixedly connected with the inner bottom of the axial tension-compression loading frame 1.

[0064] The top end of the asphalt test piece 3 is provided with an upper clamp 5, and the low end of the asphalt test piece 3 is provided with a lower clamp 6. The top of the upper clamp 5 is connected with the top of the axial tension-compression loading frame 1 through a loading rod, and a displacement sensor 7 for monitoring the displacement of the upper clamp 5 is arranged on the axial tension-compression loading frame 1. The bottom of the lower clamp 6 is fixedly connected with the rigid base 4.

[0065] As a specific manufacturing method of the asphalt test piece 3, aluminum end heads 8 are arranged at both ends of the asphalt test piece 3, and the contact surfaces of each aluminum end head 8 and the asphalt test piece 3 are roughened. The two ends of the asphalt test piece 3 are fixedly connected with the upper clamp 5 and the lower clamp 6 through the aluminum end heads 8. The aluminum end heads 8 can facilitate the fixed connection of the two ends of the asphalt test piece 3 with the upper clamp 5 and the lower clamp 6, and the contact surfaces of the aluminum end heads 8 and the asphalt test piece 3 are roughened to ensure the bonding of the asphalt.

[0066] As shown in Figure 2 As a specific arrangement of the asphalt test piece 3, the asphalt test piece 3 is in a rectangular block structure, and a semicircular notch is symmetrically arranged at the center of the length direction of both sides of the asphalt test piece 3. Figure 3The main reason for the asymmetric fracture location is that the initial defects in the specimen are randomly distributed at the beginning of the specimen forming. During the loading process, the stress tends to accumulate at the location of the most severe initial defects. In the case of the uniform cross-section specimen (uniform cross-section), the maximum tensile stress of the specimen is symmetrically distributed in a wedge-shaped area of the specimen, which is about 1 / 4 of the full length of the specimen from the end; while the center of the specimen is a large area of uniform tensile stress, which is about 90% of the maximum stress. The stress distribution of the uniform cross-section specimen shows that in most tensile or restrained thermal shrinkage tests using this specimen, the fracture location is often not in the center of the specimen, but near the end face of the specimen; in this stress distribution state, any congenital defects in the specimen, such as bubbles, cavities, impurities and micro-cracks, can cause stress concentration, change the failure temperature, fracture location and maximum stress.

[0067] For the specimen with a notch (non-uniform cross-section), the maximum stress is concentrated in the central ligament, especially at the edge of the ligament. In the full cross-section part of the specimen, the tensile stress is uniformly distributed, but the tensile stress is only 10% to 30% of the maximum tensile stress. The notch design induces temperature stress concentration to the pre-designed weak area in the center of the specimen to promote the occurrence of failure, which can effectively improve the repeatability and efficiency of the test, and also reduces the risk of congenital defects affecting the results.

[0068] More importantly, the use of the asphalt specimen 3 with a non-uniform cross-section (i.e. a notch) in the present application also has another advantage, which can control the shrinkage strain of the asphalt to match the actual shrinkage strain of the pavement asphalt by adjusting the cross-sectional area of the notch cross-section. Therefore, a semicircular notch is designed in the asphalt specimen 3, which has the advantage of gradually changing the width of the specimen, thereby minimizing the stress concentration at the cross-section transition. The test results of the asphalt stress and cracking temperature measuring device in the present application have better repeatability and low dispersion. The new device significantly improves the temperature and load control system, and uses the asphalt specimen 3 with a pre-cut notch, which greatly reduces the randomness of the crack occurrence position, thereby reducing the variability of the results. The coefficient of variation of the asphalt cracking temperature stress measured by this method is at least 50% less than that of the existing measurement method.

[0069] Specifically, the minimum width of the two notch corresponding areas of the asphalt specimen 3 is 5.6 mm; the radius of the two notches is 3.6 mm; the total length of the asphalt specimen 3 is 45 mm, the effective length of the asphalt specimen 3 is 25 mm, the effective length is the length of the asphalt specimen 3 between the two aluminum end heads 8, the width of the asphalt specimen 3 is 12.8 mm, and the height of the asphalt specimen 3 is 4.8 mm. The asphalt specimen 3 with the above dimensions can better simulate the low-temperature temperature stress of the asphalt in the field pavement.

[0070] The process of determining the asphalt test piece 3 is: after the asphalt is heated, pour it into the double-edge notch asphalt test piece 3 forming mold with the pre-placed groove end mold, take it out together with the groove end mold after waiting for it to cool and solidify, adjust the distance between the upper and lower non-expansion steel clamps 6 to the effective length of the asphalt test piece 3, and fix the asphalt test piece 3 with the groove end mold in the upper and lower clamps 6, ensure that the central axes of the upper and lower clamps 6 are aligned with the long axis of the asphalt test piece 3, gradually reduce the temperature of the asphalt test piece 3 through the programmable temperature control box 2, the test piece shrinks and deforms, the top end of the upper clamp 5 is displaced downward, and the displacement sensor 7 monitors the displacement and feeds back to the axial tension and compression loading frame 1, which provides an upward pulling force to the upper clamp 5, lengthens the asphalt test piece 3, compensates for the displacement of the upper clamp 5, and keeps the dynamic displacement of the top end of the upper clamp 5 at 0, and records the change of the pulling force with the temperature to calculate the low-temperature stress inside the asphalt test piece 3, when the pulling force value is suddenly unloaded, it is proved that the test piece is broken, and the temperature at the moment of breaking is recorded as the low-temperature cracking temperature; The preliminary test results show that the asphalt stress and cracking temperature measuring device is a reliable, simple and fast device for evaluating the potential cracking risk of modified or unmodified binders at low temperature. The coefficient of variation of the thermal stress results is at least 50% less than that of the traditional thermal cracking measurement method. The asphalt stress and cracking temperature measuring device in the present application directly measures and evaluates the thermal cracking of asphalt. There is no longer a need for indirect and complex theoretical calculation of cracking stress and cracking temperature through creep test and viscoelastic modeling. By adjusting the cooling rate and geometry of the asphalt test piece 3, the actual thermal shrinkage of the field pavement asphalt can be well simulated.

[0071] The present application also provides a determination method of the asphalt stress and cracking temperature measuring device, which comprises the following steps:

[0072] Step 1: make the asphalt test piece 3 according to the preset size;

[0073] Step 2: install the asphalt test piece 3: adjust the gap between the upper clamp 5 and the lower clamp 6 to 25 mm, fix the two ends of the asphalt test piece 3 to the upper clamp 5 and the lower clamp 6 through the aluminum end 8, and align the long axis of the asphalt test piece 3 with the central axes of the upper clamp 5 and the lower clamp 6;

[0074] Step 3: the programmable temperature control box 2 starts to reduce the temperature at a set cooling rate, gradually reduces the temperature of the asphalt test piece 3, the asphalt test piece 3 shrinks, pulls the top end of the upper clamp 5 downward, and the displacement sensor 7 monitors the downward displacement of the upper clamp 5; the loading rod applies an upward pulling force to the upper clamp 5, lengthens the asphalt test piece 3, compensates for the downward displacement of the upper clamp 5, and keeps the dynamic displacement of the top end of the upper clamp 5 at 0;

[0075] Step 4: record the tensile force value of the loading rod and the temperature value of the programmable temperature chamber 2 in real time, and calculate the low-temperature temperature stress inside the asphalt test piece 3 through the tensile force value and the temperature value;

[0076] Step 5: when the tensile force value is suddenly unloaded, the asphalt test piece 3 is fractured at the cutout, and the current temperature of the programmable temperature chamber 2 is recorded as the low-temperature cracking temperature of the asphalt test piece 3.

[0077] Further, in step 1, the method for making the asphalt test piece 3 is: a silica gel mold is made in advance, aluminum end heads 8 are arranged at both ends inside the silica gel mold, the heated flowing asphalt is poured into the silica gel mold, then the silica gel mold is stored in 0℃ ethanol for 5-8 minutes, and after cooling is completed, the asphalt test piece 3 together with the aluminum end heads 8 is taken out from the silica gel mold.

[0078] Further, in step 4, the formula for calculating the low-temperature temperature stress inside the asphalt test piece 3 is:

[0079]

[0080] In the formula, σ(T) is the temperature stress accumulated in the asphalt test piece 3 when the temperature is T; P(T) is the tensile force applied by the upper clamp 5 to the asphalt test piece 3 to overcome the shrinkage deformation with the change of temperature; A b is the cross-sectional area of the asphalt test piece 3.

[0081] Further, in step 1, the method for determining the radii of the two cutouts of the asphalt test piece 3 is:

[0082] Step S1: determine the width t(θ) of the corresponding area of the two cutouts of the asphalt test piece 3:

[0083] t(θ) = w - 2r·cosθ

[0084] In the formula, r is the radius of the cutout; w is the width of the full cross-section of the asphalt test piece 3; θ is the angle corresponding to the position of the smallest cross-section of the asphalt test piece 3;

[0085] Step S2: calculate the shrinkage tensile strain ε r of the variable cross-section at the center cutout of the asphalt test piece 3 and the shrinkage tensile strain ε f of the complete cross-section (i.e. non-cutout position) of the asphalt test piece 3:

[0086]

[0087]

[0088] In the formula, ε is the maximum shrinkage tensile strain inside the asphalt test piece 3; h is the height of the asphalt test piece 3;

[0089] Step S3: according to εr and ε f , the tensile force of the upper clamp 5 applied to the asphalt test piece 3 to overcome the shrinkage deformation is calculated:

[0090]

[0091] At the same time, in order to meet the displacement balance requirement of the upper clamp 5, the elastic elongation δ is:

[0092] δ = [α b + (LF U + LF L ) α NSCT ] ΔT · l

[0093] In the formula, α b is the thermal expansion coefficient of asphalt, generally 1.7 x 10 -4 (℃) -1 ; α NSCT is the thermal expansion coefficient of the upper clamp 5 and the lower clamp 6, i.e. the thermal expansion coefficient of invar, 9.55 x 10 -7 (℃) -1 ; LF0 and LF L are the length factors of the upper clamp 5 and the lower clamp 6, i.e. the length ratio of the clamp to the asphalt test piece 3, taking the values of 6.2 and 1.5; ΔT is the temperature drop rate; and l is the total length of the asphalt test piece 3.

[0094] Step S4: obtaining the relationship between the maximum shrinkage tensile strain ε of the asphalt test piece 3 and the notch radius r:

[0095]

[0096] In the formula, α b = 1.7 x 10 -4 (℃) -1 ; α NSCT = 9.55 x 10 -7 (℃) -1 ; (LF U + LF L ) = 7.7; l = 0.025 m; and w = 0.0128 m.

[0097] In the actual pavement, the asphalt contains aggregate, so the low-temperature shrinkage tensile strain ε1 of the asphalt in the actual pavement = (α b + LF agg · α agg ) · ΔT; and

[0098]

[0099] In the formula, α aggFor the thermal expansion coefficient of aggregate in actual pavement asphalt, 2-8x10-7(°C) -1 ; LF agg is the aggregate length factor, and the typical hot mix asphalt is 18; according to Figure 4 , is 2.6x10-4 / °C, the corresponding radius r of the notch of the asphalt test piece 3 is 3.6mm.

[0100] Further, in step 1, the method for determining the height of the asphalt test piece 3 is:

[0101] The asphalt in the field pavement and the asphalt test piece 3 in the test bear the same level of strain under the same temperature drop condition, which will produce similar temperature stress σ:

[0102] LF agg ·ε agg =(LF U +LF F )·ε NSCT-load

[0103]

[0104]

[0105] In the formula, A b-notch is the minimum cross-sectional area of the asphalt test piece 3, E NSCT is the elastic modulus of the upper clamp 5 and the lower clamp 6, i.e. the elastic modulus of the non-rising steel, which is 112GPa; A NSCT is the cross-sectional area of the upper clamp 5 and the lower clamp 6, which is 1.15x10 - 5 m 2 ; the height h of the asphalt test piece 3 is 4.8mm; ε NSCT-load =P / (A NSCT ·E NSCT ) is the tensile strain of the upper clamp 5 and the lower clamp 6 under the temperature load P.

Claims

1. A method for measuring asphalt stress and cracking temperature, characterized in that: The asphalt stress and cracking temperature measuring device includes an axial tension and compression loading frame, a programmable temperature control box is provided in the axial tension and compression loading frame, and an asphalt test piece is provided in the programmable temperature control box; a rigid base is provided at the bottom of the programmable temperature control box, and the bottom of the rigid base is fixedly connected to the bottom of the axial tension and compression loading frame; an upper clamp is provided at the top of the asphalt test piece, and a lower clamp is provided at the bottom of the asphalt test piece, and the top of the upper clamp is connected to the top of the axial tension and compression loading frame through a loading rod, and a displacement sensor for monitoring the displacement of the upper clamp is provided on the axial tension and compression loading frame; the bottom of the lower clamp is fixedly connected to the rigid base; both ends of the asphalt test piece are provided with aluminum end caps. The contact surface between each aluminum end cap and the asphalt specimen is roughened; the two ends of the asphalt specimen are fixedly connected to the upper fixture and the lower fixture respectively through the aluminum end caps; the asphalt specimen is a rectangular block structure, and a semicircular incision is symmetrically provided at the center of both sides in the length direction of the asphalt specimen; the minimum width of the corresponding area of ​​the two incisions of the asphalt specimen is 5.6mm; the radius of the two incisions is 3.6mm; the total length of the asphalt specimen is 45mm, the effective length of the asphalt specimen is 25mm, the effective length is the length of the asphalt specimen between the two aluminum end caps, the width of the asphalt specimen is 12.8mm, and the height of the asphalt specimen is 4.8mm; The measuring method comprises the following steps: Step 1: Prepare an asphalt specimen according to the preset dimensions. The method for determining the two cut radiuses of the asphalt specimen is as follows: Step S1: Determine the width of the area corresponding to the two cuts of the asphalt specimen : Where r is the radius of the cut; is the width of the full cross section of the asphalt specimen; It is the angle corresponding to the position of the cross section with the smallest area of ​​the asphalt specimen; Step S2: Calculate the shrinkage and tensile strain at the center of the asphalt specimen and the shrinkage tensile strain of the complete cross section of the asphalt specimen, i.e. the non-cut position : Where, is the maximum shrinkage tensile strain inside the asphalt specimen; is the height of the asphalt specimen; Step S3: According to and , calculate the tension applied by the fixture to the asphalt specimen to overcome the shrinkage deformation, causing the elastic elongation of the asphalt specimen : At the same time, in order to meet the displacement balance requirements of the upper fixture, elastic elongation for: Where, is the thermal expansion coefficient of asphalt; is the thermal expansion coefficient of the upper and lower fixtures, that is, the thermal expansion coefficient of the non-expanding steel; and is the length factor of the upper and lower fixtures, that is, the ratio of the length of the fixture to the asphalt specimen; ΔT is the cooling rate; is the total length of the asphalt specimen; Step S4: Obtain the maximum shrinkage tensile strain of the asphalt specimen With the cut radius r Relationship: In the actual pavement, asphalt contains aggregates, so the low-temperature shrinkage and tensile strain of asphalt in the actual pavement is ;but Where, is the thermal expansion coefficient of aggregate in actual pavement asphalt; is the aggregate length factor; the radius r of the asphalt specimen cut is calculated according to the above formula; Step 2: Install the asphalt specimen: Adjust the gap between the upper and lower fixtures to 25 mm. Securely connect the two ends of the asphalt specimen to the upper and lower fixtures with aluminum end caps. Align the long axis of the asphalt specimen with the center axis of the upper and lower fixtures. Step 3: The programmable temperature-controlled chamber begins cooling at a set rate, gradually lowering the temperature of the asphalt specimen. The specimen shrinks, pulling the top of the upper fixture downward. The displacement sensor monitors the downward displacement of the upper fixture. The loading rod applies an upward force to the upper fixture, stretching the asphalt specimen and compensating for the downward displacement of the upper fixture, keeping the dynamic displacement of the top of the upper fixture at zero. Step 4: Record the tension value of the loading rod and the temperature value of the programmable temperature control box in real time, and calculate the low-temperature stress inside the asphalt specimen based on the tension value and temperature value; Step 5: When the tensile force is suddenly unloaded, the asphalt specimen breaks at the cut, and the current temperature of the programmable temperature control box is recorded as the low-temperature cracking temperature of the asphalt specimen.

2. The measuring method of the asphalt stress and cracking temperature measuring device according to claim 1, characterized in that: In step 1, the method for making an asphalt specimen is as follows: a silicone mold is made in advance, and aluminum end caps are respectively provided at both ends of the silicone mold. The heated and flowing asphalt is poured into the silicone mold, and then the silicone mold is stored in ethanol at 0°C for 5 to 8 minutes. After cooling is completed, the asphalt specimen is taken out of the silicone mold together with the aluminum end caps.

3. The measuring method of the asphalt stress and cracking temperature measuring device according to claim 2, characterized in that: In step 4, the formula for calculating the low-temperature stress inside the asphalt specimen is: （1） Where, For temperature T The accumulated temperature stress in the asphalt specimen when ; The upper fixture changes with temperature and applies a tensile force to the asphalt specimen to overcome shrinkage deformation; is the cross-sectional area of ​​the asphalt specimen.

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