1、Standard Method of Test for Determining the Fracture Potential of Asphalt Mixtures Using Semicircular Bend Geometry (SCB) at Intermediate Temperature AASHTO Designation: TP 124-161Release: Group 3 (August 2016) American Association of State Highway and Transportation Officials 444 North Capitol Stre
2、et N.W., Suite 249 Washington, D.C. 20001 TS-2d TP 124-1 AASHTO Standard Method of Test for Determining the Fracture Potential of Asphalt Mixtures Using Semicircular Bend Geometry (SCB) at Intermediate Temperature AASHTO Designation: TP 124-161Release: Group 3 (August 2016) 1. SCOPE 1.1. This test m
3、ethod covers the determination of the fracture energy (Gf) of asphalt mixtures using the semicircular bend (SCB) geometry at an intermediate test temperature. The method also includes procedures for calculating other relevant parameters derived from the load-displacement curve. These parameters, in
4、conjunction with field performance, can be used to develop a Flexibility Index (FI) to predict an asphalt mixtures damage resistance. The index can be used as part of the asphalt mixture approval process. 1.2. These procedures apply to test specimens having a nominal maximum aggregate size (NMAS) of
5、 19 mm or less. Lab compacted and field core specimens can be used. Lab compacted specimens shall be 150 1 mm in diameter and 50 1 mm thick. When field cores are used, specimens shall be 150 8 mm in diameter and 25 to 50 mm thick. A thickness correction factor may be applied for field cores tested a
6、t thickness less than 45 mm. 1.3. A vertical notch parallel to the loading axis shall be cut on the SCB specimen. The SCB specimen is a half disc with a notch parallel to the loading and the vertical axis of the semicircular disc. 1.4. This standard does not purport to address all of the safety conc
7、erns, if any, associated with its use. It is the responsibility of the user of this standard to establish and follow appropriate health and safety practices and determine the applicability of regulatory limitations prior to use. 2. REFERENCED DOCUMENTS 2.1. AASHTO Standards: T 166, Bulk Specic Gravi
8、ty (Gmb) of Compacted Hot Mix Asphalt (HMA) Using Saturated Surface-Dry Specimens T 209, Theoretical Maximum Specific Gravity (Gmm) and Density of Hot Mix Asphalt (HMA) T 269, Percent Air Voids in Compacted Dense and Open Asphalt Mixtures T 283, Resistance of Compacted Asphalt Mixtures to Moisture-I
9、nduced Damage T 312, Preparing and Determining the Density of Asphalt Mixture Specimens by Means of the Superpave Gyratory Compactor TP 105, Determining the Fracture Energy of Asphalt Mixtures using Semicircular Bend Geometry (SCB) 2016 by the American Association of State Highway and Transportation
10、 Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d TP 124-2 AASHTO 2.2. ASTM Standards: D3549/D3549M, Standard Test Method for Thickness or Height of Compacted Bituminous Paving Mixture Specimens D5361/D5361M, Standard Practice for Sampling Compacted Bituminous Mixtur
11、es for Laboratory Testing 3. TERMINOLOGY 3.1. Definitions: 3.1.1. critical displacement, u1intersection of the post-peak slope with the displacement-axis yields. 3.1.2. displacement at peak load, u0recorded displacement at peak load. 3.1.3. final displacement, ufinalrecorded displacement at the 0.1-
12、kN cut-off load. 3.1.4. flexibility index, FIindex intended to characterize the damage resistance of asphalt mixtures. 3.1.5. fracture energy, Gfenergy required to create a unit surface area of a crack. 3.1.6. linear variable displacement transducer, LVDTsensor device for measuring linear displaceme
13、nt. 3.1.7. ligament area, Arealigcross-sectional area of specimen through which the crack propagates, calculated by multiplying ligament width (test specimen thickness) and ligament length. 3.1.8. load line displacement, LLDdisplacement measured in the direction of the load application. 3.1.9. post-
14、peak slope, mslope at the first inflection point of the load-displacement curve after the peak. 3.1.10. semicircular bend (SCB) geometrygeometry that utilizes a semicircular specimen. 3.1.11. secant stiffness, Ssecant slope is defined between the starting point of load vs. load line displacement cur
15、ve and point peak load is reached. 3.1.12. work of fracture (Wf)calculated as the area under the load versus load line displacement curve. 4. SUMMARY OF METHOD 4.1. An asphalt pavement core or Superpave Gyratory Compactor (SGC) compacted asphalt mixture specimen is cut in half to create a semicircul
16、ar test specimen. A notch is sawn in the flat side of the semicircular specimen opposite the curved edge. The semicircular specimen is positioned in the fixture with the notched side down centered on two rollers. A load is applied along the vertical radius of the specimen and the load and load line
17、displacement (LLD) are measured during the entire duration of the test. The load is applied such that a constant LLD rate of 50 mm/min is obtained and maintained for the duration of the test. The SCB test fixture and SCB specimen geometry are shown in Figure 1. 4.2. Fracture energy (Gf), secant stif
18、fness (S), post-peak slope (m), displacement at peak load (w0), and critical displacement (w1), and a flexibility index are calculated from the load and LLD results. 2016 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of appl
19、icable law.TS-2d TP 124-3 AASHTO (a) (b) Figure 1(a) SCB Test Fixture and (b) SCB Test Specimen Configuration (dimensions in millimeters) 5. SIGNIFICANCE AND USE 5.1. The SCB test is used to determine fracture resistance parameters of an asphalt mixture at an intermediate temperature. Low temperatur
20、e fracture parameters can be determined in accordance with TP 105. These parameters describe the fracture and fatigue resistance of asphalt mixtures. The calculated fracture energy indicates an asphalt mixtures overall capacity to resist cracking related damage. Generally, a mixture with higher frac
21、ture energy can resist greater stresses with higher damage resistance. It should not be directly used in structural design and analysis of pavements. It also represents the main parameter used in more complex analyses based on a theoretical crack (cohesive zone) models. In order to be used as part o
22、f a cohesive zone model, fracture energy as calculated from the experiment shall be corrected to determine energy associated with crack propagation only. A correction factor may be used to eliminate other sources of inelastic energy contributing to the total fracture energy calculated directly from
23、the experiment. 5.2. From the fracture parameters obtained at intermediate temperature, the Flexibility Index (FI) of an asphalt mixture is calculated. The Flexibility Index is calculated considering the fracture energy and slope of the load-displacement curve after the post-peak representing averag
24、e crack growth rate. The FI provides a means to identify brittle mixes that are prone to premature cracking. Flexibility Index values obtained using this procedure are used in ranking cracking resistance of alternative mixes for a given layer in a structural design. The range for an acceptable FI wi
25、ll vary according to local environmental conditions, application of mixture and expectation of service life. 5.3. This test method and flexibility index can be used to rank the cracking resistance of asphalt mixtures containing various asphalt binders, modifiers of asphalt binders, aggregate blends,
26、 fibers, and recycled materials. 5.4. The specimens can be readily obtained from SGC compacted cylinders or from eld cores with a diameter of 150 mm. LigamentLength1.5 (0.1)150.0 (1.0)a = 15.0 (1.0)50.0(1.0) 2016 by the American Association of State Highway and Transportation Officials.All rights re
27、served. Duplication is a violation of applicable law.TS-2d TP 124-4 AASHTO 6. APPARATUS 6.1. Testing MachineA semicircular bend (SCB) test system consisting of a closed-loop axial loading device, a load measuring device, a bend test xture, specimen deformation measurement devices, and a control and
28、data acquisition system. A constant displacement-rate device shall be used such as an electromechanical, screw-driven machine, or a closed loop, feedback-controlled servo-hydraulic load frame. 6.1.1. Axial Loading DeviceThe loading device shall be capable of delivering loads in compression with a re
29、solution of 10 N and a minimum capacity of 10 kN. 6.1.2. Bend Test FixtureThe xture is composed of a steel base plate, two U-shaped roller support steel blocks, two steel rollers with a diameter (D) of 25 mm and a U-shaped LVDT positioning frame (see Figure 2). The initial roller position is fixed b
30、y springs and backstops that establish the initial test spans dimension. The support rollers are allowed to rotate away from the backstops during the test; but remain in contact with the sample. The tip of the loading head has a contact curvature of 12.5 mm radius. Illustrations of the loading and s
31、upports are shown in Figure 2. Note 1The length of the two roller supports in Figure 2 shall be a minimum of 65 mm. 6.1.3. Internal Displacement Measuring DeviceThe displacement measurement can be performed using the machines stroke (position) transducer if the resolution of the stroke is sufficient
32、 (0.01 mm or lower). The fracture test displacement data may be corrected for system compliance, loading-pin penetration and specimen compression by performing a calibration of the testing system. 6.1.4. External Displacement Measuring Device If an internal displacement measuring device does not exi
33、st or has insufficient precision, an externally applied displacement measurement device such as a linear variable differential transducer (LVDT) accurate to 0.01mm can be used (Figure 2). 6.1.5. Control and Data Acquisition SystemTime and load, and load-line displacement (using external or internal
34、displacement measurement device) is recorded. The control data acquisition system is required to apply a constant load-line displacement rate at a precision of 50 1 mm/min and collect data at a minimum sampling frequency of 20 Hz in order to obtain a smooth load-load line displacement curve. 6.1.6.
35、SawLaboratory saw capable of cutting asphalt specimens; must be capable of cutting the notch described in Figure 1. 6.1.7. Conditioning ChamberEnvironmental chamber or water bath capable of maintaining specimen temperature as described in Section 10.1. 6.1.8. Measuring DeviceCaliper or ruler accurat
36、e to 1mm for specimen thickness and area measurement. 2016 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d TP 124-5 AASHTO (a) Isometric View (b) Section A-A Figure 2Isometric, Cross-Section, and Elevat
37、ion of the SCB Test Fixture (dimension in millimeters) Continued on next page. 2016 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-2d TP 124-6 AASHTO (c) Elevation Figure 2Isometric, Cross-Section, and El
38、evation of the SCB Test Fixture (dimension in millimeters) (Continued) 7. HAZARDS 7.1. Standard laboratory caution should be used in handling, compacting, and fabricating asphalt mixtures test specimens in accordance with AASHTO T 312. 8. CALIBRATION AND STANDARDIZATION 8.1. Verify the capability of
39、 the environmental chamber to maintain a constant and uniform temperature. A water bath as used in AASHTO T 283 may be used in lieu of an environmental chamber. Note 2Caution should be used if an oven is selected for sample conditioning as this will likely result in variable sample conditioning. 8.2
40、. Verify the calibration of all measurement components (such as load cells and LVDTs) of the testing system. 8.3. If any of the verications yield data that does not comply with the accuracy specied, correct the problem prior to proceeding with testing. Appropriate action may include maintenance of s
41、ystem components, calibration of system components (using an independent calibration agency, service by the manufacturer, or in-house resources), or replacement of the system components. 2016 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication i
42、s a violation of applicable law.TS-2d TP 124-7 AASHTO 9. PREPARATION OF TEST SPECIMENS AND PRELIMINARY DETERMINATIONS 9.1. Test Specimen SizeFor mixtures with nominal maximum aggregate size of 19 mm or l ess, prepare the test specimens from a lab compacted SGC cylinder or from pavement cores. The fi
43、nal SGC test cylinders shall have smooth parallel faces with a thickness of 50 1 mm and a diameter of 150 1 mm (see Figure 3). If field specimens are used, the final test specimen dimensions shall be 150 8 mm in diameter with smooth parallel faces 25 to 50 mm thick depending on available layer thick
44、ness. Note 3A typical laboratory saw for mixture specimen preparation can be used to obtain cylindrical slices with smooth parallel surfaces. Diamond-impregnated cutting faces and water cooling are recommended to minimize damage to the specimen. When cutting the SCB specimens, it is recommended not
45、to push the two halves against each other because it may create an uneven base surface of the test specimen that will affect the results. 9.1.1. SGC SpecimensPrepare one laboratory SGC sp ecimen according to T 312 in the SGC with a minimum compaction height of 160 mm. From the center of the SGC spec
46、imen, obtain two cylindrical 50 1 mm thick slices (see Figure 3). Cut each slice into two identical “halves”. This results in four SCB test specimens with target 7.0 0.5% air voids in the top and bottom slices. Note 4For laboratory comp acted specimens, if target air voids cannot be achieved for eac
47、h slice, specimen height can be increased. If specimen height cannot be increased to get target air voids in the slices, obtain a single slice from the middle of two SGC specimens. Figure 3Specimen preparation from SGC specimens 9.1.2. Field CoresObtain field cores fr om the pavement in accordance w
48、ith ASTM D5361. Obtain one 150 mm diameter pavement cores if the lift thickness is greater than or equal to 100 mm or two 150-mm diameter cores if the lift thickness is less than 100 mm. 9.1.2.1. Field SpecimensPrepare four replicate SCB test specimens using pavement cores obtained from a pavement l
49、ift, with smooth, parallel surfaces that conform to the height and diameter requirements specified herein. The thickness of test specimens in most cases for field cores may vary from 25 to 50 mm. If the lift thickness is less than 50 mm, test specimens should be prepared as thick as possible but in no case be less than two times the nominal maximum aggregate size of the mixture or 25 mm, whichever is greater. If lift thickness is greater than 50 mm, a 50-mm slice shall be prepared. Cores from pavements
