SH/T 0775-2005 Standard Test Method for Determining the Flexural Creep Stiffness of Asphalt (BBR Method)

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Card type: Test method

1. Method number / full name / English name / replacement relationship

• Method number: SH/T 0775—2005
• Chinese full name: Standard Test Method for Determining the Flexural Creep Stiffness of Asphalt (BBR Method)
• English name: Standard test method for determining the flexural creep stiffness of asphalt binder using the bending beam rheometer (BBR)
• Replacement relationship: This standard is first published (original text: This standard is first published). Modified adoption of ASTM D 6648-01 "Standard Test Method for Determining the Flexural Creep Stiffness of Asphalt Binder Using the Bending Beam Rheometer (BBR) (BBR)" (English version), redrafted according to ASTM D 6648-01.
• Main differences from ASTM D 6648-01 (original foreword): For the ASTM standards in the referenced documents that have been converted to corresponding Chinese standards, the Chinese standard names are used; the data summarized in Chapter 14 of the original text are placed in normative Annex A; the original mandatory informative annex is changed to normative Annex B; the original non-mandatory informative annex is placed in informative Annex C; content related to PG classification is deleted; 5.4, 7.5.2, 7.6.1 unrelated to the test are removed; the numbering of annex chapters, sections, and clauses is rearranged, and the remaining numbering is the same as ASTM D 6648-01.
• Publication/implementation: published on 2005-11-28, implemented on 2006-06-01; issuing authority: National Development and Reform Commission of the People's Republic of China.
• Technical administration: Technical administration by the Heavy Oil Research Institute of China University of Petroleum (East China); drafting organization: Heavy Oil Research Institute of China University of Petroleum (East China); main drafters: Zhang Xiaoying, Zhang Yonghe.

2. Scope of application and non-applicability

• 1.1 applies to measuring the flexural creep stiffness, compliance, and m value of asphalt using a bending beam rheometer. The measured flexural creep stiffness range is 20 MPa～10³ MPa (original text is "20MPa–10°MPa", requires confirmation against the source standard (source-text extraction)), and the flexural creep compliance range is 50 nPa⁻¹～10⁻³ nPa⁻¹ (original text is "50nPa-– 10-3nPa'", requires confirmation against the source standard (source-text extraction)).
• Material to be measured: unaged asphalt, or aged asphalt obtained according to GB/T 5304, SH/T 0736, SH/T 0774—2005.
• The test equipment should be capable of testing samples in the temperature range of −36 ℃～0 ℃.
• 1.2 If the specimen deformation is greater than 4 mm or less than 0.08 mm, the test result is invalid.
• 1.3 This standard does not address safety regulations related to use; users of the standard are responsible for establishing appropriate safety application procedures before use.
• Non-applicability: no explicit "non-applicability" clause is provided in the available source; only 4.1 points out that the equipment operates only in loading mode, and the bending beam rheometer cannot measure specimen recovery data.

3. Method principle

• 4.1 A bending beam rheometer is used to measure the deformation at the midpoint of a prismatic asphalt simply supported beam specimen when subjected to a constant load; the equipment operates only in loading mode and cannot measure specimen recovery data.
• 4.2 A constant load is applied to a prismatic specimen in a temperature-controlled liquid bath for 240 s; the system automatically monitors the test load (980 mN±50 mN), the midpoint deformation of the specimen, and time.
• 4.3 The maximum stress at the midpoint of the specimen is calculated from the specimen dimensions, span, and load at loading times of 8.0 s, 15.0 s, 30.0 s, 60.0 s, 120.0 s, 240.0 s; the maximum bending strain is calculated from the specimen dimensions and the deformation at the corresponding loading time; the maximum bending stress divided by the maximum bending strain gives the stiffness at the specified loading time.
• 5.2 The flexural creep stiffness or flexural creep compliance obtained from the test can be used to describe the low-temperature stress-strain-time relationship of asphalt in the linear viscoelastic region at the test temperature.
• 5.3 The low-temperature shrinkage performance of asphalt pavement is related to the flexural creep stiffness and m value of the asphalt contained in the asphalt mixture.

4. Reagents and materials (purity / preparation)

• 8.1 Plastic sheets for metal molds: clean plastic sheets with thickness 0.08 mm～0.15 mm, used as liners for the inner surfaces of the three long metal mold sections, which will not deform due to the action of hot asphalt (if demolding is not affected, plastic sheets may also be omitted).
• 8.2 Silicone rubber mold backing plate: lined between the silicone rubber mold and the glass plate; hot asphalt will not deform it, and it has sufficient stiffness to prevent movement from the glass due to asphalt shrinkage when the specimen cools; rubber plate thickness 10 mm±0.5 mm, Shore A hardness of 60.
• 8.3 Petroleum-based grease: used to adhere the plastic plate to the inner surfaces of the three long metal mold sections; non-silicone products may also be used.
• 8.4 Glycerol-talc mixture: used as a release agent on the vertical inner end faces of the metal mold; 20% chemically pure glycerol and 80% medical talc or china clay may be used.
• 8.5 Cold bath liquid: a solution that is not adsorbed by asphalt and does not affect the properties of the tested asphalt; transparent and clean, and its density at the test temperature must not exceed 1.05 g/cm³.
• 8.5.1 Suitable bath liquids include ethanol, methanol, stabilized isopropanol, glycerol-methanol-water mixture (for example 60% glycerol, 15% methanol, 25% water); other reagents may also be used, but silicone or silicone-containing mixtures must not be used.
• 9.2 Alcohols and their mixed solutions are flammable and toxic; the temperature-controlled bath should be placed in a ventilated place away from fire sources, and inhalation of alcohol vapor or skin contact with the bath liquid should be avoided.
• 9.3 At the specified low test temperature, contact of the bath liquid with the skin can cause frostbite.

5. Apparatus and equipment

• 7.1 Bending beam rheometer composition: (1) loading frame with specimen supports; (2) temperature-controlled liquid bath that maintains the specimen at the test temperature and provides buoyancy to offset the specimen weight; (3) computer-controlled data acquisition system; (4) specimen mold; (5) beam for checking and calibrating the system.
• 7.2 Loading frame: a frame composed of a sample holder, loading shaft, load cell, load zeroing element, loading element, and deformation sensor; the equipment principle is shown in Figure 1.
• 7.3 Loading system: applies an initial contact load of 35 mN±10 mN to the specimen and maintains the test load (980 mN±50 mN) within 10 mN.
• 7.3.1 Loading system requirements: the test load rise time should not be less than 0.5 s; rise time means the time required for the contact load to increase from 35 mN±10 mN to 980 mN±50 mN; the system causes the load to jump to 980 mN±50 mN within 0.5 s～5 s, and the test load should be within ±50 mN of the average test load, and then within ±10 mN of the average test load; the loading mode is shown in Figure 2.
• 7.3.2 Loading shaft: connected to and aligned with the load cell, and the deformation sensor connected to it has a spherical head of radius 6.3 mm±0.3 mm.
• 7.3.3 Load cell: used to measure the initial contact load and test load; the minimum capacity should be no less than 2.00 N, the resolution should be at least 2.5 mN, and the installation position is in line with the loading shaft and above the temperature-controlled bath liquid surface.
• 7.3.4 Linear variable differential transformer (LVDT): suitable for measuring specimen deformation, with a linear range of at least 6 mm and a deformation resolution of 2.5 μm; installed on the loading shaft and on the same axis as the loading shaft.
• 7.3.5 Sample supports: contact radius 3.0 mm±0.3 mm, two stainless steel or other corrosion-resistant metal supports, spaced at a distance of 102 mm±1.0 mm, accurate to within +0.3 μm (original text "+0.3tim", requires confirmation against the source standard (source-text extraction)); the measured support spacing is used in the calculations of Annex A; the support dimensions should ensure that the specimen is tangent to the support radius throughout the test, as shown in Figure 3.
• 7.3.5.1 The width of the specimen support in contact with the specimen is 9.50 mm±0.25 mm, as shown in Figure 3.
• 7.3.5.2 Behind each support there is a vertical alignment pin of diameter 2 mm～4 mm, used to align the specimen on the supports; the distance from the middle of each support to the front of the alignment pin should be 6.75 mm±0.25 mm, as shown in Figure 3.
• 7.4 Temperature sensor: calibrated temperature sensor, measuring range −36 ℃～0 ℃, accurate to 0.1 ℃, installed within 50 mm of the geometric center of the specimen. Note: temperature is measured with a properly calibrated platinum resistance thermometer (PRT) or thermistor, and calibration may be performed according to 11.5.
• 7.5 Temperature-controlled liquid bath: capable of maintaining the temperature at all points in its container within 0.1 ℃ of the test temperature in the range of −36 ℃～0 ℃; placing the specimen into the temperature-controlled bath may cause temperature fluctuations of ±0.2 ℃, and bath temperature fluctuations of ±0.2 ℃ are allowed during the constant-temperature adjustment process.
• 7.5.1 Liquid bath stirrer: maintains the required temperature uniformity by stirring; the liquid flow and mechanical noise generated under the stirring intensity should not affect the test sensitivity.
• 7.6 Data acquisition and control components: resolution is the minimum load 2.5 mN, minimum deformation 2.5 μm, minimum bath liquid temperature change 0.1 ℃; when the contact load is switched to the test load, the data acquisition system senses this point in time, and this time is taken as the zero load time for the test load and deformation signals, as the zero time reference point, and records the subsequent load and deformation at 8.0 s, 15.0 s, 30.0 s, 60.0 s, 120.0 s and 240.0 s.
• 7.7 Specimen mold: internal dimensions length 127 mm±5 mm, width 6.35 mm±0.05 mm, depth 12.70 mm±0.05 mm; material is aluminum or stainless steel (Figure 4), or silicone rubber (Figure 5, Figure 6).
• 7.7.1 Use a micrometer to measure the thickness of the end pieces at both ends of each mold (metal end piece ③ in Figure 4); the thickness difference between the two end pieces shall not be greater than 0.05 mm. Note: A small error in specimen thickness may cause a large error in modulus calculation, because modulus is a function of thickness (h), and the error can reach the third power (Appendix C Formula C.1.3).
• 7.8 Inspection and calibration steps: inspect and calibrate BBR according to the following steps.
• 7.8.1 Stainless steel thick beams required for measuring compliance and calibrating the load cell: length 127 mm±5 mm, width 12.7 mm±0.3 mm, thickness 6.4 mm±0.3 mm; when using these beams to measure specimen thickness according to 13.2, the beam thickness shall be accurately measured to 0.01 mm, and this data is used in the 13.2.3.1 formula to calculate specimen thickness.
• 7.8.2 Stainless steel thin beams for inspecting the entire system: length 127 mm±5 mm, width 12.7 mm±0.1 mm, thickness 1.0 mm～1.6 mm; these are the three important data that the manufacturer must report when reporting the elastic modulus of BBR; the manufacturer shall measure and report the beam thickness accurately to 0.01 mm and width accurately to 0.05 mm; during the entire test system inspection stage (see 11.3), the beam modulus is calculated from the beam dimensions.
• 7.8.3 Standard weights: use standard weights to perform the inspection and calibration of the following items.
• 7.8.3.1 Inspection of the load cell: use one or more weights with a total weight of 100 g±0.2 g and two weights each with a mass of 2.0 g±0.2 g (11.3) to inspect the accuracy of the load cell after calibration.
• 7.8.3.2 Calibration of the load cell: use four weights of known mass accurate to 0.2 g, whose masses vary uniformly within the load cell's force measurement range, see B.1.2 of Appendix B.
• 7.8.3.3 Inspection of the entire system: two or more weights, each of known mass accurate to 0.2 g, used to perform the entire system inspection specified by the manufacturer (see 11.4).
• 7.8.3.4 Accuracy of weights: perform an accuracy check on the weights in 7.8.3 at least once every three years.
• 7.8.4 Special gauge: a graduated gauge with thickness accurately measured to ±5 μm, used to calibrate the displacement sensor and to check the calibration effect of the displacement sensor (typical design see Figure 7).
• 7.9 Standard thermometers for verification: partially immersed liquid-in-glass thermometers with a suitable range, with 0 ℃ temperature points, and a scale division of 0.1 ℃, used to check temperature sensors; these thermometers shall be calibrated at least once a year.

6. Recommended operating conditions

• Test temperature range: −36 ℃～0 ℃ (1.1, 7.5).
• 13.1 Note 1: within the range of −36 ℃～0 ℃, increase or decrease the temperature from the target temperature or the middle value of the target temperature range at a gradient of 6 ℃.
• Contact load: 35 mN±10 mN (3.2.1, 7.3, 13.6, 13.7).
• Test load: 980 mN±50 mN (3.2.6, 7.3, 13.8.3).
• Test load application time: 240 s (4.2, 13.8.3).
• Seating load: 980 mN±50 mN, application time 1 s±0.1 s (13.8.1).
• Recovery time: reduce the load to the contact load of 35 mN±10 mN, and allow the specimen to recover for 20 s±0.1 s (13.8.2).
• After the specimen is kept in the bath liquid at the test temperature ±0.1 ℃ for 60 min±5 min, apply the seating load (13.1).
• Data recording time points: 8.0 s, 15.0 s, 30.0 s, 60.0 s, 120.0 s, 240.0 s (4.3, 7.6, 15.2).
• 13.8.3 The computer automatically records load values at time intervals of 0.5 s starting from 0.5 s and calculates the average recorded load; after loading, the test load shall be maintained within ±50 mN of the average test load during the initial 0.5 s and 5 s, and within ±10 mN of the average load during the remaining time.
• 13.5 During the test, the bath liquid shall be maintained at the test temperature ±0.1 ℃; otherwise, the test shall be discarded.

7. Sampling and specimen preparation

• 12.2.1 If the test material is unaged asphalt, sample according to the method of GB/T 11147; laboratory-aged samples or asphalt recovered from mixtures shall be sampled according to the appropriate test method or specification.
• 12.2.2 Heat the asphalt in an oven set at a temperature not higher than 165 ℃ until the sample flows sufficiently and is easy to pour; for modified asphalt or aged asphalt that does not flow easily, the oven temperature may be set to 180 ℃; in all cases, the heating time should be as short as possible and the heating temperature should be as low as possible; during heating, occasional stirring may be used to ensure sample uniformity, but air bubbles should be avoided from being entrained into the asphalt.
• 12.1 Mold preparation: before each casting, prepare the mold according to the provisions of 12.1.1 or 12.2.2. Note: The operator may use silicone rubber molds, but metal molds are the reference.
• 12.1.1 Metal mold preparation: clean thoroughly; apply a layer of petroleum-based grease to the inner surfaces of the three long metal parts, and use as little grease as possible to stick the plastic sheets flat onto the metal; do not use plastic sheets that deform due to the action of hot asphalt; place the plastic sheets on the metal surface and press with fingers so that they adhere to the metal surface by friction; apply a mixture of glycerol and talc to the inner surfaces of the two end pieces to prevent asphalt from sticking to the metal end pieces; assemble the mold according to Figure 4; use O-shaped rubber rings to tightly bind the mold pieces together; inspect the mold and press the plastic sheets firmly against the metal surface to squeeze out air bubbles; after assembly, place the mold at room temperature and wait to pour the asphalt.
• 12.1.2 Silicone rubber mold preparation: wipe off asphalt, grease or other residues with a clean cloth; do not clean the mold with organic solvents; assemble the silicone rubber mold according to Figure 5.
• 12.3.1 Casting the specimen (metal mold): place the mold at room temperature, pour the asphalt back and forth from one end of the mold to the other so that the asphalt is slightly above the mold; when pouring, keep the sample container 20 mm～30 mm above the top of the mold, and pour the asphalt in a single path toward the other end; place the mold filled with asphalt on the laboratory bench and cool at room temperature for 45 min～60 min; after cooling to room temperature, use a hot knife to trim off the asphalt protruding above the top surface of the mold.
• 12.3.2 Casting the specimen (silicone rubber mold): under conditions that ensure the viscosity of the asphalt binder (as judged by the operator), the silicone rubber mold in the aluminum fixture may be preheated in an oven at 135 ℃ for 30 min before casting; pour slowly and steadily from the top of the mold, taking care not to introduce air bubbles; pour the specimen to the top of the mold, and visually observe that no asphalt protrudes above the top surface; after pouring, cool the mold together with the asphalt inside at room temperature for at least 45 min.
• 12.4.1 Before the test, place the specimen in the mold at room temperature; after the specimen is cast, the test shall be completed within 4 h.
• 12.4.2 Before demolding, cool the metal mold or silicone rubber mold containing the specimen in a cooling chamber or water bath for no more than 5 min; however, the specimen must be hard enough to be easily demolded without deformation; never demold the sample within 10° of the test temperature; do not cool the mold containing the specimen in the test bath liquid, as this may cause the bath liquid temperature to fluctuate by more than 0.2 ℃.
• 12.4.3 When the specimen in the mold is hard enough and demolding does not deform the specimen, remove the metal or silicone rubber mold; to avoid specimen deformation, slide the plastic sheets and side molds off the specimen. Note: During demolding, handle the specimen carefully to avoid deformation; a deformed specimen will affect the measured stiffness and the m value.

8. Analysis procedure

• 10.1 Wipe clean the support, loading head, particles in the bath liquid, and the film. Note: At the specified test temperature, asphalt is brittle and may bring asphalt fragments into the bath liquid; debris on the support or load head will affect the measured deformation; the bath liquid may be filtered to remove fragments and maintain cleanliness.
• 10.2 Select the test temperature and adjust the bath liquid temperature to the selected temperature; before the test, equilibrate the temperature to within ±0.1 ℃ of the test temperature.
• 10.3 Turn on the loading and data acquisition system, power on according to the operating instructions, and preheat the data acquisition system and computer according to the operating instructions before operating BBR.
• 11.1 Displacement sensor check: before testing, check the displacement sensor accuracy with a graduated gauge of known dimensions similar to Figure 7; the load frame is installed in the cold bath liquid at the test temperature, remove all beams from the support, place the gauge on the reference platform below the loading shaft according to the equipment manual, apply a weight of 100 g±0.2 g to the loading shaft, and measure each step increase with the displacement sensor; compare the known gauge dimensions with the measured values displayed by the system; if the difference between the gauge dimensions and the system-measured data exceeds 15 μm, calibrate according to B.1.1 in normative Appendix B and repeat 11.1; if after calibration it still cannot meet the requirements of 11.1, the equipment shall be taken out of service pending repair.
• 11.2 Air bearing check: before operation, evaluate according to 11.2.1 and 11.2.2 whether the air bearing moves freely and whether there is friction; if the requirements cannot be met and there is friction in the air bearing, clean the shaft and clean the displacement sensor according to the equipment manual; if the friction still cannot be eliminated, the equipment shall be taken out of service pending repair. Note: friction may be caused by the displacement sensor cable bundle, asphalt on the loading shaft, oil or other particles in the air source, etc.
• 11.2.1 Place the thin steel beam from 7.8.2 on the specimen support, apply a load of 35 mN±10 mN to the beam with the zero load adjuster, and observe the LVDT data on the digital display; gently grasp the shaft and lift it upward about 5 mm to read the LVDT reading; when the shaft is released, it will immediately drift downward until contact.
• 11.2.2 Remove the beam from the support; adjust the loading shaft with the load adjuster so that it drifts freely at about the middle point of the vertical path, gently add a coin or another object of about 2 g mass to the load frame, and the shaft will gently fall freely under the mass of the coin.
• 11.3 Load cell verification:
  • 11.3.1 Contact load: before testing, verify the load cell calibration effect within the contact load range; place the 6.35 mm thick stainless steel beam from 7.8.1 on the support, apply a load of 20 mN±10 mN to the beam with the zero load adjuster, and add 2.0 g±0.2 g weights to the load platform; the load increase displayed by the system is 20 mN±5 mN; then add 2.0 g±0.2 g weights to the load platform, and the load increase displayed by the system is 20 mN±5 mN; if the displayed load increase is not 20 mN±5 mN, recalibrate according to B.1.2 in Appendix B and repeat 11.3.1; if after calibration it cannot meet the requirements of 11.3.1, the equipment shall be taken out of service pending repair.
  • 11.3.2 Test load: before testing, verify the load cell calibration value within the test load range; place the 6.35 mm thick stainless steel beam from 7.8.1 on the support, apply a load of 20 mN±10 mN to the beam with the zero load adjuster (contact load); apply 100 g mass to the load platform; the load increase displayed by the data acquisition system should be 980 mN±5 mN, otherwise calibrate the load according to the instructions in B.1.2 in Appendix B and repeat 11.3.2; if after calibration it still cannot meet the requirements of 11.3.2, the equipment shall be taken out of service pending repair.
• 11.4 Daily check of the entire system: before testing, install the load frame in the bath liquid and conduct a comprehensive check of the entire system operation; according to 7.8.2, place a thin stainless steel beam of known modulus and thickness 1.0 mm～1.6 mm on the specimen support; according to the manufacturer's instructions, place the beam on the support and apply a total mass of 50 g or 100 g±0.2 g (491 mN or 980 mN±50 mN) to the beam to ensure that the beam is properly placed and in full contact with the support; according to the equipment manual, apply another additional load of 100.0 g～300.0 g±0.2 g to the beam; the equipment software can use the load change and deformation to calculate the modulus of the beam, accurate to three significant figures; the modulus error reported by the software should be within 10% of the modulus provided by the manufacturer; otherwise the entire BBR shall be regarded as unreliable and the equipment manufacturer shall be consulted.
• 11.5 Temperature sensor check: before any test, when the test temperature changes, check the temperature probe calibration effect with a calibrated thermometer as described in 7.9; place the load frame in the bath liquid, place the thermometer in the bath liquid near the temperature sensor, and compare the temperature displayed by the thermometer with the temperature displayed by the system; if the difference between the system-displayed temperature and the thermometer-displayed temperature is not within ±0.1 ℃, calibrate according to the requirements of B.1.3 in Appendix B.
• 11.6 Loading shaft alignment check: installation or handling of the instrument may cause displacement of the loading shaft; the loading shaft and specimen support can be calibrated with the calibration gauge provided by the manufacturer or by the following method: cut a piece of white paper strip about 25 mm in length and slightly narrower than the beam width, and tape it to the beam; remove the frame from the cold bath liquid, place the beam on the support, place a piece of carbon paper on the white paper strip, pressurize the air bearing, and push the shaft down so that the carbon paper leaves a mark on the white paper; remove the beam and measure the distance from the center of the mark to each edge of the beam with a vernier caliper; the difference between the two measurements should be less than or equal to 1.0 mm; if this requirement cannot be met, the equipment shall be repaired.
• 13.1 According to the actual test requirements, select the appropriate test temperature according to Note 1; immediately after demolding, place the specimen in the test bath liquid; after the specimen is kept in the bath liquid at the test temperature ±0.1 ℃ for 60 min±5 min, apply the seating load to the specimen. Note 2: when the specimen is at low temperature, the asphalt will harden, and this process is reversible; to obtain test results with good repeatability, the storage and handling time must be carefully controlled.
• 13.2 Specimen thickness measurement: the specimen thickness is the thickness of the mold end piece, see 7.7.1.
  • 13.2.1 For metal molds, the method for measuring the thickness of the end piece may be referenced (7.7.1); for silicone rubber molds, method 13.2.2 or 13.2.3 must be used. Note: measuring the specimen thickness according to 13.2.2 or 13.2.3 can reduce errors; however, when performing 13.2.2 and 13.2.3, be careful not to bend or deform the specimen.
  • 13.2.2 Direct measurement method: measure the specimen thickness with a thickness gauge or similar device; during measurement, the specimen shall be kept within the test temperature ±0.2 ℃; measure the thickness at the midpoint of the specimen, accurate to 2.5 μm, and the operator shall input the measured value into the software for calculating the specimen stiffness.
  • 13.2.3 Displacement sensor measurement method: the thickness can be measured with the displacement sensor, either by manual calculation from the displacement reading displayed by the instrument or by inputting it into the software for automatic calculation; the thickness shall be accurate to 50 μm.
  • 13.2.3.1 Place the 6.35 mm thick stainless steel beam specified in 7.8.1 on the support, and determine the displacement reading corresponding to the top of the support; apply a contact load of 35 mN±10 mN to the stainless steel beam; record the displacement sensor reading as Rₐ₁; turn the stainless steel beam over to obtain another reading Rₐ₂, and record the average of the two readings as Rₐ; calculate the displacement sensor reading corresponding to the top of the support (see Figure 8): R₀ = Rₐ + tₑ (1). Where: R₀ is the displacement sensor reading corresponding to the top of the support; Rₐ is the average of the two displacement sensor readings obtained when the displacement sensor contacts the top of the stainless steel beam specimen; tₑ is the measured thickness of the stainless steel beam (see 7.8.1).
  • 13.2.3.2 Measure the specimen thickness before placing the specimen on the support; apply a contact load of 35 mN±10 mN to the specimen, record the displacement sensor reading as Rₛ₁, and turn the specimen over to obtain another reading Rₛ₂; if the difference between the two readings is less than 1.0 mm, average the two values to obtain Rₛ; if the difference between the two readings is greater than 1.0 mm, the specimen flatness is insufficient and the specimen shall be discarded (as shown in Figure 8). The specimen thickness calculation formula: tₛ = R₀ − Rₛ (2). Where: tₛ is the calculated specimen thickness; R₀ is the displacement sensor reading corresponding to the top of the support calculated by equation 1; Rₛ is the average of the two displacement sensor readings obtained when the displacement sensor contacts the top of the specimen.
• 13.3 Contact load and test load check: before measuring each batch of specimens, check the contact load and test load with a 6.35 mm thick stainless steel beam. Note: do not use thin steel beams and asphalt specimens for the check.
  • 13.3.1 Place the thick steel beam at the beam support position, and gently apply force to the beam with the test load adjuster until 980 mN±50 mN.
  • 13.3.2 Convert the test load to contact load, and adjust the force on the beam to 35 mN±10 mN; perform the conversion between test load and contact load four times according to this procedure.
  • 13.3.3 When performing the conversion between test load and contact load, observe the vertical movement of the loading shaft and platform: the loading shaft and the steel beam shall remain in contact; the contact load and test load shall be maintained at 35 mN±10 mN and 980 mN±50 mN, respectively.
  • 13.3.4 If the requirements of 13.3.1～13.3.3 cannot be met, the equipment shall be calibrated according to B.1.2 in Appendix B, or the loading shaft is dirty, or calibration is required (see 11.2); after calibration, cleaning, or other corrections, if the requirements of 13.3.1～13.3.3 still cannot be met, the equipment shall be taken out of service pending repair.
• 13.4 Input the relevant information into the computer according to the report content, see 15.
• 13.5 After storage and handling, place the specimen on the support so that the rear part of the specimen rests gently against the alignment pin; start the test as described in 13.6; during the test, the bath liquid shall be maintained at the test temperature ±0.1 ℃, otherwise the test shall be abandoned.
• 13.6 Manually apply a contact load of 35 mN±10 mN to the specimen; the load application time shall not exceed 1 min to ensure contact between the specimen and the load head. Note: the contact load ensures that there is no gap between the loading shaft, end supports, and specimen; within the required load range, discontinuous contact will lead to erroneous results; excessive contact time will affect the reported stiffness and m value.
• 13.7 When applying the contact load, it should be gently increased to 35 mN±10 mN; when the contact load is applied, the load on the beam should not exceed 45 mN.
• 13.8 Apply the contact load to the specimen, start the automatic test system, and the system program will perform the following operations:
  • 13.8.1 Apply a seating load of 980 mN±50 mN for a duration of 1 s±0.1 s. Note: A computer-controlled system should automatically load and unload.
  • 13.8.2 Reduce the load to the contact load of 35 mN±10 mN and allow the specimen to recover for 20 s±0.1 s; at the end of the seating load, the operator monitors the computer screen to verify that the load has returned to 35 mN±10 mN. If it has not returned to 35 mN±10 mN, abandon the test.
  • 13.8.3 Apply a test load of 980 mN±50 mN to the specimen, with a loading time of 240 s; the computer will automatically record load values at intervals of 0.5 s starting from 0.5 s and calculate the average of the recorded loads; during the initial 0.5 s and 5 s after loading, the test load should remain within ±50 mN of the average test load, and for the remainder of the time the test load should remain within ±10 mN of the average load; calculate the specimen stress from the measured actual load; see Figure 2 for the applied load pattern.
  • 13.8.4 Remove the test load and return to the contact load of 35 mN±10 mN.
• 13.9 Remove the specimen from the support for the next test.
• 14 Data reduction (see normative appendix A).

9. Qualitative and quantitative and calculation formulas

• A.2 Based on the specimen dimensions, measured load, and deformation, use the following formula to calculate the stiffness of the tested specimen when the applied load times are 8.0 s, 15.0 s, 30.0 s, 60.0 s, 120.0 s, and 240.0 s:
  • Sₘ(t) = PL³ / (4bh²δ(t))（3）
  • Where: Sₘ(t) is the measured flexural creep stiffness at time t, MPa; P is the measured load, mN; L is the support distance, mm; b is the specimen width, mm; h is the specimen height, mm; δ(t) is the specimen deformation at time t.
• A.2.1 Do not use load values and deformations before the applied load 8 s to calculate stiffness, because due to the effects of applying dynamic load and the rise time of the applied load, creep test data obtained immediately after applying the load are unreliable.
• A.3.1 Plot on two-dimensional coordinates between the logarithm of stiffness values and the logarithm of applied load time using the following calculations (the original formula contains source-text extraction noise, coefficients and summation terms require confirmation (source-text extraction)):
  • A = [S_y(S_x2S_x4 − S_x3²) + S_xy(S_x2S_x3 − S_x1S_x4) + S_xy2(S_x1S_x3 − S_x2²)] / D
  • B = [S_y(S_x1S_x4 − S_x2S_x3) + S_xy(S_x1S_x2 − S_x3) + S_xy2(S_x2² − S_x1S_x3)] / D (the original contains source-text extraction noise, requires confirmation (source-text extraction))
  • C = [S_y(S_x1S_x3 − S_x2²) + S_xy(S_x1S_x2 − S_x3) + S_xy2(S_x2² − S_x1S_x3)] / D (the original contains source-text extraction noise, requires confirmation (source-text extraction))
  • D = 6(S_x2S_x4 − S_x3²) + S_x1(S_x2S_x3 − S_x1S_x4) + S_x2(S_x1S_x3 − S_x2²) (the original contains source-text extraction noise, requires confirmation (source-text extraction))
  • When the applied load times are 8.0 s, 15.0 s, 30.0 s, 60.0 s, 120.0 s, and 240.0 s:
    • S_y = log Sₘ(8) + log Sₘ(15) + … + log Sₘ(240)
    • S_x1 = log(8) + log(15) + … + log(240)
    • S_x2 = [log(8)]² + [log(15)]² + … + [log(240)]²
    • S_x3 = [log(8)]³ + [log(15)]³ + … + [log(240)]³
    • S_x4 = [log(8)]⁴ + [log(15)]⁴ + … + [log(240)]⁴
    • S_xy = [log Sₘ(8)][log(8)] + [log Sₘ(15)][log(15)] + … + [log Sₘ(240)][log(240)]
    • S_xy2 = [log Sₘ(8)][log(8)]² + [log Sₘ(15)][log(15)]² + … + [log Sₘ(240)][log(240)]² (the two S_xy2 expressions in the original are identical, requires confirmation (source-text extraction))
• A.3.2 Calculate the coefficient of variation of stiffness using the following quadratic formula: R = [6AS_y + 6BS_xy + 6CS_xy2 − 8S_y² − S_y2] (the original formula contains source-text extraction noise, requires confirmation (source-text extraction)). Where: S_y2 = [log S(8)]² + [log S(15)]² + … + [log S(240)]² (the original is S_yz, requires confirmation (source-text extraction)).
• A.4 Use the following formula to calculate the stiffness values when the loading times are 8.0 s, 15.0 s, 30.0 s, 60.0 s, 120.0 s, and 240.0 s:
  • log Sₘ(t) = A + B[log(t)] + C[log(t)]²（4）
  • Where: A, B, and C are the regression coefficients determined in A.3.1; t is the applied load time.
• A.4.1 The difference between the calculated stiffness and the measured stiffness should be within 2%, otherwise the test result is not acceptable