Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

PUBLICATION NO. FHWA-HRT-21-056 OCTOBER 2021

FlexMATTM Asphalt Pavement Analysis 

Tools Summary and FlexMAT 

Specified Data Structure for Stress 

Sweep Rutting Test 
Introduction - 1 - October 2021 

FLEXMAT ASPHALT PAVEMENT 

ANALYSIS TOOLS SUMMARY

FLEXMAT ASPHALT PAVEMENT ANALYSIS TOOLS

FlexMAT for Cracking is a Microsoft® Excel-based analysis tool for characterizing dynamic 

modulus and cracking of asphalt pavement mixtures from Asphalt Mixture Performance Tester 

(AMPT) test results. This includes determining the 2S2P1D dynamic modulus mastercurve 

coefficients and time-temperature shift factors. It analyzes the AMPT cyclic fatigue test results 

(AASHTO TP133) to provide inputs to FlexPAVETM cracking evaluation. These inputs include 

damage characteristic curve coefficients, failure criteria coefficients, ageing parameters, and the 

coefficient of thermal contraction. The analysis also supports balanced mix design and 

production evaluation by calculating the Sapp index cracking parameter from the test data. 

Additional information can be found in the TechBrief, Cyclic Fatigue Index Parameter (Sapp) 

for Asphalt Performance Engineered Mixture Design, 2019. (FHWA-HIF-19-091; 

https://www.fhwa.dot.gov/pavement/pubs/hif19091.pdf.) 

1. "FlexMAT Cracking v2.1.xlsm" (file): Actual FlexMAT v2.1 file, cracking analysis 

version, ready for testing using the new universal data structure format requirement. 

FlexMAT for Rutting is an Excel-based analysis tool for characterizing rutting of asphalt

pavement mixtures from AMPT test results. This includes analyzing the AMPT stress sweep 

rutting (SSR) test results (AASHTO TP134) to provide shift permanent deformation model 

coefficients to FlexPAVE rutting evaluation. The analysis also supports balanced mix design and 

production evaluation by calculating the Rutting Strain Index (RSI) parameter from the test data. 

Additional information can be found in the TechBrief, Rutting Strain Index (RSI) Parameter for 

Asphalt Balanced Mixture Design, published in 2021. (FHWA-HRT-21-044; 

https://www.fhwa.dot.gov/publications/research/infrastructure/pavements/21044/index.cfm) 

2. "FlexMAT Rutting v2.1.xlsm" (file): Actual FlexMAT v2.1 file, rutting analysis version, 

ready for testing using the new universal data structure format requirement. 

CLIMATIC DATA FILES

Use .csv files with the FlexMAT Rutting v2.1 analysis tool to successfully perform the asphalt 

pavement rutting analysis. This set of files includes an expanded MERRA2 temperature database 

from 2,798 locations across the United States, and 20 yr (1996–2015) of temperature 

information. 

3. "Rutting Climate Files.zip" (files): Contains the updated temperature database required 

for RSI calculations in FlexMAT v2.1 for Rutting. 

Extract the .csv files within the .zip file into a folder. If the folder is inside the same overarching 

folder as the FlexMAT Rutting v2.1 file, it will be automatically identified. If not, the user will 

be prompted to identify the folder’s location.
Introduction - 2 - October 2021 

DATA STRUCTURE REQUIREMENT DOCUMENTS

Use these three documents to understand the AMPT test data and universal format required for 

FlexMAT to successfully import the test data and conduct the analysis. Each document will 

show the required data format the equipment manufacturers must use and report from their test 

equipment. These universal requirements allow the test data to be directly imported into 

FlexMAT from the equipment. If your equipment is not reporting data in the specified format, 

use these documents to verify requirements with the manufacturer: 

4. "AMPT-DM v1.0_final.docx" (file): Contains the FlexMAT specified data structure for 

the dynamic modulus test using the AMPT. The test data output must follow the data 

structure for import to FlexMAT.

5. "AMPT-Fatigue v1.0_final.docx" (file): Contains the FlexMAT specified data structure 

for the cyclic fatigue test using the AMPT. The test data output must follow the data 

structure for import to FlexMAT.

6. "AMPT-Rutting v1.0_final.docx" (file): Contains the FlexMAT specified data structure 

for the stress sweep rutting test using the AMPT. The test data output must follow the 

data structure for import to FlexMAT.

Upon exporting files from the equipment, users should not open or modify the files using text 

editors or Microsoft® Excel because such operations may corrupt the files and make them 

unreadable in FlexMAT.

EXAMPLE TEST DATA REPORT FILES

Use these files as examples of test data output files from the equipment that adhere to the 

universal data structure requirements to successfully use and analyze the data in FlexMAT.

7. "Input Examples" (folder): Contains dummy examples of Dynamic Modulus, Cyclic 

Fatigue, and SSR tests in new (universal) formats (compatible with FlexMAT v2.1).

CONTACT

Comments and questions can be forwarded to: 

Matthew Corrigan, Research Engineer 

Federal Highway Administration

matthew.corrigan@dot.gov
AMPT Stress Sweep Rutting Data Requirements v1.0 - 3 - October 2021 

FLEXMAT SPECIFIED DATA STRUCTURE FOR STRESS SWEEP RUTTING TEST

GENERAL DATA STRUCTURE ITEMS

• The data files must be in ASCII text format. No specific file extension or file naming convention is required. FlexMAT will browse all the 

files in selected folder to identify relevant file(s).

• The informational items (those that are not used by FlexMAT) and data (FlexMAT required) are separated by sections. Sections have unique

and defined key words enclosed in square brackets ([ ]). 

• The comment character is an exclamation point (!). 

• Any comments or nondata items within required sections must be preceded by the comment character (!) as the first character in the line. 

Blank lines are acceptable within any of the sections and will be skipped when reading. 

• Section headers are not case-sensitive, but it is recommended to use of upper-case letters for ease of readability and consistency. 

• The decimal point or period (“.”) must be the decimal separator symbol in numeric data fields.

• Within the test data, a comma, space, or tab must be used as field separator. Consecutive field separators (comma and space, for example) will 

be treated as a single-field separator.

• The positive (+) and negative (–) signs must apply to all loads, actuator displacements, and linear variable differential transformer

displacements, and must be shown as: 

o Loads: Compression positive (+), Tension negative (–). 

o Displacements: Extension positive (+), Retraction negative (–). 

o Pressures positive (+), Vacuum negative (–). 

• The current version of FlexMAT can accept input of both SI and U.S. units; however, the rutting analysis is based on SI units. SI units should be 

considered for all data items and units indicated in Section G [UNIT]. 

RUTTING TEST FILE LOCATION

The program input requires a separate folder for high and low temperatures. Within that folder for each specimen, the program looks for the file with 

keyword “AMPT-RUT-HT” for high temperature and “AMPT-RUT-LT” for low temperature in the first line.

Note of Caution: If the folder contains any redundant or surplus files with the specified data format, the program uses data from the first set of files 

read in the calculation. Therefore, the folder must limit the number of files with the specified data format. The number of files in the folder with the 

specified data format must match the number of specimens indicated in the FlexMAT Worksheet. (Typically, four total specimens are used —two 

specimens tested at high temperature and two specimens tested at low temperature— with the high and low temperature data placed in two separate

folders, respectively.) However, there can be any number of files in other data formats or file types within the folder.
AMPT Stress Sweep Rutting Data Requirements v1.0 - 4 - October 2021 

DATA STRUCTURE— RUTTING TEST DATA

ITEM DATA DESCRIPTION

A AMPT-RUT-HT and AMPT-RUT-LT

A - Required. First line in the file must have this 

key word for this data file. FlexMAT browses the 

files a folder to identify files to read.

B [VERSION]

1.0 B - Required. Must have this section completed 

and only data item is the version number, 

currently 1.0, of the file format. Used to track 

version of file when data format is updated in the 

future. Not to be confused with FlexMAT 

version. Any informational item in this section 

must start with comment character (!).

C [SUPPLEMENTARY INFORMATION]

Project name, device name, device software version, operator, target temperature, or 

any other test-related information

C - Optional. Optional section to document 

supplementary information. Not required but 

recommended to include project name, device 

name, device software version, and operator as a 

minimum. Can contain any information and not 

used in the analysis. If available, FlexMAT will

read and include it in the report section for 

reference.

D [SPECIMEN INFORMATION]

Dimensions, conditioning time, properties, or any other specimen-related information D - Optional. Optional section to document 

specimen information. Can be included or 

excluded and can contain any information. 

FlexMAT does not read and skips this section.

E [SPECIMEN IDENTIFICATION]

! Specimen identification number (maximum 80 characters)

ABCDEFG

E - Required. Must have this section completed. 

Only data item is the specimen ID. Any 
AMPT Stress Sweep Rutting Data Requirements v1.0 - 5 - October 2021 

informational item in this section must start with 

comment character (!).

F [FIELD SEPARATOR]

! 0=comma (“,”), 1=space (“ ”), 2=tab

! If comma and space are used together, please select 0 (comma) 

! If tab and space are used together, please select 2 (tab) 

0

F - Required. Must have this section completed. 

Only data item is the character type used as the 

field separator (delimiter) to separate fields 

(numeric data values) in each input record (line). 

Any informational item in this section must start 

with comment character (!).

G [UNIT]

! 0=U.S. Units, 1=SI 

! U.S.: Deviator and Contact Stress in psi, Confining Pressure in psi, Temperature in F

! SI: Deviator and Contact Stress in kPa, Confining Pressure in kPa, Temperature in C 

1

G - Required. Must have this section completed. 

Only data item is the unit type used. Must be 0 

(U.S. Units - Deviator and Contact Stress in psi, 

Confining Pressure in psi, Temperature in F) or 1 

(SI Units – Deviator and Contact Stress in kPa, 

Confining Pressure in kPa, Temperature in C). 

For the selected unit type, the parameters should 

be reported in the stated magnitude. For example, 

if SI units is selected, Deviator Stress should be 

reported in kPa not MPa. Any informational item 

in this section must start with comment character 

(!).

H [TEST PARAMETERS]

Temperature, Confining Pressure, Contact Stress, Number of Increment Loading 

Blocks, Number of Cycles per Increment Loading Block, Load Time per Cycle, Rest 

Time per Cycle, Rest Time between Increment Loading Block, or any other test 

parameters.

H - Optional. Optional section to document test 

parameters. Can be included or excluded and can 

contain any information. FlexMAT does not read 

and skips this section.

I [TEST DATA]

Supplemental test data such as Average Deviator Stress, Average Permanent Strain, 

Average Temperature, Average Confining Pressure, Permanent Deformation, or any 

other test data.

I - Optional. Optional section to document 

supplemental test data. Can be included or 

excluded and can contain any information. 

FlexMAT does not read but and skips this 

section.
AMPT Stress Sweep Rutting Data Requirements v1.0 - 6 - October 2021 

J [RUTTING TEST DATA]

! Increment Loading Block, Cycle, Deviator Stress, Permanent Strain, Confining 

Pressure...

1, 1, 672.5, 674, 69.0, 0.8, 40.0, 6.4 

1, 2, 690.0, 1062, 69.0, 0.8, 40.0, 6.4 

1, 3, 689.5, 1362, 69.0, 0.8, 40.0, 6.5 

1, 4, 689.3, 1612, 69.0, 0.8, 40.0, 6.5 

....

J - Required. Must have this section completed. 

Test data in the units specified in the [UNIT] 

section, one line per cycle within the increment 

loading block, data items separated by comma, 

space, or tab, with the following sequence per 

line: Increment Loading Block, Cycle, Deviator 

Stress (kPa or psi), Permanent Strain 

(microstrain), Confining Pressure (kPa or psi), 

Load Standard Error (%), Temperature (C or F), 

Contact Stress (kPa or psi). Any informational 

item in this section must start with comment 

character (!).
HRDI-20/10-21(WEB)E Recycled

Recyclable