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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-07-052
Date: September 2007
This LTPP Protocol describes a method for determining the moisture susceptibility of asphalt concrete specimens. This test is based primarily on AASHTO T283. The test shall be performed on compacted specimens obtained from projects included within the LTPP experiments.
This method involves the evaluation of changes in tensile strength resulting from the effects of saturation and accelerated water conditioning of compacted bituminous mixtures. The results can be used as indicators of the long term stripping susceptibility of bituminous mixtures. In this procedure, stripping is defined as the breaking of the bond between the asphalt cement and the aggregate surfaces resulting in exposed aggregate surfaces with minimal or no asphalt cement coating. The extent of stripping is established on a 102-mm (4-in) core split in indirect tension.
1.2 Summary of Test Method
Eight test specimens are required from each asphalt concrete bulk sample. Two of the cores will be used to establish the vacuum saturation technique outlined in Section 8.3. The remaining six cores shall be divided into two equal subsets of three specimens each. The first subset is tested in the dry condition for indirect tension. The second subset is subjected to vacuum saturation followed by freeze and warm-water soaking cycles and then tested in indirect tension to determine the tensile strength. Numerical indices of tensile strength properties including mean values (Y), standard deviation (Sd) and coefficient of variation (i.e., CV = 100 Sd/Y) are computed from the test data obtained from the two subsets; dry and conditioned.
1.3 Significance and Use
This method involves an evaluation of the effects of saturation and accelerated water conditioning on the tensile strength of bituminous mixtures compacted in the laboratory. In particular, this method will be used to investigate bituminous mixtures produced for use in the LTPP experiments.
Numerical indices of retained indirect tensile properties are obtained by comparing the tensile properties of saturated, accelerated water-conditioned laboratory specimens with similar properties of dry specimens.
1.4 Sample Storage
AC cores should be stored flat side down, fully supported, and between 5°C (40°F) and 21°C (70°F) in an environmentally protected (enclosed area not subject to the natural elements) storeroom.
Each sample shall have a label or tag attached that clearly identifies the material, the project number/test section from which it was recovered and the sample number, as a minimum.
In this protocol, the International System of Units (SI - The Modernized Metric System) is regarded as the standard. Units are expressed first in their "soft" metric form followed, in parenthesis, by their U.S. Customary unit equivalent.
2.1 Moisture Susceptibility testing shall be conducted after; (1) approval by the FHWA COTR to begin AC moisture susceptibility testing, (2) approval of Form L04 by the FHWA-LTPP Region, (3) visual examination and thickness of AC cores and thickness determination of layers within AC cores using Protocol P01, and (4) final layer assignment based on the P01 test results (corrected form L04, if needed) have been completed. To attain approval under item (1), the laboratory must; (a) submit and obtain approval of the QC/QA plan for the moisture susceptibility testing, and (b) demonstrate that their testing equipment meets or exceeds the specifications contained in this protocol.
2.2 Test Sample Locations and Assignment of Laboratory Test Numbers
The test shall be performed on recompacted test specimens of asphalt concrete retrieved from LTPP test sections as dictated by the sampling plans for the particular project.
The test results shall be reported separately for test samples obtained from the beginning, middle, and end of a test section as follows:
The following definitions are used throughout this protocol:
3.1 Layer: That part of the pavement produced with similar material and placed with similar equipment and techniques. The material within a particular layer is assumed to be homogeneous.
3.2 Test Specimen: That part of the layer which is used for the specified test.
4.1 AASHTO Standards
T166 Bulk Specific Gravity of Compacted Bituminous Mixtures
T167 Compressive Strength of Bituminous Mixtures
T168 Sampling Bituminous Paving Mixtures
T209 Maximum Specific Gravity of Bituminous Paving Mixtures
T245 Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus
T246 Resistance to Deformation and Cohesion of Bituminous Mixtures by Means of Hveem Apparatus
T247 Preparation of Test Specimens of Bituminous Mixtures by Means of California Kneading Compactor
T283 Resistance of Compacted Bituminous Mixture to Moisture Induced Damage
4.2 ASTM Standards
D3387 Test for Compaction and Shear Properties of Bituminous Mixtures by Means of the U.S. Corps of Engineers Gyratory Testing Machine (GTM)
4.3 LTPP Protocols
Protocol P01 -Visual Examination and Thickness of Asphaltic Concrete Cores
Protocol P02 -Bulk Specific Gravity of Asphaltic Concrete
Protocol P03 -Maximum Specific Gravity of Asphaltic Concrete
5.1 Equipment for preparing and compacting specimens from one of the following AASHTO Methods: T245 and T247, or ASTM Method D3387.
5.2 Vacuum Container from AASHTO T209 and vacuum pump or water aspirator from AASHTO T209 including manometer or vacuum gauge.
5.3 Balance and water bath from AASHTO T166.
5.4 Water bath capable of maintaining a temperature of 60 ± 1°C (140 ± 1.8°F).
5.5 Freezer maintained at -18 ± 3°C (0 ± 5°F).
5.6 A supply of plastic film for wrapping, heavy-duty leak proof plastic bags to enclose the saturated specimens and masking tape.
5.7 10-ml graduated cylinder.
5.8 Aluminum pans having a surface area of 485 to 645 cm2 (75 to 100 in2) in the bottom and a depth of approximately 25 mm (1 in).
5.9 Forced air draft oven capable of maintaining a temperature of 60 ± 1°C (140 ± 1.8°F).
5.10 Loading jack and ring dynamometer from AASHTO T245, or a mechanical or hydraulic testing machine from AASHTO T167 to provide a range of accurately controllable rates of vertical deformation including 51 mm per minute (2 inches per minute).
5.11 Loading Strips - Steel loading strips with a concave surface having a radius of curvature equal to the nominal radius of the test specimen. For specimens 102 mm (4 in) in diameter, the loading strips shall be 12.7-mm (0.5-in) wide. The length of the loading strips shall exceed the thickness of the specimens. The edges of the curved portion of the loading strips shall be rounded by grinding to remove the sharp edge in order to not cut into the sample during testing.
5.12 Recording Device - An X-Y plotter or real time computer generated plot shall be used to record the maximum compressive load applied to each test specimen.
6.1 Specimens with a nominal diameter of 102 mm (4 in) and a height of 63.5 mm (2.5 in) are to be prepared. Aggregate particles larger than 25.4 mm (1 in) should be scalped out prior to specimen preparation. The specimens should represent only one layer of the pavement structure.
6.2 After mixing, the mixture shall be placed in an aluminum pan having a bottom surface area of 485 to 645 cm2 (75 to 100 in2) and a depth of approximately 25 mm (1 in) and cooled at room temperature for 2 ± 0.5 hours. The mixture shall then be placed in a 60°C (140°F) oven for a curing period of 16 hours.
6.3 After curing, the mixture shall be placed in an oven at 135°C (275°F) for 2 hours prior to compaction. The mixture shall then be compacted to 7 ± 1.0 percent air voids or a specific void level expected in the field. The level of voids can be obtained by either adjusting the number of blows in AASHTO Method T245; adjusting foot pressure, number of tamps, leveling load, or some combination of these in AASHTO Method T247; or adjusting the number of revolutions in ASTM D3387. The exact procedure must be determined experimentally for each mixture before the eight specimens are prepared. In all cases, ASTM D3387 is the preferred method of compaction. AASHTO T245 or T247 shall only be used if the necessary equipment or expertise is not available to perform ASTM D3387.
6.4 After the specimen is extracted from the mold, it shall be subjected to an adequate cool-down period. A centering device (see Figure 1) shall then be used to scribe diametral lines on the front and back faces of the test specimens. The lines shall be scribed to pass through the center of the test specimen and will represent the axis for indirect tensile testing. It is very important to insure that the diametral marks on the front and back faces of the specimen lie in the same vertical plane. If access is limited at the rear face of the specimen, the alignment of the scribed line on the back face can be checked with the use of a mirror.
Note: Sample Number Convention. When retrieved from the field the bulk sample of asphaltic material will have a Location Number similar to "B##" and a Sample Number similar to "BV##", "BR##", "BA##" or "BT##." After compaction, each individual compacted sample shall be assigned a new Sample Number. This Sample Number should begin with the letter "D" (representing a molded sample), followed by the letter "A" (representing asphaltic material) or "T" (representing asphalt treated material), as appropriate. The last two digits should be a number assigned consecutively from one to the number of samples molded from a given bulk sample. Generally, the Sample Numbers for a given bulk sample of AC will be DA01, DA02, DA03, DA04, DA05, DA06, DA07 and DA08. This Sample Number shall follow the molded sample through all subsequent phases of materials testing. The Location Number shall remain as specified for the bulk sample.
6.5 The test specimens shall be stored for 72 to 96 hours at room temperature.
7.1 The theoretical maximum specific gravity of the mixture shall be determined (using a separate uncompacted portion of the AC mixture) using LTPP Protocol P03.
7.2 The specimen thickness shall be obtained in accordance with LTPP Protocol P01.
7.3 The bulk specific gravity of each compacted specimen shall be determined using LTPP Protocol P02.
7.4 The estimated air voids of each specimen shall be calculated using AASHTO T269.
7.5 The specimens shall be sorted into two equal subsets of three samples each. The average air voids of the two subsets shall be approximately equal.
Of the original eight samples, three each are to be tested as dry and conditioned specimens. The remaining two samples are to be used in establishing the proper saturation method in order to avoid damaging any of the three conditioned samples. If an acceptable saturation level cannot be achieved using the two extra samples, then one of the three samples to be conditioned can be used to attempt a third combination of time and vacuum pressure.
Figure 1. Illustration of suitable specimen marking device
8.1 One subset will be tested dry and the other will be preconditioned before testing.
8.2 The dry subset will be stored at room temperature until testing. When the specimens are ready to be tested, they shall be wrapped with plastic or placed in a heavy-duty leak proof plastic bag. The specimens shall then be placed in a 25°C (77°F) water bath for 2 hours and then tested.
8.3 The other subset shall be conditioned as follows:
8.3.1 The specimen shall be placed in the vacuum container supported above the container bottom by a spacer. The container shall be filled with distilled water at room temperature so that the specimens have at least one inch of water above their surface. A vacuum pressure of 508 mm of Hg (20 in of Hg) shall be applied for five minutes. The vacuum shall be removed but the specimen will remain submerged in water for 30 minutes.
8.3.2 The bulk specific gravity shall be determined by LTPP Protocol P02. Compare the saturated surface-dry weight with the saturated surface-dry weight determined in Section 7.3. The volume of absorbed water shall be calculated.
8.3.3 The degree of saturation shall be determined by comparing the volume of absorbed water with the volume of air voids from Section 7.4. If the volume of water is between 55 and 80 percent of the volume of air, proceed to Section 8.3.4. If volume of water is less than 55 percent, repeat the procedure beginning with Section 8.3.1 using more vacuum and/or time. If the volume of water is more than 80 percent, the specimen has been damaged and shall be discarded. Repeat the procedure beginning with Section 8.3.1 using less vacuum and/or time.
8.3.4 The vacuum saturated specimens shall be tightly covered with a plastic film (saran wrap or equivalent). Each wrapped specimen shall be placed in a plastic bag containing 10 ml of water and the bag shall be sealed.
8.3.5 The plastic bag containing the specimen shall be placed in a freezer at -18 ± 3°C (0 ± 5°F) for 16 hours.
8.3.6 After 16 hours, the specimens shall be placed in a 60 ± 1°C (140 ± 1.8°F) water bath for 24 hours. As soon as possible after placement in the water bath, remove the plastic bag and film from the specimens.
8.3.7 Remove the specimens after 24 hours in the 60°C (140°F) water bath, and place them in a water bath at 25 ± 0.5°C (77 ± 1°F) for 2 hours. It may be necessary to add ice to the water bath to prevent the water temperature from rising above 25°C (77°F). No more than 15 minutes should be required for the water bath to reach 25°C (77°F). The specimens shall then be tested as described in Section 9.
9.1 Determine the tensile strength of all specimens (dry and conditioned) at 25°C (77°F) in accordance with Section 9.2. The order of specimen testing shall be randomized using an appropriate randomization scheme.
9.2 The specimen shall then be removed from the 25°C (77°F) water bath and placed between the two loading strips in the testing machine using the diametral scribed markings. Care must be taken to insure that the load will be applied along the diametral axis of the specimen as illustrated in Figure 2.
The diametral markings shall be used to insure that the specimen is aligned from top to bottom, front to back. The alignment of the front face of the specimen can be checked by insuring that the diametral marking is centered on the top and bottom loading strips. With the use of a mirror, the back face can be similarly aligned. After specimen placement is assured, a compressive load shall be applied at a controlled deformation rate of 51 mm (2 in) per minute along the diametral axis of the test specimen.
9.3 The maximum compressive load observed during testing shall be recorded but the loading will continue until a vertical crack appears along at least two-thirds of the test specimen. Remove the specimen from the machine and separate into halves at the crack interface. If the specimen cannot be separated (or split) by hand after a crack has developed over at least ? of the length, it should be reinserted in the tensile test machine and the loading continued until the crack increases in length or width to the extent that the specimen can be separated by hand. In no case should any equipment other than the testing machine be used to split the specimen. The interior surface shall be inspected for stripping and the observations recorded. Using a magnifying glass, estimate the amount of coarse aggregate (that material retained on the 6.3-mm [¼-inch] sieve - pieces larger than approximately 6.3 mm [0.25 in]) in the broken face of the sample that has been stripped. Similarly, estimate, (using a stereozoom microscope, if available) the amount of fine aggregate (that material passing the 6.3-mm [¼-inch] sieve - pieces smaller than approximately 6.3 mm [0.25 in]) that has been stripped. Figure 3 shall be used to estimate the relative stripping percentages of the coarse aggregate. For estimating stripping percentages of the fine aggregate, representative areas of the surface may be chosen for making "counts" of coated and uncoated aggregate, which can be used to calculate the percent stripping of fine aggregate.
9.4 The core may be disposed of at the conclusion of the visual examination using appropriate procedures.
10.1 Calculate the indirect tensile strength of each specimen as follows:
where: St = Indirect tensile strength, kPa
P0 = Maximum load sustained by the specimen, N
t = Specimen thickness, mm
D = Specimen diameter, mm
Figure 2. Proper alignment of specimen within the loading strips for the indirect tensile test.
Figure 3. Chart for visual percentage estimation.
10.2 Calculate the numerical index of the asphalt mixture's response to the detrimental effect of water as follows:
where: Yd = average tensile strength of dry subset
Yc = average tensile strength of conditioned subset.
TSR values near one are indicative of mixtures which will have very low susceptibility to stripping after exposure to moisture and freeze-thaw conditions.
10.3 Calculate the Relative Variation in Strength (RVS) as follows:
where: CVc = coefficient of variation for conditioned subset (Sc/Yc)
CVd = coefficient of variation for dry subset (Sd/Yd)
where Sc and Sd are calculated using the following equations:
where: Stc1 = tensile strength of the first conditioned specimen
Stc2 = tensile strength of the second conditioned specimen
Stc3 = tensile strength of the third conditioned specimen
Yc = (Stc1 + Stc2 + Stc3)/3
where: Std1 = tensile strength of first dry specimen
Std2 = tensile strength of second dry specimen
Std3 = tensile strength of third dry specimen
Yd = (Std1 + Std2 + Std3)/3
The following information is to be recorded on Form T05:
11.1 Sample identification shall include: Laboratory Identification Code, LTPP Region, State, State Code, SHRP ID, Layer Number, Field Set Number, Sample Area Number, Sample Location Number, and LTPP Sample Number.
11.2 Test identification shall include: LTPP Test designation, LTPP Protocol Number, Laboratory Test Number, and Test Date.
11.3 Test Results
Report the following:
11.3.1 Maximum specific gravity of the uncompacted AC mixture.
11.3.2 Average test specimen height to the nearest 2.5 mm.
11.3.3 Average test specimen diameter to the nearest 0.25 mm.
11.3.4 Method of compaction.
11.3.5 Bulk specific gravity of each test specimen after molding and prior to conditioning.
11.3.6 Percent air voids calculated for each specimen.
11.3.7 Bulk specific gravity of each "conditioned" test specimen after vacuum saturation.
11.3.8 Total maximum load sustained by each specimen during the indirect tensile strength test in N to the nearest whole number.
11.3.9 Tensile strength of each specimen to the nearest kPa.
11.3.10 Average tensile strength of the three "control" specimens and the average tensile strength of the three "conditioned" samples (Yd and Yc respectively), kPa.
11.3.11 Standard deviation of tensile strength for the "control" and "conditioned" subsets (Sd and Sc, respectively).
11.3.12 Tensile strength ratio calculated for the specimens.
11.3.13 Relative variation in strength for the specimens (RVS).
11.3.14 Record the estimated percent coarse and fine aggregate stripped.
11.3.15 Comments shall include LTPP standard comment code(s) as shown in Section 4.3 of this Guide and any other note as needed.
11.3.16 Test date.
LTPP LABORATORY MATERIAL HANDLING AND TESTING
LABORATORY MATERIAL TEST DATA
LAB DATA SHEET T05
ASPHALT CONCRETE LAYER (ASPHALTIC CONCRETE PROPERTIES)
LTPP TEST DESIGNATION AC05/LTPP PROTOCOL P05
LABORATORY PERFORMING TEST:_____________________________________________________________________
LABORATORY IDENTIFICATION CODE:__ __ __ __
SHRP REGION _________________ STATE ___________________ STATE CODE __ __
EXPERIMENT NO _____ SAMPLED BY: ______________________________________________ FIELD SET NO. __
DATE SAMPLED: __ __-__ __-__ __ __ __
|1. LAYER NUMBER||____||2. LABORATORY TEST NUMBER||__|
|3. SHRP ID||__ __ __ __||4. LOCATION NUMBER||__ __ __|
|5. SAMPLING AREA NUMBER (SA-)||__ __||6. MAXIMUM SPECIFIC GRAVITY OF MIX||__ . __ __ __|
|7. METHOD OF COMPACTION||________________________|
8. TEST RESULTS
|DATA ITEM||UNCONDITIONED (DRY)||CONDITIONED|
|LTPP SAMPLE NO.||_ _ _ _||_ _ _ _||_ _ _ _||_ _ _ _||_ _ _ _||_ _ _ _|
|AVG. SPEC. HGT.||_ _ _._||_ _ _._||_ _ _._||_ _ _._||_ _ _._||_ _ _._|
|AVG. SPEC. DIAM.||_ _ _._||_ _ _._||_ _ _._||_ _ _._||_ _ _._||_ _ _._|
|BSG AFTER MOLDING||_._ _ _||_._ _ _||_._ _ _||_._ _ _||_._ _ _||_._ _ _|
|% AIR VOIDS||_ _._||_ _._||_ _._||_ _._||_ _._||_ _._|
|BSG AFTER VAC. SAT.||_._ _ _||_._ _ _||_._ _ _|
|MAX. LOAD||_ _ _ _ _.||_ _ _ _ _.||_ _ _ _ _.||_ _ _ _ _.||_ _ _ _ _.||_ _ _ _ _.|
|INDIRECT TENS. STR.||_ _ _ _.||_ _ _ _.||_ _ _ _.||_ _ _ _.||_ _ _ _.||_ _ _ _.|
|AVG. INDIRECT TENS. STR.||_ _ _ _.||_ _ _ _.|
|STD. INDIRECT TENS. STR.||_ _ _.||_ _ _.|
|TENSILE STRENGTH RATIO||__ . __ __|
|RELATIVE VAR. IN STR.||__ __ __|
|COARSE AGG. STRIPPED, %||__ __ __.||__ __ __.||__ __ __.|
|FINE AGG. STRIPPED, %||__ __ __.||__ __ __.||__ __ __.|
|COMMENT CODES||__ __, __ __, __ __, __ __, __ __, __ __|
|SUBMITTED BY, DATE||CHECKED AND APPROVED, DATE|
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