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Appendix E
Standard Test Procedure for Measuring
Interface Bond Strength in the Laboratory
Using the LISST
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Proposed Standard Method of Test for
DETERMINING THE INTERLAYER
SHEAR STRENGTH OF ASPHALT
PAVEMENT LAYERS
AASHTO Designation: TP XX-XX
Proposed test method under review before submitting to AASHTO
Subcommittee on Materials
American Association of State Highway and Transportation Officials
444 North Capitol Street N.W., Suite 249
Washington, D.C. 20001
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Proposed test method under review before submitting to AASHTO
Subcommittee on Materials Month Day, 20XX
Proposed Standard Method of Test for
DETERMINING THE INTERLAYER SHEAR
STRENGTH OF ASPHALT PAVEMENT LAYERS
AASHTO Designation: TP XX-XX
1. SCOPE
1.1. This test method covers the determination of the interlayer shear strength of asphalt
concrete layers using laboratory prepared or core samples.
1.2. This test can be performed on 150-mm (5.9-in.) or 100-mm (3.9-in.) diameter specimens of
asphalt concrete.
1.3. This test is applicable if both the asphalt overlay layer and the base layer thickness are
50 ± 5 mm (1.97 ± 0.2 in.), each. The total specimen thickness must not exceed 150
mm (5.9 in). Layers may be saw cut to the recommended layer thickness.
1.4. This standard may involve hazardous material, operations, and equipment. This
standard does not purport to address all safety problems associated with its use. It is the
responsibility of the user of this procedure to establish appropriate safety and health
practices and to determine the applicability of regulatory limitations prior to use.
2. REFERENCED DOCUMENTS
2.1. AASHTO Standards:
T 166, Bulk Specific Gravity Of Compacted Hot Mix Asphalt Using Saturated Surface-
Dry Specimens
T 168, Sampling Bituminous Paving Mixtures
T 209, Theoretical Maximum Specific Gravity and Density of Hot Mix Asphalt
T 269, Percent Air Voids in Compacted Dense and Open Asphalt Mixtures
T 312, Preparing and Determining the Density of Hot Mix Asphalt (Hma) Specimens By
Means Of the Superpave Gyratory Compactor
2.2. ASTM Standards:
D 3549, Standard Test Method for Thickness or Height of Compacted Bituminous
Paving Mixture Specimens
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3. TERMINOLOGY
3.1. Interlayer Shear Strength (ISS) The maximum capacity of the interface to resist failure
due to shearing stresses.
4. SUMMARY OF METHOD
4.1. The Louisiana Interlayer Shear Strength Tester (LISST) was developed for the
characterization of interface shear strength of cylindrical specimens. The device
(Figure 1) consists of two main parts, a shearing frame, and a reaction frame. Only the
shearing frame is allowed to move while the reaction frame is stationary. A cylindrical
specimen is placed inside the shearing and reaction frames and is locked in place with
collars. Loading is then applied to the shearing frame. As the vertical load is gradually
increased, shear failure occurs at the interface.
5. SIGNIFICANCE AND USE
5.1. Tack coats are applied on a pavement surface before overlay construction to ensure
adequate interface bond strength between two layers. If the interface cannot provide
enough strength to resist stresses due to traffic and environmental loading, shear failure
may occur at the interface. Poor interface bond strength may also accelerate the
appearance of other distresses such as slippage and surface cracks.
6. APPARATUS
6.1. Interlayer Shear Strength Tester - The device used for the interlayer shear strength test
shall be designed such that it adapts to any universal testing machine, has a nearly
frictionless linear bearing to maintain vertical travel, accommodates sensors that
measure the vertical and horizontal displacements, provides specimen locking
mechanism, applies consistent normal loads, and accommodates 100- and 150-mm
sample diameters. The gap between the loading frame and the reaction frame shall be
12.7 mm (1/2 in.). The device is illustrated in Figures 1 and 2.
6.2. Loading Machine - The loading machine shall produce a uniform vertical movement of
2.54 mm (0.1 in.) per minute. Universal mechanical or hydraulic testing machine may
be used such that it can provide a displacement rate of 2.54 mm (0.1 in.) per minute.
The loading device shall be capable of meeting the minimum requirements specified in
Table 1.
Table 1- Minimum Test System Requirements
RANGE ACCURACY (PERCENT)
LOAD (N) 0 25000 ± 1.0
LOADING RAM LVDT (MM) 0 150 ± 0.5
VERTICAL, HORIZONTAL LVDTS
02 ± 0.5
(MM)
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6.3. Air compressor - capable of supplying 7.5 SCFM at 40 psi to operate the pneumatic
normal load actuator.
6.4. Wet masonry saw.
Horizontal Sensor
Vertical Sensors
Normal Load Actuator Loading Frame
Reaction Frame
Figure 1. 3-D Illustration Of The Louisiana Interface Shear Strength Tester (LISST) Device.
Figure 2.Front and side view of the LISST device.
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7. HAZARDS
7.1. Standard laboratory safety precautions must be observed when preparing and testing
asphalt concrete specimens.
8. TEST SPECIMENS
8.1. Test specimens may be either laboratory-compacted HMA or sampled from HMA
pavements.
8.2. Samples cored from HMA pavement:
8.2.1. Mark the direction of traffic on the roadway surface before coring so that it can
be identified once the core is removed.
8.2.2. Cores shall be taken full depth so that no prying action is needed to extract the
cores from the pavement. Care shall be taken to avoid stress or damage to the
interface during coring, handling, and transportation. If a core debonds at the
interface of interest during the coring operation, make note of it on the coring
report.
8.2.3. Label core specimens with a paint pen.
8.2.4. Roadway core specimens shall be approximately 150 mm (5.906 in.) diameter
with all surface of the perimeter perpendicular to the surface of the core within
6 mm (¼ in.). If the height of the core above or below the interface being tested
is greater than 50 mm (1.969 in.), it shall be trimmed with a wet masonry saw to
a height of approximately 50 mm (1.969 in.).
8.2.5 Mark the location of the interface layer with white or silver paint.
8.3. Laboratory-compacted HMA samples:
8.3.1. To prepare laboratory samples, compact a cylindrical specimen 150 mm in
diameter with a thickness of 50 mm using the Superpave Gyratory Compactor
by AASHTO T312. Brush the tack coat material on the top of the prepared
specimen. The amount of tack coat will be determined by the application rate.
Pour appropriate amount of HMA mixture on top of this tacked lower half. The
amount of HMA mixture should be enough to obtain a 50 mm thick "top half".
8.3.2. Measure the diameter of the specimen and the thickness of both layers to the
nearest 1 mm.
8.4. Number of Test Specimens a single test shall consist of at least three specimens.
9. PROCEDURE
9.1. Specimen conditioning The specimens shall be allowed to stabilize at each test
temperature of 4.4, 25.0, and 60.0±1°C (40, 77, and 140 ±2 °F) for a minimum of 2
hours.
9.2. Specimen positioning Orient the core in the interlayer shear strength tester device so
that the direction of traffic marked on the core is vertical.
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9.3. The specimen should be loaded in such a manner that the interlayer is located directly
in the middle of the gap between the loading and the reaction frames. The loading
frame is the frame that can move up and down and the reaction frame is the stationary
portion of the apparatus, Figures 1 and 2.
9.4. Normal load, if required, can be applied by means of normal load actuator. The normal
load actuator should be able to apply normal pressure up to 206.84 kPa (30 psi) on a
150-mm diameter sample.
9.5. Rate of displacement Apply the displacement continuously and without shock, at a
constant displacement rate of 2.54 mm (0.1 in.) per minute until failure. Record the
resulting ultimate load, Pult, vertical, and horizontal deformations, Figure 3.
10. CALCULATIONS
10.1. Calculate the interlayer shear strength, ISS, as follows:
Pult
ISS =
D 2
4
where:
ISS = interlayer shear strength, Pa
Pult = ultimate load applied to specimen, N
D = diameter of test specimen, m
11. REPORT
11.1. Report the following for each specimen tested:
11.2. Core identification.
11.3. Report the failure surface location. Failure should occur at the interface of the two
material layers.
11.4. Note the appearance of the interface including any contaminants, milling striations,
stripping, tack coat streaks, etc.
11.5. Test results.
11.6. Specimen dimensions including thickness of the overlay asphalt, thickness of existing
layer, and diameter of specimen.
11.7. Ultimate load applied.
11.8. Interlayer shear strength, nearest Pa.
11.9. Corresponding vertical and horizontal deformations.
11.10. Average and standard deviation of interlayer shear strength for the set of cores.
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12. PRECISION AND BIAS
12.1. The precision and bias statements for this method have not been determined.
13. KEYWORDS
13.1. Interlayer Shear Strength, Asphalt Overlay, Tack Coat, Shear Strength, Slippage Failure.
3.5
Interface Shear Load (kN) a
3.0 Pult
2.5
2.0
1.5
1.0
0.5
0.0
0 1 2 3 4 5 6 7
Displacement (mm)
Figure 3. Typical LISST test result.
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