Interpreting Asphalt
Test Results
Danny Gierhart, P.E.
Asphalt Institute
Regional Engineer
Tuttle, Oklahoma
[email protected]
On almost every roadway project,
the component materials are tested.
Quality Control - testing that helps
the producer ensure that they are
providing a quality product
Quality Assurance - testing that
helps the owner ensure that they are
receiving a quality product
Regardless of whether you are
looking at tests performed for the
purpose of Quality Control or Quality
Assurance, it’s very important to be
able to properly interpret the test
results and understand what they are
telling you.
Stay on top of tests - act sooner rather than later
Test results are too often used to
simply document the degree out of
specification so that a corresponding
financial penalty can be assessed.
A better use of test results is to track
the ongoing quality of a project and
make immediate corrections as
necessary to ensure a quality
pavement.
4 Questions to Ask for Each Test
•
What is the test designed to tell you?
•
What are the potential ramifications of
a failing test result?
•
Are the potential ramifications minor
or major?
•
For this test, how much could the
degree out of specification affect
pavement performance?
Over the next 90 minutes, we’ll
evaluate the most common
QC/QA tests reported for:
• Aggregate
• Hot Mix (& Warm Mix) Asphalt
Aggregate
• Gradation
• Sand Equivalent
Hot (& Warm) Mix Asphalt
• Asphalt Binder Content
• Lab-Molded Air Voids
• Voids in Mineral Aggregate (VMA)
• Roadway Density
• Moisture Susceptibility
Aggregate
Gradation or Particle Size Analysis:
AASHTO T 30
ASTM D 5444
When you might see this test:
This test is typically run on
asphalt mix samples after the
binder has been extracted or
burned off
Aggregate Test: Gradation
Test Overview:
After the binder from the HMA or WMA mixture has
been removed by extraction or ignition, the remaining
aggregate is dried to a constant weight.
The sample is then washed over a No. 200 sieve
(0.075 mm) and again dried to a constant weight.
Next, the sample is passed through a nest of sieves
by a prescribed level of agitation. Particles that are
small enough to fall through the openings in the top
sieve continue to fall through the nest until they reach
a sieve whose openings are too small through which
to pass.
The contents of each sieve are then weighed, and the
cumulative percent passing by mass of each sieve
size is then calculated.
Aggregate Test: Gradation
Example Gradation Results
Sieve Size
Cumulative
Weight (g)
% Retained
% Passing
JMF
1” (25 mm)
0.0
0.0
100
100
3/4” (19 mm)
140.5
8.8
91
96
1/2” (12.5 mm)
256.9
16.2
84
87
3/8” (9.5 mm)
440.9
27.7
72
77
No. 4 (4.75 mm)
791.1
49.8
50
54
No. 8 (2.36 mm)
962.5
60.5
39
40
No. 16 (1.18 mm)
1063.0
66.9
33
30
No. 30 (0.600 mm)
1146.8
72.1
28
23
No. 50 (0.300 mm)
1288.1
81.0
19
14
No. 100 (0.150 mm)
1399.9
88.0
12
10
No. 200 (0.075 mm)
1480.7
93.13
6.9
5.5
Gradation: What to focus on Example Gradation Results
Sieve Size
Cumulative
Weight (g)
% Retained
% Passing
JMF
1” (25 mm)
0.0
0.0
100
100
3/4” (19 mm)
140.5
8.8
91
96
1/2” (12.5 mm)
256.9
16.2
84
87
3/8” (9.5 mm)
440.9
27.7
72
77
No. 4 (4.75 mm)
791.1
49.8
50
54
No. 8 (2.36 mm)
962.5
60.5
39
40
No. 16 (1.18 mm)
1063.0
66.9
33
30
No. 30 (0.600 mm)
1146.8
72.1
28
23
No. 50 (0.300 mm)
1288.1
81.0
19
14
No. 100 (0.150 mm)
1399.9
88.0
12
10
No. 200 (0.075 mm)
1480.7
93.13
6.9
5.5
Sample % Passing vs. Job Mix Formula
•
What do the test results tell you?
How well the plant duplicated the aggregate
proportions that were designed in the lab. Failing
gradations take the form of a sample % passing
ending up outside the allowable tolerance from JMF.
•
What are the potential ramifications of
a failing test result?
Potential problems include segregation, harsh/tender
mix, and out-of-balance volumetrics.
Sample % Passing vs. Job Mix Formula
•
Are the potential ramifications minor
or major?
If the mix volumetrics are still in specification, an out-oftolerance gradation shouldn’t affect the mix quality
much.
• How much does the degree out of
spec affect pavement performance?
If the gradation strays too far out of tolerance, other mix
criteria will be affected, such as binder content, air
voids, and VMA.
Gradation: What to focus on Example Gradation Results
Sieve Size
Cumulative
Weight (g)
% Retained
% Passing
JMF
1” (25 mm)
0.0
0.0
100
100
3/4” (19 mm)
140.5
8.8
91
96
1/2” (12.5 mm)
256.9
16.2
84
87
3/8” (9.5 mm)
440.9
27.7
72
77
No. 4 (4.75 mm)
791.1
49.8
50
54
No. 8 (2.36 mm)
962.5
60.5
39
40
No. 16 (1.18 mm)
1063.0
66.9
33
30
No. 30 (0.600 mm)
1146.8
72.1
28
23
No. 50 (0.300 mm)
1288.1
81.0
19
14
No. 100 (0.150 mm)
1399.9
88.0
12
10
No. 200 (0.075 mm)
1480.7
93.13
6.9
5.5
Maximum Aggregate Size
•
What do the test results tell you?
The maximum aggregate size influences allowable
pavement thickness, susceptibility to segregation,
asphalt content, and volumetric requirements.
•
What are the potential ramifications of
a failing test result?
A gradation on the maximum sieve size that is less than
100% indicates the presence of over-sized particles.
This could result in compaction difficulties, segregation,
and asphalt content problems.
Maximum Aggregate Size
Extra large particles may
cause drag marks in the mat
Extra coarse aggregate will
make the mix easier to
segregate
Maximum Aggregate Size
•
Are the potential ramifications minor
or major?
Gradations results showing less that 100% passing the
maximum sieve size are rare and usually only slightly
less than 100%. Potential ramifications are minimal.
•
How much does the degree out of
spec affect pavement performance?
Results deviating by more than 2 or 3 percentage
points on this sieve indicate that a coarser mix
category of HMA was sampled than expected.
Gradation: What to focus on Example Gradation Results
Sieve Size
Cumulative
Weight (g)
% Retained
% Passing
JMF
1” (25 mm)
0.0
0.0
100
100
3/4” (19 mm)
140.5
8.8
91
96
1/2” (12.5 mm)
256.9
16.2
84
87
3/8” (9.5 mm)
440.9
27.7
72
77
No. 4 (4.75 mm)
791.1
49.8
50
54
No. 8 (2.36 mm)
962.5
60.5
39
40
No. 16 (1.18 mm)
1063.0
66.9
33
30
No. 30 (0.600 mm)
1146.8
72.1
28
23
No. 50 (0.300 mm)
1288.1
81.0
19
14
No. 100 (0.150 mm)
1399.9
88.0
12
10
No. 200 (0.075 mm)
1480.7
93.13
6.9
5.5
Percent Passing the No. 200 Sieve
•
What do the test results tell you?
The % passing the # 200 sieve influences
compaction, asphalt content, and volumetric
requirements.
•
What are the potential ramifications of
a failing test result?
A low % passing the # 200 sieve may result in high air
voids, permeability, and a lower AC demand. A high
value may result in low air voids and VMA, a higher AC
demand, and a tender mix.
Check cracking as a result of a high % passing the
#200 sieve
High % passing the #200 gives the HMA a lighter appearance,
indicating a lower film thickness and lower durability.
Percent Passing the No. 200 Sieve
•
Are the potential ramifications minor
or major?
High dust portions can have major ramifications on the
longevity of a roadway. They are typically accompanied
by lower than necessary binder contents which reduce
pavement durability.
•
How much does the degree out of
spec affect pavement performance?
Deviations of less than about 2% typically don’t have
too much of an effect. Deviations greater than 2%
should be addressed immediately.
Gradation: What to focus on Example Gradation Results
Sieve Size
Cumulative
Weight (g)
% Retained
% Passing
JMF
1” (25 mm)
0.0
0.0
100
100
3/4” (19 mm)
140.5
8.8
91
96
1/2” (12.5 mm)
256.9
16.2
84
87
3/8” (9.5 mm)
440.9
27.7
72
77
No. 4 (4.75 mm)
791.1
49.8
50
54
No. 8 (2.36 mm)
962.5
60.5
39
40
No. 16 (1.18 mm)
1063.0
66.9
33
30
No. 30 (0.600 mm)
1146.8
72.1
28
23
No. 50 (0.300 mm)
1288.1
81.0
19
14
No. 100 (0.150 mm)
1399.9
88.0
12
10
No. 200 (0.075 mm)
1480.7
93.13
6.9
5.5
Percent Passing the No. 8 Sieve
•
What do the test results tell you?
The % passing the # 8 sieve has a major influence on
voids and permeability.
•
What are the potential ramifications of
a failing test result?
A low % passing the # 8 sieve may result in an
inherently permeable mix. A high value is not as
worrisome as a low value.
Low % passing the # 8 indicates a mix that is subject to
permeability, which in turn can lead to stripping.
Superpave Gradation Requirements - AASHTO M 323
Table 3 - Aggregate Gradation Control Points
Nominal Maximum Aggregate Size - Control Points (Percent Passing)
37.5 mm
25.0 mm
19.0 mm
12.5 mm
9.5 mm
4.75 mm
Sieve
Size
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
Min
Max
50.0 mm
100
-
-
-
-
-
-
-
-
-
-
-
37.5 mm
90
100
100
-
-
-
-
-
-
-
-
-
25.0 mm
-
90
90
100
100
-
-
-
-
-
-
-
19.0 mm
-
-
-
90
90
100
100
-
-
-
-
-
12.5 mm
-
-
-
-
-
90
90
100
100
-
100
-
9.5 mm
-
-
-
-
-
-
-
90
90
100
95
100
4.75 mm
-
-
-
-
-
-
-
-
-
90
90
100
2.36 mm
15
41
19
45
23
49
28
58
32
67
-
-
30
60
6
12
1.18 mm
0.075mm
Be- wary -of JMF- gradations
near
these
lower
boundaries
- - the mix
may6be inherently
permeable
0
1
7
2
8
2
10
2
10
Percent Passing the No. 8 Sieve
•
Are the potential ramifications minor
or major?
A low % passing the # 8 sieve can have major
ramifications on the longevity of a roadway. They are
often accompanied by permeability issues which can
lead to stripping.
•
How much does the degree out of
spec affect pavement performance?
Gradations near the lower broad band are especially
of concern when accompanied by a low % passing the
# 200 sieve.
Aggregate
Sand Equivalent Test:
AASHTO T 176
ASTM D 2419
When you might see this test:
Many agencies specify this test
as part of their aggregate quality
assurance program.
Aggregate Test: Sand Equivalent
Flocculating
Solution
Suspended
Clay
Clay Reading
Sand
Reading
Sedimented
Aggregate
Test Overview:
This test is an indicator of the amount of
detrimental clay or clay-like fines
present in the aggregate.
A sample of aggregate passing the No.
4 sieve is placed in a flocculating
solution, which allows the heavier
particles to settle to the bottom of the
graduated cylinder, but keeps the fines
in suspension.
The sand equivalent is the ratio of the
sand reading to the clay reading,
expressed as a percentage. High
amounts of fines result in low SEs.
Aggregate Test: Sand Equivalent
•
What do the test results tell you?
The test is intended to show the relative proportions of
fine dust or clay-like material in the aggregate.
•
What are the potential ramifications of
a failing test result?
A low sand equivalent result indicates a high
percentage by volume of clay-like fines. These fines
may cause the asphalt binder to debond from the
aggregate in the presence of moisture.
Low sand equivalents indicate a high percentage of clay-like
fines, which can cause a poor bond with the asphalt binder.
Aggregate Test: Sand Equivalent
•
Are the potential ramifications minor
or major?
A failing sand equivalent test indicates a very poor
aggregate quality, which can have major ramifications
to the longevity of the pavement.
•
How much does the degree out of
spec affect pavement performance?
Sand equivalent results do not have to be much out of
spec to be a cause for concern.
Hot Mix Asphalt
Moisture Susceptibility Test:
AASHTO T 283
ASTM D 4867
When you might see this test:
Many agencies specify this test
as part of their quality assurance
program.
Moisture Susceptibility
A mix that is susceptible to moisture may reveal itself
in a number of different ways.
It might begin as staining of the surface as in the top
picture.
It might show up as a rutting problem.
It might show up as potholes in the roadway.
Each situation can be checked by coring the roadway.
Sometimes the core will not come out in one piece.
With total stripping, it might not come out at all.
Instead you’ll find what looks like loose virgin
aggregate in the core hole
HMA Test: Moisture Susceptibility
• Test Overview
– Prepare set of 6 specimens
• 6.5 to 7.5% voids
– Represents anticipated in-service voids
– Use 3 specimens as controls
– Remaining 3 specimens are vacuum saturated 70
to 80%
– Min. 16 hour freezing at 0oF
– 24 Hours in 140oF water bath
– Bring all specimens to test temperature (77oF)
and determine indirect tensile strength
HMA Test: Moisture Susceptibility
Saturating Specimens
Freezing Specimens
Specimens in Hot Water Bath
HMA Test: Moisture Susceptibility
Determine the indirect
tensile strengths of both
sets of 3 specimens
Calculate the
Tensile Strength
Ratio (TSR)
Avg. wet tensile strength
TSR =
Avg. dry tensile strength
Typical - Minimum of 75% required for field tests
HMA Test: Moisture Susceptibility
•
What do the test results tell you?
The test is intended to show the susceptibility of the
HMA to moisture damage.
•
What are the potential ramifications of
a failing test result?
A failing TSR indicates the mix is inherently susceptible
to stripping and raveling.
HMA Test: Moisture Susceptibility
HMA Test: Moisture Susceptibility
•
Are the potential ramifications minor
or major?
The ramifications of a failing TSR can be severe.
•
How much does the degree out of
spec affect pavement performance?
Because the test procedure is not one of the most
repeatable, results down to around 70% should be
confirmed by further testing. TSR results less than
60% would be just cause for immediate work stoppage
to investigate and fix the problem.
Hot Mix Asphalt
Asphalt Binder Content Test (Ignition
Oven):
AASHTO T 308
ASTM D 6307
When you might see this test:
Performed on almost every HMA
field sample.
HMA Test: Ignition Oven
Test Overview:
An asphalt mix sample of a specified
size is placed in a tared basket.
The basket is placed in an ignition oven.
The model pictured is equipped with an
internal scale, which shows less and
less weight as the binder burns off.
The sample stays in the oven until the
internal scale stabilizes, indicating that
all of the binder has burned off.
The %binder loss by weight can then be
calculated.
HMA Test: Ignition Oven
•
What do the test results tell you?
The test shows the percent of asphalt binder by
weight of the total mix.
•
What are the potential ramifications of
a failing test result?
A low binder content can lead to premature aging of the
pavement, stripping, or raveling. A high binder content
can lead to flushing or bleeding in the pavement.
Low binder contents can lead to raveling, stripping, and
premature aging.
High binder contents can lead to flushing or bleeding.
HMA Test: Ignition Oven
•
Are the potential ramifications minor
or major?
The ramifications would likely be different depending on
the degree out of spec. Whether any of the potential
pavement distresses occur also depend on several
other factors.
•
How much does the degree out of
spec affect pavement performance?
Binder content is one of several parameters that affect
each other. However, binder contents very far out of
spec should be a major cause for concern.
Hot Mix Asphalt
Lab-Molded Air Voids:
AASHTO T 166 & T 209
ASTM D 2726 & D 2041
When you might see this test:
Performed on almost every HMA
field sample.
HMA Test: Lab-Molded Air Voids
Test Overview:
Cylindrical specimens are prepared in a
compaction device. They are made up
of asphalt binder, aggregate, and air
voids. The bulk specific gravity (Gmb) is
then calculated for each specimen.
The maximum theoretical specific
gravity (Gmm) of the mix (where air
voids are removed by vacuum) is
calculated using uncompacted mix
samples.
The percent air voids (Va) is calculated:
Gmb
Va = 100 * Gmm
HMA Test: Lab-Molded Air Voids
•
What do the test results tell you?
The test shows the percent air voids in lab-molded
specimens. This test indicates a mix property, not inplace air voids.
• What are the potential ramifications of
a failing test result?
With a low air void content, the mix is either has high
binder content or the aggregate structure has changed
to produce lower inherent voids, which could result in
bleeding or rutting. For high air void contents, the mix
likely has a low binder content, which could result in low
durability, stripping, or raveling.
Plastic mix from low air voids & VMA
Rutting most likely caused by soft subgrade
HMA Test: Lab-Molded Air Voids
•
Are the potential ramifications minor
or major?
The ramifications can be severe depending on the
degree out of specification. Whether any of the
potential pavement distresses occur also depend on
other factors.
•
How much does the degree out of
spec affect pavement performance?
All failing void contents need some type of action.
Mixes that are 1% or more out of spec indicate a need
for immediate shutdown for more trials.
Hot Mix Asphalt
Voids in Mineral Aggregate (VMA):
AASHTO T 166 & T 209
ASTM D 2726 & D 2041
When you might see this test:
Performed on almost every HMA
field sample.
HMA Test Result: VMA
Va
VMA
Air
Vbe
AC
AC + Aggregate
Vmb
Aggregate
Test Overview:
VMA is a calculated mix
volumetric property, consisting
of the volume of air voids plus
the volume binder not
absorbed into the aggregate.
The mix needs a minimum
VMA to have enough volume
to hold the proper amount of
air voids and the proper
amount of binder.
VMA = 100 -
Gmb Ps
Gsb
HMA Test Result: VMA
•
What do the test results tell you?
VMA is calculated from the binder content test and
lab-molded specimens. It tells about the total volume
of the voids created by aggregates. Most mix
specifications require a minimum VMA.
•
What are the potential ramifications of
a failing test result?
Samples with low VMA do not have enough void space
to provide the proper amount of binder and the proper
amount of air. Low VMA, depending on the reason,
could result in many distress types.
HMA Test Result: VMA
•
Are the potential ramifications minor
or major?
The ramifications can be severe depending on the
degree out of specification. Whether any of the
potential pavement distresses occur also depend on
other factors.
•
How much does the degree out of
spec affect pavement performance?
Mixes that are 1% or more low indicate a need for
immediate shutdown for more trials. Although mixes
do not usually have any upper spec limit, problems
may occur if VMA is 2% or more high.
Hot Mix Asphalt
Roadway Density:
AASHTO T 166 & T 209
ASTM D 2726 & D 2041
When you might see this test:
Performed on almost every HMA
project.
HMA Test: Roadway Density
Test Overview:
Cores are cut from the roadway. Like
lab specimens, they are made up of
asphalt binder, aggregate, and air
voids. The bulk specific gravity (Gmb) is
then calculated for each specimen.
The maximum theoretical specific
gravity (Gmm) of the mix (where air
voids are removed by vacuum) is
calculated using uncompacted mix
samples.
The percent air voids (Va) is calculated:
Gmb
Va = 100 * Gmm
HMA Test: Roadway Density
•
What do the test results tell you?
The test indicates the in-place density achieved by the
compactive effort of the rolling operation.
•
What are the potential ramifications of
a failing test result?
Low density may result in permeability, stripping,
raveling, cracking, premature aging, and premature
failure. High density may result in rutting, flushing, or
bleeding.
Durability vs Air Voids
Stability
Cohesion
Zone of Highest
Durability
Flushing
Raveling
0
% Air Voids
HMA Test Result: VMA
•
Are the potential ramifications minor
or major?
The ramifications of failure to achieve proper density
are major. Proper density can in part make up for other
shortcomings. Improper density can cause failure even
if other parameters are good.
•
How much does the degree out of
spec affect pavement performance?
Any failing density report should cause the agency to
closely examine the rolling operation. The further out
of spec, the shorter the anticipated pavement life.
How Important is Density?
In summary, all failing test results
should be followed up on.
Some test results may be received
after all of the material is already in
place.
Some test results will be received
in time for corrective action, which
should be immediate.
Remember that a few hours time
addressing a problem may prolong
the pavement life by several years.
QUESTIONS?
Good Reference Materials on the Topic:
MS-2: Mix Design Methods
SP-2: Superpave Mix Design
MS-4: The Asphalt Handbook
MS-22: HMA Construction
http://www.asphaltinstitute.org