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The Journal of Engineering Research Vol 7, No. 1 (2010) 11-21

1. Introduction

Hot mix asphalt (HMA) is the most common material
used for paving applications. It consists primarily of an
asphalt binder and mineral aggregates. The binder acts as
a gluing agent that binds aggregate particles into a cohe-
sive mass. When bound by an asphalt binder, a mineral
aggregate acts as a stone framework that provides strength
and toughness to the system. Several mixture design
methods have been developed over time, in an effort to
create a mixture that is capable of providing acceptable
performance based on certain predefined set of criteria.
The most recently developed mixture design method
____________________________________________
* Corresponding author e-mail: alshamsi@squ.edu.om

is the Superpave method. Superpave stands for Superior
Performing Asphalt Pavements and represents a basis for
specifying component materials, asphalt mixture design
and analysis, and pavement performance prediction. It
includes several processes and decision points. A key fea-
ture in the Superpave system is laboratory compaction. In
the mix design procedure, a Superpave gyratory com-
pactor (SGC) is used to carry out the compaction of the
asphalt mixture specimens in the laboratory. SGC was
found to be effective in simulating field compaction and
ensuring that the properties of the samples compacted in

Estimating Optimum Compaction Level for Dense-Graded
Hot-Mix Asphalt Mixtures

Khalid Al Shamsi*a and Louay N. Mohammadb

*a Department of Civil and Architectural Engineering, College of Engineering, Sultan Qaboos University, P.O Box 33,
Postal Code 123, Al-Khoud, Muscat, Sultanate of Oman

b Department of Civil and Environmental Engineering and Louisiana Transportation Research Center, Louisiana State University,
4101 Gourrier Ave, Baton Rouge, LA 70808, USA

Received 3 March 2008; accepted 18 May 2009

Abstract: A critical step in the design of asphalt mixtures is laboratory compaction. Laboratory compaction
should reflect field compaction and should produce mixtures that are economical and possess high structural sta-
bility. During the compaction process, asphalt mixtures are subjected to certain amount of compaction energy
in order to achieve the required density. The Superpave volumetric mix design is based on compacting HMA
mixtures to a specified compaction level described by the number of gyrations from the Superpave gyratory
compactor (SGC). This level is termed Ndes and represents the required energy (based on the traffic level
expected) to densify the mixture to a 4% air voids level. This paper re-examines the Superpave compaction
requirements through extensive laboratory investigation of the response of a number of asphalt mixtures to the
applied compaction energy. It also presents an alternative method to estimate the number of gyrations at which
a mixture first reaches an optimum aggregate interlock and hence prevents overcompaction problems that might
result in unstable aggregate structures or dry asphalt mixtures. A total of 12 HMA mixtures were studied.
During compaction, force measurement was made using the pressure distribution analyzer (PDA). The com-
paction characteristics of the mixtures were analyzed using data from the PDA and the traditional Superpave
Gyratory Compactor (SGC) results.

Keywords: Locking point, Mix design, Asphalt mixtures, Pavement materials, Laboratory compaction

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The Journal of Engineering Research Vol 7, No. 1 (2010) 11-21

the laboratory are to some extent, similar to the mix
placed in the field (Cominsky, 1994). Figure 1 is a
schematic illustration of the SGC and Fig. 2 presents a
typical output from it. A hydraulic or mechanical system
applies a load to the loading ram, which applies 600 kPa
compaction pressure to the specimen. The loading ram
diameter nominally matches the inside diameter of the
compaction mold, which is normally 150 mm for design
purposes. The SGC base rotates at a constant rate of 30
revolutions (gyrations) per minute during compaction,
with the compaction mold positioned at a compaction
angle of 1.25º. The ram pressure is monitored by a pres-
sure gauge during compaction. As the specimen densifies,
the pressure gauge and loading ram maintain compaction
pressure. The specimen height during compaction is mon-
itored and recorded.

In the Superpave system, asphalt mixtures are designed
at a specific level of compactive effort. This is a function
of the design number of gyrations (Ndesign). The design
number of gyrations depends upon the traffic level for
which the mix is designed. Traffic is represented by the

design-equivalent single axle loads (EASLs). Four traffic
categories ranging from 0.3 to greater than 30 million
EASLs are used. Higher compaction energy is applied to
mixtures in the heavy traffic category. The analysis of the
compacted samples is done in terms of percentage of the
theoretical maximum specific gravity of the mixtures.

The data from the SGC are generally used in comput-
ing volumetric properties such as density or air void con-
tent as a function of compaction gyrations. However, sev-
eral attempts have been recently made to analyze the den-
sification curve obtained from the SGC in order to evalu-
ate the asphalt mixtures’ workability and resistance to per-
manent deformation. The initial number of gyrations
(Ninitial) and the slope of the initial portion of the SGC
compaction curve have been hypothesized to reveal cer-
tain mixture properties such as tenderness of the mixtures
and the strength of aggregate structure (McGennis, 1995).

Vavrik (Vavrik, et al. 2000) suggested the evaluation of
mixture compaction characteristics based upon the lock-
ing point or the point during compaction at which the mix-
ture exhibits a marked increase in resistance to further
densification. The Alabama Department of Transportation
(Alabama DOT, 2002) have adopted the locking point mix
design concept. They define the locking point as the point
where the sample being gyrated loses less than 0.1 mm in
height between successive gyrations. Similarly, the
Georgia Department of Transportation (Georgia DOT,
2003) use the concept of locking point in designing HMA
mixtures. They define the locking point as the number of
gyrations at which, in the first occurrence, the same height
has been recorded for the third time. For Georgia, typical
locking points are reported to be in the range between low
60's to high 80's measured with the Superpave gyratory
compactor. Recently, the Oregon Department of
Transportation changed the Superpave design gyration
levels for mixes used in the state highway system
(Asphalt, 2007). Oregon highway mixes will be designed
at 4.0 percent air voids at 65, 80, and 100 gyrations. For
highways with low truck volumes (ODOT level 2), gyra-
tions have been rolled back from 75 to 65. Mixes to sup-
port moderate truck traffic (ODOT Level 3) will be
designed at 80 gyrations instead of 100. Interstate and
other highways with heavy truck traffic (ODOT level 4)
will be built with mixes designed at 100 gyrations rather
than 125 gyrations.

Guler et al. (Guler, et al. 2000) developed a gyratory
load cell and plate assembly (GLPA) for measuring HMA
shear resistance during compaction with any SGC. It is a
thin cylindrical device that is inserted on top of the mix-
ture in the compaction mold that gives a continuous meas-
ure of shear resistance under gyratory loading during
compaction. They hypothesized that bulk shear resistance
from the GLPA is a good indicator of the compactability
of HMA mixtures and their potential resistance to rutting
under traffic. It was concluded that the device offers
potential as a low-cost tool to complement volumetric
properties from the SGC by evaluating the compactability
of asphalt mixtures as well.

Control and
data acqui-
sition panel

loading
ram

mold

reaction
frame

rotating
base

Figure 1. Superpave Gyratory Compactor (Asphalt
Institute, 2001)

Figure 2. Typical data output from the SGC

umber of Gyrationns

H
ei

gh
t

of
S

pe
ci

m
en

, m
m

The Journal of Engineering Research Vol 7, No. 1 (2010) 11-21

2. Objective and Scope

The primary objective of this study was to develop a
method for estimating the number of gyrations at which an
asphalt mixture reaches its optimum aggregate interlock.
To achieve this objective, an extensive laboratory investi-
gation was carried out to study the response of a number
of asphalt mixtures to the applied compaction energy
when compacted in the Superpave gyratory compactor
(SGC). A total of 12 HMA mixtures were evaluated.
During compaction, force measurement was made using a
small device inserted in the compaction mold called the
pressure distribution analyzer (PDA). The compaction
characteristics of the mixtures were analyzed using data
from the PDA and the traditional Superpave Gyratory
Compactor (SGC) results. The following sections describe
in detail the materials used and the methodology applied
to achieve the objective of the study.

3. Materials

An asphalt mixture is a composite material that is large-
ly made of two main components, aggregate and asphalt
cement. Sources of aggregate were selected to encompass
a wide range of aggregates commonly used in asphalt
mixtures. Three aggregate types were used. These are:

Different stockpiles from each type of aggregates were
acquired. Natural coarse sand was used whenever neces-
sary in the final design blends. Aggregates were acquired
in 50-gallons barrels and kept properly sealed from any
moisture intrusion. Detailed laboratory evaluation proce-
dures of individual stockpiles were conducted to deter-
mine the basic aggregate properties such as specific grav-
ity, gradation, and other Superpave consensus properties.

Larger-sized stockpiles were sieved into individual size
fractions. Materials retained on 1", 3/4", 1/2", 3/8", No. 4,
and passing No. 4 sieves were stored in separate contain-
ers so that the required gradations could be batched direct-
ly from the individual size fractions. This method of
aggregate separation, while somewhat time and labor-
intensive, allows for strict control and exact replication of
mixture's aggregate gradation. Three aggregate structures
were designed for each aggregate type: coarse (C), medi-
um (M), and fine (F). Figure 3 shows an example of the
aggregate structures used in the study. SB polymer-modi-
fied asphalt binders PG76-22M were used in all the mix-
tures evaluated in this study.

4. Mixture Design

Mixture design was performed on all the aggregate
structures that were formulated. The Superpave mixture
design method was followed except for the VMA require-
ment. All the mixtures were designed for high-volume
traffic (Ndes = 125 gyrations at 1.25° angle of gyration).
The optimum asphalt content was determined as the
asphalt content required to achieve 4.0 percent air voids at
Ndes. Optimum asphalt contents ranged from 3.0% to

Half inch Granite

100

Sieve Size, mm

Pe
rc

en
t P

as
sin

Coarse Medium Fine

19.0 12.5 9.5 4.75 2.36 1.18 0.60 0.30 0.150 0.075

Figure 3. Example of aggregate structure used in the study

 Hard agg regates; crushed granite (GR)
 Water-absorptive, high -friction aggregate;

sandstone (SST) and
 Low friction low-water-absorption aggregate;

limestone aggregate (LS).

The Journal of Engineering Research Vol 7, No. 1 (2010) 11-21

5.1% . The coarse mixtures had higher optimum asphalt
contents for all the aggregate types considered. Tables 1
and 2 provide the job mix formula for each of the designed
mixtures.

5. Mixtures Compactability

The densification curve obtained from the SGC was
used to evaluate mixture resistance to the compaction
energy applied by the SGC. The behavior of the mixtures
during compaction was also captured using the pressure
distribution analyzer (PDA). This is a simple accessory
that measures the force applied to the mixtures using three
load-cells equally spaced at an angle of 120°. The load-
cells allow measuring of the variation of forces during
gyration such that the position or eccentricity of the result-

ant force from the gyratory compactor can be determined
in real time. The two-dimensional distributions of the
eccentricity of the resultant force can be used to calculate
the effective moment required to overcome the internal
shear frictional resistance of mixtures when tilting the
mold to conform to the 1.25º angle. Based on the data
from the load-cells, the two components of the eccentric-
ity of the total load relative to the center of the plate (ex
and ey) can be calculated. The calculations are simply
done with general moment equilibrium equations along
two perpendicular axes passing through the center of one
of the load-cells as shown in Fig. 4 using the following
equations:

(1)

Mixture name1 LS
Coarse

LS
Medium

LS
Fine

SST
Coarse

SST
Medium

SST
Fine

GR
Coarse

GR
Medium

GR Fine

Mix type 12.5 mm 12.5 mm 12.5 mm 12.5 mm 12.5 mm 12.5 mm
12.5 mm 12.5 mm 12.5 mm

Binder type PG 76-22M PG 76-22M
PG
76-

22M
PG 76-22M PG 76-22M PG 76-22M

PG 76-22M PG 76-22M PG 76-22M

Design AC content, volumetric properties, and densification
% Gmm at NI 85.1 86.2 88.0 86.0 86.4 88.0 87.3 87.3 87.1
% Gmm at NM 97.2 97.4 97.3 97.0 97.1 97.4 97.5 97.2 97.0
Design binder

content, % 5.1 4.0 3.5 5.1 3.6 3.9 4.8 4.5 4.3
Design air void,

% 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0
VMA, % 12.8 14.5 16.6 12.3 11.8 13.8 12.7 11.3 10.9
VFA, % 71.0 62.7 58.5 69.0 50.0 54.0 66.2 62.4 60.6

Metric (U.S.)
Sieve Gradation, (% passing)
19 mm (¾

in) 100 100 100 100 100 100
100

100.0 100.0
12.5 mm (½

in) 97.1 97.0 97.2 96.0 96.6 97.2
97.3

97.7 98.3
9.5 mm (3/8

in) 80.3 80.2 81.7 80.7 83.8 86.5
79.3

82.5 86.8
4.75 mm
(No.4) 46.9 55.2 59.8 48.6 57.6 64.7

46.7
54.4 65.0

2.36 mm
(No.8) 31.5 39.6 46.1 32.8 41.6 48.4

33.2
39.5 49.0

1.18 mm
(No.16) 21.8 27.9 34.7 22.2 31.5 36.9

23.3
27.8 35.4

0.6 mm
(No.30) 15.3 19.7 25.6 16.2 23.7 27.8

16.6
19.7 25.5

0.3 mm
(No.50) 9.3 11.1 14.4 12.1 15.9 17.7

10.1
11.7 14.6

0.15 mm
(No.100) 6.6 7.4 9.3 6.7 11.2 12.1

6.6
7.4 9.0

0.075 mm
(No.200) 5.5 6.0 7.2 4.2 8.4 9.1

4.8 5.4 6.5

Table 1. Job mix formula - 12.5 mm mixes - Ndes = 125 Gyrations

1 LS: Limestone, SST: Sandstone, GR: Granite

0x yM e  

The Journal of Engineering Research Vol 7, No. 1 (2010) 11-21

(2)

(3)

P1, P2, P3 are load-cell forces; ex and ey are x- and y-
components of the eccentricity, e; and ry is the location of
the plate center point with respect to the x-axis.

The frictional shear resistance of the asphalt mixture
can be calculated using the following relationship:

(4)

where,

FR = the frictional resistance
R = Resultant Force
e = eccentricity
A = cross-section area
H = sample height at any gyration cycle.

Two specimens per mixture were tested for com-
pactability in both SGC and PDA devices.

Figures 5 and 6 present an example of the behavior of
the asphalt mixtures during the compaction process using
the data from both SGC and PDA devices. The figures
show the rate of change in specimen height (from the

SGC) and the frictional resistance (from the PDA). It is
clear that the mixture reaches a condition where applying
additional compaction energy results in little or no effect
in further densifying the mixture. The point at which the
mixture starts exhibiting this level of compaction resist-
ance is termed the locking point of the mixture and is
defined as follows:

SGC Locking Point : The SGC locking point is the num-
ber of gyrations after which the rate of change in height is
equal to or less than 0.05 mm for three consecutive gyra-
tions (Fig. 5).

PDA Locking Point: It is defined as the number of gyra-
tions at which the rate of change of frictional resistance
per gyration is less than 0.01 (Fig. 6).

The locking point data presented in Figs. 7 and 8 for
both the SGC and the PDA respectively indicate that it
takes more energy to densify coarse mixtures compared to
the medium and fine mixtures. As the aggregate gradation
becomes finer, the compactability of the mixtures general-
ly improves. Locking points are much lower than the
design number of gyrations recommended by the current
Superpave design system. The highest locking point is
about 70% of the recommended design number of gyra-
tions for the heavy-traffic category (Ndes=125). Figure 9
presents the good correlation between the locking points
determined from the SGC and those determined from the
PDA. On average, the PDA locking points were about 4
gyrations lower than those determined from the SGC data.

Mixture name LS
Coarse

LS
Medium

LS
Fine

Mix type 25.4 25.4 mm 25.4 mm
Binder type PG 76-22M PG 76-22M PG 76-22M

Design AC content, volumetric properties, and densif ication
% Gmm at NI 85.0 88.8 89.1
% Gmm at NM 97.7 97.4 97.4

Design binder content, % 3.8 3.0 3.3
Design air void, % 4.0 4.0 4.0

VMA, % 11.1 9.6 10.0
VFA, % 63.5 58.2 60.5

Metric (U.S.) Sieve Gradation, (% passing)
37.5 mm (1½ in) 100 100 100

25 mm (1 in) 92.4 92.6 95.2
19 mm (¾ in) 78.8 79.3 86.5

12.5 mm (½ in) 64.7 66.0 76.9
9.5 mm ( ? in) 52.5 56.1 65.8

4.75 mm (No.4) 36.4 43.2 50.3
2.36 mm (No.8) 24.5 32.7 38.1

1.18 mm (No.16) 15.8 24.3 28.2
0.6 mm (No.30) 10.6 17.6 20.4
0.3 mm (No.50) 7.7 8.7 10

0.15 mm (No.100) 6.1 5.2 5.9
0.075 mm (No.200) 5.1 4.2 4.8

Table 2. Job mix formula - 25.4 mmmixes - Ndes = 125 Gyrtions

0y xM e  

 22x y ye e r e  

Re
FR

AH


3/8 in)

The Journal of Engineering Research Vol 7, No. 1 (2010) 11-21

6. Physical Properties of Mixtures at their
Locking Point

A limited number of mixtures were selected for mix-

ture design using the locking point concept as opposed to
the traditional Superpave Ndes. Graphical comparisons
of some physical properties from both sets of mixtures are
presented in Figs. 10 through 12. As anticipated, com-

a b

0.00

5.00

10.00

15.00

20.00

25.00

0 50 100 150 200 250 300 350

N o . o f Gy r a t i o n s

d
Figure 4. Pressure distribution analyzer (a) The PDA device (b) Analysis of forces (c) Inserting

the PDA in the compaction mold (d) Typical results

-0.5

0.5

1.5

2.5

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210

No. of Gyrations

R
at

e
of

C
ha

ng
e

No. of Gyrations

R
at

e
of

C
ha

ng
e

Figure 5. Rate of change of the height SGC compaction

The Journal of Engineering Research Vol 7, No. 1 (2010) 11-21

-0.2

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

0 10 20 30 40 50 60 70 80 90 10
0

11
0

12
0

13
0

14
0

15
0

16
0

17
0

18
0

19
0

20
0

21
0

22
0

23
0

24
0

25
0

26
0

27
0

28
0

29
0

30
0

31
0

32
0

No. of Gyrations

R
at

e
of

C
ha

ng
e

of
F

No. of Gyrations

R
at

e
of

C
ha

ng
e

of
F

Figure 6. Rate of change of frictional resistance SGC compaction

S G C Lock ing Points

70
62

83
73 72 71 68

60 61

878787

100

1/2"
LS
C

1/2"
LS
M

1/2"
LS F

1/2"
S S T

1/2"
S S T
M

1/2"
S S T

1/2"
G R
C

1/2"
G r
M

1/2"
Gr F

1"
LS
C

1"
LS
M

1"
LS F

L
oc

k
in

g
P

oi
n

Figure 7. SGC locking point results

PDA Locking Points

64
57

73 69
79

65 67
73

51 49

82 79

100

1/2"
LS C

1/2"
LS
M

1/2"
LS F

1/2"
SST
C

1/2"
SST
M

1/2"
SST

1/2"
GR
C

1/2"
Gr
M

1/2"
Gr F

1"
LS C

1"
LS
M

1"
LS F

PD
A

L
oc

ki
ng

P
oi

nt
s

Figure 8. PDA locking point research

C
ha

ng
in

g
Po

in
ts

PD
A

L
oc

ki
ng

P
oi

nt
s

The Journal of Engineering Research Vol 7, No. 1 (2010) 11-21

y = 0.97x - 3.96
R2 = 0.85

100

40 50 60 70 80 90 100

SGC Locking Point

PD
A

L
oc

ki
ng

P
oi

Figure 9. SGC and PDA locking points correlation

4.9

4.1

5.4

4.2
3.9

3.3
3.6

5.1

3.5

4.3

GRF LSF LSC SSM 1" LSF

%
AC

Ndes
Locking Point

Figure 10. Comparison of the design asphalt content

12.2

10.5

13.7

9.2

11.1

10.0

8.4

13.5

9.4

10.9

GRF LSF LSC SSM 1" LSF

VM
A,

Ndes
Locking Point

Figure 11. Comparison of the VMA results

PD
A

L
oc

ki
ng

P
oi

nt
%

A
C

V
M

A
%

The Journal of Engineering Research Vol 7, No. 1 (2010) 11-21

pacting mixtures to their locking point yielded higher
design asphalt contents than those obtained when
Superpave-design number of gyrations was used. The
asphalt content for mixtures designed using the locking
point ranged from 3.9% to 5.4% compared to 3.3% to
5.1% for the same mixtures designed using the traditional
Ndes. It is worth noting that except for the half-inch
coarse limestone mixture, there was about 0.6% increase
in asphalt content for all other mixtures when they were
designed using their locking points at the same level of
4.0% air void. The Voids in Mineral Aggregates (VMA)
values were 1.1% to 1.2% higher for the mixtures
designed at the locking point except for the medium sand-
stone mixture in which there was 0.8% increase. Higher
asphalt contents naturally resulted in higher VFA, lower
Dust/Pbeff ratio, and hence higher effective film thickness
for the mixtures in consideration.

7. Estimating Locking Point

To facilitate the design process, a multiple linear
regression model was developed using SAS software
(SAS 2002) to estimate the locking point of the mixture
based on certain properties that are thought to influence
the performance of the mixture during compaction. The
response parameter used was the locking point (LP).
Since the compaction process is always performed at ele-
vated temperatures, the influence of aggregate structure is
thought to be more pronounced than that of the binder
although the binder will still maintain some lubrication
effect that might contribute to the mixture’s response to
the applied compaction energy. Several parameters were
first introduced in the model including different character-
istics of the gradation curves of the designed aggregate
structure as well as binder content. A stepwise variable
selection procedure was first performed on a general
model that contains those variables. The purpose of such
a procedure is to remove insignificant variables from the
general model. The regression analysis was then conduct-

ed on the reduced model using the stepwise variable selec-
tion procedure. Three parameters were used in the regres-
sion analysis which were significant when included in the
model as independent variables. These were:

The predictive model used is:

(5)

where LP, VCA, P200* AC are:

LP = Locking Point to be estimated
VCA = Volume of coarse aggregate in the aggregate

structure
P200 * AC = the interaction between the effect of the

amount of material passing #200 sieve in the aggregate
structure and the estimated asphalt content.

The results of the regression procedure are shown in
Table 3. The F- Value for the model was 45.44 with a P-
value of <0.0001. This indicates that the model is signif-
icant in describing the relationship between the response
variable and the independent variables. All the parameter
estimates for the predictor variables in the model were sig-
nificant at the 95% significance level selected for the
analysis. The model was also checked for any co-lineari-
ty between the predictor variables. When there is a perfect
linear relationship among the predictors, the estimates for

5.6

4.3

3.3

6.1

5.2

2.5

8.7

3.4

4.5

9.0

GRF LSF LSC SSM 1" LSF

Te
ff,

m
ic

ro
ns

Ndes
Locking Point

Figure 12. Comparison of the effective film thickness

 Volume of coarse aggregate in the aggregate
structure (VCA) . The designer can determine
the VCA in the dry condition of aggregate by
performing a unit weight test on the combined
material retained on No. 4 Sieve for a given
blend according to AASHTO T -19 test
procedure (AASHTO T -19, 2004) , along with
determining the combined specific gravity for
this material.

 Percent Passing #200 sieve for the aggregate
structure in consideration. This parameter is
termed as “P 200”.

 Estimated initial asphalt content (AC).

 2001 38 0 62 6 86LP . * VCA . * P * AC .  

Te
ff

, M
ic

ro
ns

The Journal of Engineering Research Vol 7, No. 1 (2010) 11-21

a regression model cannot be uniquely computed. The
term co-linearity describes two variables that are near-
perfect-linearity combinations of one another. When more
than two variables are involved, it is often called multi-co
linearity, although the two terms are often used inter-
changeably.

The primary concern is that as the degree of multi-co
linearity increases, the regression model estimates of the
coefficients become unstable and the standard errors for
the coefficients can get wildly inflated.

The 'vif' option was used to check for multi-co linear-
ity. If stands for variance inflation factor. As a rule of
thumb, a variable whose 'vif' value is greater than 10 may
merit further investigation. A comparison between the
measured and predicted response variable is shown in Fig.
13.

8. Conclusions

The several key findings of this study may be summa-
rized as follows:

Analysis of Variance

Source DF
Sum of

Squares
Mean

Square F Value Pr > F

Model 2 1413.28 706.64 45.44 <.0001

Error 11 171.08 15.55

Corrected Total 13 1584.36

Root MSE 3.94 R-Square 0.89

Dependent Mean 71.21 Adj R-Sq 0.87

Coeff Var 5.54

Parameter Estimates

Variable DF
Parameter

Estimate
Standard

Error t Value Pr > |t| Tolerance
Variance
Inflation

Intercept 1 -6.86 8.43 -0.81 0.4329 . 0

VCA 1 1.38 0.15 9.04 <.0001 0.99 1.00

DAC 1 0.62 0.18 3.45 0.0055 0.99 1.00

Table 3. Linear Regression analysis to estimate locking point

100

50 60 70 80 90 100

Measured Locking Point

P
re

d
ic

te
d

L
o

c
k
in

g
P

o
in

R2=0.89

Line of Equality

Figure 13. Accuracy of the locking point estimation model

The Journal of Engineering Research Vol 7, No. 1 (2010) 11-21

References

Alabama Department of Transportation, 2002, "Special
Provision No. 02-0360(5)-2004 - Amendment for
Section 424 ," Alabama Standard Specifications.

American Association of State Highways and
Transportation Officials, 2004," Bulk Density (Unit

Weight) and Voids in Aggregate". AASHTO
Designation T 19.

Angelo, D.J., Harman, T.P. and Paugh, C.W., 2001,
"Evaluation of Volumetric Properties and Gyratory
Compaction Slope for the Quality Control of Hot Mix
Asphalt Production," Asphalt Paving Technology:
Association of Asphalt Paving Technologists-
Proceedings of the Technical Sessions, Vol. 70, pp.
729-761.

Asphalt Institute, 2001. "Superpave Mix Design,"
Superpave Series No. 2, Asphalt Institute, Lexington,
KY.

Cominsky R J., Leahy R B. and Harrigan E T., 1994,
"Level One Mix Design: Materials Selection,
Compaction, and Conditioning," Strategic Highway
Research Program, SHRP-A-408.

Georgia Department of Transportation, 2003, "Special
Provision-Section 828-Hot Mix Asphaltic Concrete
Mixtures,".

Guler, M., Bahia,H.U., Bosscher, P, J. and Plesha, M. E.,
2000, "Device for Measuring Shear Resistance of
Hot-Mix Asphalt in Gyratory Compactor,"
Transportation Research Record No. 1723,
Transportation Research Board, Washington, DC.

SAS Institute Inc, 2002-2003, "SAS Help and
Documentation". Cary, NC, USA.

The Online Magazine of the Asphalt Institute, 2007, "
www.asphaltmagazine.com".

Vavrik, W. R. and Carpenter, S.H., 1998, "Calculating Air
Voids at Specified Number of Gyrations in Superpave
Gyratory Compactor," Transportation Research
Record 1630, Transportation Research Board,
Washington, DC, pp. 117-125.

 Data from the SGC provide valuable information
on the compactability of asphalt mixtures.

 Both SGC and PDA results suggest that coarse
mixtures are more difficult to compact compared to
the medium and fine ones. This emphasizes the
importance of relating the level of applied
compaction energy to some specific attribute of the
asphalt mixtures and not basing it solely on the
expected traffic level as currently practiced in the
Superpave mixture design methodology.

 The compaction data also suggest that the current
recommended Superpave design number of
gyrations is too high and subject the mixtures to
unnecessary high compaction loads for extended
periods of time, which might have an adverse
effect on the final mixture volumetrics.

 There was a strong correlation between the data
from the SGC and PDA. This suggests the data
from the SGC provides a good indication of
mixture compactability.

 A statistical estimation model was developed based
on parameters that have a significant effect on the
compactability of asphalt mixtures. This model
can help mix designers in establishing the
optimum compaction levels for their mixtures.