The Role of Asphalt Binder Fracture Properties in Thermal Cracking Performance of Mixtures and...

The final version of this paper was presented and published in:

Proceeding of the 57th Annual Conference of the Canadian Technical Asphalt Association (CTAA)

Vancouver, British Columbia, November 2012

https://www.ctaa.ca/wp-content/uploads/2012/02/CTAA-2012-Proceedings-Order-Brochure-rev-Oct-15.pdf

The Role of Asphalt Binder Fracture Properties in Thermal Cracking

Performance of Mixtures and Pavements

Hussain Bahia, Ph.D.

Professor

University of Wisconsin-Madison

Madison, Wisconsin

Raul Velasquez, Ph.D.

Research Associate


Madison, Wisconsin

Hassan Tabatabaee, Ph.D.

Research Associate


Madison, Wisconsin

Sebastian Puchalski

Research Assistant


Madison, Wisconsin

Acknowledgements

The authors would like to thank Professor Mihai Marasteanu and Mr. Zhijun Li for their contributions.

This research was sponsored by the Asphalt Research Consortium (ARC), which is managed by the

Federal Highway Administration (FHWA) and the Western Research Institute (WRI) and by the National

Pooled Fund Study TPF-5(132): “Investigation of Low Temperature Cracking in Asphalt Pavements

Phase-II”. This support is gratefully acknowledged. The results and opinions presented are those of the

authors and do not necessarily reflect those of the sponsoring agencies.





2 THE ROLE OF BINDER FRACTURE PROPERTIES IN THERMAL CRACKING

ABSTRACT

Significant progress in controlling low temperature cracking has been achieved by introducing the

Bending Beam Rheometer (BBR) and placing appropriate limits on stiffness and relaxation rate (i.e., S

and m-value). However, with the increase in types of modifiers and extender oils used to achieve special

low temperature grades (e.g., PG xx-28 and PG xx-34), concerns about the effects of these additives on the

fracture properties of binders have been raised. The need for a testing system that is simple and effective

in measuring these properties is clearly realized. A Single-Edge Notched Bending Test based on a

modification of the standard BBR device (BBR-SENB), was developed and used to measure fracture

properties of asphalt binders.

Comparison of BBR-SENB results with fracture properties of asphalt mixtures and field performance

indicates the need to compliment current practice of using stiffness and relaxation properties measured in

the BBR with binder fracture properties to evaluate thermal cracking of pavements. It is observed that

binders of the same low temperature PG grade can have significantly different fracture energy (Gf) and

deflection at fracture (uf), measured at the grade temperature. For example, binders graded as PG (xx-22)

showed Gf values ranging between 1 and 30 J/m2 measured at -12°C.

RÉSUMÉ

Abstracts provided in English will be translated to French.





BAHIA, VELASQUEZ, TABATABAEE & PUCHALSKI 3

1.0 INTRODUCTION

It is recognized that a better approach for thermal cracking characterization of asphalt materials is to use

fracture mechanics principles rather than to use a linear viscoelastic continuum mechanics approach.

Current test methods to address low temperature cracking, such as the Bending Beam Rheometer (BBR)

[1], characterize the material in the linear viscoelastic domain at small strain levels and, therefore, do not

provide the complete picture for thermal cracking characterization.

Previous research by Hoare and Hesp [2], Hesp [3], Chailleux and Mouillet [4], Chailleux et al. [5],

among others, have used the Single-Edge Notched Bending (SENB) test, which is a fracture mechanics-

based test commonly used in metals and other materials, to obtain the fracture properties of asphalt binders

at low temperatures. These researchers succeeded in grading a broad range of materials with different

levels of polymer modification. The BBR-SENB test follows the ASTM E399 standard [6] and assumes

that Linear Elastic Fracture Mechanics (LEFM) conditions hold for the analysis of test measurements.

Figure 1 shows a schematic of the BBR-SENB test and the parameters used for the calculation of fracture

toughness (KIC) and fracture energy (Gf).

Figure 1. Bending Beam Rheometer Single-Edge Notched Bending (BBR-SENB) Schematic and

Derived Parameters

In the BBR-SENB test, fracture toughness is calculated following ASTM E399 [6] using the maximum

load (P), the notch length (a), and the dimensions of the beam. The fracture energy, Gf is calculated as the

total area under the entire load-deflection (P-u) curve, divided by the area of the ligament. Another

important fracture index obtained in the BBR-SENB is the deflection at maximum load. Historically,

maximum deflection at fracture has been found to correlate very well with field performance and it has

been used as a parameter that indicates the ductile to brittle transition of asphalt binders.

The details of the test procedure and method of calculation is presented in an earlier paper by the authors

[7]. In this paper, to verify the need of fracture properties of binders to estimate performance, materials

collected from MnROAD, an accelerated pavement research facility in Minnesota, and from the Long






Term Pavement Performance (LTPP) sections in the United States, were characterized using the

aforementioned fracture parameters. Binder fracture properties were compared with current BBR

parameters (e.g., stiffness at 60 sec and m-value), with fracture properties of asphalt mixtures, and with

thermal cracking field performance. Experimental and field performance data was used to establish the

importance of measuring asphalt binder fracture properties and the conditions at which the BBR-SENB

are most relevant.

The need for measuring fracture properties of binders is not new and the most recent attempts to measure

such properties are seen in the development and implementation of the Direct Tension Test (DTT)

AASHTO T 314 [8] in the late 1990s and the early 2000s. Also, recently the Ontario Ministry of

Transportation considered implementing the Double Edge Notch Test (DENT) [3]. In contrast to the DTT

and the DENT, the BBR-SENB is a flexural test that utilizes a fixture-free prismatic beam specimen

consistent with the design of the sample used in the BBR standard procedure. The selection of this

geometry and loading pattern was motivated by the following challenges:

The persistent problems with repeatability and consistency faced using the simple elongation test

in the DTT;

The need for a test sample with no constraints to allow stress-free shrinkage of binders that will

prevent volume restrictions and stress build up at end pieces; and

The success of the BBR test procedure in controlling temperature, loading, and displacement

measurements. The BBR-SENB requires a relatively simple modification of the loading

mechanism of the BBR which allows for reliable measurements of both fracture properties and

standard BBR parameters (i.e., S and m).

2.0 THE BBR - SINGLE EDGE NOTCHED BENDING TEST

The BBR-SENB system which is very similar to current BBR is shown in Figure 2. The additions to the

standard BBR include a loading motor that controls the displacement rate during testing and a load cell

with a higher capacity than the regular BBR. The SENB test used in this study is run at a constant

displacement rate of 0.01 mm/sec. The effect of loading rate can be significant as shown in Figure 3 for a

typical binder. Important viscoelastic effects (e.g., stress relaxation) can be observed for very low loading

rates. The framework of Linear Elastic Fracture Mechanics (LEFM) is not strictly valid if significant non-

linear time effects are present (e.g., creep). Therefore, proper selection of loading rate and temperature is

of paramount importance.

The sample used for the BBR-SENB test is achieved by adding a notch to the beams made for the BBR

standard procedure. There is a significant advantage to this geometry as it successfully resolves the

adhesion problem encountered by using other sample preparation procedures [7]. It is important to note

that sample preparation procedure is less time consuming and relatively simple when compared to other

test methods such as the DTT. The critical dimensions of the sample can be easily controlled, and most

asphalt binder technicians in North America are well trained for preparation of BBR samples.

The notched BBR beams are prepared by making a notch of 3 mm (i.e., corresponding to 20 to 25 percent

of beam depth) in the wide side (i.e., 12.7 mm) of the BBR mold side-beams. The modified BBR mold can

still be used for regular BBR beam fabrication as the notch is very thin and can be covered with the plastic






sheets commonly used in molding (Figure 4a). This is considered another advantage as the standard BBR

molds can be used with a slight modification.

Figure 2. Bending Beam Rheometer-Single-Edge Notched Bending (BBR-SENB) System

Figure 3. Effect of Loading Rate in the BBR-SENB

The repeatability of the BBR-SENB test can be significantly improved if the position and length of the

notch is carefully controlled [9]. Alignment pins were added to the aluminum molds as indicated in Figure

4a. This new preparation setup prevent the movements of the mold end pieces relative to the notch

position, which could potentially result in off center or angled notches on the beam. Figure 4b shows an

0

1000

2000

3000

4000

0 0.2 0.4 0.6 0.8 1 1.2

Forc

e (

mN

)

Displacement (mm)

Rate 0.04 mm/sec Rate 0.01 mm/sec Rate 0.0025 mm/sec






example of SENB replicates when using the alignment pins during sample preparation. Test results show

the effectiveness of the mold alignment pins in limiting variability of the BBR-SENB [9].

(a)

(b)

Figure 4. (a) BBR-SENB Specimen Mold with Alignment Pins, (b) Replicates in BBR-SENB Testing

3.0 MATERIALS

The asphalt binders collected from MnROAD (Minnesota, USA) test sections are presented in Table 1. All

binders were subjected to short-term aging using the Rolling Thin Film Oven (RTFO). Materials from MN

County Road 112 were used in a monitored field validation site in Minnesota, USA.

0

2000

4000

6000

8000

10000

12000

14000

16000

0 0.05 0.1 0.15 0.2 0.25 0.3

Lo

ad

(m

N)

Deformation (mm)






In addition to the binders listed in Table 1, a large set of modified and unmodified binders from the

Asphalt Research Consortium (ARC) project, as well as binders from selected LTPP sections were

included in this study.

Table 1. MnROAD and County Road 112 Binders Tested in the BBR-SENB

Binder Location Description

PG 58-34 PPA MnROAD 33 Modified with Polyphosphoric Acid (PPA)

PG 58-34 SBS+PPA MnROAD 34 Modified with Styrene-Butadiene Styrene (SBS) + PPA

PG 58-34 SBS MnROAD 35 Modified with SBS

PG 58-34 Elvaloy

+Acid MnROAD 77 Modified with PPA + Elvaloy

PG 58-28 MnROAD 20 Neat

PG 58-34 MnROAD 22 Unknown Modification

PG 64-22 Wisconsin Binder used in construction of SMA pavement in

Wisconsin

PG 58-28 MN CR 112 Source “V”

PG 58-28 MN CR 112 Source “C”

PG 58-28 MN CR 112 Source “M”

PG 58-34 Elvaloy MN CR 112 Source “E” modified with Elvaloy

4.0 RESULTS AND DISCUSSION

4.1 Comparison of BBR-SENB to the BBR

To verify that the current results of the BBR test (i.e., S and m) are not sufficient to indicate fracture

properties of modified binders, the BBR-SENB and BBR measurements were compared for an extensive

set of binders, which included materials in Table 1 and binders used in the Asphalt Research Consortium

(ARC). In these tests, the S(60) and m-value of the asphalt binders were measured after 1 hr of

conditioning at the same temperatures used for the BBR-SENB testing.

The BBR-SENB parameters (i.e., Gf, KIC, and deflection at fracture) are plotted against the m-value and

S(60) in Figures 5 and 6, respectively. It should be noted that all correlations made in subsequent sections

are meant to compare the ranking capability of different low temperature performance indices and are not

for the purpose of deriving direct relationships between the indices. Although no specification for

determining pavement performance based on fracture parameters exists, intuitively one would expect

higher KIC, Gf and deflection at fracture to indicate better performance.

The trends in Figures 5 and 6 show that the m-value (m(60)) and creep stiffness (S(60)) have very poor

correlation with the fracture parameters obtained in the BBR-SENB test. It can also be seen that the BBR

m-value and creep stiffness limits fail to distinguish between binders demonstrating low values of fracture

energy and those with superior (higher) fracture energy values, especially for the range passing the






Superpave criteria (S < 300 MPa, and m > 0.300). These trends clearly confirm earlier findings by other

researchers, indicating the potential need for measuring fracture energy as a performance index [10].

Figure 5. BBR-SENB Gf and KIC plotted against BBR m-value at different temperatures (Hatched

line show Superpave BBR criteria limit; green arrow shows side passing this criterion).

(a) (b)

R² = 0.25

1

10

100

0.2 0.3 0.4 0.5 0.6

Gf

(J/m

2 )

m(60) (-)

-12 C -18 C -24 C

R² = 0.17

0

10

20

30

40

50

60

70

80

0.2 0.3 0.4 0.5 0.6K

IC (

KP

a.m

0.5)

m(60) (-)

-12 C -18 C -24 C

R² = 0.37

1

10

100

0 100 200 300 400 500 600 700

Gf

(J/m

2 )

S(60) (MPa)

-12 C -18 C -24 C

R² = 0.10

0

10

20

30

40

50

60

70

80

0 100 200 300 400 500 600 700

KIC

(K

Pa.

m0

.5)

S(60) (MPa)

-12 C -18 C -24 C

R² = 0.63

0.0

0.1

1.0

10.0

0 100 200 300 400 500 600 700Def

lect

ion

at

max

imu

m

load

(m

m)

S(60) (MPa)

-12 C -18 C -24 C






(c)

Figure 6. BBR-SENB (a) Gf, (b) KIC, and (c) Deflection at maximum load plotted against BBR creep

stiffness at different temperatures (Hatched line show Superpave BBR criteria limit; green arrow

shows side passing this criterion).

4.2 Selection of Performance Parameters for the BBR-SENB

Three parameters/indices are currently obtained in the BBR-SENB: (a) Fracture energy, (b) Fracture

toughness, and (c) Fracture deflection. The ability of these parameters to capture the brittle-ductile

transition behavior of binders (assumed to be due to material undergoing glass transition) is presented in

Figure 7. BBR-SENB results in terms of fracture energy (Gf), fracture toughness (KIC), and deflection at

fracture were plotted versus the relative distance of the testing temperature to the respective binder’s glass

transition temperature (Figure 7). The Tg measurements were obtained using a dilatometric system

described in [7].

(a) (b)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

-15.0 -10.0 -5.0 0.0 5.0 10.0

Frac

ture

De

fle

ctio

n (

mm

)

Tg-T (°C)

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

-15.0 -10.0 -5.0 0.0 5.0 10.0

Gf (

J/m

2)

Tg-T (°C)

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

-15.0 -10.0 -5.0 0.0 5.0 10.0

KI C

(K

Pa.

m0

.5)

Tg-T (°C)






(c)

Figure 7. Brittle-ductile transition behaviour using BBR-SENB parameters (a) Fracture deflection,

(b) Fracture energy, (c) Fracture toughness.

The results shown in Figure 7(a) indicate that binders tested at temperatures below their glass transition

temperature (Tg) show fracture deformation/deflection consistently at or below 0.35 mm, which could be

considered as the limit for the brittle to ductile transition. The ductile to brittle cut-off value is harder to

discern when using fracture energy (Gf), which is influenced by both fracture load as well as the fracture

deflection. Although 90 percent of the binders tested in the brittle temperatures fractured at energies at or

below 10.0 J/m2, a few binders in the ductile zone also fractured at energies below this value. Figure 7(c)

clearly shows that fracture toughness is not a good indicator of the ductile to brittle transition of asphalt

binders as the magnitude of fracture toughness for several binders are in the same range when tested below

and above their glass transition. Therefore, this index is not recommended for specifications or material

selection purposes.

4.3 Effect of Physical Hardening on BBR-SENB Parameters

In the last few years the importance of considering physical hardening has been highlighted in a few

studies [11-14]. Therefore in this study the effect of isothermal conditioning on fracture properties of

binders was also investigated. Figure 8 shows the typical trend resulting from conditioning for 72 hours.

Figure 9 depicts results of testing selected binders from Table 1 after 72 hours of conditioning at their Tg.

The results are shown in terms of normalized values (ratio of value after 72 hours of conditioning divided

by the value after 0.5 hours). Figure 9 indicates significant changes in the overall fracture response of the

selected binders after isothermal conditioning. It can be seen that the slope of the load-deflection (P-u)

curve increases significantly after conditioning. The fracture toughness also increased for all binders tested

after conditioning, however, a varying trend was observed for Gf. Fracture energy increased or was

constant for the two unmodified binders (i.e., MnROAD Cell 20 and New York, NY), while decreasing for

the three modified binders. This reduction is explained by the relative loss of strain tolerance with

conditioning time. In other words, the increase in load at fracture is offset by reduction in deflection at

break for the modified binders. The observed trend is clearly shown for the deflection parameter (uf) in

Figure 9d, in which the normalized value of failure deflection are all below one when normalized relative

to their respective values measured after 0.5 hr of isothermal conditioning.






Figure 8. Typical change in load (P) - deformation (u) curve before and after isothermal

conditioning at the glass transition temperature (Tg).

(a) (b)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

C20 C22 C34 C77 NY

No

rmal

ize

d G

f

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

C20 C22 C34 C77 NY

No

rmal

ize

d K

IC

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

C20 C22 C34 C77 NY

No

rmal

ize

d P

-u S

lop

e

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

C20 C22 C34 C77 NY

No

rmal

ize

d u

f






(c) (d)

Figure 9. BBR-SENB parameters after 72 hours of isothermal conditioning normalized to results

after 0.5 hours of conditioning, (a) Gf, (b) KIC, (c) slope of P-u curve, and (d) deflection at fracture

(uf).

4.4 Comparison of BBR-SENB Results to Mixture Fracture Tests

To verify that the BBR-SENB results are relevant to the asphalt mixture behaviour, the relationship

between binder results from BBR-SENB testing and mixture fracture properties obtained using the Semi

Circular Bending (SCB) [10] are shown in Figure 10. Comparisons are made for binders and mixtures

from MnROAD test sections. Mixture testing of MnROAD materials was conducted by the University

Minnesota as part of a Pooled Fund Study on Low Temperature Cracking [10].

Figure 11 shows a good correlation between binder and mixture fracture properties (Gf). Although sample

size is small, the correlation shows that fracture energy index from BBR-SENB testing is a good potential

indicator of mixture performance.

Figure 10. Comparison of Semi Circular Bending (SCB) and BBR-SENB fracture energy (Gf).

[mixtures were prepared with same binders]






Figure 11. Comparison of BBR-SENB load-deflection curves for LTPP binders. [Labels: LTPP code

(thermal cracking severity)]

4.5 Comparison of BBR-SENB Results to Field Data

To evaluate the potential of the BBR-SENB test results in estimating field performance, information from

the Long Term Pavement Performance (LTPP) program was used. Selected LTPP binders were tested with

the BBR-SENB procedure and results were compared to field thermal cracking performance recorded in

the LTPP database.

BBR-SENB tests on binders from each section were carried out at two temperatures close to the critical

cracking temperature reported in the database. Results of comparison presented in Table 2 show a trend of

decreased number of thermal cracks with increasing binder Gf values. In this table, the LTPP thermal

cracking performance was grouped in terms of number of observed cracks (i.e., high, moderate, and low

cracking). Similar groupings were also performed in terms of BBR-SENB fracture energy, deflection at

failure, and standard BBR parameters. The grouping criteria used throughout this paper are solely for the

purpose of comparison of the data sets and are not intended as formal BBR-SENB specification limits.

Table 2 shows that, in most cases, the BBR-SENB (Gf and uf) and LTPP field performance grouping

agree. However, the ranking based on the low PG does not agree with either field performance or fracture

parameters. These results indicate that Gf and uf from BBR-SENB can be used as indices to rank asphalt

materials in terms of thermal cracking susceptibility.

Table 2. Comparison between LTPP thermal cracking performance, PG grading, and BBR-SENB

parameters obtained at temperatures close to field cracking temperature (ranking).

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Load

(N

)

Displacement (mm)

89a902 (Low)

90903 (High)

370901 (Very High)






Section

ID

No. of

Cracks

Performance

Grouping1 PG

LT PG

Grouping

Gf

(J/m2)

uf

(mm)

Testing

Temp (°C)

SENB Gf

Grouping2

SENB uf

Grouping3

350902 0 1 PG 64-22 2 34 0.89 -12 1 1

350903 0 1 PG 58-22 2 27.5 0.90 -12 1 1

340901 2 1 PG 64-22 2 24.5 0.60 -18 1 1

370964 0 1 PG 76-22 2 25 0.74 -12 1 1

370963 0 1 PG 64-22 2 27 0.64 -12 1 1

340902 0 1 PG 58-28 1 17.5 0.51 -18 1 1

370962 0 1 PG 76-22 2 14.5 0.44 -12 2 2

340961 11 2 PG 78-28 1 15.5 0.58 -18 2 1

370960 15 2 PG 76-22 2 8.5 0.25 -12 3 3

370901 29 3 PG 64-22 2 9.7 0.34 -12 3 3

Note: PG is Performance Grade, LT is Low Temperature and SENB is Single-Edge Notched Bending. 1LTPP performance grouping: Group 1: 0-2 cracks, Group 2: 3-14 cracks, Group 3: More than 15 cracks 2SENB Gf grouping (J/m2): Group 1: Gf above 15 J/m2, Group 2: Gf from 10 to 15 J/m2, Group 3: Gf less than 10 J/m2. 3SENB uf grouping (mm): Group 1: uf above 0.5 mm, Group 2: uf from 0.35 to 0.5 mm, Group 3: uf less than 0.35 mm.

Figure 11 further demonstrates the ability of the BBR-SENB load-deflection curve to clearly differentiate

between the low temperature performance of LTPP binders. The figure shows the BBR-SENB results for

four LTPP binders from sections that have performed significantly different in terms of thermal cracking.

It can be seen that a wide range of change in deflection at fracture and consequently in Gf exists between

these LTPP binders. To further validate the relevance of the fracture properties, the field performance

from four sections in Minnesota were also compared to BBR-SENB results obtained at -12°C. The results

shown in Table 3 indicate that the BBR-SENB Gf parameter was able to distinguish the worst performing

binder properly, among binders of the same PG, but could not detect the superior performance

demonstrated by the section using PG 58-23 Elvaloy modified binder. In order to better investigate this

discrepancy, the load-deflection curves of the binders showing very similar Gf, but very different field

performance were compared. It can be seen in Figure 12 that although the area under both curves is very

similar, the Elvaloy modified binder shows a softer response but deflects much more than the PG 58-28

from source “M”. This result seems to indicate that the strain tolerance of the binder, as measured by the

deflection at break, may in fact play a critical role for field performance, and that judging binder

performance by fracture energy alone may yield misleading results. The deflection at fracture and the

ranking for MN CR 112 binders are also shown in Table 3. As shown, ranking by deflection for 3 out of 4

binders matches ranking of the field cracking performance, while the 4th binder’s poor performance was

detected by Gf. Thus it seems that a combined criteria based on both uf and Gf as measured by the BBR-

SENB is capable of ranking binders in agreement with observed thermal cracking in the field.






Table 3. Comparison between MN CR 112 thermal cracking performance and BBR-SENB results

(ranking) tested at -12°C.

Binder Gf (J/m2)

Gf

Ranking2

Deflection at

Fracture (uf)

(mm)

uf

Ranking3

Transverse

Cracking (ft)

Cracking

Performance

Ranking1

PG 58-28Source “V” 15.62 2 1.04 2 197.7 3

PG 58-28Source “C” 27.88 1 0.98 2 105.2 2

PG 58-28Source “M” 22.48 1 0.82 2 110.6 2

PG 58-34 Source “E”- Elvaloy 25.12 1 1.87 1 11.3 1 1Field performance grouping: Group 1: 0-50 ft of cracking, Group 2: 50-150 ft of cracking, Group 3: More than 150 ft of cracking 2SENB Gf grouping (J/m2): Group 1: Gf above 15 J/m2, Group 2: Gf from 10 to 15 J/m2, Group 3: Gf less than 10 J/m2. 3SENB uf grouping (mm): Group 1: uf above 1.5 mm, Group 2: uf from 0.35 to 1.5 mm, Group 3: uf less than 0.35 mm.

Figure 12. Comparison of BBR-SENB load-deflection curves for two MN CR112 binders with

similar Gf but different field performance.

5.0 SUMMARY OF FINDINGS AND CONCLUSIONS

The Bending Beam Rheometer- Single-Edge Notch Bending (BBR-SENB) test using beam samples made

with a modified version of the BBR molds is a relatively simple test that can be carried out in a time frame

similar to the current BBR test. In contrast to the BBR, it is believed that the BBR-SENB test can capture

the fracture properties of binders at low temperatures. These unique properties make the BBR-SENB test a

potentially ideal performance characterization test, or an important compliment to the BBR standard test,

as it allows for the estimation of the relaxation modulus and the fracture resistance properties of binders.

The following conclusions can be drawn from the experimental results presented in this study:

Results collected show that binders of same low temperature grade can have significantly different

fracture energy (Gf) and deflection at fracture (uf) values measured at the grade temperature. For

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 0.5 1.0 1.5 2.0 2.5

Fo

rce

(N)

Deflection (mm)

58-34 Source E-Elvaloy 58-28 Source M






example, binders graded as PG (xx-28) showed a range of 5 to 80 J/m2

measured at -12°C. This

finding indicates that performance grading at low temperature is not sufficient to rank binder

properly with respect to their fracture resistance.

Analysis of field performance data from LTPP sections and a County Road (MN CR 112) in

Minnesota indicates the potential of using Gf and deflection at maximum load (uf) to rank binders

according to field cracking significantly better than current PG grade system.

Selection of a performance indicator from the BBR-SENB results is also complex. From a

fundamental point of view, the fracture energy (Gf) is a technically sound parameter. However, in

this study it appears that strain tolerance, as measured by the deflection at fracture, is also a very

important index that correlates better to field performance. It is also clear that deflection at

fracture is a good indicator of brittleness in the glassy stage (i.e., performance below glass

transition temperature). It is therefore recommended to use both parameters by specifying a

minimum allowable value of Gf and a minimum value of deflection at fracture (uf).

Although, the fracture indices obtained in the BBR-SENB test appear to be good indicators of the

fracture resistance of binders, the complexity of the viscoelastic behavior in terms of time

dependency should be recognized. It is expected that results of the BBR-SENB will vary

significantly at different temperatures and displacement rates.

Future studies on the use of the BBR-SENB and low temperature characterization are encouraged

to focus on determination of representative testing conditions. With special emphasis on the effect

of different loading rates and temperatures on the fracture response of viscoelastic materials. For

development of a reliable prediction model, it may be necessary to characterize the materials over

a range of loading rates and temperatures rather than using only one loading rate and few selected

temperatures as done in this study.

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http://www.arc.unr.edu/

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