NATIONAL BUREAU OF STANDARDS REPORT

4233

A STUDY OF RUBBER-ASPHALT PAVING MATERIALS

fry

Edward R. Oglio

Floor ? Roof and Wall Coverings Section

Building Technology Division

to

Division of Materials. Engineer Department

District of Columbia

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

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NATIONAL BUREAU OF STANDARDS REPORT

MS PROJECT NBS REPORT

1004-30-4840 June 30, 19 55 h-233

A STUDY OF RUBBER-ASPHALT PAVING MATERIALS

by

Edward R- a Oglio

Floor 5 Roof and Wall Coverings Section

Building Technology Division

for

Division of Materials, Engineer Department

District of Columbia

The publication, rep

unless permission is

25, D, C, Such pern

cal ly prepared If th

Approved for public release by the n part| IS prohlbl,ed

Director of the National Institute of

" dards ' Washins,on

ort has been specln- Standards and Technology (NIST)

)ort for lts own use on October 9, 20 1 5.

TABLE OF CONTENTS

Page

1. INTRODUCTION 1

2 , Materials 2

2,1 Modified Asphaltic Concrete Surface

Paving Mixes 2

2 P 2 Asphalt Cement 3

2,3 Latex Rubber Emulsion 3

3o TEST METHODS 3

3.1 Flexural-Fatigue Endurance Tests 3

3.2 Tests on Bitumens 4

4 a RESULTS 6

Table 1,, Results of Flexural-Fatigue Endurance

Tests 7

Table 2* Results of Analysis of Bar Specimens 9

Table 3* Tests on Asphalt Cement and Asphalt

Cement-Rubber Blends 10

5, DISCUSSION OF RESULTS 11

5.1 Flexural-Fatigue Endurance Tests 11

5.1->1 Sampling 11

5*1*2 Preparation of Test Specimens 12

5*1.3 Type of Mix 12

5 ->1.4 Bulk Density 12

5*1*5 Oxidation 13

5.2 Tests on Bitumens 13

6 f CONCLUSION'S 15

- 1 -

REPORT ON A

STUDY OF RUBBER ASPHALT PAVING MATERIALS

1. INTRODUCTION

Considerable interest has developed over the last two decades in the use of rubber additive in asphalt paving materials

in order to improve durability as well as short-term performance

of the so-called flexible pavement (Public Roads, volo 28 , No c

4, October 195^ • ) Many types of rubber in various forms and

concentrations and using various methods of blending have been

used, both in laboratory and in experimental field section studies (ibid). The Division of Materials, Engineer Department of

the District of Columbia Government as part of its continuing

program for improving District of Columbia roads and streets

has sponsored and conducted studies in both of these phases.

This report gives the results of the laboratory phase of one

such study requested under District of Columbia Laboratory No,

3135- 55 , This study was confined to a single type of rubber latex,

in emulsion form, as used in one concentration with an asphalt

cement, and combined with a closely graded aggregate. In scope,

this report is confined to a laboratory study of the properties

of the asphalt cement and the effect of the addition of the

latex, in the recommended manner and amount, on these properties. In addition, a laboratory evaluation was made to determine the effect of the latex on the properties of a plant-made

paving mix using these constituents. A Flexural-Fatigue Endur-

- 2-

ance Test which, it is our understanding, was developed under

sponsorship of the Company furnishing the rubber latex, was

used in evaluating the plant-made mixes 0

2 0 MATERIALS

The materials included a latex rubber emulsion, an asphalt

cement and a number of modified asphaltic concrete surface

paving mixes,. All materials are identified below0 2,1 Modified Asphaltic Concrete Surface Paving Mixes:

Mix No,L Marked

1

1

R-Rubber

Controlled

2

2

R-Rubber

Controlled

3

3

R-Rubber

Controlled

4

4

R-Rubber

Controlled

5

5

R-Rubber

Controlled

6

6

R-Rubber

Controlled

7

7

R-Rubber

Controlled

8 R-Rubber

9 Controlled

10 Controlled

11

11

R-Rubber

Controlled

12 R-Rubber

No # R-Rubber

-3-

2.2 Asphalt Cement -- The asphalt cement was contained in

three, 16 oz, ointment boxes and was represented as being a product of the American Oil Company of 85/100 penetration

grade *

2.3 Latex Rubber Emulsion -- The rubber was contained in

two quart bottles and was represented as containing 50/£ rubber

and $0% water. The type of rubber was not given*

3 o TEST METHODS

3*1 Flexural-Fatigue Endurance Tests

Three mixes of each type (Control and Rubberized) were used

for the tests* Selection was random but the mixes chosen were

paired as to type. For instance, control mix no, 1 and Rubberized mix no. 1 were paired and specimens were prepared on the

same day. Flexural-Fatigue Endurance was also determined on

the same day for each pair.

Three specimens from each mix were made and tested for

Flexural-Fatigue Endurance in accordance with the procedure

described in the method submitted with the samples, except that

the preparation required reheating* Reheating was done in

accordance with A 0 S,T a M. Designation D1071+-52 T, Sect. 5 (a).

In all, 18 specimens were used.

For purposes of comparison, four specimens of a sheet

asphalt surface mix (D.C. No. 2990-55) were also prepared and

tested in the manner described.

In order to study the effect of compacting load, additional

tests were made on each type for a fourth mix (No. 11). For

these tests, compacting loads of 4,000 and 8,000 pounds were

used (Note: in the regular series the compacting load was

6,000 pounds.)

Bulk density determinations were made at 25° C after the

Flexural-Fatigue Endurance Tests, using the following equations

wto in grams

Bulk Density =

Volume was determined by calculation from the dimensions

of the specimen as measured by micrometer. The weight-in air,

weight-in water method for determining volume proved impractical because of the porosity of the specimens.

Oxygen determinations were made in order to measure the

degree of oxidation of the bitumen. These were made on a microscale on the mineral-free benzene-extracted bitumens from the

mixes, and on the original asphalt cement, by the modified Unterzaucher procedure (Anal. Chem c vol. 23, p, 1408, 1951*)

Asphalt content and sieve analysis of the separated aggregate were determined on two of the control- type specimens after

Flexural Fatigue Endurance Test, using carbon tetrachloride and

standard procedures.

3,2 Tests on Bitumens

A series of conventional tests was made on the unblended

asphalt cement, heat-treated unblended asphalt cement, and two

blends of the asphalt cement and rubber latex in the proportion

of 95 parts of asphalt cement to 10 parts of rubber emulsions
<
The asphalt cement (39’+ g) was placed in a quart can and,

under constant stirring, carefully brought to a temperature

of 190°F in an oil bath* Thirty one grams of the latex emulsion were added, the temperature of the oil bath was gradually raised until all the water was driven off and the temperature of the blend reached 300°Fo Vigorous stirring was

applied mechanically throughout the blending process.

Blend A was made in accordance with the procedure just

outlined and required 3 hours for completion. Blend B was

prepared in the same way except that, in an effort to reduce

the time for completion, a pint can was used* Some of the

mixture foamed over while adding the latex in blend B and an

estimated 10% of the emulsion was lost thereby.

For purposes of comparison, a heat treatment was given to

a 390 gram sample of the asphalt cement in a manner identical

to blend A except, of course, that no latex was added.

In addition to the conventional tests, a number of other

tests were performed on the four bitumens mentioned. These

included ordinary penetration tests (100 g load, 5 sec.) at

different temperatures, and successive penetrations at 77°F

using 100 g load.

The log of ordinary penetration (100 g, 5 sec,) was plotted

against temperature in order to obtain a measure of the temperature susceptibility of the bitumens (Public Roads, vol, 27,

No. 1, March 19^0,) Such plots gave practically straight lines

through the points at 50°F, 77°F, and 95°Fa Susceptibility

- 6 - indices were calculated from the following i

c log pen0 at 95°F - log pen ? at 77°

18

Successive penetrations, at 77°P using 100 grams load,

were made increments of 7 see* penetration time with 5 sec*

allowed between increments to allow for readings to be made

and to allow partial recovery of non-permanent deformation to

occur 0 Three 7 sec. increments of penetration time were used in

this series, for a total of 15 sec. penetration time. Finally,

penetrations were obtained, also at 77°F and 100 g load, using

a 15 second un-interrupted penetration time. For convenience

this latter is called s Individual Penetration. These special

penetration tests (Successive and Individual) were made to

determine whether the bitumens showed elastic properties,

RESULTS

The results are given in tables 1 to 3 inclusive.

foie

1.

Results

of

Flexural-Fatigue

Endurance

Tests

Table

1.

Results

of

Flexural-Fatigue

Endurance

Tests

X

•H

f- H

(Da

EH

Rubberised

C/J a w <d

CD *H

i—M 1 1 o o

•H O• Eh

i—I i

ii i OxD 1 1 call

i—1 rH 1 1

1 CM ,4 •

1 HD IN 1

I • <* 1

1 H H 1

ON | CO 1 UN I UN 1 O 1 *| H 1 H 1

i i

i i

co o co UN NO UN

H H H

See

next

page

for

footnotes

Bulk

Density

g./ml

t

.

i—I rO 1 1 CU ON 1 1 * « 1 1 H H 1 11 OvO 1

1 On 0« I

1 c e |

1 H H 1 4 10 1 ON 1 ON | » 1 e | H 1 H 1

i i

i i

i i

i i — - -- ----- -

Un cO nO

On on on

0 9 0 rH i

|

i—

-

Flexural

I

y

!

Units UN nut HrOj- r>")vO ONOO Un_4 H rH H

\0 UN rOCO O OOD ON

rOO Un CM H rH i—1 i—

H O-CO CM ON O-vO H

POUNCM,4

CM rH rH

CM O

CO CM CO UN

1 1

,

-

O- CM UN

nO o O

00 O HCM H

Control

1/

Thickness .001

in.

-

H|

i—1 L i 1 O-lCN 1 1 « c | | H rH 1 11 vO ON 1

1 UNUN 1 loci

1 H rH 1

1 no O- 1

1 UNUN 1

1 * * 1

1 1 —1 rH 1

1 1

i :

i i

i i

CM CM on CM 1 1 NO NO NO NO 1 1 <* o . « j |

i—1 rH rH rH 1 1

CM ON | Un Un l f C 1 H H 1

Bulk

Density g

.

/ml

. O O- 1 1 CX) ON i 1 * e J | H H 1 1 144 i

1 On ON 1

1 o C 1

1 rH i—1 1

1 OOnO t

1 On On i

1 » • 1

1 H H 1

1 1

E I

» »

1 1

i—1 CM nQ ON 1 | ON ONCO co 1 1 • * » ® 1 1

i—1 H H i—1 1 1

CO ON I O On i t (5 | CM rH 1

Flexural y

Units

HI

o-u-,4 CO ONH^ UN

.4- H ONCO

CM rH H i—

V\i—| ONCO

NO CO ON ON

CO 0-00 J- H rH rO CM

CM CM O- OOnnO CM CM ON O- On CM H HCO o

UN 0-1 oo co H

UNooUnUnO lrx O-nQ rOCO ON CM ONON O ON ON

Hi—1 CM H i—

O- ON | nO On 1 rH ON | CM CM 1

Specimen

r

cl b c

Average cl b cAverage

1

cu b c

[

Average Average

Standard

Deviation a b c d

Average

Standard

Deviation 4MnO| UnJ

fl) ^ U

x * •H O l—

UN O-

_ J

CO§

H|

CO £ J

H

CD

CD co
/
- 8-

1/ Specimen not fully compressed due to hold-up of cover

plate and compacting bar on one edge of mold*

2/ S.A.S, Sheet Asphalt Surface Mix, D, C, Lab. No. 2990-

55.

y At plane of fracture.

b/ Flexural Units required to attain complete severance

of specimen.

y 8^000 lb, compaction load used.

6/ b 9 000 lb. compaction load used.

2/ Oxygen content of bitumens?

Original untreated asphalt cement 0.59 %

No. 7 Control mix before reheating 0,93 %

No, 7 Rubber mix before reheating 0.86 %

No, 7 Control mix specimen c after

Flexural Fatigue Test 1,09 %

No, 7 Rubber mix specimen a after

Flexural Fatigue Test 0,9& °/°

- 9

-

Table 2 0 Results of Analysis of Bar Specimens

1/ Bitumen extracted before sieve analysis ? i.e, bitumenfree aggregate

- 10-

Table 3. Tests on Asphalt Cement and Asphalt Cement-Rubber Blends

Material

Unblended

A a 0 •

_ _

Unblended

A, Co

Blend A ’Blend B

1 Heat Treatments

Temp, , °F

Time, Hrs.

300

o

200-300

3

190-300

3

!

’ 205-300

0

1 1 -

1 Penetration, 100 g 5 sec.s

at 3 2° F

,

at 50° F,

at 77° F,

at 95°F ?

5

17

80

207

lb62

155

5o5

12

49

99

r

!|6o5

<17

i 61

; 125

j Susceptibility Index 0,0229 0o0221 0,0170 i 0,0173

! Successive Pen. lOOg, 77°F

5 sec u 10 sec.

15 sec.

80

109

130

83

99

b-9

63

7bs

! 61

1 79

1 91

Individual Penetration,

77° F , 100g. 15 sec. 125 93 68 85

Softening Point, °F 118 12b- 155 ! 154

Ductility at 77°F, cm 150+ 150+ 61.2 !5,+.o

Vole Loss at 325°F for

5 hr s o

, % 0.03 0.02 LOoOl 1 0,02

Tests on Residues

Pen. at 77° F, 100g„

5 sec

,

Softening Point, °F

Ductility at 77

d F, cm

67

120

150+

57

126

150+

47

165

70.8

j

1156

1163

'82,5
>
- 11 -

DISCUSSION OF RESULTS

5.1 Flexural-Fatigue Endurance Tests

On the average, about 475 more Flexural Units were required

to cause complete fracture in the control mixes than in the

rubberized mixes. However, the Standard Deviations (820 for the

control mixes and 520 for the rubberized mixes) show a wide

scattering in the results among individual specimens for each

type. They also show considerable over-lapping between types

and, when compared to the difference in averages, greater

differences among individual specimens of each type than between the averages for the two types* It appears that, as presently constituted, the Flexural -Fatigue Endurance Test is of

little value in differentiating between the two types of mixes

tested.

Several factors that might possibly have contributed to the

results are considered below,

l a Sampling* If the samples submitted were not homogeneous,

variable results could bo expected. However, previous long experience indicates that heterogeneity rarely occurs in this

type of mix* Indeed, comparison of the analyses of the two bar

specimens reported in Table 2 confirms this and indicates excellent homogeneity and plant control. When one compares the closeness of the analytical results on the two specimens with the

very wide difference in their Flexural-Fatigue Endurance, as

determined, one is forced to reject simple heterogeneity as a

causative factor and seek elsewhere for significant factors.

- 12-

2. Preparation of Test Specimens, In the only procedure

available under the circumstances, the mix had to be reheated

in order to mold the specimens. But in considering reheating

as a factor it should be recalled that ASTM Method D107*t permits it. Also, the tests on D, C „ No 0 2990-55 (Sheet Asphalt

Surface) gave excellent reproduc ibility, despite reheating 0 In preparing specimens, the recommended compaction load

(6,000 lbs.) was used. This may be a legimate factor for consideration since the procedure originally was developed for a different mix. The results reported under Mix No, 11, where

different compacting loads were used, indicate, however, that

mere variation of the load would be ineffectual in improving

reproduc ibility,

3o Type of mix. The type of mix used in the tests contained an appreciable proportion of somewhat coarse and platelike aggregate particles. It was observed that during the test

fracture appeared to develop around a coarse, elongated particle

and progressively continue, in serpentine fashion, around adjacent particles to complete failure. When, one considers the

close reproduc ibility obtained with the Sheet Asphalt Surface

Mix (D.C. No. 2990-55) which contains aggregate that is essentially spherical, this factor is a valid one since orientation

of aggregate particles in the latter is not a factor.

4, Bulk Density. Bulk density measurements showed no significant correlation with Flexural-Fatigue Endurance. It is

true that if the data are broken dov/n and comparisons made

13 -

be tween specimens of the same sample (e 0 g„ between specimens

b & c of Control mix 5 or between specimens b & c of Rubberized

mix 5 etc.) a trend in favor of the lower bulk density (thicker,

less dense) specimens would be indicated. In other words it

might be claimed that Flexural Fatigue Endurance was inversely

proportional to bulk density. However, the wide scattering

between individual specimens of the same samples and the results

obtained with the Sheet Asphalt Surface Mix (D.C. No, 2990-55)?

where the thinner but denser specimens showed greater endurance,

compel one to conclude that such a claim is unwarranted,

5 a Oxidation, The oxygen determinations showed that oxidation had occurred. They also indicate that approximately

3/*+ of the reaction took place during plant mixing, before the

mixes were reheated, and that both types were at similar stages

of oxidation when the Flexural-Fatigue Endurance Tests were

made. It appears, therefore, that oxidation was not a significant factor in the tests 0 5o2 Tests on Bitumens

In general, the results show that the addition of the rubber

latex, in the manner described, affected the characteristics of

the asphalt cement. They also show that, as expected, heat

treatment without addition of latex also affected these characteristics, although to a lesser extent c It may be said therefore, that the changes occurring in making the blends w^re brought

about by the addition of the latex and to a lesser extent by

the heat treatment required to effect the blend.

Both the additions of rubber and the simple heat treatment

increased the consistency (as measured by penetration) at 50° F

,

77°F ? and 95°F» and the softening point while decreasing the

ductility and the susceptibility to temperature of the original

asphalt cement* In general, the changes in consistency (penetration) Susceptibility Index and in softening point are considered desirable while the change in ductility is considered

undesirable* In regard to the change in susceptibility index,

it should be noted that all are within the range of values for

paving asphalts as reported in the literature (Public Roads,

vol. 21, No. 1 and vol. 19, No. 11),

and individual penetration

Comparison between successive penetratior/ f or 15 seconds

indicates that all four bitumens have elastic properties (D.

Mack Journal of the Society for Chemical Industry, Transactions,

vol. 58, 1939, P« 306

)

0 For a purely viscous bitumen both penetrations would be the same. A quantitative approximation on

this basis would indicate elastic qualities to be most pronounced in Blend A, followed in descending order by Blend B,

the heated Treated Asphalt Cement, with the original, untreated asphalt cement possessing the least.

As stated previously, the results of the tests on the bitumens show definite differences in physical characteristics

among them, however, final judgment should not be made until

correlation with other tests, especially the experimental field

section tests, are made. The results of the latter will probably not be known for some time. Even when these results are

available, equating the characteristics of the blends as accomplished in the laboratory with those of the rubber-asphalt blend

as it exists in the plant-made mix is not entirely valid e This

is due to the fact that mixing and blending are accomplished

over a period of hours in the laboratory, and in a matter of

minutes (approx,, 2 minutes) in the plant 0 The 2-3 hour blending process insures a better dispersion of the rubber in the

asphalt c erne nt c ^...CONCLUSIONS

1, The addition of rubber latex to the asphalt cement, in

the amount and manner described, increased its consistency (as

measured by penetration) at 50°

} 77°F, and 95°F, its softening

point, and its elastic qualities, while at the same time decreasing its ductility, susceptibility, and consistency ( as

measured by penetration) at 32°F.

2 0 The three-hour heat treatment of the asphalt cement

effected similar changes but to a lesser degree, except for

ductility, and consistency at 32°F, No effect was detected

in the latter two characteristics.

3 o The Flexural_JFatigue Endurance Test, as constituted,

did not give a reliable differentiation between the two types

of Modified Asphaltic Concrete Surface Plant Mixes (control vs

rubberized) tested.

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