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A brittle point test for low temperature studies of bitumens
Jones, P. M.
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TH1 N2lr2 no. 205 c. 2BLDG
NATIONAL
RESEARCH
COUNCIL
CANADADIVISION O F BUILDING RESEARCH
A
BRITTLE POINT TEST FOR LOW TEMPERATURE STUDIES
O F BITUMENS
BY
P.
M.JONES
REPRINTED WITH PERMISSION FROM T H E
PROCEEDINGS O F THE SEVENTH ANNUAL CONFERENCE O F CANADIAN ASPHALT ASSOCIATION, VOL. VII, NOVEMBER 1962, P. 15
-
2 4 .RESEARCH PAPER NO. 205 O F T H E
DIVISION O F BUILDING RESEARCH
PRICE 2 5 CENTS
OTTAWA
NOVEMBER 1963
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A
Brittle
Point Test
For
Low
Temperature
Studies
o
Bi turnens
By P. M. JONES" ABSTRACT
I n order t o study t h e low temperature performance of asphalts, im- provements have been niade to the Institute of Petroleum Sl)ecification I P 80/53, f o r determining the brittle point of bitun~inous materials. A new procedure for preparing the coated plaques is proposed which enables a film of asphalt of lcnown thickness t o be prepared. The apparatus has been mechanized and t h e spring steel plaque replaced with a clear polyvester film. The brittle point unit is cooled a t any desired rate using alcohol a s t h e coolant. Studies have been made of effect of film thickness and r a t e of cooling upon t h e brittle point temperature. S t u d y of t h e effect of over- heating a dead-level roofing asphalt i s shown t o be one of t h e uses of t h e instrument.
It
is desirable t h a t bitumens used in road construction and in built-up roofing possess some ductility a t t h e lowest temperature to which they a r e liable to be subjected. I t has recently been brought to t h e attention of t h e Division of Building Research t h a t many cases of splitting have occurred in built-up roofing and this prompted a study by t h e Division of the low temperature characteristics of the components of built-up roofing. Although these investigations were primarily concerned with bitumens a s used in built-up roofing, the results a r e applicable t o bitumens a s used in road construction.A t low temperatures, the brittleness of bitumen is a measure of ductility. Brittleness itself, however, is not a n absolute property because i t is dependent on t h e geometric dimension of the material and also on t h e magnitude and r a t e of defornlation. In engineering studies of these roofing failures a knowledge is required of t h e ability of t h e materials t o withstand deformation. To estimate this, a brittle point test was made on t h e bitumin- ous portion of built-up roofing.
A brittle point t e s t devised by F r m s s has been in use in Europe f o r a nuntber of years. This test is described in t h e Institute of Petroleum Specification I P 80/53 (1). In this t e s t a spring steel plaque is coated with a specific amount of bitumen and placed in a n apparatus in which i t is flexed by turning a handle operating a conical cam. The flexing apparatus is suspended in a n inner vessel and cooling is produced by blowing air over ether o r adding solid carbon dioxide to acetone. The handle is turned dur- ing t h e test, to produce one flex of t h e plaque every minute a n d the r a t e of cooling is adjusted to exactly 1 . 8 O F change per minute. The cooling and
flex in^
o ~ e r a t i o n s a.se continued until the first apaearance of a crack in t h e bitumen-coating. The temperature a t which this occurs is known as t h e "brittle point".I t is very difficult to perform all of these simultaneous operations in practice and leads to very poor reproducibility. A new apparatus was designed by Pickles (2) to simplify t h e operation of this test. I n t h e study now reported t h e apparatus used is basically t h a t designed b y Pickles with modifications to improve t h e use of t h e instrument. I t will be shown t h a t t h e thickness of bitumen applied and r a t e of cooling have a n effect upon t h e brittle point temperature recorded a n d a technique has been
*P. M. JONES, Research Ofticer, Organic Materials Section, Division of Building Research, National Research Council, Ottawa.
developed t o produce coated plaques suitable f o r obtaining reproducible results.
EXPERIMENTAL Apparatus
T h e a p p a r a t u s uses t h e same principle a s in t h e F r a a s s test. I n t h e modification by Pickles a n electrical drive was used f o r flexing, a cooling liquid was circulated t o produce the required temperature drop a n d t r a n s - parent plaques of polyester film were used
to
permit obsel-vation of t h e cracks by transmitted light. F u r t h e r modifications were made by t h e Division t o t h e bearings a n d rods of t h e flexing mechanism t o produce easier operation.T h e flexing mechanism is shown in Fig. 1. T h e Teflon plug approxi- mately 2 in. in diameter carries a central stainless steel tube a t t h e end of which is fixed t h e upper clamp f o r t h e t e s t plaque. T h e lower movable clamp is carried by twin stainless steel rods which pass through Teflon sleeve bearings in t h e upper clamp a n d in a brass bloclc mounted h i g h e r u p on t h e stainless steel tube. The rods a r e joined above t h e Teflon plug by a s t i r r u p t o which is fixed a rectangular frame. A Synchron motor with a spindle speed of 1 r p m is fitted with a circular eccentric cam whose c e n t r e is displaced 0.069 in. f r o m t h a t of the spindle. T h e cam t u r n s in t h e f r a n l e a n d causes a total separation of 0.138 in.
The plaques a r e made from Mylar polyester film 0.007 in. thiclc a n d a r e fixed to t h e top a n d bottom clamps by plugs passing through holes in t h e clamps, a n d by plates secured by single screws. A 3-degree t a p e r on t h e faces of t h e clamps i m p a r t s a slight outward curvature t o the plaque and ensures t h a t t h e plaques bend outwards when flexed. T h e plaques a r e j i g drilled t o ensure t h a t t h e holes in t h e plaques coincide with t h e pins on t h e clamps at n l a x i n ~ u m separation.
T h e temperature of t h e plaque is indicated and recorded by a copper- constantan thermocoul)le placed i n a glass well through t h e centre of t h e stainless steel tube such t h a t the thermocouple junction is located
at
t h e r e a r of t h e plaque. T h e o u t p u t of t h e thermocouple is recorded on a Leeds a n d N o r t h r u p single-point continuous recorder. T h i s facilitates determina- tion of t h e r a t e of tem1)erature drop by drawing a predetermined line upon t h e recorder c h a r t and manually controlling the r a t e of cooling to follow t h i s line.Preparation of Plaques
In t h e I n s t i t u t e of Petroleuln Specification I P 80/53 O . 4 O p . l g m of bitumen is applied t o a surface having a n a r e a of 1.27 sq. in. whereas Pickles uses a plaque having a n a r e a t o b e covered of 0.725 sq. in. If t h e 0.40 g m of bitunlen is applied to this 0.725 sq. in. in a uniform film, t h e resultant film thiclmess is 0.026 in. if a density of approximately 1.0 gm/cc is assumed f o r t h e bitumen. A s o t h e r studies upon asphalt under- taken by t h e Division use bituminous films 0.025 in. in thiclcness i t was decided t o use this value. T h e effect of film thiclcness upon t h e brittle
point will 1)c c!i:.cusscd l:~t.c!r. To prepare t h e coated plaques a hydraulic press method is used. Five plaques a r e prel)al.ed a t a t i l ~ i e ; tliesc it1.c lnasli- ed to present t h e specified urea f o r application of t h e bitumen. 'l'he bitunlen is heated in a glass heating mantel until i t is a t a pouring consistency. T h i s minimum pouring temperature i s held f o r about half a n hour t o remove a i r bubbles. T h e bitunlen is poured onto t h e plaques a n d covered with a n anti-sticlc sheet, consisting of e i t h e r Teflon-coated glass fabric or a n aluminum foil treated with silicone. T h i s arrangenlent is placed in a Carver Laboratory Press with heated platens. T h e platens a r e heated to about 2 5 ° F above t h e softening point of t h e bitumen. Spacers a r e used t o produce t h e thickness of 0.025 in. of bitumen, additional spacers being required t o account f o r t h e contraction of t h e b i t u ~ n i n o u s film a f t e r cooling t o room terriperature. A pressure of 10,000 lbs. i s applied f o r about 30
seconds t o cause t h e bitumen t o flow t o the desired thickness. A f t e r t h e samples have been cooled and trimmed thcy a r e stored in a room a t 77OF and 50 per cent relative humidity until tested.
Cooling
Pickles demonstrated t h a t direct liquid cooling was preferable t o using a i r a s t h e coolant. Two concentric vessels a r e used, t h e flexing mechanism being immersed in t h e coolant in the centre of these vessels. T h e coolant is methanol and t o reduce t h e slight solvent effect of pure methanol upon bitumens a mixture of 7 volumes of methanol and 3 volumes of water is used.
A stainless stccl liquid metel-ina pump was used t o circulate t h e coolant. T h e motor has a rating of 1/26 h.p. and is metered t o deliver 0.4 t o 26 ml per sec. T h e coolant is in a closed circuit including a coil immersed in a Dewar Vessel containing acetone and d r y ice and the vessel containing t h e flexing mechanism. I n t h e flexing chamber t h e coolant is s t i r r e d by a i r bubbling. The r a t e of cooling is predetermined on t h e recorder c h a r t and intermittent use of t h e pump ensures this steady rate.
Procedure
Following assembly of the apparatus flexing and c
t h e latter a t a r a t e of l.g°F per minute. Flexing and cooling a r e continued until the brittle point is reached. This is taken t o be a crack half-way across t h e plaque such t h a t i t is visible from both sides of the plaque.
A
plaque with a split across t h e entire surface is shown in Fig. 2. Initially 1 0 determinations of each brittle point were made b u t i t h a s been found t h a t 5 such determinations a r e sufficient. The plaques a r e cleaned w i t h toluene a f t e r use but i t is wise to discard t h e plaques a f t e r about 1 0 determinationsTest Conditions
est conditions were two built-up roof-
ing grade asphalts. T h e reproducibility under a s e t of specified conditions was studied by using a straight run 140°F softening point asphalt. Effects of film thicltness and r a t e of cooling were studied using a n oxidized 140°F softening point asphalt.
Rate of Cooling '
Using plaques coated with a film of asphalt 0.025 in. thick, values for t h e brittleness were obtained f o r cooling rates varying f r o m O.lg°F per minute t o 7.2OF per minute. T h e reported brittle point temperature is t h e arithmetic mean of five determinations. F i g u r e 3 shows t h a t t h e brittle point reaches a peak value a t a r a t e of about 1.8 to 2.7OF per minute.
Thickness of Bitumen
A series of plaques were prepared having various thicknesses of asphalt. Each s e t consisted of five plaques and series of 0.010, 0.015, 0.020, 0.025, 0.030 and 0.040 in. in thicltness were tested using a cooling r a t e of l.g°F per minute. Figure 4 shows t h e variation in t h e brittle point with film thickness, and i t would appear t o be fairly constant in t h e range of 0.015 t o 0.030 in. but large variations occur outside these limits.
Accuracy
A series of ten plaques of a 140°F softening point straight-run asphalt were tested using a thickness of 0.025 in. and a r a t e of cooling of l.g°F per minute. An average value of 20.3OF was obtained with a standard deviation of 1 . 3 O F .
Test Theory
The theory of the brittle fracture of bitumens by the F r ' s s
test
method has been studied by Rigden and Lee ( 3 ) and by Van der Poel (4). Rigden and Lee calculated stress-strain relationships in the test from an elastic theory of a bent beam. Van der Poel uses t h e breaking strength of the bitumen, but both conclude t h a t the test is a n equiviscous tempera- ture test corresponding to a viscosity of 4
X
100 poise or, expressed a s stiffness, 1.1X
100 dyne/cm2 (1.6X
104 psi). I t is thus feasible to report values directly as breaking temperature without f u r t h e r conversions ns has been done by other workers.Brittle Point of Asphalts and Tars
Seventeen bitumens were investigated to determine t h e brittle point temperature. The bitumens consisted of five 140°F softening point roofing asphalts, four 170°F softening point roofing asphalts, six 85/100 penetra- tion paving asphalts and two coal t a r roofing pitches. The bitumens and their brittle point temperatures are recorded in Table I.
The brittle point temperature was obtained as t h e mean of five deternlinations using plaques having a 0.025 in. thickness of film and tested using a rate of cooling of 1.8OF per minute.
From Table I i t can be seen t h a t the dead-level roofing asphalts vary from $20.3 to - 4 . g ° F , the 170°F softening point roofing asphalts from $11.0 to -0.4OF and the 85/100 penetration paving asphalts from +8.6 to -1.7"F. The coal t a r pitches are similar to each other in regard to the brittle point temperature, but are very different to t h e asphalts having values in the f 53°F.
Overheating
A preliminary study of t h e effects of bitumens overheating upon t h e low temperature properties has been made using several of the dead-level roofing asphalts and 170°F softening point roofing asphalts.
In this study the asphalts were heated to fusion and maintained in a
fluid state a t about 320°F until all bubbles were removed. A sample was talcen f o r softening point determinations and five plaques 0.025 in. each in thicltness were ]),repared. The asphalt was then raised in temperature to 500°F and maintained a t that temperature for one hour, another series of plaques was prepared and a softening point sample removed. The asphalt was then raised to G O O 0 F and maintained a t t h a t temperature f o r 1 hour and another set of plaques prepared, together with a softening point sample. Finally the asl~lialt was heated to green smolce and main- tained a t this condition for 1 5 minutes and again samples for brittle point and softening point determinations were removed. In each case the sample was cooled before preparing the brittle point plaques because a t too high a temperature the asphalt flows under t h e maslcing tape and uneven samples a r e obtained.
The softening point determinations were made in accordance with ASTM D36-25. Brittle point measurements were made using a cooling r a t e of 1.6"F per minute.
In Fig. 5 several of t h e asl)lialts a1.e sllown with the changes of soften- ing point after heating through the described cycle and plotted against t h e brittle point temperature.
Several points a r e demonstrated by this experiment. A definite relationship is shown to exist between softening point and brittle point f o r a particular asl)lialt. This is to be expected as each are equiviscous temperatures. The brittle point increases with an increase in softening point due to overheating. I t is also apparent that moderate overheating does not have
a
very marlted effect upon the brittle point of the asphalt a s measured in this test. One hour of heating a t 500°F has only a slight effect upon t h e brittle point of the asphalts. Values of t h e brittle pointtemperature a r e increased by 4 t o 5 ° F although t h e s t r a i g h t r u n material actually sllowed a slight apparent decrease of 0.9"F. The decrease, how- ever, is within t h e standard deviation of 1.3OF. Heating a t 500°F f o r one hour followed by one hour of heating a t G O O 0 F h a s a marlted effect upon t h e brittle point (and consequently tlie softening point), but t h e values a r e still sufficiently low to cause a reasonable degree of flexibility.
A
subsequent heating to green smoke f o r 15 minutes, however, is sufficient t o destroy t h e desirable properties of tlie asphalt and in most cases t h e samples break a t room temperature. The tenil,erature of green smoke was about G50°F f o r tile samples tested. A notable exception to tliese changes was t h e straight-run asl~lialt. This had a liigli initial brittle point but changed very little in this clial-acteristic a s tlie sample was over- Iieatcd.
The 85/100 penetrating paving asphalts show a significant difference between their brittle point tenlperatures. No ove~.heating e x p e r i i ~ ~ e n t s have been performed with these samples and considerable f u r t h e r experi- mental work is necessary before t h e brittle point t e n ~ p e r a t u r e could be
a
specified parameter in tlie design of paving asphalts.There is a wide difference between roofing grade asphalts a n d coal-L?r roofing pitch with respect t o t h e brittle point t e s t in measuring t h e equiviscous temperature of 4
X
10:' poise. The significance of t h e difference should also require additional experinlental work.Self-healing
I n t h e accumulation of information on the brittle point t e s t i t was accidentally demonstrated t h a t t h e teclinique can be used t o measure the self-healing qualities of tlie bitumens. Only two samples have been tested.
A
coal-tar pitch sample had a brittle point of 55.B°F. A t t h e conclusion of t h e t e s t tlie sample was allowed t o remain at 77OF f o r 3 h o u r s a f t e r which time i t was again tested and gave a n identical value f o r t h e brittle point tenlperature. A similar type of result was obtained f r o m a dead- level rooting asplialt having a brittle point ternl~erature of 9.3OF; a f t e r remaining a t 77OF f o r about 4 hours a value of 12.0°F was ohtaineCONCLUSION
The brittle point tempel.ature provides a useful test f o r t h e study of t h e low temperature charactel-istics of asphalts. I n tlie described procedure a satisfactory method has been developed t o obtain reliable a n d repro- ducible results. The actual value is dependent upon t h e r a t e of cooling and t o a lesser extent upon t h e thicltness of material. M e a s u r e n ~ e n t s of t h e brittle point temperature of nine roofing asphalts show considerable variation in t h e value ranging f r o m 4-21.2 to ----4.O0F. A series of six 85/100 penetration paving asphalts showed smaller variations f r o m +8.0 to -1.7OF. Two coal-tar roofing pitclies had values in the order +53.G°F wliich is considerably higher t h a n results obtained f o r asphalts. T h e brittle point temperature is a n equiviscous t e n ~ p e r a t u r e (4
X lo!) poise). Studies
of t h e effect of overheating upon roofing asphalts show t h a t all of t h e asphalts can undergo moderate overheating (one hour a t 500°F) with little change in t h e brittle point temperature, b u t lose flexibility a f t e r prolonged overheating. Heating a t green sinolte even f o r a short. time causes severe changes in the essential properties of asphalts. The t e s t should be of considerable help in assessing t h e low t e n ~ p e r a t u r e properties of t a r s a n d asphalts and in studying t h e effects of overheating a n d weather- ing upon these properties and a s a measure of self-healing.ACKNOWLEDGMENTS
T h e author is grateful f o r the assistance given by Mr. G. Quenneville in performing t h e experimental portion of this paper. This paper is a
contribution from t h e Division of Building Research, National Research Council, and is published with t h e approval of tlie Director of t h e Division.
REFERENCES
1. Institute of Petroleum Specification I P 80/53, Standard methods f o r testing petro- leum nnd i t s products.
2. Pickles, S. A n improved brittle point test f o r tars. Journal of Applied Chemistry, Vol. 9. A ~ r i l . 1959. . P. - 219-223.
3. Rigden, P. J., a n d A. R. Lee. T h e Brittle fracture of t a r s nnd bitumens. Journal of Applied Chemistry, Vol. 3, 1953, p. 62-70.
4. Van dcr Poel, C. A general system describing t l ~ c visco-elastic properties of bitumens and it8 relation to routine test data. Journal of Applied Chemistry, Vol. 4, 1954, p. 221-236.
TABLE
I
BRITTEE POINT TEMPERATURES O F BITUMENS
No.
Description Brittle Point0 l 3
I'
1 140°F
S.
Pt."
B.U.R. Oxidized Asphalt (mid Saslc.) +5.2 2 140°F S.Pt.
B.U.R. Straight-Run Asphalt (south Sask.) +20.3 3 140°F S.Pt.
B.U.R. Oxidized Asphalt (Venezuelan) -0.94 140°F S.
Pt.
B,U.R. Oxidized Asphalt (Pcmbina) -4.95 140°F S.
Pt.
B.U.R. Oxidized Asphalt (Lloydminster) +10.4 6 170°F S.Pt.
Oxidized Roofing Asphalt (mid Sask.) +l4.0 7 170°F S.Pt.
Oxidized Roofing Asphalt (Venezuelan) -0.98 170°F S.
Pt.
Oxidized Roofing Asphalt (Peinbina) +8.69 170°F S.
Pt.
Oxidized Roofing Asphalt (Lloydminster) 4-5.510 85/100 Penetration Paving Asphalt (Venezuelan) f l . 0
11 85/100 Penetration Paving Asphalt (Lloydminster) f0.5
12 85/100 Penetration Paving Asphalt (Pembina) f8.6
1 3 85/100 Penetration Paving i?spllalt (Snsk. Weyburn) f0.3
14 85/100 Penetration Paving Asphalt (south Sask.) +3.2
15 85/100 Penetration Paving Asphalt Experimental -1 7
16 Coal-Tar Pitch #1 +54.3
17 Coal-Tar Pitch #2 +50.9
Figure 1 The brittle point unit
Figure
2
Brittle point
apparatus in
FIGURE 3
EFFECT OF COOLING RATE. UPON BRITTLE POINT OF ASPHALT
THICKNESS OF ASPHALT FILM, INCHES
FIGURE 4
/
140 'F S. SASH. # 2 STRAIGHT RUN I I I I I II
140 150 160 170 180 190 ZOO SOFTENING POINT, 'F FIGURE 5SOFTENING POINT
