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METHODST7

USE OF THE VANE SHEAR APPARATUS TO DESIGN A SAND ASPHALT MIX

2

2,1

22

23

2.4

2.5

2.6

2.7

2.8

2.9

2.10

2.11

3

3.1

3.2

3.2 .1

3.2.2

SCOPE The method descrtbes the determination of the optimum bitumen or tar con­tent of mixtures of sand, bituminous binder and moisture using a vane shear apparatus (see 5.1).

APPARATUS Vane shear apparatus (Figure ST7 II) (see 5. 1).

A dial-type torque wrench calibrated to read in Nm (See 5.2).

Standard equipment for the Modified AASHTO optimum moisture/maximum density determination (TMH1, Method A7).

A stop watch~

A balance to weigh up to 10 kg to an accuracy of 1 g.

A scale to weigh up to SO kg to an accuracy ot 5 g.

A thermometer to read up 10 100 cc.

Miscellaneous items as required such as jugs, pars, spoons, etc.

Alrtlgh1 containers with preSS-In Itds, of approximateiy 18 ( capacity.

Approximately 10 ! of the bitum~nous bmder 10 be used.

A well ventilated drying oven capable of maintaining a temperature of 95 to 100 "C and 105 to 110 :C.

METHOD

Preparatory work

Determine the optimum moisture content of the matertal 10 be tested accord­ing to TMH 1. Method A 7.

Preparation ot the test specimen

Preparing the sand

By means of a riffler quarter out four portions of at least 35 kg each. To each portion In turn add suftic;ent water to obtain a moisture content 3 to 4 per cent above the optirr.um moisture content, mix thoroughly to disperse the water evenly throughout the material and place each portion in a separate airtight container.

Adding binder

Heat 1,5 { of the binder in a jug to spraying or mixing temperature (.:t 5 "C), depending on which method of construction is to be used. Empty one portion of material from the airtight container into a mixing basin, place the basin with contents on a balance and add binder to give about 2 per cent of residual bitu­men or tar binder. Mix thoroughly and return the mixture to the airtight con­tainer. This is done in turn with each of the other three fractions, increasing the binder content by 1 per cent at a time.

Special methods

Draft TMH6. Pretoria, South Atnca. 1984 25

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3,2.3 Evaporating the fluids (moisture and volatile oiis)

Use the port ion containing the lowest binder content and divIde it to obtain a sample of approximately 7 kg. Place this sample in an airtight plastic bag. Ja­bel it and set it aside for later use. Weigh the remaining portion accurately to the nearest 10 g. place it in a shallow metal tray and allow it to aerate in a ven­tilated hot room or in sunlight tn the open or in a well ventila(ed oven with the temperature not exceeding 60 ce. Stir the mixture at short intervals and weigh it from time to time.

When the material has lost approximately 1 to 2 per cent of flUid by mass, di­vide it as rapidly as possible to obtain a second sample of approximately 7 kg, P~ace this sample also in a plastic bag and label it.

Repeat the process until at least five samples have been ob1alned. The fluid contents of the samples should vary by approximalely 1 to 2 per cent by mass so that the fluid content of the last two samples is below the OMC for the materiaL

3.2.4 Compaction

Compact each of the samples obtained in accordance with 3.2,3 in a mould according to TMH1 Method A7.

3.2.5 Determining the vane shear strength

Starting with the sample wlth the lowest fluid content place the mould with compacted material on the laboratory frame of the vane shear apparatus (Fig­ure ST7/I). Assemble the apparatus as indicated. Place the anvil on the vane and. by hammering, drive the vane into the sand until the vane top is at least 12 mm belOW the surface of the sample. Using a hand-operated torque wrench, apply a steady torque to give a rate of rotation of the vane not exceeding 3¢ per second. Observe the dial of the tor· que wrench and note the maximum reading obtained (see 5.2). This maximum should be obtained in not less than 10 seconds. Determine the temperature of the sand at approximately the mid-point depth of the vane. using a steel rod of suitable diameter to punch a hote for the ther­mometer.

3.2,6 Determining the fluid and binder content

After shearing. take a representative sample from the mould and determine the fluid content of the material by drying to constant mass in an oven at 95 to 100°C. Also determine the binder content according to TMH1 Method C7.

3.2.7 Repeat the process described in 3.2.5 and 3.2.6 with the other four moulds of compacted materia! and then continue with the remaining three prepared por­tions with the higher binder content in the same manner as described in 3.2.3 to 3.2.6. Record the results on a suitable data sheet.

4 CALCULATIONS 4.1 Vane shear strength

This is calculated from the following formula (see 5.3):

26 Special methods

Draft TMH6. Pretoria. South Mica. 1984

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4.2

s= 21 1l d2 (H + ~) 1 000

where

S T

- shear strength of material (kPa)

= 10rque needed to mobilize shear strength (N.m)

d - diameter of cytinder of revolution generated by vane co: diameter of vane (m)

H - height of the cylinder of revolution =. height of the vane (m),

The shear strength can also be read off from the graph in Figure ST7111.

Express the shear strength at a partIcular temperature by using the . appro~ priate temperature/shear strength relationship factor (example in Figure 5T7/111). However, it may be preferable to determine the temperature/shear strength relationship for the particular material and binder to be used (see 5.4).

Percentage of fluids (volatife oils and water) in sample

Calcuiate the percentage of flUId in the wet sample as follows:

F:.::: (X-V) 100 % X

where

F - flUId content of mIxture at time of testing (%)

X - mass of sample of wet mixture at time of testIng (kg)

Y mass of material and residua! binder after drying to constant mass at 100 cC (kg)

It follows then that the percentage of sand plus residua! binder in the wet mix

= 100 YOlo X

4.3 Mass of dry sand in CBR mould

4.4

Using the data obtained from the above calculations. calcu late the mass of the dry sand in the mould as follows:

W 0= X {100 - F} s (100 + R)

where

Ws :::= mass of dry sand in mould {kg}

R :::: parts of residual binder per 100 parts of dry sand as determined by TMH1 Method C7

Ory density of compacted sand in mould

0.:;;;. Ws Vm

where

o = dry density of compacted sand in mould (kg/m,,)

Vm :::: volume of mould (m~

Special methods

Draft TMH6. Pretoria, South Afnca. 1984 27

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4.5

5 5. i

52

53

Graphical presentation

.Plot a graph of fluid con1ent (abscIssa) against shear strength and dry density (ordinates) for each of the different binder contents. Plot the maximum dry density and shear values against corresponding binder contents which give the maximum dry density and shear values.

The optimum binder and fluid contents are reported on a suitable data sheet to the nearest first decimal place.

NOTES

The vane shear apparatus is also designed for determinIng in situ vane shear varues of sand asphalt In that case only the field frame (see Figure ST7 II) IS

placed , with the vane inserted. over the test point. The steel pins are pushed into the material and the test procedure described in 3.2.5 followed.

With a 0 to 100 ~~.m torque wrench, the readings below 30 N.m may no! be accurate enough for determining shear strength to the required accuracy In such cases it is suggested that a torqJe wrench WIth a lower capacity and higher aCCuracy should be used.

The formula relatmg shear strength. torque and vane size can be explained as follows:

As a fIrst approximation it may be assumed that the bituminous mixture shears along the cylindrical and two end suriaces of a cylinder, the diameter and height of which equal that of the vane. It may also be assumed that the dlSlri­bU110n of shear stress IS uniform across tt1e ends of the cyiinder The moment to mobil ize the shear strength of the matenal is then found by taking moments abod the axis of the cyl inder of revolut ion generatea by the vane.

T_ Sf dH d 2 :td;: d . 1 000 - i:T

• '2 + - 4' 3)

S . :rd"H :ld')·

::: (-2- "t' 6 ;;;:. S .,d" (H + E-)

2 3

Therefore

S 2T

Where

S - shear strength of the materia! (~Pa)

T '- torque to mobilize shear strength (N.m)

d ;::;: diameter of cylinder of revolution generated by vane =- diameter of vane (m)

H = height of cy linder of revolution :::: height of vane (m)

5.4 Determination of the relationship between temperature and shear strength

5.4. 1 Establish an optimum value for binder and water content as outlined in 3.2 .1 to 4.5.

28 Special methods

Draft TMli6. Pretoria. Soutt'l tl.trica , 1984

5.4.2 Prepare a further seven sets of three samples, each w ith a mass of approxima­tely 7 kg and at optimum 'binder and water content. Compact these samples In

21 moulds. as outlined in 3.2.4.

5.4 .3 Using a cold room and ovens, bring these samples. in sets of three, to specific temperatures varying by discrete steps of 5 <:'C from 15 "C to 45 ce.

5.4.4 Determine the torque readings as outlined in 3.2.5, noting the exact tempera­ture at the time of testing. Torque readings at the lower temperatures will be higher than the torque readings at the higher temperatures.

5.4.5 Calculate the shear strength as outlined in 4.1 and determine the ratIos of shear strength measured at 40 cC to shear strength measured at a given tem­perature. These ratios become the tactors to be used to convert shear strength at a given temperature to corrected shear strength at 40 =C.

5.4.6 Tabulate these results, including the mean value for each set of three samples tested at the same temperature.

54 7 Plot these results on a graph with the temperature of mixture, at time of test. as the abscissa (x-axis) and the factor tor converting shear strength as the Of '

dinate (y-axis),

REFERENCES 1. ASTM 0 2573-72.

2. MARA!S, C p, A new techmque to control compactIon of bJtumen-sand mixes on the road using a vane shear apparatus. The Civil Engineer in South Africa (March 1966).

$peclal methods

Draft TMH6. PretOfla, South AfTlca , 1984 29

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Special methods

Draft TMH6, Pretoria, South Africa. 1984

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~ 600 fI)

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400

300

200

100

0 0 10 20 30 40 50 60 70 80 90 100

TORQUE {N.ml

(,.:) -r. FIGURE ST7l1i TORQUE VERSUS VANE SHEAR STRENGTH FOR STANDARD VANE SIZE (H == 50 mm, d :: 30 mm)

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3,0

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1,0 0,9 0,8

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0,1 5 10 15 20 25 '30 35 40

TEMPERATURE OF MIXTURE AT Tl ME Of TEST (PC)

FIGURE ST7/m TYPICAL CHART TO CONVERT SHEAR STRENGTH AT TEST TEM­PERATURE (T"C) TO EQUIVALENT SHEAR STRENGTH AT 40"C

45 50