STF22 A04339−Unrestricted

REPORT

GARAPEvaluation of different laboratory compaction methods for preparation of cyclic triaxial samples

Inge Hoff

SINTEF Civil and Environmental Engineering Roads and Transport-

October 2004

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TABLE OF CONTENTS

1 INTRODUCTION .................................................................................................................3 1.1 GARAP – research project ...............................................................................................3 1.2 Laboratory compaction.....................................................................................................3

2 TESTING EQUIPMENT ......................................................................................................3

3 SAMPLE PREPARATION ..................................................................................................4 3.1 Material ...........................................................................................................................5

4 PROCEDURES FOR TRIAXIAL TESTING.....................................................................6

5 RESULTS FROM TESTING ...............................................................................................7 5.1 Resilient behaviour...........................................................................................................7 5.2 Permanent deformation behaviour ...................................................................................8

6 CONCLUSIONS ..................................................................................................................11

7 REFERENCES.....................................................................................................................12

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1 INTRODUCTION

1.1 GARAP – research project The research project “Granular Aggregates for Road and Airport Pavements” is supported by: - Norwegian Public Road Administration - Avinor - Norwegian Rail administration - Norwegian Aggregate Producers Association - Nynas - Norwegian Research council. The aims for the project is to improve laboratory testing, material modelling and design of structures with unbound granular materials as an important structural element.

1.2 Laboratory compaction Laboratory testing of materials can only be an imitation of the real field conditions. For coarse graded material it is not possible to take undisturbed samples from the road and use for triaxial testing. Hence, the material must be compacted in the laboratory to the same density as in the field or to a range of densities if the field density varies. Several different methods for laboratory compaction have been developed over the years. The most popular method for general sample preparation is maybe the impact hammer (Proctor). This is a very simple method and no expensive equipment is necessary. On the other hand the impact hammer is not a good simulation of the compaction process in the field with a heavy roller often combined with some sort of vibration or oscillation. Other methods have been developed that are supposed to simulate the field compaction better. It is obvious that the compaction level, i.e. dry density, strongly influences the material behaviour. Some research also indicates that samples compacted to the same density with different methods will behave differently e.g. research performed in the KPG project [Hoff 1998] showed dramatically higher “CBR-values”1 for samples compacted with gyratory compactor compared to samples compacted with vibratory hammer

2 TESTING EQUIPMENT The samples were tested in a triaxial testing apparatus for 150 mm diameter samples applying cyclic deviatoric loads and constant confining pressure. The development of this equipment was started in 1975 and has been going one since. New methods for load control and strain measurements have been taken into use to secure an increasing accuracy. The equipment has the possibility to also cycle the confining pressure in phase with the deviatoric load, but this ability has not been used in the tests described here. The latest improvements as part of the GARAP-project is described in a separate report [Hoff 2004] Figure 1 shows a photo of the equipment with a sample ready for testing.

1 CBR-loading procedure was used on samples with non-standard compaction and without soaking in water.

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Figure 1 Cyclic triaxial test apparatus

3 SAMPLE PREPARATION Four different compaction methods were used for sample preparation. Some information about the different equipments is given in Table 1.

Table 1 Characteristics of the different compaction methods

Type Principle Manufacturer Characteristics Gyratory compactor Kneading

Invelop oy ICT 150 RB

0 – 2 o gyratory angle 0 – 700 kPa vertical pressure 0 – 500 cycles

Modified Proctor Impact Own Weight 4.8 kg Free fall 0.450 m

Vibratory table Vibration AEG VT 360/630 cy

Frequency: 50 Hz Amplitude: 1.0 mm Dead weight 4.95 kg

Vibratory hammer Vibration/impact Kango 950 X

Total weight 35 kg Frequency: 25 – 60 Hz Amplitude: 5 mm

The gyratory can produce samples as high as 220 mm. This height was used as target height also for the other methods. The NS-EN standard requires a 2:1 ratio between sample height and diameter. For a 150 mm diameter sample this means a height of 300 mm. To compensate for the

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low samples a special procedure was used to secure low friction against the end-platens. Teflon foils and silicon oil was used for lubrication. The samples were extruded from the mould using a special technique keeping the sample confined by internal vacuum during this process and until the confining pressure in the triaxial chamber could be applied.

3.1 Material For this limited investigation only one type of material has been used. The gneiss from Askøy, outside of Bergen, is of good quality and typical for materials used in road construction in Norway. Some of the properties are listed in Table 2.

Table 2 Characteristics of the Askøy material

Specific density 2690 kg/m3 Los Angeles value 14.5 Modified Proctor density 2186 kg/m3 Modified Proctor optimal moisture content 5 % The material is a metamorphic granitic gneiss that consists of quartz and feldspar with smaller amounts of amphibole, titanite and mica. The material was sieved and combined to a well graded curve using the Fuller curve:

n

Ddp ⎟⎠⎞

⎜⎝⎛=

Where: p = percentage passing sieve d = sieve size D = Maximal grain size, 22 mm n = 0.5

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60321911.28.06.04.02.001.000.5000.200 0.125 0.060.020.0122160.6000.2500.075

0

10

20

30

40

50

60

70

80

90

100

MediumFineMediumFineMedium CoarseCoarseCoarse

GRAVELSANDSILT

Figure 2 Grain size distribution

4 PROCEDURES FOR TRIAXIAL TESTING It is believed that the main influence of compaction method could be found for the development of permanent deformations. Because of this it was decided to follow a procedure that has been developed to determine the resistance against permanent deformation. The multi-stage procedure described in the new EN-standard [CEN 2003] is well suited for the purpose of relative comparison between different materials or as in this case different sample preparation methods. The procedure is similar to procedures used earlier at NTNU/SINTEF [HOFF 1999] The load is applied stepwise in five sequences for each level of confining pressure. Table 3 shows the load levels used for strong samples. A similar table exists for use on weaker samples with lower stress levels.

Table 3 Stress levels for multistage loading procedure (high stress) Sequence 1 Sequence 2 Sequence 3 Sequence 4 Sequence 5 Confining stress, σ3 (kPa)

Deviator stress, σd (kPa)

Confining stress, σ3 (kPa)

Deviator stress, σd (kPa)

Confining stress, σ3 (kPa)

Deviator stress, σd (kPa)

Confining stress, σ3 (kPa)

Deviator stress, σd (kPa)

Confining stress, σ3 (kPa)

Deviator stress, σd (kPa)

Constant min max Constant min max Constant min max Constant min max Constant min max20 0 50 45 0 100 70 0 120 100 0 200 150 0 200 20 0 80 45 0 180 70 0 240 100 0 300 150 0 300 20 0 110 45 0 240 70 0 320 100 0 400 150 0 400 20 0 140 45 0 300 70 0 400 100 0 500 150 0 500 20 0 170 45 0 360 70 0 480 100 0 600 150 0 600 20 0 200 45 0 420 70 0 560

For each sequence the test is interrupted when all steps are completed or an axial strain of 0.5 % is reached. The test is then continued by applying the next sequence. 10 000 continuous sine shaped load pulses are applied at a 10 Hz frequency for each step.

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This procedure will give both the resistance against permanent deformations and the resilient modulus for a range of stress levels.

5 RESULTS FROM TESTING

5.1 Resilient behaviour The resilient modulus can be calculated using the following formula for cyclic triaxial tests with constant confining pressure:

a

drE

εσ∆∆

=

Where: Er = Resilient modulus ∆σd = Applied deviatoric stress ∆εa = Measured axial strain The resilient modulus is highly stress dependent as illustrated in Figure 3. Each point represents the stabile average value from each load step (10 000 pulses)

0

100

200

300

400

500

600

700

0 50 100 150 200 250 300 350 400

Max mean stress (kPa)

Res

ilien

t mod

ulus

(MPa

)

Figure 3 Resilient modulus as a function of mean stress for the first sample compacted with vibratory hammer

Several different models for stress/strain dependent resilient models have been proposed. The simplest models use two material parameters for resilient modulus and a constant value for Poisson’s ratio. More complex models using four or more parameters have also been proposed. The results from these tests could easily be interpreted using one of these models. However, for simple comparison between the samples in this investigation the resilient modulus for maximum mean stress of 200 kPa has been plotted versus the density in Figure 4.

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0

100

200

300

400

500

600

700

1.9 1.95 2 2.05 2.1 2.15 2.2

Dry density (kg/dm3)

Res

ilien

t mod

ulus

for m

ean

stre

ss =

200

kPa

(MPa

)

GyratorVibratory tableVibratory tableImpact hammer

Figure 4 Resilient modulus as a function of dry density It seems clear from Figure 3 that there exists a correlation between the dry density of the samples and the resilient modulus. However, the different compaction methods seem to give similar values for resilient modulus. The differences observed are smaller than scatter for cyclic triaxial testing on this type of material.

5.2 Permanent deformation behaviour Characterisation of a materials resistance against permanent deformations is not straightforward. Several procedures for laboratory testing and interpretation have been proposed. One obvious problem is that the development of permanent deformation is highly dependent on the load history. This means that a new sample should be used for every single stress condition that is interesting. However, this would lead to an unpractical and expensive testing program for testing of a material for the stress conditions the material is exposed to at different locations in a pavement structure. To reduce the number of tests a multistage procedure has been developed. This procedure is now adopted in the EN-standard for cyclic triaxial testing. Figure 5 shows the development of permanent axial strain for one load sequence for one of the samples compacted with vibratory table. A similar response is recorded for the other four load sequences applied to the same sample.

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-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 1000 2000 3000 4000 5000 6000

Time (sec)

Perm

anen

t axi

al s

trai

n (o

/oo)

Figure 5 Development of permanent axial strain for one sequence Each load step are then analysed to determine if the material is in one of the following stages: A – Almost purely elastic response B – Some permanent deformation, but stabilizes towards the end of the step. C – Incremental failure Obviously the development of permanent deformations is faster in the beginning of a load step than towards the end. The average strain rate (strain per pulse) for the last 5 000 cycles in each step has been used to limit the different stages. The limits used corresponds to what Werkmeister [Werkmeister 2003] proposed using an average strain rate for cycle 3 000 to 5 000.

Table 4 Limits between permanent deformation regions

81 105.2 −⋅≤pε& Category A

71

8 100.1105.2 −− ⋅≤≤⋅ pε& Category B

71 100.1 −⋅>pε& Category C

When all steps were characterized in this way and plotted in a deviatoric stress vs. confining stress diagram (Figure 6) best fit lines for the boundary between the different stages could be found using the Mohr – Coloumb parameters (apparent attraction “a” and friction angle φfailure and ρelastic limit.) The results are illustrated in Figures 7 and 8 Because the difference in “a” was relatively small the tests were reinterpreted using a fixed value for “a” to make it easier to visualize the differences between the different compaction methods.

10

0

200

400

600

800

1000

1200

-50 0 50 100 150 200Confining pressure (kPa)

Dev

iato

ric s

tress

(kPa

)

Figure 6 Determination of failure and elastic limit lines

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

1.9 1.95 2 2.05 2.1 2.15 2.2

Density (kg/dm3)

Failu

re a

ngle

(sin

φ)

GyratorVibratory hammerVibratory tableImpact hammer

Figure 7 Failure limit angle interpreted using a=15 kPa for all samples

A

B

C

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0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.9 1.95 2 2.05 2.1 2.15 2.2

Density (kg/m3)

Ela

stic

lim

it an

gle

(sin

ρ)

GyratorVibratory hammerVibratory tableImpact hammer

Figure 8 Elastic limit angle interpreted using a=15 kPa for all samples

6 CONCLUSIONS These tests have showed that different compaction methods could give different resistance against permanent deformation for samples compacted to the same dry density. Samples compacted using the Modified Proctor hammer showed less resistance to permanent deformation compared to the samples compacted with one of the two methods based on vibration. This observation is obviously only valid for this material and it is likely that ranking between the different method could be different for other material or other conditions. However, it is likely that different compaction methods will produce significantly different results also for other materials. A more trivial conclusion is that both the resilient modulus and the resistance against permanent deformation increase with increasing compaction effort.

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7 REFERENCES CEN EN 13286-7 “Unbound and hydraulically bound mixtures – Part 7: Cyclic triaxial test for unbound granular materials” Werkmeister, S. “Permanent Deformation Behaviour of Unbound Granular Materials in Pavement Constructions” Dr. thesis. Der Fakultät Bauingenieurwesen der Technischen Universität Dresden 10. Februar 2003 Hoff, I. ”Material Properties of Unbound Aggregates for Pavement Structures” Dr. thesis Norwegian University of Science and Technology. Trondheim, 1999 Hoff, I. “GARAP - Improvement of equipment for cyclic triaxial testing” SINTEF report STF22 O04321 Trondheim 2004 Hoff, I and Leif Jørgen Bakløkk ” Materialegenskaper for grus- og pukkmaterialer” SINTEF report STF22 A98459 (Delprosjektrapport KPG 18) October 1998 (In Norwegian)

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Appendix A: Results from triaxial testing

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Vibratory hammer

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Modified Proctor hammer

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19

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Vibratory table

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Gyratory compactor

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