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“Sustainability of Our Resources” – Our Future

The IOQ and AQA NZ Annual Combined Conference 2007

Evaluation of the Repeat Load Triaxial Test and its Potential for Classifying Basecourse

Aggregates

Jason Lowe

1

Evaluation of the Repeat Load Triaxial Test and its Potential

for Classifying Basecourse Aggregates

Introduction

The Repeat Load Triaxial (RLT) test is starting to gain favour in New Zealand as a

method predicting the in service performance of Basecourse aggregates. It is hoped that

adoption of the test in NZ will enable increased confidence of the in service performance

of existing basecourse materials whilst also allowing greater opportunity to develop high

performing alternative or modified basecourse materials that may not conform to the

existing specifications.

In response to this Winstone Aggregates (WA) undertook a six month testing programme

to try to better understand both the RLT test and a comparative look at how individual

basecourses performed.

To achieve this WA agreed to lease the RLT equipment from PaveSpec Ltd, the

Company formed by Dr Greg Arnold, formerly of Transit NZ, who has spent many years

studying pavement aggregates and the application of RLT testing.

The equipment was set up at WA’s IANZ accredited Auckland Laboratory in November

2006 under the watchful eye of Dr Arnold.

Over 85 RLT tests were completed during the period of November 2006 to May 2007

covering a wide range of natural, modified and alternative basecourse materials. This

paper discusses some preliminary findings specifically related to the suitability of the

RLT test for classifying basecourse aggregates and highlights areas of further work.




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Background

Repeat Load Triaxial (RLT) testing is not by any means a new test. However it has been

recently been identified and developed as a test to simulate and determine the effect of

repeated heavy traffic on basecourse aggregate. Therefore RLT testing has the potential

to effectively identify those aggregates more suitable for high trafficked roads. A lot of

the early development work was carried out by Dr Arnold and Transit NZ (Arnold 2004).

Currently TNZ M/4 is the prescribed basecourse specification for high trafficked roads,

this is an empirical, recipe based specification and there have been concerns as to the

performance variability of basecourses conforming to the specification.

RLT is a test that could, in conjunction with source and production testing, give further

confidence of the in service performance of a basecourse. RLT testing has been

developed with the aim of providing an aggregate rutting resistance measure, as detailed

by Arnold 2003, “The RLT apparatus (Fig 1) applies repetitive loading on cylindrical

materials for a range of specified stress conditions, the output is deformation (shortening

of the cylindrical sample) versus number of load cycles (usually 50,000) for a particular

set of stress conditions. Multi-stage RLT tests are used to obtain deformation curves for a

range of stress conditions to develop models for predicting rutting”.

Fig1: PaveSpec Ltd RLT equipment




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Objectives

The objectives of the preliminary work discussed in this paper were to investigate some

of the key test parameters and understand their influence on the RLT results.

There are many potential influences on the RLT test results for the purpose of this paper

the focus will be on:

• Operator Sensitivity

• Repeatability

• Compaction

• Water Content

Each of these will be discussed and preliminary findings presented.

Procedure

A Kanga 950KV Vibrating Hammer with a 146mm diameter compaction foot was used

to compact the samples (fig2). A sample preparation calculator was used to achieve target

compaction density by outputting the correct mass of sample material to compact to a

specific height in the split mould (fig3). Compaction effort and unusual observations

were recorded and explained in each individual test report.

Fig:2 Fig:3 Fig:4 Fig:5

The compacted sample (fig4) is removed from the split mould, and a sealing membrane is

fitted (fig5). The membrane gives an air tight seal enabling loads to be applied by means




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of air pressure. The sample is mounted in the pressure cell(fig6). The cell is then sealed

on to the base. The pressure cell is aligned underneath the vertical load actuator and

stand(fig7).

The vertical load actuator and pressure cell are both powered by compressed air, and

controlled by the computer software UTS017 Cyclic load trial. This software is installed

on a late model Windows PC. The physical link between the instruments and software is

through a signals box(fig8).

The vertical actuator applies a cyclic load to the test sample at a rate of 5Hz throughout

the duration of the test. A small buffer tank(fig9) feeds compressed air directly to the

apparatus. This reduces the effect caused by variable air demand on the pressure.

Fig:6 Fig:7 Fig:8 Fig:9

The stresses used in each stage are used and recommended by Dr Greg Arnold. The

stresses are believed to be the most representative combination in the appropriate resilient

modulus envelope.




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Fig 10 below displays stresses applies in all tests.

Stage Confining (kPa)

Deviator (kPa)

1 120 90 2 66.7 100

3 41.7 100 4 90 180 5 140 330 6 110 420

Fig10:The Six Stress Stages

It is recommended that RLT test samples should be compacted to a target Maximum Dry

Density (MDD) of 95% and target Optimum Moisture Content (OMC) of 100%. To

achieve this a sample moisture content is measured and the correct moisture to be added

or evaporated is calculated by weight. A simple sample preparation calculator (fig11) was

developed to help streamline this process and reduce errors.

Material Description:

M.D.D. 2.240

O.M.C. (%) 5.000

Received M.C. 0.037

Density percentage 0.950

Target Density (kg/m3) 2.128

Sample Height (m) 0.300

Sample volume (10^-3) 5.301

Mass of Dry (kg) 11.281

Mass of H2O (kg) 0.564

Mass Received (kg) 11.702

H2O to Add (kg) 0.144

Total Weight (kg) 11.846 Weight for each Layer (kg) 2.369

Mass Before Oven (kg) 6374.000

Mass After Oven (kg) 6145.000 Fig11 :Sample Preparation Calculator




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Preliminary Results

To achieve actual rut depth performance of a sample, results data is run through finite

element analysis software developed by Dr Arnold. The magnitude of rut deformation

can then be determined. It is still possible for the raw data to be analysed and

comparisons can be drawn however it is more difficult to determine magnitudes.

A spreadsheet results template was created in to which the raw RLT data can be entered.

The template adjusts the raw data in relation to other data collected, accumulates and uses

regression analysis as a means of comparing the data. The ouput is an accumulative strain

plot (fig12) and results table(fig13). These were created and documented for every

individual test, and this allowed a uniform method for displaying test data.

Accumulative Strain

0

0.05

0.1

0.15

0.2

0.25

0.3

0 50000 100000 150000 200000 250000 300000 350000

Load cycles

Str

ain

(m

m)

Stage 1

Stage 2

Stage 6

Stage 5

Stage 4

Stage 3

Fig12 : Accumulative Strain Plot

Stage

Acc first 25k Deformation Mag

Slope after 25K

Average Resilient Modulus after 25k

1 0.05879 0.03628049 432.2 2 0.07676 0.070536353 336.3 3 0.09385 0.093850339 289.55 4 0.12323 0.107659568 444 5 0.17742 0.02625335 556.5 6 0.22275 0.501472373 539.45

Fig13 : Results Table




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WA commissioned Dr Arnold to analyse results of the testing through the finite element

model he has developed, these results are measured in terms of, million equivalent

Standard Axles (ESAs) to 10mm rut within aggregate, making it easier to interpret the

results.

Operator Sensistivity

During the course of the testing programme WA had two operators (fig14) conducting the

RLT testing.

Fig14 : Geoff and Adrian

At the outset it was identified that there was no formal procedure for conducting RLT

testing. This could have led to inconsistency between the two operators and to avoid this

Geoff Moore (WA) wrote a procedure for carrying out the RLT testing that was adopted

for the duration of the testing programme.

The procedure was strictly followed for every sample tested and this consistency of

approach effectively minimized the influence on final results of different operators.

Repeatability

RLT repeatability was looked at over a 3 month period, incorporating 5 independent

production runs of TNZ M/4 produced from a North Island greywacke.




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Although the results show some variability between samples, at this stage in the

evaluation of the test the emphasis is not so much on the actual result but more on the

ability for the test to predict the consistency with which the basecourse will meet the

performance required of it.

One way of achieving this is to create performance bands in which to rank suitability of

materials for certain applications. The figure which has been suggested for the upper

band is > 10million ESAs and therefore on the results analysed in this paper the test

would appear to be able to measure a material relatively consistently for this purpose.

Sample No 1 2 3 4 5

Date of

Test

23/02/07 13/03/07 3/04/07 10/04/07 21/05/07

Volume in

Run (m3)

3500 1500 4000 4000 4000

Million

ESAs

9.4 12.5 11.3 10.4 11.3

Source Rock: Greywacke Basecourse specification: TNZ M/4

Repeatability

0

2

4

6

8

10

12

14

1 2 3 4 5

Sample No

Mil

lio

n E

SA

s t

o 1

0m

m r

ut

wit

hin

Ag

gre

gate

Careful selection of source rock and a high level of production control through the

manufacturing process is reflected in the consistency of the material produced. This

consistency is seen through both the standard TNZ M/4 source and production tests, and




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it would appear that the RLT test is comparable, in terms of predicting consistency of

material.

There are many factors that could have influenced the individual results and further work

will be undertaken to investigate the differences between the 5 samples, as well as

increasing the sample population with further results that have been tested but not yet

analysed.

This paper does not consider repeatability between different Laboratories, but proposes

that regular inter-lab testing should be conducted to establish the degree of variability

between laboratories.

Compaction

Material was sampled from the stockpile and split to enable 3 RLT samples to be

compacted as shown in table below.

Sample Number % of Compaction Million ESAs

1 Under Compaction -15% of Target 2.3

2 Target Compaction Target = 95% of MDD 10.4

3 Over Compaction + 15% of Target 12.5




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Influence of Compaction

0

2

4

6

8

10

12

14

Under Target Over

Degree of Compaction

Mil

lio

n E

SA

s t

o 1

0m

m r

ut

wit

hin

Ag

gre

gate

As can be seen from the graph, the under compacted sample had a huge reduction in

predicted life. This changed the performance of a high quality TNZ M/4 and shows the

importance of achieving full target compaction. The relationship between compaction and

performance is critical and it is vital that attention is paid to the compaction of the RLT

samples in the lab for valid results to be achieved. This needs to be investigated further,

especially when looking at test repeatability between Laboratories as the setups and

equipment for vibrating compaction carried out to NZS 4402 – Test 4.1.3, NZ Vibrating

hammer test, do vary as highlighted by Frobel & Moulding (2006). The work to date

shows that almost regardless of the high quality and consistency of the basecourse

produced at the quarry, under compaction on site will result in a hugely reduced

performance.

The over compacted sample showed improved performance, however not to the same

magnitude above target and there was significant extra compactive effort required to

achieve the +15%.

Water Content

Previous work has highlighted the influence of water as having a significant effect on

basecourse performance.




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11

Work conducted by Arnold (2003) has shown that increasing the water content reduces

the predicted performance of the basecourse and has led to the proposal that some

basecourses may only be suitable for use in dry conditions.

The preliminary findings support the previous work and water above OMC does have a

negative influence on performance as expected and this is shown in the graph. However,

RLT testing conducted on a non plastic TNZ M/4 for the degree of saturation tested to

have less influence than was thought on the basecourse performance.

This supports the use of performance testing to categorise individual materials on their

merits rather than the empirical approach adopted by specifications such as TNZ M/4.

Influence of Water

0

2

4

6

8

10

12

14

Degree of Saturation

Mil

lio

n E

SA

s t

o 1

0m

m r

ut

wit

hin

Ag

gre

gate

Dry Wet

Further tests, covering more source rock types and including Gap40 products have been

conducted and analysis of these results will add more understanding to the effect and

sensitivity of water on individual basecourse performance.




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Conclusions

Further research is required on the topics discussed in this paper to establish the validity

of these preliminary findings and relate them to a wider range of basecourse materials.

The findings presented show that the RLT test is capable, in conjunction with other

testing, of characterising basecourses and appears to have real potential in enabling

predicted performance and therefore classification of basecourse materials.

RLT testing has allowed WA to further develop knowledge of the performance

characteristics of the existing TNZ M/4 aggregates and Gap products. It has also given

the opportunity to investigate new materials as alternative basecourses and work is

continuing on evaluating and developing these products.

The RLT test could be specified in projects to allow aggregate producers to manufacture

the most suitable product that will better meet the performance requirements.

Winstone Aggregates focus is primarily on understanding and improving the performance

of our basecourse aggregates to ensure higher quality, better performing roads.




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13

References

Arnold, G. (2004). Rutting of Granular Pavements. PhD. University of Nottingham,

England, UK.

Arnold, G. & Werkmeister, S. (2006). Performance tests for selecting aggregate for

roads – report on progress.

Butkus, F. (2004, December). Reid highway basecourse test sections construction details

and performance to November 2003. (Volume 1), Pavements Engineering report

No. 2004/17 m, Main Roads, Western Australia.

Butkus, F. and Lee (1997). Pavement moduli project, a review of repeated load triaxial

test result.s Materials & pavement technology engineering report. No 97/4M, Main

Roads, Western Australia.

Frobel, T. and Moulding, S. (2006) Errors in vibrating hammer compaction test.

Inaugural Civil Engineering Laboratories Conference

Moore, G. (2007) Repeat Load Triaxial setup and implementation. Winstone Aggregates

Transit NZ (2006) Specification for basecourse aggregate (TNZ M/4). Transit New

Zealand, Wellington, New Zealand

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