A Towards the Revision of Austroads Design of Pavements With Cemented Materials

 AP-T167/10  A P-T167/10

 AUSTROADS TECHNICAL REPORT Towards the th e Revision vis ion of Aust A ustroads roads Procedures ro cedures fo r the t he De Design si gn of Pavement vements s Containi on taining ng Cemented Materials Materials

Towards the Revisio Revisio n of Austro ads Procedures Procedures for fo r th e Design of Pavements avements Containin g Cemented Cemented Materials Materials

Towards the Revisio Revisio n of Austro ads Procedures Procedures for fo r th e Design of Pavements avements Containin g Cemented Cemented Materials Materials

Towards the Revisio Revisio n of Au stroads Procedures for the Design Design of Pavements Pavements Containing Cemented Cemented Materials Published September 2010

© Austroads Ltd. 2010 This work is copyright. Apart from any use as permitted under the Copyright Act 1968, 1968, no part may be reproduced by any process without the prior written permission of Austroads.

Towards the Revisio Revisio n of Au stroads Procedures for the Design Design o f Pavements Pavements Containing Cemented Cemented Materials ISBN 978-1-921709-39-5

 Austroads Project Project No. TT1358  Austroads Publication Publication No. AP–T167/10 AP–T167/10

Project Manager  Allan Jones Prepared by Geoff Jameson

Published by Austroads Ltd. Level 9, Robell House 287 Elizabeth Street Sydney NSW 2000 Australia Phone: +61 2 9264 7088 Fax: +61 2 9264 1657 Email: [email protected] www.austroads.com.au

 Austroads believes believes this publication publication to be correct correct at the time of printing printing and does not not accept responsibility responsibility for any consequences arising arising from the use of information herein. herein. Readers should rely on their own skill and judgement to apply information to particular issues.

Towards the Revisio n of Austroads Procedures for th e Design of Pavements Containing Cemented Materials

Sydney 2010

 Au st roads p ro fi le  Austroads’ purpose is to contribute to improved Australian and New Zealand transport outcomes by: 

providing expert advice to SCOT and ATC on road and road transport issues



facilitating collaboration between road agencies



promoting harmonisation, consistency and uniformity in road and related operations



undertaking strategic research on behalf of road agencies and communicating outcomes



promoting improved and consistent practice by road agencies.

 Au st roads m emb ers hi p  Austroads membership comprises the six state and two territory road transport and traffic authorities, the Commonwealth Department of Infrastructure, Transport, Regional Development and Local Government, the Australian Local Government Association, and New Zealand Transport  Agency. Austroads is governed by a Board consisting of the chief executive officer (or an alternative senior executive officer) of each of its 11 member organisations: 

Roads and Traffic Authority New South Wales



Roads Corporation Victoria









Department of Transport and Main Roads Queensland Main Roads Western Australia Department for Transport, Energy and Infrastructure South Australia Department of Infrastructure, Energy and Resources Tasmania



Department of Lands and Planning Northern Territory Department of Territory and Municipal Services Australian Capital Territory



Department of Infrastructure, Transport, Regional Development and Local Government







 Australian Local Government Association New Zealand Transport Agency.

The success of Austroads is derived from the collaboration of member organisations and others in the road industry. It aims to be the Australasian leader in providing high quality information, advice and fostering research in the road sector.

Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

CONTENTS 1

INTRODUCTION ......................................................................................................................1

2

1987 AUSTROADS GUIDE ......................................................................................................2

3

VICROADS ADOPTED 8TH POWER RELATIONSHIP ............................................................4

3.1

Original Analysis and Conclusion .............................................................................................4 3.1.1 Laboratory Fatigue Relationship ................................................................................ 4 3.1.2 Field Fatigue Relationships ....................................................................................... 5 Further Analysis ......................................................................................................................11 3.2.1 Modulus Dependency ........................................................................................ ...... 11

3.2 4

1997 CHANGE TO 12TH POWER RELATIONSHIP ...............................................................17

5

RECENT RESEARCH FINDINGS ........................................................................................ ..18

5.1  Austroads Project TT1065 Findings .............................................. .........................................18 5.1.1 Laboratory Testing ................................................................................................... 18 5.1.2 Fatigue Under Accelerated Loading ........................................................................ 21 5.2  Austroads TT1359 Findings.......................................................... ..........................................25 6

DISCUSSION .........................................................................................................................27

7

CONCLUSIONS ......................................... .............................. .............................................. 28

REFERENCES ................................................................................................................................29

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Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

TABLES Table 3.1: Table 3.2:

Models to predict fatigue life to half modulus..............................................................8 Models to predict fatigue life to 1.0 m/m2 cracking ...................................................11

Table 5.1: Table 5.2:

Summary for flexural moduli data ............................... ............................... ...............18 Summary for breaking strain data......................................................... ....................19

Table 5.3:

Summary of models characterising the mean reduction in modulus for each axle load........................................................ ...................................................22

Table 5.4:

Summary of Load Damage Exponents (LDEs) based on mean data at each axle load........................................................ ...................................................22

Table 5.5:

Modulus, breaking strain and fatigue relationships for each material .......................26

FIGURES Figure 3.1:

Laboratory fatigue testing of field beams ........................................................... .........5

Figure 3.2: Figure 3.3:

Reduction in cemented materials modulus with accelerated loading .........................6 Fatigue relationships to half initial cemented material modulus .................................7

Figure 3.4: Figure 3.5:

Predicted strain correlation with back-calculated modulus .........................................8 Fatigue relationships to 1 m/m2 surface cracking......................................................10

Figure 3.6:

Predicted dependence of fatigue on cemented material modulus ............................ 11

Figure 3.7: Figure 3.8: Figure 3.9:

Comparison of back-calculated moduli with measured field core and beam values .............................................................................................................12 Reanalysis of accelerated loading data ....................................................................13 Prediction error variation with cemented material modulus ......................................14

Figure 3.10:

Factors influencing the variation in predicted strain..................................................14

Figure 3.11: Figure 3.12:

Relationship for sites with initial modulus less than 10 000 MPa..............................15 Relationship for sites with initial modulus greater than 10 000 MPa.........................16

Figure 5.1: Figure 5.2:

Crushed hornfels laboratory fatigue results of laboratory beams .............................20 Crushed siltstone laboratory fatigue results of laboratory beams .............................20

Figure 5.3:

Crushed hornfels fatigue results under accelerated loading: data lives 103 to 107  ..................................................................................................................23 Crushed siltstone fatigue results under accelerated loading: data lives 103 to 107  ..................................................................................................................24

Figure 5.4: Figure 5.5: Figure 5.6:

Crushed hornfels fatigue results under accelerated loading: data with initial moduli greater than 8000 MPa ............................................................ ............24 Crushed siltstone fatigue results under accelerated loading: data with initial moduli greater than 10 000 MPa .....................................................................25

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Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

SUMMARY The laboratory characterisation of cemented materials for road pavements has recently been completed in Austroads Project TT1359 Cost-Effective Structural Treatments for Rural Highways. This project concluded there is a need for a substantial revision to the thickness design procedures for cemented materials as provided in Austroads Guide to Pavement Technology – Part 2: Pavement Structural Design (Austroads 2010). This report provides a research strategy for this revision. It reviews: 



the origins of the current design procedures results of past and recent laboratory and accelerated loading trials on cemented materials modulus and fatigue.

Based on this review a research strategy is proposed to deliver the revised Austroads procedures.

 Au st roa ds 20 10 — iii —

Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

1

INTRODUCTION

The laboratory characterisation of cemented materials for road pavements has recently been completed in Austroads Project TT1359 Cost-Effective Structural Treatments for Rural Highways. This project concluded there is a need for a substantial revision to the thickness design procedures for cemented materials as provided in Austroads Guide to Pavement Technology – Part 2: Pavement Structural Design (Austroads 2010). This report provides a research strategy for this revision. In particular: 









Section 2 outlines the origins of the 18 th power fatigue relationship adopted in 1987  Austroads Guide (NAASRA 1987). Section 3 describes the finding of the 1990/91 Mulgrave accelerated loading trial of a high quality Victoria cement treated crushed rock and the associated laboratory testing which lead to VicRoads adopting an 8 th power fatigue relationship in 1993. Section 4 outlines the origins of the 12 th power fatigue relationship adopted in the Austroads Guide in 1997 and modified for project reliability in the 2004 Guide. Section 5 summarises the findings of the two recent Austroads research projects. Section 6 summarises the issues highlighted by this research review, issues which need to be addressed in amending the design procedures in Austroads Guide to Pavement Technology – Part 2: Pavement Structural Design (Austroads 2010).

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Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

2

1987 AUSTROADS GUIDE

Part 1 Section 4.3.2 of Austroads Technical Report AP-T33-04 Technical Basis of Austroads Pavement Design Guide (Austroads 2008b) describes the origins of the fatigue relationship adopted in the Austroads Pavement Design Guide (NAASRA 1987).  At the time of developing the Guide, the leading proponent within Australia of extensive use of cemented materials was the Main Roads Department, Queensland (MRDQ). Apace with this increased use, an increased appreciation of its performance developed within MRDQ. The Working Group (WG) availed itself of this knowledge base and was guided by them in the formation of fatigue relationships for cemented materials. The performance relationship in both the 1987 and 1992 versions of the Guide is (Equation 1): 18

N = (K/)

1

where N

=

is the number of repetitions of tensile strain at the bottom of the cemented layer before fatigue failure occurs, i.e. when the level of this strain is  microstrain.

The numerator K depends on the stiffness of the material as follows: Modulus of cemented material (MPa)

Value of K

2 000

280

5 000

200

10 000

150

In the development of pavement thickness design curves for Queensland conditions based on elastic analysis methods, Baran and Aubrey (1978) noted the following relationship (Equation 2) developed by Pretorius (1969). 20.3

N = (142/)

2

The modulus of the material was not known, but assumed to be > 10 000 MPa (later confirmed in Pretorius and Monismith (1972) to be 28 000 MPa). They also noted a (graphical) relationship between strain at break and modulus for cement-treated natural weathered gravel in Walker et al. (1977). The Pretorius relationship gave a tolerable strain level of 72  for 10  repetitions which, from the Walker et al. plot corresponded to 65% of the strain at break for materials stiffer than 10 000 MPa. This ratio (tolerable strain for 6 10  repetitions)/(strain at break) = 0.65 was adopted as being applicable to materials with moduli down to 2000 MPa. For a given modulus, the corresponding strain at break was determined from the Walker et al. plot and then multiplied by 6 0.65 to give the tolerable strain for 10  repetitions. In a similar manner, values of 5 7 this ratio for 10  and 10  repetitions were determined for the Pretorius material and applied to less stiff material. 6

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Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

On this basis, fatigue relationships were developed for materials of moduli 2000, 5 7 5000, 7000, and 10 000 MPa over the range 10  to 10 strain repetitions. These relationships were then used in the development of thickness design charts by Baran and Aubrey (1978). Angell (1988) reported that the relationships for the materials with moduli 2000, 5000 and  10 000 MPa were of the form (Equation 3): K2

N = (K1/µ)

3

with the values of K1 and K2 as follows: Modulus of cemented material (MPa)

K1

K2

2 000

259

19.9

5 000

244

14.5

 10 000

152

18.3

Litwinowicz (1982) undertook a review of the basis for these relationships and found that (in Angell’s words): the general level of these relationships appeared to be appropriate but that their exact form and slope still required further investigation. The WG, in reviewing these relationships, expressed some surprise that the value of the exponent (K2) did not change monotonically with the material modulus. Further investigations were undertaken and the relationships eventually adopted by the WG for inclusion in the 1987 Guide were recommended by Litwinowicz (1984, private communication) on the basis of his investigations. Subsequent to the WG’s adoption of the relationships with exponent 18, Angell (1988), in the course of development of a pavement design manual for MRDQ, undertook a further review of the literature and reported fatigue exponents of 32, 9, 12.7, 12.2 and 12 for relationships developed by workers in four countries. In light of this, together with his proposition that, if the true exponent were 18, then cement-treated pavements in Queensland would be failing very early in their design life because of vehicle overloading, Angell opted for an exponent of 12 and derived numerator (K1) values such that the revised relationships (in his words) ‘allow approximately the same levels of strain as the relationships previously used’.  Angell developed the following relationship (Equation 4): N = (K/)

12

4

with values of K as follows: Modulus of cemented material (MPa)

K1

K2

2 000

259

19.9

5 000

244

14.5

 10 000

152

18.3

The WG was apprised of MRDQ’s intention to adopt these r evised relationships while the original (1987 version) Guide was in press.

The WG decided it was too late to make the change to the 1987 Guide, and it was not until 1997 that the Guide was changed to a 12 th power relationship as discussed in Section 4.

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Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

3

VICROADS ADOPTED 8TH POWER RELATIONSHIP

3.1

Original Analysi s and Conclus ion

In 1990/91 Austroads investigated the fatigue performance of cement treated crushed rock (Jameson et al. 1992). The project included both laboratory fatigue testing and accelerated loading of 3% cement treated crushed rock, along the alignment of a proposed arterial in Mulgrave, Melbourne. The accelerated loading was undertaken using the Accelerated Loading Facility (ALF). 3.1.1

Labor atory Fatigue Relatio nsh ip

Laboratory third-point repeated flexure testing of a field beam was undertaken after more than 12 months field curing in the test pavements. The fatigue life was defined as the number of cycles when the modulus had decreased to half the initial value, these being determined after 25 cycles of bedding-in. The mean initial flexural modulus of these field beams was about 5300 MPa. The fatigue data are plotted in Figure 3.1. In reporting the fatigue results Jameson et al. (1992) mentioned that initially regression analysis was undertaken with the logarithm of the strain as the independent variable (x axis) and the logarithm of the fatigue life as the dependent or observed variable (y axis). This is the correct procedure to enable fatigue life to be estimated from strain provided the error in strain is substantially less than the error in fatigue life. This relationship is also shown in Figure 3.1.  As argued by Jameson et al. (1992) and more recently confirmed by a more rigorous orthogonal analysis of fatigue data (Gonzalez et al., 2010), undertaking the regression with the logarithm of the fatigue life as the independent variable and the logarithm of strain as the dependent variable is more appropriate. As shown Figure 3.1, the strain dependency of this latter relationship is about 8, considerably greater than when fatigue life is the dependent variable. Consequently, the laboratory fatigue testing of the field beams indicated fatigue life was inversely related to about 7-8 power of applied strain. Note that the slope was similar when beams with fatigue lives less than 1000 were deleted. The average breaking strain of the field beams was about 600 microstrain, whereas for laboratory compacted beams the average was about 145 microstrain; the latter is consistent with recent laboratory testing of laboratory compacted beams (Section 5.2). The much higher and variable breaking strain of field beams was also reported in another recent Austroads Project TT1065 (Section 5.1). This difference in the characteristics of laboratory beams and in-service cemented material is an important consideration in the future strategy to revise the Austroads design procedures. These high breaking strains of the aged field beams suggest that the fatigue results may have been influenced by micro-cracking. This may explain why fatigue results are less dependent on strain (strain exponent about 8) than observed in recent test results of laboratory compacted and cured beams, (average strain exponent 22 as described in Section 5.2).

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Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

240 data

220

Life dependent variable Strain dependent variable

200 180

   )   n    i   a   r 160    t   s   o   r   c    i 140   m    (   n    i   a   r 120    t   s   e    l    i   s 100   n   e    T

80 60 40 20

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

Load repetitions to half initial modulus

Source: Jameson et al. 1992.

Figure 3.1:

3.1.2

Laboratory fatigue testing of field beams

Field Fatigue Relatio nsh ips

Over 2 million cycles of accelerated loading were applied to 14 field experiments using dual wheel loading of 40 kN, 60 kN and 80 kN. To develop fatigue relationships, for each chainage of each experiment the initial layer moduli were back-calculated from the measured Falling Weight Deflectometer deflections. In terms of the cement treated crushed rock, the mean initial backcalculated modulus was 10 900 MPa, significantly higher than the mean f lexural moduli of the field beams (5300 MPa): this modulus difference may have been due to the variability of modulus along the test section (Section 3.2.1, Figure 3.7). Using these layer moduli, the initial strains under the applied loading were predicted. To develop fatigue relationships, these initial strains need to be related to the observed cemented materials fatigue life. This required a definition of the condition of the material at the end of fatigue life and two definitions were adopted in the analysis. Fatigue life to half initial modulus In the construction of new pavements cemented materials where the cover the cemented material is commonly 175 mm of asphalt or more it is assumed in the Austroads Guide that when the cemented material is cracked, the cracks will readily not reflect through to the surface and the pavement has a post-cracking service life. For such pavements it is not the extent and severity of cracking in the cemented material which determines the pavement life, but commonly fatigue of the overlying asphalt. In such cases the pavement life is influenced by the magnitude of modulus of the cemented material. Jameson et al. (1992) adopted a conservative value for cemented materials fatigue life as the number of loading cycles for the modulus to decrease to half the initial value. As seen from Figure 3.2, on average this fatigue life occurred about 1/10 th the number of cycles to surface cracking.

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Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

14000

12000

10000

Cemented 8000 material modulus  (MPa) 6000

4000

2000

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

2

Ratio of cycles to cycles at 1.0 m/m  surface cracking

Source: Jameson et al. 1992.

Figure 3.2:

Reduction in cemented materials modul us with accelerated loading

The fatigue lives so calculated and the associated predicted tensile strains data are plotted in Figure 3.3. At some chainages the cemented material had not decreased to half the initial modulus at the completion of trafficking. These chainages are marked as ‘Censored data’, whereas those that did reach half modulus are marked ‘Observed data’.

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3.0

Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

550 Observed data 500

Censored data Fatigue relationship without constraining modulus dependency

450 Fatigue relationship calculated with QDOT modulus dependency Fatigue relationship calculated with Austroads modulus dependency

400

350 Tensile strain (microstrain)

300

250

200

150

100

50

0 1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+07

 Load repetitions to half the initail modulus

Source: Adapted from Jameson et al. 1992.

Figure 3.3:

Fatigue relationshi ps to half initi al cemented material modulu s

The Maximum Likelihood Estimation (MLE) procedure was used to derive possible fatigue relationships as this allowed both observed and censored data to be considered. Table 3.1 summarises three fatigue relationships derived assuming fatigue life was related to applied strain and cemented materials modulus as follows (Equation 5): Ln(N) = a + bln(Strain) + cln(E) where N

=

number of load repetitions to half initial modulus

E

=

cemented materials modulus (MPa)

Strain

=

maximum horizontal tensile strain at the base of the cemented material (microstrain).

The coefficients derived from the MLE procedure are listed in Table 3.1. The beta values indicate how well the relationship explains the variability in results: the higher t he beta value the better the fit to the data.

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5

Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

Table 3.1: Models to predict fatigue life to half modulus Constraint on modulus dependency

Constant

Modulus coefficient c

Beta

a

Strain coefficient b

Modulus unconstrained

139.7

-12.1

-7.5

9.3

 Austroads modulus coefficient

130.6

-11.5

-7.0

9.2

QDoT modulus coefficient

82.5

-7.9

-3.6

7.8

Source: Jameson et al. 1992.

One key assumption of the analysis is that predicted strain and back-calculated modulus are independent variables. However, as shown in Figure 3.4, strain values are correlated with the modulus which is not unexpected as the strains values were predicted using linear elastic modelling which included cemented material modulus as an input. Hence strain and modulus data in the analysis are not independent which contravenes the assumption in the ‘modulus unconstrained’ analysis and influences the reliability of the fatigue relationships derived. Hence Jameson et al. (1992) also undertook the MLE analysis with modulus dependency fixed to the  Austroads and Queensland Department of Transport (QDoT) values at that time. As there was doubt about whether fatigue life varied with modulus at all, the fatigue relationship calculated with QDoT modulus dependency was favoured as it was less dependent on modulus. These three fatigue relationships are plotted in Figure 3.3 for a mean back-calculated cemented material of modulus of 10 900 MPa. Rounding off the relationship determined using the QDoT modulus dependency, Jameson et al. (1992) derived the following relationship (Equation 6) to predict the mean fatigue life to half the initial modulus: 1000

   )   n    i   a   r    t   s   o   r   c    i   m    (   n 100    i   a   r    t   s   e    l    i   s   n   e    T

10 1000

10000 Cemented material modulus (MPa)

Source: Adapted from Jameson et al. 1992.

Figure 3.4:

Predicted strain corr elation with back-calculated modulu s

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100000

Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

 35,000  X=N  0.45   μεE 

8

6

where N

=

allowable number of repetitions of the load

µε

=

maximum horizontal tensile strain (microstrain)

E

=

modulus of cemented material (MPa).

The use of the 8th power relationship was supported by the results of a parallel laboratory testing program (Section 3.1.1) from which a load damage exponent of 7-8 was calculated (Figure 3.1). Note that although the unconstrained modulus analysis resulted in a strain damage exponent of 12 and this provided a better fit to the data, this relationship was not favoured as the higher modulus dependency was not supported by the literature and recent laboratory testing which indicated breaking strain as the material parameter. Based on these project findings, after allowance for the fact that the data was only obtained for one high quality material and considering the impacts of pavement thicknesses VicRoads (1993) adopted (Equation 7):

 14,100  N  RF  0.351   μεE 

8

7

where N

=

allowable number of repetitions of the load

µε

=

maximum horizontal tensile strain (microstrain)

E

=

modulus of cemented material (MPa)

RF

=

reliability factor, (e.g. 1/6 for freeways).

None of the other state road agencies adopted an 8 th power fatigue relationship based on the project findings. There was concern that the project had only tested one cemented material and hence the data was insufficient to change the 18 th power fatigue relationship (Equation 1) then in the Austroads Guide. Fatigue life to surface cracking For pavements where the cover over the cemented material is less than 175 mm of asphalt it is assumed in the Austroads Guide that when the cemented material is cracked, the cracks will readily reflect through to the surface. For such pavements the fatigue life is best related to the severity and extent of surface cracking. To provide fatigue relationships for these pavement types, the fatigue life was defined as the first appearance of fine surface cracking (1.0 m/m 2 cracking severity). At this amount of cracking, the modulus had reduced to about 10% of its initial value (Figure 3.2).

 Au st roa ds 20 10 —9—

Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

The data is plotted in Figure 3.5. At some chainages the cemented material had not reached 1.0 m/m2 cracking severity at the completion of trafficking. These chainages are marked as ‘Censored data’, whereas those that did reach 1.0 m/m 2 cracking severity are marked ‘Observed data’. 550 Observed data

500

Censored data Modulus unconstrained

450

Modulus constrained to QDoT co-efficient Modulus constrained to Austroads co-efficient

400    )   n    i 350   a   r    t   s   o   r   c 300    i   m    (   n    i   a   r 250    t   s   e    l    i   s   n 200   e    T

150

100

50

0 1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

Load repetitions to 1.0m/m2 cracking

Source: Adapted from Jameson et al. 1992.

Figure 3.5:

Fatigue relationshi ps to 1 m/m2 surface cracking

 Again, the Maximum Likelihood Estimation procedure was used to derive possible fatigue relationships as this allowed both observed and censored data to be considered. Table 3.2 summarises three fatigue relationships derived using the MLE procedure and these are plotted in Figure 3.5 for a mean modulus of 10 900 MPa. Note again the higher strain damage exponent (14.9) of the unconstrained analysis and the associated high modulus dependency. While this relationship provided a better fit to the data, this relationship was not favoured by Jameson et al. (1992) as: 



the higher modulus dependency was not supported by the literature and recent laboratory testing which indicated breaking strain as the material parameter modulus and strain variables were not independent as discussed above.

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Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

Table 3.2: Models to predict fatig ue life to 1.0 m/m2 cracking Constraint on modulus dependency

Constant

Modulus coefficient c

Beta

a

Strain coefficient b

Modulus unconstrained

171.3

-14.9

-9.6

11.6

 Austroads modulus coefficient

138.3

-12.3

-7.0

10.9

QDoT modulus coefficient

89.4

-8.7

-3.6

8.7

Source: Jameson et al. 1992.

3.2

Further Analysi s

3.2.1

Modulu s Dependenc y

The unexpected finding from the accelerated loading trial data was the very high dependency of fatigue life on modulus as seen from the modulus coefficients in Table 3.1 and Table 3.2. As illustrated in Figure 3.6, if the modulus is doubled from 5000 MPa to 10 000 MPa, the predicted fatigue life for a given strain decreases by about a factor of 100. This high dependency was not accepted by Jameson et al. 1992 and they recommended a relationship which included the modulus dependency in the QDOT relationship.  An associated unexpected finding was the high variability in surface maximum deflections and curvature (D0-D200) between experiments conducted on a 100 m long test lane with notionally identical pavement thickness and composition (Figure 3.7). The back-calculated cemented material moduli reflected this variation in measured curvature. The reason for this curvature/modulus variation needs to be understood to reconcile the varying findings of research projects. Consequently, the results were reconsidered in this review. 550

500

Observed data Censored data

450

Maximum Likelihood Estimation E 3000 MPa Maximum Likelihood Estimation E 5000 MPa Maximum Likelihood Estimation E 10,000 MPa

400

Maximum Likelihood Estimation E 15,000 MPa

   )   n    i 350   a   r    t   s   o   r   c    i 300   m    (   n    i   a   r 250    t   s   e    l    i   s   n 200   e    T

E=3000 MPa

E=5000 MPa

150

E=10,000 MPa

100

E=15,000 MPa 50

0 1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

 Load repetitions to half initi al modulus

Source: Adapted from Jameson et al. 1992.

Figure 3.6:

Predicted dependence of fatigue on cemented material modulus

 Au st roa ds 20 10 — 11 —

1.0E+07

Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

20000

0.18

18000

0.16 Exp.21

Exp.6

   ) 16000   a    P    M    (   s 14000   u    l   u    d   o   m12000    l   a    i   r   e    t   a 10000   m    d   e    t   n 8000   e   m   e    C 6000

Exp. 18 & 19

Exp. 4 & 5

Exp.20

Measured compressive modulus of field cores

   0    0

0.14    2

Measured flexural modulus field beams Back-calculated modulus

0.12

Initial 60 kN Benkelman Beam Curvatures D0-D200 0.10

0.08

0.06

0.04

4000

0.02

2000

0.00

   D      0    D   e   r   u    t   a   v   r   u    C   )   s   e   m   t   r   a   e   e    B   m   n   o   r   a   i   c   m  m    l   e   (    k   n   e    B    N    k    0    6    l   a    i    t    i   n    I

8380 8385 8390 8395 8400 84 05 8410 84 15 8420 8425 84 30 8435 8440 8 445 8450 8455 8460 8465 8470 8475 8480 8485 8490 849 5

Chainage

Source: Adapted from Jameson et al. 1992.

Figure 3.7:

Comparison of back-calcul ated modul i with measured field core and beam values

In the original analysis (Jameson et al. 1992), the Maximum Likelihood Estimation procedure was used to develop fatigue relationships as this technique enables inclusion of the censored data in the analysis. Linear regression is a simpler analysis procedure readily undertaken in spreadsheets, but is unable to consider the fact that censored data has not met the definition of fatigue failure. To assess the significance of this limitation, the half-modulus fatigue relationship obtained by linear regression was calculated for comparison with the Maximum Likelihood Estimate. The results are compared in Figure 3.8 and indicate that the linear regression line is very similar to the original Maximum Likelihood Estimation relationship. This provides confidence that analysis by linear regression (with life as the dependent variable as discussed in Section 3.1.1) is reasonable for this dataset.

 Au st roa ds 20 10 — 12 —

Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

550 Observed data

500

Censored data Original Maximum Likelihood Estimation- modulus as a variable

450

Linear regression modulus as a variable

Data deleted

400

Linear analysis without modulus and with very low life deleted

   )   n    i   a   r 350    t   s   o   r   c    i 300   m    (   n    i   a   r 250    t   s   e    l    i   s   n 200   e    T

150 100 50 0 1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+07

 Load repetitions to half ini tial modulus

Figure 3.8:

Reanalysis of accelerated loading data

Linear regression was used to derive a fatigue relationship without modulus as a variable and with the four data points with fatigue life less than 10 3 cycles deleted as follows (Equation 8):

 433  N    με 

9.18

where N

=

allowable number of repetitions of the load

µε

=

maximum horizontal tensile strain (microstrain).

Equation 8 suggests the strain dependency decreases significantly (from about 12 to about 9) when modulus is not considered in the analysis and the very low life data points are deleted. However, due to the deletion of the modulus variable this fatigue relationship is a poor fit to the data and when the ratio of the predicted fatigue lives to the observed lives is plotted against cemented materials modulus (Figure 3.9), it is apparent the relationship over-predicts the life at high modulus and under-predicts at low modulus values.

 Au st roa ds 20 10 — 13 —

8

Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

100000 Exp. 4, 150 mm, 40 kN Exp.6, 150 mm, 80 kN Exp.8, 2x90 mm, 80 kN Exp.17, 2 x 90 mm, 80 kN

   )   a    P    M    (   s   u    l   u    d   o   m    l   a    i   r   e    t   a   m    d   e    t   e   n   m   e    C

Exp.18, 150 mm, 60 kN Exp.20, 150 mm, 80 kN Exp.21, 150 mm, 80 kN

10000

1000 0.001

0.010

0.100

1.000

10.000

100.000

1000.000

Predicted life to half modulus/observed life to half modulus

Figure 3.9:

Prediction error variation with cemented material modul us

It is apparent from Figure 3.10 that the twofold increase in strain due to variation in the applied load (40 kN to 80 kN) was only a minor part of the overall variation in predicted strain, with the high variation in cemented materials modulus along the test pavements the predominant source of the strain variation. 1000

Exp. 4, 150 mm, 40 kN Exp. 6, 150 mm, 80 kN Exp.8, 2x90 mm, 80 kN Exp.17, 2 x 90 mm, 80 kN Exp.18, 150 mm, 60 kN Exp.20, 150 mm, 80 kN Exp.21, 150 mm, 80 kN

   )   n    i   a   r    t   s   o   r   c    i   m    (   n    i   a   r    t   s   e    l    i   s   n   e    T

100

10 1000

10000 Cemented material modulu s (MPa)

Figure 3.10: Factors influencin g the variation in predicted strain

 Au st roa ds 20 10 — 14 —

100000

Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

Back-calculation of modulus from measured surface deflection is an inexact process and it is recognised that this is a significant source of part of the modulus variation. Nevertheless, the surface deflection did vary markedly, indicating genuine structural variation. A possible hypothesis for the large variation in modulus is that the low moduli were due to shrinkage micro-cracking during drying due to differing thermal expansion and contraction of various components (aggregates and hardened cement paste). In addition, load-induced micro-cracking prior to the commencement of accelerated loading may have occurred during the construction of the thin asphalt surfacing about a month after the 150 mm thick cemented material was placed. Given the subgrade under the cemented material was low strength (in situ CBR about seven at time of construction), delivery trucks and rollers may have induced micro-cracking in the cemented material. Consequently, it was of interest to reanalyse the data in two sets with initial moduli less and greater than 10 000 MPa, a modulus level near the mean (10 900 MPa). The relationships are shown in Figure 3.11 and Figure 3.12 and clearly demonstrate that the higher modulus material has a higher dependence on strain than the lower modulus material. However, the question remains how should such modulus variation be addressed in routine pavement design? Does surface cracking occur at areas with initial low modulus before areas of high modulus? This issue is discussed further in Section 6. Initial cemented material modulus less than 10,000 MPa

550

500 Observed data

450 Data deleted

Censored data

400    )   n    i   a 350   r    t   s   o   r   c    i 300   m    (   n    i   a   r 250    t   s   e    l    i   s   n 200   e    T

150

100

50

0 1.0E+02

1.0E+03

1.0E+04

1.0E+05

 Load repetitions to half ini tial modulus

Figure 3.11: Relations hip for si tes with ini tial modulu s less than 10 000 MPa

 Au st roa ds 20 10 — 15 —

1.0E+06

Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

Initial cemented material modulus greater than 10,000 MPa

200 Observed data 180 Censored data

160    )   n    i   a   r 140    t   s   o   r   c    i   m    (   n 120    i   a   r    t   s   e    l    i   s 100   n   e    T

80

60

40 1.0E+03

1.0E+04

1.0E+05

1.0E+06

 Load repetitions to half initial modulus

Figure 3.12: Relations hip for si tes with ini tial modulu s greater than 10 000 MPa

 Au st roa ds 20 10 — 16 —

1.0E+07

Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

4

1997 CHANGE TO 12TH POWER RELA TIONSHIP

In 1994 an accelerated loading trial adjacent to the Monaro Highway, Cooma New South Wales, was carried out to assess the performance of deep-lift in situ stabilisation using cementitious binders (Jameson et al. 1995). The gravel chosen for stabilisation was an extremely weathered granite of subbase quality similar to that on existing flexible pavements in south-eastern New South Wales. The gravel was stabilised with 5% by mass of stabilising agent consisting of 85% ground granulated slag and 15% hydrated lime. Five experiments were carried out on stabilised pavements 250 mm, 300 mm and 360 mm thick. The report concluded that the Austroads 18 th power fatigue relationship (Equation 1) generally under-predicted the fatigue life of the trial material, whilst the Queensland Department of Main Roads (QDMR) 12 th power (Equation 4) and the VicRoads 8th power (Equation 6) relationships better predicted performance. Based on the trial findings, a literature review (Jameson 1995) and QDMR and VicRoads relationships in use in 1997, a revision to the Guide was issued by Austroads which replaced the 18th power fatigue relationships by the following QDMR 12 th power relationship (Equation 9):

N = (K/)12

9

and the numerator K depended on the stiffness of the material as follows: Modulus of cemented material (MPa)

Value of K

2 000

440

3 500

350

5 000

310

10 000

260

15 000

240

In 2004 the Austroads Pavement Design Guide was published and the above K factors were used to derive the following fatigue relationship (Equation 10) suitable for any modulus in the range 2000 MPa to 10 000 MPa:

   0.804  191      113,000/E   N  RF  με    

12

where N

=

allowable number of repetitions of the load



=

tensile strain produced by the load (microstrain)

E

=

cemented material modulus (MPa)

RF

=

reliability factor for cemented materials fatigue.

 Au st roa ds 20 10 — 17 —

10

Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

5

RECENT RESEARCH FINDINGS

5.1

Aust roads Project TT1065 Finding s

Following the publication of the 2004 Austroads Pavement Design Guide, further research on cemented materials fatigue has been undertaken. During 2005-2006, over 3.3 million load cycles have been applied to two cemented materials using the Accelerated Loading Facility (ALF) as part of an Austroads funded research project (TT1065 Influence of Vertical Loading on the Performance of Unbound and Cemented Materials). The two test pavements, one comprising a crushed siltstone from Para Hills South Australia stabilised with 4% cement and the other crushed hornfels from Lysterfield Victoria stabilised with 3% cement, were constructed in late 2004 and early 2005 and then left to cure for about six months prior to accelerated loading. 5.1.1

Labor atory Testing

 A program of laboratory testing was conducted as part of the project (Austroads, 2008c). The testing program included both field beams cut from the test site and laboratory manufactured beams. Some of the laboratory beams were tested after 28 days moisture curing, while others were tested about 20 months after manufacture. These later beams were initially cured for three months, then removed from the fog room and stored on pallets, causing them to fully dry out.  Although the samples were placed back in the fog room for a minimum 48 hours prior to testing, the wet/dry/wet cycle is thought to have influenced the material properties. For instance, the siltstone flexural modulus of the 500 days old laboratory beams was about half the value of the beams cured for 32 days. Some of the flexural modulus results are summarised in Table 5.1. Table 5.1: Summary for flexural moduli data Material

Crushed hornfels 3% cement

Crushed siltstone 4% cement

Field beams

Laboratory manufactured beams

 Age (days )

Mean modulus (MPa)

Mean relative density (%)

30

14 740

96.4

90+

3 800

34 90+

 Age (days )

Mean modulus (MPa)

Mean relative density (%)

28

16 560

96.4

-

500+

12 500

-

9 220

98.0

32

11 030

94.1

9 500

-

500+

6 800

-

Source: Austroads 2008c.

The breaking strains are summarised in Table 5.2. As seen from Table 5.2, at 28 days the hornfels field beams are more highly variable than the laboratory manufactured beams, probably due to shrinkage cracking increasing the breaking strain of some of the field beams. After further wet/dry/wet curing the breaking strain increased, but the results were more variable possible due to micro-cracking in the wet/dry/wet curing period.

 Au st roa ds 20 10 — 18 —

Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

Table 5.2: Summary for breaking strain data Material

Crushed hornfels 3% cement

Crushed siltstone 4% cement

Field beams

Laboratory manufactured beams

 Age (days )

Mean breaking strain (microstrain)

Range breaking strains (microstrain)

28

287

78-532

-

-

-

 Age (days )

Mean breaking strain (microstrain)

Range breaking strains (microstrain)

28

95

91-116

-

500+

178

129-247

-

-

28

339

177-500 (two beam tested)

-

-

500+

467

295-634

Source: Austroads 2008c.

In terms of laboratory fatigue testing there were insufficient field beam results to develop a fatigue relationship. However, fatigue data on laboratory manufactured beams was available after 28 days moist curing and about 20 months wet/dry/wet curing. The data are shown in Figure 5.1 and Figure 5.2, for the hornfels and siltstone materials respectively. As part of this review, regression analyses of the data were undertaken with the logarithm of strain as the dependent variable (refer discussion Section 3.1.1). In addition, the data was analysed in the following three ways: 





using only the beams that had reduced to half modulus during the testing, without consideration of the beams that did not reduce to half initial modulus by the end of testing using data from all beams, including those that had not reached half modulus when the loading ceased (for these beams the fatigue life was conservatively estimated as the load repetitions when the loading ceased) using data from all beams except those with observed fatigue lives less t han 1000, these were considered unreliable and below the scope of loading addressed by the Austroads design procedures.

The various fatigue relationships are plotted in Figure 5.1 and Figure 5.2. Note that the strain damage exponents were higher than those reported by Austroads (2008c) principally due to the logarithm of strain being the dependent variable in the regression analysis rather than logarithm of life (refer Section 3.1.1). For crushed hornfels, the slope of the regression was high both for the beams tested at 28 days and 500+ days. It was noted that both the breaking strains (Table 5.2) and the tolerable strains for a given life (Figure 5.1) were significantly higher for the 500+ day old sample. This may have been due to micro-cracking of the test beam during the wet/dry/wet curing process. The strain damage exponent for crushed siltstone was about 12, considerably lower than the crushed hornfels value (29.6). For a given strain, the crushed siltstone generally had higher fatigue life than the crushed hornfels consistent with its higher breaking strain (Table 5.2). Note however, that the siltstone fatigue results were variable and hence the 95% confidence limits on the strain damage exponent were high (95% lower limit 8, 95% higher limit 43). As seen from Table 5.2, the breaking strains of the 500+ day old siltstone were very high and suggest that the fatigue results may have been influenced by micro-cracking produced in the wet/dry/wet curing process. The presence of micro-cracking was also reinforced by the moduli (Table 5.1) at 500+ days being substantially lower than the values after 32 days. Such micro-cracking may explain

 Au st roa ds 20 10 — 19 —

Towards the Revision o f Austro ads Procedures for the Design of Pavements Containi ng Cemented Materials

why the strain damage exponent of the siltstone was significantly lower than that calculated from the hornfels results. Crushed hornfels 160 Lab beams, 28 days, observed data Lab beams, 28 days censored data Lab beams, 500+ days, observed data lab beams, 500+ days, censored data Regression 500+ days without censored data Regression 500+ days with censored data Regression 500+ days with censored data, lives