Updating Bituminous Stabilized Materials Guidelines: Mix Design Report, Phase II

Transcription

1 APPENDIX F Technical Memorandum Updating Bituminous Stabilized Materials Guidelines: Mix Design Report, Phase II Task 6: Advanced Classification System: Part II AUTHORS: Final Report: Sept 2008 KJ Jenkins ME Twagira

2 1 BACKGROUND AND INTRODUCTION 1.1. Terms of Reference As part of advanced BSMs classification system, additional dynamic triaxial tests were required to be performed on BSM-emulsion. Nevertheless, more specimens were procured from Shedgum road in Saudi Arabia and included Task 6. The mix compositions and test variables were selected to populate the matrix of results according to the need identified on Task 5. The advance triaxial additional testing includes the evaluation of a range of variables in relation to dynamic tests namely, Resilient Modulus and Permanent Deformation. In order to develop a more robust database to enhance reliable performance limits of BSMs, the influence of the following variables were included; frequency (0.1Hz, 0.5Hz, 1Hz, 2Hz, and 5Hz) load wave ( Havesine and square sine) rest period (short and long on havesine) Confining pressure (50kPa, 100kPa and 200kPa) Temperature (10oC, 25 o C, 40 o C and 60 o C) Stress ratio (40% and 55%) Each of these variables tested on materials collected from N7 rehabilitation project. However, Shedgum Road specimens were only tested for temperature influence. The BSM-emulsion on N7 materials and Shedgum specimens takes cognisance of RAP percentage in the mix and its influence on the performance limits. The analysis of critical sets ratio and their link to permanent deformation provide knowledge and understanding of the performance estimation of BSMs. In addition, reliable limits on robust classification can be established for heuristic pavement design model. However, variables such as binder types, aggregate types, active fillers, as well as the impact on material properties requires further investigation Task Objectives In terms of the Inception Study report, the objectives of Task 6 were to: Identify the gaps on triaxial test data in Task 5, and carry out additional advance testing on BSM-emulsion. Select the mix, which take cognisance of the RAP percentage for the influence in the limit. Investigate critical stress ratio and their link to performance estimation. Establish a more robust classification system using additional data and link the limits to the heuristic pavement design model. 2

3 2. TESTING PROGRAM 2.1. Materials and specimen preparation The unstabilised fresh materials (Hornfels-RAP) for this study were procured from the N7 road rehabilitation project. The project involved pulverisation of the existing pavement layers. The recycled layer was limited to 250mm in depth, before stabilisation the materials was collected behind recycler as shown in Figure F.1. Figure F1. Collection of fresh materials (unstabilised) from N7 site. The stabilised materials with 3.3% bitumen emulsion and 1% cement was also collected from N7 site, behind the grader before compaction. The collected materials were brought to the laboratory for preparation and testing. The grading, maximum dry density and optimum moisture content of the collected materials are reported in Task 9. The bitumen emulsion type AniB SS-60 stable grade procured from COLAS was used in recycling process. Additional specimens from Shedgum Road were cores extracted from existing pavement recycled using foamed bitumen. Laboratory specimen preparations of the collected materials including, mixing, compaction and curing are presented in detailed in Task 9. 3

4 2.2. Triaxial testing matrix The triaxial testing matrix performed on collected BSM-emulsion stabilised N7 materials are indicated in Table F1 to F3, as follows. Table F1. Summary of monotonic testing matrix performed on the site stabilised N7 materials Temperature [ o C] Strain Rate [2.1 mm/min] Confining pressure [kpa] Specimen name (two specimen, one repeat) 25 M1 M2 M3 M4 M5 M6 Table F2. Summary of short dynamic testing matrix performed on the site stabilised N7 materials. Temperature [ o C] Specimen [no.] Frequency [0.1Hz, 0.5Hz, 1.0Hz, 2.0Hz, 5.0Hz] Loading wave- haversine with no rest period Confining pressure [kpa] Stress ratio [%] Two specimen, one repeat Table F3. Summary of long dynamic testing matrix performed on the site stabilised N7 materials. Temperature [ o C] Frequency [2.0Hz] Loading wave- haversine with no rest period Confining pressure [kpa] 50 Stress ratio [%] Specimen name [two specimen, one repeat] L9 L10 (R) L11 L12 (R) 4

5 The triaxial testing performed on cores extracted from Shedgum road are indicated in Table F4 as follows. Table F4. Summary of monotonic testing matrix performed on the site stabilised N7 materials Temperature [ o C] Strain Rate [2.1 mm/min] Confining pressure [kpa] Specimen name (two specimen, one repeat) 40 LC2 ST 02 LC2 ST 04 LC2 ST10 Table F5. Summary of long dynamic testing matrix performed on the site stabilised N7 materials. Temperature Frequency [2.0Hz] [ o C] Loading wave- haversine with no rest period Confining pressure [kpa] 100 Stress ratio [%] 40 Specimen name 40 LC2 ST08 50 LC2 ST04 60 LC2 ST Triaxial testing procedure The triaxial testing procedure for monotonic, short dynamic and long dynamic were performed in accordance with Stellenbosch University testing procedure (Ebels et al. 2005). 5

6 3. TESTS RESULTS 3.1. Monotonic triaxial test Table F6. Summary of the monotonic tests results of BSM-emulsion stabilised materials collected from N7 site. Specimen σ3 maximum load displ. at failure corrected strain at failure σaf σ1 Etan Esec [kpa] [kn] [mm] [%] [kpa] [kpa] [MPa] [MPa] M M M M M M A summary of the resilient modulus of BSM-emulsion stabilised materials collected from N7 site are presented in Table F7. The results are presented in accordance with the testing matrix indicated in Table F1 above. Table F7. Summary of resilient modulus test results of BSM-emulsion from N7 materials tested at 25 o C. 6

7 4. ANALYSIS AND DISCUSSION OF RESULTS 4.1. Monotonic test results for N7 stabilised materials A summary of superimposed monotonic results for different confining pressure are indicated in Figure F2. From the graphs it can be seen that applied stress at 25kPa and 50kPa has variability. This variability might have occurred due to marginal increase in pressures, which is also difficult to control during air pressure adjustment in the triaxial cell. Nevertheless, the trend indicates that applied stress increase as confining pressure increase. This is typical behaviour for the BSMs. Figure F2. Superimposed stress-strain graphs of BSM-emulsion for N7 materials tested at 25 o C Figure F3. Mohr-Coulomb diagram of BSM-emulsion for N7 materials tested at 25 o C 7

8 From the Mohr Coulomb values Figure F3, it can be seen that the shear parameters of the field stabilised BSM-emulsion from N7 are; cohesion = 135MPa and Friction angle = 43 o. These values are possible for BSMs with inclusion of RAP materials. The Cohesion and Friction angle being reduced due aggregate encapsulate in old bitumen Resilient modulus results of N7 stabilised materials The short duration dynamic triaxial test is performed to determine the resilient stiffness of pavement materials. The studies have indicated that resilient response of granular materials under short duration dynamic loading is significantly influenced by: Stress level (confirming procure) Degree of saturation Filler content, Aggregates properties Hicks and Monismith (1971) indicted that the resilient response of untreated granular materials is most significantly affected by the stress level and ca therefore related to the confining pressure, or bulk stress, Ө= σ1 + σ2 + σ3. The stress dependent behaviour of BSM-emulsion from N7 materials is shown in Figure F4 to Figure F13. From the stiffness modulus results in Table F7, it can be seen that deviator stress ratios have insignificant influence on stiffness behaviour of BSM-emulsion with different loading frequencies. Likewise there is no trend on the stiffness behaviour with relation to confining pressure and loading frequency. However the relation between confining pressure and stress ratio indicated that stiffness of the BSM-emulsion increase as stress ratio and confining pressure increases. The ranges of stiffness of BSM-emulsion vary from 330MPa to 960Mpa depending on confinement pressure and applied stress levels. The variability in stiffness behaviour on a repeat test specimen indicates that the specimens might have different volumetric properties. The compaction levels, moisture after curing and control of air pressure at lower confinement pressure have significant influence on stiffness behaviour. The parameters of the M r -Ө model of the BSM-emulsion are indicated in Figure F4 to F13. It can be seen that good correlation were found. Similar stiffness trend in magnitude have been indicated in the past researches, Jenkins (2000) for the BSM-foam and Ebels (2008) for the BSM-emulsion and BSM-foam. 8

9 Figure F4. Stress dependent stiffness behaviour of BSM-emulsion of N7 tested at 0.1Hz and 25 o C Figure F5. Stress dependent stiffness behaviour of BSM-emulsion of N7 tested at 0.5Hz and 25 o C Figure F6. Stress dependent stiffness behaviour of BSM-emulsion of N7 tested at 1Hz and 25 o C 9

10 Figure F7. Stress dependent stiffness behaviour of BSM-emulsion of N7 tested at 2Hz and 25 o C Figure F8. Stress dependent stiffness behaviour of BSM-emulsion of N7 tested at 5Hz and 25 o C Figure F9. Stress dependent stiffness behaviour of BSM-emulsion of N7 tested at 0.1Hz and 25 o C (repeat test) 10

11 Figure F10. Stress dependent stiffness behaviour of BSM-emulsion of N7 tested at 0.1Hz and 25 o C Figure F11. Stress dependent stiffness behaviour of BSM-emulsion of N7 tested at 0.1Hz and 25 o C Figure F12. Stress dependent stiffness behaviour of BSM-emulsion of N7 tested at 0.1Hz and 25 o C 11

12 Figure F13. Stress dependent stiffness behaviour of BSM-emulsion of N7 tested at 5Hz and 25 o C 4.3. Permanent deformation results of N7 stabilised materials The permanent deformation believed to occur in three phases (Ebels 2008). In the first phase, the accumulation of permanent deformation takes place in fast rate. This is due to initial permanent strain by densification. The second phase is characterised by a constant rate of deformation. At this phase, the deformation is linear with respect to the number of load application. The third phase is one of accelerated accumulation of permanent deformation due to tertiary flow. During third phase the materials may be considered to be failing in shear under repeated loading. From Figure F14, it can be seen that BSM-emulsion from N7 materials didn t reach flow point in both deviator stress ratio of 40% and 55%. Past research on similar BSMs have indicated that tertiary flow may vary from 200,000 to However, the tested mixes indicate high stability with low rate of permanent deformation. This behaviour seemed to be inconsistence with other BSMs. However, it is believed that high stiffness of the tested specimen might have been caused by long duration of storage prior to testing, hance additional curing. Due to irregularity of the results, it was concluded that repeat test be performed on the fresh collected N7 materials at the same test variables. However, the repeat test is yet to be performed. 12

13 Figure F14. Summary of superimposed PD graphs of BSM-emulsion from N7 materials, tested at 2Hz and 25 o C Monotonic test results of cores extracted from Shedgum. The investigation of on the cores extracted from Shedgum Road from Saudi Arabia, were based on the influence of temperature variation in the accumulation of permanent deformation. According to the in situ temperature measurements carried out by Loudon International, the temperature of the foamed BSM layer can be well in excess of 40ºC in the lower-half and close to 60ºC in the upper-part of the layer. For this reason, monotonic triaxial testing was carried on cores at 40ºC in the laboratory. This was followed by long-term dynamic triaxial tests at 40ºC, 50ºC and 60ºC to establish at which critical temperature accelerated permanent deformation (rutting) would occur. Figure F15 shows summary of superimposed applied stress at different conferment and test temperature of 40 o C. From Figure F15, it is clear that applied stress at confinement of 100kPa and 150kPa is marginal. This id due to minimal increment in applied pressure, better increment could be 200kPa. 13

14 Applied stress [kpa] Confinement stress: Corrected Strain [%] Figure F15. Superimposed of all Samples LC2 at 40ºC 1.5 Shear Stress τ (MPa C = 0.134MPa φ = 33.23º R 2 = Normal Stress σ (MPa) Figure F16. Mohr Coulomb (C and Ø) ST02, ST04, & ST10 at 40ºC From the Morh-Coulomb diagram values Figure F16, the following comments are pertinent to the performance properties of the foamed BSM layer: The monotonic triaxial tests provide a Cohesion Value of C = 134 kpa and a Friction Angle = 33,2º for the BSM layer. Both of these values are plausible for such a mix, with the Cohesion being reduced by the elevated temperature and the Friction Angle being reduced by the high percentage of RAP (aggregate encapsulated in old bitumen). 14

15 4.5. Permanent deformation results of N7 stabilised materials Using the shear parameters C and φ from the monotonic triaxial, long term dynamic triaxials were carried out at a stress ratio of 40% (for 40ºC). It is clear from Figures 9 and 10 that stable behaviour in the foamed BSM layer could be expected at 40ºC or below. As the layer temperature approaches 50ºC, then unstable behaviour and accelerated deformation occur, as was experienced in the in-service behaviour of the pavement Permanent axial strain C 40C 50C Number of load repetitions [-] Figure F17: Permanent deformation at 60ºC, 50ºC & 40ºC (SR=40%; σ 3 =100 kpa for all temperatures) for ST08,04 & 06 LC Permanent axial strain C 50C 60C Number of load repetitions [-] Figure F18. Permanent deformation (log) versus load application at 60ºC, 50ºC & 40ºC 15

16 1.E-02 1.E-03 Strain rate [%/cyc 1.E-04 1.E-05 1.E-06 1.E-07 40C 50C 60C 1.E-08 1.E Permanent strain [%] Figure F19. Strain rate versus permanent strain at 60ºC, 50ºC & 40ºC (SR=0.4, σ3 =100 kpa) Modelling of In-service Traffic In order to ascertain the combination of material, temperature and loading regimes that would result in rapid in-service deformation, a sensitivity analysis was carried out of a similar pavement structure to Shedgum Road using Rubicon software. The following conclusions can be drawn: Material properties (shear parameters C and φ, layer temperature and traffic loading all contribute significantly to accelerated permanent deformation. If any one of this trinity (three factors) is well below the critical limit i.e. safe, then failure should not occur. However, if any two factors are over the limit and the third factor is close to the limit, the pavement will suffer premature distress. Modelling confirms that the set of pavement conditions i.e. the trinity of factors, assessed for Shedgum Road would result in excessive rutting early in the life of the pavement i.e. after 3 to 6 months. This agrees with what has been experienced in reality. Increasing axle loads from a standard axle to a 50% overload has an exponential damage effect to the rate of rutting of the foamed BSM layer i.e. the life of the pavement is shortened exponentially, as shown by pavement modelling. In combination with elevated temperatures and high RAP percentages, overloads inflict severe damage. Summary In order to reduce the risk of premature failure of recycled pavements in harsh conditions such as Saudi Arabia, several factors need to be considered: The material properties, which are a controllable item, need to be maintained at a high level. It is not recommended that high percentages of RAP are recycled with foamed 16

17 bitumen because high pavement temperatures reactivate the old bitumen, reducing the Friction Angle and the Cohesion of the mix Temperature is an uncontrollable factor within a layer, but can be controlled by the depth of protection above. To this end it is important to provide as much cover as possible over the recycled layer i.e. 90mm or more, where possible. The traffic loading is a controllable factor. Overloading needs to be controlled otherwise exponential damage will be brought to bear on a pavement with a resultant exponential reduction in the pavement life. 17

18 5. CONCLUSIONS AND RECOMMENDATIONS The performance and fundamental characteristics of BSMs associated with loading frequency, stress ratio, temperature variation, at different confinement have been studied through triaxial testing. Based on the data of the study, the following conclusions and recommendations are drawn: The shear parameters of the field stabilised BSM-emulsion from N7 are: Cohesion = 135MPa and Friction angle = 43 o. These values are possible for BSMs with inclusion of RAP materials. The Cohesion and Friction angle being reduced due aggregate encapsulate in old bitumen. The stress dependent behaviour of BSM-emulsion from N7 materials shown that deviator stress ratios have insignificant influence on stiffness behaviour of BSMemulsion with different loading frequencies. Likewise there is no trend on the stiffness behaviour with relation to confining pressure and loading frequency. However the relation between confining pressure and stress ratio indicated that stiffness of the BSM-emulsion increase as stress ratio and confining pressure increases. The ranges of stiffness of BSM-emulsion vary from 330MPa to 960MPa depending on confinement pressure and applied stress levels. The parameters of the M r -Ө model of the BSM-emulsion from N7 indicated good correlation of stiffness magnitude indicated in the past research on the BSM-foam and BSM-emulsion. The BSM-emulsion from N7 materials didn t reach flow point in both deviator stress ratio of 40% and 55% after load repetitions. This behaviour seemed to be inconsistent with other BSMs. However, it is believed that the high stiffness of the tested specimens might have resulted from long duration of storage prior to testing, causing additional curing. Material properties (shear parameters C and φ, layer temperature and traffic loading) all contribute significantly to accelerated permanent deformation. If any one of this trinity (three factors) is well below the critical limit i.e. safe, then failure should not occur. However, if any two factors are over the limit and the third factor is close to the limit, the pavement will suffer premature distress Recommendations Due to variability of the results, it was concluded that a greater sample of test results are required i.e. repeat tests should be performed on the fresh collected N7 materials at the same test variables. These tests, however, the repeat tests will be performed as part of PhD studies. 18

19 6. REFERENCES Ebels L. J. and Jenkins, K. J., Determination of Shear Parameters, Resilient Modulus and Permanent Deformation Behaviour of Unbound and Bound Granular Materials Using Tri-Axial Testing on 150mm Ø x 300mm High Specimens. Technical Memorandum prepared by Pave Eng cc., Stellenbosch, South Africa Ebels, L. J., Characterisation of material properties and behaviour of cold bituminous mixtures for road pavements. PhD Dissertation, University of Stellenbosch, South Africa Hicks, R. G. and Monismith, C. L., Factors influencing the resilient response of granular materials. Highway Research Record No. 345, Highway Research Board, Washington DC, USA Jenkins, K. J., Ebels, L. J. and Mathaniya. E. T., Updating Bituminous Stabilized Materials Guidelines: Mix Design Inception Study. Technical Memorandum prepared by Pave Eng cc., Stellenbosch, South Africa Jenkins, K. J., Mix design considerations for cold and half-warm bituminous mixes with emphasis on foamed bitumen. PhD dissertation University of Stellenbosch, South Africa 19