1. ANALYSING A PAVEMENT STRUCTURE WITH A CRUSHED STONE OVERLAYING ON AN EXISTING THICK ASPHALT LAYER
JP Maree: Stellenbosch University: Student (V3 Consulting Engineers)
Prof. K Jenkins: Stellenbosch University: SANRAL Chair in Pavement Engineering
S Bredenhann: SANRAL
Prof. A Molenaar: Delft University of Technology, the Netherlands

2. INDEX INTRODUCTION CASE STUDY THEORETICAL ANALYSES OF PAVEMENT
PAVEMENT STRUCTURAL BEARING CAPACITY
In this presentation, I will briefly…
Give you an Introduction into what this study was about
… I will talk about a CASE STUDY
… give you an over view of the theoretical analysis of the pavement
… we will look at the theoretical bearing capacity of the pavement models
… and to wrap-up the presentation, there will be some conclusions and recommendations that the study made.
CONCLUSIONS AND RECOMMENDATIONS

3. Inverted pavement structure
INTRODUCTION
Inverted pavement structure
Conventional vs
Study
Surfacing
Asphalt surfacing
Unbound granular Base
Crushed stone base
Bound (stabilised) Subbase
Older thick asphalt layer
Conventional – TRH4
Unconventional (This study looked at…)
… constructed on top of an older pavement structure… kept intact without modifications
Nowhere … general design guidelines… recommended…yet there are a number of roads…
Unbound gravel Selected layer
Older granular subbase and selected layers
Unbound gravel Selected layer

4. INTRODUCTION
Fundamental questions generated from an engineering perspective:
What will the advantages be?
Why is this rehabilitation option not more popular?
Will there be economical benefits?
What are the risks associated, such as:
Could water be trapped at the interface?
Could compaction be a problem?
Could slip occur at the interface, and what would this lead to?
To answer some of these questions, the inverted pavement structure was analysed
(Water trapped)…especially if a crushed stone base…
(Slip)… interface between the old and the new…
…from my experience, the question that was asked the most is…
and therefore this study gave a lot of focus to the interface between old and the new...
Lack of compaction due to limited opportunity for dissipation of pore pressures due to an intact asphalt layer below?

5. CASE STUDY (NATIONAL ROUTE N1 SECTION 17)
Info from test pits
Kroonstad
±50mm Asphalt surfacing
±130mm Crushed stone base
±200mm Older thick asphalt layer
So, I contacted SANRAL…
and I was informed that the N1 Section 17 was constructed like this… 40km
I was fortunate enough… study…design phase to be upgraded… testpit data and mechanical measurements
From the testpits…the avg. pavement structure consisted of…
Unknown (Unfortunately the construction of…)
1989 (Asphalt and crushed…)
Older granular subbase and selected layers
Ventersburg

6. CASE STUDY (NATIONAL ROUTE N1 SECTION 17)
Pavement history: Was constructed in 1989, carried approximately 18 MESA and is currently under reconstruction.
Pavement analysis:
The FWD measurements indicated the road is still in a sound condition
Back-calculation provided unrealistic results and were discarded
IRI measurements were within the warning category
The majority of measured ruts were less than 10mm, however...
FWD - 95th percentile was in the vicinity of 400microns
Backcalc (An attempt was made to…)… theoretical defl. bowl fitted to the measured defl. bowl…
IRI (…there were sections of the road… heaving…)
Avg. rut of approximately 4mm

7. CASE STUDY (NATIONAL ROUTE N1 SECTION 17)
Shoving was witnessed in the outer wheel path:
2 TP excavated… severe shoving
Both TP showed… only top 2 layers
Bottom right corner…yellow line, no modification… gravel layer…
And that the shoving occurred on joint…
Going forward, these types… theoretically…

8. THEORETICAL ANALYSES OF PAVEMENT
The methods used:
Stress-dependency method
Linear Elastic Analysis (LEA)
Finite Element Analysis (FEA)

9. THEORETICAL ANALYSES OF PAVEMENT
Stress-dependency analysis of the slip:
FWD back-calculation → unrealistic → discarded
Stress-dependency method
Bulk-stress log-log model
Calculated in mePADS
Three pavement structures (from the case study) were used in the calculations:
(base )
As previously discussed…

10. THEORETICAL ANALYSES OF PAVEMENT
Stress-dependency analysis of the slip:
The aim was to determine the modulus of the base
The modulus of the asphalt was unknown → chose a lower and upper limit (1500 MPa – 5000 MPa)
All models showed a significant reduction in base moduli when slip were introduced
The reduction in modulus is a function of the old asphalt modulus and combined thickness of the base and the old asphalt
… with no slip…
A modulus was chosen…but because the… unknown... It was decided to use a lower…
Results from the analysis…
The models were analysed… without slip… between base…
Reduction (All models showed…)
It is concluded…
These results were then plotted…

11. THEORETICAL ANALYSES OF PAVEMENT
Stress-dependency analysis of the slip:
- A trendline was fitted between the results
- For 390mm combined thickness, there was a 20% reduction in the modulus of the base
- The significance of this was however not further investigated
This graph shows the reduction in modulus of the base compared to the increase in slip
Trendlines were fitted on the results, which showed that the base moduli decreased exponentially as the slip increases.

12. THEORETICAL ANALYSES OF PAVEMENT
The influence of slip on the surface deflection (LEA)
Three case study models were evaluated in mePADS
The maximum deflections were measured:
Increased by 45%
Increased by 45%
Exponential increase…
It is therefore concluded from the LEA that:
The pavement is very sensitive to slip, regarding surface deflection and the modulus of the base.
Increased by 38%

13. THEORETICAL ANALYSES OF PAVEMENT
Finite Element Analysis (FEA) with slip
The nest part of the study…
This image shows the theoretical model that was used
The model was analysed in the program ABAQUS
Model dimensions…

14. THEORETICAL ANALYSES OF PAVEMENT
Finite Element Analysis (FEA) with slip
Drawbacks:
Homogenous-isotropic-linear-elastic
Static loading
Zoom-in
- The model was then divided into a finite number of elements, with various element sizes.
- a pressure load is applied, with a 150mm radius
- The sides of the model were constrained in all directions and moments, expect in the y-direction and z-direction, and the x-direction and the z-direction Y X Z

15. Horizontal displacement
THEORETICAL ANALYSES OF PAVEMENT
Finite Element Analysis (FEA) with slip
No slip between Base layers and Older Asphalt layer
Slip Surface
Horizontal displacement
Full slip between Base layers and Older Asphalt layer
This is a graphical presentation of the horizontal contours, before and after slip.
It is shown in the first figure that the slip surface has no influence on the horizontal placement when there is no slip
However, when slip is introduced, there is a change in the horizontal contours. There is a significant reduction in horizontal moved in the older asphalt layer and there is an increase in the horizontal movement in the base.
Slip Surface

16. THEORETICAL ANALYSES OF PAVEMENT
Finite Element Analysis (FEA) with slip
Maximum slip distance
All models showed tensile stresses in the base of more than kPa, this was deemed unrealistic and was adjusted
Model
Maximum slip horizontal distance
Distance of maximum slip from centre
N1-17_11.8
41 µm
200 mm
N1-17_25.6
44 µm
N1-17_36.4
51 µm
The table shows the actual measurement of the horizontal movement at the slip surface, all models showed the maximum movement to be at 200mm of the load centre.
In theory granular material should not be able to withstand any tensile stresses, because of the lack of the PI. However, studies have showed that there is up to 50kPa of tensile stresses due to suction forces, static forces, etc.

17. THEORETICAL ANALYSES OF PAVEMENT
Finite Element Analysis (FEA) with slip
These graphs shows the profile of the pavement with the stresses plotted on them.
Graph A – shows the Major and Minor Principal Stresses over the depth of the pavement, when there are no slip between the base and the older AC
Graph B – shows stresses when slip is introduced
It graphs show that there is a change in the behaviour of the stresses in the base when slip was introduced, and that the stresses increased. It also shows that there is a significant reduction of stresses in the older asphalt layer. It is almost as if the were no proper transfer of stresses between the base and the older AC.

18. THEORETICAL ANALYSES OF PAVEMENT
Finite Element Analysis (FEA) with slip
Graph C – which is the horizontal strain over the depth of the pavement. The results show that there is a significant increase in strain the bottom of the base when slip was introduced, and a reduction of strain in the older asphalt layer.
It is thus found that the pavement structure is very sensitive to slip.
The significance of the change in patterns of the stresses and strains, with and without slip, was however not further analysed.
The results from the analysed were only used to determine the bearing capacities of the models.

19. PAVEMENT STRUCTURAL BEARING CAPACITY
Three methods were:
Pavement Number (PN) method
Linear elastic analysis (LEA) with mePADS
Finite element analysis (FEA) in Abaqus & SA transfer functions
Results from analyses:
All models with no slip were able to carry the historical traffic loading
Full slip → up to 90% reduction in bearing capacity → base is critical layer
The bearing capacity of the pavement is very sensitive to slip
Possible that a partial or full bond was created between the base and older asphalt, as the road carried traffic successfully for 24 years

20. CONCLUSIONS AND RECOMMENDATIONS
Case study: Mechanical measurements showed that the pavement is in a good condition and severe shoving in outer wheel track was witnessed.
Linear elastic analysis: Showed significant reduction in base modulus when slip was introduced and the surface deflection increased exponentially when slip increased
Finite element analysis: When slip was introduced the stresses and strains increased in the base and decreased in the older asphalt layer
Bearing capacity analysis: Pavement could carry traffic loading without slip, but could not carry the traffic loading with slip (90% reduction), therefore it is very sensitive to slip
A bond was possibly created between the base and the older asphalt layer
A high level economic analysis found that this pavement is the cheapest, even with 10% repairs to the old asphalt layer
Based on the results of this study, the inverted pavement structure may be considered a viable construction method for rehabilitation of an existing road structure

21. CONCLUSIONS AND RECOMMENDATIONS
FEA modelling:
Apply a dynamic load
Limit the tensile stresses in the granular materials to 50kPa
Add stress-dependency properties to the material
Carry out a construction energy analysis and emission analysis to determine if this pavement structure is a more sustainable solution
Determine the risk of the inverted pavement structure in terms of maintenance and performance due to ingress of surface water
Analyse more studies of such inverted pavement structures to confirm their feasibility as an alternative pavement rehabilitation/strengthening option

22. END
QUESTIONS?