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Background
South Africa boasts an extensive and mature road network. At present, the bulk of pavement design activities in the country are aimed at preserving and upgrading the existing road infrastructure. Innovative methods of pavement rehabilitation are required to increase the service life of wearing courses and to reduce the need for traffic hampering maintenance activities. To this aim, the South African National Road Agency Limited (SANRAL) has sponsored the development of the so-called Ultra Thin Continuously Reinforced Concrete Pavement (UTCRCP). UTCRCP is intended as an overlay strategy for existing roads. The technology comprises a high performance concrete layer with a nominal thickness of approximately 50 mm. The material incorporates fibres as well as mesh reinforcement and is characterised by its ability to withstand high deflections. The technology is discussed in more detail as part of Chapter 2. During the course of this study the UTCRCP technology progressed from the development phase to the implementation phase. The methodology is now being applied as part of major highway rehabilitation projects in South Africa. The design tools for the innovative UTCRCP system are currently based on conventional concrete
pavement design methodologies.
The conventional Mechanistic-Empirical approach to concrete pavement design for fatigue makes use of Linear Elastic (LE) analysis. Both the stress in the pavement slab and the material strength are obtained assuming LE material behaviour. Non-linear, non-elastic post fracture behaviour is not taken into consideration. In these models the material strength is characterised by the Modulus of Rupture (MOR) obtained in monotonic Four Point Bending (FPB) test on beam specimens. The MOR is the stress in the extreme fibre of the specimen, calculated under the assumption of a LE stress distribution at the peak load condition. The ratio between the MOR and the stress in the pavement calculated through LE analysis, is used to predict the fatigue life of the pavement. Researchers have long established that flexural strength for concrete is not a true material property, because its value changes with specimen size (Reagel and Willis, 1931, Kellerman, 1932). The size effect phenomenon is caused by the fact that concrete is a quasi brittle material and at the peak load condition cracks will already have formed in the material. Due to the presence of a crack, the assumed LE stress distribution no longer exists in the beam. In different sizes of specimens, different amounts of energy are released into the crack front, giving rise to the observed size effect. Similarly, a LE stress distribution will not be present in a pavement slab loaded to failure, because it too will have cracked. Size effect in plain concrete has been well documented and can be predicted using fracture mechanics (Bažant and Planas, 1997). Notwithstanding these limitations, LE analysis remains the basis for fatigue prediction in state of the art concrete pavement design methods.
1 INTRODUCTION
1.1 Background
1.2 Problem statement
1.3 Objectives
1.4 Thesis statement
1.5 Scope of the work
1.6 Limitations
1.7 Contribution to the state of knowledge .
1.8 Thesis structure
2 THEORETICAL FRAMEWORK
2.1 Ultra Thin Continuously Reinforced Concrete Pavements (UTCRCP)
2.2 The mechanisms of fatigue in plain and fibre reinforced concrete
2.3 Design for fatigue in concrete pavements
2.4 Some concerns regarding the conventional concrete pavement design approach
2.5 Fracture mechanics and its application to concrete
2.6 Fracture mechanics for fatigue damage prediction
2.7 Discussion on theoretical framework
3 METHODOLOGY .
3.1 Research design
3.2 Experimental program and methods
3.3 Selection of numerical simulation methods .
3.4 Discussion on the methodology
4 FRACTURE EXPERIMENTS
4.1 Engineering properties
4.2 Presentation of monotonic flexural test results .
4.3 Size effect
4.4 Fracture energy
4.5 Analysis of fatigue tests
4.6 Discussion of fracture experiments
5 ADVANCED FRACTURE MODELS
6 CONCLUSIONS
REFERENCES
