Seminar - Ravi Prathapa - Shakedown Analysis of Road Pavements
Wednesday, October 24, 2001, 1.10 - 3.40 pm
Civil Engineering Lecture Room 3
Abstract
Pavement design process never had been an exact procedure. Design decisions primarily based on the pavement life cycle cost together with an acceptable pavement maintenance regime for the particular road in consideration. In the case of most highways functional failure i.e. increase in roughness will be an important factor in the design process although the pavement is needed to be designed against structural failure in order to carry the design loads. The design inputs range from load and traffic analysis, environment factors, material properties, improvement of available material, properties of subgrade, properties of bases and sub bases, available construction standards and equipment, surfacing material and fundamental stress- strain analysis. The design process involves selecting the optimum pavement configuration based on a series of iterations on the assumed design layer thickness etc.
The pavement design process has been identified with two broad points of view. Usually design process based on available pavement performance data of various trials and other prototypes was favoured by practicing engineers. Results of WASHO Road Test in Idaho (1952 – 54), AASHO Road Test in Illinois (1958 –60) and performance observations carried out on existing road sections in UK by TRRL from Boroughbridge (1949) to Conington, Cambridgeshire (1965) have had major influence on contemporary design concepts.
On the other hand researchers approached the problem with theoretical equations with simplified assumptions and many times they do not apply to conditions exist in the field. Even the newly adopted AUSTROAD (1992) “mechanistic” procedure is based upon a failure mechanism identified with certain critical elastic strains reaching a critical level, where as embankments, foundations are designed against failure by using plastic analysis. The current pavement design model is based on an elastic and fully recoverable stress/ strain relationship.
Therefore any attempt in utilizing plastic theory in the development of a pavement model which predicts various type of pavement failures such as rutting, shoving and pushing, cracks etc. is a clear advancement in the pavement design process.
Many authors have detailed the nature of repeated application of load, which can be much more severe on a structure than simple loading to collapse. Application of a simple load beyond the first yield load but below the static collapse load will cause plastic deformation in the body. Repeated application of similar loading cycles will cause gradual accumulation of plastic deformation.
(Boulbibane and Collins, 1998) detailed following pavement failure modes in response to repeated cyclic loading.
1) At sufficiently low loading levels the response is purely elastic and no permanent strains occur
2) At higher loading levels, the response is initially plastic but after a finite number of load applications the response is purely elastic and no further permanent strain occur. When this happens the structure is said to have “shaken down”.
3) At still higher load levels, the ultimate response cycle may be of the form of a closed loop, analogous to low cycle fatigue; a state known as “cyclic or alternating plasticity”.
4) Alternatively the permanent strain may go on increasing indefinitely, a response known as “ratcheting”. In a pavement such a response would be demonstrated by the formation of a rut or the generation of slip planes.
The critical load level below which shakedown occurs, but above which permanent strains continue to occur is called the “shakedown load”.
(Sharp and Booker, 1984) suggested shakedown load as the key design parameter for pavements and seems to be the first to do so. Since then (Raad et al 1988,1989) and (Hossain and Yu 1996) extended the calculations, using lower bound approach of Melan’s Shakedown theory presented in 1936.
In Sharp and Booker’s works, analysis was based on a trapezoidal pressure distribution under a roller assuming the resulting deformation is plane strain. The material of the half space was assumed as linear elastic and perfectly plastic. Failure of the material was simply defined by Mohr – Coulomb criterion. Most of the unbound base and sub base material can be considered as granular frictional material. This means resistance to sliding is proportional to the normal force at the contact making overall frictional resistance increases as the confining stress increases. Other important factor contributing the overall resistance in the base and sub base material is aggregate interlocking. Variations in aggregate interlocking (particle size distributions) and the strength of coarse aggregate particles plays and important part in the selection of Base and Sub Base material in all road construction specifications.
more ........at >>>>>> http://www.civil.usyd.edu.au/people/prathapa
In this presentation, it is intended to discuss a detail laboratory experiment we are planning with our Pavement Testing Facility; to examine the application of shakedown theory by means of measuring accumulated plastic deformation, after applying traffic loads below and above the shakedown load. If the shakedown theory is valid, then the measured total plastic deformation after a large number of load cycles should be very small provided the applied traffic load is less than the shakedown load. On the other hand, pavement will experience significant deformation if the applied load exceeds the shakedown load. In addition to the laboratory scale experiment the results of three comprehensive full scale road tests carried out by the Australian Road Research Board’s Accelerated Loading Facility will be reviewed with the available theoretical shakedown models. Results will be available via the Internet and presented in an interactive manner, in order to maximize the participation of all interested parties.
Presentation will include video clips on Unbound Pavement construction in Australia, some results on the analysis of Accelerated Loading Facility trial at Bennala, Victoria and Windows based Data Acquisition Software development for the Pavement Testing Facility.