The fatigue characteristics of precast concrete raft units

Abstract

This research project represents a continuation of the research programme into Precast Concrete Pavement Units, "P. C. P. U. ", which is based in the Department of Civil Engineering at the University of Newcastle upon Tyne. The units were referred to as raft units throughout the thesis which focused on the use of raft units as a full concrete paving system for aircraft parking, taxiway, and other low speed areas, at airports. The physical full scale test model was designed and constructed to represent the applied loading from one of the dual wheel legs of the design aircraft, a Boeing 727-200, when it is taxiing over a raft unit paving system. It was only possible for the test model to be provided with a contact area of 200 mm square compared to the real life of 400 mm square. A theoretical correction was applied to allow for this difference. Sixteen raft units were tested in pairs using the test model. The tests were divided into three modules to investigate the effect of the raft unit dimensions, Module(M1); the reinforcement design, Module(M2); and the raft thickness, Module(M3). The twin loading assembly applied a repetitive dynamic load which was moved manually between four different loading positions to represent the aircraft moving across the raft units. The primary aim of the experimental programme was to identify for the first time the fatigue life and failure mechanisms of the raft units under the influence of twin dynamic moving loads, and provide experimental results to enable a more refined numerical design method to emerge for raft units, as well as to determine the causes of failures and to recommend remedial measures. Observations were made of vertical deflection, concrete strains, crack widths and crack patterns, failure load, and failure modes, each of which were described in detail. The test observations showed that by increasing each of the following variables, namely, the aspect ratio, the amount of steel bar reinforcement and the thickness of the raft units, resulted in each case in an extension of raft unit life. It was found that some form of uplift restraint on the raft unit should be added to improve the fatigue life for one of the loading positions and that fibre reinforcement should not be used. The ultimate load capacity of the raft units was influenced by the loading position, the applied load level and the number of load repetitions, together with the crack patterns. Using the results from the raft units that had failed within a specific module, it was possible to predict the ultimate and reserve load capacity of raft units within the modules that were only partially fatigued. Four important conclusions have been established during the research project. Firstly, based on the test results, an empirical relationship was derived using regression analysis, relating the number of load repetitions to the aspect ratio, the amount of reinforcement, the raft thickness, and the applied loading. This will need further verification, but it should eventually be very useful when estimating the fatigue life of these specific raft unit models. Secondly, a new design method has been proposed. The design methods for raft units proposed previously by Bull, Ismail, Annang, Ackroyd, and the British Port Association were reviewed. The test results enabled a new design method to be developed which was based on Bull's method but proposed new design charts and tables for each of the raft units considered in the research project which introduced the additional variables of contact pressure and the exact loading position. Thirdly, the measured strains were used to develop strain fatigue relationships for designing raft units and estimating the reserve design life in a raft unit paving system for the purpose of maintenance management by relating the accumulated number of load repetitions of a design load to the permissible concrete strain. The strain fatigue equations were generated for each of the raft units considered in this research project. Thus, the most realistic way to control raft unit distress is through the use of a predictive fatigue model. This should prove invaluable to those involved in the maintenance of raft unit paving systems. Finally, life cycle cost analysis was conducted for three types of construction pavement (paving blocks, PQC, and raft units). The analysis demonstrated that the precast concrete raft units will become a viable alternative to conventional pavement construction and a real competitive to the concrete paving blocks.