Microcracked semi-rigid structures

Paper by NISSOUX from ISCR 5th 1986 Aachen Germany

Under present economic conditions the use of bitumen is being reduced as much as possible. For more than twenty years the application of semi-rigid pavements has revealed the drawbacks but also the advantages of such structures. The chief advantage is their low cost, which can be further reduced if the materials and the techniques of placing them are developed so as to obviate the need for thick bituminous wearing courses. On the other hand, cracking is becoming increasing unacceptable to highway managers because of the maintenance cost involved, the nuisance to road users and the operating difficulties due to frequent remedial activities. The aim is therefore twofold: - reducing the effects of cracks due to shrinkage by greatly increasing their number; - improving the surface properties of the materials treated with hydraulic binders so as to enable the wearing course to be limited to a surface coating. It is reckoned that this can be achieved by compacting the base course by internal vibration and fracturing it transversely before the 'natural' cracks have appeared and so timed that these discontinuities very only vary little in width. In this way it is intended to obtain a pavement course in which the large number of discontinuities will cause it to function in a pseudocontinuous manner. Why fracture the basecourse? On the one hand this is a technique that can be applied to compacted materials as treated by internal vibration. On the other hand, predetermining the location of the cracks by incipient fractures which reduce the cross-section of the course is not sufficient for obtaining the desired type of cracking. The research work as a whole outlined here will comprise: - Developing a finite element model of a multiplyfractured structure. Improving the present mix design and placing of stabilized gravel-sand so that the mechanical and surface properties of such material will enable it to carry high-density traffic with the help of nothing more than a surface finish. - Determining the principle and the characteristics of a prototype and then of a fully developed fracturing machine for use on the site. Fracturing should make it possible to develop a highway pavement construction technique possessing all the advantages of basecourses stabilized with hydraulic binders without the major disadvantages thereof.

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