Flexible Pavement Research

It is the purpose of this paper to point out the problems associated with flexible pavements, to summarize the research that the Joint High­ way Research Project has done in this field, and to outline present endeavors. In so doing, it must be recognized, first of all that flexiblepavement research, or the general problem of flexible-pavement design, consists of two parts. One of these is the material design of competent layers of the flex­ ible pavement, such as the subbase, base, or surface course. In addition to the strength characteristics of such layers, factors of durability also must be recognized in establishing their design. The second part of the problem is concerned with the thickness design of the component layers, wherein it is required that sufficient strength be built up to carry the imposed loads. In this connection the problem must be viewed as one in which the subgrade soil is required to carry the load, but that in order to do so it must be reinforced to the extent that stress applied to it will be so distributed as not to cause failure. In conducting research on these problems, we have initiated both laboratory and field studies, and have recognized such factors as sub­ grade soil type, type and thickness of subbase, base, and surface courses, climatic conditions, amount and type of traffic, and position of the pavement with respect to the ground surface. Of these factors, the most difficult to evaluate by laboratory or field tests alone are climatic, traffic, and position with respect to the ground surface. However, laboratory or field tests, when used in conjunction with field perform­ ance surveys, can provide excellent design information. Performance surveys of existing pavements, in addition to aiding in evaluation of soil, climate, traffic, and position variables, also bring out important features of construction procedure and practical limitations. W ith these considerations, a brief summary of the research that the Joint Highway Research Project has done and is doing on the general subject of flexible pavements is presented. It should be pointed out