OF THE ART. I: RESILIENT RESPONSE OF UNBOUND AGGREGATES
By Fredrick Lekarp,1 Ulf Isacsson,2 and Andrew Dawson3
(Reviewed by the Highway Division)
ABSTRACT: The ﬁndings of an extensive literature survey on the structural response of unbound aggregates are discussed in two companion papers. In this paper, a state of the art is presented on resilient stress-strain characteristics ofsuch materials, as well as different modeling techniques. The resilient response of aggregates is affected by several factors with varying degrees of importance. These are presented, and different views on the impact of each individual factor are discussed. Research efforts in the past have resulted in different mathematical models for predicting the resilient response of aggregates under repeatedtrafﬁc-type loading. The models found in the literature are listed, and their advantages and shortcomings are reviewed. The permanent strain characteristics of unbound aggregates are considered in a separate companion paper.
INTRODUCTION The empirical nature of traditional pavement design methods has been a topic of discussion in highway engineering for many years. These methods rely onempirical rules developed through long-term experience with certain types of pavement and certain types of pavement construction material under certain conditions. The main limitation of empirical methods is that they cannot be extrapolated with conﬁdence beyond those conditions on which they are based. The essential need for pavement design procedures that are able to cater for varying design situationshas led to widespread research efforts to develop so-called analytical or mechanistic design techniques. In the analytical approach, the road pavement is treated as a structure, and its mechanical behavior evaluated in terms of load-carrying parameters in a similar manner to that used for concrete and steelwork structures. A conditional prerequisite for the success of the mechanistic approach isthat the behavior of the constituent materials is properly understood. In ﬂexible pavements, particularly when unsurfaced or thinly surfaced, granular layers play an important structural role in the overall performance of the pavement structure. Consequently, to establish more rational pavement design and construction criteria it is essential that the response of granular layers under trafﬁcloading be thoroughly understood and taken into consideration. The stress pattern induced in a pavement due to a moving wheel load is quite complex. Fig. 1 illustrates that an element in a pavement structure is subjected to stress pulses, each consisting of vertical, horizontal, and shear components. In unbound layers, the vertical and horizontal stresses are positive, whereas the shear stress isreversed as the load passes, thus causing a rotation of the principal stress axes. The deformational response of granular layers under trafﬁc loading is conveniently characterized by a recoverable (resilient) deforma1 Swedish Nat. Rd. Admin., Constr. and Maintenance, Div. of Business and Engrg. Devel., Box 4018, SE-171 04 Solna, Sweden. E-mail: email@example.com 2 Prof., Div. of Hwy. Engrg., RoyalInst. of Technol., SE-100 44 Stockholm, Sweden. 3 Sr. Lect., Dept. of Civ. Engrg., Univ. of Nottingham, University Park, Nottingham NG7 2RD, U.K. Note. Discussion open until July 1, 2000. Separate discussions should be submitted for the individual papers in this symposium. To extend the closing date one month, a written request must be ﬁled with the ASCE Manager of Journals. The manuscript for thispaper was submitted for review and possible publication on January 20, 1999. This paper is part of the Journal of Transportation Engineering, Vol. 126, No. 1, January/ February, 2000. ASCE, ISSN 0733-947X/00/0001-0066–0075/$8.00 $.50 per page. Paper No. 20093.
tion and a residual (permanent) deformation, as illustrated in Fig. 2. However, the true nature of the deformation mechanism of...