Prediction of fatigue cracking in cement treated base courses

Stabilizing materials with cement is an attractive option to allow less suitable materials to be used as basecourse or subbase. This is especially the case when good quality natural materials are not available or scarce. The problem with cement treated materials however is that they are brittle in nature and are sensitive to overloading. Since overloading always occurs, one has to take this into account when designing pavements with a cement treated base. Fail save pavements however can only be designed when appropriate transfer functions, e.g. fatigue relationships, are available. Unfortunately there is not too much information on the fatigue performance of cement treated materials and the information that is available is often not applicable to the specific condition. This paper describes the development of a field fatigue relationship for sand cement treated bases which are often used in the Netherlands. The relationship is based on an extensive analysis of the SHRP-NL data base which contained performance data of a number of pavements with a cement treated base. Furthermore the results will be presented of extensive finite element analyses which have been made to analyze the stress conditions near cracks and to determine the effect of load transfer. Combining all these results, an endurance limit for cement treated bases could be developed that can be used for the design of long life pavements. PREDICTION OF FATIGUE CRACKING IN CEMENT TREATED BASE COURSES A.A. A. Molenaar and B. Pu Delft University of Technology, the Netherlands ABSTRACT Stabilizing materials with cement is an attractive option to allow less suitable materials to be used as basecourse or subbase. This is especially the case when good quality natural materials are not available or scarce. The problem with cement treated materials however is that they are brittle in nature and are sensitive to overloading. Since overloading always occurs, one has to take this into account when designing pavements with a cement treated base. Fail save pavements however can only be designed when appropriate transfer functions, e.g. fatigue relationships, are available. Unfortunately there is not too much information on the fatigue performance of cement treated materials and the information that is available is often not applicable to the specific condition. This paper describes the development of a field fatigue relationship for sand cement treated bases which are often used in the Netherlands. The relationship is based on an extensive analysis of the SHRP-NL data base which contained performance data of a number of pavements with a cement treated base. Furthermore the results will be presented of extensive finite element analyses which have been made to analyze the stress conditions near cracks and to determine the effect of load transfer. Combining all these results, an endurance limit for cement treated bases could be developed that can be used for the design of long life pavements.Stabilizing materials with cement is an attractive option to allow less suitable materials to be used as basecourse or subbase. This is especially the case when good quality natural materials are not available or scarce. The problem with cement treated materials however is that they are brittle in nature and are sensitive to overloading. Since overloading always occurs, one has to take this into account when designing pavements with a cement treated base. Fail save pavements however can only be designed when appropriate transfer functions, e.g. fatigue relationships, are available. Unfortunately there is not too much information on the fatigue performance of cement treated materials and the information that is available is often not applicable to the specific condition. This paper describes the development of a field fatigue relationship for sand cement treated bases which are often used in the Netherlands. The relationship is based on an extensive analysis of the SHRP-NL data base which contained performance data of a number of pavements with a cement treated base. Furthermore the results will be presented of extensive finite element analyses which have been made to analyze the stress conditions near cracks and to determine the effect of load transfer. Combining all these results, an endurance limit for cement treated bases could be developed that can be used for the design of long life pavements. INTRODUCTION Stabilizing materials with cement is an attractive option to allow less suitable materials to be used in basecourses and subbases. Because good quality aggregates are not available in the Netherlands, base courses and subbases are made of recycled materials such as mixtures of crushed concrete and crushed masonry but also by stabilizing the available sand with cement or stabilizing recycled materials with cement. The problem with cement treated base courses however, is their cracking behavior. Transverse cracks always occur due to hardening and thermal shrinkage but also traffic may be the cause of extensive cracking. Both types of cracking can results in extensive and costly maintenance. In the Netherlands, “design and build” contracts are gaining popularity. This implies that the contractor is made responsible for the design and selection of materials and has to guarantee the good performance of the pavement over a 7 years period. For obvious reasons, contractors want to use cost effective materials and stabilization of the existing sand subgrade with cement or using cement stabilized recycled materials as base course materials offer such possibilities. Because of less favorable experiences with cement treated bases, the authorities want solid proof of the contractors that the designs and materials proposed by them guarantee good performance. It is clear that such a proof is hard to give if quite some assumptions have to be made about the mechanical characteristics of these materials because of lack of good quality data. Especially lack of good quality information on the fatigue performance of cement treated materials is hampering designers to proof that their proposals result in good performing roads. The lack of fatigue data of cement treated materials is because of several reasons. First of all, stabilization is mostly done in situ resulting in quite some variation in the characteristics. Secondly, these materials are brittle in nature; this implies that a small change in the applied tensile strain has a very large effect on the number of load repetitions to failure. Both factors imply that quite a number of tests needs to be done in order to obtain a reliable fatigue relation. Thirdly, unlike for asphalt mixtures, no nomographs or equations are available to estimate the fatigue characteristics of these materials. Finally, shift factors should be applied to the laboratory derived relationships in order to make them applicable for design purposes. All this implies that fatigue testing of these materials is considered to be cumbersome and questionable and one therefore tries to avoid this type of testing. In order to be able to arrive to a fatigue relationship that could be used for practical purposes, it was decided to make an analysis of the performance of the pavements with cement treated bases that are part of the SHRP-NL data base. The SHRP-NL data base was developed in the 1990’s as a parallel project to the SHRP program in the USA. The data base contains data on the visual condition as well as falling weight deflectometer results, the pavement structures, traffic etc collected over a 10 year period. As will be shown in this paper, a field fatigue relationship could be developed for sand cement base course materials. The analysis of the data base was supported by extensive finite element analyses which were made to determine the stress conditions near cracks and to analyze the effect of load transfer. By combining the field data with the finite element results, an endurance limit for cement treated sand could be developed.