Rebecca Crane and Dr. W. Spencer Guthrie, Civil and Environmental Engineering

Cement treatment of aggregate base materials has been utilized in pavement construction since the 1930s (1). The addition of cement increases the strength and stiffness of the treated material and therefore offers greater structural support to the surface layer and greater protection of the underlying subgrade. However, the use of excessive amounts of cement can produce overly stiff base materials that exhibit brittle behavior under heavy traffic loading; the susceptibility of cement-treated base (CTB) materials to cracking increases with increasing stiffness.

Knowledge of the fatigue life of CTB layers is useful to pavement engineers during the design process, especially in situations in which the base layer will be expected to bridge over weak subgrades. Depending on the magnitude of the induced tensile stress relative to the tensile strength of the CTB material, bottom-up cracking may occur, deteriorating the pavement integrity. With the advent of computer software available for calculating stresses and strains at specific locations in pavement systems, several mechanistic-empirical models have been developed specifically for predicting the fatigue life of CTB layers

The purpose of this research was to identify models published in the literature for prediction of fatigue cracking in CTB layers, identify specific pavement parameters to which each model is sensitive, identify models that predict consistently higher or lower values than the others, and determine the expected pavement life and mode of failure associated with varying AC and CTB thicknesses and CTB and subgrade modulus values in a parametric study. Six mechanistic-empirical models for predicting fatigue of CTB layers were identified in the literature review performed in this research: 1) American Coal Ash Association, 2) Australian, 3) Kohn, 4) NCHRP, 5) South African, and 6) Uzan. A full-factorial experimental design was utilized in conjunction with a three-layer flexible pavement system in the parametric study, which included 90 different combinations of AC thickness, CTB thickness and modulus, and subgrade modulus. Computer software was utilized to calculate the critical stresses and strains needed to determine AC, CTB, and subgrade fatigue life as shown in Figure 1 using the models identified in the literature review, and statistical and ranking procedures were both employed to address the research objectives.

The results of the ANOVA indicate that the results of all six models are sensitive to every factor investigated in the analysis. The data show that the Australian and Uzan models are most sensitive to both AC thickness and CTB thickness, while the Coal Ash Association and Kohn models are least sensitive to these factors. Regarding CTB modulus, the Uzan and NCHRP models are most sensitive, while the South African and Coal Ash Association models are least sensitive. Finally, the Australian and Uzan models are most sensitive to subgrade modulus, while the Coal Ash Association and Kohn models are least sensitive. Therefore, in general, the Australian and Uzan models are most sensitive to the evaluated factors, while the Coal Ash Association and Kohn models are least sensitive. In addition, the data clearly indicate that the Uzan model most frequently predicts the highest values of CTB fatigue life, while the South African model most frequently predicts the lowest values of CTB fatigue life.

The NCHRP model was used for investigating the pavement life and mode of failure associated with each combination in the parametric study because of its average sensitivity and CTB fatigue life predictions compared to the other models and because it has been recommended for incorporation in the new American Association of State Highway and Transportation Officials pavement design guide. For CTB modulus values of 500 ksi, the pavement life is trivial for combinations involving an AC thickness of 2 in. and may still be inadequate in combinations involving an AC thickness of 6 in., especially when the subgrade modulus is only 4 ksi. In consideration of an arbitrary traffic level of 10,000,000 ESALs over the design horizon of a given pavement having an AC thickness of 2 in., CTB thicknesses of 9 and 12 in. are adequate when the CTB modulus values are 1000 ksi, 1250 ksi, or 1500 ksi regardless of the subgrade modulus; for a CTB thickness of 6 in., satisfactory performance is predicted for the same CTB modulus values in every case except when the subgrade modulus is 4 ksi. When the AC thickness is increased to 6 in., the predicted ESALs exceed 10,000,000 for all CTB modulus values except 500 ksi regardless of the CTB thickness or subgrade modulus. Of the 90 evaluations, the AC layer never failed first, and the subgrade layer failed first in only two instances. These data demonstrate the ability of the CTB to effectively support the surface layer and protect the subgrade.

Although the sensitivity to specific parameters and relative proximity of model results to each other could be evaluated in this research, the true accuracy of the models could not be assessed. Field performance data are needed for validating these models; as a remarkable degree of variability exists among their predictions, some may be more applicable to certain traffic, materials, and climatic conditions than others. Furthermore, knowledge of seasonal variation in CTB layer properties would be needed to effectively utilize the charts prepared in this research for pavement design.

Reference

• State-of-the-Art Report on Soil Cement. Publication ACI 230.1R-90. ACI Committee 230, American Concrete Institute, Farmington Hills, MI, 1990.