Characterization of Cementitiously Stabilized Layers for Use in Pavement Design and Analysis (2014)

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4C H A P T E R 2 This chapter summarizes the findings from the literature review regarding the distresses of asphalt and concrete pave- ments built with CSL and the properties of CSM that contribute to these distresses (details are provided in Appendix A). Distresses of Hot-Mix Asphalt Pavements Block Cracking in Hot-Mix Asphalt Block cracking often is reported in hot-mix asphalt (HMA) pavements with CSL. This cracking is caused by shrinkage of the underlying stabilized base (Scullion 2002) that results from the loss of moisture and temperature variation. The shrinkage typically occurs shortly after construction and con- tinues thereafter. Also, block cracking occurs in CSM with high unconfined compressive strength (UCS) (Zube et al. 1969), likely due to the high shrinkage caused by the high binder content generally used in these materials. Transverse Cracking Some of the transverse cracking in the HMA surface layer results from the shrinkage of the stabilized base (Atkinson 1990, Chen 2007). This cracking starts from the bottom of the surface layer and propagates through the surface layer. Such cracking also can be due to the low bond between the surface layer and the stabilized base (George 2002). Trans- verse cracking in pavements with a granular base and a sta- bilized subbase does not occur until later in the life of the pavement (Ramsey and Lund 1959). Shrinkage cracking of the subbase causes stress concentrations at the crack loca- tions and eventually affects the stress distribution in the surface layer. The survey results (presented in Appendix A) indicate that state agencies consider transverse and block cracking to be the most severe distress types in pavements constructed with CSL. Longitudinal Cracking CSL provide strong support to HMA surface layer that reduces tension at the bottom of the surface layer and helps reduce bottom-up fatigue of the surface layer. Thus, use of a stabilized base will reduce alligator cracking in HMA. How- ever, the HMA surface layer of asphalt pavements with high stiffness CSL as the base is prone to top-down fatigue crack- ing in the wheel-path (ARA 2004, Scullion et al. 2003). This fatigue cracking is due to the high shear/tension at the surface of the HMA contributed by the high stiffness of the CSL. Dry-land longitudinal cracking outside the wheel-path has also been reported for pavements with CSL but it is gener- ally caused by the shrinkage of expansive soils and not by CSL (Luo and Prozzi 2008, Wise and Hudson 1971, Syed et al. 2000, Chen 2007, Atkinson 1990). This type of cracking is not considered in this research. Bottom-up Cracking (Alligator Cracking) of HMA Layer Bottom-up cracking may occur due to erosion or fatigue of the CSL as described in the following list: • Erosion of the surface of the stabilized base layer can create a layer of loose material between the HMA and the CSL at the base (Li et al. 1999, Meng et al. 2004, Thogersen and Bjulf 2005). This erosion increases the strain level at the bottom of the HMA layer leading to alligator cracking. In addition, the loss of fines in the loose material generated by the erosion of the stabilized materials may cause pumping (De Beer 1985). • Alligator cracking in HMA pavements can be induced by fatigue cracking of the stabilized base or subbase due to repeated traffic loads (Pretorius and Monismith 1972, Scullion and Harris 1998, Li et al. 1999). Under these loads, microcracks are initiated at the bottom of the CSL Pavement Distresses

5 due to tensile stress/strain and then propagate upwards. The fatigue of CSL leads to a decrease in the modulus val- ues of the CSL thereby increasing the tensile strain at the bottom of the HMA layer and causing fatigue cracking in the HMA layer. When a CSL is subjected to freeze–thaw and/or wet–dry cycling, the modulus value and strength are reduced and, consequently, the fatigue resistance is also reduced (Naji and Zaman 2005). • The rutting potential in asphalt pavements with CSL is reduced because of the relatively high stiffness of CSL (Von Quintus et al. 2005). However, rutting in the asphalt layer may occur because of the resulting high shear stress in the HMA layer, the erosion of the CSL, or the failure of the CSL as described below: – Rutting induced by high shear stress—The high stiff- ness of the CSL influences the stress/strain state distri- bution such that shear strain may occur in the HMA layer, increasing the potential for HMA rutting (Meng et al. 2004, Bonnot 1991). – Rutting induced by erosion—Erosion of the CSL results from repeated shear stress caused by the water move- ment that occurs due to repeated traffic loads (De Beer 1985). Also the fines in CSL could become detached and form a soft layer between the CSL and upper layer (Metcalf et al. 2001). When cracks occur, the fines can be pumped out of the pavement through the cracks leading to a soft layer and/or voids that contribute to rutting in the asphalt pavement. – Rutting induced by fatigue failure—For thick CSL, compression/crushing fatigue of the CSL could result from repeated compression at the top of CSL. For such layers, the tensile strain at the bottom of the CSL is too small to cause tensile fatigue, but the compressive strain is so high that it causes crushing in the top 2 to 3 in., especially in the presence of excessive moisture (De Beer 1990). An increase in the UCS of CSM reduces the com- pression strain and increases the crushing fatigue life (Theyse et al. 1996). A compressive strain of 1% has been suggested as the failure strain for compression fatigue. Top-down compressive fatigue often occurs when a thin asphalt layer is placed on top of a lightly stabilized, thick layer. Heave The swelling of expansive soils can cause heaving in the pavement; expansive soil often is stabilized to mitigate such swelling. However, the use of calcium-based binder for soils with high sulfate concentration leads to ettringite formation that causes a significant volume change when hydrated (Chen et al. 2005, Si 2008, Little and Nair 2007) and heaving in the asphalt pavement. Expansive soils are generally removed and replaced with non-expansive soils; therefore, swelling is not addressed in this research. Distresses of Concrete Pavements Numerous studies have indicated the contributions of sta- bilized base layers to reduced faulting, pumping, and crack- ing of concrete pavements (ARA 2004, Selezneva et al. 2000, Nussbaum and Childs 1975, Neal and Woodstrom 1977, Ruiz et al. 2005). However, shrinkage cracking of CSL and the bond between the concrete layer and stabilized base could contrib- ute to early-stage cracking of concrete pavement (Mallela et al. 2007). Erosion of CSL also contributes to cracking and joint faulting in concrete pavements. Pumping of fines leads to voids under- neath the concrete slab under repeated traffic loads, resulting in stress concentrations and cracking. The movement of the loosened materials from one joint side to the other may cause joint faulting (Jung et al. 2009, ARA 2004). Table 2-1 provides a summary of the relationship of CSL properties to the distress in asphalt and concrete pavements.

6CSL Property Pavement Distress Stiffness/ Modulus Strength Durability (freeze–thaw, wet–dry) Fatigue Resistance Erodibility Resistance Shrinkage Resistance Swell Resistance Rutting in Asphalt Layer (+) CSL Base (+) CSL Base (−) CSL Base (−) CSL Base Block Cracking in Asphalt Layer (−) CSL Base Bottom-Up Alligator Cracking of Asphalt Layer (−) CSL Base/Subbase (−) CSL Base (−) CSL Base/Subbase (−) CSL Base/Subbase (−) CSL Base Transverse Cracking in Asphalt Layer (−) CSL Base Top-Down Longitudinal Cracking in Wheel-Path (+) CSL Base/Subbase (+) CSL Base Heaving (−) CSL Base/Subbase Transverse Cracking of Concrete Pavement (−) CSL Base (−) CSL Base Faulting of Concrete Pavement (−) CSL Base Note: (+) indicates a positive relationship (e.g., the rutting potential in the asphalt layer increases as the modulus value increases) and (−) indicates a negative relationship. Table 2-1. Relationship of CSL properties to pavement distress.