Number of Equivalent 10-ton Axles (million)

E = 2,500 MPa (362.6 ksi) 

Asphalt 

surface and 

binder 

Thickness in mm (in) 

60 

(2.5) 

(3.5) 

μstr  65 57 51   

CTB with 

E

initial

12,000 MPa 

Required thickness, mm (in) 

(8.5) (9.3) (9.6) 

Allowable initial strain, 

75 69 62 57 52 47 

150 165 180 190 205 225 

CTB with 

E

initial

16,000 MPa 

Required thickness, mm (in) 

Gravel base 

E = 300 MPa (43.5 ksi) 

Thickness 150 mm (5.9 in) 

Subbase 

E = 100 MPa (14.5 ksi) 

Thickness minimum 200 mm (7.9 in) 

Subgrade 

E = 40 MPa (5.8 ksi) 

- 

 

A composite pavement structure, throughout its service life, may develop different types 

(1979), are very similar to those of flexible pavements because of the exposure that the asphalt 

concrete layer has in the composite structure.  The distresses may be grouped into three major 

categories: fracture (cracking), distortion, and disintegration.  All of the mentioned distresses 

could potentially affect the performance and structural capacity of composite pavements.   

However, the majority could be mitigated with a high-quality HMA mix, adequate overall 

structural design, and appropriate constructive procedures.   

Several research studies (Von Quintus, 1979; Smith et al., 1984; NCHRP, 2004) have 

composite pavements.  Reflective cracks are cracks that occur in the asphalt surface course of the 

composite pavement and that coincide with cracks with appreciable width or joints in the 

underlying layer.  They are caused by the relative horizontal and vertical movements of these 

cracks or joints caused by temperature cycles and/or traffic loading. 

Reflective cracks are undesirable in a composite pavement structure as they tend to 

beneath.  This may cause raveling and disintegration of the asphalt surfacing adjacent to the 

cracks (Breemen, 1963).  When a crack has a considerable width, it acts as a joint and high stress 

intensity is generated at this location.  The contraction and expansion of the rigid layer tends to 

open and close this “joint” causing a significant change in width; as a result, the tensile stresses 

induced at the bottom of the HMA surface layer exceed the strength of the asphalt overlay and a 

reflective crack is initiated. 

 13

When a chemically stabilized material (CSM) is used as the rigid base (e.g., CTB), drying 

shrinkage during the curing period is a major cause for the cracking of the base.  The reasons that 

contribute to shrinkage cracking occurrence, which then lead to reflective cracks, include 

material characteristics, construction procedures, traffic loading, and restraint imposed on the 

base by the subgrade (Adaska and Luhr, 2004).  

 

The proposed Mechanistic-Empirical Pavement Design Guide (MEPDG) mentions the 

following points regarding the use of CSM base layers (NCHRP, 2004): (1) if there is an HMA 

surface course (composite pavement scenario), any fatigue cracking in the CSM layer will result 

in a fraction of the cracking reflected through the HMA layer; and (2) if a crack relief layer (e.g., 

unbound granular layer) is placed between the HMA and CSM layer, it is possible to minimize 

or potentially eliminate reflective cracking through the HMA layer. 

 

To mitigate and control reflective cracks, various methods and techniques could be used.  

These include the use of crack relief layers, pre-cracking (microcracking) of the cemented base, 

and use of geotextiles (paving fabrics) (Adaska and Luhr, 2004). 

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