Table 6.  Parameters Used for the Design of Composite Structures

 15

Table 7.  Typical Material Properties for the Composite Pavement Layers 

Layer 

No. 

Material 

Elastic Modulus   

MPa      (psi)

 

 

Poisson’s 

Ratio 

Modulus of Rupture 

MPa (psi) 

1 

HMA 

3,448   (500,000)

†

 0.35 N/A 

2 PCC 

or 

RCC or 

Lean mix concrete or 

CTB or 

Soil Cement 

24,138   (4,000,000) 

13,793   (3,500,000) 

6,896   (2,000,000) 

3,448   (1,000,000) 

3,448   (500,000) 

0.15 

0.15 

0.15 

0.20 

0.20 

4.48   (650) 

4.14   (600) 

3.10   (450) 

1.38   (200) 

0.69   (100) 

3 Base 

and/or 

Subbase 

207   (30,000) 

138   (20,000) 

0.35 

0.35 

N/A 

N/A 

4 Subgrade 

(compacted, 

CBR=5%) 

51.7   (7,500)

*

 0.40 

N/A 

             † 

Typical value at an average service temperature. 

 

For the two AASHTO alternatives, a simple spreadsheet was created to compute all the 

values obtained from the AASHTO 1993 guide.  The IDOT alternative was computed using the 

tables, formulas, and nomographs published in their 2002 Pavement Design Guide (IDOT, 2002).  

The U.S. Air Force and Army alternative was designed using the PCASE pavement design 

software available from their website (PCASE, 2007).  The Danish design was based on the 

Danish Road Institute mechanistic design table with a 75% reliability (Thogersen et al., 2004).  

The U.K. design thicknesses were obtained using Equation 9.  Figure 3 compares the cross 

sections of all the composite structures designed using the various procedures. 

 

 

 

Figure 3.  Comparison of All Designed Composite Pavement Structures 

 16

The composite structures shown in Figure 3 ranged from a total thickness of 20 to 28 in.  

These structures may be grouped into three design groups according to similar thicknesses in the 

HMA and rigid base layer:  

 

•  Group 1, composed by the two AASHTO alternatives that resulted in an 8-in HMA 

surface course and a 10-in rigid base.  In the AASHTO 1 alternative, flexible 

pavement procedure with a CTB, the structural coefficient of the HMA, a

1

 = 0.47, 

was greater than the CTB, a

2

 = 0.27.  In the AASHTO 2 alternative, rigid pavement 

procedure with HMA rehabilitation, the structural package was similar to the 

AASHTO 1 alternative, with the exception of a thinner HMA layer.  

 

•  Group 2, composed the U.K. and IDOT designs that resulted in very similar designs 

consisting of a HMA of 175 mm (7 in) and a rigid base of 200 mm (8 in).  The rigid 

bases in these two designs were a lean-mix concrete and a PCC for the U.K. and 

IDOT procedures, respectively.  The layers’ designed thicknesses obtained by 

following the design procedure of these transportation agencies were chosen as the 

typical composite pavement to be analyzed through the mechanistic modeling.  The 

main reason why this design was selected is because of the experience in the U.K., 

which, according to the literature, is one of the countries that has the most experience 

investigating, designing, and constructing composite pavement systems in the last two 

decades. 

 

•  Group 3, composed by the military and Danish designs, which had the lowest 

thicknesses for the HMA surface layer.  Although the thickness of these layers are 

lower than for the other cases, the Washington State Department of Transportation 

specifies, based on experience, that a 100 mm (4-in) HMA thickness is thought to be 

thick enough to retard reflective cracking (WSDOT, 2007).  The Danish alternative is 

the only one that proposes the use of a granular base layer underneath the rigid base 

and above the subbase layer.  However, the presence of this granular base layer could 

be due to the lower modulus of the subgrade (40 MPa [5,800 psi]) used as fixed 

values in their design table (Table 5).  In addition, this alternative had the lowest 

HMA surface thickness (87.5 mm [3.5 in]). 

 

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