Synthesis of design and construction practices
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[123doc] - phat-trien-nguon-nhan-luc-cua-cong-ty-co-phan-misa-doc
| Reflective Cracking.
The reflective cracking modeling was based on a mechanistic- empirical overlay design method for reflective cracking proposed by Sousa et al. (2002). The study focused on the modeling of reflective cracking above cracks in the underlying pavement surface. Both dense-graded HMA and gap-graded asphalt rubber (wet process) mixes were studied in the laboratory and field to derive mechanistic relationships and statistically based equations. The measured versus predicted crack activity, both before and after the overlay was placed, was investigated. The Von Mises strain, necessary for the modeling, was developed as the following:
) ( ) b 6 VM 1 10 a Overlay Thickness (m) − ε × = × (24)
20 where
VM ε = Von Mises strain a, b = coefficients obtained experimentally.
The model was calibrated using iterative processes. Three adjustment factors were developed: aging adjustment factor (AAF), temperature adjustment factor (TAF), and a field adjustment factor (FAF). All of these factors affected the value of VM ε , which was used to determine the number of ESALs that can be sustained by the HMA overlay before the onset of reflective cracking. The final model was the following:
Asphalt rubber mix: ( ) 4.9761 19 6 VM ESALs 4.1245 10 1 10
− − ⎡ ⎤ = × × ε × ⎣ ⎦ (25)
Dense-graded mix: ( ) 5.93 20 6 VM ESALs 4.1245 10 1 10 −
⎡ ⎤ = × × ε
× ⎣ ⎦ (26)
The number of ESALs obtained from Equations (25) and (26), need to be multiplied by the FAF to obtain the final design ESALs required for the overlay to reach a specific percentage of reflective cracking. Deflections
Composite pavements have been known to provide greater structural support than traditional flexible pavements, while sharing similar noise, friction, and smoothness properties. High structural support of a pavement structure has been traditionally associated with low deflections at the surface (i.e., deflection measurements are known to be reduced when the bearing capacity of the road is high). In addition, a reduction of deflection under an applied load reduces the traffic-induced stresses and strains within the layers of the structure (Nunn et al., 1997). Therefore, a structure that provides lower deflection measurements would tend to reduce the layers’ state of stress and strain, causing the pavement structure to be less affected (damaged) by the loading conditions. The deflection analysis performed is shown in Figure 4.
The figure shows that the modeled deflections at the pavement surface are greatly reduced as the stiffness of the base increases. In this case, the stiffness or elastic modulus (E) of the base increased from soil cement (E = 3,448 MPa [500,000 psi]) to PCC (E = 27,586 MPa [4,000,000 psi]). The maximum deflection predicted when the granular base was used was 0.49 mm (19.2 mils).
Table 8 shows the percent reduction of deflections, when comparing rigid bases to the granular one. As the rigidity of the base increases, the deflections of the pavement structure decrease. This reduction in deflection suggests a reduction of stresses and strains in the various pavement layers, especially in the HMA.
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