Pavement structure mechanics response of flexible on semi-flexible overlay that based on the old cement concrete pavement damage

China academy of transportation sciences Beijing 100021 Abstract: The old cement pavement damage status directly affect the design of the paving renovation. Based on the state of the old road investigation, combined with the research data at home and abroad, use the control index that average deflection, deflection value and CBR value to determine the reasonable time to overlay. Draw up the typical pavement structure according to the principle of combination of old cement pavement overlay structure design, and calculated that the tensile stress and shear stress in asphalt layer ,semi-flexible layer and the tensile in the old cement pavement adopting BISA3.0 statics finite element analysis model when modulus in the old road was diminishing. Use the computed result to analyses the influence of old road damage condition the influence of pavement structure.


introduction
The highway and city road of cement pavement-based in domestic, it appeared a variety of damage phenomenon in the early construction of the pavement.This phenomenon lead the bearing capacity is insufficient of old pavement structure, and poor driving comfort.If we reconstructed directly, it will serious impact on the performance of road traffic that waste of time and extension of time limit for a project.
If take the way that overlay the asphalt layer and don't take invisible old cement concrete pavement.It will effective utilization of old cement concrete pavement and reflect the advantage of rapid and economic.But the old cement pavement usually exist all kinds of diseases, such as pit slot, cracks, etc.
The massive character of old cement pavement was impact by the threat situation of old cement pavement.Therefore, through the analysis of the mechanical response, to research the impact of semi-flexible overlay structures design by the threat situation of old cement pavement.

CBR value
It was put forward the old road sugared CBR range and at the grass-roots level and total thickness on the surface that the old cement pavement need to overlay by abroad of construction experience.By analyzing the deflection value shows that the two kinds of combination, it is very harmful that the modulus is less than 700MPa in the old cement pavement, the pavement deflection value will be higher than 0.25 mm. that is very harmful for overlay.
Because the shear stress is very big, and the wheel load repeatedly function will produce cumulative deformation leading to rutting.
Generally speaking when modulus attenuation serious, it is not suitable for direct overlay on old cement concrete pavement, should take certain reinforcing measures.

2
The premise of overlay A large number of domestic and foreign researches show that before the old pavement overlay the asphalt, should accurate judge the strength of the old cement pavement.And accurately determine what the paving layer to be done, as a base or as a sub base.It would be as far as possible to effective using the strength of original road.At present, the strength evaluation of old cement pavement mainly include: deflection differential and average deflection value and the CBR value. 1 The deflection differential Joint of old concrete pavement is the weak part; therefore most of the cement concrete pavement damage occurred near it.The major reason of reflection cracks on asphalt paving layer is the deflection differential of old pavement plate joint (or cracks) edge.the American Association Institute (AI) in order to the deflection differential of old pavement plate joint (or cracks) as the one of indicator of the design control index on old cement concrete DOI: 10.1051/ C Owned by the authors, published by EDP Sciences, 2015 overlay.And proposes the 0.05 as deflection difference threshold, it also not make overlay on fatigue life of asphalt layer attenuation threshold, the research results as shown in figure 2.1.

Fig2. 2 3Fig 3 . 1
Fig2.2 Suitable for overlay scope Combining with domestic and abroad control index, this article selects the average deflection value and CBR two indicators to control the timing of the overlay.Among them, using CBR value judgment whether need treatment was carried out on the old cement concrete pavement, and uses the average deflection values determine what kind of treatment.

Fig 4 . 1
Fig 4.1 Pavement calculated here According to the code for design of highway asphalt pavement (JTG D50-2006), the old cement concrete pavement is a whole.In order to facilitate stability of the old cement concrete pavement can be reflecting by the old cement road surface modulus decay.When the old cement concrete pavement modulus from 20000MPa attenuation to 700MPa, single mechanics response of asphalt road surface pavement structure is shown in table 4.2.

Figure 4 . 2
Figure 4.2 Stress and the modulus of old road history Figure 4.3 Deflection value modulus relation curve and the old road Double-layer structure mechanics response of asphalt road surface conditions is shown in table 4.3.

Figure 4 . 4 6 In figure 4 .2 and figure 4 . 4 ,From the figure 4 . 4 , 5 Conclusions
Figure 4.4 Stress and the modulus of old road history Figure 4.5 Deflection value modulus relation curve and the old road

Table 4 .
2 the old cement concrete pavement modulus changes, single layer asphalt layer structure inner stress condition

Table 4 .
3 the old cement concrete pavement modulus changes, double layer asphalt layer structure inner stress condition