The Research Of Controlling Reflective Crack By Glass-Fiber Grid In The Asphalt Overlay

The crack problem has already been one of the worst pavement diseases especially at the district of low temperature and bad overload. Because of the high tension, good compatibility with asphalt mixture and the limitation of aggregates, the glass-fiber grid is widely used in road repairing. Basing on the typical highway structure in cold district Ningxia, the temperature field, force field and coupling field are simulated by Ansys. The simulation solved the problem of coupling field in the project with crack. Then the important theories are received. The stress strength factor is changing with the temperature changing in one day, so the dynamic stress strength factor is proposed. From the changing law, two principles are found in the anti-cracking design. It is that the lowest temperature must be considered and the heaviest axle load must be independently design. Whether having the glass fiber grid or not, stress strength factor is in inverse proportion to the overlay thickness. It means that enlarging the thickness of overlay can control the crack reflecting. The varying gradient of stress strength factor with the thickness of overlay is -1.48×l0⁴Pa.m^1/2/crn, the one with modulus of AG layer is 1.44×10⁴Pa.m^1/2/MPa. Basing on the equivalence principle of the bottom stress of overlay, the thickness of overlay without the glass-fiber grid is half of thickness with the glass-fiber grid. The ratio of the bottom stress of overlay in the vehicle and coupling field can reach 5.4 while the one of the stress at the crack point can reach 2.8. So the temperature load can not be ignored or simply calculated in the cold district. The time for the lowest temperature in the road is later than at the road face and the temperature gradient is 20 minutes / cm . The stress at the crack point is twice than the bottom stress of the overlay. It means that the glass-fiber grid can absorb the stress of the crack point.For the reason of the fact that the fatigue test can not be matched with the road, a new test way is found. By the new test way the theories are found. Whether having the glass-fiber grid or not, enlarging the thickness of overlay is a good way to stop crack developing. But it cost the money. The fatigue life of overlay with glass-fiber grid can increase 500 to 50 percent comparing with the one without glass-fiber grid. The effect for improving the fatigue life is very obvious at the low temperature; while at the status of high temperature and thick overlay the effect is not obvious. Meanwhile the developing law of crack is very different. The crack with fiber grid develops lately and slowly; the one without fiber grid develops early and quickly. Without gird layer the specimen is ruptured into two dependent parts; in the other way, the specimen is not at eventual failure and the upper parts are connected by the grid.If there is not fiber grid, the fatigue life is proportion to temperature, in the other way the fatigue life of pavement is inverse proportion to temperature. So considering the economy and practicality, adding grid layer is a good way to increase the fatigue life at the cold district. From all of that a new way for the pavement overlay design with the glass-fiber grid is gained. Firstly the thickness of the overlay is designed according to the norm methods. Then the practical thickness is about half of the design one.