| The Freeze-thaw damage of rock material is the main factor contributing to the the cold region engineering failure。Limestone is commonly used as subgrade material in Iowa. After the alternating between winter and spring over many years, the subgrade is often affected by heave-thaw damage, such as the occurrence of frost boiling, crack formation and other damage phenomena, which does harm to engineering and further cause huge economic losses.PTN is a new joint material, which is a composite polymer impervious material composed of polyurethane, asphalt and other agents.Because of its good cementing property and waterproof property, it has written into Chinese technical specification(GB/T50600-2010)as recommended impervious and joint material.Entrusted by Civil Engineering Department of Iowa State University,we used PTN to seal the limestone for avoiding moisture infiltration into the limestone and the failure of frost heaving.Further more,we can apply this protective measures to decrease the subgrade frost damage in cold environment.In this article, we analysised and summarised the deterioration model of limestone freeze-thaw damage by operating large span low temperature cycle test from-70℃ to +20℃ and SEM electron microscopic scanning. We discussed the mechanism of PTN material sealing limestone to improve the frost resistance and summarized the low temperature bonding properties of PTN with 3 new interface agents as the base materials.The main results are as follows:(1)Freeze-thaw Deterioration model of limestone:During the earlier stage and middle stage of freeze-thaw, there were development and breakthrough of pores and fissures representing the development mode of fracture; During the later stage, with the increase of freezing and thawing cycles, there were expansion and softening effect of micro cracks on the surface of limestone, leading to the weak cementation of mineral particles representing the spalling mode of mineral particles. These two modes both happened during the freeze-thaw deterioration process of limestone.(2)During the large span low temperature cycle test from-70℃ to +20℃,The quality of limestone first increased and then decreased,it is mainly because the combined action of moisture migration in the earlier stage and spalling particles effects in the late stage.The strength of limestone decreased gradually, after 30 times of freeze-thawing cycles, the freeze-thaw coefficient of limestone is 0.49. We obtained the fitting curve of uniaxial compressive strength R unit and times freeze-thaw cycles X, which is R=Rinitial*(1-0.01734*X).(3)Though with different initial water contents, after 30 cycles of freeze-thawing cycles, the freeze-thaw coefficient is similar. It showed that water content did not play a great role in the frost resistance of limestone.Moreover, Long term continuous freezing did not cause great damage to the limestone.(4)PTN material has a good waterproof performance during the continuous freezing process and freeze-thawing, which can alleviate the freeze-thaw deterioration of limestone. Using PTN as sealing material can increase the strength of limestone by 60-70%, it can greatly improve the frost resistance of limestone.(5) Using three new types of interface agents: Jlwaterri(J), Polyurea bottom material(N), PTN-A materia, Xylene(E) can improve the stretching and adhesive performance of PTN in the continuous low temperature and freeze-thawing cycles. Among them, in the continuous low temperature, adding J base material, PTN has tensile strength average value of 0.94 MPa and elongation at break of the average value of 350.5%. In the freeze-thawing cycles, adding N and E base material, PTN has the tensile strength average value of 0.36 MPa, elongation at break of the average value of 165%. The above data can provide guidance for the cold engineering construction in different conditions. |
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