Bonding Degradation And Damage Mechanism Of Tunnel Fireproof Coating Under Freeze-thaw Cycles

With the further development of national infrastructure, the development of High Speed Rail and the safety awareness enhanced when used in the tunnel. The market has put forward higher requirements, especially for the bond performance and damage of tunnel fireproof coating under freeze-thaw cycles. In the macro level, the development laws of the weight, bond strength, lateral fundamental frequency and ultrasonic sound velocity of tunnel fireproof coating under freeze-thaw cycles were studied. In the micro level, combined with the nitrogen adsorption method and MATLAB image processing technology, the macro and the micro pore structure of tunnel fireproof coating were studied. The affect mechanism of redispersible powder, polypropylene fiber and air-entraining agent to the fireproof coating and the damage of the coating under freeze-thaw cycles were studied. The research shows that:1. When the content of redispersible powder increases, the initial bond strength of the coating increases, and both the cumulative pore volume of initial micro-pores and the average diameter of macro-pores are reduced. Redispersible powder has obvious effect on the micro-pores under different freeze-thaw cycles. When the content increased, the cumulative pore volume of initial micro-pores, accumulated pore volume and the average diameter of the macro-pores were decreased in the same freeze-thaw cycles. The proportion of harmless holes increases and the freeze-thaw resistant performance of the coating improved.2. When the content of polypropylene fiber increases by 0%,2%,4%, the initial bond strength of the coating were 0.204 MPa,0.353 MPa,0.388 MPa, and the cumulative pore volume of initial micro-pores and the average diameter of macro-pores were increased. The cumulative pore volume of initial micro-pores and the average diameter of the macro-pores were decreased in the same freeze-thaw cycles when the content of polypropylene fiber increased. When freeze-thaw cycles up to 30 times, the polypropylene fibers had no obvious effect on the micro-pores, but still effectively inhibit the deterioration of the macro-pores; when its content is greater than 0.2%, the freeze-thaw resistant performance of the coating did not improve.3. When the content of air-entraining agent increased, the initial bond strength of the coatings, the cumulative pore volumes of initial micro-pores were reduced, and the average diameter of macro-pores and the porosity increased. And the cumulative pore volume of initial micro-pores and the average diameter of the macro-pores were decreased in the same freeze-thaw cycles when the content of air-entraining agent increased. When its content was greater than 0.015%, the freeze-thaw resistant performance of the coating did not improve.4. As the freeze-thaw cycle went on, the decrease of the bond strength, lateral fundamental frequency and ultrasonic sound velocity of tunnel fireproof coating increased. The air-entraining agent had more obvious beneficial effects on the reduction of the damage of the coating than the redispersible powder and polypropylene fibers. The results are basically the same when the damage of the coating evaluated by the lateral fundamental frequency and the ultrasonic sound velocity.5. The residual strength model deduced by theoretical was checked out that the bond strength of the coating and freeze-thaw cycles were fit well with the test data. The relationship between residual bond strength and the degree of damage by fitting can be used to estimate the residual bond strength of the coating in the actual project and the examples for engineering application were provided.