Experimental Study On Weathering Damage Mechanism Of Ancient Tuff Bridge Under Freeze-thaw-salt Couplin

The stone ancient bridge with natural rock as the building material is an important witness of the development of human history,and its spatial structure and construction methods are diverse,and it has rich historical and artistic value.Due to the long-term exposure of the stone ancient bridge to the natural environment,along with the environmental factors and change of climatic conditions,its surface layer often produces different degrees of weathering,deterioration,peeling and other diseases.Shaoxing is a famous bridge township in China,with more than 600 stone ancient bridges in its territory.The results of preliminary research show that the surface layers of these ancient bridges have suffered varying degrees of weathering damage.Therefore,it is of great theoretical and practical significance to carry out the research on the weathering damage mechanism and protective measures of the surface of the ancient stone bridge.In this paper,we will take the Bazi Bridge,a typical tuff ancient bridge in Shaoxing area,as an example,analyze the main influencing factors of weathering damage of the ancient tuff bridge through field investigation,and construct a judgment model based on analytic hierarchy method.Carry out the tuff weathering damage test under freeze-thaw-salt conditions,and quantitatively study the corrosion evolution process and physical mechanism of magnesium sulfate and sodium sulfate on tuff by high-precision CT scanning technology.The stress-strain relationship of tuff considering the freeze-thaw-salt coupling condition was derived,and the neural network prediction model of tuff stress-strain relationship under freeze-thaw-salt coupling condition was established,and the model prediction value and the indoor test value were compared and analyzed.Furthermore,relevant tuff surface weathering restoration measures are proposed.The main conclusions are as follows:(1)The main influencing factors of weathering damage of ancient tuff bridges include temperature,moisture and load.The number of freeze-thaw times,external load,rock moisture content and river p H had great influences on the safe and stable operation of the ancient tuff bridge,and the weight coefficients were 0.140,0.159,0.140 and 0.107,respectively.(2)When the erosion days reached 60 days,the sample size immersed in saturated magnesium sulfate was 50 mm × the mass loss rate of 100 mm tuff was1.56 %,while the mass of the sodium sulfate group increased by 0.33 %;This is due to the increase in mass due to salt crystal adhesion,increased porosity,and more water ingress.(3)When the number of freeze-thaw cycles reached 60 times,the sample size soaked in distilled water,sodium sulfate and magnesium sulfate solution was 50 mm× and the weathering loss mass of 100 mm tuff gradually increased.3.82 %;3.53 %and 3.58 %;(4)Under the open system,the mass loss rate caused by freeze-thaw-salt coupling erosion of tuff is greater than that of the closed system.After 21 freeze-thaw cycles,the mass loss rates of 25 mm×50 mm tuff samples immersed in distilled water,magnesium sulfate and sodium sulfate solution under open and closed systems were3.46%,1.96% and 3.81%,respectively.2.39 %,1.43 %,2.79 %.(5)When the number of days of salt erosion increased from 10 days to 60 days,the porosity of magnesium sulfate and sodium sulfate test groups increased by 6.59 %and 4.14 %,respectively.The porosity of the samples soaked in magnesium persulfate and sodium sulfate increased by 2.25 % and 2.18 %,respectively,when the number of freeze-thaw cycles increased from 10 to 60.It shows that salt corrosion has a greater influence on the weathering process of tuff than freeze-thaw.(6)The results of quantitative CT scanning showed that when the salt erosion time increased from 10 days to 60 days,the spalling volume of the outer layer of tuff saturated with magnesium sulfate was about 11973.2 mm3.The outer layer of tuff saturated with sodium sulfate had a spalling volume of about 6031.9 mm3.The freeze-thaw salt coupling erosion increased with the increase of the number of freeze-thaw cycles,and when the number of freeze-thaw cycles increased from 10 to60,the outer layer of tuff soaked in magnesium persulfate and sodium sulfate solution was about 4467.17 mm3 and 20807.4 mm3(7)The stress-strain neural network model of uniaxial compression of saturated tuff under freeze-thaw-salt coupling conditions can be preliminarily applied to the estimation,and the theoretical prediction value and the experimental measurement value have good consistency,and the prediction accuracy can reach 82 %.