Experimental Study On The Performance Of Polypropylene Fiber Concrete Under Salt And Frost Erosion

With the large-scale construction of water diversion and transfer projects in the northwest of China,the problem of ecological vulnerability of the region has been better solved.However,hydraulic buildings in which are often in a harsh special environment,resulting in post-maintenance work and increasing costs,so there is an urgent need for concrete durability,to enhance the safety of hydraulic buildings in extreme environments to carry out research related to service life.In order to improve the durability of plain concrete,a commonly used method is to add fibers to enhance the properties of the concrete matrix in a targeted manner using its material properties.As a synthetic fiber,polypropylene fiber has superior performance,simple production process and low cost,which makes it widely used in different fields.According to a large number of tests to prove that polypropylene fiber in the matrix distribution is more uniform,the right amount of the addition can enhance the concrete internal matrix interface bonding strength,and at the same time when the concrete is cracked by tensile,compressive stress and impact and other loads,due to the presence of fibers can effectively prevent the further development of cracks.Therefore,it is of practical importance to study the durability deterioration of polypropylene fiber concrete under the coupling effect of salt attack and freeze-thaw,and to describe the evolution of salt-frost damage qualitatively and quantitatively,so as to provide the relevant theory for the durability design,mechanical performance prediction and safe service life estimation of polypropylene fiber concrete materials in cold and arid regions.To this end,this paper takes the Northwest Jingdian irrigation area as the study area,carries out field investigation in the study area,collects soil and water samples,and conducts laboratory analysis on the spalling of concrete buildings to obtain the composition of corrosion media;on this basis,simulates the actual environment in which the concrete buildings are located in the project,and selects clear water,3%Na Cl,and 5%Na₂SO₄ as the main freeze-thaw media for simulation tests.To this end,this paper takes the Northwest Jingdian irrigation area as the study area,and determines the freeze-thaw medium as clear water,3%Na Cl,and 5%Na₂SO₄ by conducting field surveys of the study area as well as collecting soil and water samples and spalling analysis of concrete buildings for inspection and analysis to derive the erosion medium content,and simulating the service environment of concrete buildings in the field.Different doses of polypropylene（0,0.6 kg/m³,0.9 kg/m³,1.2 kg/m³,1.5 kg/m³）were designed to test the salt freezing resistance of fiber concrete using the"fast freezing method".The different properties of concrete were tested at macro and micro levels to clarify the freezing process of fiber concrete in different freezing and thawing media at different dosing levels and to reveal the damage effect.To establish a salt-frost damage evolution model for concrete based on damage theory and Weber distribution theory,through which the concrete damage state can be described and the service life of the structure can be predicted.Considering the effects of different environmental factors,a high-precision concrete macro-mechanical property prediction model is constructed using artificial intelligence neural network,which can predict the strength values of fiber concrete under different salt-freeze times.The main conclusions of this paper are as follows:（1）The effect of different polypropylene fibre admixtures on the durability of concrete was investigated,revealing the mechanism from the macro and micro scales as well as energy dissipation.The results show that the incorporation of polypropylene fibers can effectively improve the degree of concrete apparent morphological damage,enhance the strength of concrete matrix to a certain extent,delay the damage process of the matrix,and enhance the overall salt-frost erosion resistance of concrete,but ultimately still cannot prevent the failure of the matrix.From the point of view of energy dissipation,the fiber increases the interface inside the matrix,when the external energy input process,the fiber’s own strength and friction can offset part of the energy for the matrix,while due to the fiber"bridging effect",so that the increase in the internal dissipation of concrete energy conversion rate is slow,delaying the arrival of crack penetration time,improve the ductile deformation and ductile damage characteristics of the specimen.However,the performance of the concrete was not enhanced with the increase of fiber admixture,and the experimental results proved that the optimum admixture of polypropylene fiber in this study was 1.2 kg/m³.（2）The durability tests of polypropylene fiber concrete under the action of different freeze-thaw media（clear water,3%Na Cl,5%Na₂SO₄ solution）were carried out,and the mechanism of the performance damage was analyzed at both physical and chemical levels.The results show that the most serious damage to concrete in the three freeze-thaw media is in the chloride salt solution,followed by the sulfate solution,and the smallest degree of damage to clear water.This is because the main source of damage from freezing and thawing of clear water is caused by the process of freezing and expansion of water molecules and their migration within the matrix;while salt intrusion-freeze-thaw damage is based on physical damage,the salt solution in the matrix internal physical and chemical reactions to generate corrosive and expansive products,the pore boundaries continue to produce crystallization pressure,expansion pressure,making concrete damage more significant.The incorporation of polypropylene fibers reduces the size and number of internal pore sizes and the fluid content in the pores,effectively improving the pore structure inside the concrete and making the damage distribution of the matrix more uniform after salt-freezing.（3）A salt-frost damage model for polypropylene fiber concrete with the dynamic elastic modulus as the damage degree and the number of freeze-thaws as the random variable was established,which can predict the service life of concrete in different environments and obtain the maximum improvement of its structural service life by about 23.93%.And based on this,the strength decay deterioration model of polypropylene fiber concrete in two salt solutions is established,and the high correlation between material damage degree and strength value can be seen through the expression of the relationship between its damage state and strength value under the coupling effect of different solutions and freeze-thaw cycles.（4）A model for predicting the mechanical properties of polypropylene fiber concrete based on the bionic intelligence algorithm was constructed,which improves its generalization ability and application scope compared with the traditional neural network prediction model,effectively solves the situation that the traditional model tends to fall into local optimum and the lack of ability to deal with nonlinearity,more accurately predicts the strength change of concrete under the influence of nonlinear characteristics and external multi-factors,and solves the problem that it takes a lot of time and cost to obtain the targeted results from the laboratory-developed mix design.