Study On Salt-Freeze Resistance Of Modified Pervious Concrete

Pervious concrete is an important material for the construction of sponge city pavement,but the unique pore structure of pervious concrete makes it vulnerable to the erosion of the external environment,and thus the popularity of pervious concrete is limited.In the north of China,the extreme weather is obvious;the water in the external environment often invades the inside of the permeable concrete,and the transparent concrete is often deteriorated and damaged by the freeze-thaw cycle.At the same time,a large number of Na₂SO₄ ions are distributed in the saline-alkali soil in the north of China,and the soil contains a large amount of salt.The rich Na₂SO₄ ions will erode the pervious concrete,which is also the cause of deterioration and damage to the pervious concrete.It can be seen that pervious concrete in the north of China is in a complex environment,and the deterioration and destruction of pervious concrete are often the result of many coupling factors.Among all kinds of external environmental effects,sulfate erosion and the freeze-thaw cycle are the most common external factors leading to the deterioration of pervious concrete structures.Therefore,the study on the resistance of pervious concrete to sulfate and freeze-thaw coupling is of great significance for the development and application of pervious concrete in northern China.In this paper,based on the actual climate in northern China,the resistance of pervious concrete to sulfate and freeze-thaw cyclic erosion（hereinafter referred to as salt-freeze resistance）in different environments of sulfate freeze-thaw solution was studied through modification of pervious concrete.The basic physical properties of pervious concrete,such as porosity,permeability coefficient,compressive strength,and flexural strength,were measured according to relevant test specifications.At the same time,combined with the actual environment in the north,a sodium sulfate solution with three concentration gradients of 0%,5%,and 10%was set as a freeze-thaw solution,and the mass loss rate and relative elastic modulus of pervious concrete were measured by the quick-freezing method as the indexes of salt-freeze resistance,so as to systematically study the salt-freeze resistance of pervious concrete.The main research conclusions are as follows:（1）An orthogonal experiment with four factors and three levels was used for the basic physical and mechanical properties test:fly ash,mineral powder,silica fume,and PVA fiber（Polyvinyl alcohol fiber）.By means of range and variance analysis,it is obtained that the important degree of influence of four factors on porosity,permeability coefficient,compressive strength,and flexural strength is as follows:mineral powder>silica fume>fiber>fly ash,and the optimal material composition under each factor is obtained within the test dosage range.At the same time,the mathematical fitting relationships of porosity and permeability coefficient,compressive strength,and flexural strength were established by a quadratic polynomial.The mathematical fitting results were good,and the fitting coefficients R2 were greater than 0.9.（2）For the durability test of salt-freeze resistance,it is found that fly ash,mineral powder,silica fume,and PVA fiber are all beneficial to the improvement of the salt-freeze resistance of pervious concrete when using range and variance analysis.Among them,silica fume sensitivity is the highest and improves the most.Compared with fly ash,mineral powder and PVA fiber are more conducive to sulfate resistance and improve the salt freeze resistance of pervious concrete more greatly.At the same time,the optimal material composition in terms of mass loss rate and relative dynamic elastic modulus of pervious concrete in different salt-frozen environments is obtained.（3）The erosion mechanism of pervious concrete is different in different erosion environments,and its deterioration rate is also different.In the freeze-thaw environment of 0%Na₂SO₄solution,the permeable concrete mainly exhibits physical and chemical erosion,while in the freeze-thaw environment of 5%and 10%Na₂SO₄ solution,the permeable concrete mainly exhibits physical-chemical erosion.In addition,the mass loss rate of pervious concrete in different freeze-thaw environments increases with the number of freeze-thaw cycles,and the increase trend is accelerated after the 50th freeze-thaw cycle.The relative dynamic elastic modulus decreased at first with the increase in the number of freeze-thaw cycles and then slowed down after the 75th freeze-thaw cycle.It is also found that the relative dynamic elastic modulus is more sensitive to the salt-frost durability of pervious concrete than the mass loss rate.（4）A permeable concrete frost resistance durability evaluation model is established in the framework of the analytic hierarchy process（AHP）,where condition and performance indicators are layered,combining the entropy value method and the gray correlation method to calculate the P-value of correlation between groups of data in the quantitative evaluation of permeable concrete frost resistance durability.The best combination of salt-freeze resistance is A1B3C3D3,fly ash 15%,ore powder 20%,silica fume 9%,and PVA fiber 0.6%.At the same time,the durability of pervious concrete is divided into four levels:excellent,good,qualified,and unqualified.