Propagation Rules Of Corrosion Cracks And Strength Evolution Of Concrete In Seawater

In marine environment,sulfate attack is an important factor affecting the durability of concrete structures.When sulfate ions in seawater invade the concrete interior,the chemical reaction between the liquid and solid phases in the concrete pores will be triggered,thus leading to delayed ettringite formation.The nucleation and growth of delayed ettringite in concrete will generate expansion stress,which is one of the important factors leading to the nucleation and expansion of micro-cracks in the material,and also one of the fundamental reasons for the damage evolution of concrete materials.The main contents of this paper are as follows:Firstly,the evolution characteristics of cracks initiation and propagation on concrete surface under sulfate attack are studied.According to the rate equation of chemical reaction for the formation of delayed ettringite,a dynamic model of information entropy evolution for surface crack propagation was established.The universal information entropy evolution equation is obtained through normalization treatment,which can well characterize the surface crack evolution characteristics of concrete under sulfate attack,and its evolution rule has nothing to do with the water-to-cement ratio of concrete itself.Secondly,a three-point bending beam specimen with prefabricated initial crack was designed to study the influence of concrete materials on its fracture toughness in sulfate solutions of different concentrations.By means of digital image correlation technique,the displacement and strain changes in the crack area are analyzed.And the change of fracture toughness of three-point bending specimens in different solution concentration with erosion time was calculated.According to the rate equation of chemical reaction,the evolution equation of internal expansion force of concrete eroded by sulfate solution is derived.In combination with the damage theory of materials,a theoretical model of fracture toughness evolution of concrete materials removed hydration effects is obtained.The results show that the model can well characterize the evolution of fracture toughness of concrete materials.Thirdly,the change rule of macroscopic physical and mechanical properties of concrete with erosion time under different salt solutions and the detection and analysis of microscopic corrosion products were studied.And the effects of ions in different salt solutions on the strength damage of concrete were discussed.The sensitivity analysis method based on concrete strength is adopted.The sensitivity of the interaction of ions to the concrete strength removed hydration effects in multi-salt solutions is analyzed quantitatively.Quantitative results that the addition of chloride ions inhibited the erosion of concrete by sulfate ions were obtained,while the addition of magnesium ions or magnesium and chloride ions promoted the erosion effect of sulfate ions.Fourthly,under the condition of sulfate erosion,the variation rule of concrete surface expansion rate and surface crack length and width with different water-to-cement ratio,different diameters and different thicknesses was studied.And it is concluded that the water-to-cement ratio and thickness of concrete have great influence on the surface expansion rate and crack length.By using the Boltzmann distribution function,a simple equation can be obtained to describe the general law of concrete surface expansion rate under sulfate erosion,and it has no relation with the water-to-cement ratio,thickness and diameter of concrete materials.Finally,considering the temperature effect,the effects of various supplementary cementing materials on the resistance of concrete materials to sulfate erosion were studied.The mechanism of different concrete materials resisting the attack of sulfate solution under different temperature is analyzed by means of the method of macro and micro,and the effect of different supplementary cementing materials under different temperature is obtained.The main conclusion is that the sulfate resistance of silica fume is the most stable and the best;the resistance of fly ash and slag powder to sulfate attack is the second;high temperature environment is very adverse to metakaolin sulfate resistance;the ability of limestone powder to resist sulfate erosion is reduced gradually at low temperature;the sulfate resistance of steel slag powder is better under the condition of variable temperature than that under the condition of constant temperature.