Research On The Damage Of Concrete Under The Combined Action Of Freeze-thaw Cycles And Sulfate Attack

The durability of concrete involves resistance to corrosion, frost, carbonation, stress corrosion and so on in material subject have much extended meaning to be studied. However, in practice, several deterioration mechanisms exist in erosion environment. Therefore, it is necessary to study the properties of the concrete subjected to more deterioration mechanisms. In north cold regions, the degradation of concrete construction seaside or in water usually is the result of freeze–thaw cycles, and the frost resistance of concrete is one of durability significant index. The sulfate attack is a extensive erosion of physical and chemical attack, combined with frost attack to be studied to avail significative reference on deterioration theory and practice of concrete construction.The macro-mechanical damage performance of concrete subjected to freeze–thaw cycles and sodium sulfate attack in solution were investigated in this paper. Three water-cement ratio(0.4,0.55,0.7) of concrete specimens were placed in four different sulfate erosion solutions, 0%, 3%, 5% and 8% sodium sulfate concentration respectively. Compare and contrast single sulfate immersed test with sulfate attack& freeze-thaw double factors test, and the evolution damage model of the relative dynamic modulus of elasticity (Ed) of concrete is proposed based on continuum damage mechanics.For the degradation of concrete is a reflection of the micro damage mechanics, the micro-experiments include two contents, the one is the analysis of quantitative evolution of Ettringite vs. erosion time measured with the method of K value approximate calculation in XRD (X-Radial Diffraction), the second is observing the formation and growth law of Ettringite, sodium sulfate crystal etc. corrosive crystals and the evolution of concrete micro-structure in sulfate& F/T erosion in situation by ESEM (Environment Scan Electron Microscope). There are several conclusions in macro and micro aspects of sulfate& F/T erosion tests. The results of single sulfate immersed 250d test indicate the sulfate erosion is a chronically process and the evolution of Ed submit to early enhancing consistency, keeping and descending further, corresponding with the negative damage mechanics of concrete in test phase. There are certain interaction effect between the action of freeze–thaw cycles and sulfate attack., especially in the small water-cement ratio specimens, the sulfate corrosive production compensate the frost crack expanding and defer the frost harm, the debase velocity of Ed and mass lesser than the big W/C ratio specimens, that degradation of concrete priority to frost damage.The distinctions of damage performance of concrete in sulfate&frost tests are revealed by the different evolution damage models. The damage evolution equation of W/C=0.4 concrete is fit in exponential curve D = aebN, corresponding the trend of accelerated increase later, for 0.55 specimen, is fit in D = aNb,and the damage of W/C=0.7 concrete is increase in linearity approximately, due to the frost resistance fail in the unsubstantial material.In the micro-experiments, the quantity of Ettringite in sulfate&frost tests is more than sulfate immersion, and improve obviously early till keeping later. The content range is 2.0% ~3.0% in small W/C ratio concrete and 3.0%~4.0% in bigger. The effect of sulfate concentration and the interfacial crack extend rapidity along with the ratio increasing in ESEM. The primary corrosive crystal is ettringite to sodium sulfate attack, it have several different growth modality in different interspaces, the gelatinous crystals and fascicular crystals are also observed in small ratio micro- structure specimens, and that, a large volume of the minuteness needle crystals exist in broad voids or cracks in big W/C ratio specimens, and gypsum crystals also exist in high sulfate ion local. Furthermore, the physical erosion of inflated Glauber's salt crystal inside or exterior of concrete also is distinct in sulfate&frost circumstance. The significant influence of material configuration, void circumstance, and erosion medium etc. to the two damage mechanisms is complicated and need to be investigated farther.