Study on the durability deterioration law of marine concrete with nano-particles under the coupled effects of freeze-thaw cycles, flexural fatigue load and Cl- erosion

The marine concrete in cold region such as harbor, wharf and sea -bridge is inevitably subjected to the coupling effects of flexural fatigue load, freeze -thaw cycles and chloride ion (Cl - ) erosion. This paper studies the durability of unreinforced concrete under the coupling effects of three factors. And in order to highlight the effects of freeze -thaw cycles, the two -factor coupling durability test of flexural fatigue load and Cl - erosion and the three -factor coupling durability test of freeze -thaw cycles, flexural fatigue load and Cl - erosion are designed. And the microstructures of various concretes are analyzed by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results show that under the coupling effects of three factors, on the one hand the icing of pore water can improve the deformation resistance of concrete; on the other hand, the expansion pressure can accelerate the deterioration of pores and promotes the development of fatigue cracks. The coupling effects of three factors significantly reduces the flexural fatigue life of concrete, and the more the number of freeze -thaw cycles, the more significant the adverse effects on the flexural fatigue life of concrete. The addition of nano -particles (The cement in concrete is replaced by the same content of nano -particles) can improve the adverse effects of freeze -thaw cycles on the flexural fatigue life of concrete, and enhance the flexural fatigue life of concrete. When the amount of nano -particles is optimal, the fatigue life of NC10 and NS20 is increased by 47.6% and 52.3% respectively. The Cl - erosion resistance of concrete increases firstly and then decreases with the increasing amount of nano -particles. The free Cl - content of the compression zone in NC10 and NS20 can be reduced by 22.4% and 24.2% respectively, and the free Cl - content of the tensile zone in NC10 and NS20 can be reduced by 16.6% and 20% respectively. Under the coupling effects of three factors, with the continues erosion of Cl - , the Ca/Si value and the atomic percentage of Ca in interface transition zone (ITZ) of concrete gradually are decreased, which makes the decreased extent of C - S - H gel and Ca(OH) 2 in concrete. The addition of nano -particles can reduce the decrease extent of Ca atom percentage in ITZ, reduce the decomposition of C - S - H gel and Ca(OH) 2 , and improve the fatigue life and Cl - erosion resistance of concrete.