The coupling effect of bending load and salt spray erosion during the service of a sea-crossing bridge accelerates the deterioration and durability of concrete and dramatically reduces the load-carrying capacity of the bridge. The effects of nanoparticles on the durability of marine concrete exposed to bending loads and salt spray erosion were studied. In this paper, nano-SiO2 and nano-Fe2O3 were mixed into plain concrete. Free chloride ions (Cl−) were titrated at different concrete depths using a four-point loading device and a self-developed salt spray erosion test chamber. Test results showed that chloride ion levels in the tensile and compressive zones for both nanoconcretes were lower than plain concrete at the same timepoint. The optimal mixtures of the two nanoparticles were 2% and 1%, and the improvement of nano-SiO2 was more significant than nano-Fe2O3. Due to the special properties of nanomaterials, they effectively improved the microstructure of concrete and the composition of cement hydration products. This allowed concrete to become more compact, reduced crack generation, increased the difficulty of Cl− migration inside the concrete, and improved the overall durability of marine concrete upon exposure to bending loads and salt spray erosion.
The service environment of concrete in the marine environment is harsh, and demands regarding the durability of marine concrete have increased. Marine concrete in harbor and wharf areas suffers from the combined effect of fatigue load, dry–wet cycles, and Cl− erosion, which can result in spalling of the concrete surface, corrosion of the internal reinforcement, and even concrete damage. This paper reviews recent research results on the durability of concrete and reinforced concrete (RC) under the combined effect of fatigue load, dry–wet cycles, and Cl− erosion. We further assess the variation in Cl− transport properties with fatigue load, the causes behind the reduction in the carrying capacity of RC products under fatigue load, the methods of Cl− erosion on concrete under the pressures imposed by dry–wet cycles, and the damage of the protective layer of concrete due to accelerated Cl− erosion caused by the action of dry–wet cycles. Further studies are needed on the durability of concrete under the action of fatigue load, wet and dry cycles, and Cl− erosion, in addition to the testing of the durability of concrete under the combined effects of the afore-mentioned various factors.
The concrete of harbor wharf is in the environment of multifactor erosion. Under the coupled effects of dry–wet cycles and nitrate erosion, the durability of marine concrete gradually decreases. Therefore, it is an important issue to improve the durability of concrete. The changes of concrete durability are investigated under the coupled effects of dry–wet cycles and nitrate erosion, and the concrete durability are improved by adding nanoparticles. Nano‐concretes are made by adding different amounts of nano‐SiO2 and nano‐Al2O3 to plain concrete. The relative dynamic elastic modulus of nano‐concretes and the NO3− content are measured as indicators to evaluate the durability of nano‐concretes. The microstructure of nano‐concretes is analyzed through scanning electron microsope (SEM) and energy dispersive spectrometer (EDS). The test results show that relative dynamic elastic modulus of nano‐concretes is increased obviously, and NO3− content of nano‐concrete at different depths is lower than that of plain concrete under the same number of dry–wet cycles. NO3− content of nano‐concrete is lower than that of plain concrete. Nanoparticles can optimize the pore structure of concrete and generate more C‐S‐H gels. Therefore, concrete durability can be efficiently increased through adding nano‐SiO2 and nano‐Al2O3 under the coupled effects of dry–wet cycles and nitrate erosion.
In order to study the influence of nanomaterials on the carbonation resistance of marine concrete under bending loads, an appropriate amount of nano-SiO2 was added to plain concrete, and a self-developed carbonation box and bending loading device were used to conduct a coupling test. Four different stress levels were set to measure the carbonation depth of nano-concrete at different ages. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to analyze the concrete interfacial transition zone. The carbonation depth was used as the test index to evaluate the durability of nano-SiO2-based concrete under the combined action of bending load and carbonation. The test results showed that the compressive and flexural strengths of concrete remarkably improved when the nano-SiO2 concentration was 2%. Compared with regular concrete, the compressive and flexural strengths of nano-SiO2 based concrete improved by 15.5% and 15.3%, respectively. When the stress level was 0.15 and 0.6, the carbonation depths of NS20 were 20.5 and 18.4% lower than those of PC in the tensile zone and 18.9 and 23.7% lower than those of PC in the compression zone. The carbonation depth of the NS20 tensile zone was lower by 31 and 18.4% at 3 and 28 days than that of PC. Compared with PC, the carbonation depth in the compression zone of NS20 decreased by 50 and 23.7%, and the carbonation depth of nano-concrete was significantly lower than that of conventional concrete under the same stress level and age. When the stress level is constant, the carbonation depth of the tension zone and compression zone increases gradually with the increase in age, and the carbonation depth of the concrete in the first 7 d was 50% that at 28 days. Under the same age, the carbonation depth in the tension zone increased with increasing stress levels, while the carbonation depth in the compression zone decreased with increasing stress levels. When the stress level was 0.3–0.45, the slope of the carbonation depth curve significantly increased. SEM and XRD analysis results revealed that nano-SiO2 significantly improved the internal structure of concrete by reducing the width of the microcracks, the number of pores, and the number of microcracks. The number of C3S/C2S and CaCO3 crystals in nano-SiO2 based concrete was significantly less than that in plain concrete, and the amount of C-S-H gel was more than that in plain concrete. Under bending loads, the nano-SiO2 significantly improved the carbonation resistance of concrete. When the dosage of nano-SiO2 was 2%, its improvement effect was the most significant.
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