Research Of The Bonding Property Between Steel And Concrete Under Sustained Compressive Loading After Single-side Salt Freezing

Winter in northern China is usually extremely cold,and many concrete structures,such as roads treated with deicing salt,nearshore and offshore buildings,are subject not only to simple freeze-thaw damage but also to salt attack.This type of damage typically occurs on a single surface,and current research in this area is still relatively limited.In order to investigate the degradation of bond performance and the mechanism of action of steelreinforced concrete under single-sided salt freezing action,this study divided 69 steelreinforced concrete specimens into 21 groups based on different times of single-sided saltfrost cycles(T),different loading stress ratios(F),and different concrete protective layer thicknesses(D),according to relevant specifications but not limited to them.Single-sided salt freezing tests and steel bar pull-out tests were conducted sequentially,observing and recording changes in the various morphological parameters of the specimens during the experiments and analyzing and calculating changes in the various parameters of the specimens.The changes in the properties of the concrete material and the bond performance of the steel-reinforced concrete during the experiments were thus demonstrated in a more intuitive and reliable manner.The results and conclusions of this research are as follows:(1)A self-designed mold for single-sided salt freezing reinforced concrete specimens has been successfully developed.The new mold is well-suited for the innovative specimen size and testing methods used in this experiment and effectively resolves issues associated with the inconvenient and poor quality of specimen formation resulting from the recommended "two specimens in one mold" method in the code.This new mold also avoids the need for the potentially damaging step of removing the mold and cutting the specimens.Furthermore,the mold has reserved holes for reinforcing bars,which allows for casting reinforced concrete specimens,expanding the scope of research on single-sided salt freezing testing of concrete.The research team has applied for a utility model patent using this mold.(2)A new method has been found to inhibit salt freeze-thaw damage.The loading state is a crucial influencing factor,as indicated by both the apparent phenomena observed in single-sided salt freeze-thaw tests and experimental data obtained from tests such as mass loss rate and relative dynamic modulus of elasticity loss rate,as well as experimental data obtained from steel bar pullout tests,such as specimen bond strength.The loading state plays a crucial role in inhibiting single-sided salt freeze-thaw damage: its presence can reduce the porosity of the concrete structure,making it more compact,thereby affecting the specimen’s water absorption capacity,reducing the entry of salt solutions into the structure’s interior,and thus lowering the degree of frost heave force and salt erosion.On the other hand,the presence of loading can also balance the frost heave force and protect the structure.However,the experimental data obtained from steel bar pullout tests show that loading can have a detrimental effect on the bond performance of steel-reinforced concrete specimens,which is also confirmed by microscopic inspection results.Therefore,in practical engineering,most steel-reinforced concrete structures are in a loaded state,and if laboratory tests are to better guide engineering practice,the actual service state of the target structure should be appropriately considered.If the target structure is under constant compressive load,its salt freeze-thaw resistance should be improved(if it is under sustained tensile load,its salt freeze-thaw resistance will inevitably decrease).This rule can also guide engineering practice: for concrete structures that are affected by salt freeze-thaw,appropriate measures can be taken,such as applying compressive loads or setting them as prestressed members in the early design stage.(3)The recommended thickness of the protective layer of single-side salt-frozen reinforced concrete has been obtained.From the various test data obtained during the singleside salt-frozen test phase,it can be seen that the thickness of the concrete protective layer is not directly related to salt-frozen damage.However,from the various data obtained during the stage of the pull-out test of the steel bars,there is a strong positive correlation between the thickness of the concrete protective layer and the bonding performance of the specimen.This is closely related to the range of the "confinement zone" around the steel bars.Within the confinement zone,the crack propagation rate is slower,but once the crack develops to the edge of the confinement zone,the rate will increase.Therefore,the radius of the confinement zone is the most suitable thickness for the concrete protective layer.Analysis of the test data indicates that a thickness of approximately 45 mm is the most reasonable thickness for the concrete protective layer of a structure subjected to single-side salt-frozen action.