工程科学与技术 (Jan 2025)
The Bonding Performance of Steel-UHPC after High Temperature with Water Cooling
Abstract
ObjectiveUltra High Performance Concrete (UHPC) has been widely used due to its dense micro-structure, which endows it with ultra-high mechanical properties. Under earthquake action, it has strong synergistic deformation ability with steel, ensuring that the structure or components will not experience degradation in stress performance due to early interface bonding failure. Fire has become one of the important forms of urban disasters. UHPC is usually directly exposed to high temperature environments, with high internal steam pressure, resulting in a large number of cracks and corner peeling, and then causing material performance degradation. Firefighting is an indispensable measure for urban building fires. During firefighting, the ambient temperature rapidly drops. For UHPC with dense micro-structure, the internal temperature stress distribution is extremely uneven, accelerating the expansion and even bursting of concrete cracks, and then resulting in unclear mechanisms for the deterioration of the interface bonding performance between UHPC and steel. Therefore, accurately understanding the degradation mechanism of the bonding performance between the steel and UHPC interface after high temperature with water cooling is crucial for evaluating the seismic performance of building structures after fire extinguishing. Currently, there is no report on the inter-facial bonding performance of UHPC steel after high-temperature water cooling. This paper presents an experiment research on bond-slip performance for steel and UHPC after high temperature with water cooling, revealing the degradation principle of bond-slip performance.MethodsThe research findings on the mechanical properties of reinforced concrete after high temperature and the inter-facial bonding performance with steel, along with the pre-test results and extant research results, were taken into consideration. This paper conducted three test temperature and cooling conditions, including room temperature, water cooling and natural cooling. Furthermore, the temperature (T), constant temperature duration (t), steel protective layer thickness (Cs) and volume reinforcement ratio (ρv) were chosen as test design parameters. The monotonically push out tests were conducted for twelve steel-UHPC specimens after high temperature with different cooling method. The apparent characteristics and mass loss rate of the specimen after high temperature were recorded in the experiment, and the displacement slip at the loading end and free end were monitored during the loading process. Based on experiments, by analyzing the collected curve data and observed phenomena, the inter-facial bonding performance between UHPC and steel after high temperature with water cooling was analyzed, and a calculation method for inter-facial bonding strength was proposed to evaluate the bonding performance after firefighting.Results and DiscussionsThe main results and discussions as follows: (1) The surface features, including color and crack development, were observed. The mass loss rate was measured. As the temperature rises from 20 ℃ to 800 ℃, the color of the specimen changes from dark gray to gray white, and the surface cracks gradually increase. As the temperature rises from 200 ℃ to 800 ℃, the mass loss rates were 1.60%, 2.54%, 4.0 % and 4.48%. (2) The process and morphology of bond failure were observed. The characteristic of load-slip curves and the bonding strength of the corresponding characteristic points were analyzed. The surface cracks of the specimen develop outwards along the original cracks after high temperature (cracks along the 45° direction at the tip of the steel flange, cracks parallel to the flange, and cracks in the middle of the flange) and extend to the side of the concrete. The load-slip curves of all specimens roughly goes through three stages, namely the initial slip stage, the slip failure stage, and the stable slip stage, where the chemical cementation force, mechanical biting force and frictional resistance gradually come into play and successively provide the bonding force at different stages. (3) The effects of temperature(20℃, 200℃, 400℃, 600℃, 800℃), cooling method(water cooling and natural cooling), constant temperature duration(60 min, 90 min, 120 min), steel protective layer thickness(50 mm, 60 mm, 70 mm) and volume reinforcement ratio(0.160%, 0.238%, 0.317%) on the inter-facial bonding performance between UHPC and steel were studied. Compared to normal temperature conditions, after the temperature of 200 °C, 400 °C, 600 °C and 800 °C, the ultimate bonding strength decreased by 13.6%, 49.3%, 92.3% and 98.2%，the residual bonding strength decreased by 16.8%, 57.1%, 94.6% and 98.9%. Compared with the constant temperature duration of 60 minutes, when the constant temperature duration reaches 90 minutes, the ultimate bonding strength and residual bonding strength are increased by 15.4% and 36.2%. When the constant temperature duration reached 120 minutes, the ultimate bonding strength and residual bonding strength decreased by 24.7% and 31.0%. When the steel protective layer thickness increases from 50 mm to 60 mm, the ultimate bonding strength increases by 15.2%, and the residual bonding strength increases by 1.25%, increasing from 60 mm to 70 mm, the ultimate bonding strength increased by 7.75%, and the residual bonding strength increased by 11.3%. When the volume reinforcement ratio increases from 0.160% to 0.238%, the ultimate bonding strength increases by 7.7%, and the residual bonding strength increases by 25.4%, increasing from 0.238% to 0.317%, the ultimate bonding strength increased by 25.9%, and the residual bonding strength increased by 25.3%.When the temperature is 400 °C, compared with the natural cooling, the ultimate bonding strength and residual bonding strength of the specimens after water cooling are larger, which are increased by 41.7% and 64.5%. (4) Based on the experimental results, a calculation method for the bonding strength after water cooling was proposed.ConclusionsAs the temperature rises from 20 ℃ to 800 ℃, the mass loss rate gradually increases to 4.48%, where the growth rate shows a trend of increasing rapidly first and then slowly. The load-slip curves of all specimens roughly goes through three stages, namely the initial slip stage, the slip failure stage, and the stable slip stage, where the chemical cementation force, mechanical biting force and frictional resistance gradually come into play and successively provide the bonding force at different stages. The bonding strength continues to degrade, and compared to normal temperature conditions, the loss of bonding strength after 400 ℃ has exceeded 50%. As t increases, the bonding strength first increases and then decreases, with the maximum bonding strength observed at a constant temperature of 90 minutes. Increasing Cs and ρv can effectively improve the bonding strength of specimens. Compared with naturally cooled specimens, the ultimate bonding strength τu and residual bonding strength τr are both increased by about 30% after water spray cooling. The proposed calculation method can provide a basis for the performance evaluation and repair reinforcement of steel-UHPC composite structures after disasters.
