Bo Diao

Effects of Freeze-Thaw Cycles and Seawater Corrosion on the Behavior of Reinforced Air-Entrained Concrete Beams with Persistent Loads

The effects of freeze-thaw cycles and seawater corrosion on the material properties of air-entrained concrete specimens and structural behavior of reinforced air-entrained concrete beams under persistent loads were investigated in the laboratory. The air content of the specimens was designed to be 3.2 and 6.1%. Results show that when subjected to alternating actions of freeze-thaw cycles and seawater immersion, the compressive strength of concrete decreased when the number of freeze-thaw cycles increased, with the ones with higher air content showing better resistance to freeze-thaw attack. In the case of reinforced air-entrained concrete beam, the presence of persistent loads accelerated the structural behavior deterioration process. When the persistent load reached half of the ultimate load, the reduction of the ultimate load and ductility was found to be a couple of times greater than those without the persistent load. Compared to the beams with 3.2% air content, the yield load, the ultimate load, and the ductility of the beams with 6.1% air content increased by 0.5, 2.5, and 13.2%, respectively. It was concluded that air entrainment has a more considerable effect on enhancing the ductility of reinforced concrete structures in cold coastal regions.

Effects of Mixed Corrosion, Freeze-Thaw Cycles, and Persistent Loads on Behavior of Reinforced Concrete Beams

The effects of mixed corrosion and freeze-thaw cycles on the mechanical properties of concrete prism specimens and the effects of mixed corrosion, freeze-thaw cycles, and persistent loads on the structural behavior of reinforced concrete beams were experimentally studied. A mixed solution of NaCl and Na2SO4 was used as a corrosive medium. Results show that under alternating actions of freeze-thaw and mixed corrosive agents, increasing the number of freeze-thaw cycles decreases the compressive strength and the elastic modulus of concrete and increases the compressive strain corresponding to the maximum compressive stress. The degradation of concrete material properties accelerates with the increase of water-cement ratio. For reinforced concrete beams, a 4% reduction in the loading capacity is found when these are subjected to freeze-thaw cycles and mixed corrosion only. However, if these actions are coupled with persistent loading, as expected during the service life of reinforced concrete structures in cold regions, a more rapid drop in the strength and deformation capacity of the beams is identified. The degradation is enhanced by a larger persistent-loading ratio. The significance of an accurate simulation of service conditions in the durability study of reinforced concrete structures in cold regions is highlighted.

Effects of Seawater Corrosion and Freeze-Thaw Cycles on Mechanical Properties of Fatigue Damaged Reinforced Concrete Beams

The effects of seawater corrosion and freeze-thaw cycles on the structural behavior of fatigue damaged reinforced concrete (FDRC) beams were experimentally studied. Results show that the residual strength of FDRC beams reduces as the fatigue load level (the ratio of maximum fatigue load to the ultimate static load) increases. The reduction in the loading capacity of FDRC beams in atmosphere environment was about 6.5% and 17.8% for given fatigue load levels of 0.2 and 0.3, respectively. However, if FDRC beams are exposed to the environment of seawater wet-dry cycles or to the environment of alternating actions of freeze-thaw and seawater immersion, as expected during the service life of RC bridge structures in coastal regions or in cold coastal regions, a more rapid reduction in the strength and stiffness of the beams is observed. The significance of an accurate simulation of working load and service condition RC bridge structures in coastal regions and cold coastal regions is highlighted.

Mechanical Performance and Chloride Diffusivity of Cracked RC Specimens Exposed to Freeze-Thaw Cycles and Intermittent Immersion in Seawater

The effects of crack width on chloride ingress and mechanical behavior of reinforced concrete (RC) specimens were experimentally studied after exposure to 300 cycles of freeze-thaw and seawater immersion (75 times). Cracks were induced prior to exposure by an eccentric compression load which was sustained until the end of the exposure period. The maximum cracks widths induced in the four column specimens were 0, 0.06, 0.11, and 0.15 mm, respectively. Results show that when the crack width was less than 0.06 mm, the effect of cracks on chloride ingress could be neglected. However, when the crack width was more than 0.11 mm, chloride ingress was accelerated. Results of static loading tests show that both yield load and ultimate load of RC columns decreased as crack width increased. When the crack width was 0.15 mm, yield load and ultimate load of RC column specimen decreased by 17.0% and 18.9%, respectively, compared to a specimen without cracks. It was concluded that crack width significantly promoted local chloride ingress and mechanical performance degradation of RC structures in cold coastal regions or exposed to deicing salts.

Bond Effects between Concrete and Steel Bar Using Different Diameter Bars and Different Initial Crack Width

The importance of an accurate simulation of service conditions in the bond performance of reinforced concrete structures in coastal regions is highlighted. Four widths of initial crack of 0, 80, 150, and 210 microns were artificially made by inserting slice into bond specimens during concrete casting. Three bar diameters of 10 mm, 14 mm, and 18 mm were selected. At 28 days, the bond specimens were exposed to the environment of wet-dry cycles of seawater and atmosphere for another 90 days. The pull-out test was then conducted and chloride contents were tested at crack area along 40 mm depth. Results show that, for the specimen with 10 mm bar diameter, cracks width of less than 80 microns vanished rapidly during wet-dry cycles; for other specimens, cracks width of 100–150 microns decreased slightly. However the cracks of width more than 200 microns increased gradually; the chloride content decreased along the depth of concrete, and the chloride content increased as the widths of initial cracks increased or as the bar diameters increased. The ductility of bond specimens decreased as the diameter increased.

Experimental Study on the Electrothermal Effect of Concrete Reinforced with Hybrid Carbon and Steel Fiber

The electrothermal effect of the carbon fiber concrete provides a new way to eliminate the snow and ice on an airfield runway. But the electrothermal effect of concrete reinforced with hybrid carbon and steel fiber and the appropriate fiber content are still unknown. The research aims at the questions above and investigates the electrothermal effect of concrete slabs with different hybrid fiber contents. It shows that the addition of steel fiber can increase the electrothermal effect of hybrid fiber concrete when the carbon fiber content is about 0.6% ~ 0.8% of cement by weight, which means that steel fiber can help to form conductive network. But the electrothermal effect of hybrid fiber concrete is reduced when carbon fiber content reaches 1.0% of cement by weight, because the addition of steel fiber will block uniform dispersal and effective overlap of the carbon fiber and the conductive network gets worse. Air bubbles generate inside the concrete can also increase the electrical resistivity of the hybrid fiber concrete and the electrothermal effect gets worse.

IFC-Compatible Durability Controlling System for RC Structure

Industry Foundation Class, which is an international standard Building Information Model (BIM) is adopted to archive and manage RC structure life cycle durability information. An IFC-compatible durability controlling system (IFC-CDCS) for RC structure is developed, which can retrieve, resolve, create, and analyze IFC files at the level of source code. The framework of IFC-CDCS is demonstrated, and the essential module of IFC interface is elaborated by specifying the data type mapping mechanism, IFC resolving toolkit, IFC data extending methodology. An IFC project created with Autodesk Revit is loaded into the system to testify that the information from IFC file can be correctly and efficiently extracted and processed by IFC-CDCS.

RC Structure Durability Information Model Based on IFC

An extension of the BIM technology that allows integrating durability factors into designing process, appraising durability and predicting residual life of building structures is proposed. Furthermore, by integrating national durability code, achievements of durability researches and the IFC standard which is developed by International Alliance for Interoperability (IAI) with the standard number of ISO10303 to facilitate AEC information sharing and exchanging, a BIM-Based Durability Appraising Software System for Reinforcement Concrete Structure is developed. The durability information model and IFC data interface are elaborated. Then the system is verified by a case study to be capable of improving lifecycle management of AEC projects, upgrading durability level and optimizing the lifecycle cost structure of building projects.

Experimental and Analytical Studies of U-Shaped Thin-Walled RC Beams Under Combined Actions of Torsion, Flexure and Shear

U-shaped thin-walled concrete bridge beams usually suffer the combined actions of flexure, shear and torsion, but no research about the behavior of U-shaped thin-walled RC beams under combined actions has been reported in literature. Three large specimens of U-shaped thin-walled RC beams were tested under different torque–bending moment ratios (T–M ratios) of 1:5, 1:1 and 1:0 to investigate the mechanical responses such as crack patterns, reinforcement strains, failure modes and ductility. The testing results showed that ductile flexural failures occurred for all three of the U-shaped thin-walled beam specimens, although the combined shear effect of circulatory torque, warping torque and shear force increased as the T–M ratio increased from 1:5 via 1:1 to 1:0, reflected by diagonal cracks and stirrup strains. More specifically, basically symmetrical flexural failure was dominated by the bending moment when the T–M ratio was 1:5; flexural failure of the loaded half of the U-shaped thin-walled section was dominated by the combined action of the bending moment and warping moment, while there were only a few cracks on the other half of the U-shaped section when the T–M ratio was 1:1; and anti-symmetrical flexural failure was dominated by the warping moment when the T–M ratio was 1:0 (pure torsion). A simple method to calculate the ultimate load of such U-shaped thin-walled RC beams under different T–M ratios was suggested, and the calculating results were corresponding well with the experimental results.

Hysteretic Behavior of Eccentrically Loaded Reinforced Air-Entrained Concrete Columns under Combined Effects of Freeze-Thaw Cycles and Seawater Corrosion

Besides service loads, reinforced concrete structures in cold coastal seismic regions are subjected to multiple attacks of freeze-thaw cycles and seawater corrosion as well as the earthquake struck. An experimental study was conducted to investigate the seismic response of eccentrically loaded reinforced air-entrained concrete columns under alternative actions of freeze-thaw cycles and chloride corrosion. Results show that, after 300 times of freeze-thaw cycles alternated with 100 times of seawater immersion, the hysteretic behavior of the eccentrically loaded columns manifested an apparent asymmetric pattern. Under forward cyclic load, the existence of larger eccentric load rendered the reduction of the ultimate load and the ductility of a column by up to 20.3% and 46.05%, respectively, but it had a positive effect if reverse cyclic load was applied. The presence of eccentric load could have a considerable impact on the seismic behavior of reinforced air-entrained concrete columns served in an aggressive environment.