A fatigue behavioral simulation for steel fiber reinforced concrete (SFRC) subjected to high cycle repetition of loads is presented. The fatigue constitutive models for normal strength concrete developed in the past decades are extended to those for SFRC with regard to tension, compression and shear transfer along dispersed cracking. The hysteretic path-dependency of SFRC is extensively focused on as well as post-cracking tension softening, because it greatly affects the stress-strain amplitudes of SFRC inside structures. The interaction of multi-directional cracks is taken into account based upon the fixed crack approach for enabling the damage evolution under the principal stress rotation. The rate effect on the stress and strain relation is also formulated to take into account the nonlinearity related to both loading rates and numbers of cycles. These proposed models for SFRC are experimentally verified in view of S-N diagrams of flexural tension for practical use.
The impact of drying at both material and structural levels is discussed based on the multi-scale thermo-mechanistic modeling. The effects of hygral states at nano to micro scales on the macro scale mechanics are focused on. It is quanti-tatively confirmed that the moisture state-dependent tensile strength and crack-dependent moisture diffusivity models play a key role in tackling with the responses of reinforced concrete (RC) members subjected to combined hygro-mechanical actions. Full 3D thermo-mechanics simulation of shear critical RC beams with effective depths, which ranges from 250mm to 1000mm, is carried out from the onset of casting. It is found that the impact of drying increases with the increasing effective depth at the constant beam width. Overall, it is emphasized that linkage of material science and structural mechanics is indispensable for holistic understanding of RC members under coupled mechanical loading and drying. The multi-scale analytical scheme is proved to be applicable for the performance assessment of RC mem-bers at ultimate states as well.
The 2011 Tohuku tsunami on the east coast of Japan resulted in killing more than 15,000 people and missing more than 2,500 people, washing away of more than 250 coastal bridges and loss of US$235 billion. Collapse of coastal bridges due to tsunami impact represents a huge obstacle for rescue works. Therefore, in the current study, the collapse of Utatsu Ohashi bridge is numerically studied. The analysis is carried out using the Applied element Method due to its advan-tages of simulating structural progressive collapse. The AEM is a discrete crack approach, in which elements can be separated, fall and collide to other elements in a fully nonlinear dynamic scheme of computations. The Utatsu Ohashi bridge collapse was successfully simulated using AEM. It was numerically found that the amount of trapped air between deck girders during tsunami had a significant effect on the behavior of the bridge. This is attributed to the buoyant force accompanied with the trapped air. A simplified method for estimating trapped air was assumed and proved to give reasonable results compared to reality. Three different solution examples for mitigating collapse of similar existing bridges were introduced and applied to Utatsu Ohashi bridge case and found to be efficient for preventing collapse.
Cracks are always present in reinforced concrete structures. In the presented research, influence of mechanical cracks on chloride ingress is studied. A compact reinforced concrete specimen was designed, mimicking the cracking behaviour of beam elements. Cracks of different widths were induced by means of mechanical loading. These cracked specimens were then subjected to weekly cycles of wetting and drying with NaCl solution. After the exposure, the specimens were cut, and chloride distributions were determined using Laser Induced Breakdown Spectroscopy (LIBS), an innovative technique which enables simultaneous determination of different elements with high spatial resolution and minimal specimen preparation. By combining element distributions of different elements, it is possible to discriminate between coarse aggregate particles, and the mortar matrix. It was found that the wider the crack is, the higher the ingress of chloride ions. This was, however, different for two tested concrete mixes. Due to highly inhomogeneous chloride distribution around the cracks, use of fine-scale experimental techniques for chloride mapping is advised, based on the presented study.
The influences of polycarboxylate (PCE) superplasticizer on pore structure of hardened cement pastes (HCPs) and impermeability of hardened mortars were investigated. Cement pastes and mortars with different water to cement ratios (W/Cs) and varied superplasticizer dosages were prepared and cured for 7 days and 28 days respectively. Mercury intrusion porosimetry and alternating current impedance (ACI) were employed to characterize the pore structure of HCPs and the impermeability of hardened mortars. Results show that PCE significantly affects the pore structure of HCPs due to its impacts on the dispersion of cement grains in fresh cement pastes. The effects of superplasticizer on the pore structure are associated with W/C and curing age of cement pastes. At W/C of 0.29, increasing superplasticizer dosage results in obvious declines in total pore volume at early ages and reduces pore connectivity. By contrast, increases in total pore volume and higher pore connectivity are found when PCE is incorporated in the paste at W/C of 0.4. ACI reveals that the inclusion of PCE brings about a denser structure and enhanced impermeability of hardened mortars. It is believed that the enhanced impermeability is related to the reduced interfacial heterogeneity and the compacted interfacial zone when PCE is added.
This study examined the alkaline activated ground slag (GS) blended with different grades of palm oil fuel ash (POFA), namely: ground POFA (GPOFA), treated or calcined GPOFA (TPOFA) and ground TPOFA or ultrafine POFA (UPOFA) in the synthesis of alkaline activated GS-POFA mortar (AAG-POFA). The AAG-POFA mortars were prepared with 8M-NaOHaq and Na2SiO3aq (Ms = SiO2/Na2O = 3.3) activators, and then cured at 60 ℃ for 24 h. The findings showed that the grinding and calcination of POFA reduced the carbon content, loss on ignition, enhanced its fineness and increased the mineral (oxide) compositions. The grades of POFA used for the synthesis of AAG-POFA products impacted the morphologies of its microstructures, compressive strength, carbonation and amorphousity. TPOFA and UPOFA could be successfully used for the synthesis of AAG-POFA mortar with considerable structural strength. While AAG-UPOFA produced the highest mortar strength, the production of AAG-TPOFA mortar was more energy conservative. Finally, GPOFA was practically unsuitable for the synthesis of AAG-POFA as it resulted in low strength products while its mix-ture was characterized with low consistency, and poor dissolution or hydroxylation upon reacting with the alkaline acti-vators.
Sulfate resistance of mortars containing limestone powder was investigated in a laboratory in which mortar specimens were continuously immersed in 33,800 ppm of sodium and magnesium sulfate solutions for 1,700 days. Mortars made from interground limestone and limestone powder, replacing cements with different proportions (10%, and 20% limestone by weight of the blended cement), were used to compare with the control Portland Type 5 cement mortar. Expansion and weight loss were measured. Microstructural analyses such as SEM, MIP, TGA and XRD techniques were also performed on the paste samples. It was observed that interground limestone cement specimens had higher expansion than the limestone powder replacing cement specimens due to smaller average pore size and lower total porosity, providing less spaces for depositing products of expansion. Contrary to the expansion, the specimens made from interground limestone cement lost less weight than those made from the limestone powder replacing cement because of lower average pore size and total porosity, making the specimens denser. For mortar containing limestone powder specimens in MS solution, the MS decreased the system' s pH. There occurred higher dissolution of CaCO3 from the limestone powder, which contributed to the formation of gypsum, magnesite and dolomite. This reduced the conversion of C-S-H to M-S-H, which resulted in less weight loss and less severe surface etching than the specimens made from OPC.
Load-slip characteristic of shear connectors is the basis of connectors’ stiffness inputting in finite element analysis and load transfer study of shear connector groups. Based on three series and 24 groups large-scale experiments of perfobond strip shear connectors (PBL) and depended on the cutting-edge measurement techniques of distributed optical fiber sensors, mechanic characteristics and important influence factors were analyzed thoroughly, corresponding formula for ultimate bearing capacity of PBL connectors was suggested. Further load-slip characteristics under elastic limit and ultimate bearing state were induced, load-slip curve expression of PBL connectors under static complete loading was de-rived. The results show that the elastic limit load of PBL connectors in hybrid structures is about 0.35 times the ultimate bearing load, the yield load is about 0.7 times the ultimate bearing load. The complete load-slip curve of a PBL connector consists of a straight line branch in elastic phase and a curve branch following logarithmic distribution in nonlinear phase.