Weiping Zhang

Mechanical Behavior of FRP-Strengthened Concrete Columns Subjected to Concentric and Eccentric Compression Loading

This paper presents the results of a study on the mechanical behavior of concrete columns strengthened with fiber-reinforced polymer (FRP) and subjected to concentric and eccentric compression loading. A numerical analysis model was developed based on the effectively confined concrete area and the stress-strain relationship of the confined concrete established based on compression tests of circular plain concrete columns. The model was verified against the test results of square concentrically loaded plain concrete columns and square eccentrically loaded reinforced concrete columns. An analytical formula of the maximum compression load of square or rectangular fiber reinforced polymer strengthened concrete columns, with respect to the same but nonstrengthened columns, was developed based on the parametric study results by using the verified model. The analytical formula was further verified with the test results of 23 square and rectangular fiber reinforced polymer strengthened columns reported in the literature. Good agreement was achieved. It was found that the increase of the maximum compression load of FRP-strengthened concrete columns, with respect to the same but nonstrengthened columns, increases linearly with the amount of FRP sheets used and decreases linearly with the load eccentricity and exponentially with the concrete compression strength. The last observation implies that the FRP strengthening technique (through wrapping) is most suitable for low-strength concrete buildings.

Compressive Behavior of Longitudinally Cracked Timber Columns Retrofitted Using FRP Sheets

This paper presents the results of a study on the compressive behavior of timber columns with longitudinal cracks. Material property tests and full-scale compression tests were conducted to investigate the failure modes and the recovery in the load-carrying capacity of cracked timber columns with the use of fiber-reinforced polymer (FRP) sheet wrapping. Different combinations of column geometries, crack dimensions, and types and spacing of FRP sheets were considered. A finite-element method (FEM)-based model was developed and verified on the basis of the test results. A parametric study was also conducted by using the verified model to further quantify the influences of crack dimensions and types and spacing of the FRP sheets. It was found that wrapping FRP sheets around cracked timber columns can recover their load-carrying capacity by up to 20%.

Modeling of Moisture Diffusivity of Concrete at Low Temperatures

Moisture diffusivity of saturated concrete varies with decreasing temperature. The variation depends on the amount of ice formed in pores and on the extent of the damage induced by the formation of ice. Ice crystals in the pores decrease the moisture diffusivity of concrete because of the low moisture diffusivity of ice, and the damage attributable to excessive ice formation in the pores increases the diffusivity. An analytical model is developed to characterize the effect of the two opposing processes on the diffusivity of the concrete under low temperatures. Three pore configurations are defined as (1) pores with no ice, (2) pores with ice and no damage, and (3) pores with ice and damage. The effective diffusivity of concrete is a combination of the diffusivities of the three types of porous media, each containing one configuration of pores. In the current study, the three types of porous media are combined by the composite model developed by Hashin and Shtrikman. The volume fraction of each configuration of pores is evaluated by a pore-size distribution based on the adsorption isotherm and a function of freezing temperature in terms of pore sizes. The diffusivities of the three types of porous media are evaluated by considering the ice-formation and damage-development mechanisms. The Druker-Prager plasticity model was used, and pores are assumed to be spherically shaped for evaluating the internal damage. The results of model prediction show that the moisture diffusivity in concrete decreases when the solution is frozen in some of the pores. If the temperature reduces further and causes damage attributable to excessive ice formation, then the diffusivity decreases at a lower rate than that at the same temperature without damage.

Analysis of aging of piezoelectric crystal resonators

Aging of piezoelectric (quartz crystal) resonator has been identified as one of the most important quality control problems of quartz crystal products. Aging is defined as frequency change with time. Aging in quartz resonators can be due to several sources: mass transfer due to contamination inside the resonator enclosure, stress-strain in the resonator blank, quartz defect, etc. In this study, the stress-strain effect, which has been believed as a dominant factor contributing to aging, is studied. The stress-strain effect is caused mainly by the long-term viscoelastic properties of bonding adhesive that attach quartz crystal plate to the ceramic base package. With the available accelerating testing method under elevated temperatures, the stress-strain induced aging in the quartz crystal resonators can be investigated. Because of the miniaturized size of the resonator, a digital image analysis method called image intensity matching technique (IIMT) is applied to obtain deformation patterns in the quartz blank due to thermal load. Our preliminary results showed that the unsymmetric thermal deformations may be a dominant contributing factor to aging. For simulation purposes, finite-element analysis is used to investigate the deformation patterns (i.e., stress-strain distributions) and corresponding natural frequency shift in the piezoelectric resonators. The viscoelastic behavior of mounting adhesives is incorporated into the analysis to show the dominant effect of long-term behavior of stress-strain developed in the crystal resonators. Also, some geometrical aspects-such as uneven mounting supports due to distances, volumes and heights of the adhesives-are simulated in the model.

Flexural Behavior of Corroded Reinforced Concrete Beams

This paper presents the results of a study on the flexural behavior of corroded reinforced concrete beams. The corrosion of twelve beams was caused by the accelerated corrosion process whereas that of the other three was caused by natural corrosion process. It was found that high impressed current density can accelerate the deterioration process of the reinforced concrete beams before cover cracking. The degradation of mechanical properties of the beams increased with the increase of the corrosion degree. It was also found that the beams corroded via the natural process were more affected by the corrosion degree and the stiffness of the beams after accelerated corrosion process decreased more given the same rebar mass loss ratio. The cross-sectional area loss and the degradation of the mechanical properties of the corroded rebars were found to be the major reasons for the decrease of the load carrying capacity of the beams while stiffness decline was mostly attributed to the bond loss. At last, a practical model for the calculation of the residual load carrying capacity was proposed and verified based on the test results. Good agreement has been achieved.

Chloride penetration in concrete under marine atmospheric environment – analysis of the influencing factors

Abstract Marine atmospheric exposure conditions provide a severe environment for reinforced concrete structures, mainly due to the occurrence of chloride-induced reinforcement corrosion. This procedure was influenced by many parameters related to the concrete properties and to the environmental condition. In this paper, cubic concrete specimens with 150-mm edge, different types of cementitious material and different strengths, were arranged on a structure, which exposed them to a natural marine atmospheric environment. The purpose was to evaluate quantitatively the influence of different exposure conditions on the durability of concrete, measured in terms of chloride penetration into concrete. Both relative humidity (RH) and temperature were monitored in the experiment. The results indicated that the surface RH and temperature of the concrete were much different from that of the air. The diffusion coefficient and surface chloride concentration were time- and location-dependent, and were influenced by the RH, temperature, and the concrete strength. The results also indicate that error results would be made when using constant diffusion coefficient and surface chloride concentration with the air RH and temperature to predict long-term chloride penetration.

Effects of stirrup corrosion on shear behaviour of reinforced concrete beams

Abstract Corrosion of stirrups is always more severe than longitudinal steel bars, leading to possible shear failure instead of bending failure. This paper covers the authors’ investigation on the effects of stirrup corrosion on shear behaviour of reinforced concrete beams. Based on modified compression field theory, a method of the whole process analysis was developed showing shear behaviour of reinforced concrete beams with corroded stirrups. Of particular interest was the cracking damage of concrete cross sections, the loss of cross-sectional area and deterioration of mechanical properties of corroded stirrups. After comparing analytical results with available shear test results, further analysis was carried out to investigate the deterioration of shear behaviour and the effects of corrosion degree, concrete strength, stirrup ratio and shear-span ratio. Stirrup corrosion was found to have little impact on cracking load, but shear strength and ductility decreased with the increase in corrosion degree, and shear failure was determined possible for beams originally designed for bending failure. With the decrease of concrete strength or the increase of stirrup ratio and shear-span ratio, the contribution of corroded stirrups to the shear strength of beams increased, and the shear strength and ductility caused by stirrup corrosion decreased as expected.

Experimental study and theoretical prediction of aging induced frequency shift of crystal resonators and oscillators

Abstract Frequency shift, due to quartz crystal resonator aging, has been identified as one of the most important quality control problems of quartz crystal products. The problem becomes more significant due to the device miniaturization and high precision standards for telecommunication applications. Since aging induced frequency shift occurs during a long time frame, it is necessary to predict the long-term behavior of the devices based on the short-term data obtained under an accelerated environment. One the other hand, frequency shift is associated with quite large random variation, and thus, a proper probabilistic theory should be used for analyzing test data and for developing a reliable prediction model. Accelerated testing was performed for various types of crystal resonators under elevated temperatures. The frequency shifts of the devices were measured at different testing periods. Markov chain model was used to characterize the frequency shift of the devices. The obtained short-term test results were used for calibrating the probabilistic transition matrix of Markov chain model. The model can then be used for predicting the long-term frequency shift. The time–temperature superposition principle in viscoelasticity was adopted to address the shift in time under different temperatures.

Thermal and hygric assessment of an inside-insulated brick wall: 2D critical experiment and computational analysis

A combined experimental-computational approach is used for the analysis of hygrothermal performance of a brick wall provided with interior thermal insulation system. A 2D laboratory experiment is performed to determine temperature and moisture fields in a characteristic segment of the envelope over a sufficiently long period including cold winter months. Then, a computational model of moisture and heat transport is developed, using an integral two-phase balance equation capable of distinguishing between the particular phases of water and an enthalpy-based heat balance equation. A 2D computational representation of the experiment is used for model calibration and identification of unknown parameters, resulting in a very good agreement of experimental and calculated fields, with R2 between 0.9687 and 0.9888. The calibrated model is subsequently used for a long-term hygrothermal assessment of the studied detail to demonstrate the functionality of the interior thermal insulation system, as well as the applicability of the developed model.

Cyclic Loading Behavior of Weak Reinforced Concrete Beam-Column Connections

Eight weak reinforced concrete beam-column connections and two strong ones subjected to cyclic loads were tested. Effects of the one-way slab, top reinforcement ratio of the beam as well as the transverse reinforcement ratio inside the joint on the seismic behavior of reinforced concrete beam-column connections were primarily investigated. Tests results show that, when subjected to the negative moment, the beam flexural resistance increases notably due to the presence of the slab. However, failure mode of the weak connection transforms from the beam flexural failure to the joint shear failure, resulting from the increased joint shear brought by the slab participation. Besides, the presence of the slab reduces the ultimate relative rotation, ductility and energy-dissipation capacity of the connection. Generally, weak connections can receive more benefit from the one-way slab than strong connections. Connections with less amount of top reinforcement in the beam exhibit better ductility and energy-dissipation capacity, which indicates that it is not always good to strengthen the beam negative moment zones in existing structures. The transverse reinforcement inside the joint has little effect on the seismic behavior of the weak connection with a one-way slab.