Yuxi Zhao

Modeling the Amount of Steel Corrosion at the Cracking of Concrete Cover

This article tries to contribute to quantify the amount of steel corrosion at the cracking of concrete cover. The amount of steel corrosion when the concrete cover cracks, is a key factor in the life of concrete structures. During the time between steel depassivation and concrete cover cracking, the process of steel corrosion develops in three stages: free expansion of the corrosion products, stress initiation of the concrete cover, and cracking of the concrete cover. A concrete cracking model is presented here to estimate the total amount of steel corrosion at the onset of the cracking of the concrete cover. This model is applied to some test results reported by other researchers. The amount of steel corrosion predicted by the proposed model when cracking of the concrete cover occurs is in agreement with the experimentally observed results.

Cracking Behavior of CFRP Laminate-Strengthened RC Beams with Premechanical and Postmechanical Environmental Damage

The main objectives of this study are to investigate the effects of three types of predamage and postdamage on the cracking behaviors of carbon fiber-reinforced polymer (CFRP)-strengthened reinforced concrete RC beams and to develop a rational methodology for predicting the average stabilized crack spacing. The predamage is induced by either sustained loading only or by the combination of sustained loading and corrosion. The predamage involved a sustained loading with an anchor tightening system, an electrochemical process to accelerate the migration of chlorides from an external electrolyte into the tested beams, and a wetting–drying cycle process with a controlled current to accelerate the corrosion of the reinforcing steel bars in the tested beams. The postdamage was induced by wetting-drying cycles. A loading test was conducted to determine the cracking behaviors of stabilized flexural cracks in the CFRP-strengthened beams with or without damage. The crack patterns, crack spacings, and test beam widths were recorded and compared, and the related mechanism was discussed. It was found that after CFRP strengthening, the effect of predamage or postdamage on the crack spacing and width is not as distinct as in the unstrengthened cases. The sustained loading predamaged beam showed insignificant differences in crack spacing and width compared to beams without predamage. Subsequently, a model capable of evaluating the crack behaviors of CFRP-strengthened beams with or without damage was developed. The analytical approach is based on equilibrium and compatibility equations to elucidate the average stresses of concrete and the CFRP laminate of a CFRP-strengthened beam element.

Design of shear strengths of corroded reinforced concrete beams

This paper focused on the design of shear strength of corroded reinforced concrete beams. Firstly, the behaviour and shear strength of corroded reinforced concrete beams obtained from recent experimental investigations are reviewed and summarised. Secondly, the shear strengths of corroded reinforced concrete beams are compared with those predictions obtained by other researchers. The effects of corroded stirrups on the shear resistance of reinforced concrete beams are also analysed. Generally, it is shown that the shear strength of beams increases at the early stages of stirrups corrosion. The loss in shear strength commences when the corrosion of the stirrups approximately exceeds a 10% threshold level. Finally, an empirical equation is proposed to predict the shear strength of reinforced concrete beams with corroded stirrups. This proposed equation is in agreement with the experimental data and enables designers to take into account the effects of corroded stirrups.

Experimental investigation and design of corroded stud shear connectors

In this study, the effect of corrosion on the static behavior of stud shear connectors was investigated. Two series of push test specimens having different stud diameters were fabricated according to Eurocode 4. The test specimens were first corroded to different corrosion rates by electronic accelerating method. Static loading tests were then performed to obtain the load-slip curves and ultimate strengths of corroded test specimens. Material properties of the concrete and steel studs used in the test specimens were also measured. Actual corrosion rates were measured from the studs obtained from the tested specimens. Design methods specified in current design standards are used to predict the design strengths of test specimens. It is shown that the design strengths are unconservative for corroded test specimens. Therefore, a new design equation is proposed which enables the designer to consider the effect of corrosion.

Behavior and modeling of fiber-reinforced polymer-confined concrete in elliptical columns:

Conversion of rectangular columns into elliptical columns (i.e. shape modification) followed by fiber-reinforced polymer jacketing provides an attractive alternative strengthening method than direct fiber-reinforced polymer jacketing of rectangular reinforced concrete columns (with corner rounding). For this method to be widely used in practice, a design method for the fiber-reinforced polymer jacket needs to be developed, and the key to such a design method is a reliable and yet relatively simple stress–strain model for fiber-reinforced polymer-confined concrete in elliptical reinforced concrete columns. This article presents a combined experimental and finite element study from which a stress–strain model for fiber-reinforced polymer-confined concrete in elliptical columns was derived. The article first presents the experimental program designed to examine the effects of key parameters. The accuracy of a finite element model for fiber-reinforced polymer-confined elliptical concrete columns is then demonstrated through comparisons with the test results. Finally, on the basis of the test data as well as the numerical results obtained with the verified finite element model, a simple and accurate stress–strain model for fiber-reinforced polymer-confined concrete in elliptical columns is formulated for use in design.

Experimental research on concrete strength prediction by Limpet pull-off test in China

Using Limpet pull-off test for concrete strength prediction has now been accepted in standards of the UK, North America, Holland and some other countries. For its application in China, pull-off tests were carried out by using the Limpet in this study. Concrete specimens with four different mixtures and strength classes were cast, representing the normal and the high performance concretes commonly used in China. After different ages, the pull-off tensile strength was determined by using the Limpet and the compressive strength was obtained by carrying out cube crushing test using a WE-100 universal testing machine. To reflect the correlation between the pull-off tensile strength and the cube compressive strength, two types of curve were used for regression, y = axb and y = aebx respectively. The regression efficiency of the two curves was compared.

Effects of Stirrups on Bond Behavior Between Concrete and Corroded Steel Bars

Steel corrosion leads to the deterioration of bond between concrete and steel bars. The serviceability and ultimate strength of concrete elements within RC structures are hence affected. Many researchers have studied the bond behavior of corroded steel bars. However, very few studies have investigated the effects of confinements on the degradation of bond strength. The present paper proposed a new kind of beam specimen based on which the effects of stirrups on degradation of bond were investigated. The test results proved that stirrups can effectively increase the bond strength between concrete and corroded steel bars.

Seasonal Variation of Surface Chloride Ion Content and Chloride Diffusion Coefficient in a Concrete Dock

Concrete powder collected in-situ from a 5-year-old concrete dock in four seasons respectively was tested by rapid chloride test (RCT). Based on Ficks second law, the surface chloride ion content and the chloride diffusion coefficient are derived by regression analysis, and the seasonal variation laws of the two parameters are discussed. Both parameters vary with the season as single-peak laws. The surface chloride ion content reaches the peak value in summer, while the peak of the chloride diffusion coefficient occurs in spring. Possible reasons for these phenomena are discussed in this paper. Finally, the choice of inspection time for those two parameters in different engineering applications is advised and this may make contribution to more accuracy of chloride diffusion prediction in engineering applications.

Rust Distribution in Corrosion-Induced Cracking Concrete

A reinforced concrete specimen subjected to wetting–drying cycles was investigated. The rust distribution was observed by digital microscopy and scanning electron microscopy. This study observes five regions around the interface of steel and concrete: concrete, corrosion products-filled paste, mill scale, the corrosion layer (CL), and steel. This study confirms that rust did not fill the corrosion-induced cracks in the concrete cover before concrete surface cracking. The results also show that the penetration of corrosion products into the porous zone of concrete and the formation of a CL at the steel–concrete interface may proceed simultaneously after the initiation of steel corrosion.

Performance of Recycled Aggregate Concrete in a Real Project

Though a large amount of research has been carried out on recycled aggregate concrete (RAC) during recent years, the application cases of RAC in real structures are still very few in China. This paper reports the application of RAC in a real project. Two beams made of RAC and commercial natural aggregate concrete (NAC) respectively were cast in-situ to carry out the tests. The two beams were subjected to the similar service loading, so that the performance of the RAC beam could be compared with that of the NAC one. Compressive strength, degree of density and uniformity were tested for both types of concrete at representative ages, and concrete strain of the two target beams was monitored regularly from the age from 1 d to 420 d. The results from laboratory tests and on-site monitoring have proved the feasibility and reliability of the application of RAC in real structures though RAC is generally inferior to NAC.