Sulfate attack on concrete structures is a serious problem around the world, particularly in the Middle East, where sulfate is abundant in soil. Design of a cementitious material with high sulfate resistance is thus required, leading to research on the high durability of cementitious materials containing γ-Ca2SiO4 (γ-C2S). The authors have proposed the following material design for high durability: a cementitious material mixed with γ-C2S and subjected to autoclaving and accelerated carbonation, to give 1.1nm tobermorite in the autoclaved hardened body, and densification at the surface. However, the sulfate resistance of this new material has not been investigated. Therefore, the purpose of the present research is to understand the effect of adding γ-C2S, autoclaving and accelerated carbonation on the permeability of sulfate ions (crucial for sulfate resistance), from the viewpoint of the reaction products and porosity. The smallest sulfate ion penetration was a sample with 80% replacement ratio of OPC with γ-C2S. Generation of vaterite following accelerated carbonation suppressed sulfate ion penetration. In addition, dissolution of 1.1nm tobermorite in the hardened body was inhibited. Also, low-Ca/Si C-S-H at the surface may reduce the porosity of this surface and thus contribute to the suppression of sulfate ion penetration.
The present study concerns a technique for determining the chloride binding capacity, using the XRD curve containing the peaks for Friedel' s salt. The influence of chloride binding on the corrosion behaviour was also evaluated. The binding capacity of chloride ions in cement paste was determined by the water extraction method. As a result, it was found that the chloride binding capacity is strongly dependent on the W/C, binder type, curing age and total chloride concentration in the paste. It is notable that chloride binding has a crucial influence on corrosion propagation: an increase in the binding capacity resulted in a decrease in the corrosion rate at a given chloride concentration in mortar specimens. However, the impact of chloride binding on the onset of corrosion was marginal, presumably due to a release of bound chlorides into free chlorides at the stage of corrosion, accompanying a pH fall in the vicinity of the steel. A strong relation between the peak intensity for Friedel' s salt and the concentration of bound chloride was observed, which can be used to determine the bound chlorides and thus the risk of chloride-induced corrosion of steel in concrete.
In service, cracks or microcracks are usually present in concrete as a result of several mechanisms, for example the drying shrinkage, thermal gradients, freezing-thawing cycles, alkali-aggregate reaction and external loading. It has been realized that cracking can significantly accelerate the ingress of chlorides into concrete since it provides preferential flow channels and allow more chlorides to penetrate. But it is also believed that cracking plays an important role on the penetration speed of chloride. The objective of this paper is to quantify the diffusion coefficient of chloride through cracks of concrete with different crack widths by means of the mesoscale modelling method based on the available experimental results from literatures. In the numerical models, the position and opening width of cracks are artificially prescribed based on geometrical layout of the samples used in test. Additionally, the Voronoi diagram technique is adopted to discrete the domain of a specimen in order to reduce the mesh bias. On the Voronoi diagram, a randomly distributed lattice network is constructed to represent the transport process of chlorides. The range of investigated crack width is from 20 to 600 μm covering the data in experimental program. The diffusion coefficients of chloride through cracks of different width, Dcr, are numerically determined by the trial and error method. It is concluded that chloride can penetrate into cracks with a much higher speed than that in free water. When the crack width is lager than a critical value, Dcr is determined as 10000 mm2/h and independent of the crack width.
This paper describes the use of three experimental methods to detect shrinkage cracking in restrained ring specimens. These methods include monitoring the strain at the inner surface of the restraining steel ring, using passive acoustic emission, and measuring the electrical resistance of conductive materials that are applied to the surface of the mortar. The methods for detecting cracking are compared and their advantages and limitations are discussed. Both plain and steel fiber reinforced (2% steel fiber by volume) mortar were evaluated in this study. The time of through cracking detected by the three methods corresponds to the time of visible cracking. In the mixtures containing fibers the time of through cracking is delayed significantly when compared to the plain systems. The delay in the age of through cracking can be explained by the role of fibers in arresting the cracks as they develop and their role in transferring stress across the crack width that keeps the cracks from opening. The passive acoustic emission data indicates substantial damage development before a through crack forms. The time of through cracking can be detected using conductive surfaces with a sudden increase in resistance of these conductive elements. Cracking can be detected using conductive coatings when the crack is 0.02 mm or larger. Specimens with different degrees of restraint show through cracking at approximately the same stress level; however, the age at which cracking occurs decreases as the degree of restraint increases. This can be attributed to the increased micro-cracking (i.e., damage and precritical crack growth) as confirmed with acoustic emission data.
In this study, the influences of both stirrup spacing and anchorage performance on the residual strength of corroded RC beams are investigated. With the increase of stirrup spacing, the applied load is easily transferred to the anchorage region, and with the increase of the corrosion ratio of rebar, the mechanism of corroded RC beams shifts from beam action to arch action. In the case of non-uniform corrosion of the main rebar, the maximum deviation ratio of the corrosion ratio of main rebars is over 0.9, and the beam suffers flexural failure due to the yielding of rebars in the extremely corroded region. In the case of uniform corrosion of the main rebars, the maximum deviation ratio of the corrosion ratio of main rebars is below 0.9, and there are two situations. If the bottom portions of the stirrups are sufficient, the applied load is restricted in the support span, and the corroded beam presents a flexural failure mode. On the other hand, if the bottom portions of the stirrups are insufficient, the applied load is transferred to the anchorage, and the corroded beam is inclined to suffer bond failure. Moreover, when the beam suffers bond failure, the residual strength depends on the anchorage performance.
Deterioration process due to corrosion of steel in concrete affects the performance of reinforced concrete structures both in service and ultimate conditions. Corrosion reduces the rebar section, deteriorates the surrounding concrete with the oxides expansion product and it also alters bond between steel and concrete. Moreover, in structures subjected to cyclic loading, the damage due to corrosion can be combined with the mechanical action that is present in service. In this case the initial crack pattern due to load action and to the rheological phenomena is further modified by the expansion of the oxides and by the interaction among those causes. In the present work, the results of an experimental campaign on reinforced concrete elements subjected to simultaneous corrosion and cyclic loading are shown. It is put in evidence the loss of structural performance, by effect of chemical degradation and mechanical action. The key role of the combined effect of those causes of deterioration is also confirmed by static tests performed under the condition of static loading and simultaneous corrosion.
This study proposes an estimating procedure that can be used to set the optimal seismic level in the seismic retrofit design for a low-rise reinforced concrete (RC) building. Along with damage control, the cost of maintenance over the remaining service life is also considered in the estimating procedure. However, when a seismic retrofit level defined by the ground acceleration corresponding to the ultimate deformation of the single degree of freedom (SDOF) system for a selected building is specified, several seismic retrofit methods can be used to achieve the specified seismic retrofit level. Since their respective maintenance costs differ, the average value of the annual costs of maintenance for a specified seismic retrofit level was adopted. Under the condition that the average value of the annual reliability indices of a seismic retrofit level for a specified damage state needs to be equal to or greater than the allowable annual reliability index, the seismic retrofit level corresponding to the minimal value is regarded as the optimal level. Finally, a case study was used to discuss the application of the estimating procedure for the optimal seismic retrofit level proposed here. In addition, combinations of the upgrading rates in yielding acceleration and the ductility capacity of the SDOF system are also suggested to the designer of the retrofit for the selected building.
This study describes a quick and efficient screening method to practically evaluate the seismic capacity of low-rise reinforced concrete (RC) buildings composed of members controlled by both shear and flexure. This study describes curves of the required range of strength to predict earthquake damage as a function of the level of ground motion and building seismic capacity. This method was applied to low-rise RC buildings damaged by an actual earthquake to determine the method' s validity. The method was also verified by comparing its results to those produced by more detailed methods of seismic capacity evaluation: the Japanese standard for seismic capacity evaluation (second- and third-level procedures), nonlinear static analysis, and nonlinear dynamic analysis. Furthermore, the proposed method was applied to eight low-rise RC buildings in Japan and 38 in Korea; the results were compared with the structural seismic capacity index (IS = 0.6), which is the Japanese standard for the critical value required to prevent large-scale earthquake damage to structures in the presence of a ground motion acceleration of 0.23 g. The proposed evaluation method was efficient; it permitted a rapid evaluation of a building' s seismic capacity and provided a means to calculate simply the degree of structural damage to a building due to a certain earthquake strength based on the seismic capacity category. The proposed method can be easily used to calculate the required strength for a low-rise RC building composed of members controlled by both shear and flexure and produce fundamental data for the earthquake preparedness of low-rise RC buildings.
This paper describes the development of an analytical plastic hinge element that can predict flexural, axial, shear and elongation deformation in plastic hinges. The element consists of layers of longitudinal and diagonal springs that represent the behavior of concrete and reinforcement. These springs are modeled using nonlinear path-dependent cyclic stress-strain relationships of either concrete or reinforcing bars depending on their position in the cross section and also depending on the mechanisms they are representing. Through a preliminary scrutiny of the analytical results, in-depth qualitative and quantitative information on the causes and consequences of plastic hinge elongation are discussed. An extensive experimental verification of the plastic hinge element is presented in a companion paper.
This paper follows on from a companion paper which described the development of a plastic hinge element that can capture elongation of plastic hinges and its effect on other aspects of cyclic response of ductile RC members. In this paper, some experimental elongation data available in literature are used to verify the effectiveness of this newly-developed plastic hinge element. The experimental results compiled in this study are examined to determine the effect of axial force on the behavior of reinforced concrete members; especially on the amount of plastic hinge elongation. Comparisons of the analytical predictions with experimental results show that the proposed model predicts elongation of plastic hinges satisfactorily. The ability to predict elongation in plastic hinges is a significant advancement in assessing the seismic performance assessment of RC structures.
In this paper, the inelastic damage process in Engineered Cementitious Composites (ECC) in the high stress concentration zone adjacent to the head of an embedded anchor under tensile load was examined experimentally and numerically. An FEM model together with a tensile strain-hardening material model of ECC was developed to simulate the damage process leading to final failure. Experimental observations on the effect of tensile ductility on the microcracking damage process and anchor pullout performance were used to verify the numerical model. Furthermore, the influence of several material parameters, including tensile ductility, tensile strength, compressive strength and modulus of elasticity, on the anchor pullout behavior was clarified numerically. It was demonstrated that the intrinsic tensile ductility in ECC led to significant enhancement of load and displacement capacities. Once the failure mode was switched from brittle to ductile, however, the tensile strength governed the pullout load capacity. Finally, a design equation for predicting anchor pullout load capacity was proposed based on the numerical analysis and verified by experimental data.