Ki Yong Ann

Resistance of Alkali-Activated Slag Concrete to Chloride-Induced Corrosion

The corrosion resistance of steel in alkali-activated slag (AAS) mortar was evaluated by a monitoring of the galvanic current and half-cell potential with time against a chloride-contaminated environment. For chloride transport, rapid chloride penetration test was performed, and chloride binding capacity of AAS was evaluated at a given chloride. The mortar/paste specimens were manufactured with ground granulated blast-furnace slag, instead of Portland cement, and alkali activators were added in mixing water, including Ca(OH)2, KOH and NaOH, to activate hydration process. As a result, it was found that the corrosion behavior was strongly dependent on the type of alkali activator: the AAS containing the Ca(OH)2 activator was the most passive in monitoring of the galvanic corrosion and half-cell potential, while KOH, and NaOH activators indicated a similar level of corrosion to Portland cement mortar (control). Despite a lower binding of chloride ions in the paste, the AAS had quite a higher resistance to chloride transport in rapid chloride penetration, presumably due to the lower level of capillary pores, which was ensured by the pore distribution of AAS mortar in mercury intrusion porosimetry.

Factors influencing chloride transport and chloride threshold level for the prediction of service life of concrete structures

In this study, the influence of parametric values on the risk of steel corrosion in concrete was quantitatively calculated in terms of the sensitivity to service life of concrete structures subjected chloride attack. Prior to the analysis, a thorough literature review on the chloride threshold level (CTL) for steel corrosion and the rate of chloride transport was performed to define the influencing factors. As a result, it was found that the CTL can be quantitatively represented in the form of total chloride content by weight of cement, reflecting both the corrosiveness and inhibition effect. For chloride transport, diffusion is the main driving force for chloride ions in concrete, although other driving forces such as permeation and sorption simultaneously exist. Finally, the sensitivity analysis of parametric values using the Ficks second law for the prediction of service life of concrete structures due to chloride induced corrosion found that the concrete cover depth is the most influencing to the time to corrosion, followed by the surface chloride concentration, apparent diffusion coefficient and the CTL.

Detection of Reinforced Concrete Crack Using Mechano-Luminescence Paint

As a nonuniform and unisotropic material with a relatively low tensile strength in spite of high compression strength, a concrete material is vulnerable to bending and tension. Due to the mechanical properties of the current reinforced concrete structures, it is hard for concrete materials to avoid the damages caused by cracks. Although cracks are the easiest things to detect and the most effectively repairable things due to their characteristics, it is very hard to measure them efficiently. In this research, the author measured cracks by visualizing them through mechano- luminescence(ML) paint. By applying ML paint on the surface of the specimen and using the 3-point bending test, the author conducted a quantitative evaluation on the mechanical properties of cracks such as the cracking aspect and length of reinforced concrete. Through the results of this research, the author confirmed the crack propagation speed by section and the mechanical correlation such as between loads and cracks and between deflection and cracks, which means this research was quite successful in analyzing the characteristics of cracks.

Corrosion Resistance of Calcium Aluminate Cement Concrete Exposed to a Chloride Environment

The present study concerns a development of calcium aluminate cement (CAC) concrete to enhance the durability against an externally chemically aggressive environment, in particular, chloride-induced corrosion. To evaluate the inhibition effect and concrete properties, CAC was partially mixed with ordinary Portland cement (OPC), ranging from 5% to 15%, as a binder. As a result, it was found that an increase in the CAC in binder resulted in a dramatic decrease in the setting time of fresh concrete. However, the compressive strength was lower, ranging about 20 MPa, while OPC indicated about 30–35 MPa at an equivalent age. When it comes to chloride transport, there was only marginal variation in the diffusivity of chloride ions. The corrosion resistance of CAC mixture was significantly enhanced: its chloride threshold level for corrosion initiation exceeded 3.0% by weight of binder, whilst OPC and CAC concrete indicated about 0.5%–1.0%.

Effect of Electrochemical Treatment in Inhibiting Corrosion of Steel in Concrete

This paper will discuss the effectiveness of electrochemical treatment in enhancing the resistance of steel to corrosion in concrete. The current density for electrochemical treatment ranges from 250, 500, and 750 mA/m2 (0.161, 0.323, and 0.484 mA/in.2) for 1 week to 125, 250, and 500 mA/m2 (0.081, 0.161, and 0.323 mA/in.2) for 2 weeks. The steel-mortar interface was also examined using backscattered electron (BSE) images after treatment. As a result, an increase in the current density of the specimens resulted in an increase of the chloride threshold level, ranging from 0.82 to 2.72% by weight of cement, whereas the untreated specimens produced a lower threshold level, ranging from 0.35 to 1.52%. Thus, the time to corrosion was increased from 167 to 571 days. It was seen in the BSE image analysis that electrochemical treatment at a high current density resulted in the formation of cracks and large voids in the vicinity of the steel and a delay in cement hydration.

Fundamental Properties of Magnesium Phosphate Cement Mortar for Rapid Repair of Concrete

Fundamental properties of magnesium phosphate cement (MPC) were investigated in this paper. The setting time and compressive and bond (i.e., flexural and tensile bond) strengths were measured to assess the applicability, and hydration product was detected by the X-ray diffraction. The specimens were manufactured with magnesia and potassium dihydrogen phosphate (K2HPO4) was added to activate hydration process. The Borax (Na2B4O7·10H2O) was used as a retarder to mitigate overwhelming rapid hardening. Mercury intrusion porosimetry was used to examine the pore structure of MPC mortar, and simultaneously rapid chloride penetration test was performed. As a result, the compressive strength of MPC mortar was mostly achieved within 12 hours; in particular, the MPC mortar at 4.0 of M/P ranked the highest value accounting for 30.0 MPa. When it comes to tensile and flexural bond to old substrate in mortar patching, the MPS had the higher tensile and flexural strengths, accounting for 1.9 and 1.7 MPa, respectively, compared to OPC mortar patching. Unlike Portland cement mortar, the MPC mortar contained mainly air void rather than capillary pores in the pore distribution. Presumably due to reduced capillary pore in the MPC, the MPC indicated lower penetrability in the chloride penetration test.

The Influence of C3A Content in Cement on the Chloride Transport

The present study concerns the influence of C3A in cement on chloride transport in reinforced concrete. Three modified cement was manufactured in the variation of the C3A content, ranging from 6.0 and 10.5 up to 16.9%. The setting time of fresh concrete was measured immediately after mixing, together with the temperature at the time of initial set. For properties of hardened concrete in the variation in the C3A, a development of the compressive strength and chloride permeation were measured using mortar specimens. Simultaneously, chloride binding capacity was measured by the water extraction method. To ensure the influence of pore structure on chloride transport, the pore structure was examined by the mercury intrusion porosimetry. As a result, it was found that an increase in the C3A content resulted in an increase in chloride binding capacity. However, it seemed that increased binding of chlorides is related to the higher ingress of chlorides, despite denser pore structure. It may be attributed to the higher surface chloride, which could increase the gradient of chloride concentration from the surface, thereby leading to the higher level of chloride profiles. Substantially, the benefit of high C3A in resisting corrosion, arising from removal of free chlorides in the pore solution, would be offset by increased chloride ingress at a given duration, when it comes to the corrosion-free service life.

Constitutive Behavior and Finite Element Analysis of FRP Composite and Concrete Members

The present study concerns compressive and flexural constitutive models incorporated into an isoparametric beam finite element scheme for fiber reinforced polymer (FRP) and concrete composites, using their multi-axial constitutive behavior. The constitutive behavior of concrete was treated in triaxial stress states as an orthotropic hypoelasticity-based formulation to determine the confinement effect of concrete from a three-dimensional failure surface in triaxial stress states. The constitutive behavior of the FRP composite was formulated from the two-dimensional classical lamination theory. To predict the flexural behavior of circular cross-section with FRP sheet and concrete composite, a layered discretization of cross-sections was incorporated into nonlinear isoparametric beam finite elements. The predicted constitutive behavior was validated by a comparison to available experimental results in the compressive and flexural beam loading test.

Development of Cement-Free Concrete Using Finely Grained Slag for Portland Cement

Abstract. The present study concerns a development of cement-free concrete using finely grained ground granulated blast furnace slag (GGBS) rather than ordinary Portland cement (OPC) as a binder in concrete mix. The GGBS was very finely ground to the level of 10,000 cm2/g, prior to casting concrete, compared to OPC of which the Blaine value accounts for about 3,200 cm2/g. In concrete casting, the NaOH activator was added to mixing water to enhance the hydration rate for cement-free concrete. To ensure the compatibility of GGBS in concrete, a development of concrete strength, ionic penetrability and pore structure were examined. As a result, it was found that cement-free concrete using the GGBS has a higher concrete strength at all ages from 7 to 56 days. In turn, the ionic penetrability, in terms of chloride diffusivity, was slightly lower in cement-free concrete than OPC concrete, presumably due to the dense pore structure, which was confirmed by the mercury intrusion porosimetry. Simultaneously, the adiabatic temperature for cement-free concrete initially rose more rapidly, leading to an accelerated hydration process. This suggests that the cement-free concrete containing GGBS can be used for structural concrete structures, imposing an economical benefit and structural stability.

Corrosion Risk of Reinforced Concrete Structure Arising from Internal and External Chloride

The corrosion risk of internal chloride and external chloride from three different exposure conditions was evaluated. The initiation of corrosion was detected by monitoring the galvanic current between cathode metal and embedded steel. The chloride threshold was determined by measuring the corrosion rate of steel by the polarization technique for internal chloride and the chloride profiling test for external chloride. As the result, the initiation of corrosion was accelerated with a cyclic wet/dry condition, compared to the totally wet condition. In addition, it was found that an increase of the drying ratio in the exposure condition resulted in an increase of corrosion rate after initiation. The threshold level of external chloride ranged from 0.2 to 0.3% weight by cement and internal chloride shows higher range, equated to 1.59–3.10%. Based on these data, the chloride penetration with exposure condition was predicted to determine the service life of reinforced concrete structure.