Chloride Penetration Profiles Research Articles

Impact of mechanical loading on the corrosion of steel reinforcement in concrete structures

This paper presents results of an experimental investigation on the effect of mechanical loading on the corrosion of steel reinforcement in concrete. Reinforced concrete beams were cast, subjected to mechanical loadings and then exposed to corrosive environment. Successive drying and wetting cycles using 3% sodium chloride salt solution were used to accelerate the corrosion attack. Electrochemical techniques such as macrocell corrosion current, linear polarization resistance (LPR), corrosion potential (E corr ) and electrochemical impedance spectroscopic (EIS), were used to evaluate the corrosion behavior of the reinforcement. Also, chloride penetration profile and visual inspection of the beam rebars were obtained. The results shows that the pre-exposure mechanical load has no influence on the corrosion initiation or corrosion rate thereafter, unless it reaches to some limit where it develops connected cracks through which the chloride ions flow and depassivate the rebars. The load of 25% of the ultimate capacity seems to be the critical load limit where some microcracks onset to be connected, facilitate the chloride ions flow, and initiate the rebar corrosion.

Water repellent surface impregnation for extension of service life of reinforced concrete structures in marine environments: The role of cracks

Abstract The enhancement of long-term durability of marine structures is a matter of interest to many researchers. The study presented in this paper examines the effectiveness of a water reducer and chloride barrier surface impregnation of the concrete cover of reinforced concrete (RC) structures, exposed to a marine environment. Specific focuses is on how surface cracks created (1) before impregnation and (2) after impregnation, affect the effectiveness of the surface treatment. The experiments are conducted in an environment which is as close as possible to the real humid subtropical marine environment. A series of reinforced concrete (RC) prisms and concrete cylinders, each treated with various commercial surface impregnation agents, were exposed to cyclic sea water shower under an outdoor environment to accelerate the dry/wet cycles for 1 year. Six types of surface impregnation agents, including four types of silane-based water repellent agents and two types of sodium silicate-based pore blockers (water–glass) were applied. Three types of RC prisms were prepared to simulate the different cracking possibilities, which may occur in surface impregnated concrete structures, during their service life. No cracks were introduced in the first prism group, while cracks were introduced before and after surface impregnation, in the second and third groups, respectively. The time-dependent water absorption of all specimens was monitored during exposure to the dry/wet cycles. Finally the specimens were split open to measure the penetration depths of the surface impregnation agents and the chloride penetration profiles. The areas with corrosion evident in the steel reinforcement in the RC prisms were also measured. Sodium silicate-based pore blockers were found to be inefficient in preventing chloride penetration of concrete under simulated marine exposures. The long-term efficiency of water repellent agents used for surface impregnation was found to be highly dependent on the type of agent and whether impregnation was carried out before or after crack formation.

Utilization of fly ash concrete in marine environment for long term design life analysis

This paper presents the performance of 7-year fly ash concrete exposed to hot and high humidity climate in marine conditions. Control concrete and fly-ash concrete cube specimens of 200 mm were cast and steel bars of 12 mm in diameter and 50 mm in length were embedded at various cover depths. The concrete specimens were exposed to tidal zone of marine environment in the Gulf of Thailand. The concrete specimens were tested for chloride penetration profile, chloride content at the position of embedded steel bar, and corrosion of embedded steel bar after being exposed to tidal zone of sea water up to 7 years. Consequently, these experimental data were used to generate the empirical equation for predicting long term required cover depth of cement and fly ash concretes to protect against the initial corrosion of reinforcing steel in a marine environment. The results showed that the increase of fly ash replacement in concrete clearly reduced the chloride penetration, chloride penetration coefficient, and steel corrosion in concrete. Interestingly, concretes with the fly ash replacement of 25–50% by weight of binder with a W/B ratio of 0.65 did not have corrosion of embedded steel bar at 50 mm concrete cover depth at 7-year exposure in a marine environment and presented the corrosion resistance as good as the cement concrete with a W/B ratio of 0.45. In addition, the empirical model indicated that all predicted data were within ±15% error of the tested data (up to 7 years). Also, the model was verified by using the investigated data of concrete exposed to a marine environment up to 10 years from other researchers; most predicted results were within ±25% error of the investigated data.

Relation between the chloride migration coefficients of concrete from the colourimetric method and the chloride profile method

In this study the electrochemical technique is applied to accelerate chloride ion migration in concrete to determine its transport property. After the accelerated chloride migration test (ACMT; the electrochemical technique is applied to accelerate chloride ion migration), the penetration depths of the colourimetric method were detected by 0.1 N AgNO3 and the chloride penetration profiles of the profile method were obtained from the chloride content of each depth. The chloride migration coefficients were calculated from the modified Fick's second law based on the colourimetric method and the chloride profile method. In order to develop a better understanding of the relationship between the migration coefficient obtained from the colourimetric method and the chloride profile method, the migration coefficient obtained from those two methods are compared. The experimental result shows that the migration coefficients obtained from the colourimetric method and the chloride profile method are linearly correlated, and the concentration of color change boundary, Cd = 0.07 mol/L, can be used to determine the migration coefficient regardless of the concrete type.

Effects of W/B ratios and fly ash finenesses on chloride diffusion coefficient of concrete in marine environment

The objectives of this investigation were to study the effect of W/B ratios and fly ash finenesses on chloride diffusion coefficient (Dc) of concrete under marine environment. Original and classified fly ashes were used as a partial replacement of Portland cement type I at 0%, 15%, 25%, 35%, and 50% by weight of binder. Water to binder ratios (W/B) were varied as 0.45, 0.55, and 0.65. Concrete cube specimens of 200 mm were cast and removed from the molds after casting 1 day and then cured in fresh water for 27 days. After that, the specimens were placed to the tidal zone of marine environment in the Gulf of Thailand. Subsequently, the specimens were tested for chloride penetration profile after being exposed to the tidal zone for 2, 3, 4, and 5 years. The regression analysis of investigated data was carried out and Fick’s second law of diffusion was applied to calculate the chloride diffusion coefficient (Dc) and chloride concentration at concrete surface (Co) based on one-dimensional analysis. The results showed that Dc of all concrete mixtures decreased with an exposure time and the decrease of W/B ratio resulted in the decrease of Dc. When the W/B ratio of concrete was reduced, the decrease of Dc in cement concrete was higher than that of the fly ash concrete. The use of fly ash with high fineness clearly reduced the rate of chloride ingress into concrete. In addition, fly ash with high fineness has more effective on reducing of Dc in concrete with higher W/B ratio than that with lower W/B ratio.

Effects of material and environmental parameters on chloride penetration profiles in concrete structures

On the basis of the transport mechanism of chloride ion, a prediction model of chloride penetration into concrete structures has been developed. The model includes the diffusion of chloride and its dependences on temperature, age, relative humidity, chloride binding and chloride convection by moisture transport. The experimental program has been set up to verify the model developed in the present study. Several series of concrete specimens were immersed in 3.5% chloride solutions for 15 weeks, and the chloride profiles of the specimens were measured and compared to the predicted chloride profiles. In addition, field measurements have been also conducted. From 10-year-old bridge piers, the chloride profiles in concrete under tidal zone were measured and compared with the predicted chloride profiles. The effects of chloride binding, relative humidity, temperature, exposure condition, and age-dependence on the chloride penetration in concrete were clarified from the present analyses. It was found from the present study that all these variables affect greatly the chloride penetration profiles in concrete. The comparison of the laboratory and field test data with the present theory confirms that the proposed model can be realistically used to predict the penetration of chloride ions into concrete structures under sea environments. Further, these results may be efficiently used for the realistic assessment and design for durability of concrete structures.

Effect of W/C ratio on covering depth of fly ash concrete in marine environment

This investigation studied the effect of W/C ratio on covering depth required against the corrosion of embedded steel of fly ash concrete in marine environment up to 4-year exposure. Fly ash was used to partially replace Portland cement type I at 0%, 15%, 25%, 35%, and 50% by weight of cementitious material. Water to cementitious material ratios (W/C) of fly ash concretes were varied at 0.45, 0.55, and 0.65. The 200-mm concrete cube specimens were cast and steel bars with 12-mm diameter and 50 mm in length were inserted in the concrete with the covering depth of 10, 20, 50, and 75 mm. The specimens were cured in water for 28 days, and then placed to the tidal zone of marine environment in the Gulf of Thailand. Subsequently, the concrete specimens were tested for the compressive strength, chloride penetration profile and corrosion of embedded steel bar after being exposed to tidal zone for 2, 3, and 4 years. The results showed that the concrete mixed with Portland cement type I exhibited higher rate of the chloride penetration than the fly ash concrete. The chloride penetration of fly ash concrete was comparatively low and decreased with the increasing of fly ash content. The increase of fly ash replacement and the decrease of W/C ratio could reduce the covering depth required for the initial corrosion of the steel bar. Interestingly, fly ash concretes with 35% and 50% cement replacement and having W/C ratio of 0.65 provided better corrosion resistance at 4-year exposure than the control concrete with W/C ratio of 0.45. In addition, the covering depth of concrete with compressive strength of 30 MPa (W/C ratio of 0.65) could be reduced from 50 to 30 mm by the addition of fly ash up to 50%.

Characterisation of chloride transport and reinforcement corrosion in concrete under cyclic wetting and drying by electrical resistivity

Concrete prisms were made with four cement types including cements with fly ash and/or blast furnace slag and three water-to-cement (w/c) ratios. Chloride penetration and corrosion of rebars were stimulated by subjecting prisms to cyclic loading with salt solution and drying. Concrete resistivity, steel potentials and corrosion rates were measured up to one year of age. Chloride penetration profiles were determined after salt loading. It was found that the resistivity of a particular concrete reflects its properties with regard to chloride penetration, corrosion initiation (probability of corrosion) and corrosion propagation (corrosion rate). Blending the cement with blast furnace slag, fly ash or both is beneficial with regard to delaying the onset of corrosion and subsequently limiting its severity under simulated de-icing salt load. The fly ash cement shows increased resistivity compared to Portland cement from eight weeks. Cement with a high percentage of slag develops a significantly higher resistivity after one week.

A study of the effect of chloride binding on service life predictions

One of the major causes of deterioration of reinforced concrete structures is chloride-induced corrosion of the reinforcing steel. The magnitude of the damage is especially large in structures exposed to marine environments and de-icing salts. The capacity of the concrete cementitious system to bind chloride ions has an important effect on the rate of chloride ionic transport in concrete and on the corrosion initiation of the steel reinforcement. This paper reviews mathematical models used in the literature to describe chloride binding in concrete. The impact of the different binding relations on the time-dependent chloride penetration profiles is investigated by solving the chloride mass conservation statement with a finite-difference approach. Results are presented for a concrete structure subjected to two different exposure conditions: submerged in seawater and exposed to de-icing salts. The implication of accounting for chloride binding in service life estimations is discussed.

Recurrent studies of chloride ingress in uncracked marine concrete at various exposure times and elevations

Uncracked reinforced concrete slabs were field exposed mounted on a floating pontoon and partly submerged for 5 years at the Swedish west coast. The total chloride ingress was analysed at various exposure times at 3 elevations representing a submerged, a splash, and an atmospheric exposure zone. The concrete mixtures varied in w/c ratio, type of cement, and amount and type of pozzolan used in the binder. The data is unique as it represents recurrently measured total chloride penetration profiles at various exposure ages, providing a foundation for the prediction of chloride ingress in concrete in a given environment. The results after 5 years of exposure confirmed the expected inverse relationship between water-to-binder ratio and chloride ingress. The use of 5–10% silica fume in the binder had a very positive effect on reducing the chloride ingress, but little or no benefit at all was found for concrete with fly ash in the binder as compared to the use of 5% silica fume. The chloride penetration rate as expressed by a calculated effective chloride diffusivity has a tendency to decrease over time. High- performance concrete with w/c ≤ 0.4 and a minimum of 5% silica fume added as a well dispersed slurry exhibited an effective chloride diffusivity in the range of 1 × 10 −13 to 5 × 10 −13 m 2/s after 5 years exposure in the splash zone.

Study of reinforced concrete slabs after three years of cathodic protection under severe conditions

The objective of this study is to determine the influence of cathodic protection on the properties of concrete cover in concrete slabs. To accelerate deterioration, the slabs were subjected to freezing and thawing cycles and sprayed with sodium chloride solution. Cathodic protection was applied to the reinforcements for three years. Then, the test pieces underwent physico-chemical analysis. Scanning electron microscopy and X-ray spectrometry were used to study the reinforcement-concrete interface, in particular for detecting alkali-silica reaction due to the alkalization. Carbonation depths, chloride penetration profiles and examinations of the reinforcements showed the good condition of the concrete cover and the beneficial effect of the cathodic protection of embedded steel.