Expansion Of Corrosion Products Research Articles

An alternative to Portland Cement for waste encapsulation—The calcium sulfoaluminate cement system

Currently, Portland Cement (PC) is used extensively in the solidification/stabilisation of a wide variety of wastes. In the nuclear industry, low and intermediate level radioactive wastes are encapsulated or immobilised within composite PC cement systems based on high replacement with blast furnace slag or fly ash. However, the high alkalinity of these PC-based systems will corrode reactive metals found in some wastes releasing hydrogen and forming expansive corrosion products. Alternative cement systems could provide a different hydration chemistry, which would allow wastes containing these metals to be encapsulated with lower reactivity. Calcium sulfoaluminate ( C S ¯ A ) cement is one such cement. It combines economy of cost and low emission of CO 2 with rapid strength gain and compatibility with other construction materials. Hydration provides an internal pore solution where the pH is considerably lower than that of PC. The main hydration product, ettringite, can incorporate a number of ions into its crystal structure, making it an ideal candidate for waste immobilisation. This paper details some results from a commercial C S ¯ A system that examines aspects of mixing, hydration of different formulations and aluminium corrosion behaviour. The fluidity of mixes can be adjusted by changing the formulations. All designed mixes were set within 24 h with little bleeding and the pH values were in the range of 10–11.5. In addition, a significant reduction in Al corrosion was observed compared to a composite OPC system. Although these results provide encouragement for the idea that C S ¯ A cement can provide a possible alternative to PC in the immobilisation of difficult and reactive wastes, further investigation is needed.

Analytical model for time to cover cracking in RC structures due to rebar corrosion

The structural degradation of concrete structures due to reinforcement corrosion is a major worldwide problem. Reinforcement corrosion causes a volume increase due to the oxidation of metallic iron, which is mainly responsible for exerting the expansive radial pressure at the steel–concrete interface and development of hoop tensile stresses in the surrounding concrete. Cracking occurs, once the maximum hoop tensile stress exceeds the tensile strength of the concrete. The cracking begins at the steel–concrete interface and propagates outwards and eventually results in the thorough cracking of the cover concrete and this would indicate the loss of service life for the corrosion affected structures. An analytical model is proposed to predict the time required for cover cracking and the weight loss of reinforcing bar in corrosion affected reinforced concrete structures. The modelling aspects of the residual strength of cracked concrete and the stiffness contribution from the combination of reinforcement and expansive corrosion products have also been incorporated in the model. The problem is modeled as a boundary value problem and the governing equations are expressed in terms of the radial displacement. The analytical solutions are presented considering a simple two-zone model for the cover concrete, viz. cracked or uncracked. Reasonable estimation of the various parameters in the model related to the composition and properties of expansive corrosion products based on the available published experimental data has also been discussed. The performance of the proposed corrosion cracking model is then investigated through its ability to reproduce available experimental trends. Reasonably good agreement between experimental results and the analytical predictions has been obtained. It has also been found that tensile strength and initial tangent modulus of cover concrete, annual mean corrosion rate and modulus of elasticity of reinforcement plus corrosion products combined significantly influence predicted time to cover cracking. The analytical predictions of the proposed model have also been found to be in line with those of the other reported published data.

Modeling of time to corrosion-induced cover cracking in reinforced concrete structures

Service life of the concrete structures depends on the protective action provided by the cover concrete against the susceptibility of the reinforcement to the corrosive environment. Depending on the level of the oxidation of metallic iron, corrosion products may have much greater volume than the original iron that gets consumed by the process of corrosion. This volume expansion is mainly responsible for exerting the expansive radial pressure at the steel–concrete interface and development of hoop tensile stresses in the surrounding concrete resulting ultimately in the through cracking of the cover concrete. This cover cracking would indicate the loss of the service life for the corrosion-affected structures. In the present paper, an attempt has been made to develop analytical models for predicting the time to cover cracking by considering the residual strength of the cracked concrete and the stiffness provided by the combination of the reinforcement and expansive corrosion products. The problem is modeled as a boundary value problem wherein the governing equations are expressed in terms of the radial displacement and the analytical solutions are presented considering a simple 2-zone model for the cover concrete viz. cracked or uncracked. The analytical models are then evaluated through their ability to reproduce available experimental trends and subsequently a sensitivity analysis has also been carried out to show the influence of the various variable parameters of the proposed models with reference to the experimental trends.

Numerical simulation of potential distribution due to the corrosion of reinforcement in concrete structures

Corrosion of the reinforcement is one of the causes of reducing service-life of concrete structures. The methodologies used for making a decision on the repair or the replacement of damaged elements require in situ measurements for locating corroding areas. This paper is focussed on the results of the application of half-cell potential mapping for locating corroding areas on reinforced concrete pipeline and on six concrete columns. For reinforced concrete pipelines the validity of the half-cell potential measurements was confirmed by using finite-element calculations based on the distribution of the electrical field in soil. For the reinforced concrete columns, it was observed that the value of the half-cell potential can be inaccurate for locating corroding zones, when steel-concrete interface is cracked or when concrete cover is delaminated due to the expansion of corrosion products. This means that highly corroding areas are not always detected by this method. The occurrence of cracking and delamination in reinforced concrete structures was also studied by using a finite-element approach. It is shown that the absolute values of potential of steel in concrete depend on the delamination of concrete cover. So, the value of half cell potential should not be used as an indicator of corrosion. On the contrary, the potential gradient is a good indicator for locating corroding reinforcement in crack-free concrete surface. Some additional finite-element calculations were carried out to study the influence of concrete conductivity, cover thickness and delamination size on the size of concrete surface influenced by corrosion. So, this size increases with concrete cover thickness.

Analytical model of corrosion-induced cracking of concrete considering the stiffness of reinforcement

The structural deterioration of concrete structures due to reinforcement corrosion is a major worldwide problem. Service life of the age-degraded concrete structures is governed by the protective action provided by the cover concrete against the susceptibility of the reinforcement to the corrosive environment. The corrosion of steel would result in the various corrosion products, which depending on the level of the oxidation may have much greater volume than the original iron that gets consumed by the process of corrosion. This volume expansion would be responsible for exerting the expansive radial pressure at the steel-concrete interface resulting in the development of hoop tensile stresses in the surrounding cover concrete. Once the maximum hoop tensile stress exceeds the tensile strength of the concrete, cracking of cover concrete would take place. The cracking begins at the steel-concrete interface and propagates outwards and eventually resulting in the through cracking of the cover concrete. The cover cracking would indicate the loss of the service life for the corrosion-affected structures. In the present paper, analytical models have been developed considering the residual strength of the cracked concrete and the stiffness provided by the combination of the reinforcement and expansive corrosion products. The problem is modeled as a boundary value problem and the governing equations are expressed in terms of the radial displacement. The analytical solutions are presented considering a simple 2-zone model for the cover concrete viz. cracked or uncracked. A sensitivity analysis has also been carried out to show the influence of the various parameters of the proposed models. The time to cover cracking is found to be function of initial material properties of the cover concrete and reinforcement plus corrosion products combine, type of rust products, rate of corrosion and the residual strength of the cover concrete. The calculated cracking times are correlated against the published experimental and analytical reference data.

Effectiveness of Impressed Current Technique to Simulate Corrosion of Steel Reinforcement in Concrete

Accelerated corrosion by means of the impressed current technique is widely used in concrete durability tests. In this study, the influence of varying the impressed current density level between 100 and 500 mA/cm 2 on the actual degree of steel reinforcing bar corrosion as well as on the concrete strain behavior due to expansive corrosion products was experimentally investigated. Twelve reinforced-concrete prisms ~15032503300 mm! were used. The prisms were reinforced by two No. 10 reinforcing bars. Corrosion was induced by means of impressed current using electric power supplies. To depassify the steel reinforcement, 5% NaCl by weight of cement was added to the concrete mix. The strain response due to the expansion of corrosion products was measured at each face of the prisms. At the end of the corrosion phase, all the corroded reinforcing bars were removed, cleaned according to the ASTM G1-90 standard, and weighed to get the actual degree of mass loss. The results showed that, up to 7.27% mass loss, accelerated corrosion using the impressed current technique was effective in inducing corrosion of the steel reinforcement in concrete. With respect to Faraday's law, the use of different current densities has no effect on the percentage of mass loss. However, increasing the level of current density above 200 mA/ cm 2 results in a significant increase in the strain response and crack width due to corrosion of the steel reinforcement.

Modeling of Concrete Cracking Induced by Steel Expansion

When steel reinforcement corrodes in a concrete structure, the expansion of corrosion products often leads to cracking in the concrete. In this paper, a simple physical model is developed to relate the size of cracks to steel expansion. By assuming the steel/concrete interface to be perfectly smooth or perfectly bonded, upper- and lower-bound solutions can be obtained. The results indicate (1) the presence of an expansion threshold below which no crack propagation can occur; (2) a “pop-in” behavior occurring at a critical expansion that is strongly dependent on initial flaw size; and (3) the steel expansion to generate a given crack size is strongly affected by interfacial bonding. When the theoretical findings are compared to available experimental results, good agreement is obtained. Based on the theoretical results, several guidelines for controlling expansion-induced cracking can be proposed. These include proper control of initial flaw size, addition of fibers to produce an R-curve behavior, and the ...

Analysis of crack propagation and crackinitiation due to corrosion of reinforcement

Stress analysis in concrete due to corrosion of reinforcement was carried out, based on the concept ofthe linear elastic fracture mechanics (LEFM) and the boundary element method (BEM). Initiation and propagation of five patterns of cracks which propagate from the reinforcement are in focus. Hydrostatic pressure distribution and vertical pressure to the surface crack were taken into consideration to simulate the expansion of corrosion products. From the theory of elasticity, under hydrostatic pressure it was concluded that other cracks than the surface crack could be nucleated either by following the surface crack or in the case of the surface crack being arrested by aggregate. During the propagation of either a surface crack or an internal crack, it was found in the BEM analysis that a spalling crack and/or a diagonal crack could be initiated. Under only vertical pressure, the stress distribution suggested a possible initiation of the internal crack only. In the case that the internal crack propagated after the surface crack was completed, a high tensile region suggested the initiation of a vertical crack.