Stray current has become a main concern for many years due to its effect on (reinforced) concrete structures and underground infrastructures. It has been reported that stray current affects not only steel reinforcement embedded in concrete, but can also induce degradation of the cement-based matrix. Stray current causes an increase of temperature in hardening concrete due to Joule heating which will accelerate cement hydration. The accelerated cement hydration results in faster evolution of materials properties (e.g. stiffness and strength) and a faster decrease of the capillary porosity. The microstructural change due to stray current flow will affect transport properties, as well as the service life performance of cement-based materials. In case the concrete is exposed to water, leaching of alkali ions will decrease compressive strength and increase permeability and diffusion coefficient of concrete. Under stray current, leaching of alkali ions in concrete is accelerated which will increase level of structural degradation. Deterioration of concrete due to stray current involves many mechanisms including ion and mass transport, electrical conduction, heat transfer and corresponding occurrence of mechanical stresses. However, the study on the effect of stray current on material properties (e.g. microstructural, mechanical, electrical properties) and longterm performance/durability of cement based materials is still lacking. The aim of this thesis is to investigate the effects of stray current on long-term performance of cementbased materials. The results of this project will contribute to a better understanding on beneficial (positive) and/or detrimental (negative) effects of stray current on cementbased materials, which is a point of significant importance for real practice...@en

This work reports on the development of microstructural and mechanical properties of mortar cubes under the synergetic action of stray current and various environmental/curing conditions. The study refers to specimens cured for 24h only, followed by a 112 days period of partial or full submersion in water or alkaline medium. Additionally, equally prepared mortar specimens were tested in sealed conditions. The outcomes for submerged and saturated conditions were compared to sealed conditions. Three current density regimes were employed i.e. 1 A/m2, 100 mA/m2, and 10 mA/m2, simulating different levels of stray (DC) current environment. The highest level of 1A/m2 was also comparable to stray current densities, as measured in field conditions. The tests were designed in a way, so that the effects of diffu-sion-controlled transport (ions leaching due to concentration gradients), were distinguished from migration-controlled ones (ion/water transport in stray current conditions). Mechanical, microstructural and electrical properties were moni-tored throughout the test. For water-conditioned specimens, the stray current was found to accelerate degradation pro-cesses. This was reflected by decreased compressive strength, reduced electrical resistivity and increased porosity of the matrix. The results were attributed to leaching-out of alkali ions due to concentration gradients, where except diffusion, migration took place i.e. the leaching-out effect was accelerated by water and ions migration in conditions of stray cur-rent flow. In contrast, stray current flowing through mortar in sealed conditions (as well as through mortar in alkaline medium) resulted in increased compressive strength and electrical resistivity. These were accompanied by densification of the bulk matrix and reduced porosity. It can be concluded that for a cement-based material at early hydration age, both positive and negative effects of stray current flow can be expected. The level and direction of these effects are dependent on the external environment and the current density levels, where stray currents above 100 mA/m2 and in conditions of concentration gradients with the external medium, would lead to more pronounced negative effects on microstructural and micromechanical performance. @en

Stray current is an electrical current “leakage” from metal conductors and electrical installations. When it flows through cement-based materials, electrical energy is converted to thermal energy that causes increasing temperature due to Joule heating phenomena. The aim of this paper is to shed light on the influence of electrical current flow on cement hydration, thermal properties and pore structure changes of cement-based materials. Calorimetry tests show that degree of cement hydration increases as a results of temperature increase due to electrical current flow through cement-based materials. To evaluate the influence of electrical current on the thermal properties of cement paste, the specific heat of cement paste was calculated based on the degree of cement hydration and temperature development during the hydration process. MIP tests were carried out to quantify changes in the pore structure due to electrical current flow. The results shows that if no other factors are present, leaching is avoided and for relatively early cement hydration age, the electrical current flow accelerates cement hydration, leading to an initial decrease in porosity of the cement paste.@en

This paper reports the results of microstructural analysis based on image analysis subjected to electrical current as a simulation of stray current effect. The purpose is to investigate the influence of electrical current flow on the development of microstructural properties in reinforced cement-based materials. In view of the significant contribution to material performance, the characterization of cement-based microstructure in an economical and reliable way is of high relevance to permeability prediction and durability studies of cement-based materials. In this study, taking the cement paste submerged in Ca(OH)₂ conditions as specimens, the pore size distribution and percolation was derived from image analysis of ESEM micrographs. The electrical properties of mortars were measured and their microstructural characteristics were investigated using quantitative image analysis techniques. Moreover this approach is compared with other general methods such as mercury intrusion porosimetry (MIP) and the comparison shows good consistency in development of parameters characterizing the materials' microstructure.

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This work reports on the synergetic effect of water-to-cement ratio, curing conditions, varying external environment and stray current on the microstructural (porosity and pore size), electrical (resistivity) and mechanical (compressive strength) properties of 28 days-cured cement-based materials. The influence of curing on porosity and pore size, in stray current conditions, was assessed by correlating the performance of 28 days cured mortar with that of fresh (24h-cured only) mortar specimens in identical environmental medium. Three different levels of electrical current density (i.e. 10mA/m2, 100 mA/m2 and 1 A/m2) were applied to simulate stray current flow through hardened mortar specimens with water-to-cement ratio of 0.5 and 0.35. Different environmental conditions were employed i.e. sealed conditions, partly immersed, and fully submerged in water and calcium hydroxide medium. Microstructural, mechanical and electrical properties were monitored in the course of 140 days. The outcomes suggest a potentially positive effect of the stray current, where water and/or humidity exchange with the external environment is restricted. The potential for this positive effect was experimentally supported through the recorded matrix densification and increased compressive strength of mortar specimens, subjected to stray current and treated in calcium hydroxide and/or sealed conditions, compared to equally handled and treated control cases. In contrast, for water submerged mortar specimens, subjected to stray current, coarsening of the bulk matrix and reduced compressive strength were observed. The outcomes were irrespective of w/c ratio and curing conditions. The effect of stray current was found to be predominantly determined by the current density level and increased at values > 100mA/m2. This would result in compromised mechanical properties and potentially reduced performance of cement-based materials within service life. Therefore, concrete curing and conditioning on site need to include considerations for the potential effect of stray currents. @en

This work reports on the influence of stray current on the development of mechanical and electrical properties of mortar specimens in sealed and water-submerged conditions. In the absence of concentration gradients with external environment (sealed conditions) or in their presence (submerged conditions), compressive strength and electrical resistivity change due to: cement hydration alone; cement hydration, affected by diffusion (including leaching-out); or cement hydration, simultaneously influenced by diffusion and migration. The results are compared to equally conditioned control specimens, where stray current was not involved.

In view of material properties development over time, the ageing factor in relevant exposure conditions is addressed, considering reported approaches for its determination. Through implementing existing methodology and based on experimentally derived electrical resistivity values, the ageing factor for sealed conditions was determined. The apparent diffusion coefficients were calculated based on ageing factors and reported relationships, reflecting the effect of stray current on matrix diffusivity.@en

This study applies image analysis techniques and mathematical morphology and stereology approach to characterize the pore structure of mortar specimens, subjected to electrical current as simulation of cathodic protection. The purpose is to investigate the effect of electrical current flow on the development of microstructural properties in reinforced cement-based materials. In view of the significant contribution to material performance, characterization of pore microstructure in an economical and reliable way is of high relevance to permeability prediction and durability studies of cement-based materials. The pore size distribution and percolation was derived from image analysis (based on OPTIMAS software) of ESEM micrographs, captured on polished specimen sections. The methodology of applied mathematical morphology and stereological theories based on the so called ‘opening distribution’ is described and is used to calculate the pore network connectivity and thereby to estimate the permeability. The image analysis techniques are applied to mortar specimens, conditioned under the synergetic action of different stray current regimes and diverse curing environment at various hydration stages. The results reveal that apparent beneficial effects of stray current can be expected for relatively young cement-based materials which can be attributed to enhanced hydration mechanisms. In contrast, coarsening of the pore structure can be observed for a more mature matrix. Moreover this approach is compared with other general methods such as mercury intrusion porosimetry (MIP) and the comparison shows good consistency in development of parameters characterizing the materials’ microstructure. @en

This work reports on the influence of stray current flow on temperature rise in hardening cement-based materials and consequently altered cement hydration. To simulate stray current, different levels of electrical current were applied to cement paste and mortar specimens immediately after casting. Isothermal calorimetry tests were carried out to quantify the heat release and the degree of cement hydration. The results confirmedan increase in the degree of cement hydration as a resultof temperature increase due to electrical current flow. A simulation approach wasemployed to predict the temperature rise in cement-based materials when higher levels of stray current are involved. By incorporating an "electrical enhancement" factor, the degree of hydration and microstructural development of cement-based systems were simulated using HYMOSTRUC3D. These results also confirmed the expected acceleration of cement hydration within temperature rise due to electrical current flow.Densification of the bulk cement matrix would be relevant, following the aforementioned enhanced cement hydration, i.e. a positive effect of low level current densities could be at hand. However, detrimental effects e.g. current and/or temperature associated microcracking, internal stress etc., can be expected upon higher current density levels. This paper only focuses on illustrating the approach with regard "electrical enhancement" factor implementation in HYMOSTRUC3D and the validation of this approach through collected experimental data. @en

“Maturity” method is commonly used to predict concrete strength development based on the temperature history of concrete. Basically, concrete strength development is estimated using the relationship between maturity index and strength e.g. ASTM C 1074 elaborates the procedure of this standard practice, where the, maturity index can be expressed either as temperature-time factor using the Nurse-Saul equation or as the equivalent age at a specific temperature using the Arrhenius equation. According to Saul, concretes of the same mix at the same maturity have approximately the same strength whatever combination of temperature and time are relevant in order to make up that maturity. In cement-based materials, strength increases with the progress of cement hydration. The amount of hydrated cement depends on how long the concrete has been cured and at what temperature levels. When electrical current flows through the cement-based material, the temperature development will be increased compared to control conditions, initially leading to accelerated cement hydration and increased strength. Various experiments on electrical curing and maturity method of cement-based materials have been performed and reported. However, the effect of electrical current flow on the maturity of cement-based materials is still not well understood.This paper deals with the influence of stray current on the maturity levels of cement-based materials. Varying levels of electrical current density were applied to mortar specimens. The maturity levels were determined based on the relationship of electrical properties (i.e. electrical resistance), thermal properties (i.e. temperature development) and mechanical properties (i.e. strength) of cement-based materials.@en