This investigation presents a comparative study on mechanical properties, electrical resistivity and microstructure of mortar under DC current, compared to mortar in rest (no current) conditions. Monitoring was performed from 24h after casting until 84 days of cement hydration. A current density level of 10 mA/m 2 was chosen as a simulation regime. It was found that the DC current exerts microstructural changes in the bulk mortar matrix and thereby affects electrical properties and mechanical performance. Whereas the latter were slightly influenced, the former were modified to a more significant extent; the current flow was found to cause initial densification of the bulk matrix at earlier stages (until 14 days of age), whereas coarsening of the material was observed after 56 and until 84 days of cement hydration. Additionally, numerical simulation of the stray current distribution is performed, meant to serve as a basis for further elaborated modelling of the level of current density that could exert significant microstructural alterations in a bulk cement-based matrix.

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/m 2 , 100 mA/m 2 , and 10 mA/m 2 , simulating different levels of stray (DC) current environment. The highest level of 1A/m 2 was also comparable to stray current densities, as measured in field conditions. The tests were designed in a way, so that the effects of diffusioncontrolled 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 monitored throughout the test. For water-conditioned specimens, the stray current was found to accelerate degradation processes. 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 current 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/m 2 and in conditions of concentration gradients with the external medium, would lead to more pronounced negative effects on microstructural and micromechanical performance.

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/m 2 , 100 mA/m 2 and 1 A/m 2 ) 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/m 2 . This would result in compromised mechanical properties and potentially reduced performance of cementbased materials within service life. Therefore, concrete curing and conditioning on site need to include considerations for the potential effect of stray currents.

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.