Retrofitting and waterproofing of aged concrete using electromigration of nanosilica: a case study

Concrete is a common construction material used to support structures around the world. However, the durability of concrete is affected by weathering action, abrasion, and chemical attack and this may lead to reduction in desired material properties necessary to support structures. Electromigration is the transport of material in a conductor under the influence of an applied electric field. All conductors are susceptible to electromigration; therefore it is important to consider the effects the electrical current resulting from the applied field may have on the conductor. The net force exerted on a single metal ion in a conductor has two opposing contributions: a direct force and wind force. Electrochemical engineering is the branch of chemical engineering dealing with the technological applications of electrochemical phenomena, such as electrosynthesis of chemicals, electrowinning and refining of metals, flow batteries and fuel cells, surface modification by electrodeposition, electrochemical separations and corrosion. This paper presents results of two small-scale tests using electromigration process as a means of transporting nanosilica to recover cement matrix integrity of aged 32 MPa concrete samples extracted from a 40-year-old structure. A set up with two vessel was proposed, with 12 Vdc electrical font working for 48 h generating transportation of nanosilica (12 nm in diameter) into the aged concrete samples. The experiments were performed in two distinct laboratories. One at Flowtest in Brazil and one at the Research Laboratory of the George Mason University Department of Civil Engineering in the US. Thus, repeatability and reproducibility of the process can be proven under laboratory conditions. The success of the electromigration process was verified with electronic microscope (qualitative analysis), scanning electronic microscope, and X ray dispersive energy spectroscopy. The results showed that an electromigration of nanosilica into the cement matrix occurred and resulted in reduction of micro fissures. Additionally, deposition of silica on the sample surface was observed. Reduction of calcium in the matrix was verified with the development of hydrated calcium silicate, providing the recovery of cement matrix in increasing cement mechanical properties like strength and also decreasing the porosity of the concrete matrix. Another important phenomenon is the rehabilitating of the chloride contaminated concrete structure to extend its service life, an electrochemical chloride extraction (ECE) treatment with simultaneous migration of silicate ion was performed. Based on referenced literature, it can be assumed that the extraction of chlorine ions occurs simultaneously with the recovery of cement matrix by nanosilica.