Study of the Electrical Resistivity of Concrete Submitted to the Internal and External Action of Sulfates

Durability of concrete structures is essential for sustainability in construction, reducing maintenance and consumption of natural resources. Sulfate ions are aggressive agents that attack concrete structures and promote steel corrosion. They are present in cement or aggregates or are absorbed through contact with sewage, acid rain, industrial effluents, subsoil, oceans, and agricultural waters. Free sulfate ions in the aqueous solutions within concrete pores typically react over time with compounds in the cement paste, generating expansive products that crystallize within the pores. This process leads to cracking and reduces the concrete's lifespan. Magnesium sulfate is particularly harmful because it reacts with calcium hydroxide, reducing alkalinity and destabilizing the hydrated calcium silicate. The search for tests that provide indicators of these expansive reactions is crucial for achieving durable structures, particularly non-destructive tests that can be performed in situ. This study evaluates the surface electrical resistivity testing to assess the integrity of concrete samples in which magnesium sulfate was incorporated as a 1% and 10% replacement for fine aggregate (internal attack). Samples subjected to wetting and drying cycles with water containing MgSO4 at concentrations of 150 mg/L and 50,000 mg/L (external attack) were also analyzed. Compressive strength tests were conducted at ages ranging from 31 to 273 days. The results indicated that, in all studied scenarios, surface electrical resistivity increased with curing time. After 100 days, the electrical resistivity of concrete with external contamination was lower than that of uncontaminated concrete. However, sulfate attack decreased the surface electrical resistivity of the concrete containing a 10% fine aggregate replacement after only 31 days of curing. Regarding compressive strength, only the 10% fine aggregate replacement significantly reduced compressive strength at all ages studied.