Degradation mechanisms of cast-in-situ concrete subjected to internal-external combined sulfate attack

•Combined attack clearly induces faster degradation to the cast-in-situ concrete.•Chemical attack is responsible for the degradation in fully immersed conditions.•Internal sulfate attack reduces the concrete strength in the early stage.•Internal sulfate attack speeds up the external sulfate attack induced degradation. In this paper, an experimental study was conducted to reveal the degradation mechanisms of cast-in-situ concrete induced by internal-external combined sulfate attack. The internal sulfate attack of concrete was conducted by adding extra sodium sulfate in concrete mixtures. Meanwhile, the external sulfate attack of concrete was investigated by immersing samples in the sodium sulfate solutions. The dimensions, mass, compressive strength and the sulfate concentration in the tested specimens were continuously monitored. The microstructure and mineral phases of tested concrete were identified after the immersion. Results illustrate that chemical attack is the main cause of degradation for fully immersed concrete in sulfate-rich environments. Cast-in-situ concrete suffers a fast and severe degradation when subjected to internal-external combined sulfate attack. Internal sulfate attack significantly retards the strength development of concrete in the early corrosion stage. Development of crack system is remarkably accelerated by internal sulfate attack hence inducing a relatively faster penetration of external sulfate ions.