Chemical and mineralogical alterations of concrete subjected to chemical attacks in complex underground tunnel environments during 20-36 years

This paper investigates the chemical and mineralogical alterations of concrete in underground tunnel structures built from 1980 to 1996, located on the China's east seashore. The underground water around the tunnels had once been treated to be not or little aggressive. However, the complex environments in the tunnels had increased the aggressiveness of certain ingredients, thus causing chemical attacks. The chlorides in the leakage underground water cause chloride attack, and the NaCl crystallizations on the surface of the C50 pre-cast concrete segments induce a deeper chloride contamination. When the concrete suffers flowing leakage water, calcium leaching also occurs, leading to decalcification and magnesium incorporation. Under this circumstance, C50 pre-cast concrete shows a higher resistance compared with C30 cast-in-place concrete, mainly due to the low water to binder ratio. Within the C30 cast-in-place concrete facing concentrated leakage water and NaCl crystallizations, a strong magnesium chloride attack is observed. Besides the materials factors, the environmental factors, including the high concentrations of both chloride and magnesium ions, the removal timing of calcium ion, and the water saturation, are believed to take responsibility for the magnesium chloride attack.