The VANDA® Aggressiveness Index was developed to quantify how corrosively aggressive an atmosphere is to a structure.
Overall ratings are not based on a formula or an average value. The ratings are subjective and may be based entirely on one criterion. For example, if the concrete condition of a structure is solid and given a rating of 1, but the atmospheric H2S is measured to be 50 ppm and falls into the severe category, the overall rating may be higher based solely on the atmospheric readings. However, if the hydraulic rating of a structure is given a 5, but all other ratings would indicate a non-aggressive atmosphere, the hydraulic rating would not necessarily override the other ratings.