I would like to model the freeze and thaw effect on reinforced concrete bridge deck surfaces using Cox Regression. But the parameter estimate is negative which is the opposite of the reality. When the freeze and thaw cycles increase the concrete deck should degrade faster, but it has a negative sign in Cox regression. Other covariates I consider are snow days, salt used (tons/lane miles), span length, and structure length. These data are collected every year and it is not constant as well. There are some less than 20% missing values as well. A portion of the bridges is spanning on water and most with under highway. I calculated the VIF for these covariates under consideration and snow and freeze and thaw has VIF>30.
But even when the freeze and thaw are in the model on its own, the parameter estimate is negative. Could anyone point out if there is anything from the statistical point of view? Can I ignore this covariate and provide a reason that its estimate is with a negative sign which does not make sense from an engineering point of view. I have a minor in statistics but want to have your opinions.