You might choose not to classify Hagar as a wife, but then you have to say Abraham had a wife and a concubine at the same time, hardly an improvement in modern eyes.
After Sarah died, Abraham married Keturah.
It should be noted that the Bible does not rule out the possibility of Abraham having other concubines while either Sarah or Keturah were still alive. Such would have been the norm for a wealthy tribal chieftain like Abraham.
In modern times he would have had a surrogate. Abraham's wife initiated it.