40 ILCS 5/9-122

No annuity or disability benefit shall be fixed, granted, or paid under this Article before the effective date. Any employee annuitant or widow annuitant may execute a waiver of his or her right to receive any part of his or her total annuity. A waiver shall take effect upon its being filed with the board. A waiver may not be revoked after it is executed and filed, except within the first 30 days after being filed.