40 ILCS 5/13-603
If an employee who has received a refund subsequently re-enters the service and renders one year of contributing service from the date of such re-entry, the employee shall be entitled to have restored all accumulation and service credits previously forfeited by making a repayment of the refund, including interest from the date of the refund to the date of repayment at a rate equal to the higher of 8% per annum or the actuarial investment return assumption used in the Fund's most recent Annual Actuarial Statement. Repayment may be made either directly to the Fund or in a manner similar to that provided for the contributions required under Section 13-502. The service credits represented thereby, or any part thereof, shall not become effective unless the full amount due has been paid by the employee, including interest. The repayment must be made in full by the employee no later than 90 days following the date of the employee's final withdrawal from service. If the employee fails to make a full repayment, any partial amounts paid by the employee shall be refunded without interest .
40 ILCS 5/13-603