50 ILCS 505/9
When a contractor receives any payment, the contractor shall pay each subcontractor and material supplier in proportion to the work completed by each subcontractor and material supplier their application less any retention. If the contractor receives less than the full payment due under the public construction contract, the contractor shall be obligated to disburse on a pro rata basis those funds received, with the contractor, subcontractors and material suppliers each receiving a prorated portion based on the amount of payment. All interest payments received pursuant to Section 4 also shall be disbursed to subcontractors and material suppliers to whom payment has been delayed, on a pro rata basis. When, however, the public owner does not release the full payment due under the contract because there are specific areas of work or materials the contractor is rejecting or because the contractor has otherwise determined such areas are not suitable for payment, then those specific subcontractors or suppliers involved shall not be paid for that portion of the work rejected or deemed not suitable for payment and all other subcontractors and suppliers shall be paid in full.
If the contractor, without reasonable cause, fails to make any payment to his subcontractors and material suppliers within 15 days after receipt of payment under the public construction contract, the contractor shall pay to his subcontractors and material suppliers, in addition to the payment due them, interest in the amount of 2% per month, calculated from the expiration of the 15-day period until fully paid. This Section shall also apply to any payments made by subcontractors and material suppliers to their subcontractors and material suppliers and to all payments made to lower tier subcontractors and material suppliers throughout the contracting chain.
50 ILCS 505/9