Every contract shall contain a provision in regard to the time when the whole or any specified portion of the work contemplated shall be completed, and shall provide that for each day completion is delayed beyond the specified time, the contractor shall forfeit and pay to the state a specified sum of money, to be deducted from any payments due or to become due to the contractor. The sum so specified is valid as liquidated damages unless manifestly unreasonable under the circumstances existing at the time the contract was made. A contract for a road project may also provide for the payment of extra compensation to the contractor, as a bonus for completion prior to the specified time, such provision, if used, to be included in the specifications and to clearly set forth the basis for such payment.