<PAGE>   1

                                                                     EXHIBIT 4.5

                   FORM OF SERIES 2000C CONVERTIBLE DEBENTURE
                            KIEWIT MATERIALS COMPANY
                             A DELAWARE CORPORATION

Registered Debenture No.                                $
8.25% Convertible Debenture                                Due: October 31, 2010

     KIEWIT MATERIALS COMPANY, a corporation organized and existing under the
laws of the State of Delaware, and having its principal place of business in the
City of Omaha, Nebraska (hereinafter called the "Corporation"), for the value
received, hereby promises to pay to                or registered assigns
(hereinafter called the "Debentureholder") on October 31, 2010, the principal
sum of                ($          ) Dollars.

     This debenture is one of the 2000C series of registered convertible
debentures of the Corporation, due October 31, 2010 unless previously redeemed
or converted, limited to the aggregate principal amount of                and
No/100 ($          ) Dollars, all issued or to be issued pursuant to an
indenture, dated as of             , 2000 (the "Indenture"), executed and
delivered by the Corporation and UMB Bank, N.A. as Trustee, (hereinafter
referred to as "Trustee"). This debenture is subject to the terms of the
Indenture, and all indentures supplemental thereto, and reference is hereby made
to the Indenture and any such supplemental indenture for a description of the
rights, limitations, obligations and immunities of the Corporation, the holders
of the debentures and the Trustee. Capitalized terms used and not defined herein
have the meanings ascribed to those terms in the Indenture.

          (1) TYPE OF PAYMENT:  Payment of interest and principal shall be in
     any coin or currency of the United States of America which at the time of
     payment shall be legal tender for the payment of public or private debts.


          (2) INTEREST:  Except as hereinafter provided, the Corporation
     promises to pay to the registered holder hereof or his registered assigns,
     interest on the principal sum of this debenture at the rate of eight and
     twenty five hundredths (8.25%) per annum until the Corporation's obligation
     with respect to the payment of the principal amount shall have been
     discharged. Interest shall accrue upon this debenture from the most recent
     date to which interest has been paid, or if no interest has been paid, from
     the date of original issuance. In the event of conversion as provided in
     paragraph (7)(a) hereof, interest shall cease accruing on the principal
     amount of this debenture on June 30, 2004. In the event of conversion as
     provided in paragraph (7)(b) hereof, interest shall cease accruing on the
     principal amount of this debenture on the Conversion Date. Interest on this
     debenture shall be paid annually on each November 1st, commencing November
     1, 2000 to the registered owner as recorded on the Security Register
     maintained by the Corporation as of the preceding October 15.


          (3) PLACE OF PAYMENT:  The principal and interest on this debenture
     shall be payable at the office of the Corporation in Omaha, Nebraska.

          (4) DEFAULT:  This debenture shall be deemed to be in default whenever
     the principal sum and/or interest becomes payable and remains unpaid for a
     period of sixty (60) days.


          (5) REDEMPTION:  This debenture is subject to redemption at any time
     prior to the date of maturity at its principal amount plus all accrued and
     unpaid interest to the date of redemption, provided, however, that the
     entire series is redeemed. This debenture may not be redeemed during the
     thirty-one (31) day conversion period provided for in paragraph (7)(a)
     hereof. Redemption shall be preceded by notice thereof, mailed to the
     registered holder by first class mail no later than thirty (30) days
     preceding the date of redemption. The registered holder may convert this
     debenture into Common Stock during that thirty (30) day period preceding
     the date set for redemption as provided for in paragraph 7(b) hereof.

<PAGE>   2


          (6) TRANSFER:  The debenture may be transferred by the registered
     holder at the principal office of the Corporation in Omaha, Nebraska on
     registry books kept for such purpose at the office of the Trustee, upon
     surrender and cancellation of this debenture, and the payment of applicable
     charges. The Corporation and the Trustee may treat the registered holder of
     this debenture as recorded on the Security Register and the delivery of a
     form of assignment with medallion signature guarantee, or other guarantee,
     acceptable to the Trustee, as the absolute owner for all purposes.


        (7) CONVERSION

          (a) This debenture is convertible into the $0.01 par value common
     stock ("Common Stock") of the Corporation on the following basis: during
     the period commencing on October 1, 2004 and ending on October 31, 2004,
     the registered holder may convert the entire principal amount of this
     debenture, plus a cash payment equal to the product of
     multiplied by the Average Closing Market Price on the Conversion Date, into
                    (               ) shares of Common Stock, subject to
     adjustment upon the occurrence of certain events as provided in the
     Indenture.

          (b) If any of the following events occur prior to October 1, 2004,
     this debenture is convertible into Common Stock: (i) for thirty (30) days
     following the occurrence of the registered holder's death; (ii) for thirty
     (30) days following a Change of Control; and (iii) for thirty (30) days
     prior to redemption of the registered holder's debentures, as provided for
     in paragraph (5) hereof. During any such period, the registered holder or
     the registered holder's representative or estate, as the case may be, may
     convert the entire principal amount of the debenture plus a cash payment
     equal to the product of                multiplied by the Average Closing
     Market Price on the Conversion Date, into                (               )
     shares of Common Stock, subject to adjustment upon the occurrence of
     certain events as provided in the Indenture.

          (c) During the period commencing on November 1, 2004 and ending on
     October 31, 2010, the date of maturity, the debenture is not convertible,
     in whole or in part.

          (d) The conversion privilege shall be deemed exercised by submission
     of this debenture with a written request for conversion plus the required
     cash payment during the applicable conversion period at the principal
     office of the Corporation. The Corporation shall thereafter, within a sixty
     (60) day period, issue certificates registered in the name of the
     registered holder representing the shares of Common Stock into which this
     debenture has been converted.

          (8) SUBORDINATION:  In the payment of their claims, all creditors of
     the Corporation shall rank equally with the holders of convertible
     debentures. All claims of the debentureholders against earnings or assets
     are hereby made superior to those of stockholders, and the Corporation
     agrees that as long as any of its convertible debentures are outstanding,
     it will not pay any dividends on its stock until all liability for unpaid
     interest on its debentures has been paid. In the event of dissolution or
     liquidation of the Corporation, the holders of all debentures shall be
     entitled to be paid in full, both principal and interest, before any assets
     of the Corporation are distributed to any stockholder.

          (9) LIQUIDATION OF LIABILITY:  No recourse shall be had for the
     payment of the principal or interest of this debenture, against any
     incorporator, stockholder, officer, or director, past, present, or future,
     of the Corporation, all such liability being expressly waived by the owner
     of this debenture.

          (10) VALIDATION:  This debenture shall not be valid or become
     obligatory for any purpose until the certificate of authentication thereon
     shall have been signed by the Trustee.

                                        2
<PAGE>   3

     IN WITNESS WHEREOF, Kiewit Materials Company has caused this debenture to
become signed and its corporate seal to be hereunto affixed by its officers duly
authorized thereunto, all as of the      day of             , 2000.

<TABLE>
<S>                                                         <C>
ATTEST:                                                                      KIEWIT MATERIALS COMPANY

-----------------------------------------------------
                                                            By --------------------------------------------------
Assistant Secretary
                                                                                     President
</TABLE>


TRUSTEE'S CERTIFICATE OF AUTHENTICATION:  This is one of the debentures
described in the Indenture, dated as of             , 2000, by and between
Kiewit Materials Company and UMB Bank, N.A., as Trustee.


<TABLE>
<S>                                                         <C>
Dated -----------------------------------------------       By --------------------------------------------------
                                                                                Authorized Officer
</TABLE>

This Debenture and the transfer thereof are subject to restrictions which are
stated in an Agreement between the Debentureholder(s) whose name(s) appear(s)
hereon and KIEWIT MATERIALS COMPANY, dated                            , 2000.

                                        3