
<PAGE>   1
                                                                   EXHIBIT 10.3

                              AMENDMENT NUMBER ONE

                                 TO CALMAT CO.
                                CREDIT AGREEMENT
                          DATED AS OF AUGUST 20, 1991

This Amendment to the Credit Agreement described below is entered into as of
October 16, 1992.

         1.      Preliminary Statement.  Reference is made to a Credit
Agreement dated as of August 20, 1991 (which, as it may be amended, extended or
supplemented from time to time, is herein called the "Credit Agreement") among
CalMat Co. (the "Borrower"), the lenders which are a party thereto (the
"Lenders") and The First National Bank of Chicago, as agent for the Lenders
(the "Agent").  On August 26, 1992 the Borrower announced a charge of
$15,000,000 relating to certain real estate assets of the Borrower and a charge
of $10,000,000 relating to the implementation of Statement of Accounting
Standards No. 106 concerning post-retirement benefits other than pensions.  The
Borrower has requested that the Lenders amend the Credit Agreement in certain
respects to take into account these charges.

         2.      Amendments.  The Borrower, the Required Lenders and the Agent
                 hereby amend the Credit Agreement as follows:

         A.      Section 6.10(iii) of the Credit Agreement is hereby amended by
                 adding at the end thereof:

                          The after-tax amount of the charge taken by the
                 Borrower in August, 1992 relating to certain real estate
                 assets, in an after-tax amount not exceeding $9,000,000
                 together with the after-tax amount of the charge taken by the
                 Borrower in August, 1992 relating to implementation of
                 Statement of Accounting Standards No. 106 concerning
                 post-retirement benefits other than pensions, in an after-tax
                 amount not exceeding $6,000,000, shall be added back to
                 Consolidated Net Income, measured on a cumulative basis from
                 January 1, 1992, (but only to the extent previously deducted)
                 for the purpose of computing compliance with the permitted
                 amount of the restricted payments as described in this Section
                 6.10(iii).

         B.      Section 6.17 of the Credit Agreement is hereby amended by
                 adding at the end thereof:

                          The amount of the pre-tax charge taken by the
                 Borrower in August, 1992 relating to certain real estate
                 assets, in an amount not exceeding $15,000,000, shall be added
                 back to Consolidated Income Before Interest and Taxes (but
                 only to the extent previously deducted) for the fiscal quarter
                 ending September 30,





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<PAGE>   2
                 1992 in computing compliance with this Section for each four-
                 quarter period which includes such quarter.

         C.      Section 6.18 of the Credit Agreement is hereby amended by
                 adding at the end thereof:

                          The amount of the pre-tax charge taken by the
                 Borrower in August, 1992 relating to certain real estate
                 assets, in an amount not exceeding $15,000,000, shall be added
                 back to Consolidated Income Before Interest and Taxes (but
                 only to the extent previously deducted) for the fiscal quarter
                 ending September 30, 1992 in computing compliance with this
                 Section for each four-quarter period which includes such
                 quarter.

         D.      Section 6.19 of the Credit Agreement is hereby-amended by
                 adding at the end thereof:

                          The after-tax amount of the charge taken by the
                 Borrower in August, 1992 relating to implementation of
                 Statement of Accounting Standards No. 106 concerning
                 post-retirement benefits other than pensions, in an after-tax
                 amount not exceeding $6,000,000, shall be added to
                 Consolidated Tangible Net Worth (but only to the extent that
                 Consolidated Tangible Net Worth was reduced by such charge) in
                 computing compliance with this Section.

         3.      Representations.  In order to induce the Lenders to enter into
this Amendment the Borrower represents and warrants that:

                 A.       The representations and warranties set forth in
         Section 5 of the Credit Agreement, as hereby amended, are true,
         correct and complete on the date hereof as if made on and as of the
         date hereof and that there exists no Default or Unmatured Default on
         the date hereof;

                 B.       The execution and delivery by the Borrower of this
         Amendment have been duly authorized by proper corporate proceedings
         and this Amendment, and the Credit Agreement, as amended by this
         Amendment, constitute valid and binding obligations of the Borrower;
         and

                 C.       Neither the execution and delivery by the Borrower of
         this Amendment, the consummation of the transactions herein
         contemplated, nor compliance with the provisions hereof will violate
         any law, rule, regulation, order, writ, judgment, injunction, decree
         or award binding on the Borrower or the Borrower's articles of
         incorporation or by-laws or the provisions of any indenture,
         instrument or agreement to which the Borrower is a party or is
         subject, or by which it or its property, is bound, or conflict with or
         constitute a default thereunder.





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         4.      Closing Conditions.  This Amendment shall be effective upon
Receipt by the Agent from the Required Lenders and the Borrower of executed
counterparts of this Amendment or of telex or telecopied confirmation of their
execution and mailing of this Amendment.

         5.      Definitions.  Unless the context shall otherwise require, all
terms used herein which are defined in the Credit Agreement shall have the
meanings assigned to them therein.

         6.      Counterparts.  This Amendment may be executed by the parties
hereto individually, or in any combinations of the parties hereto in several
counterparts, all of which taken together shall constitute one and the same
amendment.

         7.      Ratification.  Except as expressly amended hereby, all of the
representations, warranties, provisions, covenants, terms and conditions of the
Credit Agreement shall remain unaltered and in full force and effect and, as
amended hereby, the Credit Agreement is in all respects agreed to, ratified and
confirmed by the Borrower and the Lenders.

         8.      Expenses.  The Borrower shall pay all expenses of the Agent
(including reasonable charges for in-house counsel) incurred by the Agent in
the preparation of this Amendment.

         9.      Reference to and Effect on the Loan Documents.  Upon the
effectiveness of this Amendment, each reference in the Credit Agreement and the
other Loan Documents to "this Credit Agreement," "hereunder," "hereof,"
"herein" or words of like import referring to the Credit Agreement, and each
reference in the Notes and the other Loan Documents to the "Agreement,"
"thereunder," "thereof," or words of like import referring to the Agreement,
shall mean and be a reference to the Credit Agreement, as amended hereby.

         CALMAT CO.


         By:/s/ Frederick T. Sauer             By:/s/ Ronald C. Hadfield
            -------------------------------       --------------------------
            Its: Vice President                   Its: Executive VP/CFO
                 - Treasurer

         THE FIRST NATIONAL BANK OF CHICAGO
              Individually and as Agent

         By:/s/ Gene Benke                    
            -------------------------------
            Its: Senior Vice President





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