
<PAGE>   1
                                                                  EXHIBIT 10.4

                              AMENDMENT NUMBER TWO

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

This Amendment to the Credit Agreement described below is entered into as of
December 28, 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").  In December, 1992 the Borrower announced a charge of
approximately $10,000,000 relating to certain of its operations in Arizona, San
Diego and Los Angeles.  The Borrower has requested that the Lenders amend the
Credit Agreement in certain respects to take into account these charges.  The
Borrower has also requested that the Lenders amend the Credit Agreement to
extend the Revolving Credit Termination Date by one year.

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

         A.      Article I of the Credit Agreement is hereby amending the
                 definition of Revolving Credit Termination Date to read:

                          "Revolving Credit Termination Date" means August 20,
                          1994.

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

                                  In addition to the adjustment described in
                          the preceding sentence, the amount of the pre-tax
                          charge taken by the Borrower in December, 1992
                          relating to certain operations in Arizona, San Diego
                          and Los Angeles, in an amount not exceeding
                          $10,000,000, shall be added back to Consolidated Net
                          Income Before Interest and Taxes (but only to the
                          extent previously deducted) for the fiscal quarter
                          ending December 31, 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:

                                  In addition to the adjustment described in
                          the preceding sentence, the amount of the pre-tax
                          charge taken by the Borrower in December, 1992





LWLA3\22501.1

<PAGE>   2
                          relating to certain operations in Arizona, San Diego 
                          and Los Angeles, in an amount not exceeding 
                          $10,000,000, shall be added back to Consolidated 
                          Income Before Interest and Taxes (but only to the 
                          extent previously deducted) for the fiscal quarter 
                          ending December 31, 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 in its
                 entirety to read as follows:

                          6.19. Tangible Net Worth.  The Borrower will maintain
                          Consolidated Tangible Net Worth at all times equal to
                          (i) $280,000,000 plus (ii) 50% of Consolidated Net
                          Income for each fiscal quarter, commencing with the
                          fiscal quarter ending March 31, 1993, but without
                          deduction for any quarter in which there is a loss.

         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.

         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.





LWLA3\22501.1                             2

<PAGE>   3
         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         
                                          -----------------------------------
                                          Its: Vice President-Treasurer


                                      By: /s/ Ronald C. Hadfield   
                                          -----------------------------------
                                          Its: Executive Vice-President

                                      THE FIRST NATIONAL BANK OF CHICAGO
                                           Individually and as Agent


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





LWLA3\22501.1                          

