<PAGE>

                                                                   EXHIBIT 10.20

                                   AGREEMENT


     This Agreement ("Agreement") is effective as of December 15, 1996, between
CALMAT CO., a Delaware corporation (hereinafter called "Company") and H. JAMES
GALLAGHER (hereinafter called "Executive").

                                    RECITALS

     A.   The Executive presently serves as Executive Vice President-Finance and
Chief Financial Officer of the Company.

     B.   In the event the Company becomes subject to any proposed or threatened
change in control, it is imperative that the Company and the Board of Directors
be able to rely upon Executive's advice as to the best interests of the Company
and its stockholders without concern that the Executive might be distracted by
the personal uncertainties and risks created by such a proposal or threat.

     C.   In the event the Company receives any such proposal or threat,
Executive may, in addition to Executive's regular duties, be called upon to
assist in the assessment of such matters, advise management and the Board of
Directors as to whether such proposals or other matters would be in the best
interests of the Company and its stockholders, and to take such other actions as
the Board of Directors might determine to be appropriate.

     D.   Accordingly, the Management Development and Compensation Committee
(the "Compensation Committee") has authorized the Company to enter into this
Agreement with Executive to assure the Company that it will have the continued
dedication of Executive and the availability of Executive's advice and counsel,
notwithstanding the possibility, threat or occurrence of an effort to take over
control of the Company, and to induce Executive to remain in the employ of the
Company.

                                       1
<PAGE>

                                 AGREEMENT

     NOW, THEREFORE, in consideration of the foregoing recitals and the promises
and conditions herein contained, it is agreed that the following terms of
employment which are set forth in this Agreement shall take effect automatically
without any further action by the Company, Board of Directors or Compensation
Committee on the day preceding any change in Control of the Company (as defined
in Section 3.4 below), but such terms of employment shall have no force or
effect unless a Change in Control occurs.

     This Agreement shall remain in effect unless otherwise terminated by
resolution of the Compensation Committee or the Board of Directors.
Notwithstanding the foregoing, in no event shall this Agreement terminate within
three (3) years after a Change in Control of the Company without the written
consent of Executive.  It is the Company's intention to provide to Executive the
benefits set forth herein if the Company is subject to any Change in Control and
the other applicable conditions of this Agreement are satisfied.  The Company
shall notify Executive in writing at least three (3) years prior to the
effective date of termination if the Compensation Committee or Board of
Directors determines to terminate this Agreement.

     Unless this Agreement is terminated as provided above, the following terms
of employment which are set forth in this Agreement shall become effective on
the day preceding any Change in Control of the Company.

     Until the terms of employment set forth in this Agreement become effective,
the letter agreement with respect to severance benefits in the event of
termination without cause dated August 10, 1993, previously entered into by the
Company and Executive, as such agreement may be amended from time to time
(hereinafter referred to as the "Severance Agreement") shall remain in effect.
The Severance Agreement shall terminate upon a Change in Control of the Company
and thereafter shall have no further force or effect.  In no event shall the
amounts and benefits under the Severance Agreement and this Agreement be
additive to one another.

                                       2
<PAGE>

                                   ARTICLE I

                                   EMPLOYMENT

Section 1.1:  Position, Duties and Responsibilities of Executive
              --------------------------------------------------

     The Company shall employ the Executive as Executive Vice President-Finance
and Chief Financial Officer. In such position, the Executive shall have at least
such responsibilities and powers as set forth in Article IV, Section 7 of the
Bylaws of the Company in effect on the date hereof. During his employment
hereunder, the Executive shall devote his full energies, interest, abilities and
productive time during normal business hours to the performance of this
Agreement and shall, without the Company's prior written consent in each
instance, refrain from rendering services of any kind to others for compensation
or which would materially interfere with the performance of his duties under
this Agreement. Notwithstanding the foregoing, the Executive may serve as a
director of other companies with the consent of the Company's Board of
Directors.

Section 1.2:  Term of Employment
              ------------------

     (a) The Executive shall be employed for a term commencing on the day
preceding any Change in Control of the Company, and ending upon such termination
date as is set forth in a written notice given by either party terminating this
Agreement, provided that such termination date will occur not less than three
years subsequent to the date upon which such notice is given.  For example, this
Agreement could be terminated on the third anniversary of a Change in Control by
written notice given by either party on the date of the Change in Control.

     (b) During the term of employment, this Agreement for the Executive's
employment shall not be subject to termination at the instance of the Company
for any reason except for Cause pursuant to Section 1.2(c).  Except in the event
of termination of Executive's employment for Cause pursuant to Section 1.2(c),
the Executive's compensation and other benefits as provided herein shall
continue unabated during the full term of employment under this Agreement and
otherwise as provided herein; provided, however, that the Board of Directors of
the Company shall have the right,

                                       3
<PAGE>

acting in accordance with the Bylaws of the Company, to remove the Executive
from office as Executive Vice President-Finance and Chief Financial Officer of
the Company in the event the Executive acts in any way which has a direct,
substantial and adverse effect upon the reputation or condition of the Company,
in which event the Executive may terminate his employment pursuant to Section
1.5. The Board of Directors of the Company may also terminate Executive's
employment for Cause pursuant to Section 1.2(c).

     (c) Notwithstanding any other provision of this Agreement, the Board of
Directors of the Company may terminate Executive's employment for Cause pursuant
to this Section 1.2(c). The Company shall have the right to terminate
Executive's employment with the Company for Cause (A) immediately upon written
notice from the Company, if the Board of Directors shall reasonably determine
that the conduct on cause specified in such notice is not curable, or (B) upon
thirty days' written notice from the Company, if the Board of Directors shall
determine that the conduct or cause specified in such notice is curable, unless
the Executive has commenced to cure the conduct or cause specified in such
notice within 10 days following the date of such notice and has completed the
cure within 30 days following the date of such notice.

     For the purposes of this Agreement, "Cause" means:  (i) willful malfeasance
or gross negligence by Executive in the performance of his duties under this
Agreement, (ii) any act of fraud, insubordination or other conduct by Executive
which demonstrates gross unfitness for service, or (iii) Executive's conviction
(or entry of a plea of guilty, nolo contendere or the equivalent) for any crime
involving moral turpitude, dishonesty or breach of trust or any felony which is
punishable by imprisonment in the jurisdiction involved.

     If the Executive's employment is terminated for Cause, the Company will pay
Executive his annual base salary and continue to provide the benefits described
in Section 3.2 through the date of termination and thereafter will have no
further obligations under this Agreement.

                                       4
<PAGE>

Section 1.3:   Termination of Agreement Upon Disability of Executive
               -----------------------------------------------------

     If at the end of any calendar month the Executive is and has, for six (6)
full months, continuously been unable, due to mental or physical illness or
injury, to perform his duties under this Agreement in his normal and regular
manner, this Agreement shall be terminated subject to the provisions of Section
3.1.

Section 1.4:   Termination of Agreement Upon Death of Executive
               ------------------------------------------------

     If the Executive dies, this Agreement shall be terminated on the last day
of the calendar month of his death.

Section 1.5:   Termination of Employment by the Executive
               ------------------------------------------

     If, without the Executive's express written consent, the Company (i)
significantly reduces the importance of the functions, duties, responsibilities
or authority of the Executive, (ii) reduces the Executive's compensation or
benefits, or (iii) relocates the principal executive office of the Company to a
location outside of Los Angeles County or reassigns the Executive to a location
other than the principal executive office of the Company (except for required
travel on the Company's business to an extent substantially consistent with the
Executive's travel obligations existing on the date of this Agreement), then the
Executive may inform the Company that such action by the Company constitutes
constructive notification under Section 1.2 that this Agreement will be
terminated three years thereafter.  After any such constructive notice or actual
notice to the Executive of the termination of this Agreement pursuant to Section
1.2 other than for Cause, the Executive may at any time terminate his employment
with the Company.  In the event of such a termination of employment by the
Executive, the Company shall within 72 hours after such termination make a
payment to the Executive of the lump-sum value (without any present-value
discount or adjustment for inflation) of the salary and benefits that would have
been provided to the Executive for the period from the date of such termination
until the date which is three years from the date of such constructive or actual
notice, and the Executive shall be free to seek and obtain other employment

                                       5
<PAGE>

or arrangements for the rendition of personal services with or to others,
notwithstanding the provisions of Section 1.1, and without a reduction of his
compensation and benefits hereunder.

                                   ARTICLE II

                                  COMPENSATION

Section 2.1:  Salary
              ------

     The Company shall pay a salary to the Executive for the term hereof at the
minimum rate which was in effect on the day preceding the Change in Control of
the Company.  During the term hereof, the Executive's salary may be increased by
the Company's Board of Directors, in which event this Agreement shall be deemed
amended to reflect such increases.

Section 2.2:  Travel and Entertainment Expense
              --------------------------------

     The Company shall reimburse the Executive for reasonable travel and
entertainment expenses incurred in behalf of the Company in the performance of
his duties hereunder.

                                  ARTICLE III

                                 OTHER BENEFITS

Section 3.1:  Payments on Account of Disability of Executive
              ----------------------------------------------

     If this Agreement is terminated under Section 1.3 after a Change in Control
of the Company, the Company shall continue to pay to the Executive on account of
his disability the biweekly or semimonthly salary installments under Section 2.1
at the salary rate in effect on the date of said termination (subject to
adjustment pursuant to Section 3.3) and to provide him with benefits as
described in Section 3.2, until three years after the Executive is given notice
of such termination; provided, however, that in the event of the Executive's
death such monthly installments as shall be

                                       6
<PAGE>

payable hereunder shall terminate. The payments made hereunder will be reduced
by any disability benefits paid to Executive under disability plans or insurance
provided by the Company.

Section 3.2:  Participation in Other Company Benefits
              ---------------------------------------

     In addition to the benefits provided in this Agreement the Executive shall
throughout the term hereof (prior to death) be entitled to and shall receive all
other benefits generally available to other executives of the Company, including
(without limitation) benefits under the Company's medical, health, disability,
death benefit, profit sharing and bonus and other incentive compensation plans
(other than stock option plans).  The Executive shall be entitled to receive
benefits which are at least as great in scope and amount as those which he
received immediately prior to the Change in Control. The Executive shall also be
entitled to benefits under the Company's Supplemental Executive Retirement Plan
("SERP"), whether or not the SERP is available to other executives.  If the
Executive's employment is terminated after a Change in Control pursuant to
Section 1.2(b), Section 1.3 or Section 1.5: (i) the amount the Executive
otherwise would have received as bonus payments for the period after the
termination shall be determined assuming that he would be entitled to an annual
bonus or bonuses at an annual rate at least equal to the average of his annual
bonuses for the preceding two years, and that he would be entitled to receive a
pro-rata portion of such amount for any period shorter than a full calendar
year; (ii) the amount of the profit sharing plan contributions otherwise made on
behalf of the Executive shall be determined assuming that the level of Company
contributions to the plan would equal the average of such levels for the
preceding two years; (iii) the payment for the value of welfare benefits shall
be based on the current cost of such coverage to the Company and shall take into
account the Executive's entitlement to participate in the Company's welfare
benefit plans for retirees (if he would have qualified for such participation
assuming he had an additional three years of service); (iv) the amount to which
the Executive would otherwise be entitled under the SERP shall be determined (A)
based on his age at the date he would otherwise have terminated employment, (B)
taking into account the years of vesting service and the salary and bonuses he
would have earned through such date based on his salary rate in effect at his
actual termination date and assuming that he would be entitled to bonuses as
provided in (i) above, increased using the 6.5% compensation scale assumption
adopted by the SERP for funding the SERP trusts, (C) deeming the amount of the
Executive's Employer Contribution Benefit (as defined in the

                                       7
<PAGE>

SERP) to include the amount in (ii) above, (D) assuming payments of SERP
benefits would not have commenced prior to the date the Executive would
otherwise have terminated employment, and (E) permitting the Executive to elect
to receive (without a penalty) a lump sum payment upon his termination of
employment which is the Actuarial Equivalent (as defined in the SERP) of the
lump sum payment which would have been payable to Executive (if he made a timely
election to receive a lump sum payment) on the date he would have otherwise
terminated employment; and (v) the Executive's vested interest under all Company
plans shall be determined assuming he had an additional three years of service.

Section 3.3:  Cost of Living Adjustment
              -------------------------

     The biweekly or semimonthly payments to be paid to or on account of the
Executive in any year under Section 3.1 shall be increased on January 1 of each
year after Executive terminates employment (the "adjustment date") in the same
proportion as the proportional difference between the "Consumer Price Index for
Urban Wage Earners, Clerical Workers, all items (Los Angeles -Long Beach -
Anaheim areas)," published by the United States Department of Labor, Bureau of
Labor Statistics (the "CPI"), in effect on the adjustment date and the CPI in
effect on January of the year in which Executive terminates employment.  Should
the Bureau of Labor Statistics discontinue publication of the CPI, or publish
the same less frequently, or alter the same in any manner, then the Company may
adopt a substitute index or substitute procedure which reasonably reflects and
monitors consumer prices.  A decline in the CPI shall not serve as a basis for a
reduction in the biweekly or semimonthly payments to be paid to or on account of
the Executive.

Section 3.4:  Stock Options
              -------------

     The Company shall amend all outstanding Stock Option Agreements with the
Executive to provide, and shall provide in future options under the Company's
Stock Option Plans or future stock option plans, that the Executive may
accelerate the exercisability of all options to acquire shares covered by such
Stock Option Agreements and by future agreements (i) on the termination of the
Executive's employment by the Company after a Change in Control of the Company
for any reason other than Cause (as such terms are defined in this Agreement) or
(ii) the occurrence of a "Change

                                       8
<PAGE>

in Control of the Company." For the purposes of this Agreement, a "Change in
Control of the Company" or "Change in Control" shall be deemed to have occurred
if (i) any "person" (as such term is used in Sections 13(d) and 14(d) of the
Securities Exchange Act of 1934, as amended (the "Exchange Act")), other than a
trustee or other fiduciary holding securities under an employee benefit of the
Company, is or becomes the "beneficial owner" (as defined in Rule l3d-3 under
the Exchange Act), directly or indirectly, of securities of the Company
representing 50% or more of the combined voting power of the Company's then
outstanding securities regardless of whether or not the Board of Directors shall
have approved such Change in Control, (ii) the shareholders of the Company
approve (a) a merger or consolidation of the Company with any other corporation
regardless of which entity is the surviving company, other than a merger or
consolidation which would result in the voting securities of the Company
outstanding immediately prior thereto continuing to represent (either by
remaining outstanding or by being converted into voting securities of the
surviving entity) more than 50% of the combined voting power of the voting
securities of the Company or such surviving entity outstanding immediately after
such merger or consolidation or (b) a plan of complete liquidation of the
Company or agreement for the sale or disposition by the Company of all or
substantially all of the Company's assets, or (iii) during any period of two
consecutive years, individuals who at the beginning of any such period
constitute the Directors of the Company cease for any reason to constitute at
least a majority thereof unless the election, or the nomination for election by
the Company's stockholders, of each new Director of the Company was approved by
a vote of at least a majority of such Directors of the Company then still in
office who were Directors of the Company at the beginning of any such period.

                                   ARTICLE IV

                               GENERAL PROVISIONS

Section 4.1:   Assignability of Agreement
               --------------------------

     The rights and duties of the parties hereunder shall not be assignable by
either party, except that this Agreement and all rights and obligations
hereunder shall be assigned by the Company and its express assumption in writing
required by any corporation or other business entity which succeeds

                                       9
<PAGE>

to all or substantially all of the business of the Company through merger,
consolidation, corporate reorganization, or by acquisition of all or
substantially all of the assets of the Company.

Section 4.2:   Integration
               -----------

     Except for the Severance Agreement and as provided in Sections 2.1 and 3.2
hereof, this Agreement contains the entire agreement between the parties and
supersedes all prior agreements, understandings, commitments and practices,
whether written or oral.  No amendments to this Agreement may be made except by
a writing signed by both parties.  This Agreement shall be construed according
to the laws of the State of California.

Section 4.3:   Notices
               -------

     Any notice to the Company required or permitted hereunder shall be given in
writing to the Secretary of the Company either by personal service or by
registered mail, postage prepaid, addressed to the Company at its principal
place of business.  Any such notice to the Executive shall be given in a like
manner and if mailed shall be addressed to the Executive at his home address
then shown in the files of the Company.  For the purpose of determining
compliance with any time element herein, a notice shall be deemed given on the
postmarked date.

Section 4.4:   Indemnification Provisions
               --------------------------

     The indemnification provisions set forth in Article VII of the Bylaws of
the Company in effect on the date hereof are hereby incorporated by reference as
a material term of this Agreement, constitute material consideration for the
Executive's agreement hereunder and will, as in effect on the date hereof,
continue to be a material term of this Agreement without regard to their future
amendment or repeal unless the Executive expressly agrees in writing to accept
such amendment or repeal as an amendment of this Agreement.

                                       10
<PAGE>

Section 4.5:   Additional Payments to the Executive
               ------------------------------------

     In the event that any amounts or benefits which are paid to the Executive
by the Company or any other person or entity pursuant to this Agreement or any
deferred compensation plan or agreement, stock option plan or other plan,
agreement or arrangement are subject to the excise tax imposed by Section 4999
of the Internal Revenue Code of 1986, as amended (the "Code") or any similar
state tax provision, the Company shall pay the Executive an additional amount
(the "Gross-Up Payment") such that the net amount of the payments and benefits
retained by the Executive, after deduction of any excise tax thereon and any
interest payable with respect to such excise tax, and any federal and state
income tax and any excise tax upon the Gross-Up Payment, shall be equal to the
payments and benefits which the Executive would have received absent the
application of such excise tax.  The Gross-Up Payment shall be paid to the
Executive upon the earlier of (i) the time at which the Company withholds such
excise tax from any payments to the Executive or (ii) 30 days after the
Executive notifies the Company that the Executive has filed a tax return which
takes the position that such excise tax is due and payable in reliance on a
written opinion of the Executive's tax counsel that it is more likely than not
that such excise tax is due and payable.  If the Executive makes any payment
with respect to any such excise tax as a result of an adjustment to the
Executive's tax liability by any federal, state or local authority, the Company
will pay such Gross-Up Payment within 30 days after the Executive notifies the
Company of such payment.  Without limiting the obligation of the Company
hereunder, the Executive agrees, in the event the Executive makes any payment
pursuant to the preceding sentence, to negotiate with the Company in good faith
with respect to procedures reasonably requested by the Company which would
afford the Company the ability to contest the imposition of such excise tax;
provided, however, that the Executive will not be required to afford the Company
any right to contest the applicability of any such excise tax to the extent that
the Executive reasonably determines that such contest is inconsistent with the
overall tax interests of the Executive.  The Company agrees to hold in
confidence and not to disclose, without the Executive's prior written consent,
any information with regard to the Executive's tax position which the Company
obtains pursuant to this Section 4.5.

                                       11
<PAGE>

Section 4.6:   Right to Arbitrate
               ------------------

     The Executive shall have the right, in addition to all other rights and
remedies in law or in equity, at his election, to seek arbitration in Los
Angeles County, California, under the Employment Dispute Resolution Rules of the
American Arbitration Association, in the event of any dispute concerning this
Agreement.

Section 4.7:   Attorneys' and Accountants' Fees
               --------------------------------

     The Company shall pay to the Executive all legal and accounting expenses
and fees incurred by Executive in seeking to obtain or enforce any right or
benefit provided by this Agreement after a Change in Control of the Company or
in connection with any tax audit or proceeding to the extent attributable to the
application of Section 4999 of the Code to any payment or benefit under this
Agreement, without regard to whether the Executive prevails in obtaining or
enforcing such right or benefit.  The Company shall pay to the Executive any
expenses and fees incurred in any such proceeding promptly after receipt by the
Company of a written notice by the Executive that he has incurred such fees or
expenses.

     EXECUTED by the parties this 30th day of December, 1996.


H. JAMES GALLAGHER            CALMAT CO.

"Executive"                         "Company"



/s/ H. JAMES GALLAGHER        By /s/ A. FREDERICK GERSTELL
----------------------           ----------------------------
                                   Chairman of the Board and
                                   Chief Executive Officer



                              By /s/ PAUL STANFORD
                                 -----------------
                                     Secretary


                                       12