Concrete sculpture

Isabelle Leirens <isabelle.leirens [at] kikirpa__be> writes

>I would like to thank Christopher Weeks and N.J. Bud Goldstone for
>their interesting information. I would like to ask Mr. Goldstone
>some more questions about the information he sent:
>
>    what recipe of mixture of  cement and sand did you develop and
>    use?
>
>    which system did you use to localise the place of the
>    reinforcements in the sculpture (metal detector,...)?

We used dozens of x-rays in unbroken areas or simply opened up
cracks (with diamond cutting blades on a rotary saw) to examine the
condition of reinforcements. X-ray costs are reasonable if the depth
of the part to be looked into is 8 inches or less and opening the
sculpture is difficult. Most of the Watts Towers members are 5
inches or less wide. The x-ray should show voids in the material,
poor or missing bonds between the steel and sculpture, and steel
damage. Larger (deeper) parts may require dangerous levels of
radiation. Ask the x-ray company. Opening the cracks and probing may
be your answer, particularly if the part is already broken open so
one may see inside with the naked eye!

The following is verbatim from our Watts Towers Handbook:

    SAND IN CEMENT/MORTAR MIXTURE
        A series of compression tests run in 1990 and 1991 at
        Smith-Emery Company, showed 2700 psi using sand sizes as
        specified, below.

        The sand grain sizes specified in the State of California
        DPR document (Ehrenkrantz Report) for repair mortar are as
        follows:

        AGGREGATE: Clean sharp natural sand conforming to the
        following size gradations and the requirements of ASTM C144

            Sieve Size                    percent passing
            No. 4                            100
            No. 8                         95 to 100
            No. 16                        70 to 100
            No. 30                        40 to 75
            No. 50                        20 to 35
            No. 100                        2 to 15
            No. 200                        0 to  2

        WATER: Potable, free of oils, acids, alkalies and organic
        matter.

    PIGMENT ADDED TO CEMENT MIXTURE

        Tests in 1990 at Smith-Emery showed a 22% increase in
        compression strength of the sand-Portland cement mixture by
        addition of a small amount of color pigment (3288 psi vs
        2700 psi).

    JAHN MORTAR COMPRESSION STRENGTH REDUCTION

        Tests in 1991 on Jahn M70 and M90 restoration mortar samples
        prepared by the Cultural Affairs' staff showed compression
        strengths of approximately 2280 psi, 35% less than published
        in the Jahn literature. This value is also 18% less than
        that for the cement mortar mixture.

        Method B: Non critical members determined to require
        replacement due to reinforcement loss or major cracks,
        spalls or failed portions   (Ornamental arcs and bands,
        finials).

        See Method A., steps 1. through 5., above.

    6/91 Crack filling with Jahn mortar or cement.

        Structural cracks, other than hairline cracks, may be filled
        with the approved Portland cement mixture (see 4.1.5) or
        Jahn mortar.  Hairline cracks may be filled (injected) with
        Jahn mortar M30 or may be opened wide enough to fill with
        Portland cement or Jahn mortar.

        Jahn mortar M70 application instructions: Mixing. It is
        unnecessary and unadvisable to add aggregate to the Jahn
        mortar. Smoothness can be controlled during the stage at
        which the surface is worked and by the type of tool used for
        the purpose.

    TESTS HAVE SHOWN A 50 PERCENT LOSS OF STRENGTH BY ADDING SAND.

    6/9 Application. [PHASE I]. Proper application requires pre
    wetting all surfaces and maintaining a moist environment at the
    repair for 5 days minimum. After the mixing is completed, trowel
    the mortar onto the dampened area of repair. The mortar should
    be applied in a bigger shape than necessary so it can later be
    modelled down after the skin forms. Certain shapes can be
    blocked out at this stage. For example, areas where there were
    formerly shells in the surface can be imprinted with a shell to
    give the general appearance of the original. It is especially
    important to pay attention to this when the repair follows a
    regular pattern like on the main spire base of the Ship of Marco
    Polo). One wants to avoid unwittingly producing a new pattern
    that would conflict with the one produced by Rodia.

    Deeply cut lines in the mortar or deep designs (like a tool
    pattern) can be blocked out at this stage also. Immediately
    after application, the surface of the repair mortar should be
    sprayed with water until the water begins to drip off the fill.
    The wet mortar should then be wrapped with damp cloths and
    plastic wrap. Any plastic that will keep the damp cloth from
    drying out overnight will suffice (saran, polyethylene, etc.).
    It would be best to apply this on a day when there is another
    work day the next day; for example, on the first day of a two
    day work week.

        Conservation Handbook for Rodia Towers in Watts
        Page 17

    Application. [PHASE II]. The day after the mortar is applied, a
    second integral phase is required in order for the mortar to
    effectively cure. While essential, these steps can be carried
    out while other aspects of the work are being done. After the
    plastic is unwrapped and the damp cloths are removed, the mortar
    should be cut with a spatula or trowel to its proper shape. Care
    should be taken to follow the shape of the element and cut the
    shape accordingly so it is not too high at the edges. It is
    important to take note of the shape of individual planar
    surfaces: sometimes a seemingly round shape is actually squared
    off in places. Cutting the shape at this stage also removes the
    skin that will form on the surface of the mortar as part of the
    curing process. It is essential to remove the skin in order to
    get the proper surface appearance. However, this step is
    critical because it directly impacts the strength of the
    material due to its ability to absorb water. Once the proper
    shape is achieved, the mortar should be repeatedly watered with
    a spray bottle until it no longer accepts water. The strength of
    the mortar is directly dependent on the amount of water absorbed
    at this stage. As a general rule, the repair should be sprayed
    three times an hour on this second day, but no less than 15
    times. If one sprays the surface too soon, the mortar will sag
    and run off the surface. However, it is generally impossible to
    overwater the surface if the watering is done the day after
    application. If the mortar is applied and left for more than
    three or four days, this process be carried out must still once
    it is unwrapped. Otherwise the mortar will be very soft and
    weak. The watering should begin once the desired shape and
    design are achieved. To create a complicated design, block out
    the grid  on the first day and then carve out the more
    complicated elements on the second day.

    Pigmenting and sealing. The Jahn mortars can be pigmented from
    the surface. This makes it easier to achieve a proper color with
    a minimum amount of mixing and alteration of the original
    properties of the mortars. The color is applied as a mixture of
    dry pigments in water. The colors will vary in the pigmented
    mortars, but the greyish mortar is achieved with black, white,
    raw

    PIGMENT ADDED TO CEMENT MIXTURE

        Tests in 1990 at Smith-Emery showed a 22% increase in
        compression strength of the sand-Portland cement mixture by
        addition of a small amount of color pigment (3288 psi vs
        2700 psi).

    JAHN MORTAR COMPRESSION STRENGTH REDUCTION

        Tests in 1991 on Jahn M70 and M90 restoration mortar samples
        prepared by the Cultural Affairs' staff showed compression
        strengths of approximately 2280 psi, 35% less than published
        in the Jahn literature. This value is also 18% less than
        that for the cement mortar mixture.

    MORTAR: repair and replacement mortar consists of

        Cement: 1 part by volume
        Lime  : 1/10 to 1/4 part by volume
        Sand  : 3 parts by volume, dry
        Water : minimum to form a workable mix.
        Acryl 60 must be diluted prior to adding to dry mix.

    4/90 Note: 1 1/4 part cement used for repair of "heart" on post
    02 of South Wall 3/90

    Mixing procedures: measure materials by volume or equivalent
    weight. Use known measure (not a shovel). Record measures in
    Project log. Mix mortar using a clean mechanical batcher. Mix
    dry ingredients for one minute prior to the addition of water.
    Thoroughly mix for three minutes after the addition of water.
    Place mortar within two hours of mixing. Do not retemper or use
    partially hardened material.

    LIME: ASTM C207, hydrated mason's lime, type S PORTLAND CEMENT:
    ASTM C150, Type I

    AGGREGATE: Clean sharp natural sand conforming to the following
    size gradations and the requirements of ASTM C144

        Sieve Size                    percent passing
        No. 4                            100
        No. 8                         95 to 100
        No. 16                        70 to 100
        No. 30                        40 to 75
        No. 50                        20 to 35
        No. 100                        2 to 15
        No. 200                        0 to  2

    WATER: Potable, free of oils, acids, alkalies and organic
    matter.

    4/90  MORTAR: M70  18A grey and M70  11 red  w/pigments added
    4/92  add M70 (yellow to be made by Jahn)

        Jahn Engineering
        17106 Kloosterweg
        34 3232 Le Brielle Postbus 161 3230
        Ad Brielle, Holland, The Netherlands
        Cathedral Stone Company, Inc.
        2505 Reed Street
        Washington, D.C. 20018
        202-832-1135    Mr. Dennis Rude

N. J. Bud Goldstone, conservation engineer
Co-author "The Los Angeles Watts Towers"
Project Engineer Las Pozas and Watts Towers

                                  ***
                  Conservation DistList Instance 12:60
                 Distributed: Friday, January 22, 1999
                       Message Id: cdl-12-60-004
                                  ***