Museum of Art - Balboa Park
San Diego, SD County, California
Replication and Replacement of an Existing Historic Decorative Cast Stone Veneer using Moldmaking, Casting, and Restoration Techniques.
The Museum of Art and the California Tower are major architectural elements of the historic core of Balboa Park in San Diego, site of the 1915 California Exposition. Earthquakes and constant exposure to a salt-laden marine environment had combined to cause significant deterioration to this Churriguersque entry surround, and to require replacement of much of the original historic fabric with new matching cast stone.
Wikipedia link: http://en.wikipedia.org/wiki/San_Diego_Museum_of_Art
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Mild steel reinforcement placed within the cast stone matrix for strength during the original manufacturing process ultiimately resulted in the stress and failure of much of the facade material, posing a visually unsatisfying appearance as well as a life-safety hazard. |
Work also took place at the California Tower nearby, but its problems were much less advanced in terms of decay. |
As with many other cast stone installations from this period, the builders did not take into account the tremendous expansive force of the decaying internal iron reinforcing rods. |
This process is also described in some of our other projects listed elsewhere on this site, but the results were equally destructive, and the damaged material posed a risk severe enough that the building entry remained swathed in protective scaffolding for a number of years prior to the commencement of restoration work. |
An additional hazard was posed by the old hollow clay tile backup wall that supported the decaying original cast stone. In any seismic event this structure would pose a serious liklihood of collapse. |
Careful dis-assembly of the original facade was executed using diamond-tipped chain saws and circular saws to help ensure that enough original material could be preserved for the use of the new cast stone manufacturer's mould makers. |
Each piece that was relatively intact was carefully relieved from its setting bed mortar prior to be hoisted off the facade of the building. |
Heavy lifting systems made it possible to selectively remove the masonry units without damaging adjacent stones. |
The removal process was facilitated through the use of overhead beams and dollies to allow safe movement of the stones, some of which weighed up to 1,500 pounds. |
Work of this kind is a curious blend of brawn, finesse, and techniqe - damaging the pieces in the removal process was sometimes unavoidable because of the inherent degradation, but every effort to avoid this damage was made at all steps in the selective removal process. |
Fortunately, the iron-jacked pieces were limited to a small portion of the overall number of pieces, allowing us to control the process and avoid safety hazards. |
The preservation of our worker's health during the removal process went hand-in-hand with the preservation of the original material slated for reproduction. |
Unfortunately, several of the stones from the original 1915 construction were in an advanced state of decay from the the internal iron-jacking and were removed in pieces. |
Some stones were unfortunately so far degraded that original surface material (needed for the reproduction process) was missing and had to be later recreated by artisans during the mould making process. |
The worst cases of decay were carefullly removed and documented piece by piece. |
Several stones were ready to discorporate but were successfully removed in one piece after careful handling. Again, preservation of all possible original surface material was a paramount ingredient in the successful completion of the replication process. |
On the whole, the mix design and quality of the original facade material was good, and the intrinsic cohesiveness of the cast stone units allowed us to remove pieces as large as this 3/4 ton unit with no damage. |
Careful documentation and lableling of each removed cast stone unit was cross-referenced with a previously prepared elevation drawing, using a pre-determined designation code, to make sure that all units were documented and accounted for. |
Each unit was transported to the new cast stone manufacturer's facility for the mould making process. After silicon rubber impressions were formed, stainless steel reinforcing bar was used to provide the internal strength. New Portland cement mortar was then poured into each mould to produce the new replacement cast stone which matched the original exactly in shape, profile, detail and overall appearance. |
For the new material installation, we chose to use a small portable crane for the ease and facility of the installation crew. |
A sophisticated structural design scheme had us installing a steel I-Beam primary support bolted to the new replacement shotcrete wall. The new cast stone units would then be attached to this primary support and minimize the need for penetration into the new shotcrete backup wall, with its predominant internal steel reinforcing skeleton - drilling through rebar would have been counter-productive. |
A standard rule used in masonry preservation is that no ferrous anchoring material should be in direct contact with the masonry. Following this precept, all anchors and support brackets in direct contact with the new cast stone were fabricated from 1/2" stainless steel angle and plate. |
Our primary and secondary anchor scenario allowed us to hide these rust-proof components in a way that ensured safe and durable supports and anchors without endangering the adjacent original material not replaced. |
In terms of sheer complexity, this project was as demanding a preservation challenge as any we have encountered. |
Reinstallation of the new cast stone units utilized a conceptually simple procedure, placing setting mortar as a bedding material underneath the newly placed stone, and then attaching the stainless steel anchors to the previously installed primary anchor to complete the process. Over 350 pieces of new cast stone were placed in this manner. |
Notice that the material is solid cast stone rather than the more common GFRC (Graphite Fiber Reinforced Concrete) so prevalalent these days, with walls usually no thicker than one inch. This was mandated so that the new material would match the original in all respects. |
While the mass of these new cast stone units was much greater than that of a lighter GFRC alternative, it also allowed the use of an extremely stable and durable anchoring system, ensuring that this new replacement facade would last much longer that any thin-shell alternative. |
For the first time in years, the main entry to the Museum of Art is unencumbered by protective scaffolding and allowed to express the decorative language and vision of a previous age. |