“Central Park Masonry Arch Restoration Program”

American Institute for Conservation, Objects Group, Norfolk, VA, June 1995.

Author: Mark Rabinowitz

ABSTRACT

In the 1980’s, the Central Park Conservancy began the project of restoring the 33 masonry arches that are an important functional and decorative component of Central Park. These structures were designed by Calvert Vaux with the assistance of Jacob Wrey Mould to carry the carriage road over cross drives, pedestrian paths, bridal trials and water bodies.  The separation of traffic into independent systems for different uses is one of the recognized innovations of the Parks landmark design.  12 of the masonry arches have been substantially rebuilt under this program.  Along with improvements in the adjacent landscapes, the installation of a waterproofing system and the replacement or re-creation of historic fabric have been the primary goals of each restoration.  The fact that we have been able to return to what is essentially the same project scope for each arch over a dozen years has provided us with a unique opportunity to learn from our mistakes and refine our restoration designs and methods.  In addition the training and development of the Historic Preservation Crews, our in-house work force, has been a major secondary benefit of this process.  This paper will describe our restoration designs and improvements and the training program.

The masonry arches, together with the cast-iron and rustic wood bridges and the Park’s decorative and functional architecture, constitute a cohesive collection of Victorian design of un-paralleled beauty.  Most arches were built during the first construction campaign, between 1859 and 1863.  Additional structures were added, (and removed) as the city grew and uses of the park changed.  Vaux integrated the vocabulary of Gothic Revival and Rustic styles into the overall landscape conceptions, fulfilling not only the functional needs of the separation of traffic that is one of the innovations of the design of Central Park but providing an architectural counterpoint to the character of the landscape designs.  The arches frame views and their masses and traceries establish human scale and presence in the dells and vales of the park.  Vaux established a hierarchy of structural types that related both to the arches’ functions; transverse road, carriage drive, bridal trails or pedestrian paths, and to the landscape types, formal, pastoral or picturesque.

The masonry arches are mostly designed as low, compound-curved brick vaults with exterior decorative stone facades.  Other designs in use are true arch and iron beam and brick barrel vaulting. .  Buttressed ribbing supports the lateral thrust of the vaults at the facades and evenly spaced along the vault.  The Philadelphia brick vaults was laid in triple depths and the vault interiors are relieved with coffering, decorative bonds, stone string courses or colored brick inserts.  The interior spaces were intended as respites from rain or heat and were provided with benches and occasionally drinking or decorative fountains.  These spaces are not now often seen as inviting and restoration designs include interior lighting and clear sight lines along path approaches to increase the sense of security for park users.

The facades are provided with a lively set of variations on the themes of decorative voissiors, intrados, balustrades, reventments, and wing walls.   Facades are  carved and figured, often of mixed stone, with New Brunswick Sandstone being the most commonly used material.  Other stones include locally quarried Manhattan Schist, Westchester Gneiss and Inwood Marble.  Balustrades are of stone, cast iron or a combination of both.

Despite various repair and restoration campaigns the collection of structures had suffered serious deterioration by the end of the 1970’s along with the entire Park infastructure.  The masonry arches had deteriorated in several ways.  The brick faces within the vaults spalled due to migrating road salt re-crystallization, through impact and from voids open up behind the facing courses or between the facades and the vaulting from loss of mortar adhesion exacerbated by freeze/thaw action.  In some instances, the low arches developed hinges at the center and spring lines.  In rare instances this hinging dislodged entire bricks.  Damage was common in areas adjacent to roadways where balustrades and copings were frequently hit by vehicles.  Generally, however, the collection remained structurally sound while suffering more from surface decays and lack of proper maintenance than anything else.

Original designs combined far sighted long range planning on the one hand with an odd lack of concern about times effects on the other.  The excellent structural detailing meant that the arches have survived as well as they have.  All foundations remain true and level with minimal hinging of the vaults.  This is not surprising from an architect who introduced his only publication, “Villas and Cottages” with a design for a self-flushing outhouse.  Vaux always dealt with the fundamentals first.  The soft New Brunswick sandstone that he favored, however, readily erodes and much of the detailed carvings have been erased.  This characteristic was known at the time of its use but Vaux selected it despite that because he preferred its color and its ready supply.  The steep embankments they also favored readily eroded, exposing foundation courses.  Steep grading also called for steep angles on some wing wall copings that could shifted and fell when the natural cement mortars failed.  The delicate sandstone carvings broke and were replaced with crude cementitious repairs if at all.  Similar problems effected the cast iron railing systems which were replaced with steel railings or cyclone fencing.

Restoration planning

Fortunately, the NYC municipal archives hold a good collection of original drawings for most of these structures.  Combined with numerous historic photographs from the Park’s well documented early phases and remaining physical evidence a reasonable understanding of original design intentions could be deduced for each arch.  The restoration goals were established to be:

• Waterproofing the upper surface of the brick vaults to prevent further migration of water and salts.
• Creation of a second waterproofed concrete umbrella pitched to carry surface water away from the vaults
• Installation of an integral drainage system to direct and collect runoff before it can attack either the arches or the adjacent landscapes.
• Recreate historic paving, curbing and gutter details.
• Repair or replace spalled or damaged brick faces.
• Reset existing elements and integrate replacements.
• Recreate damaged or missing facade and balustrade elements.
• Clean all masonry surfaces.

The original design was developed by John Belle of Beyer, Blinder, Belle, architects in 1980. It was applied to Greywacke and Trefoil arches. Changes and improvements in method evolved as the Conservancy treated Driprock, Greengap, Playmates, Glade, Glenspan, Huddlestone, Winterdale, Dalehead, Greyshot and Dipway arches.

WATERPROOFING

The waterproofing was begun by excavating the roadway to expose the back of the brick vault to the depth of the tops of the buttresses.  The masonry was scraped clean and parged with latex modified cement.  The excavation revealed remnants of the original waterproofing; a rough shingling of slate fragments in mortar and fragments of  a bituminous layer. ( The excavation also exposed interesting structural variations, for example, Dalehead is built in a in-filled swampy part of the Park.  The low-lying terrain continued to plague the site with flooding.  The existing drawings show a counter arch lying below grade between the footings.  Its existence was not clearly explained until we discovered a framing of iron bars and rods imbedded in the masonry, compressing the lateral forces of the double arches.  In essence, the arch is a donut stood on end and floating in the mud.)

The voids between the buttresses were infilled with concrete castings so as to provide a roof that extends from brick edge to brick edge of the interior vault.  This “roof” was coated with a bituminous membrane backed with a permeable protection board.  The board is provided with channeling that allows water to flow across the top of the membrane and drain into gravel pits at either end of the arch.  Later improvements in the design included the drilling of horizontal weep holes through the base of the arch to direct this water back out to the gutters and drains within the vault.  Another improvement that proved very effective was the addition to the vault waterproofing of a through wall flashing system beneath the coping course.  Earlier projects had only waterproofed to the sidewalk or coping course which had allowed for continued failures at the joint with the facade.  This additional waterproofing carried the  membrane through to the bridge edges and provided an effective method of protecting the facades from descending water.

SECOND WATERPROOFED CONCRETE UMBRELLA

Gravel is added to bring up the grade and a reinforced concrete umbrella was cast atop the fill.  This too is waterproofed with the same membrane system.  This “belt and suspenders” duplication of the waterproofing system, although sound in theory, may be eliminated in future designs.  A recent regularly scheduled grinding and  re-asphalting  of the roadway cut through this membrane before we got information to the contractors to notify them about the need to restrict their grinding to the upper paving zone.  It seems unlikely that we will always be able to communicate this need to future contractors.

BRICK REPAIR

Spalled and decayed bricks were repaired with cementeous patching compounds or replaced with custom specified new units.  The first campaign attempted unit replacements only which resulted in unacceptably large joints due to the difficulty of installation from the vault interior.  Olmsted had reported with a great deal of pride on a construction innovation they had used in which small iron facings were placed between bricks as they were laid up to prevent mortar from accumulating on the brick faces while they were resting on the wooden form work during construction.  To better maintain these tight joint lines patching compounds were selected instead of unit replacements for later projects.  Bricks were cut back to sound substrates and built up individually.  Joint lines were maintained to full depths for later pointing.  Overhead repairs were reinforced with nylon rods set in “Akemi” polyester adhesive.  We employed “Jahn” custom formulated patching mix for several arches until the combination of high cost, a tendency to lighten in our working conditions and unreliable delivery forced us to experiment with developing our own mixture.  Integrated Conservation Resources performed technical analysis on the bricks of the most recent projects and developed formulated “N” type mortars served well to overcome the previous problems.  It remains to be seen whether these mixes will perform as well over time but, as the vaults are enclosed and now waterproofed the bricks are not subject to the same stresses as exterior facings would be.

The technical analysis of the bricks had the further advantage of identifying the extremely high levels of salt deposits that remained within the bricks.  These had continued to leach out after earlier restorations even though no new salts were entering from the roadway.  Ground humidity had continued to re-dissolve and re-deposit these existing salts on the new surfaces. During the last two restorations, we repeated a treatment of saturating the vaults with hot water pressure-washing followed by cold water rinses to dissolve and pull the salts to the surface.  We found this quicker and more effective than poulticing.  The salt crystals that formed after a couple of days of drying were scraped off the surfaces or cut off along with the spalled brick faces during patching preparation.

FACADE RESTORATION

In most cases, the major restoration requirements were in repair or replacement of balustrade elements.  Brittle cast iron broke from impact or heaving,  surfaces were encrusted with numerous paint layers or rusted from lack of maintenance.  New Brunswick sandstone weathers poorly and suffered from surface losses through erosion, spalling or de-lamination and damage from impact.  Copings at the roadway heaved and shifted as their mortar beds disintegrated.  Steeply curving wing wall copings were more susceptible to displacement as they were set without pinning.  Plant materials grew against and through masonry.  All exterior surfaces were soiled and required re-pointing.

The preservation of remaining historic fabric was the highest priority goal.  Recreations were based on remaining fabric, historic images and other existing bridges, certain design motifs having been re-cycled on different structures.   Repair and replacement methods developed as the nature of the program changed.

Cast iron repairs have been contracted out and little has developed in our approaches.  New patterns were created and castings produced at the original assembly break points.  Paint analysis was performed on any existing fragments and new schemes devised to attempt to match.  Discussions continue as to the appropriate paint layers to match as Vaux remained at the park during a long enough time period that it seems likely he oversaw early revisions in the finishing.  Railings consistently were first coated with muted earth tones and then refinished with polychrome schemes that included gilding.

Our restoration of stone work has changed during the course of these projects.  On            Greywacke Arch the missing stone balusters were replaced with cast-stone facsimiles.  We wished to introduce in-kind replacements in later projects and switched to utilizing New Brunswick sandstone or other natural stones.  For the New Brunswick stone the original quarry sites were located and a local firm engaged to begin cutting new stone which closely matches the characteristics of the original.  At first we ordered pre-finished units from carvers at the quarry.   As the skills of our crews developed we switched to having the more and more of the carving done in-house and on site.  During the most recent projects we were able to have large complex carved panels completed on site by our staff in a manner that mirrors the Park’s original phase in process as well as appearance.

The development of the skills of our stone carving crews has been a gratifying result of this program.  The training program was developed to comply with a provision of our contract with the City of New York Department of Parks and Recreation whereby, as general contractors a percentage of work must be performed by staff rather than sub-contractors.  Beginning with the relatively simple tasks of brick and stone re-pointing and composite patching, we have gradually specialized and improved the technical know-how of our crews so that during the most recent restorations they provided all of the restoration and replacement work.  The learning was facilitated by providing a sequence of work of increasing complexity.

We started with Dutchman repairs on relatively simple units.  The crew then did all of the carving for a section of the perimeter wall whose historic finish of hand tooled broaching and borders allowed for a wide margin of treatments.  The variations from one hand to another actually increased the historical fidelity of our work over previous units which had been carved to contract documents and tended towards uniformity.  More recent projects required them to carve highly figured Dutchmen for the Bethesda Terrace and Maine Monument. They now are competent in decorative painting, masonry patching, cleaning and stone carving and repair.  A major advantage for the Park has been that the skills gained through these projects continue to serve our preservation needs after these projects end.  We have been able to provide for our on-going historic preservation maintenance through the skills developed through this system.

A similar process was used on the masonry cleaning.  Our graffiti crew adapted their familiarity with pressure washing and chemical removers application to the cleaning of the arches.  We relied on Prosoco Heavy Duty Restoration Cleaner and moderate pressure water blasting to remove most of the soiling. Additional stains were cleaned with organic acids and, in some cases, selective air abrasion.  In general, we left some soiling rather than resort to more aggressive methods.

In many ways, Central Park is a unique site.  The combination of such a large concentration of similar architectural elements and the premier location within New York as an attraction for potential funding is unusual.  Nevertheless, some form of this method of development of in-house resources should be applicable to other public sites.  We are now benefiting from the training programs as our skilled artisans are dedicating themselves full-time to historic maintenance, hopefully assuring that these fine structures will not again fall into decline.  Seeing the conversion of the arches from sites of negligence and threatening decay to elegant and welcoming counterpoints of the landscape they were designed to be has been a gratifying experience as too has been the obvious increase in visitor’s use after restoration.