Balancing History and Structural Restoration
Built in 1925, the Ca D’Zan mansion in Sarasota, Fla., is a 32-room, two-story, Venetian-style home, replete with an 8,000-square-foot Italian marble terrace with preeminent views of Sarasota Bay. Considered “the last of the Gilded Age mansions,” Ca D’Zan cost $1.5 million to construct. A prominent feature of the property was an exquisite dock.
By 2009, the Ca D’Zan mansion was one of Florida’s most visited historic tourist attractions. Although millions of dollars of restoration work recently had been completed, the dock had been untouched. The Italian marble covered dock had deteriorated, deemed structurally unsound, and closed to public access. Over time the slab had eroded, which caused the decorative precast on the slab edge and the marble on top to fall apart. If the slab had been allowed to further deteriorate, the remaining precast and marble would have soon fallen into the ocean. Repairing the dock to meet structural requirements was critical, but the repair had to match the historic look-and-feel of the original dock.
Evaluating the Existing Structure
To begin, the STRUCTURAL project team performed a full survey of the dock’s underside. The inspections, which had to be performed in the early morning hours due to the tides, required employees to wear chest-high waders while dealing with the ocean tides. Signs of concrete deterioration were quickly discovered, but unfortunately the repairs for this section were not in the current budget. STRUCTURAL’s initial report to the owners stated that future repairs should be scheduled on the Ca D’Zan dock underside within the next five years. For the time being though, the underside slab was in adequate enough condition to perform the much needed repairs to the topside.
The topside survey of the Ca D’Zan dock required STRUCTURAL to evaluate two separate portions of the delicate historic marble and the concrete to which the marble was grouted. In any historic restoration project, the goal is to preserve and reuse as much of the original material, especially the material that is placed for aesthetic purposes. The survey of the marble revealed that approximately 90% of the existing marble could be left in place or temporarily removed and re-installed later. Any marble tile that was substantially cracked or incomplete would be removed and replaced. Much of the tile on the south side of the dock was already missing due to severe deterioration of the concrete substrate.
All areas of exposed concrete on the topside of the dock were sounded for structural integrity. However, almost all of the concrete on the south side of the dock was deteriorated, and it was determined by the team that the first 2 inches of concrete had to be removed on the entire southern dock before any marble could be placed back. Also the perimeter of the dock was deteriorated in large portions and caution would have to be exercised in dealing with the decorative precast that still existed. In addition, the steel in much of the dock perimeter had suffered substantial sectional loss and would need to be replaced.
The repair to the Ca D’Zan dock had three significant challenges. First, the work occurred over an ocean bay that had fluctuating tides. In addition to this safety hazard, the dock lacked railings. A comprehensive safety plan was needed to keep employees protected from falling into the water, as well as to give ample working time for critical work tasks.
Second, this dock was a notable historic landmark. Therefore, the repair became much more unique and challenging than an ordinary concrete slab project. Not only did the marble and fading decorative precast need to be preserved, but the STRUCTURAL team would have to blend in the new marble to the existing.
Finally, the Ca D’Zan mansion and its terrace were to remain open during the project. Although visitors were not allowed to walk on the open dock portions, they were located just a few feet away and directly above the worksite. Therefore, safety considerations were not only for employees, but also for the hundreds that visited each day.
Demolition and Containment
None of the material removed could fall into the ocean water below, therefore, containment was the first step taken in the demolition process. The first barrier put in place was a 150-foot-long floatable turbidity barrier extending from the southeast corner of the dock to the center portion. This barrier was later moved to the northern section when the demolition work phased in that direction. This type of turbidity barrier is designed to prevent any floating material from leaving the confines of the perimeter of the project.
The second containment method featured a fine mesh snow fence securely installed on the underside of the dock. Any concrete falling during demolition became trapped in this mesh barrier and removed each day to prevent inclement weather or waves from knocking captured debris into the water.
In order to achieve safe access around the dock, scaffolding towers were erected securely in the water and 12-foot planks were installed to span each tower. This provided an even working platform adjacent to the dock and gave the team necessary hand rails. Where scaffolding was not erected, slab grabbers with handrails were used instead.
The demolition began with careful removal of all marble pieces marked during the survey. Special care was taken to ensure the integrity of any remaining historical marble. Where possible, plywood sheets were placed over the marble so they could be traversed without sustaining damage. This protection became increasingly important since the site dumpster was located on the north side of the project (out of sight of visitors), while most of the demolition took place on the southern side of the dock.
A total of 65 cubic feet of concrete was removed from 28 locations. These locations were mostly on the dock’s southern side, but several were in areas adjacent to existing decorative precast or between two existing marble pieces that were difficult to access.
Because maximum bonding of the new concrete to the existing concrete is crucial, the steel and concrete had to be properly prepared. The crew used chipping hammers and 7000-psi pressure washers to achieve a course and clean surface. This process also revealed the condition of the exposed rebar, which had suffered a large amount of sectional loss. Much of the rebar along the dock edge had completely deteriorated, requiring full-depth demolition of the concrete in these locations. Then the cleaned steel was inspected, and new #5 rebar doweled into the slab edge and added to any location that displayed significant sectional loss in the steel.
Careful Formwork Installation
The formwork was very challenging, because it was supported by shoring based on the ocean floor. Because the tides allowed work in the water only in the early morning, these shores were checked each morning to verify they had not moved, and double checked the day of the pour. In addition, the formwork needed to be constructed to a very fine tolerance. The marble placed on top of the new concrete needed to be installed at a very specific height and slope to match the existing marble. Also, the new decorative precast for the slab edge was later installed at a different elevation. The precision in the formwork was achieved through an iterative process using string lines and 1×2-foot lumber.
A total of 127 cubic feet of 4000-psi concrete (43/4 cubic yards) was placed the day of the challenging pour. The repairs were located throughout 150 feet of the dock and often repairs existed between historic marble. The pump operator couldn’t see the crew’s hand signals—a huge challenge as just five extra seconds of pumping could have resulted in an overflow into the ocean. In the end, a crew of four manned the hose line, vibrated and troweled the concrete, and protected the existing marble. The project manager was stationed on the upper terrace—allowing simultaneous communication with the crew and the pump operator. After the pour, the entire team used wet rags to clean any spilled concrete off of the existing marble.
The original dock featured a beautiful decorative precast edge that needed attention. STRUCTURAL planned to replace portions of this precast, as needed, matching the color, texture, and decorative shape. After the mockup of the precast was approved by the owner, 4-foot sections were grouted in place where needed.
The last step of the project was the installation of the new marble pieces by the marble tile subcontractor. Fortunately this subcontractor had performed much of the tile work on the Ca D’Zan grounds and was familiar with the patterns and the nature of the marble. They were able to use marble pieces stored several years earlier for the final step in the restoration of the dock.
An impressive transformation, visitors are able to walk on the terrace at the Ca D’Zan mansion just as they did 85 years ago—enjoying the romantically classical gateway to Sarasota Bay.