Replicating Classical Decorative Elements in Exposed-Aggregate Concrete

Tuesday, November 3, 2015: 1:55 PM
Presenter: Amy Hollis , Worcester Eisenbrandt, Inc., Baltimore, MD
Meridian Hill Park, in northwest Washington DC, was conceived and constructed during the City Beautiful Movement of the early 20th century.  Built over the course of 20 years, the 12-acre hillside was transformed into a terraced oasis on one of the city’s major arteries.  Designers George Burnap and Horace Peaslee used a classical architectural vocabulary and aristocratic landscape design to furnish an open urban park, accessible to the public.

While classical architecture was traditionally produced in stone, builder John J. Early constructed the terraces, walls, balustrades, and fountains with exposed-aggregate concrete, in a combination of precast pieces and large poured-in-place foundations.  There are over 20 different aggregate mixtures in the park, as well as several colors of cement, producing a highly decorative environment.

The National Park Service is currently undertaking a multi-phase plan to restore Meridian Hill.  As part of that restoration, Worcester Eisenbrandt was contracted to clean and repair the Grotto area.  The grotto consists of a monumental entrance, an enclosed staircase from 16th Street to the upper lawn of the top terrace, and the plaza above.  Because of deteriorated elements suspended over the entrance, the grotto area had been closed to the public for a year prior to construction.  The steel reinforcement in the mutules had begun to corrode, breaking up the concrete units, pieces of which had fallen to the sidewalk and stairs below.  WEI removed five of the six mutules due to damage, and then proceeded to replicate the remaining unit.

There are many products and methods currently used for precast exposed-aggregate concrete, but most of those are used to produce flat panels or simple columns for new construction.  To produce classical mutules with details on four sides, WEI required precision in the following tasks:

  1. Matching the aggregate size and color mix, ultimately using three different aggregates from different suppliers, and sieving for the correct size range.  Two different size ranges were used for each unit.
  2. Matching the color of the cement matrix, using the best sand color available and augmenting the mix with small amounts of pigment.
  3. Finding the right concrete retarder to produce multiple levels of etch on each unit.
  4. Constructing the molds so that each area could be released and rinsed according to its own depth of etch without disturbing the areas that needed more time in the mold.  This required several different materials and multiple trials to achieve the final mold design.

The entire process took 9 months to produce accurately replicated units, from aggregate searches to installation.  The project took the skills of a number of WEI staff to complete (conservators, masons, carpenters, historians, and managers).  From the sidewalk below, the new mutules are indistinguishable from the clean surrounding units.