The Role of Concrete + Cement in Historic Preservation

Ron Sturm

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Aug 18, 2020

When we think of historic structures, our minds may jump to an ancient castle or a church which is hundreds of years old. And while you’re not wrong, there are many more recently built structures which are also deemed ‘historic’. The qualifications for how a structure is deemed historic differs slightly around the world. In the United States, a 50+ year old structure is eligible for landmark status if it meets several requirements. Examples of some modern, concrete historic structures in the United States include Frank Lloyd Wright’s Guggenheim Museum (1959) in New York City; Unity Temple in Oak Park, Illinois (1908), considered to be the first “modern” concrete building; and State Farm Center sports dome (1963) at the University of Illinois at Urbana-Champaign.

Frank Loyd Wright's Guggenheim Museum, New York City

The preservation of historic structures involves many different disciplines including historical research, architectural design, materials evaluation and structural engineering. A firm understanding of the materials used to build the structure and the environment to which the structure is exposed are also important considerations. Historic structures may be constructed of reinforced concrete, assembled pre-cast concrete elements, masonry or a combination of these. But what happens when distress and degradation take place in historic buildings? And what challenges arise with preservation?

Degradation of Historic Concrete Structures

Many historic structures in the US built in the beginning and middle of the 20th century are beginning to show signs of degradation and distress. Crumbling facades, cracks in walls and flatwork, and excessive loss of surface texture and detail are ways the degradation may manifest itself. Effective maintenance may keep these problems at bay, but eventually action will have to be taken to remedy aging concrete.

Environmental exposure conditions and other factors can accelerate the degradation of concrete over time. These may include cyclic freezing and thawing, exposure to salts and aggressive chemicals, internal chemical reactions, and corrosion of steel reinforcement. These factors, as well as anticipated exposure conditions, need to be considered when planning a repair of historic concrete structures.

Replicate and Repair Historic Concrete Structures

Often, the most difficult task is to try and match the concrete materials used in the original structure. Materials applied today could be incompatible with the original materials used in the construction of the structure. Another aspect is matching the texture and colors of the two respective concretes, which is usually linked with sourcing similar or identical materials within the concrete. Preserving and repairing go hand-in-hand for historic structures and maintaining the integrity of the structure is often just as important as repairing it.

To find compatible concrete and to aesthetically match it with the original materials is a daunting task. Concrete is made of aggregate, usually rock or gravel, plus sand bound in a hardened cementitious matrix. These raw materials are finite resources, and once they’ve been used up – which is probable in the 50+ years since the original structure was built – they’re likely not available. Our task is to replicate a material that’s as old as the structure we’re trying to preserve but using repair materials that are available today.

Condition survey. First and foremost, the building should be inspected to evaluate the condition of the structure so as to determine where repairs are needed. Historical records should be reviewed, and past repairs should be identified. At this time, samples of the concrete, representative of the concrete in damaged and undamaged areas of the structure, should be collected.

Analysis of materials. Through specialized laboratory analysis, which may include petrographic examination and chemical analysis, the composition and characteristics of the concrete and its mineral constituents are determined. These analyses also may ascertain the environmental factors and mechanism of distress that are damaging the concrete. Findings of these and other tests provide invaluable information about types and proportions of aggregate and cementitious constituent used in the concrete. This can then be used to help select new aggregate, cementitious materials and other additives that closely match the original constituent of the historic concrete.

Repair material evaluation. Once a repair mix is designed, the concrete should be tested for strength development, volumetric stability, color and appearance, adhesion, and other properties to make the concrete physically compatible and visually consistent with the original historic concrete.

Removal and repair. Damaged and degraded portions of the concrete are removed. The resulting exposed surfaces may also need to be conditioned or prepared in a manner to properly receive repair materials. Repair concrete is then cast and bonded to these surfaces. The outer surfaces of the repair may need to be conditioned or treated to provide a visual match to the color, texture, profile and overall appearance of the surrounding historic concrete.

While historic preservation of concrete is challenging, it can be successful if the project team and the owner work through appropriate investigation, analysis, design, mock-up and implementation efforts. Comprehensive understanding of cementitious materials and aggregates, both historic and modern, is key to an effective, durable preservation project. When executed correctly, a historic concrete preservation project results in the extension of the life of culturally and architecturally important structures.