Conserving Concrete

For anyone doubting the appropriateness of concrete as a subject for conservation (perhaps wishing away over a century of architecture), the introduction to Concrete: Case Studies in Conversation Practice provides a rejoinder.

Concrete is one of the most ubiquitous materials of the twentieth century; therefore, anyone involved in conserving modern heritage needs some understanding of the material, its deterioration, and its effective repair. Some 150 years of development of reinforced concrete has produced an extraordinary legacy of structures and buildings.[i]

Concrete is an ancient building material, used since ancient times, refined by the Romans and revived for the development of Portland cement in the 1820s, since when it has been used almost continually.

Regardless of the building typology or construction date, the decay mechanism common to all the projects comes down to the basic issue of the steady progress of carbonation that leads to the eventual corrosion of the reinforcement. The rate of carbonation is affected by factors such as the depth of cover to reinforcement, poor workmanship, and low cement content, sometimes in combination, in each case influenced by the environmental conditions arising from the location.[ii]

The key difficulty for many béton brut buildings is that concrete is structurally integral and also the undecorated surface, thus serious deterioation of the concrete cannot be left untreated and any treatment cannot but alter the appearance of the building. It was only in the late 1980s that a programmatic, scientific approach to preservation of concrete structures was developed by the architecture profession – surprisingly late considering the considerable evidence of deterioration which had been documented before then.

Part of the Conserving Modern Heritage series from Getty Publications, Concrete: Case Studies in Conversation Practice brings together essays by conservators who have worked on projects to conserve and restore concrete buildings. Their experiences provide us with greater understanding of the problems and solutions for this issue which grows increasingly relevant and serious as some classic Modernist buildings are reaching a state of decrepitude. Strategies for corrosion protection and repair include patching, partial replacement and resurfacing. These have the obvious drawbacks of introducing ahistorical or incongruous surfaces. Corrosion inhibiting solutions and electrochemical realkalization are other approaches[iii].  

One ingenious solution which retards corrosion of rebar is remote sacrificial anode (RSA). The electro-chemical process of water corroding steel is delayed by the attachment of a section of grounded metal to the concrete. The electrical charge is thereby transferred to the RSA which corrodes instead of the steel in the concrete. However, electrical resistance of concrete means that it produces a localised effect rather than a solution for large areas. The RSA can be replaced when it is corroded, every 30 years or so. New techniques are being pioneered that involve the installation of meshes that are inside the concrete close to the rebar and connected to an RSA, effectively wiring the entire structure to stabilise corrosion but this seems to be construction application rather than a remedial system. Non-corroding anodes using direct current electricity are alternatives but require mains power or sustainable electrical generators, which are not always suitable or cost effective.

This volume provides 14 case studies of remedial action to restore and renovate concrete exposed to the elements. The first is structures from the 1920s and the last is a Donald Judd sculpture of 1988-91. The examples are free-standing objects, enclosed buildings and exposed open-air structures. They range from utilitarian structures, a church, market, warehouse, zoo, bathing station, school, university, theatre and residence.  Some of these are world-famous structures (the Marseille Unité d’Habitation of Le Corbusier), while others are lesser known. Architects include Oscar Niemeyer, Pier Luigi Nervi and Eugène Freyssinet. 

The structures featured are superb examples of their type, hence the efforts to conserve them. Some techniques could be used to extend the life of less important concrete structures. Halles du Boulingrin, the Reims covered market, features soaring thin-ribbed parabolic arches forming a curved roof – dramatic, aesthetically satisfying and practical. (Such a design had already been used by the architect for aircraft hangars at Orly, Paris.) It is a fine example of Art Deco architecture, built to replace a building destroyed during the Great War bombardment which practically erased the medieval town. It was fully restored and an acrylic-resin membrane applied to the outside to waterproof it. In this case the original building had been painted, so extensive restoration could be done, effectively resurfacing the entire roof and painting it. The installation of micro-processor controlled fans combatted the issue of condensation, which had dogged the original design.

A uniquely complex structure is Villa Girasole, Verona, which incorporated house which that revolved 360° on a circular base of concrete. Constructed between 1929 and 1935, the building was part residence, part art work, part proof of concept. The functioning of the building/machine/sculpture caused the damage to the platform. Regrettably, the essayists do not describe if the house is currently functional. In the case of Niemeyer’s Brazilian church, much of the water damage was caused by builders omitting to include the specified expansion joints. At the Morse and Ezra Stiles Colleges, Yale University, one problem was steel window frame anchorages corroding within concrete walls. The famous Unité d’Habitation in Marseille suffered due to some substandard concrete in places and the thinness of certain details (especially the balcony balustrades) led to cracking and spalling due to corrosion of internal steel. The pilotti were corroded and required resurfacing.

The essays by experts in the projects provide background, assessment of the problems, the potential solutions, conservation work undertaken and the general findings. Technical information is given (including plans, models, diagrams), though not at too great a length. Even the data is understandable to the non-specialist reader. Numerous photographs give us an idea of the original appearance of the buildings, their damage and their current state. There is a glossary included.

Evidence is that building conservators adhere to the principles of minimal intervention, maintenance rather than alteration, avoiding “improvement”, making changes noticeable but not jarring, attempting to keep interventions reversible and other standards that are shared with the best practice of art conservation. This publication shows that there is much that can be done to remedy unsightly, inconvenient and dangerous degradation of concrete structures – though the cost and difficulty may be in some cases prohibitive. Traditionalists, hoping demolition should be the only just destiny for Modernist structures, will be disappointed; for the rest of us, the documented solutions seem pragmatic and acceptable ways to extend the life spans of concrete structures. 

Catherine Croft, Susan Macdonald (eds.), Concrete: Case Studies in Conversation Practice, Getty Publications, 2019, paperback, 236pp, fully illus., $59.95/£45, ISBN 978 1 60606 576 1

(c) 2020 Alexander Adams

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[i] Concrete, p. 11

[ii] Concrete, p. 17-8

[iii] Note François Botton’s caution: “[…] it appears the effectiveness of the realkanization procedure may depend on the level of corrosion before treatment, or it may simply not last longer than twelve to thirty months.” Concrete, p. 123