Show Summary Details

Page of

Printed from Grove Art Online. Grove is a registered trademark. Under the terms of the licence agreement, an individual user may print out a single article for personal use (for details see Privacy Policy).

date: 02 December 2022

Conservation of concretefree

Conservation of concretefree

  • Beril Bicer-Simsir
  •  and Simeon Wilkie

One of the most significant transformations in the construction industry began with the invention of Portland cement at the beginning of the 19th century. This transformation continued by the subsequent developments of Portland cements (e.g. specialty and blended Portland cements) and concretes (e.g. reinforced, pre-stressed, post-stressed, prefabricated, etc.), and resulted in an extensive collection of historically significant concrete architecture. Similarly, Portland-cement based material developments also influenced art and artistic works, and many examples of concrete art including decorative elements, wall paintings, and sculptures became an important part of our built heritage. In many cases, the lack of understanding of the relationships between the properties of concrete and its performance (e.g. the use of improper mix constituents and proportions, mixing, curing, and design criteria) and the lack of established quality control have also become the main sources of deterioration and durability issues, which historically significant concrete art and architecture face today and therefore require conservation. In other cases, the change of use, function, and significance of the concrete architecture as well as the intent to extend the design life of the concrete art and architecture necessitate the conservation of concrete.

See also Concrete.

1. Causes of deterioration.

While concrete is considered to be a very durable construction material, it is still susceptible to deterioration due to several different mechanisms that are generally grouped under chemical, physical, and biological mechanisms. Chemical mechanisms include carbonation, chloride contamination, alkali-aggregate reaction (alkali-silica and alkali-carbonate reactivity), sulfate attack, and attacks by acids and bases. Physical mechanisms are environmental effects such as freeze-thaw cycles, weathering (thermal effects and erosion due to abrasion and cavitation), salt crystallization, structural loading (e.g. overloading, settlement, and earthquakes), impact loading (e.g. blast), shrinkage, and expansion. Biological mechanisms include the propagation of invasive plants and microorganisms on the concrete surface. All these deterioration mechanisms result in a wide range of damage types that require a variety of intervention methods. Most common damage types are cracking (surface and/or internal), surface distress (scaling, honeycombing, disintegration, pop-outs, spalling), delamination, surface discoloration (rust staining, efflorescence, seepage or leakage through joints and cracks, biological growth), and other distresses, such as deflections, heaving, curling, and settlement.

One of the most common deteriorations of historic concrete is the corrosion of the steel reinforcement. When the reinforcement corrodes, the pressure created by the expansive corrosion products causes damage to the surrounding concrete. While corrosion of the steel can occur due to moisture ingress through existing defects and cracks in the concrete surface, it can also occur or speed up due to other mechanisms including carbonation and chloride contamination. These two mechanisms are particularly common in historic concrete. Calcium hydroxide (portlandite) present in fresh concrete is the source of the alkalinity of the pore fluid in concrete and is also the source of the oxide layer on the steel reinforcement protecting it from corrosion. When concrete is exposed to air, calcium hydroxide changes into calcium carbonate and over time the alkalinity in concrete reduces. When the depth of carbonated concrete reaches the steel reinforcement, the protective oxide layer diminishes and the corrosion process easily begins and develops at a higher rate when moisture is available. The presence of chloride ions can also locally break down the protective oxide layer. Chloride-induced corrosion usually occurs due to ingress from external sources (de-icing salts and marine environments), but it can also occur as a result of internal contamination from the use of calcium chloride as an accelerating admixture (pre-1976), unwashed aggregate, and contaminated mixing water in historic concrete structures. In many cases, carbonation- and chloride-induced corrosion may cause no visible damage on the concrete surface until the corrosion advances to the point that the highly expansive corrosion products cause cracking, delamination, and spalling of the surrounding concrete, which encourages moisture ingress and increases the rate of corrosion.

Another common deterioration mechanism seen in historic concrete structures, especially those built before the 1960s, is alkali–aggregate reaction (AAR). AAR occurs when alkaline pore fluid reacts with minerals present in certain aggregates to form a gel that undergoes volumetric expansion in the presence of water, creating stresses in the concrete that can result in cracking. AAR is a particularly severe form of deterioration, as the reactive aggregates are dispersed throughout the concrete and the expansion can only be prevented by stopping moisture from penetrating the surface of the concrete, and often full replacement of the concrete is required.

2. Conservation methodology.

There are several paths and objectives to consider for conservation of concrete. These include preservation (i.e. the process of maintaining the present condition and arresting further deterioration), rehabilitation (i.e. the process of modifying to a desired useful condition), repair (i.e. the replacement or correction of deteriorated, damaged or faulty materials, components, or elements), restoration (i.e. the process of re-establishing the materials, form, and appearance to those of a particular era), and strengthening (i.e. the process of increasing the load-bearing capacity of a structure or portion thereof).

As an example, the prime objective of preventive conservation can be the exclusion of moisture. Depending on the specific situation, this can be achieved in a variety of ways, including ensuring adequate drainage of flat roofs and adequate ventilation, or by the use of protective coatings that keep the concrete dry, yet allow it to breathe. When extensive cracking, delamination, and spalling exist due to corrosion, major repairs are necessary. In this case, the affected concrete has to be removed, the reinforcement rods are cleaned or replaced, and then the concrete itself is replaced, resulting in the loss of the original fabric.

A systematic and well-defined condition assessment is crucial to identify conservation objectives and to determine the most suitable intervention method for the specific case. The focus of the condition assessment is to identify the condition of existing concrete (both original and repairs), understand the past and current exposure conditions, document and map deterioration symptoms, and understand the related deterioration mechanisms, combined with material analysis to characterize existing materials. A variety of nondestructive, minimally destructive, or in some cases more destructive diagnostic tools can be used for the condition survey in the field and laboratory. A number of analytical laboratory tests can be performed on collected samples (cores, powder, chips, etc.) to study the chemical, physical, and mechanical characteristics of the materials to be treated and to identify existing deterioration mechanisms. These include elemental analysis (e.g. X-ray fluorescence [XRF] and scanning electron microscopy with energy dispersive X-ray spectroscopy [SEM-EDS]), compositional analysis (e.g. X-ray diffraction [XRD], thermogravimetric analysis–mass spectrometry [TGA–MS], and petrography), wet chemical analysis, sieve analysis, salt analysis (e.g. ion chromatography [IC]), porosity, water absorption, vapor permeability, and strength measurements. A number of test methods can be used to detect damage or the extent of it in the field. For example, to detect delaminations and voids, sonic tomography or ground-penetrating radar can be used. Thermal imaging can be used to identify moisture issues. While there is no guaranteed method of identifying corrosion without exposing the embedded steel, the likelihood of corrosion can be assessed by measuring the half-cell potential of the steel.

Based on the information collected during the condition assessment, the need for intervention methods (e.g. crack stabilization, surface repairs, cleaning, strengthening, and protection), the desired properties of the intervention materials, and the development of custom-mixed materials or selection of commercial proprietary materials that will be compatible with the existing concrete can be determined. These decisions should be made by an experienced interdisciplinary team including conservators, conservation scientists, conservation architects, and structural engineers. The selection and implementation of intervention materials are crucial for the success of the conservation and to avoid the need for frequent repeated interventions.

Following laboratory testing of candidate intervention materials, field trials and mockup tests should be conducted prior to implementation. Following field trials, complications with the process should be identified and necessary precautions should be taken. If the performance of the intervention material is not satisfactory, modifications to custom-mixed intervention materials can be made and different commercial intervention materials can be tested. Conservation interventions are challenging procedures and should only be undertaken by experienced conservators and conservation professionals. Periodic post-treatment monitoring using nondestructive diagnostic tools should be carried out to ensure the durability of the treatment over time and to identify the changes in the condition of concrete.

Bibliography

  • Technical Report No. 50—Guide to Surface Treatments for Protection and Enhancement of Concrete. Blackwater: Concrete Society, 1997.
  • Technical Report No. 54—Diagnosis of Deterioration in Concrete Structures—Identification of Defects, Evaluation and Development of Remedial Action. Blackwater: Concrete Society, 2000.
  • Sutherland, R., Humm, Dawn, and Chrimes, Mike, eds. Historic Concrete: The Background to Appraisal. London and Reston, VA: Thomas Telford, 2001.
  • Macdonald, S., ed. Concrete: Building Pathology; foreword by David Watt and Peter Swallow. Oxford and Malden, MA: Blackwell Science, 2003.
  • Odgers, D., ed.. Practical Building Conservation: Concrete, English Heritage. Farnham and Burlington, VT: Ashgate Publishing, 2012.
  • CS173—Visual Concrete: Weathering, Stains and Efflorescence. Blackwater: Concrete Society, 2013.
  • Macdonald, Susan and Ostergren, G., eds. Conserving Twentieth-Century Built Heritage: A Bibliography. Los Angeles, CA: Getty Conservation Institute, 2/2013. Available at: http://hdl.handle.net/10020/gci_pubs/twentieth_centruy_built_heritage (accessed June 1, 2022).
  • Normandin, K. C. and Macdonald, Susan. A Colloquium to Advance the Practice of Conserving Modern Heritage: March 6–7, 2013: Meeting Report. Los Angeles: The Getty Conservation Institute. Available at: http://hdl.handle.net/10020/gci_pubs/colloquium_report (accessed June 1, 2022).
  • ACI 563.1R-18—Specification for Repairs of Concrete in Buildings. Farmington Hills, MI: American Concrete Institute, 2014.
  • Urquhart, D. Historic Concrete in Scotland Part 3: Maintenance and Repair of Historic Concrete Structures. Edinburgh: Historic Scotland, 2014.
  • Custance-Baker, A., Crevello, Gina, Macdonald, Susan, and Normandin, Kyle C., eds. Conserving Concrete Heritage: An Annotated Bibliography. Los Angeles: Getty Conservation Institute, 2015. Available at: http://hdl.handle.net/10020/gci_pubs/concrete_biblio (accessed June 1, 2022).
  • Croft, C., Macdonald, Susan, and Ostergren, Gail, eds. Concrete: Case Studies in Conservation Practice. Los Angeles, CA: Getty Conservation Institute, 2018.
  • ACI 364.1R-19—Guide for Assessment of Concrete Structures before Rehabilitation. Farmington Hills, MI: American Concrete Institute, 2019.