Keywords

1 Introduction

Within the field of conservation, concrete heritage is becoming an area of growing concern as the number of culturally significant concrete structures which require ongoing repair and maintenance continues to increase. The development of reinforced concrete through the late-19th and 20th centuries provided a new material that revolutionized the construction industry and provided a means of achieving new architectural and structural feats. The arrival of this material came at a time of unprecedented technological, social, and economic change, with huge-scale rebuilding following two world wars, rapid population growth, mass migration and the expanse and development of urban societies [1]. This new material was quickly and widely adopted in numerous different architectural styles and functions and provided not only a new tool for artistic expression but also for social reform through the creation of new spaces to live and work. As a result, there is now a diverse wealth of culturally significant 20th-century concrete structures around the world that present new and unique conservation challenges.

In the past, concrete durability issues were not as well understood [2]. In many early publications and promotional materials, it was widely believed that this new material would be permanent and maintenance-free. Unfortunately, this is not the case and a combination of factors, such as corrosion of reinforcement, high water/cement ratios (w/c), the use of reactive aggregates and unsuitable mix designs, have resulted in concrete deterioration at many of these structures requiring ongoing interventions. While there is now a general acceptance of the importance of concrete heritage from the 20th century [3], there are few widely accepted guidelines on the approach to its preservation and conservation [4,5,6,7]. While it is understood that the fundamentals of best practices and performance criteria in concrete repair also apply to heritage concrete, the many studies and published guidance [8,9,10,11,12,13,14] on the performance criteria of concrete repairs are not always familiar to the conservation field and, in some cases, promote irreversible changes to the structure that are in conflict with traditional conservation principles such as minimal intervention and retreatability.

There are few studies on the long-term performance of patch repairs designed to preserve the aesthetic significance of the original fabric. As a result, there has been a history of repairs carried out without application of good concrete repair fundamentals and had an unacceptable impact on the aesthetic significance of the concrete structure. As a result, these poor-quality repairs have short life cycles and, ultimately, cause physical damage to the heritage structure (Fig. 1).

Fig. 1.
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Example of a repair which has failed to weather and has a negative impact on the historic aesthetic of a culturally significant concrete structure.

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2 Project Background

An international experts’ meeting focused on conserving concrete heritage took place at the GCI, Los Angeles, in 2014 [15], during which the materials and techniques used to carry out patch repairs on culturally significant concrete structures were determined to be key issues that require further investigation. More specifically, the experts agreed that, within the field of concrete conservation, there was a need for both an improved understanding of the patch repair process, and for better information on repair materials and their long-term performance.

While conservation work often aims for ‘like-for-like’ repairs [4], which replicate the original material in both composition and aesthetics, this is not always feasible [15]. For example, when compared to polymer-modified and proprietary repair materials, some like-for-like repairs may not have mechanical properties which are as desirable, may have more issues bonding to the original substrate, and may not provide the same level of durability or protection to the steel reinforcement. Additionally, while conservation work aims to integrate the principles of minimal intervention and retreatability, this can be at odds with contemporary concrete repair guidance [9] which may recommend sacrificing more of the original fabric in exchange for increased performance of the repair.

There needs to be additional guidance for the conservation community on selection of repair materials and the approach to patch repair work on culturally significant concrete heritage. As a response to this specific challenge, three institutions, the GCI, Historic England (HE) and the Laboratoire de Recherche des Monuments Historiques (LRMH) commenced work on an international collaborative research project, ‘Performance Evaluation of Patch Repairs on Historic Concrete Structures’ (PEPS). The project team is also joined by expert consultants from Rowan Technologies (England) and Wiss, Janney, Elstner Associates, Inc. (USA). Begun in 2018, the PEPS project aims to produce practical guidance to help those repairing historic concrete through the process of the selection of appropriate repair approaches, procedures, and materials. The scope of work includes the assessment of case studies in the USA, England, and France within a variety of climatic and environmental conditions, typologies, and repair approaches.

3 Phase I: Project Development

The first phase of the research began in 2018, and had 3 key outcomes:

3.1 The Development of the Research Proposal

After reviewing the existing needs of the field, it was determined that the project should specifically aim to produce better understanding on:

  • Current repair methods and practices;

  • Performance of patch repairs typically undertaken on historic concrete;

  • Efficacy and durability of patch repair materials currently used in historic concrete;

  • Efficacy of current repair techniques.

Additionally, it was agreed that, in the long-term, the project could service the concrete conservation community through the development and dissemination of:

  • Guidance on selection of appropriate repair materials;

  • Practical guidance on repair methods;

  • A methodology for recording, monitoring, and evaluating repairs to historic reinforced concrete.

3.2 The Development of the Assessment Methodology

While there is clear guidance on carrying out assessments of concrete structures [16,17,18], there were no pre-existing protocols that detailed the scope of work required of the PEPS research. As such, a new assessment methodology had to be designed specifically for the project. This methodology, developed by an interdisciplinary team of professionals working in the field of concrete conservation, includes a variety of traditional and non-traditional, non-destructive, mechanical, and chemical and electrochemical characterization and diagnostic techniques. In addition to the specification of individual evaluation protocols, a shared way of recording results and reporting observations also had to be developed to ensure data was collected consistently and could be unified for further analysis.

The methodology was developed with aim of answering the following questions:

  • What materials and protection systems were used in the repair and adjacent substrate?

  • How was the repair area prepared and how has this impacted the repair bond?

  • How is the repair performing technically?

  • How is the repair performing aesthetically?

3.3 The Selection of Case Studies

Up to ten case studies were identified in each country (USA, England, and France) where patch repairs had been applied to culturally significant concrete structures and both the aesthetic and technical performance had been a consideration in the repair strategy. It was concluded that the case studies selected should include repairs in a wide range of conditions and environments and using different materials and techniques – reflecting the diversity of approaches and contexts for repair that currently exist. Specific criteria for selection included:

  • Concrete buildings or structures, where the surface is exposed or untreated;

  • Repairs performed with the goal of conserving historic and aesthetic values;

  • Access to documentation, and professionals who worked on the repair (architects, engineers, tradesmen, and craftsmen);

  • Easy access to the site and patches to be evaluated;

  • Consent must be given from site owners/managers for access to site and data;

  • Case studies should have diverse ages of original concrete and ages of repair;

  • Include repairs made with various levels of craftsmanship;

  • Include repairs that used different materials (concrete, mortar, like-for-like, proprietary, etc.) and placement techniques (hand troweled, poured);

  • Case studies can present combined use of patch repairs and other protection or treatment techniques;

  • Complementary techniques applied should not obscure patches, such as coatings;

  • Case studies should represent a good range of environmental conditions (coastal, urban, etc.) and aspects.

4 Phase II: Preliminary Assessment

4.1 Establish Case History

During the preliminary assessment, all of the selected case studies were evaluated. Before conducting site visits, desk studies were undertaken on each site to establish each site’s case history and provide data on how the repair strategies were developed and executed, and support the interpretation of current conditions observed on site. The desk study included a review of existing building specifications and repair documentation, complemented by interviews with owners, asset managers and other relevant individuals who could validate the history of the repairs.

4.2 Initial Site Visits

Preliminary Investigations. During the first site visits, approximately 10 patches were assessed per case study using only non-destructive tests (NDT) and documentation. The selection of patch repairs to include in the study was done following an initial walk through the site to identify, whenever possible, patches representing different degrees of deterioration, exposure, and vintages. The field assessment was then completed using the following techniques:

  • Visual and tactile observations;

  • Sounding to determine delamination, poor consolidation, or voids;

  • Photographic documentation using scale, color checker, and crack gauge;

  • Water spray to test repellence and possibility of hydrophobic treatments;

  • Rebound hammer and scratch test to determine differences in surface hardness between repairs and adjacent substrate;

  • Covermeter survey of patch and adjacent substrate to identify reinforcement location and depth of cover.

Determination of Patch Performance. Upon completion of the Phase II site visits, each patch was assigned a designation (good, fair, poor, very poor) based on its aesthetic and technical performance. However, there are no widely accepted criteria for judging both the aesthetic and technical performance, so an agreed criteria had to be developed. The criteria are described in detail in Table 1, and examples of patches with different performances are shown in Fig. 2.

Table 1. Criteria for judging the aesthetic and technical performance of patch repairs
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Fig. 2.
figure 2

Examples of a technically and aesthetically good patch (top left), an aesthetically good but technically poor patch (top right), technically good but aesthetically very poor patches (bottom left), and a patch that is aesthetically poor and technically very poor (bottom right).

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5 Phase III: Detailed Diagnostic

Following a review of the results of Phase II, 5 English, 3 American, and 4 French case studies were selected for further assessment in Phase III. The aim of this phase is to carry out a more detailed investigation to detect underlying deterioration which may not have been identified during Phase II and to identify material characteristics that could help explain the performance of the repairs. Phase III testing is being performed on 5 patches chosen from the group assessed previously in Phase II and their adjacent areas to detect any significant differences in behavior between the repair and concrete substrate. During these site visits, a detailed procedure including both NDT and invasive testing is being conducted, and samples are being removed for laboratory analyses. Laboratory analyses were carried out internally at the labs of LRMH and the GCI, with some additional testing carried out by external laboratories in the USA and France.

5.1 Detailed Site Investigation

Detailed site investigation focused on adding more measurements to preliminary investigations and introducing additional evaluation techniques on key aspects of aesthetic and technical performance. Evaluation of historic concrete structures can be challenging due to a lack of reliable information about the materials and techniques used. Furthermore, removing material from culturally significant structures for testing in the laboratory can be a challenge due to legal, ethical and financial restrictions on the removal of material, and as the amount of material that can be removed is often minimal, this can result in laboratory samples which are not representative of the material as a whole [19]. As such, it was determined that the testing methodology should utilize non-destructive means where possible to maximize the amount of data obtained without causing unnecessary damage to the structure. However, it was also recognized that it was critical to include some invasive techniques to properly assess the technical performance of patch repairs, and so sites were selected where this would be possible (Table 2).

Table 2. Overview of field tests carried out.
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While the site testing included NDT techniques and tools which are standard for concrete investigations, such as sounding, rebound hammer, covermeter and electrical resistivity (Wenner probe), it also included additional tests which are commonly applied to cultural heritage. Colorimetric measurements were taken on-site using Konica Minolta CR-410 colorimeters with D65 CIE standard illuminant, 2° CIE standard colorimetric observer angle, and measurement area of ø 50 mm, with the results recorded in CIELAB. The number of measurements taken per site was dependent on the size of the patch and the variability of the material. However, a minimum of 6 representative areas were measured per patch, with a further minimum of 6 measurements taken on the original concrete adjacent to each patch. This allowed a more objective determination of differences in color between the patch repairs and the original concrete. Surface observations were also recorded using Dino-Lite field microscopes (20–220x), which aided in determining the causes of color variation.

Initial water absorption was determined by the contact sponge method. Contact sponge measurements were carried out on 5 patches per site, with 3 measurements per patch, and a further 3 measurements taken on the original concrete adjacent to each of the 5 patches. While the amount of water and contact time of the sponge must be determined on a case-by-case basis depending on the material, a time of 90 s with 4 ml of water was typically found to be suitable in this study. This provides a much quicker alternative to traditional water absorption techniques, such as the Karsten/RILEM tube.

Necessary invasive procedures included traditional electrochemical techniques, half-cell potential and linear polarization resistance, which were carried out on-site to assess the probability of reinforcement corrosion – one of the key causes of repair failure due to the expansion of corroding steel, which induces tensile stress in concrete, in turn causing cracking and spalling of the concrete cover. In addition to these, pull-off tests were conducted to assess the bond strength of the repair materials, and inspection openings allowed visual observations of the steel to determine its condition and whether any protective coatings had been applied (Table 3).

Table 3. Overview of samples to be removed.
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5.2 Laboratory Testing

Analyses of the samples removed from the sites were carried out by the authors in the laboratories of both LRMH and the GCI, with petrographic examinations of thin sections performed by specialists at WJE, and some additional specialist analyses subcontracted to external laboratories in France.

Analyses of historic concrete in the laboratory are complicated due to the limited amount of material that is often available and both the chemical and physical alterations which occur over time [19]. As a result, several of the test procedures had to be slightly modified to account for this, and an optimized system had to be developed to maximize the amount of data obtained from a minimal number and volume of samples. In most cases, the modification of the test procedure was limited to using samples that were below the size requirements or older than the age limit specified. In all tests where the relevant standard required the samples to be dried, this was done at no higher than 45 ℃ to prevent any further chemical or physical degradation or the test would be considered destructive. Where temperatures above this were absolutely necessary, these tests were performed last, and the sample was not used in any further testing.

Additional complications arose from the fact that two fundamentally different types of patch repair were encountered on the sites studied – ‘form-and-pour concrete’ and ‘hand-applied mortars’ – which resulted in significant differences in the depth of repairs and size of aggregates used. As such, two slightly different processes for analyzing samples had to be adopted to accommodate these differences.

An overview of the specific laboratory tests carried out and the relevant standards is given in Table 4.

Table 4. Overview of laboratory tests being carried out.
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6 Conclusion

The PEPS project goal is to improve repair of heritage concrete by producing practical guidance to help those repairing historic concrete in the process of selecting appropriate repair approaches, procedures, and materials. It is assumed this will be built on the basis of good concrete repair practice and craftsmanship. This will be informed by field and laboratory assessments of previous repairs that have been carried out on culturally significant concrete structures in England, France, and the USA. This paper provides an overview of the project methodology and outlines the project background, development, and assessment phases currently underway, while final results and conclusions will be presented in future publications.