Abstract

Strategic reuse of demounted concrete elements in new buildings may be one of the solutions that will support the transition to circular construction. To ensure wider application of concrete reuse, RISE developed a methodology for the assessment of the structural condition of existing buildings, and the selection of elements suitable for reuse, including guidelines for their disassembly, storage, and installation. However, one of the main obstacles for wide application of concrete reuse is the uncertainty concerning the remaining service-life of concrete elements and evaluation of quality over the future service-life in a new building. This paper describes a methodology for material and structural assessments which combine non-destructive, on-site testing with traditional laboratory tests of samples extracted from the structures. The results are intended to support the decision-making process on reuse and give a technical basis for the design of new buildings. Great consideration is put on various deterioration mechanisms for concrete and steel corrosion affecting structural condition of housing and office buildings. To assess the impact of degradation processes, theoretical models are considered, while the remaining service life is estimated by means of a simplified approach that provides the basis for evaluation of likelihood and severity of consequences entailed by material degradation on the structural performance. The proposed approach was validated on the results from three pilot projects, where real buildings in Stockholm and Uppsala, Sweden, were reused or prepared for reuse to different extent. The analysed buildings had different functions (housing, office, parking) and structures (prefabricated elements and in-situ casted concrete), being representative for Swedish building stock. One of the buildings has been already dissembled and the prefabricated, where prestressed hollow-core slabs have been successfully reused for a new office building construction. Based on these experiences, a simple classification system for quality of concrete elements for reuse was proposed with three main parameters, namely calculation of remaining service-life, extent of cracking and the target exposure class. The proposed system is not complete and must be further validated for various types of elements and structures by wider group of market actors.

Introduction

When a structural element is dismantled from a building, it loses its legal status as a building part and becomes waste. The social and economic perception of waste has evolved in the last decades and its potential as a resource for reuse, recycling, or energy production (depending on its material properties, ease to reuse or recycle, financial, and environmental value) has gained considerable attention. Reuse is the second step in the waste pyramid after prolongation of the service-life due to preservation of the embodied value and, as such, it has minimal impact on the environment. To accelerate the transition from linear to circular economy, it is mandatory to ensure the quality of the concrete elements for reuse in terms of mechanical performance, as well as fire-safety, acoustic or thermal properties, and longevity required by building standards. The most logical approach to achieve that is to apply existing standards and regulations for new structures. Mechanical properties or other functional requirements can be assessed with the same experimental methods used for new materials or structures. However, this approach is not applicable for durability, which is normally ensured by design than evaluated by testing on building and element levels. Moreover, most of current regulations and standards are formally restricted to production of new materials and elements, and do not apply to the assessment of existing ones. Therefore, current building norms and standards have to be updated to meet the challenges posed by circular economy.

Conclusions and future work

Durability and assessment of remaining service-life are main technical hinders in reuse of structural concrete elements due to the standards defining it by design approaches rather than performance-based regulations. The updates of Eurocode 2 may bring new opportunities in these terms.

The performed literature review of degradation models for concrete and steel corrosion indicated that cracking and relative humidity have major influence for the degradation and consequences for service-life and should be handled with special attention.

Prefabricated concrete elements from the 70′ and 80′ in Sweden have usually higher quality and resistance against carbonation than in-situ casted elements, thus being more attractive candidates for reuse in new buildings. Additionally, prestressed elements tend to be uncracked, which increases their durability and makes disassembly much easier.