Analytical stress-strain law of FRP confined masonry in compression: Literature review and design provisions

The use of Fibre Reinforced Polymer (FRP) wraps has become common in practical applications to retrofit existing columns with poor structural features. Wrapping the member with one or more FRP sheets makes it possible to induce confinement action and enhance strength and ductility. This application has been widely studied and adopted in reinforced concrete members for about twenty years, while its suitability to masonry columns and piers has been investigated during the last decade. The results of several studies were summarized in different design expressions for calculating effective confinement pressure, ultimate compressive stress and strain, and the overall trend of the stress-strain curve. Despite this huge amount of research, some results seem actually to be in contrast with the experimental results available in the literature, due to the different key variables arising in this application. This paper presents a comparative analysis of the main existing theoretical models to predict the stress-strain curve of FRP confined masonry in compression. Clay brick masonry is studied and criteria to obtain the strength increase are initially analyzed and compared with experimental results, evaluating the reliability of each expression. In the second part, models to calculate the overall constitutive law in compression are examined and the average error and coefficient of variation are calculated for each model.