This SP is the result of two technical sessions held during the 2017 ACI Spring Convention in Detroit, MI. Via presentations and the resulting collection of papers, it was the intention of the sponsoring committees (ACI Committees 549 and 562 together with Rilem TC 250) to bring to the attention of the technical community the progress being made on a new class of repair/strengthening materials for concrete and masonry structures. These materials are characterized by a cementitious matrix made of hydraulic or lime-based binders, which embeds reinforcement in the form of one or more fabrics also known as textiles. The great variability of fabric architectures (for example, cross sectional area, strand spacing, and fiber impregnation with organic resin) coupled with the types of material used (aramid, basalt, carbon, glass, polyparaphenylene benzobisoxazole (PBO) and coated ultra-high strength steel) makes the characterization, validation, and design of these systems rather challenging. Irrespective of the reinforcement type (synthetic or ultra-high strength steel), the impregnating mortar is applied by trowel or spray-up. It should also be noted that fabric reinforced cementitious matrix and steel reinforced grout, in particular, are very different from other repair technologies such as FRC (fiber reinforced concrete) and UHPC (Ultra High-Performance Concrete) in that they utilize continuous and oriented reinforcement. In a sense FRCM and SRG can be viewed as the modern evolution of ferrocement.

Repair and strengthening of concrete and masonry structures using fabric-reinforced cementitious matrix (FRCM) and streel-reinforced grout (SRG) are emerging technologies in the industry allowing engineers and contractors to effectively remove deficiencies, improve structural performance and prolong life of existing concrete or masonry structures. FRCM is a composite consisting of one or more layers of cement- or hydraulic-based matrix reinforced with dry fibers in the form of open fabric. Similarly, SRG consists of a matrix reinforced with cords of twisted micro steel wires woven to form a fabric (mesh). Acceptance Criteria AC434 was published to provide guidelines for the evaluation of FRCM/SRG strengthening of concrete and masonry structural elements because the building codes in the USA do not have requirements for testing and determination of structural capacity, reliability and serviceability of this class of composite technologies. AC434 establishes requirements for testing and calculations that can lead to the issuance of a product research reports as evidence of a product’s building code compliance. This paper summarizes and presents the key features of AC434 and its relationship to ACI committee 549.4R, the guide to design and construction of externally bonded FRCM and SRG systems for repair and strengthening concrete and masonry structures.

Unreinforced masonry construction are typically prone to brittle failures due to the nature of their constituent materials, and in many cases their strength is related to the shear strength of the primary walls. In regions affected by intense seismic events, the presence of heritage construction made by poor masonry, strongly enhance this type of vulnerability. The recent earthquakes that occurred in the past ten years in the Eurasian regions drew the attention of researchers and engineers in this sense, since entire cities formed by ancient masonry buildings were affected by extensive disasters and human losses. In order to find an effective solution to these important structural problems, composite materials in forms of Fiber Reinforced Polymers (FRP) were found to be effective and attractive in many cases, showing a good applicability both in reinforced concrete (RC) buildings and masonry construction. In this last case they are well accepted in modern masonry construction, but the use of epoxy resin as matrix and adhesive, combined with high performance fibers (i.e. carbon) have shown some limitations in the field of heritage masonry construction, in which the substrate is very poor. In this perspective two main issues obstacle an effective use of FRP: the mechanical compatibility and the saturation of the surface respect to transpiration of humidity. For these reasons in Europe there are some real applications of heritage masonry buildings in which the use of FRP is not welcome by the authorities that are asked to evaluate the strengthening proposals The problem of the mechanical compatibility is due to the differences between the stiffness and strength of the fibers (typically carbon) and the properties of the masonry substrate which may be 10-3 times those of the fibers. The problem of breathability is due to the fact that polymeric resins create an impermeable jacket which interrupts the cycle of humidity transpiration through the masonry. This may lead to a degradation of the masonry, in the long terms, because of the saline formations. For these reasons the use of a new generation of fibrous materials named as Fiber/Fabric Reinforced Cementitious Matrix (FRCM) was introduced, in order to use long reinforcing fibers into an inorganic matrix based on lime or cement mixes. These new materials have lower mechanical properties respect to FRP composites, but they show higher compatibility with poor masonry. The present manuscript illustrates the results of an extensive experimental campaign, in which masonry panels, made with limestone and poor hydraulic mortar, were tested under diagonal shear forces, until failure. The panels were tested in unreinforced configuration, then different FRCM reinforcement systems were applied and tested to compare the respective results. Both single wall and double wall panels were tested in order to represent different cases found in real applications. Totally thirty specimens were tested. The results, illustrated and discussed in the paper, show the significant increase in terms of mechanical properties that was measured in all cases of FRCMstrengthened walls. Due to the use of different mortars and fibrous systems some differences in terms of failure modes and damage at failure will be shown, even if it is reasonable to believe that for the type of masonry tested herein, all the strengthening methods resulted tremendously effective in terms of load capacity and energy dissipation, without showing a sudden brittle collapse.

An innovative option to reinforce existing masonry buildings or to increase the load-bearing capacity of new ones subjected to lateral loading caused by wind or earth pressure is to apply textile reinforcement in render on the masonry surface or in mortar in the bed-joint. This idea is based on the new material “textile reinforced concrete” (TRC). However, due to the specific characteristics of masonry compared to concrete, it is necessary to find suitable textiles for the use in combination with the masonry unit, mortar and render. To achieve a deeper knowledge on the performance of this composite material, an extensive experimental study is currently carried out. The main objectives are to identify suitable reinforcing materials as well as to describe the load-bearing and deformation behavior of textile reinforced masonry under lateral load and hence to derive a design model. Within this study tests are conducted on small-scale composite specimens under tensile, shear and flexural load, from which the needed parameters for the design model shall be defined. Basic part of these tests is the investigation of the bond behavior between textile reinforcement and mortar/rendering under tensile load, for which a new test method for TRC was implemented. From large scale tests on masonry walls subjected to lateral loading, the effectiveness of strengthening masonry externally with textile reinforced render will be assessed.

A shake table investigation was carried out on a full-scale U-shaped masonry assemblage to study the effectiveness of Steel Reinforced Grout (SRG) for the improvement of the out-of-plane seismic capacity of masonry walls. Natural accelerograms were applied with increasing scale factor up to failure. A first session of tests was performed on the unreinforced specimen, that collapsed by out-of-plane overturning. Steel tie bars were then installed to prevent overturning. In this case, severe damage developed due to bending. Finally, the wall was retrofitted with horizontal strips of Ultra High Tensile Strength Steel cords, externally bonded to the masonry with lime based mortar, and steel connectors. SRG led to a significant improvement of the seismic capacity, strongly limited damage development, and entailed small modifications of the dynamic properties of the specimen. Since the reinforcement had a thickness of less than 10mm, it is suitable for applications within the plaster layer during the maintenance work of the façades without modifying their appearance.