FRCM/TRM systems
Textile Reinforced Mortar (TRM) composites are an innovative and particularly promising solution for the repair and strengthening of structures. They comprise a high strength textile, which is applied to the surface of structural members by a mortar matrix. Textiles are made of either continuous fibres of carbon, glass, basalt, and PBO, arranged in the form of open meshes, or steel cords or ropes. Cement, lime or geopolymer mortar matrices (possibly enriched with short fibres and/or polymeric additives) can be used. Since many TRM systems comprise a cementitious mortar, the acronym FRCM (Fabric Reinforced Cementitious Matrix) is also widely used in the scientific literature. Differently, the acronym SRG (Steel Reinforced Grout) is generally adopted for the systems with steel textiles.
The idea of embedding a high strength mesh into a mortar matrix dates back to the late 1840’s, at the origin of reinforced concrete. Josef-Louis Lambot proposed the use of iron wires set into a cement mortar for the construction of boats, water tanks, etc. Nearly one century later, Pier Luigi Nervi effectively employed the ferrocemento, a sort of reinforced mortar consisting of a cement plaster applied over one or more layers of a thin steel net, to build thin, hard shell structures for roofs or water tanks. His patent was registered in 1943 during the self-sufficient period in Italy and this technique was extensively used for a number of well-known engineering works. More recently, a similar technology, known as Textile Reinforced Concrete (TRC), was developed. TRC systems make use of alkali resistant glass, carbon or aramid fabrics, combined with high performance finely grained cement concrete and, thanks to their high strength in both compression and tension, are precast to build thin structural elements, such as permanent formworks, façades and pedestrian bridges.
In the last decade, TRM systems have been proposed and developed as externally bonded reinforcements. They offer the same advantages of Fibre Reinforced Polymers (FRPs), such as high strength-to-weight ratio, relatively fast and easy installation, and versatility, making them an effective, practical and cost-efficient solution for the upgrade, seismic retrofitting, and repair of structures. The inorganic matrix employed in TRMs in place of the epoxy resins of FRPs provides better fire resistance, easier and faster installation on uneven or wet substrates, and no risks for the workers caused by toxic volatile compounds. The use of lime-based mortars ensures the additional benefits (vapour permeability and physical/chemical compatibility with masonry substrates) required for applications to historic structures.
In order to determine the mechanical properties of TRM composites and derive acceptance and design parameters, experimental tests are carried out in the laboratory, including direct tensile tests on bare textile specimens and composite coupons, and shear bond tests, with either single-lap or double-lap schemes.
Direct tensile tests provide the stress-strain response curve, which is generally characterized by three response stages (uncracked, crack development, and cracked), the stress and strain values of the transition points between one stage and the following one, and those of the peak, the modules of elasticity of the three stages, and the crack pattern.
TRM-to-substrate shear bond tests provide the stress-slip response and the corresponding failure mode. Detachment may occur by cohesive debonding within the substrate, by detachment at the textile-to-matrix or mortar-to-substrate interface, by slippage of the textile within the matrix, or, finally, by tensile rupture of the unbonded textile.