When concrete or masonry structures are found to have insufficient strength, externally bonded reinforcement has proven to be a good option. Over the last decade, externally bonded fiber-reinforced polymer (FRP) composite systems have been the best alternative to conventional methods for strengthening and retrofit. But despite its effectiveness in a strengthening system, the epoxy matrix of FRP systems is susceptible to failure in high-temperature or aggressive environments.

Fabric-reinforced cementitious matrix (FRCM) systems are an emerging and expanding tool that could become a viable alternative to FRP systems. FRCM systems developed as an evolution of FRP with the epoxy matrix being replaced with a cementitious matrix. Because of the cementitious matrix, FRCM systems can offer better performance under elevated temperatures, humidity, and ultraviolet radiation than FRP systems. The cementitious matrix can also contribute to the durability of the strengthening system in moist and chemically aggressive environments, or in applications where vapor permeability of the strengthening system is required.

Characteristics of FRCM

FRCM systems are externally bonded strengthening systems consisting of a fabric mesh embedded in a cementitious matrix. Similar to FRP systems, the fabric in an FRCM system carries the stresses transferred from the concrete or masonry substrate through the cementitious matrix. The fabric commonly consists of meshes made of carbon, alkali-resistant glass, basalt, or polymeric fibers (such as polyparaphenylene benzobisoxazole, PBO), or hybrid systems.

The cementitious matrix used in FRCM is designed to be non-shrinkable and workable so it can be easily trowelled and will penetrate the fabric mesh openings. Both hydraulic and non-hydraulic cements are used. Finely graded sands (grain size smaller than 0.02 inch) help improve the workability of the fresh mix and the impregnation of the fabric mesh. The water-to-mortar ratio by weight typically ranges between 15% and 25%. The mortar mix can include chopped fibers to reduce plastic shrinkage cracking. Organic compounds can also be used to control the hardening rate and the workability of the fresh mix, to improve the bond to the fabric mesh, and to enhance the mechanical properties. The fiber content is generally limited to less than 5% by weight of cement to obtain a fireproof matrix.

FRCM versus FRP systems

Essentially, both FRCM and FRP systems can be used in strengthening applications, such as flexural and shear strengthening of concrete and masonry members, and to provide confinement to concrete columns. Because of the different chemical composition of the matrices, though, the two composite systems exhibit some important differences.

One obvious difference is the fabrics used. In FRP systems, the polymer matrix is basically a fluid that ensures full impregnation (wetting) of the fiber sheets; in FRCM, the cementitious matrix is unable to penetrate the finely woven strands of fiber sheets. So, to ensure transfer of stresses from the matrix to the fiber, the fiber sheets that are normally used in FRP are replaced with open fabric meshes in FRCM. The mesh-type fabrics ensure that the fibers are embedded in the cementitious matrix.

For masonry strengthening, a polymer matrix can act as a barrier that can trap moisture within the masonry and lead to debonding of the strengthening material. A cementitious matrix of FRCM is more compatible with masonry substrates than a polymer matrix because of the porosity and vapor permeability, which allow moisture to migrate through the masonry.

Lastly, a fundamental difference is the resistance to elevated temperatures. FRCM systems perform much better than FRP systems under elevated temperatures due to their inherent noncombustibility. The cementitious matrix in FRCM will not undergo severe degradation and will preserve its mechanical properties at elevated temperatures. With FRP systems, on the other hand, the maximum service temperature is limited by the glass transition temperature of the polymer matrix, which typically ranges from 140° F to 180° F.

Applications in structural strengthening

Generally, the structural performance of reinforced concrete or masonry elements strengthened with FRCM systems is comparable to similar elements strengthened with FRP systems. FRCM systems can be used for flexural and shear strengthening of concrete and masonry elements, or to provide confinement to concrete columns. In reinforced concrete beams and flat slabs, FRCM systems can be used to increase the flexural strength, by placing it on the tension zone with the primary direction of the fabric in the direction of the tensile stresses (along the span of the beam or slab).

FRCM systems can also be used as shear reinforcement in beams. This is accomplished by fully wrapping or U wrapping the beams at the critical sections with the primary direction of the fabric in the transverse direction. Strengthening of columns with FRCM systems requires wrapping around the column, with the primary direction of the fabric in the direction transverse to the axial load. Strengthening of masonry walls is accomplished using a bi-directional fabric mesh to provide flexural and shear resistance due to the combined action of out-of-plane and in-plane loads.