University of Miami
FRCM components: the matrix (a) and the fabric mesh (b).
Design and building code compliance
FRCM evolved from ferrocement, where the mortar matrix is reinforced with open meshes of continuous dry fibers. For this reason, ACI Committee 549, Thin Reinforced Cementitious Products and Ferrocement, undertook the task of developing design guidelines for strengthening of concrete and masonry with FRCM. Last year, the committee published ACI 549.4R13, Guide to Design and Construction of Externally Bonded FRCM Systems for Repair and Strengthening Concrete and Masonry Structures. This guide gives recommendations for the design, installation, and quality assurance of FRCM systems.
The acceptance criteria for evaluation and the characterization of the FRCM were established by the International Code Council Evaluation Service (ICC-ES) in the document titled, “Acceptance Criteria for Masonry and Concrete Strengthening Using Fiber-Reinforced Cementitious Matrix (FRCM) Composite Systems” (AC434 2013). An FRCM system that has been evaluated according to these acceptance criteria and that has received a product research report from ICC-ES can be accepted as building code-compliant by building officials.
Installation
FRCM systems are installed using the hand layup method. The installation procedure is simple and is similar to installing stucco or cement plaster with a metal mesh or a cementitious parge coat with a fiber mesh. Contractors typically do not need any special training to perform this work. Mixing and applying the cementitious matrix is similar to conventional hand-applied repair mortars used in concrete repairs. The surface preparation requires minimal surface roughening, leveling, and moistening to achieve the required saturated surface dry condition.
Mechanical Behavior and Failure Mechanisms
One difference between fiber-reinforced polymer (FRP) and fabric-reinforced cementitious matrix (FRCM) systems is the mechanical behavior in tension of the composite. FRP systems are linear elastic until failure, and their ultimate tensile strain is limited by the ultimate strain of the fibers. Read More
First, a thin layer of cementitious matrix (about 0.2 inch thick) is uniformly applied to the substrate (either trowel- or spray-applied). The fabric mesh is then pressed into the matrix. Finally, a second thin layer of cementitious matrix is applied on top of the mesh. Multiple layers of mesh can be applied following the same procedure—however many are specified.
The cementitious matrix in the first mesh layers does not need to be fully cured in order to apply subsequent layers. Primers, fillers, or protective coatings may be required for some substrates or matrices.
Temperature at the time of installation can affect the cementitious matrix, though. High temperatures (95° F to 130° F) may reduce the workability, while low temperatures (39° F to 43° F) may slow setting considerably. Because FRCM is inherently noncombustible, it can be used unprotected.
Final thoughts
FRCM systems are currently being introduced in the structural repair and rehabilitation industry as a new strengthening technology. Due to their superior performance at high temperatures, better compatibility with the substrate, and improved durability, FRCM systems are a good alternative to FRP systems. Now, with the availability of the ACI 549 guidelines, FRCM systems for concrete and masonry strengthening will become a more common tool for designers and contractors.
Antonio De Luca, Ph.D. staff engineer, and Gustavo Tumialan, Ph.D., P.E., senior project manager, are both with Simpson Gumpertz & Heger in Waltham, Mass.
