Flexural and Shear Strengthening of URM Walls with FRP Systems

1- FRP Flexural Strengthening

The structural performance of URM walls subject to out-of-plane loading is highly dependent on the tensile strength of masonry and magnitude of the in-plane (axial) compressive forces acting on the wall. URM walls can span in one or two directions depending on the boundary conditions. When subject to out-of-plane loads, initial cracks form at or near the ends of the walls. These cracks are typically located at the base of the wall. After the initial cracking occurs, the wall behaves as a simply-supported element until further cracks develop about mid-height. These cracks typically extend along the mortar bed joints and lead to failure of the wall.

FRP systems can significantly increase the flexural strength of URM walls. Because of the reversed stresses caused by out-of-plane loads due to high wind pressures (windward and leeward) and earthquake excitations, FRP flexural strengthening of URM walls require installation of FRP systems on both wall faces.

FRP composite systems are very effective for strengthening of URM walls that can behave as simply- supported elements, or very nearly so, such as the case of slender walls. FRP composites can also be effective for strengthening of “stocky” walls provided that these are not built between rigid supports (reinforced concrete or steel beams), or that floor or roof structure framing into the wall create a simply-supported boundary conditions (e.g. floor framing consisting of metal deck supported by open-web joists).

Walls with low h/t ratios, typically less than 12, and built between rigid supports can develop arching action. Basically, as a stocky wall bends due to the out-of plane loads, the wall is restrained from rotation at the supports. This action induces an inplane compressive force, which depending on the stiffness of the supports can significantly increase the wall resistance to out-of-plane loads. Walls able to develop arching do not typically require to be strengthened, and therefore are not addressed by ACI 440.7R-10.

2- FRP Shear Strengthening

The structural behavior of URM walls subject to in-plane loads depends on several parameters including geometry such as dimensions and thickness, construction type such as bond pattern, and type and strength of masonry units and mortar; axial loading, and boundary conditions. Walls under in-plane loads can have the following four modes of failure: diagonal tension, bed-joint sliding, toe crushing and rocking. The first three modes of failure are force-controlled failures, whereas rocking is a deformation-controlled failure. Deformation-controlled failures are more ductile than force-controlled failures. A URM wall failing due to rocking (deformation-controlled failure) has the ability to absorb more energy after initial cracking than URM walls failing due to force-controlled failures. Thus, any approach for strengthening of URM walls should be aimed at precluding the occurrence of force-controlled failures, or in its defect it should provide ductility to the wall so this is able to significantly before failure.

FRP composite systems are effective to increase the shear strength of URM walls by preventing or delaying the occurrence of bed-joint sliding failure, if the cracks are oriented along the wall diagonal (i.e. stepped cracking), and diagonal tension failure. FRP composites can also provide pseudo-ductility to URM walls.

There is limited research on infill walls (walls built within reinforced concrete or steel frames) strengthened with FRP; and therefore, strengthening of infill walls is not covered by ACI 440.7R-10. In general the in-plane behavior of infill walls is dependent of the relative stiffness ratio between the masonry and the surrounding frame, aspect ratio of the wall, and occurrence of premature failures such as crushing of corners in contact with the frame.

3- Detailing and Construction Considerations

Detailing and construction considerations for FRP overlays include the following.

• Surface preparation: The surface of the masonry needs to be cleaned of loose material and finishes that prevent proper adhesion. Sandblasting of the masonry is not usually necessary; a wire brush is used instead. (See the following figure).
• Complete overlay vs. strips: Both in research and in practice, both complete overlays over the full surface of the wall and use of strips are found. Vertical strips are used when only improvement to out-of-plane resistance is needed. Diagonal strips have been used to resist diagonal tension stresses from in-plane shear.
• One side or both sides of wall: Applying fiber to both sides of the wall improves performance, particularly for out-of-plane resistance.
• Continuity of fiber at top and bottom: providing load transfer with the fiber can challenge, particularly at floor-to-wall interfaces. If the fiber is being used to resist out-of-plane loads and is transferring these loads back into the floor diaphragm, special details may be needed to turn the vertical fiber overlay into the horizontal diaphragm. Fiber cannot be bent at 90 degrees rather a radius is needed. Sometimes, steel reinforcing plates are used to stiffen the turns. If the fiber is being used to transfer in-plane loads from one story to the next, continuity past the floor is needed and will require special details as does shear transfer out of the diaphragm into the wall