Wire Ropes and CFRP Strips to Provide Masonry Walls with Out-Of-Plane Strengthening
- N is the normal force that develops as a reaction to the weight force P, exerted by the body at rest in Figure 2 (body 1): N is equal and opposite to P;
- A is the frictional force that develops as a reaction to the shear force T, exerted by the hanging body in Figure 2 (body 2): A is equal and opposite to T as long as body 1 is at rest;
- Φ is the resultant of the active forces, N and A, and is applied to body 1;
- F is the resultant of the reactive forces, P and T, and is applied to the support plane.
2. The Straps/Strips Combined Technique
- It consists of a three-dimensional continuous strengthening system that leads to a box-type behavior of the retrofitted building (Figure 6).
- It establishes good transversal connections, which are particularly useful in cases of multi-layer masonry walls with weak connections between the vertical layers.
- It allows the straps to form closed loops that cross the thickness of the masonry wall.
- It is an active reinforcement technique, since the fastening system provides a pre-tension to the straps. Thanks to the pre-tension, the straps do not require any damage to begin to post-compress the masonry enclosed within them.
- It makes use of special protective elements at the loop corners, to avoid damage due to concentration of stresses at the corners.
- It is easily concealable under a plaster layer because the thicknesses of the straps and the protective elements are of the same order of magnitude as the thickness of the plaster. Therefore, from an aesthetic point of view it is minimally invasive.
- It overcomes the irregularities of the walls easily, making it possible to strengthen even ornamented or complex-shaped walls.
- It minimizes the increase in the total weight of the structure, making it possible to avoid further attraction of seismic forces.
- It continues to wrap the wall even after masonry crushing, allowing the damaged building not to collapse. This high degree of ductility (Figure 7) allows the combined technique to survive structural damage, acting as both a reinforcement system and a protection device.
2.1. First Combined Technique: Straps Made of Steel Ribbons
2.2. Second Combined Technique: Straps Made of Steel Wire Ropes
3. Experimental Program
3.1. Bricks and Mortar
3.2. Protective Funnel-Shaped Plates and Rounded Angles
3.3. Mechanical Characterization of the Steel Wire Ropes
3.4. Mechanical Characterization of the Jionts
- 1 ferrule (Specimen 1, Figure 26);
- 2 ferrules in succession (Specimen 2, Figure 27);
- 1 clip (Specimen 3, Figure 28);
- 2 clips in succession (Specimen 4, Figure 29);
- 1 ferrule and 2 clips, in succession, starting from the Flemish eye (Specimen 5, Figure 30);
- 1 ferrule, 1 clip, a second ferrule, and a second clip, in succession, starting from the Flemish eye (Specimen 6, Figure 31);
- The load of Specimen 3 increased linearly up to a value of about 2.4 kN. Then, the slope of the load/displacement diagram decreased due to the yield behavior of the steel wires and the load continued to increase monotonically up to a value of about 2.9 kN. At this point, the specimen suffered a load drop due to the squashing of the Flemish eyes. Once the squashing of the Flemish eyes was over, the load started to rise again, up to its maximum value. Afterwards, the fraying of the steel wires quickly led the specimen to failure. It is worth noting that the fraying started from one of the two clips used to close the Flemish eyes (Figure 34). In fact, the eccentricity of the load supplied to the steel wire rope—due to the non-perfect coaxiality between the turnbuckle and the “live” side—caused the flat bearing seat of the clip to rotate, until its edge came into direct contact with the steel wires. This caused the pinching and abrading of the steel wires and, consequently, their failures in rapid succession.
- The linear behavior of the load/displacement diagram of Specimen 4 terminated for a load value of about 3.4 kN, which corresponds to approximately 142% of the load at the end of the linear branch of Specimen 1. The yield behavior of the steel wires and squashing of the thimbles took place simultaneously from this moment forward, decreasing the slope of the load/displacement diagram but without causing any load drop. The slope of the linear branch is greater than the slope of the linear branch for Specimen 3, which means that Specimen 4 is stiffer than Specimen 3. Actually, the stiffness of Specimen 4 is comparable to the stiffness of the steel wire rope without a joint. The yield behavior and squashing processes terminated with the fraying of the steel wires, which is responsible for the “step behavior” of the last part of the load/displacement diagram: Each load drop in this final part is the consequence of the failure of one or more steel wires. The fraying started from a clip, the first from the Flemish eye (Figure 35). As for Specimen 3, the cause for this lies in the non-perfect coaxiality between the turnbuckle and the “live” end (Figure 25). However, the second clip—that forces the part of the “dead” side between the two clips to bear load—partially eliminates the torsion of the first clip, delaying the fraying. This could also be the reason for the greater stiffness and maximum load of Specimen 4.
- The purpose of the fifth fastening scheme was to eliminate the torsion of the first clip of the fourth fastening scheme, that is, the clip closest to the Flemish eye. In other words, the function of the ferrule was to center the load on the two clips (Figure 36). Specimen 5 did not actually fray near the first clip (Figure 37): It frayed near the second clip. The improved load centering allowed the specimen to withstand a higher ultimate load, comparable to the ultimate load of the steel wire rope without a joint. However, the ferrule caused an excessive deformation of the Flemish eyes, as for Specimens 1 and 2. This greatly reduced the stiffness of the specimen.
- The sixth fastening scheme introduces an additional ferrule between the two clips to eliminate even the rotation of the second clip, with the aim of preventing the specimen from fraying near both clips. The second ferrule actually further improved the load centering, eliminating fraying near both clips. However, this concentrated the deformation phenomena on the thimble that twisted, cutting off the steel wires (Figure 38). The twisting of the thimble occurred due to the excessive squashing of the Flemish eye. In fact, once the two ends of the thimble come into contact, the further squashing of the Flemish eye is possible only by forcing the two ends of the thimble to slide one over the other in the direction orthogonal to the load. This causes the twisting borders of the thimble to cut the steel wires. Even in Specimen 5 the excessive deformation of the Flemish eye caused a twist of the thimble (Figure 35), but this did not lead to damage to the steel wires. Lastly, the concentration of the deformations on the Flemish eyes greatly reduced the stiffness of Specimen 6, as for Specimen 5.
3.5. Three-Point Bending Flexural Test on a Masonry Specimen
3.5.1. Preparation of the Specimen and Test Setup
- Removal of the specimen from the testing machine;
- Removal of all the straps from the specimen;
- Overturning in the vertical configuration of the two parts resulting from the failure;
- Restoration of the cavity for the passage of the straps near the disconnected cross-section, inserting a steel tube of the same external diameter as the drilled holes;
- Restoration of the disconnected mortar bed joint (Figure 41a), after lifting and holding of the upper part of the specimen in position with a girder crane;
- Maturing of the mortar on the restored mortar bed joint;
- Application of longitudinal CFRP strips (50 mm × 1.2 mm), on both main faces of the restored specimen (Figure 41b);
- Application of the straps on the restored specimen (Figure 42).
3.5.2. Results and Discussion
5. Future Developments
- In order to avoid that the breakage of one or more funnel-shaped elements interrupts the chain of the longitudinal straps, it may be useful to re-design the 3D-printed elements or use a more resistant material for the protective elements.
- In order to avoid that the geometric effects due to the relative rotations around the inner hinge frustrate the action of the longitudinal straps on the transverse straps, it may be useful to decrease the length of the loops near the middle cross-section, where the inner hinge has the maximum probability of localization.
- In order to avoid that excessive post-delamination fraying leads to the collapse of the structural element, it may be useful to use longitudinal stainless steel ribbons in addition to the longitudinal steel wire ropes, at least near the middle cross-section. Since the function of the additional steel ribbons is only to safeguard life, they do not need a pre-tension.
- In order to avoid excessive load drops and dangerous release of energy at the time of delamination—which are peculiarities of epoxy resins—it may be useful to replace the epoxy resin (organic) with a mortar matrix (inorganic).
- First ascending branch: The specimen remains un-cracked.
- Horizontal branch (constant stress at increasing strains): Multiple cracks develop in the mortar, after the first cracking. During this phase, the area of the resistant cross-section of the mortar decreases progressively [75,76,77,78,79,80] due to the gradual development of the crack pattern that causes a progressive transfer of stress from the mortar to the fibers of the open-mesh textile.
- Second ascending branch, characterized by a reduced slope with respect to that of the first branch: The crack pattern has reached its maximum development and the residual stiffness is due to the fibers of the textile, up to the break.
Conflicts of Interest
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|Specimen||Dimensions [mm]||Weight [g]||Breaking Load [N]||Compressive Strength [N/mm2]||Normalized Compressive Strength [N/mm2]|
|PA1||55 × 54 × 55||296.10||116,436||39.632||34.480|
|PA2||57 × 57 × 55||317.80||165,730||50.911||44.293|
|PB1||55 × 53 × 55||297.50||146,733||49.624||43.173|
|PB2||56 × 55 × 57||319.20||142,681||46.099||40.106|
|PC1||56 × 53 × 56||310.50||144,933||47.777||41.566|
|PC2||56 × 55 × 56||317.10||149,422||48.148||41.888|
|Specimens of the Flexural Tests||Dimensions [mm]||Weight [g]||Breaking Load in Bending [N]||Flexural Strength [N/mm2]||Specimens of the Compression Tests||Breaking Load in Compression [N]||Compressive Strength [N/mm2]|
|P1||40 × 40 × 160||466.42||1758||4.120||P1A||30,530||19.080|
|P2||40 × 40 × 160||469.81||1838||4.310||P2A||30,980||19.360|
|P3||40 × 40 × 160||470.42||1443||3.380||P3A||27,500||17.190|
|P4||40 × 40 × 160||459.63||1885||4.420||P4A||34,544||21.590|
|P5||40 × 40 × 160||463.81||1990||4.660||P5A||33,880||21.180|
|P6||40 × 40 × 160||462.01||1598||3.750||P6A||30,400||19.000|
|Specimen||Maximum Load [kN]|
|Without a joint||5.186|
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Ferretti, E. Wire Ropes and CFRP Strips to Provide Masonry Walls with Out-Of-Plane Strengthening. Materials 2019, 12, 2712. https://doi.org/10.3390/ma12172712
Ferretti E. Wire Ropes and CFRP Strips to Provide Masonry Walls with Out-Of-Plane Strengthening. Materials. 2019; 12(17):2712. https://doi.org/10.3390/ma12172712Chicago/Turabian Style
Ferretti, Elena. 2019. "Wire Ropes and CFRP Strips to Provide Masonry Walls with Out-Of-Plane Strengthening" Materials 12, no. 17: 2712. https://doi.org/10.3390/ma12172712