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  1. Experimental Evaluation of the Behaviour of Masonry Walls Strengthened Using Textile-Mortar System
  2. Seismic Strengthening of Bond-Critical Regions In Rectangular RC Columns Using FRP Wraps
  3. Seismic FRP Retrofit of Bond-Critical Regions in Circular RC Columns: Validation of Proposed Design Methods




Experimental Evaluation of the Behaviour of Masonry Walls Strengthened Using Textile-Mortar System
By: M. Harajli, & H. El Khatib – American University of Beirut, Lebanon
J. Tomas San-Jose – Labein Foundation, Spain

ABSTRACT: This work constitutes a part of a comprehensive research project (OPERHA) aimed at developing and testing a system for strengthening historical buildings in Europe and the Mediterranean Area. The system is composed of a combination of textile fiber mesh and mortar. In order to validate the proposed strengthening system, representative wall specimens were tested for their out-of-plane load resistance and behaviour under static and cyclic loading. Several parameters were investigated. These include the type of masonry wall (concrete block, sandstone, brick), the type of mortar (natural lime, cement-based), and type of textile (bitumen coated E-Glass, basalt, coated basalt). The cement mortar and the basalt textile were developed by Fyfe Europe specifically for the purpose of the OPERHA project. Companion specimens, strengthened using steel wire mesh, were also tested for comparison. All textile reinforced masonry (TRM) specimens failed in a combination of transverse detachment of the textile-mortar matrix due to the transverse or shear displacement of the blocks relative to each other, and combined transverse shear - tension fracturing of the textile fibers. The transverse separation of the textile-mortar matrix and the partial or total shearing off (tearing) of the fibers associated with the transverse displacement of the blocks during load application reduced the load capacity of the walls that would have been mobilized otherwise. Nevertheless, regardless of the mode of failure, it was found that reinforcing masonry walls with textile-mortar leads to a considerable increase in their out-of-plane load and deformation capacities under static loading. When subjected to cyclic loading, the TRM specimens exhibited a stable hysteresis response, low stiffness and strength degradation with number of cycles, and considerable energy absorption and dissipation capacities, leading in effect to a substantially improved seismic performance. The wire mesh reinforced (WRM) wall specimens mobilized the highest load capacity but were the least ductile when compared to the TRM specimens.

Keywords: retrofit; strengthening; fiber-reinforced polymers; masonry; walls; seismic.


 Procedure for TRM strengthening
Procedure for TRM strengthening

ACKNOWLEDGMENTS

This research constitutes a part of OPERHA project. The project is supported by the International Co-operation (INCO-MED) Program-European Commission, 6th Framework Program for Research, Technological Development and Demonstration, Contract No. 517765 (INCO). The authors are most grateful for this support and to the Faculty of Engineering and Architecture at the American University of Beirut (AUB) for providing the laboratory facilities.

CONCLUSIONS

Based on the results of this experimental investigation, the following conclusions can be drawn:

  1. When subjected to out-of-plane bending, textile reinforced masonry (TRM) walls fail in a combination of the following modes: excessive widening of one or two cracks, transverse detachment of the textile-mortar matrix due to the transverse displacement of the blocks relative to each other, and combined transverse shear - tension fracturing of the textile fibers. The specimens which developed one single crack, mainly those with stiff cement based mortars (TYFO), failed prematurely due to the shear fracturing of the fibers at the crack location leading to a lower load capacity of the wall when compared with their companion specimens.

  2. Irrespective of the type of stone, mortar, or textile used, in all TRM wall specimens, the progressive breaking of the fibers in a combined shear-tension mode due to excessive crack widening and possible transverse displacements of the masonry stones resulted in a lower load capacity of the walls than the one that would have been expected should the fibers mobilize their full tension capacity.

  3. Regardless of the causes of failure, reinforcing masonry walls with textile leads to a considerable increase of their flexural strength and lateral deformation capacity when subjected to static loading. The corresponding improvements were independent of the type of masonry stones used in this investigation.

  4. When subjected to cyclic loading, the TRM wall specimens exhibited a substantially increased strength, a stable hysteresis behaviour, low stiffness and strength degradation with number of cycles, and considerable energy absorption and dissipation capacity, leading to a substantially improved seismic performance when compared to unreinforced masonry walls (URM).

  5. While the WRM specimens with two layers of steel wire mesh acquired the highest strength gain compared to the other strengthened walls, they failed in a rather brittle mode by fracturing of the wires early in the response and after the lapse of only few cycles, leading to quick stiffness degradation and sudden loss in load capacity.

  6. Because of partial fracturing of the textile mesh when TYFO mortar was used, Bitumen coated – E Glass textile reinforced specimens using lime mortar (low strength or high strength) exhibited a more stable behaviour and slightly larger energy absorption and dissipation capacity than specimens prepared using TYFO cement mortar.

  7. The increase in load capacity for the TRM specimens varied between a minimum of 7 times and a maximum of 12 times the self-weight of the walls. Because the tensile capacity of the steel wire mesh was completely mobilized at flexural failure of the walls, the WRM specimens with two wire meshes attained a sizably high load capacity of about 20 times the self-weight of the wall.

  8. The lateral drift capacity of the walls relative to the wall height of the TRM specimens varied between a minimum of about 1.0% to a maximum of 3.5%. For the WRM, the displacement at peak load was less than 0.4% for specimens with two wire meshes and less than 0.2% for specimens with one single mesh.

  9. Among all the different strengthening systems evaluated in this investigation, the use of coated basalt textile with lime mortar for strengthening masonry walls leads to the most stable cyclic response of the walls, the highest energy absorption and dissipation capacity and the best combination of wall strength and ductility possible.
Table 7 - Summary of Wall Specimens Designation and Test Parameters Table 8 - Summary of Test Data Fig. 1 – Types of masonry stones used in the investigation Fig. 2 – Typical details of the wall specimens
Fig. 3- Samples of fiber textile and wire mesh Fig. 4 –Procedure for TRM strengthening Fig. 5 – Test setup Fig. 6 – Loading protocol for the cyclic test
Fig. 7 – Typical modes of failure Fig. 8a– Static load-displacement response of the strengthened block wall specimens Fig. 8b Static load-displacement response (continued) Fig. 8c – Static load-displacement response (continued)
Fig.9– Static load-displacement response of the strengthened brick wall specimens Fig. 10 –Cyclic load-displacement response of the TRM strengthened sandstone wall specimens Fig.10 (continued) Fig.10 (continued)
Fig.10 (continued) Fig. 11 –Cyclic load-displacement response of the TRM strengthened brick wall specimens Fig. 11 (Continued) Fig. 11 (Continued)
Fig. 11 (Continued) Fig. 12- Envelope load-deflection response of the TRM sandstone specimens in the cyclic test Fig. 13- Envelope load-deflection response of the TRM brick specimens in the cyclic test






Seismic Strengthening of Bond-Critical Regions in Rectangular RC Columns using FRP Wraps
By: M. H. Harajli and F. Dagher

Synopsis: The use of external fiber reinforced polymer (FRP) wraps for bond strengthening of spliced reinforcement in rectangular reinforced concrete (RC) columns and the consequent effect on the seismic response of the columns is experimentally investigated. Full scale unconfined and FRP confined column specimens with lap-spliced reinforcement at the base were tested. Also, companion columns with continuous reinforcement and with internal steel confinement to satisfy the ACI Building code requirements for regions of high seismic hazard (earthquake resistant columns) were tested for comparison. It was found that confining the spliced zone with FRP wraps increased the bond strength of the spliced bars, reduced the bond deterioration and pinching under cyclic loading, and increased the lateral load resistance and ductility of the columns. The corresponding improvements approached those of the earthquake resistant columns. The lateral strain developed in the FRP increased with decreasing ratio of concrete cover to splice diameter and with increasing area of FRP wraps. The predictions of bond strength of spliced bars in FRP confined concrete using available design expressions were in good agreement with the test data.

Keywords: bond strength, fiber reinforced polymer, cyclic load, seismic strengthening

ACKNOWLEDGMENTS

This research is supported by the Lebanese National Council for Scientific Research (NCSR) under grant No. 113010-32305. The authors are most grateful for that support and to the Faculty of Engineering and Architecture at the American University of Beirut (AUB) for providing the test facilities.


CONCLUSIONS

The effect of bond strengthening of spliced column reinforcement using FRP wraps on the seismic response of rectangular columns is experimentally investigated. Column specimens with continuous reinforcement and confined with closely spaced ties (earthquake resistant specimens) were also tested for comparison. The following conclusions can be drawn from this study:

  1. For the unconfined columns, the spliced column reinforcement suffered splitting bond failure before yielding, leading to considerable strength reduction and stiffness degradation of the columns in the first cycle following splitting.

  2. The modes of failure of the FRP confined specimens were completely or partially due to bond splitting of the spliced bars. Splitting failure caused large slip of the starter bars and concentration of deformation at the base of the columns. The failure mode of the earthquake resistant columns occurred by crushing of the confined concrete core and buckling of the reinforcement.

  3. Due to bond failure and cyclic bond degradation, the unconfined columns experienced dramatic loss in load resistance and considerable stiffness degradation in the first few cycles following bond failure. The strength and stiffness degradation of the unconfined specimens under cyclic loading were more pronounced for the columns with smaller ratios of concrete cover to bar diameter .

  4. Confining the splice zone with external FRP confinement restricted the growth of the splitting cracks leading to enhanced envelope bond resistance, increase in the steel stress that can be mobilized at bond failure (up to yield) and consequently increase in the envelope lateral load capacity of the columns.

  5. The maximum increase in lateral load capacity due to FRP confinement was limited by yielding of the spliced reinforcement. Generally, the increases grew larger with decreasing and with increasing area of FRP sheets. Compared to the companion unconfined columns, the increase in lateral load capacity due to one or two FRP wraps were 9% and 16% for the columns in series C14, 14% and 11% for series C16, and 44% and 60% for series C20.

  6. Confining the concrete with FRP wraps reduced the bond degradation of the spliced bars leading to less reduction in load resistance with increasing drift ratios, better energy absorption and dissipation capacities, less pinching and consequently improved seismic response. With the exception of the FRP confined specimens in series C20 which have the lowest , the seismic performance of the FRP confined specimens in series C14 (C14FP1/FP2) and C16 (C16FP1/FP2) were practically similar to their companion earthquake resistant specimens.

  7. The lateral strain developed in the FRP sheets at the envelope peak lateral load varied between a minimum of 100 to a maximum of 1300 which are considerably lower than the fracture strain of the FRP sheets. The corresponding FRP strains decreased with increasing area of FRP sheets and increased with decreasing ratio of concrete cover to bar diameter.

  8. For the FRP confined specimens, most of the lateral drift is attributed to the slip of the starter bars and associated fixed end rotation at the base of the column. In other words, the contribution of the inelastic deformation due to the penetration of reinforcement yield into the plastic hinge zone was relatively small.

  9. FRP bond design expressions (Eq. 3) can be used accurately and efficiently for estimating the thickness of FRP jacket required for strengthening of bond-critical regions in RC members.
Table 1 – Summary of test parameters and relevant test results Table 2 – Comparison of predicted bond/steel stress with test results Fig. 1 – Column specimen used in the experimental investigation (1 cm = 0.4 in.) Fig. 2 – Test setup (1cm = 0.4 in.)
Fig. 3 – Lateral load history of the column specimens Fig. 4 – Typical failure mode of the unconfined and earthquake resistant columns Fig. 5 – Failure mode of the FRP confined columns (after stripping the FRP sheets) Fig. 6 – Lateral load – drift ratio response of the columns in series C14 (1 kN = 0.224 kip)
Fig. 7 – Lateral load – drift ratio response of the columns in series C16 (1 kN = 0.224 kip) Fig. 8 – Lateral load – drift ratio response of the columns in series C20 (1 kN = 0.224 kip) Fig. 9 – Energy absorbed and dissipated (1 kN-m = 8.8 k-in) Fig. 10 – Typical response of moment - “slip” of outermost column bars(1 kN-m = 8.8 k-in, 1 mm = 0.04 in)
Fig. 11 – Typical response of FRP strain with lateral load (1 kN = 0.224 kip) Fig. 12 – Contribution of and to the drift ratios of the column specimens






Seismic FRP Retrofit of Bond-Critical Regions in Circular RC Columns: Validation of Proposed Design Methods
By: Mohamed H. Harajli and Zeinab Khalil

ABSTRACT: A general expression was derived (Harajli, 2005) for designing the thickness of external FRP jacket required for bond strengthening of spliced reinforcement within the critical hinging region of reinforced concrete (RC) columns. The reliability of the expression has been verified against recent experimental data of rectangular column sections (Harajli and Dagher, 2008). The objective of the current investigation is to evaluate the application of the corresponding design expression for circular column sections vis-à-vis other design methods proposed in the literature. Full scale circular columns with spliced reinforcement were tested under lateral load reversals. It was found that the mechanism by which the FRP confinement improves the bond strength of spliced bars in circular columns is similar to that in rectangular columns. All the unconfined columns suffered premature splitting bond failure leading to almost total strength and stiffness degradation. Confining the splice zone with external FRP jackets, within the range of thicknesses predicted by the proposed design expression under evaluation, lead to a substantial improvement in seismic performance.

Keywords: bond strength, fiber reinforced polymer, cyclic load, seismic retrofit.


 Typical Failure mode of the specimens of the current study
Typical Failure mode of the specimens of the current study

ACKNOWLEDGMENTS

This research is supported by the Lebanese National Council for Scientific Research (LNCSR) under grant No. 113010-32305. The authors are most grateful for that support and for the American University of Beirut (AUB) for providing the test facilities.


CONCLUSIONS

Based on the results of this study the following conclusions can be drawn:

  1. Without adequate splice length and internal confinement by steel ties, the spliced reinforcement at the critical hinging region of columns (circular in this particular study) are likely to experience bond splitting failure before mobilization of full flexural strength. Splitting failure results in a sudden and almost complete load and stiffness degradation.

  2. The use of external FRP confinement improved the bond strength, increased the splice stress up to yield and helped the columns in mobilizing their full flexural capacity. The increases in maximum load resistance relative to the unconfined columns varied in this particular test between a minimum of 5% to a maximum of 38% for the columns confined with one FRP layer, and between 11% and 40% for the columns confined with two layers.

  3. The FRP confined columns experienced more stable hysteresis response, lower strength and stiffness degradation, and larger energy absorption and dissipation capacities when compared to the unconfined columns.

  4. The average stress developed in the FRP jacket varied between a minimum of 2% to a maximum of 15% of the fracture tensile strength of the FRP sheets. The corresponding FRP stress decreased with increasing thickness of the FRP jacket.

  5. The improvement in bond strength due to external FRP confinement is not sensitive to the shape of the column section (rectangular, circular) as does the axial load capacity of the columns.

  6. Compared to other proposed design methods, Eq. (13) predicts a realistic and reasonably conservative jacket thickness, it accounts for all parameters that have long been known to influence the splice strength of steel bars in tension, and most importantly it is applicable for both rectangular and circular sections.
Table 1 – Summary of Test Parameters Table 2 – Estimated jacket thickness for bond strengthening Table 3 – Summary of relevant test results Table 4 – Performance Indicators
Fig. 1 – Splice Parameters used in Eqs. (3), (4) and (13) Fig. 3 – Details of the column specimens (1mm = 0.04 in.) Fig. 4 – Cyclic load history (1mm = 0.04 in.) Fig. 6 – Typical failure mode of the specimens of the current study
Fig. 7 - Typical failure mode of the pilot specimens of Harajli (2008) Fig. 9 – Moment-displacement response of the columns in series Fig.10 – Moment-displacement response of the columns in Series II (1kN-m = 8.8 k-in.; 1mm = 0.04 in.) Fig.11 – Moment-displacement response of the columns in Series III (1kN-m = 8.8 k-in.; 1mm = 0.04 in.)
Fig.12 – Moment-displacement response of the pilot specimens of Harajli (2008) (1kN-m = 8.8 k-in.; 1mm = 0.04 in.) Fig.13 – Typical cysclic response of FRP strain (1kN-m = 8.8 k-in.)