Unreinforced masonry (URM) structures represent a significant portion of existing historical structures around the world. Recent earthquakes have shown the need for seismic retrofitting for URM structures. Various types of strengthening methods have been used for URM structures. In particular, a strengthening technique using externally bonded (EB) fiber reinforced polymer (FRP) composites has attracted engineers since EB FRP materials effectively enhance the shear strength of URM walls with negligible change to cross-sectional area and weight of the walls. Research has been extensively conducted to determine characteristics of URM walls strengthened with EB FRP materials. However, it is still difficult to determine an appropriate retrofitting level due to the complexity of mechanical behavior of strengthened URM walls. In this study, in-plane behavior under lateral loading was, therefore, investigated on a full-scale nonstrengthened URM wall and URM walls retrofitted with two different FRP materials: carbon (CFRP) and hybrid (HFRP) sheets. The test results indicated that both FRP composites were effective in increasing shear strength in comparison with the control specimen. However, better performance was obtained with HFRP compared to CFRP. In addition, an equation for estimating effective strain was proposed, and the theoretical results were in good agreement with the experimental ones.
In general, masonry structures are considered to be optimal for low-rise structures in many countries due to easy and fast construction, abundant material, and no special technique for construction. Although masonry structures are strong enough to resist large compressive stress, these structures have poor ductility and thus are vulnerable under dynamic loading such as earthquake. For instance, unreinforced masonry (URM) structures have been prohibited for public structures including schools since the Long Beach earthquake, of 1993, in California, USA. Even though structures were constructed to meet the high level seismic requirements of New Zealand, many of those were severely damaged and collapsed due to consecutive earthquakes, in 2010 and 2011. This resulted in a great deal of humans and property losses [
Recently, the risk of earthquake events has increased in many countries that have a low probability of earthquake occurrences. For example, the number of earthquakes in South Korea increased by 54.3% in the recent three years. As with many countries, there are many masonry structures constructed without meeting current seismic requirements and strengthening, especially in South Korea where there is even an obvious probability of earthquake. More specifically, low-rise masonry structures in South Korea are 30% of all domestic structures, over 40% of all domestic houses, and substantially vulnerable to earthquake [
Due to the aforementioned reasons, research on strengthening URM walls has been extremely conducted. FEMA 356 suggests design guidelines of URM walls to resist lateral force and evaluation of existing structures on the basis of existing research results. In addition, FEMA 356 [
In particular, research on URM walls retrofitted with externally bonded (EB) fiber reinforced polymer (FRP) composite materials has been substantially conducted due to the well-known advantages of FRP materials (i.e., good corrosion resistance, light weight, ease of installation, and high specific stiffness and strength). In terms of the material properties of FRP composites, substantial research has been conducted. For instance, research on the effect of temperature has been carried out [
In addition to research at the material level, the structural behavior of URM walls strengthened with FRP composites has been considerably investigated. The common failure modes of URM walls strengthened in shear are the debonding of EB FRP composites, the rupture of FRP composites, or the failure of URM wall. In many tests, the debonding of EB FRP composites was observed [
Although EB FRP composites do not reach their ultimate states, structures can fail by the debonding of composites from concrete substrate due to shear or flexural palling at the end of composite materials. Similarly, the deformation of composites is caused after concrete substrate deforms since EB FRP composites are bonded to the substrate, which is called passive strengthening technique [
As mentioned above, considerable research has been conducted on URM walls strengthened with various FRP composites such as CFRP, GFRP, AFRP, and BFRP composites. However, research on the in-plane behavior of URM walls retrofitted with hybrid FRP (HFRP) is significantly limited. Therefore, the objective of this study is to investigate the in-plane behavior of URM walls strengthened with CFRP and HFRP (GFRP plus AFRP) sheets under cyclic loading. Furthermore, an equation is proposed to estimate accurate effective strain and thus the shear strength of URM walls retrofitted with EB FRP composite materials.
Behavior of URM walls is quite different from that of reinforced masonry walls. In particular, failure modes of URM walls are substantially crucial since strengthening material FRP sheets have their own directional natures. Moreover, it is essential to know the strength of existing URM walls for determining the proper strengthening level. Thus, failure modes of URM walls have been divided into four categories in this study. Strength capacity of URM walls in each category was estimated in accordance with FEMA 356 [
Failure modes of URM walls can be divided into shear and flexure categories, and then each category can be subdivided into deformation and force controlled actions. Failure modes can be determined depending on the length-to-height ratio (
Failure mode of unreinforced masonry wall by aspect ratio.
Deformation controlled action | Force controlled action | |
---|---|---|
|
Rocking | Toe crushing |
|
Bed joint sliding | Diagonal tension |
Failure mode of unreinforced masonry wall.
Bed joint sliding
Rocking or toe crushing
Diagonal tension
Shear strength of URM walls can be predicted using the estimation equations by FEMA 356 [
Studies were conducted to predict the shear strength of URM walls strengthened with FRP composite materials. For instance, ElGawady [
AC 125 [
In this study, the in-plane behavior of URM walls strengthened with unidirectional FRP sheet applied to one side of the walls was investigated to quantify the strengthening effectiveness of FRP composites. To achieve the purpose, three full-scale specimens were designed. One (URM-0.92) was nonstrengthened to serve as a control specimen and the other two (RTM-CFS-SF and RTM-HBRD-SF) were strengthened with CFRP and HFRP sheets, respectively. The aspect ratio (
As mentioned above, two types of FRP composites were used. One is CFRP, a widely used strengthening material, and the other is HFRP, newly developed. HFRP was made of GFRP and AFRP to introduce advantages of the two FRP composites. The mechanical properties of the CFRP and HFRP composites were obtained experimentally in the laboratory and are provided in Table
Material properties of FRP and resin.
Type |
|
|
|
|
|
---|---|---|---|---|---|
GFPR sheet | #1 | 88.98 | 2709.01 | 159.47 | 1.69 |
#2 | 96.45 | 2867.75 | 166.26 | 1.72 | |
#3 | 93.57 | 2838.24 | 169.27 | 1.68 | |
|
|||||
Hybrid sheet | #1 | 139.89 | 2322.27 | 64.14 | 3.62 |
#2 | 150.76 | 2490.39 | 76.24 | 3.27 | |
#3 | 144.35 | 2510.44 | 72.65 | 3.46 | |
|
|||||
Resin1 | Tensile strength (MPa) | Tensile modulus (GP) | Elongation at break [%] | Density (g/cm3) | |
|
|||||
Epoxy | 85 | 10.5 | 0.8 | 1.2 |
Stress-strain relationship of FRP sheet.
Since the masonry wall specimens were full scale, commercially available cement bricks with dimensions of 190 × 90 × 57 mm were used. Bed joints of 10 mm and 1.0 B thickness were chosen. The average compressive strength of the bricks was obtained as 15.7 MPa following the test method per KS F 4004 (Table
Material properties of URM.
Compressive strength of cement brick [MPa] | Compressive strength of mortar [MPa] | Compressive strength of prism [MPa] | |||
---|---|---|---|---|---|
#1 | 14.29 | #1 | 7.54 | #1 | 11.92 |
#2 | 15.94 | #2 | 8.64 | #2 | 12.77 |
#3 | 15.97 | #3 | 9.02 | #3 | 12.86 |
The parts between the walls and their bases were strengthened with FRP composites in the vertical direction to avoid early flexural failure due to low axial force and aspect ratio. The strengthening amount to resist flexure was determined following the sectional analysis used for RC walls as depicted in Figure
List of specimens.
Specimen |
|
|
Aspect ratio |
|
Retrofit material |
|
FRP sheet layer | Brick element [mm] | Vertical reinforcement [mm] |
---|---|---|---|---|---|---|---|---|---|
URM-0.92 | 2380 | 2400 | 0.92 | 190 |
|
— | |||
RTM- |
CFRP | 0.16 | 1 | 60 | |||||
RTM- |
Hybrid | 0.17 | 1 | 45 |
Flexural strength calculation of retrofitted specimen.
Specimen dimensions (unit: mm).
URM-0.92
Retrofitted specimen (RTM-CFS-SF, RTM-HBRD-SF)
As shown in Figure
Test setup.
The masonry wall specimens were tested using displacement control. Loading histories are depicted in Figure
Applied displacement history.
Figure
Summary of test results.
Specimens |
|
|
|
|
|
|
|
|
|
|
|
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URM-0.92 | Pos. | 13 | 18 | 23 | 1.47 | 2.83 | 12.6 | 0.06 | 0.5 | 2.8 | — |
Neg. | −5 | −9 | −12 | −1.78 | −9.8 | −9.8 | −0.08 | −0.1 | 1.25 | — | |
|
|||||||||||
RTM-CFS-SF | Pos. | 74 | 74 | 99 | 14.3 | 17.7 | 33.1 | 0.57 | 0.7 | 1.2 | 4.3 |
Neg. | −54 | −81 | −108 | −19 | −28.1 | −33 | −0.94 | −1.3 | 1.5 | 9 | |
|
|||||||||||
RTM-HBRD-SF | Pos. | 63 | 104 | 139 | 17.6 | 32.8 | 43.4 | 0.65 | 1.3 | 1.8 | 6 |
Neg. | −49 | −90 | −121 | −22.6 | −33.2 | −43.3 | −0.92 | −1.4 | 1.5 | 1.2 |
All estimates associated with moment and shear computed based on actual material properties.
Crack pattern and failure mode.
URM-0.92
RTM-CFS-SF
RTM-HBRD-SF
Load-displacement relationship.
URM-0.92
RTM-CFS-SF
RTM-HBRD-SF
Backbone curve
As a control specimen, URM-0.92 was a nonstrengthened masonry wall. After initial cracks formed, no additional load was transferred between the URM wall and base due to cracks in the mortar between the URM wall and base, resulting in lifting of the URM wall with an ultimate load of 23 kN at a 0.2% drift ratio. After a 0.5% drift ratio, it was observed that displacement continuously increased without load increase due to the wall rotation. Thus, it appeared that, after the ultimate load was recorded, failure occurred due to the wall lifting at a drift ratio of 0.4%.
The specimen RTM-CFS-SF, strengthened with CFRP sheet, reached an ultimate load of 99 kN at a drift ratio of +0.69%. RTM-CFS-SF showed approximately 330% larger load-carrying capacity in comparison with URM-0.92. When the strengthened specimen reached the ultimate load, rupture of FRP sheet applied between the wall and base occurred with a loud sound. This was attributed to stress concentration at the debonding area of the FRP sheet from the wall. Then, the load-carrying capacity of the strengthened specimen rapidly decreased. After wall lifting was observed, failure of the FRP sheet propagated, and ultimately RTM-CFS-SF failed due to the crushing of the brick at the bottom of the masonry wall.
The other strengthened specimen with HFRP, RTM-HBRD-SF, presented an ultimate load of 139 kN at a drift ratio of +1.31%. RTM-HBRD-SF indicated approximately 504% and 40% larger load-carrying capacity than URM-0.92 and RTM-CFS-SF, respectively. In addition, unlike RTM-CFS-SF, RTM-HBRD-SF showed continuous load resistance capability and gradual decrease of load-carrying capacity after the ultimate load was reached. Due to the mechanical properties of HFRP (low modulus of elasticity and large ultimate strain), there was no rupture of the FRP sheet between the masonry wall and base. However, due to propagation of the diagonal crack following the mortar face, RTM-HBRD-SF failed with signs of HFRP sheet debonding from the masonry wall after a drift ratio of +1.5%.
To evaluate the contribution of the FRP sheet to shear strength improvement, strains were measured in the FRP sheets in the horizontal and vertical directions. Figure
FRP sheet strain (vertical direction).
RTM-CFS-SF
RTM-HBRD-SF
Figure
FRP sheet strain (horizontal direction).
RTM-CFS-SF
RTM-HBRD-SF
Figure
Evaluation of shear strength.
As stated before, the shear strength model by Triantafillou showed poor agreement with the test results. Effective strain was a critical factor of the shear strength model and obtained through nonlinear regression analysis on the basis of experimental data. In this study, a theoretical study was, therefore, conducted through nonlinear regression analysis to find a better effective strain and thus to estimate more accurate shear strength for a URM wall strengthened with FRP sheet.
As seen in (
To consider the stress concentration phenomenon and early failure of an FRP sheet, existing experimental data were collected from studies [
Effectiveness strain distribution according to retrofit ratio.
Therefore, an equation for URM walls strengthened with FRP composites was derived through nonlinear regression analysis using an exponential function type (
The test results were compared with the ones estimated using (
Evaluation of proposed equation.
Specimen |
|
| |
---|---|---|---|
|
| ||
RTM-CFS-SF | 518 | 80 | 99 |
RTM-HBRD-SF | 308 | 130 | 139 |
In this study, the in-plane behavior of URM walls strengthened with EB FRP sheets was investigated to assess the strengthening effectiveness of FRP sheets on URM walls. Three full-scale masonry wall specimens were examined. The following conclusions can be drawn.
The FRP sheets improved the structural integrity of URM walls. Both CFRP and HFRP were effective in increasing the strength of URM walls by 4.3 and 6 times in comparison with the control specimen.
When FRP composites are used as strengthening materials, debonding and rupture of FRP composites significantly affect the lateral resistance of specimens strengthened with FRP materials. Both phenomena occurred in RTM-CFS-SF, resulting in rapid strength decrease. On the contrary, there was no rupture of HFRP in RTM-HBRD-SF, and thus, gradual strength degradation was obtained after the ultimate. Therefore, HFRP consisting of GFRP and AFRP appears to be superior to CFRP from the standpoint of strength and material usage.
The strength of the URM walls strengthened with FRP sheets was estimated using the shear strength model for RC beams by Triantafillou. The accuracy of the model for RC beams was significantly low for the strengthened URM walls since the measured strain of the URM walls retrofitted with FRP sheets was much smaller than that of RC beams used for the shear strength model. Therefore, effective strain is an essential variable for the design of URM walls strengthened with FRP sheet.
An equation is, herein, proposed to estimate an accurate effective strain and consequently the shear strength of URM walls retrofitted with FRP sheet. The suggested model was in good agreement with the test results by indicating a small difference less than 10%. However, it should be noted that the proposed model needs to be applied with care since the number of data used to derive the equation is insufficient.
The cross-sectional area of FRP
The bonding area of mortar
The elastic modulus of FRP
The effective strength of FRP material
The strength of URM walls
The axial compressive stress (axial compressive force/area of wall)
The diagonal tension stress
The ultimate tensile strength of FRP sheet
The axial force of FRP sheet
The compressive strength of masonry
The wall height
The wall length
The expected axial compressive force on wall
The shear strength of URM wall
The glass transition temperature
The wall thickness
The shear strength in case of bed joint sliding
The shear strength in case of bed rocking
The shear strength in case of toe crushing
The shear strength in case of diagonal tension
The shear stress in case of bed joint sliding
The boundary condition constant (0.5 and 1.0 for cantilever and fixed at both ends, resp.)
The effective strain of FRP
The strengthening ratio in horizontal direction.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by Chungwoon University Foundation Grant 2015 and the National Research Foundation of Korea (NRF; 2013R1A1A2010717) grant funded by the Korea government (15CTAP-C097470-01).