1. Introduction
Unreinforced masonry is one of representative structural systems for buildings that have been damaged severely in most major earthquakes worldwide. Recently, two major earthquakes occurred in Gyeongju and Pohang, respectively, located in South Korea caused significant damage to many buildings although those earthquakes have a moderate level of local magnitude, 5.8 and 5.4, respectively. Unexceptionally, bearing walls in unreinforced masonry buildings and external masonry veneers were reported as building components damaged the most significantly in the two earthquakes.
The mechanical property of masonry is difficult to predict because masonry is composed of different materials, i.e., masonry units and type or material of joint, and strongly influenced by workmanship strongly [
1]. In particular, masonry units have different dimensions and shapes and are made of diverse materials, such as adobe and concrete. Masonry walls also fail in diverse modes such as diagonal cracking, horizontal sliding, rocking, and corner crushing. The mechanical characteristics of masonry have been investigated through experimental works by many researchers. One of the representative reasons for this is that the mechanical characteristics of masonry are considerably influenced by local materials. Narrowing down the scope of the literature to the Korean masonry materials, the following research works are available in the literature. Yu and Kwon (2011) [
2] compared mechanical characteristics of masonry prisms produced using either cement or lime for joint mortar based on compression and diagonal tension tests. Kim et al. (2001) [
3] performed compression and diagonal tension tests for the masonry prisms produced using joint mortar with various mixing ratios, and proposed empirical equations for the compressive strength of joint mortar and elastic modulus of masonry prisms. Yang et al. (2019) [
4] proposed a shear stress–strain relationship through diagonal tension tests of masonry prisms for numerical analysis of masonry walls.
Most Korean masonry buildings are low-rise buildings and have not been designed seismically, even though those were constructed after the Korean Building Acts introduced seismic design provisions in 1988. The structural system of Korean masonry buildings is usually unreinforced masonry, of which vulnerability to earthquakes is well-known, as mentioned above. As a result, social demand for strengthening methods that can be installed easily with low cost is increasing in Korea after the two recent earthquakes. Choi et al. (2010) [
5] studied seismic retrofit of unreinforced masonry walls using shotcrete overlay, which was made of Engineered Cementitious Composite (ECC) containing Polyvinyl Alcohol (PVA) fibers. They validated the effect of shotcrete overlay to enhance both strength and deformability experimentally. Taghdi et al. (2000) [
6] showed that the shear strength and ductility of masonry walls and reinforced concrete walls were increased by attaching steel strips with concrete anchors. Ghiassi et al. (2012) investigated bond-slip behavior for masonry strengthened with different FRP materials both numerically and experimentally [
7]. In addition, Ismail et al. (2011) [
8] proposed a method to stitch masonry units with twisted steel bars that are placed in the groove made on the surface of the masonry and filled with mortar. Darbhanzi et al. (2014) [
9] conducted a seismic retrofit of unreinforced masonry walls, which comprises the addition of two vertical steel ties on both edges of the walls, and anchored to the top and bottom of the walls. Silva et al. (2014) [
10] strengthened unreinforced masonry by injecting lime grout, confirming that the strength and deformation capacity of the masonry increase two times and three times, respectively. In addition, as with this study, several researchers have conducted experimental studies to strengthen unreinforced masonry using overlay methods. Almeida et al. (2015) [
11] strengthened unreinforced masonry by overlaying polypropylene fiber reinforced mortar (PFRM) and carbon fiber mesh (CFM). D’Ambrisi et al. (2013) [
12] strengthened unreinforced masonry walls with three types of polymeric mesh attached with mortar plastered on the walls. Benedetti (2019) [
13] strengthened masonry walls using glass fiber mesh-reinforced mortar (G-FRM) so that the shear strength of the masonry walls increased by more than 100%, even with a 15 mm or smaller thickness of strengthening layers on the two wall faces. In most preceding studies, fiber mesh or wire mesh is usually used in the overlaying method for strengthening masonry walls.
In this study, an efficient method to strengthen unreinforced masonry walls using a steel fiber-reinforced mortar (SFRM) overlay. The proposed overlaying method has the advantage of constructability during residence of occupants. In addition, plastering work does not require high workmanship and can be applied to the vicinity of openings without cutting or splicing. Above all, mortar can be integrated with masonry, considering that it has been used for the joint and finishing of masonry. However, the brittle behavior of plain mortar is similar to masonry. It is necessary to add reinforcement appropriate for plastering material to improve mechanical behavior of masonry walls after retrofit. SFRM is a cement composite incorporating steel fibers which improves brittle behavior of mortar as well as mechanical properties such as tensile strength and toughness. Generally, steel fibers are known to have better strength and stiffness than organic fibers such as Polyethylene (PE), Polypropylene (PP), Nylon, etc., but relatively long and thick dimensions make those difficult to apply steel fibers to thin mortar overlay. On the other hand straight or hook-type steel fibers commonly used in construction fields are relatively stiff and difficult to plaster in a thin layer without balls and protrusion, of which the latter may cause human injury. As a result, this study adopted Amorphous Steel Fibers (ASF), which reinforces mortar effectively in spite of its relatively short length due to better bonding characteristics owing to the thin shape and surface roughness.
In this study, the effect of SFRM for strengthening masonry walls was assessed based on the compressive and diagonal tension strength for masonry prisms. Those strength parameters are used to define the lateral force-resisting capacity of masonry walls in a major failure mode such as toe-crushing or diagonal tension failure, as stipulated in ASCE 41-17 [
14]. In order to build strengthened prism specimens, a recommended SFRM mixing ratio with both strengthening efficiency and overlay constructability is derived based on constructability tests for fresh mortar, compression and tension tests for hardened mortar specimens for various mixing ratios, since the characteristics of SFRM strongly depend on the type of fiber, fiber volume fraction, and mixing ratio of mortar. Then, the performance of the strengthening method is evaluated by compressive and diagonal tension tests of masonry prisms with and without SFRM overlay to which the recommended mixing ratio of the mortar. This study concentrates on two representative types of unreinforced masonry. One is made of red bricks and usually used as an exterior veneer. The other is concrete bricks, usually used for bearing walls. For easy construction, only SFRM was used, without additional glass fiber or wire mesh. Finally, the applicability of existing design equations is investigated based on the measured strengths of prism specimens.
4. Estimation of Enhanced Shear Strength
The shear strength observed in the tests are compared with those predicted using design equations. ACI 549.4R-13 [
39] provides that shear strength of masonry walls strengthened with Fabric Reinforced Cement Matrix (FRCM) is calculated by combining the contribution of the masonry and the FRCM composite material, respectively, as follows
where
,
and
are the nominal shear strength, the contribution of the masonry, and the contribution of the FRCM composite material. The FRCM is a kind of mortar reinforced with grid type mesh, while SFRM is mortar reinforced with steel fibers. In spite of difference between the two types of reinforcement, the shear strength of masonry walls strengthened by SFRM is supposed to be calculated using Equation (2) considering that both FRCM and SFRM are essentially the same material for strengthening masonry walls.
The masonry prisms failed in the diagonal tension failure mode as shown in
Figure 17 and
Figure 18. For such a case, the shear strength of the masonry wall,
, can be calculated using Equation (3) proposed by Li et al. (2005) [
40] for the diagonal tension failure
where
is the angle between horizontal and main diagonal of the wall,
is the net area of the cross section of the masonry wall (
), and
is the tensile strength of the masonry calculated as
and
, in which
is the compressive strength of the masonry (MPa), for red clay bricks and concrete bricks, respectively (MPa), in accordance with Silva et al. (2008) [
41]. However, diagonal tension test result for masonry prisms without strengthening was used for
instead of calculation by Equation (3), considering the tensile strength of bricks has much uncertainty. From the peak load in the diagonal tension test, only the component parallel to the horizontal joints in the masonry prism was decomposed and assigned to
. On the other hand, Equation (3) was used to calculate the contribution of the SFRM overlay to the shear strength, denoted by
. In Almeida et al. (2015) [
11], masonry prisms were strengthened with mortar overlay reinforced by mesh, and the contribution of mortar overlay except mesh was calculated using Equation (3). This is a reasonable approximation when the mortar joints between bricks are relatively strong and the failure is governed by masonry units such as bricks. The tensile strength of SFRM in
Figure 6 was used for
in Equation (3).
In addition, Sagar et al. (2017) [
42] conducted experiments on the diagonal tension strength and out-of-plane flexural strength for masonry walls strengthened with FRCM, and proposed Equation (4) to calculate the contribution to the shear strength by FRCM
where
is the shear strength of the engineered cementitious composite (ECC) section (kN) and was used as shear strength of the SFRM in this study,
is the thickness of the overlay (mm),
is the lever arm assumed as 0.87 times the length of the specimen (mm),
and
are the tensile and compressive strengths of the overlay, respectively (MPa), and
is the length of the wall (mm).
The shear strengths of masonry prisms strengthened with SFRM overlays obtained from the tests and calculations using Equations (2)–(4) are summarized for comparison in
Table 11, in which the contribution of the masonry and that of the SFRM are given separately. It is noted that all the calculated shear strengths for strengthened masonry prisms are consistently higher than the corresponding experimental strengths and Equation (4) tends to overestimate the shear strengths more significantly than Equation (3).
Equation (2) assumes that both masonry and mortar overlay for strengthening fail simultaneously. However, SFRM overlay detached from the interface without damage, as shown in
Figure 17c. The premature detachment of the SFRM overlay causes Equation (2) to overestimate the strength of the strengthened masonry prisms. To improve the accuracy in predicted shear strength, it is necessary to take into account the bond strength between SFRM and masonry, which requires an additional test program. Otherwise, it can be anticipated that error in strength estimation using Equations (2) and (3) will be reduced if any technique to improve the bond strength or shear connectors between masonry and SFRM overlay are applied.
In case of RB0 and CB0, the strengthened masonry prism fail with the mortar overlay cracked, as shown in
Figure 17b. Therefore, the error in strength estimation given in
Table 11 does not result from the premature detachment of the mortar overlay. In this case, the error may be caused by the use of Equation (3) or (4), in which material strength is for the masonry unit and mortar joint stronger than the masonry unit is assumed to exist.