The idea of base isolation is to uncouple the masonry building and the foundation by placing flexible pads between them, thus preventing the earthquake motions from transmitting up through the building, or at least reducing them greatly [7], as demonstrated in Fig. (1). When the ground shakes, only a small portion of the shaking from the base will be transmitted to the superstructure. Previous research has presented that the appropriately chosen flexible pads can reduce the forces induced by earthquake 5 to 6 times compared with the structures without using base isolation. In the experimental result of [8], the masonry part retrofitted with isolators experiences a 2.8 up to 24 times smaller displacement and the forces are reduced from 1.5 to 15 times compared with the one registered in the fixed foundation. The base isolation technique can be perfectly applied in low to mid-rise masonry buildings. Particularly, the base isolation is a suitable retrofitting strategy for the heritages of historical importance as it can preserve their original appearance while the conventional rehabilitation would be destructive to the appearance of the buildings [9]. However, it is so far not easy to implement this technique under the existing building. The base isolation system is usually seen as a presentation of maximum capacity to resist seismic loads, which could be achieved for the structure without additional invasive retrofitting measures [10]. [11] retrofitted an old masonry chapel building using base isolation technology, which used the laminated rubber bearing and damper together. It was found out that this feasible technique could resist seismic loading and keep its architectural feature. Similarly, [12] found that the retrofitted masonry building using pure friction base isolation system has a 50% reduction in maximum roof acceleration in comparison to the conventional fixed base structure. The base isolation technique could be applied in combination with other strengthening materials, Fibre Reinforced Polymer (FRP) for instance, [13] concluded that the improvement of this technique alone is not sufficient, while the use of horizontal and vertical CFRP laminate strips greatly improves the seismic behaviour. This technique cannot be only applied to the structural masonry buildings, but also to the heavy non-structural monolithic objects, such as the pinnacles. [14] implemented this system on a pinnacle to resist earthquake action expressed through base isolation, whose spectrum is compatible with the design seismic action. The best application of this technique is on the newly built building as the construction of the flexible pad would be easier. Though it would be perfect if this technique can be implemented on the masonry heritage as it can preserve its authenticity, the mechanical work would be cumbersome which may destroy the whole structure out of a sudden. Besides, the cost would be huge as the expense to install a hybrid base isolation system can be as much as 3% of the total cost of the building [15]. It also should be noted that this technique cannot be implemented on tall buildings.

Fig. (1). The technique of base isolation.

The seismic damper is a mechanical device to dissipate the energy caused by the earthquake in a building. During an earthquake, the seismic energy will be transmitted from the substructure to the superstructure. A portion of the energy will be absorbed and dissipated by dampers, and in such case, the shaking of the building is damped. Damper was first used in tall buildings to resist wind effects. Later it was introduced to the buildings against earthquake effects. Quite a lot of different types of seismic dampers have been developed and applied, and the most commonly applied types in low-rise buildings include viscous dampers, friction dampers and yielding dampers. In an earthquake, some energy will be dissipated in the forms of heat and friction by the viscous dampers, thus resulting in less likelihood of failure. For the friction damper, it maintains the integrity by redressing the floors back to their relative positions in the beginning, while yielding dampers absorb the energy and yield before the floors do. In such a way, the structural failure of the building itself will be prevented. The use of seismic dampers can significantly reduce seismic vulnerability as well as encounter the complicated effects of the unknown as well as uncertain interventions during the building’s lifetime [16]. So far, most of the application of seismic dampers are conducted on the framed structure, only a small number of research is carried out on masonry buildings [17, 18] as it is suitable for insertion within a chevron bracing system. [19] improved the seismic response on a historical chimney by using Tuned Mass Damper (TMD) and this technique improves the seismic response in terms of compressive stress value, base shear and top displacement [20]. rehabilitated an old unreinforced masonry building using hysteric dampers system (Fig. 2). [21] has also proposed this technique to retrofit stone masonry buildings, and significant improvement has been achieved. Conventional retrofitting with seismic damper requires heavy demolition and long construction times on masonry buildings, therefore, [21] proposed an alternative damper called Added Damping and Stiffness (ADAS) dampers, which is characterized by the addition of new external concrete walls equipped with ADAS dampers, thus reducing the intervention on the initial building. Moreover, [22] retrofitted a masonry building using a combination of steel bracing and dampers. The results found that the dampers dissipated a huge amount of seismic energy and prevented the masonry of excessive deformation and cracking. However, [13] argued that this technique might not be efficient as the seismic damper requires large deformations to be efficient, while the masonry structure is generally rigid. So far, as previously stated, the seismic dampers are more commonly applied in rehabilitating framed structures but not that popular in masonry structures.

The general surface treatment involves attaching strengthening materials to the original structure and tied together by using mortar or steel links. The most frequently used approach in surface treatment is shotcrete and ferrocement.

Shotcrete is applied by spraying shotcrete over a mesh of wire installed on the surface of the masonry wall. (Fig. 3a). In general, the thickness of the overlay ranges from 70mm to 150mm [23, 24]. Normally, the shear dowels need to be used as well for the sake of transferring shear between the masonry-shotcrete interface. Before the application of shotcrete, the removal of wythes of bricks and filling the voids should be carried out first. [25] did a series of experiments on masonry wall panels using this technique. The ultimate lateral strength was increased by nearly 3.6 times. Furthermore, the stiffness at peak loading was increased by a factor of 3, though the initial stiffness was unaffected by this method. [24] found that shotcrete jacketing on both surfaces can reduce masonry tension on an average of 50% while one-sided shotcrete jacketing reduces the tension by about one-third. Besides, the roughness of the masonry surface plays an important role as well in determining the effectiveness of retrofitting. The retrofitting of shotcrete will be improved if the substrate surface is particularly rough after removal of loose or deteriorate portions [26].

Fig. (3). Surface treatment. (a) Shotcrete [25] (b) Ferrocement [29].

The ferrocement consists of closely spaced multiple layers of hardware embedded in a high strength (15-30MPa) cement mortar layer (10-15mm thickness) [27], shown in Fig. (3b). The mechanical properties of ferrocement depend on mesh properties as the mesh improves the in-plane inelastic deformation capacity by confining the masonry units after cracking. In a static cyclic test [28], this retrofitting technique increased the in-plane lateral resistance about 150%. [29] found that only 0.29% reinforcement in the longitudinal direction can increase the strength of masonry wall panel in out-of-plane direction by more than 10 times.

In general, the surface treatment method can significantly improve the strength and stiffness of masonry structure. Furthermore, they both increase the wall height-to-ratio, the in-plane lateral resistance, out-of-plane stability and arching action are increased accordingly [30]. Obviously, this technique is suitable for the vertical masonry element and it will be harmful if being implemented on the horizontal masonry elements like arches. Nevertheless, the shortcomings of this method are the much time consumption in the application and it destroys the original aesthetics. Therefore, this technique is not suitable for the retrofitting of masonry heritage,

Sometimes, the masonry units in the buildings are still of good quality but the mortar is poor or it was not fully filled. Therefore, the mortar could be replaced or refilled by new bonding material with higher strength. Grout injection and re-pointing are the most often used techniques.

Grout injection is implemented by filling the voids and cracks [31] developed different types of grouts for filling spaces ranging in size from very narrow cracks to large voids and empty joints. This technique has been found to be effective at restoring the initial stiffness and strength of masonry, but no significant improvement in the initial stiffness or strength. Though the grout can be replaced with material of higher strength, the improvement is still not remarkable [32] found that the addition of 2% Ordinary Portland Cement to the mortar made little or no difference to the ultimate acceleration resistance. However, the effectiveness of this technique can be improved if it is used in combination with other techniques [33] conducted a study by combining FRP rods and re-pointing technique on masonry structure. The results displayed that using re-pointing technique combined with FRP laminates is the most effective retrofitting technique. It should be noted that this approach will work efficiently only if the mechanical property of the mix and its physical and chemical compatibility with the masonry to be retrofitted has been achieved [34].

In the retrofitting of masonry heritages, the preservation of the original aesthetics and the compatibility in terms of physicochemical and mechanical characteristics are the most important concerns [35]. The former means that the authenticity of masonry heritages need to be preversed after retrofitting while the latter means that masonry and the retrofitting material should have a good compatibility in terms of physicochemical and mechanical performance. The use of incompatible retrofitting materials may initiate decay mechanisms or even lead to catastrophic results [36]. The application of grout injection and re-pointing can preserve the original appearance of masonry heritage. As previously stated, the physical and chemical compatibility between masonry heritage and retrofitting materials is critical, while the interaction between retrofitting material and masonry is still not known clearly. Therefore, recent research regarding the design and selection or restoration mortar is interlinked with compatibility assessment to ensure the long-term durability of masonry heritage [35] presented a methodological approach for the selection of restoration mortars regarding fragility analysis. In the paper, selection of the optimum mortar, complying with the set compatibility and performance requirement can be accomplished by setting requirements during the characterization of the retrofitting materials and the investigation of masonry heritage.

This technique is suitable for most masonry buildings, especially for the masonry heritage as the authenticity can be preversed after retrofitting. The prerequisite is that the retrofitting mortar does not have a detrimental effect on the initial masonry. Another ideal area of application is multi-leaf masonry walls where the connection between different layers are poor as well as the voids in the dry rubble stone’s inner core. This method becomes popular and practical because of its minimal cost and ease of implementation, and most importantly, its sustainability.

The application of this technique is to install steel elements next to the original masonry element, which might be tied together or not. During an earthquake, small cracks are expected to occur and they will develop and propagate if external loading exceeds its load carrying capacity. However, the new steel system has a considerably large stiffness and will stop the cracking on the masonry wall from propagating [37, 38]. In such cases, the external load will be carried by the stronger steel system while the initial masonry system may work as a structural element instead of carrying loads. [39] conducted a research by attaching steel members directly on the masonry wall, and the results presented that the lateral strength in in-plane direction of the reinforced wall was improved about 4.5 times. Other studies concluded that this steel reinforcing system is significantly effective in improving the masonry structure’s resistance, ductility, and energy absorption [38, 39]. This technique is very effective in improving the load resistance of a structure as steel is a strong retrofitting material. Therefore, this approach is applicable for the weak masonry structures or the structures that need to be improved remarkably. However, as the appearance of steel will change the aesthetics of the original masonry structure, thus, it is not a suitable retrofitting approach for masonry heritage. Furthermore, the high cost is another concern of its implementation in developing countries.

In the post-tensioning strengthening method, the pre-stressed reinforcements are placed along the vertical elements for the sake of improving the strength and ductility of the lateral load resisting frame of the structure [37]. In detail, this method is carried out by drilling a hole through the masonry wallets, and vertically placing pre-stressed reinforcement in the drilled hole. The compressive force will be provided by the pre-stressed reinforcement, which can counteract the tensile force occurred in masonry wall, thus improving its load carrying capacity. Experiments illustrated that the lateral load resistance of masonry walls can be doubled [40, 41] implemented this approach on masonry walls in out-of-plane direction to analyse the flexural behaviour. Though the results presented that the ductility of the reinforced masonry panel was not improved, the strength and stiffness were increased remarkably. However, in the results of experiments conducted by [42], some difference regarding the ductility was found. The maximum strength can be increased by a factor of 2.1 to 2.8, while the ductility by values was improved with an average value of 2.7 times. Similarly, [43] found that the shear capacity and ductility can be improved significantly, while the ability of energy dissipation is also increased remarkably. Besides the application on bare masonry panels, this technique can be used to improve the seismic performance of RC frame infilled with masonry wall [44] conducted such a study to find that the engagement between RC frames and masonry infill has been improved through this retrofitting technique, thus leading to a postponed failure mechanism. It should be noted that the axial force provided by the pre-stressing bar and the vertical load should be smaller than a certain limit. Exceeding the limit, the ductility will decrease [43].

The center core is similar to the post-tensioning technique to some extent, which involves of installing a reinforced core in the vertical direction of the masonry walls. The differences between this technique and post-tensioning are that the steel bar is not pre-stressed and the drilled hole is much bigger. The idea of this technique is to improve the masonry’s ability to resist cracking and increase the ductility while keeps the stiffness unchanged [45]. This method has successfully doubled the strength of a masonry panel in a static cyclic test [28, 46] has successfully applied this technique on more than 60 projects to mitigate the earthquake hazard.

Some of the shortcomings in the above approaches, adding mass, for instance, can be overcome by using mesh reinforcement. FRP is the most commonly used mesh reinforcement to reinforce URM structures (Fig. 4). FRP composite was first used to retrofit or strengthen the existing concrete structures. It has been extended and applied on other (masonry, timber) structures and extensively studied as well [47].

In general, strengthening/retrofitting of URM walls using FRP composites can improve the strength of masonry wallets about 1.1 to 3 times [34, 48] found that the resistance of the wallets can be improved by 13-84% by doing an analysis on masonry wallets retrofitted with carbon fibre. The improvement might be much more dependent on the to be retrofitted structure. In the study of [49], FRP has been found to improve the shear resistance of the masonry buildings by 3.25 times. The study concluded that if the economy and mechanical behaviour are both concerned, it is better to choose unidirectional FRP laminates or fabric strips instead of using two-dimensional fabrics [50] carried out research to investigate the effectiveness of the FRP with different configurations: grid arrangement and diagonal strips. It is noted that the asymmetrical application of the reinforcing is not effective in improving the shear resistance of masonry walls. Moreover, it is found that the grid strips provide a better stress redistribution, which results in a less brittle failure, while the diagonal strips are more effective in enhancing the shear capacity. FRP can be applied in combination with other strengthening/retrofitting materials [51] found that the combined use of FRP and PP-band performed much better than the application of either individual usage. In terms of the failure type of masonry panels retrofitted with FRP, the detachment of FRP from masonry surface plays an important role [51]. The effectiveness of retrofitting will be lost if the FRP starts debonding from the masonry surface [52] found that the failure modes are masonry crushing, FRP rupture and debonding.

Fig. (4). FRP strengthening approach on masonry wall.

The reinforcing of masonry panels using FRP possesses the merits of little added mass, low disturbance and relatively high improvement in strength. Nevertheless, the shortcomings of this technique are that it is expensive, requires high technical skill and changes the structure’s appearance. The initial cost of FRP material is about 5 to 10 times more expensive than steel [53], which is a big concern in choosing the retrofitting approaches. In addition, the property and performance, especially their long-term behaviour, of FRP materials have not been thoroughly understood [54]. Moreover, the FRP is normally applied by externally attaching the strips or sheets on the surface of masonry wall, and a water-proof barrier and prevention of the natural transpiration of the masonry structure might be created. Finally, this type of strengthened structures will be particularly weak if the epoxy-based bonding material is used in the strengthening with FRP composites [30].

If FRP is too expensive to afford in the developing countries, the Polypropylene (PP) band and bamboo meshes can be alternatives. PP band is a universal cheap packing material having considerable elongation capacity, which has been introduced as a cheap reinforcing approach in Japan. [55] did a test on both retrofitted and unretrofitted masonry panels, and the result displayed that the reinforced panel with PP mesh provide a higher residual strength after the occurrence of cracks. This strengthening approach is often used in reinforcing adobe masonry structure. [56] applied this technique on the non-engineered (adobe) masonry in rural Nepal. This retrofitting approach has found to be helpful in preventing the loss of material and maintaining wall integrity. This approach possesses the same merit that it is suitable on the low-strength masonry structure/element. With the application on high-strength masonry structure, the effectiveness will be much less significant. [51] retrofitted a brick masonry house using this technique, and the results showed that the retrofitted masonry structure was unable to withstand a severe shaking condition. With the bamboo meshes. [57] conducted a research on retrofitting an adobe house using bamboo band meshes. The retrofitted adobe house could withstand over twice larger input energy than the non-retrofitted specimen. The advantages of pp-band and bamboo meshes are their low cost and easy availability.

The mesh reinforcement is not only effective in reinforcing the vertical masonry structures but also significant in the strengthening of horizontal elements like vaults and arches, especially the composite materials made with textile fibers both with polymeric and cementitious matrices [58] strengthened masonry arches and vaults with different composites (TRM, SRG and FRP), all the experimental results showed that all the reinforcement systems were very efficient in increasing the maximum load [59] used the post-strengthening method with C-FRP on masonry vaults, and a similar result was found. More research about retrofitting/strengthening on masonry vaults and arches can be found in the work of [58, 60, 61] reviewed the strengthening of masonry arches using composite regarding the reinforcement position, and the result pointed out that the best type of reinforcement position is continuous at the intrados and extrados of the arches.

Though the polymer-reinforces fibers are the most commonly used to strengthen vaults and arches, they still possess the disadvantages of lack of water vapor permeability [62] proposed an alternative method by embedding long steel fibers and basalt textiles in the mortar to provide a steel-basalt reinforce mortar-based composite. The retrofitting results are compared with the polymer composites and both cases are effective in strengthening masonry vaults in terms of increasing of load and deformation capacity [6] agrees with the work of [62] that basalt on a mortar matrix provides both higher capacity and better ductility compared with polymer matrix.

The reticulatus system, was recently proposed by [63] to retrofit/reinforce rubble stone masonry. This system is carried out by inserting a continuous mesh of high strength reinforcement in the mortar joints, which are striped off about 40-60mm. Then, the mesh of reinforcement will be anchored to the masonry panel with transverse metal bars with the number of 5-6 per m². After that, the reinforcement and anchoring bars will be covered by re-pointing mortar back to the joints. The dimension of the reinforcement mesh normally ranges from 300-500mm, and it must be smaller than the thickness of the panel [64]. The detailed configuration of a typical reticulatus system is displayed in Fig. (5).

Fig. (5). Configuration of reticulatus system (extracted from [63]).

In the study of [65], the reticulatus system has been applied on historic masonry to investigate the flexural strengthening. The results have proved the improvement and the potential application of this technique. However, the improvement of bending force and the initial stiffness was realized only under appropriate pre-tension. In the work of [66], the reticulatus system made of fibre-reinforcement was used to retrofit an ancient building to provide a cross-interlock to resist against tensile strength produced by lateral forces. Though the compression, shear and flexural strength of stone or rubble masonry wall reinforced by the reticulatus system can be increased, the effectiveness of this reinforcing technique relies on the reinforcement mesh embedded in the mortar joints [63]. Besides the difference of improvement on retrofitting materials, the improvement of same retrofitting material on different masonry structures varies as well. [64] found that the shear strength of the retrofitted pebble masonry was improved about 40% while 17% for stone masonry by using the same retrofitting technique.

As this reinforcing system can keep the original aesthetic of the building, it is therefore suitable in reinforcing the fair-face masonry or the masonry heritages. Furthermore, this technique fits the masonry panels of both regular and irregular shape. Similar with FRP, the reticulatus system does not put on much extra load. So far, this technique is only applied on the stone/rubble/pebble masonry structure. Further investigation of the application of this technique on other masonry structures, such as brick masonry, should be conducted.

This technique involves constructional columns confining the masonry walls at all corners and wall intersections as well as the vertical borders of door and windows openings [27]. The integrity will be improved much more remarkably if the constructional columns relate to ring beams at floors levels. Both the constructional columns and ring beams confine the masonry structure at the same storey. This method could improve the resistance ability in both directions (out-of-plane and in-plane) [67] found that this approach could improve the lateral resistance about 1.5 times and the lateral deformations and energy dissipation about 50%. [68] has investigated this technique on half-scale specimens under cyclic loading, and the tests demonstrated that the energy dissipation of the wall has been improved as well as the deformability in in-plane direction. It is recommended to use this confined system for newly built masonry structure in Eurocode 8 as the integrity of the building can be guaranteed. The application of this technique on existed buildings will be hard and high cost.

The reinforced concrete ring-beams were normally used in masonry structures to improve its mechanical behaviour. The masonry structures confined with constructional columns and ring beams are expected to perform well in earthquakes [69] concluded in a study on confined masonry structures that the mechanical performance (ductility and strength) of the masonry panels are maintained mostly by the confining elements. Furthermore, more strength of the masonry structure will be preserved during an earthquake with higher reinforcement ratio and more confining elements. In some cases, if the existing ring beam is damaged or weak initially, retrofitting/strengthening can be done on the ring beam to restore its original function [70] retrofitted the masonry building with masonry ring-beam reinforced with composites. The result showed that the masonry ring-beam reinforced with composites performs well in terms of load carrying capacity. Similarly with constructional columns, this technique is easy to install on newly built buildings.

Tie bars can also be applied to increase the integrity of the masonry building. The function of the tie bar is to provide compression stress on the masonry wall horizontally or vertically, which is quite similar with the post-tension technique. In some cases where the foundation settled unevenly and building inclined, and the tie bars can be applied to redress the inclined parts back to its original level [42] did a series of tests on masonry panels retrofitted with vertical steel ties. The outcomes presented that the vertical ties can remarkably increase the seismic capacity of masonry structure both in strength and ductility. It should be noted that surface treatment of the bar should be carried out carefully to avoid corrosion.

Generally, the mortar in a masonry structure is too weak to consider its tensile strength. Therefore, the tensile and flexural strength of a masonry element are always ignored compared with its compressive strength. Mortar mixed with fibre/textile can possibly be used to improve the tension and flexural resistance, and accordingly improve the integrity of a masonry structure. Fibre/textile additives used in the mortar help to improve mortar’s tensile strength. [71] strengthened the masonry infill walls using plaster and hybrid glass fibres. The results demonstrated that it is not only effective in avoiding expulsion in out-of-plane direction of masonry panels, but also reducing the global in-plane damage. Similarly, [72] applied Textile-Reinforced Mortar technique (TRM) for the sake of preventing brittle failure. The result displayed that the ductility has been enhanced as well as the strength improved in out-of-plane direction. However, it should be noted that the improvement in integrity is not as remarkable as the above-mentioned methods. [73] employed Steel Reinforced Grout (SRG) which is made by embedding ultra-high tensile strength steel chords in the mortar in a convex masonry substrate. However, as the performance of SRG depends on the roughness of the masonry surface, as well as the curing conditions. Therefore, the result of this study was proved to be insufficient for a comprehensive understanding of this technique. This technique is very similar with the re-pointing and grout injection approaches.