Methods for strengthening masonry structures

Thread Content

From the Internet for everyone’s learning purposes – Methods for strengthening masonry structures 1 Introduction In the context of the market economy, the one-sided pursuit of economic benefits often leads to construction quality accidents. Reinforcing structures with quality issues is a problem that needs to be addressed frequently. And the reinforcement of masonry structures is the most common issue. Practical reinforcement of masonry structures is divided into two categories: direct reinforcement and indirect reinforcement. During design, the appropriate method can be selected based on actual conditions and usage requirements. 　　 　　2 Direct reinforcement methods suitable for masonry structures 　　　2.1 Reinforcement method using reinforced concrete overlays This method is a type of composite section reinforcement technique. Its advantages include a simple construction process and strong adaptability; the load-bearing capacity increases significantly after the masonry is reinforced, and there is mature design and construction experience available ； Suitable for the reinforcement of columns and walls with partitions ； Its disadvantage is that the wet construction time on site is long, which has a certain impact on production and daily life, and the clear height of the reinforced building is reduced to some extent. 　　2.2 Reinforcement method using a reinforced cement mortar overlay The reinforcement method using a reinforced cement mortar overlay involves removing the plaster layer from the surface of the brick wall that needs to be reinforced, attaching φ4–8mm steel mesh sheets to both sides of the wall, and then applying a cement mortar cover layer, as shown in Figure 1. 　　This method belongs to a type of composite section reinforcement method. Its advantages are similar to those of the reinforced concrete overlay reinforcement method, but it does not improve load-bearing capacity as much as the former ； It is suitable for the reinforcement of masonry walls, and is sometimes also used to seal the stirrups passing through the walls on both sides when reinforcing wall columns in reinforced concrete overlays. 　　2.3 Additional diaphragm wall columns for reinforcement The form of the concrete diaphragm walls is shown in Figure 2. The connection between the concrete counterweight column and the original wall is very important. 　　This method belongs to the category of reinforcement methods that increase the cross-section. Its advantages are similar to those of the reinforced concrete overlay reinforcement method, but the increase in load-bearing capacity is limited, and it is more difficult to meet seismic requirements; it is generally used only in non-seismic areas. 　　 　　3 Indirect reinforcement methods suitable for masonry structures 　　　3.1 Unbonded external steel reinforcement method Cement mortar is used to attach angle steel around the load-bearing brick columns, which are then secured with clamps; subsequently, gussets are welded to the angle steel to form a single unit. The clamps are removed, and cement mortar is applied to protect the angle steel, as shown in Figure 3. 　　This method belongs to the traditional reinforcement techniques; its advantages include simple construction, less on-site work and wet work, as well as relatively reliable stress performance ； Suitable for the reinforcement of masonry columns where it is not allowed to increase the cross-sectional size of the original members, yet a significant increase in the cross-sectional strength is required ； Its disadvantages are high reinforcement costs and the need for protective measures similar to those used in steel structures. 　　3.2 Prestressed strut reinforcement method This method can significantly increase the load-bearing capacity of masonry columns, and the reinforcement effect is reliable ； Suitable for strengthening masonry structures that are under high stress and high strain conditions ； Its drawback is that it cannot be used in environments with temperatures above 600°C. 　　 　　4 Structural Strengthening and Repair of Masonry Structures 　　　　 4.1 Adding Ring Beams for Strengthening When the ring beams are not installed in accordance with current design codes, or when there are significant defects at the junctions of vertical and horizontal walls, or when the overall integrity of the building is poor, ring beams should be added to reinforce it. 　　4.2 Adding beam pads for reinforcement When the brick masonry under the beam is partially crushed or vertical cracks appear in the wall beneath the beam, beam pads should be added for reinforcement. 　　4.3 Partial demolition of masonry structures When a part of a building is damaged, but the cause of the damage has been identified and it does not affect the structure’s load-bearing capacity or safety, the damaged wall section can be demolished, and then filled in with whole bricks using mortar of one higher strength grade. 　　4.4 Repair of masonry cracks Before carrying out crack repairs, it is necessary to determine the cause of the cracks in the masonry, taking into account factors such as the stress conditions of the masonry elements and the characteristics of the cracks. This allows for targeted crack repairs or the implementation of appropriate reinforcement measures. There are methods such as filling and repairing with cement mortar, filling and repairing with reinforced cement mortar, and repair through grouting. 　5. An example: A building in Yisuhé, Xiangtan City, which is a six-story brick-concrete structure with a construction area of over 5,000 square meters. When the construction of the main structure reached the sixth floor, vertical cracks appeared in the load-bearing transverse walls of the first and second floors. More than 80% of the load-bearing transverse walls on the lower level are cracked; the spacing between vertical cracks is approximately 800–1200 mm, while the length of these cracks is around 600–1000 mm. The cracks extend not only along the mortar joints but also through the individual blocks themselves. The width of these cracks is about 1–2 mm ； More than 60% of the load-bearing transverse walls on the second floor are cracked; the spacing between the vertical cracks is 1000–1500 mm, and these cracks extend along the grout lines as well as through the building blocks themselves. The width of these cracks ranges from 0.5 to 1 mm. From the lower floor, there are occasional sounds of blocks cracking, indicating that the building on the lower floor is about to collapse. Fortunately, the construction team took urgent action: they used over 100 fir strips to attach to the side walls in order to support the floor slabs. Once supported, the cracks were brought under control, and no further expansion of those cracks occurred after these remedial measures were taken. The construction unit immediately reported to the Municipal Construction Project Quality Supervision Station, and subsequently to the Municipal Construction Committee and the Provincial Construction Committee. Investigations have revealed that the direct cause of the serious safety hazards in the building’s structure was severe cutbacks in construction quality. The original design for this masonry structure called for the use of MU7.5 small concrete hollow blocks, bonded together with M5 mixed mortar, in the ground floor. According to sampling tests, the mortar strength grade of this layer is M1, and the strength grade of the bricks is MU5.0, **which is below the design requirements. Upon recalculation, the compressive strength of masonry made with M5 mortar and MU7.5 blocks meets the safety requirements, whereas the compressive strength of masonry made with M1 mortar and MU5.0 blocks only reaches 65%–70% of the required value. The indirect cause of the quality accident was management violations; the project did not go through the necessary quality supervision procedures, and there was no oversight. Moreover, the constructor lacks professional technicians for on-site inspections. Depending on the severity of the issue, the construction authorities have ordered demolition and reconstruction, and plan to hold a provincial meeting on quality and safety at the site. At that time, over 2 million yuan had already been spent on this project; after demolition, only some of the precast hollow panels could be recovered, and their value was roughly equal to the costs of demolition and site cleanup. Thus, the investment of over 2 million yuan was lost with no return. However, the funds for this project came from employee contributions, and the construction contractor had no capacity to repay them; it was preparing to accept criminal penalties to settle the matter. At the repeated requests of the construction unit, and with the approval of the relevant construction authorities, experts from Xiangtan University, Hunan University and other institutions were tasked with carrying out the reinforcement design as well as overseeing and guiding the implementation of the reinforcement work. The cost of final settlement for strengthening the building was 250,000 yuan. The cost of 250,000 yuan was sufficient to offset losses of over 2 million yuan, demonstrating that strengthening dilapidated buildings is economically much more advantageous than demolishing them and rebuilding them. After reinforcement, the buildings passed the safety inspections and met all safety requirements; they have been in safe use for 10 years to date. 　　The following describes the reinforcement plan and the main calculations: 1. Reinforcement Plan Before carrying out the reinforcement design, the entire set of drawings was reviewed, and both the masonry elements as well as the concrete beams and columns other than those made of masonry were inspected and their calculations rechecked, to ensure that while the load-bearing transverse walls become safe through reinforcement, no safety hazards remain in other structural elements. Reinforcement measures are taken simultaneously for the identified problems, and the solutions are as follows: (1) Reduce the number of floors by removing load; the main building is reduced from seven floors to six floors ； 　　(2) The first and second floor walls are reinforced with reinforced concrete layers on both sides ； 　　(3) The local compressive strength of the hollow blocks is insufficient; therefore, shims are placed at the ends of the beams or pressure grouting is used to fill the voids in the blocks at those ends ； 　　(4) Structural columns are added at the exterior wall corners to reinforce the stability of the walls ； 　　(5) The cast-in-place reinforced concrete slabs are strengthened by reducing the span, adding secondary beams, and thickening the surface layer by 40 mm ； 　　(6) Add load-bearing walls under beams with insufficient bearing capacity. 　　Here, the reinforcement scheme for the load-bearing transverse walls is highlighted. Since the walls on the ground floor are on the verge of failure, a concrete composite masonry approach is adopted: 60-mm-thick reinforced concrete walls are cast in place on each side of the original 240-mm-wide walls. The vertical reinforcement uses φ8 bars with a spacing of 150 mm, while the horizontal reinforcement uses φ6 bars with a spacing of 200 mm. The concrete used is C20 fine-grained concrete. The concrete on both sides of the walls is connected by φ6 bars running through the walls, which are placed at the brick joints, with a spacing of 900 mm in both directions. Due to the severe damage to one layer of the wall, its load-bearing capacity is ignored in the reinforcement calculations; thus, all the loads are borne by the concrete walls on both sides ； Due to the minor cracks in the second-layer wall, reinforcement was carried out by applying a cement mortar layer with rebar mesh on both sides. The load-bearing capacity of the original wall was taken into account during the reinforcement calculations; the cement mortar used was of M10 grade with a thickness of 3 mm per side, and the rebar mesh configuration was the same as that of the lower layer. 　　2. Reinforcement calculations 　　(1) Reinforcement calculations for the bottom wall 　　A transverse wall 1m wide is considered; the design value of the axial pressure, as determined through statistical calculations, is 310 KN. 　　Calculation of the height-to-thickness ratio of the transverse wall: In the calculations above, the load-bearing capacity of the original wall was not taken into account; it is evident that the reinforcement effect of the composite masonry is very safe. According to the calculations, the concrete layer on both sides can be made thinner, but a thickness of less than 60 mm would pose difficulties during construction. Therefore, the reinforcement is carried out according to the original design specifications. 　　(2) Strengthening calculation for the double-layer wall: A transverse wall 1 m wide is considered, and the design value of the axial pressure is 260 KN as determined through statistical calculations. 　　Calculation of wall height-to-thickness ratio Thickness of composite masonry wall: Height-to-thickness ratio: Calculation of the bearing capacity of composite masonry: The above calculations show that a reinforced cement mortar surface layer also provides an excellent strengthening effect, and it is easy to apply in construction. 　　 　　6 Conclusions 　　　6.1 All engineering and technical personnel involved in construction projects should first pay attention to the quality of building products, ensuring that no quality incidents occur ； 　　6.2 For construction projects that require reinforcement, there are many available reinforcement techniques. Different techniques have varying degrees of suitability, and in practical applications it is very important to choose the appropriate reinforcement scheme. This requires taking into account the specific requirements of different projects and working conditions, as well as economic considerations, in order to make the best choice. 　　6.3 When carrying out reinforcement work, we should strictly follow the procedures of conducting inspections and assessments first, then selecting and designing the reinforcement plan, and finally organizing construction inspections and approvals. 　　6.4 Practical experience in structural reinforcement shows that using reinforced cement mortar to reinforce masonry walls yields excellent results, and the construction process is straightforward.