In construction and large-scale industries, reinforced concrete structures often play a key role, serving as frames, ceilings and functional platforms for various buildings. They carry multi-ton loads acting both in static and dynamic modes. Over time, stress cannot but affect the state of the structure. As a result, reinforcement of reinforced concrete structures is required in one way or another. The specific methodology for carrying out such operations depends on the operating conditions of the facility, technical and physical parameters and planning requirements.
What is reinforced concrete structures?
First of all, you should decide what a reinforced concrete structure is in principle. In capital construction, thispart of the structure that takes on increased operational loads. The basis of the structure is formed by a concrete structure, and reinforcing bars are used as its basic reinforcement. At the same time, strengthening and restoration of reinforced concrete structures can be carried out comprehensively and partially. If a defective area on the surface was identified by diagnostics, then the repair will primarily concern this part, although the causes of destruction must first be investigated, which can justify the feasibility of reconstructing other sections of the structure.
What is meant by amplification as such? This is a technical operation in construction, due to which the operational life of buildings and individual structures in particular is extended. There are different methods of repair and strengthening of reinforced concrete structures. All of them, to varying degrees, involve the solution of the following tasks:
- Increasing the strength of nodes and load-bearing components of the structure by including new elements. The latter can be beams, lintels, cantilever parts, stiffeners, etc.
- Unloading or redistributing the mass that acts on the reinforced concrete base. In this case, the arrangement of structures that mechanically affect the target area of fortification is affected. Unloading reduces the requirement for reinforced concrete structures.
- Increasing the basic strength characteristics of an object and its elements by replacing.
When it is necessary to reinforce the RC-designs?
Even at the stage of erection of the supporting frame, technical solutions and building materials are selected in accordance with future loads with the expectation of long-term operation. Over time, due to various factors, the technical condition of the structure deteriorates and there is a need to support its critical elements. Full reinforcement of reinforced concrete structures should be performed in the following cases:
- Loss of design strength due to aging and fatigue of materials. This is especially true of the concrete structure, which is subjected to negative chemical influences and natural mechanical stress.
- Remodeling of a building, as a result of which the configuration of load-bearing walls, beams, columns, trusses and consoles is changed. Strengthening or mass relief may be required at structural anchor points.
- Changing the number of floors. There is also a redistribution of weight over columns, ceilings and walls, which requires a revision and the bearing capacity of the elements of the structure.
- Ground movements that have either already deformed or changed the configuration of the impact on the foundation, and consequently on the load-bearing nodes of the frame. Restoration of force balance between structures is also required.
- Destruction or partial damage to load-bearing parts or individual elements due to accidents, natural disasters, earthquakes, man-made disasters.
- When errors are detected at the design stage or identified already during the operation of the building.
In this case, the main and mostcommon reasons that entail the need to strengthen reinforced concrete structures in one way or another. The specific nature of wear or damage should be determined during a comprehensive survey, on the basis of which a project for strengthening the structure is developed and the best way to implement it is selected.
Diagnosis and Troubleshooting of Design
Technical inspection is carried out in accordance with the schedule or unscheduled in case of obvious signs of the destruction of the building. This part of the activities is regulated by non-destructive testing standards in accordance with GOST 22690 and 17624. The assessment based on the results of the survey is made in accordance with the set of rules (SP) on the strengthening of reinforced concrete structures under the number 63.13330.
Diagnostic procedures begin with a visual inspection, during which external damage is detected - defects, chips, cracks, etc. To detect hidden damage, non-destructive testing methods are used. Such tasks are solved by means of special equipment, for example, with the help of electromagnetic or ultrasonic flaw detectors. In particular, ultrasonic devices operating by georadar and echo-pulse methods are more often used for the troubleshooting of reinforced concrete. During the inspection, voids, the presence of aggressive components in the structure, the destruction of reinforcing rods, traces of corrosion, etc.
Based on the data obtained, a further strategy is developed to eliminate damage, repair, restore orredistribution of loads. At the same stage, defectologists can give recommendations on strengthening reinforced concrete structures, taking into account the specifics of damage that can only be fixed by non-destructive testing tools. An important role in determining how to strengthen the structure will be played by specific technical and physical parameters on which the structure is operated.
Gain specifications
The parameters of the reinforcements may vary depending on the configuration of the application of the additional force and the specific requirements for supporting the structure. The most common characteristics are the modulus of elasticity of the support and tensile strength. Thus, the optimal reinforcement of reinforced concrete structures with composite materials on average provides elasticity in the range of 70,000-640,000 MPa and tensile strength indicators - from 1500 to 5000 MPa. Of course, it is not necessary in every case to strive for maximum performance. The choice of a specific power potential of the support and strengthening elements depends on the current state of the reinforced concrete structure.
As for the dimensional parameters, they will depend on the strengthening scheme, which is compiled on the basis of the planning solution. For example, fragmentary reinforcement of a reinforced concrete slab can be performed by additional support for a 300 mm thick monolithic beamless module. Reinforcing columns usually have an average section of 400x400 mm and are placed under the ceiling in increments of 5-7.5 m.determined by the stress-strain state of floors and load-bearing walls.
In a complex form, for example, the reinforcement of reinforced concrete structures with carbon fiber can have the following technical characteristics:
- The thickness of the element is 0.3 mm.
- Width - 300 mm.
- Weight - 500 g/m2.
- Modulus of elasticity – 230000 N/mm2.
- Density – 1.7 g/cm3.
- Tensile strength - 4000 N/mm2.
- Shear strength of structure - 7 N/mm2.
- Compressive strength of material - 70 N/mm2.
- Deformation at break of structure – 1.6%.
- Adhesion of composite fiber to concrete structure - 4 N/mm2.
- Young's modulus - 400 N/mm2.
The specificity of the use of modern composite materials is due to the fact that the adhesive composition plays a significant role in the assembly operations with them. Often it acts as an independent sealing and restoring means of strengthening the concrete structure. For example, epoxy compounds may well perform the functions of sealing technological seams and joints.
Regulations
In the process of calculating, designing and performing installation work, one should be guided by several GOSTs, among which are 31937, 22690 and 28570. These documents to varying degrees regulate the maintenance and reconstruction of buildings and structures. It is also necessary to take into account the standards of the document SP 63.13330, which gives specific instructions onorganization and implementation of repair and restoration measures, including the strengthening of reinforced concrete structures with composite materials. SP 164.1325800 will also help in the use of other plastic and fiberglass materials for reinforcement. General regulations to look out for are as follows:
- The development of the reinforcement project should be carried out only on the basis of data from the field survey of structures.
- By the time of calculation for materials and configuration of installation work, information about the size of the target object, its condition, methods of reinforcement, concrete strength, etc. should be prepared.
- After the examination, a fundamental decision is made on the admissibility of the structure for repair with further operation.
- Reinforcing measures should be carried out so that composite fibers or metal rods provide joint load work with the concrete structure.
- It is not allowed to reinforce structures in which there are pockets of corrosion damage.
- In the process of preparing a project, it is also important to calculate the need to provide additional protective properties of the material, for example, to include refractory or moisture-resistant coatings in the structure.
Concrete Reinforcement Benefits
Along with the rules governing the repair and restoration of building structures, it would be useful to initially prepare a base of methodological materials that will helpsolve the tasks in practice. To date, there are many visual instructions that describe step by step and visually the technologies for applying specific methods for the reconstruction of certain structures. For example, LLC "Interaqua" and "NIIZHB" offer a comprehensive guide to strengthening reinforced concrete structures with composite materials based on the set of rules SP 52-101-2003. The material describes the choice of structural solutions, the principles of calculating the strengthening of walls and ceilings, as well as technological methods for the use of carbon parts.
If we are talking about industrial facilities, then highly specialized manuals can be used, which also focus on the special operating conditions of structures. In particular, Far East PromstroyNIIproject LLC offers instructions for strengthening reinforced concrete structures of the 1.400.1-18 series. This material highlights the nuances of strengthening load-bearing walls and ceilings in the structure of industrial buildings.
Development of a structural reinforcement project
The main task of this stage is to propose a specific technical solution for the implementation of strengthening the structure of the target object. During the development process, specialists are guided by data on the characteristics of construction materials, their geometric parameters, operating conditions and existing damage. At the moment, the following design principles for the reinforcement of reinforced concrete structures have been developed:
- Interconnection of components. A common mistake that occurs during construction is to consider the work sitein an isolated format. That is, a load-bearing wall, for example, will be calculated based on direct loads on it without focusing on nearby factors of influence. In fact, a high-quality and durable system can only be designed with a comprehensive consideration of all operating factors.
- Optimization. The tasks of strengthening structures can be solved in different ways, and in almost every case there is a solution that will allow the facility to maintain a high working life. But at the same time, it is desirable to strive to minimize the amount of work, the mass of auxiliary supporting parts and rationalize the use of consumables. The lower the degree of intervention in the structure of the structure, the higher its reliability. By the way, modern technologies for reinforcing reinforced concrete structures with composite materials, which are smaller in size and weight compared to metal counterparts, just allow minimizing the volume of inclusion of foreign elements.
- Economic rationalization. Even if it is possible to use large financial resources in the implementation of the reinforcement project, it is important to consider that complex and massive technical solutions always require high costs already in the process of maintenance during the operation of the structure.
- Compliance with established requirements. Each design stage must take into account both the general normative rules and the specific requirements of the technical and structural device in relation to the target building.
Rules for calculating the reinforcement of reinforced concrete structures
Technical calculation of structures is the basis of design work, during which the actual loads are correlated with the power potentials of the materials used for reinforcement. The initial data for the complex calculation are taken from the design scheme, its dimensions, acting loads and the nature of damage. Separate articles in the assessment of materials for reinforcing reinforced concrete structures are calculated indicators for compressive strength, compression zone height, stability along inclined sections, etc.
The fundamental value of the design, which determines the ability to cope with actual loads, will be the moment of maximum bending. For its calculation, the reliability factors for the material and load are used. The nature of the distribution of damage over the cross section of the structure is also determined, taking into account the degree of its elasticity. If the initial maximum bending moment exceeds the process of cracking along the section, then the calculation should be carried out in the same way as for a section with cracks, without taking into account the potential for deformation development.
Constant values of target materials are also used in calculations to reinforce structures. Modern guidelines for strengthening reinforced concrete structures, in particular, rely on the following indicators:
- Strength - range from 1000 to 1500 MPa, but not less.
- Modulus of elasticity - from 50 to 150 GPa.
- Glass transition temperature (used for composites) - not less than 40 °С.
Dimensional parameters and installation configuration are determined individually in relation to a specificdesigns.
Classification of reinforcement methods
Modern technologies make it possible to use an extensive list of means of technical strengthening of various structures, adapting to specific operating conditions. At the basic level, it is worth dividing all methods of strengthening reinforced concrete structures on the basis of physical condition. In particular, liquid, woven and solid elements can be distinguished. In the first case, the strengthening will be carried out according to the method of repairing external damage. This can be the elimination of cracks by means of a sand-cement mortar, and the sealing of joints with adhesive building compounds. Fabric materials are used less frequently and mostly as a reinforcing agent, which is applied to the poured area with the same strengthening solutions.
As for solids, they are structural parts that are somehow integrated or superimposed on a damaged structure. In this case, the methods of reinforcing reinforced concrete structures can be divided both by the type of material used (metal, composites, stone) and by the installation configuration. The most popular method of reinforcing with solid products is belt reinforcement, in which profiled pads clamp the damaged area. But this is not the only way to use such products.
Basic methods of strengthening reinforced concrete structures
Depending on the results of the initial survey and based on the design decisionthe following methods of reinforcing reinforced concrete structures can be used:
- Laying repair plaster in order to restore the structure of the concrete surface. If there are open areas for the passage of reinforcement, they are also sealed with primer mixtures or plaster.
- Introducing concrete mortar into cavities, cracks, voids and other internal structural defects detected by non-destructive testing.
- Shotcrete with concrete mix. Concrete mortar is applied to the surface with special guns at high speed. This mechanics of treatment of damaged areas allows the formation of dense reinforcing layers of high strength.
- Strengthening the foundation on which the structure rests. This is done through reinforced concrete clips, metal belts, anchor ties and other solid elements.
- Strengthening reinforced concrete columns, beams and walls through the installation of complex reinforcing clips, frames and shirts. In such a device, elements of reinforcement, formwork and shotcrete can be used. Since this method involves the creation of rather significant additional structures, recommendations for strengthening reinforced concrete structures recommend carefully calculating the maximum load on the ceiling. Otherwise, after some time it will be possible to detect cracks already in the structure of the lower level load-bearing elements.
- Point increase in the durability of crossbars, beams, posts and supporting elements with composites. For such purposes, small-format, but durable parts made of carbon fiber, Kevlar, carbon andetc.
As practice shows, the most effective solution in supporting the power potential of reinforced concrete structures is precisely the structural change in their foundation. The addition of walls and ceilings with third-party support elements such as struts, on the contrary, is considered ineffective and technologically inexpedient. But again, specific decisions should be made on the basis of a comprehensive survey and calculation.
Reinforcement with steel and composites - which is better?
The fundamental division in many ways to strengthen building structures is based on the type of material used. Power solid-state rods and structural elements are the most common reinforcing fittings, but it can be made on the basis of traditional steel alloys, and using modern plastics. Which is better?
The advantages of metal include its versatility, high strength and affordable cost. By the way, the reinforcement of reinforced concrete structures with carbon fiber, with all the positive technical and physical qualities, can cost 20-30% more than using even high-quality stainless steel. What justifies such costs? Yet composites exhibit unsurpassed tensile strengths that surpass even steel. Also, unlike concrete, carbon fiber is characterized by a higher fatigue strength resource, which eliminates intermediate restoration measures during the long-term operation of the building. Are there any disadvantages of composites besides the high price? There are nuances of ecological properties, since inplastic is still the basis of the material, but the significance of the influence of synthetic additives is minimal in terms of danger to humans.
Conclusion
Measures for the repair, restoration and strengthening of reinforced concrete structures, as a rule, require a lot of organizational and financial costs. This is due to the complexity of their design and the technological problems of performing installation operations. Even minor cosmetic procedures must be carried out in several stages - from troubleshooting with the preparation of an object for work to the direct elimination of damage or an increase in the strength qualities of materials. Therefore, in the recommendations for the design of reinforcement of reinforced concrete structures, experts note the need to consider the most technically flexible options for solving the problem. For example, the simplest replacement of steel reinforcement with a diameter of 12 mm by a carbon fiber rod 8 mm thick with the same reinforcement effect will minimize up to 50% of power costs. But of course, such optimization is not always possible. In any case, the principles of maintaining the required strength, elasticity and rigidity of structures should come to the fore. Following the normative plans and high-quality installation schemes will make it possible to rationally perform the strengthening, as much as possible postponing the time for the need to complete the reconstruction of the building with the replacement of the reinforced concrete structure.
