What is Steel plate bonding strengthening of concrete?
Strengthening through steel plate bonding is a structural enhancement technique that involves attaching steel plates to concrete surfaces using high-strength adhesives, primarily epoxy. This method, pioneered in the late 1960s, significantly increases the flexural strength or stiffness of concrete structures. By bonding steel plates to specific areas of beams and slabs, it forms a unified composite structure that improves the overall strength and stiffness of the existing concrete element. The success of this method depends on the quality of both the concrete and the bond achieved between the steel plate and the concrete surface.
What is Steel plate bonding strengthening of concrete used for?
The primary use of steel plate bonding strengthening is to enhance the flexural load-bearing capacity and stiffness of concrete structures, making it particularly valuable for seismic reinforcement, adapting structures to new uses or requirements, addressing deficiencies in construction quality, and correcting design or construction flaws.
This technique can be used to correct insufficient flexural capacity of concrete structures arising due to under-design, construction faults or increased loading.
How does Steel plate bonding strengthening of concrete work?
The addition of steel plates to concrete elements for strengthening purposes leverages the high flexural strength of steel to significantly enhance the flexural capacity of the concrete. Unreinforced concrete inherently has a much lower flexural strength, typically about 10 to 15 percent of its compressive. When concrete is subjected to bending, the tensile stresses generated on the tensile face can easily lead to cracking and failure, given that concrete's tensile strength is significantly lower than its compressive strength.
By bonding steel plates to the tension face of concrete beams or slabs, the process effectively creates a composite section where the steel plate acts in tandem with the concrete, thereby increasing the overall flexural strength of the member. The steel plates do not change the loading profile of the structure in a traditional sense but redistribute the internal stresses more favourably, allowing the concrete element to withstand greater moments and shear forces than it could on its own.
This composite action between the steel plates and the concrete improves the ductility of the structure, enabling it to undergo larger deformations without failing, which is particularly advantageous in seismic applications. The key to achieving this synergy between the steel and concrete lies in the effective bonding of the steel plates to the concrete surface, ensuring that the load is adequately transferred and shared between the two materials.
Flexural strength, or the ability of a material to resist deformation under load, is critical in the design of structural elements subjected to bending. For concrete, this capacity is significantly enhanced through the strategic application of steel plates, making steel plate bonding an effective solution for improving the structural performance of existing concrete structures facing increased load demands or requiring retrofitting to meet current engineering standards.
How do I repair concrete structures using Steel plate strengthening of concrete?
A step-by-step procedure to carry out steel plate bonding for strengthening of concrete structures to comply with BS EN 1504-4:2004 involves the following:
Conduct a detailed assessment of the concrete structure to identify the need for strengthening and to ensure the concrete has adequate quality with good surface condition and sufficient compressive and tensile strength.
Perform structural analysis to determine the load-bearing capacity required.
Design the strengthening scheme including what type of steel plates, their required dimensions and the optimal positioning.
Select the most suitable bonding compound based on concrete condition, the forces involved and the environment in which it is to be used. Typically, a form of epoxy.
Prepare the concrete surface meticulously to achieve the necessary profile and cleanliness, ensuring a strong bond between the steel plates and concrete. Repair any areas with poor concrete quality and fill in cracks.
Apply the selected high-performance adhesive evenly on the steel plates or directly on the concrete surface as specified by the product guidelines.
Carefully position and fix the steel plates onto the prepared concrete surface, ensuring proper alignment and pressure to achieve a uniform bond.
Allow the adhesive to cure as per the manufacturer’s specifications, ensuring the environment is within the suitable temperature and humidity ranges for optimal curing.
Conduct quality control tests such as pull-off tests to verify the adhesive bond strength between the steel plates and the concrete surface.
Document the entire process, materials used, and test results for future reference and compliance verification.
Perform a final inspection of the bonded steel plates to ensure the strengthening work meets all specified requirements and standards.
What equipment and expertise are required for repairing concrete structures using Steel plate strengthening of concrete?
Minimal specialised equipment is required to strengthen concrete using steel plate bonding. The simple tools needed include:
Surface preparation tools such as grinders and sandblasters.
Adhesive application equipment such as rollers, brushes and notched trowels.
Clamps and weights for securing steel plates during curing.
Quality control testing equipment like pull-off testers.
Significant expertise is however required in the assessment of structural load bearing capacity, calculation of the tensile and compressive loading of each element and designing optimal reinforcement schedules to increase the flexural strength of the structure. Skill and experience in surface preparation techniques for optimal adhesive bonding, the application of structural adhesives and placement of steel plates and proficiency in conducting quality control tests and interpreting results are also required.
What are the advantages of repairing concrete structures using Steel plate strengthening of concrete?
The method has proven its ability to provide durable and reliable repairs that meet design requirements for strength and rigidity over long periods, supported by 30-year aging tests for adhesives.
The repair is quick and efficient, offering rapid completion, enabling projects to continue without production halts. This fast-paced construction reduces overall project timelines.
The process is clean, requiring no wet work, and does not significantly alter the structure's appearance, maintaining architectural integrity.
Compared to other reinforcement methods steel plate bonding saves both time and money.
The method is competitive compared to other strengthening techniques due to its lower installation costs which makes steel plate strengthening a viable option for a wide range of project budgets.
The technique can be applied with little to no disruption to the ongoing services or usage of the structure, particularly advantageous in environments where downtime needs to be minimized, such as in industrial facilities or essential infrastructure.
What are the disadvantages of repairing concrete structures using Steel plate strengthening of concrete?
The application process for steel plates is highly sensitive to environmental factors. Moisture can significantly affect the quality of the bond between the steel plates and the concrete, while temperature variations can impact the curing time of the adhesives used, potentially compromising the effectiveness of the strengthening.
Steel elements are susceptible to corrosion over time, necessitating additional measures for corrosion protection. This can add to the overall cost and maintenance requirements of the strengthened structure, diminishing some of the cost advantages.
Steel is extremely dense and adds more weight to the structure than comparable reinforcement method such FRP plates.
What are the limitations of repairing concrete structures using Steel plate strengthening of concrete?
Structural issues may arise from peeling stresses that occur at the ends of the steel plates. These stresses can lead to debonding and failure of the strengthening system if not properly addressed during design and installation.
Certain codes and standards may require the steel plates to be bolted to the concrete in addition to being bonded. This requirement can increase both the construction time and the cost of the project, making it less appealing compared to other methods that do not have this requirement.
Steel plate strengthening systems must also consider protection against fire, as steel can lose its structural integrity at high temperatures. The need for fireproofing can add to the complexity and cost of the project.
The success of steel plate bonding heavily relies on the quality of the existing concrete. Adequate concrete strength and surface condition are crucial for ensuring a strong and durable bond. Poor concrete quality may limit the applicability of this strengthening method.
Ancillary information
Service disruption: Yes
Preliminary works: Yes
Posterior works: Yes
Time consumption: Medium
Cost: Medium
References and further information
ISO 8501-1: 2007, Preparation of steel substrates before application of paints and related products;