Increased Section Reinforcement Method, which involves enlarging the cross-sectional size of components, naturally enhances their load-bearing capacity. It is suitable for general components such as beams, slabs, columns, walls, and foundations. The advantages include effective reinforcement, cost-effectiveness, and broad applicability. However, the disadvantages include complex construction, substantial wet work, long project duration, and potential impact on clearances and aesthetics. For instance, this method is typically used when the reinforcement in components exceeds limits or when the axial compression ratio of wall columns is beyond the limit.

Section 1: Chongqing Reinforcement Company Design Regulations

From the load-bearing perspective, this method is applicable to components under compression and bending; the concrete strength of the original component should not be lower than C10; it can be calculated as a whole cross-section at once; it conforms to the plane section assumption, suitable for calculation of the cross-section.

Enhanced Calculation

1. Calculation Principles: When reinforcing bending members, the method involves increasing the cross-sectional area in the tension zone and the compression zone. When increasing the cross-sectional area in the compression zone, it becomes a composite bending member, and thus, the calculation method for composite bending members as per the current concrete code is adopted. For increasing the cross-sectional area in the tension zone, the reinforcement specification provides calculation formulas for both the normal and oblique sections, similar to those in the concrete code. However, a reduction factor is introduced for the added steel and concrete strength. The reduction factor for steel under normal bending is 0.9; for steel under shear in oblique sections, it is 0.9, and for concrete, it is 0.7. When calculating the normal section under axial compression, both the steel and concrete reduction factors are 0.8; under eccentric compression, they are both 0.9. Theoretical analysis indicates that the stress characteristics of the reinforced members are related to whether the reinforcement is unloaded or partially unloaded during construction. If not unloaded or only partially unloaded, the working condition of the reinforced member after reinforcement is considered a secondary load, with strain lag issues. When completely unloaded (which is difficult to achieve in actual engineering), although the working condition is a primary load, the section is still not as good as a new member constructed in one go due to the influence of secondary construction. The additional main steel is inevitably affected by various factors in terms of connection and stress state, which may lead to its strength not being fully utilized. Since the reinforcement design is calculated based on the entire section (primary load), considering these reduction factors is necessary.

2. Practical Calculation Method: Due to the complexity of the calculation formulas provided by the reinforcement specifications for designers, the use of computer programs is necessary. The 08 version of the PKPM series programs has added reinforcement design, whereas the PKPM software itself lacks this feature, necessitating the use of simple approximation methods. These methods should be both straightforward and highly operational, with a minimal degree of approximation to meet engineering design requirements. Approximation Method: Utilize the PKPM series software for calculations, but treat the new concrete as being of the same grade as the old concrete during the calculation. The design drawings show the new concrete to be one grade higher than the old, thereby partially or fully offsetting the reduction in concrete strength. Since the new added section typically occupies a small portion of the total section, the calculated area of reinforcement needed for reinforcement is generally increased appropriately during the shear calculation of the inclined section to compensate for the insufficient concrete strength reduction. Additionally, consider a reduction of 0.9 for the steel strength when subjected to bending, shearing, and eccentric compression. Furthermore, use the flat cross-section assumption to reduce the strength or section of the old steel during bending, thereby determining the new steel section area.

III. Construction Regulations

1. The minimum thickness for new concrete layers: slabs should not be less than 40; beams and columns should be not less than 60 when manually cast, and not less than 50 when shotcrete.

2. Reinforcement bars shall be hot-rolled, with the diameter of the tensile steel bars on plates not less than 8; the diameter of the tensile steel bars on beams not less than 12; the diameter of the tensile steel bars on columns not less than 14; the diameter of the anchor-type stirrups not less than 8, with the diameter of the U-shaped stirrups being the same as the original stirrups; the diameter of the distribution bars not less than 6.

3. The added longitudinal reinforcement of梁 should be securely anchored at both ends, with different simple yet reliable anchoring methods selected based on the nature of the beam and the type of support. The added longitudinal reinforcement of the column should extend into the foundation and meet anchoring requirements, with the top end passing through the floor to connect with the foot of the upper column or topped and anchored at the roof deck. The reliable anchoring of the reinforced steel is crucial, as poor anchoring can hinder the full utilization of the added reinforcement.

4. The connection between the longitudinal reinforcement and the original longitudinal reinforcement can be achieved through short steel bar welding. The diameter of the short steel bar should not be less than 20mm, and its length should not be less than 5 times its diameter. The center distance between the centers of each short bar should not exceed 500mm. Refer to Specification Page 28, Section 5.5.3.

5. When adding tie rods in the shape of a U-bolt, the U-bolt should be welded onto the existing tie rod, or the tie rod can be anchored using the anchoring method; when reinforcing with a sleeve, an annular tie or a胶anchor-type tie rod should be installed.

IV. Construction Requirements

1. The increased cross-section reinforcement method calculates as a whole section. Therefore, the new and old concrete must be able to work together like a single type of concrete. To meet this requirement, the bonding quality at the interface between the new and old concrete must be ensured, thus, certain surface treatment requirements for the original concrete components are necessary. It is required to remove the plaster layer from the component surfaces, clean the defects in the concrete surface to dense areas, and roughen the surface to create a pattern of ridges or grooves. The depth of the pits and grooves should not be less than 6mm, with not fewer than 5 pits per 100mm x 100mm area, and the spacing of the grooves should not exceed the spacing of the rebars or 200mm. The component surfaces should then be washed clean with pressured water and coated with a layer of high-bonding section bonding agent. When a higher bonding quality is required at the interface between new and old concrete, small pits can be drilled into the bonding surface, with φ10 short rebars embedded, extending 100~150mm, half in and half out of the pit, with a spacing of 200~300mm, arranged in a plum blossom pattern. Expansion-type anchor bolts can also be anchored into the interface between new and old concrete.

2. Reinforcing Steel Connection Enhancement: When using short steel bars for welding, the concrete protective layer should be chiseled away to expose at least half of the steel section. Weld the reinforcing steel, short steel bars, and original steel bars together. During welding, the process should be done by sections, segments, and layers, starting from the middle towards both ends, ensuring full welds. Before welding, consider the load-bearing condition of the reinforced component, and if necessary, implement unloading or temporary support measures.

3. Concrete Mortar Protective Layer: When longitudinal rebars are connected with short rebars, it is generally used to apply a concrete mortar protective layer. The construction requirements are as follows: After the base is treated, apply a thin layer of 1:1 cement mortar mixed with 10% construction adhesive, approximately 3mm thick. After 24 hours, proceed with the plastering. Before plastering, moisten the base with water. The plastering should be done in layers and multiple coats. Typically, it is divided into the base layer, middle layer, and surface layer, with each layer being between 6-10mm thick. Latex and other materials can be added to the mortar. The curing period should generally last for no less than 3 days.

4. The maximum particle size of coarse aggregate in concrete should not exceed 20mm. For concrete with a thickness less than 100mm, it is advisable to use fine aggregate concrete.