Steel Reinforcement Materials
Steel reinforcement provides tensile strength to concrete structures, compensating for concrete’s inherent weakness in tension. , and proper bond development between steel and concrete should be verified during constructionDeformed reinforcing bars with surface ribs create mechanical bond with the surrounding concrete, transferring forces between the two materials. The most common grades are Grade 60 with a yield strength of 60,000 psi and Grade 75 with a yield strength of 75,000 psi. The bar size designation system uses numbers from 3 through 18, with each number representing the bar diameter in eighths of an inch. A number 4 bar has a diameter of 1/2 inch, while a number 8 bar has a diameter of 1 inch.
Welded wire reinforcement consists of cold-drawn wire welded into a grid pattern for use in slabs, walls, and pavements. The wire spacing and diameter are specified to provide the required steel area in each direction. Steel fibers are increasingly used as secondary reinforcement in concrete to control cracking and improve toughness. Fiber-reinforced concrete does not replace primary reinforcement but can reduce the amount of conventional reinforcement needed for crack control in certain applications.
Design Principles
The design of reinforced concrete follows the basic principle that steel resists tension while concrete resists compression. The neutral axis separates the compression zone from the tension zone in a flexural member. Understanding concrete cover requirements is essential for achieving quality results in this aspect of construction. Understanding mechanical bar splices is essential for achieving quality results in this aspect of construction.Understanding minimum reinforcement requirements is essential for achieving quality results in this aspect of construction.Reinforcement must be placed on the tension side of the member, which is the bottom of simply supported beams and the top of continuous beams over supports. The required reinforcement area is calculated based on the design moment, material strengths, and section geometry using the principles of strain compatibility and force equilibrium.
Minimum reinforcement requirements ensure that reinforced members do not fail suddenly when cracking occurs. ACI 318 requires that the provided reinforcement be sufficient to resist at least 1.2 times the cracking moment of the section. This ensures ductile behavior and provides warning before failure. Maximum reinforcement limits prevent over-reinforced sections where concrete crushing would occur before steel yields, resulting in brittle failure. The balanced reinforcement ratio represents the condition where steel yielding and concrete crushing occur simultaneously.
Placement and Spacing Requirements
Proper reinforcement placement is essential for structural performance but is often compromised in the field due to congestion and access limitations. Clear spacing between parallel bars must be at least 1 inch, 1.33 times the maximum aggregate size, or 1.5 times the bar diameter, whichever is greater. These spacing requirements ensure that concrete can flow around and between bars during placement, preventing voids and ensuring bond development. For columns, the clear spacing between longitudinal bars must be at least 1.5 times the bar diameter or 1.5 inches.
Concrete cover requirements protect reinforcement from corrosion and fire. The minimum cover depends on the exposure condition and member type. Interior beams and columns require 1.5 inches of cover, while exterior members exposed to weather require 2 inches. Slabs and walls cast against earth require 3 inches of cover. Prestressed members and members exposed to corrosive environments require additional cover. Proper cover is maintained using bar supports, commonly called chairs, bolsters, and spacers, that hold the reinforcement at the correct elevation during concrete placement.
Bond Development and Splices
The bond between steel and concrete transfers forces and prevents slippage. Development length is the length of embedment required to develop the full yield strength of the bar. Development length depends on bar diameter, concrete strength, bar coating, and confinement conditions. Epoxy-coated bars, used for corrosion protection in corrosive environments, require 1.2 to 1.5 times the development length of uncoated bars due to reduced bond.
Lap splices are the most common method of connecting reinforcing bars. The lap length must be sufficient to transfer the force between the two bars through bond. Tension lap splices are classified as Class A or Class B depending on the percentage of bars spliced at the same location and the amount of reinforcement provided. Class B splices require 1.3 times the development length and are used when more than half the bars are spliced at one section. Mechanical splices using couplers provide an alternative to lap splices, particularly for large bars where lap lengths would be excessive.