Steel reinforcement plays a crucial role in the strength and stability of concrete foundations. Concrete, while strong in compression, is weak in tension, and steel reinforcement (rebar) is added to counteract this weakness. Proper placement of both horizontal and vertical rebar in a footing is essential for the overall performance of the structure. This article explores the key aspects of rebar placement, including minimum concrete cover, reinforcement location, support, tying, and the potential impact of incorrect practices.
Placement of Rebar in Footings
The placement of reinforcement in concrete footings involves several important steps. These include ensuring proper concrete cover, correctly locating the reinforcement, tying the rebars together, and using appropriate supports to maintain the proper position. Each of these activities is vital to achieving the desired strength and durability of the foundation.
Minimum Concrete Cover
Concrete cover is the thickness of concrete that protects the reinforcing steel from environmental factors, such as moisture and chemical exposure, which can lead to corrosion. In addition to providing corrosion protection, adequate cover is essential for ensuring proper bonding between the steel and the concrete, allowing the steel to effectively contribute to the load-bearing capacity of the structure.
The American Concrete Institute (ACI) 318 Building Code provides specific guidelines for minimum concrete cover based on the type of exposure and the reinforcement used:
- For footings (cast against and permanently exposed to earth), the minimum cover is typically 3 inches.
- For concrete exposed to weather or earth, such as basement walls, the minimum cover is:
- #6 bars and larger: 2 inches
- #5 bars or smaller: 1½ inches
- For concrete not exposed to weather or earth, such as slabs, walls, and joists:
- #14 and #18 bars: ½ inch cover
- #11 bars and smaller: ¾ inch cover
- Beams and columns: 1½ inches cover
Maintaining the correct concrete cover is essential for ensuring the longevity and structural performance of the footing.
Placement of Rebars
The placement of rebars is a critical aspect of rebar placement in footings, affecting the load-carrying capacity and overall stability of the structure. A general rule of thumb is that reinforcement should be placed at the tension side at the bottom of the footing. This ensures that the steel will resist tensile forces, which is particularly important when the footing is subjected to bending.
Square Footings:
In square footings, rebars are typically arranged uniformly in both directions to ensure an even distribution of forces across the footing. According to ACI guidelines, rebars should not be placed more than 18 inches apart.
Rectangular Footings:
For rectangular footings, the rebars in the long direction are placed uniformly, but in the short direction, the placement is slightly more complex. A portion of the reinforcement in the short direction must be placed within a band that matches the width of the footing in the short direction. This ensures that the footing can effectively distribute loads in both directions.
Improper rebar placement can have serious consequences on the structural performance of the footing. For example, if the top bars are placed too low or the bottom bars are raised too high (by even ½ inch more than specified), the footing’s load-carrying capacity could decrease by as much as 20%, particularly in a shallow slab.
Reinforcement Supports
To maintain the correct position of rebar, reinforcement supports are used. These supports prevent the rebars from making contact with the soil or ground, which could lead to corrosion over time. They also ensure that the rebars stay in the correct position throughout the construction process.
Bar supports are essential for maintaining the correct concrete cover during placement. These supports are available in various materials, including steel wire, precast concrete, and plastic, and come in a range of sizes and heights to suit different rebar configurations. The height of the support depends on the size of the reinforcement bars being used.
It is important to note that bar supports should not be used as a foundation for construction equipment such as concrete pumps, buggies, or laser screeds. These heavy tools can damage or misalign the supports, compromising the structural integrity of the reinforcement.
Reinforcement Tying
Once the reinforcement bars are placed, they need to be securely tied to prevent displacement during construction activities and concrete pouring. Although tie steel does not contribute directly to the strength of the structure, it plays a crucial role in maintaining the position of the rebars.
Typically, 16½ or 16-gauge black, soft, annealed wire is used for tying the rebars, though heavier gauge wire (15 or 14 gauge) may be required for larger or heavier reinforcement bars. It’s not necessary to tie every intersection of rebar; tying every fourth or fifth intersection is usually sufficient. However, care should be taken to ensure the ends of the tie wires do not make contact with the concrete surface, as they could rust over time and lead to structural issues.
There are different methods of tying rebar, each suited to specific situations:
- Snap Tie: The simplest tie, typically used for horizontal rebar in flat positions.
- Wrap and Snap Tie: Used for tying vertical wall reinforcement, helping to hold bars securely in place.
- Saddle Tie: A more complex tie used for securing bars at column corners and stirrups at beam corners.
- Wrap and Saddle Tie: Similar to the saddle tie, but with an additional wrap around the first bar to secure it better.
- Figure Eight Tie: Often used in walls to secure heavy mats of reinforcement.
These tying techniques ensure that the reinforcement bars stay in place, minimizing the risk of displacement during the pouring and curing of concrete.
Additional Considerations for Footing Rebar Placement
- Bar Support Spacing: The spacing of bar supports depends on the size and type of reinforcement being used. For instance, high chairs may be placed 4 feet apart for #5 bars, and 3 feet apart for #4 bars in solid slabs.
- Prohibited Practices: Certain practices should be avoided during reinforcement placement, such as placing rebar directly on fresh concrete or adjusting the position of reinforcement bars while concrete is being poured. The practice of “hooking,” which involves pulling up reinforcement bars from the subgrade during concrete placement, should be strictly avoided, as it can lead to improper rebar positioning.
- Spacers for Vertical Concrete: Side form spacers, such as dobies (precast concrete blocks) or plastic shapes, may be used to maintain the correct cover for vertical concrete applications like walls.
- Responsibilities: The proper placement of rebar is a shared responsibility among several parties, including the ironworker, foreman, contractor, and inspector. All must ensure that the reinforcement is positioned according to the design specifications to prevent structural issues.
- Allowable Deviation: The ACI code allows a certain degree of deviation in rebar placement. For slabs and walls, a tolerance of ±3 inches is allowed. For stirrups and ties, the deviation depends on the beam depth (inches divided by 12) and column width (inches divided by 12).
Conclusion
Proper placement, support, and tying of steel reinforcement in concrete footings are vital to ensuring the strength, durability, and safety of a structure. Following guidelines for minimum concrete cover, accurate rebar placement, and using appropriate tying and supporting techniques will help to achieve optimal performance. By understanding and implementing these practices, engineers and construction workers can avoid common mistakes that could compromise the integrity of a foundation.