How to Replace Reinforcing Steel?
In larger projects the iron-workers will place the reinforcing steel, most contractors place some reinforcement. What is important is to get it in the right place and keeping it there during concrete placement which plays a critical role to the structure's performance. Reinforcement must be placed as planned on the placing drawings. In the placing drawings, the detailer will indicate the number of bars, bar lengths, bends, and positions.
One of the most important reason for placing the reinforcing steel in the right order is to get the right amount of concrete cover—which is the amount of concrete between the reinforcing steel and the surface of the concrete member. The other really important factor when it comes to protecting reinforcing steel from corrosion is the cover.
It is also necessary to have a cover to make sure that the steel bonds to the concrete well enough to develop its strength. The cover requirements are mostlymentioned in the project specifications or are even shown on the drawings. In case it is not specified then theACI 318 Building Code must be referred to know about the minimum cover for cast-in-place concrete.
Minimum concrete cover:
In case of concrete cast against and permanently exposed to earth (such as footings): 3 inches
In case of concrete exposed to weather or earth (such as basement walls):
6 bars and larger: 2 inches
5 bars or smaller: 1½ inches
For concrete not exposed to weather or in contact with ground:
Slabs, walls, and joists: 14 and 18 bars: ½ inch
Slabs, walls, and joists: 11 bars and smaller: ¾ inch
Beams and columns: 1½ inches
The design of the structure depends on having the steel in the right place, that is something one must remember. Incorrect reinforcing steel placement must never take place as it can and has led to serious concrete structural failures.
Such as, by lowering the top bars or raising the bottom bars by ½ inch more than that specified in a 6-inch-deep slab could lead to a reduction in its load-carrying capacity by 20%.
It is inacceptable if you decide to position by placing reinforcement atop a layer of fresh concrete and then pouring more on top of it. Reinforcing bar that are made of steel wire, precast concrete, or even plastic, must be used for support.The chairs and supports can be found in various heights which can support specific reinforcing bar sizes and positions. As it is already known that when it comes to expense plastic accessories are less expensive than metal supports.
You can refer to The Concrete Reinforcing Steel Institute's Ready Reference Reinforcing Steel Resource Guide or the classic Placing Reinforcing Bars which has three tables using which you can understand and learn about most of the currently available supports in the various materials along with the description of situation where each is most effectively used.
Just by simply placing the bars on supports cannot be enough. It is important to secure the reinforcing steel to avoid any displacement during construction activities and concrete placement. This problem is most commonly solved by using tie wire. The tie wire comes in 3- or 4-pound coils.
The way to use it is by placing the wires in a wire holder or a reel is suspended from the worker's belt for accessibility. The usual gauge of the wire is 16½- or 16-gauge black, soft, annealed wire, but in case of heavier reinforcement there might be the need to use 15- or 14-gauge wire to hold the proper position of the rebar.
There a number of different types of tie types (ties are basically wire twists for connecting intersecting bars), ranging from snap ties to saddle ties, which are then used in the concrete reinforcing industry. To understand even more clearly you can check the CRSI's Placing Reinforcing Bars which illustrates the types of ties and describes the situation where each is most effectively used.
If you want to tie epoxy-coated bars then you must use PVC ties. You can also easily find proprietary snap-on ties which are available, for example the Speed-Clip Rebar Tie from Con-Tie Inc. It is an extremely simple device using which you can attach rebar in parallel or at any angle by hand. You also do not require any tools in the process.
You do not have to tie every section when you are tying bars as every fourth or fifth is normally enough. You must keep in mind that the tie contributes no strength to the structure, therefore, you can only need more in case the steel might become displaced in the process of concrete placement.
You must make sure that the tie wires are away from the surface of the concrete where there is a chance for them to rust. You can tie a number of intersections in preassembled mats or reinforcing steel just to make the assembly rigid enough for placing, you can do this in every intersection around the outside and every other in the middle of the mat. You must not tack weld the intersections as it is not usually allowed, as that will help in the reduction of cross-sections in the bar.
Regarding the tolerances on placement
There will always be a scope for little variation even though bars should be placed as close to the specified position as possible. As defined by ACI 117 about the tolerance of rebar positioning, “Tolerances for Concrete Construction and Materials,” which are shown in the table.
Remember what this means: According to ACI 117, a tolerance is the permitted variation from a given dimension which truly means that how far off the rebar actually is from what is shown in the drawings. Therefore, if the clear distance between the outside of a reinforcing bar and the face of a 6-inch-wide concrete beam is specified to be 2 inches, then the tolerance allows it to be no less than l 5/8 inches.
You will also find that the tolerance on the position of longitudinal bars, is quite loose which is around ±3 inches. The reason for this position variation is because the exact position is not that critical for as long as the proper cover is maintained and the specified number of bars are included.
Here are some things to remember about placing rebar:
- For construction equipment such as concrete pumps, buggies, or laser screeds bar supports are not provided.
- The size of the reinforcing bar for support determines the spacing of bar support. Such as, for a one-way solid slab with 5 temperature-shrinkage bars, along with high chairs are used at 4 feet on center; for 4 bars, you will find that the high chairs would be placed 3 feet on center.
- The placement of reinforcement onto layers of fresh concrete or adjusting the position of bars or welded wire reinforcement during concrete placement are not allowed. Hooking is the process in which the placementof reinforcement is on the subgrade and it is pulled up during concrete placement.
- The responsibility to check that the reinforcing bars in concrete construction are placed correctly falls on the ironworker, ironworker foreman, contractor, and inspector.
- These are the deviation from specified location: In slabs and walls, other than stirrups and ties ±3 in. Stirrups: depth of beam in inches divided by 12. Ties: width of column in inches divided by 12.