Planning a concrete slab, patio or driveway? Are you trying to figure out if you need concrete rebar for your project? There are several factors that affect the design of the pouring process and the need for rebar.
Concrete supporting heavy loads, large vehicles or structures must be reinforced with steel rebar to improve its tensile strength. Pads 5 inches or thicker should also be reinforced, as should bridge doors. Concrete that rests on layers of poor soil or is exposed to freeze-thaw cycles also needs it.
In this article, we explain what rebar is, when it should be used, the types and sizes of rebar and how to install them. Also, we discuss wire meshes and when to use them. Our goal is to provide you with the information you need to determine if you need welded rebar or mesh for your project.
- What is concrete reinforcement?
- Does concrete always need reinforcement?
- When do you need to use rebar in concrete?
- concrete terrace
- concrete sidewalk
- Go away
- small concrete slab
- types of concrete reinforcement
- carbon steel rebar
- galvanized reinforcement
- Reinforcement steel made from stainless steel
- Epoxy coated rebar
- Reinforcement vs. wire mesh
- Can you pour concrete without reinforcement?
- Does rebar in concrete rust?
- What size reinforcement do I need for a concrete slab?
- Where does reinforcement go in a concrete slab?
- How to estimate the amount of reinforcement?
- How to reinforce concrete with rebar
- When do you use chicken wire on concrete?
What is concrete reinforcement?
Rebar or rebar are steel bars used to reduce the likelihood of concrete cracking due to tensile forces. Concrete has great compressive strength, but it can crack or fracture if not fully supported from below or from within. Reinforcing steel has been used to improve the tensile strength of concrete for over 150 years.
Rebar is manufactured from hot-rolled steel in various gauges and is used to reinforce concrete from the inside. It is commonly placed in a grid pattern, formed into a steel mesh, or arranged in parallel strands. The thicker the bar and the more times it is laid, the stronger the finished product.
Steel increases the ability of concrete to withstand forces or stresses acting against it horizontally or vertically. It distributes the forces through the concrete instead of centering and cracking it. Reinforced concrete is much stronger and more stable than unreinforced concrete. Rebar is often referred to as deformed steel because of the burrs that allow it to bond tightly to concrete.
Does concrete always need reinforcement?
Concrete slabs or slabs used for patios or driveways do not need reinforcement for cars, light trucks or deckchairs. When concrete needs to withstand heavy machinery, heavy vehicles, hot tub, spa or concrete pizza oven, reinforcement is recommended. Concrete supported on embankment or other disturbed or weak soil would also benefit from reinforcement.
Rebar helps reduce cracking in concrete, making it stronger and more stable. Reinforcing steel allows concrete to withstand increased weight, stress, vibration and even displacement within soil layers. It helps to prolong the useful life of the concrete, minimizing displacements, cracks and fractures.
Reinforced concrete can crack, but steel prevents horizontal, vertical, and lateral separation or movement. Concrete without steel reinforcement is more likely to crack and separate under tensile stresses and forces of extension and contraction. Rebar allows for larger and thicker castings, minimizing the forces acting on the concrete.
When do you need to use rebar in concrete?
The use of rebar in all concrete slabs is an unnecessary expense, although there are times when a steel rebar mesh should be used. Large pads often require steel reinforcement to prevent bending and tearing due to freezing and thawing or other pulling forces. If in doubt, it is best to consult a civil engineer or similar professional. Here are some reasons why reinforcement is needed:
Slabs thicker than 5 inches must have rebar screen to prevent cracking. Those that can support structures built on their perimeter or inside must also be reinforced.
Also, reinforce slabs spanning weak or disturbed soils, or over drains or trenches. Plates that support heavy equipment, dump trucks, RVs or materials such as building materials or drums of liquid must also be reinforced.
Terraces close to buildings often link backfill soil and should have additional tensile strength, as should those on slopes or weak soil. If you're planning a hot tub, spa, outdoor kitchen, fireplace or pizza oven, a grab bar is a must. Terraces that could one day be enclosed or support a permanent structure such as a pergola must also be reinforced.
A 4-inch-thick concrete driveway for a 1/2-ton car or truck doesn't require hot-rolled steel to last for decades. However, thicker concrete, sidewalks with poor ground support, or those that carry heavy loads or vehicles regularly should be reinforced.
Walkways usually have expansion cracks cut or put into them and they usually tear there. Sidewalks that cover weak soil, tree roots or drainage areas all benefit from rebar.
Many walkways that cross sidewalks or are frequently traversed by heavy equipment or loads must also contain steel rebar. Sidewalks with a lot of foot traffic, particularly walks or step walks, should also be reinforced.
small concrete slab
The purpose of a small concrete slab will determine whether reinforcement is required. A small 3" x 4" pad in front of a door should not need steel as long as the base of the floor is solid, while one that supports a ladder should have steel.
However, a small 4-inch-thick pad that supports a chimney or hot tub must be reinforced. Remember that steel will prevent cracking if weight is distributed unevenly or if the board is not well supported.
types of concrete reinforcement
Reinforcing steel has been used in concrete for over 150 years and has continued to improve over time. While there are other ways to strengthen concrete, the focus of this article is on steel and how it is used to improve the tensile strength of slabs, slabs, sidewalks, and driveways. Rebar is used to form the skeletal framework that helps concrete resist bending and stretching forces to keep it strong.
Reinforcement is either "smooth" or "warped". Smooth profile is smooth and is usually pressed into concrete before setting. The deformed profile has ribs to engage or grip the concrete as it is poured. The two profiles have specific purposes, but work similarly to reinforce concrete.
There are different diameters and grades of rebar designed for different purposes and structures. Before casting, the steel is cut, folded, and tied together to form a woven, mesh, or wire mesh-like skeleton inside the molds. The thicker the rebar and the smaller the spacing between the wires, the greater the strength it imparts to the concrete.
Structures that require rebar are generally inspected prior to pouring to ensure acceptable spacing and location. Improper spacing or placement, even an inch, can weaken resistance by 20%. In addition to spacing and placement, the type of reinforcement is important.
carbon steel rebar
Hot rolled carbon steel or “black bar” is most commonly used. It is ribbed but can be bent and provides structural strength to the concrete. It's relatively inexpensive, but worth the cost for the extra strength. When carbon steel is exposed to the weather through cracks, it corrodes faster than other types of steel.
Galvanized rebar, like anything galvanized, is more resistant to corrosion than carbon steel. In fact, it is 40 times more resistant to corrosion and therefore more expensive. Steel is coated with zinc for protection by cold or hot plating or electroplating. It is commonly used in road or bridge construction where salt corrosion can be an issue.
Reinforcement steel made from stainless steel
Stainless steel rebar is the most expensive steel and is only used when others cannot. It has the best corrosion resistance and is very strong. It contains higher levels of chromium and nickel and is used in structures where exposure to salt or magnetism is an issue - in ship structures or MRI machines. In addition, stainless steel rebar is better suited for seismic, impact, fire and safety purposes than other carbon or alloy rebar.
Epoxy coated rebar
Epoxy coated rebar or "green bar" is carbon steel rebar that has been coated with an epoxy coating to resist corrosion. It was used on bridges and roads exposed to salt water or deicing salts.
Unfortunately, epoxy coating is less durable than galvanized coating and even a drilling can lead to corrosion. Epoxy coated rebar has been banned in some jurisdictions and is under review in others.
Wire mesh is a grid of rebar welded or woven together to form a wire mesh or checkerboard pattern to reinforce concrete. The blankets are available in different bar thicknesses and spacing, in smooth and deformed profiles and in different types of steel.
Plain carbon steel wire mesh is common in residential panels and driveways, while roads and bridges may use galvanized mesh and other stainless steel construction.
Reinforcement vs. wire mesh
Rebar is used to strengthen concrete and reduce the likelihood of cracking and breaking. The choice between rebar or wire mesh generally depends on the desired strength of the final product. Some contractors prefer to use both in their concrete slabs - rebar between the top 1/2 and 1/3 and mesh between the top 1/3 and 1/4 of a 4-inch thick pad.
Rebar offers greater tensile strength than wire mesh, while wire mesh provides a smaller, stronger grid to connect the top layers of concrete and minimizes the risk of cracking. The rebar needs to be mounted on 2" plastic chairs or other equipment to hold it in place, which is an extra step.
Rebar does not bend easily and provides more support when a slab is bridging weak or compromised soil. Additionally, rebar provides more support for concrete that supports heavy loads, machinery or equipment.
The wire mesh can rest on the ground or rebar grid before pouring and be lifted through the concrete before it begins to set so that it is an inch below the surface. Alternatively, for small pads, the mesh can be sunk 1 inch into the concrete after pouring but before construction begins.
Mesh bends easily, which is a plus for modeling, but can affect the surface finish or even affect the finish. For concrete slabs that must only withstand light loads, mesh offers a less expensive option for reinforcement to reduce cracking and prevent cracks from splitting.
Can you pour concrete without reinforcement?
The purpose of concrete reinforcing bars is to give concrete greater tensile strength to minimize cracking. Concrete slabs that are poured into the ground with a properly prepared and compacted base and must not withstand heavy loads do not require reinforcement.
the size andcast thicknessAlso determine if steel bar reinforcement should be used. Panels 5 inches or thicker and large pads must be reinforced. Plates that support structures, heavy equipment, machinery or supplies must also contain steel reinforcement to prevent cracking, fracturing or separation at expansion joints.
Reinforcing bars are not necessary for concrete slabs, but they greatly improve their strength and service life. Some contractors do not build slabs without steel, thinking they offer greater strength and versatility. The additional costs are comparatively small, and as patios and walkways are converted to support structures, it may make sense to incorporate rebar.
Does rebar in concrete rust?
Steel encased in concrete is relatively safe from corrosion due to concrete's relative impermeability and alkaline nature. The high pH of concrete helps protect the steel with a thin layer of oxide. Unfortunately, the chloride ions in sea salt and road salt can penetrate concrete and attack steel.
Rebar that is not completely surrounded by concrete or that has been exposed due to cracking or delamination will corrode. So make sure repairs are made to protect the steel. The rebar or mesh oxidizes along the strands and expands, causing the concrete to crack from the inside. Rust stains on the concrete surface usually indicate rebar rust unless something left them in the concrete.
Proper preparation and installation of steel prior to casting is just as important as maintaining placement during casting. Steel that is too close to the edges, top or bottom of the concrete is at risk of corrosive forces. Using rocks or pieces of wood to support rebar can also expose the rebar to corrosion from groundwater and salt.
What size reinforcement do I need for a concrete slab?
Most jurisdictions require inspection of rebar prior to casting, so it is best to checklocal building codes. Rebar size and placement based on slab length, width and thickness should be identified. Bar placement and bar diameter are important to the structural strength of concrete.
Most housing boards use #3 with a 3/8 inch diameter (3/8 in² cross section) or #4 with a 1/2 inch diameter (1/2 in² cross section). Housing slabs that carry heavy loads can use a 5 or 5/8 inch diameter. In a 4-inch slab of 3,000 PSI concrete, #3 rebar typically strengthens the pad to 6,600 PSI, while the 1/8-inch increase in #4 can increase yield strength to 11,780 PSI.
Rebar thickness should be no more than 1/8 of the slab, so a 4" slab should be no more than 4" or 1/2" of bar steel. It is best to check with a structural engineer to determine if your slab is bridged or cantilevered.
For 4-inch concrete slabs used for driveways and patios, #3 rebar is common. Sidewalks that support daily heavy truck traffic often use 1/2″ or #4 steel bars. Pads connecting culverts, ditches, or cantilevers may require #5 rebar or grid placement closer to #4.
Where does reinforcement go in a concrete slab?
The spacing or placement of the rebars is just as important as the size. Distance from the leak edge, placement between surface and ground, and grid size or spacing greatly affect rebar strength. Rebar run as low as 1 inch can reduce strength by up to 20%, so it's important to make sure the steel stays where it's supposed to be.
Section R506.2.4 of the 2018 IRC states that steel reinforcement in concrete slabs must be in the middle of the upper third level of the leak. Supporting rebar on plastic benches or supports is a common method of holding steel in place. It is important to secure or tie the rebar where it crosses to keep it in place during pouring and placement of concrete.
Rebar is usually placed 3 inches from the perimeter to provide edge support. A 4-inch-thick concrete driveway that supports two vehicles usually has an 18-inch steel #3 grid and a patio a 24-inch grid. Heavy loads or thicker pads may require different steel or placement. So consult the experts.
How to estimate the amount of reinforcement?
Rebar is available in lengths up to 60 feet, but is most commonly supplied in 8-foot, 12-foot, or 16-foot lengths. Different retailers offer different lengths depending on usage. Oreinforcement weightit is usually a factor in shipping costs so be sure to check that too when ordering or picking up steel.
Calculating the amount of rebar can be done with simple math, drawing a plan on paper or using an online rebar calculator. The dimensions of the slab determine the amount of rebar required. Pieces that are too short need to overlap by 30 times the diameter, so #3 by 12 inches or more and #4 by at least 15 inches. So, if possible, try lengths that don't need to overlap.
To calculate the amount of rebar, convert the length and width of the pad to inches. Subtract 6 inches each to allow for a 3-inch inset from the edges. Divide each dimension by the intended grid spacing, round to whole numbers, and add an extra piece for the "0" or initial. You can add 10% for overlays.
Here are steps to calculate rebar for a 20 x 44 foot double driveway with a 16 inch rail:
- 20' x 12 = 240” – 6” = 234” ÷ 16” = 14,625 pieces or 15 pieces + 1 = 16 for length.
- 44' x 12 = 528” – 6” = 522” ÷ 16” = 32,625 parts or 33 parts + 1 = 34 for latitude.
- Rebar can be ordered to required lengths and shipped to site or cut to length with an angle grinder or saw. Alternatively, order lengths you can carry and add 12" or 15" for each overlap, depending on your rod diameter.
- 19.5" = one 12' + one 10' including 30" for one round or three lengths of 8' plus 54" for two rounds.
- 43.5' = four 12' lengths allow for 54" for three rounds or six and a half 8' pieces to have enough overlap - 90" for six rounds.
- 1495 feet of #3 rebar weighs approximately 560 pounds and #4 approximately 1000 pounds (including 10% for overlap or slump).
Concrete slabs for most residential purposes don't require rebar grids smaller than 12 inches, so you're unlikely to use a lot of steel. The concrete must surround and encapsulate the steel. So as long as there's room for it to flow around the steel, you're good to go.
How to reinforce concrete with rebar
Cracking often occurs due to temperature issues such as freeze-thaw cycles, concrete shrinkage, applied loads, or soil settling. Most concrete slabs crack, so it is common for large or long slabs to insert or cut expansion joints or seams every 10 feet to control where cracks occur. Concrete slabs are generally reinforced to prevent crack propagation or to provide tensile strength against stress cracking.
Rebar or wire mesh do not prevent all cracks, they minimize the crack size and provide support against loads and ground settling that can cause stress cracking. Rebar should be used for 5-inch or thicker slabs, but not necessary for thinner pads. However, the additional cost will help protect your investment from soil changes or settlements, tree roots, crack propagation, heavy loads and other unforeseen events.
To reinforce a 4" concrete slab or driveway with steel rebar, determine grid spacing and rebar size - #3 or #4 on a 12" to 18" grid. Use 2-inch rebar chairs to support the rebar in the middle layers of the base. Overlap the joints by at least 12 inches for #3 and 15 inches for #4, tie with cable ties and keep the steel 3 inches from the perimeter. It is important to keep the grid level and in the place during pouring.
Here is a video showing how to install a concrete slab reinforcement mesh:
When do you use chicken wire on concrete?
Chicken wire can be used independently on concrete slabs or in conjunction with rebar. Mesh prevents crack expansion, provides bending resistance, and makes the pad more durable. It also provides tensile strength in case the floor shifts or settles.
The mesh provides a uniform steel grid in the concrete to hold it together. Consider use when the pad can only support light weight or the ground base is well compacted. Add it an inch above rebar to provide increased tensile strength and crack resistance for pads carrying heavy loads.
Rebar improves concrete's resistance to freezing and thawing, weathering, tree roots, soil settling or displacement, and heavy loads. Concrete 5 inches thick or more must contain rebar, and thinner ones also benefit from improved tensile strength and prevention of crack separation.
I hope you have a better understanding of what rebar is, when and where to use it, and are better prepared to plan your project.
A handyman for most of his life, Eugene loves to be creative while inspiring the creativity of others. He is passionate about DIY, remodeling and woodworking.