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Do slabs need rebar?

Concrete slabs poured on the ground with a properly prepared and compacted base, and that isn't expected to support heavy loads, doesn't require rebar. The size and thickness of the pour also determine if steel bar reinforcing should be used. Slabs 5” or greater in thickness and large pads should be reinforced.

plasticinehouse.com - Rebar in Concrete: Does Concrete Slab, Patio, Driveway Need Rebar?

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Are you planning a concrete slab, patio, or driveway? Trying to figure out if you need rebar in concrete for your project? There are different factors that affect the design of the pour and whether rebar is necessary. Concrete supporting heavy loads, large vehicles, or structures should be reinforced with steel rebar to improve its tensile strength. Pads 5” or thicker should also be reinforced, as should those bridging openings. Concrete resting on poor ground strata or exposed to freeze-thaw needs it too. In this article, we’ll explain what rebar is, when it should be used, different types and sizes of rebar, and how to install it. Additionally, we’ll discuss wire mesh and when to use it. Our aim is to provide you with the information necessary to determine if you need concrete reinforcing bar or mesh for your project.

What Is Concrete Rebar?

Reinforcing bar, or rebar, is steel bars used to reduce the likelihood concrete will crack due to tension forces. Concrete has great compression strength but can crack or break if not supported fully underneath or from within. Rebar has been used for more than 150 years to improve the tensile strength of concrete. Rebar is made in varying gauges of hot-rolled steel and used to strengthen concrete from within. It is commonly laid in a grid pattern, formed into a steel web, or placed in parallel strands. The thicker the bar and more frequent its placement, the stronger the finished product. The steel improves the concrete’s ability to withstand forces or stresses applied horizontally or vertically against it. It distributes the forces through the concrete instead of allowing it to centralize and crack. Steel reinforced concrete is much stronger and more stable than concrete without the rebar. Rebar is often known as deformed steel due to the ridges that allow it to bond firmly with the concrete.

Does Concrete Always Need Rebar?

Concrete slabs or pads used for patios or driveways don’t need reinforcing for cars, light trucks, or lounge chairs. If the concrete will need to support heavy machinery, heavy-duty vehicles, a hot tub, spa, or concrete pizza oven, rebar is recommended. Concrete that will rest on backfill or other disturbed or weak ground would also benefit from reinforcement. Rebar helps reduce cracking in concrete and makes it stronger and more stable. The reinforcing steel allows concrete to withstand more weight, tensile stresses, vibration, and even shifts within the soil strata. It helps extend the life of the concrete by minimizing shifting, cracking, and breaking. Reinforced concrete may crack, but the steel prevents horizontal, vertical, and lateral separation or movement. Concrete without steel reinforcing is more likely to crack and separate under tensile stresses and expansion and contraction forces. Rebar allows for larger and thicker pours by minimizing the forces that prey on concrete.

When Do You Need to Use Rebar in Concrete?

Using rebar in all concrete slabs is an unnecessary expense, although there are times when a web of steel rebar should be used. Large pads commonly require steel reinforcing to prevent bending and cracking due to freeze-thaw or other tensile forces. It is best to check with a Structural Engineer or similar professional if unsure. Here are some reasons when rebar is needed:

Slab

Slabs thicker than 5” should have a web of rebar to prevent it from cracking. Those that may support structures erected on their perimeter or within it should also be reinforced. Additionally, reinforce slabs that span weak or disturbed ground, or over drainage pipes or ditches. Slabs that will support heavy equipment, dump trucks, RVs, or materials such as building supplies or barrels of liquids should be strengthened too.

Concrete Patio

Patios near buildings commonly bridge backfilled ground and should have added tensile strength, as should those on slopes or weak ground. If planning for a hot tub, spa, outdoor kitchen, fireplace, or pizza oven, reinforcing bar is a must. Patios that may one day be enclosed or carry a permanent structure like a pergola should be strengthened too.

Concrete Driveway

A 4”-thick concrete driveway used for automobiles or ½-ton trucks doesn’t require hot-rolled steel to last for decades. However, thicker concrete, driveways over poor ground support, or those that regularly support heavy loads or vehicles should be reinforced.

Walkway

Walkways commonly have expansion cracks cut or placed in them and usually will crack there. Sidewalks that span weak ground, tree roots, or drainage areas benefit from rebar. Many walkways that cross driveways or are frequently crossed by heavy equipment or loads should also contain steel rebar. Walkways that have a lot of foot traffic, especially in-step walking or marching should be reinforced as well.

Small Concrete Slab

The purpose of a small concrete slab determines if rebar is needed. A small 3’x4’ pad outside a door shouldn’t require steel provided the ground base is solid, while one supporting a flight of stairs should have steel. A small 4”-thick pad supporting a fireplace or hot tub, however, should be reinforced. Remember, the steel helps prevent cracking when weight is unevenly distributed, or the slab isn’t well supported.

Types of Reinforcement in Concrete

Reinforcing steel has been used for over 150 years in concrete and has undergone improvements throughout that time. Although there are other ways to reinforce concrete, the focus of this article is on steel, and how it is used to improve the tensile strength of pads, slabs, driveways, and walkways. Rebar is used to form the skeletal structure that helps concrete withstand bending and stretching forces to keep it strong. Rebar is either ‘plain’ or ‘deformed’. The plain profile is smooth and is commonly pushed into concrete before it sets. The deformed profile has ridges to bond with or grip the concrete as it is poured. The two profiles have specific purposes but work similarly to strengthen concrete. There are different diameters and grades of rebar designed for varying purposes and structures. Prior to a pour, the steel is cut, bent, and tied together to form a wire fabric-like weave, web, or skeleton within the forms. The thicker the rebar and closer the spacing between strands, the greater the strength it brings to the concrete. Structures that require rebar are commonly inspected prior to the pour to ensure spacing and location are acceptable. Improper spacing or placement by even an inch can weaken the strength by 20%. Along with spacing and placement, the type of rebar is important.

Carbon Steel Rebar

Hot-rolled carbon steel rebar, or ‘black bar’, is the most frequently used. It is ridged, yet can be bent, and provides structural strength to concrete. It is reasonably low cost but well worth the expense for the added strength. If exposed to the elements through cracks, carbon steel will corrode more quickly than other types of steel.

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Galvanized Rebar

Galvanized rebar, like anything galvanized, has greater resistance to corrosion than carbon steel. It’s actually 40-times more resistant to corrosion, and thus more expensive. The steel is coated with zinc by cold or hot plating or electroplating for protection. It is often used for road or bridge construction where salt corrosion may be an issue.

Stainless Steel Rebar

Stainless steel rebar is the most expensive steel and is only used when others can’t be. It has the best resistance to corrosion and is very strong. It contains a higher content of chromium and nickel and is used in structures where exposure to salt or magnetism is a concern – marine structures or MRI machines. Additionally, stainless steel rebar is better suited for seismic, impact, fire, and security 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 a layer of epoxy to resist corrosion. It has been used in bridges and roadways subject to saltwater or de-icing salts. Unfortunately, the epoxy coating is less durable than the galvanized coating and even a pinhole perforation can lead to corrosion. Epoxy-coated rebar has been banned in some jurisdictions and is under review in others.

Wire Mesh

Wire mesh is a grid panel of rebar welded or woven together to form a wire fabric or plaid-like pattern to reinforce concrete. The mesh is available in different rebar thicknesses and spacings, in plain and deformed profiles, and in different types of steel. Plain carbon steel wire mesh is common in residential slabs and driveways, while roads and bridges may use galvanized mesh, and other structures stainless steel.

Rebar vs Wire Mesh

Reinforcing steel is used to strengthen concrete and reduce the chances of it cracking and pulling apart. The choice between rebar or wire mesh often depends on the strength desired in the end product. Some contractors prefer to use both in their concrete slabs – rebar between the 1/2 and upper 1/3rd and mesh between the upper 1/3rd and 1/4 of a 4” thick pad. Rebar provides greater tensile strength than mesh, while the mesh offers a smaller fixed grid for bonding the upper concrete strata and minimizing the risk of cracking. Rebar needs to be set on 2” plastic chairs or other devices to keep it in place which is an additional step. Reinforcing bar doesn’t bend easily and offers more support when a slab bridges weak or compromised ground. Additionally, rebar offers more support to concrete bearing heavy loads, machinery, or equipment. Wire mesh may rest on the ground or rebar grid prior to the pour and be lifted through the concrete before it begins to set so it is an inch below the surface. Alternatively, for small pads, the mesh can be sunk an inch into the concrete after it is poured, but before it starts to set up. Mesh bends easily which is a plus for shaping but can interfere with surface finishing, or even compromise the finish. For concrete slabs expected to only carry light loads, mesh offers a less expensive option to rebar for reducing cracking and preventing cracks from separating.

Can You Pour Concrete Without Rebar?

The purpose of concrete reinforcing bar is to provide concrete with greater tensile strength to minimize cracking. Concrete slabs poured on the ground with a properly prepared and compacted base, and that isn’t expected to support heavy loads, doesn’t require rebar. The size and thickness of the pour also determine if steel bar reinforcing should be used. Slabs 5” or greater in thickness and large pads should be reinforced. Slabs that will support structures, heavy equipment, machinery, or supplies should also contain steel reinforcing to prevent cracks, breaks, or separation at expansion joints. Rebar isn’t necessary for concrete slabs but greatly improves its strength and lifespan. Some contractors won’t build slabs without steel, identifying that it offers greater strength and versatility. The added expense is comparatively low, and with patios and driveways being repurposed to support structures, it may make sense to include rebar.

Does Rebar Rust in Concrete?

Steel encased in concrete is reasonably safe from corrosion due to the relative impermeability of concrete and its alkaline nature. The high pH value in concrete helps protect the steel with a thin oxide layer. Unfortunately, the chloride ions in marine salt and road salts can permeate into the concrete and attack the steel. Rebar that isn’t fully surrounded by concrete, or that has been exposed due to cracking or delamination will corrode, so ensure repairs are done to protect the steel. Rebar or mesh will oxidize along the strands and expand, causing the concrete to break from within. Rust marks on the surface of concrete, unless left by something placed on the concrete, often indicate rusting of reinforcing steel. Proper preparation and installation of steel prior to pouring is as important as maintaining the placement during the pour. 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 by groundwater and salt too.

What Size Rebar Do I Need for a Concrete Slab?

Most jurisdictions require an inspection of the rebar prior to the pour, so it’s best to check the local Building Codes. The size and placement of rebar based on the slab length, width, and thickness should be identified. Placement and bar diameter are important to the structural strength of the concrete. Most residential slabs use #3 which is 3/8” diameter (3/8in² cross-section) or #4 which is 1/2″ diameter (1/2in² cross-section). Residential slabs bearing heavy loads may use #5 or 5/8” diameter. In a 4” slab of 3,000 PSI concrete, #3 rebar commonly strengthens the pad to 6,600 PSI, while the 1/8” increase to #4 can bump the strength yield to 11,780 PSI. The thickness of rebar should be no more than 1/8 that of the slab, so a 4” slab shouldn’t have steel greater than #4 or 1/2″ bar. It’s best to check with a Structural Engineer if your slab will bridge or cantilever. For 4” concrete slabs used for driveways and patios, #3 rebar is common. Driveways that will support daily traffic by heavy trucks frequently use 1/2″ or #4 steel bar. Pads that bridge culverts or ditches or cantilever may require #5 rebar or a tighter grid placement of #4.

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Where Does Rebar Go on a Concrete Slab?

The spacing or placement of reinforcing bar is as important as the size. The distance from the edge of the pour, placement between the surface and ground, and the grid size or spacing greatly affect the strength provided by the rebar. Rebar misplaced by as little as 1” can reduce strength by up to 20%, so it is necessary to ensure the steel stays where it’s supposed to be. Section R506.2.4 of the 2018 IRC states that steel reinforcement in concrete slabs on grade must be within the center to the upper third of the pour. Supporting the rebar on plastic chairs or stands is a common way to keep the steel in place. It is important to wire or fasten the rebar together where it intersects to keep it positioned during the pouring and placement of the concrete. Rebar often is placed 3” in from the perimeter to provide edge support. A 4” thick concrete driveway supporting two vehicles often has an 18” grid pattern of #3 steel, and a patio a 24” grid. Heavy loads or thicker pads may require different steel or placement, so check with the experts.

How to Estimate Rebar Quantity?

Rebar is available in lengths up to 60-feet but is more commonly delivered in 8’, 12’, or 16’ lengths. Different retailers offer different lengths depending on use. The weight of the rebar is often a factor in delivery cost, so you may want to check that out too when ordering or picking up the steel. Calculating the amount of rebar can be done with some simple math, drawing a plan on paper, or using an on-line rebar calculator. The dimensions of the slab determine the amount of reinforcing steel needed. Pieces that are too short need to overlap by 30 times the diameter, so #3 by 12” or more, and #4 by at least 15”, so if possible, try for 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” from each to allow for 3” inset from the edges. Divide each dimension by the planned grid spacing, and round up to whole numbers, and add one extra piece for the ‘0’ or starter. You may want to add 10% for overlaps. Here are the steps to calculate the rebar for a double wide driveway 20’ x 44’ using a 16” spaced grid: 20’ x 12 = 240” – 6” = 234” ÷ 16” = 14.625 pieces or 15 pieces + 1 = 16 for the longitude. 44’ x 12 = 528” – 6” = 522” ÷ 16” = 32.625 pieces or 33 pieces + 1 = 34 for the latitude. Rebar can be ordered in the required lengths and delivered to the worksite or cut to length using an angle grinder or hacksaw. Alternatively, order lengths that you can transport and add the 12” or 15” for each overlap depending on rebar diameter. 19.5” = one 12’ + one 10’ including 30” for the one overlap or three 8’ lengths with 54” for two overlaps. 43.5’ = four 12’ lengths allow 54” for three overlaps, or six and a half 8’ pieces to have enough to overlap – 90” for six overlaps. 1495 feet of #3 rebar will weigh about 560-pounds and #4 approximately 1000-pounds (includes 10% for overlap or waste). Concrete slabs for most residential purposes don’t require rebar grids smaller than 12”, so it isn’t likely you’ll use too much steel. Concrete needs to surround and encapsulate the steel, so as long there is space for it to flow around the steel, you’re good to go.

How To Reinforce Concrete With Rebar

Cracking commonly occurs due to temperature issues like freeze-thaw, concrete shrinkage, loads applied, or ground settling. Most concrete slabs will crack, so it is common practice to insert or cut expansion joints or seams every 10’ on large or long slabs to control where cracking occurs. Concrete slabs are normally reinforced to prevent cracks from expanding or to provide tensile strength against stress force cracking. Rebar or wire mesh won’t stop all cracking, they will minimize the size of cracks and provide support against loads and ground settlement which can cause stress cracking. Rebar must be used for slabs 5” or thicker but isn’t necessary for thinner pads. However, the added expense helps protect your investment against ground shifting or settling, tree roots, cracks expanding, heavy loads, and other unforeseen occurrences. To reinforce a 4” concrete slab or driveway with steel rebar, determine the grid spacing and rebar size – #3 or #4 in a 12” to 18” grid. Use 2” rebar chairs to support the rebar in the center strata of the pad. Overlap joints by a minimum of 12” for #3 and 15” for #4, bind with wire ties, and keep the steel 3” back from the perimeter. It is important to keep the grid uniform and in place during the pour. Here’s a video that shows how to install rebar grid for concrete slab:

When to Use Wire Mesh in Concrete?

Wire mesh can be used independently in concrete slabs or in conjunction with reinforcing bars. The mesh helps prevent cracks from expanding, provides flexural strength, and makes the pad more durable. It also offers tensile strength should the ground shift or settle. Mesh provides a uniform grid of steel in the concrete to hold it together. Consider using it if the pad will only support lightweight, or the ground-base has been well compacted. Add it an inch above rebar to offer greater tensile support and crack resistance for pads that will support heavy loads.

Conclusion

Rebar improves concrete’s resistance to freeze-thaw, washouts, tree roots, ground settling or shifting, and heavy loads. Concrete 5” or thicker must include reinforcing steel, and those thinner also benefit from the improved tensile strength and crack separation prevention. Hopefully, you have a better understanding of what rebar is, when and where to use it, and are better prepared to plan your project.

plasticinehouse.com - Rebar in Concrete: Does Concrete Slab, Patio, Driveway Need Rebar?