10 Things to Consider When Buying Reinforcing Wire Rod for Precast Elements

Concrete is one of the most common construction materials. Because of its durability, low maintenance requirements, fire resistance, and ease of use, people all over the world use concrete for many projects.

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But concrete has one potentially fatal flaw. If concrete is under a particular type of force, it will break&#;quickly.

Thankfully, there&#;s a way to combat that fatal flaw: use reinforcement, such as rebar.

Here at Gra-Rock, we carry a full line of concrete reinforcement supplies, including rebar, because we understand how catastrophic it can be if your concrete isn't properly reinforced!

In this article, we'll explain why reinforcement is necessary and how to reinforce concrete using rebar.

Let&#;s get started!

Why Does Concrete Require Rebar?

Most concrete requires some type of reinforcement.

Why?

While concrete is very strong in some ways, it also has a few devastating weaknesses. But to understand these weaknesses, we first need to understand the different types of stress that can be placed on objects.

1. Compressive stress. Compressive stress is a force that is placed upon an object that shortens or compresses the object. For example, if an elephant steps on your toe, you will experience compressive stress.

2. Shear stress. Shear stress occurs when forces are applied perpendicularly to one another. If you lock your fingers together and pull against yourself, you are experiencing shear stress.

3. Tensile stress. Tensile stress is a force exerted on an object that lengthens or stretches that object. When you swing on a rope swing and jump into a swimming hole, you exert tensile stress on the rope.

Concrete is very strong under compressive stress and shear stress, but it performs poorly under tensile strength. In fact, the tensile strength of concrete is only about 10-15% of its compressive strength.

Enter rebar.

Rebar is used primarily to increase the tensile strength of concrete.

(To learn more about concrete strength, read our related blog post: Understanding Concrete Strength: From PSI To Tips For Pouring Concrete)

What Is Concrete Rebar?

Rebar (short for reinforcing bar) is a steel rod used to strengthen concrete.

The rods come in various lengths and thicknesses and usually have ridges or bumps, so they bond well with the concrete.

Rebar is made from steel. Steel is very strong and expands and contracts in changing temperatures at nearly the same rate as concrete.

What Does Rebar Do For Concrete?

As we already mentioned, concrete handles compressive stress well but does poorly under tensile strength.

Since almost every structure experiences more than one force acting on it, this is a problem.

Take, for example, the classic beam.

When a beam experiences compressive stress on the top, it bends. And when a beam bends from compressive stress on the top, the beam's bottom stretches.

That means the bottom of the beam experiences tensile stress.

So, since concrete doesn't do well with tensile stress, it doesn't always make a good structural material on its own.

But, when we add rebar, two things happen.

First: When rebar is placed in concrete, it creates a composite material. The concrete protects against compressive stress, and the rebar protects against tensile stress. This composite material is extremely strong.

In fact, concrete that includes rebar has a breaking point nearly double that of concrete without rebar.

Second: When rebar is placed in concrete, it gives warning signs before the concrete breaks apart completely.

Concrete without rebar is considered brittle. As the pressure increases on pure concrete, it will suddenly break without warning.

On the other hand, concrete that includes rebar is considered ductile. That means that as pressure increases, small fissures and cracks can be seen forming in the concrete.

This is positive in two ways:

• Concrete that contains rebar remains strong even with small cracks

• There is a warning signal before the concrete completely fails.

When is Rebar Necessary?

Does every single concrete job need rebar?

Not necessarily.

Concrete surfaces that support large trucks, heavy machinery, or steady traffic need concrete rebar reinforcement, and any structural concrete, like walls in buildings, should definitely include rebar.

But if you&#;re pouring a concrete driveway as a place to park your family minivan, heavy reinforcement using rebar may be overkill.

When in doubt, though, use rebar. No matter how large or small the concrete pour is that you are doing, rebar will make your concrete stronger. At the very least, rebar will dramatically decrease the number of cracks in the concrete.

Bonus tip: If you are doing a small residential concrete bar and steel rebar rods feel like overkill, you can use welded wire fabric. Mesh is thinner than rebar, so it&#;s not as strong, but it&#;s also cheaper.

8 Main Types of Rebar

As we just discussed, welded wire fabric is a type of rebar ideal for certain applications.

Maybe you&#;re wondering: Are there other types of rebar that are ideal for specific situations?

Yes, there are!

Here are some of the different types of rebar you may wish to use.

Carbon Steel Rebar: This is the most common type of rebar and is sometimes referred to as a "black bar." It's incredibly versatile but corrodes more easily than other types. This makes it less than ideal for areas that are subject to high humidity or in structures frequently exposed to water.

Welded Wire Fabric: Welded wire fabric (WWF) is made from a series of steel wires arranged at right angles and electrically welded at all steel wire crossings.

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It is useful in slab-on-ground slabs where the ground has been well compacted. A heavier fabrication of welded wire fabric can be used in walls and structural floor slabs. This is commonly used in road pavement, box culverts, drainage structures, and small concrete canals.

Epoxy-Coated Rebar: Epoxy-coated rebars are simply rebars coated with a thin layer of epoxy. This makes them up to 1,700 more times resistant to corrosion than standard carbon steel rebars. As a result, they are often used in areas in contact with salt water or where a corrosion problem is imminent.

The only problem is that the coating can be very delicate, so bars should be ordered from a reputable supplier.

A particular concern with epoxy-coated rebars is that they can be susceptible to severe corrosion. If the epoxy is damaged in small spots, all the corrosion is concentrated in that one spot.

Galvanized Rebar: Galvanized rebars are 40 times more resistant to corrosion than carbon steel rebars, and they are much harder to damage than epoxy-coated rebars.

This makes it an excellent alternative to epoxy-coated rebars if you need something less prone to corrosion.

Unfortunately, galvanized rebar is about 40% more expensive than epoxy-coated rebar.

Sheet-Metal Reinforcing Bars: Sheet-metal reinforcement is commonly used in floor slabs, stairs, and roof construction. Sheet-metal reinforcing bars are composed of annealed sheet steel pieces bent into corrugations about one-sixteenth of an inch deep with holes punched at regular intervals.

European Rebar: The advantage of European rebar is its low cost. European rebar is made primarily of manganese, which makes it cheap and easy to bend.

This flexibility makes European rebar easy to work with, but it's generally not recommended for areas that experience earthquakes or for projects that require substantial structural integrity from its rebar.

Stainless Steel Rebar: Stainless steel rebar is quite expensive - about eight times the price of epoxy-coated rebar!

It is the highest quality rebar available for most projects. However, using stainless steel in all but the most unique of circumstances is often overkill, not to mention very expensive!

But, for those who have a reason to use it, stainless steel rebars are 1,500 times more resistant to corrosion than black bars. Stainless steel rebars can also be bent in the field, which is very convenient.

Glass-Fiber-Reinforced-Polymer (GFRP) Rebar: Like carbon fiber, GFRP rebars will not corrode &#; ever, under any conditions. But that feature comes at a significant cost. These rebars can run at ten times the cost of epoxy-coated rebars!

If you read over that list of rebar types and still have questions about which one is best for your project, that's ok. Reach out to a rebar manufacturer or local concrete provider to get advice on which kind of rebar you should be using.

Choosing the Right Size of Rebar

There aren't just different types of rebar; there are also different sizes of rebar!

The size of the rebar you'll use for a particular job depends on the amount of strength you need. When you need more strength, you'll use bigger pieces of rebar.

In the United States, rebar is categorized by a number reflecting the solid diameter of the rebar. The numbers range from # 3 (smallest) to # 18 (largest).

For example, The # 3 bar size is 3/8&#; diameter of the solid section, the # 4 bar size is 4/8&#; diameter of the solid section, and the # 5 bar size is 5/8&#; diameter of the solid section.

The three different sizes of rebar used for home projects are usually # 3, # 4 and # 5.

Rebar size # 3 is used for driveways and patios. For walls and columns, # 4 rebar is better, as these structures require more strength. For footers and foundations, it's best to use the # 5 rebar.

How to Place Rebar in Concrete

Once you know the type and size of rebar you need, it's time to place it in concrete!

There is no simple formula for placing rebar correctly. A lot depends on the variables of your particular build. For example, how much force will be exerted on the concrete? Will the concrete be freezing and thawing over the seasons?

If you are doing a simple pour around your home, talk to your local concrete contractor or some other knowledgeable person about how to place the rebar.

When it comes to bigger commercial pours, the rebar specifications should be detailed in the blueprints. An engineer has carefully figured out exactly how much rebar is needed and how it should be spaced, so follow the directions carefully.

Thought and care must be put into how the rebar is placed, or the concrete's structural integrity could be compromised.

For example, if the engineer calls for rebar spaced every 4 inches, you need to place three bars for every 12 inches of the form.

If the steel placer is a little sloppy and places the bars at 5-inch spacing rather than 4-inch spacing, the product's strength will be reduced by 20%. Yes, concrete's structural integrity can be compromised that easily!

Bending and Cutting Rebar

You may know precisely how far apart to place your rebar, but what if your bars are too long? Or what if your structure requires bent rebar?

Some rebar comes pre-bent, but in general, be prepared to cut and bend the rebar so you can place it properly.

If you have the right tools, the process is easy.

First, let's talk about cutting rebar. There are several tools to use for this task.

A hacksaw or bolt cutter is a good option if the rebar is thin enough and if you aren&#;t cutting a large quantity. If you are doing a job of significant size, an angle cutter with a cutting wheel does a great job.

With all the tools listed, it&#;s important to note that you don&#;t need to cut through the entire rebar. You only need to cut through half of it, and you can break it in half easily. Use this little hack, and you&#;ll end up saving yourself a lot of time.

Sometimes, rebar needs to be tied. That&#;s a whole topic in itself, but if you&#;d like to learn more about tying rebar, your local concrete contractor is a great place to start.

Conclusion

Concrete is an essential material in construction. However, without rebar, concrete loses much of its value.

Thankfully, you don&#;t need to be an engineering expert to be able to understand how to use rebar. Next time you want to pour concrete, you can be confident in choosing the correct type and size of rebar and installing the rebar.

If you are looking for rebar or a ready-mix concrete supplier in Northern Indiana, contact us at Gra-Rock for the concrete rebar that you need.

We have over 15 years of concrete experience, and we want to help you with whatever project you are working on!

We also offer:

We've also written some helpful articles you may be interested in if you want to learn more about concrete projects.

We'd love to help you out&#;contact us today if you have any questions or would like to schedule your next concrete delivery!

Above and below: Properly placed/supported reinforcement will result in properly located reinforcement in the slab. Consult manufacturers' literature for maximum chair and other support spacing and use a 12-inch minimum reinforcement spacing so workers can avoid walking on the reinforcement.

Most slabs-on-ground are unreinforced or nominally reinforced for crack-width control. When positioned in the upper or top portion of the slab thickness, steel reinforcement limits the widths of random cracks that may occur because of concrete shrinkage and temperature restraints, subbase settlement, applied loads or other issues.

This type of reinforcement is commonly called shrinkage and temperature reinforcement.

Shrinkage and temperature reinforcement is different than structural reinforcement. Structural reinforcement is typically placed in the bottom portion of the slab thickness to increase the slab's load capacity. Most structural slabs-on-ground have both top and bottom layers of reinforcement for controlling crack-widths and increasing load capacities. Because of constructability issues and costs associated with two layers of reinforcement, structural slabs-on-ground are not as common as nonstructural slabs.

While there are several reinforcing options for nonstructural slabs-on-ground, this article focuses on steel reinforcing bars and welded wire reinforcement for crack width control.

The basics

Steel reinforcing bars and welded wire reinforcement will not prevent cracking. Reinforcement is basically dormant until the concrete cracks. After cracking, it becomes active and controls crack widths by restricting crack growth.

If slabs are placed on high quality subbases with uniform support and consist of low shrinkage concrete with joints properly installed with spacings of 15 feet or less, reinforcement is generally unnecessary. Most likely, there will be few random or out-of-joint cracking. If random cracks do occur, they should remain fairly tight because of the limited joint spacing and low concrete shrinkage thereby limiting future serviceability or maintenance issues.

When slabs are placed on problematic subbases with risks of non-uniform support or consist of moderate to high shrinkage concrete or joint spacings exceed 15 feet, then reinforcement is necessary to limit widths of cracks should they occur. As crack widths grow and approach about 35 mils (0.035 inches), the efficiency of load transfer through aggregate interlock diminishes and differential vertical movements across cracks or slab "rocking" can occur. When this happens, crack edges become exposed and edge spalling will likely occur, especially if the slab is exposed to wheeled traffic and especially hard-wheeled lift trucks. Once spalling starts, crack widths at the surface become wider and slab deterioration along cracks increase significantly.

When contraction joints are unacceptable and not installed, shrinkage and temperature reinforcement is required. This design approach is sometimes referred to as continuously reinforced or joint-less slabs and allows numerous, closely spaced (3 to 6 feet), fine cracks to occur throughout the slab.

Crack control options

In general, there are two options for controlling cracks in slabs-on-ground: 1) control the location of cracking by installing contraction joints (does not control crack widths) or 2) control crack widths by installing reinforcement (does not control crack location).

With Option 1, we tell the slab where to crack and widths of contraction joints or cracks in the joints are largely controlled by the joint spacing and concrete shrinkage. As joint spacings and concrete shrinkage increases, joint widths increase. Similar to cracks, if joint widths approach about 35 mils, the efficiency of the aggregate interlock to transfer loads and prevent differential vertical movements across joints can be significantly reduced. For this reason, many designers use load-transfer devices including steel dowels, plates or continuous reinforcement through contraction joints to ensure positive load transfer and to restrict differential vertical movements across joints.

With Option 2, we allow slabs to crack randomly but control crack widths with steel reinforcing bars or welded wire reinforcement. Typically, contraction joints are not installed with this option. Instead, cracking occurs randomly forming numerous, tightly held together cracks. Because of appearance, this crack control option should always be discussed with the owner.

Cutting reinforcement at joints

Use caution when using both crack control options in the same slab. If too much reinforcement passes through contraction joints, joints become too stiff and may not crack and open as designed. When contraction joints fail to activate (i.e., crack and open) because of reinforcement, out-of-joint or random cracking typically occurs. If both options are used, it is necessary to limit the amount of reinforcement passing through joints to ensure proper activation.

Some designers specify to cut all the reinforcement at contraction joints while others may specify to cut every other bar or wire. By cutting every other bar or wire, the remaining reinforcement will help provide load-transfer and minimize differential panel movements but not restrict joints from activating. If the specifications and construction drawings do not indicate what to do with temperature and shrinkage reinforcement at joints, contractors should submit a request for information. Many times contractors are inappropriately blamed for out-of-joint cracking associated with this design issue.

Location of reinforcement

Steel reinforcing bars and welded wire reinforcement should be positioned in the upper third of the slab thickness because shrinkage and temperature cracks originate at the surface of the slab. Cracks are wider at the surface and narrow with depth. So, crack-control reinforcement should never be positioned below the slab's mid-depth. Reinforcement should also be placed low enough so saw cutting does not cut the reinforcement. For welded wire reinforcement, the Wire Reinforcement Institute recommends steel placement 2 inches below the surface or within the upper third of the slab thickness, whichever is closer to the surface. Designers typically specify the reinforcement position by specifying concrete cover (1 1/2 to 2 inches) for the reinforcement.

Positioning a single layer of reinforcement in the center or at mid-depth of the slab is not recommended (except for 4-inch-thick slabs). This is an all-purpose location where the designer hopes to increase the load capacity of the slab in addition to provide crack-width control. However, positioning reinforcement in the middle of the slab will not effectively accomplish either objective.

Steel reinforcing and welded wire reinforcement should be supported and sufficiently tied together to minimize movements during concrete placing and finishing operations. Otherwise, reinforcement may not be properly located in the slab. Support reinforcement with chairs or precast-concrete bar supports. Chairs should have sand or base plates and bars should have at least a 4-inch square base to ensure they don't sink into the subbase. Use support spacings that ensure reinforcement does not sag between supports or is not pushed down by foot traffic or fresh concrete. Flexible reinforcement including welded wire reinforcement requires closer support spacing. In addition to specifying the type and amount of reinforcement, designers should specify the type and spacing of supports to ensure proper positioning of the reinforcement.

Welded wire reinforcement should never be placed on the ground and pulled into position after concrete placement. The "hook and pull" technique always results in improperly positioned reinforcement. How can workers uniformly "hook and pull" welded wire reinforcement into the specified location while standing on the reinforcement?

Placement tolerances

Vertical placement tolerance for reinforcement in slabs-on-ground is ± 3/4 inch from the specified location. For slab thicknesses 12 inches or less, the concrete cover tolerance is - 3/8 inch measured perpendicular to the concrete surface and the reduction in cover cannot exceed one-third of the specified cover. In many cases, the cover tolerance overrides the vertical placement tolerance. Properly placing and supporting reinforcement will help ensure compliance with these vertical placement tolerances.

This article was originally published on Feb. 25, . 

References:

ACI 117-06. "Specification for Tolerances for Concrete Construction and Materials"

ACI 302.1R-04. "Guide for Concrete Floor and Slab Construction"

ACI 360R-06. "Deign of Slabs-on-ground"

ASCC Position Statement #2. "Location of Rolled Welded Wire Fabric in Concrete"

WRI Tech Facts. "Supports Are Needed for Long-Term Performance of Welded Wire Reinforcement in Slab-on-grade" (TF 702-R-08)

WRI Tech Facts. "How to Specify, Order and Use Welded Wire Reinforcement" (TF 202-R-03)

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