What Does Rebar Do for Concrete?
Concrete is a type of construction material that is used for long years ago. Generally, slabs, patios, and foundations are made of concrete.
Concrete is used for, small projects as well as large projects like dams, retaining walls, etc. Small projects like countertops, and fireplace surroundings are also made of concrete.
Concrete generally provides huge compressive strength that’s why it can withstand very high compressive loads without cracking but concrete is very weak in tension. Due to a small tensile force, it will show some serious cracks.
To overcome this situation we add some extra materials like steel reinforcement which creates a steel skeleton inside the concrete.
The main purpose of steel reinforcement is to take tension but it also helps to create a good bond between concrete and reinforcement and helps to increase the internal strength.
By using steel reinforcement it can able to withstand higher loads and it can be easily used in longer beams. Generally, all the concrete structures in recent time is a reinforcement steel concrete structures that can withstand higher loads.
Concrete is obtained by mixing cement, sand, aggregates, and water with the required amount and it generates a paste that dries and hardened. After that, it needs curing operation for completion of the hydration process for full strength gain.
The curing process is done for 28 days. In this period, concrete hardened gained higher compressive strength but it is very weak in tension too.
Cement and water are the main ingredients of the concrete and sand & aggregates act as filler materials that provide strength to concrete.
Concrete absorbs water because it evaporates water at the time of curing; so it will create some tube-like pores. The pores are responsible for absorbing water and concrete acts like a sponge-like structure.
Generally concrete provides huge compressive strength that’s why it can withstand very high compressive loads without cracking but concrete is very weak in tension. A small tensile load will create some major cracks in concrete.
Those pores which are created at the time of hydration are responsible for weakness because cracks will generate at that path. A good example of this is a long concrete beam that will show failure in the middle due to its self-weight even if it is rigidly fixed at the support region.
The strength of concrete is measured by a compressive strength testing machine and the unit is psi (pound per square inch). As an example, concrete of 3500 psi strength can withstand a compressive force of 3500 psi but it can not resist any amount of tensile strength.
By changing the size of aggregate, type & grading aggregate we can achieve our desirable compressive strength of concrete. Steel reinforcement, fiberglass, wire mesh, etc materials can increase the tensile strength of concrete.
Rebar generally provides additional strength and it distributes weight properly throughout the beam. The rebar prevents cracks and resists structural damage.
Concrete is used for construction purposes many years ago but reinforced cement culture is not invented at that time. Concrete generally provides huge compressive strength that’s why it can withstand very high compressive loads without cracking but concrete is very weak in tension.
Due to a small tensile force, it will show some serious cracks. That’s why concrete structures are not so long in the general case.
The strength of concrete is measured by a compressive strength testing machine and the unit is psi (pound per square inch). As an example, a concrete of 5000 psi strength can withstand a compressive force of 3500 psi but it only resists 500 psi amount of tensile stress.
Rebar also helps to bear compressive load which is widely used in recent times. In doubly reinforced beams this type of combination is observed.
Rebar is the internal material of any structure; so rebar is the also important material or condition as well as the exterior loading condition of any structural element.
Characteristics of Rebar:
Concrete has extremely good compressive strength but it has one weakness which is it is very weak in tension. This property makes concrete useless in building construction.
Without rebar, the concrete can not handle those huge loads that’s why all the building structure construction rebar is used along with concrete. By this method, concrete can resist bending and stretching effects because rebar has excellent properties.
Rebars are available in a wide range of varieties in the marketplace with grades #3 to #18. The numbers are generally referred to as the diameter of the rebar.
Thicker diameter rebar can provide more strength with compare to thinner diameter rebar and engineers select the grade of rebar according to the requirement of the work.
Rebar has high bending strength and it creates a good quality bond with concrete, that’s why the bond strength of concrete increases along with the bending strength.
Generally, in reinforced concrete structure, rebar is tied together and create a skeleton-like structure; after that, we pour concrete into that. One thing you should always remember is that the rebar must be tied properly.
The most critical thing is the spacing of the rebar. If you do not provide the required spacing then it can reduce the strength of concrete and it can be reduced to 20%. The rebar inspection is done separately because it is responsible for the strength of the structure.
In previous time plain mild steel bars are used as rebar but it is weak in creating a bond that’s why in recent times HYSD bars are used which provides better bond strength along with bending.
Why Rebar Makes Concrete Stronger?
In any concrete structure compression and tension, will appear but concrete can not support all those forces alone that’s why rebar comes to help concrete and it takes all the tensile loads.
If you add some loads on a concrete beam it can easily resist all the compressive loads at the support regions but at the middle part of the beam there will be a tension zone and tensile forces will appear on that portion but concrete can not resist that & it will show tension cracks and finally the beam will fail.
Concrete is very against vibration because in the vibration contraction and expansion take place & it is not good for concrete. Rebar helps to take all those stresses which concrete can not take and make an all-rounder material.
Types of Rebar
There are generally two types of rebar are available, those are in the following below-
Plain rebar and deformed rebar. Deformed rebar has ridges on the surface area which helps to create a good bond with concrete but plain rebar does not have that property. Other types of rebar are shown below-
- Glass fiber reinforced polymer (GFRP): This type of polymer is a good substitute for rebar. It has resin wrapped coating with fiberglass. The corrosion will not take place in this type of rebar but it is expensive.
- Carbon steel: This is a common type of rebar that is comparatively low cost and provides good strength but is prone to corrosion when exposed to weather conditions.
- Epoxy-coated: The other name of the epoxy-coated rebar is a green bar. The main application of this type of bar is in bridge structures or in wet climate conditions because it is very much resistant to corrosion. In some places where the epoxy-coated bar is not a good option, those are earth-shifting places, vibration, cracks, etc because it will damage the bar coating.
- European: It is a very cheap type of reinforcement where manganese is present in major form. In America, it is used as a solid structural element.
- Galvanized rebar: It is the same as epoxy coating rebar but much more expensive and highly resistant to corrosion. Here, zinc coating is used for galvanization but for hot plating, gold plating, and electroplating processes. Zinc plays as a protective shield against the steel.
- Stainless steel rebar: It is the most expensive type of rebar where zinc or epoxy coating is not used but is highly resistant to corrosion and withstand huge loads.
In the market, rebar is available in a wide range of sizes. If we increase the grade of rebar then the rebar diameter will increase. In the United States, imperial sizes are used instead of metric sizes.
The use of rebar in concrete is always determined by an engineer or an architect. All the details of rebar diameter, the number of rebar are specified on the detailed drawing of the structure.
So, the engineer of the architect must follow the required amount of rebar. Always use the mentioned amount of rebar and mentioned number of the grade of rebar.
Rebar size chart-
|IMPERIAL BAR SIZE||SOFT METRIC SIZE||WEIGHT PER UNIT LENGTH (lb/ft)||MASS PER UNIT LENGTH||DIAMETER||DIAMETER||AREA||AREA|
Compressive Vs Tensile Strength:
The strength of concrete is compressive strength which can resist the structure against external loading and self-weight. The compressive strength is generally measured in megapascals (MPa) or pounds per square inch (psi).
Every concrete has different strengths according to the cement grade and the location of use. In the strength gaining of concrete water plays an important role.
The compressive strength of concrete varies from 17.5 MPa (3500 psi) to 34.5 MPa (5000 psi). This strength is observed after curing for 28 days.
The tensile of concrete is nearly 1/10 th of the compressive strength of that concrete if the compressive strength of concrete is 20N/mm2 then the tensile strength of that concrete will be 2 N/mm2. To overcome this effect we use steel rebar into it.
Adding rebar increases the tensile strength of that concrete. The best example is a parking garage where re4bars are used in concrete otherwise the structure will fail. It resists deflection and cracking.
Generally, for rigid pavement on highways, rebar is not used because loads are directly transferred to subsoil but if a sinkhole is detected then we must use rebar to resist the deflection of a concrete slab.
Generally base material is a type of layer that is laid beneath the concrete. After placing the base material the concrete will be poured. It rests just above the soil and also acts as a foundation. It provides a flat base for the concrete.
There are some factors that we need to keep in our mind. Expansion and contraction: For temperature variation concrete will show contraction and expansion. For this reason, it will create huge pressure on a concrete structure that’s why rebar is placed at the expansion joint to bear this tensile stress.
Changes: The property of the subsoil may change, there will be some differential settlement beneath the structure. To overcome this type of situation rebars are provided to rest the deflection.
Overload: Sometimes structure will face some overload conditions due to external loading. In that situation, the structure will face some problems if rebar is not provided perfectly. If rebar is provided then that can take those loads.
The Thickness of Concrete:
Generally, 4 inches thickness is provided in concrete. In this situation, wire mesh is provided instead of rebar because rebar will take up more space. In 5-inch concrete, you can easily provide rebar. If the concrete is going to be thicker then it withstand more load and more rebar and thicker bar are needed.
Rebar cages are built in those cases where the thickness of the concrete is very higher. More rebars are needed to hold those structures. Generally in any type of reinforced concrete structure vertical and horizontal rebar is provided. Sometimes we also wire mesh and other chemicals to increase the strength of that.
You need to fix your rebar properly otherwise it will move laterally and the whole structure will be hampered. That’s why the position of rebar is more important and little dispute in positioning will hamper the whole strength of the structure.
Every works need a different type of rebar positioning that’s why a fixed position count is not possible. It is determined by the architect or the engineer according to the structure.
So, previously make a rebar cage and make sure that the binding of the rebar is otherwise it will dislocate from its original position. Most of the structure will fail due to the wrong placement of the rebar. The steps for placing rebar are in below-
- Firstly make sure that the correct size of your rebar.
- Secondly, examine every bar position.
- Thirdly, check the bonding or joining of two or more rebar.
- Finally, you start your concrete pouring work.
Wire Mesh Vs Rebar:
In concrete structures, rebar provides strength to the structure. In all large projects, rebar is used along with concrete but in some specified small work normal concrete is used. Like if we construct 4” thick concrete pavement for the sidewalk then rebar is not needed but 12” thick sidewalk pavement sometime needs rebar.
Steel mesh is made of stainless steel wire in the grid which comes in a roll with a specified length and width. Generally, the length of this type of wire mesh is 100 feet.
These wire mesh sheets are embedded into concrete to increase the strength of concrete. Though wire mesh does not increase the strength like rebar if you construct a bent or circular shaped structure then you need to use the wire mesh.
- Rebar generally adds more strength but wire mesh generally increases the ductility property of the concrete.
- Wire mesh insertion is comparatively easier than rebar positioning.
- Wire mesh is used to resist cracks in small structures but rebar is used to prevent cracks in large structures.