Reinforcing bar (or reinforcement/rebar) is a critical construction element in the built environment. Anything from your driveway to 100+ storey buildings rely upon reinforcement to give them strength and durability. But why does rebar have ridges?
There are two reasons why modern rebar is manufactured with ridges or ribs (also called deformations), firstly the ridges provide a roughened surface to the bar which increase its friction coefficient allowing it to bond more effectively with concrete, secondly the manufacturing process of introducing the deformations increases the bars tensile strength through the process of work hardening.
Lets take a closer look at these two unique characteristics of deformed reinforcing bars, how they effect Structural Engineering design, and how a bar with ridges differs from round (or flat) reinforcement…
How do Ridges or Deformations on Rebar help it Bond with Concrete.
Concrete structures are required to be built in stages. Each stage of construction often needs to be structurally tied to the next subsequent stage so the final built form behaves as one single entity.
The point at which two stages meet is often referred to as a “construction joint”. Inadequate detailing at a construction joint interface was a primary reason why the FIU Pedestrian bridge in Florida collapsed. To see a thorough analysis into why and how the FIU bridge collapsed from a Structural Engineers perspective, see the article located at THIS link.
Some examples of connections between subsequent stages of a structure may include:
- A column base attached to its foundation
- Two adjacent slab pours where the concrete volumes need to be poured over several days.
- A slab connected to a shear wall
- The columns and walls from one level connected to the level below/above it.
The bond between the reinforcing bar and the concrete is what allows different stages of construction to be tied together. But how does this bond work?
Lets consider a deformed reinforcing bar imbedded into a concrete mass. Also lets consider applying a tension force at the end of the bar pulling it away from the concrete mass.
If we were to increase the applied force on this hypothetical bar, failure will eventually occur. Failure of the system will occur one of three ways:
- The shear/friction interface between the rebar and the concrete fails and the bar slips out of the concrete.
- The concrete mass fails and pulls out of the surrounding concrete forming a cone or pull-out failure.
- The rebar fractures under the tension load.
The distance the reinforcing bar protrudes into the concrete mass is referred to its development length (or embedment length).
The mechanisms of failure of our example above is dependant on the development length which has been provided to the bar within the concrete mass.
Often the Structural Engineer will specify the embedment length of the reinforcement such that full development of the bar is achieved. This maximises the strength of the connection as the full strength of the bar is utilised…
Full development of a reinforcing bar is when it is imbedded an adequate distance into a concrete mass to cause failure through fracture of the reinforcement rather than failure of the friction/shear interface of the bar and concrete or concrete cone failure.
From this explanation above, we can determine that two of the three failure mechanisms of a reinforcing bar being pulled out of a concert mass is caused by in-sufficient embedment:
|Development / Embedment
|Type 1: Shear/Friction failure of rebar against concrete
|Type 2: Concrete cone (or pull-out) failure
|Type 3: Fracture of reinforcement
|Fully Developed/Adequate Embedment
The deformations (or ridges/ribs) on a reinforcing bar allows a shorter embedment length to achieve full development of the bar. The ridges on the bar increases the contact surface area of the concrete against the bar surface and also increase the friction coefficient of the interface. This means that a shorter embedment length is required for a deformed bar with ridges compared to a round bar.
Deforming the Bar with Ribs or Ridges Adds Strength.
Reinforcement comes in different shapes and sizes. Two main types of reinforcement used in Australia and around the world are:
- Deformed Bar (identifiable by distinct ridges or ribs)
- Round bar (bar with a smooth surface)
Take a look at Best Bar’s website, a local reinforcement supplier in Australia for coverage on sizes and bar types.
Adding the ribs or deformations to reinforcing bar has the added benefit in increasing its tensile strength.
To understand how this works, we need to look at the structure of steel at an atomic level.
At sub-microscopic level, the atoms within steel arrange themselves in a tightly packed formation. This arrangement is referred to as a crystal structure. Crystal not being the transparent glass you may be familiar with but:
A piece of a homogeneous solid substance having a natural geometrically regular form with symmetrically arranged plane faces
The atoms within steel arrange themselves in regular planes. When steel is bent beyond its elastic limit these planes of atoms slide against one-another which causes the deformation.
Something interesting occurs while the atomic structure undergoes deformation. Dislocations can form in the planar arrangement due to deformation. The more the steel is deformed the more dislocations are produced in the atomic arrangement of the steel.
The dislocations can then join and intertwine with each other. The network of dislocations prevent the crystal arrangement of atoms from sliding along each other easily which gives rise to an increased strength of the steel.
The image below illustrates what a single dislocation may look like within the crystal structure of steel at the atomic level (dislocation is running in and out of your screen at the location denoted as “D”). The dislocation produces a disruption in the regular crystal pattern:
Blacksmiths have intuitively known this property in steel before we understood atomic theory. This phenomenon of steel becoming stronger as its deformed is called “work hardening”.
Japanese swordsmiths have used this technique to produce samurai swords for centuries. The process involving heating, folding and “kneading” the steel dramatically increase its strength.
During the manufacturing of deformed reinforcement, the bar is crimped or drawn while still mailable to give the distinct ribbed characteristic we see in modern reinforcing steel. This introduces a deformation to the steel necessary to produce the dislocations which increase its strength.
You can try this yourself with a mini experiment at home. Next time you finish a soft drink from of a can, squash it then bend it back and forth several times and notice how it becomes stiffer before it fractures. Similarly with production of reinforcing bar, the right amount of work hardening an heat treatment is required in order to give it adequate strength as well as ductility characteristics.
This process, as well as heat treatment gives deformed bars around double the tensile strength of conventional round bar reinforcement. This means that around half the amount of deformed bars are required to achieve the same strength as a round bar.
|Deformed Bar Tensile Strength
|Round Bar Tensile Strength
|500MPa (72 kip/in2)
|250MPa (36 kip/in2)