Before we try and understand the working of steel and concrete, we need to understand the need for them to work together.Do we really understand why a RCC structure is designed and used? Lets start by understanding the forces that a RCC structure have to resist during its working.
a) Why are RCC (reinforced cement concrete) structures designed and used?
Most RCC members like beams, slabs, columns and retaining walls have to resist the following forces.
Tensile Force
Compressive Force
Shear Force
Torsional Force
Moments
RCC structure are made up of concrete and steel and these two materials are put together because they compliment each-others properties.
Concrete:-
Concrete is a very good material which can withstand compressive forces.
It can withstand the elements of nature quite efficiently.
But,
Concrete has no or negligible tensile strength.
It is brittle and cracks easily due to shrinkage, temperature variation and creep.
When subjected to bending moments, plain cement concrete, it will break the same way a biscuit would.
Steel:-
Steel has a very high tensile strength and are also good at compressive strength.
Steel is very ductile, a property which concrete lacks.
But,
Steel is highly corrosive when compared to concrete and thus requires higher maintenance.
The combination of forces as stated earlier cannot be tackled by using concrete or steel alone in a cost effective way. Thus the composite material RCC is used and designed, so as to be cost effective, easier to construct and long lasting with minimum maintenance.
Steel as transverse reinforcement (eg. stir-ups, ties, spirals etc) supplement the shear and torsional capacity of a structure.
Why concrete needs reinforcement?
I want my readers to know and understand that concrete is a mass which is brittle, and factors like shrinkage and temperature causes crack formation in plain cement concrete (PCC). They develop these cracks even when not subjected to the earlier stated forces and moments.
A minimum steel concept is used to avoid or limit these crack formation. Sometimes steel meshes or 6 mm steel bars are used.
Now, I would try and explain how steel and concrete work together in an RCC structure.
For the purpose of explanation, I will take the example of a Singly Reinforced RCC beam, as I find it to be a section which functions a simple task, but is subjected to many different types of forces.
Also, I will not get into the calculation part of the explanations as I am writing this blog to clear the concepts of the reader. You would find many books where you would find such calculations if you are interested, also explaining calculations would really need a chapter dedicated to itself.
Concrete is very capable of resisting the compressive stress developed within the beam, but requires the help of steel to resist the tensile stresses developed in the lower portion.
The dotted lines represent the deformation of the beam when a load is applied to the beam. This is how a wooden ruler would bend. The following will be the result of the application of load as shown in the diagram.
The top portion of the beam is compressed. This causes compressive stresses to develop in this portion of the beam.
The bottom portion of the beam tends to elongate. In other words, this portion of the beam is subjected to tensile forces.
As explained in the earlier section of this blog, concrete is very capable of resisting the compressive stress developed within the beam, but requires the help of steel to resist the tensile stresses developed in the lower portion. Steel also performs the duty of holding the concrete mass in place, thus preventing shrinkage, creep and cracks due to deformation. Thus steel and concrete, two different materials work together as one composite material as an RCC beam.
Most commonly used steel now a days is the hot rolled deformed bars. These bars have corrugations or grips which helps concrete adhere to the steel better.
Stirrups in a beam functions as a grip or hold for the concrete mass. but the main function is to resist shear forces developed in the member.
Even a Singly reinforced steel bar will have secondary bars at the top compressive portion of the beam which aids as an anchor and grip for concrete and stirrups.
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