What is RCC and Basics of RCC
Concrete is, without a doubt, the most commonly used building material today. However, plain concrete has some glaring flaws. That is why we use RCC (Reinforced Cement Concrete) in most of the load-bearing members of a structure. Today, we will talk about the basics of RCC or reinforced concrete details.
Issue with plain concrete?
The problem of plain cement concrete, or PCC, is that it has great compressive strength, but very little tensile strength. In fact, the tensile strength of plain concrete is close to only a tenth of the compressive strength of the same mix. That is why PCC is very much useless in structural members which bear tensile as well as compressive loads. For example, a horizontal beam will crack at the bottom while its topside is still capable of bearing ten times more than the failure load.
What is RCC
To combat this, steel bars are inserted at the portions of concrete where tensile loads are expected, like on the bottom of a beam. This can make the concrete bear ten times more tensile loads than without bars. These steel bars are called reinforcement bars, and the member which has been strengthened this way is called a RCC member (like a RCC beam or RCC slab).
How much steel is good in RCC
Steel bars are incredibly strong. Volume being equal, they can take 28 times more compressive loads than standard plain concrete, and can resist tensions as much as 280 times more! That’s incredible – so why don’t we just build structures out of steel?
The answer to that is hidden deep in the pockets. Per unit steel is around 60 times costlier than per unit of concrete, considering everything! So, obviously for general purpose and high-volume static work, reinforced concrete is the best choice between economy and structural strength. We have to carefully determine how much reinforcement the loads need, and not an inch more.
Methods of designing RCC structures
There are two methods of designing reinforcement in concrete structures:
1. Elastic method
2. Ultimate load method
2. Indirect leveling
Each method gives somewhat different results. Let’s see what they are below.
Elastic or working load method
This is the more traditional way of figuring out the reinforcements and as such it is most widely used too. In this, the loads expected to be working on the structure is figured out first. Then the criterion for the strength of the structure becomes the forces working on it and its capacity to sustain those forces without breaking.
Ultimate load or load factor method
This is the rather newer method of designing the steel reinforcement in concrete members. In this method, a final load is determined at which the structure will fail inevitably, and then a safety margin is added to those values. This then becomes the designing factor of the reinforcement.
Mathematical factors in Reinforcement Calculation
Factor of safety
This is the relation between the ultimate stress at which the structural member will surely fail, and the maximum allowable stress. This is assumed 3 in case of the concrete and 2 in case of the steel.
Modulus of elasticity
This is the number indicating how strong the steel or concrete is, that is, its elastic property. Mathematically, it is expressed as the ratio between the stress caused by the load on the material and the deformation that happens due to that stress. That is,
Modulus of elasticity = stress/deformation
This is the defining factor of an RCC design. It tells us how much steel must be used in the design in respect to the concrete. It is the ratio of the moduli of elasticity of steel and concrete used in the member, and is indicated by the letter ‘m’.
While the steel quality remains more or less the same throughout construction, the modulus of elasticity is taken as a constant value (200 kN/mm2). However, the quality of concrete varies wildly (there are concrete grades in use through M5 to M200). For this reason, the modulus of elasticity in the concrete is the only real variable in practice here.
For calculation, use this formula for determining the modular ratio in your RCC structure:
Modular ratio = 2800 / 3 x fc
Where fc is the maximum compressive strength in your type of concrete before bending, expressed in kg/m2. There should be charts in building code books about the expected values of ‘m’ for each grade of concrete, though. You won’t have to calculate unless you’re using some special concrete grade.
Let us take the example of an RCC beam here, with steel bars embedded at the bottom of it. The stress is applied to the beam and, for permissible stress values, there will be no displacement of the steel inside the concrete. Therefore, the steel and the concrete are being pushed and pulled as one.
Therefore, according to the above theory, the stress in the steel will be ‘m’ times greater than the stress in the concrete. Put in other words, the steel will carry ‘m’ times more loads than the concrete will.
Since the load in the steel in much greater than the load in concrete in the same member, you need not consider the concrete stress. Because if the load of steel is ever shared to the concrete, it will surely break down (hence we only consider fc and keep steel modulus a constant). To be realistic, the concrete on the bottom side will always crack, whether it is visible to the naked eye or not. Don’t worry, the steel is carrying the loads and the concrete at the bottom of the beam is pretty much useless.
One thing you should note here that due to chemical changes, weather, inequality at the time of drying, the concrete strength varies even within the same member, slightly. So, it is best to rather allow for a safety margin rather than aim for the perfect values. Better be stronger than weaker.