GENERAL PROVISIONS FOR THE DESIGN OF REINFORCED CONCRETE MEMBERS
“Feel the Structure” MSA
This blog is dedicated to those who is a beginner in the field of reinforced cement concrete.It will help them to gather key note for RCC design,detailing & site execution.
Before designing reinforced cement concrete(RCC) structural members, it is essential to know the environmental factors in which designed member will be exposed through out it’s life & second aspect for RCC design is it’s strength against shear, bending and torsion which depends upon the characteristics & quantity of its ingredient/components.
How environmental condition will affect proposed grade of concrete, it is mandatory to know the characteristics of different components ,water cement ratio & their proportion in mix design of concrete and reaction of C-H-S gel of concrete in green state & in hardened state with certain deleterious chemicals like chloride & sulphate.
Basically, concrete is made up of cement, sand , aggregate & water. When ingredient like cement, sand & aggregates are mixed with water in proper proportion , plain cement concrete is formed.This is usually called as PCC (Plain Cement Concrete).When reinforcement or rebar is cast along with PCC, then it is called as reinforced cement concrete (RCC) as reinforcement (rebar) reinforces flexural strength of concrte. It is well known fact and experimentally proved that PCC is weak in bending/flexure and strong in compression.Let say, if we are using M25 grade of concrete then this grade of concrete will have characteristic strength as 25N/mm2 which is compressive in nature. As per IS456-2000,flexural strength of M25 is 0.7x (sqaure root of grade of concrete) i.e 0.7X (25)^0.5= 3.5N/mm2. Now you can see, how flexural strength of concrete is much lower than compressive strength .
Hence, when you are asked to design RCC members, then your first priority shall be to calculate shear force and bending moment and accordingly you have to find out which fibre of concrete member is under tension and which one is under compression.On the basis of that, you need to place calculated reinforcement in terms of numbers or spacing to improve tensile strength of concrete. For example, a cantilever slab & beam will have top surface undergoing tension whereas bottom of same will be under compression.As we know that flexural/bending tensile strength of concrete is weak,hence,main reinforcement shall be placed at top whereas nominal reinforcement shall be at bottom of this slab as prescribed by code to control cracks. You can better understand from from the following images.
From above three images like,Fig-1,Fig-2 & Fig-3, i have walked you through the concept of tensile bending & how designed reinforcements are placed on tension face of RCC structural members. If you see Fig-1, which is a typical section for cantilever slab where main reinforcement has been placed at top face of concrete slab as top of this concrete slab is under tension/tensile bending whereas bottom slab is under compression.Hence, main reinforcement has placed at top face of slab whereas distribution bar has been placed at bottom face of slab.As a structural engineer, our main task is to calculate bending moment and accordingly reinforcements in terms of numbers or spacing are placed on tension face. Rest of distribution bars or binder bars(which are minimum) are placed as prescribed in standard code for concrete design to control cracks.
let us see Fig-2 which is a typical section for footing. In this case, when a column is subjected to load(Axial Load with uni- axial or bi-axial moment), a base pressure from soil will be generated in upward direction, hence bending of footing slab will start taking place in the same direction.Bottom face of this footing will be like parabolic curve in upward direction.Here, you can analog a footing as a inverted cantilever slab. Being bottom of footing in tension, main reinforcement has been placed at bottom. It is not mandatory to provide compression reinforcement at top of footing as it is in compression. However, we sometime provide nominal or minimum reinforcement prescribed in code to control cracks.
Now to understand better about tensile bending and placement of main reinforcement in tension area,i would refer you to see fig-3.In Fig-3, i have shown a typical detail of cantilever retaining wall in which two faces of wall have been shown.Left face is towards earth whereas right face is towards road side. As this retaining wall is a vertical cantilever, hence, left face will be in tension whereas right face will be in compression.As earth has retained in left side of this wall, force exerted by earth will try to bend wall in right direction creating tension in left face.Hence, main reinforcement has been placed in left face of the wall.Main reinforcement can be placed in both faces of wall only when free end (top of wall) is hinged or fixed.
Now to deal same concept in more detail, i have come up with Fig-4 ,which is a liquid retaining structure in which reinforcement in RCC wall has been shown in the both faces of wall as main reinforcement. If you see closely W-2 & W-3, same face of wall may be sometime in tension & sometime in compression depending upon whether tank is full or tank is empty. Hence, main reinforcement has been placed in both faces of wall.
Above discussion is all about the detailing part only as far as bending strength is concerned.But this is not enough.In order to have a safe,durable & aesthetically appealing structure we need to check certain codal provisions and criteria at design as well as at construction stage. Point wise these are explained below.
- Grade of Concrete: As per the exposure of proposed structure throughout it’s life and to reduce section size in high rise buildings, grade of concrete is decided.For example, if you are going to design a marine structure then minimum grade of concrete shall be M30 as per IS:456-2000 whereas for any other structure, we can start our design with a minimum garde of M20. Apart from grade of concrete, water cement ratio shall be different in different component of same building like foundation, column,beams & slabs.Sometimes, soil having sulphate, sulphate resisting cement(SRC) is used instead of normal PPC( Portland Pozzolana Cement ) & OPC ( Ordinary Portland Cement ) for durability.As sulphate voluminize concrete up 300% after reacting with C-H-S gel(Complex hydration product of cement) & hence huge cracks are formed which are further source of deterioration of reinforcement & concrete.Furthermore,if there is high congestion of reinforcement and column sizes are big like (1.5m x 1.5m, 2m x 2m and so one),self compacting concrete is used to avoid mechanical vibrator as it will be very difficult to compact such a mass concrete. Also, self compacting or high flow concrete (which are also known as special concrete) is used when there is high congestion at junction of column & beams. In this case if normal concrete is used , then concrete will not be compacted properly even after using mechanical vibrator. This happens because it is very difficult to insert needle of mechanical vibrator inside heavily congested reinforcement and therefore, honey comb is formed.Due to formation of honey comb , strength of RCC member is reduced. If honey comb is formed due to slurry leakage, lack of compaction, improper mixing & placement,this shall be immediately repaired by injection grouting or any other approved method.
Above explanation on concrete was only on selection of concrete in design stage. Until & unless, proper mixing, placing and curing of concrete is done at site, we can not ascertain durability & strength.As bad quality of shuttering (leading to slurry leakage),improper placing & compaction will lead to the formation of honey comb. On the other hand, improper curing will lead to formation of weaker concrete in hardened state as hydration process will be completed because of lack of water.If we say,water is blood of concrete then it will not be an exaggerated statement.
Mohammad Sohel Akhtar(MSA)