CLEAR COVER FOR DIFFERENT R.C.C. MEMBERS

What Is Clear Cover (or) Cover in Concrete?

The clear cover (or) Cover in RCC, is the distance between the exposed concrete surface (without plaster and other finishes) to the nearest surface of the reinforcement bar i.e., Steel rod.

Mostly, it is known as "clear cover" or "minimum concrete cover" in general terms on a construction site.

Why clear cover is important?

Cover is provided to protect the reinforcing bar (rebar) from corrosion due to weather and fire. If we don't leave a cover, then the reinforcing bar will stick to the surface of the shuttering and cause rust in the bar after we casting the column or beam and also damage the strength of the column or beam.

For example, in the case of fire in a building, the metal begins to melt and lose its strength. Therefore, the clear cover acts as an insulator and prevents from melting.

This gives more strength to the bars, reduces the chances of slipping and also increases the durability of the bars. This provides adequate embedding for the reinforcing bars to withstand stress without loss of grip.

How to Define Nominal Cover

As per IS 456(Clause 26.4.1), the term clear cover has been replaced by the term Nominal cover.

The nominal cover is the distance between the exposed concrete surface to nearest reinforcement bar ( it can be any bar I.e., main bar, longitudinal bar and even links or stirrups).

What Is Effective Cover in Concrete?

The Effective cover is the distance measured betweenthe exposed concrete surface (without plaster and other finishes) to the centre of the area ( centroid ) of reinforcement, i.e., tension or compression reinforcement. 

Effective cover = overall depth – effective depth.    (Or). 

Effective cover = Clear cover + Diameter of Stirrup + (Diameter of main reinforcement bar ÷ 2).

Member	Clear cover (Normal Conditions)	wet climatic environment or sea side location
Foundation	75 mm
Raft Foundation	Top = 50 mm Bottom = 75 mm Sides = 75 mm
Strap Beam	50 mm
Beam	25 mm	35 to 40 mm
Column	40 mm	50 mm
Flat Slab	20 mm
Slab	15 mm

DRAIN DESIGN DRAWINGS

 

For Drawings PDF click here 

TYPES OF BEAMS -- Based on reinforcement

Singly Reinforced Beams:

A singly reinforced beam is a beam provided with longitudinal reinforcement in the tension zone only.

The analysis and design of a reinforced concrete member subjected to bending are based on the following assumptions: 

1) Plane sections transverse to the centre line of a member before bending remain plane sections after bending.

2) Elastic modulus of concrete has the same value within the limits of deformation of the member.

3) Elastic modulus for steel has the same value within the limits of deformation of the member.

4) The reinforcement does not slip from concrete surrounding it.

5) Tension is entirely by steel.

6) The steel is free from initial stresses when embedded in concrete.

7) There is no resultant thrust on any transverse section of the member.

Doubly Reinforced Beams:

Beams reinforced with steel in compression and tension zones are called Doubly reinforced beams.  

CONCRETE SAMPLES

Sampling Procedure: 

         A random sampling procedure is adopted to ensure that each concrete batch shall have a reasonable chance of being tested i.e., sampling is done to the entire period of concreting and cover all mixing units.

Frequency:

The minimum frequency of sampling of concrete of each grade shall be accordance with the following:

Quantity of concrete in the work (m3)	Number of samples
1-5	1
6-15	2
16-30	3
31-50	4
51 and above	4 Plus one additional sample for each additional 50 m3

Strength test of concrete (IS 456):

          Samples from fresh concrete shall be taken and cubes shall be made and tested at 28 days. The test strength shall be the average of the strength of 3 specimen. The individual variation should not be more than 15 percent (+15% or -15%) of the average.

Different Grades of Concrete -- Concrete Mix Design

Different grades of Concrete:

          Based on different concrete mix proportions (or) mix ratio’s, concrete mixer is divided into different grades. As per IS 456:2000, concrete mix design has the following mix proportions:

Type of Concrete	Concrete Grade	Mix Ratio (or) Proportion	Characteristic Compressive strength of Concrete @ 28 Days in N/mm2
Ordinary concrete	M5	1:5:10	5 N/mm2
	M7.5	1:4:8	7.5 N/mm2
	M10	1:3:6	10 N/mm2
	M15	1:2:4	15 N/mm2
	M20	1:1.5:3	20 N/mm2
Standard Concrete	M25	1:1:2	25 N/mm2
	M30	Design Mix	30 N/mm2
	M35	Design Mix	35 N/mm2
	M40	Design Mix	40 N/mm2
	M45	Design Mix	45 N/mm2
	M50	Design Mix	50 N/mm2
High Strength Concrete	M55	Design Mix	55 N/mm2
	M60	Design Mix	60 N/mm2
	M65	Design Mix	65 N/mm2
	M70	Design Mix	70 N/mm2

Note:-

In Example: M5

M represents "Mix"

5 represents the compressive strength achieved after 28 days of curing.

GRADES OF CONCRETE