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Reinforced Cement Concrete(RCC)

Reinforced Cement Concrete is a combination of concrete and steel to build a structure instead of using only concrete.

Brief History

François Coignet was a French industrialist of the nineteenth century, a pioneer in the development of structural, prefabricated and reinforced concrete.

In 1853 Coignet built the first iron reinforced concrete structure, a four story house in Paris.

Ernest L. Ransome, was an innovator of the reinforced concrete techniques in the end of the 19th century

Uses of RCC

It is used in the construction of Columns, Beams, Footings, Slabs etc.

It is used in storage structures like Dams, Water Tanks, Tunnels etc.

It is used to build heavy structures like Bridges, Walls, Towers, Under water structures.

It is used in tall structures and sky scrapers.

Why it is essential?

High relative strength

High toleration of tensile strain

Good bond to the concrete, irrespective of pH, moisture, and similar factors

Thermal compatibility, not causing unacceptable stresses in response to changing temperatures.

Durability in the concrete environment, irrespective of corrosion or sustained stress for example.

Merits of Reinforced Concrete

1. Good Binding Between Steel and Concretethere is a very good development of bond between steel and concrete.

2. Stable StructureConcrete is strong in compression but week in tension and steel as strong in tension so their combination give a strong stable structure.

3. Less Chances of BucklingConcrete members are not slim like steel members so chances of buckling are much less.

4. Aestheticsconcrete structures are aesthetically good and cladding is not required

5. Lesser Chances of Rustingsteel reinforcement is enclosed in concrete so chances of rusting are reduced.

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Short Reinforced Concrete Compression Members

Short - slenderness does not need to be considered –column will not buckle

Only axial load

L

Cross-sectional Areas:As = Area of steelAc = Area of concreteAg = Total area

Fs = stress in steelFc = stress in concrete

From Equilibrium:P = Acfc + Asfs

P

L

P

If bond is maintained εs = εc

Reinforced Concrete

Mechanism of Load

Transfer

Load

Roof Surface

Roof Slab

Beams

Column

Foundation

Sub Soil

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Function of structure is to transfer all the loads safely to ground.

A particular structural member transfers load to other structural member.

Design Loads

Dead Load“The loads which do not change their magnitude and position w.r.t. time within the life of structure”Dead load mainly consist of superimposed loads and self load of structure.

Self LoadIt is the load of structural member due to its own weight.

Superimposed LoadIt is the load supported by a structural member. For instance self weight of column is self load and load of beam and slab over it is superimposed load.

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Design Loads (contd…)

Live Load“Live loads consist chiefly of occupancy loads in buildings and traffic loads on bridges”

They may be either fully or partially in place or not present at all, and may also change in location.

Their magnitude and distribution at any given time are uncertain, and even their maximum intensities throughout the life time of the structure are not known with precision.

The minimum live loads for which the floor and roof of a building should be designed are usually specified in the building codes that governs at the site construction.

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Objectives of DesignerThere are two main objectives

1. Safety

2. Economy

SafetyThe structure should be safe enough to carry all the applied throughout the life.

EconomyStructures should be economical. Lighter structures are more economical.

Economy α 1/self weight (More valid for Steel Structures)

In concrete Structures overall cost of construction decides the economy, not just the self weight.

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Load CombinationsTo combine various loads in such a way to get a critical situation.

Load Factor = Factor by which a load is to be increased x probability of occurrence

1. 1.2D + 1.6L

2. 1.4D

3. 1.2D + 1.6L + 0.5Lr

4. 1.2D + 1.6Lr + (1.0L or 0.8W)

Where

D = Dead load

L = Live load on intermediate floors

Lr = Live load on roof

W = Wind Load

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Shrinkage

“Shrinkage is reduction in volume of concrete due to loss of water”

Coefficient of shrinkage varies with time. Coefficient of shortening is:

0.00025 at 28 days

0.00035 at 3 months

0.0005 at 12 months

Shrinkage = Shrinkage coefficient x Length

Excessive shrinkage can be avoided by proper curing during first 28 days because half of the total shrinkage takes place during this period

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Creep

“creep is the slow deformation of material

over considerable lengths of time at constant stress or

load”Creep deformations for a given concrete are practically proportional to the magnitude of the applied stress; at any given stress, high strength concreteshow less creep than lower strength concrete.

Compressive strength

Specific Creep

(MPa) 10-6 per MPa

20 145

30 116

40 80

55 58

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Plain & Reinforced Concrete

Creep (contd…)

How to calculate shortenings due to creep?Consider a column of 3m which is under sustained load for several years.

Compressive strength, fc’ = 30 MPa

Sustained stress due to load = 10 MPa

Specific creep for 28 MPa fc’ = 116 x 10-6 per MPa

Creep Strain = 10 x 116 x 10-6 = 116 x 10-5

Shortening due to creep = 3000 x 116 x 10-5

= 3.48 mm

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Strength measurement

Specified Compressive Strength Concrete, fc’

“28 days cylinder strength of concrete”

The cylinder has 150mm dia and 300mm length.

According to ASTM standards at least two cylinders should be tested and their average is to be taken.

ACI 5.1.1: for concrete designed and constructed in accordance with ACI code, fc’ shall not be less than 17 Mpa (2500 psi)

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Plain & Reinforced Concrete-1

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Concrete Cylinder Concrete Cube

Beginnings of Reinforced

Concrete

CAUSES OF FAILURES IN RCC BUILDING

EARTHQUAKE

CORROSION

When the earthquake forces exceed the designparameters, the alternating forces of the earthquakefirst break the concrete on one side of the column andsubsequently on the other side.

Building A

Building B

Building A :- has thick and stiff floors and

slender supporting columns.

During a earthquake, the whole building will pancake.the bottom columns receive the largest forces andbend; walls crack

Building B :- has a ductile floor design.

During Earthquake, Floors will be waving andcracking, but the building would not collapse.

CORROSION

CAUSES OF CORROSION

Moisture

Oxygen diffusion

Temperature

Humidity

How to avoid corrosion?

Careful detailing to protect from water

Use stainless steel

Protect steel with galvanizing (zinc coating) or other protective coating

Corrosion of Steel

Every 90 seconds, across the world, one ton of steel turns to rust; of every two tons of steel made, one is to replace rust.

Most concrete used for construction is a combination of concrete and reinforcement that is called reinforced concrete.

Steel is the most common material used as reinforcement, but other materials such as fiber-reinforced polymer (FRP)

are also used

Reinforcement in a concrete column

REINFORCEMENT USED IN RCC BUILDING

Fiber reinforcement:Fiber-reinforced concrete (FRC) is concrete with the addition of discrete reinforcing fibers made of steel, glass, synthetic(nylon, polyester, and polypropylene), and natural fiber materials.

Synthetic fibers can be delivered to the mixing system in preweighed, degradable bags that break down during the mixing cycle. Steel fibers are introduced to the rotating mixer via conveyor belt, either at the same time as the coarse aggregate or on their own after all the conventional ingredients have been added.

1. The major applications of FRC are slab-on-grade construction, precast concrete, and shotcrete.

2. Some examples of slab-on-grade construction are airport runways, residential, commercial, and industrial floor slabs, and hydraulic structures

3. Fiber- reinforced shotcrete is used for rock slope stabilization, tunnel liners, hydraulic structures, and maintenance of existing concrete.

4. FRC is also used in repair applications, such as repair of bridge decks, piers, and parapets.

Steel reinforcement:Steel reinforcement is available in the form of plain steel bars,

deformed steel bars, cold-drawn wire, welded wire fabric, and deformed welded wire fabric.

1. Deformed steel bars:—Deformed bars are round steelbars with lugs, or deformations, rolled into the surface of the bar during manufacturing

2. Threaded steel bars:—Threaded steel bars are made by several manufacturers in different grades They are used as an alternative to lapping standard deformed bars when long bar lengths are required

3.Welded wire fabric:—Welded wire fabric reinforcement also known as welded wire reinforcement is a square or rectangular mesh of wires.

Typical deformed reinforcing barWelded wire reinforcement sheets

TYPES OF CONCRETE1.Prestressed concrete: Prestressed concrete is structural concrete in which internal stresses

have been introduced to reduce potential tensile stresses in the concrete resulting from loads.

Applications

a. To resist internal pressures in circular structures like tank, pipe

b. To limit cracking in bridge decks and slabs-on-grade.

c. To improve capacity of columns and piles.

d. To reduce long-term deflections.

2.Plain concrete: Plain concrete is structural concrete without reinforcement

It is sometimes used in slabs-on grade ,pavement, basement walls, small foundations, and curb-and-gutter.

3.Pretensioned concrete: Pretensioning is usually performed in a factory (or

precasting yard). The tendons are held in place and tensioned against the ends of the casting bed before the concrete is placed.

Advantages of pretensioned concrete are that it

tendons are bonded to the concrete over their entire length.

4.Post-tensioned concrete: Post-tensioning is usually performed at the job site. Post-

tensioning tendons are usually internal but can be external.

Some of the advantages of post-tensioning are that it does not require the large temporary anchorages required for pretensioning,

It allows for larger members than are possible in a precasting plant.

Plain & Reinforced Concrete

Reinforced Cement Concrete (RCC) contd..

Mix Proportion

Cement : Sand : Crush

1 : 1.5 : 3

1 : 2 : 4

1 : 4 : 8

Water Cement Ratio (W/C)

W/C = 0.5 – 0.6

For a mix proportion of 1:2:4 and W/C = 0.5, if cement is 50 kg

Sand = 2 x 50 = 100 Kg

Crush = 4 x 50 = 200 Kg Batching By Weight

Water = 50 x 0.5 = 25 Kg

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Structural Member of RCC

Slabs

Beams

Columns

Foundation

Slabs It is better to provide a max spacing of 200mm(8”) for

main bars and 250mm(10”) in order to control the crack width and spacing.

A min. of 0.24% shall be used for the roof slabs since it is subjected to higher temperature. Variations than the floor slabs. This is required to take care of temp. differences.

It is advisable to not to use 6mm bars as main bars as this size available in the local market is of inferior not only with respect to size but also the quality since like TATA and SAIL are not producing this size of bar.

Beams

A min. of 0.2% is to be provided for the compression bars in order to take care of the deflection.

The stirrups shall be minimum size of 8mm in the case of lateral load resistance .

The hooks shall be bent to 135 degree.

Columns

Minimum cross-sectional dimension for a Column.

Longitudinal Reinforcement

Transverse reinforcement

Helical Reinforcement

Foundation Minimum size of foundation for a single storey of

G+1 building, where soil safe bearing capacity is 30 tonnes per square meter, and the oncoming load on the column does not exceed 30 tonnes.

Reinforcing bar details

Foundation Minimum size of foundation for a single storey of

G+1 building, where soil safe bearing capacity is 30 tonnes per square meter, and the oncoming load on the column does not exceed 30 tonnes.

Reinforcing bar details

Arrangement of reinforcement in various structural members :

R.C.C. is used as a structural element, the common structural elements in a building where

R.C.C. is used are:

(a) Footings (b) Columns

(c) Beams and lintels (d) roofs and slabs.

1) Footings :In rectangular footing the reinforcement parallel to the long direction shall be distributed uniformly across the width of the footing. In short direction, since the support provided to the Footing by the column is concentrated near the middle, the moment per unit length is largest i.e., the curvature of the footing is sharpest immediately under the column and decreases in the long direction with the increasing distance from the column. For this reason larger steel area is needed in the central portion and is determined in accordance with the equation given below.

2) Columns :

The main reinforcement in columns in longitudional, parallel to the direction to the direction of the load and consists of bars arranged in square, rectangular or spherical shape.

Main steel is provided to resist the compression load along with the concrete.

The bar shall not be less than 12mm in diameter

Nominal max. Size of coarse aggregte is5mm.

The no of bars in columns are varies from 10, 12, 14, 16 with varying diameter.

3) Beams :

Generally a beam consists of following types of reinforcements :

Longitudinal reinforcement .

Shear reinforcements.

Side face reinforcement in the web of the beam is provided when the depth of the web in a beam exceeds 750 mm.

Arrangements of bars in a beam should confirm to the requirements of clause given in 8.1and 8.2of SP34.Bars of size 6,8,10,12,16,20,25,32,50 mm are available in market.

Thickness of the slab is decided based on span to depth ratio . Min reinforcement is 0.12% for HYSD bars and 0.15% for mild steel bars. The diameter of bar generally used in slabs are: 6 mm, 8 mm, 10 mm, 12 mm and 16 mm.

The maximum diameter of bar used in slab should not exceed 1/8 of the total thickness of slab. Maximum spacing of main bar is restricted to 3 times effective depth . For distribution bars the maximum spacing is specified as 5 times the effective depth .

4) Slabs :

Minimum clear cover to reinforcements in slab depends on the durability criteria . Generally 15 mm to 20 mm cover is provided for the main reinforcements.

Torsion reinforcement shall be provided at any corner where the slab is simply supported on both edges meeting at that corner.It shall consist of top and bottom reinforcement, each with layer of bars placed parallel to the sides of the slab and extending from the edges a minimum distance of one fifth of the shorter span.