Reinforced Concrete Construction RCC - Civil Engineering - vk

Reinforced Concrete Construction RCC 

- Civil Engineering

A nominal mix is designed based on the ratios given in code IS:456 - 2000 and can be hand mixed on site. Nominal mixes are done for strengths up to 20MPa (i.e. M20).

Grade of Concrete and The quantity of water per 50kg of cement

• M5         60

• M7.5 45

• M10 34

• M15 32

• M20 30

The above water content may be changed for better workability keeping the w/c ratio constant.

Water content is an important parameter deciding the strength

Different types of excavating equipment.

• Power shovel, backhoe, dragline and clamshell, all are used as excavation equipment under a different set of condition and requirements.
• The table below shows a comparison between all different types of excavating equipment.

• .

Item of Comparison	Power Shovel	Back Hoe	Drag Time	Clam Shell
Excavation in hard soil or rock	Good	Good	Not Good	Poor
Excavation in wet soil or mud	Poor	Poor	Moderately good	Moderately good
Distance between footing and digging	Small	Small	Long	Long
Loading Efficiency	Very Good	Good	Moderately good	Precise but slow
Cycle time	Short  vk study civil Enginering	Sightly more than power shovel	More than power shovel	More than the other equipment.

• Power shell is known as a hoe and Dipper shovel is known as a backhoe.

Concrete in Sea-Water

When sea water is used for preparation of concrete, strength of concrete is reduced by about 10-20% and setting time accelerates. Corrosion of reinforcement is not a problem if concrete is having less permeability and proper cover to ensure good quality.

Consider the following statement in making concrete with sea water:

• 1. Strength of concrete is reduced by about 10-20%.
• 2. Setting time accelerates.
• 3. Corrosion of reinforcement is not a problem if concrete is of good quality.

all options are correct

Geopolymer

• Geopolymer is an inorganic alumino-silicate polymer, synthesized from predominantly silicon and aluminium material such as fly ash.

• The geo-polymer gel binds the loose coarse and fine aggregates to form geopolymer concrete. Geopolymer gel replaces the C-S-H gel in cement concrete. Chemical reaction period for this concrete is sustainably fast, and the required curing period may be within 24 to 48 hours.

Concrete Strength

• Split tensile strength is about 2/3 of Modulus of Rupture

• It is observed that the Cube strength of concrete is nearly 1.25 times the cylinder strength.

• Tensile strength of concrete is much less than compressive strength of concrete. 

• ∴ Split tensile strength < modulus of rupture < Cylinder strength < Cube strength

Limit State Method for RCC

• The acceptable limit for the safety and serviceability requirements before failure occurs is called a “limit state.

• In the limit state method, the structure shall be designed to withstand safely all loads liable to act on it throughout its life, it shall also satisfy the serviceability requirements, such as deflection and cracking.

There are two limit states:

i) Limit state of collapse: 

• The limit state of collapse of the structure or part of the structure could be assessed from rupture of one or more critical sections and from buckling due to elastic or plastic instability or overturning.

• The resistance to bending, shear, torsion and axial load at every section shall not be less than the appropriate values at that section produced by the most unfavourable combination of loads on the structures using the appropriate partial safety factors.

ii) Limit state of serviceability: 

• It consists of a limit state of deflection, cracking, and vibration.

Limit state of Collapse	Limit State of Serviceability
It considers imaginary behaviour of structure at the time of collapse	It considers actual behaviour of structure under working or service loads.
The various limit state of collapse as per IS 456:2000 are: 1. Flexure 2. Shear 3. Compression 4. Stability 5. Torsion	The various limit state of serviceability as per IS 456:2000 are: 1. Cracking 2. Deflection 3. Corrosion 4. Vibration

Notes

• As per clause No. 38.1 of IS 456:2000, the maximum strain in concrete at the outermost compression fibre is taken as 0.0035 in bending.

• While, as per clause No. 39.1 of IS 456:2000, the maximum compressive strain in concrete in axial compression is taken as 0.002.

Retarder

A retarder is a chemical agent that slows down a chemical reaction. For example, retarders are used to slow the chemical hardening of plastic materials such as wallboard, concrete, and adhesives. 

Admixture

Accelerator	Retarder	Air Entraining Admixture	Plasticizer	Super Plasticizer
Calcium Chloride	Calcium Sulphate	Wood Resins	Lignosulphate	Modified Lignosulphate
Silicate	Tartaric Acid	Plant & Animal fatty acid	Carbohydrates	Sulphonated Nalane Formaldehyde
Flourosilicate	Sugar	Steric Acid	Polyglycol Easter
Triethanolamine	Starch	Oleic Acid	Hydroxylated Carbolic Acid
	Cellulose	Aluminium Powder
	Gypsum	Hydrogen Peroxide

Water-tightness Fluoro-silicate

 Mineral Admixture  Surkhi

Water Cement Ratio

• Water cement ratio plays an important role in the design strength of concrete mix.

• The water cement ratio is measured in terms of weights of water to the weight of cement.

• The range for w/c ratio is typically from 0.3 to 0.8 for all types of mixes where, below 0.3 will make concrete very stiff and above 0.8 will makes a wet and fairly weak strength concrete

• Generally, for hand mixes, w/c is taken moderate to high i.e. 0.5 - 0.6

Important points:

• More water results into more spaces between the cement particles which leads to lesser strength.

• Concrete with a higher w/c ratio is also more susceptible to cracking and shrinkage. Shrinkage leads to micro-cracks, which are zones of weakness.

• Once the fresh concrete is placed, excess water is squeezed out of the paste by the weight of the aggregate and the cement paste itself. When there is an excess of water, that water bleeds out onto the surface.

•  The micro channels and passages that were created inside the concrete to allow that water to flow become weak zones and micro-cracks.

As per IS 456: 2000:

• For design purpose, compressive strength of concrete is assumed to be 0.67 times the characteristic strength of concrete. The partial factor of safety of 1.5 is also to be applied in addition to this.

• Characteristic Strength of concrete Cube = fck

• Characteristic strength of concrete in actual structure = 0.67fck

• Design strength of concrete in flexural compression = 0.67fck/ partial FOS = 0.67fck/ 1.5 = 0.45f­ck

Footing is designed as per shear criteria.

There are two types of shear

• One way shear
• Two way shear

In one way shear the critical section for shear is:

• At a distance d from the face of the wall or column as the case may be.
• At a distance d/2 from the face of the wall or column if the footing slab is on piles.

Two way/punching shear:

• It shall be checked around the column a perimeter half the effective depth of footing slab away from the face of column or pedestal.
• The critical section for two-way shear or punching shear is at a distance of d/2 from the face of the column.

Curing: 

• It is the process of hardening the concrete mixes by keeping its surface moist for a certain period, in order to enable the concrete to gain more strength.

• Curing plays an important role in strength development and durability of concrete. Curing takes place immediately after concrete placing and finishing, and involves maintenance of desired moisture and temperature conditions, both at depth and near the surface, for extended periods of time.

• Properly cured concrete has an adequate amount of moisture for continued hydration which leads to:
• Development of strength
• volume stability,
• resistance to freezing and thawing,
• abrasion and scaling resistance.

Fly ash

• The use of fly ash in portland cement concrete (PCC) has many benefits and improves concrete performance in both the fresh and hardened state. Generally, fly ash benefits fresh concrete by reducing the mixing water requirement and improving the paste flow behavior.

Fly ash benefits to fresh concrete are:

1. Improved workability: 

• The spherical shaped particles of fly ash act as miniature ball bearings within the concrete mix, thus providing a lubricant effect. This same effect also improves concrete pumpability by reducing frictional losses during the pumping process and flat work finishability.

2. Decreased water demand: 

• The replacement of cement by fly ash reduces the water demand for a given slump. When fly ash is used at about 20 percent of the total cementitious, water demand is reduced by approximately 10 percent. Higher fly ash contents will yield higher water reductions. The decreased water demand has little or no effect on drying shrinkage/cracking. Some fly ash is known to reduce drying shrinkage in certain situations.

3. Reduced heat of hydration: 

• Replacing cement with the same amount of fly ash can reduce the heat of hydration of concrete. This reduction in the heat of hydration does not sacrifice long-term strength gain or durability. The reduced heat of hydration lessens heat rise problems in mass concrete placements.

Fly ash benefits to Hardened concrete are:

1. Increased ultimate strength: 

• The additional binder produced by the fly ash reaction with available lime allows fly ash concrete to continue to gain strength over time. Mixtures designed to produce equivalent strength at early ages (less than 90 days) will ultimately exceed the strength of straight cement concrete mixes

2. Reduced permeability: 

• The decrease in water content combined with the production of additional cementitious compounds reduces the pore interconnectivity of concrete, thus decreasing permeability. The reduced permeability results in improved long-term durability and resistance to various forms of deterioration

3. Improved durability: 

• The decrease in free lime and the resulting increase in cementitious compounds, combined with the reduction in permeability enhance concrete durability.

Reinforcement Requirements in One Way Slab:-

• Mild steel requirement in either direction of the slab shall not be less than 0.15% of the total cross-sectional area (bD). However for HYSD bars in either direction, the reinforcement shall not be less than 0.12% of the gross cross-sectional area.

• Maximum dia of reinforcing bars shall not exceed 1/8of the total thickness of slab. { e.g. t = 80 mm dia = 10 mm}

The reduction Coefficient of Long column

The strength reduction coefficient method is used for the design of a long column. It is valid for Working Stress Method only.

Strength Reduction coefficient is given as

• Cr=1.25−ℓeff/48b

• Where, b = least lateral dimension of core and for the circular column with helical reinforcement it is the diameter of the core.

Self Compacted Concrete

• Placing self-compacted concrete is easier than conventional concrete. Formwork used must be in good conditions to prevent leakage, the following rules are to be followed to minimize the risk of segregation:

• i) Limit of vertical free fall distance to 5 meters.

• ii) Limit the height of pour lifts to 500 meters.

• iii) Limit of the permissible distance of horizontal flow from point of discharge to 10 meters.

• 1) Water/powder ratio by volume is to be 0.80 to 1.00
• 2) Total powder content to be 160 to 240 liters (400-600 kg) per m3
• 3) The sand content may be more than 38% of the mortar volume.
• 4) Coarse aggregate content should normally be 28 to 35% volume of the mix.
• 5) Water/cement ratio is selected based on the strength required. In any case, water content should not exceed 200 liters/m3.
• ∴ Self – compacting concrete (SCC) is characterized by High powder component and less coarse aggregate.

Lightweight concrete: 

• It can be defined as a type of concrete which includes an expanding agent in it, so that it increases the volume of the mixture while giving additional qualities such as lessened the dead weight.

Light weight concrete can be prepared by using Lightweight aggregates:

• Lightweight aggregate concrete can be produced using a variety of lightweight aggregates. Lightweight aggregates originate from either:

Natural materials, like volcanic pumice.

• The thermal treatment of natural raw materials like clay, slate or shale i.e. Leca.

• Manufacture from industrial by-products such as fly ash, i.e. Lytag.

• Processing of industrial by-products such as pelletised expanded slab, i.e. Pellite.

• If little structural requirement, but high thermal insulation properties, are needed then a light, weak aggregate can be used. This will result in relatively low strength concrete.​

Aerated Concrete:

• Aerated concrete is made by introducing air or gas into a slurry composed of Portland cement or lime and finely crushed siliceous filler so that when the mix sets and hardens, a uniformly cellular structure is formed.

There are several ways in which aerated concrete can be manufactured.

• By the formation of gas by chemical reaction within the mass during liquid or plastic state.
• By mixing preformed stable foam with the slurry.
• By using finely powdered metal (usually aluminum powder) with the slurry and made to react with the calcium hydroxide liberated during the hydration process, to give out large quantity of hydrogen gas. This hydrogen gas when contained in the slurry mix, gives the cellular structure.

No-fines Concrete

• This process omits the fines from conventional concrete. No-fines concrete as the term implies, is a kind of concrete from which the fine aggregate fraction has been omitted.

• This concrete is made up of only coarse aggregate, cement and water. Very often only single sized coarse aggregate, of size passing through 20 mm retained on 10 mm is used.

• No-fines concrete is becoming popular because of some of the advantages it possesses over the 

Conventional concrete.

• The single sized aggregates make a good no-fines concrete, which in addition to having large voids and hence light in weight, also offers architecturally attractive look.

The degree of workability in vee bee test

It is classified based on the time taken in seconds as shown below:

• i) For very low workability, Vee bee time must be greater than 20 seconds.

• ii) For low workability, Vee bee time lies between 6-12 seconds.

• iii) For medium workability, Vee bee time lies between 3-6 seconds.

• iv) For High workability, Vee bee time lies between 0-3 seconds.

Maturity of Concrete

• Correct Expression for maturity of the concrete is:
• M = ∑ (Time × Temperature)
• -11°C is taken as a datum line for computing maturity.

Pumped Concrete

• Generally, almost all pumped concrete is conveyed through 125 mm pipeline. 
• General rule is that the pipe diameter should be between 3 to 4 times the largest size of aggregate.
• Concrete has been pumped to a height over 400 m and a horizontal distance of over 2000 m. 
• But the diameter of the pipeline used for transportation of concrete by pumps should not exceed 30 cm.

Shear Stress Distribution in RCC sections

• In a homogenous rectangular section of concrete, the shear stress distribution is parabolic throughout the depth.

• But RCC section is a non-homogenous section and for analysis purpose, we assume that the concrete section below the neutral axis is under tension and would not be able to resist any load and we ignore this part.

• Further, the section above the neutral axis is in compression and homogenous as no steel is present in the upper portion, so shear stress distribution will be parabolic till the neutral axis and thereafter till the depth of reinforcement it will be rectangular.

Concrete in Sea-water (as per IS 456: 2000)

• Concrete in sea-water or exposed directly along the sea-coast shall be at least M 20 Grade in the case of plain concrete and M 30 in case of reinforced concrete. The use of slag or pozzolana cement is advantageous under such conditions.

 

 

Exposure condition	Environmental  condition	Minimum  grade required  for  reinforced concrete
Mild	Concrete surface protected  against aggressive conditions.	M20
Moderate	Concrete surface sheltered  from severe rain or continuously underwater	M25
Severe	Concrete surface: 1. Exposed to severe rain  and alternate wetting and drying 2. Completely immersed in sea water 3. Exposed to coastal environment	M30
Very severe	Concrete surface exposed  to sea water spray	M35
Extreme	Surface members in tidal zone	M40

 

 

IS Codes

• IS 4926:2003 - Ready-Mixed Concrete - Code of Practice

• IS 4634:1991 - Methods for testing performance of batch-type concrete mixers (first revision)

• IS 8142:1976 - Method of test for determining setting time of concrete by penetration resistance

• IS 4082:1996 - Recommendations on stacking and storage of construction materials and components at site (second revision)

Fiber reinforced concrete

• Plain concrete possesses a very low tensile strength, limited ductility and little resistance to cracking. Poor tensile strength is due to the propagation of such microcracks, eventually leading to brittle fracture of the concrete.

• In order to increase the tensile strength of the concrete, the conventional reinforced steel concrete is practiced, but they do not increase the inherent tensile strength of the concrete.

• Whereas by addition of small, closely spaced and uniformly dispersed fiber to concrete would act as a crack arrester and would substantially increase the static and dynamic tensile strength of the concrete.

• Fiber reinforced concrete can be defined as the composite material consisting a mixture of cement, mortar, or concrete and uniformly dispersed suitable fibers.

Note:

• Glass fiber used in making fiber concrete has very high tensile strength of 1020 – 4080 N/mm2.

• Carbon fiber used in making fiber concrete has also very high tensile strength of 2110 – 2185 N/mm2.

• Asbestos is also a mineral fiber used in making fiber concrete having tensile strength of 560 – 980 N/mm2.

Form-works and its Removal

Type of Formwork	Formwork removal time
Walls, columns and vertical sides of beams	1 to 2 Days
Slabs (props left under)	3 Days
Removal of props to slabs (Spanning up to 4.5 m)	7 Days
Removal of props to slabs (Spanning over 4.5 m)	14 Days
Beam soffits (props left under)	7 Days
Removal of props to beams and arches (Spanning upto 6 m)	14 Days
Removal of props to beams and arches (Spanning over 6 m)	21 Days

High performance concrete possesses

• high workability
• high strength
• high modulus of elasticity
• high density
• high dimensional stability
• low permeability
• resistance to chemical attack

Load taken by column column

•  for column with lateral or circular ties.
• Pu = 0.40 fck Ac+ 0.67 × Fy × ASC

Where, Pu = ultimate load

fck = Characteristic strength of concrete

AC, ASC = Area of concrete & steel respectively

Fy = yield strength of steel

• When helical reinforcement is used, ultimate load capacity is increased by 5%
• Pu = 1.05 (0.40 fck AC + 0.67 Fy × ASC)

• This formula is valid only when emin ≤ 0.05 × D
• emin = minimum eccentricity =maximum{(lunsupported/500 + D/30 ) or 20 mm}

Sand or Fine Aggregate

• The sand particles consist of small grains of silica (SiO2). It is formed by the decomposition of sandstones due to various effects of weather.

According to the natural sources from which the sand is obtained, it is of the following three types:

(1) Pit sand: 

• This sand is found as deposits in soil and it is obtained by forming pits into soils. It is excavated from a depth of about 1 m to 2 m from ground level and it is washed to remove clay and silt.

• The pit sand consists of sharp angular grains which are free from salts and it proves to be excellent material for mortar or concrete work. For making mortar, the clean pit sand free from organic matter and clay should only be used.

(2) River sand: 

• This sand is obtained from banks or beds of rivers. The river sand consists of fine rounded grains probably due to mutual attrition under the action of water current. The colour of river sand is almost white. As river sand is usually available in clean condition, it is widely used for all purposes.

(3) Sea sand: 

• This sand is obtained from seashores. The sea sand, like river sand, consists of fine rounded grains. The colour of sea sand is light brown. The sea sand contains salts. These salts attract moisture from the atmosphere. Such absorption causes dampness, efflorescence and disintegration of work. The sea sand also retards the setting action of cement.

The recommendations of IS 456: 2000 related to different critical section in an isolated footing are listed in following table:

Cl No.	Recommendation for	Location of critical section
34.2.3.2	Bending Moment	At the face of the column
31.6.1	Two way shear or Punching shear	At a section d/2 from the face of the column
34.2.4.1	One way shear or Beam Shear	At a section d from the face of the column
34.2.4.3	Development length	At face of the column or where abrupt change in section occurs

Cover for Reinforcement

• The minimum clear cover depends upon the exposure condition and fire resistance.
• Nominal Cover to Meet Fire Resistance:
• The nominal cover is the design depth of concrete cover to all-steel reinforcements.
• It shall be not less than the diameter of the bar.
• Minimum values of the nominal cover of normal-weight aggregate concrete to be provided to all reinforcement to meet the specified period of fire resistance shall be given in the table below 

Member	Minimum Cover
Slab	20 mm
Beam	20 mm
Column	40 mm for d > 12 mm 25 mm for d < 12 mm
Footing	50 mm

Nominal Cover to Meet Durability Requirements:

Exposure	Nominal concrete cover  (in mm) not less than
Mild	20
Moderate	30
Severe	45
Very Severe	50
Extreme	75

The strength reduction coefficient method is used for the design of a long column. It is valid for Working Stress Method only.

Strength Reduction coefficient is given as

Cr=1.25−ℓeff48b

A counterfort retaining wall is a cantilever wall with counterforts, or buttresses, attached to the inside face of the wall to further resist lateral thrust.

In the counterfort retaining wall, the stem and the base and the base slab are tied together by counterforts, at suitable intervals

Because of the provision of counterforts, the vertical stem, as well as the heel slab, acts as a continuous slab, in contrast to the cantilevers of cantilever retaining wall

Counterforts are firmly attached to the face slab as well as the base slab; The earth pressure acting on the face slab is transferred to the counter forts which deflect as vertical cantilevers

The back of the rear counter forts comes in tension and their front face is under compression

Hence the inclined (back) face of rear counter forts should be provided with main reinforcement and in front counterforts, the tension develops at the bottom face so it is provided with main reinforcement

Counterfort retaining walls are economical for height over about 6m

∴ In a counterfort retaining wall, the main reinforcement is provided on the bottom face in the front counterfort and the inclined face in the back counterfort.

As per IS 875 Part 2, clause 3.1, Imposed Floor load for residential building are:

S.No	Residential Buildings (Dwelling houses)	U.D.L (kN/m2)
1.	All rooms and kitchens	2.0
2.	Toilet and Bath rooms	2.0
3.	Corridors, passages, staircases including tire escapes and store rooms	3.0
4.	Balconies	3.0

As per codal provisions of IS 3370:

• Minimum grade of concrete for the R.C.C water tank is M30.
• Maximum cement content is 400 kg/m3 to take care of shrinkage effect.
• Minimum grade of concrete is 320 kg/m3.
• Minimum grade of concrete for P.C.C is M20.
• Maximum w/c ratio is 0.45.
• Minimum nominal cover is 45 mm.
• Maximum allowed crack width is 0.2 mm in the LSM design.
• To reduce cracking due to temperature, shrinkage, and moisture loss at an early stage of concrete, curing should be done for at least 14 days.
• Permeability of concrete must be least so use leaser value of w/c ratio.
• No porous aggregate should be used.
• Part of structure retaining liquid and enclosing space above liquid should be taken under server exposure condition.
• All the structures to be designed shall be designed for both empty and full condition.
• Cracking of concrete can be controlled to some extent by maintaining a slope filling rate of 1 m in 24 hours at the first time of filling.
• Permissible stress of the material is as follows:
• a) Mild Steel - 115 N/mm2 and HYSD bar - 130 N/mm2
• b) Concrete

Grade	Direct tension	Bending Tension
M25	1.3 N/mm2	1.8 N/mm2
M30	1.5 N/mm2	2.0 N/mm2

• If the thickness is more than 200 mm, the reinforcement is provided in 2 layers, one on each face
• Minimum steel is 0.64% and 0.4% of the surface zone for mild steel and HYSD respectively
• The above percentage values can be reduced to 0.35% and 0.24% for tanks with no dimension more than 15 m.

As per IS 800: 2007, Clause No. 3.8,

Slenderness ratio of steel sections is limited as per the following:

:

Member	Maximum Effective Slenderness ratio
Pure Compression Member	180
Pure tension Member	400
Tension Member subjected to reversal of stress due to loads other than wind and earthquake.	180
Tension Member subjected to reversal of stress due to wind or earthquake load.	350
A member subjected to compression forces resulting from wind or earthquake forces provided that deformation of such member does not affect the stress in any part of structure.	250
Compression flange of beam against lateral torsional buckling	300

As per IS 456:2000, Clause 23.3;

Slenderness limit for lateral stability of beam
For Simply supported or continuous beam	Minimum of 60b or 250b2/d
For Cantilever	Minimum of 25b or 100b2/d

Where,

• d is the effective depth of the beam and b the breadth of the compression face midway between the lateral restraints.

• R.C.C beam not provided with shear reinforcement may leads to failure in its bottom inclined roughly to 45° to the horizontal due to diagonal tension.
• The tension which is caused in the tensile zone of the beam (i.e. at the bottom of beam) due to shear, near the supports is called as diagonal tension, which cannot be resisted by concrete alone. So shear reinforcement is provided in the R.C.C. beams to take up diagonal tension and prevent cracking of beam.

Why 45°

• At the supports, where bending stress is practically zero, the value of principal stress equals to the shear stress. As the maximum shear stress plane is 45° to the principal plane, it acts diagonally at 45° to the horizontal.

Partial safety factor for shop welding and field welding

• As per IS 800 : 2007 codal provisions partial factor safety kept for shop welding is 1.25 and for filed welding is 1.5. The reason for considering more partial factor of safety for field welding is that in field quality control in welding is less as compared to welding done at factories or shops. 

Important Points:

Description	FOS as per IS 800: 2007
Bolted Connection	1.5
Welded Connection	1.5 (field) 1.25 (Shop)
Steel Member (yielding)	1.1
Steel Member (Rupture)	1.5

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