REQUIREMENTS GOVERNING REINFORCEMENT AND DETAILING

General :-
Reinforcing steelof sametypeand grade shall be used
as main reinforcement in a structural member.
However.simultaneous use of two different types or
grades of steel for main and secondary reinforcement
respectively is permissiblte.

Bars may be arranged singly. or in pairs in
contact, or in groups of three or four bars bundled in
contact. Bundledbarsshallbeenclosedwithinstirrups
or ties. Bundled bars shall be tied together to ensure
the bars remaining together. Bars larger than 32 mm
diametershall not be bundled. except in columns.


The recommendations for detailing for
earthquake-resistant construction given in IS 13920
should be taken into consideration, where applicable
(see also IS 4326).

Development of Stress in Reinforcement

The calculated tension or compression in any bar at

any section shall be developed on each side of the

section by an appropriate developmentlength or end

anchorage or by a combination thereof.

Development Length ofBars

The development lengthLd is given by

L _ ,a. (1---

4fbd

whereL= nominaldiameter of the bar,

a =stressinbaratthe sectionconsideredat design

load, and

t hd =design bond stress given in 26.Z.1.1.

NOTES

1 The development IcnJlhincludca anchomae vDlues of hooks

in tension reinforcement.

2 For bars of sections otherthan cireular. the development

lenathshould be sufficient to develop the stras in the bar

TRANSPORTING, PLACING, COMPACTION AND CURING

(1) Transporting and Handling:-

After mixing, concrete shall be transported to the
formwork asrapidlyas possiblebymethods whichwill
prevent the segregation Of Jossof any of the ingredients
or ingress of foreign matter or water and maintaining
(he required workability
13.1.1 During hoi Of cold weather. concrete shall be
transportedindeep containers. Othersuitable methods
to reduce the loss of water hy evaporation in hot
weather and heat loss in cold weather may also be
adopted

(2) Placing:-

TIleconcrete shall he depositedas nearlyas practicable
in its final position to avoid rehandling. The concrete
shall he placed and compacted hefore initial setting of
concrete commences and should not be subsequently
disturbed. Methods of placing should be such as
to preclude scgreg.rtion. Care should be taken to
avoid displacement of reinforcement or movement
of Iorrnwork . As a general guidance, the maxi ...
mum permissible free fall of concrete may be taken
as 1.5 Ill.

(3) COmpaction:-

Concrete should he thoroughly compacted and fully
worked around the reinforcement, around embedded
fixtures and intocorners of the fonnwork.
13.3.1 Concrete shall be compacted using mechanical
vibrators complying with IS 2505, IS 2506. IS 2514
and IS 4656. Over vibration and under vibration of
concreteare harmful and should beavoided. Vibration
of very wet mixesshould also he avoided.
Whenever vibration has to be applied externally, the
design of formwork and the disposition of vibrators
shouldreceivespecialconsiderationto ensure efficient
compaction and to avoid surface blemishes.

(4) Curing:-

Curing isthe process of preventing the Jossof moisture
from the concrete whilst maintaining a satisfactory
temperature regime. The prevention of moisture loss
from the concrete is panicularly importantifthe water￾cement ratio IS low, ifthe cement has a high rate of
strength development, if the concrete contains
granulated blast furnace slag or pulverised fuel ash.
TIlecuringregimeshould alsoprevent the. development
of high temperature gradients within the concrete.
The rate of strength development at early ages of
concrete made with supersulphated cement is
significantly reduced at" lower temperatures.
Supersulphated cement concrete is seriously affected
by inadequate curing and the surface has to be kept
moist for at least seven days.
13.5.1 Moist Curing
Exposed surfaces of concrete shall be kept
continuously in a damp Of wet condition by pending
or by covering with a layer of sacking, canvas, hessian
or similar materials and kept constantly wet for at least
seven days from the date of placing concrete in case

What is the creep of concrete?

Creep Of Concrete:

Creep can be defined as the elastic and long-term deformation of concrete under a continuous load. Generally, a long term pressure changes the shape of concrete structure and the deformation occurs along the direction of the applied load. When the continuous load is removed, the strain is decreased immediately. The amount of the decreased strain is equal to the elastic strain at the given age. This quick recovery is then followed by a continuous decrease in strain, known as creep recovery that is a part of total creep strain suffered by the concrete.

Creep Coefficient:

The ratio of the ultimate creep strain to the elastic strain at the age of loading is termed as creep coefficient.

Photo Source: Theconstructor

Factors Affecting Creep Of Concrete:

The factors that affect creep of concrete are similar to the factors affecting shrinkage, which are as following:

1. Water-cement Ratio:

The rate of creep is increased with increasing water cement ratio.

2. Humidity:

It is influenced by humidity and drying condition of the atmosphere.

3. Age Of Concrete:

The rate of creep rapidly decreases with time. The time taken by a concrete structure to attained creep is 5 years.

4. Aggregate:

Aggregates with moisture movement and low elastic modulus cause a large amount of creep. The rate of creep generally decreases with the increase of the size of aggregates.

5. Admixtures:

Some admixtures (mainly accelerators) are also responsible for causing creep in concrete.

Other Factors:

• Types of cement.
• Entrained air.
• Concrete strength.
• Improper curing etc.

PLAIN AND REINFORCED CONCRETE

Plain concrete: 

It is a mixture of sand , gravel,cement , and water which results in a solid mass .

Concrete is strong in compression but weak in tension. its tensile strength aprox , one tenth of compressive strength plain concrete is mostly used in mass concrete work. ( As in dams) 

Rainforced concrete : 

• It is a concrete with reinforcement embedded in it . The embedded reinforcement makes it capable of resisting tension also.
• Steel bars embedded in the tension zone of concrete, relieves concrete of any tension and takes all tension without separating from concrete 
• The bond between steel and surrounding concrete ensures strain compatibility i.e. the strain at any points in the steel is equal to that in the adjoining concrete.
• Reinforcing steel imparts ductility to concrete which is otherwise brittle material
• Here ductility means large deflection owing to yielding of steel , thereby giving ample warning of impending collapse 
• Tensile stress in concrete arises on account of direct tension, flexural tension , diagonal tension , temperature and shrinkage effect , restraint to deformation 
• Under these conditions, reinforcement must be provided across potential tensile crack .

Materials used for R.C.C. Work

1. Cement 
2. Aggregate
3. Steel
4. Water

• The material which is passed through BIS test sieve no 480 is termed as fine aggregate . Usually , the natural river sand is used as a fine aggregate.
• The material which is retained on BIS test sieve no. 480 is termed as a coarse aggregate . The broken stone is generally used as a coarse aggregate .
• For thin slabs and walls, the maximum size of coarse aggregate should be limited to one-third the thickness of concrete section 
• It may be notes that sometimes ingredients other then above are added in concrete to give it certain improve qualities or for changing different physical propertis in its fresh and hardened stages. These ingredients or substances are known as the admixtures . The additional of an admixture may improve the concrete with respect to its strength , hardness , workability , water resisting power etc.

Cement concrete

The cement concrete is mixture of cement, send , pebbles or crushed rock and water , which when placed in the skeleton of forms and allowed to cure becomes hard like a stone

Properties of cement concrete : 

1. It has high compressive strength 
2. It is free from corrosion and there is no appreciable effect of atmospheric agents on it 
3. It hardens with age and the process of hardening continues for a long time after the concrete has attained sufficient strength 
4. It is provided to be more economical than steel 
5. It binds rapidly with steel and as it is weak in tension , the steel reinforcement is placed in cement concrete at suitable places to take up the tensile stresses .this is termed as the R C C 
6. It forms a hard surface , capable of resisting abrasion.
7. Its final strength and quality depends entirely on local conditions and persons handling it.