4A Shrinkage and Creep
4A Shrinkage and Creep
4A Shrinkage and Creep
and Creep of
Concrete
Presentors:
Barotilla, Josam B.
BSCE 4-A
CREEP
(Sometimes called cold flow) is the tendency
of a solid material to move slowly or deform
permanently under the influence of mechanical
stresses. It can occur as a result of long-term
exposure to high levels of stress that are still below
the yield strength of the material. Creep is more
severe in materials that are subjected to heat for
long periods, and generally increases as they near
their melting point.
CREEP
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
Creep Coefficient:
The ratio of the ultimate creep strain to the elastic strain at the age of
loading is termed as creep coefficient. The assumed data of creep
coefficient are given below:
Age of Loading Creep Coefficient
7 days 2.2
28 days 1.6
1 year 1.1
FACTORS THAT ATTRIBUTES TO
CREEP OF CONCRETE
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.
FACTORS THAT ATTRIBUTES TO
CREEP OF CONCRETE
OTHER FACTORS:
Types of cement.
Entrained air.
Concrete strength.
Improper curing etc.
Effects of Creep on Concrete and
Reinforced Concrete
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TYPES OF SHRINKAGE
1. PLASTIC SHRINKAGE:
Plastic shrinkage occurs very soon after pouring the concrete in the
forms. The hydration of cement results in a reduction in the volume of
concrete due to evaporation from the surface of concrete, which leads
to cracking.
2. DRYING SHRINKAGE:
The shrinkage that appears after the setting and hardening of the
concrete mixture due to loss of capillary water is known as drying
shrinkage. Drying shrinkage generally occurs in the first few months
and decreases with time.
TYPES OF SHRINKAGE
3. CARBONATION SHRINKAGE:
Carbonation shrinkage occurs due to the reaction of carbon dioxide
(CO2) with the hydrated cement minerals, carbonating Ca(OH)2 to
CaCo3. The carbonation slowly penetrates the outer surface of the
concrete. This type of shrinkage mainly occurs at medium humidities
and results increased strength and reduced permeability.
4. AUTOGENOUS SHRINKAGE:
Autogenous shrinkage occurs due to no moisture movement from
concrete paste under constant temperature. It is a minor problem of
concrete and can be ignored.
FACTORS AFFECTING
SHRINKAGE
1. WATER-CEMENT RATIO:
shrinkage is mostly influenced by the water cement ratio of concrete. It
increases with the increases in the water-cement ratio.
2. ENVIRONMENTAL CONDITION:
It is one of the major factors that affect the total volume of shrinkage.
Shrinkage is mostly occurred due to the drying condition of the
atmosphere. It increases with the decrease in the humidity.
3. TIME:
The rate of shrinkage rapidly decreases with time. It is found that 14-
34% of the 20 years shrinkage occurs in two weeks, 40-80% shrinkage
occurs in three months and the rest 66-85% shrinkage occurs in one
year.
FACTORS AFFECTING
SHRINKAGE
4. TYPE OF AGGREGATE:
Aggregates with moisture movement and low elastic modulus cause
large shrinkage. The rate of shrinkage generally decreases with the
increase of the size of aggregates. It is found that concrete made from
sandstone shrinks twice than the concrete of limestone.
5. ADMIXTURES:
The shrinkage increases with the addition of accelerating admixtures
due to the presence of calcium chloride (CaCl2) in it and it can be
reduced by lime replacement.
FACTORS AFFECTING
SHRINKAGE
Other Factors:
The type and quantity of cement.
Granular and microbiological composition of aggregates.
The strength of concrete.
The method of curing.
The dimension of elements etc.
STRUCTURAL EFFECTS OF SHRINKAGE ON
CONCRETE STRUCTURES
1. JOINTS
Shrinkage of concrete between movement joints causes joints to open
or makes it wider. Therefore joints must be designed to accommodate
the widening caused by shrinkage.
2. OTHER MATERIALS
Where other materials, such as ceramic tiles, are fixed on top of
concrete surface, shrinkage of the concrete causes relative movement
between the different materials. The resulting stresses can cause
failure at the interface.
3. CRACKING
If shrinkage is restrained, the concrete is put into tension and when
tensile stress becomes equal to tensile strength, the concrete cracks.
Structural designers can design structures to minimize restraint,
prestress the concrete to prevent tensile stress, or use reinforcement to
control cracking.
STRUCTURAL EFFECTS OF SHRINKAGE ON
CONCRETE STRUCTURES
4. LOSS OF PRESTRESSING FORCE
Shrinkage causes a reduction in prestressing force. When calculating
prestresing forces, designers take into account to ensure that residual stress is
structurally adequate.
5. REPAIRS
If concrete is used to fill a cavity in old concrete, shrinkage of the new concrete
is restrained by the surrounding old concrete. Repair concretes and mortars
must be specially formulated (by incorporating a polymer material) to prevent
cracking caused by this restraint.
6. BOND STRENGTH
Shrinkage of the concrete causes the concrete to grip reinforcing bars more
tightly. This increases friction between concrete and steel and so improves
bond strength, especially for plain bars.
7. DEFLECTIONS
The deflection of flexural members is increased by shrinkage. This is because
the lightly reinforced compression zone is free to shrink more than heavily
reinforced tension zone.
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