Shrinkage & Creep
Shrinkage & Creep
Shrinkage & Creep
FACULTY OF ENGINEERING
DEPARTMENT OF CIVIL ENGINEERING
GRADUATE STUDIE
BY
BADREDIN M. AMMAR
Registration Number : 2216103
Lecturer
.
Fall 2016
CE 605 ADVANCED R.C DESIGN
1-1 Shrinkage
Shrinkage is the property of the properties of concrete, which becomes hardened
in the air. It does not cause shrinkage problems unless there is a restriction on the
movement, which causes tension stresses within the concrete, which leads to crack it
and can minimize the harmful effects of shrinkage by :
1. Effective curing
2. Movement joints
3. Shrinkage Reinforcements
1. Drop hard parts in the mixture and the loss of free water from the fresh
concrete, causing what is known as plastic shrinkage.
2. Chemical union between cement and water leads to a Autogenous shrinkage .
3. Drying concrete as a result of water loss causes a drying shrinkage.
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CE 605 ADVANCED R.C DESIGN
1-4 Many factors influence the shrinkage of concrete caused by the variations
in moisture conditions :
1. Cement and Water Content. The more cement or water content in the concrete
mix, the greater the shrinkage.
2. Composition and Fineness of Cement. High-early-strength and low-heat
cements show more shrinkage than normal portland cement. The finer the
cement, the greater the expansion under moist conditions.
3. Type, Amount, and Gradation of Aggregate. The smaller the size of aggregate
particles, the greater the shrinkage. The greater the aggregate content, the
smaller the shrinkage .
4. Ambient Conditions, Moisture, and Temperature. Concrete specimens
subjected to moist conditions undergo an expansion of 200 to 300 106, but
if they are left to dry in air, they shrink. High temperature speeds the
evaporation of water and, consequently, increases shrinkage.
5. Admixtures. Admixtures that increase the water requirement of concrete
increase the shrink-age value.
6. Size and Shape of Specimen. As shrinkage takes place in a reinforced concrete
member, tension stresses develop in the concrete, and equal compressive
stresses develop in the steel. These stresses are added to those developed by
the loading action. Therefore, cracks may develop in concrete when a high
percentage of steel is used. Proper distribution of reinforcement, by producing
better distribution of tensile stresses in concrete, can reduce differential
internal stresses. The values of final shrinkage for ordinary concrete vary
between 200 and 700 106.For normal-weight concrete, a value of 300
106 may be used. The British Code gives a value of 500 106, which
represents an unrestrained shrinkage of 1.5 mm in a 3-m length of thin, plain
concrete sections. If the member is restrained, a tensile stress of about 10
N/mm2 (9.8 N/mm2) arises. If concrete is kept moist for a certain period after
setting, shrinkage is reduced; therefore, it is important to cure the concrete for
a period of no fewer than 7 days.
Exposure of concrete to wind increases the shrinkage rate on the upwind side.
Generally, concrete shrinks at a high rate during the initial period of hardening,
but at later stages the rate diminishes gradually. It can be said that 15 to 30% of the
shrinkage value occurs in 2 weeks, 40 to 80% occurs in 1 month, and 70 to 85%
occurs in 1 year.
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CE 605 ADVANCED R.C DESIGN
2-1 Creep
The ultimate magnitude of creep varies between 0.2 106 and 2 106
per unit stress (lb/in2) per unit length. A value of 1 106 can be used in practice.
The ratio of creep strain to elastic strain may be as high as 4.
Creep takes place in the hardened cement matrix around the strong aggregate.
It may be attributed to slippage along planes within the crystal lattice, internal
stresses caused by changes in the crystal lattice, and gradual loss of water from the
cement gel in the concrete.
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CE 605 ADVANCED R.C DESIGN
2-3 The different factors that affect the creep of concrete can be summarized as
follows :
1. Level of Stress. Creep increases with an increase of stress in specimens made
from concrete of the same strength and with the same duration of load.
2. Duration of Loading. Creep increases with the loading period. About 80% of the
creep occurs within the first 4 months; 90% occurs after about 2 years.
3. Strength and Age of Concrete. Creep tends to be smaller if concrete is loaded at a
late age. Also, creep of (14 N/mm2) strength concrete is about 1.41106,
whereas that of (28 N/mm2) strength concrete is about 0.8106 per unit
stress and length of time.
4. Ambient Conditions. Creep is reduced with an increase in the humidity of the
ambient air.
5. Rate of Loading. Creep increases with an increase in the rate of loading when
followed by prolonged loading.
6. Percentage and Distribution of Steel Reinforcement in Reinforced Concrete
Member . Creep tends to be smaller for higher proportion or better distribution of
steel.
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CE 605 ADVANCED R.C DESIGN
7. Size of Concrete Mass. Creep decreases with an increase in the size of the teste
specimen.
8. Type, Fineness, and Content of Cement. The amount of cement greatly affects the
final creep of concrete, as cement creeps about 15 times as much as concrete.
9. WaterCement Ratio. Creep increases with an increase in the water cement
ratio.
10. Type and Grading of Aggregate. Well-graded aggregate will produce dense
concrete and consequently a reduction in creep.
11. Type of Curing. High-temperature steam curing of concrete, as well as the proper
use of a plasticizer, will reduce the amount of creep.
Creep develops not only in compression but also in tension, bending, and torsion.
The ratio of the rate of creep in tension to that in compression will be greater than
1 in the first 2 weeks, but this ratio decreases over longer periods .
For normal concrete loaded after 28 days, Cr = 0.133t ,where Cr = creep strain
per unit stress per unit length. Creep augments the deflection of reinforced concrete
beams appreciably with time. In the design of reinforced concrete members, long-
term deflection may be critical and has to be considered in proper design. Extensive
deformation may influence the stability of the structure.
Sustained loads affect the strength as well as the deformation of concrete. A
reduction of up to 30% of the strength of unreinforced concrete may be expected
when concrete is subjected to a concentric sustained load for 1 year.
The fatigue strength of concrete is much smaller than its static strength. Repeated
loading and unloading cycles in compression lead to a gradual accumulation of plastic
deformations. If concrete in compression is subjected to about 2 million cycles, its
fatigue limit is about 50 to 60% of the static compression strength. In beams, the
fatigue limit of concrete is about 55% of its static strength.
References