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Shrinkage and Creep of Concrete CE 3420 Concrete Technology CE 3420 Concrete Technology Prof. Ravindra Gettu Prof. Ravindra Gettu IIT Madras IIT Madras

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Shrinkage & Creep of Concrete

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Page 1: Shrinkage Creep

Shrinkage and Creep of Concrete

CE 3420 Concrete TechnologyCE 3420 Concrete TechnologyProf. Ravindra GettuProf. Ravindra Gettu

IIT MadrasIIT Madras

Page 2: Shrinkage Creep

Nature of Shrinkage and Creep• Creep and shrinkage are time-dependent strains that

involve the movement of water• Shrinkage strains occur when water is lost• Creep strains occur when water is forced to move by stress• These strains are not completely recoverable when the

load is removed or the concrete is re-saturated

Young et al.Young et al.

Page 3: Shrinkage Creep

Effects of Shrinkage and Creep

• Axial strains increase with time; e.g., in columns under compressive loads and bridge piers.

• Deflections increase; e.g., beams and girders in flexure.

• Stress relaxation; e.g., the prestressing force decreases with time as the concrete shrinks and creeps.

• Cracks can occur in elements that are restrained and develop tensile stresses; e.g., in pavements and slabs-on-grade.

Page 4: Shrinkage Creep

Types of Shrinkage• Plastic shrinkage: Due to the loss of water in the plastic

state due to evaporation.

• Autogenous shrinkage: Chemical shrinkage (lower volume of hydrates than cement and water) + Self-desiccation (reduction in the pore water due to hydration).

• Thermal contraction (or thermal shrinkage): Due to the decrease in temperature after setting.

• Drying shrinkage: Due to the loss of water to the environment in the hardened state.

• Carbonation shrinkage: Volume reduction due to the

reaction of hydrated cement paste with CO2 in the

presence of moisture.

Page 5: Shrinkage Creep

Shrinkage

hours days weeks months years

Time

Plastic

Thermal(contraction)

Autogenous

Drying

Carbonation

Page 6: Shrinkage Creep

Ways of Reducing Plastic Shrinkage

• Reduce the rate of evaporation of water (by decreasing the temperature of the concrete, placing the concrete rapidly, curing as soon as possible). Evaporation rates should not exceed 1 kg/m2/hour.

• Reduction of the cement content (by optimizing the paste volume, using complementary materials).

• Utilization of shrinkage-reducing admixtures.

Page 7: Shrinkage Creep

Autogenous Shrinkage

• The volume of the hydrates is about 8-12% lower than the sum of the volumes of the cement and water that take part in the reaction. This is known as chemical shrinkage.

• Further shrinkage can occur when water is removed from capillary pores due to the hydration of the cement. This process is called self-desiccation.

• Chemical shrinkage and self-desiccation lead to autogenous shrinkage (without any loss of moisture to the environment).

• In concrete, the autogenous shrinkage is restrained by the aggregates and the already hydrated cement. Consequently, it is an order of magnitude less than in cement paste.

Page 8: Shrinkage Creep

Autogenous Shrinkage

• Autogenous shrinkage increases with cement content, fineness of cement, concrete temperature, and with the C3A and C4AF content of the cement.

• Typical values of autogenous shrinkage strain are about 40×10-6 at the age of one month and 100×10-6 after five years.

• At low w/c, autogenous shrinkage is higher. For a concrete with w/c = 0.17, an autogenous shrinkage strain of 700×10-6 has been reported.

• In usual concretes, the strain due to autogenous shrinkage is neglected or included in the drying shrinkage strain.

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Page 9: Shrinkage Creep

Swelling

• Concrete cured continuously under water exhibits an increase in volume and mass.

• The expansion is due to the absorption of water by the gel, which pushes the gel particles further apart. There is also a decrease in the surface tension of the gel water that further increases the expansion.

• The expansion or swelling strain in cement paste can be about 1300×10-6 after 100 days of submerged curing. However, the swelling in concrete is much lower (in the order of 100×10-6 after six months).

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Page 10: Shrinkage Creep

Drying Shrinkage

Young et al.Young et al.

• Removal of water due to exposure to unsaturated air causes drying shrinkage. Part of this is irreversable.

• The change in volume due to drying is less than the volume of the water removed from the concrete.

• Initially, the removal of free water causes little or no shrinkage.

• As concrete dries, water is lost from the capillary pores, creating menisci leading to capillary stresses in the hydrated cement paste.

• As further loss of humidity occurs, menisci are created in smaller and smaller pores, leading to larger and larger stresses.

• Additionally, the C-S-H structure is compacted.

• Also, the water moves from the micropores to maintain hygral equilibrium, decreasing the disjoining forces within the C-S-H.

Page 11: Shrinkage Creep

Drying Shrinkage

Data for concrete wet-cured for 28 days and then maintained at different relative humidities

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Page 12: Shrinkage Creep

Carbonation Shrinkage

• The surface zone of concrete undergoes shrinkage due to the carbonation of the hydrated cement paste.

• Carbonation shrinkage is probably caused by the dissolving of the Ca(OH)2 crystals and the depositing of CaCO3 in spaces free from stress.

• Carbonation increases the shrinkage at intermediate relative humidities. At low humidities, there is insufficient water in the pores to form carbonic acid with the CO2. On the other hand, when the pores are saturated, the diffusion of CO2 is slow.

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Page 13: Shrinkage Creep

Static Fatigue or Creep

Effect of constant compressive loads (Rüsch, 1960)

Young et al.Young et al.

Page 14: Shrinkage Creep

Creep

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• Creep is the gradual increase in strain due to the applied load.

• Creep is defined in terms of the time-dependent strain due to a sustained load

• When concrete is restrained, creep is manifested as relaxation (or a gradual lowering of the stress).

Relaxation under constant strain of 360×10-6

Page 15: Shrinkage Creep

Types of Creep

• Basic Creep: Creep strain that occurs without any loss of moisture to the environment.

• Drying creep: Creep strain that can be attributed to the effect of drying (= total creep - basic creep)

• Creep recovery: When sustained load is removed, there is an instantaneous decrease in strain, followed by a gradual decrease, called creep recovery

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Creep recovery of mortar maintained in fog room under 15 MPa stress and then unloaded

Page 16: Shrinkage Creep

• When load is applied, the different components of the hardened cement paste take different extents of the stress. The capillary pores will transmit very little stress whereas the C-S-H is under the highest stress.

• The water in the micropores within the C-S-H is under high stress and tends to diffuse to regions of lower stress. This results in the contraction of the micropores.

• The C-S-H structure gradually densifies through a viscoelastic process.

• The C-S-H agglomerations slip at a decreasing rate. As the water is lost, van der Waals and chemical bonding increases.

Young et al.Young et al.

Basic Creep

Page 17: Shrinkage Creep

• Drying enhances the creep strain; i.e., drying induces additional drying creep.

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Drying Creep

Page 18: Shrinkage Creep

• There is a direct proportionality between the creep strain and the applied stress, except for very young concretes.

• The upper limit of proportionality is reached when concrete starts cracking. Therefore, the proportionality applies until about 0.6 times the compressive strength, i.e., within the normal range of stresses in structures in service.

• Above the limit of proportionality, the creep strain increases at an increasing rate with increasing stress. This is called nonlinear creep.

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Influence of Stress on Creep

Page 19: Shrinkage Creep

Factors Affecting Shrinkage and Creep

Influence of the Aggregates

• Contraction occurs in the hardened cement paste due to creep and shrinkage, while the aggregates act as stiffer inclusions.

• The restraining effect of the aggregates gives rise to the following relationship between the concrete and paste strains:

con = p (1-Va)n

where con and p are the strains in the concrete and paste, Va is the volume fraction of aggregates in the concrete, and n is a parameter that depends on the elastic constants of the aggregates (usually about 1.8).

Young et al.Young et al.

Page 20: Shrinkage Creep

Factors Affecting Shrinkage and Creep

Influence of the Aggregates on Drying Shrinkage

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Page 21: Shrinkage Creep

Factors Affecting Shrinkage and Creep

Young et al.Young et al.

Influence of the Paste Properties

• Creep and shrinkage strains are lower when the paste properties are such that:

• C-S-H particles cannot slip and move relative to each other due to changes in the nature of their bonding

• Capillary porosity is lower

• Lower paste content and w/c, ageing, higher hydration rates and pre-drying decrease creep and shrinkage.

Page 22: Shrinkage Creep

Factors Affecting Shrinkage and Creep

Influence of the Mix Proportions on Drying Shrinkage

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Page 23: Shrinkage Creep

Factors Affecting Shrinkage and Creep

Young et al.Young et al.

Water Transport in Drying Concrete

• For concrete to dry out, water must move from the interior to the surface. This is governed by diffusion.

• Initially the water moves by bulk diffusion but once menisci have formed, movement is by vapour transport and diffusion coefficients are smaller.

• Movement through the dry surface layer is through vapour diffusion.

• However, in elements with high surface-volume ratios, evaporation rates and, consequently, shrinkage can be greater.

Page 24: Shrinkage Creep

Combined Effect of Shrinkage and Creep

• Creep strains are always opposed to the applied stress; i.e., creep will cause strains in the direction of the stress. They are always additive with the elastic strains.

• The Poisson’s ratio is about the same in creep as in the elastic regime. So, lateral expansion will increase due to creep.

• Shrinkage strains are volumetric; i.e., shrinkage strain is same in all directions.

• Under uniaxial loading, the elastic, creep and shrinkage strains will lead to axial contraction, whereas shrinkage and creep may compensate each other in the lateral direction.

Page 25: Shrinkage Creep

Combined Effect of Shrinkage and Creep

• Under conditions of restrained shrinkage, creep will lead to a decrease in the stress (i.e., relaxation) resulting in a reduction of cracking.

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Page 26: Shrinkage Creep

Study of Shrinkage and Creep in Hardened Concrete

Age

Str

ain

Curing

Ins

tant

aneo

us s

trai

n =

i

i + Basic creep strain

i + Basic + Drying creep strains

Drying shrinkage strain

Autogenous shrinkage strain

sealed

sides exposed

Page 27: Shrinkage Creep

Shrinkage Testing

Some part of the autogenous shrinkage is neglected. This maybe significant in high strength concrete

AïtcinAïtcin

Page 28: Shrinkage Creep

Creep Testing

Usual Exposure of Specimens

Standard curing is used when the drying or total shrinkage is to be determined. Basic creep is determined using sealed specimens.

AïtcinAïtcin

Page 29: Shrinkage Creep

Results of Creep and Shrinkage Testing

• Characterization of the concrete• Shrinkage (autogenous & drying) strain for a

certain age and drying period (normally one year)• Creep (basic & drying) coefficient for a certain age

and loading period, which is normally one year; drying and loading periods normally coincide. The creep coefficient is the ratio between the creep strain and the initial instantaneous strain.

• Test results serve for developing and/or verifying models used in structural design

Page 30: Shrinkage Creep

References

• Concrete, S. Mindess and J.F. Young, Prentice-Hall, USA, 1981

• Properties of Concrete, A.M. Neville, Pearson Education, Delhi, 2004

• The Science and Technology of Civil Engineering Materials, J.F. Young, S. Mindess, R.J. Gray and A. Bentur, Prentice Hall, 1998

• Concrete: Microstructure, properties and materials, P.K. Mehta and P.J.M. Monteiro, Indian Concrete Institute, Chennai, 1999

• High-Performance Concrete, P.-C. Aïtcin, E&FN Spon, London, 1998