Dimesional Stability - Creep

Cement

Aggregates

Admixtures

Mixture Design

Fresh Concrete

Hardened Concrete

Dimensional Stability

Durability

 

 

Concrete undergoes various time-related deformations, the primary ones being creep and shrinkage. Both creep and shrinkage involve the movement of water in concrete. While creep involves the forcible movement of water when a sustained load is applied, shrinkage results from the loss of water by drying. The strains due to creep and shrinkage are not completely recovered when the load is removed or the concrete is wetted.

    Because of creep and shrinkage:
  • 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 occurs; 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.

Creep

Creep is the deformation of a material due to sustained stress. On the other hand, relaxation is the reduction over time of stresses generated in the material due to a sustained deformation or strain.

Sustained loading causes the rearrangement of hydrated cement paste (especially layered products like CSH), and expulsion of water, both of which result in a gradual change in volume, causing creep. The expulsion of water can be higher in drying conditions. Thus, the overall creep of a concrete element can be differentiated into basic creep and drying creep. Figure 1 shows the development of long term strain due to basic creep and free shrinkage (no load) that corresponds to the drying effect.

Figure 1. Additive effect of creep due to sustained load and drying

Basic 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.

The general variation of basic creep strain with respect to time can be represented by the schematic shown in Figure 2. Three regions can be broadly outlined – the initial elastic response region (I), the delayed elastic response region (II), and the long-term flow region (III). Upon unloading, an instantaneous recovery of the strain corresponding to the elastic response occurs. With time, recovery of the strain associated with the delayed elastic response, called creep recovery, also takes place. The strain built up in the material due to the long-term flow is permanently maintained, and cannot be recovered.

Figure 2. Strain variation corresponding to a sustained stress

Factors affecting creep of concrete

Amount and quality of aggregate in concrete

Among the constituents of concrete, only the paste shows creep. The presence of aggregates tends to reduce the creep. The creep of concrete is related to the creep of cement paste by the following empirical equation:
Log (cp/c) = a log {1/(1-g-u)}, where
cp = creep of cement paste, c = creep of concrete, g = volume fraction of aggregate, u = volume fraction of unhydrated cement, and a is a factor that depends on the deformability (Poisson’s ratio and modulus of elasticity of the aggregate and paste); a decreases as Eaggregate increases.

The other aggregate parameters that have a bearing on creep are the grading, maximum size, shape, porosity (since as porosity increases, Eaggregate decreases), and mineralogy.

Applied stress and strength of concrete

Creep is found to be almost directly proportional to the ratio of applied stress to strength of concrete, up to a limit of about 0.6. Above this ratio, creep increases with stress at an increasing rate. The degree of microcrack formation and coalescence is the factor responsible for this behaviour.

Concrete with higher strength has lower creep. The rate of strength gain also affects creep. Strains related to creep are also the highest at early ages of concrete, since removal of moisture is easier.

Ambient conditions

Creep is higher at lower relative humidity (RH). Here, the basic creep that is primarily a load related effect, is aggravated by the removal of water from concrete, leading to drying creep. The rate of creep also increases with temperature. Of course, the lower strength of concrete at high temperatures also contributes towards higher creep.

Specimen size

Creep decreases with an increase in size of the specimen. This is important from the perspective of mass concrete, where the creep strains in the interior may be far different from the creep strains on the exterior of concrete (drying effects are more severe on the exterior).

Effects of creep

Negative or adverse effects:

  • Quicker approach to failure strain occurs due to creep.
  • Creep may cause cracking in mass concrete structures, where the rate of creep is different in the interior and exterior.
  • Creep can lead to excessive deflections.
  • In prestressed concrete, creep of the concrete can lead to a gradual loss in the prestressing force.

Positive or beneficial effects:

  • In columns, a gradual transfer of loading between concrete and steel can take place because of creep. However, in eccentrically loaded columns, creep would not have a good effect.
  • Creep (in combination with relaxation) can lead to a reduction in stress concentration induced by shrinkage, temperature changes, or support movement.

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