Concrete deforms over time through two distinct mechanisms: creep, which is stress-dependent strain under sustained load, and shrinkage, which occurs independently of stress. Both influence the long-term behaviour of reinforced concrete structures and must be accounted for during design.
Creep is defined as stress-dependent strain in concrete, representing the difference between strain increases over time in specimens exposed to constant stress versus unloaded specimens measured under load-independent conditions. When you load a concrete element, it continues to deform even when the load remains constant. Creep depends on several material and environmental factors, including cement paste content, capillary porosity, water-cement ratio, age at loading, and element thickness.
Shrinkage refers to strain that develops without external load. Time-dependent deformation of concrete can be divided into stress-dependent (creep) and stress-independent (shrinkage) categories. Unlike creep, shrinkage occurs even in unloaded members.
Several prediction models are currently used for estimating creep and shrinkage. These include EN 1992-1-1 (2004), RILEM Model B3 (1995), MC 2010, GL Model 2000, ACI 209R (1992), AS 3600 (1988), and BS 8110 (1985). Each model employs different formulations and simplifying assumptions, leading to significant differences in predicted values. Studies based on concrete slabs of different thicknesses and strengths, with loading introduced at seven and 28 days, indicate that the B3 model serves as a reliable benchmark, while models such as ACI 209R, BS 8110, and AS 3600 are less recommended due to their simplifications.
The total strain at any time t in a concrete element exposed to continuous uniaxial stress can be expressed as the sum of initial elastic strain, creep strain, shrinkage strain, and temperature-related strain.
