Creep and shrinkage cause significant long-term deformation critically affecting concrete structure serviceability throughout service life. Creep is time-dependent inelastic deformation occurring under sustained load through stress redistribution within cement paste and gradual moisture movement; creep deformation can equal or exceed immediate elastic deformation over years. Shrinkage is volume reduction occurring upon drying through loss of water from cement paste and aggregate movement creating tensile stress and potential cracking. Combined creep and shrinkage effects can create dramatic deflections exceeding design limits, crack widening creating durability concerns, and prestress loss reducing safety margin in prestressed structures. Comprehensive testing quantifies these effects enabling accurate design predictions and specification of concrete properties minimizing long-term deformation. Prediction models incorporate concrete composition factors including cement type and water-cement ratio, member geometry including effective thickness, curing conditions determining initial development, and environmental exposure factors including temperature and relative humidity.
Creep Testing Methods
Standard creep testing applies sustained load to concrete specimens measuring deformation over extended periods typically 365+ days. ASTM C512 standard specifies testing procedures using sustained load of 40% of 28-day compressive strength representing typical service-level loading below proportional limit where creep is linearly proportional to applied stress. Specimens are typically 100mm diameter by 200mm height cylinders loaded at 7 days age after initial curing allowing sufficient strength development to safely support sustained load without failure. Loading age selection affects creep magnitude; earlier loading (7 days) produces greater total creep than later loading (28 or 90 days) because cement hydration continues under load. Deformation measurements begin immediately upon loading and continue at regular intervals: 24 hours post-loading, 7 days, 28 days, 56 days, 90 days, 180 days, and 365+ days establishing complete time-dependent deformation history. Strain gauge installation on specimen surface measures specimen deformation directly; precision measurements to 0.001mm enable detection of small deformation increments. Loading frame design applies constant load throughout testing preventing load fluctuation that would affect creep development. Temperature and humidity control throughout testing maintains constant environment preventing environmental effects from confounding creep measurement. Creep coefficient is calculated from measured strain by dividing creep strain (strain under sustained load minus elastic strain) by elastic strain at loading producing dimensionless coefficient. High creep coefficients (2.0-3.0) indicate high creep susceptibility while low coefficients (0.5-1.0) indicate low creep susceptibility.
Shrinkage Testing Procedures
Shrinkage testing measures volume change occurring upon drying through loss of water from concrete. Standard specimens include 100mm diameter by 200mm height cylinders or 25x25x285mm prisms allowing flexibility in testing setup. After initial curing (typically 7 days), specimens are demoulded and initial length is measured using precision comparator apparatus capable of 0.01mm resolution. Specimens are then exposed to controlled laboratory environment typically maintained at 23°C and 50% relative humidity simulating indoor exposure. Subsequent length measurements occur at regular intervals: 1, 3, 7, 14, 28, 56, 90, 180, and 365+ days establishing shrinkage progression curve. Drying shrinkage development is rapid initially reaching approximately 50% of total shrinkage by 28 days, 80% by 90 days, and essentially asymptotic by 365 days where further shrinkage continues very slowly. Companion non-drying control specimens stored continuously at 100% relative humidity show no drying shrinkage and serve as baseline for measuring actual shrinkage minus autogenous shrinkage occurring during initial hydration even without drying. Temperature and humidity control during testing is critical because temperature variations affect shrinkage magnitude and humidity changes cause measurement variability. Shrinkage strain is calculated from measured length change divided by original length producing strain value (mm/mm) typically ranging from 300-800 microstrains for normal concrete. Drying shrinkage potential varies significantly based on cement type (higher with Type I, lower with Type III), water-cement ratio (higher w/c produces more shrinkage), curing conditions (shorter initial moist curing allows more shrinkage), and member thickness (thicker members prevent complete drying producing lower shrinkage).
Prediction Models and Design Application
Eurocode 2 and ACI 209 provide prediction models for creep and shrinkage enabling design application before testing results available. Prediction models incorporate multiple concrete composition factors including cement type (Type I produces more creep than Type III due to slower hydration), water-cement ratio (higher w/c produces more creep due to higher porosity), cement replacement materials such as fly ash or slag (low-reactivity materials reduce creep), and curing history (longer moist curing reduces creep by advancing hydration). Member geometry effects include effective thickness (mean thickness = 2 × volume/perimeter) which controls drying rate; thinner members dry faster producing higher shrinkage while thicker members dry slowly producing lower shrinkage. Curing conditions including duration of moist curing and temperature during initial hydration significantly affect creep; longer moist curing reduces creep by completing early-age hydration. Loading age adjustments recognize that earlier loading produces more creep than later loading; loading at 7 days produces greater creep than loading at 28 days due to ongoing hydration under load. Environmental exposure factors including relative humidity and ambient temperature affect creep and shrinkage; high humidity reduces drying shrinkage while high temperatures increase drying rate. Prediction equations typically provide estimates within ±30% accuracy reflecting variability in concrete properties and environmental conditions. Design applications include long-term deflection calculations incorporating immediate elastic deflection plus creep deflection (elastic deflection × creep coefficient), prestress loss calculations considering creep-induced stress relaxation in prestressing steel, and crack width assessment accounting for increased crack width from sustained load and shrinkage restraint.
Design Considerations and Mitigation
Creep and shrinkage require careful design consideration to ensure structures meet serviceability requirements throughout service life. Design deflections must account for immediate elastic deflection plus time-dependent creep and shrinkage contributions; total deflection limits per applicable codes (typically L/250 for live load deflection) must accommodate all components. Crack width calculations must include creep effects which increase crack widths under sustained load; design typically limits maximum crack width to 0.3mm for exposure to water or de-icing chemicals and 0.4mm for sheltered exposure. Prestress loss evaluation must account for creep-induced stress relaxation in prestressing steel which may lose 10-20% of initial prestress over 40-year service life; design prestress losses are estimated and incorporated into prestress design reducing initial prestress requirement. Design compositional strategies to minimize creep and shrinkage include selection of low water-cement ratio (0.40-0.45) producing lower creep, use of slow-hydrating cement types such as Type II or blended cements producing lower creep, incorporation of supplementary cementitious materials such as fly ash (10-30%) producing lower creep, and use of fine aggregate with low fines content reducing water demand. Curing protocol importance is recognized through longer initial moist curing (7-28 days) advancing hydration and reducing subsequent creep. Environmental protection measures including shading and windbreaks during initial curing reduce drying rate and subsequent shrinkage. Reinforcement detailing with appropriate crack control reinforcement reduces crack width from shrinkage and sustained load. Member geometry optimization using lighter and thinner sections where possible reduces dead load deformation while maintaining adequate strength.
Applicable Standards
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