CONCRETE

Creep & Shrinkage (Concrete)

Time-dependent deformation of concrete under sustained load

Also calledcreepshrinkageconcrete creepdrying shrinkageautogenous shrinkage

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Definition

Creep is the time-dependent increase in strain of concrete under sustained stress; shrinkage is the time-dependent decrease in volume due to moisture loss from the hardened concrete. Both are critical long-term phenomena that affect deflection, prestress loss, and crack development in RCC and pre-stressed structures. IS 456:2000 Cl. 6.2.5 and IS 1343:2012 specify creep coefficients and shrinkage strains for design.

Creep coefficient θ (= creep strain ÷ initial elastic strain) for normal-weight concrete in moderate humidity reaches 1.6-2.5 over the structure's lifetime, with 50% occurring within the first 90 days, 65% within 1 year, and 80-90% within 5 years. Higher relative humidity reduces creep; higher temperature, lower w/c, and higher cement content increase creep. Shrinkage strain εsh ≈ 300 × 10⁻⁶ for normal-weight concrete (Cl. 6.2.4.1), again predominantly occurring within the first 90-180 days. For thin members in dry climates (low RH), shrinkage can reach 600 × 10⁻⁶.

Design implications: long-term deflection is approximately 1.5-2.5× short-term elastic deflection due to creep — IS 456 Cl. 23.2 specifies span/250 final deflection limit, with multiplier on short-term value. For pre-stressed concrete, creep loss in pre-stressing strand is typically 10-15% of initial prestress over 1-2 years; combined creep + shrinkage loss can reach 20-25% in extreme cases. Structures highly sensitive to creep-shrinkage: long-span post-tensioned slabs (deflection grows over years), tall buildings (axial shortening of columns differs from shear walls — must be modelled), and pre-stressed bridge girders. Routine residential RCC structures rarely experience problematic creep-shrinkage if design follows IS 456 deflection limits and contraction joints are provided at 6-9 m intervals.

Typical values

Creep coefficient (final)θ ≈ 1.6-2.5

Creep at 90 days≈ 50% of final

Creep at 1 year≈ 65% of final

Shrinkage strain (normal humidity)εsh ≈ 300 × 10⁻⁶

Shrinkage strain (dry climate)up to 600 × 10⁻⁶

Combined PSC loss (creep + shrinkage)10-25% of initial prestress

Where used

Long-term deflection check — IS 456 Cl. 23.2.1 multiplier
Pre-stressed concrete design — losses estimation in IS 1343 Cl. 18
Post-tensioned slabs — deflection prediction over 5-10 years
Tall buildings — differential axial shortening between cores and columns
Bridge girders — pre-camber to compensate long-term deflection

Acceptance / threshold

Per IS 456 Cl. 23.2: total long-term deflection ≤ span/250 (ground floor) or span/350 (with sensitive partitions). For pre-stressed concrete per IS 1343 Cl. 18, prestress losses including creep + shrinkage must be ≤ 25% of initial prestress.

Site example

Site reality: a Bengaluru office project's 9 m post-tensioned slab developed 28 mm deflection at year 3 versus design predicted 18 mm. Forensic analysis revealed the contractor had applied prestress 5 days after pouring (instead of 10 days as specified), so the concrete creep coefficient was higher than design. The lesson: prestressing schedule is not a contractor convenience — IS 1343 Cl. 18 mandates minimum age before stressing and the design assumes that age.

Frequently asked

What is creep in concrete?

Creep is the slow, time-dependent increase in strain of concrete under sustained compressive stress. Even when stress remains constant, the concrete continues to deform — typically 50% of total creep strain occurs within 90 days, 80% within 5 years. Creep coefficient θ = creep strain ÷ elastic strain ≈ 1.6-2.5 for normal concrete. IS 456 Cl. 6.2.5 provides design values.

What causes shrinkage in concrete?

Shrinkage is volume reduction due to moisture loss from hardened concrete. Three components: (1) plastic shrinkage in fresh concrete from rapid evaporation, (2) drying shrinkage in hardened concrete from interior moisture loss to atmosphere, (3) carbonation shrinkage from CO₂ reaction over years. Drying shrinkage dominates and reaches εsh ≈ 300 × 10⁻⁶ at moderate humidity; higher in arid climates. IS 456 Cl. 6.2.4 gives design values.

How is long-term deflection calculated for RCC?

Per IS 456 Cl. 23.2.1: total long-term deflection = short-term elastic deflection × (1 + creep multiplier) + shrinkage curvature deflection. Creep multiplier depends on age at first loading, w/c ratio, RH; typically 1.6-2.5. Shrinkage curvature depends on shrinkage strain, top/bottom reinforcement ratio, and section depth. Modern software (ETABS, SAFE, STAAD) computes both automatically; check manually for critical long-span members.

Related concrete terms

Concrete Mix Design

Proportioning of cement, water, aggregates per IS 10262

Concrete Grades (M15-M80)

M20=20 N/mm² characteristic strength at 28 days

Water-Cement Ratio (w/c)

Mass of water to mass of cement. 0.40-0.55 typical.

Concrete Cover

Min concrete from rebar to surface (15-50mm per exposure)

Concrete Curing

Min 7 days for OPC, 10 days for PPC, 14 days for sulphate-resistant

Slump Test / Workability