Shrinkage is the reduction in volume of concrete due to moisture loss and other physicochemical reactions. Unlike creep, shrinkage occurs even in the absence of external load.

a. Plastic Shrinkage:
- Occurs within few hours of placing.
- Caused by rapid evaporation of surface water.
- Results in surface cracking.

b. Drying Shrinkage:
- Most common type.
- Due to moisture loss from hardened concrete.
- Significant in the first few months after curing.

c. Autogenous Shrinkage:
- Results from internal chemical reactions.
- Prominent in low water-cement ratio mixes.

d. Carbonation Shrinkage:
- Due to reaction between CO₂ and calcium hydroxide.
- Occurs at the surface over long periods.

• Water-cement ratio: Higher w/c = more shrinkage.
• Aggregate content and type: More aggregate = less shrinkage.
• Curing method: Proper curing reduces shrinkage.
• Environmental conditions: Low humidity and high temperature increase shrinkage.

• Cracking: Especially in restrained conditions (e.g., slabs).
• Loss of serviceability: Affects alignment and surface finish.
• Loss of bond: Can reduce reinforcement-concrete bond strength.

Shrinkage does not always result in visible damage. However, when movement is restrained — either externally (e.g., by foundation, reinforcement, or adjacent structural elements) or internally (e.g., differential drying) — cracks can form.

Cracking Mechanism:
- Restraint introduces tensile stress in the concrete as it tries to shrink.
- If tensile stress exceeds the tensile strength of concrete, cracks form.
- Typically, these cracks are:
- Random or map-like on slabs and pavements.
- Parallel and spaced regularly in walls or beams.
- Surface-wide microcracks in high-shrinkage pastes.

High-Risk Zones:
- Slabs-on-grade.
- Long retaining walls.
- Tunnel linings.
- Precast members with low reinforcement.
- Concrete exposed to sun and wind during curing.

Engineers and contractors adopt various strategies to minimize shrinkage and control cracking, enhancing long-term durability and aesthetics.

a. Mix Design Optimization:
- Use low water-cement ratio (<0.5).
- Add shrinkage-reducing admixtures (SRA).
- Replace part of cement with fly ash or slag to reduce heat and self-desiccation.
- Use well-graded aggregates in higher proportion to minimize paste volume.

b. Curing Techniques:
- Begin curing immediately after finishing.
- Use wet coverings, fogging, or sprinkling for moist curing.
- Apply curing compounds (liquid membranes) on exposed surfaces.
- Extended curing (at least 7–14 days) for large structures.

c. Structural Detailing:
- Provide adequate reinforcement to resist tensile stresses.
- Use control joints or contraction joints in slabs and walls at proper intervals.
- Provide expansion joints in long structures.
- Ensure smooth transitions between new and existing concrete.

Proper characterization of creep and shrinkage is essential for prediction models and quality control.

a. Shrinkage Testing (IS 1199 / ASTM C157):
- Drying Shrinkage: Standard prism (e.g., 75×75×285 mm) is measured for length change over 28, 56, 90 days.
- Autogenous Shrinkage: Specimens are sealed and monitored for volume change.
- Plastic Shrinkage Test: Measures cracking susceptibility under controlled drying conditions.

b. Creep Testing (IS 516 / ASTM C512):
- Cylindrical specimens are subjected to constant axial compressive stress (usually 30-40% of compressive strength).
- Strain gauges or extensometers measure deformation over time.
- Simultaneous control specimens are kept unloaded to isolate creep from shrinkage.

Modern structural codes require engineers to account for time-dependent deformations in long-term deflection and stress analysis.

Key Models and Standards:
- ACI 209R – Prediction models for creep and shrinkage.
- CEB-FIP Model Code – European design guidelines.
- IS 456:2000 – Indian code provisions for creep coefficient and shrinkage strain.
- Eurocode 2 – Time functions for creep, shrinkage, and modulus of elasticity.

Creep Coefficient (φ):
Defined as:
ε (t,t )
ϕ(t,t )= cr 0

0 ε (t )
e
Where:
- ε = creep strain,
cr
- ε = elastic strain at loading.
e
Total Strain Equation (under sustained load):
ε (t)=ε +ε (t)+ε (t)
total e cr sh
Where:
- ε = Immediate elastic strain,
e
- ε (t) = Creep strain,
cr
- ε (t) = Shrinkage strain.
sh
These equations help in deflection prediction and serviceability checks in beams, slabs, and tall structures.

Engineers must evaluate site conditions, environmental exposure, and structural function to balance performance and cost.

Precast vs. Cast-In-Situ:
- Precast members must control shrinkage and creep due to storage, transport, and erection loads.
- In cast-in-place elements, long-term differential shrinkage between old and new concrete is critical.

Tall Buildings and Bridges:
- Long columns and bridge piers suffer differential creep, leading to misalignment.
- Post-tensioned bridges must account for loss of prestress due to creep and shrinkage.

Mass Concrete:
- Shrinkage cracking is minimized by temperature control, low heat cement, and mass placement planning.

• Engineered Cementitious Composites (ECC): Exhibit microcracking and self-healing.
• Use of nanomaterials (e.g., nano-silica) to control shrinkage and improve packing density.
• 3D printed concrete — new methods are being developed to predict shrinkage and deformation during printing.
• Machine learning models are now used to predict shrinkage and creep using big data from field and lab tests.