Shrinkage of Concrete – Types, Causes, Magnitude and Control

A fresh concrete slab in your city just finished being cast. Three months later, it has cracks running across it — yet it was never overloaded. The culprit? Shrinkage. Concrete shrinks as it loses moisture and cools after hydration. In structures that are restrained from moving freely (like slabs on ground or walls), this shrinkage induces tensile stresses that cause cracking. Understanding shrinkage is essential for designing crack-free, durable concrete structures.

1. Types of Shrinkage

Concrete can shrink at different stages for different reasons. The main types are:

• Plastic Shrinkage: Occurs before the concrete has set — in the fresh, plastic state — when the surface evaporation rate exceeds the rate of bleeding.
• Drying Shrinkage: The most significant type for structural design — occurs after hardening as adsorbed water evaporates from the C-S-H gel pores over months and years.
• Autogenous Shrinkage: Occurs during early hydration when water is consumed chemically (self-desiccation), reducing internal humidity. Significant in low W/C (<0.40) high-performance concrete.
• Thermal Shrinkage: Contraction after the concrete cools down from the peak temperature reached during heat of hydration. Critical in mass concrete.
• Carbonation Shrinkage: CO&sub2; from the atmosphere reacts with Ca(OH)&sub2;, producing CaCO&sub3; which has a smaller volume than the reactants. Also lowers pH — a durability concern.

2. Drying Shrinkage — The Most Important Type

Drying shrinkage is a gradual long-term process. As the concrete dries, water is lost in a specific sequence from the most weakly held to the most strongly held:

1. Free pore water (capillary water) evaporates first — causes little shrinkage
2. Adsorbed water (held on C-S-H gel surfaces) evaporates — this causes significant shrinkage as the gel layers collapse together
3. Interlayer water (within C-S-H sheets) lost at very low humidity — causes additional shrinkage

The process is partly reversible: if concrete that has dried is re-wetted, it expands (swells) partially back. However, the first drying cycle always causes some permanent irreversible shrinkage. This is why IS 456 design considers a long-term net shrinkage strain.

3. Plastic Shrinkage — The Surface Crack Problem

Plastic shrinkage cracks are particularly frustrating because they form while you’re still standing next to the slab. They appear as random surface cracks on freshly placed concrete exposed to hot, dry, or windy conditions — typically within the first 1–4 hours after casting.

Cause

When the rate of evaporation from the concrete surface exceeds the rate of bleed water rising to replace it, the surface layer undergoes rapid moisture loss. Since the concrete hasn’t hardened yet, it can’t resist the tensile stress from this surface shrinkage, and it cracks.

Critical evaporation rate

If evaporation rate > 1 kg/m²/h, plastic shrinkage cracking is likely. Influenced by: air temperature, concrete temperature, relative humidity, and wind speed.

Prevention

• Mist fogging the surface to increase relative humidity above the concrete
• Windbreak screens to reduce wind velocity
• Shade structures to reduce solar radiation and concrete temperature
• Apply curing compound or wet burlap immediately after finishing
• Avoid placing concrete in very hot, dry, or windy conditions if possible

4. Magnitude of Shrinkage Strains

5. Factors Affecting Drying Shrinkage

• Water content: Higher water content per m³ → more water to lose → higher shrinkage. Water content is the most important variable. Reducing water by 20% (using superplasticizer) can reduce shrinkage by 20–30%.
• W/C ratio: Higher W/C → more capillary pores → more drying → more shrinkage.
• Cement content: Cement paste shrinks; aggregate doesn’t. Higher cement content → more paste → more shrinkage potential. Well-graded aggregate restrains paste shrinkage.
• Type of cement: High C&sub3;A cements develop more ettringite → slightly more shrinkage. Low heat cements typically have lower early shrinkage.
• Aggregate type and content: High aggregate content → less paste → less shrinkage. Hard, rigid aggregates (granite, quartzite) provide better internal restraint of paste shrinkage than soft aggregates.
• Member size: Thin members dry faster and shrink faster. Thick members develop moisture gradients — surface dries and cracks while interior remains moist.
• Relative humidity of environment: Lower RH → more drying → more shrinkage. In very humid climates, shrinkage cracking is less of a problem.

6. Restrained Shrinkage and Cracking

If concrete could shrink freely (like a rubber band you let go of), there would be no problem — it just gets shorter. The problem arises when shrinkage is restrained by:

• Reinforcement bars embedded in the concrete
• Adjacent structural elements (columns, walls at slab edges)
• Friction between slab and ground
• Differential drying (surface dries faster than interior)

When restrained, the shortening tendency induces tensile stress. If this tensile stress exceeds the tensile strength of concrete at that age (which is developing simultaneously), cracks form.

Critical period: Early age (1–14 days) is most vulnerable because concrete is gaining strength but also developing significant shrinkage, and the tensile strength is still low.

7. Control of Shrinkage

• Reduce water content: Use superplasticizer to reduce mix water by 20–30% at same workability. Most effective single measure for reducing drying shrinkage.
• Use shrinkage-reducing admixtures (SRA): Propylene glycol-based SRAs reduce surface tension of pore water, reducing capillary stresses. Can reduce drying shrinkage by 20–50%.
• Adequate curing: Delays onset of drying shrinkage — more hydration occurs before drying begins, leading to denser paste with better restraint against shrinkage.
• Maximum aggregate content: Aggregates restrain paste shrinkage. Use maximum aggregate size and aggregate content consistent with workability requirements.
• Construction joints and movement joints: Divide large slabs into smaller bays with planned joints. Shrinkage occurs in each bay independently without restraint.
• Reinforcement: Doesn’t prevent shrinkage but distributes crack widths — converting one large crack into many fine, harmless hairline cracks.
• SCMs (fly ash, GGBS): Generally reduce drying shrinkage due to lower heat and finer pore structure.

8. Diagram — Types and Causes of Shrinkage

9. Exam Tips (RTMNU)

• ✅ Five types of shrinkage: plastic, drying, autogenous, thermal, carbonation — list all five with brief descriptions for 5-mark answers.
• ✅ Drying shrinkage magnitude: 200–800 × 10³ microstrain — this range is frequently asked.
• ✅ IS 456:2000 design shrinkage strain: 0.0003 (300 microstrain) — cite this value.
• ✅ Plastic shrinkage: evaporation > bleeding rate → surface cracks within 1–4 hours — explain clearly.
• ✅ Most important factor: water content per m³ (not just W/C ratio) governs drying shrinkage.
• ✅ Restrained shrinkage → tensile stress → cracking — connect this chain of events in answers.

10. Key Takeaways

• Concrete shrinks due to moisture loss, chemical reactions, and temperature changes.
• Five types: plastic, drying (most important structurally), autogenous, thermal, carbonation.
• Drying shrinkage: 200–800 microstrain over months/years. IS 456 design value: 300 microstrain.
• Most important factor: water content per m³ of concrete. More water → more shrinkage.
• Restrained shrinkage induces tensile stresses → cracking. Control by joints, reinforcement, and SRAs.

11. FAQs

Q1. What is drying shrinkage of concrete?

Drying shrinkage is the volumetric contraction of hardened concrete caused by the evaporation of adsorbed water from the C-S-H gel pores as the concrete dries. It develops progressively over months and years, with typical strain values of 200–800 microstrain. IS 456:2000 uses a design value of 0.0003 (300 microstrain) for long-term calculations.

Q2. What causes plastic shrinkage cracking?

Plastic shrinkage cracking occurs when the rate of surface evaporation exceeds the rate of bleeding water rising to the surface. The surface concrete dries and contracts while the underlying concrete is still fluid, generating tensile stress in the surface layer that exceeds its very low tensile strength at this early stage. Random surface cracks form, typically 0.1–2 mm wide, within 1–4 hours of casting.

Q3. What is the IS 456 design value for shrinkage strain?

IS 456:2000 Clause 6.2.4 specifies a total design shrinkage strain of 0.0003 (300 microstrain) for ordinary concrete. This value is used in calculating long-term deflections of beams and slabs and in designing reinforcement for crack width control.

Q4. How does reinforcement control shrinkage cracking?

Reinforcement doesn’t prevent shrinkage from occurring — it restrains the concrete from contracting freely, which creates tensile stresses. However, by distributing the tensile force over many small zones, reinforcement converts what would be one or two wide, damaging cracks into many fine hairline cracks that are structurally acceptable and aesthetically tolerable. IS 456:2000 specifies minimum shrinkage and temperature reinforcement ratios for this purpose.

Q5. How do you reduce drying shrinkage of concrete?

Most effective measures: (1) Reduce water content using superplasticizer. (2) Use shrinkage-reducing admixtures (SRAs based on propylene glycol). (3) Maximise aggregate content — aggregates restrain paste shrinkage. (4) Use SCMs (fly ash, GGBS) which generally reduce shrinkage. (5) Provide adequate curing to delay onset. (6) Provide construction joints and movement joints in large slabs.

🔗 Related: Creep of Concrete – Causes, Factors and Effects

🔗 Related: Durability of Concrete – IS 456 Requirements

📚 Reference: IS 456:2000 Clause 6.2.4 – Shrinkage of Concrete, BIS