Understand why concrete moves — and how to control it on every project
A complete 2026 guide to concrete shrinkage and expansion: types, causes, calculation formulas, measurement methods, and proven prevention strategies for engineers, builders, and contractors.
Essential knowledge for designing crack-free, durable concrete structures in 2026
Concrete is never truly static. From the moment it is placed, internal chemical reactions, moisture changes, and temperature fluctuations all cause the material to change in volume. Understanding concrete shrinkage and expansion is the foundation of durable structural design — ignoring it leads directly to cracking, joint failure, and costly repairs.
Not all shrinkage is the same. Plastic shrinkage occurs before the concrete sets, drying shrinkage develops over months as moisture evaporates, autogenous shrinkage happens through self-desiccation in low water-cement mixes, and carbonation shrinkage occurs over years as CO₂ reacts with the cement paste. Each type demands a different design and management response.
Concrete shrinkage and expansion can be predicted with engineering formulas and measured on-site using ASTM and AS standards. With the right mix design, joint placement, curing regime, and reinforcement strategy, volume change can be managed to prevent structural damage — even in high-temperature or high-humidity environments common across Australia and South Asia in 2026.
Concrete shrinkage is a reduction in volume that occurs as concrete loses moisture, undergoes internal chemical reactions, or cools from elevated temperatures. It is one of the most significant durability concerns in concrete construction because unrestricted shrinkage generates tensile stress — and concrete is weak in tension, typically cracking at strains as low as 100–200 microstrain.
Shrinkage is measured in microstrain (με) or millimetres per metre (mm/m). A typical unreinforced concrete slab may experience total drying shrinkage of 400–800 με over its service life. For a 10-metre slab, that means up to 8 mm of total shortening — enough to cause significant cracking if not controlled through joints, reinforcement, or shrinkage-reducing admixtures.
Shrinkage itself does not cause cracking — restrained shrinkage does. When concrete is free to move, it simply shortens. When movement is restrained by friction, reinforcement, or adjacent structure, tensile stress builds until the concrete cracks. This is why joint design and subgrade friction are so critical in slab-on-ground construction.
Plastic shrinkage occurs in the first few hours after placing, before the concrete has set. If the rate of surface evaporation exceeds the rate at which bleed water reaches the surface, the surface dries faster than the interior and cracks form. Plastic shrinkage cracks are typically shallow, irregular, and parallel — common on flat slabs in hot, dry, or windy conditions.
Drying shrinkage is the most significant and widely studied form. It develops over weeks, months, and even years as free and adsorbed water evaporates from the hardened cement paste. The rate is highest in the first 3–6 months but continues at a slower rate indefinitely. Drying shrinkage is directly proportional to the water-cement ratio — reducing w/c from 0.60 to 0.45 can reduce long-term shrinkage by 20–30%.
According to structural assessment guidelines, drying shrinkage damage is one of the most commonly observed defects in existing concrete structures, particularly in elements cast without adequate contraction joints.
Autogenous shrinkage results from self-desiccation — the consumption of water by cement hydration reactions in sealed or very low w/c concrete. It is most significant in high-performance and ultra-high-strength concretes with water-cement ratios below 0.40. Unlike drying shrinkage, autogenous shrinkage occurs even in the interior of thick sections where moisture cannot escape, making it a critical consideration in bridge decks, tunnels, and mass concrete elements.
Over decades, atmospheric CO₂ penetrates the concrete surface and reacts with calcium hydroxide in the cement paste to form calcium carbonate. This carbonation reaction causes a small but cumulative volume reduction in the surface zone. While carbonation shrinkage is minor compared to drying shrinkage, it contributes to surface microcracking and can accelerate reinforcement corrosion by reducing the alkalinity that normally protects steel.
During hydration, cement generates significant heat. In mass concrete pours — foundations, dams, and large columns — core temperatures can exceed 70°C. When this heat dissipates, the concrete contracts. If the outer surface has already cooled and stiffened, the contracting core creates tensile stress at the surface, leading to thermal cracking. This is a primary concern in mass concrete construction and requires thermal management through mix design and pour sequencing.
Typical magnitude ranges in microstrain (με). Actual values depend on mix design, environment, and element geometry.
While shrinkage is more common, concrete can also expand under certain conditions. Thermal expansion occurs as temperature rises — the coefficient of thermal expansion for concrete is approximately 10–12 × 10⁻⁶ /°C, meaning a 30°C temperature rise in a 20-metre element causes roughly 6–7 mm of elongation. Expansion joints in bridges, pavements, and large floor slabs are specifically designed to accommodate this movement.
Alkali-silica reaction (ASR) is a chemical expansion mechanism where reactive silica in certain aggregates reacts with alkalis in the cement paste to form a hygroscopic gel. This gel absorbs moisture and swells, creating internal pressure that eventually causes the characteristic map cracking (crazing) visible on affected structures. ASR expansion can be several hundred microstrain over decades and is irreversible once initiated.
Engineering standards provide empirical formulas for predicting concrete shrinkage and thermal movement. These are essential for joint design, reinforcement detailing, and long-term serviceability assessment.
Where k1 = factor for time and hypothetical thickness, k4 = 0.7 for interior exposure / 1.0 for exterior, εcs.b = basic shrinkage strain (typically 800 × 10⁻⁶)
Where: ΔL = change in length (mm), α = coefficient of thermal expansion ≈ 10–12 × 10⁻⁶ /°C, L = member length (mm), ΔT = temperature change (°C)
Where: t = time after end of curing (days), f = 35 for moist-cured concrete (ACI 209), εsh,u = ultimate shrinkage strain (typically 780 × 10⁻⁶ for standard mixes)
| Shrinkage / Movement Type | Typical Magnitude | Onset Timing | Primary Driver | Key Control Method |
|---|---|---|---|---|
| Plastic Shrinkage | 50–300 με | 0–6 hours after placing | Surface evaporation > bleeding | Evaporation retarder, wind breaks, misting |
| Drying Shrinkage | 400–800 με | Days to years | Moisture loss from hardened paste | Low w/c ratio, contraction joints, curing |
| Autogenous Shrinkage | 50–200 με | Hours to weeks | Self-desiccation (w/c < 0.40) | Internal curing (pre-wetted LWA), SRA |
| Thermal Shrinkage | 100–500 με | Days (cooling phase) | Heat of hydration dissipation | SCM replacement, pipe cooling, insulation |
| Carbonation Shrinkage | 50–100 με | Years to decades | CO₂ + Ca(OH)₂ → CaCO₃ | Low permeability mix, surface coating |
| Thermal Expansion | 100–500 με (reversible) | Any time (temperature rise) | Temperature increase | Expansion joints at calculated spacing |
| ASR Expansion | 200–1000+ με | Years to decades | Alkali-silica gel swelling | Non-reactive aggregate, SCMs, low alkali cement |
The single most influential mix design parameter. Higher w/c means more evaporable water and greater long-term drying shrinkage. Reducing w/c from 0.60 to 0.45 typically reduces total drying shrinkage by 20–35%. Every litre of water saved per cubic metre is approximately 10–15 με of shrinkage reduction.
Aggregate does not shrink — only the cement paste does. Higher aggregate volume fraction restrains paste shrinkage, reducing overall concrete shrinkage. Stiff, hard aggregates (granite, basalt, quartzite) provide better internal restraint than softer aggregates. Maximising aggregate content within workability limits is a practical shrinkage control strategy.
Adequate early-age moist curing delays the onset of drying shrinkage and reduces its ultimate magnitude by extending the period of hydration. AS 3600 and ACI 308 both specify minimum 7-day moist curing for normal-strength concrete. Premature curing termination — common on fast-track projects — significantly increases shrinkage cracking risk.
The hypothetical thickness (2 × cross-sectional area / exposed perimeter) controls the drying rate. Thick elements dry slowly from the core, producing a moisture gradient that generates differential shrinkage and internal stress. Thin slabs dry quickly and uniformly. The AS 3600 k1 factor explicitly accounts for element size in shrinkage prediction.
Supplementary cementitious materials (SCMs) such as fly ash and slag generally reduce drying shrinkage by lowering paste content and heat generation. However, silica fume increases autogenous shrinkage due to its very low w/c demand. High-C₃A cements generate more heat and may increase thermal shrinkage. Blended cements are generally preferred for shrinkage-sensitive applications.
Shrinkage-reducing admixtures (SRAs) work by lowering the surface tension of pore water, reducing the capillary tension forces that drive shrinkage. They can reduce total drying shrinkage by 20–50%. Expansive cements and shrinkage-compensating admixtures (Type K) create controlled early expansion to pre-compress the concrete, offsetting subsequent drying shrinkage.
Concrete shrinkage is measured using standardised prism or bar specimens with embedded or surface-mounted length comparator gauges. In Australia, AS 1012.13 specifies the test method using 75 × 75 × 285 mm prisms stored at 23°C and 50% relative humidity. In North America, ASTM C157 is the equivalent standard. Length change is measured periodically from the time of demoulding and expressed as a percentage or in microstrain.
Effective control of concrete shrinkage and expansion requires a coordinated approach across mix design, detailing, and construction practice. No single measure is sufficient — the most durable structures combine multiple strategies. The principles described below apply equally to industrial floors, bridge decks, retaining walls, and residential slabs.
Contraction (control) joints create planes of weakness that channel unavoidable shrinkage cracking to predictable, manageable locations. For unreinforced slabs-on-ground, joint spacing should not exceed 30–36 times the slab thickness. Expansion joints accommodate thermal movement and must be sized using the thermal formula: ΔL = α · L · ΔT. For more on joint placement in relation to supporting structures, see the backfilling around concrete foundations guide, where joint continuity at the slab-foundation interface is discussed.
Conventional steel reinforcement does not prevent shrinkage — it controls crack width by distributing cracks more finely. A minimum reinforcement ratio of 0.25% (temperature and shrinkage steel) is required by AS 3600 for slabs exposed to shrinkage. Synthetic macro-fibres and steel fibres provide three-dimensional crack control throughout the section, making them particularly effective for industrial floor slabs and suspended slabs where joint-free construction is desired.
Shrinkage-optimised mix design targets the lowest practicable water content, maximised aggregate volume, appropriate use of SCMs, and inclusion of SRA where warranted. For high-performance concrete with w/c below 0.40 — common in 2026 for infrastructure and commercial construction — internal curing using pre-wetted lightweight aggregate or superabsorbent polymers (SAP) is increasingly specified to mitigate autogenous shrinkage.
Concrete elements experience both shrinkage and expansion depending on moisture state and thermal conditions — often simultaneously in different zones of the same element.
Ground-supported slabs are the most crack-prone concrete elements due to the high friction restraint provided by the subgrade. Drying shrinkage is the dominant mechanism. Effective slab design requires careful attention to subgrade preparation, vapour barrier placement, joint layout, and reinforcement detailing. Refer to the concrete floors acoustic performance guide for related considerations in floor slab specification, where surface regularity and crack control both affect acoustic outcomes.
Retaining walls are highly susceptible to restrained thermal and drying shrinkage because the base is cast first and fully cured before the wall is poured. The wall is then rigidly restrained at the base and shrinks against this restraint, producing vertical through-cracks at regular intervals. Vertical contraction joints at 3–5 m spacing and horizontal temperature-and-shrinkage reinforcement are standard mitigation measures. The selection of backfill materials also affects the drainage and moisture conditions that influence long-term shrinkage — see the backfill materials for retaining walls guide for details.
Bridge decks experience all forms of shrinkage simultaneously — drying from the top surface, autogenous shrinkage in high-performance concrete, and thermal cycling from daily and seasonal temperature variation. Differential shrinkage between a composite deck and its steel or concrete girders generates hogging moments and transverse tension. This is why bridge deck specifications in 2026 routinely include SRA, internal curing, and low-heat blended cements alongside detailed expansion joint design.
Australian Standard for concrete structures. Contains the definitive shrinkage prediction model (εcs), thermal expansion coefficient, and joint design requirements used across Australia and referenced in New Zealand and Southeast Asia.
View Standard →ACI Committee 209 report comparing multiple time-dependent shrinkage prediction models including ACI 209, B3, GL2000, and CEB MC90-99. Essential reference for structural engineers designing for long-term deformation in 2026.
View ACI Resource →Standard test method for length change of hardened hydraulic cement mortar and concrete. Covers specimen preparation, curing, storage conditions, and measurement procedures for laboratory shrinkage determination.
View ASTM Standard →