Result
—
Full breakdown below
Identify, distinguish, prevent and manage both cracking mechanisms in residential and commercial concrete
A complete 2026 guide to drying shrinkage cracking and thermal cracking in concrete — covering the physics of each mechanism, how to tell them apart on site, control joint design, mix design strategies, curing requirements, crack width assessment, and remediation options for Australian conditions.
Every concrete element cracks unless designed, mixed, placed, and cured to manage the two primary cracking forces — shrinkage and thermal movement
Drying shrinkage cracking occurs when concrete loses moisture to the atmosphere as it hardens and dries beyond its initial set. As the cement paste dries, it contracts volumetrically — but the aggregate and any surrounding or underlying restraint (soil, reinforcement, adjacent elements) resist this contraction. When the resulting tensile stress exceeds the concrete's tensile strength — typically 1.5–4.0 MPa for standard concrete — cracks form. Thermal cracking occurs due to temperature changes in the concrete — either the heat of hydration during early-age curing (particularly in mass concrete) causing the surface to cool and crack while the core is still hot, or seasonal/diurnal temperature cycles in hardened concrete causing expansion and contraction stresses in restrained elements. Both produce cracks, but at different times, in different patterns, and requiring different prevention strategies.
Australia's climate makes both cracking mechanisms particularly relevant. The hot, dry conditions across most of the continent — high evaporation rates, low humidity, and strong winds — create an extremely aggressive environment for drying shrinkage, particularly during summer pours. A concrete surface in Sydney or Brisbane on a 35°C, low-humidity day with a 20 km/h wind can lose moisture at rates of 1.5–2.0 kg/m²/hr — well above the 1.0 kg/m²/hr threshold at which plastic shrinkage cracking becomes likely. For thermal cracking, the diurnal temperature range in Australia (up to 20–25°C in inland areas) combined with large concrete pavements, retaining walls, and bridge elements creates significant restrained thermal movement. Understanding both mechanisms is essential for anyone specifying, placing, or inspecting concrete in 2026.
Repairing cracked concrete is significantly more expensive than preventing cracks in the first place. A shrinkage crack in a residential driveway that was preventable with correct joint placement costs $150–$400 per crack to saw-cut and epoxy inject — but the joint costs $5–$15 per lineal metre to install during construction. For structural elements, a thermal crack that exceeds the design crack width limit of 0.3 mm for normal exposure (0.2 mm for aggressive exposure) under AS 3600:2018 may require expensive crack injection, surface sealing, or in severe cases element remediation. The fundamental message of this guide is that both cracking types are largely predictable and preventable with good design, mix selection, placement practice, and curing — not afterthoughts to be managed during defect liability periods.
The most important skill for anyone working with concrete is being able to distinguish drying shrinkage cracking from thermal cracking — and from other crack types — by visual inspection alone. The two mechanisms produce cracks with characteristically different patterns, timing, locations, and widths. The comparison below covers all key distinguishing parameters.
Figure 1 — Approximate onset timing of different concrete crack mechanisms after placement (2026)
Drying shrinkage is the reduction in volume that occurs in concrete as the excess water not consumed by cement hydration evaporates from the hardened cement paste. The shrinkage is not uniform across the section — the surface loses moisture faster than the interior, creating a moisture gradient that produces tensile stress at the surface restrained by the less-dry interior. The ultimate drying shrinkage strain of standard Australian concrete is typically in the range of 400–800 microstrain (0.04–0.08%) — equivalent to a contraction of 4–8 mm per 10 m of concrete length. For a 20 m long unreinforced concrete pavement slab with no joints, this would produce a total contraction of 8–16 mm, generating tensile stresses far exceeding the concrete's tensile capacity.
The magnitude of drying shrinkage is controlled primarily by the cement paste content and water content of the mix — more paste means more shrinkable material, and more water means more evaporable moisture. Key factors that increase shrinkage include: high water-cement ratio (above 0.50), high cement content (above 400 kg/m³), fine aggregate with high fines content, inadequate or absent curing, low relative humidity during curing, high ambient temperature during early drying, and use of pure Portland cement rather than blended cement (fly ash and slag reduce shrinkage by 15–30%). In contrast, increasing the coarse aggregate volume fraction reduces shrinkage — the aggregate provides internal restraint against cement paste contraction.
Plastic shrinkage cracking must be distinguished from drying shrinkage cracking — it occurs in the first 0–6 hours after placement, before the concrete has reached initial set, when the evaporation rate from the surface exceeds the rate at which bleed water rises to replace it. The result is diagonal or random cracks 0.5–3 mm wide on the surface, typically parallel to each other and spaced 200–600 mm apart. The trigger is an evaporation rate above 1.0 kg/m²/hr — calculated using the Menzel nomograph from wind speed, air temperature, concrete temperature, and relative humidity. Prevention requires wind breaks, shade cloth, evaporation retarder spray, and fog misting — not simply covering with plastic after the cracks have already formed.
AS 3600:2018 specifies maximum design crack widths for reinforced concrete elements based on exposure class: 0.3 mm for Exposure Classes A1 and A2 (typical interior and exterior residential), and 0.2 mm for Classes B1, B2, and C (near-coast and marine environments). These limits apply to the calculated crack width at the surface of the concrete, not the maximum possible crack. Achieving these limits requires minimum reinforcement ratios, bar spacing limits (maximum 300 mm in flexural elements), and in some cases specific crack control bars. For unreinforced slabs, crack control relies entirely on correctly spaced joints.
Reinforcement does not prevent shrinkage — it distributes the cracking into many fine cracks rather than allowing fewer wide cracks to form. A slab without reinforcement will develop one or a few wide cracks; the same slab with adequate distributed reinforcement will develop many fine cracks (below 0.3 mm) that are functionally and aesthetically acceptable. The minimum reinforcement ratio for crack control in AS 3600 is typically 0.0025 × bh (0.25% gross cross-section area). Synthetic macro-fibres (polypropylene or steel fibres at 2–5 kg/m³) can supplement conventional reinforcement for plastic shrinkage control and early drying shrinkage, but are not a substitute for bar reinforcement in structural elements.
Thermal cracking in concrete arises from two distinct scenarios that must be treated differently. The first is early-age thermal cracking in mass concrete — the heat released by cement hydration raises the internal temperature of thick concrete elements (walls, footings, raft slabs, bridge piers) by 30–60°C within the first 24–72 hours. The surface cools by convection and radiation while the core remains hot; the resulting temperature differential causes the surface to contract while the core restrains it, generating tensile stress at the surface. The second scenario is long-term thermal movement — hardened concrete expands and contracts with ambient temperature at a coefficient of thermal expansion (CTE) of approximately 10 microstrain/°C; in restrained elements (retaining walls, pavements, bridge decks), this seasonal and diurnal movement generates cyclic tensile and compressive stresses that eventually cause cracking if not accommodated by expansion joints.
Estimate shrinkage strain, thermal stress, and control joint spacing for your concrete element
Control joints — also called contraction joints or crack inducers — are the primary tool for managing drying shrinkage cracking in concrete slabs and pavements. They work by creating a pre-defined plane of weakness across the slab section, so that when the shrinkage crack inevitably forms, it forms at the joint rather than at a random location. A correctly designed and spaced control joint system does not prevent cracking — it controls where the crack occurs, keeping it at the joint where it is shallow, sealed, and aesthetically acceptable. Without joints, the same total crack width occurs, but distributed as random, full-depth cracks across the slab surface.
Contraction joints are saw-cut or formed grooves that weaken the slab section to induce cracking at the joint plane. Saw-cut joints must be made within 4–12 hours of pour (or before 25% of the 28-day strength is reached) — cutting too late means the crack has already formed randomly. The saw cut depth must be a minimum of one-quarter (25%) of the slab thickness — for a 100 mm slab, minimum 25 mm deep, with 30–35 mm preferred. Formed grooves (cast into the slab top during finishing) require minimum 25% depth and must be clean, well-defined, and continuous across the full slab width without interruption by mesh or bars.
Expansion joints provide a full separation between adjacent concrete elements to accommodate thermal expansion and relative movement without transferring load. They are required at: all junctions between a slab and a fixed structure (column base, wall, step, drain); where slabs of different thicknesses meet; around circular or non-rectangular protrusions; and at maximum spacing of 6–8 m in exposed concrete pavements in Australian climates with high diurnal temperature ranges. Expansion joints are formed using a compressible filler board (closed-cell polyethylene or polystyrene, minimum 10 mm thick) placed against the structure before the slab is poured, with a sealant applied to the top 20–25 mm after curing.
Construction joints are formed at the end of each day's pour or between separate concrete placements. They must be designed and detailed carefully — an unplanned construction joint is essentially a random crack through the full section. Where reinforcement passes through a construction joint, continuity of the reinforcement maintains structural integrity. The joint face must be cleaned (sandblasted or wire-brushed), all laitance removed, and the surface pre-wetted (not saturated) before the next pour is placed. For slabs, construction joints should coincide with control joint locations wherever possible to avoid creating a structurally weak section at an unintended location.
The maximum spacing between contraction joints depends on the slab thickness, reinforcement level, concrete grade, and exposure conditions. Standard guidance for Australian conditions: unreinforced concrete paths and driveways — maximum 3.0 m × 3.0 m panels (or 25 × slab thickness, whichever is less); mesh-reinforced residential slabs — maximum 4.5 m × 4.5 m; bar-reinforced industrial pavements — maximum 6.0 m × 6.0 m. The panel aspect ratio should not exceed 1.5:1 (length to width). Square panels are preferred — long, narrow panels concentrate tensile stress in the longitudinal direction and crack along the centre line regardless of joint spacing. Use the Joint Spacing calculator above for your specific conditions.
Exposed control joints must be cleaned and sealed after curing to prevent incompressibles entering the joint, moisture ingress, and edge spalling. For residential concrete driveways and paths, a polyurethane sealant (e.g. Sika Flex, MasterSeal) is the most widely used — it has sufficient elasticity to accommodate crack movement, strong adhesion, and good UV and fuel resistance. For industrial floors, a two-part epoxy or polyurea sealant provides a harder, more durable surface. The joint must be dry and clean before sealant application — prime with manufacturer's primer if required. Tooled to 6–8 mm depth, with the sealant bead slightly below flush to prevent damage by foot or vehicle traffic.
Both polypropylene microfibre (0.3–0.9 kg/m³) and steel fibre (25–35 kg/m³) can supplement control joint design by reducing plastic shrinkage cracking and improving post-crack tensile capacity. Polypropylene microfibre is primarily effective against plastic shrinkage cracking in the first 24 hours — it bridges the early microcracks before they widen. Steel fibre and macro-polypropylene fibre (2–6 kg/m³) are effective for drying shrinkage control — they provide residual tensile capacity after cracking, limiting crack widths. However, fibres are not a substitute for control joints in unreinforced slabs — they reduce crack frequency and width but do not eliminate cracking if joints are omitted or incorrectly spaced.
Both drying shrinkage and thermal cracking can be significantly reduced by selecting the right concrete mix for the application. The key principle is to minimise the total cementitious paste volume while maintaining adequate workability, strength, and durability — because it is the cement paste, not the aggregate, that shrinks and generates heat. The following reference table summarises the mix design parameters most relevant to crack control in 2026.
| Mix Parameter | Effect on Shrinkage | Effect on Thermal Risk | Recommended Limit / Target | Notes |
|---|---|---|---|---|
| Water-cement ratio (w/c) | ↑ Higher w/c = more shrinkage | Minimal direct effect | ≤ 0.50 for crack-controlled | Lower w/c always reduces shrinkage — use plasticiser to maintain workability |
| Cement content (kg/m³) | ↑ More cement = more shrinkage | ↑ More cement = more heat | 280–360 kg/m³ typical residential | Minimise cement content while meeting strength and durability requirements |
| Fly ash replacement (%) | ↓ 25–40% FA reduces shrinkage 15–25% | ↓ Significantly reduces heat of hydration | 25–35% for standard; up to 40% for mass | Slows strength gain — confirm 28-day strength with supplier; extend curing |
| Ground slag (GGBFS, %) | ↓ 40–50% slag reduces shrinkage 10–20% | ↓ Moderate heat reduction | 30–50% typical | Improves durability in sulphate/chloride conditions; slower early strength |
| Aggregate content and type | ↓ More coarse agg = less shrinkage | Minimal | Maximise coarse aggregate volume | Higher aggregate / paste ratio is always better for crack control |
| Shrinkage-reducing admixture (SRA) | ↓ Reduces drying shrinkage 20–40% | Minimal | 1–2% by cement weight | Effective but adds cost (~$15–30/m³) — best value for high-specification floors |
| Nominal max aggregate size | ↓ Larger aggregate = less shrinkage | Minimal | 20 mm nominal standard; 40 mm mass | 40 mm max aggregate in mass concrete reduces paste content and heat |
The primary Australian Standard for concrete structure design — Section 3 covers shrinkage strain calculation, Section 8 covers crack control requirements including maximum crack widths, minimum reinforcement ratios for crack control, and bar spacing limits for residential and commercial concrete elements.
Standards Australia →The Australian Standard for measuring the drying shrinkage of concrete test specimens — the basis for comparing mix shrinkage performance and specifying maximum shrinkage limits for floor slabs, industrial pavements, and other crack-sensitive applications.
Standards Australia →CCAA Data Sheet — Drying Shrinkage of Cement and Concrete: the industry reference for Australian concrete shrinkage data, typical values, factors affecting shrinkage, and mix design strategies. Free download from the CCAA technical library.
CCAA Technical Library →Concrete Institute of Australia recommended practice Z16: Crack Control of Mass Concrete — covers temperature rise calculation, critical differentials, mix design for thermal management, temperature monitoring, and formwork insulation requirements for mass concrete pours in Australian conditions.
Concrete Institute of Australia →