How temperature and shrinkage steel controls cracking in concrete slabs, walls, and structural members
A complete guide to temperature reinforcement in concrete for 2026. Understand why temperature steel is required, how to calculate minimum reinforcement ratios per AS 3600 and ACI 318, correct bar spacing, placement rules, and practical design examples for slabs, walls, and pavements.
Essential knowledge for structural engineers, drafters, and concrete practitioners designing crack-controlled concrete elements in 2026
Temperature reinforcement — also called shrinkage and temperature (S&T) reinforcement — is steel placed in concrete members perpendicular to the primary structural reinforcement. Its purpose is not to carry design bending moments or shear forces, but to control and distribute the cracking caused by concrete shrinkage, thermal expansion and contraction, and restrained deformation. Without adequate temperature steel, concrete slabs and walls develop wide, irregular cracks that compromise durability, aesthetics, and serviceability in 2026.
Plain concrete has virtually zero tensile strength relative to the forces generated by temperature change and drying shrinkage. When a concrete slab cures, it shrinks by approximately 300–600 microstrain. If restrained at its edges or base, tensile stresses build up and the concrete cracks. Temperature reinforcement does not prevent cracking — it controls crack width and spacing, ensuring cracks remain narrow, evenly distributed, and structurally acceptable rather than forming a few wide, harmful cracks that allow moisture and chloride ingress.
Temperature and shrinkage reinforcement is required in all concrete elements that are not fully designed for flexure in every direction — primarily one-way slabs (in the transverse direction), concrete walls (horizontal bars for vertical shrinkage), slabs-on-ground, retaining walls, bridge decks, and any large concrete element subject to significant temperature differentials. Two-way slabs and beams have structural reinforcement in both directions that typically satisfies the minimum temperature steel requirement simultaneously.
Temperature reinforcement in concrete is the minimum steel provided to resist internal tensile stresses caused by thermal movements and drying shrinkage — not by applied structural loads. When concrete is placed, it undergoes two primary volume changes: drying shrinkage (loss of free water during curing, continuing for months to years) and thermal movement (expansion and contraction with ambient temperature changes). In a restrained slab or wall, these volume changes generate tensile stresses that can exceed the tensile strength of concrete, causing cracking.
The role of temperature steel is to bridge these cracks — distributing the tensile force across many fine cracks rather than allowing it to concentrate at one location. This is called crack control. The American Concrete Institute (ACI 318) and Australian Standard AS 3600 both specify minimum reinforcement ratios for temperature and shrinkage control, defining the least amount of steel that must be present in the non-structural direction of any concrete member to maintain acceptable serviceability performance.
One-Way Slab Cross-Section — Reinforcement Layout
Temperature bars run perpendicular to structural (flexural) bars. In a one-way slab spanning left-right, structural bars run left-right, and temperature bars run front-to-back. Together they form a reinforcement mesh that controls cracking in all directions.
Concrete undergoes significant volume changes throughout its life. During the first 28 days of curing, drying shrinkage causes concrete to contract by 200–400 microstrain, equivalent to a shortening of 0.2 to 0.4 mm per metre of slab length. Over a 10-metre slab, this represents 2–4 mm of potential shortening — enough to cause visible cracking if the slab is restrained. Temperature reinforcement does not eliminate this movement; it ensures the energy is dissipated through many small, controlled cracks rather than a few large ones.
Long-term thermal cycling adds further strain. In outdoor environments, the temperature differential between summer and winter can exceed 40°C, producing thermal strains of up to 400 microstrain in a concrete element. For more on how concrete responds to environmental exposure, including how cracking affects acoustic and structural performance, see the guide on acoustic performance of concrete floors, which discusses how slab integrity relates to sound transmission and crack control.
Concrete expands and contracts with temperature at a coefficient of thermal expansion of approximately 10 × 10⁻⁶ per °C. A 20-metre-long slab exposed to a 30°C temperature range experiences a free thermal movement of 6 mm. When restrained at the supports, this produces tensile stresses of up to 1.5–2.5 MPa — exceeding the flexural tensile strength of most concrete grades and causing thermal cracking without adequate temperature steel.
Drying shrinkage occurs as water evaporates from the concrete matrix after placement. Ultimate shrinkage strains for normal-density concrete range from 300 to 600 microstrain depending on water-cement ratio, aggregate type, humidity, and member thickness. Thinner slabs and those exposed to dry conditions shrink faster and more severely. High shrinkage concrete mixes — those with high water content, fine aggregates, or supplementary cementitious materials — require closer temperature bar spacing to maintain crack control.
Temperature and shrinkage stresses only develop when the concrete is restrained from moving freely. A slab sitting on a smooth frictionless surface would shrink without cracking. In practice, slabs are restrained by friction with the subbase, bond to supporting beams and walls, and connection to adjacent structural elements. The degree of restraint — expressed as a restraint factor R from 0 to 1 — directly determines the magnitude of tensile stress that temperature reinforcement must control.
In mass concrete elements — thick foundations, bridge piers, and large transfer slabs — the heat of hydration during the first 3–7 days causes the core temperature to rise by 30–70°C above the surface temperature. When the core cools and contracts while the surface is already set and rigid, severe thermal gradient cracking can develop. Temperature reinforcement in mass concrete must be specifically designed for this early thermal phase, often requiring air-entrained concrete mixes and thermal management plans.
Plastic shrinkage cracking occurs in the first few hours after concrete placement, before the concrete has gained sufficient tensile strength to resist surface drying stresses. It is most severe when evaporation rates exceed 1.0 kg/m²/hr — typically in hot, dry, or windy conditions. While temperature reinforcement helps once the concrete has hardened, plastic shrinkage is primarily controlled through curing practices, evaporation retarders, and windbreaks during placement — not by reinforcement alone.
Design codes limit crack widths in reinforced concrete to protect durability and appearance. AS 3600 and ACI 318 both target a maximum crack width of 0.3 mm for interior exposure conditions and 0.2 mm for aggressive environments. Temperature reinforcement achieves these limits by reducing the crack spacing — more bars mean more, narrower cracks. The relationship between steel ratio, bar diameter, bar spacing, and crack width is defined by crack spacing models in each standard, forming the basis for temperature reinforcement design.
Design standards specify minimum reinforcement ratios for temperature and shrinkage control. These are absolute minimum values — in practice, additional steel may be required for crack width limits, structural continuity, or robustness. The ratios below apply to the gross cross-sectional area of the concrete member (b × h) in the direction being checked.
| Standard | Member Type | Steel Grade | Min. Ratio (ρ) | As Formula | Max Spacing |
|---|---|---|---|---|---|
| ACI 318-19 | Slabs (S&T direction) | Grade 60 (420 MPa) | 0.0018 | 0.0018 × b × h | min(5h, 450 mm) |
| ACI 318-19 | Slabs (S&T direction) | Grade 40 (280 MPa) | 0.0020 | 0.0020 × b × h | min(5h, 450 mm) |
| ACI 318-19 | Slabs (S&T direction) | Welded wire fabric | 0.0018 | 0.0018 × b × h | min(5h, 450 mm) |
| AS 3600-2018 | One-way slabs | D500N (500 MPa) | 0.0025 | 0.0025 × b × D | min(2.5D, 500 mm) |
| AS 3600-2018 | Walls (each face) | D500N (500 MPa) | 0.0015 | 0.0015 × b × D | min(2.5t_w, 500 mm) |
| Eurocode 2 | Slabs / walls | B500 (500 MPa) | Variable | kc × k × fct,eff / σs × Act | min(3h, 400 mm) |
| ACI 318-19 | Walls (each face) | Grade 60 (420 MPa) | 0.0025 (vertical) 0.0025 (horizontal) |
0.0025 × b × h | min(3h, 450 mm) |
| AS 3600-2018 | Slabs-on-ground | D500N (500 MPa) | 0.0060 (crack control) | 0.0060 × b × D | Depends on joint spacing |
Designing temperature reinforcement involves three steps: determining the required steel area from the minimum ratio, selecting a bar size and spacing that delivers that area, and confirming the spacing does not exceed code limits. The process is straightforward for standard slabs and walls but requires more care for unusual geometries, aggressive exposures, or elements with close construction joints.
Multiply the minimum reinforcement ratio by the gross concrete area in the temperature steel direction. For a 200 mm thick slab, 1000 mm wide design strip, using ACI 318 with Grade 60 bars: As = 0.0018 × 1000 × 200 = 360 mm² per metre width. This is the minimum required — you must always check whether the structural design requires more steel in this direction and use the larger value.
Given: 180 mm thick one-way slab, D500N reinforcement, 1000 mm wide design strip.
Required: Temperature & shrinkage steel in transverse direction.
Minimum ratio (AS 3600 Cl. 9.4.3): ρ = 0.0025
As_min = 0.0025 × 1000 × 180 = 450 mm²/m
Select: N12 @ 250 mm crs → As = 452 mm²/m ✅ (just satisfies)
Check spacing: max = min(2.5 × 180, 500) = min(450, 500) = 450 mm — 250 mm ✅
Result: N12 @ 250 mm centres in the transverse direction of the slab.
Choose the smallest practical bar diameter and the widest spacing that still delivers the required steel area, without exceeding the code maximum spacing. Smaller bars at closer spacing give better crack control than larger bars at wider spacing — the same total steel area distributed more evenly produces finer, more numerous cracks. As a practical rule, N10 or N12 bars at 200–300 mm centres satisfy temperature reinforcement requirements in most standard residential and commercial concrete slabs in Australia under AS 3600.
Codes impose a maximum spacing on temperature reinforcement to ensure the crack control benefit is achieved uniformly across the slab surface. Under AS 3600, the maximum spacing is the lesser of 2.5 times the overall slab thickness (D) or 500 mm. Under ACI 318, the maximum is 5 times the slab thickness or 450 mm, whichever is less. These limits prevent large unreinforced zones where uncontrolled cracking could develop between widely spaced bars — particularly important in long slabs and slabs restrained at both ends.
In a one-way slab, structural flexural reinforcement runs in the spanning direction (typically the short direction). Temperature and shrinkage bars run perpendicular — in the long direction — at the minimum ratio. Both faces of the slab require temperature steel if the slab is thicker than about 200 mm and subject to significant thermal exposure, or if the slab is expected to crack on the tension face. In practice, most one-way slabs have a single layer of temperature bars near the top of the slab in the transverse direction, supplementing the main bars at the bottom in the spanning direction.
Concrete walls require both vertical and horizontal reinforcement on each face. The horizontal bars act as temperature and shrinkage reinforcement, preventing vertical cracking from horizontal restraint. AS 3600 requires a minimum of 0.0015 × b × D horizontal steel on each face of a wall. ACI 318 requires 0.0025 of the gross wall cross-section in both vertical and horizontal directions when deformed bars are used. Long walls — retaining walls, basement walls, bridge abutments — are particularly susceptible to thermal cracking and benefit from closer horizontal bar spacing and strategically placed contraction joints to limit crack formation. For more on retaining wall materials and fill behaviour, see the guide on backfilling around concrete foundations.
Industrial and commercial slabs-on-ground face the most severe temperature and shrinkage conditions — large areas, significant restraint from subbase friction, and high live loads. Temperature reinforcement in slabs-on-ground is designed to control crack widths between saw-cut contraction joints, not to span across joints. AS 3600 specifies minimum reinforcement based on joint spacing: for joint spacings beyond 6 m, steel ratios of 0.0060 or higher are required to keep crack widths within acceptable limits. Closer joint spacing allows lower steel ratios, while greater joint spacing demands more temperature steel. Welded wire mesh (WWM) or deformed bar on chairs is typically specified for slab-on-ground temperature reinforcement.
It is important to distinguish between temperature reinforcement and structural reinforcement in concrete design. They serve entirely different purposes and are governed by separate code clauses, yet both must be satisfied simultaneously in every concrete member.
Structural reinforcement is designed to carry the calculated bending moments, shear forces, and axial loads from applied dead and live loads. It is sized by structural analysis and section design calculations using the ultimate limit state. Its amount depends on the loads and the concrete and steel strengths — it can range from the minimum steel ratio up to the maximum (balanced or ductility-limited) ratio. Structural reinforcement is placed in the direction of primary stress — typically the spanning direction in slabs and the vertical direction in columns.
Temperature and shrinkage reinforcement is designed for the serviceability limit state — crack control, not load capacity. Its minimum amount is fixed by code regardless of the applied loads, because it must control thermally and shrinkage-induced stresses that exist in every concrete member. Temperature steel is always placed perpendicular to the structural reinforcement in one-way systems. It does not contribute to the structural load capacity of the section (conservatively) but does improve ductility and robustness at ultimate limit state as a secondary benefit.
In two-way slabs, beams, and columns where structural reinforcement is provided in all directions, the structural steel typically exceeds the minimum temperature steel requirement. In this case, no separate temperature-only bars are needed — the structural bars perform both functions simultaneously. The designer must still check the structural steel ratio against the temperature minimum in each direction to confirm compliance. In lightly loaded two-way slabs, it is possible for the structural reinforcement to fall below the temperature minimum, requiring additional bars.
ACI 318-19 Chapter 7 (One-Way Slabs) and Chapter 11 (Walls) provide the minimum temperature and shrinkage reinforcement requirements for US practice. Section 7.6.1 specifies the 0.0018 ratio for Grade 60 deformed bars and the maximum spacing rules for temperature steel in slabs. ACI also publishes ACI 207 series documents for mass concrete thermal crack control — essential for thick foundations and transfer structures with significant heat of hydration effects.
ACI International →In cold climates and freeze-thaw environments, temperature reinforcement works alongside air-entrained concrete mixes to manage thermal cracking. Air entrainment reduces the tensile stress buildup from freeze-thaw cycling by providing void space for ice crystal expansion. Understanding how air-entrained concrete behaves under temperature change is essential for correctly sizing temperature reinforcement in exposed slabs, pavements, and bridge decks subject to winter conditions.
Read the Guide →When temperature reinforcement has been insufficient — or omitted — existing concrete structures develop wide, irregular cracks that require assessment and remediation. Understanding how to evaluate crack patterns, measure crack widths, assess the cause (thermal vs structural vs settlement), and design appropriate repairs is a critical skill for engineers working with existing concrete in 2026. Our assessment guide covers crack mapping, non-destructive testing, and repair strategy selection.
Read the Guide →