Joint movement = α × panel length × temperature range. A 90 m slab with joints every 30 m and a 40 °C range moves 12 mm per joint (± 6 mm), so a ± 25% sealant needs a joint at least 24 mm wide. Slabs on ground also need contraction joints at 24–36 × thickness: 3.6–5.4 m for 150 mm.
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Expansion joints
2→ 3 panels of 30.00 m
Movement at each joint
12.0 mm (± 6.0 mm)
Minimum joint width
24.0 mm for a ± 25% sealant
Contraction joint spacing (24–36 t)
3.6–5.4 m use ≈ 4.5 m
Contraction joints per panel
6 at 4.29 m each way
Saw-cut depth (¼ of slab)
37.5 mm
Joint layout is a design decision: follow the joints on the structural drawings. Also add joints at changes in shape, re-entrant corners and where the building changes height.
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ΔL = α × L × ΔT = 10.0×10⁻⁶ × 30.00 m × 40.0 °C = 12.0 mm
Width = (ΔL ÷ 2) ÷ 25% = 24.0 mm (sealant moves ± about the installation width)
ACI 360R: contraction joints at 24–36 × 150.0 mm = 3.60–5.40 m; many specifications cap this at about 4.5 m (15 ft)
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90 m long slab on ground, 150 mm thick, expansion joints at 30 m, ΔT = 40 °C, α = 10 × 10⁻⁶/°C, ± 25% sealant:
2 expansion joints, three 30 m panels
ΔL = 10⁻⁵ × 30 × 40 = 12 mm (± 6 mm)
Minimum joint width = 6 ÷ 0.25 = 24 mm
Contraction joints at 3.6–5.4 m → use about 4.3–4.5 m, sawn 37.5 mm deep (¼ of the slab)
Movement and Joint Width (ΔT 40 °C, α 10 × 10⁻⁶, ± 25% sealant)
Joint spacing
Movement
Minimum width
20 m
8 mm
16 mm
30 m
12 mm
24 mm
40 m
16 mm
32 mm
50 m
20 mm
40 mm
Contraction Joint Spacing (ACI 360R, 24–36 t)
Slab
Spacing range
100 mm (4 in)
2.4–3.6 m (8–12 ft)
125 mm (5 in)
3.0–4.5 m (10–15 ft)
150 mm (6 in)
3.6–5.4 m (12–18 ft)
200 mm (8 in)
4.8–7.2 m, often capped near 4.5 m
Frequently Asked Questions
How far apart should expansion joints be in concrete?
In buildings, commonly every 30–45 m (100–150 ft), closer in hot climates, on exposed roofs and in long parapets. Joint spacing is a design decision shown on the structural drawings.
How far apart should control joints be in a slab?
ACI 360R guidance is 24 to 36 times the slab thickness (3.6–5.4 m for a 150 mm slab) with many specifications capping it at about 4.5 m (15 ft). Keep panels close to square.
How wide should an expansion joint be?
Wide enough that the sealant stays within its movement capability. Joint width = (total movement ÷ 2) ÷ capability: 12 mm of movement with a ± 25% sealant needs 24 mm.
References
ACI 360R: Guide to Design of Slabs-on-Ground: contraction joint spacing 24–36 × slab thickness
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