Concrete Thermal Expansion Calculator 2026 - ACI & AASHTO Joint Design Tool

The standard CTE for normal-weight concrete is 5.5 x 10⁻⁶ /°F (9.9 x 10⁻⁶ /°C) per ACI 318 and AASHTO T336. The value ranges from 4.5 (limestone) to 7.0 (quartzite) per degree Fahrenheit depending on aggregate type. For bridge and pavement design, AASHTO T336 lab testing is required rather than using the ACI default value.

Use the formula ΔL = α × L × ΔT. Example: a 100-ft gravel concrete slab (CTE = 5.5) with a 70°F seasonal swing: 5.5e-6 × 1,200 in × 70 = 0.462 in of expansion. The concrete PSI calculator helps determine your concrete's elastic modulus for the thermal stress check.

ACI 360R states the minimum joint width is 3/4 inch for exterior slabs. The required width equals the calculated thermal movement times a safety factor of 1.25 to 1.50. For a 100-ft outdoor slab with a 70°F swing, the calculated movement is about 0.46 inches, so a 3/4-inch joint (0.75 in) with the 1.5x factor provides adequate capacity. Never size joints for expansion only - they must also accommodate thermal contraction and any drying shrinkage.

ACI 360R recommends expansion joint spacing of 2 to 3 times the slab thickness in feet. A 4-inch slab needs joints every 8-12 feet; a 6-inch slab, every 12-18 feet; an 8-inch slab, every 16-24 feet. For outdoor slabs in harsh climates (temperature swings over 80°F), stay closer to the lower end of these ranges to avoid overstress.

Control joints (contraction joints) are shallow saw cuts (1/4 slab depth) that create a weak plane for shrinkage cracks to follow - they only work in one direction. Expansion joints are full-depth separations filled with compressible material that accommodate both expansion AND contraction. Exterior slabs exposed to large temperature swings need true expansion joints, not just control joints. Control joints placed at 2-3 times slab thickness will catch shrinkage cracks but not prevent thermal expansion damage.

The total movement is the same - it equals CTE × length × ΔT regardless of direction. However, contraction during winter is usually more damaging because concrete's tensile strength (400-600 PSI) is much lower than its compressive strength (3,000-8,000 PSI). Expansion in summer can cause joint "blow-up" if joints are too narrow or filled with incompressible debris. Both directions must be accounted for in your joint design. Check the concrete set time calculator to understand how curing temperature establishes the "zero movement" baseline.

Aggregate type controls CTE more than any other mix variable. Quartzite and siliceous gravel concrete has a CTE of 6.5 to 7.0 x 10⁻⁶ /°F - about 40 percent higher than limestone concrete at 4.5 to 5.0. This matters most for bridge decks and pavements. AASHTO T336 requires direct lab measurement of CTE for pavement design to avoid under- or over-designing joints. Using the wrong CTE value is one of the leading causes of premature pavement distress in the USA.

For exterior slabs, use a self-leveling polyurethane sealant (ASTM C920 Type S, Grade P) for horizontal joints, or a backer rod plus polyurethane caulk for vertical joints. Pre-molded cork, foam, or neoprene fillers work for joints in high-movement areas. Never use rigid mortar to fill expansion joints - it defeats the entire purpose. The joint filler must compress to at least 50 percent of joint width when the concrete reaches maximum expansion temperature.