Question: We are placing an 8-inch thick concrete parking lot with the joints at 18 feet apart. I thought we needed some expansion joints to account for the thermal expansion when the concrete gets hot in the summer. The engineer, though, insists expansion joints aren’t necessary. Why not?

Answer: The reason expansion joints aren’t needed is simply that drying shrinkage of the concrete after placement is greater than any anticipated thermal expansion. Start by reading Scott Tarr’s article on concrete shrinkage. You’ll note that he says even low-shrinkage concrete shrinks about 0.05%. So with the joints 18 feet apart, the slab will shrink 0.05% x 18 feet x 12 inches/foot = 0.12 inches. Thermal expansion of concrete is about 5.5 x 10-6/° F. Say you placed the slab at 70° F and it heats up to 150° F in the summer. That’s an 80° F temperature change. So the expansion would be 80 x 5.5 x 10-6 x 18 feet = 0.095 inches. And the sawed joints will at least 1/8-inch (0.125 inch) wide which provides an extra measure of safety. So even under these severe conditions, the expansion is less than the shrinkage so no expansion joint is needed.