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Calculus is a specific field of mathematics, mostly to do with limits, integrals and derivatives. Those all feature very heavily in working out loads and stresses.

But it's unfair to say the Roman didn't have calculus. It wasn't formal calculus, but they absolutely had mathematics, and the Greeks worked out the exhaustion method a century before the Via Appia was even started.

You don't need calculus to do some very impressive building, you can go very very far with experience, rules of thumb and basic maths. Hell, ask any civil engineer and they'll gladly show you some common formula that makes physicists cry.