  Gravitational waves (GWs) from merging black holes allow for unprecedented
probes of strong-field gravity. This requires accurate predictions of GWs in
extensions of General Relativity. We compute waveforms in a subset of quadratic
gravity theories motivated by the low-energy limit of quantum gravity
paradigms. Working in the first post-Newtonian approximation, where the new
effects of curvature nonlinearities beyond a scalar field first appear, we
quantify the impact on the GW phasing and parameter dependence. Our results lay
an important foundation for future precision tests of gravity.
