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In order to describe inflation in general relativity, scalar fields must inevitably be used, with all the setbacks of that description. On the other hand, $f(R)$ gravity and other modified gravity theories seem to provide a unified description of early and late-time dynamics without resorting to scalar or phantom theories. The question is, can modified gravity affect directly the mysterious radiation domination era? Addressing this question is the focus in this work, and we shall consider the case for which in the early stages of the radiation domination era, namely during the reheating era, the background equation of state parameter is different from $w=1/3$. As we show, in the context of $f(R)$ gravity, an abnormal reheating era can affect the primordial gravitational wave energy spectrum today. Since future interferometers will exactly probe this era, which consists of subhorizon modes that reentered the horizon during the early stages of the radiation domination era, the focus in this work is how a short abnormal reheating era that deviates from the standard perfect fluid pattern with $w\neq 1/3$, and generated by higher order curvature terms, can affect the primordial gravitational wave energy spectrum. Using a WKB approach, we calculate the effect of an $f(R)$ gravity generated abnormal reheating era, and as we show the primordial gravitational wave spectrum is significantly amplified, a result which is in contrast to the general relativistic case, where the effect is minor.