In this paper, we address the problem of optimal relay placement in a cellular network assuming network densification, with the aim of maximizing cell capacity. In our model, a fraction of radio resources is dedicated to the base-station (BS)/relay nodes (RN) communication. In the remaining resources, BS and RN transmit simultaneously to users. During this phase, the network is densified in the sense that the transmitters density and so network capacity are increased. Intra- and inter-cell interference is taken into account in Signal to Interference plus Noise Ratio (SINR) simple formulas derived from a fluid model for heterogeneous network. Optimization can then be quickly performed using Simulated Annealing. Performance results show that cell capacity is boosted thanks to densification despite a degradation of the signal quality. Bounds are also provided on the fraction of resources dedicated to the BS-RN link.