The ability to create and steer beams, and the availability of large bandwidths have opened up the possibility of using mmWave 5G networks for radar-like sensing applications. In this paper, we introduce a signal processing pipeline that is able to process reflected OFDM-based communications waveforms and create radar images without affecting communications protocols or data throughput. An experimental demonstration system for this concept comprising a prototype basestation transmitter and an auxiliary imaging receiver is also presented. These two components are implemented with Si-based 28-GHz, 64-element phased array transceiver modules and software-defined radios. Measurement results show 3D radar images of indoor scenes with 2° angular and 15 cm ranging resolution using 5G-like communications waveforms at 28-GHz, without any effect on communication functionality.