This paper presents an ultra-compact and high efficiency rectenna which can power wireless sensing nodes fully embedded in a concrete structure. The innovative proposed design is optimized in term of efficiency and compactness for the targeted application. The rectenna is compact, which a volume of 56 mm x 32 mm x 10 mm, (0.0004) and can reach a maximum RF to dc conversion efficiency of 70% at 868 MHz for 10kΩ load and for an illuminating RF power density of 4μW/cm². The use of a metallic cavity increases the rectenna performances and limits the negative impact of the concrete material. Thus, the rectenna into the cavity harvest a dc voltage up to 1.15V when the rectenna is illuminated with a RF power density of 1 μW/cm². Experimental results of the rectenna embedded in the concrete beam confirms the feasibility of powering a wireless sensor by wireless power transmission.