Development of early age properties and strength gain in concrete are intimately related to the hydration of cement. In this paper, a nondestructive technique for monitoring the setting and hardening process of portland cement concrete, currently under development, is presented. The technique is based on measuring the ultrasonic wave reflection factor (WRF) between the hardening concrete and a steel interface. The measured WRF is intimately related to the physical changes in hardening concrete produced by hydration and provides a means to assess the in-place property development of concrete from a very early age. To test the applicability of the WRF technique in predicting the strength gain in a structure, a laboratory evaluation of the WRF technique on large laboratory-sized specimens was performed. Three slabs with different compressive strengths reflecting various pavement and/or bridge mixes were cast. Each slab was instrumented to obtain WRF profiles with time. In addition, cores were obtained at regular intervals to evaluate compressive strength of the slabs. A good correlation between the experimental trends in WRF measurements and the in-place strength development is shown. Finally, the percentage change in the WRF reading is shown to be very close to the percentage change in the strength gain in the slabs.