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Ultrasonic waves propagating in solids have stress-dependent velocities. Therelation between stress (or strain) and velocity forms the basis of non-linearacoustics. In homogeneous solids, conventional time-of-flight techniques havemeasured this dependence with spectacular precision. In heterogeneous medialike concrete, the direct (ballistic) wave around 500 kHz is stronglyattenuated and conventional techniques are less efficient. In this manuscript,the effect of weak stress changes on the late arrivals constituting theacoustic diffuse coda is tracked. A resolution of $2.10^{-5}$ in relativevelocity change is attained which corresponds to a sensitivity to stress changeof better than 50 kPa. Therefore the technique described here provides anoriginal way to measure the non-linear parameter with stress variations on theorder of tens of kPa.

Monitoring stress related velocity variation in concrete with a $2.10^{-5}$ relative resolution using diffuse ultrasound