Monitoring compressive strength of concrete by nuclear magnetic resonance, ultrasound, and rebound hammer

The relationship between T₁ and T₂ nuclear magnetic resonance relaxation times and compressive strength of concrete is determined and compared with those obtained using ultrasonic pulse velocity and rebound hammer. Concrete cylinders measuring 150 mm (6 in.) in diameter and 300 mm (12 in.) in length were cast from three concrete mixtures produced by a local precast company and tested to obtain T₁ and T₂ relaxation times, ultrasonic pulse velocity, rebound number, moisture content, and compressive strength at 1, 3, 7, 14, 28, and 56 days of age. Estimation of the compressive strength from T₁ and T₂ relaxation times has the same reliability as those produced by the rebound number and the ultrasonic pulse velocity, respectively. The relaxation times measurements in concrete, particularly T₂, are an alternative way to obtain information that can be related not only to compressive strength, but also to the pore size distribution and crack width.