Permeability of concrete in a marine environment

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Monitoring the setting and early hardening of concrete using an ultrasonic waveguide

The use of an embedded waveguide to monitor the setting and early hardening of concrete has been investigated. The technique uses measurements of the attenuation of a guided wave propagating along the waveguide to obtain the shear velocity, and hence shear modulus, of the concrete. Tests have been carried out on mortar and concrete specimens monitored continuously over 48 hours. Over the first few hours, while the mortar/concrete was still fluid, the absolute rate of increase in shear modulus was low. From around 5 hours to around 20 hours, shear modulus increased linearly. Thereafter, shear modulus increased, but at an ever-decreasing rate. The shear velocity of the mortar was calculated to be 1720 m/s after two days, corresponding to a shear modulus of 5.6 GPa, while the 48 hour-old concrete had a shear modulus of 16.5 GPa. Repeated tests showed similar end states, but some differences in hardening rate; these were attributed either to temperature variations or to differences in mould geometry. The technique worked equally well on mortar and concrete, indicating that the presence of the large aggregate particles in concrete does not adversely affect the results