Sodium chloride de-icing salts are widely used for winter maintenance but can lead to corrosion of reinforcing steel and spalling of concrete. Urea is an alternative de-icer that is used on large steel bridges, airport runways and the Midland Links motorway viaducts. This report reviews the literature and describes experiments to test whether urea has a deleterious effect on concrete or reinforcing steel. The report does not consider the environmental consequences or the cost-benefits of using urea instead of sodium chloride. Concrete specimens were pretreated with 5% sodium chloride solution for one year before being exposed to 5% and to saturated urea solutions in order to simulate the influence of urea on concrete previously exposed to rock salt deicer. Other specimens were exposed only to 5% and to saturated urea solutions to simulate the influence of urea on new concrete. The mechanical properties of concrete cubes, exposed to weekly cycles of drying and submersion in a variety of urea and salt solutions for 30 months were investigated by ultrasonic pulse velocity and cube crushing at regular intervals. The microstructure of the concrete was examined by mercury intrusion porosimetry and scanning electron microscopy. The resistance to freezing and thawing of concrete prisms, after 12 months exposure to weekly cycles of drying and submersion in a variety of urea and salt solutions was studied by recording weight loss, dynamic modulus of elasticity and ultrasonic pulse velocity. The effects of urea on reinforcement corrosion were investigated by monitoring changes in corrosion potentials and corrosion rates of steel electrodes embedded at four depths of cover in the concrete slab specimens are shown. These specimens were exposed to various forms of conditioning with urea and salt solutions. Similar unreinforced concrete slab specimens were used to monitor changes in the composition of the pore solution of the concrete as a function of depth and time. The experiments showed that the use of urea on concrete not previously exposed to sodium chloride did not cause corrosion of the reinforcing steel or damage the concrete, with one proviso, namely: that surface scaling can occur where urea solutions crystallise on concrete surfaces that are not washed by rain. This should be borne in mind even though it is unlikely to occur on bridges. For concrete previously treated with sodium chloride solution subsequent exposure to urea did not produce a measurable reduction in corrosion rate of the steel, although there was no increase in the corrosion rate. This work was carried out under contract by the Department of Civil Engineering and Construction, University of Aston for the Bridges Division of the Transport and Road Research Laboratory (TRRL).

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