Failure Of Prestressing Steel Induced By Crevice Corrosion In Prestressed Concrete Structures

The application of prestressed concrete technology in civil structures is relatively recent. Therefore, the existing prestressed concrete members in such structure are still relatively young, and the corrosion and the concrete deterioration problems associated with this type of concrete members have became clearly evident only in the early 1980s. Damage during service, frequently appearing after several years or decades of exploitation, is usually most consequential. The lacking or insufficiently alkaline protection already from the beginning or its loss due to carbonation and/or depassivation after chloride attack are the major causes of later damage or even of a failure. Steel wire strands, due to their structure, usually consisting of steel wires helically laid about a core centre, frequently another wire, may be subjected to crevice corrosion even if they are well grouted with uncarbonated mortar but in presence of a little amount of chloride ions. Crevice can occur also in cable to duct (metal or plastic ducts) contacts. Crevice condition easily promotes a drastic change in local environmental composition, that is a pH reduction and an increase of chloride concentration. In such a condition steel strand failure due to hydrogen embrittlement can occur. Experimental results obtained in calcium hydroxide solution with added potassium chloride in simulated crevice condition on stressed steel strands, showed crack formation after only 1250 hours. Chloride concentration was lower than the critical amount to induce pitting corrosion or generalized corrosion at that pH (12.4), whilst was enough to induce extended crevice corrosion. In some case crevice attack was so intense to reduce drastically steel wire cross section.