Durability of Lightweight Concrete for Bridge Decks

As concrete durability becomes a higher priority for departments of transportation’s acceptability of concrete for bridge decks, consideration of resistance to the penetration of chloride ions has become a key consideration in defining durability. For years the “Rapid Chloride Ion Penetration (RCP) test has been the accepted standard to measure penetrability of concrete to chloride ions. With the movement to use chloride ion penetrability as a part of acceptance criteria for a concrete mix, use of the newer and easier Surface Resistivity (SR) test as an alternative to the slower, more complicated “Rapid Chloride Ion” test became a desirable option. Most of the test results reported in this thesis were from SR tests.

This thesis reports the results of extensive tests on concrete specimens cast in the lab and in the field from work on the I-40 Bridge over the French Broad River. While the emphasis was on testing with the SR meter, both SR and RCP tests were performed. When evaluating lightweight concrete, several of the variables that affect both of these tests became evident. The specimen saturation, cement brand, curing method, and time of curing all affected the results. The research concluded that lightweight concrete can be as durable as normal weight concrete, but measured values of chloride ion permeability take longer to reach a predetermined acceptance level. The benefit of internal curing helps lightweight curing make up for the porous aggregate. However, this research did raise important questions about the higher quality control needed for lightweight concrete in mixing, placing and finishing. When evaluating lightweight concrete using Surface Resistivity testing, several precautions should be taken to ensure consistent, reliable and accurate results, including the important consideration of proper curing until the time that SR readings are taken.