A New Test to Address Problems with Concrete Bridge Deck Overlays

This paper describes how concrete bridge deck overlays have experienced problems in Oklahoma and elsewhere throughout the country. While overlays are designed to extend the life of a concrete bridge deck by 20 years or more, it has been observed that many recent overlays are exhibiting poor bonding to the existing concrete bridge deck, as confirmed by cracking and delaminating problems. It is believed that there are about 20 new overlay projects every year in Oklahoma, and each typically costs about $220/m2 ($20/ft2). Many overlays are requiring repairs within a short period of time, sometimes as little as one month, which adds to the expense. The precise cause of the distresses to the overlays is unknown. Possible causes include weaknesses or defects in the existing concrete or inadequate surface preparation prior to placement of the overlay. A study will be underway soon to examine concrete bridge deck overlays that have been built since 2000 to gain an understanding of why some recent overlays perform well and why others experience early bond problems. The study will involve the collection of extensive data on concrete bridge deck overlays to identify any variables that may be causing poor bond between the new and old concrete. Information will include bridge location, type of bridge structure, mean daily traffic date of original bridge construction, date of overly construction, cost of the overlay, overlay construction procedures, traffic control during overlay construction, surface preparation prior to placement of the overlay, overlay thickness, concrete mixture design, concrete curing procedures, fresh and hardened concrete properties, repairs since the overlay was built, and present condition of the overlay. A bond test will be performed on these bridges to provide an assessment of the bond between the new and old concrete, and also the quality of the new and old concrete. The bond test, also called the pull off test, became an ASTM specification in 2004 (ASTM C 1583), and it can be performed on concrete bridge decks prior to placement of an overlay as well as after placement of an overlay. The test is minimally destructive, as it involves attaching a small disk to the concrete surface with epoxy, drilling a partial core the same diameter as the disk into the concrete surface, and then applying a force to pull off the disk and a quantity of concrete attached to the disk. Concrete that has a high permeability and a high content of chlorides will likely have a low bond test value. Concrete that was somehow damaged during surface preparation will also likely have a low bond test value. There may be a high correlation between overlay performance and bond test data. It may be found that there is a minimum bond value that is necessary for adequate durability of the concrete bridge deck overlay. A bond test performed prior to placement of an overlay may be useful as a guide to the kind of bridge deck repair that is necessary. If the existing concrete does not meet a minimum bond value, then it is likely that additional depth should be removed from the bridge deck to reach sound concrete or an alternative surface preparation should be employee prior to placement of the overlay. Very low bond values may indicate that a full depth bridge deck replacement is necessary. Preliminary studies were conducted to examine two brands of bond test instruments, several different brands and types of epoxy for attaching disks to concrete, and disk size, both 50 mm (2 in) and 75 mm (3 in) diameters. Though ASTM C 1583 specifies 50 mm (2 in) disks, the larger 75 mm (3 in) disks produced less variability in the test results.