Well, it’s as simple as it sounds. In order to identify areas of hollow sounding delaminated concrete, a chain is dragged across the surface of the concrete. An inspector, who drags the chain, delineates these hollow areas in order to map out the limits of each delamination, typically for future repairs. This practice is historically considered as the accepted practice for assessing the condition of concrete bridge decks without asphalt overlays. With some training, chain drag can be performed by most able-bodied workers.
However, there are drawbacks to this traditional means of concrete evaluation. In the application of bridge deck evaluation, chain drag testing requires a lane closure and disruption of normal traffic patterns. Depending on the size of the deck and the number of inspectors performing chain drag, these closures could last for a significant amount of time. Lane closures are expensive and can cause major traffic delays.
There is also the question of the accuracy and repeatability of chain drag. In 2001, the Federal Highway Administration (FHWA) published a report titled “Reliability of Visual Inspection for Highway Bridges, Volume I: Final Report”, in which the reliability of common Routine Inspections and In-Depth Inspections of highway structures was evaluated. As part of this study, 22 different teams performed an In-Depth Inspection including delamination survey by the chain drag method on the same bridge deck. The results of these In-Depth inspections are quite revealing, and illustrate just how subjective and imperfect the chain drag method can be for evaluating the condition of a bridge deck.
The Nondestructive Evaluation (NDE) Validation Center at the FHWA’s research center, the Turner Fairbank Highway Research Center (TFHRC), provided an estimate of the bridge deck delamination at 19% of the inspected deck area, which was then validated with core samples. Of the 22 teams that performed In-Depth Inspections, 20 used the chain drag method, and only 5 teams detected delamination quantities within 5 percentage points of the TFHRC estimate (i.e., between 14% and 24% of the inspected deck area).
A range of factors was considered by the study when comparing the results of these chain drag surveys. One such factor was the experience level of each inspection team. The study noted that 16 teams had at least marginal experience performing chain drag surveys, and 7 teams had little or no experience performing chain drag surveys. The average results for the experienced teams was 14% delamination quantity, and the average results for the inexperienced teams was 10% delamination quantity, illustrating the effect that experience can have on the results of a chain drag survey.
MORE TO CONSIDER
Other factors were recorded and considered in the study, including the ambient temperature during the inspection, the time of day of the inspection, and the duration of the inspection. All these factors can lead to subjectivity in the survey and variations in the results. Other factors, such as traffic and noise levels, can change from bridge to bridge and affect the results of chain drag surveys. Finally, the individual inspector’s hearing range can play an important role in the reliability and repeatability of the results as those with poor hearing could miss areas of delamination that others may detect.
The FHWA reliability study indicates that the chain drag method is a relatively simple method for detecting delaminations and requires minimal training, but the inspections themselves can require considerable time and resources in the form of lane closures and traffic delays, and the results can be subject to considerable variation and inaccuracy. Other NDE techniques offer more efficient surveys with more objective and accurate data and results.
CLICK TO LEARN ABOUT THESE ALTERNATIVE INSPECTION METHODS
infrasense.com/bridge-deck-scanning
Chain Drag 