Electrical resistivity measurements to determine the steel fiber content of concrete

Although steel fiber reinforced concrete is becoming increasingly popular in the construction industry, its application is currently limited to certain areas. One reason is the lack of an economical non‐destructive testing method to determine the content, distribution, and orientation of steel fibers in fresh and hardened concrete. However, these parameters are decisive for the assessment of the static performance of a building component. In this article a new test method is proposed, which is based on the electrical conductivity of concrete. Using a two‐electrode experimental set‐up, measurements of the electrical conductivity were done on hardened concrete cubes with defined fiber content and a correlation between electrical conductivity and fiber content could be identified. Within the scope of extensive test series, the fiber content and the age of specimens were varied, and several identical series were produced and observed to ensure statistically verified results. At this stage of research the test method based on electrical conductivity measurement provides reliable results on the fiber content of preconditioned concrete cubes. Based on the results a new model was developed to quantify the fiber content of the examined cubes. This model is based on the estimation of the relative conductivity, the so‐called increase of conductivity. Undesired influences caused by aging, respectively hydration in the young concrete age were eliminated, by relating the conductivity of fiber reinforced concrete to an unreinforced concrete. Further investigations with focus on the influence of concrete composition and other specimen sizes are going to be conducted as next steps for the development of an in situ test setup for the diagnosis of concrete structures.