Post-Tension Cable Characteristics
Example of Conquest GPR 3D visualization of rebar and post-tensions cables embedded in a concrete slab.
Overview
Post-tension (PT) cables are used in concrete construction to allow thinner slabs and greater span lengths between support columns. These cables, composed of steel wires inserted into a plastic sheath, are subsequently "tensioned" and grouted after the concrete is poured.
Problem
Contractors want to avoid hitting PT cables when drilling or cutting for rehabilitation or renovation. The tremendous cable tension makes cutting a cable dangerous for the operator and compromises of the structure's integrity. GPR can readily locate embedded metal structure; identifying PT cables when a rebar mat is present is challenging.
GPR Contribution to Solution
Unique aspects of PT cables allow GPR imaging to identify these features with more certainty. A prior understanding of construction practice is extremely helpful. The following are two common features of PT cables:
- They droop lower in the slab between support columns
- The cables are often bundled together in a grouping
In GPR images, thes attributes indicate patterns to look for. First, if there is a systematic cable grouping, looking for that pattern in the GPR image is highly diagnostic. Second, drooping cables will appear at different locations on GPR depth images from the adjacent depths. Scanning multiple depth slices looking for features that plunge or rise is diagnostic.
Conquests multiple depth image displays are specificaly designed for the field operator to quickly look for these characterstics on site.
A typcial group pattern from PTcables bundled together in a grouping.
GPR cross section collected along a traverse directly over a PT cables in a concrete deck structure showing the PT cable drooping between support columns
Discerning a post-tension cable from rebar generally requires scanning a larger area to properly understand the layout of structural elements in a slab.
In the below example a 2'x 8' Conquest grid scan captured several cables in one grid.
3” Depth Slice showing the rebar.
8” Depth Slice showing the PT cables.
When in doubt, expanding the survey area grid size to get the big picture always helps.
Results & Benefits
This case study shows how understanding the structure can enhance the diagnostic value of GPR for concrete imaging. Some key benefits are:
- Grid imaging is key to seeing the unique attributes of PT cables.
- Real-time depth imaging is essential for cost-effective site evaluation
- Conquest provides a compact, portable easy-to-use on site imaging capability. Operation is simple and intuitive
- Interactive viewing of depth slices is highly beneficial
- marking out a target and profiling along its axis can define the droop pattern unique to PT cables.
- Imaging over larger areas to see patterns and trends is a best-practice.
GPR responses vary greatly depending on the target being sought and the host material. GPR response variability can be challenging to new GPR users. When learning about GPR, the best practice is to review several similar case studies to develop an understanding of variability. Check for other insightful information on the resources tab to learn more. Use Contact Us or Ask-the-Expert to reach our Application Specialists who can help you tap into Sensors & Software's vast array of technical information.