Fellows,
I need to interpret GPR 2D sections processed by using Reflex-2D. the scope of work is to detect air gaps or voids under the reinforced concrete slab. Please share your ideas and attach some papers regarding this.
Thank you Thomas Sir!
Have a look an attached sections on which I've picked anomalies. Survey ground is made up of concrete slab of 5 by 5m and the slab joints are visible on the section. There are small hyperbolic reflections within the slab below the surface joints at 0.25m depth. These feature seems me as an air gap or small voids.
Add your suggestions!
You will need to understand the construction. You have highlighted the base of the slab at 350-400mm. This may actually be the bottom of a subbase layer. It is not unusual to have a roller compacted concrete subbase below a concrete pavement. I suspect the bottom of the slab is more likely at approximately 270mm (assuming you are using the correct velocity for concrete). You may want to try using a migration reduce the signal effects from the reinforcement. This should help to show up construction layering and voiding below the slab in some cases. Depending on the width of the joints it is not uncommon to get edge effects from the bottom of the joints. What you need to look out for is variations in amplitude or width of the hyperbolas which may indicate voiding. If they are all relatively consistant then it is unlikely that they are all voids. Some of the small scale voids you highlighted are more likely to be steel bars near the bottom of the slab as appear higher up. It is possible that they may be tie bars adjacent a longitudinal joint although they are much lower than expected. The sub-slab void indicated may be voiding. However, it may equally well be a slab reconstruction material change, a culvert or high moisture within the subbase. It is easy to jump to conclusions and therefore it is always prudent to carry out some physical ground truthing to determine the true nature of signal anomalies. Highlight your most likely candidates for further investigation. There may also be vissible indications of voiding such pumping, rocking, stepping, spalling or cracking which I assume is the reason behind this investigation.
Hi,
When I check your GPR profiles, there are some anomalies you indicated as void; but I think they don’t represent the void/s. For example, the anomaly located at around 16 m of distance of your first GPR profile seems to me as ringing because it begins almost from zero and oscillate through time or y-axis. This would be caused metallic object on the surface or very close to the surface.
Your data sets are very crowded with especially rebar reflections that can hide some reflections, may cause hardness. If you have a chance to conduct ground-truth survey, please do it.
Also, as we know, the modeling could be the other option to analyze the reflections. The peaks of the reflected signals could be used to characterize the reflecting surfaces. For example, set some objects such as voids, water filled voids and metallic objects. Then pick individual signals corresponding to each object and check the first peaks of the reflections or (polarity). The long and the short of what I am talking, please run “Trace Analysis” using synthetic data sets. This could help you to define and characterize the reflections observed on your field data sets.
It is a long time for me to not studying with Reflex; but, as far as I remember, the software covers phase follower option that you can use to interpret your data.
Kind Regards
Thank You all for your valuable answers!
Dear Val Sir and Aycan ma'am,
I haven't done any ground truth survey yet. Could you please share some papers regarding 'ground-truth survey' and 'Trace analysis' so that I can get an idea to conduct this survey.
Regards
Pranjal
Hi;
Below are some references I would like to share. Please check GPR radargrams included in the references, especially interpreted radargrams. I think they could give some ideas that will help in your work, Sir.
In addition to the basic data processing steps such as background removal, I would like to suggest you to apply de-convolution and migration to reduce your data. It seems to me both data processing steps would make your data more interpretable.
I hope the references given below could be helpful
Kind Regards
REFERENCES
Xu, X., Xia, T., Venkatachalam, A., 2012. Development of high-speed ultrawideband ground-penetrating radar for rebar detection. J. Eng. Mech. 139, 272–285. doi:http://dx.doi.org/10.1061/(ASCE)EM.1943-7889.0000458
Yehia, S., Abudayyeh, O., Abdel-Qader, I., Zalt, A., 2008. Ground-Penetrating Radar, Chain Drag, and Ground Truth: Correlation of Bridge Deck Assessment Data. Transp. Res. Rec. J. Transp. Res. Board 2044, 39–50. doi:10.3141/2044-05
URL 1 https://fhwaapps.fhwa.dot.gov/ndep/DisplayTechnology.aspx?tech_id=24
URL 2 http://www.ijser.org/paper/Thickness-evaluation-of-asphalt-and-base-layers-of-some-major-and-minor-arterial-roads-in-Kumasi-using-ground-penetrating-radar.html
URL 3 http://foundationperformance.org/pastpresentations/gehrig_paper_march2004.pdf
URL 4 http://ntl.bts.gov/lib/43000/43000/43073/134431_FR.pdf
URL 5 http://www.sciencedirect.com/science/article/pii/S0963869504001021
URL 6 http://www.mdpi.com/1424-8220/9/3/1454/htm
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