GPRS vs. Competitors
We received a call this week from a customer who was concerned about piers that had been recently poured. There was concern over whether the rebar inside the piers had been placed correctly, or had perhaps shifted during the pour. The customer needed to know the depth of the horizontal rebar and the distance from each bar to the top of the pier. Because of concerns over who was at fault in the situation, another party had already had the piers scanned by their own GPR company, which will go unnamed. Keep reading to see how our results compared with the competitor’s results.
1. The Age of the Concrete
The concrete was relatively fresh, only a few weeks old, so the concrete had very low GPR visibility. Concrete needs to age for about a year to achieve full GPR visibility, though decent visibility is achieved after a few months. Concrete that is less than a month old has very low visibility because some moisture is still trapped inside.
2. Surface irregularity.
The side of the pier was not smooth. It was very rough, so the GPR could not easily pass along the surface. There are two issues with rough surfaces. The first is that the GPR needs to remain coupled to the surface for the best signal. If the antenna lifts off the surface more than a quarter inch, data quality is reduced. The second issue is that the survey wheel cannot track well, and thus it is easy to lose track of where we are in the data.
The competitor had previously scanned the piers and marked them with black markings. To distinguish our results, we marked in red.
We quickly realized that the slab would be difficult to scan with the 1600 MHz antenna, which is the standard GPR antenna in the industry. The 1600 MHz antenna is large and has four wheels, so it is difficult to keep the antenna flush to the surface, and the frequency does not work very well on moist concrete.
Instead, we switched to the 2GHz antenna, which is smaller, with only one larger survey wheel. It is much easier to keep the palm antenna flush to the surface, and the higher frequency can also see deeper through moist slabs.
Scanning the piers, we confirmed the competitor’s markings (though our markings were slightly different from theirs), and found numerous additional bars that were apparently too deep for them to locate (see photo). The bars they had missed were critical, and would have resulted in one or more piers being ripped out or repaired unnecessarily. We spoke to the general contractor and the picture became more clear. The GC had spoken to the previous scanners and he said they had used the same base GPR unit, only without the 2GHz palm antenna attachment. The technicians had actually commented during that locate that they lacked the attachment.
It suddenly made sense why our markings were different from theirs. They had been using a 4-wheeled unit, so their survey wheel would not have tracked along the surface as well. It also made sense that they had missed the deeper rebar, since they were using a different frequency.
The Bigger Issue
One final comment by the general contractor was revelatory. He said that the competitor had claimed their unit was effective to 12"-18", but they had in fact missed rebar at 6". Missing that rebar was not in itself surprising, since the slab was so moist, but it was surprising to hear that they had not warned the GC about their reduced effective depth, owing to the newness of the concrete. They had left the GC with the impression that they saw to 12" and that there was no rebar to that depth. In fact, they should have known that their effective depth was much less. Every technician is limited to what his GPR unit is capable of, and different frequency antennas all have pros and cons. But communicating 12" as the effective depth was an avoidable mistake that almost had costly implications. Luckily, we found the extra rebar, saved two of the piers from unnecessary repairs, and corrected the misunderstanding about the effective depth of the competitor’s device to about 4.5" (they missed all rebar deeper than about 4.5") and we explained that our own effective depth was about 6"
GPRS has equipped all of our technicians with both 1600 MHz antennas and 2 GHz palm antennas. So when you call us for a scan, we use our extensive training to assess the right antenna for each job.
To get a quote, call us at 214.471.9001, download our app, or visit www.gprsinc.com
GPRS does not provide geophysical, geological, land surveying or engineering services. If you need such services, please contact an appropriate professional.