Anyone in the construction industry knows that building codes exist for a reason. So, if the engineer says that rebar must be placed a specific distance apart in a concrete masonry unit (CMU) wall, the structural integrity of the building depends on contractors following those specifications to the letter.
But, sometimes, unscrupulous contractors looking to save a buck or two decide to ignore the specs and take shortcuts to get the job done cheaper. That’s exactly what we encountered at a Veterans Administration hospital project where our scanning expertise was required to provide rebar verification services to check whether the building’s previously installed rebar was up to code.
Buildings that have concrete foundations and walls are structurally reinforced with rebar, and the size, grade, and placement of these steel reinforcement bars are all critical components of any construction plan. Typically, contractors who install the rebar are required to have their work inspected prior to pouring the concrete. This ensures that the structural elements meet the engineer’s plan and the governing code.
But occasionally, contractors fail to call for rebar inspection, leading to other components of the structure—like concrete and roofing—to be installed over the rebar, effectively hiding it from visual inspection. This is where our GPR expertise comes in.
All projects have specific requirements to ensure the structural integrity of the building, but with a government project, those requirements are not only numerous, but extremely stringent. There is almost no wiggle room when it comes to meeting their specifications.
Rebar verification through ground penetrating radar (GPR), allows us to determine with precision where each steel reinforcement bar is located without affecting the completed work. It’s an immensely valuable service for owners who need to demonstrate that their buildings are up to code without starting over from scratch.
Most CMU buildings have two cells in each wall block, some that are grouted with concrete and rebar, some that are grouted with concrete only, and some that are ungrouted. In our VA project, though, every single cell was grouted with rebar. This made the structure tremendously stable, but there were some issues with rebar placement that made our GPR services necessary.
The government-approved engineering plans for this building mandated that the rebar be spaced out at specific intervals to increase the strength of the CMU walls. It is the mason’s job to place the rebar at the specified intervals, pour the concrete, and ensure the rebar stays spaced correctly throughout the process. Best practice is to use specially designed rebar spacers to keep the rebar in position as the concrete is poured and set.
Whether to save money on spacers or to expedite the job, the mason on this project had elected not to use rebar spacers. Instead, he instructed his workers to move the bars apart as they poured the concrete to make sure the rebar wasn’t touching. This inexact process meant that some bars may have been installed too close together while others may not have been close enough.
But, because the mason didn’t call for concrete inspection before the roof was added, there was no way to visually determine if the work met the engineer’s specs or not. In fact, government inspectors told the contractors they may need to tear the building down and start over to ensure the rebar was installed properly. Needless to say, this warning was not well-received.
Enhanced Scanning was called in to use our GPR equipment to scan the walls and determine if the rebar had been properly spaced. To get the most accurate data possible, we had to use our GPR’s dielectric constant estimates and our own groundtruthing skills to determine the exact depth of a CMU block.
In most cases, the dielectric constant of concrete is about 6 on a scale of 1 to 81, with 1 being airand 81 being water. This number tells us how well an electromagnetic pulse (EMP) will move through a substance and is necessary for precise GPR location. But, since our equipment only provides estimates for the dielectric constant, we have to use direct observation to calibrate the GPR antenna to the exact CMU depth. Known as “groundtruthing,” this skill is vital for ensuring maximum accuracy.
Once we determined the scanning depth, we proceeded to scan the walls while standing on an A-frame ladder instead of our usual scissor lift, which is preferable for safety and efficiency. Unfortunately, a scissor lift wouldn’t fit in our working space, so we had to climb up the ladder, scan as far as we could reach, step down one rung on the ladder, move the GPR antenna to the next section, and repeat the process.
Under normal rebar verification circumstances, owners typically want two or three small sample areas scanned so an overall rebar pattern can be established. This was no ordinary job, though. To avoid the worst case scenario of tearing down the building and starting over, we had to scan numerous large areas of the structure to gather sufficient data to appease the inspector. It was rebar verification on steroids, if you will. Following the meticulous scanning work, we provided a comprehensive report detailing the results to inform critical inspection decisions that would need to be made.
Radargram from GPR scan of the CMU wall
The results of our scan weren’t what the project managers wanted, as they showed that the rebar was not properly spaced in the walls. In fact, the rebar was nearly touching in some places, which confirmed the inspector’s concerns. But, without our technology, thoroughness, and skills, the only other way they could have obtained this information was to tear the building down.
We don’t always know the outcome of our projects, and this is one of those situations. It was our job to provide the most thorough, detailed information possible to help the inspectors make their decisions. While we suspect the two sides were able to come to a resolution that didn’t involve the complete destruction of the building, we don’t really know.
What we do know, though, is that we were able to pinpoint the rebar in the walls without disturbing the concrete, saving everyone involved time and money. It was the most in-depth and time-consuming rebar verification project we’ve ever been involved in, and its success highlights our diligence and GPR expertise.
Whether a project is government-regulated or not, Enhanced Scanning provides thorough and accurate results for rebar verification and other GPR services. Contact us today to get the information you need from GPR scanning to inform crucial decisions about your property.
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