At Penhall Technologies, a division of Penhall company, we recently incorporated a new service into our portfolio, Digital Concrete X-Ray. In summary, digital concrete x-ray provides images of embedded objects in concrete slabs, walls, or ceilings. It is also known as a complementary service to concrete-cutting or concrete-coring projects. Knowing precisely what is in the concrete is beneficial in terms of cost reduction and damage prevention. As a result, with digital concrete x-ray, you can be assured that your project is advancing at minimal level of risk, and most importantly, in the right direction.
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Penhall technicians who perform concrete x-ray service undergo special training and certification. Due to the service nature, our analysts are fully aware of different on-the-job hazards and the safety procedures to be followed. Regardless of the project scope, we always come prepared and ready to take on the challenge.
What is Concrete X-Ray?
Concrete x-ray is the non-destructive application of hard x-rays or gamma rays to image the interior of a concrete target to identify and locate rebar, conduit, post tension cables and other embedded objects. It is considered a branch of industrial radiography. According to the EPA, radiography is useful when you want to avoid damaging the material being tested. Practically the most common targets for concrete x-ray are suspended slabs or concrete walls that may be renovated or retrofitted as part of a larger structure by cutting new openings.
Although cutting through rebar will weaken a structure it can often be achieved safely and within structural tolerance limits. Cutting through post tension cables poses more serious issues and is rarely deliberately executed. Likewise conduit should nearly always be avoided and accidental cuts can necessitate costly repairs, safety concerns and project slowdowns.
At Penhall Technologies, we offer both GPR (Ground Penetrating Radar) scanning and concrete x-ray. In general, both technologies can assist you in locating rebar, detecting what inside the concrete slab, etc. If you are unsure what technologies to use, we can have our account managers or analysts to get in touch with you for consulting purposes. In addition, upon request, we can also provide job walk to help you gain a better insight into the scope of your project.
Difference between concrete X-ray and concrete GPR
X-ray is often considered superior to GPR (ground penetrating radar) for imaging the interior contents of a concrete slab due to the clarity and accuracy the image. X-rays are also inherently easier to interpret. However, in practical field applications, GPR is a more common approach. X-ray imaging will always require access to both sides of the concrete target – so a slab on grade concrete target cannot use x-ray at all. Learn More about the difference between the 2 services.
How Concrete X-ray Works
An x-ray image is essentially a shadow or a projection of the density of objects that are targeted. As x-rays strike a target, the photons will pass unimpeded through the softer less dense material, but will scatter or be absorbed by denser material. Steel will absorb more energy that concrete – leading to less x-rays hitting the detector directly in the straight-line from the emitting source to the detector. In effect a shadow is cast and recorded. As negatives are commonly examined rebar will show as a lighter patch (the inverse) – although with digital imaging it is very easy to reverse the contrast and show rebar and denser materials as darker. With film a chemical reaction occurs when the x-rays meet the film surface – and with DDA’s an electrical change is generated, which can be captured and quantified.
Taking X-ray Images of Concrete
A typical concrete x-ray set up requires 2 operators – one for the detector and one for the source. While capturing the concrete imaging, the 2 operators are on different floors – each making sure that the exclusion zone is clearly marked and no accidental intrusions occur. The detector is most often on the upper floor facing down while the x-ray source is below facing up, although an inverted setup is also possible. It is important to line up the detector with the source to ensure the emitted rays are directly striking the detector (perpendicular) and not coming from an angle. Complete remote control is possible, or the source operator will manually switch on the x-ray beam first (assuming a x-ray tube is used) and then call the detector operator to switch open the panel and take an image. When isotopes are used the source operator cranks out the isotope from its secure container exposing it the air and effectively firing gamma rays towards the detector.
Safety measures are vital and necessitate both operators to use a combination of survey meters and dosimeters to read the volume of radiation in their specific location and enforcing exclusion zones. Governing bodies set limits for exposure at a national and state level.
After an exposure the film is either developed in a dark room, or if a detector panel is used the image is captured immediately and displayed on a computer screen for enhancement and editing.
Accuracy of X-ray Images of Concrete
X-rays are part of the electromagnetic spectrum (the same as visible light) and travel in straight lines through vacuums (for this purpose air behaves much like a vacuum). As they are generated from a source they spread out (like light from a bulb) and the intensity naturally decreases with distance from the source. The further away the source is from the target the weaker the signal (it follows an inverse square rule). For much of industrial radiography being close to the target poses no problems – particularly when the purpose is to find the presence, or identity of an object rather than it’s position or measurement. With concrete x-ray the customer is usually looking to make an alteration by a core or cut and requires x-ray imaging to avoid hitting an embedded object. In this case, it is actually better to place the x-ray source further from the target to ensure that the wave front hitting the target is as close as possible to being parallel to the target’s surface and the direction of x-ray travel is perpendicular. The advantage of this set up is that it minimizes dilation of the image from the center point – although some dilation is inevitable and cannot be avoided. The simple way to imagine this effect is to visualize your own shadow created by a point light source near to you, versus one created by rays originating from far away (such as the sun’s rays).
Countering this objective is the inverse square fall off in intensity – so taking an image too far away becomes impractical. It may not be possible to expose the film or detector to enough radiation to render any image and the longer exposure will disperse more radiation increasing the danger. So with concrete x-ray there is a need to strike a balance in the position of the source with respect to the detector.
Roughly – for an image taken from 8” to 9” away there will be 5% to 10% dilation from the center of the image. The actual dilation will depend on the height/depth of the object in the concrete. With digital x-ray and image editing it is possible to make some correction for the dilation – but without precise knowledge of the exact depth of the object the correction can only be based on an assumption and is imprecise. So the safest approach when using an x-ray image to determine position for cutting is to leave a safe zone around each observed object. Most operators suggest a 2” zone around an object and a 2” zone around the image perimeter.
Cost of Concrete X-ray
Concrete x-ray is more expensive than GPR. The equipment is more expensive and a minimum of 2 operators is needed at all times. As a rule of thumb, concrete x-ray will cost double the hourly rate of GPR – although pricing by image may be more attractive for both parties. $100 per image is fairly standard, combined with a mobilization charge of several hundred dollars. Night and weekend work is standard which can lead to overtime rates too.
X-ray Dosage – Safety Guidelines
The standard unit for measuring exposure is the Sievert, which equates to an effective joule of energy dissipated in a kilogram (S.I. units). Sieverts differ from the older unit of grays by an adjustment to make the dosage the biologically effective equivalent amount. Natural background radiation varies by location but roughly equates to 3 mSv a year (0.003 Sv per year). A total dose of 1 Sievert is estimated to increase the risk of developing cancer by 5%. Based on purely background radiation (mostly from naturally occurring radon gas) that would take over 300 years to achieve. Occupational dose limits are set by the United States Nuclear Regulatory Commission (USNRC) at 0.05 Sv per year.
Protection from radiation is an important aspect of industrial radiography. The International Atomic Energy Authority (IAEA) had developed basic safety guidelines regarding the practice.
- Ensure equipment is in good working condition before using it
- Post warning signs around work area to prevent accidental exposure
- Before turning on equipment, confirm no people are in work area
- Audibly signal that the source is going to be exposed
- Confirm the source is turned off and no longer emitting radiation before allowing anyone to access the work area
Objects exposed during testing do not retain radiation and can be immediately handled when the testing is complete.