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.
Typically the x-ray source is an isotope, usually iridium-192 or cobalt-60, or the x-rays can be generated by an x-ray tube. The detector is either film or a digital detector panel. Concrete x-ray is usually practiced in the field as the objective is to reveal the contents of a concrete target without moving or harming the target. Practically the most common targets 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.
Concrete x-ray is frequently specified by structural engineers to give the clearest indication of hidden objects and hazards before cutting commences on a 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. X-ray also needs at least 2 operators – effectively doubling the cost due to the need to safely observe and cordon off 2 areas separated by the concrete target. Frequently the 2 areas will be separate floors with a suspended slab between them as the target. While GPR uses radar – part of the electromagnetic spectrum – the wavelength is comparatively long meaning the energy is low. A cell phone emits greater energy and when using GPR no exclusion zone is required. By contrast x-rays have a much, much shorter wavelength and exposure must be minimized. The dangers are considered cumulative – so limits are typically set both by rate (amount per hour) and also in aggregate. Jobsites are evacuated to the limits of a preset exclusion zone when using x-ray – so practically most field work is conducted at night or at weekends – particularly when the jobsite is a still an active, occupied building during normal work hours.
How Concrete X-ray Works
An x-ray image is essentially a shadow or 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. In a human body, bones contain calcium that acts a comparatively stronger absorber than muscle, fat and flesh. Fewer x-ray photons passes through the bones so leading to a pattern of higher and lower exposure on the film – and thus an effective image. The same principle applies to concrete x-ray – although both concrete and metal (usually steel) are much denser that the human body the relative difference in absorption enables an image to be formed. 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-rays Images of Concrete
A typical set up requires 2 operators – one for the detector and one for the source. When a floor or ceiling is imaged 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.
Industrial X-ray Sources
Industrial x-ray sources are drawn either from radioactive isotopes or from an x-ray tube. The most common isotopes used are iridium-192 and cobalt-60. Iridium is weaker, meaning less ability to penetrate thicker concrete. It also has a shorter half life (74 days) than cobalt (5 years) and so must be safely deposed of and replenished more frequently. Every sample decays exponentially so with any isotope the emitting volume declines with age. With isotopes the emitted radiation is often called gamma radiation rather than x-ray radiation – but the distinction can be somewhat arbitrary. The term gamma is more commonly used with active isotopes and associated with shorter radiation wavelengths and higher energy photons. The energy of each photon once emitted will not decline with age of the sample – rather the volume of emitted photons. Individual photon energy is correlated with penetrating power – the ability to go through thicker and denser samples. However with a weak volume image quality is poor. An analogy would be taking a photographic image in very dark conditions. Once radioactive, an isotope cannot be ‘switched off’ – meaning it will emit indefinitely (although with a declining volume). Both iridium and cobalt are considered highly dangerous and consequently must be kept in very secure and protected locations. When not used, such isotopes are housed in a chamber, or box made from very dense and radiation absorbing materials. Lead and tungsten are frequently used to shield the source and with cobalt a depleted uranium box is used. Uranium is 19 times more dense than water so a simple 1’ x 1’ box can weigh over 350 lbs. The actual radioactive sample is surprisingly small – around the size of a grain of rice. Oddly the storage box is frequently referred to as the “camera” – although it contains the emitting source rather than the film. To expose the sample the “camera” has a mechanical snake, operated at a safe distance to push and pull the small isotopes in and out of the containment box.
X-rays can also be generated by an electronic tube, which fires high-energy electrons into a target (molybdenum with graphite, or a tungsten alloy) which in turn then emits x-rays. The electrons are fired at high voltage, over 100 keV and sometimes higher. While much larger and more powerful tubes are technically possible, they become unwieldy and impractical in the field so a 300 keV limit is the norm. Such a tube is about 30” tall and weighs around 70 lbs. X-ray tubes will always require an electrical supply, but most field tubes can use a standard mains supply and step the voltage up to the required levels. One great advantage of the x-ray tube is that when it is not switched on it cannot emit any radiation – significantly improving safety over an isotope. Consequently an x-ray tube does not need to be shielded when it is switched off – making storage and transportation much easier and safer.
Comet X-ray Source
Penhall uses the Comet PXS EVO 300D tube as the source for x-rays. This is an air-cooled model capable of producing 300 keV rays at 3.0 mA for a continuous hour. However such a long exposure time is rarely if ever used as when the tube is paired with a digital detector an 8 inch concrete slab can be imaged in as little as 10 seconds. The focal spot is 3mm – which is ideal for imaging objects embedded in concrete. (A smaller focal spot of 1mm with a weaker output is better suited to examining welds and measuring thin fractures). This tube was designed specifically for fieldwork, for example pipelines and refinery work as well as general construction sites. The x-ray beam is emitted at a 40 degrees by 60 degrees spread although collimators can refine this. The Comet tube is combined with a digital control unit that can be positioned a safe distance from the source, or operated with a delay.
Digital X-ray Versus Analog X-ray
Just as with standard photography the x-ray imaging industry is moving away from analog imaging to digital platforms. Digital refers not to the source but the detecting medium. Most technological and engineering development with digital detection came first through medical research and then became ruggedized to work effectively in the oil and gas industry. Analog x-ray really refers to the use of traditional x-ray film – with a standard 14” x 17” template. X-ray film suffers from being single use and is actually not very sensitive, requiring long exposure times, even when using high energy sources such as cobalt. A 10” slab can require an exposure time of 15 to 20 minutes and a 12’ slab can require 30 minutes. A rough rule of thumb is that for every 2” of extra concrete thickness the exposure time is doubled. Analog film also requires chemical development. At a jobsite a mobile darkroom is required, so film is taken from the target to the darkroom and another 20 to 25 minutes of development is needed. Unfortunately any mistakes require the whole process to start again from the beginning. After development the x-ray film is kept as the negative and then tracing paper overlaid to sketch out embedded objects for a customer. As a negative image – a dense object like rebar will actually show as a light object. A void will show as a dark object. Concrete itself absorbs x-rays – so the gray scale is relative – and plain concrete will show as a medium gray, depending on the film development time.
Digital detectors fall into 2 distinct groups. Computed radiography using a reusable imaging plate made with photo stimulated phosphor or digital detector arrays which are solid state and also reusable. In computed radiography the imaging plate is read by a scanner, creating a digital image, which is then stored and manipulated on a computer. The plate is then reset so it can be used again. Scanning only takes about 30 seconds but requires a very sophisticated specialized scanner. Computed imaging plates are about 2x as sensitive as film – effectively halving the exposure time.
Digital Detector Arrays (DDAs) are usually the same size as standard x-ray film (14” x 17”) and about 0.5” thick. Currently they are relatively expensive and made in small numbers – so their use in industrial settings is limited. However, from a performance perspective DDA’s are clearly superior to any other detecting medium. Their advantage comes from their very high sensitivity – around 50x more than film, which in turn can lead to much quicker exposure times (and in turn much less radiation being emitted). DDA’s will also generate and communicate images to computers within seconds, giving near real time imaging. Digital detector panels do not need to be scanned and newer versions simply transmit data wirelessly over several hundred feet of range. Smaller plate formats are also made which can be used for confined space imaging or when mobility and weight are critical. When paired with a small x-ray tube (a pulse operated version is very lightweight) the entire set-up can be carried in a backpack. Mobile security is another application such as x-raying suspicious packages without having to touch or move them.
NOVO X-ray detector
Penhall uses the NOVO-DR digital detector panel with a 22” diagonal plate. The detector is shielded and ruggedized for field applications and come with military spec connectors. The images created are 16 bit and when enlarged can show details not possible with traditional film. The DDA works with both x-ray and gamma rays and so can be paired with a range of sources. In addition the combination of the DDA with an x-ray tube such as the Comet 300D is the safest set-up for concrete x-ray as it minimizes the radiation necessary at a jobsite. The system also uses specific imaging software to take a low contrast image and apply differential leveling and gain to enhance the quality to a more recognizable display.
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.