Figures 1-5

Introduction
Radiographic testing of concrete slab floors in multistory buildings can provide valuable information for building renovation and engineering evaluation projects. A typical renovation project will require holes cored through abovegrade floors for connections to utility services, duct work, communication lines and anchor bolts. In an engineering evaluation, concrete reinforcement size and placement information may be required to establish floor load carrying capacity. Radiographic testing can reveal the internal features of abovegrade floor slabs in order to avoid severing post tension cable and electrical conduits in core cutting operations. These items may be critical to current building tenants and require expensive repairs if damaged. Repairs for post tension cable can cost tens of thousands of dollars. Electrical service interruptions could be costly in terms of lost work time due to downed computers or office lighting. Post tension cable may, when cut, pop out of floors
and cause injuries. Coring personnel have been shocked when live wires in electrical conduits were cut through. Radiographic testing is applicable in preempting costly litigations that could arise from such accidents. In keeping with the standardization trend of the NDT industry, the development of defensible techniques for radiographic testing of abovegrade concrete floors is indicated. Independent NDT labs offering this service should consider treating this subject as seriously as any code radiography æ in other words, with the highest degree of integrity. It is hoped that the information presented here will advance this highly visible application of radiographic testing.

---

Radiographic testing of concrete slab floors can provide invaluable information in building renovation. 

 

---

Safety Considerations
No discussion of radiographic testing, especially when it is to be performed in public areas, should begin without a thorough treatment of safety considerations. Radiographic testing operations must, by law, adhere to the radiation licensee's operating and emergency procedures deriving from the agreement state agency or the Nuclear Regulatory Commission rules. It may be advisable to include a section in these procedures dealing expressly with concrete radiography due to the potential proximity of the public. The safety goal is the limitation of the radiation dose to levels as low as reasonably achievable. Radiographic testing crews must be especially vigilant in their safe operating practices in public environments. Radiographic testing to be performed in specific locations may require special security measures. Examples of such locations are hotel lobbies, schools, hospitals and airports. Some sites will require additional personnel beyond the mandatory two person radiographic testing crew posted to guard the entrances of restricted areas.

Faced with a job at a highly vulnerable site, a useful strategy is to call the Nuclear Regulatory Commission or the agreement state agency having jurisdiction over the operation and request a site audit. Agency audits can benefit radiographers by documenting their adherence to regulations. These agencies gain valuable experience when responding to such radiographic testing licensee requests. The firsthand information gained by the agency may avert a public relations disaster or lawsuit and cooperation between regulatory agencies and their licensees boosts the credibility of the radiographic testing community. In planning radiographic testing operations, the public must always be considered first. Proper protocol demands routing notifications of scheduled radiographic testing operations to all interested parties through building management. Tenants of spaces to be entered for source or film placement or radiation surveys need to be informed well in advance of the work. After hours security and cleaning workers must also be informed of the radiographic testing schedule. Besides these notifications, agency approved barricades should be placed as necessary and guards stationed at strategic entry points to prevent unauthorized persons from entering restricted radiation areas. Mistakenly assuming that everyone has been informed and will cooperate can lead to extremely compromising situations.

Members of the public must be guaranteed unrestricted areas, where dosage levels do not exceed regulatory limits. It is prudent to assume that minors may be in the population when considering these dosage limits. If a space near a radiographic testing operation is not surveyed, the radiographer is thereby assuming that the radiation levels will be below regulatory limits. Of course, the agencies require that the radiographers perform physical radiation surveys of restricted area boundaries. Radiation levels in office buildings can be especially unpredictable compared to more familiar industrial sites. Authorized security personnel should be enlisted to ensure that relevant areas
are truly unoccupied. Assuming that any space is vacant without personal verification by the radiographer in charge can become a regrettable error. Only authorized radiographic testing personnel should have access to radiation areas. Under normal circumstances, everyone should get permission from the designated radiographer in charge before entering posted radiation areas. Radiographic testing personnel, besides their usual array of dosimetry and survey meters, should ideally be equipped with twoway radios. Instantaneous communication is essential to safe radiographic testing operations where personnel are typically out of each other's sight. The cost of the radios will be negligible compared to the enhanced safety and security they afford. Placement of radiographic testing personnel and control of the public aside, proper source exposure techniques can limit the radiation field to a great degree. Agency regulations specify that a collimator be used. To further attenuate extraneous radiation, lead shotbags may be packed around a collimator placed on a suitably stout stand. This extra shielding isn't always necessary but can be very useful when needed. The majority of the techniques presented here assume placing the source above and the film below the floor under investigation. The utility of this arrangement will become apparent later as the need for surface markers is developed. Film above/source below techniques are quite possible, though perhaps more cumbersome.

 

Radiation Source Selection
The radiation source used for a particular job should ideally be selected on the basis of the ability to complete a properly set up exposure as quickly as possible. Safety, concrete thickness, size of cores, number of planned exposures, site access and overall costs are also factors involved in radiation source selection. Two isotope radiation sources are used for the bulk of commercial radiographic testing, Ir-192 and Co-60. Ir-192 can handle up to 203 mm (8 in.) thickness, while Co-60 can handle up to 406 mm (16 in.). Co-60 may, of course, be used on thinner floors, especially where there are many exposures to be made. Site access may limit a radiographer's isotope choices. An Ir-192 exposure device weighing less than 22 kg (50 lb) may be hand carried, while a Co-60 device may weigh 222 kg (500 lb). Co-60 costs are higher than those for Ir-192. However, the greater energy and radiation output of Co-60 over Ir-192 may reduce costs in cases where many exposures are planned or the concrete thickness makes Ir-192 exposures excessively long. Co-60 can typically be setup at much greater source to film distances than Ir-192, which can reduce image distortion.

 

Distortion in Concrete Coring
Two factors creating undesirable distortion in concrete coring radiography are image enlargement and shift. A reinforcing bar close to the sourceside of a floor will appear enlarged relative to one closer to the filmside. Objects farther from the center of the exposure will appear shifted from their true locations. These effects trouble many radiographers because of the unfamiliarly thick sections encountered in concrete structures. However, confidence may be gained through the use of readily understandable geometry and consistent practice in proper techniques. The geometry of distortion demands that a larger diameter core or thicker concrete require a greater source to film distance. In some instances, a larger film area, supplied by multiple cassettes, can substitute for source to film distances. A greater source to film distance will always minimize distortion. Thinner concrete will produce a less distorted image for a given exposure setup. When planning exposures, there are certain factors and issues to be kept in mind. Typical exposure geometry is illustrated in Figure 1.

 

Calculating Core Diameter
Radiation proceeds through the floor in a cone shaped pattern as indicated in Figure 1. The central ray is perpendicular to the floor and the film plane. An area of the floor surface is thus enlarged onto the film below. Note that the size of the radiograph does not directly indicate the area of the floor surface imaged. This discrepancy must be dealt with or assumptions will be made about the contents (or lack thereof) in areas not imaged by the radiograph.

Concrete thickness, source to object distance and total source to film distance must be factored into a calculation that will predict the maximum core diameter imaged by a given film size. This sourceside surface area may be termed the actual film size. An equation is used to find the actual film size that a given technique will yield. A correction factor, which can be used to calculate the dimensions of floor surface area imaged, is found as follows: source to object distance divided by the total source to film distance equals the correction factor. The correction factor multiplied times a film dimension, length or width, yields the actual floor surface imaged in that direction. Hence, if one multiplies the film length by the correction factor, then multiplies the film width by the correction factor, then takes the products of both equations and multiplies them, one can determine the actual film size. Correcting to the actual film size yields the length and width of sourceside floor surface imaged. The proper source to film distance will produce an image comfortably containing an anticipated core diameter and allow for minor film misalignment in a reasonably short exposure time. This area will always be smaller than the dimensions of the film in the cassette. This process is illustrated in Figure 2.

Here, the resulting actual film size equals approximately 295 by 359 mm (11.6 by 14.1 in.). With the technique illustrated above, a 203 mm (8 in.) diameter core could be imaged with room to spare on a properly centered film. As to physically centering a film for exposure, only the perpendicular central ray of a radiation source goes straight through a floor. Lacking a nearby penetration in the floor from which to measure, lining up a cassette within an inch or so of the core's center can be a real challenge. Plumb bobs, long tapes, careful measurements and maps are useful tools in this vital step. Some radiographers have reported the use of magnets and compasses. Tapping with a hammer and listening may get you close. Convenient pilot holes, made with hammer drills and small diameter bits, have been known to cut through the very items the radiographic testing was intended to protect. Anyone who has looked at a misaligned radiograph will agree that however it is accomplished, accurate alignment is absolutely essential to the process.

Multiple, overlapped films may be placed to increase actual film size when using a short source to film distance, imaging a large diameter core or attempting to compensate for questionable film alignment. Increased distortion in both image enlargement and shift will result from a shorter source to film distance. This condition may be acceptable, especially where thinner floors or smaller holes make distortion less of an issue. Thus, the source to film distance can be maximized to increase actual film size or minimized to shorten the exposure time. In any case, image enlargement is the main concern here. One must ensure that the source to film distance selected, having taken into account floor thickness and core diameter, will produce an actual film size large enough to contain the entire core volume.

 

Tricks of the Trade
Besides minimizing distortion, the use of sourceside markers ensures that holes may be marked for coring with a reasonable safety margin. To prove that a technique is providing adequate radiographic data to core safely is of paramount importance. The potential for liability demands doing the math beforehand. Calculations aside, the efficacy of any setup may be proven empirically by using sourceside markers. Therefore, on the source side of the floor the outline of the core should be marked with a lead ring. This ring can be formed from a strip cut from readily available 3 mm (0.1 in.) lead sheet. This ring, the most enlarged object in the radiograph, defines the conical volume of the core. Any contents revealed within the image of the ring lie either in the path of the core bit or very close. The ring and any other markers used should be indexed to the floor quite carefully. Lead letters (include some that cannot be read reversed in the image, such as the letter R), should be used to identify the film. For the best future reference, the whole surface marker arrangement may be preserved with spray paint. These tricks of the trade can be readily appreciated and understood by all involved in the coring operation, once they view the finished radiograph, even if the preexposure math escapes them.

 

Image Shift
Image shift is another concern altogether, but no less important. Ideally, the lead ring should be centered up below the source on the floor. This will center the ring's image on a properly placed film. A plumb bob may be used to align the center of the lead ring with the central ray of the radiation source. A suitable lead marker, such as a fishing sinker, should be placed on this center. This marker can also indicate a compass direction on the film for orientation purposes when interpreting the results. The lead ring and center marker on the floor surface will be the objects whose images are the most enlarged (both) and/or shifted (the ring). The central ray images the only nonshifted point on the radiograph. The degree of shift of a feature is dependent upon height in the floor and the distance of the feature from the central ray. These relationships are illustrated in Figure 3. The farther an item is from the film plane or from the central ray, the more the image of that feature will be shifted in the radiograph. Once again, the parameter most readily under the radiographer's control is source to film distance. The greater the source to film distance, the less the radiographic image will be shifted and enlarged. This principle is illustrated in Figure 4.

Keeping these geometrical facts in mind simplifies the interpretation of the radiograph. As previously mentioned, using the surface markers takes the worry out of the process. If no item (conduit, cable, rebar) is revealed within the circumference of the image of the lead ring, the floor may be cored with relative impunity. If items are present in the circumference of the lead ring, the proposed hole can be relocated and subsequent exposures may reveal a clear area. The image shift mechanism is predictable and quantifiable with geometric calculations if enough of the variables involved are known. An application of this principle is the determination of depth of discontinuities in a thick section. This procedure is described in the Nondestructive Testing Handbook, second edition: Volume 9, Special Nondestructive Testing Methods.

 

Rebar Depth Determination
A floor's load carrying capacity can be determined by depth and size of rebar. Building occupancy permits typically hinge on a professional engineer's certification of the floor load carrying capacity. If archival blueprints cannot be produced, radiographic testing may be used to image the structural concrete reinforcements. Using the aforementioned techniques and careful workmanship, reinforcement bar in abovegrade slabs can be located, sized and its depth in the floor determined. To begin the radiographic testing data gathering, it is a good idea to start with one conventional exposure through the floor. This will verify exposure time and orientation of rebar to be studied. The bars must be running perpendicular to the direction the source will be moving and also perpendicular to a line drawn between the surface markers. Using the technique described in the Nondestructive Testing Handbook, a double exposure or parallax radiograph is produced. Source side markers are placed so that during the first half of the exposure, one will be imaged through the floor locating the source's central ray and the other marker will be shifted. On the second half of the exposure, after moving the source, the central ray will image the previously shifted marker and the original central ray marker will be shifted. Comparison of the shifts and careful measurements taken from the setup can yield depth location of features. Once the rebar depth is known, divide the source to rebar distance by the total source to film distance. This yields a correction factor, which, when multiplied times the measured rebar image, yields the actual rebar size. In this technique, the floor thickness must be known. In the alternate technique illustrated in Figure 5, the floor thickness need not be known initially. One radiograph is produced to establish an exposure time, film alignment and subject rebar orientation. The second exposure (on a fresh film) is set up according to Figure 5. Accurate measurements and data records are necessary for successful calculations.

For Figure 5, assume that M and L/3 are known and T is unknown. Using the rule of triangles,

 

(1)

we can determine that

(2)

 

(3)

 

(4)

 

(5)

Tan q, D_c and M are known; thus T can be calculated using the equation

(6)

or

(7)

While depth determinations are fairly rigorous, interpreting radiographs for safe core cutting locations involves little more than identifying surface markers and images of internal items in the floor. Post tension cable resembles roughly stranded rope: a distinctly nonhomogenous, striated density with no terminations. Rebar may show upsets (bumps) in profile, tie wire, terminations and a homogenous density throughout. Plastic conduits will evidence an increase in film density due to the void, with lighter copper conductor wire visible inside. A film density of 1.8, measured through the concrete alone, is usually sufficient for interpretation. Clients will also be able to view lower density film using commonly available lighting. Some clients prefer to read films themselves, although most will leave the interpretation to the radiographers. A white china marker is useful to indicate shot numbers, floor numbers, center markers and core outlines. Critical items such as post tension cable and conduits should not be marked so closely as to obscure their outlines. Due to the possibility for onerous and avoidable communication mistakes, the radiographer in charge should always make sure that clients have all of the information they need to core with confidence. It must be emphasized that once the indexing marks on the floor are gone, the radiographic testing data may be useless.

 

Conclusion
One last word is appropriate to those unfamiliar with concrete radiographic testing. Concrete is a variable radiation absorber compared to a fairly predictable one such as steel. To establish and maintain proper exposure times with the chosen radiation source, especially where exact floor thickness is unknown or variable, a direct reading pocket dosimeter may be placed behind the film to record a relative dose. The dosimeter reading necessary to achieve a proper film density will have to be determined experimentally. Generally, the fastest lead screen industrial radiography films available are ideal for concrete radiographic testing. The lead screens themselves do not have to be code quality and a 0.3 mm (0.01 in.) thickness front and back will suffice. If the exposure geometry is correct as outlined above, the radiographic sensitivity using fast film with lead screens will be adequate for interpretation. Successful fluorescent screen techniques have been reported. However, due to the cost of the screens themselves and their usually required hard cassettes, the savings in exposure times may not save money overall. Handling of these heavy cassettes may well be a formidable challenge to the radiographer hauling armloads of them up flights of stairs.

 

Acknowledgments
Many of the techniques described here evolved over more than a decade of practical radiographic testing experience. The author extends recognition to the management, technicians and engineers at Reinhart and Associates, Inc., in Austin, Texas.

 

* BWXT Pantex, LLC, PO Box 576, Claude, TX, 79019; (806) 477-4165; fax (806) 477-4147; e-mail <jforbis@pantex.com>.

 

Copyright © 2001 by the American Society for Nondestructive Testing, Inc. All rights reserved.

[ Materials Evaluation ]