Figure 1-3
Figure 4-6
Figure 7-9

Introduction
The increased use of nonmetallic construction products and materials creates a special type of problem when it becomes necessary to locate these materials embedded in concrete slabs. Holes are often later bored through floor slabs to run additional utility lines or to make structural modifications to buildings. When these holes are bored, it is very important to not cut through existing power lines, fiber optic communication lines or other utilities. A severed fiber optic line at an airport could result in a major communication dilemma until repairs to the service line could be completed. Similar conditions exist at banks and other types of office buildings.

Locating plastic piping containing fiber optic lines inside a concrete floor slab can be very difficult. Construction design drawings are not always accurate because last minute changes to cable and piping routing are often made but not recorded. The routing of lines is of little importance to the installer as long as the equipment is connected to the cable and it works properly. Furthermore, when concrete is placed, the lines may be moved around by the weight of the concrete as it flows and is vibrated into place. As a result, the exact location of the fiber optic cable or polyvinyl chloride (PVC) piping will be unknown.

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Locating plastic pipe in concrete can be difficult.  

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The old style copper or metallic communication wires were much easier to detect because copper is very dense compared to the surrounding concrete and shows up well on X-ray film (Hager, 1999). The new fiber optic cables, which are made of nonmetallic materials and located inside PVC pipe, are often placed near the bottom of the slab and may be hidden by concrete reinforcement, aggregate or seams in metal stay in place concrete forms. This increases the difficulty in detecting the cables using NDT equipment. In addition, the tubes containing fiber optic cables are usually of a very small diameter.

Although there are several different NDT techniques that may be capable of locating objects in concrete slabs, X-ray is often the preferred method because it gives the positive identification of an object on a developed X-ray film. Many contractors and inspectors trust X-ray results more than the other methods because it has been used for many years with proven results and accuracy. The film may also be stored for later examination. Additionally, building and inspection codes consider radiography an acceptable method for detecting objects in concrete slabs.

 

Scope of Solution
Because radiography is such an important and commonly used method of inspecting the interior of concrete structures, this project focused on improving X-ray imaging of PVC piping inside concrete. The scope of this paper will concentrate on three solutions to the previously mentioned problems:

• Develop a method of standardizing identification markings on PVC piping commonly used inside of concrete slabs so they may be distinguished from one another when viewed on X-ray film.
• Create an image quality indicator that verifies that the radiography was conducted correctly. This indicator must also verify that if PVC piping with standardized markings were present, its image would appear on the X-ray film.
• Create an imaging system that is capable of determining the approximate depth of an object using only one exposure of radiation on the film. Current methods of determining object depth normally require two or three exposures to determine its depth.

In order to meet these goals, experiments were conducted using plastic pipe 22 mm (0.9 in.) in diameter placed inside concrete slabs that were 114 mm (4.5 in.), 165 mm (6.5 in.), 203 mm (8 in.) and 216 mm (8.5 in.) thick. The technique developed was tested with two types of radiography equipment: traditional radiography equipment using an Ir-192 radioactive source and X-ray film and a digital radiography system that uses computer enhanced imaging. Both types of radiography equipment showed that the new imaging techniques developed were capable of accurately identifying PVC piping and estimating its depth. Only the results from the traditional radiography are included in this paper.

 

General Radiography Theory
Radiography technology is one of the most attractive forms of nondestructive testing because it allows inspectors to see into the interior of the structure being examined. Radiographic theory is based on two principles:

• Materials with different densities or thicknesses absorb different amounts of radiation.
• When a photographic film is exposed to a radioactive source, the film will be darkened producing a negative image. In the new digital X-ray systems, the film is replaced with a digitized computer screen that can produce either a positive or negative image.

When a 200 mm (8 in.) concrete slab is examined using radiography, a small void such as a small diameter plastic tube is often very difficult to see on the developed X-ray film. This is because the difference in density and volume is small compared to the overall mass of the concrete slab. The image of the plastic tube blends in with the surrounding concrete (Hager, 1999; Picker, 1991).

 

Applications of Radiography
Radiography may be used to image any dense material and is commonly used to examine steel, concrete, machine parts, castings, human anatomy or other materials in which the presence of internal discontinuities are suspected. Objects that are inspected by radiography are sensitive to the orientation of the lines of radiation. In addition, objects with very similar densities and thicknesses may be hard to distinguish from each other on X-ray film (Picker, 1991).

Reinforced concrete is often inspected using radiography to determine the location of steel reinforcement, electrical conduit and prestressing cables (ACI-311, 1983). Areas with voids in the concrete or missing reinforcement may be located with radiographic inspection methods before a slab is loaded to its full capacity. This is especially important for large structures where thousands of pieces of steel reinforcement may be placed and the possibility of omission is high (ASTM E-94-93, 1993).

One of the greatest difficulties encountered when inspecting concrete slabs is locating nonmetallic materials. When it is necessary to locate fiber optic lines or small plastic conduits, the much greater density of the surrounding concrete tends to block out the image of the smaller, less dense object.

 

Identification of PVC piping
PVC piping is intended for different applications. For example, a small diameter PVC pipe passing through a concrete slab may be used as a water supply pipe, a conduit for electrical wire or a conduit for fiber optic cable. To improve X-ray imaging of such a plastic pipe, it is necessary to establish a method of marking the pipe so it will show up clearly on X-ray film. These markings should be different for each intended use of the conduit and allow the inspector to distinguish between conduits used for different utilities.

In this research, a series of copper wire rings were located at different standardized spacings on PVC piping. The spacing can identify the intended use of the tubes. The wider spacing could be used for less important applications such as water pipes and the narrower spacing used to identify more important conduits such as fiber optic lines. Although the use of these copper markings will greatly increase the cost of the pipe, in certain applications this cost is negligible when a positive identification can be made during rehabilitation or restoration projects. It is certain that there is indeed a lot of existing concrete infrastructure where this increased cost, in hindsight, would not be significant.

 

Image Quality Indicators and Verification of Results
In order to verify the accuracy of results on a developed X-ray film, a device such as an image quality indicator (IQI) can be placed within the exposed area of the film. This device will show up on the film when the radiography is conducted correctly and will prove whether or not the object inside the concrete slab is in the area viewed on the exposed X-ray film. If the image quality indicator is not visible on the developed film, yet is known to be present, then the test area must be reinspected and a new radiograph made. In this research, a ladder shaped bar was used with a known standardized spacing. This bar allows the inspector to measure the image of the bar on the developed X-ray film and helps determine the size of objects viewed on the film. It is also used in helping determine object depth as well as slab thickness, which will be discussed later. Figure 1 shows a schematic of a ladder bar used as an image quality indicator.

 

Determination of Depth of an Object
This research project also develops a method of using ladder bar image quality indicators to estimate the depth of a marked PVC pipe with only one radiograph exposure. This is accomplished by placing one ladder bar on top of the concrete slab and another bar on the bottom of the slab perpendicular to the top bar. When the developed film is viewed, the image of the top bar will have wider spacing than the image of the lower bar. The image of the spacing of the rings on the cable conduit may then be scaled in proportion to the ladder bar spacings and the depth of the conduit may be estimated by similar triangles. This technique is further explained in experiment 3.

 

Current Method of Determining Object Depth Using Radiography
When conventional X-ray techniques are used, it is very difficult to determine the depth of an object with one exposure of the film. The standard method of determining the depth of an object is to take several exposures on the same film to show two or more images in different positions. This is accomplished by moving the source of radiation at measured intervals and using geometry and similar-triangle calculations for depth determination. This method is time consuming and exposes the surrounding area and workers to potentially higher levels of radiation than when using only one exposure. Figure 2 is a schematic of a typical arrangement of X-ray equipment used to create multiple exposure images on the same X-ray film.

The double exposure technique most commonly used to estimate the depth of an object is described in the ASME Boiler and Pressure Vessel Code (1992). From Figure 2, Equation 1 can be used to determine object depth:

 

(1)

Where X₁ is the spacing between radiation source locations, X₂ is the spacing between images on X-ray film, Y₁ is the distance from S₂ perpendicular to film and Y₂ is the object height above the X-ray film. Although using two exposures of radiation to the film can be time consuming and expensive, this method of determining the depth of an object has been used for many years with reliable results.

 

Experimental Results
Six experiments were conducted to verify the technique of the proposed imaging system. Three experiments were conducted at a remote field location with a portable radiography truck and the assistance of a professional NDT inspector. These three experiments used Ir-192 as a source of radiation to expose conventional X-ray film. The film was then developed at the site using the darkroom in back of the truck. The other three experiments were conducted at a laboratory using a modern digital X-ray imaging system. This X-ray system used a digitized screen instead of X-ray film, eliminating the need for a darkroom and development of the film. When digital X-ray equipment is used, the image is visible instantly. The imaging screen has controls to adjust the brightness and contrast of the image, allowing the operator to create many different views of the image. The image can then be stored on a computer disk for review in the future. Only the results from the conventional source radiography are presented in this paper. For complete documentation of this project, refer to Howanick (1999).

The first experiment determined the required size of wire that should be used for the IQI and self identification system (SIS). The IQI is used to verify the existence or nonexistence of an object within a concrete slab and to verify that the radiography was conducted properly. If the image of the IQI is visible on the developed X-ray film (or the screen of a digital X-ray system), it helps verify that the proper radiography procedure was followed. The second experiment determined if the SIS was capable of positively identifying plastic piping located inside a concrete slab. The third experiment tested the 3D depth finding capability of the IQI and SIS system.

 

Experiment 1 - Minimum Wire Diameter Required for IQI and SIS
For the first experiment, five copper wires and one steel wire varying in diameter from 0.5 mm (0.02 in.) to 3 mm (0.13 in.) were placed diagonally through a test section of concrete slab. The wires were aligned from the top of one side of the concrete mold to the bottom of the other side so that its depth of concrete cover would vary. A metal frame was built to hold the wires in place utilizing large steel washers. These washers would show up on the developed X-ray film and secured each end of the wires. This allowed the location of the ends of the wires to be known even if the wire diameter was so small that the wire itself did not appear on the developed film. Figure 3 shows a schematic of the experiment.

The concrete was then carefully placed in the mold to form a slab of concrete 200 mm (8 in.) thick. The test slab was then imaged using conventional Ir-192 radiography with a source-to-film distance of 2030 mm (80 in.). After the film was developed, the smallest wires that showed up adequately on the film were observed to be solid copper or steel wire 3 mm (0.13 in.) in diameter. This wire could be identified at all depths of concrete cover. This was the size and type of wire used for the construction of the SIS rings around the PVC test specimens and the IQI ladder bars in the following two experiments.

 

Experiment 2 - Self Identification System Experiment
The second experiment involved casting in place three hollow 22 mm (0.9 in.) diameter plastic tubes with self identification system (SIS) rings inside a concrete slab and verifying detectability. The SIS rings were spaced at 25 mm (1 in.) on the first tube, 50 mm (2 in.) on the second tube and at 75 mm (3 in.) on the third tube. The tubes were then placed in a 200 mm (8 in.) thick concrete slab 127 mm (5 in.) from the top.

The slab was then imaged using conventional Ir-192 radiography and the developed film showed that the PVC tubes with SIS rings could easily be distinguished from each other as shown in Figure 4. Hollow plastic tubes of this size are normally very difficult to see on X-ray film. The SIS rings made these tubes visible and allowed the different tubes to be distinguished from each other. These images were produced with a source to film distance of 2 m (80 in.).

 

Experiment 3 - Three Dimensional Imaging Using the AIQI System
The third experiment demonstrated the ability of the IQI system (the ladder bars) to determine the depth of an object within a concrete slab with one radiation exposure. In addition, the thickness of the slab can be determined. The IQI bars were made of steel wire 3 mm (0.13 in.) in diameter, as determined in experiment 1 as the minimum diameter wire that would produce a visible image on the X-ray film throughout the depth of a 200 mm (8 in.) thick concrete slab. Figure 1 shows a schematic of the ladder bars utilized throughout. The actual spacing of the SIS rings, X₁ and hence the utility type, can be determined by comparing the ring spacing on the film to the spacing of the upper and lower IQI bars as follows:

(2)	If LIQI < X2 < UIQI then X1 = 50 mm
or	If X2 < LIQI then X1 = 25 mm
or	If X2 > UIQI then X1 = 75 mm

where X₁ is the actual spacing of the SIS rings, X₂ is the projected spacing of the SIS rings on film, UIQI is the upper image quality indicator and LIQI is the lower image quality indicator.

The distance of the object to the bottom of the slab can then be determined from similar triangles as follows:

(3)

where h is the distance from object to bottom of slab and H is the film to source distance.

Experiment 3 involved placing two PVC tubes 22 mm (0.9 in.) diameter with SIS rings having a spacing of 50 mm (2 in.) at different depths inside the slab. The center of the top tube was placed 75 mm (3 in.) from the top of the slab. With the film placed at the bottom of the slab, the actual distance from the centerline of the tube to the film, h_actual, would be 127 mm (5 in.). The IQI ladder bars were placed on the top and bottom of the slab and the slab was imaged with conventional Ir-192 radiography using a source to film distance of 635 mm (25 in.). Figure 5 shows the arrangement of the source of radiation, the film, the plastic tubes with SIS rings and the IQI ladder bars. Figure 6 shows a plan view arrangement of the IQI ladder bars and the plastic tube with SIS rings. Figure 7 shows the images produced on the X-ray film.

To estimate the distance from the upper tube to the bottom of the slab with the SIS rings the following procedure is followed. First the images on the developed film are carefully measured as shown in Figure 7. The type of utility can be determined from Equation 2. In this project, the actual ring spacing is 25, 50 or 75 mm (1, 2 or 3 in.), depending on the utility type. To ensure that the depth of the plastic conduit is measured at the tube centerline, the outside edge of the SIS ring image must be measured. Using Equation 2 and referring to Figure 7, the actual ring spacing is determined as follows: X₂ = 63 mm (2.5 in.), UIQI = 74 mm (3 in.), LIQI = 50 mm (2 in.), 50 mm (2 in.) < 63 mm (2.5 in.) < 74 mm (3 in.)    X₁ = 50 mm (2 in.)

The distance from the tube to the bottom of the slab h may then be determined by using Equation 3. Figures 8 and 9 show a graphical illustration of Equation 3. With a film to source distance H of 635 mm (25 in.), the distance from the object to the bottom of the slab can be determined from Equation 3 as follows (distance given in millimeters):

(4)

This experiment estimated the location of the top tube very near to the actual depth as h_actual was constructed at approximately 127 mm (5.0 in.). Similarly, one can estimate the lower tube location following the same procedure.

 

Slab Thickness Determination
The previous results show that this technique is capable of helping inspectors locate, identify and estimate the depth of PVC pipe inside of concrete slabs with one exposure of radiography. Similarly, this method may also be used to determine the thickness of a concrete slab with one exposure of radiography. The difference in size between the images of the upper and lower ladder bars may be measured and the thickness of the slab may be determined by using Equation 3. In this calculation, X₁ is the spacing of the LIQI, X₂ is the spacing of the UIQI, H is the film to source distance and h is the slab thickness. If X₁ = 50 mm (2.0 in.), X₂ = 74 mm (2.9 in.) and H = 635 mm (25 in.) then

(5)

The numbers above are expressed in millimeters. The actual depth was constructed to be at approximately 200 mm (8 in.).

 

Summary and Recommendations
This project showed that for normal concrete structures of up to 200 mm (8 in.) thick the minimum visible metallic wire is of the order of 3 mm (0.13 in.) in diameter using conventional film and Ir-192 source radiography. Therefore, 3 mm (0.13 in.) diameter wire was used to construct both the IQI ladder bar and the SIS rings for the PVC piping. Utilizing the IQI and PVC piping equipped with SIS rings, normally invisible piping can be located in a 3D aspect with only one exposure of radiation. In addition, the thickness of a slab can be determined, which proves to be a useful feature because in many finished construction situations the slab edge is not available for direct measurement.

The authors acknowledge that the cost of manufacturing plastic conduit with SIS rings built in will be expensive when compared to normal piping. However, in applications where communication lines or other vital utilities are at issue, the increase in price would be justified by the confidence in positive identification.

It is necessary to standardize the spacing of the SIS rings so that a specific spacing always identifies a tube's intended use for this system to be used effectively. The tube may then be positively identified as a fiber optic conduit, electrical conduit, water pipe or other utility on the radiograph. The spacing of the IQI ladder bars must also be standardized so all inspectors have the same tool to work with when estimating the size and depth of objects being viewed. With complete standardization of this technique, it has the potential of being a very effective and useful tool for engineers and inspectors utilizing radiographic technology. As more nonmetallic composite materials are introduced into the construction and manufacturing environment, it becomes increasingly important to develop new techniques that allow identification and inspection of these products. This research is a step toward the development of such a technique for NDT inspection of concrete infrastructure in certain building applications where the additional cost is warranted.

 

References
American Concrete Institute, 311-83 Standard for Concrete Care, 1983.

American Society for Mechanical Engineers, ASME Boiler and Pressure Vessel Code, July 1992.

American Society for Testing and Materials, ASTM E-94-93, Standard Guide for Radiographic Testing, September 1993.

Hager, Craig, personal interviews, 1999.

Howanick, D., Advanced Radiographic Imaging for Assessment of Civil Engineering Infrastructure, Masters Thesis, University of Colorado at Denver, 1999.

Picker X-Ray Corporation, Guide to Radiographic Inspection and Interpretation of Radiographic Film, 1991.

 

 

* University of Colorado at Denver, Department of Civil Engineering, Denver, Colorado 80217-3364; (303) 556-8017; fax (303) 556-2368; e-mail <krens@carbon.cudenver.edu>.

^† Transportation Techniques, 1705 East 39th Avenue, Denver, Colorado 80205; (303) 382-1041.

^‡ Albert Knott and Associates, 7500 East Dartmouth, #27, Denver, Colorado 80231; (303) 745-4586; e-mail <aknott@ix.netcom.com>.

 

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