Bridge – FHWA InfoTechnology

Bridge - Radiography (RAD Void)

Target of Investigation

Radiography can be used for the following applications:

Determining the positions, diameters, and condition of tendons and strands in post-tensioned and prestressed concrete girders.
Detecting voids in cast-in-place, precast prestressed concrete girders.
Assessing the grouting condition in the post-tensioning ducts.

Description

Concrete radiography is similar to radiography used to produce medical x rays. This method involves gaining information in a test object by subjecting it to high energy electromagnetic radiation. The propagation of radiation through the test object is captured on a special photographic film on the opposite side of the specimen to produce a photograph like representation of the internal structure of the concrete.

Physical Principle

The basic concept of gamma-ray radiography is illustrated in figure 1. A source transfers gamma-ray energy to a collimator through a guide tube by means of a telecontrol unit so that gamma rays impinge on the sample to be inspected. On the other side of the sample, an image plate registers the gamma-ray intensity traversing the sample. The lower the density of the material is, the higher the intensity of the gamma-ray energy that reaches the plate will be.

Gamma-Ray Radiography Concept. The three-dimensional drawing shows the basic concept of gamma-ray radiography. From left to right, the items depicted are control unit cable housing, source container, guide tube, collimator, sample, and image plate. Gamma-ray energy is transferred from the source container, through the guide tube, to the collimator, which directs the energy through the sample, creating on the image plate an image of the gamma-ray intensity. — Figure 1. Illustration. Gamma-Ray Radiography Concept.

Since concrete is about 2.5 times denser than biological tissues, x rays used in medicine are not suitable for concrete members. For this reason, concrete radiography usually employs gamma-ray sources instead of x ray generators such as are used in medicine. Another advantage of gamma rays over x rays is gamma rays are emitted spontaneously by certain substances so electrical power is not required. An additional advantage of the ¹⁹²Ir source is it can be housed in a small and relatively light container (~ 20 kg, 44lb).

The penetration of the gamma ray energy depends on its wavelength and energy level. The higher the source energy is, the greater the penetration of the gamma ray energy will be.

Figure 2 shows how voids are readily identified by using the radiographic method. An empty soda bottle embedded in a concrete block irradiated with gamma rays from a ¹⁹²Ir source can be readily identified. The attenuation produced by voids (air) is negligible. Hence radiation transmitted through a void in the interior of a concrete sample will produce a “shadow” darker than the areas where radiation traverses the full thickness of the concrete sample.

Empty Soda Bottle Embedded in a Concrete Block Irradiated with Gamma Rays From a superscript 192 end superscript Ir Source. The figure is rectangular. The image of an embedded empty soda bottle, upright but with a slight tilt to the left, is clearly visible. The image of the embedded empty soda bottle is darker than the image of the surrounding concrete. — Figure 2. Radiograph. Empty Soda Bottle Embedded in a Concrete Block Irradiated with Gamma Rays From a ¹⁹²Ir Source.

Since steel attenuates gamma ray energy more than concrete, steel elements such as rebars or strands embedded in a concrete sample project a lighter “shadow” on the image plate. A severely corroded rebar with a nonuniform cross section projects on the image plate non-uniformly (figure 3). These are radiographs of a slab (left) and a tank wall (right) obtained with a ¹⁹²Ir source. The radiograph on the left hand side also shows an electrical conduit to the right of a corroded rebar. The red lines in the lower part display the photographic density along the “cuts,” indicated by the blue lines on the radiograph. They help visualize the reduced cross section due to corrosion.

Radiography of a Distorted Rebar Profile. The figure has two rectangular radiograph images. The image on the right shows a vertical rebar or similar embedded object in a tank wall. The rebar image is lighter than the image of the surrounding wall. The image on the left shows a vertical rebar embedded in a slab. The rebar image is lighter than the image of the surrounding slab. To the right of the rebar in the slab is a darker almost vertical image of an electrical conduit, which is tilted slightly to the right. Two horizontal blue lines through the slab indicate “cuts” where photographic density was measured. Below the slab are two irregular red lines displaying the photographic density along the “cuts.” Both red lines have valleys corresponding to where the blue lines cross the rebar. — Figure 3. Composite Graph. Radiograph of a Distorted Rebar Profile.

Data Acquisition

Image plates, either conventional or more modern computed radiography (CR) image plates, record the gamma ray energy penetrated to the opposite side of a sample emitted from a ¹⁹²Ir, ¹³⁷Cs, or ⁶⁰Co source. Conventional image plates must be developed after exposure, while CR image plates are converted into digitized images. The recording resolution of the CR image plates is about 5 times greater than that of conventional image plates. Plate size is usually 35 cm by 43 cm (14 inches by 17 inches).

Data Processing

Figure 4 provides an example of a concrete wall with honeycombing. Visual inspection of the image plate is usually enough for detecting voids. A more detailed quantitative analysis can be carried out by examining the photographic density D(x,z) (i.e., the “gray” level) along different z_i-lines.

Concrete Wall with Honeycombing. The rectangular image has several almost vertical lighter areas tilted slightly to the right. The top portion of the image and especially the top right portion have a number of irregular lighter areas. The horizontal axis of the image is labeled X. The vertical axis is labeled Z. — Figure 4. Radiograph. Concrete Wall with Honeycombing.⁽¹⁾

Data Interpretation

Data interpretation for detecting voids is straightforward. Voids visually manifest as relatively darker areas in the radiograph of a concrete sample. However, other phenomena can result in relatively darker images. For instance, relative to the lighter shadow projected by reinforcing bars, the rest of the plate will appear darker. These situations are easily identified since the rebars project a well-defined shape. Another, more subtle effect is due to the intensity of the radiation falling onto the plate being not only a function of the attenuation of the materials it traverses but also of the inverse of the square of the gamma-ray path length d. If d is comparable to the dimensions (x,z) of the plate, the radiation intensity will be noticeably higher, producing a darker zone near the point at which d is smallest. That is the point (x_o,z_o) at which the line from the source perpendicular to the plate intersects the plate. In these cases, the photographic density D(x,z) on the plate will diminish regularly as (x,z) is farther from (x_o,z_o), exhibiting circles of equal photographic density.

Figure 5 shows an example where these two effects combine to make identifying a void difficult. The radiograph in figure 5 corresponds to a concrete structure with several rebars (numbered vertical light gray bands) and a few stirrups (horizontal light gray lines). The small white rectangle with black dots, the label above it and the thin vertical metallic segment are external reference elements.

The source-to-plate distance in this measurement was only 22 cm. In order to determine whether the darker area at the center of the radiograph corresponds to a void or not, the photographic density along the dotted line was measured (red data points in the lower plot) and compared with the expected (calculated) photographic density.

The green and magenta lines on the graph in figure 5 are the calculated photographic densities with and without steel bars. The good agreement between the measured data and the calculations indicates that the dark areas extending from near the center towards the borders of the radiograph are not due to a void but rather to the variation of the gamma-ray path length among the different points

Radiograph and Data Interpretation. The figure has an radiograph image and a graph. The radiograph is of a concrete structure with several rebars and a few stirrups. The rebars are vertical light gray bands numbered, from left to right, 4, 9, 5, 2, 6, 7, and 10. The stirrups are unnumbered horizontal light gray lines. Between rebars 5 and 2 is a dark area. The graph is titled Photographic Density in Plate 2. The y-axis of the graph is labeled Density and ranges from 0.0 to 2.5. The x-axis is labeled Position x and is centimeters, ranging from 9 to 49. The plotted dots form an irregular hyperbola. A green line and a magenta line on the graph are the calculated photographic densities with and without steel bars and roughly trace the irregular plot of the dots. — Figure 5. Composite Graph. Radiograph and Data Interpretation.⁽²⁾

Advantages

Produces “photographic” quality results and precise locating of reinforcement for condition assessment, void detection, and grout condition assessment.
Small, portable source systems.
Some source systems, such as those employed with ¹⁹²Ir source, are easy to shield and Can be used in the field without interrupting traffic or causing disturbances to the general public.

Limitations

Requires precautionary measures against gamma rays. Public exposure to radiations must always be below the limits set by the Nuclear Regulatory Commission and State regulations.
Small, portable gamma ray source systems can only penetrate concrete elements of the thickness up to about 300 mm. Systems that can emit more energy must be used for thicker elements.

References

Mariscotti, M.A.J., et. al., “Gamma-Ray Imaging for Void and Corrosion Assessment,” Concrete International, Vol. 31, No. 11, November 2009, pp.48-53.
The data shown in this article are taken from studies carried out by THASA (www.thasa.com).