USE OF THE MEGASCAN™ IMAGING PROCESS IN THE INSPECTION OF RAIL
BRIDGES AND OTHER MAJOR STRUCTURES

K.D Brown
Material Measurements Ltd
Inspection House
61 Albert Road North
Reigate, Surrey. RH2 9RS, UK

ABSTRACT
This paper reviews the Megascan™ Imaging Capture System, which is a radiographic technique used by Material Measurements Ltd (MML) in the investigation and assessment of railway and other civil engineering structures. Reference is made to the key role that the Megascan ™ Imaging Capture System currently plays in post-construction inspection and assessment.

INTRODUCTION
The long-term safety, durability and performance of railway and civil engineering structures such as bridges, mainly depend upon good detail design, the quality of materials used and the standard of workmanship achieved on site. Unfortunately, defects do occur in structures for a wide variety of reasons, ranging from poor detailing to the lack of maintenance. The investigation and assessment of defects is a demanding task which in many cases requires the use of experienced staff, supported where necessary by specialist inspection techniques. One such specialist inspection technique is the Megascan ™ Imaging Capture System, which is a capable radiographic Non Destructive Testing (NDT) process. The Megascan™ has been successfully employed in the investigation of the internal integrity of various structural components, where for example, it has played a vital role in examining the condition of post- tensioning tendons and the positioning of reinforcement. The success of the Megascan ™ radiographic capture system results from major improvement and development work carried out by MML during the past five years. This work has yielded improvements both in safety and in radiographic technique, while also providing advances in the speed of image capture and material penetrating capabilities.

Megascan ™ Imaging Capture System
The Megascan™ comprises four main components: - radiographic head, power pack, control panel and capture plate.

Figure 1: Megascan™ Equipment

Radiographic Head:

Two units are available with different levels of energy and dose output. The smaller is rated at 6 MeV, has a power of 0.1 to 3 rads and weighs 90 kg. The larger is rated at 7.5 MeV, has a power of 0.1 to 7.3 rads and weighs 115 kg. The external dimensions of both units measure 380 x 240 x 580 mm. The larger 7.5 MeV unit has over twice the dose output of the smaller 6 MeV unit, this means that the larger unit is faster in capturing a given image. The higher energy output means that the 7.5 MeV unit can for example, penetrate concrete up to a thickness of 1500 mm, while the 6 MeV unit can penetrate concrete up to thickness of 700 mm.

Power Pack: This component transforms mains voltage up to the required voltage level to power the system. The power pack’s external dimensions measure 400 x 550 x 350 mm and it weighs 46 kg.

Control Panel: This component allows the dose, energy level and shot times to be set, it also monitors the accumulated dose output for any given radiographic exposure. The control panel measures 200 x 150 x 100 mm and weighs 1.5 kg.

Capture Plate: This component is available in different sizes and various radiographic speeds. The radiographic speed relates to the time the plates are exposed and the amount of radiographic dose required to produce a radiographic image that has sufficient information to allow interpretation. Most work undertaken by MML uses a 350 x 430 mm plate size, mounted in a 10 mm thick plate holder. Multiple plates can be used to cover an area of up to 860 x 700mm.

Depending on the film type used, it is now possible to capture an image through 1000 mm thick reinforced concrete in approximately 20 minutes. Similar settings can obtain images through 650 mm of concrete in 30 seconds depending on the density of the concrete and the amount of reinforcement within the section.

IMAGE CAPTURE PLATE
There have been major improvements in the image capture system with regard to the quality and speed of capture, resulting in improved image clarity. Images are normally ready for interpretation within ten minutes of capture. Post-image processing may also be used to further aid interpretation. Through close collaboration between MML and industrial film suppliers, MML have available a range of plate types to suite many applications. For example, a range of capture plate types is available and these vary from high speed, low radiographic contrast to slow speed high radiographic contrast. The final choice of capture plate type depends on the information to be obtained. Fastest plate types are used mainly in the initial stages of an inspection to locate areas of interest. In recent use of a digital capture system, MML have successfully maintained similar capture times to those required for conventional film plates. This has major cost saving and environmental advantages, given that the capture plates may be reused up to 4000 times before requiring replacement. In addition, post-processing can be completed as the image is produced using pre-set perimeters, this together with other tools such as dimensioning, inversion, inclusion of text, image archiving and magnification make this system invaluable on large contracts.

Figure 2 Typical Image

The film plate capture system may still in some instances result in the best method of image capture and assessment. The benefits of digital image manipulation may still be achieved with the film plate capture system, provided that each capture plate is digitised as a separate operation.

Figure 3: Examples of head location and the use of the structure as a collimator

HEALTH, SAFETY AND ENVIRONMENTAL CONSIDERATIONS
Health, Safety and Environmental considerations are of paramount importance when specifying radiography and for which all parties involved will have some responsibility. For all contracts a site survey is undertaken prior to the use of the Megascan™ and a notification is sent to the Health and Safety Executive informing them of the intention to carry out radiography. This notification and any related safety issues are conducted by MML. The site radiographer also carries out a risk assessment and produces method statements for a safe system of work. MML can also assist in the incorporation of these documents into other related site safety documents. It is accepted that there is an increasing radiation risk as the output of the source rises. However, the high power output of the Megascan™ has some practical advantages over conventional radiographic systems, due to the systems high output and frequency, it produces very little low energy scattered radiation. As the Megascan™ is electrically operated, it has the major advantage over the use of radiographic isotopes, in that it can be immediately turned off and made safe. This is achieved either by switching off at the control unit, or terminating the power supply or by use of the remote hand held safety switches.

Prior to any radiographic inspection activity a safety plan must be produced. This requires careful preparation in advance, together with the submission of formal notification to the Health & Safety Executive. Safety plan preparation is a simple but nevertheless thorough operation and should not be seen as an obstacle, as this type of work is commonly undertaken in the general engineering and aviation industries. There are strict guidelines laid down under the Ionising Radiation Regulations 1999 for safe working practices, part of which requires the use of a fully trained and experienced work force. MML can meet such requirements and have been conducting this type of work for many years

Due to the small overall size and portability of the Megascan™, it is easy to control and hence has only a limited effect on surrounding areas, so generally allowing other works or access to continue. As the point from which radiation is emitted is very small, this enables the head to be positioned much closer to the item being radiographed. This results in radiographic quality being maintained, while exposures are much shorter than those required by other types of radiographic units.

Other useful features of the Megascan™ are that it can be powered by either a 110 volt or 240 volt, 16 amp electrical supply. It is fail safe in operation by virtue of the fact that when the electrical supply is cut, this will immediately terminate the exposure. In addition, there is no residual radiation after termination of the exposure and the surface under investigation could be touched immediately, without fear of contamination by radiation.

Figure 4: Safe distances

Where there is a requirement to maintain access to a specific area, the Megascan™ has an integral dose monitoring device which gives the option of running the unit intermittently but still achieving the overall dose required by the capture plate.

For example, in the case of a road that has to be kept open to traffic, traffic lights can be use to stop traffic when the exposure is on, then the exposure can be stopped for a period to allow traffic to flow. When suitable, the traffic can again be stopped and the exposure reinstated. This can be repeated until the capture plate has received the correct dose to achieve the image required. Additional shielding can also be easily employed, materials such as lead or concrete blocks or water filled containers can be used to create an enclosure around the head. The structure under inspection can also be utilised to provide shielding by putting the unit head inside the structure as seen in figure 3. The use of digital or fast captures plates can also be used as an additional safety benefit, by either reducing capture times or dose rate outputs.

On a recent contract using the Megascan™ to obtain images through 600 mm of concrete, traffic on the opposite side of the concrete deck was kept flowing as illustrated by figure 5. Behind the Megascan™ the safe working zone was also reduced to less than 5 meters from the unit, simply by the use of mobile shielding. Figure 4 also gives examples of safe distances where ‘T’ = the thickness of the penetrated or surrounding material thickness.

Figure 5: Bridge deck setup

* Thickness, safe distances and exposure times are given for the 7.5 MeV unit only.

TYPICAL APPLICATIONS
The Megascan™ system is a powerful non destructive testing method and should be viewed as an important part of the tool kit available to the investigation team, where access is available to both sides of the element to be inspected. The Megascan™ system will have greater influence if targeted on particular problem areas and if used in conjunction with other inspection methods. For example, Ground Penetrating Radar could be undertaken to initially locate post-tensioning ducts. This could be followed by use of the Megascan™ to verify if the duct is voided or if the tendon is positioned correctly within the duct, or even to check if the duct is filled with air, grout or water. Such inspection work does not involve any damage to the structure and thus does not leave a possible weak point for attack over future years. However, in some cases the use of minor invasive investigation together with the use of an endoscope can be an invaluable way of calibrating results obtained using the Megascan™ system. Such criteria can then be applied across the inspection and assessment of an entire structure.

Tasks undertaken by the Megascan™ system on previous contracts:

1. Locating reinforcement in various layers to facilitate coring through seismic sensitive reinforced concrete structures.
2. Mapping reinforcement positions, size and integrity in a structure so that load carrying capacity could be assessed.
3. Locating material density changes due to chemical attack or differences in material composition.
4. Location of voiding within post-tensioned tendon ducts in new or existing structures as a means of verifying structural integrity.
5. Identification of corroded or broken wires within a water contaminated post-tensioned tendon duct
6. Location of wire breaks within a bridge beam that had been stuck by a vehicle.
7. To check bond of concrete to steel within a structure subjected to fire.
8. Verification that a structure has been built to the correct design requirements
9. To locate missing crack inducers.

Summary: Use of the Megascan™ Image Capture System

1. Megascan™ Image Capture System is a reliable method of determining internal defects. It is able to penetrate concrete thicknesses of up to 1500mm
2. Interpretation will usually be possible within a few minutes of termination of the exposure
3. The method is non-destructive and is suitable for the verification of new build construction
4. The new technological advances incorporated into the Megascan™ system results in increased detail being shown on radiographs, reduced safe areas being required, better equipment portability and greater control of safety

Material Measurements Ltd have a quality management system audited by the National Quality Assurance, NQA, applicable to Non Destructive Testing against the provisions of BS EN ISO 9002 : 1994. It includes detailed procedures and a contingency plan for radiation safety, all of which are issued to the Health & Safety Executive.

All MML technicians have undergone rigorous training as industrial radiographers and are qualified to international standards. Safety standards are maintained by the implementation of continuation training for all radiographers and other staff by our independently appointed Radiation Protection Advisor.

A site survey is undertaken prior to all contracts when using the Megascan™. The site radiographers also carry out a Risk Assessment and can provide Method Statements for a safe system of work.

The service is available at short notice, although time to arrange a site survey and discussion of the Safety Plan is always paramount to achieving the most cost-effective results.