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Non Destructive Testing is a new addition to the technology of condition monitoring and asset management. NDT process offers low cost and simple solution to the testing and evaluation of material strength. This is a method whereas the condition of a given material is tested without altering the operational capability of that material. This feature is appealing to various sectors.
History of NDT is fairly recent and it was mainly used in manufacturing industry. With the advancement and evolution of technology, the scope of NDT is widening. This research provides an insight of implementing NDT technology in construction industry. Majority of the tall civil structures are built of concrete. Numerous factors such as nature, inadequate concrete quality and faults in design lead to loss of strength and defects in those structures. In addition to that, the facilities available for testing vertical concrete structures are limited to date. For that reason this research preliminary aims at developing a battery powered robot that is capable of climbing vertical concrete structures to carry out Non Destructive Testing (NDT).
Based on the described situation, this report provides an assessment of the potential for utilizing NDT technology in condition monitoring of civil structures. The aim of this report is to establish the key principles behind the feasibility of such technology. In order to do so, the background of NDT is investigated to highlight the current the state of the technology as well as the aspects of requirements for developing such system.
This report examines the feasibility of developing a mobile robot for NDT purposes. The factors involved in implementing robotic NDT are discussed in chapter 3 after outlining a selective overview of current analysis of the problem in chapter 2. The requirement analysis of designing such system is presented in chapter 4. A draft layout of the proposed system is described in chapter 5. Key points and lessons identified from this review that could assist in further development proposed ideas are emphasized in chapter 6. In general, the scope of this report is to determine the feasibility of designing a robot capable of climb vertically to carry out NDT process in concrete structures by classifying the challenges in design process together with the summery of recommendation and further work.
2.0 General Background
Concrete has been used as a widely popular building material for years. The material and structural properties of concrete make it superior than other building materials such as steel, cement, brick, etc. In addition to that, concrete is environmentally friendly as research established that the manufacturing and building process of concrete emit less CO2 than other materials  as it consume less energy during the production process. The required production energy of other materials is given below:
Figure 1: Required production energy for common building materials .
However, plain concrete surface has low tensile strength and ductility. Hence the technique of using reinforcement substance such as steel bar with the concrete has been introduced to withstand the tensile and shear force that concrete structures experience.
2.1 Defects in Concrete Structures
Although reinforced concrete has been used extensively to build civil structures like tall buildings, bridges, dams, etc. These structures have long lifecycle which ranges from 50 to 100 years. But due to the exposure of rain, snow and moisture, various defects build up n these structures. These defects significantly reduce the life expectancy of concrete bodies.
Defects in concrete structures are usually caused by :
Due to those reasons the most common defects that occur in concrete are:
Cracks: Cracks could be a constructional defect. Air and moisture gain easier access to reinforcement bars (rebar) through cracks. As a result the structural strength and durability is greatly reduced.
Figure 2: Cracks on a concrete slab .
Delamination: Delamination is a kind of horizontal crack which separates the slab and the upper surface of the concrete. Delamination affects a larger area than common cracks. Periodical freezing and thawing of the concrete surface is the root cause of delamination.
Figure 3: Delamination of concrete slab .
Rebar corrosion: This kind of defect impacts the integrity and strength of any concrete structure to a great extent. Chemical reaction between the steel rebars and the water or moisture that gets entrance through surface cracks causes the rebars to corrode. The end result is growth of corrosion product along the steel body. The extra pressure created by that causes the concrete to crack even further and fall of leaving the rebars widely exposed. The rebar corrosion process could be illustrated by the following diagram.
Figure 4: Rebar corrosion process .
2.2 Current State of Technology
In order to ensure structural safety, many governmental regulations have been devised. These regulations urge for regular evaluation and testing of civil structures. In this process Non Destructive Testing (NDT) has gained popularity in recent years for its efficiency. But till now manual NDT of concrete structures is widely used. Most often it is seen the inspection is carried out by human technicians using handheld Non Destructive Equipment (NDE). Moreover the task is even more daunting is the inspected structure is vertical. In that case the overall testing process becomes complex and expensive as scaffoldings are to be made for access to the targeted area. The picture below shows the scaffolding built around the Tay Road Bridge in Dundee, Scotland. It is apparent from picture that manual NDT is less effective in terms of complexity and resource consumption.
Figure 5: Scaffoldings around Tay Road Bridge column.
Based on the current status, it could be concluded that robotic NDT of vertical concrete structures will provide:
Better scalability and coverage: As a robot could cover a larger are than a human operator.
Effective resource consumption: Robotic NDT reduces the set up time and cost.
Increased health and safety: As a robot will be used to carry out the inspection, human operator could afford to stay in safety.
2.3 Market Situation
NDT techniques are becoming popular amongst end users as it can provide early warnings of any structural damage without compromising the working capability of the structures. Moreover the governmental safety regulations are becoming more rigorous to ensure civilians safety. These conditions have opened a wide window of NDT in the commercial market. Various reports have suggested a remarkable growth potential of NDT in recent years. A report produced by Global Industry Analysts Inc. estimated NDT industry to reach $1.4 billion by 2017 worldwide ["Nondestructive Test Equipment: A Global Strategic Business Report" announced by Global Industry Analysts Inc.] It also projected continuous growth in the years after as novel technologies and improvements in existing technologies will provide improved and rapid results. According to the research findings, Asia-Pacific region is the largest market for NDT industry. A raid expansion of infrastructure industry in Eastern Europe promises a significant market share of NDT in this region.
Another report published by the Research and Market looked at the market trend of NDT in the previous years. In this research, Frost & Sullivan's specialist analysts thoroughly studied the market from a geographic point of view as: North America, Europe, Asia Pacific, and Rest of World. It found an increase of 4.7% in world market for NDT industry in the year 2009 [World Nondestructive Test Inspection Services Market, Research and Market]. The key finding of that report was that NDT market continued to growth even during the economic crisis.
Figure: Total NDT market based on geographic location (2005-2013) [ref].
Current market features of show potential and scope of research and new technology in NDT industry. Hence the resources spent in this current research will make it worthwhile.
3.0 Dimensions of Feasibility
In this report various parameters of feasibility are looked at in terms of technical, operational, economical and scheduler. This will assist to determine whether the required technologies exist and are those technologies accessible. This investigation will also establish the fact whether this proposed solution to the current problem will actually solve the situations. Another key question will be is it possible to develop the proposed robotic system within the given time frame. The proposed solution could be derived from the challenges and general considerations that might be faced in developing a concrete climber.
3.1 Technological Considerations
The utmost challenge for this research would be to find and modify a suitable adhesion technology for the robot. It was presumed that the robot will be able to climb vertical concrete surface by utilizing zero or less energy. Based on that initial design criterion finding a natural adhesion mechanism for the robot to carry NDE is vital.
Recently many novel ideas such as suction and vortex mechanism have been introduced to develop wall climbing robot [K.Ikeda, T.Yano,: "Development of a Wall Climbing Robot with Scanning Type Suction Cups", Proceedings of International Mechanical Engineering Congress Sydney]. But these technologies have the limitation of high power consumption. Therefore the implementation of natural magnets could be considered in this mater.
3.1.1 Adhesion Mechanism
The concrete is reinforced by steel bars. Steel is a ferromagnetic material which means if it is suspended in a magnetic field the steel bar will be magnetized and attracted by the magnet. So based on this technique permanent magnets could be used as the medium of adhesion between the robot and concrete surface.
Neodymium rare earth magnets are considered the strongest magnet ever. They come in different sizes and grades depending on the magnetic power. According to manufacturer's datasheet, a 65Ã-65Ã-30 mm dimension N42 grade neodymium magnet has a holding force of 1200 kg [****add ref]. Therefore in this particular research the manipulation of the magnetic force of the rare earth neodymium magnets could provide the secret of zero powered adhesion technique for the climbing robot.
3.1.2 Non Destructive Equipments (NDE)
Once the optimum adhesion technique is identified, the next challenge would be to categorize correct types of testing equipments. These equipments have to be precise in data acquisition as well as strong enough to penetrate the concrete surface for defect detection. The most common and effective equipments discussed below could be regarded as considerable in this project.
Ultrasonic Pulse Velocity Method: This method is put into use by generating repetitive pulses of 80 kHz n the concrete surface. A receiver is used in conjunction with the transducer to detect the rebound signal and measure the required time. Then the velocity of that generated pulse is measured by using the following equation:
Pulse velocity = Path Length ÷ Travel Time
The uniformity of concrete joint and any presence of cracks or spalling are then assessed by the results obtained. A higher rate of velocity represents a strong and uniform concrete joint. The instrumentation of ultrasonic pulse velocity technique could be drawn as below:
Figure 5: Pulse propagation on ultra pulse velocity method.
Ground Penetrating RADAR (GPR): GPR makes the use of electromagnetic waves typically in the region of 10-2000 MHz frequency. Waves are generated by an antenna which is carried along the concrete surface. Whenever the waves hit an obstacle, in this case detect any defects, a portion of that reflected pulse travel back to the surface and get detected by the receiving antenna. GPR has a clear edge over other NDE for this particular project as the GPR could be carried easily by the robot along the vertical surface for inspection.
The application of GPR differs in terms of frequency. Higher antenna frequency provides better resolution but lower depth penetration whilst lower frequency leads to larger penetration but lower resolution. The table below shows the resolution and depth penetrations provided by higher and lower frequency GPR.
Blind zone, m
Table 1: Relationship between resolution and depth penetration [day5].
3.1.3 Experimental Setup
Before the first prototype of the wall climbing robot could be built, a vast number of experimental data have to gathered and analyzed for better optimization of initial assumption. Verily carrying out these experiments is time consuming and rather expensive. Bearing that in mind, Computer Aided Design (CAD) could provide better alternative than physical experiments. Finite Element Analysis (FEA) model method could be implemented in the experimental process to simulate the holding force of neodymium magnets in various configurations. In one hand this would save the money spent in buying magnets of different sizes, this could also offer a greater flexibility in choosing experimental parameters.
Figure : A sample FEA model of neodymium magnet.
However, few considerations have to be made prior to choosing the right alternatives.
Subscription cost: Most of the FEA software are specialized software and could cost a lot to subscribe. Fortunately London South Bank University holds a license subscription of Comsol Multi Physics software. It is a leader in the FEA market with great technical support. This subscription will reduce the overall development cost of the proposed wall climbing robot.
Training needs: The University has appointed a specialist technician who has in-depth knowledge of Comsol Multi Physics. Moreover, Comsol Company itself runs various workshops which provide hands on training in using Comsol suite for complex model simulation. Most of these workshops are free for subscribers. So it can be said that the training needs are met efficiently.
3.3 Requirement Analysis and Development Cost
A requirement analysis has been carried out to determine all the elements required for successful completion of this project. A range of design and feasibility parameters were looked at in devising the design specification. By figuring out the fundamental aspects of the design and development process, a detailed system design specification could be obtained as below:
Purpose: Design a robot capable of climbing vertical concrete structures and carry out Non Destructive Testing (NDT).
The robot must be battery operated and should have an operating time of 2-3 hours.
It must be able to carry a payload of 15 kg.
The robot must have a zero or low powered adhesion mechanism to couple with vertical concrete surface.
It must have an actuation system to carry the NDT equipments vertically.
The robot should be able to locate rebar, surface crack, delamination and spalling.
What will it look like?
What materials will be used?
What will be the physical dimensions?
What are the special features?
The robot could be the shape of a model toy car with DC servo motor actuation system. It will implement neodymium rare earth magnets as the adhesive to concrete surface. The robot will be equipped with onboard WiFi module for remote communication. The body of the robot could be made of plastic or carbon fibre to reduce the gross weight of the body. Ground Penetrating RADAR (GRP) will be used as primary rebar locator while ultrasonic and infra-red sensors will also be employed as sensing circuitry.
What is the type and purpose of the system?
What will it do?
How will it do this?
The robot will be able to climb vertical concrete surfaces. Using GPR, it will locate rebars buried under the surface and adhere itself with the surface using permanent magnets. It will be controlled wirelessly along the surface. During that process the robot will collect real time data by implementing various sensing systems and transmit them to control base on the ground.
Who is the system for?
What do they want this system to do?
Where and why will they use it?
The robot must be small in size so that it could be deployed in tall structures. As this robot will be able to carry out inspection on concrete structures, the main used of this system will be civil and construction engineers, structural maintenance technicians and surveyors.
How much would this system cost to manufacture?
As this is a research project therefore, the aim is to make a prototype rather than a mass sale production. The initial expenditure should not exceed more than £500 margin. A detail cost analysis is carried out in later part of this report.
Table 2: System design specification.
Having the specification been laid out, all the materials needed to meet those requirements were represented in a Bill Of Material chart. In this chart all discrete components required were included with the manufacturer's name and quoted price.
DC Servo Motor
Ground Penetrating RADAR (GPR)
Scan Mini HR
54 kHz Ultrasonic Sensor for concrete surface
Table 3: Bill Of Material (BOM) chart.
To facilitate enough time for each step of the research, managing the time effectively is crucial. Proper time management will ensure all critical paths and requirements are addressed and met at the end of the project. A Gantt chart provides the best option when it comes to time management. A Gantt chart has been produced to allocate sufficient amount of time for each task to be carried out. All the critical steps of the whole process are highlighted with milestones to identify the progress along the line.
Completion of literature review.
Finishing of first prototype design.
Complete Testing of the prototype.
Concrete Wall Climbing Robot ready for deployment.
Figure : Gantt Chart with Milestones.