A STUDY OF NDE-PERFORMANCE IN THE CONDITION ASSESSMENT OF REINFORCED CONCRETE STRUCTURES

Transcription

1 A STUDY OF NDE-PERFORMANCE IN THE CONDITION ASSESSMENT OF REINFORCED CONCRETE STRUCTURES Peter Shaw Force Institute Denmark Abstract In order to understand the condition of concrete structures with a view to effective maintenance planning then inspections should be customised to meet individual needs. If inspections are to provide more than superficial information then non-destructive testing methods will be required. The recent interest in non-destructive testing has been considerable, yet it s application limited. The methods are considered to be mainly qualitative, and they are sometimes regarded as being ambiguous as well as not capable of performing sufficiently well in real situations. The demands placed on non-destructive testing method capability are usually expressed in simple terms, such as the ability to detect a defined size of void. There has been little done to express the inspection needs in terms of typical problem cases, with a clear definition of defect type, acceptance criteria and limiting conditions. In turn there are few accepted procedures for applying non-destructive testing, as there are few examples of typical cases. Emphasis should perhaps be placed on focussing the available technology on welldefined problem cases and dealing with these from an engineering viewpoint. Recent work with High Energy Radiography combined with a digital imaging system is presented here together with a new ultrasonic pulse echo technique. Both of these have meant considerable advances in NDT-techniques and are presented briefly in this paper. 63

2 1. Introduction Experience in Northern Europe during the last decade has shown that the awareness of how concrete structures should be managed is extremely varied. Considering ten different large projects, which have included both general condition surveys and specific problem cases, it was found that all the structures were to various degrees insufficiently robust to have prevented some deterioration from occurring. Considering that all of these structures were 20 to 30 years old at the time of inspection then this is not surprising, as the knowledge available on the topic was not at all good at the time of construction. However, in seven of the ten cases the critical, life-limiting problems were caused by poor workmanship, insufficient control during construction combined sometimes with major defects such as that caused by mechanical damage at an early age. The motivation for making an investigation is varied of course, but usually follows a pattern. In the larger sectors of industry or public works there is transfer of information, which means that if a problem is identified at one location then it will be dealt with successively at others. It is, however, often after 20 or 30 years that the first investigations are made. Defects can be either visible (surface) or hidden (internal). Visible indications of damage are often motivation for closer investigations, but are often not sufficient to prompt an investigation. Even visible defects remain hidden if not observed. Considering the frequency of unexpected defects found during routine investigations then the question must be asked as to how many undetected and serious defects there may be in the thousands of structures, which do not even undergo regular visual examination. Apart from the deterioration processes which progress from the concrete surface and which can normally be observed and quantified with relative ease using more conventional methods, there is also a danger of internal damage and deterioration. Even today problems commonly arise due to lack of proper concrete compaction and for instance settlement of form-work during casting. This can lead to serious corrosion problems at a later stage, as well as causing local reductions in strength. The fact that seven of the ten cases mentioned were found to have significant defects originating from construction is surely evidence of the need for more control. It is rare that non-destructive testing is applied to establishing the condition in terms of the remaining lifetime of a structure. However, if used even moderately during and after construction then the chances of detecting serious or potentially serious defects would be greatly increased. In the twelve cases considered below in which non-destructive testing has been used then the problem has been associated with construction defects (voids and cracks) or 64

3 simply to determine thickness and reinforcing details. The investigations were, with one exception prompted by visual evidence of damage or lack of available drawings. Structure Problem case NDE methods Result used: success* Bridge Casting defects UPV, IE, SASW 3 Bridge Concrete repair quality UPV, IE, SASW 2 Bridge Casting defects (settlement) UPV, IE, SASW 2/3 Bridge Casting defects (voiding) UPV, IE, SASW 3 Bridge Mechanical dam (crack depth) UPV, IE, SASW 2/3 Bridge Cable duct condition (voids) HER, Rad 1/2 Bridge Post-check reinf cover Rad, CM 1 Roof slab Thickness & reinf quantity Rad, IE 1 Floor slab Thickness IE 1 Concrete tank Homogeneity IE, UPV 2/3 NPP Casting defects (voids) IE 4 NPP Reinf details HER, Rad, CM 1 The following grading has been used: 1 Quantitative with high degree of accuracy 2 Qualitative and to some degree quantitative 3 Qualitative (able to delineate) 4 Ambiguous The abbreviations used are as follows: HER High Energy radiography (7.5 MeV Betatron) Rad High Frequency radar (1 GHz) CM - Covermeter IE - Impact Echo SASW - Spectral Analysis of Surface Waves UPV - Ultra-sonic Pulse Velocity As can be seen from the table then useful and accurate information about reinforcement details can be obtained with available methods. A simple problem case such as measuring concrete thickness crops up from time to time and can be determined accurately with Impact Echo. This has wider implications, e.g. the same principles can be used to measure the quality of bond between concrete layers and to measure the depth to voids and cracks. 65

4 In those cases in which the problem is less well defined, such as suspected casting defects, then seismic methods are used primarily for their speed and the ability to measure from one side of the structure. They generally provide qualitative information although in some cases they can be analysed to estimate physical properties, e.g. of different layers of concrete. In all of the above cases (graded 2 or 3), with one exception, it was necessary to drill cores from selected points to be able to confidently describe the concrete condition in terms of the test results, or simply to interpret the signals. Perhaps this illustrates more clearly the need to establish data about the expected response of seismic tests in particular problem situations, and not least to establish testing procedures and criteria. Improvements will, however, have to be made one way or another, as seismic methods are often still regarded as a bit ambiquous by the client, despite the enthusiasm of the inspector. Also, the result of non-destructive testing often do not meet the client s expectations. This suggests that the whole process from planning to execution and not least visualisation and presentation of results must be improved. 2. Some NDE developments A non-destructive examination of large structures requires methods that can rapidly survey to locate problem areas and having found them to describe them. By their very nature seismic techniques are fairly rapid and can penetrate concrete to great depths, although the information they provide can be quite limited and also sometimes confusing, particularily if nothing is known about the structure or if the structure geometry is complicated. Radiography will provide very detailed information and high-energy units can cope with concrete up to 1500 mm thick, but it cannot be practically applied to other than point investigations. Some recent advances have been made in both seismic and radiographic techniques, which make them attractive alternatives, bearing in mind what is stated above. A series of tests were made on concrete mock-ups with well-defined defects and reinforcement configurations. Some results of these results are presented here. 3. The reference block This is shown in Fig.1. It consists of a 16 meter square and 800 mm thick reinforced concrete slab on supporting legs. The slab is reinforced on top and bottom sides with φ 16 mm bars at 200 mm centers. Inside the slab are a number of both cubical and 66

5 spherical voids, as well as various crack configurations. The concrete used is described below: Strength (28 day) 40 MPa (measured 52.4 MPa) Density 2393 Kg/m3 Water/cement-ratio 0.37 Air content 4.6 % Aggregate type Crushed Dalby granite Aggregate size Max 25 mm Test sequence Ultrasonic Pulse Echo The block was scanned at intervals of 300 mm as depicted by the blue lines in Fig.1. Along each line a separation of 100 mm was used. This produced B-scans of the entire block. Also, each defect was scanned at test intervals of 20 mm and in some case 10 mm and at various frequencies. 4. The Ultrasonic Pulse Echo unit The unit is manufactured in Russia and named A 1220 Ultrasonic Flaw Detector. It consists of a transducer unit with an array of 24 pointed and spring-loaded transducers plus a hand-held computer. The equipment can be operated easily by one person. 67

6 The transducer produces shear wave pulses and acts as a sending-receiving unit. The frequency of the signals can be varied from 33 KHz to 250 KHz. The transducer is used without a contact medium and does not require surface preparation. A B-scan file consisting of up to 24 x 40 tests can be stored and transferred to a laptop on site in a matter of minutes. The A-scans can be examined individually. In our case a B-scan consisting of 320 points was made and the data cube displayed in less than one hour. Test results For the test block mentioned above the maximum penetration (transmission-reflection) was found to be approximately 1000 mm. In a series of tests through solid concrete the coefficient of variation of full thickness velocities was found to be 1.3 %, which demonstrates both homogeneous concrete and a high degree of measurement accuracy and consistency. The results were found to be unambiguous for both solid and voided concrete. A blind test of the Block was made and all defects were detected. Likewise solid concrete was established correctly as being solid. A-scans were made at close intervals across each defect (void and crack). Depth measurement was within ± 20 mm, but usually better. Three criteria can be used to establish wether the concrete is solid or contains voids: 1) The existence or not of a back-wall echo at the test point 2) The directly reflected signals from defects 3) Multiple echoes from defects (useful in congested areas) 68

7 Fig.2. A-scan at 33 KHz showing a 100 mm cubic void at 305 mm below a layer of reinforcing. In Figure 2 it can be seen that the direct reflections from the 100 mm void are clearly distinguishable from the upper reinforcing. The multiple echoes from the void can be seen as can the window in the back-wall echo at 800 mm. Fig. 3: The linear reflection to the right is from a 200 mm cubic void at 220 mm. The hyperbolae to the left side are caused by 16 mm φ reinforcing bars. 69

8 The width of reflectors can be established with good accuracy. This is due to the nature of the transmitted wave fronts, which appear to have little spread from the transducer. Three factors may be used to establish the width of reflectors: 1) The width of the reflected image 2) The width of the back-wall echo window 3) The width of the multiple echo (where applicable) It is thought that with reasonably experienced use then defect size can be estimated very accurately, particularly in the case of flat reflectors such as parallel cracks and square voids. Fig.4: The indication to the right is from a 200 mm cubic void at 500 mm. The hyperbolae to the left are from reinforcing bars, as in Fig.3. 70

9 Fig.5: The indication to the right in the picture is from a 6 mm encast steel plate, behind which there are two voids of approximately 130 mm diameter. The voids are placed adjacent to the steel plate. The distance from the testing surface to the steel plate is 240 mm. The indications to the left in the picture are from reinforcing bars. The Ultra-sonic Pulse echo equipment has performed extremely well in a number of different situations and problem cases. This has included the delineation of voids and cracks, location of reinforcement and pre-stressed cable ducts, as well as location of voids in ducts. The equipment must be seen as a major step forward in non-destructive testing technology, not least because of its simple application and speed. 5. High Energy Radiography and the Agfa Strukturix Digital Phosphour System High Energy Radiography using a 7.5 MeV Betatron has been used successfully on major projects to determine exact reinforcing details and to inspect the condition of pre-stressed cable ducts. Until recently then a traditional film was used, typically FD8p in large format. The limiting factor has been exposure time which is approximately 60 minutes for 1000 mm thick concrete. This might define an upper practical limit of concrete thickness. Other difficulties arise using film, e.g. lack of dynamic range and difficulties in presentation. The Agfa Strukturix DPS system Instead of traditional film this system uses imaging plates which when exposed to radiation stimulate phosphor crystals on the plates to a higher energy level which is dependant on the accumulated radiation dose. The number and level of excitation at 71

10 different locations on the imaging plate is, thus determined by the density of the various parts of the object under test. Using a scanner the equivalent entrapped energy on the exposed plates is converted to light, which is captured by a photo-multiplier and converted into digital data. This data is then post-processed in a work-station. The imaging plates may be used many hundreds of times. Test set up Mock-ups with well-defined reinforcement configurations and voided pre-stressed cable ducts were constructed. A sufficient number of concrete blocks were made to enable concrete thickness to be varied between 300 mm and 1500 mm. Comparisons were made between the Agfa Strukturix DPS-imaging plates and both FD8p and D7 film. The pre-stressed cable duct was injected with grout and voids were made in the grout with sizes from 15 mm to 70 mm. The duct was then cast into a 300 mm thick concrete block. A focus-film distance (FFD) of 1600 mm was used in all tests. Full radiation energy (7.5 MeV) was used. In these tests an object-film (plate) distance (OFD) of approximately 150 mm can be assumed. Objectives of the tests One of the primary objectives was to determine if the imaging plates could provide image quality equivalent to film in the same, or preferably shorter exposure times. The maximum practical concrete thickness was also of interest. In addition the sensitivity of the imaging plates and film in detecting voids was examined. Small voids inside pre-stressed cable ducts can cause corrosion and initially a target of detecting a 20 mm void in 1000 mm reinforced concrete was set. Further tests were made to determine how accurately bar diameter could be measured, as well as bar section reduction and relative depth of bars. 72

11 Fig. 6: Image of 300 mm thick concrete using the DPS-system. The duct can be seen together with reinforcing bars. The 15, 20 and 30 mm voids are clearly visible. In figures 6 and 7 can be seen the results of tests on 300 and 1000 mm thick concrete. The image quality decreases with concrete thickness and the level of contrast (difference in image density) between solid and voided concret decreases with increasing thickness. Despite this it was possible to detect all voids, down to 15 mm in concret ewith thickness up to 1200 mm using the DPS-imaging plates. Reinforcement bar size can be determined with very good accuracy. This requires knowledge of bar position (depth) to make allowance for geometrical effects (image projection). Alternatively then two shots from different angles may be taken to determine bar depth. Near-side and far-side bars can be distinguished by comparing image density (steel bar:concrete) with the width of the projected image of the bars. 73

12 Fig.7: Image of 1000 mm thick concrete. The vertical lines are caused by reinforcing bars and to the right hand side of the picture can be seen the projected image of a reinforcing bar from the focus-side of the test blocks. The visible voids have dimensions 60 and 70 mm. Fig. 8: The image shows 300 mm thick concrete beams with a slotted 16 mm bar placed behind. The slots are placed at right angles (left) and parallel with (right) the direction of radiation., Attempts were made to detect and measure bar reduction due to corrosion. This was done in three ways: 1) By inspection of a concrete block with suspected corrosion 2) By casting a severely corroded bar in one of the concrete blocks 3) By placing a slotted bar behind concrete blocks (Fig.8). 74

13 In the first case it was found to be difficult to distinguish corrosion on small diameter bars. suspected corrosion was found to be mistakenly confused with local voiding or pores in the concrete adjacent to the bars. In the second case then it was possible to visibly detect a 50% loss of section on a 12 mm bar. It should be pointed out that the bar had been thoroughly cleaned before casting into the block, so there were no corrosion products adjacent to the bar. In the third case then as expected it was possible with ease to detect all bar reductions down to 10 % (90 % remaining) by visual examination (that is without making use of the data processing facilities) if the loss of section is oriented perpendicular to the direction of radiation. For the other case, that is parallel orientation, then a 20 % reduction can be detected visibly. The second case is the more realistic of the two as it would not normally be possible to radiograph parallel with the corrosion plane of real reinforcing bars. Comparing traditional film with the DPS-system for detection of reinforcement corrosion then the latter is clearly more suitable. If over-exposed then reinforcing bars on film may falsely appear to have reduced section. The DPS-system has the benefit of wide dynamic range, which can be used to establish wether there is bar reduction or not. Further experiments will be conducted on realistic mock-ups, that is with actual corroding bars cast in concrete to determine what effect corrosion products and eventual cracking and voiding at the point of corrosion will have on visibility. The most significant advantage of the DPS-system over traditional film has been found to be the reduction in exposure times, which is approximately 60 % compared with FD8- film. This advantage is greatest for concrete thickness around mm. Film exposure times for 1200 mm thick concrete are prohibitive. The maximum possible thickness of concrete which may be radiographed with the 7.5 Mev Betatron using the DPS-system, has been found to be 1500 mm. 6. Conclusion It would be of great benefit to the civil engineering industry if quality control beyond visual inspection of new reinforced concrete structures was to include checks of internal integrity and conformity both during and after completion and construction. In this way it would be possible to detect and remedy defects which might in the future become critical and life-limiting. This requires the use of non-destructive testing, which can be executed efficiently and confidently. Not least the testing methods must produce results which can be easily understood by both user and client. It must also be a goal to make non-destructive 75

14 techniques available on a wider scale, rather than being restricted to a relatively small group of specialists. The combination of High Energy Radiography and the Digital Phosphour System enables typical concrete thickness to be examined on site in reasonable exposure times. The results can now be visualised and reported in a rapid and more understandable manner. Seismic testing requires both improved capability, but also the ability to test, process and visualise a large number of points rapidly. The development of this new ultrasonic pulse echo equipment has gone a long way in meeting these needs. Acknowledgement This work has been partly sponsored by The Nordic Industrial Foundation. 76