Bundesanstalt für Materialforschung und -prüfung

International Symposium (NDT-CE 2003)

Non-Destructive Testing in Civil Engineering 2003
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Nondestructive evaluation of concrete structures
- A case study in detailed inspection using 24-year-old reinforced concrete -

Satoko Watanabe, Technology Center of Metropolitan Expressway, Tokyo, Japan
Sudhir Misra, Visiting Professor, ICUS, IIS, University of Tokyo, Japan
Taketo uomoto, Professor and Director, ICUS, IIS, University of Tokyo, Japan

ABSTRACT

Deterioration observed in concrete structures has clearly highlighted the need for periodic inspection and maintenance. The Japan Society of Civil Engineers has recently published specifications for the maintenance of concrete structures, which lay down the framework for initial, periodic and detailed inspection. A 3m long piece from a 1.2m high sidewall removed from its location during a highway realignment was moved to the laboratory and extensively studied using various nondestructive methods. This paper presents the results obtained in the framework of maintenance procedure that could be followed in the inspection and maintenance of structures.

INTRODUCTION

Concrete structure had long been considered to be maintenance-free, but that 'belief' has been found to be grossly misplaced by the signs of aging and deterioration that have been seen on concrete structures all over the world. Though considerable research in mechanisms of deterioration, such as, alkali aggregate reaction, chloride induced reinforcement corrosion, freezing and thawing, etc. has substantially improved our design and construction procedures, the task of 'maintaining' existing structures has become a daunting challenge to civil engineers. In Japan, accidents involving spalling of concrete from a tunnel and a bridge have attracted attention and considerable concern for the 'third party' safety and serviceability of the structures has been expressed[1].

In large cities like Tokyo, it is nearly impossible to shut down infrastructure facilities (e.g. highways) even for a short period, and therefore inspection and possibly even maintenance action needs to be carried out keeping in mind the operational requirements. Thus, there is an urgent need to evaluate existing tools and develop appropriate methods and procedures, keeping in mind factors such as non-availability of trained manpower and developments in other areas of engineering and technology. This paper presents the results obtained from extensive nondestructive testing carried out using a concrete sidewall removed from a highway.

INSPECTION OFRC STRUCTURES

Usually owner organizations carry out periodic inspection of concrete structures under their charge using the means at their disposal. Such inspection is often limited to visual observations and only when deemed necessary, is measurement using specific tools (Schmidt hammer, etc.) carried out. The procedures presently followed often require experienced manpower and apart from the subjectivity involved, often suffer from lack of reproducibility and the results can hardly be appropriately recorded and used for comparisons subsequently.

Fig 1: Overall maintenance methodology.

Figure 1 is a representation based on the methodology laid down by the Japan Society of Civil Engineers for inspection of concrete structures[2]. An effort has been made to streamline inspection procedures and link initial inspection, to be carried out after completion of the construction, to the repair and maintenance action, that may be required. Of course, given the diversity in the nature and environment of the structures, issues like frequency of inspection have been left to individual agencies.

TEST PIECE USED IN PRESENT INVESTIGATION

Fig 2: Test piece used in present investigation.

A part of the metropolitan expressway built in 1977 for a traffic of 61000 cars/day in the outskirts of Tokyo (about 1.5km from the seacoast) was to be removed during a realignment of the expressways. A 3m x 1.2m piece from the RC sidewalls of this part was brought to the laboratory for detailed investigation[3]. A front view and the cross-section of the piece are shown in Figure 2. The outside of the piece was the target of the present investigations. The choice of this face was governed by considerations including

  1. this face, being exposed to the south side, was more susceptible to carbonation,
  2. any damage or spalling to this side constituted a greater hazard from the point of view of risk to a third-party, and,
  3. given service conditions, only this surface can be used for monitoring or nondestructive testing.

Though it was not possible to locate the design and construction records, details of the specifications generally followed at the time of construction, are shown in Table 1[4].

Design strength 300kgf/cm2
CementOPC
Gmax25mm
Slump8±2.5cm
Air4±1%
Dia of bars13mm (deformed)
Cover30mm
Table 1: General Specifications.

NONDESTRUCTIVE TESTS CARRIED OUT

Visual inspection
When observed from a distance, as is likely to be expected during an actual inspection, it was difficult to extract any conclusive evidence, except that the surface appeared roughened and exposed aggregates could be seen at the surface.

Figure 3 shows a close-up of the surface, and it is clear that about 4mm of the cement paste from the surface has been eroded, exposing coarse aggregates. Observations from a closer range also showed evidence of about eight cracks originating at the top surface, extending to varying lengths towards the bottom. Of course, an accurate assessment of the location, length and width of the cracks was difficult, though the maximum width at the surface was estimated to be about 0.5mm. No estimate about the depth of these cracks was at all possible. A schematic representation of the cracks traced from the concrete surface is given in Figure 4. The figure also shows the 20cm grid marked on the surface to facilitate comparison with results from other investigations.


Fig 3: Surface of concrete with aggregate exposed.

Fig 4: Schematic view of cracks and measured width

Using digital photographs
Figure 5 shows a representation of the digital photograph of the test piece using a digital camera. It can be seen from the figure that the cracks observed on the surface, can be identified in the photograph also. Using the resolution of camera, the width of these cracks was estimated and the results showed that the lengths and widths could be reasonably accurately estimated from the digital data, though it was not possible to discern cracks smaller than 0.1mm in width.

Fig 5: Digital photograph of test piece showing cracks detected. Fig 6: Representation of test piece using infrared thermography.

Using infrared thermography
Infrared thermography has been proposed as one of the tools to study the distributions of surface temperature in concrete and relate the differences to the defects. An infrared camera, was used to study the surface temperatures of the test piece over a period of 6 hours (10am to 4pm) on a clear day, from a distance of about 4.6 m. A typical distribution is given in Figure 6. It can be seen that distribution of observed temperature is relatively uniform, and the method may not help in identify the cracks mentioned above. Also, given the nature of the method, though an instantaneous measure of the surface roughness is hard to obtain, the method could be used to follow changes in surface characteristics over time.

Crack depth estimation using ultrasonic method
Using the principle of wave of diffraction from the edge of a crack, and using two sensors at different distances across the crack, the depth of cracks shown in figures 4 and 5 was measured and the results are given in Table 2. The locations for the measurement of the depth are as shown in Figure 4. From the observations, it was found that though the cracks do not penetrate the full depth of the 200mm section, the depth is much larger than the provided cover of about 30mm. In other words, the reinforcement is rendered susceptible to corrosion, though signs of corrosion are yet not seen on the surface. Besides the effect on the integrity of the reinforcement, information on depth of cracks is invaluable in cases grouting of cracks is to be undertaken.

Location 1-1 1-2 1-3 2-1 2-2 3-1 4-1 5-1 6-1 6-2 6-3 6-4 7-1 8-1
Depth7576276217759082124110130191580
Table 2: Crack depth estimated by ultrasonic method. [unit;mm]

Determination of reinforcement location and cover thickness using radar method
Fig 7: Location of reinforcement using radar scanning.
By scanning the concrete surface using radar equipment, the location of the bars and the depth of concrete cover were found. It was found that the average spacing in the main reinforcement was about 95mm, whereas spacing in the lateral reinforcement varied between 100 and 140mm. A schematic representation of the layout of reinforcement using radar scanning is shown in Figure 7. As far as the depth of the concrete cover is concerned, it was found to vary between 30 and 40mm with an average of about 37mm.

Studying reinforcement corrosion using measurement of natural potentials
Fig 8: Distribution of natural potentials.
Having got the layout of the bars, natural potential was recorded at a spacing of 5cm using
Cu-CuSO4 electrode and the results in terms of the contours are plotted in Figure 8.

The results were evaluated on the basis of the guidelines of the research committee of the JSCE, summarized in Table 4[5]. It may be noted that these guidelines are a slight modification of the more popular ASTM guidelines, which has a very wide window of natural potentials about which no definite assessment can be made. Thus, the corrosion activity in the test piece was determined to be 'slight'.

Natural potential (E) Corrosion level Remarks
-250mV£EIBlack surface with no corrosion.
-350mV£E£-250mVIIOnly spots of rust on rebar surface
-450mV£E£-350mVIIIThin rust layer with corrosion products adhering to concrete
E£-450mVIVFormation of expansive corrosion products, with little loss in cross-section
VFormation of expansive corrosion products, with loss in cross-section
Table 4: Evaluation of natural potential data (Reference 5).

Estimates of compressive strength
An effort was made to estimate the compressive strength of the concrete using the Schmidt rebound hammer and compare them with actual strength obtained using cores. The results from the rebound hammer are given in Figure 9, in terms of the raw data and the estimated value of the compressive strength.

Fig 9: Estimates of compressive strength.

The rebound numbers were calculated using 20 values determined at each target area, and using appropriate corrections for direction, etc. Though the average value of the strength could be estimated to be about 33N/mm2, the figure shows that the values are quite different for the top and the bottom half, where the individual averages were 26N/mm2 and 39.5N/mm2 respectively. This difference can be attributed to difference in the thickness of the wall in the two portions.

DISCUSSION OF RESULTS

Verification of results from nondestructive inspections
To physically verify the results obtained using the methods described above, the concrete was chipped at three locations from the wall to reveal reinforcement, and the following observations can be made:

  1. Compressive strength vs. estimates using Schmidt hammer:
    The compressive strength obtained using 3 cores from the top portion of 100mm diameter was found to be 32 N/mm2, with the individual values Figure 10 Estimated and being 33, 33 and 30 N/mm2. Now, compared with the measured cover thickness average of 26N/mm2 from the results of the Schmidt hammer in that portion, it is clear that the results are not in very close agreement.

  2. Cracking and crack width:
    Though it was not possible to get any information about the cracks from the studies using thermographic data, the data from digital camera, showed promise in terms of being able to identify cracks, and even provide a reasonably accurate estimate of the width.

  3. Corrosion of reinforcement using natural potential:
    Though interpretation using the ASTM guidelines was not conclusive, but using the guideline developed by the research committee of the JSCE [5], only slight corrosion activity was expected, and this was found to be true to the extent that the direct observation of the reinforcement suggested in areas where the concrete was chipped out.

  4. Estimates of cover thickness and reinforcement location radar scanning:
    The actual cover to the reinforcing bars was measured at different locations, and Figure 10 shows a comparison of the estimated and actual cover values. It can be seen that results are basically in agreement, though difficulty was encountered in measuring cover in the bottom half of the test piece having dense reinforcement.

    Information about rebar location is often required for drawing cores, measurement of natural potential, etc. It may be pointed out that though the scanning yielded good results in the upper about 600mm, but there was too much interference from adjacent reinforcement to enable accurate mapping of the bars in the bottom part. Improvement in the results in this part could be expected by changing the frequency of the equipment, which was maintained at a level of 1GHz in the present work.

Fig 10: Estimated and measured cover thickness.

USING NONDESTRUCTIVE TESTS FOR EVALUATION AND INSPECTION

On the basis of the results of this study, it can be said that tools such as thermographic and digital images can be used during periodic inspection to obtain and record reliable data during periodic inspection. These methods do not need close access to the structure and yield information that can be easily stored and used as a basis for comparison at the subsequent inspection(s). Also, the radar scanning method could be used in cases when studies the location and/or cover to the reinforcement is required, though the method requires close access to the structure. However, the method could be effectively used in initial inspection when actual data for cover, etc. is required.

However, tests such as the measurement of natural potentials, etc. may not be conclusive in obtaining information about the corrosion of the reinforcement. More work thus needs to be carried out to develop an appropriate methodology to accurately understand the present state and rate of deterioration.

CONCLUDING REMARKS

The need to develop appropriate nondestructive testing methods for existing concrete structures cannot be over-emphasized. In addition to visual inspection, various new methods were used with the view to determine their effectiveness and range of applicability. Within the limitations of the results and the experimental conditions, it can be said that methods such as digital photography and thermography can be used to effectively compliment visual inspection and the records used for comparison at the time of subsequent inspection, without having to get physically close to the structure. Methods such as Schmidt rebound hammer, radar scanning, ultrasonics, natural potential, etc. could provide valuable information but can be used only in cases when the structure is physically accessible, or an appropriate arrangement is made. However, the results from the present investigation highlight the need for additional work before these methods can be effectively used as part of a comprehensive strategy for detailed inspection.

ACKNOWLEDGEMENTS

The results discussed here form a part of the Report of the Research Committee on Non-Destructive Evaluation for Concrete Structures of the Institute of Industrial Science, University of Tokyo, and the support of all the members of the committee is gratefully acknowledged.

REFERENCES

  1. Uomoto, T.; 'Towards Safer and Securer Built Environment in Mega Cities', Seisan Kenkyu, University of Tokyo, Vol.55, No.2, 2003
  2. Standard Specifications for Concrete Structures (Maintenance); JSCE, 2001
  3. IIS Committee (RC-7) Report; Non-Destructive Evaluation for Concrete Structures, IIS, University of Tokyo, 1999-2002
  4. For example, Standard Specification for Design and Construction of Concrete Structures; JSCE , 1974
  5. Concrete Engineering Series 26; Present status and future trend in research related to reinforcement corrosion, corrosion protection, and repair, JSCE, 1997
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