Application of Ultrasonic Pulse Velocity to Detect Concrete Flaws

Alexandre LORENZI 1, Luciane Fonseca CAETANO 1, João Luiz CAMPAGNOLO 1, Luciani Somensi LORENZI 1, Luiz Carlos Pinto da SILVA FILHO 1

1 Laboratorio de Ensaios e Modelos Estruturais, Programa de Pós-graduação em Engenharia Civil, Universidade

Federal do Rio Grande do Sul; Porto Alegre, Brasil Phone: +55 51 33089547, e-mail: alexandre.lorenzi@ufrgs.br

Abstract Concrete structures are normally subject to various aggressive actions derived from the loads or the environmental agents, which make the concrete condition to change over time. The development of controlling methods to determine the condition and ascertain the quality of concrete is critical. In order to prevent and control the deterioration of concrete structures it is therefore interesting to establish continuous monitoring strategies, which might be a powerful tool to increase the service life of concrete. This implies the development of control methods capable of determining the quality and the condition state of concrete. Ultrasonic Pulse Velocity (UPV) testing can play an important role in this area, since they allow us to monitor the density and homogeneity of the material, providing information about the strength evolution and about the existence of internal flaws and defects. UPV is ease to applicants in concrete analysis, not damage the concrete structure to be inspect and its effectiveness in estimating the concrete conditions. This method show great potential in research structures. Thus, it can be conclude that UPV can contribute to the success in control and quality deterioration of concrete structures. Damage may result in a poorly adjusted reduction of service life of the structure. This article shows how they can be employ for UPV testing of control structures Keywords: Concrete, Inspection, Ultrasonic Pulse Velocity 1. Introduction The increasing number of deterioration problems found in relatively new concrete structures provides a strong argument for the development and validation of techniques that would allow monitoring the condition state of elements and provide data on their potential service life. Non-destructive test methods (NDT) have been proven to be very suitable for these tasks, since they are quick, accurate and do not cause any considerable damages to the structure, [1]. NDT methods are very important components of inspection procedures of infrastructure objects: bridges, highways, tunnels, and other civil and industrial structures. Aging infrastructure requires regular inspections for optimal planning of repairing and reconditioning and for assurance of human life safety. Majority of infrastructure objects is complex systems and their variety does not permit to work out a common standard approach to inspection procedures. Over the last decades, several NDT methods have evolved from laboratory curiosities to vital quality control tools in different productive fields. These methods are nowadays commonly used to check variations in structure, changes in surface, presence of cracks or other physical discontinuities, measure the thickness of materials and determine other characteristics of industrial products. According to CEB-FIP Model Code 1990 [2], concrete structures should be designed, constructed and operated in such way that, under the expected environmental influence, they maintain their safety, serviceability and acceptable appearance during an explicit or implicit time without requiring unforeseen high costs for maintenance and repair. NDT tests can be particularly important at concrete structures to improve an in site quality control. Structures monitoring is a necessary procedure, since early detection of problems allows the use of simpler and cost-effective methods for evaluation and repair.

2. Nondestructive Testing Concrete structures form the infrastructure of several countries. Due to the nature of their use, concrete structures are normally subjected to various aggressive actions derived from the loads or the environmental agents, which make the concrete´s condition to change over time. Therefore, the development of controlling methods to determine the condition and ascertain the quality of concrete is critical. The concrete constructions are approaching the end of its useful life of project. Consequently, there is a concern about the state of deterioration and safety. In the current stage, where the quality requirements are sign, it is importance the development alternatives to assess the quality of concrete structures. The dynamic nature of concrete is one of the very reasons why the development of control methods to determine the condition and ascertain the quality of concrete is critical [3]. The application of NDT is an interesting strategy to check and control these structures. The most used property to control concrete structures is the strength. NDT application in civil engineering has become a topic of interest in several countries. This happens because NDT can be used to quality control of new structures [4]. Regarding the durability of civil engineering structures, NDT characterization is a high-performance method to obtain in situ data and hence improve the evaluation of the potential remaining service life of real structures [5]. NDT has been defined as comprising those test methods used to examine an object, material or system without impairing its future usefulness. NDT methods have developed from a laboratory curiosity to an indispensable tool of production. NDT methods can be used to check variations in internal structure; to detect changes in surface conditions, the presence of cracks or other physical discontinuities; to measure the thickness or determine other characteristics of industrial products, [6]. The use of NDT for structural components in civil engineering depends on the application reliability of the testing-methods, the knowledge about the application itself and the economical usage. To lead these methods to a wide acceptance in building industry, performance demonstrations have to be implemented. To use these methods for quality control it is necessary to know exactly the performance and usability of the methods [7]. The deterioration of concrete in structures is a result of simultaneous action of several degradation mechanisms and results in a decrease in the integrity of the structure. The state of deterioration is often hidden from view and is only evident at a stage where there is a significant reduction in the load carrying capacity [8]. NDT is essential in the inspection of alteration, repair and new construction in construction industry [9]. The TCS-17 document, produced by the International Atomic Energy Agency [10] points out several specific situations associated with the construction in which the use of NDT methods can be considered attractive. NDT methods testing can be used in those situations as a preliminary to subsequent coring. Typical situations where non-destructive testing may be useful are, as follows:

a) quality control of pre-cast units or construction in situ; b) removing uncertainties about the acceptability of the material supplied owing to

apparent non-compliance with specification; c) confirming or negating doubt concerning the workmanship involved in batching,

mixing, placing, compacting or curing of concrete; d) monitoring of strength development in relation to formwork removal, cessation of

curing, prestressing, load application or similar purpose; e) location and determination of the extent of cracks, voids, honeycombing and similar

defects within a concrete structure;

f) determining the concrete uniformity, possibly preliminary to core cutting, load testing or other more expensive or disruptive tests;

g) determining the position, quantity or condition of reinforcement; h) increasing the confidence level of a smaller number of destructive tests; i) determining the extent of concrete variability in order to help in the selection of

sample locations representative of the quality to be assessed; j) confirming or locating suspected deterioration of concrete resulting from such factors

as overloading, fatigue, external or internal chemical attack or change, fire, explosion, environmental effects;

k) assessing the potential durability of the concrete; l) monitoring long term changes in concrete properties; m) providing information for any proposed change of use of a structure for insurance or

for change of ownership. One of the principal objectives of the development of NDT techniques is a reliable assessment of defects of concrete members even when they are accessible only from a single surface. This is now the main goal of employment NDT techniques in construction [11]. Using NDT can work in order to check the deterioration monitoring. NDT contribute to success at quality control of concrete structures. NDT methods are becoming very important parts at the inspection procedures for several infrastructure objects: dams, bridges, highways, tunnels, and other industrial structures. This demand will probably increase since aging infrastructure requires regular inspections for optimal planning of repair and intervention, in order to ensure performance and human safety [12]. 2. Ultrasonic Tests Ultrasonic Pulse Velocity (UPV) method is particularly effective, powerful and flexible, allowing in-depth analysis of material homogeneity. It is possible to determine concrete uniformity, to control its quality, to follow up the deterioration, to check the presence of internal flaws and voids using UPV, and, making comparisons with reference specimens, it may estimate potential compressive strength. Finally, when regularly used, it may provide data on the development of problems [13]. UPV can be used effectively for detection of cracks and flaws in hardened concrete [10]. In the case of concrete, the standards establishes procedures for carrying out the tests, which is performed in order to determine concrete uniformity, to control its quality, to follow up the deterioration, to detect internal flaws and voids, to estimate modulus of elasticity and monitor the variations at concrete characteristics [14]. Using UPV, and to make comparisons with reference specimens, it may estimate potential compressive strength. For concrete tests, the method is normally based on the use of portable equipment, composed by the source/detector unit and handheld surface transducers, which works in the frequency range of 25 to 60 kHz [15]. The time between the issuance of the wave and its reception is measured to an accuracy of at least 0,1µs [16]. The evaluation of UPV results is a highly specialized and complex activity, which requires careful data collection and expert knowledge and sensitivity to obtain reliable diagnosis. In order to map the homogeneity of a structure, it is necessary to interpret and connect a large number of UPV readings. Accurate analysis requires reliable interpretation strategies, but there is not yet a proper widely accepted method to analyse this data [13].

UPV method is not adjusted to measure the structure strength, but it is useful to analyze the concrete properties and to determine its homogeneity. For analysis of results, it is necessary experience, and support of software that carry through one adequate analysis of ultrasonic results. To map the homogeneity of one structure it is necessary a great number of ultrasonic readings. The research related in this work aimed to investigate the concrete homogeneity in a dam and contribute to analyse the concrete quality using 3D mapping techniques. 3. Inspection The main objective of this study was to verify concrete homogeneity of the dams by UPV mapping the distribution of UPV. The analysis of the surface contour maps created using the UPV data indicated that the use of the values obtained from indirect measurements seemed to create images that were better fits to the known position of the defects. Variations of UPV were graphically map using a software that generates contour lines for view concrete homogeneity of each element. The generated images shows variations in concrete homogeneity. These variations can be associated to the presence of defects or flaws. UPV propagation at concrete is very sensitive to the presence of flaws, cracks or other discontinuities. The mapping is performed by indirect measurements technique which has proved appropriate to map the surfaces allowing detect voids and defects. The method involve the UPV determination between measurement points located on a mesh created at surfaces of concrete structures under consideration. The map was conduct by processing the readings obtained by use a grid that varied according to the size of the analysed structure. The grid range from 20x20cm to 100x100cm depending on the structure under analysis. For the implementation of the readings was used grease as couplant material, which ensured proper contact between the transducers and the tested surface. This prevent the presence of surface irregularities that interferes at the collected data, Picture 1.

Picture 1 – Structure inspected.

3. Results Pictures 2, 4 and 7 shows the surfaces generated by the UPV results in the concrete structures. In the pictures is possible see differences at concrete that was used in structure. The analysis of these differences shows the different regions of concrete. Pictures 2 and 3 shows the UPV results to structure A. Analysing graphically the results obtained it is possible observe the velocity map on both sides of the structure. The results indicates that the general condition of concrete structure is good. It is possible observed in some regions of concrete structure low UPV values. These values indicate regions that need repair before start dam operation.

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Picture 2 – Surface mapping - structure A1.

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Picture 3 – Surface mapping - structure A2.

Picture 4 shows the results of visual examination of core extracted in regions characterized by different UPV values. By analyzing the cores it is possible improve the diagnosis, allowing define the results range that correspond to unacceptable voids or defects in the structure.

Picture 4 – Visual examination of structure A cores. The results of structure B map indicates that there are large amounts of voids at structure. The UPV mapping of this structure indicates several regions with low UPV values that indicates the structure needs intervention. Figure 5 shows the UPV distribution in the structure B. At structure B was decided to perform a complete scan of the concrete structure. Densification difficulties during produce the structure B cause many surface defects. This result indicates that the structure needs repairs.

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Picture 5 – Surface mapping - structure B.

In addition, a core extraction indicates the presence of a large number of voids. Pictures 6 and 7 shows the results of visual examination that was made at concrete cores extracted from regions characterized by low UPV. The analysis of cores allows improving the diagnosis by setting a range of results that correspond to unacceptable voids at structure.

Picture 6 – Visual examination of cores at structure B.

Picture 7 – Visual examination of cores at structure B.

The results of structure C map indicates that in general the concrete conditions are acceptable. Some regions that UPV results are lower but are located between 10 to 12 meters high, where there was possibility to have a concreting interruption. UPV results indicates that the concrete condition of the element is very good. It is not necessary to make one significant intervention to repair. Specific interventions were made to correct local problems detected by UPV. Picture 8 shows the UPV distribution on both sides of structure C.

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Picture 8 – Surface mapping - structure C. 4. Conclusions NDT tests can be used to decision-making and establish strategies to proceed an intervention, as demonstrated in this case study. The concrete condition investigation in structures demonstrated the capability of UPV to check the conditions concrete structure. The UPV tests can contribute to the quality control of concrete structures. This study has demonstrated that UPV tests can be used to located relatively small defects in concrete structures, which could represent problems in terms of structural performance. UPV tests are very sensitive to homogeneity and density variations and can provide important data for decision making about the conditions of concrete structures. The use of a contour mapping technique was show to be an adequate way to organize and interpret the results of UPV tests, in order to detect suspicious zones. The use of grids to collect data is seen as a good way to structure the test and make sure that no region is overseen. This research demonstrated the capability of the use UPV for check the condition of concrete structures at dams. In general, the work carried out indicates that UPV tests can play an important role in determining the existence of defects in concrete structures. References

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