Keywords: Detection, Cover Meter, Localisation, Magnetic Method, Prestressed Elements, Radar,Rebar, Reinforcement
This paper was presented at the International Symposium Non-DestructiveTesting in Civil Engineering (NDT-CE) 26.-28.09.1995 in Berlin. NDT-CE, Full Program or the Ultrasound Part

Abstract

The paper describes a method of linking a distance-controlled covermeter with a radar system to an efficient non destructive testing method for exact location of prestressed elements under multiple rebar layers. During investigation, rebars or other metal objects near the surface are located by the covermeter and marked graphically within the radar profiles. The gain of information improves the interpretation of radar profiles without any additional efforts. Besides localisation of prestressed elements, the system can be used for all radar investigations where the knowledge of metal objects near to the surface helps analysing radar profiles or when a differentiation between precious and base metals is necessary.
Table of contents
1. Introduction
2. The Measurement Principle of the HOCHTIEF Covermeter
3. The Measurement Principle of Radar
4. Combination of a Covermeter and Radar
5. Bench Tests
6. Interpretation Example
7. Practice Investigations
8. Further Application Possibilities

1. Introduction

Non destructive measurement procedures are necessary for assessment of the stability of old and new constructions and for alteration of existing structures. Damage of prestressed elements during drilling procedures involves high cost for repairs. Damage and costs can be prevented if the exact positions of the prestressed elements are known. Also for investigation of a single prestressed element in case of corrosion or cracks, damage to a structure through removing concrete can be reduced or completely prevented by knowing the exact position of the prestressed element. Even if building documentation's are available, the exact position of prestressed elements can only be determined by in situ investigations. Radar survey is on principle applicable for location of prestressed elements below rebars [2]. The interpretation of the recorded signal (radar profile) however is complicated by overlying layers of rebars. Improvement of interpretation can be achieved by separating the signals of different types of reinforcement. The following chapters describe the measurement principles of the covermeter and radar. Chapter 4 explains how both systems can be combined. Finally chapters 5 to 9 deal with applications and results of the combined system.

2. The Measurement Principle of the HOCHTIEF Covermeter

The HOCHTIEF covermeter is a device to locate rebars and to measure the concrete cover of rebars [1]. The covermeter creates a low frequency electromagnetic field with a coil. The inductance of this coil changes when metal objects are put near to the coil. The change of inductance depends of the type of steel, the quantity and the distance between the steel and the coil. The influence of magnetic metals (base metals) is much greater than the influence of non magnetic metals (precious metals). The type of steel is known, since in practice mainly base metals are used for armament near to the surface. The mass of steel can be determined by the diameter of the rebars. The diameter of the rebars is constant within one object and mostly known. If however, the diameter is unknown, it can be determined by a small opening of the concrete. Since the quantity and the sort of steel are known and constant, the distance between the rebar and the coil, which is equivalent to the concrete cover, can be determined from the change of inductance.

A measurement is performed by rolling the covermeter across the surface of the concrete. The measured values are transferred to a laptop and displayed in a diagram. Figure 1 shows a concrete specimen with rebars and the corresponding diagram of a measurement. The horizontal axis correlates with the measured distance, while the vertical axis correlates with the measured cover. The highest points of the curve show the positions of rebars. These peaks correspond to the minimal distance between the coil and the rebars and are equal to the amount of concrete cover [3]. In a different mode, with no laptop connected, the built in microcomputer of the HOCHTIEF covermeter detects these peaks on-line during measurement. Every time a peak is detected, the covermeter creates a short audible and visible signal. This function can be used to easily determine the lateral positions of the rebars.

Figure 1: Diagram of a covermeter measurement

3. The Measurement Principle of Radar

In civil engineering, radar investigations are used among other things for localisation of impurities in concrete structures such as armament or hollow areas [2]. In practice, a pulsed radar system is applied. It consists of a transportable computer, a small monitor with a built-in keyboard and one ore more antennas. The antennas are connected to the computer through special input/output ports. During measurement, the antenna is moved across the surface of the concrete by hand or with a rail system. Figure 2 shows a specimen with a rebar in the middle and the corresponding simplified radar profile underneath. The antenna is moved in direction of the arrow. The antenna emits short impulses of electromagnetic waves (pulse length=1 to 2 ns, frequency approx. 1 GHz) Electromagnetic waves spread spherical in homogeneous materials. Because of the composition of the antenna, the spherical spreading is focused to a shape that can roughly be approximated by the shape of a cone (see figure 2). If the electromagnetic waves hit an interface between two materials, a part of the wave is reflected. The amount of reflection (equation 1) depends on the difference of the dielectric constants of the two materials [4].

Figure 2: Radar profile of a specimen

Equation 1 Reflection coefficient (RC) of an electromagnetic wave hitting the interface between a material with a dielectric constant and a material with a dielectric constant . Figure 3: Example of a reflection A wave with the intensity A, crosses the interface from air to concrete . This results in an RC=0,33. This means that 33% of the wave is reflected (RC·A), while 67% penetrates the concrete (1-RC)·A.

The reflected parts of the wave are received by the antenna. The intensity of the received reflections of one pulse are recorded in a single line along the vertical axis of the radar profile (figure 3). Waves reflected from interfaces near the antenna are received earlier and therefore displayed higher in the radar profile than waves reflected from interfaces far away (equation 2). The intensity of the reflections is displayed in form of different colours. The recording of all received reflections of one pulse is called a scan. When the antenna is moved, more scans are generated by the survey wheel and displayed next to the previous scan. The sequential scans form the complete radar profile. The horizontal axis of the radar profile corresponds to the measured distance, while the vertical axis corresponds to the depth of the measured object. Because of the cone shaped radiation of the electromagnetic waves, interference patterns like the rebar in the middle of the specimen in figure 2 form a hyperbolic structure in the radar profile. The multiple hyperbolas are caused by multiple reflection of the wave between the rebar and the antenna.

Vew =	c ------			Equation 2 Vew = speed of an electromagnetic wave in a material with an dielectric constant .
d =	Vew	x	dt ------ 2	d = distance between antenna and point of reflection. dt = time between emitted and received wave.

4. Combination of a Covermeter and Radar

The covermeter is an efficient tool to locate rebars near to the surface. The interpretation of the signals is not very difficult, since the covermeter only detects base metals. These advantages, however, limit the range of applications of the covermeter. The gauging depth is 10 cm. A covermeter can only detect the first rebar layer and some parts of a second rebar layer, due to magnetic shielding. Radar has a wide range of applications. It can be used for localisation of armament, holes and other objects inside of concrete objects. The gauging range of radar exceeds 50 cm depending on antenna and concrete type. The interpretation of the signals however is difficult, because radar responds to almost all inhomogeneities. The area near to the antenna - about 5 cm with the 1 GHz antenna - cannot be seen clearly, because reflected waves are jammed by the emitted pulse. Experience and information about the basic construction of an object are important to interpret the origin of hyperbolas in radar profiles during an investigation In practice, the depth of rebars is less then 8 cm and can be localised clearly with a covermeter. Prestressed elements are positioned deeper. They can be localised with radar. By assemblage and synchronisation of a covermeter and a radar antenna, HOCHTIEF developed a sensor which combines the advantages of both measurement methods. This CM-Radar sensor marks rebars localised by the covermeter graphically in the radar profile.

Figure 4: Arrangement of the CM-Radar sensor

Figure 4 shows the arrangement of the CM-Radar Sensor. It consists of a covermeter, an antenna and a microcomputer with an attached survey wheel. Because the information of the covermeter is added to the radar profile, the coil of the covermeter is positioned in front of the antenna. Every time the covermeter detects a rebar, it sends a pulse to the microcomputer. The microcomputer stores this information until the center of the antenna reaches the point where the covermeter detected the rebar. The microcomputer then sends a pulse to the radar system to mark the position in the radar profile with a vertical line. To synchronise the radar system with the covermeter, the microcomputer generates all necessary impulses via the information received from the survey wheel. The use of a microcomputer makes the system adaptable to future developments.

5. Bench Tests Different specimens where produced to test the system. Figure 5 shows a specimen with 3 prestressed element under a layer of rebars and the result of the measurement. The dashed vertical lines show the positions of the rebars, the 3 hyperbolas show the position of the prestressed elements. The first rebar on the left is not marked because the coil is positioned in front of the antenna.

Figure 5: Bench test with the CM-Radar sensor

6. Interpretation Example

The following example discusses the interpretation difficulty of radar profiles and shows, how the information of the CM-Radar sensor helps finding the origins of hyperbolas. Figure 6 shows a radar profile of a heavily armed concrete structure. The profile shows 5 objects. One large reflector on the left and right side which could be caused by prestressed elements and three small reflectors in the middle probably caused by rebars, while it is not sure whether their position is determined by the hyperbolas A or B (see figure 6).

Fig 6: Normal radar profile

Figure 7 shows the same radar profile with the information of the covermeter. The large reflector on the right side obviously results from a reflection of two close rebars. The reflector on the right side must be a prestressed element, since there is only one rebar near to it. The positions of the rebars in the middle are clearly determined by the markers.

Fig 7: Radar profile with CM-Radar

7. Practice Investigations

The following cases are examples where the system has been used successfully in practice.
• Bridge "Hammerbachtalbrücke" near Passau
  For an assessment of the bridge stability, among other things the condition of prestressed elements had to be evaluated. For this evaluation a non destructive crack sensor system of the technical university of Berlin was applied. For the application of this crack sensor system the lateral prestressed elements had to be localised exactly. With the CM-Radar sensor all necessary prestressed elements were localised and marked on the bridge deck.
• Swimming pool on the island of Föhr
  Because of stricter regulations of the water quality of swimming pools, additional freshwater jets had to be placed in the floor and the sides of a suspended swimming pool. Besides the surface near rebars, the floor of the swimming pool was reinforced with two longitudinal layers and one lateral layer of prestressed elements. The positions planned for the freshwater jets were already marked on the floor. The prestressed elements of all three layers near to the marking were localised with the CM-Radar sensor. Afterwards the positions of the markings could be corrected. The correction prevented damage to the prestressed element during the following drilling procedures.
• Bridge in Gifhorn
  To reduce the traffic noise, baffle boards had to be installed along a road through Gifhorn. For the installation of the baffle boards on the bridge during the necessary drilling procedures, all lateral and two longitudinal prestressed elements had to be localised and marked on the bridge deck. Due to the successful localisation of the prestressed elements with the CM-Radar sensor, no element was hit during the installation of the baffle boards.

8. Further Application Possibilities

The CM-Radar sensor can be used to detect whether a rebar is made of base or precious metals. This application can be used for the investigation of the slab constructions built in the last 40 years in the former GDR. The face panels of these sandwich constructions are normally attached to the bearing panels by special rebars made of precious metals. For an assessment of the stability, it is necessary to determine whether precious or base metals were used for the attachment of the face slabs. The covermeter only senses rebars made of base metals. Radar senses all type of metals. Through the evaluation of both measurement principles, it is possible to differentiate between base metals and precious metals.

Literatur

1. A.Schaab, C.Flohrer, B.Hillemeier Die zerstörungsfreie Prüfung der Betondeckung Beton- und Stahlbetonbau 11/1989 und 12/1989
2. C.Flohrer, B.Bernhardt Das Orten von Spannbewehrung unter einer mehrlagigen Stahlbetonbewehrung DGZfP, Symposium Zerstörungsfreie Prüfung im Bauwesen 27.2.-1.3.1991 in Berlin
3. C. Flohrer, B. Hillemeier Zerstörungsfreie Prüfung der Betondeckung der Bewehrung DGZfP, Symposium Zerstörungsfreie Prüfung im Bauwesen 27.2.-1.3.1991 in Berlin
4. Meinke, Gundlach Taschenbuch der Hochfrequenztechnik, Band 1: Grundlagen Springer Verlag, 1992

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