A STUDY ON THE APPLICABILITY OF COVER METER AND GPR SURVEY FOR COVER THICKNESS AND IDENTIFCATION IN REINFORCED STRUCTURES

Bhaskar SANGOJU#, and Vasanthakumar, S.& #Scientist, &Project Assistant, CSIR-Structural Engineering Research Centre (SERC), Taramni, Chennai, 600113, T.N., India E-mail address of corresponding author: [email protected]

ABSTRACT Adequate cover to the reinforcement bar (rebar) is essential to protect the rebar from aggressive chloride or carbon-di-oxide induced environments. Ascertaining the cover depth is important to implement durability http://www.ndt.net/?id=22341 standards to the structure under construction. For the condition assessment of deteriorated (RC) structures, it becomes essential to locate the , know the cover thickness, rebar diameter, etc. in existing structures, where, structural drawings may not be available. The present study explores the possibilities of Cover meter and Ground Penetrating Radar (GPR), two prominent non-destructive testing (NDT) techniques, for mapping of the rebars, identification of rebar position and estimation of cover thickness. The results indicate that GPR is very good in identifying the rebar position and estimating the cover depth even in heavily reinforced concrete structures.

More info about this article: Keywords: NDT, Cover meter, GPR, , Rebars.

1.0 INTRODUCTION Reinforcement corrosion is a major concern that causes extensive damage to the reinforced concrete (RC) structures. For new or freshly constructed RC structure, adequate concrete cover to reinforcement bar (rebar) is essential to protect it from external aggressive chloride or carbon-di-oxide induced environments. For the quality assessment of RC structures, it becomes essential to locate the rebars, know the cover thickness and rebar diameter. Also, for core sampling and extraction, rebar location is important to avoid cutting of the rebar. A reliable estimation of rebar location, concrete cover thickness and rebar diameter, non-destructively, is very much important for the quality assurance of new and for the condition assessment of existing RC structures.

Various non-destructive testing (NDT) techniques such as Rebound hammer, Ultrasonic pulse velocity, Cover meter, Impact echo (IE), Pulse echo (PE), Infrared thermography (IRT), Ground penetrating radar (GPR), Linear polarization resistance (LPR), etc., are being used for the condition assessment of reinforced concrete structures [1-2]. Among various NDT techniques, two techniques that are most commonly used in the field for the rebar identification, cover thickness and rebar diameter estimation are Cover meter and GPR techniques.

The commercially available Cover meter (Profoscope+), working principle, application, limitation, etc., are described in Proceq manual [3]. Advanced versions of Profoscopes are being released by Proceq. It is to be noted here that Cover meter probe setting (detectable range), bar diameter setting, etc., shall affect the cover thickness measurement. Limited research is reported on the reliable applicability of Cover meter for cover thickness and rebar diameter estimation. A laboratory study on concrete blocks with single embedded rebar reported that bar diameter can be estimated with an error percent between 1% to 35% for bar diameters 16mm and above for cover thickness up to 50mm [4]. The study carried out by Reuben et al. [5] reported that for cover thickness measurement, bar diameter setting and range setting (low or high) has greater influence on the cover thickness estimation. It is

1

reported in the Proceq manual that Cover meter survey is not affected by the presence of non- conductive materials such as wood, plastics, etc., however, any kind of conductive materials within the magnetic field will have an influence on the measurement [3]. GPR is a new technique being used for subsurface investigation, rebar identification and mapping [6-8]. GPR uses electromagnetic pulses to image the subsurface. The commercially available GPR (StructureScan Mini) and its working principle is described in GSSI manual [9]. Accurate estimation of depth, or cover requires a reliable knowledge of the dielectric properties of the concrete [9, 10]. It is observed from the literature that these two techniques have their own merits and demerits, which depend on the depth of cover, spacing between rebars and diameter of rebars, etc. [3, 5,11-13].

Many times during field investigations, it is experienced difficulty in the (i) rebar identification, (ii) cover thickness, and (iii) diameter estimation using the Cover meter. This could be due to uneven/rough surface, deep concrete cover, congestion of rebars, etc. Hence, a laboratory parametric study is proposed to explore the reliable applicability of Cover meter and GPR for the rebar identification and estimation of cover thickness in concrete members considering different cover thicknesses and rebar diameters. The present study is also extended to identify the rebar position and cover thickness in a strong reaction floor concrete wall, available at CSIR-SERC, Chennai.

2.0 EXPERIMENTAL PROGRAMME 2.1 Specimen Details For the present study, number of test specimens/blocks (B1 to B5) with different cover thicknesses and rebar diameters are used for the data generation and reliable interpretation. The concrete block is of 150mm thickness with a size of 400×250mm. For easy interpretation and also to avoid interference effect from the adjacent rebars, single bar is placed in each block. For easy casting, an effective cover of 50mm from top is maintained for all the blocks. Rebars with different diameters viz., 32mm, 25mm, 20mm, 16mm, and 12mm are used to get different clear covers. To represent deep cover to the rebars in situations such as isolated foundations, irrigation dams, etc., a big square block (S) of size 1500×1500mm with 500mm thick is cast. Three rebars are placed in the block at sufficient spacing (i.e., more than 200mm) among them, to avoid interference effect from the adjacent rebars while doing scanning [3, 14]. Three diameters, viz., 16mm, 20mm and 25mm are placed with an effective cover of 160mm from the top surface. The bars are placed at a spacing more than 200mm to avoid interference effect from the adjacent rebars and also to get reliable GPR radagram for identifying location and spacing of rebars [14]. Table 1 illustrates the details of different test specimens. Figure 1 (a) and 1 (b) show the scanning of a typical concrete block using Cover meter and GPR.

2

(a) using Cover meter (b) using GPR Figure 1 Scanning on a typical block specimen

Table 1: Details of different block specimens Diameter of Effective Effective cover Concrete rebar used cover from from the block (mm) the top (mm) bottom (mm) B1 12 50 100 B2 16 50 100 B3 20 50 100 B4 25 50 100 B5 32 50 100 25 160 340 S 20 160 340 16 160 340

2.2 Cover Thickness Estimation using Cover meter To determine the cover thickness, scanning is carried out on the concrete blocks B1 to B5. To get a reliable cover thickness, the rebar diameter needs to be known and has to be feeded in the Cover meter device [3]. In general, many times in the field, rebar details may not be available, especially for old structures and sometimes not possible to estimate the rebar diameter due to reasons such as congestion of rebars, thick concrete cover, etc. In such cases, an appropriate diameter is to be assumed based on the history of the structure and experience. Keeping that as back ground, in the present study, a parametric study has been carried out for reliable cover thickness estimation by assuming different input diameters (four diameters/trials used). Also, to understand more about the applicability of Cover meter, both top and bottom covers of block specimens (blocks) are proposed to be estimated.

Four input diameters/trials are used for cover thickness estimation. Among the four input diameters, two are less than the actual, one is equal and the other is higher than the actual rebar diameter. Table 2 presents the details of input diameters assumed. The clear cover thicknesses are estimated using Cover meter and Figure 2 shows the plot of effective cover thicknesses for different input diameters. It is observed from Figure 2 that for the rebar diameters between 16mm and 32mm, the cover thickness is estimated reliably, only when setting the input diameter as actual bar diameter. It is also observed that when the input diameter is set as a value higher than the actual, the estimated cover is higher and vice versa. This has been observed for both top and bottom covers. From Figure 2, it can be inferred that the accuracy of cover thickness greatly depends on the set value of input diameter in the Cover meter device. The maximum error observed in the present study is about 21% for

3 different incorrect input diameters. Therefore, in order to have reliable and correct cover thickness estimations, it is desirable to collect the rebar diameter details first, if available or estimate the rebar diameter using Cover meter before going for cover thickness estimation/measurement.

Cover thickness estimations are also made on a big square block, ‘S’ for three different embedded rebars (16mm, 20mm and 25mm diameter). Cover thickness could not be estimated using Cover meter, which could be due to the embedment of rebars at a deep cover (of about 150mm). However, Proceq manual states that the measuring range is dependent on the bar size and for 25mm rebar diameter, the thickness measuring range is up to 160mm [3].

Table 2: Different blocks-actual and assumed input/set diameters

Concrete Actual rebar Assumed input block diameter (mm) diameters (mm) B1 12 8 10 12 16 B2 16 10 12 16 20 B3 20 12 16 20 25 B4 25 16 20 25 32 B5 32 20 25 32 36

Note: Effective cover is equal to clear cover in mm + d/2; where, ‘d’ is diameter of rebar in mm

Figure 2 Cover thickness variation for different input diameters

4

2.3 Cover Thickness Estimation using GPR A portable GPR (StructureScan Mini) with inbuilt antenna (2.6GHz frequency) is used in the present study [13]. Appropriate dielectric constant value has been evaluated by following methods [9]. • On site calibration over the specimens • Hyperbolic fitting using the migration processing method.

GPR scanning is carried out on each of the concrete block specimens across the rebar position. Typical radargram images are shown in Figure 3. The hyperbola peak in radargram indicate the location/position of rebar. To get the cover thickness to the rebar, the radargram is processed for time zero correction using RADAN software [13]. The vertical position of the rebar i.e., the clear cover to the rebar can also be observed by studying the hyperbola along with its O-scope. It can be seen that there is a change in the signal polarity in the O-Scope corresponding to the peak of the hyperbola. This represent the position of rebar from the surface [13]. The cover thickness results estimated using GPR are closely matching with the actual cover thicknesses. Figure 4 shows the processed radargram image for square block ‘S’. It is clear that the three hyperbola peaks indicate the position of three embedded rebars. The cover thickness is estimated by studying the hyperbola along with its O-scope and the estimated cover thicknesses are closely matching with that of the actual cover thicknesses. Table 3 presents the comparison of cover thickness estimations by using Cover meter and GPR. The error in estimation of the cover thickness is calculated with reference to actual clear cover thickness. For smaller or shallow cover depths up to 40mm by using the Cover meter, it can be observed that, the error in estimated cover thickness is about 1% to 4% for embedded rebars with diameters more than 16mm and is about 11% to 15% for embedded rebars with diameters 12mm/or less. However, using Cover meter, cover thickness could not be estimated for embedded rebars in block, ‘S’, wherein the rebars are placed at an effective cover thickness of 160mm from surface. Nevertheless, with GPR, the cover thickness estimation is possible for shallow as well as deeper cover ranges and also the error in estimation is less. The additional advantage of GPR is that the radargram images can be used to study the internal features of concrete, rebars, mapping/arrangement of rebars, etc. [8,15,16].

Figure 3 Radargram for typical block specimen B2; top and bottom face

5

Figure 4 Radargram with hyperbolas for square block, ‘S’

Table 3 Estimated cover thickness using Cover meter and GPR

Cover meter GPR Estimated Diameter of Estimated Estimated Estimated Block cover from rebar (mm) cover from cover from cover from bottom top face bottom face top face face B1 12 49 (44)* 108 (94) 45 (44) 92 (94) B2 16 43 (42) 95 (92) 41 (42) 91 (92) B3 20 41 (40) 93 (90) 39 (40) 90 (90) B4 25 38 (37.5) 89.3 (87.5) 37 (37.5) 88 (87.5) B5 32 34 (34) 85 (82) 34 (34) 82 (82) S 16 Could not estimate due to 153.8 (152) 20 higher cover thickness 151.0 (150) ----- 25 (about 150mm) 157.7 (147.5)

Note: * Clear cover/cover is equal to (i) 50mm - d/2; (ii) 160mm - d/2; where, ‘d’ is diameter of rebar in mm

3.0 FIELD STUDIES ON STRONG FLOOR REACTION WALL Field studies are carried out to identify the rebar position and estimate the cover thickness of newly constructed strong floor reaction wall (heavily reinforced) in the CSIR-SERC campus. The reinforcement is placed in two layers on both faces of the wall. Some of the internal details are as follows: thickness of concrete wall, 750mm; horizontal reinforcement, 16mm diameter and vertical (main) reinforcement, 32mm diameter, spaced at 150 to 200mm; inserts @ 1000mm c/c; clear cover to the 32mm dia. main bars is 40mm and 16mm dia. bars is 72mm (40+32=72mm). GPR survey and Cover meter survey is carried out at selected locations to identify the position/location of rebars in the reaction wall. Figure 5 (a) shows the concrete reaction wall with inserts.

Initially, Cover meter survey is carried out to identify the position of rebars between inserts. The rebar identification could not be found with Cover meter and this could be due to the presence of metal inserts, congestion of reinforcement, more cover thickness (72mm), etc. Figure 5 (b) shows the GPR radargram with hyperbolas. As it is reported earlier, the hyperbola peaks in the radargram indicate rebar locations between four inserts. It is observed that the horizontal and vertical reinforcement could be identified using GPR and is matching

6

closely with the available drawings. This shows the limitation of Cover meter in identifying the rebars in heavily reinforced concrete structure such as reaction floor wall.

(a) Reaction wall (b) Radargram with hyperbolas Fig. 8 Strong floor reaction wall

4.0 CONCLUSIONS Following conclusions can be drawn from the limited experimental study: • Cover meter and GPR techniques can be used for rebar identification and cover thickness estimation. In general, Cover meter technique is simple to use and easy to interpret the results when compared to that of GPR technique. • In laboratory cast single rebar specimens, rebar diameter estimation using Cover meter is reliable for cover depths up to 40 to 50mm. Consistent and reliable diameter could not be estimated when the rebar diameter is 12mm/or less. • To get more reliable and correct cover thickness estimations using Cover meter, it is desirable to get the rebar diameter details, if available or estimate the rebar diameter using Cover meter before going for cover thickness estimation/measurement. • Structural members with deep cover such as foundations and heavily reinforced sections with metal inserts such as reaction floor walls, rebar identification using Cover meter is difficult. In such cases, GPR is effective in identifying rebar locations and cover thickness estimations.

Acknowledgements Authors would like to acknowledge K. Sivasubramanian, Scientist, ACTEL, CSIR-SERC for the support during the experimental data collection.

References [1]. V. M. Malhotra, and N. J. Carino, (Eds.), Handbook on Nondestructive testing of concrete, CRS Press, Washington DC, 2004. [2]. J. H. Bungey, S. G. Millar, and M. G. Grantham, “Testing of concrete in structures”, 4th edition, 2006, Taylor & Francis, NY, USA. [3]. Proceq “Profoscope operating instructions”, www.proceq.com, Switzerland, 2013. [4]. K. Sivasubramanian, K.P. Jaya and M. Neelemegam, International Journal of Civil and Structural Engineering, 3 (3), (2013).

7

[5]. B. Reuben and Z. Tony, The e-Journal of Non-destructive Testing, www.ndt.net; December (2008). [6]. ASTM Standard D 4748-10, “Standard Test Method for Determining the thickness of bound pavement layers using Short-Pulse Radar”, Annual book of ASTM standards, American Society of Testing and Materials, USA, 2010. [7]. ASTM D 6432 - Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation, Annual book of ASTM standards, American Society of Testing and Materials, USA, 2011. [8]. P. Srinivasan, K. Ravisankar, and S. Thirugnanasambandam, IUP Journal of Structural Engineering, V (4), pp. 43-52, (2012). [9]. GSSI “Manual for RADARTM 7”, Geophysical Survey Systems, Inc.Salem, New Hampshire, USA, 2013. [10]. S.Bhaskar and K. Ramanjaneyulu, "Estimation of rebar diameter in concrete structural elements using ground penetrating radar." 25th National Seminar & International Exhibition on Non-Destructive Evaluation (NDE 2015), Hyderabad, 2015 (CD format). [11]. J.Helal, M. Sofi and P. Mendis, Special Issue: Electronic Journal of Structural Engineering, 14 (1), pp. 97-105, (2015). [12]. British Standard BS1881 Part 204 "Recommendations on the use of electromagnetic Cover meters". British Standards Institution, London, 1998. [13]. “GSSI Handbook for RADAR Inspection of concrete”, Geophysical Survey Systems, Inc.Salem, New Hampshire, USA, August 2006. [14]. C. W. Chang, H. L. Chen and S. L. Hung, Construction and Building Materials, 23 (2), pp. 1057-1063, (2009). [15]. D.M. McCann and M.C. Forde, NDT and E International, 34, pp. 71–84, (2001). [16]. J. H. Bungey, and S. G. Millard, “Radar inspection of structures”, Proceedings of the International in Civil Engineering Structures and Buildings, pp. 173-178, (1993).

8