Chinese Journal of Engineering

Volume 2016, Article ID 9714381, 7 pages

http://dx.doi.org/10.1155/2016/9714381

## An Experimental and Numerical Study on Embedded Rebar Diameter in Concrete Using Ground Penetrating Radar

Department of Civil Engineering, University of Texas at Arlington, P.O. Box 19308, Arlington, TX 76019, USA

Received 1 March 2016; Revised 23 June 2016; Accepted 21 July 2016

Academic Editor: Pierre-yves Manach

Copyright © 2016 Md Istiaque Hasan and Nur Yazdani. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

High frequency ground penetrating radar (GPR) has been widely used to detect and locate rebars in concrete. In this paper, a method of estimating the diameter of steel rebars in concrete with GPR is investigated. The relationship between the maximum normalized positive GPR amplitude from embedded rebars and the rebar diameter was established. Concrete samples with rebars of different diameters were cast and the maximum normalized amplitudes were recorded using a 2.6 GHz GPR antenna. Numerical models using GPRMAX software were developed and verified with the experimental data. The numerical models were then used to investigate the effect of dielectric constant of concrete and concrete cover on the maximum normalized amplitude. The results showed that there is an approximate linear relationship between the rebar diameter and the maximum GPR normalized amplitude. The developed models can be conveniently used to estimate the embedded rebar diameters in existing concrete with GPR scanning; if the concrete is homogeneous, the cover depth is known and the concrete dielectric constant is also known. The models will be highly beneficial in forensic investigations of existing concrete structures with unknown rebar sizes and locations.

#### 1. Introduction

As a nondestructive evaluation tool, ground penetrating radar (GPR) has been used for subsurface imaging of soil, pavement, and concrete and in many other fields. Use of GPR in concrete evaluation was started in early 1990s. GPR has been used to find concrete cover and thickness of bridge decks [1, 2]. GPR has also been widely used for bridge deck deterioration mapping with high degree of success [3, 4]. The uses of GPR in evaluating the thickness of concrete and asphalt pavement and detection of voids in pavements are also reported [5–7]. Several past studies have explored the extraction of additional information about the rebar embedded in concrete, such as the diameter of the rebar. Normally GPR responses from any cylindrical target are hyperbolic in shape. Therefore, a GPR scan does not provide direct information about the diameter of the target. If quantitative information about the rebar, such as diameter, can be retrieved from a GPR scan, it will be an excellent addition to the existing usage of a GPR. Rebar diameter is an important parameter in determining the various strength, safety, and serviceability properties of concrete structures. In forensic evaluation of concrete structures, the embedded rebar size may not be known because as-built drawings could be absent or nonreliable. In these situations, destructive techniques are normally used to determine the embedded rebar diameter. Determination of such diameters with nondestructive techniques such as GPR scan can be a very useful process. The ability of GPR to scan large distances in a short time period could be conveniently employed in rebar diameter estimation. The hyperbola that results from the GPR trace of a rebar embedded in concrete can be represented by mathematical models. A study on hyperbola curve fitting demonstrated a mathematical model that can predict the diameter from the equation of the hyperbola [8]. Another empirical study proposed a physical model of rebar scanning embedded in concrete with a GPR antenna [9]. A study on Radar Cross Section (RCS) of the cylindrical rebar in concrete showed that the ratio of RCS in copolar and cross polar direction was related to the rebar diameter [10]. Another study used the ratio of the amplitudes obtained from two different antenna orientations to predict the diameter of the rebar in concrete [11]. None of the aforementioned methods were simple or accurate enough to predict the diameter of the rebar. Another study used 2 GHz and 4 GHz GPR antennae and found the correlation between the rebar diameter and the maximum amplitude from rebar with two different antenna orientations [12]. It was shown that the maximum amplitude increased with the increase of rebar diameter for both numerical and experimental data. Although this method [12] was relatively simple and easy to follow, it did not address the effect of changing concrete properties and the numerical model was not verified with experimental work. The maximum amplitude of GPR signal from the rebar is a parameter that can be easily and quickly retrieved from the GPR scans. An approach to correlate the GPR maximum amplitude with rebar diameter in concrete will, therefore, be a very useful tool. In this study, the diameter of the embedded rebar in concrete was correlated with the maximum possible normalized amplitude from a GPR scan.

#### 2. Experimental Setup

Six experimental concrete blocks were constructed herein using normal weight concrete with a water cement ratio of 0.40 and a maximum aggregate size of 3/4 in. (19 mm) with a target 28-day compressive strength of 4000 psi (27.5 MPa). All beams were cast at the same time to ensure homogenous dielectric constant in all the six samples. Dielectric constant is an electromagnetic property of a material which controls the propagation of radar waves through the materials. The beam dimensions were 54 in. (1370 mm) long, 10 in. (250 mm) wide, and 6 in. (150 mm) deep. Three different concrete covers were used [1 in. (25 mm), 2 in. (50 mm), and 3 in. (75 mm)] in the blocks to see the variation of GPR response with the depth of rebar in concrete. A schematic diagram of the concrete block sample is shown in Figure 1.