Cracks in concrete or cement based materials present a great threat to any civil structures; they are very dangerous and have caused a lot of destruction and damage. Even small cracks that look insignificant can grow and may eventually lead to severe structural failure. Besides manual inspection that is ineffective and time-consuming, several nondestructive evaluation techniques have been used for crack detection such as ultrasonic technique, vibration technique, and strain-based technique; however, some of the sensors used are either too large in size or limited in resolution. A high resolution microwave imaging technique with ultrawideband signal for crack detection in concrete structures is proposed. A combination of the delay-and-sum beamformer with full-view mounted antennas constitutes the image reconstruction algorithm. Various anomaly scenarios in cement bricks were simulated using FDTD, constructed, and measured in the lab. The reconstructed images showed a high similarity between the simulation and the experiment with a resolution of
Many buildings and civil structures are made up of reinforced concretes or cement based materials. These structures are designed to carry certain amount of load under certain conditions and for a specific period. Environmental exposure and loadings are ways through which deterioration and damage are introduced inside a functioning civil structure or cement based materials during service. For example, a structure can deteriorate when it is loaded with more than what it has been originally designed for. This gradual deterioration and damage to the material usually appear in the form of a crack or other anomalies. The anomaly presents a great threat to any civil structures; it is very dangerous and has caused a lot of destruction and damage. Even small cracks that look insignificant can grow and may eventually lead to severe structural failure. Whatever may be the genesis of these cracks, either being micro or macro in nature, the side effects of such a defect affect not only the structural properties of such buildings but also more importantly their mechanical behaviour, integrity, and permeability characteristics [
Besides manual inspection that is ineffective and time-consuming, several nondestructive evaluation techniques have been used for crack detection such as ultrasonic technique, vibration technique, and strain-based technique; however, some of the sensors used either are too large in size or suffer from poor resolution [
In this paper, we present the results using the proposed technique for cement based bricks. A total of 16 antennas were mounted surrounding the material producing a full-view configuration. A homogeneous material with
The 2D domain is considered in this work for the imaging of cracks on the bricks cross section. The 2D domain is considered in this work for the imaging of cracks on the bricks’ cross section because it saves memory space optimally and demands less tasking compared to 3D domain that is computationally time demanding. The imaging space is made up of rectangular block placed at the center of the domain surrounded on every side by transceiver points numbered from 1 to 16 to give full-view scanning of the brick phantom. The brick phantom without crack in which the dielectric properties are known (homogeneous material) is used as the background for the imaging domain. As a result of this, each sensor sends UWB pulse signals to the domain and other sensors now serve as observation points with each of them recording the received signal as the resulting response from the field.
The main aim of using FDTD model is to be able to test and to also verify the capability of the imaging modality in detecting and differentiating cracks of different orientation and sizes from a brick and the constructed structure. In this work, several models with different crack size, positions, and orientation were simulated using this model. Holes of different diameters and lengths were used to characterise different crack sizes. These holes were positioned in the brick model at
The image reconstruction is based on delay-and-sum beamforming technique as indicated in (
To test the resolution of the system, several FDTD models with different crack size, positions, and orientation were simulated. Firstly, a two-dimensional model of a brick with a single 5 mm or
The FDTD simulation of a single hole with diameter of 5 mm. (a) The FDTD model. (b) The reconstructed image.
The same defect model was constructed and measured in the lab using the configuration shown in Figure
The experimental setup.
From the first experimental result, it can be concluded that the reconstructed image from the measurement as displayed in Figure
The experiment of a single hole with diameter of 5 mm. (a) The brick with the 16 antennas. (b) The reconstructed image.
To study the localization performance of the system, multiple holes were simulated and measured. This time the model consisted of three holes, each with diameter of 5 mm or
(a) The FDTD model with three holes. (b) The simulation result. (c) The brick with three holes. (d) The experimental result.
A real crack in a building could be modelled with a line air gap with width of 5 mm as depicted in Figure
(a) The FDTD model with a 5 mm line crack. (b) The simulation result. (c) The broken and reassembled brick. (d) The experimental result.
Since civil structures are normally made up of multiple bricks combined together and laid on each other to form a building, the next experiment is aimed at testing the ability of the signal to penetrate through a constructed building structure. The constructed structure is made of three bricks laid side by side and three bricks lay on top producing a 220 mm thick square beam which is 330 mm high. The antennas were mounted at a distance of 60 mm apart from each other with four on each side. The measurement setup remains the same as that of the single brick. A hole with a diameter of 5 mm was drilled at the center of the beam as displayed in Figure
(a) Experiment with a constructed brick structure. (b) Top view of the brick structure. (c) Reconstructed image for the structure with hole.
From Figure
The effectiveness of this technique can be very much appreciated when compared with other forms of nondestructive measurements techniques. Firstly, according to [
From the simulation and experimental results of various anomaly scenarios, it could be concluded that microwave imaging technique has a high potential to be applied to defect detection of cement based materials. The achieved resolution of
The authors declare that they have no competing interests.