Non-Destructive Testing for Building Evaluation

1. Introduction

As of recent, non-destructive testing (NDT) has been used more frequently to evaluate and assess the condition of buildings. This evaluation includes a wide range of activities, including simple visual inspection; thermography; optical laser treatment; acoustic, radiological, and electromagnetic methods; and advanced tomography. At the construction stage, the materials and parts of the buildings are tested on site or in a laboratory. At the maintenance stage, buildings are more often monitored. Special efforts have been placed on the complementary application of various NDT techniques into an integrated structural health monitoring (SHM) system. More advanced techniques and algorithms are used to correlate the obtained signals and parameters from NDT measurements.

The main objective of this Special Issue is to share, present, and discuss recent applications of NDT for the evaluation and assessment of the condition of buildings. Particular focus was placed on real-world applications. The Special Issue starts with a compressive review of recent literature from the point of view of the evaluation of the adhesion between overlays and substrates in concrete floors, as well as non-destructive and semi-destructive testing methods used for this purpose [1]. This article also specifies some research gaps and possible future perspectives.

The rest of the Special Issue mainly contains articles that extend knowledge in the field of non-destructive testing for the evaluation of buildings with regards to the applications of ultrasonic, optical, thermographic, analytical, numerical, and laboratory methods.

2. Application of Ultrasonic Methods

Article [2] starts with interesting exemplary photos of cracks in a concrete cover after fire, as well as pictures of concrete surfaces after fires. Moreover, it discusses the color changes in concrete after fire. Samples for tests were taken after a fire from a 64 apartment building under construction. Core samples were tested across the sample’s height using the ultrasonic pulse velocity (UPV) method. The UPV method was validated with regards to the compressive strength of concrete, which was established based on the destructive strength tests. Afterwards, the distribution of the post-fire concrete strength across the sample’s height was evaluated using the UPV method. The presented method showed to be accurate as it can estimate the thickness of the concrete layer that is necessary to be removed or reinforced after being subjected to fire.

Another application of ultrasonic methods is presented in [3]. This time, the UPV method was used to evaluate the properties of cement-based materials modified with marble powder. The replacement ratios ranged from 5 to 15% of the amount of cement when compared to an unmodified reference sample. The UPV method showed to be a useful tool to indirectly evaluate the compressive strength and the apparent density of cement-based materials modified with marble powder. To support this statement, relationships between each of these parameters were also presented in the article.

3. Application of Optical Methods

Article [4] shows an application of close range photogrammetry (CRP) for the measurement of dynamic displacements of a three-dimensional one-story building frame model placed on a one-dimensional shake table. The performance of the CRP method was positively verified using a traditional method of attached accelerometers, as well as numerical models. The CRP method is also beneficial due to the fact that it measures displacements in three independent directions at the same time. Moreover, the cost and required time for instrumentation and analysis are relatively low when compared to other methods.

The dynamic analysis of elements made of simple monolithic glass is presented in [5]. This analysis was performed based on non-destructive laboratory tests, which were enhanced with operational modal analysis, numerical simulations, classical analytical models, and video-tracking techniques. In this research, the video-tracking technique showed a certain potential for dynamic investigations.

4. Application of Thermographic Methods

Article [6] proposes an experimental procedure which uses infrared thermography (IRT) to measure the surface temperature of building elements. Thanks to the proposed procedure, it is possible to approximate the thermal transmittance of the U-value. The application of IRT, in combination with the dynamic method (DYNM), could be successfully used for in situ tests. This is due to the fact that the DYNM does not need to have stable boundary conditions. Furthermore, the proposed procedure can also be used for a relatively fast and inexpensive approximation of the U-value without the use of expensive equipment that is needed in other methods (e.g., heat flux sensors).

Another interesting case study presents the application of thermographic methods in order to find structural elements and hidden openings in historic buildings in Osijek [7]. The first step of this research was to apply a steady-state thermographic method to analyze a building’s envelope. After that, evaluation was carried out using step heating.

5. Analytical, Numerical, and Laboratory Methods

Article [8] presents a new model for estimating the shear capacity of support zones that were reinforced longitudinally using fiber-reinforced polymer (FRP) bars instead of typical shear reinforcement. According to the authors, the shear strength, with regards to the proposed model, can be estimated using an accompanying (non-destructive) method for the empirical determination of the shear resistance of FRP bars.

The next valuable application of numerical methods is presented in [9]. This paper describes a case study of the dynamic characterization of a grandstand of a stadium in Lisbon. The grandstand was made of reinforced concrete. Data for the model, in the form of the natural frequencies and vibration modes, were taken from the performed ambient vibration tests on the analyzed grandstand. A sensitivity analysis provided evidence of the numerical fitting of the experimental results.