Journal of Materials in Civil Engineering

There is currently a need to improve the nondestructive testing techniques that are used to inspect grouted steel reinforcing tendons, anchors, and rock bolts for corrosion and fracture. A method of inspection using guided ultrasonic waves has been proposed, which uses a pulse-echo technique carried out from the free end of the structure. The maximum inspection range is determined by the amount of attenuation that the wave experiences as a result of leakage into the embedding material and material losses. However, previous work has identified high frequency modes that have low attenuation and so increase the inspection range. Research has been carried out with a focus on the inspection of the posttensioning tendons used to reinforce concrete. The research presented in this paper uses experimental techniques to measure the attenuation in short lengths of grouted tendons, to evaluate the reflection coefficient of the modes from different geometry breaks, and to assess the impact of tendon curvature. The outcome of this research shows that the inspection range for tendons is limited, but the outlook for the inspection of the larger diameter grouted bolts and rebars that are used in the construction industry is promising. Considerable success has already been achieved in the testing of epoxy bonded rock bolts using this method.

Health monitoring of steel strands is the subject of much research in the nondestructive evaluation and civil engineering communities. This paper deals with a guided stress wave method for stress monitoring and defect detection in seven-wire strands. A simplified acoustoelastic formulation of the Pochhammer-Chree vibrations in cylindrical waveguides is derived in the framework of the partial wave representation for guided waves. Magnetostrictive transducers are used to excite and detect the waves in the experiments. Results from acoustoelastic measurements on single wires and on strands are presented, showing the feasibility of the method for stress measurement, although an anomalous behavior of the strands at low stress levels remains the subject of current investigation. Improvements to the inherently low sensitivity of acoustoelastic stress measurements are suggested by adding the effect of strand elongation. The role of the strand anchorages is also examined in the context of wave attenuation. Finally, the suitability of the guided wave method for the detection of indentations and broken wires in the strands is demonstrated, including the possibility of inspecting the critical anchored regions.

The remanent magnetism method allows the identification of potentially unsafe conditions in pretensioned and posttensioned concrete structures by locating fractures in the prestressing steel. This nondestructive method identifies fractures of single wires, even when they are bundled with intact wires. The magnetic field of tendons is measured at the concrete surface, once they have been premagnetized with an electromagnet. Fractures produce characteristic magnetic leakage fields, which can be measured with appropriate sensors at the concrete surface. The parameters associated with fractured wires have been quantitatively identified in the laboratory and have been confirmed in the field. The knowledge of these parameters allows us to draw conclusions about the reduction of a cross-sectional area or the number of fractured wires in a tendon. The method has been successfully applied outside of the laboratory on full size bonded and unbonded posttensioned structures.

Ultrasonic reconstruction by the synthetic aperture focusing technique (SAFT) has a great potential to image concrete elements and detect embedded objects. Its algorithm focuses ultrasonic signals received at many aperture points by coherent superposition, yielding a high-resolution image of the region of interest. Using this approach, several problems caused by the strongly inhomogeneous structure of concrete are diminished, where scattering of transmitted pulses leads to disturbing phenomena such as attenuation and structural noise. This contribution is intended to review the work of the writers on the application of SAFT reconstruction to concrete testing. First, consequences of scattering of ultrasonic waves in concrete are qualitatively explained. Then the use of SAFT is discussed in comparison to traditional A-scan and B-scan techniques. Different reconstruction algorithms and implementations are presented for one-, two-, and three-dimensional SAFT. Pulse-echo measurement systems are described, which are able to acquire large sets of data on linear and planar apertures employing single transducer, transducer array, and scanning laser Doppler vibrometer arrangements. To illustrate the application of the SAFT techniques, examples from laboratory and field experiments are described comprising imaging of back walls, tendon ducts containing faults, layers, and reinforcement in concrete elements.

The in situ testing of early age concrete strength is crucial for determining the time of form removal from concrete elements, opening highways to traffic, or applying prestress to steel reinforcement. A nondestructive ultrasonic technique, which measures the reflection loss of ultrasonic transverse waves at a concrete-steel interface, is presented in this paper. The aim is to compare wave reflection measurements on mortar and concrete to strength. It is shown that the reflection loss is linearly related to the strength gain of mortar and concrete at early ages. The experiments have revealed a relationship between the homogeneity of the tested materials and the consistency of the reflection measurements. The repetition of simultaneous measurements of wave reflections and compressive strength on mortar results in similar strength-reflection loss relationships. Multiple measurements on the same concrete gave multiple strength-reflection loss relationships. The accuracy of the strength predictions made with the proposed method is discussed and compared to that of other nondestructive test methods.

The paper deals with the rheological characterization of different heterogeneous civil engineering materials that have time-dependent properties. Studies on cellular concrete, mortar, hydraulic concrete, and cold bituminous mixes are presented. The properties are monitored throughout setting and hardening. Such materials are heterogeneous mixtures containing solid particles, binder, fluids, and gas bubbles. For all of these materials, different formulations were investigated, such as water content, binder proportioning, and compaction level. A nondestructive device allows the setting to be monitored and to be clearly characterized. This device makes it possible to simultaneously study the propagation of compressional and shear waves in the same sample of material. To prevent diffraction of waves from heterogeneities, a condition of a long wavelength compared to the heterogeneity size is required, so low frequencies are used (20–1,000 Hz). Velocity and damping coefficients versus time are deduced, then related to rheological viscoelastic properties by an inverse analysis. The linear behavior of these materials under small strains is proved, and the measurements are very repetitive and sensitive to the nature and the component proportioning of the materials. The rheological evolutions of the different materials are presented and compared. It is shown that the normalized evolution of hydraulic concretes follows a single master curve for all of the different formulations investigated. Similar results are established for cellular concretes and mortars.

Fiber-reinforced polymer (FRP) composites, in the form of pultruded laminates or built-up woven fabrics, are being used widely to strengthen existing concrete and masonry structures. The success of these materials in performing their intended functions depends, to a large extent, on how well they are bonded to themselves and to the substrate. There is a need for an efficient and reliable method to detect and characterize defects at the substrate interface and within multi-ply systems. Infrared thermography is well suited for this purpose because it is inherently sensitive to the presence of near-surface defects and can interrogate large areas efficiently. Before infrared thermography can be developed into a standard methodology, however, an understanding is needed of the effects of testing parameters and different types of defects. A multiyear study is under way to develop this understanding through the use of controlled-flaw experiments and finite-element modeling. This paper reports on the initial phases of this study. An experimental setup is described for measuring the emissivity of a carbon FRP composite and for obtaining a well-defined heat pulse. Good agreement was found between experimental thermal response parameters and those calculated from finite-element models of controlled-flaw specimens. This agreement provides assurance of the validity of parametric studies based on numerical simulations.

The acoustic emission (AE) technique can be a useful method for the investigation of local damage in materials. One of the advantages compared to other techniques is the recording of the damage process during the entire load history without any disturbance to the specimen. Differences between the traditional parameter-based and newer signal-based techniques are described along with some examples of measurements to study the steel-concrete interaction in reinforced-concrete cubes. Signal-based procedures, such as accurate 3D localization of damage sources, solutions for fault plane orientation, and moment tensor inversion, are described with respect to applications in civil engineering. The more quantitative analysis of the signals is based on a 3D localization of AE sources (hypocenters) and the recordings obtained from a sensor network. Using moment tensor inversion methods, the radiation pattern of acoustic emission sources and the seismic moment (as an equivalent to the emitted energy), as well as the type (Mode I, Mode II, and mixed modes) and orientation of the cracks, can be determined.

Concrete bridges in the United Kingdom represent a major legacy that is starting to show signs of distress. Therefore, the need for monitoring them is an urgent task. The acoustic emission (AE) technique was proposed as a valid method for monitoring these bridges but more study is needed to develop methods of analyzing the data recorded during the monitoring. The writers would like to propose a b-value analysis as a possible way to process AE data obtained during a local monitoring. The b-value is defined as the log-linear slope of the frequency-magnitude distribution of acoustic emissions. This paper presents the results of a b-value analysis carried out on data recorded during a laboratory test on a reinforced concrete beam designed as representative of a bridge beam. During the experiment, the specimen was loaded cyclically and it was continuously monitored with an AE system. The data obtained were processed and a b-value analysis was carried out. The b-value was compared with the applied load, with a damage parameter, and with the cracks appearing on the beam. The damage parameter represents the cumulative damage in terms of total sum of acoustic emissions. The results showed a good agreement with the development of the fracture process of the concrete. From a study of the b-value calculated for a whole loading cycle and for each channel, some quantitative conclusions were also drawn. Further development work is needed to make the b-value technique suitable for practical use on a real bridge.

In recent years the use of ground penetrating radar (GPR) at frequencies from 500 MHz to 2.5 GHz has yielded very good results for inspection of concrete structures. The possibility of performing nondestructive measurements quickly and with convenient recording of the measurement results is particularly beneficial. The technique is well-suited for locating tendon ducts at depths down to 50 cm, detecting voids and detachments, and measuring thickness of structures that are only accessible from one side. This paper presents the basics of GPR, its limits, and the results of laboratory investigations and case studies. It also shows that GPR can be used for regular inspection, searching for the cause of damage, and quality assessment of civil engineering structures.

Assessing the condition of a structure is necessary to determine its safety and reliability. Ideally, structural health monitoring should be similar to medical health monitoring of the body. In medical health monitoring, the life signs such as pulse and blood pressure give an overall indication of the overall health of the body. This is analogous to global structural health monitoring, in which damage to the structure can be identified by measuring changes in the global properties of the structure. Once the body signs show an anomaly, we do a battery of tests to determine the cause of the anomaly. Analogously in structural health monitoring, nondestructive evaluation (NDE) can be used to determine the nature of the damage. NDE methods to determine local damage are also becoming more accepted in practice. This paper describes some of the recent and current National Science Foundation projects in this area of research. Promising areas for NDE are identified.