S. Aparicio

On the measurement of frequency-dependent ultrasonic attenuation in strongly heterogeneous materials

This paper deals with the measurement of frequency-dependent ultrasonic attenuation in strongly heterogeneous materials, such as cementitious materials. To improve the measurement of this parameter on this kind of materials, a linear swept-frequency signal is used to drive an emitter transducer to conduct a through-transmission inspection in immersion. To filter out undesirable frequency content, time-frequency filtering and detection process are performed. The use of this method has been compared with two excitation techniques, the broadband and the narrowband pulses. The results obtained using the swept-frequency excitation together with the time-frequency filtering, allows the determination of the attenuation curves with high accuracy over a wide frequency range without the need for complicated equipment, and improves the effective bandwidth by using a unique pair of transducers.

A Model for Scale-Free Networks: Application to Twitter

In the last few years, complex networks have become an increasingly relevant research topic due to the large number of fields of application. Particularly, complex networks are especially significant in the area of modern online social networks (OSNs). OSNs are actually a challenge for complex network analysis, as they present some characteristics that hinder topology processing. Concretely, social networks’ volume is exceedingly big, as they have a high number of nodes and links. One of the most popular and influential OSNs is Twitter. In this paper, we present a model to describe the growth of scale-free networks. This model is applied to Twitter after checking that it can be considered a “scale-free” complex network fulfilling the small world property. Checking this property involves the calculation of the shortest path between any two nodes of the network. Given the difficulty of this computation for large networks, a new heuristic method is also proposed to find the upper bounds of the path lengths instead of computing the exact length.

Monitoring of Freeze-Thaw Cycles in Concrete Using Embedded Sensors and Ultrasonic Imaging

This paper deals with the study of damage produced during freeze-thaw (F-T) cycles using two non-destructive measurement approaches—the first approach devoted to continuous monitoring using embedded sensors during the cycles, and the second one, performing ultrasonic imaging before and after the cycles. Both methodologies have been tested in two different types of concrete specimens, with and without air-entraining agents. Using the first measurement approach, the size and distribution of pores were estimated using a thermoporometrical model and continuous measurements of temperature and ultrasonic velocity along cycles. These estimates have been compared with the results obtained using mercury porosimetry testing. In the second approach, the damage due to F-T cycles has been evaluated by automated ultrasonic transmission and pulse-echo inspections made before and after the cycles. With these inspections the variations in the dimensions, velocity and attenuation caused by the accelerated F-T cycles were determined.

Influence of aggregates and air voids on the ultrasonic velocity and attenuation in cementitious materials

ABSTRACT This paper examines the influence of the size and volume fraction of both aggregates and air voids on the ultrasonic phase velocity and attenuation coefficient measured in cementitious specimens. A multiphase approach of the Waterman-Truell (WT) model was used to study theoretically how the size and volume fraction of the constituents in cementitious materials affect velocity and attenuation profiles. To verify the theoretical results obtained by the model, ultrasonic measurements were performed in several cementitious specimens with different granulometries of elastic aggregates (glass microspheres). Both theoretical and experimental results showed that velocity information can largely identify changes in the volume fraction of aggregates, whereas attenuation information is more sensible to variations in the aggregate size.

Measurement of the degraded depth in cementitious materials by automatic digital image processing

The combination of a staining method and an automatic digital image-processing algorithm is presented here, to measure degradation depths in cementitious materials. The measurement of those degraded depths is usually made by direct visual measurements, resulting in many errors and low reproducibility. The automatic digital image analysis (ADIA) method proposed here is mainly based on the differentiation of the degraded zone and the sound zone on the basis of the image histogram. The method comprises several steps, such as sample alignment, image calibration, background subtraction, image filtering, automatic segmentation and final measurement of the degraded depth. The algorithm developed has been used to measure the degraded depths of a set of decalcified cement mortars, made from different cement types and with varying w/b ratios. Relative to previous methods, this automatic procedure improves the precision (about 0.03 mm) and the statistical representation of the measurements. The results obtained by ADIA were compared with direct visual measurements with a very good correlation (R2 = 0.96) and a mean error of 6%.

Routing Topologies of Wireless Sensor Networks for Health Monitoring of a Cultural Heritage Site

This paper provides a performance evaluation of tree and mesh routing topologies of wireless sensor networks (WSNs) in a cultural heritage site. The historical site selected was San Juan Bautista church in Talamanca de Jarama (Madrid, Spain). We report the preliminary analysis required to study the effects of heating in this historical location using WSNs to monitor the temperature and humidity conditions during periods of weeks. To test which routing topology was better for this kind of application, the WSNs were first deployed on the upper floor of the CAEND institute in Arganda del Rey simulating the church deployment, but in the former scenario there was no direct line of sight between the WSN elements. Two parameters were selected to evaluate the performance of the routing topologies of WSNs: the percentage of received messages and the lifetime of the wireless sensor network. To analyze in more detail which topology gave the best performance, other communication parameters were also measured. The tree topology used was the collection tree protocol and the mesh topology was the XMESH provided by MEMSIC (Andover, MA, USA). For the scenarios presented in this paper, it can be concluded that the tree topology lost fewer messages than the mesh topology.

The Use of Wireless Sensor Networks to Monitor the Setting and Hardening Processes of Self-Compacting Concrete

Traditionally, the study of cementitious materials has been performed using wired sensor technologies. Because these technologies are expensive and difficult to install, the use of wireless sensor networks has gained increasing importance. In this paper, the study of setting and hardening processes for two different types of self-compacting concrete (SCC) using a wireless monitoring system is reported. The monitoring system used to perform such study consists of a wireless sensor network using Cricket motes. These motes were purchased from Crossbow Technologies. For our research, the most important capability of Cricket motes is that they host a transmitter/receiver in the ultrasonic wavelength region. For monitoring the setting and hardening processes, the velocity of the ultrasonic pulse traveling across the material was measured, along with the humidity and temperature values both inside and outside the concrete sample. Multi-hop data transmission techniques were considered to monitor the velocity data.

Damage Assessment by Ultrasonic Images in Concrete Subjected to Freeze-Thaw Cycles

This paper reports on damage evaluation of concrete subjected to freeze-thaw cycles using ultrasonic images. The use of automated ultrasonic inspection systems supplies velocity and attenuation maps, providing the spatial variations in velocity and attenuation in concrete specimens with high resolution. Assessment of damage in concrete specimens is conducted defining several parameters extracted from the ultrasonic images obtained before and after the freeze-thaw cycles. These parameters are related to surface scaling and internal cracking. Experimental trials were carried out using different concrete specimens with and without air-entraining agents. Comparisons between the parameters obtained from the ultrasonic images and the failure criterion defined by the standard ASTM C666/C666M-03 are performed. As a result, this work shows the feasibility of using ultrasonic images as an effective tool for evaluating freeze-thaw damage in concrete.