Engineered Materials Abstracts

Paper provides an attempt to create a methodology for automated structure health monitoring procedures using vibration spectrum analysis. There is an option to use autoregressive (AR) spectral analysis to extract information from frequency spectra when conventional Fast Fourier transformation (FFT) analysis cannot give relevant information. An autoregressive spectrum analysis is widely used in optics and medicine; however, it can be applied for different purposes, such as spectra analysis in electronics or mechanical vibration. This paper presents an automated structural health monitoring approach based on the algorithm-driven definition of the first resonant frequency value from a noisy signal, acquired from trafficcreated bridge vibrations. We implemented the AR procedure and developed a peak detection algorithm for experimental data processing. The functionality of the proposed methodology was evaluated by performing research on six bridges in Vilnius (Lithuania). We compared three methods of data processing: FFT, filtered FFT and AR. Bridges vibrations under different excitation conditions (wind, impulse and traffic) in normal direction were measured using accelerometers. AR provided one peak representing the lowest resonant frequency in all cases, while FFT and filtered FFT provided up to 12 peaks with similar frequency values. Such results allow implementing our method for remote automated structures health monitoring and ensure structures safety using a convenient and straightforward diagnostic method.

Distributed Optical Fiber Sensors (DOFS) are novel and increasingly popular strain monitoring tools recently applied to the Structural Health Monitoring (SHM) of Reinforced Concrete (RC) structures. Up to now, most applications have seen the instrumenting of the latter's external surfaces yet, in few circumstances, this technique has also been adopted with the scope of measuring the strains present on the embedded reinforcement bars (rebars). Before the advent of DOFS, due to the lack of tools able to perform such investigation in an accurate, completely-distributed and un-intrusive fashion, structural analyses that rely on the knowledge of the rebars’ strains (such as tension stiffening) have always resorted to theoretical, empirical or numerical solutions. Yet, with the potential provided by DOFS, such insight is finally acquirable and represents the start of a new way of understanding the composite behavior of RC structures. The experimental campaign, topic of the present article, intends on taking full advantage of such potential to study the bond stress and slip present on the surface between concrete and steel rebars in differently sized cracking and non-cracking RC tensile members. These are key parameters for the development of any stress transfer approach-based RC structures’ serviceability analysis, thus the importance of using DOFS for this novel application. The DOFS extracted bond/slip laws are further compared with the Model Code 2010’s predictions and seems to provide consistently higher bond stresses per similar slip than the latter.

Cracking effects are the intrinsic attribute of serviceability analysis of reinforced concrete elements. These effects are the research object of numerous studies, which have been carried out for more than half a century. Despite extensive research, there is no commonly accepted methodology for cracking analysis. The existing approaches employ empirical models customised using results of laboratory specimens with simplified geometry that are not able to represent common cracking tendencies adequately. The focus of this experimental study is to examine the influence of the cross-section parameters on the cracking behaviour of concrete elements subjected to tension. The test program encompasses 30 concrete prisms reinforced with multiple bars. Two lengths of the specimens were investigated verifying the comparability of the cracking results of relatively short (500 mm) and long (1210 mm) prisms. All specimens had an identical 150×150 mm cross-section. The number and diameter of bars and cover depth were among the test variables. The bars were distributed in the cross-section ensuring 15 mm, 30 mm, 40 mm and 50 mm depth of concrete cover. The cracking results do not support the generally accepted concept that relates crack widths to crack spacing following which the formation of the maximum crack is expected close to the uncracked block having the maximum length. In the tested specimens, the maximum crack was located next to the uncracked block of maximum length in 60% of the considered prisms; the maximum crack occurred between the two blocks whose sum is maximum in only one case. The adequacy of the crack prediction model from Model Code 2010 was also examined, identifying tendencies that point out potential improvements in the model. The modification should address the evidenced inability of the model to predict the correlation between the cover depth and crack width satisfactorily.

Rigid polyurethane foams (RPUFs) were successfully modified with different weight ratios (0.5 wt%, 1 wt%, 2 wt%) of nanosilica and nanosilica suspension (nanosilica-enhanced with ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6]). The resulting foams were evaluated by their processing parameters, chemical structure (Fourier Transform Infrared Spectroscopy Analysis, FTIR), morphology (Scanning Electron Microscopy Analysis, SEM), mechanical properties (compressive test, three-point bending test), viscoelastic behavior (Dynamic Mechanical Analysis, DMA), thermal property (Thermogravimetric Analysis, TGA, thermal conductivity) and application properties (water absorption, dimensional stability). The results showed that the morphology of modified foams is significantly affected by the type of the filler and filler content, which resulted in inhomogeneous, irregular, large cell shapes and further affected the physical and mechanical properties of resulting materials. RPUFs modified with nanosilica suspension represent better mechanical and thermal properties comparing to the RPUFs modified with pure nanosilica. The results showed that the best results were obtained for RPUFs modified with 1 wt% of nanosilica suspension. In comparison with unfilled foam, compositions modified with 1 wt% of nanosilica suspension provide higher density (44 kg/m3), greater compression strength (258 kPa measured parallel and 162 kPa measured perpendicular), and lower thermal conductivity (0.026 W/m·K). The results obtained in this study suggest that the introduction of nanosilica and nanosilica suspension over a certain optimal level (such as 2 wt%) has a negative effect on the cell morphology, which leads to deterioration of physico-mechanical properties of the modified foams.

Practical interest in 'off-site construction' has risen remarkably over the last decade, and with it there has been a burgeoning of academic research in the field. Complementing this research, a number of literature reviews have been conducted. None, however, are systematic. This study addresses this lack, offering the first bibliometric study to explore the state of off-site construction research (OCR). A quantitative approach using 'science mapping' techniques is employed to examine 501 top-ranked construction journal articles. Longitudinal trends in publishing are identified, as are dominant research sub-fields, their connectedness with other areas of study, as well as citation patterns, publication journal areas of focus, key research institutions, key research persons, along with the extent to which these interact with each other in research networks. The findings are instructive in identifying the deficiencies in current research. Among these is a bias towards product research over operations and management, and a sharp compartmentalization of sub-fields, with little or no cross-fertilization between researcher areas, the researchers themselves, nor the research institutions. Clearly, this awareness will inform industry, journal editors and researchers of the need for a deeper exchange of ideas in any future research efforts.

The paper considers the velocities of air movement in the ventilated air gaps of walls and focuses on pressure fields in both wall models arranged in a climatic chamber and exploited houses. The article investigates the influence of air movement on heat transfer through walls applying numerical modelling methods and conducting experiments in the climatic chamber. The thermal effects of air flows have been described with reference to the Nusselt number defined as the ratio of average convective and conductive heat fluxes and heat flux through still air.

Experimental tests have determined the effect of slag, opoka (silica-calcite sedimentary rock), silica fume (SiO2) suspension, dolomite dust and sodium silicate solution (NaSS) together with the polycarboxylatether based plasticizing admixture on the yield stress and viscosity of Portland cement paste the rheological properties of which have been defined applying a rotational viscometer with co-axial cylinders. The tests have revealed that slag, opoka, silica fume suspension and dolomite dust added to cement paste by replacing 10% of Portland cement (by weight) have an effect on the yield stress and viscosity of the paste subject to the form and fineness of additive particles. When 10wt% of Portland cement is replaced with slag cement, the yield stress of Portland cement paste reduces by about 25.9%, and viscosity increases by about 3.5 times compared with the yield stress and viscosity of reference cement paste. The yield stress of Portland cement paste with 0.5% NaSS admixture increases insignificantly, and viscosity grows approximately twice compared with reference cement paste. The tests have also showed that the modifying admixtures of Portland cement paste enable to control the rheological properties of cement paste.

Every year, colossal amounts of used and non-biodegradable rubber tyres are accumulated in the world. Experience shows that the most efficient way to increase the concrete fracture energy GF (N/m) is to use metal or polypropylene fibres. The optimal content of fibre increases concrete resistance to stress (especially tensile stress under bending force). Concrete fracture is not brittle; concrete continues deforming after maximum stresses and is able to resist certain stresses, there is no abrupt decrease in loading. The research has proved that crumb rubber can be used in concretes as an alternative to metal and polypropylene fibres. The investigation has found that rubber waste additives, through their specific properties can partly take up tensile stresses in concrete and make the concrete fracture more plastic; besides, such concrete requires a significantly higher fracture energy and concrete samples can withstand much higher residual strength at 500 mm crack mouth opening displacement (CMOD) and deflection.

Numerous alternatives exist for foundation systems and construction technologies. The systems can be described by different criteria values which are incorporated in the conventional design process. Decision on the most suitable construction technology is vital for success and depends on many effectiveness criteria. The business success depends on the right choice. The mandate of a construction management researcher is to use rational, systematic, science-based techniques to inform and improve various decisions. The paper presents multiple criteria decision making model for selection of a pile-column technology. The technological criteria are determined by an experimental study. Based on in-situ investigation of natural soil conditions, criteria values are determined. The decision making model incorporates five different methods and techniques. To solve a problem, it uses three multiple criteria decision making methods. Integrated criteria weights are determined by using the analytic hierarchy process and the expert judgement method. This model could be used to solve complicated problems pertaining to the selection of a construction technology.

This paper describes a technique and mechatronical system for testing drills of a small diameter at different stages of production. The goal is to realize a system for drill testing which automatically increases the load applied to a drill under testing conditions and measure the drill's breaking torsion moment and deflection angle before a break occurs. The system's apparatus part and algorithms for the control of actuators and data acquisition from sensors are explained in the article. Also, a testing technique was applied in theoretical investigations to define the stress concentrations in dangerous places of the drill. The proposed technique and system have been verified by testing the drills of a small diameter at different stages of production-after thermal, mechanical treatment, and for quality control of the finished product.