BENDING, BUCKLING AND FREE VIBRATION ANALYSES OF NANOBEAM-SUBSTRATE MEDIUM SYSTEMS

This study presents a newly developed size-dependent beam-substrate medium model for bending, buckling, and free-vibration analyses of nanobeams resting on elastic substrate media. The Euler-Bernoulli beam theory describes the beam-section kinematics and the Winkler-foundation model represents interaction between the beam and its underlying substrate medium. The reformulated strain-gradient elasticity theory possessing three non-classical material constants is employed to address the beam-bulk material small-scale effect. The first and second constants is associated with the strain-gradient and couple-stress effects, respectively while the third constant is related to the velocity-gradient effect. The Gurtin-Murdoch surface elasticity theory is adopted to account for the surface-free energy. To obtain the system governing equation as well as corresponding boundary conditions, Hamilton’s principle is called for. Three numerical simulations are presented to characterize the influences of the material small-scale effect, the surface-energy effect, and the surrounding substrate medium on bending, buckling, and free vibration responses of nanobeam-substrate medium systems. The first simulation focuses on the bending response and shows the ability of the proposed model to eliminate the paradoxical characteristic inherent to nanobeam models proposed in the literature. The second and third simulations perform the sensitivity investigation of the system parameters on the buckling load and the natural frequency, respectively. All analytical results reveal that both material small-scale and surface-energy effects consistently stiffen the system response while the velocity-gradient effect weakens the system response. Furthermore, these sized-scale effects are more pronounced when the underlying substrate medium becomes softer

Thermal and Acoustic Properties of Sustainable Structural Lightweight Aggregate Rubberized Concrete

This study investigated the effect crumb rubber recycled from wasted tires on properties of structural lightweight aggregate concrete (LWAC). Two types of concrete were tested: control LWAC and rubberized lightweight aggregate concrete (RLWAC). The control LWAC consisted of cement, fine aggregate (river sand), and lightweight coarse aggregate (porous aggregates). For the RLWAC, the fine aggregate was replaced by crumb rubber at the rate of 10, 20, 30, 40, and 50% by volume. The water to cement ratio for both concrete types was set at 0.35. The experiment series consisted of density (ASTM C567), compressive strength (ASTM C39), flexural strength (ASTM C78), thermal conductivity (ASTM C518), and sound absorption coefficient (ISO 10534-2). Results showed the decrease in density of about 10%, compressive strength of 21.4%, and flexural strength of 35.4% with the increasing crumb rubber replacement ratio up to 50%. For thermal and sound properties, the increasing crumb rubber content of up to 50% improved the thermal insulation of concrete, as seen by the decrease in thermal conductivity by about 14.6%. RLWAC also exhibited superior sound insulating properties to LWAC as seen by higher sound absorption coefficient over the working sound frequency range. In order to satisfy the requirements of ASTM C330 and ACI 318, the optimum crumb rubber replacement was recommended at less than 10%

Use of buffer treatment to utilize local non-alkali tolerant bacteria in microbial induced calcium carbonate sedimentation in concrete crack repair

Concrete often suffers cracks due to its low tensile strength. The repair process can vary ranging from surface coating, grouting, and strengthening. Microbial induced calcium carbonate sedimentation process (MICP) is a process of utilizing non-pathogenic bacteria to produce calcium carbonate through its urease activity in crack repair (filling). It is known as an alternative crack repair method that does not utilize Portland cement. In general, the bacteria used in MICP are alkali tolerant bacteria that have a higher chance of surviving the high alkalinity environment in concrete. However, in some regions, alkali tolerant bacteria are difficult to acquire and unavailable locally. This study introduced a technique to utilize non-alkali tolerant bacteria in MICP using buffer treatment. Instead of injecting bacteria directly onto the crack surface, the buffer solution was applied onto the crack surface prior to the bacteria injection. Results from the laboratory indicated a higher bacteria survival rate when the buffer treatment was applied to the medium. For the crack filling, with the buffer treatment, the crack was completely filled within 21–28 days. The microstructure results also showed that the crystal deposits from both laboratory and crack surface were similar in both physical appearance and phase composition

Seismic strengthening of low strength concrete columns using high ductile metal strap confinement : a case study of Kindergarten school in Northern Thailand

The 2014 Chaing Rai earthquake (Thailand) caused extensive damage in many reinforced concrete (RC) buildings built before the introduction of modern seismic design guidelines. Much of the damage on these buildings was attributed to the inadequate capacity and/or ductility of columns. As a result, suitable and cost-effective strengthening techniques for such substandard elements are necessary. This article presents a case study on the seismic strengthening of a one-story RC kindergarten school located in Ampor Pan, Chaing Rai province. The building was partially damaged during the afore-mentioned earthquake, which led to cracking in walls, columns, and beam-column joints. As part of the initial assessment, innovative repair solutions were sought to minimize construction time, labor, and material cost. Accordingly, an innovative strengthening technique that uses Post-tension Metal Strapping (PTMS) was proposed to strengthen the damaged RC elements. This article presents details of the structural assessment performed on the building, as well as details of the PTMS strengthening strategy, which was applied for the first time in a real full-scale structure. This article contributes towards the validation and application of the PTMS strengthening on real structures, which had not been possible until now

A practical macro-mechanical model for the bend capacity of fibre-reinforced polymer bars

Bent fibre-reinforced polymer bars embedded in reinforced concrete elements resist lower forces than straight counterparts due to strength losses at the bend, and such losses are difficult to calculate. This paper reports on an investigation into the effect of section geometry and bond, which led to a new macro-mechanical model to calculate the bend capacity of fibre-reinforced polymer bars. The proposed model uses a Tsai-Hill failure criterion and accounts for factors known to influence the bend capacity of the bars. A section factor, ignored in existing models, also accounts for the strength degradation due to the change in geometry at the bent portion of the bar. The model was calibrated using a set of 80 tests found in the literature and performed by the authors. The results indicated that, compared to existing equations, the proposed model predicts the bend strength of bars more accurately, with an average prediction to experiment ratio of 1.0 and a standard deviation of 0.25. Following validation and verification, appropriate values for the model parameters are recommended for design. The proposed model can lead to more economic design, by up to 15%