Publications

Engineering Surface Wettability to Alleviate Membrane Scaling in Photothermal Membrane Distillation

To address the growing demand for fresh water with a smaller carbon footprint, photothermal membrane distillation (PMD) has been proposed by combining membrane distillation with solar irradiation. Due to its low level of energy consumption and portability, PMD is becoming increasingly attractive for water production in off-grid areas. However, it still suffers from several challenges, including membrane scaling. To address this issue, we developed photothermal membranes with different wettabilities. #PCNT-2 with a hydrophobic surface was obtained by electrospinning a polyvinylidene fluoride (PVDF) nanofibrous substrate and a hydrophobic PVDF/carbon nanotube (CNT) surface layer. The surface was then modified by depositing a polydopamine (PDA) layer to make it hydrophilic (#PCNT-D). It was found that #PCNT-D exhibited the highest surface temperature under both dry and wet conditions and the highest evaporation rate due to its broad light absorption and high photothermal efficiency. PMD tests show that #PCNT-D with a hydrophilic surface possessed the best antiscaling performance under illumination. This can be attributed to it having the highest surface temperature and lowest mass transfer resistance, the highest salt solubility on a membrane surface, and the best heat transfer efficiency between the heated membrane surface and absorbed water in the PDA layer.

A finite element for modelling the nonlinear behavior of shear connectors in composite structures

Effect of air-entrainment and phase transition on chloride diffusion in partially frozen concrete

In some regions concrete is partially frozen for several months each year, yet there is paucity of information about the effect of air entrainment on chloride diffusivity and binding in partially frozen concrete. To address this issue, concrete without and with entrained air were subjected to chloride diffusion under constant exposure temperatures of +5, 0, −5, and −15 °C and their total and free chloride concentrations were determined. The entrained air reduced chloride diffusivity by 18% when the concrete evaporable water was either unfrozen or nearly fully frozen, but when it was partially frozen it reduced it by only 3%. Non-air entrained concrete exhibited higher binding capacity than air entrained concrete under all the test temperatures. After freezing, the effect of temperature on concrete diffusivity cannot be captured by Arrhenius law alone. A procedure is proposed to account for this deviation and the predicted diffusion coefficients agree reasonably with the experimental values.

Rational Method for Calculating Deflection of Continuous FRP R/C Beams

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Analysis of curved multicell box girder assemblages

A method of analysis is proposed for curved multicell box girder grillages. The method can be used to analyze box girder grillages comprising straight and/or curved segments. Each segment can be modelled by a number of beam elements. Each element has three nodes and the nodal degrees of freedom (DOF) consist of the six DOF for a conventional beam plus DOF to account for torsional warping, distortion, distortional warping, and shear lag. This element is an extension of a straight element that was developed earlier. For a more realistic analysis of the intersection regions of non-colinear box girder segments, the concept of a rigid connector is introduced, and the compatibility requirements between adjoining elements in those regions are discussed. The results of the analysis showed good agreement with the shell finite element results, but the proposed method of analysis needs a fraction of the time and effort compared to the shell finite element analysis.

Durability of GFRP and BFRP reinforcing bars in simulated seawater sea sand calcium sulfoaluminate cement concrete pore solution

Thanks to the much smaller carbon footprint of calcium sulfoaluminate cement (SAC) concrete and the corrosion resistance of fiber reinforced polymer (FRP) reinforcement, FRP-reinforced SAC concrete can possibly replace conventional reinforced concrete in some cases. In this study, for the first time, the durability of glass FRP (GFRP) and basalt FRP (BFRP) bars, made with epoxy resin, is investigated in simulated pore solutions of concretes made with (a) SAC and fresh water, (b) SAC and seawater and sea sand. The bars were immersed in each solution and kept at constant temperature of 30 °C, 45 °C and 60 °C for up to 180 days. Bar samples were tested for their retained tensile strength and physiochemical degradation after 30, 60, 90, and 180 days of immersion. Scanning electron microscopy (SEM), X-ray dispersive spectroscopy (EDS), and Fourier transform infrared (FTIR) spectroscopy were used to identify the underlying damage mechanisms. Results show that solution (b) is less harmful to BFRP than solution (a), while the opposite is true in the case of GFRP. Both solutions caused the dissolution of the epoxy resin and the leaching of the Fe in the basalt fiber. Overall, GFRP exhibited higher durability than BFRP. Based on image analysis, the deteriorated zone within the bar cross-section is not a uniform ring, but its area correlates with the loss of tensile strength. Inspired by the Avrami-Erofe'ev Equation and the Eyring's Transition State Theory, a novel time-temperature-dependent FRP bar degradation model is proposed. The proposed model shows close agreement between the predicted and measured tensile strengths.

Effect of sub-zero versus sub-freezing temperature on chloride diffusion and binding in concrete

Effect of temperature on chloride diffusion in concrete has been extensively studied by others and it is found to follow Arrhenius law. However, the lowest exposure temperature studied to date has been −4 °C, but due to supercooling the concrete pore solution is not expected to freeze at −4 °C. It is hypothesized here that below the supercooling temperature, some of the pore water will freeze and pore ice will form, which increases the pores tortuosity, and reduces their connectivity, resulting in reduced diffusivity. To test this hypothesis, concrete samples are subjected to natural diffusion in a 10% NaCl solution under 5, 0, −5, or −15 °C for 3 and 6 months. It is discovered that Arrhenius law can capture the effect of temperature on chloride diffusion, but when the temperature falls below the supercooling temperature, the effect of pore ice must be also considered. A method is proposed for capturing the latter effect. The diffusion coefficients of the frozen specimens computed by the proposed method differ maximum 15% from the corresponding experimental values.

Damage Detection Monitoring and Evaluation in Concrete Structures

Debonding evolution in nonlinear FRP-retrofitted RC beams with cohesive interface

Reliability-based load factors for blast design

In this study the concepts of reliability are used to derive blast load factors. First, some objective criteria are proposed for the proper interpretation of pressure data gathered in arena tests. These criteria are applied to the pressure–time histories recorded during field tests involving live explosive detonated in contact with the ground. Three major shock wavefront parameters, including peak pressure, impulse, and positive phase duration are calculated. Next, statistical analysis is performed on these metrics to estimate their probability density functions and goodness-of-fit tests are carried out to gauge the appropriateness of each estimate. Using the best-fitting distribution for each wavefront metric, load factors are derived on the basis of two approaches. The first approach employs the percentiles of the three load metrics, each estimated using the pertinent probability distribution. The second approach uses concepts of reliability and presents load factors for low, medium, and high level of protection. The two sets of load factors are compared and the limitations of each approach are discussed.

Utilization of carbon nanotubes (CNTs) in concrete for structural health monitoring (SHM) purposes: A review

Carbon nanotubes (CNT) as a functional filler can increase the electrical conductivity property of concrete and thus provide intrinsic self-sensing properties with no need for external sensors to monitor the behavior of concrete infrastructure and structures containing CNT. CNT-cement composites also improve mechanical strength and have higher energy absorption capacity. The sensitivity of their electrical conductivity to external physical parameters, such as strain, stress, load, temperature, displacement, and pressure, makes them suitable for structural health monitoring (SHM) applications. This paper presents a comprehensive review of the CNT properties, fabrication process, composition, and sensing characteristics as well as challenges for applying CNT concrete as part of a self-sensing structure. Furthermore, the self-healing property of CNT, as an integral feature of future smart concrete infrastructure is discussed.

Durability of GFRP and CFRP Bars in the Pore Solution of Calcium Sulfoaluminate Cement Concrete Made with Fresh or Seawater

Calcium sulfoaluminate cement concrete (CSAC) reinforced by fiber-reinforced polymer (FRP) bars, termed bars for brevity, is a good alternative to steel-reinforced concrete in marine environments due to the corrosion resistance of FRP and the lower pH of CSAC. For the first time, multi-mechanical tests are conducted to compare the durability of glass FRP (GFRP) to that of carbon FRP (CFRP) after exposure to CSAC pore solution. The bars were immersed in a simulated pore solution of CSAC made with either fresh water and river sand or with seawater and sea sand. Solution temperature was held constant at 30 °C, 45 °C or 60 °C for 30, 60, 90 and 180 days of immersion. Tensile, horizontal and transverse shear tests, as well as detailed microstructural analyses, were conducted to determine the level and mechanisms of degradation for each type of bar. Sea salt increases the degradation of both bars, but it degrades GFRP more than CFRP. The bars’ retained tensile strength is a reliable indicator of their durability, while their post-exposure horizontal and transverse shear strengths are found inconsistent and counter intuitive. In the GFRP, the fiber, the epoxy matrix and their interface suffered damage, but in the CFRP, the carbon fiber was not damaged. Under the test conditions in this study, the maximum reduction in the tensile strength of the GFRP was 56.9% while that of CFRP was 15.1%. Based on the relevant ASTM standard, the CFRP bar satisfies the alkaline resistance requirement of the standard in the CSAC pore solution with and without salt, whereas the GFRP bar does not meet the same requirement in the above pore solution with salt.

Time-Response Analysis of Arching Unreinforced Concrete Block Walls Subjected to Blast Loads

The new ASCE59-11 standards currently limit the use of unreinforced masonry (URM) walls in blast-resistant construction, regardless of the wall boundary conditions. This is attributable in part to the lack of experimental and analytical studies focusing on evaluating the response of URM walls under blast when the walls are forced to arch between the surrounding frame members. In this paper, the out-of-plane displacement response and structural stability of one-way vertical arching URM walls subjected to blast loads are investigated. A simple bilinear moment-rotation relationship is developed to simulate the arching wall responses. The model takes into account the masonry material strength, thrust forces, and wall geometry. Time-response analyses were performed using both single-degree-of-freedom (SDOF) and two-degrees-of-freedom (2DOF) models. Both models take into account the rocking phenomenon and second-order effects. Responses generated by both models were validated using experimental data reported previously. For preliminary design, performance charts were developed to correlate the effects of the wall slenderness ratio, masonry strength, and block size to the wall response under different levels of blast loads. The developed model and charts can be used as simple and quick calculation tools to estimate the required thickness, height, and strength of the wall under an expected blast threat when hardening of URM walls is necessary, with arching being considered as one of the alternatives.

Canadian research on FRP in construction

Canadian research on FRP applications in construction is described. The research is carried out mainly in Canadian universities, or in some Canadian federal government laboratories, and is focused on the design, construction, and repair/strengthening of reinforced concrete structures. The research on new structures focuses primarily on bridges, particularly on bridge decks, while research related to repair/strengthening involves bridges and buildings. The FRP of choice in Canada is CFRP, with some research involving GFRP. The research results have led to the development of FRP design codes for both buildings and bridges and to a number of demonstration projects and field applications. To date, two highway bridges have been constructed in Canada in which some of the bridge girders are prestressed with CFRP tendons. In addition, a number of other bridges have been constructed where the deck slab and barrier walls have been partially or entirely reinforced with CFRP or GFRP bars. CFRP sheets and laminates have been used to repair/strengthen concrete structures for increased flexural, axial, and shear strength.

Use of polymer grids for short- and long-term crack control in concrete slabs

Shrinkage cracks in Portland cement concrete can cause serious problems when cracked reinforced concrete is subjected to corrosive agents such as salts. The seepage of salty solutions through the cracks into the reinforcing steel can lead to corrosion of the steel and ultimately failure of the structure. This paper presents the results of an extensive experimental investigation designed to evaluate the potential of using polymer grids to minimize shrinkage cracks and their subsequent effects on corrosion of steel reinforcement. The investigation consisted of a short-term and a long-term programme. In the short-term programme, 120 × 600 × 2000 mm concrete slabs were subjected to drying shrinkage. In the long-term programme, 100 × 500 × 3200 mm concrete slabs were loaded and unloaded under severe conditions of deicing salt applications. The test results showed that the use of polymer grids can minimize shrinkage cracking. Also, in the occurrence of a crack, the presence of polymer grids as secondary reinforcement resulted in controlling the crack width.

Effects of a red mud mineralogical composition versus calcination on its pozzolanicity

Past research indicates that red mud (RM) has negligible pozzolanicity unless calcined. An iron-rich and silicate-poor RM was calcinated at 600, 800 and 1000 °C to investigate this issue. Tests were performed on mortars made with Portland cement (control) and with 85 wt% cement and 15 wt% RM. At 91 days, the mortar containing non-calcined RM reached 94% of the compressive strength of the control mortar, but the ones containing calcined RM attained much lower strength. Mineralogical, microstructural, and thermodynamic analyses are performed to find the mechanisms undergirding this behavior. The crucial influences of Al/Si ratio and crystalline transformations are highlighted.

Mechanisms Contributing to the Performance of Uncalcined Bayer's Red Mud as a Supplementary Cementitious Material in High Quality Concrete

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Effectiveness of FRP stirrups in concrete beams subject to shear

No abstract is provided for this article.

Shear strength of reinforced recycled concrete beams without stirrups

A new method of mixture proportioning is used for investigating the shear performance of reinforced concrete (RC) beams made with coarse recycled concrete aggregate (RCA). In this method, RCA is treated as a two-phase material comprising mortar and natural aggregate therefore to proportion the concrete mixture with RCA, the relative amount and properties of each phase are considered separately. Using the new mix proportioning method, several beams were designed and tested to study the effect of a number of parameters including the shear span-to-depth ratio and beam size on the serviceability and strength of RCA concrete beams without shear reinforcement. For each beam its load–deflection curve, shear deformations, diagonal cracking load, crack pattern, ultimate shear strength and failure mode were determined. The results showed that the shear performance of RC beams made with RCA can be comparable, or even superior, to that of beams made entirely with natural aggregates at both serviceability and ultimate limit states, provided the proposed mixture proportioning method is used. Furthermore, the simplified methods of ACI and CSA standards as well as Eurocode 2 were found applicable to all reinforced RCA-concrete beams.