Various aspects of a general finite‐element program for the nonlinear analysis of steel‐and‐concrete structures is described. The program accounts for the nonlinear behavior of concrete, steel, and shear connectors. Concrete is treated as an orthotropic nonlinear material. The concept of equivalent strain is used to establish independent stress‐strain relationships in the directions of orthotropy. Steel is modeled as an elastoplastic strain‐hardening material, and classical theory of plasticity together with the von Mises failure criterion is applied. For shear connectors, an empirical nonlinear shear force—slip relationship is used. The accuracy and reliability of the program are demonstrated by the analysis of two composite beams and a multigirder bridge over the entire loading range up to failure. The analytical results are compared with the corresponding experimental and/or field data with good agreement between the two. The reported results demonstrate the feasibility and reliability of the nonlinear finite‐element method as an expedient alternative to costly experimental work in some situations.
Accurate prediction of corrosion initiation period and service life of reinforced concrete structures requires accurate determination of their chloride diffusion coefficient and binding capacity. Temperature and exposure duration affect these properties, but there is paucity of knowledge regarding the effects of long-term exposure to sub-zero temperatures. Hence, the effects of sub-zero temperature, T, and exposure duration, t, on the chloride diffusion decay index, m, and binding in ordinary Portland cement concrete are investigated. Non-steady natural diffusion tests are conducted at + 5, 0, −5 and −15 °C for 3, 6, 12 and 18 months, and at the end of each period, the total and free chloride concentration profiles are determined. The index m is found to vary over a wide range, it being directly proportional to T and inversely to t. It drastically deviates from the commonly assumed value of 0.2 in the literature. Chloride binding is found to obey the Freundlich isotherm. The coefficient α of the isotherm equals 1.0 ± 0.16 and is only slightly dependent on T or t while its exponent β varies from 0.178 to 0.502, without exhibiting a systematic relationship with either parameter. Binding is noticed to increase with drop in temperature for the first three months of exposure, but over longer exposure durations, no systematic relationship is observed.
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The behavior of reinforced concrete panels, or slabs, retrofitted with glass fiber reinforced polymer (GFRP) composite, and subjected to blast load is investigated. Eight 1000×1000×70mm panels were made of 40MPa concrete and reinforced with top and bottom steel meshes. Five of the panels were used as control while the remaining four were retrofitted with adhesively bonded 500mm wide GFRP laminate strips on both faces, one in each direction parallel to the panel edges. The panels were subjected to blast loads generated by the detonation of either 22.4kg or 33.4kg ANFO explosive charge located at a 3-m standoff. Blast wave characteristics, including incident and reflected pressures and impulses, as well as panel central deflection and strain in steel and on concrete/FRP surfaces were measured. The post-blast damage and mode of failure of each panel was observed, and those panels that were not completely damaged by the blast were subsequently statically tested to find their residual strength. It was determined that overall the GFRP retrofitted panels performed better than the companion control panels while one retrofitted panel experienced severe damage and could not be tested statically after the blast. The latter finding is consistent with previous reports which have shown that at relatively close range the blast pressure due to nominally similar charges and standoff distance can vary significantly, thus producing different levels of damage.
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Four 6.5 m long two span continuous beams were tested to investigate the feasibility of using FRP grids as shear reinforcement. The beams were longitudinally reinforced with equal amounts of CFRP reinforcement while transversely two beams were reinforced with steel stirrups and two with CFRP grids. The two spans of each beam had identical amount and disposition of reinforcement, and were symmetrically loaded with a point load at the center of each span. For the beams with steel stirrups, the so-called Av/s = 2.0 mm while for the ones with CFRP grids the equivalent quantity was 1.75 mm. The elastic modulus of the grid was half the elastic modulus of the steel. The beams were monotonically loaded to destruction and they all failed in flexure. Despite their lower shear reinforcement ratio, the beams with the CFRP grid performed as well as those with the steel stirrups. They actually failed at 10% higher load than the beams with steel stirrups. Strain values of up to 0.4% were measured in the grids. Recognizing the practical convenience of grid shear reinforcement over custom manufacturing of stirrups, and considering their satisfactory performance, as observed in the present testing program, the use of FRP grids as shear reinforcement is indeed feasible.
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Membrane scaling is unavoidable when treating highly concentrated and even supersaturated brine in the continuous membrane distillation (MD) process. Omniphobic membranes with higher nucleation energy barriers could partially mitigate the membrane scaling. Herein, we fabricated omniphobic corrugated membranes to further alleviate membrane scaling via the synergistic effects of omniphobicity and corrugation on the membrane surface. The scaling behaviors of omniphobic flat membrane (#PVDF-FF), omniphobic corrugated membrane (#PVDF-CF) at parallel (PL) and perpendicular (PD) modes were investigated. Different modes represented the relative position between the feed flow direction and the pattern of corrugated membrane. The results showed that the modified corrugated membranes had water contact angles over 150° and anti-wettability against low surface tension liquids. Compared to #PVDF-FF, #PVDF-CF at the PL mode exhibited a better slippery property while the PD-mode #PVDF-CF had higher slippery resistance. The membrane omniphobicity was confirmed to be effective against membrane scaling when treating a 3.5 wt% NaCl feed solution. However, the scaling resistance of #PVDF-CF at both PL and PD modes was much better than that of #PVDF-FF when 25 wt% NaCl or 20 mM supersaturated gypsum solutions were used as feeds. The better anti-scaling properties of PL-mode #PVDF-CF should be attributed to a higher flow rate and shorter residence time of feed flow parallel to the corrugated patterns, as well as the alleviated concentration polarization. The turbulent effect induced by the perpendicular feed flow on the corrugated patterns and the consequent alleviation of concentration polarization endowed the PD-mode #PVDF-CF with the best scaling resistance.
Results from finite element analyses and laboratory tests are presented which suggest that in the presence of construction induced cracks in new asphalt overlays, geogrid reinforcement will be ineffective in preventing reflective cracking. As is customary, the reinforcement layer was placed at the interface of the overlay and the existing pavement surface. However, when through improved compaction, by means of a newly developed compactor, called AMIR, construction induced cracks were minimized, the same reinforcement proved to be effective in controlling reflective cracking. (A) For the covering abstract see IRRD 857794.
Twelve identical concrete prisms were strengthened with Carbon Fibre-Reinforced Polymer (CFRP) laminate strips on two opposite faces and the laminates were anchored using the newly developed CFRP π-anchor. Prisms were tested in tension to investigate the effectiveness of the anchor to possibly delay delamination and/or to prevent the complete separation of the laminate from the prism. The salient feature of the anchor is its wide head to resist high interfacial shear stresses and its shanks that were inserted in predrilled holes to provide mechanical anchorage and to resist pull-out. The anchor doubled the tensile load-carrying capacity, effectively delayed delamination and prevented the CFRP laminate from full separation. Furthermore, the strengthened prisms experienced noticeable deformation.
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Membrane wetting and fouling are two major challenges in membrane distillation (MD), especially when the feed has components with low surface tension. A series of membranes with different wettability were developed herein to provide rational guidelines for membrane selection to treat these wastewaters. The membranes with different wettability, and Janus membranes composing of same hydrophilic surface but different substrates (hydrophobicity, superhydrophobicity, superomniphobicity), were made by electrospinning and modifications. It was found that when the feeds had anionic and cationic surfactants, the superhydrophobic and superomniphobic modifications improved their anti-fouling/wetting properties. However, both membranes were rapidly wetted when the feeds had nonionic surfactant Tween-20. The Janus membranes could not delay membrane fouling/wetting when treating the feeds with free surfactants. It even worsened membrane performance as the hydrophilic layer absorbed the surfactants and accelerated membrane fouling/wetting. Regarding the emulsified oily wastewaters, the hydrophilic layer on Janus membranes did show obvious improvement in their anti-fouling/wetting abilities. The underwater hydrophobic layer formed a protective layer and impeded oil contact with the underlying substrate, but the choice of the substrate still needed attention. The superomniphobic substrate showed the most excellent stability as it could reject surfactants even if some managed to pass through.
This work utilized fertilizer driven forward osmosis (FDFO) to extract fresh water from acid mine drainage (AMD) wastewater for irrigation, and investigated the effects of draw solutes on membrane fouling behaviours. The carbon nanotubes (CNTs) and polydopamine (PDA) were coated on the thin film nanofibrous composite (TFNC) membrane #FO-0 as anti-fouling modification. When 1 M NH4H2PO4 (MAP) and 500 mg/L copper AMD wastewater (pH=3.0) used as draw and feed solutions, #FO-0 and PDA-coated TFNC membrane (#FO-PDA) exhibited rapid declines of water fluxes due to the dense fouling layer. In contrast, the CNT-coated membrane #FO-CNT exhibited the lowest flux decline of 26.0% attributed to better hydrophilicity, the lowest roughness and neutral surface charge. All the TFNC membranes exhibited insignificant membrane fouling when (NH4)2SO4 (SOA) was used as draw solute. Additionally, the osmotic backwashing could effectively recover water flux. The discharged draw solution promoted plant growth without posed potential health threat.
