Apparent Water Permeability Research Articles

Multi-performance experimental assessment of autogenous and crystalline admixture-stimulated self-healing in UHPFRCCs: Validation and reliability analysis through an inter-laboratory study

The huge benefits brought by the use of Ultra High-Performance Fibre-Reinforced Cementitious Composites (UHPFRCCs) include their high “intrinsic” durability, which is guaranteed by (1) the compact microstructure and (2) the positive interaction between stable multiple-cracking response and autogenous self-healing capability. Hence, self-healing capability must be properly characterized addressing different performances, thus providing all the tools for completely exploiting such large potential. Within this context, the need is clear for a well-established protocol for self-healing characterization. To this end, in the framework of the Cost Action CA15202 SARCOS, six Round Robin Tests involving 30 partners all around Europe were launched addressing different materials, spanning from ordinary concrete to UHPFRCC, and employing different self-healing technologies. In this paper, the tailored experimental methodology is presented and discussed for the specific case of autogenous and crystalline-admixture stimulated healing of UHPFRCC, starting from the comparison of the results from seven different laboratories. The methodology is based on chloride penetration and water permeability tests in cracked disks together with flexural tests on small beams. The latter ones are specifically aimed at assessing the flexural performance recovery of UHPFRCCs, which stands as their signature design “parameter” according to the most recent internationally recognized design approaches. This multi-fold test approach allows to address both inherent durability properties, such as through-crack chloride penetration and apparent water permeability, and more structural/mechanical aspects, such as flexural strength and stiffness.

Is salt the solution? Interactions between trace-metal uptake and salinity in the freshwater crabPotamonautes warreniCalman, 1918 (Decapoda: Brachyura: Potamonautidae)

AbstractCrustaceans inhabiting freshwater are more vulnerable to increased loading with dissolved trace metals than those from more saline conditions for both physicochemical and physiological reasons. Potamonautes warreni (Calman, 1918) is a true freshwater crab inhabiting rivers in proximity to mining activities in South Africa. In order to assess whether the uptake of zinc (Zn2+) and cadmium (Cd2+) were associated with Na+- and Ca2+-uptake routes, individuals of P. warreni were acclimated to different Na+ or Ca2+ concentrations before measuring trace-metal uptake into the haemolymph and tissues in an intermediate freshwater. Cd2+ uptake into the haemolymph was elevated following acclimation to ‘Na+ replete’ conditions, when compared with ‘Na+ deplete’ conditions, indicating that a physiological response to the acclimation salinity had influenced Cd2+ uptake. Zn2+ uptake by the whole body was elevated in P. warreni individuals from the ‘Na+ replete’ group when compared with crabs acclimated to Na+ free water. A general change in permeability may have occurred and indeed, the apparent water permeability (water efflux) of P. warreni was increased in elevated Na+ and Ca2+ concentrations compared with that in softer water. A comparison of the osmoregulatory ability between P. warreni and the catadromous crab Eriocheir sinensisH. Milne Edwards, 1853 highlighted the greater permeability of the latter. Lower Zn2+ and Cd2+ uptake rates were measured in P. warreni in comparison to those measured in E. sinensis by other authors, indicating that the more well adapted a species is to life in freshwater, the less susceptible they will be to metal uptake. Future investigations to understand further the mechanisms influencing Zn2+ and Cd2+ uptake are required to understand the role that salinity can play in protecting a species from trace-metals released into the environment.

Aquaporin (AQP) channels in the spiny dogfish, Squalus acanthias I: Characterization of AQP3 and AQP15 function and expression, and localization of the proteins in gill and spiral valve intestine.

Complementary DNAs (cDNAs) for two aquaporin water channel genes (AQP3 and AQP15) were amplified cloned and sequenced to initiate this study. Northern blot analysis was carried out to confirm the mRNA sizes of these AQP genes with AQP3 mRNA bands exhibiting sizes of 1.2 and 1.6k bases and AQP15 had a mRNA band of 2.1k bases. Northern blot analysis was also performed on kidney and esophagus total RNA samples from fish acclimated to 75%, 100% or 120% seawater (SW). The level of AQP15 mRNA expression was shown to significantly decrease following salinity acclimation from 100 to 120% SW. An opposite but non-significantly different trend was observed for AQP3 mRNA levels. Full length cDNAs were then used to generate AQP3 and AQP15 mRNAs for microinjection into Xenopus oocytes. Both AQP3- and AQP15- microinjected oocytes exhibited significantly elevated apparent water permeability compared to control oocytes at neutral pH. The apparent water permeability was mercury-inhibitable, significantly so in the case of AQP3. AQP3 microinjected oocytes showed pH sensitivity in their apparent water permeability, showing a lack of permeability at acidic pH values. The Carboxyl-terminal derived amino acid sequences of AQP3 and AQP15 were used to generate rabbit affinity-purified polyclonal antibodies. Western blots with the antibodies showed a band of 31.3kDa for AQP3 in the kidney, with minor bands at 26, 24 and 21kDa. For AQP15 a band of 26kDa was seen in gill and kidney. Fainter bands at 28 and 24kDa were also seen in the kidney. There was also some higher molecular weight banding. None of the bands were seen when the antibodies were pre- blocked with their peptide antigens. Immunohistochemical localization studies were also performed in the gill and spiral valve intestine. In the gill, AQP15 antibody staining was seen sporadically in the membranes of surface epithelial cells of the secondary lamellae. Tyramide amplification of signals was employed in the spiral valve intestine. Tyramide-amplified AQP3 antibody staining was observed in the basal membrane of the invaginated epithelial cell layer of secondary intestinal folds in luminal surface of either the side wall of the spiral valve intestine or in internal valve tissue 'flaps'. For the AQP15 antibody, tyramide-amplified staining was instead found on the apical and to a lesser extent the lateral membranes of the same invaginated epithelial cell layer. The localization of AQP3 and AQP15 in the spiral valve intestine suggests that a trans-cellular water absorption pathway may exist in this tissue.

Extending the life of water reuse reverse osmosis membranes using chlorination

Numerous efforts have been made over the years to extend the lifespan of reverse osmosis (RO) membranes. End-of-life RO membranes are periodically replaced and usually discarded in landfills. Periodic membrane modification using chlorination may be an alternative to recover their productivity without compromising process safety. In this research, RO membranes from an engineering-scale ultrafiltration-RO system treating reclaimed water were exposed five times to 2000 ppm-h of chlorine immediately after chemical cleaning. Water, conductivity, ion, and organic permeability coefficients and rejection were related to the chlorine dose. The breakthrough of six naturally occurring viruses with different levels of persistence to wastewater treatment was also monitored. After five chlorine doses, the apparent water permeability was recovered to 1.0–1.5 L m−2 h−1 bar−1, a 3.1-fold increase compared to the end-of-life membranes, with only a 2% decrease in observed salt rejection. Interestingly, apparent conductivity and ion permeability slightly decreased after the first and second chlorine dose, likely because the chlorine removed irreversible fouling/scaling and thus reduced concentration polarization. After the third chlorine dose, as the RO membrane surface oxidized, more monovalent ions permeated through the membrane, while observed divalent ion rejection remained relatively high and constant (>97%). Similarly, the RO permeate dissolved organic carbon concentration and total fluorescence intensity decreased between end-of-life membrane and the second chlorine dose, followed by an increase after the third dose, and only humic substances and building block compounds (<2 kDa) were detected. All viruses were below or near the detection limit, indicating that oxidation did not compromise the integrity of the membrane in its ability to reject virus-sized particles. The results suggest that targeted chlorination may extend RO membrane lifespan by increasing apparent water permeability while maintaining RO-like selectivity.

J0270201 張力下における脂質膜の水分子透過性変化の分子動力学解析 : ドラッグデリバリ用リポソーム開発に向けて

In order to develop drug delivery systems (DDS) using liposomes, it is important to understand the effects of mechanical stresses on the water permeability of liposomes. We perform a series of molecular dynamics simulations of stretched palmitoyl-oleoyl phosphatidylcoline (POPC) lipid bilayers, which is the fundamental shell component of the liposomes. The stretched bilayers are simulated by constant temperature and bilayer normal pressure MD simulations with various constant areas. Under stretching, the bilayer thickness becomes thin. In the core of the bilayer, the lipid density increases, resulted in the smaller diffusion coefficient and the larger potential of mean force of water in the core region. This leads to the increase in the local resistance for water permeation. However, the apparent water permeability, which is the inverse of the integrated value of the resistance profile across the bilayer, shows the increase trend as the thickness decreases, although it depends on the applied stretch. This indicates that the water permeability and the permeation mechanism might be affected by mechanical stresses. As the DDS liposomes experience various mechanical stresses during blood circulation, it may be important to evaluate the leakage of drugs from the liposome considering the history of the stresses and the apparent permeability change.

Water permeability in hydrate‐bearing sediments: A pore‐scale study

AbstractPermeability is a critical parameter governing methane flux and fluid flow in hydrate‐bearing sediments; however, limited valid data are available due to experimental challenges. Here we investigate the relationship between apparent water permeability (k′) and hydrate saturation (Sh), accounting for hydrate pore‐scale growth habit and meso‐scale heterogeneity. Results from capillary tube models rely on cross‐sectional tube shapes and hydrate pore habits, thus are appropriate only for sediments with uniform hydrate distribution and known hydrate pore character. Given our pore network modeling results showing that accumulating hydrate in sediments decreases sediment porosity and increases hydraulic tortuosity, we propose a modified Kozeny‐Carman model to characterize water permeability in hydrate‐bearing sediments. This model agrees well with experimental results and can be easily implemented in reservoir simulators with no empirical variables other than Sh. Results are also relevant to flow through other natural sediments that undergo diagenesis, salt precipitation, or bio‐clogging.

Permeation Properties and Pore Structure of Surface Layer of Fly Ash Concrete.

This paper presents an experimental study on the nature of permeation properties and pore structure of concrete surface layers containing fly ash. Concretes containing different dosages of fly ash as a replacement for cement (15% and 30% by weight of total cement materials, respectively) were investigated. Concrete without any fly ash added was also employed as the reference specimen. Laboratory tests were conducted to determine the surface layer properties of concrete including chloride transport, apparent water permeability and pore structure. The results demonstrate that incorporation of fly ash, for the early test period, promotes the chloride ingress at the surface layer of concrete but substituting proportions of fly ash may have little impact on it. With the process of chloride immersion, the chloride concentration at the surface layer of concrete with or without fly ash was found to be nearly the same. In addition, it is suggested that the water permeability at the concrete surface area is closely related to the fly ash contents as well as the chloride exposure time. Pore structure was characterized by means of mercury intrusion porosimetry (MIP) test and the scanning electron microscopy (SEM) images. The modification of pore structure of concrete submersed in distilled water is determined by the pozzolanic reaction of fly ash and the calcium leaching effect. The pozzolanic reaction was more dominant at the immersion time of 180 days while the calcium leaching effect became more evident after 270 days.

The Water Permeability Reduction After Successive Hypo-Osmotic Shocks in Kidney Principal Cells is Apically Regulated

Background/Aims: Renal principal cells maintain their intracellular water and electrolyte content despite significant fluctuations of the extracellular water and salt concentrations. Their water permeability decreases rapidly (within a few seconds) after successive hypo-osmotic shocks. Our aim was to investigate the contribution of the apical and basolateral surface to this effect and the potential influence of fast reduction in AQP-2, -3 or -4 plasma membrane content. Methods: Rat principal cells of kidney collecting duct fragments underwent hypo-osmotic challenge applied apically or basolaterally and the regulatory volume decrease (RVD) was measured by the calcein quenching method. The AQP -2, -3 and -4 content of the plasma membrane fraction was quantified by Western blotting. Results: The hypo-osmotic shock applied apically causes rapid swelling with high apparent water permeability and fast RVD. An identical successive shock after 15-20 sec causes significantly lower swelling rate with 3-fold reduction in apparent water permeability. This reaction is accompanied by AQP2 decrease in the plasma membrane while AQP3 and AQP4 are unaffected. The contribution of the basolateral cell surface to RVD is significantly lower than the apical. Conclusion: These results indicate that in principal cells the effective mechanism of RVD is mainly regulated by the apical cell plasma membrane.

A 3D lattice Boltzmann effective media study: understanding the role of C-S-H and water saturation on the permeability of cement paste

A 3D lattice Boltzmann model is developed and used to calculate the water and gas permeabilities of model cement pastes at different degrees of water saturation. In addition to permeable micron-sized capillary pores and impermeable solid inclusions, the lattice Boltzmann model comprises weakly-permeable nano-porous calcium silicate hydrate (C-S-H). The multi-scale problem is addressed by using an effective media approach based on the idea of partial bounce-back. The model cement paste microstructures are generated with the platform µic. The critical parameters, C-S-H density and capillary porosity, are taken from 1H nuclear magnetic resonance relaxation analysis. The distribution of water and air is defined according to the Kelvin–Laplace law. It is found that when the capillary porosity is completely saturated with a fluid (either water or gas), the calculated intrinsic permeability is in good agreement with measurements of gas permeability on dried samples (10−17–10−16 m2). However, as the water saturation is reduced, the calculated apparent water permeability decreases and spans the full range of experimentally measured values (10−16–10−22 m2). It is concluded that the degree of capillary water saturation is the major cause for variation in experimental permeability measurements. It is further concluded that the role of the weakly-permeable C-S-H, omitted in earlier modelling studies, is critical for determining the permeability at low capillary saturation.

Factors affecting flux performance of forward osmosis systems

The performance of a forward osmosis (FO) system may be characterised by the assessment parameters: FO–RO flux ratio (Jw/Jw(RO)), apparent FO water permeability (Jw/(πds−πml)), and the newly developed flux efficiency factor (Jw,ob/Jw,re). The former two parameters offer information on extent of internal concentration polarisation and driving force utilisation, respectively. The Jw,ob/Jw,re factor has practical relevance, and reveals the inevitable trade-off between flux and recovery (φ) for a FO system. The derived Jw,ob/Jw,re factors corresponded well to experimental observations. High water permeability, low salt-to-water permeability ratio, and large mass transfer coefficient improve the performance of a FO system, but these may also be influenced by operational and fouling effects, such as draw solute transmission, fouling resistance and cake-enhanced concentration polarisation. It was shown that membrane properties also play a significant role in fouling behaviour. Fouling amelioration factors include aeration and osmotic backwash. A thin-film composite membrane showed potential for FO application with favourable intrinsic transport parameters. It was demonstrated that a FO system could achieve stable water production with both relatively high flux efficiency (Jw,ob/Jw,re=0.8) and high recovery (φ=95.8%), which attested to the technology potential.

Water permeation through gas diffusion layers of proton exchange membrane fuel cells

Water transport through gas diffusion layer of proton exchange membrane fuels cells is investigated experimentally. A filtration cell is designed and the permeation threshold and the apparent water permeability of several carbon papers are investigated. Similar carbon paper with different thicknesses and different Teflon loadings are tested to study the effects of geometrical and surface properties on the water transport. Permeation threshold increases with both GDL thickness and Teflon loading. In addition, a hysteresis effect exists in GDLs and the permeation threshold reduces as the samples are retested. Moreover, several compressed GDLs are tested and the results show that compression does not affect the breakthrough pressure significantly. The measured values of apparent permeability indicate that the majority of pores in GDLs are not filled with water and the reactant access to the catalyst layer is not hindered.

Aquaporin-4 Gene Disruption in Mice Reduces Brain Swelling and Mortality in Pneumococcal Meningitis

The astroglial water channel aquaporin-4 (AQP4) facilitates water movement into and out of brain parenchyma. To investigate the role of AQP4 in meningitis-induced brain edema, Streptococcus pneumoniae was injected into cerebrospinal fluid (CSF) in wild type and AQP4 null mice. AQP4-deficient mice had remarkably lower intracranial pressure (9 +/- 1 versus 25 +/- 5 cm H2O) and brain water accumulation (2 +/- 1 versus 9 +/- 1 microl) at 30 h, and improved survival (80 versus 0% survival) at 60 h, through comparable CSF bacterial and white cell counts. Meningitis produced marked astrocyte foot process swelling in wild type but not AQP4 null mice, and slowed diffusion of an inert macromolecule in brain extracellular space. AQP4 protein was strongly up-regulated in meningitis, resulting in a approximately 5-fold higher water permeability (P(f)) across the blood-brain barrier compared with non-infected wild type mice. Mathematical modeling using measured P(f) and CSF dynamics accurately simulated the elevated lower intracranial pressure and brain water produced by meningitis and predicted a beneficial effect of prevention of AQP4 upregulation. Our findings provide a novel molecular mechanism for the pathogenesis of brain edema in acute bacterial meningitis, and suggest that inhibition of AQP4 function or up-regulation may dramatically improve clinical outcome.

Physicochemistry or physiology: cadmium uptake and effects of salinity and osmolality in three crabs of different ecologies

The effects of salinity and osmolality differences on the uptake rates of dissolved cadmium were investigated in 3 crabs of different ecologies — the euryhaline common shore crab Carcinus maenas, the extremely euryhaline Chinese mitten crab Eriocheir sinensis, and a more stenohaline crab, the velvet swimming crab Necora puber. This study tests predictions derived from the interpretations of the authors from a study of Zn uptake by the same crabs, in terms of the balance between physicochemical and counteractive physiological effects controlling trace metal uptake rates from solution. As predicted, reduced salinities caused increases in the cadmium uptake rate of E. sinensis as expected from the free metal ion model, with increased free cadmium ion availabilities in conditions of reduced chloride complexation. In the cases of C. maenas and N. puber, however, decreased salinity was associated with reduced cadmium uptake, a result interpreted in terms of a physiological response by these crabs to low salinity offsetting the physicochemical effect of increased free cadmium ion availability. This physiological response may include reductions in apparent water permeability with reduced salinities. Osmolality experiments showed that the physiological responses made by the crabs to decreases in salinity could not be explained simply by responses to changes in osmotic pressure of the medium. The interaction of physiology and physico- chemistry controls rates of trace metal uptake from solution by these ecophysiologically distinct crab species.

Inter-population differences in Cd, Cr, Se, and Zn accumulation by the green mussel Perna viridis acclimated at different salinities

Trace metal (Cd, Cr, Se, and Zn) uptake from the dissolved phase, assimilation efficiency from the dietary phase, and metal body burden, as well as the clearance rate, apparent water permeability and gill surface area were measured in the green mussel Perna viridis collected from two contrasting salinity sites in Hong Kong coastal waters with following acclimation in the laboratory at different salinities. The concentrations of metals were 1.2–6.4× greater in mussels collected from the low salinity site, i.e. Tai O, as compared with Tap Mun (high salinity) mussels. Influx of Cd and Zn from the dissolved phase increased with decreased salinity. Furthermore, Cr(VI) and Se (selenite) influx were also affected by decreased salinity, in particular below 17 psu, suggesting that speciation was not the only factor to effect metal uptake. Long-term acclimation to different salinity also had an effect on metal uptake. Mussels collected from Tap Mun (high salinity) accumulated metals 1.2–2.2× faster than mussels from Tai O (low salinity) at intermediate and high experimental salinities >17 psu. Metal assimilation efficiencies were unaffected by salinity variation, but gut passage times were significantly longer at low (10 psu) salinities, indicating some effect of lowered salinity on mussels gut physiology. Gill surface area, which was similar in both populations of mussels, and filtration rate, which was generally similar or higher in those groups with low metal uptake, could not explain the inter-population difference in metal accumulation. Although there was no significant difference, due to high inter-individual variability, the apparent water permeability of the Tap Mun population was on average about 1.6× greater compared with Tai O mussels, and may partially account for the difference in metal uptake between these two populations acclimated at salinity >17 psu. Thus, the effect of salinity on metal uptake is dependent on metal biogeochemistry as well as a range of physiological responses. There was a clear effect of acclimation to lowered salinity on metal uptake, which may have important implications for metal accumulation modelling, biomonitoring, and toxicity studies.

Experimental study of the osmotic behaviour of reverse osmosis membranes for different NaCl solutions and hydrostatic pressure differences

The osmotic behaviour of five different present-day reverse osmosis membranes was studied, using different NaCl solutions (<6 wt.%) and applying different hydrostatic pressure differences (<5.2 bar) between the high and low concentration sides. The osmotic water flux through the membranes was found to increase non-linearly with respect to the increasing concentration difference over the membrane, leading to non-constant values for apparent water permeability. The polyamide composite membranes studied resulted only in very weak osmosis for even relatively small hydrostatic pressure differences and for strong NaCl solutions. Asymmetric cellulose acetate membranes showed somewhat better osmotic behaviour. However, all the osmotic water fluxes measured were still far too small for practical application in the production of energy.

Effects of changes in salinity and osmolality on the rate of uptake of zinc by three crabs of different ecologies

The effects of salinity and osmolality differences on the uptake rates of dissolved zinc were investigated in 3 crabs of different ecologies —the euryhaline common shore crab Carcinus maenas, the extremely euryhaline Chinese mitten crab Eriocheir sinensis, and a more stenohaline marine crab, the velvet swimming crab Necora puber. Reduced salinities caused increases in the zinc uptake rate of E. sinensis as expected from the free metal ion model, with increased free zinc ion availabilities in conditions of reduced chloride complexation. In the cases of C. maenas and N. puber, however, reduced salinity was associated with reduced zinc uptake, a result interpreted in terms of a physiological response by these crabs to low salinity offsetting the physicochemical effect of increased free zinc ion availability. Results can be partly explained by reported changes in apparent water permeability (AWP) made by the crabs to low salinity, although experiments manipulating solution osmotic pressures independently of salinity (and therefore chloride concentrations) indicate that other physiological responses may also be coming into effect. The interaction of physiology and physicochemistry in controlling trace metal uptake from solution clearly varies between species.

Effects of changes in salinity on the apparent water permeability of three crab species: Carcinus maenas, Eriocheir sinensis and Necora puber

This study has investigated the apparent water permeabilities (AWP) at different salinities of three crabs Eriocheir sinensis, Carcinus maenas and Necora puber from predominantly freshwater, estuarine and marine habitats, respectively. AWP of the crabs increased significantly in the order E. sinensis, C. maenas and N. puber in correlation with the salinity of their typical habitats. C. maenas is able to alter AWP in response to decreases in salinity. Crabs acclimated for 7 days to salinities of 20 ppt or below had significantly increased water efflux times over crabs acclimated to 33 ppt. Nonacclimated crabs, examined immediately (within 5 h) after transfer from 33 ppt to lower salinities, did not alter the half-times of water efflux significantly, but did alter water influx half-times. E. sinensis did not significantly change its apparent water permeability in response to a lowering of salinity from 33 to 5 ppt. Its permeability is generally very low, and thus the crab is well adapted to life in fresh and estuarine waters. N. puber can reduce its high apparent water permeability (AWP) after 1-day acclimation from 33 ppt to the lower salinity of 25 ppt.

Organotin and osmoregulation: quantifying the effects of environmental concentrations of sediment-associated TBT and TPhT on the freshwater-adapted European flounder, Platichthys flesus (L.).

Chronic (5 weeks) exposure of freshwater-adapted European flounder, Platichthys flesus (L.), to environmental concentrations of sediment-associated tri-n-butyltin chloride (TBTCl) and triphenyltin chloride (TPhTCl) caused significant changes to hydromineral fluxes and membrane permeability, mechanisms that maintain osmotic homeostasis. The half-time of exchange of tritiated water (THO) in TBTCl- and TPhTCl-exposed fish was significantly increased during the first 2 weeks of the experiment and then decreased steadily, eventually reaching the level that the control group had constantly maintained throughout the experiment. This change in apparent water permeability was accompanied by a significant decrease in diffusional water flux across the membranes. Passive Na(+)-efflux across the gills was increased significantly but effluxes in the control group were near constant over the same time span. Drinking rates in the organotin groups increased significantly while the rate of urine production did not change. This lead to an increased net water balance in the organotin groups and consequently to a significant reduction of the blood osmolality of both organotin groups when compared to a control. There would appear to be a metabolic cost attached to the changes produced by exposure to environmental levels of organotin compounds which are manifested as a minimal increase in body length compared to the controls.

Effects of cadmium exposure on volume regulation in the lugworm, Arenicola marina

Effects of hypo-osmotic stress and Cd (cadmium) exposure, applied singly or in combination, on volume regulation were investigated in Arenicola marina, the common European lugworm. In short-term experiments, the combined exposure to Cd and hypo-osmotic stress mainly affected the worm’s capacity for regulating the coelomic fluid volume without significantly affecting the regulation of intracellular fluid volume. Exposure to Cd increased the worm’s sensitivity to hypo-osmotic stress noted as increased mortality compared to the mortality in groups exposed only to Cd or hypo-osmotic stress. In long-term Cd exposure experiments, the capacity for coelomic fluid volume regulation was affected even at constant external osmolarity and the tissue water concentration decreased slightly. The results could not be explained by changes in Apparent Water Permeability (AWP) as Cd did not significantly affect this. The glycogen content of the chloragogenous tissue did, however, increase during long-term Cd exposure suggesting a metabolic effect. The results demonstrate that two stressors, each without serious effects when applied alone, may have fatal effects when applied in combination. The kind of osmotic stress applied in these experiments is common in many biotopes inhabited by the lugworm. The severe effects of the combination with Cd exposure indicate that classical ecotoxicology testing could conceivably underestimate the toxicity of chemical compounds due to not considering combined effects of natural and anthropogenic stressors.

Effect of Shrinkage and Load-Induced Cracking on Water Permeability of Concrete

Research on the durability problems of concrete structures has focused on the efficacy of concrete as a barrier against aggressive environments. Research laboratories have used permeability tests on intact concrete for assessment and effectively ignored in-service cracking such as shrinkage and load-induced cracks on the apparent water permeability. Shrinkage cracking was achieved by rapid oven drying concrete specimens at 105 deg C; load-induced cracking was achieved by loading 200-mm-by 100-mm diameter specimens in uniaxial compression levels to stress of 0.3, 0.5, 0.7, and 0.8 of the average cylinder crushing strength. Water permeability was chosen as a measure of the barrier quality. Experimental variables that included five different mixes yielded standard deviations of nominally identical samples that exceeded apparent differences in behavior attributed to the imposed stress. Significant differences in coefficient of permeability were obtained between virgin and oven-dried and resaturated specimens. These results support the view that orientation and distribution of shrinkage cracks were effectively isotropic, whereas load-induced cracks were strongly oriented and localized but did not have a measurable effect on the permeability of specimens that were unloaded when tested, thus indicating significant elastic recovery of the cracked concrete, which caused cracks to close.