The effects of air content on permeability of lightweight concrete

Air entraining agent is used to control the floatation of lightweight aggregate (LWA) in lightweight aggregate concrete (LWAC), therefore reducing the segregation of LWAC. At the same time, using an air entraining agent will affect the water sorption of the concrete. In this paper, two lightweight concrete mixes of density 1000 kg/m³ and air content of 13.5% and 31.9% were compared and the effects of entrained air on the strength, surface sorptivity, and chloride permeability of LWAC are presented. Results show that the use of porous LWA would not lower the permeability resistance of concrete. Entrained air had little effect on sorptivity but a major effect on chloride permeability. The weaker pores' network in the cement paste is the basic cause for the high chloride permeability of concrete than the use of porous LWA. Although chloride permeability of low density LWAC concrete decreased with age of concrete, it was found that the concrete was not dense enough to stop the chloride ion to penetrate through the concrete before the concrete mature at 90 days.     

Effects of the air–steam mixture on the permeability of damaged concrete

Massive concrete structures such as the containments of nuclear power plant must maintain their tightness at any circumstances to prevent the escape of radioactive fission products into the environment. In the event of an accident like a Loss of Coolant Accident (LOCA), the concrete wall is submitted to both hydric and mechanical loadings. A new experimental device reproducing these extreme conditions (water vapor transfer, 140 °C and 5 bars) is developed in the GeM Laboratory to determine the effect of the saturation degree, the mechanical loading and the flowing fluid type on the concrete transfer properties. The experimental tests show that the previous parameters significantly affect the concrete permeability and the gas leakage rate. Their evolution as a function of the mechanical loading is characterized by two phases that are directly related to concrete microstructure and crack development.     

Water retention and gas relative permeability of two industrial concretes

This experimental study aims at identifying the water retention properties of two industrial concretes to be used for long term underground nuclear waste storage structures. Together with water retention, gas transfer properties are identified at varying water saturation level, i.e. relative gas permeability is assessed directly as a function of water saturation level S_w. The influence of the initial de-sorption path and of the subsequent re-saturation are analysed both in terms of water retention and gas transfer properties. Also, the influence of concrete microstructure upon water retention and relative gas permeability is assessed, using porosity measurements, analysis of the BET theory from water retention properties, and MIP. Finally, a single relative gas permeability curve is proposed for each concrete, based on Van Genuchten–Mualem's statistical model, to be used for continuous modelling approaches of concrete structures, both during drying and imbibition.     

Modelling chemical degradation of concrete during leaching with rain and soil water types

Percolation of external water through concrete results in the degradation of cement and changes the concrete pore water and solid phase composition. The assessment of long-term degradation of concrete is possible by means of model simulation. This paper describes simulations of chemical degradation of cement for different types of rain and soil water at an ambient earth surface temperature (10 °C). Rain and soil water types were derived using generic equations and measurement of atmospheric boundary conditions representative for North-Belgium. An up-to-date and consistent thermodynamic model is used to calculate the geochemical changes during chemical degradation of the concrete. A general pattern of four degradation stages was simulated with the third stage being the geochemically most complex stage involving reactions with calcium-silicate hydrates, AFm and AFt phases. Whereas the sequence of the dissolution reactions was relatively insensitive to the composition of the percolating water, the duration of the different reactions depends strongly on the percolating water composition. Major identified factors influencing the velocity of cement degradation are the effect of dry deposition and biological activity increasing the partial pressure of CO_2(g) in the soil air phase (and thus increasing the inorganic carbon content in the percolating water). Soil weathering processes have only a minor impact, at least for the relatively inert sandy material considered in this study.     

Identification and quantification of cracks in concrete by optical fluorescent microscopy

Cracks in concrete structures can indicate major structural problems and can damage the appearance of monolithic construction. Cracking of concrete is a major factor affecting for the material strength and durability. The development of a crack pattern can contribute to increasing the permeability and the diffusivity of concrete, which is generally connected with a substantial reduction of its durability. This paper describes a method for identification and quantification of crack patterns in concrete by means of optical fluorescent microscopy and image analysis. Results obtained for undamaged and deteriorated specimens are presented. The range of investigation included several concrete mixes made in the laboratory. In order to induce cracks, concrete mixes were exposed to freezing action 0, 1 and 2 h after mixing. The concrete cubes of 100-mm and 150-mm size were frozen for 0 (reference specimens) and 2 days. Investigation of compressive strength, water permeability, chloride migration and analysis of cracks was made after 28 days. The low-temperature deteriorated specimens showed a significant reduction of compressive strength and resistance to water and chloride penetration in concrete. Correlations between density of cracks and compressive strength, depth of water penetration and depth of chloride penetration have been proposed.     

On the mechanism of polypropylene fibres in preventing fire spalling in self-compacting and high-performance cement paste

With the increasing application of self-compacting concrete (SCC) in construction and infrastructure, the fire spalling behavior of SCC has been attracting due attention. In high performance concrete (HPC), addition of polypropylene fibers (PP fibers) is widely used as an effective method to prevent explosive spalling. Hence, it would be useful to investigate whether the PP fibers are also efficient in SCC to avoid explosive spalling. However, no universal agreement exists concerning the fundamental mechanism of reducing the spalling risk by adding PP fiber. For SCC, the reduction of flowability should be considered when adding a significant amount of fibres.In this investigation, both the micro-level and macro-level properties of pastes with different fiber contents were studied in order to investigate the role of PP fiber at elevated temperature in self-compacting cement paste samples. The micro properties were studied by backscattering electron microscopy (BSE) and mercury intrusion porosimetry (MIP) tests. The modification of the pore structure at elevated temperature was investigated as well as the morphology of the PP fibers. Some macro properties were measured, such as the gas permeability of self-compacting cement paste after heating at different temperatures. The factors influencing gas permeability were analyzed.It is shown that with the melting of PP fiber, no significant increase in total pore volume is obtained. However, the connectivity of isolated pores increases, leading to an increase of gas permeability. With the increase of temperature, the addition of PP fibers reduces the damage of cement pastes, as seen from the total pore volume and the threshold pore diameter changes. From this investigation, it is concluded that the connectivity of pores as well as the creation of micro cracks are the major factors which determine the gas permeability after exposure to high temperatures. Furthermore, the connectivity of the pores acts as a dominant factor for temperatures below 300 °C. For higher temperatures micro cracks are becoming the major factor which influences the gas permeability.     

The permeability of Portland limestone cement concrete

The effect of limestone addition on the air permeability, water permeability, sorptivity, and porosity of limestone cement concrete has been investigated. Six Portland limestone cements (PLCs) with different limestone content (10-35% w/w) were produced by intergrinding clinker, gypsum, and limestone. A water-to-cement ratio (w/c) of 0.70-0.62—depending on the cement strength class—was used to prepare concrete of the compressive strength class C20/25 of EN 206-1. A modified commercial triaxial cell for 100-mm-diameter samples was used for the determination of the gas (N₂) and the water permeability of concretes. In addition, the sorptivity and porosity of the samples were measured, while thin sections of the concrete specimens were examined by means of optical microscopy. It is concluded that the PLC concrete indicates competitive properties with the ordinary Portland cement (OPC) concrete. Furthermore, the limestone addition has a positive effect on the water permeability and the sorptivity of concrete.     

Effect of coarse aggregate and water/cement ratio on intrinsic permeability of concrete subject to drying

Intrinsic permeability of two types of concrete to nitrogen gas was tested; each had water/cement ratios of 0.40 and 0.50 and a volume fraction of coarse aggregate of 0.41 ± 0.01. Two coarse aggregates of 10mm single size were used, one a crushed limestone of low porosity, the other a sintered fly ash of high porosity. Test specimens (100mm diameter by 50mm thick) were water cured, then dried, during which weight loss and permeability were measured. All concretes had low permeability but consistently higher weight loss and permeability were found in the lightweight aggregate concrete specimens, indicating that accessible pores became available fairly quickly, despite the good quality of mortar used. Such behaviour may result from too rapid drying of the lightweight concrete test specimens. For a more realistic indication of permeability it is recommended that testing should be done in a laboratory environment that would, ideally, simulate insitu conditions. For similar 28 day strength both types of concrete had comparable permeability.     

Elaboration of new tubular ceramic membrane from local Moroccan Perlite for microfiltration process. Application to treatment of industrial wastewaters

In this study, an original microfiltration tubular membrane (M1) made from local Moroccan Perlite was used to treat three wastewater types: effluents coming from beamhouse section of tannery (effluent A), textile effluent coming from jeans washing process (effluent B), and dicing wafer effluent generated by electronic industries (effluent C). The prepared membrane is composed of two layers of Perlite with two different granulometries: a macroporous support with a pore diameter centered near 6.6 μm and porosity of about 42%, and a microfiltration layer, performed by slip casting method, with a mean pore size of 0.27 μm. The water permeability determined of the membrane is 815 L/h m² bar. Tangential microfiltration using Perlite membrane proved to be effective in removing pollutants from the three effluents with almost the same efficiencies than that obtained with a commercial Alumina membrane (M2) with a pore diameter of 0.2 μm and a water permeability of 1022 L/h m² bar. Tangential microfiltration process operated at lower pressure (1 bar) was seen to remove turbidity from the three feeds completely. Perlite membrane allowed significant reduction of Chemical Oxygen Demand COD (50–54%) and Total Kjeldahl Nitrogen TKN (56%) of beamhouse effluent. It showed a significant decrease of COD (54–57%) and a complete discoloration of textile wastewater.     

Ozone utilisation for discharge printing of reactive dyed cotton

Environmental pollution is one of the major concerns of the textile finishing sector. The reduction or substitution of the harsh chemicals used during dyeing and printing processes is necessary. In this study, the use of ozone for the discharge printing process was examined in order to substitute the use of reductive agent and caustic soda by ozone gas. The reactive dyed cotton samples were wetted by water and some selected solutions at 25%, 40% and 60% pick up were used and subjected to ozone gas treatment. The gas flow rates were 5 and 10 l/min for 5 and 10 min treatment times, respectively. The results were compared with that of conventional discharge printed samples. Colour discharge (%), colour difference (ΔE), strength, washing and rubbing fastness and chemical oxygen demand (COD) values were compared and reported. Colour discharge increased at higher gas flow rates and prolonged treatment times. Although ozone printing could not attain the contour sharpness of conventional discharge printing, the addition of selected chemicals affected colour discharge and the contour sharpness. Strength tests did not show a significant decrease when using ozone treatment. Fastness tests results (washing and rubbing) were slightly higher compared with conventional discharge printed samples. COD values were much lower for ozone treatment compared with conventional discharge printing effluent. Consequently, it was demonstrated that ozone may be an environmentally friendly substitute for discharge printing.     

Characterization of a water-based paint for corrosion protection

Corrosion of steel rebars in reinforced concrete is one of the major problems in the construction industry. Carbonation reactions of concrete with carbon dioxide and, mainly, the chloride salts action are the main causes responsible for concrete degradation. Protective coatings help to improve the durability of concrete structures by acting as a physical barrier against the corrosion agents. Waterborne paints are usually used for concrete protection rather than solvent-based paints since they are less pollutant. The aim of this work is to investigate the influence of the pore size and porosity on the permeability of the paints films toward sodium chloride. Three characterization methods from membrane science were implemented to characterize paint coatings. The time-lag method was used to determine the permeability toward the sodium chloride and toward helium and argon, these for approximately 100% relative humidity. From the seven waterborne paints formulated, only one was found to be suitable for surface protection of reinforced concrete, since its permeability toward NaCl was smaller than 10⁻¹⁴ m² s⁻¹, the threshold value required by National Laboratory of Civil Engineering (LNEC) in Portugal. For the formulated paints, it was observed that the average pore size correlates well with the permeability toward sodium chloride. This is an important result since obtaining the permeability toward sodium chloride of corrosion protective paints is very time consuming, while the average pore size can be obtained in a much shorter time.     

Factors affecting measurement of hydraulic conductivity in low-strength cementitious materials

The hydraulic conductivity (water permeability) is one of the most significant transport properties of concrete and measuring it is a key step in predicting the performance of concrete as a barrier to the movement of fluids and ions. The transport properties are critical for the performance of the cover layer in protecting embedded reinforcement as waste containments barriers (which are considered in this paper) and other applications such as dams. The measurements are difficult to interpret due to experimental effects of sample size and changes of flow with time and the chemistry of the fluid used.The intrinsic permeability to water and synthetic leachate was determined and the relationship between the eluted volume passing and permeability was established for mortar mixtures having compressive strengths ranging from 5 to 20 MPa. Two mortar mixtures containing portland cement and one without portland cement and incorporating cement kiln dust, lagoon ash, and Ferrosilicate slag were tested. The effects of the sample size were also investigated.The results indicate a decrease in hydraulic conductivity for lower strength mixtures and a slight increase in permeability coefficient for the higher strength mixtures with increasing permeating volumes. Increasing the testing specimen size also slightly increased the coefficient of permeability in lower strength mixtures and decreased the coefficient in higher strength mixtures. The permeability coefficient did not change significantly with pore solution pressure.     

国内氯化亚砜生产与市场

比较了氯化亚砜几种生产工艺的优缺点,指出二氧化硫气相连续法工艺的潜力较大.国内近2年氯化亚砜进口量约为国内总产量的30%左右,而出口量极小,说明国内市场现状为供不应求,具有很大的市场潜力.而且目前氯磺酸法氯化亚砜生产能力占国内总能力的44%以上,由于该技术将被逐步被淘汰,国内实际生产总量将进一步降低.国内相关企业应抓住时机,进行技改、扩建或新建.     

Experimental evidence of a moisture clog effect in cement-based materials under temperature

This study is an original contribution to the understanding of the hydraulic behaviour of cement-based materials when subjected to temperature rises. Permeability is measured continuously during heating by injecting inert gas into a sample at homogeneous temperature. Using a confining cell especially designed in our laboratory, the sample is submitted to a constant heating rate, up to 200 °C, superimposed to hydrostatic pressure (at ca. 5 MPa). In parallel with a normalised CEM II mortar (water-to-cement ratio (W/C) of 0.5), a CEM V-cement-based concrete, used in nuclear waste storage applications, is studied. For normalised mortar, gas retention is evidenced, depending on the sample size (scale effect), water saturation level S_w, and heating rate. For dry normalised mortar, permeability may be divided by two during heating. In conjunction with thermo-gravimetry analysis (TGA) results, such evolution is attributed to the dehydration of C–S–H around 150 °C. Indeed, mass loss after heat cycling is substantially higher than that due to free water release solely: mortar loses structural, bound water during the process. For partially-saturated and long mortar samples, a gas retention phenomenon is recorded when heating at a rate of ca. 4.9 °C/min. Our analysis is that free water inside the macropores, as well as bound water released from the C–S–H, dilates or vaporizes, and obstructs the interconnected porous network. Due to moisture clogging, no more gas is allowed through the material pore network: a so-called gas retention phenomenon occurs. Most interestingly, although loosing structural water like normalised mortar, yet over a wider temperature range, dry CEM V concrete displays good temperature resistance, as its permeability remains constant during heating. For highly partially-saturated concrete, a gas retention effect is recorded. As a conclusion, observed phenomena at the laboratory scale testify of potentially strong gas retention effects upon engineering structures subjected to temperature gradients over time. Indeed, quite low temperature rises (and heating rates) are able to induce moisture clogging inside partially-saturated materials. It is also concluded that cement-based material composition, i.e. bound water release ability, is influential in gas transport phenomena under temperature.     

Correlation between water permeability of latex-modified concrete (LMC) and water diffusion coefficient of latex film

Cement-based materials are generally known as weak materials in flexure and tension in comparison with compression. Polymers are used in cement-based materials to improve their flexural and tensile behaviors. The composite is called as polymer-modified concrete/mortar. Furthermore, polymers decrease permeability of water into cementitious matrices. Polymers are usually used as admixtures in concretes in form of latexes. Latexes are water-based polymers, which are consistent with water-based concrete matrices. On this basis, these kinds of products are called latex-modified concretes (LMCs). However, it has been found that chemical composition, particle size distribution, molecular weight, physical/mechanical properties of latexes affect performance of modified concretes. In this investigation, six latexes in three categories (acrylic, SBR and polyvinyl acetate) were used as concrete admixtures. They were characterized for chemical composition (by FTIR analysis), minimum film formation temperature, pH, glass transition temperature (T _g), particle size and particle size distribution to evaluate the effect of each property on LMC performance. Due to the formation of latex film in the microcracks and pores of concrete microstructure, it was suggested that diffusion of water into films controls permeability of whole concrete structures. On this basis, the diffusion coefficient of the latex films subjected to water was measured using a new method (continuous FTIR analysis). Capillary water absorption test was performed on concrete specimens to verify validity of the suggestion. It was found that there is a correlation between capillary water absorption of LMCs and water diffusion coefficient of latex films.     

Self-consolidating concrete subjected to high temperature: Mechanical and physicochemical properties

This paper presents an experimental study on the performance of self-consolidating concrete (SCC) subjected to high temperature. Two SCC mixtures and one vibrated concrete were tested. These concrete mixes were developed in the French National Project B@P. Mechanical and microstructural properties were studied at ambient temperature and after heating. We studied compressive strength, flexural strength, bulk modulus of elasticity, porosity and permeability. For each test, the specimens were heated at a rate of 1 °C/min up to different temperatures (150, 300, 450 and 600 °C). In order to ensure a uniform temperature throughout the specimen, the temperature was held constant at the target temperature for 1 h before cooling. In addition, the specimen mass was measured before and after heating in order to determine the loss of water during the test. The results allowed us to analyze the degradation of SCC and vibrated concretes due to heating.     

Experimental discussion on the mechanisms behind the fire spalling of concrete

The behaviour of six concretes at high temperature (600 °C) and in particular the risk of fire spalling is studied. Tests are performed with two sizes of samples: small samples (300 × 300 × 120 mm³) and small slabs (700 × 600 × 150 mm³). Different storage conditions (pre‐drying at 80 °C, air and water storing) are used to highlight the effect of the initial water content. Thanks to different scenarios of heating, the influence of the heating curve is studied. Results enabled to identify parameters that highly influence the risk of fire spalling: initial water content and concrete permeability during heating. The permeability of concrete can increase during heating due to the melting of the polypropylene fibres or by thermal damage. This thermal damage is important when heating is violent (ISO 834 or increased hydrocarbon fire), or when concrete is made with silico‐calcareous aggregates (flint). Fire spalling cannot be explained by either the only thermo‐mechanical behaviour of concrete, or only by the appearance of high pore gas pressure. Based on the recent hypothesis of the critical zone, the formation of a saturated layer of liquid water is consistent with the results obtained. Copyright © 2014 John Wiley & Sons, Ltd.     

Water permeability of reinforced concrete with and without fiber subjected to static and constant tensile loading

In this study, an innovative permeability device allowing permeability measurement simultaneously to loading was used to investigate the water permeability and self-healing of reinforced concrete. The experimental conditions focused on normal strength concrete (NSC) and fiber reinforced concrete (FRC) tie specimens under static and constant tensile loadings. Crack pattern and crack openings under the same loadings were measured on companion specimens. Experimental results emphasized the positive contribution of fibers to the durability of reinforced concrete. Under static tensile loading, the FRC tie specimens were 60% to 70% less permeable than the NSC tie specimens at the same level of stress in the reinforcement. After 6 days of constant loading, the FRC showed greater self-healing capacity with a reduction in water penetration of 70% in comparison to 50% for the NSC. The main cause of self-healing was the formation of calcium carbonate (CaCO₃).     

High-flux whisker layer ceramic membrane prepared by gel spin-coating method for low-pressure oil/water emulsion filtration

Insufficient permeability and membrane fouling significantly influence the efficiency of ceramic microfiltration (MF) membranes in oil/water emulsion treatment. In this study, a high-flux whisker layer ceramic MF membrane with super-hydrophilicity was successfully fabricated through gel-spin coating method and a low-temperature oxidation method, which was used to separate oil/water emulsion. The effects of the whisker layer and surface wettability were systematically investigated, and the mechanism of in-situ gelling and pore size distribution was proposed. The super-hydrophilic ceramic MF membrane with an average pore size of 250 nm exhibited a high gas flux of 934 m³/(m²·h·bar) and excellent pure water flux of 9754 L/(m² h bar). Even after a long-term circulating filtration process, the super-hydrophilic ceramic MF membrane still maintained a high water flux of over 50 L/(m²·h) at a transmembrane pressure of 5 KPa during the treatment of oil-in-water emulsion with a concentration of 1000 mg/L. Overall, the developed ceramic MF membrane demonstrated high permeability and excellent anti-fouling performance, making it a promising candidate for oil/water emulsion wastewater treatment.     

Permeability measurement on high strength concrete without and with polypropylene fibers at elevated temperatures using a new test setup

A new test setup for permeability measurement at room and high temperature is presented. The experimental results obtained by employing the new setup are reported and validated. The experiments are performed on high performance concrete, without and with addition of polypropylene fibers under temperatures ranging from 20 °C to 300 °C as well as after cooling of previously heated specimens to the room temperature. The results show that plain concrete exhibits steady increase in permeability with increasing temperature, whereas concrete with fibers exhibit a sudden increase of permeability at temperatures between 80 °C and 130 °C. The results confirm the governing role of permeability on explosive spalling and suggest the existence of mechanisms of pressure relief other than just melting of fibers. The microstructure of concrete with fibers is investigated using SEM before and after exposure to high temperature. It is observed that the melted polypropylene flows only into the micro-cracks and does not penetrate into cement paste.