Study on water sorptivity of the surface layer of concrete

This paper presents the results from a study of water sorptivity of concrete surface layer. The sorptivity is characterized by a surface sorptivity index as measured by Autoclam. In this study, different types of concrete were immersed in ultrapure water and NaCl solution prior to the sorptivity test. The influences of several factors on the value and evolution of concrete surface sorptivity index are discussed. It is found that: concrete surface sorptivity is a function of the pore structure, higher porosity and lower tortuosity lead to higher surface sorptivity; as cured in moist condition for 1 month, the surface sorptivity is an increasing function of w/c in plain cement concretes, and an increasing function of fly ash replacement if w/b is kept constant; surface sorptivity increases as immersed in ultrapure water in the first month of immersion due to leaching, and decreases thereafter as the continuous hydration of cementitious materials makes the pore structure finer and finer; the immersion in NaCl solution limits the effect of leaching because of the formation of calcium oxychloride compounds, and results in lower long-term surface sorptivity index as compared with the ultrapure water immersion, due to the formation of Friedel’s salt which reduces the pore volume and blocks the pore network.     

The influence of reusing ‘Formtex’ controlled permeability formwork on strength and durability of concrete

The effects of the reuse of ‘Formtex’ Controlled Permeability Formwork (CPF) liner on strength and durability properties of concrete were investigated at two different water-cement ratios and the results are reported in this paper. Test blocks were cast using the CPF on one side and impermeable formwork (IF) on the opposite side of the mould so that direct comparisons could be made between the two. The strength was assessed using the Limpet pull-off tester and both the air permeability and the water absorption (sorptivity) were measured using the Autoclam Permeability System. Both these instruments measured the ‘covercrete’ properties. In addition, cores cut from the test specimens were subjected to an accelerated carbonation test and a chloride exposure test. The results showed that the ‘Formtex’ CPF increases the surface strength and the durability of concrete compared to the IF. There was an almost complete elimination of blowholes. The permeability of concrete decreased and its resistance to the ingress of both carbon dioxide and chlorides increased when CPF was used. The beneficial effects of the Formtex CPF were most evident in concrete of higher water-cement ratio. With the reuse of the Formtex liner twice, that is a total of three uses, the performance of the CPF to improve the properties of concrete remained almost the same. In this research the CPF liner was cleaned thoroughly between each use, which must be adhered to for site applications for reproducing the beneficial effects observed in the laboratory.     

Comparative performance of chloride attenuating and corrosion inhibiting systems for reinforced concrete

This paper reports a laboratory-based study carried out to compare the performance of various proprietary concrete protection systems, designed to reduce chloride ingress and reinforcement corrosion. These include: controlled permeability formwork (CPF), a silane/siloxane hydrophobic surface treatment (S/S), an integral liquid waterproofing admixture (WP) and a corrosion-inhibiting chemical admixture (CI). Tests were carried out on a Portland cement (PC) concrete (40 N/mm² design strength) for chloride diffusion index (using a two-cell compartment accelerated test) and, under cyclic wetting and drying conditions, total chloride content at cover depth (25 mm) and corrosion of carbon steel in reinforced concrete specimens (using half-cell potential and corrosion current density (polarisation resistance) measurements). The results indicate that for all protection systems, resistance to chloride ingress was improved, with the greatest benefits noted for the S/S and CPF concretes. Corrosion levels occurring for these generally followed the ranking of chloride ingress rates. The CI was found to reduce the rate of chloride ingress and to give lowest corrosion current densities in relation to chloride contents. This system appeared to provide best overall performance. The practical implications of the results are considered in terms of equivalence of the systems to an increase in design strength or cover depth,i.e. parameters used for specifying concrete durability in standards, and the wider issues relating to their selection and use are reviewed.     

Influence of temporal resolution and processing of exposure data on modeling of chloride ingress and reinforcement corrosion in concrete

The impacts of temporal resolution and processing of exposure data on the long-term chloride ingress and reinforcement corrosion in concrete were studied. Exposure data from one simulated and two real climates was processed to create boundary conditions for a one-dimensional geometry studied using a numerical heat and mass transport model that includes full coupling of heat, moisture and ion transport. Heat, moisture, and chloride concentration distributions were passed to a simplified reinforcement corrosion initiation and propagation model. The numerical study indicates that processing and temporal resolution of the exposure data has a considerable impact on long-term hygrothermal distribution, chloride ingress, and reinforcement section loss results. Use of time-averaged exposure data in the heat and mass transport model reduces the rate of chloride ingress in concrete and affects prediction of reinforcement corrosion initiation and propagation. Randomly sampled exposure data at daily, weekly, or monthly resolution yields prediction of reinforcement corrosion initiation and propagation closer to original resolution results than time-averaged exposure data.     

Comparative study on strength,sorptivity, and chloride ingress characteristics of air-cured and water-cured concretes modified with metakaolin

This paper reports an investigation in which the performance of plain and metakaolin (MK)-modified concretes were studied under two different curing regimes. The purpose of this study is to evaluate the effectiveness of MK in enhancing the strength and permeation properties of concrete. MK was used to replace 0–20% of Portland cement by weight in concrete with two water-binder (w/b) ratios of 0.35 and 0.55. The change in compressive strength, sorptivity, and chloride ingress with age at all cement replacement levels under both air and water curing are compared with those of the control concrete. The results indicated that the inclusion of MK greatly reduced sorptivity and chloride permeability of concrete in varying magnitudes, depending mainly on replacement level of MK, w/b ratio, curing condition, and chloride exposure period. It was found that under the inadequate or poor curing, MK-modified concretes suffered a more severe loss of compressive strength and permeability-related durability than the plain concretes.     

Effect of hydrophobic surface treatment on freeze-thaw durability of concrete

The effect of surface treatment using silanes on the frost durability is investigated on both laboratory and field specimens in an accelerated laboratory test. Measurements include moisture uptake during the pre-saturation and F-T stages, cumulative mass loss and internal bulk cracking under frost salt/water exposure. It is found silane treatment substantially reduces surface scaling, but cannot prevent bulk moisture uptake or the occurrence of the internal frost damage when concrete is insufficiently air entrained. Salt scaling is dominated by the capillary suction process in the thin surface region under freezing which can be curtailed by the pore lining effect from silanes creating a hydrophobic barrier to the ingress of external liquid. This in turn suppresses ice growth in the surface region, evidenced by the complete elimination of sub-freezing dilation in a length-change measurement of small-scale concrete specimens with surface treatment. However, internal frost damage is controlled by the universal degree of pore saturation which in turn is dependent on the bulk moisture uptake.     

Chloride ingress data from field and laboratory exposure – Influence of salinity and temperature

Marine exposure conditions provide an aggressive environment for reinforced concrete structures, mainly due to the occurrence of chloride-induced reinforcement corrosion. There are several influencing factors, but despite a lot of research little is known about the influence from variations in the exposure conditions. Therefore an exposure programme has been made, where concrete specimens, made from one single concrete composition, have been exposed at 12 marine locations around the world, along with a parallel study under laboratory conditions. The specimens have been constantly submerged during the exposure to get well-defined exposure conditions. The purpose was to get a quantitative measure of how different exposure conditions influence the durability of reinforced concrete, measured in terms of chloride ingress into the concrete. The results indicate that the exposure conditions (salinity and temperature of seawater), as expected, influenced the chloride ingress. This is further analysed and discussed in the paper.     

Poly(methyl methacrylate) capsules as an alternative to the ‘’proof-of-concept’’ glass capsules used in self-healing concrete

Development of suitable capsules is essential to achieve self-healing by encapsulation. In the context of self-healing concrete, capsules that can be easily mixed into concrete and release the healing agent when cracking occurs are ideally required. The optimization of these properties would allow for a successful implementation at large scale in practical (concrete) applications. In the present work, the suitability of polymeric cylindrical capsules made of poly(methyl methacrylate) (PMMA) to carry healing agent in self-healing concrete has been evaluated. An innovative method to assess more easily the capsules survival during concrete mixing was developed. This method is based on the evaluation of the setting behavior of concrete containing capsules filled with setting accelerator. Capsules with a wall thickness of 0.7 mm were able to resist the concrete mixing process and to rupture at relatively small crack widths (116 μm) after applying a surface treatment to increase the adhesion between the capsules and the cementitious matrix. Next, the self-healing efficiency of the encapsulation materials (glass or PMMA) was evaluated on real-scale concrete beams. The results showed that cracked concrete beams with mixed-in capsules (glass or PMMA) filled with water-repellent agent showed higher resistance against chloride ingress compared to plain cracked concrete beams. PMMA capsules showed a lower self-healing efficiency (in relation to chloride ingress) compared to glass due to a less favorable distribution of the capsules in the concrete. However, concrete containing glass capsules is susceptible towards alkali-silica reaction.Although optimization of the PMMA capsules is still necessary to improve their distribution in concrete and achieve higher self-healing efficiency, the obtained results indicate that these capsules could be a promising solution towards self-healing concrete.     

Properties of high volume glass powder concrete

Mechanical and durability properties of concrete with cement replaced by finely grounded glass powder in high volume up to 60% were investigated. XRD and TGA analyses indicated that the fine glass powder reacted with calcium hydroxide to form calcium-silicate-hydrates. As such, the microstructures of concrete were more compact and homogeneous, especially at the interfacial transition zone. Concrete with cement replaced by 15% and 30% glass powder exhibited the highest strength increase and correspondingly the lowest porosity. Beyond a replacement of 30%, calcium hydroxide became insufficient for the pozzolanic reaction of glass powder. However, the high volume glass powder concrete retained distinct resistance against water and chloride ingress, due to the reduction in pore size and connectively. Reductions of 77%, 83%, 96%, 91% and 92% were observed respectively for water penetration depth, sorptivity, conductivity, chloride diffusion and migration coefficients in concrete with cement replaced by glass powder by 60%.     

Effects of inorganic surface treatment on water permeability of cement-based materials

The permeability of the cement-based materials can be used as an important indicator of their durability. Surface treatment is a simple way to reduce permeability and improve durability of cement-based materials. This paper studied the effects of fluosilicate and sodium silicate surface treatments on the permeability of cement-based materials using the Autoclam water permeability and water absorption testing method. The experimental results showed that both fluosilicate and sodium silicate surface treatments could effectively reduce the permeability of cement-based materials. However, fluosilicate worked within the first 28days after treatment, while sodium silicate showed more obvious effect at later ages. Autoclam water permeability index exhibited an exponential relationship with the water absorption of the cement-based materials. In addition, mercury intrusion porosimetry result suggested that these inorganic surface treatment agents could reduce the porosity of surface layer of cement-based materials.     

Exploitation of poor Greek kaolins: Durability of metakaolin concrete

In this paper the effect of metakaolin on concrete durability is investigated. A Greek kaolin of low kaolinite content was thermally treated at defined conditions and the produced metakaolin was finely ground. In addition, a commercial metakaolin of high purity was used. Eight mixture proportions were used to produce high performance concrete, where metakaolin replaced either cement or sand in percentages 10% or 20% by weight of the control cement content. Durability of metakaolin concrete was evaluated by means of resistance to chloride penetration, air permeability, sorptivity, porosity and pore size distribution. Metakaolin concrete exhibits significantly lower chloride permeability, gas permeability and sorptivity. The addition of metakaolin refines the pore system of concrete, leading to a decreased mean pore size and improved uniformity of the pore size distribution. The produced metakaolin, derived from the poor Greek kaolin, imparts similar behavior to that of the commercial metakaolin, with respect to the concrete durability.     

Chloride penetration into concrete in marine environment—Part I: Main parameters affecting chloride penetration

The durability of concrete structures exposed to marine environment depends mainly on the ability of concrete to resist to chloride ingress. This complex phenomenon depends on many parameters related to the concrete properties and to the micro-environmental characteristics. This paper presents the results of an experimental study where fifty four 1.0×0.5×0.12 m concrete panels were exposed to marine environment for three to five years. A study of three concrete mixes in five different exposure conditions was conducted and the results show that the chloride penetration is strongly dependent on both the concrete quality and the exposure conditions. Both the diffusion coefficientsD and the surface chloride concentrationCs derived from the chloride profiles show a clear time dependence. This effect has serious implications for long term predictions of chloride penetration, therefore if constant values ofD andCs are assumed gross errors can be made.     

Lattice modeling of chloride diffusion in sound and cracked concrete

Reinforced concrete structures are frequently exposed to aggressive environmental conditions. Most notably, chloride ions from sea water or de-icing salts are potentially harmful since they promote corrosion of steel reinforcement. Concrete cover of sufficient quality and depth can ensure protection of the steel reinforcement. However, it is necessary to study the effects of material heterogeneity and cracking on chloride ingress in concrete. This is done herein by proposing a three-dimensional lattice model capable of simulating chloride transport in saturated sound and cracked concrete. Means of computationally determining transport properties of individual phases in heterogeneous concrete (aggregate, mortar, and interface), knowing the concrete composition and its averaged transport properties, are presented and discussed. Based on numerical experimentation and available literature, a relation between the effective diffusion coefficient of cracked lattice elements and the crack width was adopted. The proposed model is coupled with a lattice fracture model to enable simulation of chloride ingress in cracked concrete. The model was validated on data from the literature, showing good agreement with experimental results.     

混凝土养护期间HRB400钢筋钝化行为研究

为了研究HRB400钢筋在模拟养护阶段混凝土孔隙液中表面钝化膜的生长过程,采用3种电化学测试技术(开路电位、电化学阻抗谱、动电位极化曲线)研究钢筋表面钝化膜的性能随着浸泡时间的变化特征,此外通过XPS对稳定钝化膜的成分与结构进行分析。结果表明,同传统钢筋材料一样,HRB400钢筋在模拟混凝土养护条件的溶液中展现出良好的耐蚀性能,浸泡5 d后便能够生成稳定钝化膜。XPS分析发现钢筋钝化膜为双层结构,内层以二价铁离子化合物为主,外层主要由三价铁离子化合物组成。     

Modeling influence of hysteretic moisture behavior on distribution of chlorides in concrete

Aggressive environmental conditions, such as exposure to the sea climate or use of de-icing salts, can have a considerable influence on the durability of reinforced concrete structures due to corrosion-induced damage of reinforcement. Recently, the coupled 3D chemo-hygro-thermo-mechanical (CHTM) model for simulation of processes related to the chloride induced corrosion of steel reinforcement in concrete was developed. In the model, it is assumed that for wetting and drying of concrete, the transport of water is controlled by a single sorption curve. However, it is well known that concrete exhibits a hysteretic moisture behaviour, which significantly influences the distribution of moisture and chlorides. To account for the hysteretic moisture behaviour of concrete and for simulating a more realistic time and space distribution of moisture, the CHTM model was further improved. The proposed hysteretic model is implemented into a 3D finite element code and it is validated using a numerical example, which shows reasonably good agreement with the available test results. Similar to what is observed in the experimental tests, it is shown that due to the wetting and drying of the concrete surface, the peak concentration of chloride moves progressively deeper into the concrete specimen.     

Analysis technique for restrained shrinkage of concrete containing chlorides

Cracks in concrete containing chlorides easily occur due to restraint conditions and they can be the main reasons of durability and safety issues. In this paper, analysis technique which can handle mixed chloride and its effect on restrained drying shrinkage is proposed. For the evaluation of stress development and cracking time due to restrained drying shrinkage, free and restrained drying shrinkage test are carried out for concrete specimens containing different sodium chloride (NaCl) content. The results show that mixed chloride content increases restraint stress but does not increase strength. Considering the effect of chloride on shrinkage based on the test results, effective restraint stress development and cracking of concrete specimens containing different level of chloride are evaluated through utilizing previously developed models for behaviors in early-age concrete like hydration and moisture transport. The results from this proposed technique are verified by comparison with test results.     

Durability assessment of alkali activated slag (AAS) concrete

The environmental impact from the production of cement has prompted research into the development of concretes using 100% replacement materials activated by alkali solutions. This paper reports research into the durability of AAS concrete. The durability properties of AAS have been studied for a range of sodium oxide dosages and activator modulus. Properties investigated have included measurements of workability, compressive strength, water sorptivity, depth of carbonation and rapid chloride permeability. Microstructure studies have been conducted using scanning electron microscopy and energy dispersive X-ray spectroscopy. It was concluded that an activator modulus of between 1.0 and 1.25 was identified as providing the optimum performance for a sodium oxide dosage of 5% and that AAS concretes can exhibit comparable strength to concrete currently produced using Portland cement (PC) and blended cements. However, with regards to the durability properties such as water sorptivity, chloride and carbonation resistance; the AAS concretes exhibited lower durability properties than PC and blended concretes. This, in part, can be attributed to surface microcracking in the AAS concretes.     

Protection provided by surface treatments against chloride induced corrosion

Corrosion of reinforcement bars due to chloride ingress is a well known problem in reinforced concrete. Several methods have been adopted to protect reinforced concretc, and one of them is to provide added protection to the concrete surface in the form of surface treatments such as silanes and siloxanes. They react with the cement matrix and form a hydrophobic layer on the walls of the pores within the concrete. This protects the concrete from the ingress of water and water-born salts. Moisture is required for reaction of silanes and siloxanes with the hydrated cement matrix. However, too much water in the pores of concrete will prevent the treatments from penetrating deeper into the pores. A study has been carried out to investigate the variation, due to the presence of moisture, on the effectiveness of the protection. It has been found that irrespective of the moisture content at the time of application of treatment, silanes and siloxanes provided good protection against chloride ingress and corrosion of reinforcement bars. However, the depth of penetration and therefore the durability of the treatment is adversely affected when the concrete is near saturation.

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Résumé Dans le béton armé, la corrosion des armatures de renforcement due à la pénétration du chlorure est un problème bien connu. Plusieurs méthodes ont été adoptées pour protéger le béton armé, dont la protection supplémentaire au revêtement du béton sous forme de traitement à la surface par des silanes et des siloxanes. Ces produits réagissent avec la matrice cimentaire et forment une couche hydrophobe sur les parois des pores à l'intérieur du béton, empêchant la pénétration de l'eau et des sels véhiculés par l'eau. L'humidité est nécessaire pour obtenir la réaction des silanes et des siloxanes avec la matrice cimentaire hydratée. Cependant, trop d'eau dans les pores du béton empêche les traitements de pénétrer plus profondément dans les pores. Une étude a été menée sur l'efficacité de la protection en fonction des variations de la présence de l'humidité. On a constaté que, quelle que soit la teneur en humidité au moment de l'application du traitement, les silanes et les siloxanes fournissent une bonne protection contre l'entrée du chlorure et la corrosion des armatures. Néanmoins, la profondeur de la pénétration, et donc la résistance du traitement, sont défavorablement modifiées lorsque le béton est près de la saturation.

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Editorial note Prof. David J. Cleland is a RILEM Senior Member and the Secretary of TC 145-WSM (Workability of Special Concrete Mixes). Prof. A. E. Long is a RILEM Senior Member and a member of TCs 126-IPT (In-place Testing of Hardened Concrete) and 163-TPZ (Interfacial Transition Zone and Properties of Transfer).     

持压荷载与干湿循环作用下再生混凝土氯盐侵蚀行为

采用干湿比为3∶1和质量分数为5wt%的NaCl溶液,开展了持压荷载与干湿循环共同作用下不同再生粗骨料取代率(r=0%、30%、50%、100%)混凝土的氯离子传输试验,分析了持压应力水平(λ_c=0.1、0.3、0.5)对氯盐侵蚀性能的影响。基于非饱和混凝土的氯离子对流-扩散模型,提出了考虑应力水平和再生骨料取代率影响的水分和氯离子扩散系数模型,并验证了该模型的有效性。结果表明：相同再生粗骨料取代率的混凝土内自由氯离子含量、氯离子扩散系数和表面氯离子浓度均随应力水平的增加呈先减小后增大的趋势,同一应力水平下与再生粗骨料取代率呈正相关,再生粗骨料取代率为100%的试件承受0.1f_c、0.3f_c、0.5f_c(f_c为再生混凝土(RAC)立方体抗压强度值)应力作用的氯离子扩散系数分别是无应力状态的0.97、0.88和1.48倍;所建立的持压荷载与干湿循环作用下RAC氯离子传输模型,为再生混凝土耐久性分析提供理论依据。     

Computational results of a model for chloride ingress in concrete including convection, drying-wetting cycles and carbonation

Over the past decades a considerable research effort has been attributed to the modelling of chloride ingress into reinforced concrete in order to predict its durability. Traditional models are based on the error-function. In the present paper computational results of an advanced model are presented, which takes environmental temperature and humidity fluctuations, chloride binding, diffusion and convection, as well as carbonation effects into account. These qualifications make the model particularly suitable for simulating drying-wetting cycles, an example of which is included. The good performance of the moisture sub-model is demonstrated in an example concerning drying cement paste. Another example deals with a simple submersion case. The examples are based on laboratory and real-life experiments with documented concrete or paste compositions and environmental conditions, as well as the eventual measured chloride profiles. In general, the computational results are in good agreement with the measurements. The paper concludes with a comparison between simulation results of the present model and the error-function model for the case of cyclic drying-wetting exposure. It appeared that the implicit negligence of the moisture fluctuations by the error-function model, forced that model to deploy strongly deviating material characteristics to reach agreement with the measured chloride profile.