Size effect of concrete samples on the kinetics of external sulfate attack

The external sulfate attack (ESA) of concrete is a disease related to expansive sulfate hydrate formation in a hardened cement matrix. The aim of this research is to study how the choice of a concrete sample size can impact on the kinetics of ESA, by exposing different types of specimen to constant immersion in a solution dosed with 5% Na₂SO₄⋅10H₂O. Monitoring involves mass, dynamic modulus and expansion measurements. It is concluded that 4 × 8 cm concrete cylinders (cored from 11 × 22 cm concrete cylinders) are more quickly damaged by ESA than usual sample types (11 × 22 cm concrete cylinders and 4 × 4 × 16 cm mortar prisms). For all sample types, damage is always limited to the periphery of the sample in the short run. The thickness of the damaged zone is in the region of the size of the largest aggregates. For 4 × 8 cm concrete cylinders, this periphery corresponds to the entire sample because the maximum aggregate size is of the order of the size of the specimen. In this situation, the percolating crack network resulting from swelling is assumed to dramatically damage the cement matrix and to give sulfate solution access to the whole sample. Hence, by using this original type of cored samples, the concrete resistance to sulfate attack can be studied under reliable conditions (concrete formulations and not mortar ones, good sensitivity to ultrasonic tests) and advantage can be taken of the increased kinetics of degradation.     

Dissolution and solubility behavior of fenofibrate in sodium lauryl sulfate solutions

The solubility of fenofibrate in pH 6.8 McIlvaine buffers containing varying concentrations of sodium lauryl sulfate was determined. The dissolution behavior of fenofibrate was also examined in the same solutions with rotating disk experiments. It was observed that the enhancement in intrinsic dissolution rate was approximately 500-fold and the enhancement in solubility was approximately 2000-fold in a pH 6.8 buffer containing 2% (w/v) sodium lauryl sulfate compared to that in buffer alone. The micellar solubilization equilibrium coefficient (k*) was estimated from the solubility data and found to be 30884 ± 213 L/mol. The diffusivity for the free solute, 7.15 × 10^- 6 cm²/s, was calculated using Schroeder's additive molal volume estimates and Hayduk-Laurie correlation. The diffusivity of the drug-loaded micelle, estimated from the experimental solubility and dissolution data and the calculated value for free solute diffusivity, was 0.86 × 10^- 6 cm²/s. Thus, the much lower enhancement in dissolution of fenofibrate compared to its enhancement in solubility in surfactant solutions appears to be consistent with the contribution to the total transport due to enhanced micellar solubilization as well as a large decrease ( ∼ 8-fold) in the diffusivity of the drug-loaded micelle.     

Durability of self-consolidating concrete to different exposure regimes of sodium sulfate attack

This study investigates the durability of a wide scope of self-consolidating concrete (SCC) mixture designs to sodium sulfate attack. The mixture design variables included the type of binder (single, binary, ternary and quaternary), air-entrainment, sand-to-aggregate ratio and hybrid fibre reinforcement. Since current standard test methods (e.g. ASTM C 1012) generally do not address various sulfate attack exposure scenarios that may exist under field conditions, three different sulfate attack exposure regimes (full immersion, wetting-drying and partial immersion) were investigated in the current study. In the wetting-drying and partial immersion exposure regimes, results of the physico-mechanical properties revealed performance risks associated with some SCC mixture designs. Such risks were not captured by the full immersion exposure. Thermal, mineralogical and microscopy studies elucidated the complexity of degradation mechanisms, which in some cases varied at different locations of the same specimen. Findings from this study emphasize the need for performance standard tests that can better simulate various realistic field exposure regimes in order to achieve a more reliable and comprehensive evaluation of the resistance of concrete to sulfate attack.     

Phenomenological aspect of concrete durability theory

It is shown that concrete destruction by freezing and thawing, in dry and hot climate, under sulphate and potassium aggression occurs because of high inner tensile stresses appearing as a result of bonding in concrete conglomerate the components with widely different deformations under the action of corrosion. The problem of concrete frost resistance is considered in detail. The model of concrete as a material of “conglomerate in conglomerate” type, divided into three structural levels is proposed. The measurement of damage to concrete, being characterized by the ratio of maximum total inner stresses to local strength of structure is introduced. It is shown that the developed approach makes it possible to explain from a single viewpoint the majority of experimental facts, known in the field of concrete durability.     

锈蚀对钢筋混凝土结构耐久性的影响研究

钢筋混凝土结构在工程建设中得到广泛的应用,然而结构中的钢筋锈蚀是影响钢筋混凝土结构耐久性的主要因素。文章阐述了混凝土结构的耐久性理论,锈蚀影响混凝土耐久性的因素以及预防锈蚀的措施等。     

Effects on concrete durability of using recycled ceramic aggregates

Ceramic waste from ceramic and construction industries is one of the most important parts in the global volume of construction and demolition waste (CDW). Ceramic waste may have several uses, one of which as coarse aggregate for concrete artefacts. Within a research campaign in course at Instituto Superior Técnico (IST), concerning the reuse and recycling of CDW, the viability of replacing primary limestone aggregates with ceramic waste on the production of concrete pavement slabs has been studied. Compression and bending tests previously performed have shown the mechanical suitability of replacing, at least partially, limestone aggregates with ceramic recycled ones. In this paper, the results of the water absorption tests, either by capillarity or by immersion, and the results of the abrasion resistance tests are presented, all related to long-term concrete durability.     

Influence of the purification of sodium dodecyl sulfate on the physical properties of solutions and O/W emulsions

The effect of the purification of sodium dodecyl sulfate (SDS) has been studied in aqueous solutions and in o/w emulsions. The physical properties (surface tension, conductivity, light dispersion, viscosity, and granulometry) depend on the degree of purity of surfactant. This effect is less important within the emulsion technology when thickener or cosurfactant agents are added to the emulsion.     

Effect of sludge water from ready-mixed concrete plant on properties and durability of concrete

Besides the increasing disposal cost, sludge water, a wastewater washout from ready-mixed concrete plant, has caused environmental impact problems. This paper investigates the utilization and recycling of sludge water as mixing water for concrete production. The basic properties of sludge water were obtained according to ASTM standards. The properties of dry sludge powder such as chemical compositions and physical properties were investigated. The properties of fresh concrete studied were unit weight, slump, and temperature rise. The mechanical properties of concrete, such as compressive strength and modulus of elasticity, were studied. The durability aspects, such as drying shrinkage and weight loss due to acid attack, were investigated. For parametric study, sludge water was used as a replacement of tap water varying from 0% to 100% by weight. The water-to-cement ratios were 0.5, 0.6, and 0.7, respectively. In this study the sludge water tested has a high alkalinity and the total solids content exceeding the limit of ASTM C94, contributing to the more porous and weaker matrix. As a result, when increasing the percentage of sludge water in mixing water, the drying shrinkage and weight loss due to acid attacks increased, and the slump and strength decreased. However, the unit weight and temperature of fresh concrete were not affected by the use of sludge water.     

Effect of elevated curing temperatures on the strength and durability of concrete

In recent years, high core temperatures of 77–93°C of mass concrete elements during curing have been recorded by the Florida Department of Transportation (FDOT). These curing temperatures have been recorded while still maintaining the maximum differential temperature of 20°C between the core and exterior of mass concrete elements as specified by the FDOT. This study was conducted to evaluate the strength and durability of concrete subjected to elevated curing temperatures. Using typical FDOT class IV mass concrete mixes; it is shown that elevated curing temperature resulted in lower later-age strengths. Blending the cement with fly ash and slag, resulted in increased strength and durability when compared to the plain cement mixes.

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Résumé Ces dernières années, le département des transports de la Floride—Florida Department of Transportation, FDOT—a enregistré des températures élevées de 77 à 93 degrés Celcius dans des blocs de béton massif en cours de séchage. Comme le précise le FDOT, ces températures de séchage ont été enregistrées alors qu’une température différentielle maximum de 20°C était maintenue entre l’intérieur et l’extérieur de ces blocs de béton massif. Cette étude fut conduite pour évaluer la solidité et la durabilité d’un béton exposé à des températures de séchage élevées. En utilisant du béton massif issue de mélanges typiques, classe IV, du FDOT, on démontre que la température de traitement élevée a eu comme conséquence des forces inférieures à un age plus avancé. Les mélanges comportant une addition de scories et de cendres volantes se sont avérés plus solides et plus durables que ceux restés purs.

     

Theoretical analysis of the measurement of polarisation resistance in reinforced concrete

Polarisation resistance (R_p) technique based on Stern–Geary equation is one of the most widely used methods of measuring corrosion rate of reinforcement in the field. With the aid of a “sensorised guard ring”, this electrochemical technique is claimed to be able to determine corrosion rate (I_corr) within a given measuring area. However, there are three theoretical problems in the application of this technique: (1) the original Stern–Geary equation is applicable in a uniform corrosion system at its corrosion potential, whereas the reinforced concrete structure may be subjected to non-uniform corrosion or strong polarisation by macro-cell galvanic effects or imposed currents; (2) the value of the parameter B in the original Stern–Geary equation has been estimated to fall within the range 25–52 mV. This may not be suitable for all the corrosion cases of reinforced concrete structures; (3) The polarised surface area of steel may theoretically not always be fully confined by the sensorised guard ring when the cover concrete is too thick.

This paper aims at discussing the theoretical problems. A general relationship between the dissolution rate of steel reinforcement and the measured polarisation resistance is deduced under general conditions. The range of B value is also analysed based on all the possible corrosion situations of reinforced concrete. Furthermore, unsatisfactory confinement by sensorised guard ring on a thick cover concrete is demonstrated.     

Effect of metakaolin and silica fume on the durability of self-consolidating concrete

Metakaolin (MK) is a valuable admixture for concrete/cement applications that can enhance the performance of cementitious composites through high pozzolanic reactivity, much like silica fume (SF). While SF concrete is characterized by superior mechanical and durability performance, concrete containing MK achieves comparable properties at a lower price and with better workability. The objective of this study is to investigate the effect of cement replacement by MK on the durability of self-consolidating concrete (SCC); the effect of SF at similar levels of MK replacement has also been included for comparison. The durability performance of SCC was evaluated based on the results of drying shrinkage, freezing and thawing, salt scaling, and rapid chloride permeability tests. The results of these tests indicate that highly durable SCC mixtures can be produced using a high MK content with an optimum percentage of around 20%. The results also show that the durability of SCC, especially with high MK content, is higher than that of SCC containing SF.     

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.     

混凝土结构耐久性设计方法与寿命预测研究进展

为量化钢筋锈蚀与余震对钢筋混凝土结构抗震性能的影响,选取两栋按我国现行规范设计的中国东南沿海地区钢筋混凝土框架结构为研究对象,考虑未锈蚀和由低到高三种不同锈蚀率(5%、10%和15%)的四种工况,采用真实主余震序列作为输入,选取Park-Ang损伤指数作为结构损伤指标,开展了主余震序列作用下的未锈蚀与锈蚀钢筋混凝土框架结构的地震损伤评估与易损性分析。计算结果表明：锈蚀率的提高加剧了主余震序列作用下的结构累积损伤,其增长率最大可超过50%。由钢筋锈蚀单一因素引起的结构损伤在主余震累积损伤中的占比最高可超过30%。此外,钢筋锈蚀因素会导致结构的主余震易损性曲线发生显著提升。当锈蚀率较大时,钢筋锈蚀对结构易损性的影响与余震对结构易损性的影响相接近。钢筋锈蚀和余震两个因素的耦合作用会使结构的易损性水平发生更为显著的提升。因此,十分有必要在既有钢筋混凝土结构抗主余震性能评估中考虑钢筋锈蚀因素的影响。     

Considerations on the Italian method for freezing and thawing durability of concrete

The identity of test systems in the different laboratories is essential because the results on the freezing behaviour of concrete should be comparable. In that aim, after many tests, the Working Group “Permeability and Durability” of Unicemento has published a standard on concrete freezing. Series of tests have been specially carried out in order to determine the influence of different parameters on test results (size of specimens, saturation degree, time of single cycle phases, velocity of freezing and thawing, etc...). On the basis of the results obtained, the enclosed standard is proposed.     

Effect of magnesium sulfate on the durability of limestone mortars based on quaternary blended cements

Currently, the use of blended cements incorporating various supplementary cementing materials, preserved in aggressive environments has become common. This paper describes the investigation results conducted on the evaluation of the resistance to magnesium sulfate solution (MgSO₄) of limestone mortars containing simultaneously; limestone filler, blast furnace slag and natural pozzolan. In this study, the deterioration of limestone mortars due to sulfate attack was evaluated by measuring changes in weight, length and compressive strength at the ages of 30, 60, 90, 120 and 180 days of immersion in exposure environments. The X-ray diffraction was also used in order to determine the different mineral phases. It is noteworthy that, the pH variation of the conservation solutions has been monitored during tests. The exposure solution was renewed monthly until the end of tests. The results showed that, the resistance to sulfate attack of mortars made with quaternary binders was better than that of mortars based on ordinary Portland cement.     

Increasing durability of lightweight concrete through FRP wrap

The purpose of this study was to investigate the effects of Fiber Reinforced Polymer (FRP) wraps on reducing chloride ingress and increasing the durability of lightweight concrete. Several prior studies have dealt with FRP application on normal weight concrete. However, the literature on externally bonded FRP application on lightweight concrete is sparse. This study involved an accelerated testing to determine concrete degradation and decrease in chloride ingress and associated increased durability that FRP wrapping can provide to lightweight concrete. Forty two cylinders were subjected to electricity induced accelerated testing in a saline solution for 50 days. Samples were removed from the immersion tank after specific failures and analyzed for chloride content and failure modes. Both lightweight and normal weight concrete greatly benefited from FRP wrapping in terms of increased time to failure and reduced concrete chloride. Lightweight concrete generally performed better with Carbon FRP (CFRP), and normal weight concrete with Glass FRP (GFRP). CFRP wraps produced the best overall protection. Multiple wraps were more effective at reducing chloride ingress than single wraps. Lightweight concrete had greater chloride permeability reductions, whereas normal weight concrete benefited more from FRP confinement effects. The majority of sample failures were caused by cracking in the concrete substrate.     

A design consideration for durability of high-performance concrete

Environmental factors, especially the climate, have significant influence on concrete structure. This paper aims to investigate the harmful effects of maritime climate on the durability of concrete structures built in coastal areas. Singly reinforced beam specimens of traditional design and those of densified mixture design algorithm (DMDA) were employed to study the potential problems of concrete structure. Results indicate that cracks on the concrete structure, if go unnoticed, may cause failures. Thus, it is important to know the methodology of achieving high strength and durable concrete in order to avoid formation of cracks in the structural member.