Physical Salt Attack Research Articles

The effect of pH and carbonation on the partially immersed mortar exposed to physical salt attack

In saline-alkali or salt-lake areas, the influence of pH value of underground aqueous solution and carbonation on the deterioration of cement-based materials remains an open question. This study utilizes various techniques to investigate the physical properties and degradation mechanism of partially immersed mortar incorporated with different dosages of fly ash exposed to combined action of salt solutions (10 wt% Na2SO4, 3.5 wt% NaCl + 10 wt% Na2SO4, and tap water) at different pH values (pH 5, 7, and 9) and carbonation (accelerated carbonation: 20 vol% CO2 and natural carbonation: 0.035 vol% CO2). The results indicate that the degree of deterioration of mortar increases with dosage of fly ash and pH value of salt solution, with the maximum mass loss exceeding 25 %. Physical sulfate attack (PSA) is responsible for the partially immersed mortar exposed to combined action of salt attack at various pH values and natural carbonation. However, when carbonation is accelerated, a more severe deterioration occurs, with the durability time of mortar reduced by nearly 60 %. PSA and decomposition of C-S-H are mainly responsible for the deterioration of mortar, while the formation of thaumasite and natrite is partially responsible for the deterioration of mortar. The presence of NaCl in mixed salt solution inhibits the precipitation of Na2SO4 crystals; the precipitation of NaCl clogs the pore network of mortar, which impedes the progressive penetration of CO2. The presence of NaCl in mixed salt solution is beneficial to salt resistance and carbonation resistance of mortar.

Influence of chlorides and salt concentration on salt crystallization damage of cement-based materials

Durability of concrete exposed to physical salt attack (PSA) has primarily been studied on specimens partially submerged in sulfate solution. However, in sulfate-containing soils, chlorides are often present. The aim of this study is to investigate the influence of salt concentration and the presence of chlorides on the deterioration of cement-based materials caused by PSA. Cement mortars partially submerged in three different concentrations of sodium sulfate solutions (5%, 9% and 13%) and two mixed Na2SO4–NaCl solutions (5%–2% and 5%–5%) and exposed to cyclic drying-wetting were investigated. The crystallization behavior of salt solutions and the alteration in the macroscopic properties and microstructure of the specimens were observed to understand the differences in deterioration. The results showed that the presence of chloride ions significantly reduced the surface scaling above the solution level. Microstructural characterization suggested that thenardite tended to precipitate homogeneously in the specimens partially submerged in solutions containing chlorides. Consequently, crystallization pressure is less likely to develop in such a case. On the contrary, for the specimens partially submerged in pure sodium sulfate solutions, a higher concentration of salt solution facilitated more thenardite accumulation in a thin layer beneath the surfaces, which in turn accelerated the damage process. The findings may improve guidance on the evaluation of concrete resistance to PSA when chlorides are detected in sulfate-bearing environments.

Effect of Temperature on the Physical Salt Attack of Cement Mortars under Repeated Partial Immersion in Sodium Sulfate Solution.

Physical salt attack (PSA) is one of the dominant durability issues of cement-based materials, where salt crystallization pressure is the driving force inducing damage. However, research on the temperature-related deterioration behavior of cement-based materials is limited. In this study, salt-contaminated cement mortars were rewetted at different temperatures. The assessment criteria were based on the visual appearance, weight evolution and size distribution of scaled materials, and the alterations in the microstructure were investigated by microscopy, thermal and mineralogical analyses. The results indicated that more severe damage developed at 5 °C than that at 20 °C due to the greater crystallization pressure caused by the conversion from thenardite (Na2SO4) to mirabilite (Na2SO4·10H2O) at the lower temperature. No damage was observed at 35 °C, since the repeated dissolution and re-crystallization of thenardite were harmless for the specimens. In addition, two distinct damage patterns were observed for PSA performed at 5 °C and 20 °C, namely, granular disintegration and contour scaling.

Reliability of the damage rating index to assess the condition of concrete affected by external sulfate attack

The damage rating index (DRI) has proven to be a quite reliable microscopic technique enabling the condition assessment of concrete affected by internal swelling reactions. Yet, very little research has been conducted on the use of the DRI to appraise induced deterioration caused by physical salt attack (PSA) and external sulfate attack (ESA). This study aims to assess the condition of concrete subjected to different salt solutions (i.e. sodium sulfate (Na2SO4), sodium chloride (NaCl), seawater and limewater), and exposure conditions (i.e. partially and fully immersed) through the DRI. Concrete specimens displaying two distinct water/cement ratios (i.e. 0.45 and 0.65) were manufactured and exposed to the above salt solutions for 12 months. Then, the samples were removed from the exposure conditions and prepared for microscopic analysis. The DRI was shown to be an effective technique to assess the condition of PSA- and ESA-affected concrete since different degrees of damage and features were captured for the various exposure conditions studied.

Performance of repaired concrete piles partially embedded in Sulfate-Bearing soil

Physical salt attack (PSA) is a key damage mechanism of concrete in salt-bearing environments. Multiple case studies were reported throughout literature on PSA of concrete; yet, without any case on the performance of repaired concrete elements. The current paper presents a case study analyzing the condition of repaired concrete piles in 47-year-old administrative building in Winnipeg, MB, Canada, after 22 years from repair works. Deterioration of the concrete piles was visually observed above the ground level and repair zones, in the form of scaling, crumbling and reduction of cross section. In addition to fluid transport and pore structural features tests, mineralogical, thermal and microscopy analyses were performed on concrete chunks/cores extracted from the piles to elucidate the underlying mechanisms of damage. The repair concrete was capable of mitigating chemical and physical sulfate attacks; however, it could not stop the progress of PSA on the concrete above it, urging for careful design of repair strategies for elements subjected to PSA. Recommendations for future repair in this case are also presented.

The Impact of Long-Term Physical Salt Attack and Multicycle Temperature Gradient on the Mechanical Properties of Spun Concrete.

The article is focused on spun concrete made with different chemical admixtures under long-term exposure to aggressive salt-saturated ground water and a cyclic temperature gradient. Over a long-term experimental investigation, 64 prismatic spun concrete specimens were subjected to multicycle (75–120) processing under combined aggressive ambient conditions. Prismatic specimens were soaked in water or saline and dried at a temperature of 45–50 °C. The long-term multi-cycle effect of the temperature gradient and physical salt attack on the compressive strength, Young’s modulus and durability of concrete was found to be negative. Chemical admixtures, though, improved the structure of spun concrete, thus having a significant positive effect on its physical-mechanical properties and durability.

Performance of concrete under accelerated physical salt attack and carbonation

Conventional testing of physical salt attack (PSA) on concrete does not consider concomitant factors that may exist in service and alter the mechanisms and kinetics of PSA. This study adopted a combined testing approach where concrete was concurrently investigated under accelerated PSA and carbonation, simulating elements serving in heavy traffic and industrial zones, while implementing ambient conditions similar to that in geographic locations with previous cases of PSA. Based on the tested mixture design parameters [water-to-binder ratio, cement type, and supplementary cementitious materials], potential performance improvement and risks were identified. Thermal, mineralogical, and microscopy analyses elucidated the co-occurrence of complex degradation processes in concrete subjected to accelerated PSA and carbonation, which were distinctive from that induced by the single-factor PSA exposure. The synoptic results from this study may informatively improve guidance on mixture design of concrete when dual exposure to PSA and carbonation is expected in the field.

The Reinforced Spun Concrete Poles under Physical Salt Attack and Temperature: A Case Study of the Effectiveness of Chemical Admixtures.

The present paper focused on the investigation of the effectiveness of using various chemical admixtures and their effect on the strength and deformability of the reinforced spun concrete members—the supporting poles of the overhead power transmission lines—under the unfavorable long-term combined action of the aggressive salt-saturated groundwater and the temperature changes. According to the long-term experimental program, 96 prismatic spun concrete specimens were subjected to multi-cycle (25-50-75 cycles) processing under the combined aggressive environmental conditions. It has been found that chemical admixtures which decrease the initial water-cement ratio produce a considerable positive effect on the mechanical properties of spun concrete used in hot and arid climates and exposed to physical salt attack (PSA). Superplasticizers decrease the initial water-cement ratio the most, and, due to a unique concrete compaction method used, they produce the most homogeneous and dense concrete structure. They can be recommended as most effective in increasing the durability of spun concrete used under the above-mentioned aggressive environmental conditions.

Durability of Concrete Superficially Treated with Nano-Silica and Silane/Nano-Clay Coatings

Protection of the surface layer of concrete is essential for achieving durability and functionality of concrete elements during their service life. In this paper, an effort is made to utilize colloidal nano-silica (5%–50%) and a synthesized nanocomposite as superficial treatments for concrete; silane was used as the neat resin to disperse nano-montmorillonite particles at different dosages (5% and 10%). The coatings were applied to a typical concrete mixture used for residential concrete in North America. The transport properties of the treated concrete were characterized using the rapid chloride penetrability test and the absorption/desorption percentages. Moreover, concrete was evaluated under severe durability exposure involving physical salt attack (PSA), which is a wetting/drying regime responsible for surface damage of concrete elements subjected to continuous salt supply along with cyclic ambient conditions. Deterioration was visually assessed and quantified using mass change. In addition, thermal and microscopy analyses were performed on concrete specimens to elucidate the mechanisms of enhancement by surface treatment. The results showed that increasing the concentration of nano-silica particles in the colloid led to an improved performance of concrete, with the 50% loading ratio achieving the least penetration depth, absorption/desorption percentage, and mass loss of concrete under aggravated PSA. For the silane/nano-clay composite, the low dosage of nano-clay was adequate to mitigate the damage caused by PSA on concrete.

Silane and methyl-methacrylate based nanocomposites as coatings for concrete exposed to salt solutions and cyclic environments

This study aimed at investigating the efficiency of polymeric nanocomposites as superficial treatments for concrete subjected to aggravated exposures. Silane and methyl-methacrylate were used as base resins, in which nano-clay and nano-silica particles were mixed at different concentrations (0, 5, and 10%). The nanocomposites were applied to inferior and good quality [water-to-binder ratios of 0.60 and 0.40, respectively] concretes, and their effects on transport/barrier properties were evaluated in terms of penetrability and absorption/desorption percentages. Moreover, the durability of coated concrete was assessed under severe conditions: physical salt attack (PSA) and salt-frost scaling. The degradation mechanisms and coatings’ functionality were studied by mineralogical, thermal, and microstructural tests. Results showed that silane/nanocomposites were effective at enhancing the barrier properties of concrete, thus mitigating PSA and salt-frost scaling for sound and cracked concretes. Methyl-methacrylate/nanocomposites, especially at 5% dosage, were efficient at resisting PSA of concrete; but, they failed in the salt-frost scaling exposure.

Effect of temperature on durability of cement-based material to physical sulfate attack

Physical salt attack (PSA) is a deterioration mechanism caused by crystallization of salts in pores. Many experimental studies have shown that pore size distribution influences cement-based materials susceptibility to PSA. However, the deterioration of concrete due to PSA is also governed by environmental conditions. In this paper, mortars with water to cement ratios of 0.4, 0.5, and 0.6 were subjected to cycles of salt contamination, immersion in 5% sodium sulfate solution separately at 5 ℃ and 20 ℃, and then dried at 50 ℃. The assessment methods were based on visual appearance and weight change. In the meantime, X-ray diffraction, scanning electron microscopy equipped with energy dispersive spectroscopy, and X-ray computed tomography were used to elucidate the damage mechanism. The results showed that the damage pattern depended on the temperature: uniform surface crumbling at 5 °C; edges and corners scaling at 20 ℃, and the damage was more severe at 20 ℃ than at 5 ℃. Besides, the evolution of salt uptake indicated that the crystallization of sodium sulfate occurred deeper below the surface of the specimens when the temperature of the sodium sulfate solution was higher. Moreover, lower water to cement ratio was more beneficial to prevent PSA.

Influence of Nanosilica on Physical Salt Attack Resistance of Portland Cement Mortar

Influence of Nanosilica on Physical Salt Attack Resistance of Portland Cement Mortar

Effect of Coatings on Concrete Resistance to Physical Salt Attack

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Potential accelerated test methods for physical sulfate attack on concrete

There is currently no standard test method to determine the physical salt attack (PSA) resistance of cement-based materials. In this study, several accelerated test methods for PSA were evaluated. This was accomplished by testing concrete mixtures in partial and full immersion in sodium sulfate solutions at different concentrations, temperature, and relative humidity cycling. A total of nine concrete mixtures made from three types of portland cement at three different water-to-cement ratios (w/c) were used in this research to compare each method with materials of different qualities. The results showed that a 5% sodium sulfate solution was too dilute to be used in accelerated PSA testing, whereas increasing the concentration to 10% showed better results. In addition, it was found that damage occurred during low temperatures at high relative humidity compared to high temperatures at low relative humidity. Testing fully immersed specimens in 30% sodium sulfate solution gave the highest mass loss, but the damage was located at the bottom third of the specimens because of a concentration gradient that formed in the solution.

Durability of Portland-limestone cement-based materials to physical salt attack

Because salt solution ingress is governed by permeability and diffusivity and because salt crystallization pressure is directly related to pore size, changes in cement composition and hydrated microstructure may influence the durability of cement-based materials in salt-laden environments in complex ways. Here, performance of portland limestone cement (PLC) and ordinary portland cement (OPC) mortars exposed to sodium sulfate and calcium nitrate salt solutions was assessed, relative to their pore structure and sorptivity. At high w/b (0.60), increased salt crystallization damage was observed in the PLC mortar and was linked to its refined pore structure. At low w/b (0.40) performance was similar and was attributed to higher tensile strength and the inability of the salts to penetrate smaller pores. These results show that a w/b of 0.40 should provide adequate protection from capillary rise and evaporation-induced salt crystallization in both PLC and OPC mortar, but that at higher w/b differences in performance are possible.

The engineering geology of concrete in hot drylands

In hot drylands the main causes of concrete deterioration are physical salt attack by aggressive evaporite salts, reinforcement corrosion (e.g. carbonation, chloride ingress) and aggregate unsoundness. The manufacture of concrete presents more problems than in temperate countries due to the higher ambient temperatures and drying winds, plus windblown salts (mainly chlorides and sulphates). However, it is not just aridity that is significant to concrete in hot drylands, but also the availability of moisture and the duration of moist conditions. This paper draws upon the lifetime's experience gained by the lead author (PGF) and aims to provide a basic introduction to the ‘engineering geology of concrete' for hot drylands. The focus is on the Middle East and North Africa, but the general messages have broader applicability.

Sulfate attack on residential concrete foundations in Japan

Deterioration of concrete structures caused by external sulfate attack was found in many residential concrete foundations in Japan. In residential concrete foundations constructed on sulfate-bearing ground, sodium sulfate solution contained in the soil penetrates the above-ground area of the concrete, and a scaling of the area occurs with white crystals of thenardite or mirabilite. The deterioration mechanism is known as “physical sulfate attack,” “physical salt attack,” “salt weathering,” and so on. The deterioration cases were usually observed in sites where the ground comprised of marine deposits that contained sulfides; these soil conditions are common in large part of Japan. This paper describes the concrete deterioration phenomenon based on field investigations and analysis results of soil and concrete samples. A rapid test method to reproduce the physical sulfate attack is proposed.

The role of carbonation in the occurrence of MgSO4 crystallization distress on concrete

Based on the definition of physical salt attack, the Guide to Durable Concrete (ACI 201.2R) suggests that MgSO4 cannot lead to crystallization distress on concrete. In this paper, concretes consisting of 30% fly ash (FA), 20% limestone powder (LP), 50% slag (SG) and pure Portland cement (PC) were acceleratingly carbonated for 20d before being partially immersed in a 10% MgSO4 solution. After 150d of partial exposure, MgSO4 crystallization was attributed to the damage of the outer layer of carbonated FA, SG, LP concrete specimens in the evaporation zone. The outer layer of FA and LP concretes used as reference had also become carbonated and detached due to MgSO4 crystallization. The test results supported the idea that MgSO4 crystallization distress can occur in concrete through carbonation, and that concrete carbonation should be considered when analyzing physical salt attack on concrete.

Exploring effects of supplementary cementitious materials in concrete exposed to physical salt attack

It has been well established that supplementary cementitious materials (SCMs) can significantly enhance the resistance of concrete to chemical sulfate attack. However, the effect of SCMs on the durability of concrete exposed to physical salt attack is still controversial. To date, there have been only limited studies that have investigated this concrete durability issue. Therefore, the present study investigates the effects of using different types of SCMs, including silica fume, fly ash and metakaolin, in concrete subjected to environments prone to physical salt attack. Results indicate that the damage of concrete escalates as the addition level of SCMs increases. An attempt is made to delineate this problem and explain the mechanisms controlling this behaviour, which could have implications for existing design codes. The findings call for caution when SCMs are specified for concrete subjected to environments conducive to physical sulfate attack.

Damage mechanisms of two-stage concrete exposed to chemical and physical sulfate attack

There is currently dearth of information on the durability of two-stage concrete (TSC) to physical and chemical sulfate exposure. TSC differs from conventional concrete in many ways including its placement technique, high aggregate content and the use of very flowable grout. Therefore, available data on the behavior of conventional concrete under sulfate attack may not be applicable to TSC. For instance, the effects of different parameters such as the addition of supplementary cementitious materials (SCMs) on the resistance to sulfate attack are well documented for conventional concrete; however, for TSC this has not been duly investigated. In this study, the behavior in sodium sulfate laden environments of TSC mixtures incorporating different SCMs as partial replacement for ordinary Portland cement (OPC) was investigated. Two different sodium sulfate exposure regimes were studied: full immersion simulating chemical sulfate attack, and partial immersion combined with cyclic temperature and relative humidity, which is conducive to physical salt attack. Fully immersed TSC specimens incorporating fly ash or metakaolin exhibited high sulfate resistance. Surprisingly, TSC specimens incorporating silica fume exhibited significant damage due to thaumasite formation. Under exposure to physical salt attack, TSC specimens incorporating fly ash and/or silica fume incurred severe surface scaling at the evaporative front, while those made with metakaolin unveiled adequate resistance to surface scaling. An attempt has been made to delineate the mechanisms which explain such observed behavior.