1 ACI Materials Journal/November 2020 ACI MATERIALS JOURNAL TECHNICAL PAPER Physical salt attack (PSA) is a potential deterioration mechanism in porous materials, including concrete, exposed to salt-laden environments. Damage occurs as salt crystals grow in the near- surface pores causing tensile stresses on the pore walls higher than the tensile capacity of concrete, which can lead to surface scaling similar in appearance to that of freezing-and-thawing damage. This paper compiles, synthesizes, and analyzes current knowledge/ research on this topic in terms of the mechanisms of damage, test procedures, damage assessment methods, most influential factors, protection against PSA, and code/guideline provisions. Moreover, key aspects that require further investigation are highlighted, along with a proposed classification for the resistance of concrete to PSA and mitigation strategy. Keywords: durability; physical salt attack; protection; salt hydration distress; salt weathering; testing. INTRODUCTION The topic of physical salt attack (PSA), also known as salt weathering or salt hydration distress, has been a subject of extensive research on stones and rocks for a relatively long time. 1 Also, PSA was recognized under a range of environ- ments as a potential cause of concrete deterioration that leads to progressive scaling and flaking of concrete surfaces. 2 PSA occurrence is due to the presence of certain salts, notably sodium sulfate, in the exposure environment, while the ambient conditions (temperature and relative humidity [RH]) are favorable for salt crystallization. Damage is observed on the evaporative surfaces of concrete in contact with salt-rich media (for example, soil, groundwater, or seawater), such as the exposed portions of basement walls, abutments, piers, slabs on grade, tunnels, and so on (Fig. 1 and 2). Sodium sulfate, carbonate, and chloride, in order of aggression, have been reported to cause PSA. 2,3 The damage from PSA is mainly physical in nature; that is, limited or no chemical alteration of hydration products is involved. 4-6 While in the classical form of external sulfate attack, sulfates react with the hydration products, producing gypsum, ettringite, thaumasite, or brucite 7 ; in the case of specific environments conducive to PSA, compounds from these same sulfate salts crystallize in the near-surface pores, producing deleterious pressure to concrete. Flatt 6 reported that the value of this pressure can reach 10 to 20 MPa (0.15 to 0.3 ksi), which is far greater than the tensile strength of normal and even high-performance concrete. However, it must be recognized that both chemical and physical attack may result from the same exposure to external sources of sulfates; therefore, they do not necessarily occur in isola- tion from each other. In case of efflorescence depositing on the surface or minor surface scaling, PSA can be only of aesthetic concern. However, in severe PSA cases, significant spalling of concrete cover occurs, causing a reduction in the effective cross section and provoking corrosion of steel rein- forcement, which in turn would compromise the structural capacity of concrete. RESEARCH SIGNIFICANCE A new chapter on PSA has been added to ACI 201.2R- 16, 8 “Guide to Durable Concrete.” This paper is a group effort by ACI TG2 (Physical Salt Attack) of ACI Committee 201, Durability of Concrete, to provide a basis for updating the current ACI 201.2R-16 document by providing further understanding of PSA, including occurrence, mechanisms, compilation of published/unpublished data on field/labo- ratory testing, discussion of influential parameters, and comparisons of codes/guidelines and protection/coatings. Based on a synthesis of literature, a classification for the resistance of concrete to PSA and a mitigation strategy are proposed. PSA OCCURRENCE As a deterioration mechanism in concrete, PSA has only recently appeared as a distinct damage mechanism in the latest edition of ACI 201.2R-16. 8 This late recognition of PSA is likely the result of the confusion of PSA with chem- ical sulfate attack, as both mechanisms result from external sulfate exposure. Part of that was from the field studies commenced in 1982 by Portland Cement Association (PCA) on sulfate resistance of concrete, where concrete specimens were partially embedded in sulfate-rich soil (10% sodium sulfate) located in Sacramento, CA. In a report issued by PCA in 1989, the distress in specimens was misidentified as chemical sulfate attack 9 ; however, a subsequent report in 2002 indicated that specimens’ distress was mainly located only above the level of sulfate-saturated sand, which was not characteristic of chemical attack, with only minor damage below ground level. Stark 10 attributed the main cause of damage to PSA, and it was stated that the physical damage was far more prominent than that resulting from the chem- ical attack. MS No. M-2020-029 Physical Salt Attack on Concrete: Mechanisms, Influential Factors, and Protection by M. R. Sakr, M. T. Bassuoni, R. D. Hooton, T. Drimalas, H. Haynes, and K. J. Folliard ACI Materials Journal, V. 117, No. 6, November 2020. MS No. M-2020-029, doi: 10.14359/51727015, received January 20, 2020, and reviewed under Institute publication policies. Copyright © 2020, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including author’s closure, if any, will be published ten months from this journal’s date if the discussion is received within four months of the paper’s print publication.
Physical Salt Attack on Concrete: Mechanisms, Influential Factors, and Protection
May 01, 2023
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