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The durability of a concrete structure is highly dependent on the strength and permeability of the surface layer, as it is the surface layer that must prevent the entry of materials that can initiate or enhance the harmful effects on concrete. Sulfates are one of the most common destructive factors in concrete in most parts of Iran, especially in the southern regions of the country where concrete is exposed to seawater (which contains sulfate compounds). Also, the lack of compaction of the surface layer, due to the difficulty of vibration in limited spaces between molds and rebars and other accessories, is one of the main reasons for the poor durability of reinforced concrete structures exposed to environmental factors. Naturally, incorrect vibration results (worming, detachment, dehydration) have stronger negative effects on permeability and therefore durability. Self-compacting concrete with suitable properties is free from these defects and as a result, materials with less inconsistency and uniform permeability have less weaknesses for environmental harmful factors and, therefore, have better durability. Therefore, considering that the "torsion" test shows good sensitivity to surface changes of concrete, so in this study, using the "torsion" test, the effect of magnesium sulfate on the surface strength of self-compacting concretes has been investigated. In the "twist" test, a 5 cm diameter metal cylinder is glued to the surface of the test site using epoxy resin adhesive. Then, using a conventional hand-held tachometer, a torsion anchor is inserted into the metal cylinder to break the test object. The equipment used in the "twist" test is very cheap, simple and accessible compared to other corresponding tests. The damage from the "torsion" test is very superficial and minor, and by causing failure in the test object itself, it directly determines its strength. Self-compacting concrete mixing designs were studied by replacing 25, 35 and 45% cement with fly ash filler, and a conventional concrete mixing design as a control to study the effect of magnesium sulfate on the strength of self-compacting concrete. Self-compacting concrete tests including slump flow test, slump flow time of 50 cm, V-shaped funnel, V-shaped funnel increase time (5 minutes) and L-shaped mold were performed on self-compacting concrete acceptance criteria on self-compacting concrete mixing designs. . The results indicate that sulfated water not only does not have a negative effect on the surface strength of self-compacting concrete containing fly ash, but also provides better curing conditions for these samples. The use of fly ash also makes the magnesium sulfate solution a more suitable medium than ordinary water for the surface strength of self-compacting concretes. The process of obtaining surface strength in almost all self-compacting samples treated in magnesium sulfate solution is more than that in ordinary water. However, in the case of ordinary concrete, the process of obtaining the surface strength of all samples placed in magnesium sulfate solution is less. For self-compacting samples treated in magnesium sulfate solution, with increasing the percentage of fly ash, the surface resistance of 3 and 7 days decreases. But the 28-day surface resistance increases.

Keywords: Surface strength, Twist-off, Self-compacting concrete, Magnesium sulfate, Fly ash.

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