Experimental Evaluation of the Concrete Damage and Pore Characteristics under Salt-Freezing Cycles

Abstract

Herein, ordinary silicate concrete specimens are prepared to study the damage law of a cement-concrete material under the effects of salt erosion and a freeze–thaw environment. NaCl, NaHCO3, and Na2SO4 solutions are separately produced, according to the characteristics of saline soil, to conduct an experimental study on the concrete characteristics during quick salt freezing cycles, and to analyse the changes in its compressive strength, mass loss, and dynamic elastic modulus (DEM) under freeze–thaw cycles. Low-field nuclear magnetic resonance (NMR) and scanning electronic microscopy are used to investigate the change in the microstructure of concrete specimens under salt freeze–thaw cycles (FTCs). The results show the loss in compressive strength, mass, DEM, and NMR spectrum signal increased by 1.5–3 times, 3–5 times, 1.5–2.5 times, and 2–4 times, respectively, for concrete specimens under 50–100 FTCs in 6.8% composite salt solution, in comparison to fresh water. Apparent spalling, decreases in the DEM, and reductions in the compressive strength occur in concrete when increasing the number of salt FTCs. The number of internal cracks in the concrete structure increase under the combined action of salt crystallization, moisture absorption, and freeze–thaw. The changes in the internal microscopic pore volume in concrete structures exhibit the same trend with changes in the macro mechanical properties of concrete. The correlation coefficients between the changes in each peak in the NUR spectrum and the changes in the compressive strength of concrete specimens under FTCs in freshwater or low-concentration salt solutions are both larger than 0.7, calculated using the grey correlation degree method. Therefore, these changes could be used as a potential evaluation index for salt frozen damage to concrete structures.

Influence of Variatropy on the Evaluation of Strength Properties and Structure Formation of Concrete under Freeze-Thaw Cycles

“…Let us consider the obtained results and discuss them from the point of view of the pore structure. As already mentioned by us earlier and confirmed by numerous studies by other authors [31][32][33][34][35][36]56], vibrated concrete with a simple structure has an open porosity and is thus subject to rather facilitated penetration of moisture from the outside, through the outer layer of concrete and, thereby, is at risk of less resistance to freezing and thawing. This is due, firstly, to easier penetration of moisture through the pores in the outer layer, and thus, unclosed porosity creates a capillary structure of concrete, in which, when frozen, the moisture particles contained in the capillaries lead to the destruction of concrete from the inside.…”

“…A rational combination of technological and prescription factors directly affects the structure of the concrete composite at all its levels. Porosity parameters "under freezing and thawing conditions are important for assessing the frost resistance of concrete" [31,32]. The porosity of the HCP is divided into several types, namely: the total porosity of the HCP, the porosity of hydrated cement, capillary porosity, open and closed porosity [33,34].…”

Evaluation of fly ash concrete in salt environment

“…Concrete damage occurs when NaCl evaporates so that crystals appear in the pores of the concrete, which can push through the pores of the concrete walls. Thus, the concrete can be damaged and broken into pieces [4].…”