The use of deicer salts in solid or solution form onto pavements is a frequent practice in cold climate regions, since these salts are able to decrease the freezing temperatures of solutions [1][2], which may speed the melting process of existing ice and also reduce the ice formation on streets, sidewalks, parking lots and other pavement surfaces. However, damages on both conventional and pervious concrete pavements have been reported after the application of deicers, including magnesium chloride [3], which might be explained by complex physical and chemical interactions between the deicer and cementitious components in the concrete [4][5].
There are uncertainties with regards to the use of pervious concrete in cold climate areas, not only because of the possible harms caused by deicers, but due to the freezing and thawing effect, too. These uncertainties are led by the assumption that those two factors might produce more damages, or at least damages that are harder to fix, on pervious concrete than on conventional concrete, due to the porous nature of the material. Impacts caused by freezing and thawing might be more significant on pervious concrete because its large percentage of void content allows the rainwater to pass through the pavement structure, and a considerable volume of rainwater may freeze within these voids. With respect to the deicer attacks, deicer solutions may accumulate in areas of difficult access, into the pores of the pervious concrete, and a high concentration of salt might stay in contact with the material for long periods of time, since part of the water in the deicer solution may evaporate or drain through the subbase layer and the soil.
Although the freezing and thawing effect contributes to the damages caused on pervious concrete, the structural impacts caused by deicers, including magnesium chloride, are more significant than the freezing and thawing effect alone [6][2]. It should also be noted that water expands after freezing and magnesium chloride deicers, specifically, reduces this icing pressure, in pervious concrete, more significantly if compared to other deicers, such as calcium chloride (CaCl2) and sodium chloride (NaCl), or even in comparison to pure water [2]. In other words, that means that the use of deicers is beneficial to the impacts caused by the ice formation and consequent freezing and thawing effect. However, the damages caused by the deicers themselves may be significant and deserve attention.
Some studies investigate the resistance of pervious concrete to the negative impacts of deicers. One of these studies states that deicer attacks to concrete might be reduced if the amount of hydroxides is also reduced in the cementitious components of it [5], since the reactions between deicers and hydroxides may form non-cementitious materials, weakening the concrete [7][8]. In concrete, these hydroxides are usually calcium-based, formed after the hydration of free calcium oxide (CaO) and other calcium oxide phases, such as alite (C3S), belite (C2S) and aluminate (C3A). Because free calcium oxide and its related phases represent a significant percentage of a typical Type I cement, it is suggested that using supplementary cementitious materials (SCMs) or accelerating the concrete carbonation may reduce the formation of hydroxides [5].
Carbonation of concrete is typically produced by carbon dioxide sequestration from the air into the concrete [9], thus calcium-based carbonates are formed, replacing existing calcium-based hydroxides. Based on aqueous complexation, it is assumed that this process could prevent the concrete cementitious components from reacting with deicers, however, natural carbonation is a long process that might take years until deeper portions of the concrete get significantly carbonated. For this reason, a novel method was proposed [10] and initially applied [11] with the aim to accelerate the concrete carbonation. The method consists of a topical treatment with a sodium bicarbonate (NaHCO3) solution poured onto pervious concrete specimens, to promote chemical exchanges between the carbonate components of NaHCO3 and the free hydroxides of the cement paste (see Equation 1), accelerating the concrete carbonation and perhaps increasing its resistance to one specific deicer attack, magnesium chloride.
It was demonstrated that the treatment with sodium bicarbonate solution is effective in terms of the concrete carbonation acceleration [10], through pH changes in the solution and thermogravimetric analyses (TGA) of the concrete. However, there are still uncertainties with regards to possible harms also caused by the treatment itself or by the combination of the treatment with the magnesium chloride deicer. Some studies included the treatment of pervious concrete with sodium bicarbonate and combined it with magnesium chloride applications [11][12], and results reinforced this uncertainty, since it was not clear if the changes in the concrete’s compressive strength were caused by the NaHCO3 solution, by the MgCl2 deicer or by a combination of treatment and deicer applications.
Due to this uncertainty, another study isolated the treatment with sodium bicarbonate on pervious concrete [13], without the deicer as an additional variable, and a relevant finding was that the aging factor, before and after the treatment, contributes to the material’s changes in compressive strength [13][11][12]. It was demonstrated that the NaHCO3 treatment affects the concrete negatively if it starts too early after casting (about a month) [11], when compared with other specimens that were treated 2 months after casting [11][12]. In addition, specimens with higher compressive strength averages were presented, if more time was given between the end of the treatment and the compressive strength test performance [13]. There is also interest in treating pervious concrete with sodium bicarbonate for moss control, so studies into its impact on the concrete are useful for this additional application [14].
This article aims to test groups of pervious concrete specimens, summed with part of previously presented results [13], in order to produce statistically significant data with respect to the influence of the aging factor over the sodium bicarbonate treatment onto pervious concrete. That is to say, this article aims to find how much time is needed post curing before treatment to not harm the pervious concrete, as well as to verify if giving more time after the treatment may perhaps benefit the material.