Alkaline Hydrogels—Multifunctional Materials for Concrete Rehabilitation

Steel reinforced concrete forms a good composite material, as the properties of the individual components complement each other ideally. The steel reinforcement is very ductile and, thus, can strengthen the concrete with its tensile properties. The cement, in turn, protects the steel by passivating the surface, which is due to the high alkaline pH of the concrete [1]. These excellent properties of the composite and the good availability of the compounds leads to the fact that steel reinforced concrete is the most used material in the world after water [2]. The typical lifespan of a steel-reinforced structure is estimated to be about 50–60 years. The main reason for the shortening of this lifetime is the corrosion of the steel reinforcement by two environmental factors. One of them is the carbonation of the concrete [3] and the other is the ingress of chloride ions down to the layer of the steel reinforcement [4]. In the carbonation process, the corrosion is occurring on a large area following a chemical reaction of the concrete with atmospheric carbon dioxide, which reduces the pH value of the cementitious matrix and dissolves the passive layer of the steel [5]. The ingress of chloride is a phenomenon triggered mainly by de-icing salts or sea spray. The dissolved chloride ions diffuse through the porous matrix of the concrete to the steel reinforcement [6]. In this process, the protective passive layer is dissolved only locally, resulting in pitting corrosion. Consequently, the cross-section of the reinforcing steel is reduced, leading to a global failure of the composite [7]. Additionally, cracks can further reduce the lifespan of the affected buildings by increase the ingress of carbon dioxide and chlorides.

Prevention of corrosion and maintenance of existing structures is of major interest in the context of the climate change. To maintain the carbonated structure, the pH value of the concrete is raised over a certain threshold again to prevent the corrosion. This process is called realkalisation. Yet, the drawbacks of these methods are either their high invasive procedure, permanent application, or the quick loss of the alkaline environment after the treatment [8]. For the chloride ingress maintenance, the most commonly used method is the electrochemical chloride extraction (ECE) [9]. In this method, an external electrode is coupled to the steel reinforcement and placed on the concrete surface. Then the steel is cathodically polarized, and chloride is extracted from the surface by the driving voltage of 40–50 V. The high voltage is a result of the poor bonding of the currently used coupling material to the concrete surface. Further disadvantages of this method are that chloride gas generated at the anode can be poorly captured, and the coupling material must be frequently re-watered by hand to maintain conductivity. This leads to high personnel costs for such treatments.

Therefore, in the presented work, a new type of organic compound will be presented that opens novel pathways for structure maintenance. A highly alkaline diallyldimethylammonium hydroxide (DADMAOH) hydrogel consisting of a monomer, a comonomer, and a crosslinker will be demonstrated. It will be shown that there are several methods for producing the DADMAOH monomer and what advantages this charged molecule brings for the maintenance of concrete. In addition, it will be presented how the use of different comonomers and crosslinkers can influence the properties of the resulting gels in order to tune macroscopic properties relevant for applications such as viscosity and stickiness, as well as the four major advantages for use in building rehabilitation: 1. The ability to realkalise carbonated concrete [10], 2. The passivating effect on reinforcing steel, 3. The sealing of water-bearing cracks in concrete [11] and 4. The high conductivity, which makes the material an ideal coupling material for the electrochemical chloride extraction [12]. The latter is demonstrated with a practical field test on chloride‐infested car‐park columns.

The monomer DADMAOH polymerizes radically under ring closure and, due to the cationic charge of the ammonium group, provides a high charge density and thus also a high alkali density in the resulting polymer. To date, there are two working methods to prepare the highly alkaline hydroxide form of the DADMA hydrogel: The first on is ion exchange starting from the commercially available diallyldimethylammonium chloride (DADMAC) solution [10]. However, exchange of the monomer solution without ion exchange resin in a NaOH solution leads to Hoffmann-like decomposition of the ammonium compound to the amine, resulting in loss of cationic charges of the resulting gels. On the other hand, completely polymerized DADMAC gel can be converted to the hydroxide form by equilibration with NaOH solution, as the gel is significantly more stable against hydrolytic decomposition than the monomer [13] (Fig. 1).

A crosslinker is required to bridge the polymer chains to form a swellable gel network. In this context, the gels using N,N-methylenebisacrylamide (BIS) as crosslinker are well documented. However, BIS comes with the disadvantages of low solubility and low alkali resistance. By using tetraallylammonium-based crosslinkers such as tetraallylammonium bromide (TAAB) or tetraallyltrimethylenedipiperidine (TAMPB), much more homogeneous networks can be obtained, due to the similarity of the chemical structure to DADMAOH. This leads to stable gel formation while achieving higher degrees of swelling, even with lower amounts of crosslinker [14]. Furthermore, it can be shown that DADMAOH hydrogels with tetraallylammoinium-based crosslinkers are significantly more resistant to hydrolysis in the alkaline milieu than those with BIS [15]. This is of particular importance for subsequent applications in the construction research.

The third element in the gel formulation is the comonomer. The choice of molecule can be used to control the macroscopic and rheological properties of the resulting gels, such as degree of swelling, adhesion, stiffness, or curing time of the polymerization solution. The changes in properties are mainly due to intramolecular interactions. For example, the use of methylacrylamide allows the formation of hydrogen bonds, while the use of acrylic acid leads to ionic interactions, which additionally physically crosslink the gel network and thus lead to a stiffening of the structure [11].

In contrast to the small hydroxide ions, the polycationic backbone of the hydrogel is immobile, so that it is able to exchange anions with the surrounding medium. This property can be used in the context of maintenance of steel concrete structures. The concrete of these buildings increasingly loses its alkali buffer due to the action of carbon dioxide and water forming calcium carbonate. As a result, the pH of the concrete decreases and the passive layer of the embedded reinforcing steel is destroyed. Common repair methods in this case are the removal of the old concrete and the application of alkaline fresh concrete. In contrast, the DADMAOH hydrogel can be applied to the surface of the carbonated concrete, as shown in Fig. 2a.

The cationic polymer backbone remains on the surface as a stationary phase, while the hydroxide ions can penetrate the concrete by diffusion driven by the concentration gradient. The water required for diffusion is provided by the gel itself, which also fills the pore spaces of the concrete, so that only concretes with a sufficient pore volume are suitable for this application. The carbonate ions of the concrete migrate into the hydrogel according to the gradient. An additional driving force for this is achieved by the high flexibility of the polymer chains, since these are capable to complex the bivalent carbonate ions and thus bind them considerably stronger than the monovalent hydroxide ions. In order to verify this thesis, concrete samples were made from a CEM I with a w/c ratio of 0.6. After 28 days of storage, the samples were transferred to a climate chamber with elevated CO₂ pressure and were fully carbonated after 6 months. The verification of complete carbonation was performed using phenolphthalein according to DIN EN 14630:2007-01. Subsequently, the samples were loaded with gel on the surface and sealed with foil to protect them from the influence of air. After 28 days, a realkalization depth of approx. 1 cm was observed by phenolphthalein test. Samples for thermogravimetric analysis of the concrete were taken from the fresh sample after storage (Fig. 2b red), after carbonation (Fig. 2b black) and after realkalization (Fig. 2b blue). The thermogravimetric analysis was performed between 30 ℃ and 900 ℃ under N₂ and the resulting curves were normalized to the value at 300 ℃ in order to neglect the weight loss of the previously evaporated water. Figure 2b shows a decrease in mass at about 500 ℃ for the uncarbonated fresh concrete, which is due to the decomposition of calcium hydroxide to calcium oxide. Between 650 ℃ and 700 ℃ the mass decrease of calcium carbonate can be observed. After carbonation, the shoulder of calcium hydroxide disappears, whereas significantly more calcium carbonate is found in the sample. By realkalisation with DADMAOH hydrogel, the calcium hydroxide fraction was lowered and a calcium hydroxide content was detected. This allows the conclusion that the gel is able to restore a partial amount of the alkali buffer.

Since high pH values are capable of passivating steel, the DADMAOH hydrogel should also be able to achieve this effect in the case of direct contact, such as cracks in concrete structures. To verify this assumption, an experiment in accordance with DIN EN 480-14 was prepared. Technically, a polarisation experiment with 500 mV for 24 h should be performed in a corrosive environment. This experiment is crucial in civil engineering application, as it shows, whether the used material could potentially corrode the steel reinforcement. Therefore, it was necessary to perform this experiment on the poly(DADMAOH-co-acrylic acid) gels and thereby verify its non-corroding nature. A technical drawing of the experimental setup is shown in Fig. 3a. Mortar cubes with the dimension of 100 × 100 × 100 mm³ were prepared from CEM I 42.5 with a w/c ratio of 0.5. On one side of the samples a mixed-metal oxide coated titanium mesh (MMO) was embedded in a distance of 10 mm to the surface. After curing the cubes for 28 days, a 20 mm hole was drilled in the middle of the sample and filled with Gel. For the potentiostatic experiments, the entire sample was first placed in a bucket of water, since the conductivity of dry mortar is not sufficient. Furthermore, a 3-electrode setup was used. The reference electrode consisted of a MMO mesh, which was positioned in the water outside of the sample (reference electrode and water bath not shown in Fig. 3a). The embedded MMO mesh was used as the counter electrode and an 8 mm diameter steel pin placed in the drill hole and surrounded in DADMAOH gel served as the working electrode. Lastly, the surface of the Gel was covered with silicon oil to prevent moisture exchange with the surrounding air. An open circuit delay was measured to determine the resting potential between the electrodes before the potentiostatic experiments were performed. The results of the potentiostatic measurement can be seen in Fig. 3b [11].

To observe possible corrosion, it was chosen to increase the potential between the electrodes stepwise. Therefore, 100 mV/day steps were used until the potential reached 500 mV. The resulting current must not reach a value above 10 µA to fullfill the requirements of the DIN EN 480-14. During the whole polarization procedure described, no significant current outside the signal-to-noise ratio was detected. Even after 5 days of maximum polarization of 500 mV, all values remain below 5µA. Therefore, it appears that even under these electrochemical conditions, the alkaline environment of the gel prevents the steel from depassivation. This measurement also shows that the remaining chloride ions, which may have been left in the Hydrogel by the ion exchange process of gel production (< 5%), do not corrode steel-reinforced concrete [11].

In addition to the strongly alkaline pH value, the high charge density also offers further advantages: the cationic charges interact very well with surfaces containing silicates, i.e. mainly negatively charged surfaces. The gel is therefore able to adhere to concrete. Furthermore, the gel matrix swells when water is added and could therefore be well suited to seal water-bearing cracks. This was tested according to WTA-Merkblatt 4–6 “sealing of structural elements in contact with soil at a later stage” under moderate impact of pressing water (exposure class W2.1-E) on mortar prisms. For this application mortar prisms were used with dimensions of 160 × 40 × 40 mm³. In order to test for the above mentioned realkalisation feature, the mortar prisms were fully carbonated. To obtain a defined crack, the samples were broken in a 3-point bending test and the two halves were brought back together with a screw rack. The crack was adjusted to a typical water-bearing crack width in cementitious materials of 0.3 ± 0.03 mm. An unpolymerized DADMAOH reaction solution was applied into the crack by using an injection packer and cured there for 7 days. Next, a hose coupling was attached to the samples and then connected to a water reservoir through a tube. Further, the reservoir was put in a height of 5 m to ensure a water pressure difference of around 0.5 bar. The schematic presentation is shown in Fig. 4 [11].

According to WTA procedure the sealing test was performed for 28 days and the rear side of the samples had been observed to check upon the occurrence of leakages. During the whole time, no leakage has occurred for all samples. This observation could be correlated with the gel-blocking effect, which would seal the water leaking crack. After this successful test of the material, the sealed mortar samples were broken and the mortar surface then cautiously wiped off to remove residues of the gel. A phenolphthalein test was performed to examine the mortar for realkalisation. It was obtained that the previously carbonated samples were realkalised for the most part, leaving only the edges untouched, due to the pressureless filling method of the curing solution. If the carbonated sample would have had a steel reinforcement, the injection of the gel, as a maintenance procedure, would be an effective 3-in-1-system to seal, realkalise and passivate the steel reinforcement [11].

The fourth major advantage of DADMAOH gel is its high conductivity. This, together with the good bonding to concrete, enables the gel to serve as an effective coupling material in the electrochemical chloride extraction, by which chloride ions are extracted from the pore system of the concrete using an externally applied voltage. In preliminary laboratory tests, it was shown that with a DADMAOH-co-methacrylamide gel voltages of 1 V are sufficient to obtain extraction of chloride ions with this coupling material, whereas previous commercial methods used 40–50 V. Furthermore, any chlorine gas that is formed at the anode is immediately trapped and bound by the highly alkaline gel, preventing it from leaking into the environment. As a practical application example, a field test was performed on four chloride-infested car-park columns using this novel coupling system. For economic reasons, the gel was mixed with water‐saturated hydroxyethyl cellulose in a 1:2 ratio. The gel was applied to the columns with an MMO mesh and then wrapped with PE film to protect it from drying out (Fig. 5a). The extraction was planned in cycles comprising 21 days of ECE and 7 days of rest. To observe the evolution of the chloride concentration, samples were taken from three depths during the resting periods. In the first cycle the voltage was set to 5 V, to achieve a current density of 1 A/m². As the concrete resistance increased with the application time, due to the continued extraction of charge carriers, the voltage had to be increased to 8 V in the second cycle to maintain the desired current density. After two of these ECE Cycles three of four treated columns were successfully reduced by more than 95% of the original chloride content. The fourth column required an additional 21‐day period, in which the voltage was raised to 15 V, potentially due to a lower porosity and/or lower moisture content. The results of this procedure are shown in Fig. 5b [12].

As already observed during the mortar experiments, the chloride ions close to the surface were first extracted from the concrete during the ECE application. In the first cycle, about 70% of the chloride close to the surface was extracted within 21 days. This was also observed regarding the 20–40 mm profile where the decrease was slightly lower. In the depths close to the reinforcement, the decrease was significantly lower than for the other profiles. Hence, the chloride close to the surface could be extracted more efficiently. In the second cycle, the chloride at the surface was again extracted more efficiently than from deeper layers. Only a small change could be observed for the second profile. In the third cycle, it is clearly seen that the procedure is more efficient than a standard approach. More than 95% of the chloride could be extracted from the columns. Finalizing, the mixture of the gel/hydroxy cellulose could be used in the ECE maintenance while applying voltages of about 1/10 of typical application. Additionally, moistening has not been needed during the process, which significantly reduces the personnel cost [12].

The presented highly alkaline diallyldimethylammonium hydrogel proves to be a true multitool in the field of maintenance. The material was shown to have four fundamental advantages: a) realkalizing carbonated concrete, b) repassivating exposed reinforcing steel, c) sealing water-bearing cracks, and d) electrolytic coupling to concrete surface in electrochemical chloride extraction. Furthermore, due to the chemical structure, a tailor-made material for the respective application can be produced by a clever choice of comonomers such as acrylic acid or methacrylamide. The above-mentioned application possibilities of the material were verified by water leakage test (WTA W2.1-E) and polarization test according to DIN EN 480-14. In addition, electrochemical chloride extraction with the new gel-containing coupling system was successfully performed in a field test on chloride-contaminated parking garage columns. The operating period for these highly alkaline hydrogels in maintenance depends on the area of application. For realkalization, it is recommended to perform the treatment as soon as the covering 1 cm of concrete is completely carbonated. Concrete with a compressive strength class C20/25 reaches this carbonation depth after approx. 7 years, whereas a concrete C30/37 will take approx. 30 years. The crack injection solution should be applied in the case of water-bearing crack formation, while the ECE should be performed in the presence of elevated chloride contents of approx. 4 mass percent.