A Case Study: Mitigating Leakage in a Water Treatment Plant with Crystallizing Additive Technology
Leonardo Otto Coutinho (1); Luiz Fellipe B. E. Campello (2)
(1) MSc in Transportation Engineering (COPPE/UFRJ), Civil Engineer at Alchemco® Brazil, Civil Engineer (UFRJ), E-mail: leonardo.coutinho@alchemco.com.br
(2) Civil Engineer, E-mail: fellipe@alchemco.com.br
Summary
This article aims to present the results obtained through a visual inspection of a case study conducted at a water treatment plant that used a crystallizing additive in reinforced concrete. During its initial filling, this water treatment plant exhibited several points of water leakage. Over time, visual inspections revealed the formation of crystals that, in many cases, successfully sealed cracks and fissures in the structure. The purpose of this case study is to visually examine the reaction triggered by the use of crystallizing additives, specifically the formation of an insoluble crystalline structure within the pores and capillaries of the concrete, and its capacity to block water penetration through capillary action, microcracks, or larger fissures in the reinforced concrete structure. The study results demonstrated that the crystallizing additive effectively mitigated and blocked water infiltration within the reinforced concrete structure. Over time, the crystallization process evolved, further strengthening the structure's resistance to water penetration. However, some technical limitations of the additive were identified, particularly in addressing cracks that exceeded its performance threshold. Keywords: self-healing concrete, concrete protection, permeability reducer, degradation mechanisms, pathological manifestations.
1 Introduction
The construction industry increasingly faces challenges related to the durability of reinforced concrete structures. These structures are critical for ensuring that buildings and infrastructure can be utilized for longer periods, extending their service life and reducing the frequency of maintenance for industrial, infrastructure, commercial, or residential projects. One of the most significant challenges lies in maintaining the integrity of reinforced concrete structures, which, in many cases, allow water to penetrate their framework, initiating the process of degradation. This water infiltration can occur through capillary and gel pores or, over time, through cracks and fissures that may develop. According to NEVILLE (2011), voids in reinforced concrete structures, such as porosity, defects, discontinuities, and microcracks, greatly influence the strength and durability of these structures. Many authors associate higher compressive strength as a key factor in protecting concrete structures against chemical attacks. METHA (2014) notes that concrete with higher compressive strength can better withstand imposed loads without failure, maintaining the structure's integrity. However, compressive strength should not be the sole factor considered in evaluating the durability of reinforced concrete structures. Other factors, such as the thickness of the concrete cover, the void index of the concrete, and the structure's age, are also significant when analyzing exposure to chemical attacks, as highlighted by HELENE (2003). Additionally, shrinkage of the structure must be carefully evaluated, as it directly impacts cracking caused by stresses induced by restrained volumetric changes due to temperature variations, as noted by NUNES (2007).
Water-related pathologies are among the primary factors contributing to the reduced service life of structures and are a leading cause of maintenance. QUERUZ (2007) points out that, directly or indirectly, water is one of the main agents of degradation in reinforced concrete structures. To address these challenges, several solutions have been developed to protect reinforced concrete structures from the adverse effects of environmental aggressiveness. Among these, permeability-reducing admixtures—also known as crystallizing additives or self-healing concrete—are widely used to protect reinforced concrete structures from the detrimental effects of water ingress. By preventing water infiltration, this solution significantly extends the service life of these structures and reduces the need for future maintenance.
2 Objective and Characteristics of the Crystallizing Additive
Reinforced concrete structures often develop cracks and fissures that, when exposed to water or severe environmental aggressors, accelerate the degradation process, significantly reducing the structure's service life and necessitating early maintenance. The objective of the case study presented in this article is to visually demonstrate how the crystallization process (self-healing) can effectively protect reinforced concrete structures against water penetration and its harmful effects. This process provides enhanced protection and extends the service life of the structure. Several factors contribute to achieving denser reinforced concrete structures with lower porosity. Among these are the water-to-cement (w/c) ratio and the compressive strength of the concrete, where higher compressive strength typically corresponds to lower void content. MOREIRA (2001) establishes a direct correlation between axial compressive strength and resistance to chemical attacks in reinforced concrete structures. Regarding concrete structural designs, NBR 6118 (ABNT, 2014) defines, in its Table 13.4, the characteristic crack width limits to ensure adequate protection and service life for reinforcements. For example, in an environment with environmental aggressiveness class I (CAA I), such as rural or submerged environments, the crack width limit (ELS-W) is set at 0.4 mm. In contrast, for urban environments (CAA II), the limit is reduced to 0.3 mm. It is crucial to assess the environmental aggressiveness of the location where the reinforced concrete structure is being built and take all necessary precautions to ensure that crack openings remain within the limits defined by NBR 6118 (ABNT, 2014). This approach ensures the structure's appropriate service life. The use of permeability-reducing admixtures or crystallizing additives in reinforced concrete structures helps prevent cracks from allowing water ingress into the concrete matrix, thereby mitigating severe structural degradation. Crystallizing additives are powdered products added to concrete either in a concrete mixer truck or at a batching plant, incorporated into the concrete mix before placement. After placement and curing, the active chemical compounds within the crystallizing additive remain dormant until they come into contact with moisture. This moisture may result from water passing through the concrete's porosity, cracks, or fissures. When activated by moisture, these concentrated active chemical compounds react with the water and cement hydration byproducts to form an insoluble crystalline structure within the concrete's pores, voids, and capillaries. This process effectively blocks water passage and ensures the watertightness of the concrete structure.
This material can be used to waterproof structures subjected to both positive and negative pressure. The crystallization process occurs when the active chemical compounds in the crystallizing additive react with moisture and free calcium in the concrete, forming a continuous barrier of insoluble crystals that penetrate deeply into the concrete's capillary structure. This process blocks voids in the concrete, preventing water passage. The speed of crystal formation depends on several factors, such as the void ratio, the amount of cement used, and the water flow within the structure. Complete crystallization can take anywhere from a few days to several weeks. The manufacturer specifies a limitation for the crystallization process regarding the crack width limit (ELS-W), which is a maximum of 0.4 mm, as defined by NBR 6118 (ABNT, 2014). The technical recommendation for using crystallizing additives is to apply them as a supplementary (backup) waterproofing method for reinforced concrete structures, especially in buried structures. While the additive creates a crystalline structure, it does not have the high capacity to resist certain stresses in reinforced concrete structures, particularly those caused by thermal expansion in exposed structures. Therefore, this solution is recommended for environments such as water treatment plants (ETAs), sewage treatment plants (ETEs), tunnels, swimming pools, underground slabs, retaining walls, concrete pipes, elevator shafts, and similar areas.
3 Case Study
The objective of this case study is to assess, through visual inspection, the effects of using a crystallizing additive in reinforced concrete at a Water Treatment Plant (WTP) located within an industrial facility. The concrete structure was poured in April 2022. This visual inspection aims to observe the progression of an insoluble crystalline structure forming in the cracks and fissures of the concrete, effectively blocking water penetration that occurred after the tank was filled. Given that this reservoir is situated in an industrial area, the NBR 6118 (ABNT, 2014) standard specifies two potential crack width limit (ELS-W) classifications based on the level of environmental aggressiveness: CAA III: Industrial area with strong aggressiveness, posing a significant risk of structural deterioration, with an ELS-W limit of 0.3 mm. CAA VI: Industrial area with very strong aggressiveness, posing a high risk of structural deterioration, with an ELS-W limit of 0.2 mm. The WTP was built using concrete with a compressive strength (fck) of 40 MPa and wall thicknesses of 0.50 m. Some walls of the reinforced concrete structure were prestressed. After curing, the structure was filled with water. Notably, no internal waterproofing system was applied to the WTP structure. This case study provides an opportunity to evaluate the performance of crystallizing additives in preventing water penetration and supporting long-term durability under aggressive industrial environmental conditions. After the tank was filled, multiple points of moisture were observed, as the cracks and fissures significantly exceeded the crack width limits (ELS-W) defined by NBR 6118 (ABNT, 2014), with some fissures measuring more than 2.5 mm in width. Given the industrial setting, the tank could not be emptied initially due to its immediate integration into the production process. Any required treatment of these cracks and fissures had to be performed externally. The case study began with visual inspections of the external face of the reinforced concrete structure to monitor the behavior and effectiveness of the crystallization process achieved through the application of a crystallizing additive in the concrete matrix. This case study includes three visual inspections conducted on distinct dates, supported by photographic evidence, to document the progression and performance of the crystallization process. This process occurs through the reaction of the concentrated active chemical compounds in the crystallizing additive with moisture, which originates from water passing through the reinforced concrete structure. On May 30, 2022, the reservoir was fully filled, and on May 31, 2022, the first visual inspection was conducted to evaluate how the reinforced concrete structure of the WTP was behaving regarding water percolation. The findings from this inspection are shown in Image 1.
It was observed on May 31, 2022, that the structure exhibited significant water percolation at various points of the reinforced concrete, primarily due to a high level of cracking. The cracks and fissures varied in size and exceeded the crack width limits (ELS-W) defined by NBR 6118 (ABNT, 2014). According to the technical data sheet provided by the manufacturer of the crystallizing additive used in this project, the material is limited to sealing cracks up to 0.4 mm in width, which aligns with the maximum ELS-W limit established by NBR 6118 (ABNT, 2014). In this case study, cracks exceeding 2.5 mm were identified, making it initially unlikely that the crystallization process would lead to a significant improvement, as these cracks were far beyond the maximum sealing capacity of the additive specified in the manufacturer’s data sheet. As the formation of insoluble crystals resulting from the reaction between the active chemical compounds in the crystallizing additive, moisture, and free calcium in the concrete is a slow process, a waiting period of 38 days was observed. On July 8, 2022, a second visual inspection was conducted to assess the extent of the crystallization process’s progression during this time. The results of this progression can be seen in Image 2.
A significant reduction in water flow can be observed at the points shown in Image 1 compared to those in Image 2, indicating that the crystallization process was progressing steadily. Cracks and fissures were identified where water flow had completely ceased, leaving the areas entirely dry. In these instances, the formed crystals were visibly noticeable within the cracks and fissures. In other cases, as shown in Image 2, a reduction in moisture was evident, although several areas still exhibited substantial amounts of residual moisture. The third visual inspection was conducted 14 days later, on July 22, 2022, to reassess the behavior of water percolation in the reinforced concrete structure and evaluate the progression of the crystallization process during this period. The results are shown in Image 3.
In Image 3, a significant improvement can be observed compared to Image 2, with a substantial reduction in the points of moisture identified 14 days earlier. When compared to Image 1, Image 3 clearly demonstrates how the crystallization process can aid in waterproofing and ensuring watertightness in accordance with NBR 9575 (ABNT, 2010). Since this is a chemical reaction, it requires time for the crystals to fully form. This timeframe varies depending on factors such as crack or fissure width, water flow, and other conditions. In Images 4, 5, and 6, the progression of the crystallization process will be presented, showing how the concentrated active chemical compounds reacted with water and cement hydration byproducts to form insoluble crystals in a specific area of the WTP. The same inspection dates as Images 1, 2, and 3 were maintained to simplify the identification of the evolution and results of the insoluble crystal formation process.
Images 4, 5, and 6 clearly illustrate the effectiveness, importance, and performance of the crystallizing additive in sealing cracks and fissures in a reinforced concrete structure. This solution is increasingly being adopted in such structures to protect and support them against harmful actions, especially in areas of severe environmental aggressiveness. Image 7 shows the outcome of the crystallization process, highlighting a crack filled with crystals formed by the reaction of the crystallizing additive with moisture and free calcium present in the concrete.
4 Conclusion
The speed of crystal formation in the crystallization process is directly influenced by the water flow to which the area is exposed. Higher water flow tends to accelerate the chemical reaction between the active compounds in the crystallizing additive and the free calcium in the concrete. The performance of the crystallization process observed over the 52 days between Image 1 and Image 3 is evident, demonstrating the importance of seeking solutions that enhance the performance of reinforced concrete structures. The technology embedded in the crystallization process allows the structure itself to protect against certain pathological manifestations, such as water percolation, as demonstrated in this case study. Research by LIU (2016) highlights how the use of crystallizing additives in reinforced concrete structures improves durability and performance by reducing water and aggressive agent penetration. Crystallizing additives are essential for reinforced concrete structures, as even when a structure is well-designed and properly constructed, cracks or fissures may still form, accelerating the degradation process. If the structure incorporates a crystallizing additive, these degradation mechanisms can often be mitigated or entirely prevented after the crystallization process is complete. WONG (2015) underscores the self-healing capability of cracks over time. In this study, permeability and compression tests revealed that the mechanisms responsible for reducing water penetration depth in the self-healing of cracks occur primarily through the formation of CSH gel from unreacted clinker grains or through gel formation from magnesium-based additives. Another critical aspect is the crack and fissure filling capacity of the crystallizing additive used. According to the manufacturer's technical data sheet, the material is rated for passive cracks up to 0.4 mm. However, after 52 days with water flow, cracks exceeding 2.5 mm were successfully sealed, surpassing the additive's stated limit by more than six times. Without the use of the crystallizing additive in this WTP, deeper interventions with longer timelines and significantly higher costs would have been required to resolve the extensive construction defects. While some areas of the structure required localized treatment, the crystallizing additive performed efficiently in the vast majority of cases.
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