Computational assessment regarding structural rehabilitation of a reinforced concrete decanter tank

The article describes the analysis of an industrial structure (reinforced concrete decanter tank for the drinking water’s storage in the Iasi – Romania area) in order to justify the proposals for structural rehabilitation to ensure structural stability. In this regard, a computational model was created in Graitec Advanced Design, using the physico-mechanical characteristics of the materials (concrete, reinforcement) from the initial design. Subsequently, using the data obtained from the technical assessment report and the non-destructive tests carried out on the structural elements, it was found a depreciation of the concrete grade (C25/30 → C16/20), at the current stage. At this stage (the structure’s service state) a modelling of the degraded structure was achieved by comparing the results obtained by Graitec Advanced Design and Axis VM, in order to realise an extended structural analysis between initial and current state. A solution for the rehabilitation of degraded elements using polymeric composite materials reinforced with carbon fibers was proposed, which was validated by the Sika CarboDur Composite systems (software program for the design of consolidation solutions).


Introduction
A civil/industrial structure has to fulfil the service, strength and stability requirements throughout the lifecycle, without any significant loss of functionality and unforeseeable maintenance work [1,2].
In the current context, the requirements of strength, stability and safety in use must be handled with particular attention to reinforced concrete construction due to particular operating conditions [6,7].
The operating conditions of the decanter tanks are assessed in accordance with the data collected in relation to the projected technological process taking place inside the basins, the operation of the technological equipment and the characteristics of the aggressive environment inside and outside the basins [3].
Generally, the static actions applied to the decanter tanks during operation, the long-term influence of a series of climatic factors (humidity, variations in temperature) as well as the aggressive action of the environmental agents contribute significantly to the reduction of the service life.

Particularities regarding the static behaviour of the decanter tanks
In order to store liquids and to ensure the structural stability, circular shape is mainly used for reinforced concrete tanks.The radial pressure given by the stored liquid (water) is evenly distributed in the circular sections producing axial forces.
Another factor to be considered for operational safety is represented by the dimensions of the structure, which are limited by deformation and cracking conditions.The specific deformation is independent of the decanter diameter, the risk of cracking being directly proportional to the diameter of the tank [4].The site is regularly checked in regard to stability (levelling measurements on the decanter tank and the emplacement).In case of decanter tanks, the protection against infiltrations will be represented by using an effective waterproofing system.
To assure a structure's proper functionality current operating conditions have to be considered in the structural rehabilitation design stage: environmental conditions, charging / discharging of the decanter tank, pipes protection against corrosion, the utilities functionality.

Case study 3.1. Structural system's description
The analysed construction is a suspension decanter with a volume of 10,000 cubic meters, semi-buried, having a circular shape, within the Chirița Treatment Plant, Iasi city (Figure 1 The constructive structure of the decanter is in the form of a reinforced concrete basin divided into the reaction chamber and the clarifying compartment (Figure 2).The decanter's basin consists of the following structural elements: perimeter walls (1), raft foundation (2), the central structure (having the role to support the central rotor with pallets and delimiting the mixing spaces (3) and the supporting beam of the scrapping bridge (4).

Numerical analysis by Finite Element Method of the decanter in initial stage
The structures analysis (static, dynamic) in general, and of decanter tanks in particular, has been carried out using the Finite Element Method (F.E.M.).The use of F.E.M. allows the determination of stress and displacements for all load categories, considering special structural shapes [4,5].
For decanter tanks, the static analysis aims to static linear analysis, static nonlinear analysis and identification of the failure mechanism and safety factor assessment; For the considered case study, linear static analysis was performed.At the initial stage of the analysis, the mechanical characteristics of the materials (concrete, reinforcements) were collected from the technical project (Table 1): The studied structure was modelled and analysed using the Graitec Advanced Design software program, and the static linear analysis was performed to calculate the maximum internal forces according to the next structural model (Figure 3).The considered structural model was achieved by using bar-type elements.Reinforced concrete columns were considered to be embedded at the foundation's level, respectively hinged at the circular beam's level.
The circular beam (the columns supporting beam of the scrapping bridge) was calculated taking into account two hypotheses: -continuous beam (to obtain the maximum values of the bending moment in the support); -hinged beam (to obtain the maximum values of the bending moment in the field).The reinforcement of the structural elements section in the computer program using the data from the initial project was carried out for the columns (Fig. 4) and the circular beam respectively (Figure 5).The accuracy of the calculation depends on the considered structural model, which must be in accordance with the static scheme (nature of the links between the component elements).The loads involved in calculating the structure of the supporting beam of the scrapping bridge are: -self-weight of the structure (circular beam, columns, perimeter wall, raft foundation, stored water); -self-weight of pipes (supported by columns); -self-weight of the scrapping bridge; -seismic loading (it is considered that the scrapping bridge is stationary).
After performing the linear static analysis, the following values of the internal forces were obtained (Figure 6 ÷ 12, Table 2)  62.16

Visual inspection and identification of degradations
Following the in-situ inspection, at the level of the rolling frame of the scrapping bridge, the following degradations / nonconformities of the structural elements were found: -concrete surfaces with areas where concrete is segregated and exfoliated / delaminated (Figure 13  From the analysis of the degradations, it can be concluded that they are caused mainly by the action of aggressive environmental agents, water in the decanter and the efforts resulting from the braking of the scrapping bridge, as well as due to possible execution defects (reinforcements insufficient coverage, the use or inefficient waterproofing system).
In this respect, in order to ensure the requirements of mechanical strength and stability, a series of non-destructive tests (sclerometer test) were carried out on the structural elements (columns, respectively circular beam) to determine the mechanical characteristics of the supporting beam of the scrapping bridge (current stage), the resulting values for the concrete grade being corresponding to the C16/20 class (decreasing compared to the value of the concrete's class in the initial project -C25/30).
Also, due to chlorinated water actions from the interior surface of the structural elements, the concrete was degraded due to carbonation.

Comparative structural analysis for degraded structure
The linear static analysis was performed according to the same structural model (Figure 14) in two software programs (Graitec Advanced Design and Axis VM) in order to realise an extended structural analysis between initial and current state.Thus, the linear analysis by two software programs could provide more accurate results.3.In this regard, the numerical values obtained in the actual stage (using two software programs), taking into account the analysis results, structural rehabilitation is highly recommended.

Structural rehabilitation using CFRP materials 4.1. General remarks regarding retrofitting solutions
Structural rehabilitation is the main component in terms of increasing the safety in operation and the sustainability characteristics of these types of structures.
The causes that generate the state of degradation of the special water storage structures (design / execution errors, accidental actions, lack of current maintenance and repair works, the quality of the materials used, aggressive environmental agents, the long service life, etc.), can ultimately lead to the accelerated degradation of the structural elements and hence the transfer from the service state of the structure (the state of operation of the structure) to the non-compliant state.
The structural rehabilitation solutions used can be the classic ones (surfaces jacketing using special mortars -grouting) or by using modern materials of CFRP type (polymeric composite reinforced with fibers).
For reinforced concrete structures, in addition to composite materials, there are used other available methods of rehabilitation, for example: • Shotcrete (sprayed concrete) consolidation: This method involves applying a layer of sprayed concrete onto the existing structure.This additional layer of concrete improves the load-bearing capacity and crack resistance of the structure.• Chemical anchors: Consolidation of reinforced concrete structures using chemical anchors involves injecting a chemical material into drilled holes, which solidifies and creates a bond between the existing concrete and additional elements such as bars or plates.• Injection of epoxy resins: This method entails injecting epoxy resins into cracks and voids in the concrete structure to enhance strength and waterproofing.The resins penetrate the pores and cracks of the concrete, solidify, and reinforce the structure, reducing the risk of further cracking and water infiltration.• Surface restoration and protection: Various techniques can be used to repair deteriorated surfaces of the concrete structure, such as repairing with polymer mortars or applying surface protection systems like paints or waterproof coatings.These methods help restore the integrity and aesthetic appearance of the concrete and protect the structure from harsh environmental factors.It is important to evaluate and choose the appropriate method based on the type and degree of deterioration of the reinforced concrete structure, as well as the specific needs of the rehabilitation project.Consequently, when it comes to rehabilitating reinforced concrete structures, there are several methods available beyond composite materials.Each method offers unique advantages and is suitable for different types and extents of damage.The choice of rehabilitation method should be based on a thorough assessment of the structural condition and the specific requirements of the project.
Materials such as shotcrete, chemical anchors, and epoxy resins provide effective solutions for enhancing the load-bearing capacity, crack resistance, and waterproofing properties of reinforced concrete structures.These methods address both structural integrity and durability concerns, ensuring the long-term performance of the rehabilitated structure.
Furthermore, surface restoration and protection techniques play a vital role in preserving the aesthetics and safeguarding the concrete from environmental degradation.By repairing damaged surfaces and applying protective coatings, the structure's appearance is restored, and its resistance to aggressive factors like moisture, chemicals, and weathering is improved.
Ultimately, the success of a rehabilitation project for reinforced concrete structures relies on selecting the most suitable method and implementing it with proper expertise and quality materials.This ensures the safety, and functionality of the structure, prolonging its service life and minimizing the need for future interventions.
CFRP materials are multiphase systems, obtained artificially, by the association of at least two chemically distinct materials, with a clear separation interface between the components and the resulting compound material is created in order to obtain properties that cannot be obtained by any of the components working individually [9,10] The use of modern consolidation systems with polymeric composite materials reinforced with fibers in the structural rehabilitation of reinforced concrete elements and / or structures has well-known and documented advantages, such as: resistance to the action of corrosive agents (chlorine ions), high durability, superior mechanical characteristics (compared to classical materials), reduced execution time, etc.
In this case, in order to comply with the requirements of mechanical strength and stability, it was chosen the consolidation systems with polymeric composite materials reinforced with carbon fibers, using the SIKA CarboDur automatic calculation program.

Numerical calculus using SIKA CarboDur software program
Sika® Carbodur® FRP Design is a software program for designing consolidation solutions using Sika CarboDur Composite systems to increase the bending and shear capacity, respectively the sealing of the reinforced concrete structures and structural elements itself.
Following the model carried out by F.E.M. and the selection of the chosen structural rehabilitation method, the solution for consolidating the structural elements made of reinforced concrete (columns, circular beam) at bending and shearing by having discrete strips and continuous jacketing with CFRP was calculated.In both situations, it was opted for the use of carbon fiber fabric.

Columns
In the case of pillar's reinforcement (400 x 400 mm) the following calculation efforts (Table 4) were taken into account, respectively the capable efforts (Table 5) for the unconsolidated structure.For the pillar's consolidation, carbon fiber reinforced polymeric composite (CFRP) was used, having the following mechanical characteristics (Table 6).As a result of the calculation performed, the following numerical values for the bending check (Table 7) and the shear (Table 8) respectively resulted.To increase the bending load-bearing capacity (bending moment) two layers of unidirectional strip arranged longitudinally on each side of the pillar over the entire height of the element will be used (Figure 22 -blue colour).In case of increasing the load-bearing shear capacity (shear force), a layer of unidirectional strip with a width of 150 mm at the interaxial distance of 300 mm perpendicular to the axis of the column, arranged in the form of discrete strips, shall be used (Figure 22 -red colour);

Circular beam
In the case of the circular beam's reinforcement (600 x 350 mm), the following calculation efforts (Table 9) were taken into account, respectively the capable efforts (Table 10) for the unconsolidated structure.For columns consolidation, the following system of composite material reinforced with carbon fiber was used, having the following mechanical characteristics (Table 11).] As a result of the calculation performed, the following numerical values obtained for the bending consolidation (Table 12) and the shear consolidation (Table 13) respectively for both situations resulted, considering a minimum capacity increase imposed of 70% (the effective consolidation increase being of 85%).Based on the numerical values obtained, the consolidation of the circular beam will be carried out with polymeric composite materials on reinforced with carbon fibers as is shown in Figure 23.
In order to increase the bending load-bearing capacity (bending moment), a layer of unidirectional strip, connected longitudinally at the beam's bottom (Figure 23 -blue colour) will be used, and to increase the bearing capacity at shear (shear force), a layer of unidirectional strip will be used, connected in a transverse direction on three sides (bottom and the lateral faces of the beam) in continuous U-shape strips (Figure 23 -red colour).

Final remarks
In the current stage, the results obtained using two software programs shows that the most stressed parts of the structural elements are common in both models (Table 14), and based on the numerical values obtained, the structure have to be rehabilitated with CFRP systems.Taking in consideration that each FEM software program has its proper manner to introduce the structural model corroborated with the fact there are very small differences in the resulting values (less than 5%), this research can be suitable for the linear static analysis of the reinforced concrete structures.
In the same time, obtained results shows that both computer programs are compatible in order to be used for the structural analysis of the reinforced concrete water tank, assuming that are realised from homogeneous material.
A proper numerical analysis based on conclusive data correlated with the detailed investigation of durability factors (destructive and / or non-destructive tests, in-situ and in authorised laboratories) represents an effective way to simulate the degradations level and to ensure a safe service of the structure.

Figure 1 .
.a, 1.b, 1.c).The decanter was put into operation in 1994, as a result of a project to extend the treatment plant.a).Side view b).Top view c).General view of the structural elements (including scrapping bridge) Decanter tank no. 2 -Chirița treatment plant

Figure 3 .
Figure 3. Structural model -the supporting beam of the scrapping bridge

Figure 6 . 7 .Figure 8 . 6 Figure 10 .Figure 11 .
Figure 6.Min, max envelope -Axial force Nx Figure 7. Min, max envelope -Shear force Vy .a); -the presence of cracks in the supporting columns of the supporting beam on the opposite side of the running direction of the scrapping bridge; the cracks were identified next to the longitudinal and transverse reinforcements, respectively, having openings of 1-2 mm (Figure 13.b); -delamination of the concrete coating, respectively uncovered and / or corroded reinforcement bars (Figure 13.c).

Figure 13 .
Degradations to the supporting beam of the scrapping bridge

Table 2 .
Values of the internal forces -initial stage Structural element Internal effort Maximum value Columns Axial force Nx [KN] 137.24Shear force Vy [KN] 16.33 Shear force Vz [KN]

Table 3 .
Maximum values -actual stage

Table 5 .
Shear and bending capacity -unrehabilitated structure

Table 7 .
Design moments

Table 8 .
Design shear force Thickness of the carbon fiber tf = 0.172[mm]

Table 10 .
Shear and bending capacity -unrehabilitated structure

Table 11 .
Mechanical characteristics of the CFRP system 2

Table 12 .
Design moments