Technical solutions for WWTP and WTP

Transcription

1 Technical solutions for WWTP and WTP Crack-bridging ability of coatings Iuri André Castelo-Branco Miranda Dias Instituto Superior Técnico, Lisbon, Portugal October 2016 Abstract The concrete structures can have in the course of time deferred behavior. The imposed deformations result from the evolution of the hydration reactions and interaction with the environment. Restricted support conditions or connection to other structural elements (e.g. walls) induce significant axial stresses that, superimposed to the effects of direct loads, can lead to the occurrence of cracking. The assessment of a reservoir service behavior should include the binomial waterproofing solutions/control of cracking in concrete. Failing to follow this evaluation is believed to be the reason for frequent failure in that scope. The aim of the present study was to carry out a survey of the main waterproofing systems currently available on the market to be applied in waste-water treatment plants (WWTP) and water treatment plants (WTP) reservoirs, their associated costs and crack-bridging abilities (CBA), among others. Additionally, it was made an economic assessment, in terms of initial cost, of waterproofing solutions compatible with different cracks width in relation to the solutions without coating, dimensioned in accordance with the permeability control for these structures. The results show that the waterproofing solutions are less economically competitive when compared to solutions without coatings. This inequality decrease as the restrictions imposed deformations increase. However, in the selection process of the most adequate solution, other long-term benefits of the coatings should be considered, such as protection and impermeability against vapors and corrosive products that can accelerate structural degradation. Keywords: Imposed deformations, Behavior of the walls, Waterproofing systems, Crackbridging ability, Cracking, Tanks. 1. Introduction In general, the assessment of the safety at rupture of reinforced concrete structures practically guarantees a null probability of collapse. Therefore, it is mainly necessary to ensure their good service behavior, particularly for reservoirs. In this respect, the structural design, at the level of the definition of the quantities of reinforcement to be used, not only passes by an additional rupture safety check, but also by the 1

2 specific evaluation of safety in service to ensure the functionality requirements. In the case of reservoirs this concerns the limitation of crack opening. One reason for the lower quality of service performance is caused by imposed deformations (indirect actions) in structures that, being hyperstatic, cannot deform freely. The restriction to the free movement of the structure can cause the development of stresses that, when exceed the tensile strength of concrete, give rise to cracking, often transversal to the whole section. The imposed deformations should be mainly considered in the verification of serviceability limit states (S.L.S). In terms of the ultimate limit states (U.L.S), if there is enough plastic deformation capacity in structures, i.e., ductility, the imposed deformation near the rupture will not generate internal forces that interfere with their safety. It is known that a proper structural conception and design allows the minimization of cracking effects from direct and indirect actions. However, this requires, in general, higher reinforcement ratio and, therefore, higher associated costs. This work mainly focuses on the evaluation of possibilities to conjugate waterproofing solutions with the control of concrete cracking, in view of more efficient global solutions. 2. Risk assessment of cracking in reservoirs For the assessment of the good behavior in service of the several structural elements it is necessary to perform control of cracking, taking into account durability requirements, aesthetics and water tightness. The last is particularly important for reservoirs EC 2 - part 1-1 [2] and EC2 part 3 [3] approach According to the EC 2 - Part 1-1 [3], the initial approach of cracks width takes into account the visual appearance and exposure class of the structure in terms of durability (Table 2.1) Table 2.1 Recommended values for crack width, w max (mm) [2] Exposure Class Reinforced members and Prestressed menbers with prestressed members with bonded tendons unbonded tendons Quasi-permanent load combination Frequent load combination X0,XC XC2, XC3, XC XD1, XD2, XS1, XS2, 0.3 XS3, Decompression Note 1: For X0, XC1 exposure classes, crack width has no influence on durability and this limit is set to guarantee acceptable appearance. In the absence of appearance conditions this limit may be relaxed. Note 2: For these exposure classes, in addition, decompression should be checked under the quasipermanent combination of loads. This approach is considered insufficient, because it doesn t take into account the demands of tightness to reservoir functionality. In structures such as reservoirs, in which tightness requirements are important, a distinction should be made between transversal cracks on the whole section or on parts of it. As far as 2

3 transversal cracks are implicated, the recommendations above are not enough to ensure proper functioning. For that case, the EC2 part 3 [3] specific recommendations should be followed. Non-transversal cracks are usually limited and, in principle, do not affect the permeability of structural elements, as long as there is a compressed concrete zone with a thickness of 50 mm or more, or 0.2h (smallest of these two values, considering as zero the tensile strength of the concrete, and h as the thickness of the element). In EC2-part 3 [3] it is recommended that transversal cracks (w k1) should be controlled according to the permeability requirements for a specific type of liquid, hydrostatic pressure (p), and wall thickness (h). For class 1, for example, the formula is: p/h 5 w k1 =0.2 mm; p/h 35 w k1 =0.05 mm; For intermediate values of p/h, may be made a linear interpolation between 0.2 and 0.05 mm as proposed by Figure 2.1. p Figure 2.1 Recommended values for width, w k1 p For classes with higher permeability requirements, all predictable transversal cracks should be avoided unless measurements similar to those as coatings are adopted Minimize the effects of the imposed deformation in reservoirs using waterproofing systems This work is closely linked to crack-bridging ability (CBA) of coatings, and its possible compatibility with different structural solutions at service level (cracking). The information regarding the CBA of the coating fully bound to substrate can be obtained from crackbridging tests, whether by static measuring (EN [7] Method A) (Figure 2.2) or dynamic measurement (EN [7] Method B). Figure 2.2 Test scheme of static crack-bridging ability (Method A) 3

4 In this work the focus will be only on current waterproofing solutions for WTP (water treatment plants) and WWTP (waste-water treatment plants), listed below. Epoxy resins based systems: most of the times with a CBA of reduced expression, little more than 0.1 mm or even no crack-bridging ability provided by their manufacturers. In some cases, the CBA can be increased for values of about 0.5 mm by the inclusion of glass fiber reinforcement. Polyurea based systems: in general, exhibit a good CBA. At static level, this can be greater than mm and, at dynamic level, 0.2 to 0.5 mm of crack movements. Polyurethane based systems: they have ability to resist at good levels of cracking. For these products, the CBA at static level is usually higher than 0.5 mm, and can reach values of mm at positive temperatures. At dynamic level, this coating has a CBA from 0.10 to 0.15 mm of crack movements. Cementitious mortars based systems: at static level they present a CBA of at least mm, and may reach, in most cases, cracks larger than 0.5 mm or even belonging to the highest class. For situations of negative temperatures (-10 C), the CBA of most products are higher than 0.5 mm. At dynamic level, the coatings exhibit an ability to contemplate crack movements from 0.10 to 0.30 mm. 3. Isolated axial imposed deformations An externally imposed deformation (such as temperature variation), is applied to the entire section, both steel and concrete. An internally imposed deformation, as is the case of shrinkage, is only applied to one of the section materials (concrete), while the steel acts a restriction element to the phenomenon. In Figure 3.1 shows a summary of the structural response characteristics of both types of imposed deformations. N Axial Force Fracture Strain Non- Cracked Cracked Fracture Phases Force a) b) Average Strain Figure 3.1 Comparison of results between imposed external (a) [1] and internally deformations (b) [6] 4

5 In a situation of exterior imposed deformation, each new crack due to the elastic response is formed according to a value of axial force near N cr. In the case of internally imposed deformation the resulting axial force needed to form each new crack has a tendency to be less than N cr and is generally smaller than the value of the previous crack. This fact is explained by the restrictive effect of the reinforcement on the shrinkage of the concrete, creating selfbalancing stresses in the non-cracked section, with the concrete in tension and steel in compression. These stresses in the concrete, which increase as the phenomenon of shrinkage occurs, are such that they decrease the reserve needed to attain the resistance stress, and consequently the axial force value for each new crack as well. Studies carried by Camara et al. [5] show that, although the level of stresses in steel is lower for internal imposed deformations (as is the case of the shrinkage of concrete), the crack opening values are of the same order of magnitude as the ones from external imposed deformation. This fact happens because the shrinking of concrete in relation to the reinforcement, in the area between cracks, also contributes to increased opening of cracks. 4. Design Criteria 4.1. Superposition of effects of direct actions with indirect axial actions The overlay of effects (direct actions + indirect actions) should not be obtained through the sum of calculated forces to each isolated actuating action. This is only applicable to indirect actions in non-cracked structures. The forces due to imposed deformations depend on the state of elements stiffness. If these are cracked due to the effect of the vertical loads, the rigidity for the action of the imposed deformation will be lower. Thus, the forces that will be developed due to the imposed deformations will also be lower, as represented in Figure 4.1 for the case of an external and internal imposed deformation. Average Strain Average Strain Figure 4.1 External and internal imposed deformations, with and without superposition of effects [6] In the case of superposition of effects with vertical loads, the axial forces due to imposed deformations in terms of service, should be obtained through a percentage of elastic forces, which can be described as follows, considering the reduction coefficient ξ: 5

6 N id = N id elastic ξ (4.1) Where, N id axial forces induced by imposed deformations; elastic N id elastic axial forces induced by imposed deformations (It should be at most equal to N cr ); ξ global reduction coefficient (ξ = k ξ T + (1 k) ξ cs, com k [0,1]). Camara et al. [4] defined the reduction coefficients, ξ T and ξ cs according to percentage of reinforcement (ρ) and the level of imposed deformation. The estimated values are summarized in Table 4.1: Table 4.1 Reduction coefficients ξ for evaluation of axial imposed deformation effects ξ T ρ [%] Global imposed deformation Shrinkage imposed deformation ξ cs The ξ variation with reinforcement percentages is not very significant. Therefore, Luís [6] suggested a simplified design reference, which consists in taking the bold values on Table 4.1 independently of the steel reinforcement percentage. As a simpler and more direct alternative, Camara et al. [4] propose to consider shrinkage as an equivalent to temperature variation. This situation leads to higher stress levels, while with a reasonable estimation of the crack opening. To do this we need to determine the equivalent temperature range, T eq, which also covers the effect of shrinkage. This is obtained as follows: Where, T eq = T + ε cs α, with α = (4.2) ε cs is the total shrinkage strain of concrete; T is the temperature variation of concrete. Additionally to the axial forces, the bending moments resulting from the imposed deformations were also considered. Although small, these bending moments should not be completely disregarded because they have an expression in the connections between walls. In this work, the bending force was calculated with the reduction coefficient (ξ) as calculated for correspondent axial forces. 6

7 4.2. Methodology of structural design Luís [6] suggested a method of structural design for the situation of superposition of effects of direct actions with indirect axial actions based on elastic analysis. The proposed methodology is summarized in the following steps: 1. Design for Ultimate Limit States (ULS) without consideration of the imposed deformations, ensuring a minimal axial tension reinforcement; 2. Placing of at least the minimum longitudinal reinforcement in areas where restriction effects to the imposed deformations are expected to be important; After defining the distribution of reinforcement, one should perform the stress analysis, taking into account the superposition effect. For this, the following aspects should be followed: 3. For vertical loads use the quasi-permanent combination; 4. To assess the level of axial force generated by the restriction of free shortening apply first the imposed deformations, ie, shrinkage and/or temperature variation in the structural model, with an adjusted modulus of elasticity. 5. Define alignments for section analysis and evaluate areas where the elastic axial force, combined with the bending moment, conducts to cracking; 6. Set the level of reduction of axial force to consider. In areas of the structure where the axial force previously estimated is greater than Ncr use the value of Ncr and apply the reduction factor ξ. In areas where the axial force estimated at point 3 is less than Ncr, apply ξ to that axial force; 7. Evaluate the level of tensions in reinforcement for the pair of forces (N, M) in section with cracks, defining the suitability of the percentage of reinforcement placed in accordance with the regulatory criteria stipulated; 8. According to the results, adjust the longitudinal reinforcement quantities defined in the first phase, in order to limit the cracking for different case studies, in the areas indicated by the analysis. These were the general methodology guidelines applied to the case study described in chapter Case study In this study, the goal was to analyze the behavior of the side walls of a reservoir submitted to indirect actions overlapping with direct actions, as well as the cost variability of different rates of reinforcement for different project crack openings. The study was performed for a rectangular tank, with a configuration as in Figure

8 Main Elevation Side Elevation Figure 5.1 Model of the reservoir in study (without roof) The model reservoir is 40x20 m, with a height of 4 m and a wall thickness of 0.30 m (in orange in Figure 5.1). The reinforced concrete cover (not represented in Figure 5.1 for clarity) has a thickness of 0.22 m. It is supported by beams of 1x0.4 m (in cyan in Figure 5.1) and circular section columns of 0.4 m of diameter (yellow in Figure 5.1). The beams are spaced by 7 m in general, and 6 m from the edges of the reservoir. The bottom slab (see Figure 5.2) was modeled as simply supported, with the soil not imposing any horizontal restriction on slab. In practice, there is always some friction, which can be reduced by making use of specific membranes between the ground and the foundation. S500 E s =200 GPa C35/45 E c =34 GPa Figure 5.2 Cross section between central pillars and materials The study showed that, in terms of ultimate limit states security (U.L.S), the wall reinforcements of the model tank were viable. The reinforcements will be only affected by the service requirements, which, in the case of reservoirs, would be primarily the limitation of cracking. The serviceability limit states (S.L.S.) analysis was performed according the methodology in chapter 4.2, with an adjusted modulus of elasticity of Ec,28/3 (as suggest by Appleton [8]), and a T eq of -50ºC (expression (4.2)). The distribution of elastic axial forces due to imposed deformation in main elevation wall is represented in Figure N x [KN/m] Figure 5.3 Elastic axial force N 11 due to imposed deformations across the main elevation wall (40x4m) 8

9 In order to assess the possibility of defining design criteria of reinforced concrete (control of cracking) conjugated with different waterproofing solutions, it was decided to analyze the horizontal reinforcement needed for four scenarios. In the first scenario, it was only defined the minimum horizontally reinforcement, to observe the obtained crack opening. In the second scenario was evaluated the necessary amount of horizontal reinforcement to comply with the tightness Class 1 of EC2 - Part 3 [3]. In third and fourth scenarios it was evaluated the necessary amount of horizontal reinforcement, to comply with a crack width equal to 0.30 to 0.50 mm, respectively. In Table 5.1 is presented an economic assessment, in terms of initial cost, of waterproofing solutions (chapter 2.2) compatible with different cracks width in relation to the solutions without coating, dimensioned in accordance with the permeability control for these structures. Table 5.1 Economic balance of the solutions using the coatings (in /m 2 ) compared to the 2nd scenario (uncoated) Section Elevation Compatible coatings 4ª scenario (w k 0.5 mm) Epoxy Cementitious mortars Polyurea Polyurethane with reinforcement High C.R. Low C.R. Edge Main Middle Main Edge Side Middle Side Section Elevation 3ª scenario (w k 0.3 mm) Cementitious mortars Compatible coatings < 2ª scenario (w k 0.16 mm and w k 0.19 mm) Epoxy High C.R. Low C.R. without reinforcement Edge Main >18.5 Middle Main >18.5 Edge Side >18.5 Middle Side > Conclusions Note: C.R. Chemical resistance In this study the goal was the evaluation of the behavior of the reinforced concrete reservoir walls resulting from the imposed deformations actions superposed with the direct actions, particularly in terms of their service behavior (cracks openings). As a result was found that the ratio of reinforcement to be used is not directly proportional to the desired crack opening, i.e. a reduction of the crack width by half doesn t mean a requirement of the double of reinforcement rate previously observed. It was observed that the regulatory thickness requirements for reservoirs (EC 2 - Part 3 [3]) without recourse to waterproofing systems, require quantities of reinforcement clearly above the 9

10 minimum, as defined in EC2 - Part 1-1 [2], for zones where there is likely to occur major axial stresses because of imposed deformations. The coatings based on epoxy resin (without reinforcement), in general, are those that have a lower or even no crack-bridging ability (>0.1mm), and the polyurea based systems the largest (>1250mm). Subsequently it was performed a behavioral analysis at the level of crack-bridging ability and possible economic gain, of the different waterproofing solutions when compared to solutions that don t preconized any coating, for comparable tightness. It was found that the solutions with coatings are generally less economically advantageous in terms of construction investment. It is noted that, as the restrictions imposed deformations increase, i.e. in the zones most located on the middle of the side walls, the coatings becoming more economically competitive compared to the uncoated solution. The solutions based on polyurea, polyurethane and epoxy resin, normally by having a higher chemical resistance tend to be more recommended for wastewater treatment plants (WWTP). Cementitious mortars based systems, are the most recommended solutions for water treatment plants (in drinking water reservoirs), although some products based on polyurethane and epoxy resin (without solvents) also may be. It should also be noted that the choice of the solution to be adopted, in addition to its construction cost, should also take into account other benefit that the coatings can provide, such as chemical resistance and effective waterproofing against vapors and corrosive products that can damage the concrete structures, and consequently reduce the life time of the reservoir, among other things. The long term economic analysis was not within the scope of the present project. References [1] Favre, R., Jaccoud, J.-P., Burdet, O., Charif, H. Traité de génie civil, volume 8: Dimensionnement des structures en béton - Aptitude au service et éléments de structures, École Polytechnique Fédérale de Lausanne, Lausanne, 1997; [2] EN Eurocódigo 2: Projecto de estruturas de betão parte 1.1: Regras gerais e regras para edifícios, CEN, March, 2010; [3] EN Eurocódigo 2: Projecto de estruturas de betão parte 3: Silos e Reservatórios, CEN, June, 2006; [4] Camara, J., Luís, R. Structural response and design criteria for imposed deformations superimposed to vertical loads, fib Congress, Naples, 2006; [5] Camara, J., Luís, R. Crack Control for Imposed Deformations, Research Article, Laussane, 2007; [6] Luís, R. Análise e dimensionamento de estruturas de betão com sobreposição de cargas e Deformações Impostas, MSc. Thesis in Structural Engineering. Instituto Superior Técnico. Lisbon, 2005; [7] DRAFT pren Paints and varnishes - Coating materials and coating systems for exterior masonry and concrete Part.7:Determination of crack-bridging properties - Test methods, CEN, August, 2002; [8] Appleton, J. Estruturas de Betão Volume 1, Edições Orion, 2013; 10