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cement ratio in concretes exposed in sewage treatment plants
Construction and Building Materials 229 (2019) 116842
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Influence of the cement type and water/cement ratio in concretes exposed in sewage treatment plants Ana Paula Brandão Capraro a,⇑, Marcos Antonio Cheremeta b, Marcos Paulo Galvão Gonçalves b, Claiton Cremonez b, Marcelo Henrique Farias de Medeiros a a b
UFPR – Programa de Pós-Graduação em Engenharia de Construção Civil, Av. Cel. Francisco H. Dos Santos, 100, CEP: 81530-000 Curitiba, PR, Brazil UNIFACEAR, Av. Thomaz Coelho, CEP: 83707-067 Araucária, PR, Brazil
h i g h l i g h t s Chemical, physical and mechanical properties of produced specimens were evaluated. A year of exposure was enough to identify mechanisms of gases attack. The cement CP V was more susceptible to the attack of sewage gases, tested in compression strength, SEM and XRD tests. The SEM images showed the presence of deleterious products, like ettringite. At the end of the study all the series presented visible signs of degradation.
a r t i c l e
i n f o
Article history: Received 7 June 2019 Received in revised form 5 August 2019 Accepted 31 August 2019
Keywords: Concrete Sewage treatment plant Disintegration Compressive strength drop Deleterious products
a b s t r a c t The objective of this paper is to evaluate the influence of the cement type (CP V, CP II and CP IV) and water/cement radio (0.40, 0.50 and 0.65) in concretes exposed in sewage treatment plant. The parameters monitored for one year were: compressive strength, pulse velocity, electrical resistivity, SEM e XRD. Some characteristics of aggressiveness, such a disintegration of part of the concrete, became visible in the visual inspections. The 0.65 water/cement ratio series, for all cements, was the only one to present compressive strength drop. The SEM and XRD proved the occurrence of the attack, with the identification of deleterious products. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Concrete corrosion is one of the most significant failure mechanisms of sewer pipes and can reduce the sewer service life significantly. To facilitate the management and maintenance of sewers, it is essential to obtain reliable prediction of the expected service life of sewers, especially if that is based on limited environmental conditions [1]. Corrosion causes the loss of concrete mass and structural capacity, eventually leading to ultimate structural collapse [2]. The rehabilitation and replacement of damaged structures sewers cost, in the USA, approximately $14 billion per year [3], in Germany, runs to about 100 billion US$ [4], and tend to increase service life he aging of the infrastructure [5].
⇑ Corresponding author. E-mail addresses: [email protected] (A.P.B. Capraro), macheremeta@ sanepar.com.br (M.A. Cheremeta). https://doi.org/10.1016/j.conbuildmat.2019.116842 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
According to Stanaszek-Tomal and Fiertak [6], treatment plants are hardly ever completely filled with sewage, and as a result there is a lot of space above the water line for bacterial development and growth and accumulation of gas products from sewage decomposition. The most commonly occurring bacteria in sewage treatment systems include bacteria of the genus Thiobacillus, those grow and develop best at 25–35 °C. They are able to oxidize sulphide, elemental sulphur, thiosulphate and polythionates. Bacteria of the Thiobacillus species can be divided into two groups. Group one consists of those that only grow at neutral pH values, those are responsible for the conversion of the elemental sulphur to sulphuric acid. The other group grow at lower pH values and are able to use Fe2+ as an electron donor. In this other group, a representative of which is the Thiobacillus thiooxidans species, of great significance is the growth at acidic pH values. They develop best at pH from 2 to 5 and in under aerobic conditions. Thiobacillus intermedius is most active at pH in the range from 3 to 7, and oxidizes thiosulphate ions
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(S2O23 ). Thiobacillus ferrooxidans occurs in aerobic conditions and has a range of pH growth from 1.5 to 5 [7]. The presence of sulfur compounds and sulfate reducing bacteria (desulfatation), in the anaerobic environment, leads to the production of hydrogen sulfide (H2S) and carbon dioxide (CO2). Hydrogen sulfide, when released from the effluent in the form of hydrogen sulphide (H2S), reacts with the oxygen of atmospheric air (abiotically), forming water (H2O) and sulfur compounds (S2O-2 3 ; S4O6-2; SO-2 4 ). The other portion of hydrogen sulfide (H2S) (by acid attack) and carbon dioxide (CO2) (by carbonation), dissolve in the water contained in the pores of the concrete, gradually reducing the pH of the hydrated matrix [8]. The term biogenous sulfuric attack is used for processes in sewerage systems where microorganisms form sulfuric acid (H2SO4) and cause the cement paste matrix to be attacked until it dissolves [4]. According to Grengg et al. [9] the gaseous compounds produced are liberated into the atmosphere of the concrete pipes and manholes, where they partly dissolve into the condensates along the concrete walls and subsequently diffuse into the concrete pore structure. These conditions, added to high temperatures, humidity, pH, nutrients and CO2 availability facilitated the degradation in cementitious materials [10]. In order to prevent the attack on the concrete subject to the sewage treatment plants, the Brazilian concrete standards (NBR 6118 and NBR 12655) [11,12] presents minimum requirements for concretes to be used for this purpose. According to the standards, the classification of concrete exposed to sewage treatment plants is the most aggressive, class IV, with the requirements: water/cement 0.45, compressive strength at 28 days 40 MPa, minimum covering for reinforcement slab equal to 45 mm and minimum consumption of cement of 360 kg/m3. Although the limitations presented by the standard, there is still little information about the influence of the type of cement and the use of additions on the concrete of sewage treatment plants. However, researches such as Baldermann et al. [13], indicate the use of replacement cement by supplementary cementitious materials (SCM) as an alternative to durability in sewage treatment plants. Therefore, the study of the different types of cement, as well as different types of additions, can aid in the study of performance of structures subject to sewage treatment plants. In a city in Brazil, in 2016, the covered slab of the anaerobic fluidized sludge reactor (AFSR), with 11 years old, collapsed, during the maintenance of the structures of the sewage treatment plant. The covered slab is shown in Fig. 1, before the collapse (A) and after the collapse (B). Situations such as the one presented above highlight the need for studies in the area, since collapses in these structures lead to a high cost of repair in addition to paralyzing activities, which are essential to the population.
Moradian et al. [14] emphasize the importance of concrete degradation analysis through different parameters of durability, such as chloride content, water absorption, compressive strength, chemical constituents (SEM and XRD), carbonation and corrosion. The objective of this work was to compare the properties, chemical constituents, pulse velocity, electrical resistivity and compression strength, of different exposed concretes in an AFSR. For this, three Brazilian commercial cements, allowed per norm for the studied use, and three water / cement ratios were chosen. In addition, the study evaluated the influence of the replacement of 10% of cement with active silica, since the literature reports the use of additions as mitigation to the attack. 2. Experimental 2.1. Materials and mixture characteristics Three cement were used in the research: CP V ARI – NBR 16,697 [15] equivalent to Type III – ASTM C150 [16], CP II F – NBR 16,697 [15] equivalent to Type II – ASTM C150 [16] and CP IV – NBR 16,697 [15] equivalent to Type IV – ASTM C150 [16]. The CP V, CP II and CP IV cements have specific mass of 3.12 g/cm3, 3.11 g/ cm3 and 2.82 g/cm3, and Blaine specific surface area of 4078 cm2/ g, 3398 cm2/g and 4290 cm2/g, respectively. Although the three cements mentioned are used for different purposes (Type III for high initial strengths, Type IV for slower reactions, Type II for general use, especially when moderate resistance to sulphate or moderate heat of hydration is desired), Brazilian standards, NBR 6118 and NBR 12,655 [11,12], do not establish a specific type of cement for use in sewage treatment plants. The considerations of the standards are regarding the minimum strength, maximum water/cement ratio and minimum cement consumption, leaving the designer to choose the cement. Evaluating the fineness requirement of the cements studied, by NBR 16,697 [15], note that the type III has the most finer particles, followed by the Type IV and then Type II. The difference between the cements is related to the milling process and the additions used, which interfere with the packing of the particles and the speed of the hydration reactions. Chemical composition of three Portland cement were determined by X-ray fluorescence (XRF) and presented in Table 1. Part of the studied groups had 10% of the cement replaced by silica fume, to evaluate the filler effect, associated with the pozzolanic effect of high reactivity. The silica fume, because of the pozzolanic reaction, fills the voids and pores, reducing the porosity of composites [17], and this may be an interesting effect to amortize the gas attack. The silica fume particles are spherical, vitreous, have an average diameter of 0.20 mm, specific surface area between 15,000 and 25,000 m2/kg and specific mass 2200 kg/m3.
Fig. 1. Covered slab of the anaerobic fluidized sludge reactor. A – Before de collapse; B- After the collapse with corrosion signs.
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A.P.B. Capraro et al. / Construction and Building Materials 229 (2019) 116842 Table 1 Chemical characterization (%) of three cements.
CaO SiO2 Al2O3 Fe2O3 MgO SO3 Free CaO Ignition loss Insoluble waste Alkaline equivalent
CP V
CP II
CP IV
60.89 18.91 4.39 2.69 4.74 2.86 0.99 2.99 0.92 0.62
60.47 19.19 4.07 2.59 4.79 2.62 1.06 3.32 1.28 0.61
45.12 28.94 9.97 3.72 3.18 2.49 0.74 3.34 1.45 1.05
The fine aggregate was obtained from the mixture of 75% of medium natural sand and 25% of fine natural sand. The coarse aggregates were obtained from the mixture of 15% of 12.5 mm gravel and 85% of 19.0 mm gravel, from dolomitic origin. The Table 2 shows the characteristics of the aggregates. The characteristics of the aggregates were obtained in accordance of ASTM C128 [18], ASTM C127 [19] and NBR NM 46 [20]. As the water/cement radio was a varied parameter it was necessary to use a superplasticizer to standardize the slump (100 ± 20 mm) of the mixtures. The plasticizer used was the MasterPolyheeed, composed of substances which act as dispersants of the material agglomerate, providing a high reduction of without changing the holding time of the concrete, with has a specific mass of 1.07 g/cm3. Given the range suggested by the manufacturer, the content employed was 0.9% with respect to the mass of the binder. The reference concrete mix proportion was adopted in compliance with the requirements of NBR 6118 [11] (compressive strength at 28 days 40 MPa, cement consumption 360 kg/m3 and water/cement ratio 0.45). Two water/cement ratio were also adopted for other two series, higher than allowed by the standard (0.50 and 0.65). The fixed parameters between the mix proportion were the mortar content equal to 52% and the slump equal to 100 ± 20 mm. The amount of water in the mixtures was corrected, taking into account the humidity of the aggregates and the water portion of the additive used. Six mix proportion were employed (Table 3) for each of the three cements, resulting in 18 different mixtures. The group Ref. (0.40) refers to the series in accordance with the standard. The (0.50) and (0.65) were the groups with water/cement ratio equal 0.50 and 0.65, respectively. The groups with ‘‘+S” refer
to the series with replacement of cement by silica fume. Table 3 shows that cement consumption is not constant for the groups, since the water/cement ratio was an altered parameter. Thus, this work will not evaluate the compressive strength of the series in a comparative way between them, only of them with themselves over the time. 2.2. Experimental procedure 2.2.1. Specimen preparation For the compressive strength, pulses velocity and electrical resistivity tests, cylindrical samples with 10 cm of diameter and 20 cm of height were molded, in conformity with NBR 5738 [21]. To statistically analyze the results, using the Tukey test (95% confidence), 3 specimens were molded for each group at each age analyzed. For the SEM and XRD tests, fragments of the cylindrical specimens exposed to an aggressive condition were used, which were removed after the compression strength test. The concretes were prepared following the recommendations of NBR 12,655 [12]. After the mixture, the concrete was poured in the molds gradually, in three steps. After each step, the material was manually compacted, with a compacting rod. The specimens were demolded 24 h later. After that they were submerged in lime saturated water, for 28 days. This was adopted to produce an advanced hydrated condition for the concretes. Altogether, 378 specimens were molded, considering 6 mix proportion for each type of cement, 4 ages, 3 specimens for statistical analysis and 2 exposure conditions. 2.2.2. Exposure conditions After 28 days, in submerged cure time, the specimens were separated into two exposure conditions, a reference, which consisted of remaining half of them in a tank with water saturated with lime, and in the aggressive condition exposure (exposed in the sewage treatment plant). Fig. 2 shows the two adopted conditions, reference exposure (Fig. 2A) and aggressive conditions (Fig. 2B). The gas chamber was chosen for accommodation of the specimens because the simultaneous presence of deleterious agents to the concrete (gases) and humidity, essential conditions to the attack [10]. For accommodation of the specimens HDPE boxes were fixed with stainless steel bolts (Fig. 3). The materials were chosen because of the resistance and durability under the conditions of exposure to the sun and gases.
Table 2 Characteristics of the aggregates. Aggregates
Fineness module
Clay content (%)
Specific mass (g/cm3)
Water absorption (%)
Content of powdery material (%)
Medium natural sand Fine natural sand 12.5 mm gravel 19.0 mm gravel
2.27 1.38 5.90 7.04
0.00 0.00 0.00 0.00
2.63 2.82 2.75 2.77
0.40 0.70 0.40 0.20
8.90 4.10 3.30 2.06
Table 3 Mix proportions (m3) for each cement (CP V, CP II and CP IV). Materials (kg/m3)
Ref. (0.40)
(0.50)
(0.65)
Ref. (0.40) + S
(0.50) + S
(0.65) + S
Cement Silica fume Medium natural sand Fine natural sand 12.5 mm gravel 19.0 mm gravel Water
425 – 511 219 320 746 170
340 – 563 241 320 746 170
261 – 612 262 320 746 170
382.5 42.5 511 219 320 746 170
306 34 563 241 320 746 170
234.9 26.1 612 262 320 746 170
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Fig. 2. Exposure conditions adopted for the study. A – Standard exposure – tank with lime saturated water; B- Aggressive exposure – Gas chamber of the anaerobic fluidized sludge reactor.
of them, for 24 h, in pure ethyl alcohol and later in the permanence of them, for another 24 h, in an oven at 40 °C. The procedure followed that described by Pan et al. [25]. Microscopy images were obtained with a Tescan SEM FEG microscope, Mira 3 and Oxford X-Max 50 X-ray analytical microprobe (EDS). First, the samples were glued to the stub with graphite enamel, for identification of the elements by EDS. Afterwards, they were removed from the equipment, metalized with gold, and the images were obtained. The metallization was not carried out from the beginning to avoid overlapping of elements during the obtaining of the EDS spectra. The XRD analysis were carried out with powder samples, obtained by spraying the concrete fragments. The equipment used for the test is from the brand AMTU and its power is equal to 40 kV/30 mA. The reading was performed between the positions 5 ° 2h and 75 ° 2Th, with step size of 0.0170 (°2h) and scan step size of 40 (s). In parallel to the tests, visual inspections were carried out in the samples, in order to direct the SEM and XRD tests to the cases of greater degradation. 3. Results and discussion
Fig. 3. Specimens accommodated in the gas chamber of the anaerobic fluidized sludge reactor.
2.2.3. Performed tests For 360 days, the cylindrical specimens of concrete were evaluated by laboratory tests, compressive strength, electrical resistivity and ultrasonic velocity. In all of them the specimens were tested on the saturated dry surface condition. The analysis ages were: 28, 84, 168 and 360 days. The first tests to be performed were pulse velocity and electrical resistivity, because they were non-destructive, and then, the compressive strength one. For the pulse velocity test, Proceq Ultrasonic Pulse Velocity – Pundit Lab was used. The 54 kHz transducer was used. The test was performed in accordance with ASTM C597 [22]. The equipment used for the electrical resistance analysis, Proceq Resipod, has the operating principle of the four Wenner electrodes. The test consists of placing four electrodes on the surface of the test specimens, where they are located and equidistant from the others. The test was carried out in accordance with the Spanish standard [23]. After that, compression strength test was performed, in accordance with NBR 5739 [24], in an EMIC hydraulic compression machine with load capacity equal to 100tf. Samples used for the SEM and XRD tests underwent a stoppage process at the test ages (168 and 360 days for the SEM test and 360 days for the XRD test). The process consisted in the immersion
3.1. Visual analysis At 168 days, it was possible to notice stains and occasional disintegrations in all series, Fig. 4 (A). The stains could be found more easily in the bases and tops of the specimens, because they are places with greater availability of gases and humidity. At 360 days, the degradation can be observed throughout the entire specimens, being more evident for the series with water/ cement ratio equal to 0.65 (Fig. 4 B). 3.2. SEM and XRD analysis According to Sulikowski and Kozubal [26], the sulphur oxidation by Thiobacilus thiooxidans, that occurs in sewage treatment plants, produced acid attacks to the calcium hydroxide, resulting in the gipsite formation. The gipsite at crystallization increases its volume by 130%, moreover, it can react with tricalcium aluminate creating ettringite. Because they are consequences of the attack, the identification of gipsite and ettringite crystals in cementitious compounds in advanced ages may have proved the event of deleterious reactions. Already with 168 days of the materials (140 days of exposure in sewage), the identification of crystals similar to gipsite and ettringite in all series with water/cement ratio equal to 0.65 (the most critical), without silica fume, was possible. Fig. 5 shows the SEM images for these three series.
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Fig. 4. Specimens conditions during the study. A – Stains in the bases and tops of all specimens at 168 days of age; B – Specimens degradation in series with water/cement ratio 0.65 at 360 days of age.
Fig. 5. SEM images obtained of the 0.65 water/cement ratio series, without silica fume, at 168 days of age. A – cement CP V; B – cement CP II; C – cement CP IV.
Fig. 5 A presents the group with cement CP V (0.65), and in the image, it is possible to see the pore filling with acicular crystals (indicated by the arrow), identified by EDS analysis as ettringite crystals. The presence of this crystal, in that quantity, at 168 days of age of the samples, indicates the occurrence of the attack, because at that age, without an external source of sulphate, a healthy concrete would have its entire ettringite transformed in monosulfate hydrate [27]. Fig. 5 B shows the group with cement CP II (0.65), and in the image it is possible to see the pore filling with some acicular crystals (ettringite). The group with cement CP IV (0.65), Fig. 5 C, also presents, in a punctual way, the presence of ettringite. However, in this series a more irregular aspect of the matrix and some circular crystals, similar to gipsite, were observed. The presence of gipsite and even occasional presence of ettringite shows, compared to the previous cement, that this group still was at an early stage of the attack. The irregular aspect and the early stage of the attack can be explained by the use of cement with a high content of pozzolanic additions (up to 50% of fly ash), which has slower hydration reactions [28]. As the series with CP V and CP II were the ones, at 168 days, that presented more crystals of ettringite and easier to find them, the hypothesis of greater susceptibility of these cements to the attack raises, since these types have more quantity of Portlandite available. At 360 days of age, a great advance in the attack was observed, as well as the one observed in the visual analysis. The series with CP V and CP II (Fig. 6 A and B, respectively) presented deposition of products with irregular appearance, identified as EDS with great amount of calcium and sulfur (Table 4), confirming the occurrence of the attack. Also, at this age, the two series, CP V (0.65) and CP II
(0.65) without silica fume, presented cracks in the matrix (indicated by the arrows), providing the greater fragility of the material. The CP IV (0.65) series continued to indicate the presence of deleterious products, with the appearance of gipsite (Fig. 6 C), in the pores, as evidenced by EDS (Table 4). However, the observed degree of filling at 360 days was much higher than the one observed at 168 days. The XRD analysis were performed on the same SEM samples at the age of 360 days. Fig. 7 A, B and C show, respectively, the diffractograms obtained for the series CP V (0.65), CP II (0.65) and CP IV (0.65). The behavior of the CP V and CP II series were similar, as observe in the SEM analyzes. The main products found for theses series were quartz (SiO2) and basanite (2CaSO4H2O). Basanite is the main constituent of thermally treated sewage sludge [29]. Other researches, such as Li et al. [30] and Huang et al. [31], pointed the presence of basanite in concrete exposed in acid media, such as sewage treatment plants. However, the CP IV (0.65) series, confirming the one observed in the SEM analysis, presented (Fig. 7 C) as constituents the quartz (SiO2) and gipsite (CaSO4.2H2O), as observed in the research by Hoppe Filho et al. [32]. 3.3. Pulse velocity The Figs. 8 and the 9 show the pulse velocity results for the reference and aggressive exposure, respectively. In general, the tendency of the samples exposed to the reference condition were of increase of the parameter along the ages analyzed. The behavior was expected, since the samples were conditioned in an environment without aggressive agents and the increase in the pulse
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Fig. 6. SEM images obtained of the 0.65 water/cement ratio series, without silica fume, at 360 days of age. A – cement CP V; B – cement CP II; C – cement CP IV.
Table 4 Analysis of EDS in the points indicated in Fig. 6 (A, B and C).
*
Elements
Wt%* of the point of Fig. 6 A
Wt%* of the point of Fig. 6 B
Wt%* of the point of Fig. 6 C
Oxygen Calcium Sulfur Carbon Silica Aluminum
45.1 25.6 24.2 4.1 0.9 0.1
44.6 25.1 23.8 4.7 1.5 0.3
35.9 29.1 29.0 3.5 2.5 –
Wt% – Percentage, by weight of the elements, at the analyzed point.
velocity indicates a greater compactness, and therefore greater hydration of the cement. However, from 28 to 84 days, there was a decrease in the pulse propagation velocity in some series (Fig. 8), in general, those with higher addition contents, such as Ref. (0.40) CP IV + S and Ref. (0.40) CP II + S. At 168 and 360 days, these series already recovered a growth pattern, and this initial variation could be related to the initial hydration of the matrix, slower for cements with higher percentage of addition. Comparing the series of the same cement and the same water/cement ratio between themselves, analyzing only the influence of the 10% of silica (Ref. (0.40) CP V versus Ref. (0.40) CP V + S, for example), in all cases a small increase of the pulse was observed with the use of the addition, that was approximately 2%. Analyzing the graphics in both figures there is a tendency of green dots on the upper part and red on the lower part, indicating higher values for CP IV and lower for CP II. Opposite to the growth tendency observed in the reference exposure condition, in the aggressive condition (Fig. 9), the tendency was of a decrease in velocity, for all series, between all ages. The decrease in velocity indicates the greater presence of voids in the matrix and, consequently, greater fragility. According to Jiang et al. [33] the formation of gipsite and ettringite, consequence of the attack, was the factor responsible for the appearance of crack in the matrix, by the generated tensions. Thus, as observed in SEM images, the pulse test was sensitive to indicate the occurrence of the attack and damage brought by exposure of the samples in the aggressive environment. 3.4. Electrical resistivity Fig. 10 shows the results of electrical resistivity for reference condition. It could be noted, once again, that CP IV cement was
responsible for the higher resistivity values in all ages, as shown by the study of Medeiros Junior et al. [34], Medeiros Junior e Lima [35] and Mendes et al. [36] as the effect of pozzolanic additions. All series increased the resistivity considerably from 28 days to 360 days, which increased from approximately 4.5 for CP II, 3.5 for CP IV and 3 for CP V. The values of electrical resistivity in the aggressive condition (Fig. 11) could be considered higher than those observed in the reference condition, comparing the same series in the two conditions. The series that presented the greatest difference between the condition was (0.50) CP IV, being the aggressive condition 323% higher than the reference one. The higher values of resistivity presented in Figs. 10 and 11 are for the CP IV with active silica. This occurs because that is the group with lower content of portlandite, since it has lower clinker content and the pozzolanic effect, which tends to consume calcium hydroxide. Ca(OH)2 tends to dissociate in Ca+2 and OH– in the presence of water, generating an electrolyte with high electrical conductivity. Thus, if the concrete has a low amount of portlandite by the pozzolanic effect, the electrical resistivity of this concrete tends to be larger, as evidenced by the work of Medeiros et al. [37]. The fact that the electrical resistivity is higher for the concrete exposed to the aggressive environment, compared to the reference exposure (in lime water) is also related to the portlandite. Ca(OH)2 is one of the first elements to be transformed into the mechanisms of degradation by biogenic sulfuric acid, forming crystals of ettringite and gypsum. Thus, the pore solution of the concrete becomes less conductive as a result of the degradation at the sewage treatment plant. This is an example of how the electrical resistivity data interpretation needs to be done by a trained and experienced professional, since the tendency is to interpret that the more resistive the concrete, the better the integrity condition it is. However, in this case the opposite occurs. 3.5. Compressive strength Fig. 12 shows the evolution of the compressive strength for the samples in the reference condition. It is observed that between the water/cement ratio series 0.40 the ones that presented the greatest compressive strength throughout the study were Ref. (0.40) CP V and Ref. (0.40) CP V + S. At 28 days of age, all series with a water/ cement ratio of 0.65 were below 40 MPa, and were not in accordance with the minimum compressive strength required by NBR 6118 [9]. However, all groups had increased compressive strength over time, and the only one that did not reach 40 MPa, in the reference condition, at 360 days of age, was (0.65) CP II.
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Fig. 7. XRD analysis obtained of the 0.65 water/cement ratio series, without silica fume, at 360 days of age. A – cement CP V; B – cement CP II; C – cement CP IV.
Contrary to that observed in the previous condition, the samples subjected to aggressive condition presented a decrease in compressive strength, all of them at 360 days of age, below
40 MPa (Fig. 13). The decrease in this property corroborates to the results obtained in the pulse velocity, SEM images and XRD, indicating fragility of the cement matrices and proving
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5800
Pulse velocity (m/s)
5650 5500 5350 5200 5050 4900 4750 0
40
80 120 160 200 240 280 320 360 400
Age (days)
Ref. (0.40) CP V (0.50) CP V (0.65) CP V Ref. (0.40) CP V + S (0.50) CP V + S (0.65) CP V +S Ref. (0.40) CP II (0.50) CP II (0.65) CP II Ref. (0.40) CP II + S (0.50) CP II + S (0.65) CP II +S Ref. (0.40) CP IV (0.50) CP IV (0.65) CP IV Ref. (0.40) CP IV + S (0.50) CP IV + S (0.65) CP IV +S
Fig. 8. Pulse velocity monitoring for the series in reference exposure (tank with saturated lime water).
Pulse velocity (m/s)
5800 5650 5500 5350 5200 5050 4900 4750 0
40
80
120 160 200 240 280 320 360 400
Ref. (0.40) CP V (0.50) CP V (0.65) CP V Ref. (0.40) CP V + S (0.50) CP V + S (0.65) CP V +S Ref. (0.40) CP II (0.50) CP II (0.65) CP II Ref. (0.40) CP II + S (0.50) CP II + S (0.65) CP II +S Ref. (0.40) CP IV (0.50) CP IV (0.65) CP IV Ref. (0.40) CP IV + S (0.50) CP IV + S (0.65) CP IV +S
Age (days) Fig. 9. Pulse velocity monitoring for the series in aggressive exposure (sewage treatment plant).
Ref. (0.40) CP V
Electrical Resistivity (Ωm)
300
(0.50) CP V (0.65) CP V
250
Ref. (0.40) CP V + S (0.50) CP V + S (0.65) CP V +S
200
Ref. (0.40) CP II (0.50) CP II
150
(0.65) CP II Ref. (0.40) CP II + S
100
(0.50) CP II + S (0.65) CP II +S
50
Ref. (0.40) CP IV (0.50) CP IV (0.65) CP IV
0 0
40
80
120
160
200
240
Age (days)
280
320
360
400
Ref. (0.40) CP IV + S (0.50) CP IV + S (0.65) CP IV +S
Fig. 10. Electrical resistivity monitoring for the series in reference exposure (tank with saturated lime water).
the aggressiveness of the studied environment. Even the concrete that met the initial requirements of the standard (relation water/cement, cement consumption and compressive strength at 28 days) showed a decrease in its properties at 360 days.
The damage observed in the mechanical properties of the concretes exposed to the aggressive condition is in accordance with the one presented by Grengg et al. [9], who noticed in their study that the crystallization of products from the attack causes a decrease in the mechanical strength of concrete.
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Ref. (0.40) CP V
Electrical Resistivity (Ωm)
300
(0.50) CP V (0.65) CP V Ref. (0.40) CP V + S
250
(0.50) CP V + S (0.65) CP V +S
200
Ref. (0.40) CP II (0.50) CP II
150
(0.65) CP II Ref. (0.40) CP II + S
100
(0.50) CP II + S (0.65) CP II +S Ref. (0.40) CP IV
50
(0.50) CP IV (0.65) CP IV
0 0
40
80
120
160
200
240
280
320
360
400
Ref. (0.40) CP IV + S (0.50) CP IV + S (0.65) CP IV +S
Age (days)
Compressive Strength (MPa)
Fig. 11. Electrical resistivity monitoring for the series in aggressive exposure (sewage treatment plant).
100 90 80 70 60 50 40 30 20 10 0 0
40
80
120
160
200
240
280
320
360
400
Age (days)
Ref. (0.40) CP V (0.50) CP V (0.65) CP V Ref. (0.40) CP V + S (0.50) CP V + S (0.65) CP V +S Ref. (0.40) CP II (0.50) CP II (0.65) CP II Ref. (0.40) CP II + S (0.50) CP II + S (0.65) CP II +S Ref. (0.40) CP IV (0.50) CP IV (0.65) CP IV Ref. (0.40) CP IV + S (0.50) CP IV + S (0.65) CP IV +S
Compressive Strength (MPa)
Fig. 12. Compressive strength monitoring for the series in reference exposure (tank with saturated lime water).
100 90 80 70 60 50 40 30 20 10 0 0
40
80
120
160
200
240
Age (days)
280
320
360
400
Ref. (0.40) CP V (0.50) CP V (0.65) CP V Ref. (0.40) CP V + S (0.50) CP V + S (0.65) CP V +S Ref. (0.40) CP II (0.50) CP II (0.65) CP II Ref. (0.40) CP II + S (0.50) CP II + S (0.65) CP II +S Ref. (0.40) CP IV (0.50) CP IV (0.65) CP IV Ref. (0.40) CP IV + S (0.50) CP IV + S (0.65) CP IV +S
Fig. 13. Compressive strength monitoring for the series in aggressive exposure (sewage treatment plant).
4. Conclusions The exposure of the samples in the gas chamber of sewage treatment plant allowed to evaluate the characteristics of the concrete under a condition of extreme aggressiveness. The development of the research described in this work allows the presentation of the following conclusions: - At 168 days of age of the samples, it was possible to notice difference between the samples of the different cements. The SEM
test identified a higher concentration of deleterious products (gypsum and ettringite) in cements with lower percentages of addition (CPV and CPII) confirming their susceptibility to attack. The result obtained in the image test indicate the need for the correct specification of cement for the different types of exposure of concrete structures; - The pulse velocity test was sensitive to differentiate the series from the reference and aggressive condition, the last being responsible for the lower velocities. Therefore, the applied technique proves to be viable from an investigative point of
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view, which may be an inspection tool to evaluate the quality of concrete structures subjected to biogenic sulfate attack; The electrical resistivity test indicated the highest values for the series exposed in the aggressive condition, being explained by the physical alterations of pore filling by the degrading products, as observed in the SEM test. The test used also proved to be a structural assessment tool, since it was sensitive in the differentiation of the studied series; All series exposed to aggressive condition showed a decrease in compressive strength when compared the ages 28 and 360 days. The series that presented the greatest drop was Ref. (0.40) CP V + S, being 63%, followed by (0.50) CP V + S, being 53%; The use of active silica in the evaluated content, despite indicating higher filling at early ages by the tests of pulse velocity and resistivity, did not prevent the entry of gases and the occurrence of the attack, as evidenced by the decrease in the compressive strength. The study of the use of different silica contents could be interesting in future works by determining the ideal content to improve concrete properties; The mixture that met the standard requirements (water/cement ratio, cement consumption and compressive strength at 28 days), as well as the others, showed a decrease in its properties, alerting to the strong aggressiveness of the environment. This result reinforces the need for the correct choice of cement type, an observation not contemplated by the Brazilian concrete standard; Finally, the study highlights the need to research new standards limits and protective alternatives to structures subject to the aggressive condition of sewage treatment plants.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors thank the support, as scholarships, by Foundation Araucária, as well as the availability of the infrastructure and human resources made available by the Federal University of Paraná (PPGECC/UFPR-Brazil). Our gratitude also to the Center of Electron Microscopy of UFPR (CME/UFPR), by the support during the tests of SEM, to the NOVAMIX, by the support during the concrete mixtures and the company responsible for the sewage treatment plant, SANEPAR, by the availability of the structure. References [1] X. Li, F. Khademi, Y. Liu, M. Akbari, C. Wang, P.L. Bond, J. Keller, G. Jiang, Evaluation of data-driven models for predicting the service life of concrete sewer pipes subjected to corrosion, J. Environ. Manage. 234 (2019) 431–439. [2] G. Jiang, J. Keller, P.L. Bond, Z. Yuan, Predicting concrete corrosion of sewers using artificial neural network, Water Res. 92 (2016) 52–60. [3] M.P.H. Brongers, P.Y. Virmani, J.H. Payer, Drinking water and sewer systems in corrosion costs and preventative strategies in the United States, United States Department of Transportation Federal Highway Administration, 2002. [4] W. Kaempfer, M. Berndt, Estimation of service life of concrete pipes in sewer networks, Durability of Building Materials and Components 8 (1999) 36–45. [5] M.G. Alexander, A. Bertron, N. De Belie, Performance of cement-based materials in aggressive aqueous environments State-of-the-Art Report, Springer, RILEM TC 211 – PAE, 2013. [6] E. Stanaszek-Tomal, M. Fiertak, Biological corrosion in the sewage system and the sewage treatment plant, Procedia Eng. 161 (2016) 116–120.
[7] R.L. Islander, J.S. Devinny, F. Mansfeld, A. Postyn, H. Shih, Microbial ecology of crown corrosion in sewers, J. Environ. Eng. 117 (1991) 751–770. [8] J. Monteny, E. Vincke, A. Beeldens, N. De Belie, L. Taerwe, D. Van Gemert, W. Verstraete, Chemical, microbiological, and in situ test methods for biogenic sulfuric acid corrosion of concrete, Cem. Concr. Res. 30 (4) (2000) 623–634. [9] C. Grengg, F. Mittermayr, G. Koraimann, F. Konrad, M. Szabó, A. Demeny, M. Dietzel, The decisive role of acidophilic bacteria in concrete sewer networks: A new model for fast progressing microbial concrete corrosion, Cem. Concr. Res. 101 (2017) 93–101. [10] W.J. Meyer, Case study of prediction of sulphide generation and corrosion in sewers, J. WPCF 52 (11) (1980) 2666–2674. [11] Brazilian Association of technical standards. NBR 6118 – Design of structural concrete - Procedure. Rio de Janeiro, 2014. [12] Brazilian Association of technical standards. NBR 12655 – Portland cement concrete – Preparation, control, receipt and acceptance - Procedure. Rio de Janeiro, 2014. [13] A. Baldermann, M. Rezvani, T. Proske, C. Grengg, F. Steindl, M. Sakoparning, C. Baldermann, I. Galan, F. Emmerich, F. Mittermayr, Effect of very high limestone content and quality on the sulfate resistance of blended cements, Constr. Build. Mater. 188 (2018) 1065–1076. [14] M. Moradian, M. Shekarchi, F. Pargar, M. Valipour, Deterioration of concrete caused by complex attack in sewage treatment plant environment. American Society of Civil Engineers, J. Perform. Constr. Facil 26 (2012) 124–134. [15] Brazilian Association of technical standards. NBR 16697- Portland cement – Requirements. Rio de Janeiro, 2018. [16] American Society for Testing and Materials. ASTM C150 – Standard specification for Portland cement. 2018. [17] A. Chainpanich, R. Rianyoi, T. Nochaiya, The effect of carbon nanotubes and silica fume on compressive strength and flexural strength of cement mortars, Mater. Todays: Proc. 4 (2017) 6065–6071. [18] American Society for Testing and Materials – ASTM C128. Standard test method for relative density and absorption of fine aggregate, 2015. [19] American Society for Testing and Materials – ASTM C127. Standard test method for relative density and absorption of coarse aggregate, 2015. [20] Brazilian Association of technical standards. NBR NM 46 – Aggregates – Determination of material finer than 75um sieve by washing. Rio de Janeiro, 2003. [21] Brazilian Association of technical standards. NBR 5738 – Concrete – Procedure for molding and curing concrete test specimens. Rio de Janeiro, 2015. [22] American Society for Testing and Materials – ASTM C597. Standard test method for pulse velocity through concrete, 2016. [23] Spanish standardization and certification association - UNE 83988-2: Durabilidad del hormigón – determinación de la resistividad – Parte 2: Método de las cuatro puntas o de Wenner. AENOR, Madrid, 2012. [24] Brazilian Association of technical standards. NBR 5739 – Concrete – Compression test of cylindrical specimens. Rio de Janeiro, 2018. [25] Z. Pan, L. Cheng, Y. Lu, N. Yang, Hydration products of alkali-activated slag-red mud cementitious material, Cem. Concr. Res. 32 (2002) 357–362. [26] J. Sulikowski, J. Kozubal, The durability of a concrete sewer pipeline under deterioration by sulphete and chloride corrosion, Procedia Eng. 153 (2016) 698–705. [27] C. Shi, G. Zhang, T. He, Y. Li, Effects of superplasticizers on the stability and morphology of ettringite, Constr. Build. Mater. 112 (2016) 261–266. [28] S.K. Nath, S. Kumar, Role of alkali concentration on reaction kinetics of fly ash geopolymerization, J. Non-Cryst. Solids 505 (2019) 241–251. [29] Z. Pavilik, J. Fort, M. Zaleska, M. Pavlikova, T. Anton, I. Medved, M. Keppert, P.G. Koutsoukos, R. Cerny, Energy-efficient thermal treatment of sewage sludge for its application in blended cements, J. Cleaner Prod. 112 (2016) 409–419. [30] J. Li, Z. Chen, Q. Wang, L. Fang, Q. Xue, C.R. Cheeseman, S. Donatello, L. Liu, C.S. Poon, Change I re-use value of incinerated sewage sludge ash due to chemical extraction of phosphorus, Waste Manage. 74 (2018) 404–412. [31] X.M. Huang, F.A. Perras, S. Manzano, I.I. Slowing, A. McNabb, M. Pruski, The anomalous solidification of concrete grindings from acid treatment, Cem. Concr. Res. 116 (2019) 65–69. [32] J. Hoppe Filho, B. Rheinheimer, S.S. Khoe, L.V. Artigas, A.F. Sabbag, M.H.F. Medeiros, Degradation of concrete from a sewage treatment plant (STP) by biogenic sulfuric acid, Alconpat J. 4 (2014) 84–96. [33] G. Jiang, E. Wightman, B.C. Donose, Z. Yuan, P.L. Bond, J. Keller, The role of iron in sulfide induced corrosion of sewer concrete, Water Res. 49 (2014) 166–174. [34] R.A. Medeiros Junior, M.G. Lima, M.H.F. Medeiros, L.V. Real, Investigation of the compressive strength and electrical resistivity of concrete with different types of cement, ALCONPAT J. 4 (2014) 116–128. [35] R.A. Medeiros-Junior, M.G. Lima, Electrical resistivity of unsaturated concrete using different types of cement, Constr. Build. Mater. 107 (2016) 11–16. [36] S.E.S. Mendes, R.L.N. Oliveira, C. Cremonez, E. Pereira, R.A. Medeiros-Junior, Electrical resistivity as a durability parameter for concrete design: Experimental data versus estimation by mathematical model, Constr. Build. Mater. 192 (2018) 610–620. [37] M.H.F. Medeiros, J. Hoppe Filho, E. Pereira, P. Helene, G.C. Isaia, High-volume fly ash concrete with and without hydrated lime: chloride diffusion coefficient from accelerated test, J. Mater. Civ. Eng. 25 (2013) 411–418.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of the cement type and water/cement ratio in concretes exposed in sewage treatment plants Ana Paula Brandão Capraro a,⇑, Marcos Antonio Cheremeta b, Marcos Paulo Galvão Gonçalves b, Claiton Cremonez b, Marcelo Henrique Farias de Medeiros a a b
UFPR – Programa de Pós-Graduação em Engenharia de Construção Civil, Av. Cel. Francisco H. Dos Santos, 100, CEP: 81530-000 Curitiba, PR, Brazil UNIFACEAR, Av. Thomaz Coelho, CEP: 83707-067 Araucária, PR, Brazil
h i g h l i g h t s Chemical, physical and mechanical properties of produced specimens were evaluated. A year of exposure was enough to identify mechanisms of gases attack. The cement CP V was more susceptible to the attack of sewage gases, tested in compression strength, SEM and XRD tests. The SEM images showed the presence of deleterious products, like ettringite. At the end of the study all the series presented visible signs of degradation.
a r t i c l e
i n f o
Article history: Received 7 June 2019 Received in revised form 5 August 2019 Accepted 31 August 2019
Keywords: Concrete Sewage treatment plant Disintegration Compressive strength drop Deleterious products
a b s t r a c t The objective of this paper is to evaluate the influence of the cement type (CP V, CP II and CP IV) and water/cement radio (0.40, 0.50 and 0.65) in concretes exposed in sewage treatment plant. The parameters monitored for one year were: compressive strength, pulse velocity, electrical resistivity, SEM e XRD. Some characteristics of aggressiveness, such a disintegration of part of the concrete, became visible in the visual inspections. The 0.65 water/cement ratio series, for all cements, was the only one to present compressive strength drop. The SEM and XRD proved the occurrence of the attack, with the identification of deleterious products. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Concrete corrosion is one of the most significant failure mechanisms of sewer pipes and can reduce the sewer service life significantly. To facilitate the management and maintenance of sewers, it is essential to obtain reliable prediction of the expected service life of sewers, especially if that is based on limited environmental conditions [1]. Corrosion causes the loss of concrete mass and structural capacity, eventually leading to ultimate structural collapse [2]. The rehabilitation and replacement of damaged structures sewers cost, in the USA, approximately $14 billion per year [3], in Germany, runs to about 100 billion US$ [4], and tend to increase service life he aging of the infrastructure [5].
⇑ Corresponding author. E-mail addresses: [email protected] (A.P.B. Capraro), macheremeta@ sanepar.com.br (M.A. Cheremeta). https://doi.org/10.1016/j.conbuildmat.2019.116842 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
According to Stanaszek-Tomal and Fiertak [6], treatment plants are hardly ever completely filled with sewage, and as a result there is a lot of space above the water line for bacterial development and growth and accumulation of gas products from sewage decomposition. The most commonly occurring bacteria in sewage treatment systems include bacteria of the genus Thiobacillus, those grow and develop best at 25–35 °C. They are able to oxidize sulphide, elemental sulphur, thiosulphate and polythionates. Bacteria of the Thiobacillus species can be divided into two groups. Group one consists of those that only grow at neutral pH values, those are responsible for the conversion of the elemental sulphur to sulphuric acid. The other group grow at lower pH values and are able to use Fe2+ as an electron donor. In this other group, a representative of which is the Thiobacillus thiooxidans species, of great significance is the growth at acidic pH values. They develop best at pH from 2 to 5 and in under aerobic conditions. Thiobacillus intermedius is most active at pH in the range from 3 to 7, and oxidizes thiosulphate ions
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(S2O23 ). Thiobacillus ferrooxidans occurs in aerobic conditions and has a range of pH growth from 1.5 to 5 [7]. The presence of sulfur compounds and sulfate reducing bacteria (desulfatation), in the anaerobic environment, leads to the production of hydrogen sulfide (H2S) and carbon dioxide (CO2). Hydrogen sulfide, when released from the effluent in the form of hydrogen sulphide (H2S), reacts with the oxygen of atmospheric air (abiotically), forming water (H2O) and sulfur compounds (S2O-2 3 ; S4O6-2; SO-2 4 ). The other portion of hydrogen sulfide (H2S) (by acid attack) and carbon dioxide (CO2) (by carbonation), dissolve in the water contained in the pores of the concrete, gradually reducing the pH of the hydrated matrix [8]. The term biogenous sulfuric attack is used for processes in sewerage systems where microorganisms form sulfuric acid (H2SO4) and cause the cement paste matrix to be attacked until it dissolves [4]. According to Grengg et al. [9] the gaseous compounds produced are liberated into the atmosphere of the concrete pipes and manholes, where they partly dissolve into the condensates along the concrete walls and subsequently diffuse into the concrete pore structure. These conditions, added to high temperatures, humidity, pH, nutrients and CO2 availability facilitated the degradation in cementitious materials [10]. In order to prevent the attack on the concrete subject to the sewage treatment plants, the Brazilian concrete standards (NBR 6118 and NBR 12655) [11,12] presents minimum requirements for concretes to be used for this purpose. According to the standards, the classification of concrete exposed to sewage treatment plants is the most aggressive, class IV, with the requirements: water/cement 0.45, compressive strength at 28 days 40 MPa, minimum covering for reinforcement slab equal to 45 mm and minimum consumption of cement of 360 kg/m3. Although the limitations presented by the standard, there is still little information about the influence of the type of cement and the use of additions on the concrete of sewage treatment plants. However, researches such as Baldermann et al. [13], indicate the use of replacement cement by supplementary cementitious materials (SCM) as an alternative to durability in sewage treatment plants. Therefore, the study of the different types of cement, as well as different types of additions, can aid in the study of performance of structures subject to sewage treatment plants. In a city in Brazil, in 2016, the covered slab of the anaerobic fluidized sludge reactor (AFSR), with 11 years old, collapsed, during the maintenance of the structures of the sewage treatment plant. The covered slab is shown in Fig. 1, before the collapse (A) and after the collapse (B). Situations such as the one presented above highlight the need for studies in the area, since collapses in these structures lead to a high cost of repair in addition to paralyzing activities, which are essential to the population.
Moradian et al. [14] emphasize the importance of concrete degradation analysis through different parameters of durability, such as chloride content, water absorption, compressive strength, chemical constituents (SEM and XRD), carbonation and corrosion. The objective of this work was to compare the properties, chemical constituents, pulse velocity, electrical resistivity and compression strength, of different exposed concretes in an AFSR. For this, three Brazilian commercial cements, allowed per norm for the studied use, and three water / cement ratios were chosen. In addition, the study evaluated the influence of the replacement of 10% of cement with active silica, since the literature reports the use of additions as mitigation to the attack. 2. Experimental 2.1. Materials and mixture characteristics Three cement were used in the research: CP V ARI – NBR 16,697 [15] equivalent to Type III – ASTM C150 [16], CP II F – NBR 16,697 [15] equivalent to Type II – ASTM C150 [16] and CP IV – NBR 16,697 [15] equivalent to Type IV – ASTM C150 [16]. The CP V, CP II and CP IV cements have specific mass of 3.12 g/cm3, 3.11 g/ cm3 and 2.82 g/cm3, and Blaine specific surface area of 4078 cm2/ g, 3398 cm2/g and 4290 cm2/g, respectively. Although the three cements mentioned are used for different purposes (Type III for high initial strengths, Type IV for slower reactions, Type II for general use, especially when moderate resistance to sulphate or moderate heat of hydration is desired), Brazilian standards, NBR 6118 and NBR 12,655 [11,12], do not establish a specific type of cement for use in sewage treatment plants. The considerations of the standards are regarding the minimum strength, maximum water/cement ratio and minimum cement consumption, leaving the designer to choose the cement. Evaluating the fineness requirement of the cements studied, by NBR 16,697 [15], note that the type III has the most finer particles, followed by the Type IV and then Type II. The difference between the cements is related to the milling process and the additions used, which interfere with the packing of the particles and the speed of the hydration reactions. Chemical composition of three Portland cement were determined by X-ray fluorescence (XRF) and presented in Table 1. Part of the studied groups had 10% of the cement replaced by silica fume, to evaluate the filler effect, associated with the pozzolanic effect of high reactivity. The silica fume, because of the pozzolanic reaction, fills the voids and pores, reducing the porosity of composites [17], and this may be an interesting effect to amortize the gas attack. The silica fume particles are spherical, vitreous, have an average diameter of 0.20 mm, specific surface area between 15,000 and 25,000 m2/kg and specific mass 2200 kg/m3.
Fig. 1. Covered slab of the anaerobic fluidized sludge reactor. A – Before de collapse; B- After the collapse with corrosion signs.
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A.P.B. Capraro et al. / Construction and Building Materials 229 (2019) 116842 Table 1 Chemical characterization (%) of three cements.
CaO SiO2 Al2O3 Fe2O3 MgO SO3 Free CaO Ignition loss Insoluble waste Alkaline equivalent
CP V
CP II
CP IV
60.89 18.91 4.39 2.69 4.74 2.86 0.99 2.99 0.92 0.62
60.47 19.19 4.07 2.59 4.79 2.62 1.06 3.32 1.28 0.61
45.12 28.94 9.97 3.72 3.18 2.49 0.74 3.34 1.45 1.05
The fine aggregate was obtained from the mixture of 75% of medium natural sand and 25% of fine natural sand. The coarse aggregates were obtained from the mixture of 15% of 12.5 mm gravel and 85% of 19.0 mm gravel, from dolomitic origin. The Table 2 shows the characteristics of the aggregates. The characteristics of the aggregates were obtained in accordance of ASTM C128 [18], ASTM C127 [19] and NBR NM 46 [20]. As the water/cement radio was a varied parameter it was necessary to use a superplasticizer to standardize the slump (100 ± 20 mm) of the mixtures. The plasticizer used was the MasterPolyheeed, composed of substances which act as dispersants of the material agglomerate, providing a high reduction of without changing the holding time of the concrete, with has a specific mass of 1.07 g/cm3. Given the range suggested by the manufacturer, the content employed was 0.9% with respect to the mass of the binder. The reference concrete mix proportion was adopted in compliance with the requirements of NBR 6118 [11] (compressive strength at 28 days 40 MPa, cement consumption 360 kg/m3 and water/cement ratio 0.45). Two water/cement ratio were also adopted for other two series, higher than allowed by the standard (0.50 and 0.65). The fixed parameters between the mix proportion were the mortar content equal to 52% and the slump equal to 100 ± 20 mm. The amount of water in the mixtures was corrected, taking into account the humidity of the aggregates and the water portion of the additive used. Six mix proportion were employed (Table 3) for each of the three cements, resulting in 18 different mixtures. The group Ref. (0.40) refers to the series in accordance with the standard. The (0.50) and (0.65) were the groups with water/cement ratio equal 0.50 and 0.65, respectively. The groups with ‘‘+S” refer
to the series with replacement of cement by silica fume. Table 3 shows that cement consumption is not constant for the groups, since the water/cement ratio was an altered parameter. Thus, this work will not evaluate the compressive strength of the series in a comparative way between them, only of them with themselves over the time. 2.2. Experimental procedure 2.2.1. Specimen preparation For the compressive strength, pulses velocity and electrical resistivity tests, cylindrical samples with 10 cm of diameter and 20 cm of height were molded, in conformity with NBR 5738 [21]. To statistically analyze the results, using the Tukey test (95% confidence), 3 specimens were molded for each group at each age analyzed. For the SEM and XRD tests, fragments of the cylindrical specimens exposed to an aggressive condition were used, which were removed after the compression strength test. The concretes were prepared following the recommendations of NBR 12,655 [12]. After the mixture, the concrete was poured in the molds gradually, in three steps. After each step, the material was manually compacted, with a compacting rod. The specimens were demolded 24 h later. After that they were submerged in lime saturated water, for 28 days. This was adopted to produce an advanced hydrated condition for the concretes. Altogether, 378 specimens were molded, considering 6 mix proportion for each type of cement, 4 ages, 3 specimens for statistical analysis and 2 exposure conditions. 2.2.2. Exposure conditions After 28 days, in submerged cure time, the specimens were separated into two exposure conditions, a reference, which consisted of remaining half of them in a tank with water saturated with lime, and in the aggressive condition exposure (exposed in the sewage treatment plant). Fig. 2 shows the two adopted conditions, reference exposure (Fig. 2A) and aggressive conditions (Fig. 2B). The gas chamber was chosen for accommodation of the specimens because the simultaneous presence of deleterious agents to the concrete (gases) and humidity, essential conditions to the attack [10]. For accommodation of the specimens HDPE boxes were fixed with stainless steel bolts (Fig. 3). The materials were chosen because of the resistance and durability under the conditions of exposure to the sun and gases.
Table 2 Characteristics of the aggregates. Aggregates
Fineness module
Clay content (%)
Specific mass (g/cm3)
Water absorption (%)
Content of powdery material (%)
Medium natural sand Fine natural sand 12.5 mm gravel 19.0 mm gravel
2.27 1.38 5.90 7.04
0.00 0.00 0.00 0.00
2.63 2.82 2.75 2.77
0.40 0.70 0.40 0.20
8.90 4.10 3.30 2.06
Table 3 Mix proportions (m3) for each cement (CP V, CP II and CP IV). Materials (kg/m3)
Ref. (0.40)
(0.50)
(0.65)
Ref. (0.40) + S
(0.50) + S
(0.65) + S
Cement Silica fume Medium natural sand Fine natural sand 12.5 mm gravel 19.0 mm gravel Water
425 – 511 219 320 746 170
340 – 563 241 320 746 170
261 – 612 262 320 746 170
382.5 42.5 511 219 320 746 170
306 34 563 241 320 746 170
234.9 26.1 612 262 320 746 170
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Fig. 2. Exposure conditions adopted for the study. A – Standard exposure – tank with lime saturated water; B- Aggressive exposure – Gas chamber of the anaerobic fluidized sludge reactor.
of them, for 24 h, in pure ethyl alcohol and later in the permanence of them, for another 24 h, in an oven at 40 °C. The procedure followed that described by Pan et al. [25]. Microscopy images were obtained with a Tescan SEM FEG microscope, Mira 3 and Oxford X-Max 50 X-ray analytical microprobe (EDS). First, the samples were glued to the stub with graphite enamel, for identification of the elements by EDS. Afterwards, they were removed from the equipment, metalized with gold, and the images were obtained. The metallization was not carried out from the beginning to avoid overlapping of elements during the obtaining of the EDS spectra. The XRD analysis were carried out with powder samples, obtained by spraying the concrete fragments. The equipment used for the test is from the brand AMTU and its power is equal to 40 kV/30 mA. The reading was performed between the positions 5 ° 2h and 75 ° 2Th, with step size of 0.0170 (°2h) and scan step size of 40 (s). In parallel to the tests, visual inspections were carried out in the samples, in order to direct the SEM and XRD tests to the cases of greater degradation. 3. Results and discussion
Fig. 3. Specimens accommodated in the gas chamber of the anaerobic fluidized sludge reactor.
2.2.3. Performed tests For 360 days, the cylindrical specimens of concrete were evaluated by laboratory tests, compressive strength, electrical resistivity and ultrasonic velocity. In all of them the specimens were tested on the saturated dry surface condition. The analysis ages were: 28, 84, 168 and 360 days. The first tests to be performed were pulse velocity and electrical resistivity, because they were non-destructive, and then, the compressive strength one. For the pulse velocity test, Proceq Ultrasonic Pulse Velocity – Pundit Lab was used. The 54 kHz transducer was used. The test was performed in accordance with ASTM C597 [22]. The equipment used for the electrical resistance analysis, Proceq Resipod, has the operating principle of the four Wenner electrodes. The test consists of placing four electrodes on the surface of the test specimens, where they are located and equidistant from the others. The test was carried out in accordance with the Spanish standard [23]. After that, compression strength test was performed, in accordance with NBR 5739 [24], in an EMIC hydraulic compression machine with load capacity equal to 100tf. Samples used for the SEM and XRD tests underwent a stoppage process at the test ages (168 and 360 days for the SEM test and 360 days for the XRD test). The process consisted in the immersion
3.1. Visual analysis At 168 days, it was possible to notice stains and occasional disintegrations in all series, Fig. 4 (A). The stains could be found more easily in the bases and tops of the specimens, because they are places with greater availability of gases and humidity. At 360 days, the degradation can be observed throughout the entire specimens, being more evident for the series with water/ cement ratio equal to 0.65 (Fig. 4 B). 3.2. SEM and XRD analysis According to Sulikowski and Kozubal [26], the sulphur oxidation by Thiobacilus thiooxidans, that occurs in sewage treatment plants, produced acid attacks to the calcium hydroxide, resulting in the gipsite formation. The gipsite at crystallization increases its volume by 130%, moreover, it can react with tricalcium aluminate creating ettringite. Because they are consequences of the attack, the identification of gipsite and ettringite crystals in cementitious compounds in advanced ages may have proved the event of deleterious reactions. Already with 168 days of the materials (140 days of exposure in sewage), the identification of crystals similar to gipsite and ettringite in all series with water/cement ratio equal to 0.65 (the most critical), without silica fume, was possible. Fig. 5 shows the SEM images for these three series.
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Fig. 4. Specimens conditions during the study. A – Stains in the bases and tops of all specimens at 168 days of age; B – Specimens degradation in series with water/cement ratio 0.65 at 360 days of age.
Fig. 5. SEM images obtained of the 0.65 water/cement ratio series, without silica fume, at 168 days of age. A – cement CP V; B – cement CP II; C – cement CP IV.
Fig. 5 A presents the group with cement CP V (0.65), and in the image, it is possible to see the pore filling with acicular crystals (indicated by the arrow), identified by EDS analysis as ettringite crystals. The presence of this crystal, in that quantity, at 168 days of age of the samples, indicates the occurrence of the attack, because at that age, without an external source of sulphate, a healthy concrete would have its entire ettringite transformed in monosulfate hydrate [27]. Fig. 5 B shows the group with cement CP II (0.65), and in the image it is possible to see the pore filling with some acicular crystals (ettringite). The group with cement CP IV (0.65), Fig. 5 C, also presents, in a punctual way, the presence of ettringite. However, in this series a more irregular aspect of the matrix and some circular crystals, similar to gipsite, were observed. The presence of gipsite and even occasional presence of ettringite shows, compared to the previous cement, that this group still was at an early stage of the attack. The irregular aspect and the early stage of the attack can be explained by the use of cement with a high content of pozzolanic additions (up to 50% of fly ash), which has slower hydration reactions [28]. As the series with CP V and CP II were the ones, at 168 days, that presented more crystals of ettringite and easier to find them, the hypothesis of greater susceptibility of these cements to the attack raises, since these types have more quantity of Portlandite available. At 360 days of age, a great advance in the attack was observed, as well as the one observed in the visual analysis. The series with CP V and CP II (Fig. 6 A and B, respectively) presented deposition of products with irregular appearance, identified as EDS with great amount of calcium and sulfur (Table 4), confirming the occurrence of the attack. Also, at this age, the two series, CP V (0.65) and CP II
(0.65) without silica fume, presented cracks in the matrix (indicated by the arrows), providing the greater fragility of the material. The CP IV (0.65) series continued to indicate the presence of deleterious products, with the appearance of gipsite (Fig. 6 C), in the pores, as evidenced by EDS (Table 4). However, the observed degree of filling at 360 days was much higher than the one observed at 168 days. The XRD analysis were performed on the same SEM samples at the age of 360 days. Fig. 7 A, B and C show, respectively, the diffractograms obtained for the series CP V (0.65), CP II (0.65) and CP IV (0.65). The behavior of the CP V and CP II series were similar, as observe in the SEM analyzes. The main products found for theses series were quartz (SiO2) and basanite (2CaSO4H2O). Basanite is the main constituent of thermally treated sewage sludge [29]. Other researches, such as Li et al. [30] and Huang et al. [31], pointed the presence of basanite in concrete exposed in acid media, such as sewage treatment plants. However, the CP IV (0.65) series, confirming the one observed in the SEM analysis, presented (Fig. 7 C) as constituents the quartz (SiO2) and gipsite (CaSO4.2H2O), as observed in the research by Hoppe Filho et al. [32]. 3.3. Pulse velocity The Figs. 8 and the 9 show the pulse velocity results for the reference and aggressive exposure, respectively. In general, the tendency of the samples exposed to the reference condition were of increase of the parameter along the ages analyzed. The behavior was expected, since the samples were conditioned in an environment without aggressive agents and the increase in the pulse
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Fig. 6. SEM images obtained of the 0.65 water/cement ratio series, without silica fume, at 360 days of age. A – cement CP V; B – cement CP II; C – cement CP IV.
Table 4 Analysis of EDS in the points indicated in Fig. 6 (A, B and C).
*
Elements
Wt%* of the point of Fig. 6 A
Wt%* of the point of Fig. 6 B
Wt%* of the point of Fig. 6 C
Oxygen Calcium Sulfur Carbon Silica Aluminum
45.1 25.6 24.2 4.1 0.9 0.1
44.6 25.1 23.8 4.7 1.5 0.3
35.9 29.1 29.0 3.5 2.5 –
Wt% – Percentage, by weight of the elements, at the analyzed point.
velocity indicates a greater compactness, and therefore greater hydration of the cement. However, from 28 to 84 days, there was a decrease in the pulse propagation velocity in some series (Fig. 8), in general, those with higher addition contents, such as Ref. (0.40) CP IV + S and Ref. (0.40) CP II + S. At 168 and 360 days, these series already recovered a growth pattern, and this initial variation could be related to the initial hydration of the matrix, slower for cements with higher percentage of addition. Comparing the series of the same cement and the same water/cement ratio between themselves, analyzing only the influence of the 10% of silica (Ref. (0.40) CP V versus Ref. (0.40) CP V + S, for example), in all cases a small increase of the pulse was observed with the use of the addition, that was approximately 2%. Analyzing the graphics in both figures there is a tendency of green dots on the upper part and red on the lower part, indicating higher values for CP IV and lower for CP II. Opposite to the growth tendency observed in the reference exposure condition, in the aggressive condition (Fig. 9), the tendency was of a decrease in velocity, for all series, between all ages. The decrease in velocity indicates the greater presence of voids in the matrix and, consequently, greater fragility. According to Jiang et al. [33] the formation of gipsite and ettringite, consequence of the attack, was the factor responsible for the appearance of crack in the matrix, by the generated tensions. Thus, as observed in SEM images, the pulse test was sensitive to indicate the occurrence of the attack and damage brought by exposure of the samples in the aggressive environment. 3.4. Electrical resistivity Fig. 10 shows the results of electrical resistivity for reference condition. It could be noted, once again, that CP IV cement was
responsible for the higher resistivity values in all ages, as shown by the study of Medeiros Junior et al. [34], Medeiros Junior e Lima [35] and Mendes et al. [36] as the effect of pozzolanic additions. All series increased the resistivity considerably from 28 days to 360 days, which increased from approximately 4.5 for CP II, 3.5 for CP IV and 3 for CP V. The values of electrical resistivity in the aggressive condition (Fig. 11) could be considered higher than those observed in the reference condition, comparing the same series in the two conditions. The series that presented the greatest difference between the condition was (0.50) CP IV, being the aggressive condition 323% higher than the reference one. The higher values of resistivity presented in Figs. 10 and 11 are for the CP IV with active silica. This occurs because that is the group with lower content of portlandite, since it has lower clinker content and the pozzolanic effect, which tends to consume calcium hydroxide. Ca(OH)2 tends to dissociate in Ca+2 and OH– in the presence of water, generating an electrolyte with high electrical conductivity. Thus, if the concrete has a low amount of portlandite by the pozzolanic effect, the electrical resistivity of this concrete tends to be larger, as evidenced by the work of Medeiros et al. [37]. The fact that the electrical resistivity is higher for the concrete exposed to the aggressive environment, compared to the reference exposure (in lime water) is also related to the portlandite. Ca(OH)2 is one of the first elements to be transformed into the mechanisms of degradation by biogenic sulfuric acid, forming crystals of ettringite and gypsum. Thus, the pore solution of the concrete becomes less conductive as a result of the degradation at the sewage treatment plant. This is an example of how the electrical resistivity data interpretation needs to be done by a trained and experienced professional, since the tendency is to interpret that the more resistive the concrete, the better the integrity condition it is. However, in this case the opposite occurs. 3.5. Compressive strength Fig. 12 shows the evolution of the compressive strength for the samples in the reference condition. It is observed that between the water/cement ratio series 0.40 the ones that presented the greatest compressive strength throughout the study were Ref. (0.40) CP V and Ref. (0.40) CP V + S. At 28 days of age, all series with a water/ cement ratio of 0.65 were below 40 MPa, and were not in accordance with the minimum compressive strength required by NBR 6118 [9]. However, all groups had increased compressive strength over time, and the only one that did not reach 40 MPa, in the reference condition, at 360 days of age, was (0.65) CP II.
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Fig. 7. XRD analysis obtained of the 0.65 water/cement ratio series, without silica fume, at 360 days of age. A – cement CP V; B – cement CP II; C – cement CP IV.
Contrary to that observed in the previous condition, the samples subjected to aggressive condition presented a decrease in compressive strength, all of them at 360 days of age, below
40 MPa (Fig. 13). The decrease in this property corroborates to the results obtained in the pulse velocity, SEM images and XRD, indicating fragility of the cement matrices and proving
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5800
Pulse velocity (m/s)
5650 5500 5350 5200 5050 4900 4750 0
40
80 120 160 200 240 280 320 360 400
Age (days)
Ref. (0.40) CP V (0.50) CP V (0.65) CP V Ref. (0.40) CP V + S (0.50) CP V + S (0.65) CP V +S Ref. (0.40) CP II (0.50) CP II (0.65) CP II Ref. (0.40) CP II + S (0.50) CP II + S (0.65) CP II +S Ref. (0.40) CP IV (0.50) CP IV (0.65) CP IV Ref. (0.40) CP IV + S (0.50) CP IV + S (0.65) CP IV +S
Fig. 8. Pulse velocity monitoring for the series in reference exposure (tank with saturated lime water).
Pulse velocity (m/s)
5800 5650 5500 5350 5200 5050 4900 4750 0
40
80
120 160 200 240 280 320 360 400
Ref. (0.40) CP V (0.50) CP V (0.65) CP V Ref. (0.40) CP V + S (0.50) CP V + S (0.65) CP V +S Ref. (0.40) CP II (0.50) CP II (0.65) CP II Ref. (0.40) CP II + S (0.50) CP II + S (0.65) CP II +S Ref. (0.40) CP IV (0.50) CP IV (0.65) CP IV Ref. (0.40) CP IV + S (0.50) CP IV + S (0.65) CP IV +S
Age (days) Fig. 9. Pulse velocity monitoring for the series in aggressive exposure (sewage treatment plant).
Ref. (0.40) CP V
Electrical Resistivity (Ωm)
300
(0.50) CP V (0.65) CP V
250
Ref. (0.40) CP V + S (0.50) CP V + S (0.65) CP V +S
200
Ref. (0.40) CP II (0.50) CP II
150
(0.65) CP II Ref. (0.40) CP II + S
100
(0.50) CP II + S (0.65) CP II +S
50
Ref. (0.40) CP IV (0.50) CP IV (0.65) CP IV
0 0
40
80
120
160
200
240
Age (days)
280
320
360
400
Ref. (0.40) CP IV + S (0.50) CP IV + S (0.65) CP IV +S
Fig. 10. Electrical resistivity monitoring for the series in reference exposure (tank with saturated lime water).
the aggressiveness of the studied environment. Even the concrete that met the initial requirements of the standard (relation water/cement, cement consumption and compressive strength at 28 days) showed a decrease in its properties at 360 days.
The damage observed in the mechanical properties of the concretes exposed to the aggressive condition is in accordance with the one presented by Grengg et al. [9], who noticed in their study that the crystallization of products from the attack causes a decrease in the mechanical strength of concrete.
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Ref. (0.40) CP V
Electrical Resistivity (Ωm)
300
(0.50) CP V (0.65) CP V Ref. (0.40) CP V + S
250
(0.50) CP V + S (0.65) CP V +S
200
Ref. (0.40) CP II (0.50) CP II
150
(0.65) CP II Ref. (0.40) CP II + S
100
(0.50) CP II + S (0.65) CP II +S Ref. (0.40) CP IV
50
(0.50) CP IV (0.65) CP IV
0 0
40
80
120
160
200
240
280
320
360
400
Ref. (0.40) CP IV + S (0.50) CP IV + S (0.65) CP IV +S
Age (days)
Compressive Strength (MPa)
Fig. 11. Electrical resistivity monitoring for the series in aggressive exposure (sewage treatment plant).
100 90 80 70 60 50 40 30 20 10 0 0
40
80
120
160
200
240
280
320
360
400
Age (days)
Ref. (0.40) CP V (0.50) CP V (0.65) CP V Ref. (0.40) CP V + S (0.50) CP V + S (0.65) CP V +S Ref. (0.40) CP II (0.50) CP II (0.65) CP II Ref. (0.40) CP II + S (0.50) CP II + S (0.65) CP II +S Ref. (0.40) CP IV (0.50) CP IV (0.65) CP IV Ref. (0.40) CP IV + S (0.50) CP IV + S (0.65) CP IV +S
Compressive Strength (MPa)
Fig. 12. Compressive strength monitoring for the series in reference exposure (tank with saturated lime water).
100 90 80 70 60 50 40 30 20 10 0 0
40
80
120
160
200
240
Age (days)
280
320
360
400
Ref. (0.40) CP V (0.50) CP V (0.65) CP V Ref. (0.40) CP V + S (0.50) CP V + S (0.65) CP V +S Ref. (0.40) CP II (0.50) CP II (0.65) CP II Ref. (0.40) CP II + S (0.50) CP II + S (0.65) CP II +S Ref. (0.40) CP IV (0.50) CP IV (0.65) CP IV Ref. (0.40) CP IV + S (0.50) CP IV + S (0.65) CP IV +S
Fig. 13. Compressive strength monitoring for the series in aggressive exposure (sewage treatment plant).
4. Conclusions The exposure of the samples in the gas chamber of sewage treatment plant allowed to evaluate the characteristics of the concrete under a condition of extreme aggressiveness. The development of the research described in this work allows the presentation of the following conclusions: - At 168 days of age of the samples, it was possible to notice difference between the samples of the different cements. The SEM
test identified a higher concentration of deleterious products (gypsum and ettringite) in cements with lower percentages of addition (CPV and CPII) confirming their susceptibility to attack. The result obtained in the image test indicate the need for the correct specification of cement for the different types of exposure of concrete structures; - The pulse velocity test was sensitive to differentiate the series from the reference and aggressive condition, the last being responsible for the lower velocities. Therefore, the applied technique proves to be viable from an investigative point of
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view, which may be an inspection tool to evaluate the quality of concrete structures subjected to biogenic sulfate attack; The electrical resistivity test indicated the highest values for the series exposed in the aggressive condition, being explained by the physical alterations of pore filling by the degrading products, as observed in the SEM test. The test used also proved to be a structural assessment tool, since it was sensitive in the differentiation of the studied series; All series exposed to aggressive condition showed a decrease in compressive strength when compared the ages 28 and 360 days. The series that presented the greatest drop was Ref. (0.40) CP V + S, being 63%, followed by (0.50) CP V + S, being 53%; The use of active silica in the evaluated content, despite indicating higher filling at early ages by the tests of pulse velocity and resistivity, did not prevent the entry of gases and the occurrence of the attack, as evidenced by the decrease in the compressive strength. The study of the use of different silica contents could be interesting in future works by determining the ideal content to improve concrete properties; The mixture that met the standard requirements (water/cement ratio, cement consumption and compressive strength at 28 days), as well as the others, showed a decrease in its properties, alerting to the strong aggressiveness of the environment. This result reinforces the need for the correct choice of cement type, an observation not contemplated by the Brazilian concrete standard; Finally, the study highlights the need to research new standards limits and protective alternatives to structures subject to the aggressive condition of sewage treatment plants.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors thank the support, as scholarships, by Foundation Araucária, as well as the availability of the infrastructure and human resources made available by the Federal University of Paraná (PPGECC/UFPR-Brazil). Our gratitude also to the Center of Electron Microscopy of UFPR (CME/UFPR), by the support during the tests of SEM, to the NOVAMIX, by the support during the concrete mixtures and the company responsible for the sewage treatment plant, SANEPAR, by the availability of the structure. References [1] X. Li, F. Khademi, Y. Liu, M. Akbari, C. Wang, P.L. Bond, J. Keller, G. Jiang, Evaluation of data-driven models for predicting the service life of concrete sewer pipes subjected to corrosion, J. Environ. Manage. 234 (2019) 431–439. [2] G. Jiang, J. Keller, P.L. Bond, Z. Yuan, Predicting concrete corrosion of sewers using artificial neural network, Water Res. 92 (2016) 52–60. [3] M.P.H. Brongers, P.Y. Virmani, J.H. Payer, Drinking water and sewer systems in corrosion costs and preventative strategies in the United States, United States Department of Transportation Federal Highway Administration, 2002. [4] W. Kaempfer, M. Berndt, Estimation of service life of concrete pipes in sewer networks, Durability of Building Materials and Components 8 (1999) 36–45. [5] M.G. Alexander, A. Bertron, N. De Belie, Performance of cement-based materials in aggressive aqueous environments State-of-the-Art Report, Springer, RILEM TC 211 – PAE, 2013. [6] E. Stanaszek-Tomal, M. Fiertak, Biological corrosion in the sewage system and the sewage treatment plant, Procedia Eng. 161 (2016) 116–120.
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