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Preliminary experimental study on the effects of surface-applied photocatalytic products on the durability of reinforced concrete
Construction and Building Materials 48 (2013) 137–143
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Preliminary experimental study on the effects of surface-applied photocatalytic products on the durability of reinforced concrete Alessandra Fiore a,⇑, Giuseppe Carlo Marano a, Pietro Monaco a, Alessandro Morbi b a b
Technical University of Bari, DICAR, Via Orabona 4, 70125 Bari, Italy Italcementi Group S.p.A, Bergamo, Italy
h i g h l i g h t s The effect of photocatalytic products on concrete durability was tested. The photocatalytic activity of titanium dioxide was used. The durability of concrete in terms of carbonation resistance was improved. Surface-applied photocatalytic products provide protection for steel bars in concrete.
a r t i c l e
i n f o
Article history: Received 5 September 2012 Received in revised form 10 May 2013 Accepted 17 June 2013
Keywords: Photocatalytic products Reinforced concrete structures Durability Carbonation Reinforcing bar corrosion Experimental tests
a b s t r a c t The aim of this paper is to assess, by the results of suitable experimental tests, the durability performances of surface-applied concrete layers that incorporate a photocatalytic material such as titanium dioxide. The use of photocatalytic materials for air purification has been developing rapidly in the last decades. Within this framework, the proposed experimental study is particularly significant considering that, although there are many advantages in applying photocatalytic construction materials, during the TiO2-photocatalysis a large variety of organics, viruses, bacteria, fungi, algae can be totally degraded and mineralized to CO2, H2O and harmless inorganic anions. The mineralized amounts of CO2, deriving from both the photocatalytic oxidation itself and the external ambient, can progressively activate a chemical deterioration due to carbonation. It is then important to investigate the resistance properties of photocatalytic concrete products against carbonation. The effects of the photocatalytic activity on the durability of concrete have been assessed by using accelerated carbonation and corrosion of reinforcing steel tests. In order to keep the photocatalytic process ongoing, specimens with finish coatings containing TiO2 were put in direct contact with air (water in the form of humidity) and were subjected to light radiation. Results show that photocatalytic concrete products applied on the surface of concrete improve the property of carbonation resistance and reduce the corrosion propagation rate of reinforcing bars with respect to the case of cement coatings without any photocatalysts. The adopted experimental procedure can give a first contribution in order to understand if the photocatalytic activity of titanium dioxide creates a photocatalysed system acting as a protective barrier against the deterioration processes. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Reinforced concrete (RC) has been used as the main structural material in the construction industry for many years. Nevertheless in the last decades the rapid deterioration of RC structures due to alkali-aggregate reaction, chloride-induced corrosion and carbonation has caused engineers to seek new ways to rehabilitate aging structure and to improve durability properties of concrete under aggressive environments. ⇑ Corresponding author. Tel.: +39 080 5963743; fax: +39 080 5963719. E-mail address: a.fi[email protected] (A. Fiore). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.06.058
Among the countermeasures adopted against the deterioration of reinforced concrete structures, the use of composite materials has shown great potential in the area of durability of RC structures. The cementitious-polymeric composite materials are used for the modification of concrete surfaces and include finish coatings, barrier penetrants, linings, liquid-applied membrane waterproofing materials and permanent forms, which allow to eliminate or control chemical degradation factors in RC structures [1]. Considerable research has been carried out on concretes containing binary cements based on fly ash or silica fume which have shown advantages such as improved durability [2,3]. Polymer-coated reinforcing bars, including epoxy-coated reinforcing bars, FRP reinforcements
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and continuous fiber reinforcing materials, are employed as corrosion-resistant materials [4,5]. Also the use of corrosion inhibitors is resulted to be an effective method in order to control rebar corrosion [6,7]. Only recently technical codes have introduced the durability and reliability issues, which both rank amongst the most decisive structural performance characteristics. In particular Model Code 2010 specializes in the durability design of concrete structures [8]. The objective is to identify agreed durability related models and to prepare the framework for the standardization of performance-based durability design approaches. According to this innovative approach, structural design should take into account environmental actions leading to the degradation of concrete and embedded steel. In this framework this paper presents the results of a systematic study on assessing the effects of photocatalytic concrete products on the durability of reinforced concrete. Until now photocatalytic materials have been applied on concrete pavement surfaces and external building surfaces in urban areas mainly to reduce air pollution caused by road traffic and industry. In fact the use of titanium dioxide as photocatalyst mainly allows, under the UV-A part of the sunlight, the photochemical conversion of nitrogen oxides (NOx), emitted in the atmosphere by car traffic and transport, to low concentrated nitrates due to heterogeneous photocatalytic oxidation [9,10]. The reaction products in the form of nitrate compounds are water soluble and will be flushed from the active concrete surface by rain. The nitrate compounds can be finally extracted from the rain water by a standard sewage plant. In addition the nature of the cement matrix is particularly suitable for incorporating titanium dioxide (TiO2) powders and other photo-oxidation products. As a result a variety of products containing TiO2 are already available on the European market and their capacity to mitigate air pollution is widely proven. Differently from the above approaches, this paper presents the results of experimental tests carried out in order to evaluate the durability properties of photocatalytic concrete products in terms of carbonation of concrete and corrosion of reinforcing steel [11–13]. In fact the photocatalytic oxidation activated by the titanium dioxide not only allows the conversion of nitrogen oxides, but in general is capable of degrading a wide range of pollutants, both of organic and inorganic nature, under the influence of UV or solar light. In particular the photocatalytic oxidation can decompose the volatile organic compounds to harmless substances such as CO2 and H2O [10]. Part of these reaction products can be transferred to solid CaCO3 in the outer pore system and the mineralized CO2 can be consumed by the carbonation. This effect is reduced by the contemporary production of H2O, so that it is difficult to establish the quantity of CO2 involved in the carbonation process. On this topic it is worth noting that a special phenomenon, termed ‘‘photoinduced superhydrophilicity’’ has been discovered in the last years, according to which trapping of holes at the TiO2 surface causes an high wettability [14]. In conclusion cover concrete during the photocatalytic degradation process can be exposed to some amounts of CO2, deriving both from the photocatalytic oxidation itself and the external ambient [15], which can activate a chemical deterioration due to carbonation. It is then important to assess the carbonation resistance of photocatalytic concrete products in order to be sure to use their airpurifying properties without reducing the durability performance of the structure. This topic can be investigated just by suitable experimental tests, due to the different internal and external factors which can affect the amount of carbonation occurring with a given cement paste composition, such as water/cement ratio, chemical composition of the cement paste, curing time, internal and/or external relative humidity, external temperature, carbon dioxide concentration and texture of the surface [16,17]. The goal of the study is to assess the carbonation resistance of photocata-
lytic concrete coatings during the photocatalytic activity itself, reproducing real external ambient conditions. In the experimental setup photoactivity is produced thanks to the interaction between cement mortar surface containing TiO2, UV radiation and H2O (water in the form of humidity). In this case the reaction products are represented by hydroxyl radicals, hydrogen and/or superoxide ions [10]. The described experimental procedure can give a first contribution in order to understand if the application of photocatalysts in coatings of reinforced concrete structures can create a photocatalysed system acting as a protective barrier against the deterioration processes. 2. Experimental details 2.1. Concrete mix and casting of test specimens Sixteen cubes (10 10 10 cm3, Type A specimen) and three reinforced slabs (20 10 5 cm3, Type B specimen) were cast for this experimental investigation (Fig. 1). Type CEM 32.5R II-A/LL Portland Cement (with Matera limestone) was used in the concrete mix for the test specimens. Table 1 gives the chemical properties of the concrete. Figs. 2 and 3 show the sections of Type A and B specimens respectively.
Fig. 1. Casting of test specimens: (a) Type A specimens; and (b) Type B specimens. Table 1 Mixture proportions of concrete. Mix components
Weight (%)
Weight (kg/m3)
CEM 32.5R II-A/LL Matera Sand (0–4)VC Fine aggregate (4–10)VC Course aggregate (10–20)VC Water CRETV-M [03–1] additive
100.0 58.0 13.0 29.0 0.70 0.33
300 983.1 225.6 489.6 210 1.00
Fig. 2. Section of Type A specimen.
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Fig. 3. Section of (a) N or TX and (b) TQ Type B specimens.
After casting, the sides of eight cubic specimens and of one Type B sample were covered with cement (N) mortar and the sides of eight cubic specimens and of one Type B sample were covered with photocatalytic (TX) cement mortar, with thickness of 3 mm (s1) for all specimens except for two Type A specimens with N mortar and two with TX mortar, for which a thickness of 5 mm (s2) was adopted. Type FINICEM 6 BIANCO IDRO mortars containing 25% white cement were produced. In the TX mortars a 5% TiO2 content was added. On the third slab (TQ) no finish coatings were applied.
still highly alkaline, a purple-red colour was obtained. In the carbonated part of the specimen where the alkalinity of concrete is reduced, no colouration occurred [22]. According to code UNI EN 13295:2005 [23], the average depth of the colourless phenolphthalein region was measured from three points for each side, on both the two edges of the split face, about 1 h after spraying the indicator (Fig. 5).
2.2. Accelerated carbonation Carbonation is a neutralizing reaction during which hydroxide components of the cement paste react with the mild carbonic acid created by the carbon dioxide gas when it dissociates in the pore water. As a result carbonation reduces the hydroxide concentration in the pore solution, destroying the passivity of the embedded reinforcement. Since carbonation reactions are very slow in natural environments, accelerated methods can be adopted for measurement of non-time dependent relationships in reasonably short time frames. In the case herein analyzed, response has been accelerated incubating specimens in an environment with an elevated concentration of carbon dioxide (5%) so that the reaction is not limited by the amount available. The concentration of 5% for the CO2 air content is widely used in accelerated carbonation tests [18,19]. Focusing on the accelerated testing protocol herein adopted, after curing for 6 days, the specimens were transferred to a sealed chamber and subjected to accelerated carbonation at 35 °C in temperature, a CO2 concentration of 5% and relative humidity varying between 30% and 80% every 12 h. The aim was to reproduce the alternate wet and dry conditions characterizing aggressive marine environments. The choice concerning relative humidity can be clarified considering that the most conducive humidity values, for maximum carbonation, range from 50% to 70%; similarly carbonation rates decrease at 100% relative humidity since carbon dioxide cannot easily penetrate saturated pores [20,21]. Fluorescent lamps were finally used to provide UV radiation. The test specimens were taken out of the carbonation chamber at 14, 21 and 28 days, in order to investigate the evolution in time of deterioration. Fig. 4 shows the carbonation chamber used for the accelerated experimentation.
3. Concrete carbonation test The following test specimens were taken out of the carbonation chamber at 14 days: two Type A specimens with N mortar (with s1 thickness) and two Type A specimens with TX mortar (with s1 thickness). All the other Type A specimens were taken out of the carbonation chamber at 21 days. After splitting the concrete specimens, the freshly split surface was cleaned and sprayed with a phenolphthalein pH indicator. In the noncarbonated part of the specimen, where the concrete was
Fig. 4. Accelerated carbonation chamber.
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Fig. 5. Concrete carbonation test; measurement of the average depth of the colourless phenolphthalein region.
Fig. 6. Concrete carbonation test at 14 days on Type A specimens with s1 thickness: (a and c) with N mortar; and (b and d) with TX mortar.
Table 2 Average depth of the colourless phenolphthalein region for specimen 1 N.
Carbonation test – 14th day – specimen 2 N
Carbonation test – 14th day – specimen 1 N Right section – dk,1 3 (mm) a1 = 36.60 b1 = 35.21 a2 = 31.45 b2 = 33.50 a3 = 33.52 b3 = 35.95 dk,a = 33.85 dk,b = 34.88 P dk(a d),right = dk,right = 39.94 mm Left section – dk,1 3 (mm) a1 = 37.67 b1 = 37.36 a2 = 33.67 b2 = 33.01 a3 = 34.23 b3 = 37.03 dk,a = 35.19 dk,b = 35.80 P dk(a d),left = dk,left = 41.10 mm dk = 40.52 mm
Table 3 Average depth of the colourless phenolphthalein region for specimen 2 N.
c1 = 45.95 c2 = 46.41 c3 = 50.60 dk,c = 47.65
d1 = 43.37 d2 = 39.18 d3 = 47.60 dk,d = 43.38
c1 = 43.92 c2 = 40.96 c3 = 44.61 dk,c = 43.16
d1 = 51.81 d2 = 47.98 d3 = 51.01 dk,d = 50.26
Right section – dk,1 3 (mm) a1 = 33.87 b1 = 37.34 a2 = 32.00 b2 = 33.44 a3 = 33.70 b3 = 35.32 dk,a = 33.19 dk,b = 35.36 P dk(a d),right = dk,right = 35.22 mm Left section – dk,1 3 (mm) a1 = 36.36 b1 = 35.18 a2 = 33.19 b2 = 35.59 a3 = 33.53 b3 = 37.45 dk,a = 34.36 dk,b = 36.07 P dk(a d),left = dk,left = 34.96 mm dk = 35.09 mm
c1 = 38.48 c2 = 31.88 c3 = 36.08 dk,c = 35.48
d1 = 35.40 d2 = 33.41 d3 = 41.69 dk,d = 36.83
c1 = 35.33 c2 = 33.38 c3 = 37.84 dk,c = 35.51
d1 = 35.7 d2 = 31.24 d3 = 34.76 dk,d = 33.9
A. Fiore et al. / Construction and Building Materials 48 (2013) 137–143 Table 4 Average depth of the colourless phenolphthalein region for specimen 1 TX. Carbonation test – 14th day – specimen 1 TX Right section – dk,1 3 (mm) a1 = 33.36 b1 = 38.23 a2 = 31.69 b2 = 32.67 a3 = 34.53 b3 = 31.36 dk,b = 34.08 dk,a = 33.86 P dk(a d),right = dk,right = 33.17 mm Left section – dk,1 3 (mm) a1 = 40.1 b1 = 35.98 a2 = 30.51 b2 = 31.17 a3 = 34.99 b3 = 37.53 dk,a = 35.2 dk,b = 34.89 P dk(a d),left = dk,left = 34.21 mm dk = 33.69 mm
c1 = 30.22 c2 = 29.8 c3 = 31.21 dk,c = 30.41
d1 = 34.05 d2 = 31.19 d3 = 37.81 dk,d = 34.35
c1 = 33.93 c2 = 30.51 c3 = 31.52 dk,c = 31.98
d1 = 35.34 d2 = 34.16 d3 = 34.84 dk,d = 34.78
Carbonation test – 14th day – specimen 2 TX
Left section – dk,1 3 (mm) a1 = 31.37 b1 = 31.72 a2 = 28.72 b2 = 29.02 a3 = 32.92 b3 = 34.06 dk,a = 31.003 dk,b = 31.6 P dk(a d),left = dk,left = 31.83 mm dk = 31.56 mm
specimens with TX mortar is approximately on average 0.86 the depth of specimens with N mortar. At 21 days the surfaces of all the other specimens have been fully carbonated (Fig. 7); this result is due to the highly aggressive conditions created in the carbonation chamber. The carbonation depths of Type A specimens are shown in Fig. 8 versus the CO2 exposure period. 4. Test of reinforcing bar corrosion
Table 5 Average depth of the colourless phenolphthalein region for specimen 2 TX.
Right section – dk,1 3 (mm) a1 = 32.16 b1 = 31.31 a2 = 30.89 b2 = 28.56 a3 = 32.6 b3 = 29.95 dk,a = 31.88 dk,b = 29.94 P dk(a d),right = dk,right = 31.29 mm
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c1 = 31.48 c2 = 30.45 c3 = 33.14 dk,c = 31.69
d1 = 33.01 d2 = 30.44 d3 = 31.59 dk,d = 31.68
c1 = 35.96 c2 = 30.11 c3 = 33.71 dk,c = 33.26
d1 = 31.3 d2 = 29.81 d3 = 33.26 dk,d = 31.45
3.1. Results and discussion Fig. 6 shows the results obtained for the two Type A specimens with N mortar and the two with TX mortar taken out of the carbonation chamber at 14 days. The corresponding carbonation depths are reported in Tables 2–5, according to the dimensional parameters introduced in Fig. 5. It is evident that carbonation is more intensive in specimens without TiO2. The carbonation depth of
The corrosion of reinforcing steel in concrete exposed to aggressive environment leads to cracking of concrete and subsequent loss in the load-carrying capacity of a reinforced concrete member [24,25]. In the experimental study herein presented the corrosion mechanism of reinforcing bars induced by carbonation of concrete has been investigated. Steel embedded in good quality concrete is chemically protected by the high alkalinity of pore water, while the dense and relatively impermeable structure of concrete provides the physical protection. The alkaline compounds, mainly calcium and to a certain extent potassium and sodium, in the cement contribute to the high alkalinity of the pore solution, which in the presence of oxygen, passivates the steel. The loss of alkalinity due to carbonation of concrete can destroy the passive film. The test to assess the properties of surface-applied photocatalytic products on protection of steel bars, has interested the three Type B specimens described in Section 2.1. Each specimen contained four 12 mm diameter reinforcing bars with four different concrete covers (C1 = 1.5 cm; C2 = 2.0 cm; C3 = 2.5 cm; C4 = 3.0 cm), as shown in Fig. 3. The three slabs were taken out of the carbonation chamber at 21 days and sprayed with a phenolphthalein pH indicator. On the non-passivated steel a purple-red colour was obtained; similarly when the passive layer was broken down no colouration occurred. The progress of corrosion was evaluated by observing on reinforcing bars the colourless phenolphthalein region. Fig. 9 illustrates some images of the test. 4.1. Results and discussion Fig. 10 shows the results obtained with reference to the TQ slab. It is evident that the level of corrosion decreases as the concrete
Fig. 7. Concrete carbonation test at 21 days on Type A specimens: (a) with N mortar and s1 thickness; (b) with N mortar and s2 thickness; (c) with TX mortar and s1 thickness; and (d) with TX mortar and s2 thickness.
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Fig. 8. Carbonation resistance of Type A specimens.
Fig. 10. Test of reinforcing bar corrosion for TQ Type B specimen.
Fig. 9. Test of reinforcing bar corrosion for TQ, N and TX Type B specimens taken out of the carbonation chamber at 21 days.
cover increases. Similarly Fig. 11 shows the comparison between the three Type B specimens with reference to all the C1–C4 concrete covers. The corrosion propagation rate of reinforcing bars is visibly lower in the TX slab with respect to the TQ and N ones. The obtained measurements are summarized in Fig. 12a and b, where the percentages of passivated steel and carbonated area are respectively shown in function of the concrete cover for each Type B specimen. It is evident that in all specimens the percentages of passivated steel and carbonated area decrease with increasing the concrete cover. The test results also clearly indicate that TX specimens are characterized by higher resistances against carbonation and barcorrosion in comparison with N and TQ ones.
Fig. 11. Test of reinforcing bar corrosion for TX, N and TQ Type B specimens at 21 days: (a) with C1 concrete cover; (b) with C2 concrete cover; (c) with C3 concrete cover; and (d) with C4 concrete cover.
In particular Fig. 12a shows that the reinforcing bars in the TQ specimen are more susceptible to localized corrosion than the bars in the N and TX slabs. In the case of C1 concrete cover, the percentage of passivated steel reaches a value of approximately 84% in the TQ specimen, while is equal to about 46% and 41% in the N and TX slabs respectively. This attests the effectiveness of using mortar finish coatings in order to improve the durability of RC structures.
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corrosion propagation rate of reinforcing bars decreases as the concrete cover increases. Acknowledgment Prof. Marcello Di Marzo and Ing. Enrico Vitobello are gratefully acknowledged for their contribution and devices during the experimental program. References
Fig. 12. Percentages of (a) passivated steel and (b) carbonated area versus the concrete cover for Type B specimens taken out of the carbonation chamber at 21 days.
Finally observing Fig. 12b it emerges that in correspondence of the concrete cover C4 the carbonated zone in the TX slab is about 0.7 the ones of the N and TQ slabs. This difference is less evident as the concrete cover decreases, but is anyway significant: in correspondence of the concrete cover C1 the carbonated zone in the TX slab is about 0.9 the ones of the N and TQ slabs. 5. Conclusions The experimental study herein presented deals with the design topic of durability of reinforced concrete elements. In this field, the effects of applying concrete surface layers incorporating a photocatalytic material have been investigated, analyzing the efficiency in terms of carbonation of concrete and corrosion of reinforcing bars. For sake of clearness, the results obtained on specimens covered by photocatalytic cement mortar have been compared with the ones obtained on specimens simply covered by cement mortar. The results of the experimental tests have shown that the concrete carbonation depth can be significantly reduced by adopting photocatalytic surface layers. The results have also indicated that the application of titanium dioxide modified cementitious materials on the external surface of reinforced concrete elements improves the corrosion performance of reinforcing bars in presence of carbonation of concrete. In particular it also emerged that the
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Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Preliminary experimental study on the effects of surface-applied photocatalytic products on the durability of reinforced concrete Alessandra Fiore a,⇑, Giuseppe Carlo Marano a, Pietro Monaco a, Alessandro Morbi b a b
Technical University of Bari, DICAR, Via Orabona 4, 70125 Bari, Italy Italcementi Group S.p.A, Bergamo, Italy
h i g h l i g h t s The effect of photocatalytic products on concrete durability was tested. The photocatalytic activity of titanium dioxide was used. The durability of concrete in terms of carbonation resistance was improved. Surface-applied photocatalytic products provide protection for steel bars in concrete.
a r t i c l e
i n f o
Article history: Received 5 September 2012 Received in revised form 10 May 2013 Accepted 17 June 2013
Keywords: Photocatalytic products Reinforced concrete structures Durability Carbonation Reinforcing bar corrosion Experimental tests
a b s t r a c t The aim of this paper is to assess, by the results of suitable experimental tests, the durability performances of surface-applied concrete layers that incorporate a photocatalytic material such as titanium dioxide. The use of photocatalytic materials for air purification has been developing rapidly in the last decades. Within this framework, the proposed experimental study is particularly significant considering that, although there are many advantages in applying photocatalytic construction materials, during the TiO2-photocatalysis a large variety of organics, viruses, bacteria, fungi, algae can be totally degraded and mineralized to CO2, H2O and harmless inorganic anions. The mineralized amounts of CO2, deriving from both the photocatalytic oxidation itself and the external ambient, can progressively activate a chemical deterioration due to carbonation. It is then important to investigate the resistance properties of photocatalytic concrete products against carbonation. The effects of the photocatalytic activity on the durability of concrete have been assessed by using accelerated carbonation and corrosion of reinforcing steel tests. In order to keep the photocatalytic process ongoing, specimens with finish coatings containing TiO2 were put in direct contact with air (water in the form of humidity) and were subjected to light radiation. Results show that photocatalytic concrete products applied on the surface of concrete improve the property of carbonation resistance and reduce the corrosion propagation rate of reinforcing bars with respect to the case of cement coatings without any photocatalysts. The adopted experimental procedure can give a first contribution in order to understand if the photocatalytic activity of titanium dioxide creates a photocatalysed system acting as a protective barrier against the deterioration processes. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Reinforced concrete (RC) has been used as the main structural material in the construction industry for many years. Nevertheless in the last decades the rapid deterioration of RC structures due to alkali-aggregate reaction, chloride-induced corrosion and carbonation has caused engineers to seek new ways to rehabilitate aging structure and to improve durability properties of concrete under aggressive environments. ⇑ Corresponding author. Tel.: +39 080 5963743; fax: +39 080 5963719. E-mail address: a.fi[email protected] (A. Fiore). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.06.058
Among the countermeasures adopted against the deterioration of reinforced concrete structures, the use of composite materials has shown great potential in the area of durability of RC structures. The cementitious-polymeric composite materials are used for the modification of concrete surfaces and include finish coatings, barrier penetrants, linings, liquid-applied membrane waterproofing materials and permanent forms, which allow to eliminate or control chemical degradation factors in RC structures [1]. Considerable research has been carried out on concretes containing binary cements based on fly ash or silica fume which have shown advantages such as improved durability [2,3]. Polymer-coated reinforcing bars, including epoxy-coated reinforcing bars, FRP reinforcements
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and continuous fiber reinforcing materials, are employed as corrosion-resistant materials [4,5]. Also the use of corrosion inhibitors is resulted to be an effective method in order to control rebar corrosion [6,7]. Only recently technical codes have introduced the durability and reliability issues, which both rank amongst the most decisive structural performance characteristics. In particular Model Code 2010 specializes in the durability design of concrete structures [8]. The objective is to identify agreed durability related models and to prepare the framework for the standardization of performance-based durability design approaches. According to this innovative approach, structural design should take into account environmental actions leading to the degradation of concrete and embedded steel. In this framework this paper presents the results of a systematic study on assessing the effects of photocatalytic concrete products on the durability of reinforced concrete. Until now photocatalytic materials have been applied on concrete pavement surfaces and external building surfaces in urban areas mainly to reduce air pollution caused by road traffic and industry. In fact the use of titanium dioxide as photocatalyst mainly allows, under the UV-A part of the sunlight, the photochemical conversion of nitrogen oxides (NOx), emitted in the atmosphere by car traffic and transport, to low concentrated nitrates due to heterogeneous photocatalytic oxidation [9,10]. The reaction products in the form of nitrate compounds are water soluble and will be flushed from the active concrete surface by rain. The nitrate compounds can be finally extracted from the rain water by a standard sewage plant. In addition the nature of the cement matrix is particularly suitable for incorporating titanium dioxide (TiO2) powders and other photo-oxidation products. As a result a variety of products containing TiO2 are already available on the European market and their capacity to mitigate air pollution is widely proven. Differently from the above approaches, this paper presents the results of experimental tests carried out in order to evaluate the durability properties of photocatalytic concrete products in terms of carbonation of concrete and corrosion of reinforcing steel [11–13]. In fact the photocatalytic oxidation activated by the titanium dioxide not only allows the conversion of nitrogen oxides, but in general is capable of degrading a wide range of pollutants, both of organic and inorganic nature, under the influence of UV or solar light. In particular the photocatalytic oxidation can decompose the volatile organic compounds to harmless substances such as CO2 and H2O [10]. Part of these reaction products can be transferred to solid CaCO3 in the outer pore system and the mineralized CO2 can be consumed by the carbonation. This effect is reduced by the contemporary production of H2O, so that it is difficult to establish the quantity of CO2 involved in the carbonation process. On this topic it is worth noting that a special phenomenon, termed ‘‘photoinduced superhydrophilicity’’ has been discovered in the last years, according to which trapping of holes at the TiO2 surface causes an high wettability [14]. In conclusion cover concrete during the photocatalytic degradation process can be exposed to some amounts of CO2, deriving both from the photocatalytic oxidation itself and the external ambient [15], which can activate a chemical deterioration due to carbonation. It is then important to assess the carbonation resistance of photocatalytic concrete products in order to be sure to use their airpurifying properties without reducing the durability performance of the structure. This topic can be investigated just by suitable experimental tests, due to the different internal and external factors which can affect the amount of carbonation occurring with a given cement paste composition, such as water/cement ratio, chemical composition of the cement paste, curing time, internal and/or external relative humidity, external temperature, carbon dioxide concentration and texture of the surface [16,17]. The goal of the study is to assess the carbonation resistance of photocata-
lytic concrete coatings during the photocatalytic activity itself, reproducing real external ambient conditions. In the experimental setup photoactivity is produced thanks to the interaction between cement mortar surface containing TiO2, UV radiation and H2O (water in the form of humidity). In this case the reaction products are represented by hydroxyl radicals, hydrogen and/or superoxide ions [10]. The described experimental procedure can give a first contribution in order to understand if the application of photocatalysts in coatings of reinforced concrete structures can create a photocatalysed system acting as a protective barrier against the deterioration processes. 2. Experimental details 2.1. Concrete mix and casting of test specimens Sixteen cubes (10 10 10 cm3, Type A specimen) and three reinforced slabs (20 10 5 cm3, Type B specimen) were cast for this experimental investigation (Fig. 1). Type CEM 32.5R II-A/LL Portland Cement (with Matera limestone) was used in the concrete mix for the test specimens. Table 1 gives the chemical properties of the concrete. Figs. 2 and 3 show the sections of Type A and B specimens respectively.
Fig. 1. Casting of test specimens: (a) Type A specimens; and (b) Type B specimens. Table 1 Mixture proportions of concrete. Mix components
Weight (%)
Weight (kg/m3)
CEM 32.5R II-A/LL Matera Sand (0–4)VC Fine aggregate (4–10)VC Course aggregate (10–20)VC Water CRETV-M [03–1] additive
100.0 58.0 13.0 29.0 0.70 0.33
300 983.1 225.6 489.6 210 1.00
Fig. 2. Section of Type A specimen.
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Fig. 3. Section of (a) N or TX and (b) TQ Type B specimens.
After casting, the sides of eight cubic specimens and of one Type B sample were covered with cement (N) mortar and the sides of eight cubic specimens and of one Type B sample were covered with photocatalytic (TX) cement mortar, with thickness of 3 mm (s1) for all specimens except for two Type A specimens with N mortar and two with TX mortar, for which a thickness of 5 mm (s2) was adopted. Type FINICEM 6 BIANCO IDRO mortars containing 25% white cement were produced. In the TX mortars a 5% TiO2 content was added. On the third slab (TQ) no finish coatings were applied.
still highly alkaline, a purple-red colour was obtained. In the carbonated part of the specimen where the alkalinity of concrete is reduced, no colouration occurred [22]. According to code UNI EN 13295:2005 [23], the average depth of the colourless phenolphthalein region was measured from three points for each side, on both the two edges of the split face, about 1 h after spraying the indicator (Fig. 5).
2.2. Accelerated carbonation Carbonation is a neutralizing reaction during which hydroxide components of the cement paste react with the mild carbonic acid created by the carbon dioxide gas when it dissociates in the pore water. As a result carbonation reduces the hydroxide concentration in the pore solution, destroying the passivity of the embedded reinforcement. Since carbonation reactions are very slow in natural environments, accelerated methods can be adopted for measurement of non-time dependent relationships in reasonably short time frames. In the case herein analyzed, response has been accelerated incubating specimens in an environment with an elevated concentration of carbon dioxide (5%) so that the reaction is not limited by the amount available. The concentration of 5% for the CO2 air content is widely used in accelerated carbonation tests [18,19]. Focusing on the accelerated testing protocol herein adopted, after curing for 6 days, the specimens were transferred to a sealed chamber and subjected to accelerated carbonation at 35 °C in temperature, a CO2 concentration of 5% and relative humidity varying between 30% and 80% every 12 h. The aim was to reproduce the alternate wet and dry conditions characterizing aggressive marine environments. The choice concerning relative humidity can be clarified considering that the most conducive humidity values, for maximum carbonation, range from 50% to 70%; similarly carbonation rates decrease at 100% relative humidity since carbon dioxide cannot easily penetrate saturated pores [20,21]. Fluorescent lamps were finally used to provide UV radiation. The test specimens were taken out of the carbonation chamber at 14, 21 and 28 days, in order to investigate the evolution in time of deterioration. Fig. 4 shows the carbonation chamber used for the accelerated experimentation.
3. Concrete carbonation test The following test specimens were taken out of the carbonation chamber at 14 days: two Type A specimens with N mortar (with s1 thickness) and two Type A specimens with TX mortar (with s1 thickness). All the other Type A specimens were taken out of the carbonation chamber at 21 days. After splitting the concrete specimens, the freshly split surface was cleaned and sprayed with a phenolphthalein pH indicator. In the noncarbonated part of the specimen, where the concrete was
Fig. 4. Accelerated carbonation chamber.
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Fig. 5. Concrete carbonation test; measurement of the average depth of the colourless phenolphthalein region.
Fig. 6. Concrete carbonation test at 14 days on Type A specimens with s1 thickness: (a and c) with N mortar; and (b and d) with TX mortar.
Table 2 Average depth of the colourless phenolphthalein region for specimen 1 N.
Carbonation test – 14th day – specimen 2 N
Carbonation test – 14th day – specimen 1 N Right section – dk,1 3 (mm) a1 = 36.60 b1 = 35.21 a2 = 31.45 b2 = 33.50 a3 = 33.52 b3 = 35.95 dk,a = 33.85 dk,b = 34.88 P dk(a d),right = dk,right = 39.94 mm Left section – dk,1 3 (mm) a1 = 37.67 b1 = 37.36 a2 = 33.67 b2 = 33.01 a3 = 34.23 b3 = 37.03 dk,a = 35.19 dk,b = 35.80 P dk(a d),left = dk,left = 41.10 mm dk = 40.52 mm
Table 3 Average depth of the colourless phenolphthalein region for specimen 2 N.
c1 = 45.95 c2 = 46.41 c3 = 50.60 dk,c = 47.65
d1 = 43.37 d2 = 39.18 d3 = 47.60 dk,d = 43.38
c1 = 43.92 c2 = 40.96 c3 = 44.61 dk,c = 43.16
d1 = 51.81 d2 = 47.98 d3 = 51.01 dk,d = 50.26
Right section – dk,1 3 (mm) a1 = 33.87 b1 = 37.34 a2 = 32.00 b2 = 33.44 a3 = 33.70 b3 = 35.32 dk,a = 33.19 dk,b = 35.36 P dk(a d),right = dk,right = 35.22 mm Left section – dk,1 3 (mm) a1 = 36.36 b1 = 35.18 a2 = 33.19 b2 = 35.59 a3 = 33.53 b3 = 37.45 dk,a = 34.36 dk,b = 36.07 P dk(a d),left = dk,left = 34.96 mm dk = 35.09 mm
c1 = 38.48 c2 = 31.88 c3 = 36.08 dk,c = 35.48
d1 = 35.40 d2 = 33.41 d3 = 41.69 dk,d = 36.83
c1 = 35.33 c2 = 33.38 c3 = 37.84 dk,c = 35.51
d1 = 35.7 d2 = 31.24 d3 = 34.76 dk,d = 33.9
A. Fiore et al. / Construction and Building Materials 48 (2013) 137–143 Table 4 Average depth of the colourless phenolphthalein region for specimen 1 TX. Carbonation test – 14th day – specimen 1 TX Right section – dk,1 3 (mm) a1 = 33.36 b1 = 38.23 a2 = 31.69 b2 = 32.67 a3 = 34.53 b3 = 31.36 dk,b = 34.08 dk,a = 33.86 P dk(a d),right = dk,right = 33.17 mm Left section – dk,1 3 (mm) a1 = 40.1 b1 = 35.98 a2 = 30.51 b2 = 31.17 a3 = 34.99 b3 = 37.53 dk,a = 35.2 dk,b = 34.89 P dk(a d),left = dk,left = 34.21 mm dk = 33.69 mm
c1 = 30.22 c2 = 29.8 c3 = 31.21 dk,c = 30.41
d1 = 34.05 d2 = 31.19 d3 = 37.81 dk,d = 34.35
c1 = 33.93 c2 = 30.51 c3 = 31.52 dk,c = 31.98
d1 = 35.34 d2 = 34.16 d3 = 34.84 dk,d = 34.78
Carbonation test – 14th day – specimen 2 TX
Left section – dk,1 3 (mm) a1 = 31.37 b1 = 31.72 a2 = 28.72 b2 = 29.02 a3 = 32.92 b3 = 34.06 dk,a = 31.003 dk,b = 31.6 P dk(a d),left = dk,left = 31.83 mm dk = 31.56 mm
specimens with TX mortar is approximately on average 0.86 the depth of specimens with N mortar. At 21 days the surfaces of all the other specimens have been fully carbonated (Fig. 7); this result is due to the highly aggressive conditions created in the carbonation chamber. The carbonation depths of Type A specimens are shown in Fig. 8 versus the CO2 exposure period. 4. Test of reinforcing bar corrosion
Table 5 Average depth of the colourless phenolphthalein region for specimen 2 TX.
Right section – dk,1 3 (mm) a1 = 32.16 b1 = 31.31 a2 = 30.89 b2 = 28.56 a3 = 32.6 b3 = 29.95 dk,a = 31.88 dk,b = 29.94 P dk(a d),right = dk,right = 31.29 mm
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c1 = 31.48 c2 = 30.45 c3 = 33.14 dk,c = 31.69
d1 = 33.01 d2 = 30.44 d3 = 31.59 dk,d = 31.68
c1 = 35.96 c2 = 30.11 c3 = 33.71 dk,c = 33.26
d1 = 31.3 d2 = 29.81 d3 = 33.26 dk,d = 31.45
3.1. Results and discussion Fig. 6 shows the results obtained for the two Type A specimens with N mortar and the two with TX mortar taken out of the carbonation chamber at 14 days. The corresponding carbonation depths are reported in Tables 2–5, according to the dimensional parameters introduced in Fig. 5. It is evident that carbonation is more intensive in specimens without TiO2. The carbonation depth of
The corrosion of reinforcing steel in concrete exposed to aggressive environment leads to cracking of concrete and subsequent loss in the load-carrying capacity of a reinforced concrete member [24,25]. In the experimental study herein presented the corrosion mechanism of reinforcing bars induced by carbonation of concrete has been investigated. Steel embedded in good quality concrete is chemically protected by the high alkalinity of pore water, while the dense and relatively impermeable structure of concrete provides the physical protection. The alkaline compounds, mainly calcium and to a certain extent potassium and sodium, in the cement contribute to the high alkalinity of the pore solution, which in the presence of oxygen, passivates the steel. The loss of alkalinity due to carbonation of concrete can destroy the passive film. The test to assess the properties of surface-applied photocatalytic products on protection of steel bars, has interested the three Type B specimens described in Section 2.1. Each specimen contained four 12 mm diameter reinforcing bars with four different concrete covers (C1 = 1.5 cm; C2 = 2.0 cm; C3 = 2.5 cm; C4 = 3.0 cm), as shown in Fig. 3. The three slabs were taken out of the carbonation chamber at 21 days and sprayed with a phenolphthalein pH indicator. On the non-passivated steel a purple-red colour was obtained; similarly when the passive layer was broken down no colouration occurred. The progress of corrosion was evaluated by observing on reinforcing bars the colourless phenolphthalein region. Fig. 9 illustrates some images of the test. 4.1. Results and discussion Fig. 10 shows the results obtained with reference to the TQ slab. It is evident that the level of corrosion decreases as the concrete
Fig. 7. Concrete carbonation test at 21 days on Type A specimens: (a) with N mortar and s1 thickness; (b) with N mortar and s2 thickness; (c) with TX mortar and s1 thickness; and (d) with TX mortar and s2 thickness.
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Fig. 8. Carbonation resistance of Type A specimens.
Fig. 10. Test of reinforcing bar corrosion for TQ Type B specimen.
Fig. 9. Test of reinforcing bar corrosion for TQ, N and TX Type B specimens taken out of the carbonation chamber at 21 days.
cover increases. Similarly Fig. 11 shows the comparison between the three Type B specimens with reference to all the C1–C4 concrete covers. The corrosion propagation rate of reinforcing bars is visibly lower in the TX slab with respect to the TQ and N ones. The obtained measurements are summarized in Fig. 12a and b, where the percentages of passivated steel and carbonated area are respectively shown in function of the concrete cover for each Type B specimen. It is evident that in all specimens the percentages of passivated steel and carbonated area decrease with increasing the concrete cover. The test results also clearly indicate that TX specimens are characterized by higher resistances against carbonation and barcorrosion in comparison with N and TQ ones.
Fig. 11. Test of reinforcing bar corrosion for TX, N and TQ Type B specimens at 21 days: (a) with C1 concrete cover; (b) with C2 concrete cover; (c) with C3 concrete cover; and (d) with C4 concrete cover.
In particular Fig. 12a shows that the reinforcing bars in the TQ specimen are more susceptible to localized corrosion than the bars in the N and TX slabs. In the case of C1 concrete cover, the percentage of passivated steel reaches a value of approximately 84% in the TQ specimen, while is equal to about 46% and 41% in the N and TX slabs respectively. This attests the effectiveness of using mortar finish coatings in order to improve the durability of RC structures.
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corrosion propagation rate of reinforcing bars decreases as the concrete cover increases. Acknowledgment Prof. Marcello Di Marzo and Ing. Enrico Vitobello are gratefully acknowledged for their contribution and devices during the experimental program. References
Fig. 12. Percentages of (a) passivated steel and (b) carbonated area versus the concrete cover for Type B specimens taken out of the carbonation chamber at 21 days.
Finally observing Fig. 12b it emerges that in correspondence of the concrete cover C4 the carbonated zone in the TX slab is about 0.7 the ones of the N and TQ slabs. This difference is less evident as the concrete cover decreases, but is anyway significant: in correspondence of the concrete cover C1 the carbonated zone in the TX slab is about 0.9 the ones of the N and TQ slabs. 5. Conclusions The experimental study herein presented deals with the design topic of durability of reinforced concrete elements. In this field, the effects of applying concrete surface layers incorporating a photocatalytic material have been investigated, analyzing the efficiency in terms of carbonation of concrete and corrosion of reinforcing bars. For sake of clearness, the results obtained on specimens covered by photocatalytic cement mortar have been compared with the ones obtained on specimens simply covered by cement mortar. The results of the experimental tests have shown that the concrete carbonation depth can be significantly reduced by adopting photocatalytic surface layers. The results have also indicated that the application of titanium dioxide modified cementitious materials on the external surface of reinforced concrete elements improves the corrosion performance of reinforcing bars in presence of carbonation of concrete. In particular it also emerged that the
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