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Algal colonization kinetics on roofing and façade tiles: Influence of physical parameters
Construction and Building Materials 48 (2013) 670–676
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Review
Algal colonization kinetics on roofing and façade tiles: Influence of physical parameters D. Giovannacci a,⇑, C. Leclaire a, M. Horgnies b, M. Ellmer c, J.D. Mertz d, G. Orial d, J. Chen b, F. Bousta d a
Cercle des Partenaires du Patrimoine, 29 rue de Paris, Champs sur Marne, France Lafarge Centre de Recherche, 95 Rue du Montmurier, 38291 Saint Quentin Fallavier, France c Monier Technical Centre, Sussex Manor Business Park, Crawley, West Sussex RH10 9NZ, England, United Kingdom d USR 3224 MNHN-CNRS-LRMH, 29 rue de Paris, Champs sur Marne, France b
h i g h l i g h t s A liquid film of water was a main condition to induce algal growth. The roughness should be the most significant parameters in order to retain algae. In saturated conditions, the porosity boosts colonization by playing a role of water supply. The surface chemistry that could be considered as a second order parameter.
a r t i c l e
i n f o
Article history: Received 4 June 2012 Received in revised form 16 July 2013 Accepted 21 July 2013 Available online 24 August 2013 Keywords: Tiles Algae Aesthetic Accelerated test Kinetics of colonization Porosity Roughness Concrete
a b s t r a c t Algal growth is responsible for aesthetic defects on roofing tiles. Accelerated water-streaming tests were done on different building materials. The results establish the ranges of porosity and roughness that can initiate the colonization under humid saturation. A smooth surface is then recommended to reduce the settlement. Experiments done on limestone and clay tiles demonstrate that coupling high porosity and rough surface should be banished to avoid any rapid algal colonization. However, the alkaline composition of concrete tiles can strongly affect the algal settlement, whatever their intrinsic porosity and roughness. Observations by Environmental Scanning Electron Microscopy show that calcite crystals could promote the settlement of algae on the surface. Ó 2013 Elsevier Ltd. All rights reserved.
Contents 1. 2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Test methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Mercury intrusion porosimetry (MIP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Profilometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. Environmental Scanning Electron Microscopy (ESEM). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4. Image analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4.1. Image analysis method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4.2. Threshold method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4.3. K-means method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4.4. Accuracy of the method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⇑ Corresponding author. Tel.: +33 160377780. E-mail address: [email protected] (D. Giovannacci). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.07.034
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3.
4. 5.
2.2.1. Reference glass tiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Tiles made of limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Tiles made of clay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4. Tiles made of concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Accelerated water-streaming tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Algal cultures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Accelerated laboratory set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3. Choice of strains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Algal growth kinetics on reference glass tiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Algal growth kinetics on tiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Characterization of algal settlement on concrete tiles surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Roofs and building façades (made of quarry, slate or concrete tiles) exposed to natural environments are subject to biological development, causing aesthetic degradations [15]. Visible microorganisms involved in biological settlement are first algae, followed by lichen and moss, respectively. In the last previous years, numerous observations on roof or façade tiles have shown that algae are ubiquitous and mainly responsible for the discoloration [5]. Species involved have been characterized [11] and belong to the two major classes encountered: Cyanophyceae and Chlorophyceae. Kinetics of algal growth depends on environmental conditions and on material properties such as chemical composition, roughness, porosity, surface energy, pH [2,3,11]. Those main physical and chemical parameters characterize the concept of bioreceptivity, defined as the ability of a material to be colonized [17]. This concept is correlated with the content and the localization of liquid water, which is the main requirement for algal growth [18]. Several studies [10,22] indicate that a film of liquid water is necessary for the colonization with green algae, but incertitude exists about the localization of the available water (in the porous network and/or near the top-surface). Some preventive treatments are currently performed to delay algal growth on tiles such as the use of water repellents and photo-catalytic TiO2 inclusions [7]. Curative and preventive treatments, such as biocides [15], can be also applied to remove the colonization with algae [1,21,24]. However, according to new environmental issues, the definition of threshold parameters would offer new alternatives: the limitation of the use of chemical products and the optimization of the manufacturing process to protect roof or façade in an easier and cheaper way. Because the first visible biological developments begin generally after a 1-year of natural exposure [2,11], the studies carried out on algal colonization require the implementation of accelerated tests. Several accelerated water-streaming tests have been developed in France for concrete substrates [4,12,14]. The main results have shown that the behavior of algae settlement in different moisture conditions is species-dependant. They have then established that roughness and porosity are the most influent parameters on algal growth. The assumption that a superficial film of liquid water is then sufficient to induce a perennial settlement seems to be relevant [10,22]. In order to study the influences of physical parameters, several reference tiles (made of porous sintered glass types or non-porous glass) have been initially exposed to a homemade accelerated water-streaming test. After recalling the experimental set-up and the characterization means (image analysis) carried out, kinetics of algal settlement on glass tiles are compared to those measured
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on the four types of tiles made of different building materials (limestone, clay, common pre-cast concrete and high-performance concrete, respectively). 2. Experimental 2.1. Test methods 2.1.1. Mercury intrusion porosimetry (MIP) The glass and concrete tiles were investigated by the mercury intrusion porosimetry technique (Autopore IV 9500 from Micromeritics, USA) to assess the porous distribution (with breakthrough diameter) and the overall porosity [13,20,23,25]. The pressure range of the porosimeter was from sub ambient up to 400 MPa, covering the pore diameter range from about 360 lm to 3 nm. Tests were carried out on 10 10 10 mm3 samples cut from the core of the tiles. The samples were dried in an oven at 45 °C overnight before being tested. 2.1.2. Profilometry The roughness was characterized by a SurPhase HS (from PhaseView, USA). This system couples spatial variation of the electromagnetic power with 3D shape of the illuminated object (on standard step height of 20 lm, the accuracy for step height measurement is 0.022 lm and the standard deviation (repeatability) is 0.014 lm). The arithmetic means (Ra) of the profile deviations from the mean line were calculated to compare the roughness of each kind of tile. 2.1.3. Environmental Scanning Electron Microscopy (ESEM) Tiles made of concrete were characterized after algal colonization using a highresolution field-effect gun digital scanning electron microscope (SEM FEG Quanta 400 from FEI Company, USA) with an accelerating voltage of 15 keV and a current intensity of 1 nA. The images were obtained in environmental mode (ESEM) that enables observations of hydrated and organic samples, such as microorganisms, assuring their integrity. 2.1.4. Image analysis The characterization means used to measure algal growth on tiles have to be non-destructive. One of the easiest methods seemed to be image analysis whose effectiveness has been confirmed in previous studies [4,9,12]. The colonized area could be very low; a particular attention will be so given on image analysis to obtain the best accuracy. The photographs of samples were obtained using a Hewlett Packard Scanjet 8300 scanner. This method offered identical conditions of lighting, resolution and recording parameters. 2.1.4.1. Image analysis method. Quantification of the colonized area was done using two different image analysis methods. The first one, and the more used, was the threshold method [2,12] while the second one was the k-means method [2,17]. Since accuracy of those two methods has not been assessed in these previous studies, we review the two methods and propose in this paper a method to measure it. 2.1.4.2. Threshold method. The threshold method is based on a classical signal processing method of the low/high pass frequency filter. The threshold value can be estimated or calculated on the first image (provided on wet samples) without algal spots. However, the fact remained that the main difficulty is to determine the threshold value. Sometimes the threshold value is set by operators [9], and therefore accuracy of the method may be reconsidered. In this study, this value was calculated from the Y normalized histogram of initial images (images are decomposed along the Y vectors of CMYk color space). Assuming that the distribution of pixel coloration has a
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Gaussian distribution, the threshold value was defined as the first value above the trust interval at 99.7%. Indeed, the assumption of a Gaussian distribution was verified and was therefore available. 2.1.4.3. K-means method. K-means clustering treats each object such as having a location in space. It finds partitions such that objects within each cluster are as close to each other as possible, and as far from objects of other clusters as possible. K-means clustering requires specifying the number of clusters to be partitioned [6]. Whereas specific algorithms exist to calculate the best value, we made this equal to two to simplify the problem. The choice of colonized area would be binary and automatic in this case. For more than two clusters, the operator has to determine which cluster corresponds to algal development [19]. 2.1.4.4. Accuracy of the method. To assess accuracy of both methods, green areas were measured on artificial areas. The range of the color surface was fixed between 5% and 78% and 4 different shades of green were used. The threshold value was calculated on a flat white image, which corresponds to a sample before the experiment. The threshold method seems to have a better accuracy than the k-means method, since the variation of absolute errors was lower. Therefore, this method was then used in this study (as noted before, the k-mean method would be more efficient with more than two clusters). The accuracy of the threshold measure was then estimated to be 0.4% of the overall surface of the image. Measurements below this value would then consider as insignificant. 2.2. Materials 2.2.1. Reference glass tiles Four distinct porous glass tiles (referred as PG1, PG2, PG3 and PG4) made by sintering of agglomerated Pyrex powder (Verre Labo Mula, France), were used as reference. The choice was led by the fact that glass tiles are: – Chemically neutral. – Isotropic and homogeneous for surface and volume properties point of view. The Table 1 provides the overall porosity, the breakthrough diameter and the roughness values for all these tiles. The breakthrough diameter corresponds to the percolation threshold [13], more generally called the pore access diameter, while the roughness value corresponds to the arithmetic mean of the profile deviations from the mean line (Ra). The results show that the porous distribution was unimodal; the overall porosities were all about 40% but the Ra values were significantly different (from 5 to 37 lm). Moreover, non-porous glass tiles made of pure quartz (Mondiaquartz, France) were also used. Then a sandblasting operation (using alumina powder) was done on certain of the glass tiles to increase their roughness. Table 1 shows that the non-porous glass tiles (referred as NPG1, NPG2, NPG3 and NPG4) did not present any open porosity but possessed respective range of roughness (from 0.1 to 13.5 lm). 2.2.2. Tiles made of limestone Tiles referred as ‘‘Limestone’’ were made of Tuffeau limestone, which is a biochemical sedimentary rock. As shown by Table 1, the tile made of limestone is very porous with an overall porosity of 42% for a mean diameter about 10 lm (values close those of certain glass tiles). This limestone tile is known to have a very important bioreceptivity [17]. 2.2.3. Tiles made of clay A fired Danish clay tile (referred as ‘‘Clay’’) was also tested. These tiles were based on a mix of illitic clay minerals, pressing onto formworks. Considering the data provided by Table 1, the clay tiles present middle-values of porosity and roughness, compared to those of the limestone and high performance concrete tiles. 2.2.4. Tiles made of concrete Table 1 provides the characteristics (overall porosity, breakthrough diameter and roughness) of the two different concrete tiles tested. First, a precast concrete mix-design was used (referred as ‘‘PCC’’). The tiles were prepared by mixing CEM I 52.5 R cement blended with aggregates (0–4 mm) and limestone filler (80/20). The pre-cast concrete mix was manufactured using a W/C ratio of 0.40. The mix was extruded through a purpose built machine onto aluminum pallets and then cured at 60 °C/90% of relative humidity (RH) for upwards of 6 h. This kind of PCC mix-design induced relatively porous and rough concrete tiles. Other concrete tiles (referred as ‘‘HPC’’) were prepared using a high-performance concrete mix-design. These tiles were prepared using a fresh mix (with a water to cement ratio (W/C) of 0.26) composed of ordinary Portland cement (CEM I 52.5), limestone filler, silica fumes, sand (0–1 mm), fibers, and admixture made of super-plasticizer. The HPC tiles were manufactured by pouring the fresh mixture into formworks made of polyvinylchloride (12 15 1 cm). No steps of agitation or densification were used. All the tiles were removed from their formworks after 20 h. While the PCC mix gives a porous and rough concrete tile, the HPC mix is known to induce a close packing and smooth surface [8,16].
Table 1 Physical characteristics of the tiles. Tiles Porous glass
Non-porous glass
Limestone Clay PCC HPC
Overall porosity (%)
Breakthrough diameter (lm)
Roughness Ra (lm)
PG1 PG2 PG3 PG4 NPG1
40 40 39 41 x
7 7 14 29 x
5.4 20.7 27.6 37.2 0.1
NPG2 NPG3 NPG4
x x x 42.0 25.0 20.3 8.5
x x x 9.6 0.65 0.29 0.015
5.6 8.8 13.5 20.3 15.1 25.6 1.0
2.3. Accelerated water-streaming tests 2.3.1. Algal cultures Algal settlement on material is species or classes-dependant. Main species encountered on roof belong to Chlorophyceae family, filamentous or unicellular species. Two strains, selected for this study, were representative of species frequently isolated on concrete walls [2]. Klebsormidium flaccidum (filamentous, reference ALCP 749b) and Chlorella vulgaris (unicellular, reference CCAP 211/11b) were obtained from the algal culture collection of the Museum National d’Histoire Naturelle (MNHN, France). Algal cultures were grown in flasks with 100 mL of a specific medium (BG11, classical medium for algae growth Blue Green Medium from Sigma AldrichÒ), and maintained in a climate controlled incubator (photoperiod 12 h per day, 25 °C). 2.3.2. Accelerated laboratory set-up The accelerated water-streaming test consisted of a simulated roof on which algal suspension circulates into a plastic sprinkling rail (1 cm diameter with 2 mm holes every 2 cm) that spread water on the top of the tiles, running down their entire surface, as shown on Fig. 1. The experimental device was similar to the test developed by Dubosc [11]. The support composed of a polyvinylchloride frame that was inclined at 45° (to look like a natural roof), allowed simultaneous spreading of 16 samples. A smooth plastic deck made it possible to have a continuous flow between the rail and the top of the tiles (1 6 15 cm), to limit preferential flow on tiles. The simulated roof was enclosed in a transparent chamber stored in a climatic cave with a temperature of 13 °C and 99% RH. The temperature in the chamber ranged periodically with light between 15 and 18 °C and air moisture was close to saturation. The sprinkling cycles, ensured by two aquarium pumps (400 L/h) immersed in the suspension, were done every 12 h with duration of 90 min. The chamber was filled with 40 L of BG11 medium enriched with algal suspension. Algal concentration was verified with optical density and Malassez counting chamber. Since algae need light to grow, two neon lamps ensured 12 h per day photoperiod. The power light was symmetric relative to the median axis of the chamber (along the width), and reached 1000 Lux on the upper part and 620 Lux in the lower one. For each kind of tile, the test was replicated three times. After each test, inoculum was tested, and algal culture contamination tracked with morphological analyses. If other strains (than inoculated ones) were found, the set-up was totally disinfected and the test restarted. 2.3.3. Choice of strains Some preliminary experiments were carried out to identify which algal morphology could quickly colonize the tiles. Tiles chosen for those experiments were made of clay roofing, which are known to have a high bioreceptivity [5]. Three specimens were exposed to the water-streaming test, stimulated with the two strains, the unicellular algae, on the one hand, and the filamentous one, on the other hand. Algal growth kinetics on tiles was measured by image analysis using the threshold method (the threshold value was measured on wet cleaned tiles). Results have shown that kinetics of colonization were three times faster with the filamentous species than the unicellular species. Thus, K. flaccidum was the algal strain used for all the other experiments done in this study.
3. Results 3.1. Algal growth kinetics on reference glass tiles First water-streaming experiments were done to compare colonization kinetics that occurred on the four porous glass tiles, which
D. Giovannacci et al. / Construction and Building Materials 48 (2013) 670–676
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(a) light 100 cm
Sprinkling rail
sample Plastic deck
pump
(b)
Fig. 1. (a) Image and (b) schematic description of the water streaming test chamber.
possessed a similar overall porosity (about 40%) but distinct pore distribution and roughness (values provided in Table 1). Fig. 2a shows the evolution of the colonized areas obtained after a few weeks of exposure to the water streaming. The roughness (characterized by the Ra value) seems to be the predominant parameter that influenced the algal colonization when the overall porosity values were held constant. For example, PG1 and PG2 tiles showed the same pores characteristics (overall porosity and pore distribution) and then a similar content of liquid water during the experiment (continuous water flow, 99% RH). However, the rough tile (Ra of PG2: 20.7 lm) was faster colonized by algae (colonized area about 20 times higher after 3 weeks) than the smooth tile (Ra of PG1: 5.4 lm). To assess the effect of the roughness on algal colonization, other experiments were carried out on non-porous glass tiles, which presented distinct levels of roughness (as shown in Table 1). The quantification of the colonized surface on those tiles is given in Fig. 2b. After 4 weeks of test, only a minor part of the surfaces was colonized, highlighting the interest of using non-porous and relative smooth tiles. For example, algal colonization reached less than 0.25% of the entire area for NPG1 and NPG2 tiles. The algal depositions detected by image analysis were then probably microorganisms stuck on tiles just before scanning but they cannot be recognized as a perennial colonization.
3.2. Algal growth kinetics on tiles As shown by Table 1, the limestone tiles were highly porous and rough, with values of porosity and Ra close to those of certain porous glass tiles (such as PG2, PG3 and PG4). Similarly, the water-streaming tests done on the limestone tiles highlighted a rapid algal colonization. As shown by Fig 2c, almost all the surface of limestone tiles was then covered by algae after 4 weeks of exposure. The tiles made of clay were relatively porous (25% of overall porosity) but smoother than the limestone tiles (Ra value of 15 lm). Fig 2c establishes that the clay tiles were then slowly colonized by algae. Compared to the previous results obtained on glass tiles, this result could confirm that a certain value of smoothness (above approximately 15 lm) would reduce the colonization kinetics by algae (even in case of porous tiles). As shown by Table 1, the two types of concrete tiles offered different porosity and roughness characteristics. However, Fig. 2c illustrates slow colonization kinetics that was relatively close to those of the clay tile or PG1 tile. These results were relatively conformed to those expected for the HPC tiles, which present intrinsic low overall porosity and very smooth surface. However, slow algal colonization after 28 days of exposure was not expected on the PCC tiles (due to their intrinsic high overall porosity and rough surface).
D. Giovannacci et al. / Construction and Building Materials 48 (2013) 670–676
Colonized surface, %
674
100 80 PG4 PG3 PG2 PG1
60 40 20 0 0
5
10
15
20
25
30
Time, days
Colonized surface, %
(a) 10 NPG4 NPG3 NPG2 NPG1
8 6 4 2 0 0
5
10
15
20
25
30
35
40
Time, days
Colonized surface, %
(b) 100 80 HPC Lime Clay PCC
60 40 20 0 0
5
10
15
20
25
30
Time, days
(c) Fig. 2. Time variations of colonized area by Chlorophyceae observed on tiles made of: (a) porous glass, (b) non-porous glass, and (c) limestone, clay and concrete.
Finally, we hypothesize that kinetics of colonization detected on the concrete tiles were disturbed by their specific surface composition, which was highly alkaline after demoulding. An alkaline pH is then known to inhibit the growth of algae [9] and should be taken account before considering the porosity or roughness characteristics of the concrete tiles.
surface and depended on the micro-topography of the substrate. We hypothesize also that the growth of crystals of calcite may enhance the settlement of algae but other tests should be undertaken to determine the influence of carbonation of the concrete surface. 4. Discussion
3.3. Characterization of algal settlement on concrete tiles surface To understand the algal growth kinetics on the PCC and HPC tiles, the surfaces were characterized by ESEM after algal colonization. Observations were carried out after 4 weeks of water-streaming test and images are provided in Fig. 3. Filamentous cells were easily identified on both concrete tiles. Fig. 3a and b highlighted the differences of micro-topography between PCC and HPC surfaces. Many cracks and textured areas were detected on the PCC surface while the HPC surface was very flat without any visible opened pores. In Fig. 3c and d, images showed that algae cells took root into the material whatever the type of concrete tile. Thereby, crystals of calcite were detected near several parts of filamentous cells (Fig. 3e and f). The biological dissolution and re-crystallization of calcite were long-time processes that could not be considered in this experiment; then this calcite did not result from a reaction between the acids generated by algal cells and the alkaline compounds of the hydrated cement paste. As a conclusion, these observations highlighted that the settlement of algae was heterogeneously distributed on the concrete
Considering all the tests done on the glass, limestone and clay tiles, it could remain difficult to separate exactly the influence of the roughness whatever the porosity, which was determinant for the evaporation process. The experiments done on the non-porous glass tiles showed that no algal colonization occurred until a roughness (Ra value) of 13.5 lm but we did not succeed in manufacturing glass tiles rougher than this value and presenting no open pores at the surface. However, air moisture was saturated with water in our laboratory set-up. The water was then always available and must be sufficient for algal colonization. Considering this point, only the roughness can be examined as relevant factor because this parameter can directly increase the probability of trapping the algae. Indeed, a low Ra value close to 5 lm allowed reducing strongly the colonization kinetics on the porous glass tiles (about 40% of porosity). Considering a porosity of 25%, a Ra value about 15.1 lm offered also a slow algal colonization on clay tiles. In this particular environment, a roughness equal or lower than 13.5 lm seems to be sufficient to avoid any perennial algal settlement for non-porous
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algae
(a)
(b) open porosity
(c)
algae
(d)
algae
calcite
(e)
(f )
Fig. 3. ESEM images of algal growth on PCC tile (a, c, and e) and HPC tile (b, d, and f).
glass samples. We can then hypothesize that the minimal roughness required for algal colonization is probably reached to a Ra value close to 13.5 lm. Because kinetics and colonized areas were 5 times larger if the tiles were porous and rough (as seen by comparing Fig. 2a and b), the presence of high porosity could be considered as a booster of the algal colonization when the roughness is sufficient to allow the settlement of algae. The kinetic and colonized areas were quite similar for PG1, Clay, PCC and HPC tiles. The analysis of covariance with a 95% confidence interval was necessary to differentiate them. The first group was composed of clay or concrete tiles while the second group was composed of PG1 tile. The slope of PG1 was significantly distinct and lower than for the others. The difference was probably due
to the surface chemistry, which can play a role for tiles made of clay or concrete. Indeed, the concrete tiles were specific due to their alkaline composition that could postpone the algal colonization for a while considering the level of carbonation. The areas of concrete tiles colonized by algae were very small after 28 days of water-streaming tests, whatever their intrinsic porosity and roughness. Contrary to the PCC tiles, we suppose also that the low roughness of the HPC tiles (Ra of 1.0 lm) could protect them after a complete carbonation of the surface. Other experiments should be then performed in future to compare the algal colonization on aged (and carbonated) concrete tiles to confirm this hypothesis. Such a parametrical study could be implemented by the use of design of experiment (DOE) and the Taguchi method.
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5. Conclusions
References
Water-streaming tests were performed on different samples to study the kinetics of colonization by algae on:
[1] Alum A, Rashid A, Mobasher B, Abbaszadegan M. Cement-based biocide coatings for controlling algal growth in water distribution canals. Cem Concr Compos 2008;30:839–47. [2] Barberousse H. Etude de la diversité des algues et des cyanobactéries colonisant les revêtements de façade en France et recherche des facteurs favorisant leur implantation. PhDthesis. MNHN, Paris France; 2006. [3] Barberousse H, Lombardo RJ, Tell G, Couté A. Factors involved in the colonization of building façades by algae and cyanobacteria in France. Biofouling 2006;22:69–77. [4] Barberousse H, Ruot B, Yéprémian C, Boulon G. An assessment of façade coatings against colonisation by aerial algae and cyanobacteria. Build Environ 2007;42:2555–61. [5] Berdahl P, Akbari H, Levinson R, Miller WA. Weathering of roofing materials – an overview. Constr Build Mater 2008;22:423–33. [6] Blanchet G, Charbit M. Signaux et images sous MATLAB: méthodes, applications et exercises corrigés. Hermes Science Publications; 2001. [7] Chen J, Poon CS. Photocatalytic construction and building materials: from fundamentals to applications. Build Environ 2009;44:1899–906. [8] De Larrard F. Concrete mixture proportioning. London (UK): E & FN Spon Ed; 1999. [9] De Muynck W, Ramirez AM, De Belie N, Verstraete W. Evaluation of strategies to prevent algal fouling on white architectural and cellular concrete. Int Biodeterior Biodegrad 2009;63:679–89. [10] De Winder B. The role of water retaining substrata on the photosynthetic response of three drought tolerant phototrophic microorganisms isolated from a terrestrial habitat. Arch Microbiol 1989;152:458–62. [11] Dubosc A. Etude du développement de salissures biologiques sur les parements en béton: mise au point d’essais accélérés de vieillissement. PhD thesis. INSA, Toulouse (France); 2000. [12] Dubosc A, Escadeillas G, Blanc PJ. Characterization of biological stains on external concrete walls and influence of concrete as underlying material. Cem Concr Res 2001;31:1613–7. [13] Dullien FAJ. Porous media: fluid transport. (NY) USA: Academic Press; 1979. [14] Escadeillas G, Bertron A, Blanc P, Dubosc A. Accelerated testing of biological stain growth on external concrete walls. Part 1: Development of the growth tests. Mater Struct 2007;40:1061–71. [15] Flores-Colen I, de Brito J, de Freitas VP. Stains in façades rendering – diagnosis and maintenance techniques classification. Constr Build Mater 2008;22:211–21. [16] Gueit E, Darque-Ceretti E, Tintillier P, Horgnies M. Surfactant-induced growth of a calcium hydroxide coating at the concrete surface. J Coat Tech Res 2012;9:337–46. [17] Guillitte O. Bioreceptivity: a new concept for building ecology studies. Sci Total Environ 1995;167:215–20. [18] Kennedy A-D. Water as a limiting factor in the Antarctic terrestrial environment: a biogeographical synthesis. Arctic Alpine Res 1993;25:308–15. [19] Kuo RJ, Wang HS, Hu TL, Chou SH. Application of a K-means on clustering analysis. Comput Mathem Appl 2005;50:1709–24. [20] Matthews GP, Ridgway CJ, Spearing MC. Void space modeling of mercury intrusion hysteresis in sandstone, paper coating, and other porous media. J Colloid Interf Sci 1995;171:8–27. [21] Moreau C, Verges-Belmin V, Leroux L, Orial G, Fronteau G, Barbin V. Waterrepellent and biocide treatments: assessment of the potential combinations. J Cultural Heritage 2008;9:394–400. [22] Palmer RJ, Friedmann EI. Water relations and photosynthesis in the cryptoendolithic microbial habitat of hot and cold deserts. Microb Ecol 1990;19:111–8. [23] Purcell WR. Capillary pressures. Their measurement using mercury and the calculation of permeability there from. J Petroleum Tech 1949:39–47. [24] Urzi C, De Leo F. Evaluation of the efficiency of water-repellent and biocide compounds against microbial colonization of mortars. Int Biodeterior Biodegrad 2007;60:25–34. [25] Washburn EW. The dynamics of capillary flow. Phys Rev 1921;17:273–83.
– Reference samples made of porous and non-porous glass (to the knowledge of writers, it is the first time this kind of study was done with isotropic glass samples), – Tiles made of different building materials (limestone, clay and concrete). A new methodology based on image treatment was developed to ensure the measurement of algae colonization. The influences of the porosity and roughness were then established using the tiles made of glass, limestone and clay, while the specific influence of the alkaline composition was highlighted using the concrete tiles. Indeed, considering a Ra value lower than 13.5 lm, the nonporous glass tiles were not colonized after 4 weeks of exposure to the water streaming tests. This value can be considered as a threshold value for the initialization of a perennial colonization. Besides, a relative smoothness (Ra between 5.4 and 20.7 lm) allowed reducing the potential algal colonization on the porous glass tiles (although an overall porosity of 40%). Concerning the limestone tiles (characterized by high values of overall porosity and roughness), the kinetics of algal colonization was close to those of the roughest porous glass tiles, confirming the high influence of this couple of parameters for these two distinct materials (totally different considering their chemical properties). However, the clay tiles showed middle-values of porosity and roughness (25% of overall porosity and 15.1 lm of roughness) and then a relative slow kinetics of algal colonization. Both the concrete tiles showed very slow kinetics of colonization (at least during the 4 weeks of the tests) whatever their respective porosity and roughness characteristics. We hypothesized that the concrete surfaces were protected by the alkaline composition, which was able to disturb dramatically the growth of algae. ESEM observations of the concrete tiles highlighted a potential relation between the presence of calcite crystals and the settlement of algae. Finally, our results done in very favorable conditions for algal settlement (RH about saturation) suggested that the aesthetic of roofs would be more durable by manufacturing very smooth tiles (whatever their intrinsic porosity). Other tests will be performed to evaluate if this smoothness could reduce or avoid the use of cleaning agents and biocides. Acknowledgements The authors would like to thank S. Brun, M. Dykman and C. Bouillon (Lafarge) for their helps during the preparation and the analyses of the tiles; M. Guivarc’h and A. François for their assistance.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Review
Algal colonization kinetics on roofing and façade tiles: Influence of physical parameters D. Giovannacci a,⇑, C. Leclaire a, M. Horgnies b, M. Ellmer c, J.D. Mertz d, G. Orial d, J. Chen b, F. Bousta d a
Cercle des Partenaires du Patrimoine, 29 rue de Paris, Champs sur Marne, France Lafarge Centre de Recherche, 95 Rue du Montmurier, 38291 Saint Quentin Fallavier, France c Monier Technical Centre, Sussex Manor Business Park, Crawley, West Sussex RH10 9NZ, England, United Kingdom d USR 3224 MNHN-CNRS-LRMH, 29 rue de Paris, Champs sur Marne, France b
h i g h l i g h t s A liquid film of water was a main condition to induce algal growth. The roughness should be the most significant parameters in order to retain algae. In saturated conditions, the porosity boosts colonization by playing a role of water supply. The surface chemistry that could be considered as a second order parameter.
a r t i c l e
i n f o
Article history: Received 4 June 2012 Received in revised form 16 July 2013 Accepted 21 July 2013 Available online 24 August 2013 Keywords: Tiles Algae Aesthetic Accelerated test Kinetics of colonization Porosity Roughness Concrete
a b s t r a c t Algal growth is responsible for aesthetic defects on roofing tiles. Accelerated water-streaming tests were done on different building materials. The results establish the ranges of porosity and roughness that can initiate the colonization under humid saturation. A smooth surface is then recommended to reduce the settlement. Experiments done on limestone and clay tiles demonstrate that coupling high porosity and rough surface should be banished to avoid any rapid algal colonization. However, the alkaline composition of concrete tiles can strongly affect the algal settlement, whatever their intrinsic porosity and roughness. Observations by Environmental Scanning Electron Microscopy show that calcite crystals could promote the settlement of algae on the surface. Ó 2013 Elsevier Ltd. All rights reserved.
Contents 1. 2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Test methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Mercury intrusion porosimetry (MIP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Profilometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. Environmental Scanning Electron Microscopy (ESEM). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4. Image analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4.1. Image analysis method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4.2. Threshold method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4.3. K-means method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4.4. Accuracy of the method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⇑ Corresponding author. Tel.: +33 160377780. E-mail address: [email protected] (D. Giovannacci). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.07.034
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3.
4. 5.
2.2.1. Reference glass tiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Tiles made of limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Tiles made of clay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4. Tiles made of concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Accelerated water-streaming tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Algal cultures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Accelerated laboratory set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3. Choice of strains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Algal growth kinetics on reference glass tiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Algal growth kinetics on tiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Characterization of algal settlement on concrete tiles surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Roofs and building façades (made of quarry, slate or concrete tiles) exposed to natural environments are subject to biological development, causing aesthetic degradations [15]. Visible microorganisms involved in biological settlement are first algae, followed by lichen and moss, respectively. In the last previous years, numerous observations on roof or façade tiles have shown that algae are ubiquitous and mainly responsible for the discoloration [5]. Species involved have been characterized [11] and belong to the two major classes encountered: Cyanophyceae and Chlorophyceae. Kinetics of algal growth depends on environmental conditions and on material properties such as chemical composition, roughness, porosity, surface energy, pH [2,3,11]. Those main physical and chemical parameters characterize the concept of bioreceptivity, defined as the ability of a material to be colonized [17]. This concept is correlated with the content and the localization of liquid water, which is the main requirement for algal growth [18]. Several studies [10,22] indicate that a film of liquid water is necessary for the colonization with green algae, but incertitude exists about the localization of the available water (in the porous network and/or near the top-surface). Some preventive treatments are currently performed to delay algal growth on tiles such as the use of water repellents and photo-catalytic TiO2 inclusions [7]. Curative and preventive treatments, such as biocides [15], can be also applied to remove the colonization with algae [1,21,24]. However, according to new environmental issues, the definition of threshold parameters would offer new alternatives: the limitation of the use of chemical products and the optimization of the manufacturing process to protect roof or façade in an easier and cheaper way. Because the first visible biological developments begin generally after a 1-year of natural exposure [2,11], the studies carried out on algal colonization require the implementation of accelerated tests. Several accelerated water-streaming tests have been developed in France for concrete substrates [4,12,14]. The main results have shown that the behavior of algae settlement in different moisture conditions is species-dependant. They have then established that roughness and porosity are the most influent parameters on algal growth. The assumption that a superficial film of liquid water is then sufficient to induce a perennial settlement seems to be relevant [10,22]. In order to study the influences of physical parameters, several reference tiles (made of porous sintered glass types or non-porous glass) have been initially exposed to a homemade accelerated water-streaming test. After recalling the experimental set-up and the characterization means (image analysis) carried out, kinetics of algal settlement on glass tiles are compared to those measured
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on the four types of tiles made of different building materials (limestone, clay, common pre-cast concrete and high-performance concrete, respectively). 2. Experimental 2.1. Test methods 2.1.1. Mercury intrusion porosimetry (MIP) The glass and concrete tiles were investigated by the mercury intrusion porosimetry technique (Autopore IV 9500 from Micromeritics, USA) to assess the porous distribution (with breakthrough diameter) and the overall porosity [13,20,23,25]. The pressure range of the porosimeter was from sub ambient up to 400 MPa, covering the pore diameter range from about 360 lm to 3 nm. Tests were carried out on 10 10 10 mm3 samples cut from the core of the tiles. The samples were dried in an oven at 45 °C overnight before being tested. 2.1.2. Profilometry The roughness was characterized by a SurPhase HS (from PhaseView, USA). This system couples spatial variation of the electromagnetic power with 3D shape of the illuminated object (on standard step height of 20 lm, the accuracy for step height measurement is 0.022 lm and the standard deviation (repeatability) is 0.014 lm). The arithmetic means (Ra) of the profile deviations from the mean line were calculated to compare the roughness of each kind of tile. 2.1.3. Environmental Scanning Electron Microscopy (ESEM) Tiles made of concrete were characterized after algal colonization using a highresolution field-effect gun digital scanning electron microscope (SEM FEG Quanta 400 from FEI Company, USA) with an accelerating voltage of 15 keV and a current intensity of 1 nA. The images were obtained in environmental mode (ESEM) that enables observations of hydrated and organic samples, such as microorganisms, assuring their integrity. 2.1.4. Image analysis The characterization means used to measure algal growth on tiles have to be non-destructive. One of the easiest methods seemed to be image analysis whose effectiveness has been confirmed in previous studies [4,9,12]. The colonized area could be very low; a particular attention will be so given on image analysis to obtain the best accuracy. The photographs of samples were obtained using a Hewlett Packard Scanjet 8300 scanner. This method offered identical conditions of lighting, resolution and recording parameters. 2.1.4.1. Image analysis method. Quantification of the colonized area was done using two different image analysis methods. The first one, and the more used, was the threshold method [2,12] while the second one was the k-means method [2,17]. Since accuracy of those two methods has not been assessed in these previous studies, we review the two methods and propose in this paper a method to measure it. 2.1.4.2. Threshold method. The threshold method is based on a classical signal processing method of the low/high pass frequency filter. The threshold value can be estimated or calculated on the first image (provided on wet samples) without algal spots. However, the fact remained that the main difficulty is to determine the threshold value. Sometimes the threshold value is set by operators [9], and therefore accuracy of the method may be reconsidered. In this study, this value was calculated from the Y normalized histogram of initial images (images are decomposed along the Y vectors of CMYk color space). Assuming that the distribution of pixel coloration has a
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Gaussian distribution, the threshold value was defined as the first value above the trust interval at 99.7%. Indeed, the assumption of a Gaussian distribution was verified and was therefore available. 2.1.4.3. K-means method. K-means clustering treats each object such as having a location in space. It finds partitions such that objects within each cluster are as close to each other as possible, and as far from objects of other clusters as possible. K-means clustering requires specifying the number of clusters to be partitioned [6]. Whereas specific algorithms exist to calculate the best value, we made this equal to two to simplify the problem. The choice of colonized area would be binary and automatic in this case. For more than two clusters, the operator has to determine which cluster corresponds to algal development [19]. 2.1.4.4. Accuracy of the method. To assess accuracy of both methods, green areas were measured on artificial areas. The range of the color surface was fixed between 5% and 78% and 4 different shades of green were used. The threshold value was calculated on a flat white image, which corresponds to a sample before the experiment. The threshold method seems to have a better accuracy than the k-means method, since the variation of absolute errors was lower. Therefore, this method was then used in this study (as noted before, the k-mean method would be more efficient with more than two clusters). The accuracy of the threshold measure was then estimated to be 0.4% of the overall surface of the image. Measurements below this value would then consider as insignificant. 2.2. Materials 2.2.1. Reference glass tiles Four distinct porous glass tiles (referred as PG1, PG2, PG3 and PG4) made by sintering of agglomerated Pyrex powder (Verre Labo Mula, France), were used as reference. The choice was led by the fact that glass tiles are: – Chemically neutral. – Isotropic and homogeneous for surface and volume properties point of view. The Table 1 provides the overall porosity, the breakthrough diameter and the roughness values for all these tiles. The breakthrough diameter corresponds to the percolation threshold [13], more generally called the pore access diameter, while the roughness value corresponds to the arithmetic mean of the profile deviations from the mean line (Ra). The results show that the porous distribution was unimodal; the overall porosities were all about 40% but the Ra values were significantly different (from 5 to 37 lm). Moreover, non-porous glass tiles made of pure quartz (Mondiaquartz, France) were also used. Then a sandblasting operation (using alumina powder) was done on certain of the glass tiles to increase their roughness. Table 1 shows that the non-porous glass tiles (referred as NPG1, NPG2, NPG3 and NPG4) did not present any open porosity but possessed respective range of roughness (from 0.1 to 13.5 lm). 2.2.2. Tiles made of limestone Tiles referred as ‘‘Limestone’’ were made of Tuffeau limestone, which is a biochemical sedimentary rock. As shown by Table 1, the tile made of limestone is very porous with an overall porosity of 42% for a mean diameter about 10 lm (values close those of certain glass tiles). This limestone tile is known to have a very important bioreceptivity [17]. 2.2.3. Tiles made of clay A fired Danish clay tile (referred as ‘‘Clay’’) was also tested. These tiles were based on a mix of illitic clay minerals, pressing onto formworks. Considering the data provided by Table 1, the clay tiles present middle-values of porosity and roughness, compared to those of the limestone and high performance concrete tiles. 2.2.4. Tiles made of concrete Table 1 provides the characteristics (overall porosity, breakthrough diameter and roughness) of the two different concrete tiles tested. First, a precast concrete mix-design was used (referred as ‘‘PCC’’). The tiles were prepared by mixing CEM I 52.5 R cement blended with aggregates (0–4 mm) and limestone filler (80/20). The pre-cast concrete mix was manufactured using a W/C ratio of 0.40. The mix was extruded through a purpose built machine onto aluminum pallets and then cured at 60 °C/90% of relative humidity (RH) for upwards of 6 h. This kind of PCC mix-design induced relatively porous and rough concrete tiles. Other concrete tiles (referred as ‘‘HPC’’) were prepared using a high-performance concrete mix-design. These tiles were prepared using a fresh mix (with a water to cement ratio (W/C) of 0.26) composed of ordinary Portland cement (CEM I 52.5), limestone filler, silica fumes, sand (0–1 mm), fibers, and admixture made of super-plasticizer. The HPC tiles were manufactured by pouring the fresh mixture into formworks made of polyvinylchloride (12 15 1 cm). No steps of agitation or densification were used. All the tiles were removed from their formworks after 20 h. While the PCC mix gives a porous and rough concrete tile, the HPC mix is known to induce a close packing and smooth surface [8,16].
Table 1 Physical characteristics of the tiles. Tiles Porous glass
Non-porous glass
Limestone Clay PCC HPC
Overall porosity (%)
Breakthrough diameter (lm)
Roughness Ra (lm)
PG1 PG2 PG3 PG4 NPG1
40 40 39 41 x
7 7 14 29 x
5.4 20.7 27.6 37.2 0.1
NPG2 NPG3 NPG4
x x x 42.0 25.0 20.3 8.5
x x x 9.6 0.65 0.29 0.015
5.6 8.8 13.5 20.3 15.1 25.6 1.0
2.3. Accelerated water-streaming tests 2.3.1. Algal cultures Algal settlement on material is species or classes-dependant. Main species encountered on roof belong to Chlorophyceae family, filamentous or unicellular species. Two strains, selected for this study, were representative of species frequently isolated on concrete walls [2]. Klebsormidium flaccidum (filamentous, reference ALCP 749b) and Chlorella vulgaris (unicellular, reference CCAP 211/11b) were obtained from the algal culture collection of the Museum National d’Histoire Naturelle (MNHN, France). Algal cultures were grown in flasks with 100 mL of a specific medium (BG11, classical medium for algae growth Blue Green Medium from Sigma AldrichÒ), and maintained in a climate controlled incubator (photoperiod 12 h per day, 25 °C). 2.3.2. Accelerated laboratory set-up The accelerated water-streaming test consisted of a simulated roof on which algal suspension circulates into a plastic sprinkling rail (1 cm diameter with 2 mm holes every 2 cm) that spread water on the top of the tiles, running down their entire surface, as shown on Fig. 1. The experimental device was similar to the test developed by Dubosc [11]. The support composed of a polyvinylchloride frame that was inclined at 45° (to look like a natural roof), allowed simultaneous spreading of 16 samples. A smooth plastic deck made it possible to have a continuous flow between the rail and the top of the tiles (1 6 15 cm), to limit preferential flow on tiles. The simulated roof was enclosed in a transparent chamber stored in a climatic cave with a temperature of 13 °C and 99% RH. The temperature in the chamber ranged periodically with light between 15 and 18 °C and air moisture was close to saturation. The sprinkling cycles, ensured by two aquarium pumps (400 L/h) immersed in the suspension, were done every 12 h with duration of 90 min. The chamber was filled with 40 L of BG11 medium enriched with algal suspension. Algal concentration was verified with optical density and Malassez counting chamber. Since algae need light to grow, two neon lamps ensured 12 h per day photoperiod. The power light was symmetric relative to the median axis of the chamber (along the width), and reached 1000 Lux on the upper part and 620 Lux in the lower one. For each kind of tile, the test was replicated three times. After each test, inoculum was tested, and algal culture contamination tracked with morphological analyses. If other strains (than inoculated ones) were found, the set-up was totally disinfected and the test restarted. 2.3.3. Choice of strains Some preliminary experiments were carried out to identify which algal morphology could quickly colonize the tiles. Tiles chosen for those experiments were made of clay roofing, which are known to have a high bioreceptivity [5]. Three specimens were exposed to the water-streaming test, stimulated with the two strains, the unicellular algae, on the one hand, and the filamentous one, on the other hand. Algal growth kinetics on tiles was measured by image analysis using the threshold method (the threshold value was measured on wet cleaned tiles). Results have shown that kinetics of colonization were three times faster with the filamentous species than the unicellular species. Thus, K. flaccidum was the algal strain used for all the other experiments done in this study.
3. Results 3.1. Algal growth kinetics on reference glass tiles First water-streaming experiments were done to compare colonization kinetics that occurred on the four porous glass tiles, which
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(a) light 100 cm
Sprinkling rail
sample Plastic deck
pump
(b)
Fig. 1. (a) Image and (b) schematic description of the water streaming test chamber.
possessed a similar overall porosity (about 40%) but distinct pore distribution and roughness (values provided in Table 1). Fig. 2a shows the evolution of the colonized areas obtained after a few weeks of exposure to the water streaming. The roughness (characterized by the Ra value) seems to be the predominant parameter that influenced the algal colonization when the overall porosity values were held constant. For example, PG1 and PG2 tiles showed the same pores characteristics (overall porosity and pore distribution) and then a similar content of liquid water during the experiment (continuous water flow, 99% RH). However, the rough tile (Ra of PG2: 20.7 lm) was faster colonized by algae (colonized area about 20 times higher after 3 weeks) than the smooth tile (Ra of PG1: 5.4 lm). To assess the effect of the roughness on algal colonization, other experiments were carried out on non-porous glass tiles, which presented distinct levels of roughness (as shown in Table 1). The quantification of the colonized surface on those tiles is given in Fig. 2b. After 4 weeks of test, only a minor part of the surfaces was colonized, highlighting the interest of using non-porous and relative smooth tiles. For example, algal colonization reached less than 0.25% of the entire area for NPG1 and NPG2 tiles. The algal depositions detected by image analysis were then probably microorganisms stuck on tiles just before scanning but they cannot be recognized as a perennial colonization.
3.2. Algal growth kinetics on tiles As shown by Table 1, the limestone tiles were highly porous and rough, with values of porosity and Ra close to those of certain porous glass tiles (such as PG2, PG3 and PG4). Similarly, the water-streaming tests done on the limestone tiles highlighted a rapid algal colonization. As shown by Fig 2c, almost all the surface of limestone tiles was then covered by algae after 4 weeks of exposure. The tiles made of clay were relatively porous (25% of overall porosity) but smoother than the limestone tiles (Ra value of 15 lm). Fig 2c establishes that the clay tiles were then slowly colonized by algae. Compared to the previous results obtained on glass tiles, this result could confirm that a certain value of smoothness (above approximately 15 lm) would reduce the colonization kinetics by algae (even in case of porous tiles). As shown by Table 1, the two types of concrete tiles offered different porosity and roughness characteristics. However, Fig. 2c illustrates slow colonization kinetics that was relatively close to those of the clay tile or PG1 tile. These results were relatively conformed to those expected for the HPC tiles, which present intrinsic low overall porosity and very smooth surface. However, slow algal colonization after 28 days of exposure was not expected on the PCC tiles (due to their intrinsic high overall porosity and rough surface).
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Colonized surface, %
674
100 80 PG4 PG3 PG2 PG1
60 40 20 0 0
5
10
15
20
25
30
Time, days
Colonized surface, %
(a) 10 NPG4 NPG3 NPG2 NPG1
8 6 4 2 0 0
5
10
15
20
25
30
35
40
Time, days
Colonized surface, %
(b) 100 80 HPC Lime Clay PCC
60 40 20 0 0
5
10
15
20
25
30
Time, days
(c) Fig. 2. Time variations of colonized area by Chlorophyceae observed on tiles made of: (a) porous glass, (b) non-porous glass, and (c) limestone, clay and concrete.
Finally, we hypothesize that kinetics of colonization detected on the concrete tiles were disturbed by their specific surface composition, which was highly alkaline after demoulding. An alkaline pH is then known to inhibit the growth of algae [9] and should be taken account before considering the porosity or roughness characteristics of the concrete tiles.
surface and depended on the micro-topography of the substrate. We hypothesize also that the growth of crystals of calcite may enhance the settlement of algae but other tests should be undertaken to determine the influence of carbonation of the concrete surface. 4. Discussion
3.3. Characterization of algal settlement on concrete tiles surface To understand the algal growth kinetics on the PCC and HPC tiles, the surfaces were characterized by ESEM after algal colonization. Observations were carried out after 4 weeks of water-streaming test and images are provided in Fig. 3. Filamentous cells were easily identified on both concrete tiles. Fig. 3a and b highlighted the differences of micro-topography between PCC and HPC surfaces. Many cracks and textured areas were detected on the PCC surface while the HPC surface was very flat without any visible opened pores. In Fig. 3c and d, images showed that algae cells took root into the material whatever the type of concrete tile. Thereby, crystals of calcite were detected near several parts of filamentous cells (Fig. 3e and f). The biological dissolution and re-crystallization of calcite were long-time processes that could not be considered in this experiment; then this calcite did not result from a reaction between the acids generated by algal cells and the alkaline compounds of the hydrated cement paste. As a conclusion, these observations highlighted that the settlement of algae was heterogeneously distributed on the concrete
Considering all the tests done on the glass, limestone and clay tiles, it could remain difficult to separate exactly the influence of the roughness whatever the porosity, which was determinant for the evaporation process. The experiments done on the non-porous glass tiles showed that no algal colonization occurred until a roughness (Ra value) of 13.5 lm but we did not succeed in manufacturing glass tiles rougher than this value and presenting no open pores at the surface. However, air moisture was saturated with water in our laboratory set-up. The water was then always available and must be sufficient for algal colonization. Considering this point, only the roughness can be examined as relevant factor because this parameter can directly increase the probability of trapping the algae. Indeed, a low Ra value close to 5 lm allowed reducing strongly the colonization kinetics on the porous glass tiles (about 40% of porosity). Considering a porosity of 25%, a Ra value about 15.1 lm offered also a slow algal colonization on clay tiles. In this particular environment, a roughness equal or lower than 13.5 lm seems to be sufficient to avoid any perennial algal settlement for non-porous
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algae
(a)
(b) open porosity
(c)
algae
(d)
algae
calcite
(e)
(f )
Fig. 3. ESEM images of algal growth on PCC tile (a, c, and e) and HPC tile (b, d, and f).
glass samples. We can then hypothesize that the minimal roughness required for algal colonization is probably reached to a Ra value close to 13.5 lm. Because kinetics and colonized areas were 5 times larger if the tiles were porous and rough (as seen by comparing Fig. 2a and b), the presence of high porosity could be considered as a booster of the algal colonization when the roughness is sufficient to allow the settlement of algae. The kinetic and colonized areas were quite similar for PG1, Clay, PCC and HPC tiles. The analysis of covariance with a 95% confidence interval was necessary to differentiate them. The first group was composed of clay or concrete tiles while the second group was composed of PG1 tile. The slope of PG1 was significantly distinct and lower than for the others. The difference was probably due
to the surface chemistry, which can play a role for tiles made of clay or concrete. Indeed, the concrete tiles were specific due to their alkaline composition that could postpone the algal colonization for a while considering the level of carbonation. The areas of concrete tiles colonized by algae were very small after 28 days of water-streaming tests, whatever their intrinsic porosity and roughness. Contrary to the PCC tiles, we suppose also that the low roughness of the HPC tiles (Ra of 1.0 lm) could protect them after a complete carbonation of the surface. Other experiments should be then performed in future to compare the algal colonization on aged (and carbonated) concrete tiles to confirm this hypothesis. Such a parametrical study could be implemented by the use of design of experiment (DOE) and the Taguchi method.
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5. Conclusions
References
Water-streaming tests were performed on different samples to study the kinetics of colonization by algae on:
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– Reference samples made of porous and non-porous glass (to the knowledge of writers, it is the first time this kind of study was done with isotropic glass samples), – Tiles made of different building materials (limestone, clay and concrete). A new methodology based on image treatment was developed to ensure the measurement of algae colonization. The influences of the porosity and roughness were then established using the tiles made of glass, limestone and clay, while the specific influence of the alkaline composition was highlighted using the concrete tiles. Indeed, considering a Ra value lower than 13.5 lm, the nonporous glass tiles were not colonized after 4 weeks of exposure to the water streaming tests. This value can be considered as a threshold value for the initialization of a perennial colonization. Besides, a relative smoothness (Ra between 5.4 and 20.7 lm) allowed reducing the potential algal colonization on the porous glass tiles (although an overall porosity of 40%). Concerning the limestone tiles (characterized by high values of overall porosity and roughness), the kinetics of algal colonization was close to those of the roughest porous glass tiles, confirming the high influence of this couple of parameters for these two distinct materials (totally different considering their chemical properties). However, the clay tiles showed middle-values of porosity and roughness (25% of overall porosity and 15.1 lm of roughness) and then a relative slow kinetics of algal colonization. Both the concrete tiles showed very slow kinetics of colonization (at least during the 4 weeks of the tests) whatever their respective porosity and roughness characteristics. We hypothesized that the concrete surfaces were protected by the alkaline composition, which was able to disturb dramatically the growth of algae. ESEM observations of the concrete tiles highlighted a potential relation between the presence of calcite crystals and the settlement of algae. Finally, our results done in very favorable conditions for algal settlement (RH about saturation) suggested that the aesthetic of roofs would be more durable by manufacturing very smooth tiles (whatever their intrinsic porosity). Other tests will be performed to evaluate if this smoothness could reduce or avoid the use of cleaning agents and biocides. Acknowledgements The authors would like to thank S. Brun, M. Dykman and C. Bouillon (Lafarge) for their helps during the preparation and the analyses of the tiles; M. Guivarc’h and A. François for their assistance.