Influence of concrete properties on the initial biological colonisation of marine artificial structures
Introduction
More than 70% of countries with shorelines have their largest cities built on the coast (Beck and Airoldi, 2007). Almost half of the world's population is now estimated to live in coastal areas and that figure is predicted to double by 2025 (Creel, 2003). As the coastal population has increased, more pressure has been put on coastal ecosystems through habitat conversion, increased pollution and greater demand for marine resources (Beck and Airoldi, 2007). The change from natural to artificial coastal habitats is considered one of the main threats to coastal ecosystem integrity and sustainability (Shabman and Batie, 1980). These marine developments are generally associated with fragmentation and loss of natural habitats, damaged seascapes and reduced biodiversity (Beck and Airoldi, 2007). Despite the rapid development of artificial marine structures across the world, our understanding of the colonisation of these environments by epibiotic plants and animals is limited (Dugan et al., 2011).
Natural habitats such as rocky reefs provide habitats for a large number of intertidal and subtidal species of plants and animals (Glasby and Connell, 1999), and are being replaced by artificial structures. These marine infrastructures can provide significant habitat for epibiota and also attract mobile subtidal species during high water and birds during low water (Firth et al., 2013; Martin et al., 2005). However, the epibiotic assemblages they support are often depauperate compared to natural habitats (Glasby and Connell, 1999; Bulleri and Chapman, 2010; Vaselli et al., 2008; Moschella et al., 2005). Natural habitats provide shade, moisture and refuge from predation and disturbance through their rough surfaces, pits, crevices and rockpools (Strain et al., 2018). In contrast, artificial structures typically fail to provide these textures and microhabitats, with a notable impact on the organisms that would otherwise colonise these surfaces (Aguilera et al., 2014).
Marine biofilms form rapidly on coastal structures and consist of dynamic communities of diatoms, cyanobacteria and green algae which form within a matrix of extracellular polymeric substances (Flemming and Wingender, 2010; Stewart and Franklin, 2008). These biofilms act as one of the main food sources for grazing invertebrates (Hill and Hawkins, 1991; Williams, 1964). In addition, biofilms are known to emit chemicals used as cues for settlement by the larvae of fouling organisms (Hadfield, 2011; Salta et al., 2013). They are also an important contributor to ecosystem functioning and productivity (Golléty and Crowe, 2013). Factors such as microtopographic complexity and substrate surface chemistry have been reported to influence the development and composition of biofilm communities (Sweat and Johnson, 2013; McManus et al., 2018). As the physiochemical properties of concrete may vary widely, biofilm composition may also be altered. The influence of variation in concrete properties may also vary depending on the environmental context, for example in relation to exposure to wave action. Therefore, understanding the direct effect that concrete composition will have on biofilm community structure is not only important due to their role as a food source for grazing invertebrate species, but also to understand the indirect effect that changes in biofilm community structure may have on the colonisation of invertebrate and macroalgal species as succession progresses.
It is proposed that the ecological value (i.e. local biodiversity and local biomass) of artificial marine infrastructure could be increased through careful design of pre–fabricated ecologically engineered units (Firth et al., 2014; Perkol-Finkel and Sella, 2014; Perkol-Finkel and Sella, 2015; Loke and Todd, 2016). Material selection is a crucial parameter in the design of these units. Reinforced concrete often plays an important role in the design process due to its ease of production, relatively low cost and its suitability for mass construction. Portland cement concrete is widely used (Perkol-Finkel and Sella, 2014), and has been found to offer good support for colonising organisms with calcareous skeletons (e.g. oysters, serpulid worms, barnacles and corals), as they deposit calcium carbonate onto the surface in a biogenic build-up process (Risinger, 2012). That said, the high surface alkalinity of the concrete (pH 12–13 compared to 8 of seawater) could reduce settlement of other less alkotolerant species marine organisms and result in communities dominated by a few alkotolerant taxa (Guilbeau et al., 2003). By adding pozzolanic industry by-products such as ground granulated blast-furnace slag (GGBS), fly-ash and silica fume to the concrete mix, it is possible to reduce the alkalinity of the concrete and potentially create a more suitable surface for colonisation by marine species (Bertos et al., 2004).
Concrete possesses an inherent reserve of alkalinity, and this is sometimes expressed through its capacity for neutralizing acids by hydrolysis of the products of anodic dissolution. This inhibitive property of concrete arises from its ability to neutralise the acid produced by hydrolysis of the products of anodic dissolution. This inhibitive property can be characterised by its acid neutralisation capacity (ANC). The ANC is also considered a key parameter in characterising concrete corrosion resistance (Sergi and Glass, 2000). It is also a useful measure to describe the concrete alkalinity. The cement content, binder type and free water to cement ratio play a significant role in ANC of concrete (Jung et al., 2012).
Furthermore, the addition of pozzolanic industry by-products would also enhance the chloride resistance of the concrete and make it less prone to corrosion associated with the marine environment (Pacheco-Torgal and Jalali, 2011; Sivaraja et al., 2010). Chloride ions exist in seawater and can permeate the concrete, destroying the passive layer at the surface of the concrete reinforcement. This begins once the chloride concentration reaches a certain threshold and leads to continuous corrosion of the embedded steel (Pack et al., 2010). Previous research (Pack et al., 2010; Cheewaket et al., 2010) has shown that increased GGBS replacement levels can lessen the probability of chloride induced corrosion.
Firth et al. (Firth et al., 2016) have also investigated other approaches such as enhancing the topographical complexity at a different scales and increasing the range of habitats present. Surface texture, holes, cracks pits and pools have been proven to have a significant and direct effect on increasing biodiversity on artificial marine infrastructures (Firth et al., 2014; Chapman and Blockley, 2009; Evans et al., 2016; Ido and Shimrit, 2015). Other research (Borsje et al., 2011) has shown that green algae colonisation on the rough surface of 75 cm × 50 cm slabs is faster than for smoother surfaces.
The concept of applying ecological principles to marine infrastructure has only developed within the last decade and to date has been rarely implemented in many coastal environments (Evans et al., 2019). It is important to understand the ecology of developed shorelines and to find ways to decrease the impact of urban development. Although some research has been conducted on the use of concrete as a new habitat for marine micro inhabitants (Guilbeau et al., 2003; Evans et al., 2019), they rarely consider the engineering requirements and focus instead on ecological aspects. This paper describes a collaboration between ecologists and engineers to develop concrete materials which could fulfil the structural requirements and further enhance biodiversity on artificial structures. We examine the feasibility of facilitating the growth of marine species on reinforced concrete structures by changing concrete mix design and composition. In addition to testing the engineering characteristics of a range of alternative concrete mixes, we tested whether variation in the aggregate, binder and plasticizer in concrete tiles would affect (a) strength, chloride resistance and alkalinity (b) biological colonisation of tiles deployed on an artificial rocky shore north of Dublin. We also explored the potential for variation in environmental context to alter the influence of concrete composition on biological colonisation.
Section snippets
Materials and methods
A series of mixes were designed to meet the engineering and ecological requirements. The focus of this paper is the influence of mix parameters on the concrete characteristics, and how these then correlate to environmental performance. This was assessed by designing eight different concrete compositions, and the parameters of interest were binder type, aggregate type and influence of plasticizer.
Compressive strength
The average compressive strength of the various concrete mixtures was determined at 7 and 28 days and these are presented in Fig. 2. It can be seen that higher strengths are generally associated with the limestone aggregate / CEMI concretes, and that the use of GGBS had little effect on the strength.
Resistance to chloride ingress
The results of the non-steady state migration test are presented in Table 2, leading to a series of non-steady state migration coefficients (Dnssm). The highest diffusion coefficients corresponded
Concrete characterisation
The focus of the initial concrete testing was to establish that the selected mix designs were appropriate for deployment in the marine environment. The results of the compressive strength testing showed that all candidate mixes achieved this, but with slightly lower strengths for concretes with granite aggregates. This behaviour has previously been reported by Wu et al. (Wu et al., 2001), although the opposite has also been observed (Pacheco Torgal and Castro-Gomes, 2006; Hussin and Poole, 2011
Conclusion
In this study, although we clearly show that all of the eight difference concrete mixes that were tested were capable of meeting the requirements of Eurocode 2 (in term of resistance, serviceability, durability) with respect to the selected exposure class. Only the concretes manufactured using a GGBS/CEMI binder were likely to have the required chloride resistance to provide a long service life. These findings will reassure engineers and designers that there are more environmentally- and/or
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to thank Derek Holmes and John Ryan of the UCD School of Civil Engineering, and Jennifer Coughlan, Veronica Farrugia Drakard, Caitlin Dalla Pria and Martina Caplice of the UCD School of Biological and Environmental Sciences. The support of Ally. J. Evans, Pippa J. Moore and Melanie Prentice of the Institute of Biological, Environmental & Rural Sciences at Aberystwyth University is also appreciated.
This research was funded by the Ecostructure project (part-funded by the
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2023, Marine Environmental ResearchCitation Excerpt :These structures, particularly those constructed for coastal protection, are often located in areas that experience significant wave action leading to sediment scouring/deposition, meaning they represent more stressful environments for colonizing species (Moschella et al., 2005). Furthermore, the limited availability of microhabitats and water retaining features (e.g., grooves and pools), the lower topographic complexity, and the use of materials such as concrete, the chemistry of which can impact settlement, present further challenges for organisms colonising artificial structures (Aguilera et al., 2014; Chapman, 1994; Firth et al., 2013b; Natanzi et al., 2021; Sella et al., 2018). To date, the majority of studies comparing communities across artificial structures and natural rocky shores have focussed on differences in richness, abundance and assemblage structure (Aguilera et al., 2014; Bulleri, 2005; Bulleri and Chapman, 2004; Chapman and Bulleri, 2003; Firth et al., 2013b; Lopez, 2019; Pister, 2009), with less attention given to the population structure or performance of individual species (but see Díaz-Agras et al., 2010; Farrugia Drakard et al., 2021; Drexler et al., 2014; Moreira et al., 2006).
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