Concrete durability in tunnels is essential, as failures in the past have led to the loss of life. There is a tendency in sprayed concrete, whether used in tunnel works, or in open-air applications, for biological cells and debris to accumulate on the outer and inner surfaces of concrete. This further attracts moisture, which may facilitate chemical reactions and promote the growth of bacteria (1). Tunnel excavation work for a new Ring Rail Line under the eastern runway of Helsinki-Vantaa Airport in Finland has been delayed, due to interesting and unexpected phenomena. From the ground above, breakdown products of propylene glycol were found to be seeping into the tunnel, originating from de-icing agents used on aircraft at Helsinki-Vantaa. When interacting with oxygen, these products facilitate the formation of a certain microbe population in the water seeping into the tunnel and on the tunnel walls. As a consequence, the construction budget keeps growing and according to the Ministry of Finance, the cost-estimate for the additional work has reached EUR 50M (USD 66.8M) (2).
Microorganisms such as bacteria and algae have been widely reported to be involved in the deterioration of concrete (3) (4). However, relatively few direct relationships have been established between the activities of microorganisms and synthetic fibres within concrete, in harsh environments such as tunnels. It has been reported that steel fibres within sprayed concrete were severely attacked by bacteria (1), and synthetic fibres have also been observed in a marine environment to be susceptible to microbial colonisation (5) (6).
It is claimed by other researchers that some species of bacteria may enhance the durability of concrete, however, it is the view of the author that filamentous growth is detrimental to the durability of synthetic fibre reinforced concrete in harsh environments. In related studies by the author, the degradation of synthetic fibres have been observed in a marine environment over the eight year lifetime of a sea defence scheme in the UK (5).
It is usually accepted that a watertight concrete is durable. Once water tightness is lost however, the interior of the concrete may become saturated; allowing microorganism’s carried in water to play a greater active role in deterioration. As the microorganisms are transported into the matrix of the material they will contribute successive cycles of expansion, further cracking, fibre de-bonding and liberation leading to increased permeability, see figure 1. The occurrence of bacteria in concrete tunnels was reported in 1934 (7); highlighting an interesting and serious problem, by the appearance of Crenothrix, a type of bacterium found growing in the ground water in the Hetch Hetchy water system (USA).
This seepage water, having a high mineral content and containing the Crenothrix organism, flowed through the tunnel and carried the microorganism into several lines of conveying water. After intensive study, controlling the organism with a chlorineammonia treatment was used. Algae have also been commonly observed within subsea tunnels clogging of drains (8).
DISCUSSION
The biotenacious growth on and around the micro-fibres in (Figures 2 and 3) can be likened to the natural process of retting where micro-organism’s have been used for many years, in the extraction of fibres from plant materials. Retting employs the action of bacteria to dissolve and degrade the surrounding tissue of bast-fibre, thus separating the fibres. This is essentially an assimilative process, during which organic residues are washed away, leaving the fibres intact. Degradation of fibre reinforced concrete however, is essentially a dissimilative process, but with a similar result.
Filamentous bacterial growth (Figure 3) which can occur at very low nutrient levels, not only wraps around the fibres weakening the fibre/concrete matrix bond, but condition the surface of the fibre, making it more amenable to colonisation by other micro-organisms such as algae (Figure 4). The mechanisms observed and presented in Figures 1 and 2 occur when the material bond is weakened as a direct result of the physical activity of an organism, such as its movement or growth (9).
In cement composites with fibre inclusions the matrix in the vicinity of the inclusion can be quite different in its microstructure to that of the bulk cement matrix. This modified matrix can be as great as 50µm to 100µm (10).
The higher porosity at this interface, favours the appearance and development of bioerosive forces in the form of microbial filamentous growth. Experimental evidence from other researchers indicates that porous concrete creates a favourable environment for microbial colonisation, because water-borne organisms including algae adhere to both its inner and outer surfaces (11). The observations reported in this study, particularly the attachment of filamentous organisms, the cohesion between particles, the penetration and growth (seen in figure 5) are comparable to those observed in previous work on boring activity within the concrete of historic buildings (12). This type of deterioration has also been reported in subsea tunnels in Norway (1), where steel fibre reinforced concrete used for rock support was attacked by saline ground waters along the concrete/ rock interfaces as well as the outer rough and more reactive concrete surfaces. The process had frequently led to the total disintegration of the cement paste matrix and steel fibre after less than five years exposure and was reported as closely related to the growth of biofilms.
CONCLUSION
Early bacterial biofilm formation within the matrix is the start of the marine fouling process, leading to further colonisation of algae and other microorganisms. This continual growth at the fibre/cement interface weakens the bond between fibre and cement. Filamentous microbial growth around fibrillated polymer and the penetration of microbial filaments through the material is detrimental to the overall durability of the concrete itself. Potential inluences of marine organisms on the durability of synthetic fibres in concrete used in harsh environments clearly warrant further detailed investigation
