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Peter Hughes Bridge
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A UK take on a global debate: The use of
marine dredged aggregate for concrete
Dr. Peter Hughes HPU, PRC.
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Figure 1. Examples of beach sand studied from the dredging area.
This article presents preliminary observations from an on-going investigation
into the use of beach sand as fine aggregate for marine concrete. Scanning
electron microscopy (SEM) revealed gram-positive bacteria which originated
from beach sand having survived the concrete manufacturing process and
was observed growing within the freshly hardened matrix.
Material from marine deposits around the coast of Great Britain has
been used in concrete production for several decades however; no provision
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is made in the current (UK) standard for the control of microbial growth
within or on the surface of beach sand. In the UK about 20% of natural
gravel and sand is sea dredged (Figure 1). MPA-Cement warns that washed
beach sand is generally unsuitable by itself for good quality concrete, due to
the single-sized grading, but this may be overcome by blending aggregates.
In terms of the durability of marine concrete, aggregates should be
mineralogically homogeneous, strong …..and clean.
Microorganisms are a significant component of beach sand, from which
bacteria, fungi, parasites and viruses have all been isolated. In the UK, close
to the dredging area from which the study concrete was made, intertidal
zone sediments appeared to serve as a substantial reservoir for thermophilic
campylobacters, which could contribute significantly to bacterial numbers in
surface waters, especially in rough weather (1). Sand beaches in Portugal
contained counts of Clostridium perfringens under various tidal conditions (2).
Other researchers suggested that C. perfringens could be a good index of
faecal contamination in sand sediment (3). Low levels of Campylobacter
jejuni were recorded in both coastal waters and sand on a number of Israeli
beaches, with the beach sand containing higher counts than adjacent shore
waters (4). Researchers isolated Shigella spp. from beach sand and water in
the bay of Gdansk (Poland) (5).
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Figure 2. Example of fine aggregate (beach sand) having been washed in
seawater before being used in the manufacture of the concrete studied.
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Figure 3. Filamentous bacterium cultured from samples of the
beach sand collected from the dredging area.
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Figure 4. Surface of the new concrete. Beach sand can be observed at the surface.
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Figure 5. SEM micrograph: Filamentous bacterial growth from a pore of the
surface of beach sand within freshly hardened concrete.
Beach sand samples (Figure 2) were taken from the dredging site and
examined. Bacterium was cultured (Figure 3) in the laboratory from these
sand samples. Freshly hardened concrete containing the sand (Figure 4) was
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examined by SEM. The micrograph (Figure 5) confirmed the formation of the
bacterial biofilm and is a representative image of bacterial filamentous
growth within the new concrete matrix examined. Empty water voids were
often observed within the new matrix, forming rippled effect chambers,
these pockets of water within the matrix may have been utilized by the
microorganism.
All of the designated beaches close, and including the dredging site,
continually fail the European directive imperative standards for recreational
waters. Studies have shown that microbial contamination is higher in sand
than in adjacent waters, as the sand behaves as a passive harbour for
cumulative pollution (6). A recent study in the American Journal of
Epidemiology surveyed (US) beachgoers and found that people who buried
themselves in the sand or built sandcastles were more likely to expose
themselves to harmful bacteria than those who went swimming on the same
beaches.
Relevant research found viable cells buried to a depth of 0.2m below
the sand surface in a Scottish sea loch (7). A similar finding occurred 200km
from the source of the marine aggregate used in the study mix, in Lough
Neagh (Northern Ireland), where high concentrations of living cells were
found attached to sand grains down to 0.5m below the sand surface (8).
Considering the constant mixing of sediment at a beach surface, it may
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follow that microbial endoliths could occupy some of the beach extraction
used in the production of the concrete examined in the current investigation.
While aggregates for unreinforced concrete can be washed with sea
water, as was the case in this study, for reinforced concrete the aggregates
must be carefully washed with potable water to remove excess chloride from
sea salt, however they will still retain shell fragments and organic matter
that can affect the water demand of the mix. The organic content refers only
to water-soluble organic compounds derived from decaying vegetation, tests
for which no longer appear within (UK) standards.
It is the author’s understanding that the early biofilm formation (of
bacteria), reported here, is the actual start of the marine fouling process,
before the concrete is even placed, leading to an accelerated colonisation of
algae, once the concrete is placed at site. Related studies by the author on
the effect of this fouling on fine aggregate (9) and synthetic fibres (10) are
reported elsewhere.
Work continues by the author on DNA analysis and cultured specimens
from retrieved beach sand shows promise. The preliminary results suggest
that the blanket use of marine sourced aggregates should be reconsidered
for concrete that is to remain in contact with sea water, particularly mass
(unreinforced) concrete. Washing in water may only partly remove some of
the epilithic biomass present on the surface of the aggregate, possibly
leaving endolithic microorganisms, (Figure 5) to continue and thrive. It has
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been reported some microorganisms have resting stages (spores and
zygotes) that allow cells to lay dormant in unfavourable environments, even
freezing conditions or to survive in ephemeral pools. Previous research found
that even after soils had air-dried for 35 years, cells could be cultured.
Therefore the microbial content may need to be controlled in structures
subject to permanent wetting by sea water to control growth on and inside
the matrix. BS EN 12620 is the predominant specification concerning the use
of aggregates for concrete supported by UK national guidance document PD
6682-1. This guidance should consider undesirable elements such as
microorganisms more closely and place precise limits on their presence. For
marine concrete structures there is a tangible risk that microbial growth will
remain on or within the beach sourced fine aggregate, leading to increased
colonisation and the biodeterioration of concrete.
Further discussions are invited at: [email protected]
April 2017
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References 1. Obiri-Danso, K, Jones, K. Seasonality of thermophilic Campylobacter and faecal indicators in
sediments from the intertidal zone of Morecambe Bay. Proceedings of the 9th International Workshop
on Campylobacter, Helicobacter and Related Organisms-Cape Town. Abstracts, 1997, 15-19.
2. Mendes, B, Nascimento, M.J, Oliveira, JS. Preliminary characterisation and proposal of
microbiological quality standard of sand beaches. Water Science and Technology. 27, 1993, 453–456.
3. Bonadonna, L, Dal Cero, C, Liberti, R, Pirrera, A, Santamaria, C, Volterra, L. Clostridium perfringens
come indicatore in sedimenti marini. [Clostridium perfringens as an indicatorin marine sediments.].
Ingegneria Sanitaria Ambientale. 1, 1993, 28-30.
4. Ghinsberg, R.C, Leibowitz, P, Witkin, H, Mates, A, Seinberg, Y, Bar, D.L, Nitzan, Y, Rogol, M.
Monitoring of selected bacteria and fungi in sand and seawater along the Tel-Aviv coast. United Nations
Environment Programme, Mediterranean Action Plan, pp. 65–81. (MAP Technical Reports Series No. 87),
1994, 65-81.
5. Dabrowski, J. Isolation of the Shigella genus bacteria from beach sand and water of the bay of Gdansk.
Bulletin of the Institute of Maritime and Tropical Medicine in Gdyinia. 33, 1982, 49–53.
6. Oliveira, J.S, Mendes, B.S. Water quality in Portugal. 1° Congresso da Agua, Lisbon, Portuguese
Association of Water Resources (APRH). 1992, Vol. 2, 155–179.
7. Steele, J., Munro, A., Giese, G. Environmental factors controlling the epipsammic flora on beach and
sublittoral sands.. J. mar. biol. Ass. 50, 1970, 907-918.
8. Jewson, D., Briggs, M. Benthic algae in Lough Neagh. Lough Neagh: The Ecology of a Multipurpose
Water Resource. 69, 1993, 239-244.
9. Hughes, P., Fairhurst, D., Sherrington, I., Renevier, N., Morton., L.H.G., Robbery, P., Cunningham, L.
Microscopic study into biodeterioration of marine concrete. International Biodeterioration &
Biodegradation. 79, 2013, 14-19.
10. Hughes, P., Fairhurst, D., Sherrington, I., Renevier, N., Morton., L.H.G., Robbery, P., Cunningham, L.
Microscopic examination of a new mechanism for accelerated degradation of synthetic fibre reinforced
marine concrete. Construction and Building Materials. 41, 2013, 498-504.