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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: p.hughes58@outlook.com

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.