Impact of marine biofouling on durability of marine concrete
Around Great Britain, 20% of sand and gravel is dredged from the sea. Other European countries avoid sea sands and gravel. While micro-organisms were once thought to improve concrete, new observations cast doubts. Some study samples with an expected life or 100 years are showing signs of bacteria and algal growth after four years and may not last for ten. Peter Hughes describes his study of concrete and raises concerns for the durability of concrete which is in contact with sea water.
Robyn Williams: Our PhD this week concerns concrete and how it may crumble—yes, bridges and buildings—if it's wrongly prepared.
Peter Hughes: My name is Peter Hughes, I'm a final year PhD candidate at the University of Central Lancashire in the UK, investigating marine biofouling and its implications for the durability of marine concrete. Material from marine deposits around the coast of Great Britain has been used in concrete production for several decades, even though other countries throughout Europe avoid this practice.
However, no provision is made in the current UK standards for the control of microbial growth within or on the surface of beach sand. In the UK about 20% of natural gravel and sand requirements are sea dredged. There are over 2,000 quarries operating around Australia that produce 150 million tonnes of rock, limestone, gravel and sand.
Recent research into the use of certain micro-organisms for the improvement of durability has recently drawn the attention of research groups all over the world. Most research in this field has reported on the addition of bacteria into the matrix as a self-healing addition. But this study is the first to report observations of filamentous bacterial growth within new unplaced marine concrete.
Durability of marine concrete is arguably its principal property. It is important that concrete should be capable of withstanding the harsh conditions throughout the expected life of the structure. Beach sand samples from the vicinity of the dredging site were examined under a scanning electron microscope and found to be colonised by filamentous microbial growth. Bacterium was then cultured in the lab from these sand samples and matched to bacteria found in new concrete. Freshly hardened concrete containing the sand was examined, confirming the formation of the bacterial biofilm within the new concrete matrix. Bacteria had survived the concrete making process.
I know production on some projects in southern China have been recently suspended following reports that the concrete contained marine dredged sand, illegal sea sand rather than river sand. The Shenzhen Housing and Construction Bureau held a news conference stating that it had launched an enquiry into this inferior practice. The use of concrete made from marine sand is believed to be widespread throughout the Pearl River Delta area of the Guangdong Province, one of the major hubs of industry and commerce in China.
The UK Mineral Products Association warns that washed beach sand is generally unsuitable by itself for good quality concrete due to the single size grading, but this can be overcome by blending aggregates. While aggregates for unreinforced concrete can be washed with seawater, 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 preliminary results of this ongoing study suggests that the blanket use of marine source aggregate in the UK should be reconsidered for concrete that is to remain in contact with seawater. Washing in water may only partly remove some of the biomass present on the surface of the aggregate, possibly leaving micro-organisms to continue and thrive.
After only five years monitoring at the study site, several precast units are in a very poor condition. These units have a design life of 100 years. It is doubtful if a large number of these units will last ten years. Microbial content needs to be controlled, particularly in structures subject to permanent wetting by seawater, to control growth on and inside the matrix. Guidance should consider undesirable elements such as micro-organisms 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 fine aggregate, leading to increased colonisation and the biodeterioration of concrete.
Robyn Williams: Peter Hughes is at the University of Central Lancashire. So there's a warning.
- Peter Hughes
- PhD Candidate
University of Central Lancashire
Preston Lancashire UK
- Robyn Williams
- David Fisher