Steel reinforcement corrosion is a perennial problem for concrete as a construction material. Engineers are looking at novel ways to ward off this concrete cancer.
When create connects via video link with Dr Priyan Mendis, the University of Melbourne professor is in his birth country of Sri Lanka.
He has not returned for sentimental reasons, however, or to sample the island’s renowned culinary scene or magnificent scenery. Mendis is in pursuit of what he tells us is the purest graphite in the world.
Sri Lanka’s lode of ultra-pure highly crystalline vein graphite consists of more than 98 per cent carbon, according to the Graphene Council.
That purity – and the cost-effective production of graphene that it portends – could be the secret to curing concrete cancer.
Eaten away
The irony of rebar corrosion and other forms of concrete cancer is that it results from attempts to make concrete structures stronger.
Steel reinforcements are set into concrete structures to improve their tensile strength, but even in highly corrosive coastal environments, this metal framework is often set close to the material’s surface – as little as 40 mm.
But the strengthening steel performs superbly as long as the concrete around it shields from exposure to corrosive elements. The problem begins when chemicals such as chlorides, sulfates and carbon dioxide react with moisture trapped in the usually alkaline concrete to produce acid.
“So these acids reduce the pH value, create that acidic type of environment, and destroy that passive, protective layer around the reinforcement,” Mendis explained. “That is what creates the corrosion. Corrosion creates oxides, it expands and the concrete starts cracking.”
Concrete cancer is an apt name, Mendis said.
“Because it is really dangerous,” he said. “That is the main durability problem for concrete.”
That’s where graphene comes in.
Microscopic marvel
Since graphene was first fully isolated in atom-thick sheets in 2004, engineers have recognised the potential of the carbon allotrope’s remarkable properties.
“Graphene is, compared to other materials around, thousands and thousands of times stronger,” Mendis said, “because it’s a one-layer material.”
That is one literal layer of atoms, and being composed of just a single layer of atoms removes the presence of imperfections in the material’s lattice structure, endowing it with high tensile strength.
Construction engineers took note. The University of Manchester physicists who first isolated the material by peeling it layer by layer from chunks of graphite received the Nobel Prize for their work, and Mendis himself began researching the material soon after.
He wanted to turn graphene from a substance that evinced astonishing properties in the laboratory to one that had useful applications in everyday life – on a construction site, for instance.
Water works
The first step to using graphene to strengthen construction materials is to convert it to graphene oxide.
“Graphene oxide is water soluble – we call it hydrophilic – and that is a big advantage,” Mendis explained. “Water is an ingredient in concrete, so we mix it and put [the graphene oxide] in as an additive to the concrete mix. By putting this in, the compressive strength in the concrete increased by about 20-30 per cent. Even the tensile [strength] – the pull-out load – even that increases by almost double.”
The strength of graphene-enhanced concrete makes it much more durable, including in its resistance to corrosion.
“Normally in concrete, we have these voids,” Mendis said. “If you can reduce the voids, it becomes more compact, and what the graphene oxide is doing is making it more compact and reducing those voids.”
The resulting, less porous structure restricts corrosive chemicals from seeping too far into the material, protecting the steel reinforcement inside. But the graphene oxide additive also helps during the formation of the concrete itself, when cement is mixed with water.
“Graphene oxide creates nucleation sites, and those nucleation sites help to improve the reaction, which adds to the strength of the concrete.”
Black gold
A remarkably small amount of graphene can make a big difference.
“If we need about 400 kg of cement for one cubic metre [of concrete],” Mendis said, “we have to use only 120 g of graphene oxide.”
But that remarkably small amount of graphene can put its cost out of the reach of many everyday users.
“Graphene oxide was so expensive that we couldn’t try it in the field,” Mendis said. “No-one wanted to consider it.”
Which brings us back to Mendis’s Sri Lankan quest. He’s working there, he said, with Ceylon Graphene Technologies, a supplier producing cheap graphene oxide. He hopes it will be cheap enough to convince the construction industry to make use of his enhanced, strengthened concrete.
“What we have done is to show that there’s no penalty if you use this,” he said. “Because of the durability, there’s no penalty – and it reduces carbon numbers. As we know, sustainability is very big these days. So it’s sustainable, durable, and there’s no cost penalty on the material.”
Graphene oxide is not a perfect concrete additive; it’s an ingenious solution rather than a silver bullet. The resulting concrete is less workable, and Mendis is still working on “super-plasticisers” that can counter this effect.
And while there have been other solutions to concrete cancer, Mendis is less enthused about these. Carbon nanotubes, for instance, are too expensive, he said, while cathodic protection, which involves introducing a more easily corroded “sacrificial metal” to act as an anode, is “messy”.
“We don’t have a choice sometimes, but the best way is not to have that intervention and to improve the concrete,” he said. “You can do both. There’s no problem having cathodic protection; there’s no interference.
“But if we can get rid of the cathodic protection by using high-level, advanced materials, that’s probably the best solution.”
Take your knowledge of the future of corrosion protection further with this webinar for engineers.
Concrete cancer is not a disease. It is the consequence of poor engineering, poor supervision and poor building practices.
The cause of concrete cancer is.
Insufficient cement in the concrete making the concrete porous.
Poorly graded concrete with too large of a maximum sized aggregate specified,
Too much water in the concrete making the concrete porous.
Too little water in the concrete to develop sufficient slump and allow the concrete to flow around the reinforcing steel.
Lack of compaction making the concrete porous.
Steel bars placed too closely together so that there is a lack of dense concrete where the bars are too close together.
Insufficient cover in a corrosive environment.
The cure for preventing concrete is education of engineers to design and specify the proper components of concrete, proper control, education of site control personal, and education of concrete workers.
I would like to see the photograph show the cover to the steel on the end face of the concrete.
Galvanizing the reinforcing is one surefire way of mimimizing the effects of corrosoon on pourly mixed, placed and compacted concrete.
segregation of concrete when placed temains the main cause of porous concrete