Will the Roman 2000 year product guarantee stand up for today’s concrete mix designs?
Ancient concrete mixes have stood the test of time and withstood all of the environmental attacks known to mankind for over 2000 years. Today’s concrete, however, is susceptible to environmental damage and often needs protection or repair or, in the extreme, reinstatement within thirty (30) years.
What is the secret to 2000 year old concrete?
Roman concrete mix designs were simple. No admixtures. No risk from sulphates. No issues with the penetration of water. No steel reinforcement to rust. No concrete cancer.
Ancient concrete consisted of just three elements:
- The paste – limestone (crushed and burnt)
- The “sand” – volcanic ash
- The aggregate – large lumps of rock
The burnt volcanic ash has an amorphous silica structure with many holes in the molecular network which fill with calcium hydroxide upon mixing with wet lime and becomes the paste which binds the rocks together. Sand in modern concrete however is crystalline and does not have holes in the molecular structure to accommodate the cement paste.
Ancient Romans mixed their concrete product by hand and tamped the mortar into place thus minimising the water content and therefore created a low slump and highly durable mix.
They also discovered that the volcanic ash developed hydraulic properties when mixed with lime and then realised the advantages of hydraulic lime, i.e. cement that hardens underwater.
The Trajan's Market
Modern Fly Ash Mix Designs.
Some modern concrete mixes use fly ash as a supplementary cementitious material which delivers improved workability and, like the Ancients, later age strength and high durability.
Modern Concrete Mixes.
But unlike those majestic Roman structures, today’s cities are plagued by crumbling concrete tower blocks and decaying bridges and expressways.
Today’s concrete is made using Portland cement, coarse and fine aggregates of stone and sand, and water steel reinforcement.
Admixtures are chemicals added to the concrete mix to control its setting properties and are used primarily when placing concrete during environmental extremes, such as high or low temperatures and windy conditions.
Although steel reinforced concrete is one of the most widely used construction materials around the world, it can suffer degradation over time due to the embedded steel corroding, causing the concrete to crack and “spall”.
In extreme situations, the integrity of the structure may be lost, resulting in the need for partial or complete demolition. Corrosion affects all reinforced concrete buildings and structures to some extent, with an estimated annual cost of billions of dollars to national economies. In addition, loose damaged pieces of spalled concrete falling from buildings and structures is a real safety risk.
When chlorides, carbon dioxide gas and other aggressive agents penetrate concrete, they initiate corrosion of reinforcement that typically results in cracking, spalling and weakening of the concrete infrastructure. As reinforcing bars rust, the volume of the rust products can increase to many times that of the original steel, increasing pressure on the surrounding material which cracks the concrete. The cracks can then propagate to delamination and eventually spalling of the concrete.
Usually, the most exposed elements deteriorate first but because the active corrosion may take five to 15 years to initiate cracks in the concrete, much of the actual corroded reinforcement is not visible. Such corrosion is often called “concrete cancer”, because it appears as if the structure was being eaten away from the inside.
We have now realised these issues with modern concrete, for example:
- Thousands of passengers have been travelling over Hawkesbury River Rail Bridge connecting Sydney to the Central Coast every day, despite a crucial supporting pylon being riddled with “concrete cancer”. With cracks up to two metres long, the pylon is so severely deteriorated it crumbles away like powder.
- Edgewater Towers in St Kilda’s Marine Parade is a notorious example of how costly concrete cancer can be. The modernist block of 100 apartments sprang up in 1961 but by the 90s spalling was ravaging its balconies. Issues were still being remediated as recently as 2011.
- Gold Coast’s Ageing High-Rises. Bodies corporate of these ageing buildings are now faced with the prospect of either having to spend hundreds of thousands of dollars, if not millions of dollars, rectifying concrete cancer and other building defects, or they look to cash out and sell to developers.
- It was recently revealed that one building — the landmark Focus Apartment tower — needed $2.7 million in repairs to prevent “extremely dangerous” deterioration from concrete cancer or spalling.
- The 20-storey Iluka Surfers Paradise high-rise was demolished after concrete cancer destroyed its structural integrity.
- Dozens of 40-year-old Gold Coast high rise apartment towers built in the 1970s face million-dollar concrete cancer repair jobs similar to the $215 million in repairs needed by Brisbane’s City Hall.
- When it was finished in 1978, the 67-storey MLC Centre in Martin Place, Sydney, was not only Australia’s tallest office building, but also the biggest reinforced concrete structure in the world. Thirty-three years later in 2011, the Harry Seidler-designed structure is showing its age. Its concrete facade is breaking up and the owners have agreed to spend $100 million repairing it in an operation that will go 24 hours a day, seven days a week for four years.
- The Sydney Opera House might be one of the world’s most iconic buildings but it faces potentially significant conservation challenges, a US philanthropic organisation says. The Getty Foundation has awarded the Sydney Opera House Trust $US200,000 ($A224,000) for a study of the concrete elements of the building and to develop long-term conservation strategies should it become necessary in the future.
Concrete Repair Methodology
Concrete repair methods need to provide a permanent solution in order to avoid a recurrence. FCS Concrete Repairs will Investigate, Diagnose, Test and Recommend the appropriate solution which will depend on the extent of damage and the feasibility of the repair or if necessary reinstatement of the concrete element affected.
The nature and type of repair will be determined by:
- Extent of corrosion of reinforcement
- Extent of loss of strength of reinforcement
- Extend of loss of the bond between the reinforcement and the concrete
- Extent of deficiency in concrete cover over reinforcement
- Extent of deflection due to cracking in the tensioned areas.
- Extent of honeycombing in concrete
- Extent of porosity of concrete
- Extent of damage and loss of strength due to sulphate attack.
- Extent and width of cracking