CONCRETEThe Oldest New Material there is
A building material made from a mixture of broken stone or gravel, sand, cement and water, which can be spread or poured into moulds and forms a stone-like mass on hardening.
Concrete may be one of the defining materials of modern architecture, but it has been in continuous use for around 8,500 years, first by the Nabataea Bedouins, then by the ancient Egyptians, Assyrians, Greeks and Romans and, a little later, by the modernist architects of Milton Keynes, city blocks and many a contemporary housing estate.
Concrete is essentially a mixture of two ingredients: Cement (Portland or other) and an aggregate such as crushed stone, gravel, sand, slag or even recycled concrete. The Romans also added horse hair (to reduce cracking while it hardened) and blood (to improve frost resistance) to their opus caementicium.
In 1853 François Coignet, a French industrialist, embedded iron bars in the unset mix and gave us reinforced concrete – a major advance that, as we’ll see, has its own problems…
But while concrete’s basic formulation may be simple, the variety and proportions of its mix are complex. To clean or repair any material, you first need to know exactly what that material is. And concrete is no different…
Given concrete’s ancient legacy, it should be no surprise that its care and cleaning has evolved down the centuries, right up to the present day and the introduction of advanced cleaning methods. But we are also expert in longer standing, and equally effective, techniques.
Either way, we can address discolouration, efflorescence, acid staining, etc. We have a detailed understanding of the full range of cleaning techniques (from chemical to light abrasion) and of when and how each should be applied.
Our work often incorporates more invasive treatments including flexible joint sealing and crack injection to address flaking, spalling and sealing issues.
Many concrete buildings constructed over the last 50 years or so are in trouble. Unfortunately, some contractors back then cut corners, threw up buildings with little care and left them fundamentally compromised. Some have had to be demolished. Many others are suffering from the effects of one or more of the consequences of poor quality concrete construction.
The problem generally begins with water penetrating the concrete, starting a series of chemical reactions that ‘eat’ the concrete from the inside. Once the resulting toxic solution has reached the external surface, it reacts further with the air and cracks start to appear, letting in yet more water. At the same time, stalactites form inside causing even greater structural stress.
And if the cement is too alkaline, it will react with the air’s carbon dioxide, producing tell-tale star-shaped cracks. Ironically, reinforced concrete is particularly at risk. The bars added for reinforcement will rust once the integrity of the surrounding concrete has been compromised, causing them to expand and the concrete to spall.
Of course, once concrete starts to crack (for whatever reason), it also suffers the effects of water freezing and expanding, accelerating the cracking process.
Fortunately, the industry has learned its lesson and contemporary concrete construction has eradicated these problems. Too late for many buildings, but we have the expertise to remedy those that are in trouble, but not yet past the point of no return.
Regent Street Disease
Regent Street Disease (or, if you are in Manchester, Deansgate Disorder) is another ailment now manifest in older concrete buildings. RSD affects steel-frame constructions with a non-structural but heavy facade of, say, Portland Stone, brick or faience. Once water has penetrated the facade and broached any protective coating originally applied to the steel, corrosion sets in…
While the problem has been designed and engineered out of new buildings, many older, and iconic, structures are threatened both here (The BBC Building in Portland Square for one) and abroad (many of Chicago’s skyscrapers are at risk).