It was known to the Romans, the Egyptians and to even earlier Neolithic civilisations. The foundations of the Colosseum in Rome were made of dense concrete (in 82AD), while lightweight concrete was used in some of the arches and vaults.
After the collapse of the Roman Empire its secrets were almost lost, only to be rediscovered in more recent times. Indeed its modern development spans no more than 175 years; 1824 is the date on the patent for the manufacture of the first Portland cement, one of the most important milestones in concrete’s history.
Concrete is a building material composed of cement, crushed rock or gravel, sand and water, often with chemical admixtures and other materials; this mixture eventually hardens into a stone-like material.
Cement and water are the two ingredients that do all the chemical reacting. The gravel and sand give strength.
- If you mix gravel, sand and water together, nothing will happen.
- If you mix cement, dry gravel and dry sand together, nothing will happen.
- It is only when the cement is mixed with the water that something happens!
This may merely reflect the fact that a large number of reinforced concrete structures are now coming-of-age although certain changes in materials have been identified over the past twenty years or so which might be partially responsible in some cases.
For example, in the early seventies it was believed that there were no deposits of alkali reactive aggregates in this country and yet within a few years alkali aggregate reaction became a subject of major interest as more and more cases came to light.
This upsurge in ‘alkali-silica reaction’ or ASR problems may be partially traceable to an increase in cement alkali content in the preceding years.
Furthermore, increases in the early age strength of cement may have meant that specified minimum strengths were being achieved with less cement than had previously been used with an appropriate effect on subsequent durability.
From our experience it is true to say that 90% of the problems that will be experienced in concrete repair will involve steel reinforcement corrosion as a primary problem. For the most part this will have been caused entirely by simple carbonation/low cover and/or the presence of chloride salts either from calcium chloride used as an accelerator, or even from de-icing salt in some cases.
However, in some cases, some other more subtle defect may be present, such as a shrinkable aggregate, alkali silica reaction, frost attack, sulfate attack, structural cracks or a whole variety of other possibilities.
It must be borne in mind that many of these phenomena may reveal themselves first of all in areas of low cover and carbonated concrete, perhaps because microcracking from one or other of these causes has permitted carbonation to advance more rapidly than might otherwise have been the case.
In such cases, it is all too easy to look at the effects of the problem, and not the cause; steel corrosion is the SYMPTOM, not the CAUSE. Attempting to repair concrete by simp[ly addressing the symptom may simply mean that the problem recurs in a relatively short space of time.
The concrcte specialist will know when the advice of a structural engineer is required.
The plate illustrated structural cracks at a column beam connection. If examined carefully, the column behind shows no evidence of cracking, but in this case the beam is supported on a brick wall. The wall was not intended to be load bearing, but was clearly taking some of the stress off the column On the right, a car park is showing early signs of failure due to punching shear. This causes radial cracking around column positions as the deck flexes about the support. At later stages, an annular crack appears, which can indicate the onset of failure. Pipers Row car park in Wolverhampton collapsed due to this phenomenon.
These mechanisms of these factors are unusual in that they do not attack the integrity of the concrete. Instead, aggressive chemical species pass through the pores in the concrete and attack the steel.
This is unlike a ‘normal’ deterioration process due to chemical attack on concrete. Other acids and aggressive ions such as sulphate destroy the integrity of the concrete before the steel is affected. Most forms of chemical attack are therefore concrete problems before they are corrosion problems.
Carbon dioxide and the chloride ion are very unusual in penetrating the concrete without significantly damaging it. Accounts of acid rain causing corrosion of steel embedded in concrete are unsubstantiated.
Only carbon dioxide and the chloride ion have been shown to attack the steel and not the concrete.