Common Types of Concrete Damage
From our experience it is true to say that 90% of the problems that will be encountered in concrete repair will involve steel reinforcement corrosion. This is, however, more often a symptom of the concrete damage and not the primary cause of the damage.
Below you’ll find some of the more common signs of concrete damage. It is essential to determine the cause before effecting any repair and this will ensure the appropriate repair work is carried out for a long-lasting solution.
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Common Types of Concrete Damage
The most common problem caused by corrosion is spalling of concrete cover, whereby concrete flakes off the main structure.
A man was killed in New York City by a slab of concrete which spalled off a bridge substructure due to de-icing salts, and a vehicle was badly damaged in Michigan in a similar incident.
Special metal canopies have been built around the lower floors of high rise buildings where corrosion has led to risk of falling concrete.
This enables the investigators to collect the fallen concrete at regular intervals and weigh it; in that way they can determine whether the corrosion rate is stable, increasing or decreasing.
Freeze / Thaw Damage
Concrete of inadequate durability, if subjected to a wet environment and freezing can be disrupted by freeze-thaw attack.
Water enclosed in the pores of the wet concrete will expand on freezing and the high internal stresses so created can disrupt the surface.
The effects are intensified by subsequent freeze/thaw action as minute cracks develop which, in turn, become filled with water. Concrete with a high water to cement ratio is especially vulnerable. Addition of an air entraining agent to the concrete reduces the risk of frost damage.
The problem is often characterised by parallel lines of cracking as freeze thaw damage penetrates deeper into the concrete.
Damage to concrete attributable to fire has 3 principal types
(i) Cracking and micro-cracking in the surface zone
This is usually sub-parallel to the external surface and leads to flaking and breaking away of surface layers. Cracks also commonly develop along aggregate surfaces – presumably reflecting the differences in coefficient of linear expansion between cement paste and aggregate. Larger cracks can occur, particularly where reinforcement is affected by the increase in temperature.
(ii) Alteration of the phases in aggregate and paste
The main changes occurring in aggregate and paste relate to oxidation and dehydration. Loss of moisture can be rapid and probably influences crack development. The paste generally changes colour and various colour zones can develop. A change from buff or cream to pink tends to occur at about 300°C and from pink to whiteish grey at about 600°C. Certain types of aggregate also show these colour changes which can sometimes be seen within individual aggregate particles. The change from a normal to light paste colour to pink is most marked. It occurs in some limestones and some siliceous rocks – particularly certain flints and chert. It can also be found in the feldspars of some granites and in various other rock types.
It is likely that the temperature at which the colour changes occur varies somewhat from concrete to concrete and if accurate temperature profiles are required, some calibrating experiments need to be carried out.
(iii) Dehydration of the cement hydrates
This can take place within the concrete at temperatures a little above 100°C. It is often possible to detect a broad zone of slightly porous light buff paste which represents the dehydrated zone between 100 and 300°C.
Most problems with corrosion of steel in concrete are not due to loss of steel but the growth of the oxide. This leads to cracking and spalling of the concrete cover.
Structural collapses of reinforced concrete structures due to corrosion are rare. Concrete damage would usually have to be well advanced before a reinforced concrete structure is at risk.
Particular problems arise when the corrosion product is black rust, which occurs in low oxygen situations, in very damp concrete where chlorides are present and in pre-stressed, post-tensioned structures where corrosion is difficult to detect as the tendons are enclosed in ducts. Tendon failure can be catastrophic as tendons are loaded to 50% or more of their ultimate tensile strength and modest section loss leads to failure under load.
Vertical and Horizontal Cracks
If reinforcing steel is doing its job in areas of tension in the structure, small cracks will occur as the tensile load exceeds the tensile strength of the steel. Most of these are small cracks (less than 0.5 mm) intersecting the reinforcing steel at right angles.
They do not lead to corrosion of the steel as any local ingress of chlorides, moisture and carbonation is limited and contained by the local alkalinity. Obviously there is a limit to this ‘self-healing’ ability. If large cracks stay open (greater than 0.5 mm), then corrosion can be accelerated.
Such cracks may be due to plastic shrinkage, thermal expansion or other reasons. Corrosion causes horizontal cracking along the plane of the rebar and the corner cracking around the end rebar. This leads to the loss of concrete cover.
This is the main consequence of reinforcement corrosion with its subsequent risk of falling concrete and unacceptable appearance.