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Migrating Corrosion

The Causes of Concrete Deterioration
Reinforced concrete can be a highly durable structural material requiring little or no maintenance. However, it is now recognised that without correct design, mixing, placement and curing, the durability of reinforced concrete may be impaired. Further protection should be provided immediately after construction if durability is to be achieved. Steel reinforcement in well-designed concrete structures is protected from corrosion by the passivating effect of the highly alkaline concrete cover. Concrete decay can arise from the failure of the “passivating effect”.
 

Carbonation

Neutralisation of the alkaline layer around the steel by the reaction with carbon dioxide causes carbonation of the concrete cover and results in loss of alkalinity. Loss of alkalinity at the interface between concrete and reinforcement is generally associated with:

 

  • Inadequate depth of cover
  • Permeable cover due to too high water/cement ratio. Insufficient cement in the mix.

 

Inadequate curing of the concrete. Poor compaction of the concrete. Permeable cover allows the penetration of carbon dioxide which reacts with the free lime to produce mildly acidic carbonates.

Chloride Attack

  • Penetration of the chloride ions into the concrete from de-icing salts or marine environments.
  • Incorporation of calcium chloride in the concrete mix as an accelerator.   

Chlorides attack the reinforcing steel through electrolytic action producing black coloured rust. In dense high strength concrete, a significant loss of the reinforcing bar can take place without the normal visible signs of disruption to the concrete cover, such as spalling.
 

Alkali Silica Reaction (ASR)
Cracking may be caused by the formation of an expansive gel through the reaction of alkali with reactive silica, usually caused by high levels of alkalis in permanently wet concrete.


High Alumina Cement (HAC) Conversion

Concrete containing high alumina cement with high water-cement ratio will often suffer conversion, a progressive degradation of the hardened cement paste.
 

In both HAC conversion and alkali silica reaction, there is no known cure or treatment. Any patch repairs and external protective treatment can only delay the inevitable decay.
 

In Facades where the concrete specification is often lower than for highway structures, the chance of the concrete being permeable is greater and so more prone to carbonation. Chlorides may also be a problem in areas of atmospheric pollution or where calcium chloride was used as an accelerator.
 

In Infrastructure, such as highways, the concrete specification is more rigorous resulting in dense impermeable concrete, resistant to carbonation Highway structures suffer mainly from the penetration of chloride ions from applied de-icing salts
 

Investigation

In establishing the causes of failure, concrete repair contractors will test for both carbonation and chlorides. When chlorides are present, preparation for the remedial concrete repair needs greater attention i.e. cleaning and protecting the steel. The future protection of the structure may also need to be different. For reinforced concrete suffering from both carbonation and chloride attack, it is necessary to follow the procedures for dealing with chloride decay.
 

Testing for carbonation is relatively simple -a freshly broken out piece of the concrete sprayed with phenolphthalein solution remains grey where carbonation has taken place but turns pink if still highly alkaline. Detecting for levels of chlorides will require laboratory analysis, but on-site test kits to show the presence of chlorides is available. The depth of cover can be measured using a properly calibrated cover meter.
 

 


 

 

 

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