Why Cracks appears in ConCrete?

Written on Thursday, March 31, 2011 by Jose Sanchez Marquez

1. There are more possibilities that cracks appear in high resistance concrete (Above 28 Mpa) than in normal concretes (21 Mpa).
2. High relationship between water and cement (w/c) can increase the possibilities of cracks.
3. Construction process of placement and compaction concrete, eliminating most of the air trapped in it.

When concrete is in fresh stage, the cracks that can appear are plastic settlement and plastic shrinkage. When it is in hardened state can appear physics, chemicals, thermals and structural cracks.

NON-STRUCTURAL CRACKS

• Plastic Settlement

  Occurred in the first 3 hours and it is caused mainly by the steel reinforce due to stop the free movement of concrete on the element, generating in some parts the lack of or the excess of concrete.

The best way to try to avoid it is making a good vibration meanwhile the concrete is place. Other thing is not to add more water to concrete; the water dosage is responsibility of the manufacturer. Finally we must follow the design specifications of every country about minimum coating.

  

• Plastic shrinkage

  Mainly happen for the water lost on concrete, during the (1 – 6 hours) when concrete was put. However this kind of shrinkage doesn’t represent any structural damage on the element.

These are the principal reasons than contribute with this phenomenon:

1. Quick loss of moisture by high temperatures,

2. Big temperatures differences between day and night,

3. Strong winds on the zone than can dry the concrete.

4. Concrete Water Suction by soil or granular base on rigid pavements.

1. After put concrete in fresh stage over a hardened concrete.

2. After put different layers of concretes (resistances, fc) top of each other.

3. An excess of vibrator when concretes are very fluid as consequence of the use of more water than the design dosage recommended (settlement above 7”).

4. When there are important concentrations of steel reinforced.

5. In thin layers of concrete (less than 4’’)

The best way to try to avoid it, is curing the concrete for 7 seven days constantly, after the concrete loses its shine. In columns and floors is recommended cover the concrete with plastic to try to keep the moisture and wind. Also is recommended the use of water irrigators to keep an average temperature. Wet all the surface in where in the concrete is going to be put, soil, steel, floor to reduce the risk of water suction and finally a good vibrating process.

• Thermal shrinkage

  Concretes can present volume variations due to the temperature that generates the concrete by itself when is surrounded by formwork or in contact with soil or surrounded by a previous concrete. Big volumes of concrete generates overheat during the curing process and hardening, making very difficult dissipate all the heat, reason why It can reach high temperatures higher than the environment temperature.
  This is very common when there are big changes in temperature or in massive concretes that demands big volumes at the same time.The cracks can appear during the (1-5 Days) after the concrete was put.

The best way to try to avoid is using cold water or ice on the concrete dosage. A concrete average temperature of 24°C can prevent this kind of shrinkage. It is also recommended wet the aggregates and try to starts at night or day time with a standard temperature.

 STRUCTURAL CRACKS

These are the principal reasons than contribute with this phenomenon:

1. Wrong Design

2. Change the final use or purpose of the building or structure

3. Wrong location of the reinforcing steel

4. Heavy loads on young concretes (overloads), they don't reach final resistance age (creep)

      

 FINAL RECOMENDATIONS

References:

• Noticreto 97. Noviembre - Diciembre 2009. Juntas en Pavimentos de Concreto Hidraúlico. Control de la Retracción del concreto - Por: Rodrigo Salamanca Correa
• Recomendaciones Cuando se presente fisuración del Concreto. Soluciones que Construyen un mejor pais. Version 2010. HOLCIM COLOMBIA S.A
• Blog tecnico de cemento y concreto http://www.360gradosblog.com/post/2011/02/28/Que-hacer-cuando-se-presentan-fisuras-en-el-concreto.aspx

Posted in concrete cracks, creep, curing, Plastic Settlement, Plastic shrinkage, shrinkage

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