For producing high quality concrete you should take control of problems which are commonly occurred during concreting. These problems mainly happen for improper mixing, placing and handling. Some problems also happen due to weather effect and other reasons. These problems have major impact on strength and duration of concrete First you should know about the common problems of concreting. The common problems are:
Segregation: Segregation is the tendency of separating concrete particles in fresh concrete. Due to segregation honey comb, scaling, laitance, porous layers and bond failure at construction joint etc. can happen in concrete. To avoid segregation following actions can be taken-
- Properly following concrete mix-design
- Mixing concrete ingredients properly
- Maintaining water-cement ratio
- Not dropping concrete from too high during placing
- Avoid over vibrating of concrete
Bleeding: when concrete begin to set the mixing water come out from the concrete and evaporates. If evaporation rate of water is not sufficient then water stay on the concrete surface. This the conc bleeding. Normal Bleeding is a part of concrete hardening process. But excess bleeding is harmful to concrete. It reduces the bond between reinforcement & concrete and also concrete particles with cement paste. As a result, it reduce the strength and durability of hardened concrete. Bleeding can be avoided using following technique :
Maintaining proper water-cement ratio
Using water reducing admixture with concrete mix
Using well graded fine aggregates.
Plastic Shrinkage cracking: If the evaporation rate of mixing water of concrete is higher than the bleed water of concrete then this cracking occurs in concrete surface. It happen due to hot weather. To avoid this concreting problem see “Impacts of hot weather on concreting and protective actions“.
Dusting: If the concrete surface bond is weak with subsequent layer then the surface dusts under traffic. The following measures can be taken to avoid this problem:
- Keep the water-cement ratio under reasonable level
- Use dust free aggregates
- Ensure proper hydration of concrete
- Avoid concrete finishing work with bleed water. etc
Laitance: laitance is the bad effect of concrete bleeding and segregation. Due to bleeding & segregation a thin layer of cement-sand particles appears on the concrete surface that is the laitance. It’s also harmful to concrete as it weaken the bond of subsequent layers of concrete. The following measures can be taken to prevent the occurrence of concrete laitance:
- Remove the clay, dust, silt and other organic materials from concrete aggregate before mixing.
- Don’t apply water on fresh concrete during finishing work.
- Maintain proper water cement ratio.
- Using well graded fine aggregates.
- Using water reducing admixture with concrete mix.
Scaling: Scaling is the appearing of concrete aggregate on surface due to flaking or peeling away mortar from concrete surface. Same protective actions for laitance and dusting can be taken to protect concrete scaling.
There are also some other problems happen during concreting work. But these are major concreting problem to avoid.