- · Concrete Bleeding
Bleeding means the water content within the concrete comes out to the surface of the concrete. Where can we observe bleeding? Good examples are in a highly wet mix, badly proportioned and insufficiently mixed concrete. Excessive bleeding often occurs in thin members like roof slab and concrete that is placed in sunny weather. Due to bleeding, water content within the concrete will come up and accumulate at the surface, at the same time bringing up certain amount of cement. This formation of cement paste at the top surface is known as “Laitance”. This laitance formed on roads produces dust in summer and mud in rainy season.
Concrete Bleeding: the formation of cement paste at the top surface, “Laitance”.
Water creates continuous channels during its traversing from bottom to top surface of concrete and the bleeding channels will remain continuous and unsegment. These continuous bleeding channels are the reason that causes permeability of the concrete structures. As the mixing water is coming up, if the aggregates used are flaky, it may intercept water from going up hence create high possibility that it accumulate at below. This accumulation of water results in water voids which reduce the bond between the aggregates and the paste. The weakened bond can be remedied by vibration of concrete. The formation of laitance can be reduced by delayed finishing operations.
However, there are ways to avoid the occurrence of bleeding in concrete. Proper proportioning and uniform mixing, use of fine pozzolanic materials as well as air-entraining agent, use of finer cement power or cement with low alkali content are effective to reduce bleeding. In fact, bleeding is not detrimental if the rate of evaporation of water at the concrete surface is equal to the rate of bleeding. After the water provided workability during casting process, its removal from the concrete body by bleeding is considered good for the hardening process of concrete.
- · Segregation
Segregation is the separation of the constituent materials of concrete. Concrete that is good in quality is one in which all the ingredients are uniformly distributed to make a homogeneous mixture. Nonetheless, the presence of differences in the sizes as well as specific gravities of the raw materials of concrete has to be put into consideration. It is natural that any material has the tendency to fall apart.
Types of segregation that occur more frequently:
- Coarse aggregates (gravels) separate out or settle down.
- Paste separate away from coarse aggregates.
- Water separate out due to lowest specific gravity among all ingredients.
Segregation: the constituent ingredients of concrete fall apart.
A well manufactured concrete does not exhibit any tendency for segregation. The cohesive and fatty characteristics of matrix prevent the aggregates to fall apart with the rest. Meanwhile, the matrix itself is sufficiently contained by the aggregate. Similarly, water will not come out freely.
The conditions favorable for segregation are:
- The mixture is proportioned poorly where insufficient matrix is not capable to bind the aggregates properly.
- Excess water content in the concrete mix.
- Dropping of concrete from heights as in the case of placing concrete in column concreting.
- When concrete is discharged from a badly designed mixer, or from a mixer with worn out blades.
- Conveyance of concrete by conveyor belts, wheel barrow, long distance haul by dumper, long lift by skip and hoist.
Vibration of concrete is normally used to compact concrete in building construction. An important criterion is that only comparatively dry mix should be vibrated. If the mix is excessively vibrated in wet condition, the concrete is possible to get segregated. Another thing is that vibration is advised to be continued just for required time for optimum results. A too long time of continuous vibration, especially in wet mix, will result in segregation too because of the settlement of coarse aggregates.
Incorrect mix, aggregate segregation and the mortar displaced towards the surface
Correctly composed mix, uniform distribution of the aggregate
- · Hydration in concrete
Hydration is a chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become hydrates or hydration products. Concrete gains its strength by the hydration process of cement particles which is continuing for long time. Certainly, the rate of hydration is fast to start with, but continues over a very long time at a decreasing rate. In the industry and in actual work, higher water/cement ratio is set. It is because the concrete is exposed to atmosphere, the water content in the concrete will evaporate in fast rate causing the water available in the concrete insufficient for effective hydration to take place especially in the top layer.
If the hydration is to carry on, extra water must be added as supplement of the absorbed and evaporated water. Therefore, the curing process is considered as creation of a favorable environment during the early stage for undisturbed hydration. The desirable conditions are moderate temperature and ample moisture. While hydrating, concrete releases high heat which is detrimental regarding its volume stability. Heat of hydration may also shrinkage in concrete, thus producing cracks. If the heat released is removed by some means, the adverse effect can be reduced. A good way is through a thorough water curing.
- · Air Entrainment
Air entrainment contributes to the reduction of the density and strength of concrete. Air entrainment is used to produce a number of effects in both the plastic and the hardened concrete.
- Resistance to freeze–thaw action in the hardened concrete.
- Increased cohesion, reducing the tendency to bleed and segregation in the plastic concrete.
- Compaction of low workability mixes including semi-dry concrete.
- Stability of extruded concrete.
- Cohesion and handling properties in bedding mortars.
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