Effect of Concrete Ingredients on Bleeding of Concrete
The ingredients used in the concrete mix i.e. cement, sand, aggregates, admixtures and supplementary materials if any, have a great effect on the bleeding of the concrete. Special consideration on the choice and selection of these materials can help to avoid the bleeding effect seen in concrete.
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Effect of Concrete Ingredients on Bleeding of Concrete
- Cement Content
- Water Content and Water-Cement Ratio
- Aggregate
- Supplementary Cementitious Material
- Chemical Admixtures
Effect of Water Content and Water-Cement Ratio on Concrete Bleeding
There is a predominant control on the bleeding of the concrete by the water content and water-cement ratio. The figure -2 shows the variation of bleeding with different water content. There is an increase in the water availability with an increase in the amount of water or the water- to - cementitious material ratio.
The increase in water content will increase the bleeding capacity and bleeding rate. Water content increased by one-fourth amount will bring an increase in the bleeding rate more than 2 and half times.
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| Fig.2: Relation Between The Water-cement ratio and the Bleeding of the Concrete. The bleeding is expressed as percent of mix water |
Effect of Cement Content on Concrete Bleeding
The cement content, type, chemistry, and fineness have great influence on the bleeding properties of the concrete. As the fineness of the cement increases, a number of bleeding decreases. This is shown in figure -2.
| Fig.3. Effect of Cement Fineness on the bleeding of the concrete. |
A reduction in the cement content, as it is related to the reduction of water cement ratio will bring a reduction of the bleeding.
The reactions that occur during the mixing period or during the bleeding period will affect the rate of bleeding.
There are chances for the leaching of SO3 from the cement. This has a good effect in reduction of bleeding. An increase in the content of SO3 reduces bleeding. It is not good to have a correlation between the water -soluble alkalies in the cement and bleeding. The most trend is that the bleeding will decrease with an increase the alkalies. Other effects like the precipitation of the gypsum and SO3 have an overshadowing effect.
Bleeding and Hydration Reaction
The Effect of Bleeding and heat of hydration shows that the higher degree of initial chemical reactivity that is occurring with the cement just after mixing will help in the reduction of bleeding.
Effect of Aggregates on Bleeding of Concrete
Normally it is observed that the surface area or the fineness of the sand have a little effect on the bleeding of the concrete. But those aggregates that have a higher amount of silt, clay or other materials that will pass 75µm, have a significant effect in reducing the bleeding of the concrete.
If we use aggregates that will increase the paste content significantly like the use of gap-graded aggregates, then the bleeding is increased. If the combined grading is mixing in the whole mix, i.e. 2.36mm to 9.5mm, bleeding is increased.
Effect of Chemical Admixtures on Bleeding of Concrete
The most commonly used admixtures are the water reducers and the air-entraining admixtures. The use of entrained air will reduce the bleeding.
The use of water reducers will bring the impression that it will reduce the bleeding a reduction. If the water reducers are used to reduce the demand for water content, then a decrease in the bleeding is observed. The use of high-range water retarders or plasticizers is almost similar to that of a normal range of water reducers.
The table below shows the admixture calcium chloride effect on the bleeding properties of concrete. This is mainly used as an accelerator.
Special bleeding-reducing admixtures that are based on cellulose derivatives, various absorbing resins or other various chemical formulations can help in eliminating the bleeding significantly.
These admixtures will make the concrete mix to become more cohesive and thixotropic. This is very effective in preventing the chances of segregation. Normal construction won't use these methods. But these are employed for special applications.
Effect of Supplementary Cementing Materials on Concrete Bleeding
The use of different cementing materials like silica fume, fly ash, rice husk and the natural pozzolans will reduce the bleeding property that is delivered inherently by increasing the cementitious material amount in the concrete.
As the material type and their respective source vary, all will have varying outputs.
- The use of fly ash mainly reduces the bleeding found in concrete. The use of GGBFS or ground granulated blast furnace slags will increase the bleeding capacity and the bleeding rate when compared with a mix of OPC alone. The fineness of the GGBFS is the reason behind the effect of slag on the bleeding.
- The use of natural pozzolans and calcined clay in concrete will reduce the bleeding. This is due to the fineness of the pozzolans and their effect on the water demand.
- Silica Fume with its extreme fineness will help in the reduction of bleeding. If proper precautions are taken there are chances of plastic shrinkage for concrete made from this material. This is because the silica fume will be concrete with no or very little water to cement material. This will have no water to rise above and cause bleeding. This when becoming extreme, causes plastic shrinkage.
- The Rice husk ash will reduce the bleeding proportionally with the addition of the ash in the paste. The fineness of the material is the prime factor that is responsible for the bleeding Reduction.
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