The bulk density of sand varies with moisture content and is minimum at about 5% moisture content. At this moisture content of about 5%, the surface tension of the water film around the particles makes it impossible for the particles to find the closest packing: the volume increases, the density decreases. When making sand cement blocks we want a closest packing of the particles so as to achieve maximum density and therefore maximum strength. This necessitates an increase of the water content of a mix and therefore to employ higher water cement ratios. There is a minimum amount of water which is necessary for the stability of the block in the green state.A higher water cement ratio means more capillary pores in the cement paste and therefore a lower strength. The question is, for a certain cement content of a mix, say 10% by weight what is the minimum water cement ratio for maximum strength? Is there anybody who has done anything in these lines?
Assume that you are researching on the same type of sand ( pit sand) and the same type Cement (Ordinary Portland Cement). No superplasticers are to be used since you will be employing unskilled labor in a remote location.
What about plasticiser? By using plasticiser you can decrease the water-cement-Ratio.
There is no optimum water cement ratio as such for maximum strength. The lower the w/c ratio the higher the strength. People have gone upto to 0.18, but you would require high amount of superplasticizer content to still keep the workability. Please refer to this paper: "Ultra-high performance concrete and fiber reinforced
concrete: achieving strength and ductility without
heat curing"
Without superplasticizer , The optimum w/c ratio depends on the grading of sand (i.e: the river sand for example demand a quantity of water less than the dune sand), it depends also on sand-cement (s/c) ratio.
By using superplasticiser you can decrease the water-cement-Ratio, this manner , increase the workability criterion and hens, the compressive stregth.
There is no single optimal w/c ratio for maximum strength as the strength will depend on a lot of other variables. The optimal w/c ratio depends on the sand grain sizes, quality of cement, and presence of additives and method of production (manual or mechanized).
There is no single optimal w/c ratio for maximum strength as the strength will depend on a lot of other variables. The optimal w/c ratio depends on the sand grain sizes, quality of cement,and presence of additives and method of production (manual or mechanized).
A workable W/C ratio is about 0.25 with the use of plastisizer, which may give very good strength. However, the final strength will also depend on the quality of material that is being used; for example: particle size of the cement (fineness) and percentage of hard minerals in the sand.
Try to obtain the best value experimentally. You can start with the normal consistency of the cement (See ASTM C187). In my opinion try to get your optimum value there by trying + or - values. You have to watch workability carefully. Do not forget that your sand is not the standard sand described in ASTM.
For cement sand block you may calculate the quantity of water by the standard consistency of cement.
you may calculate like this ((Standard consistency/4)+3.5)% of dry weight of cement sand mix.
i think many factors could control optimum w/c ratio, for example workability, size of sand , desired strength and even the desired density of the production, however, it is not easy to give a specific value without trial mixes for example or using super plasticizers to decrease w/c in order to get maximum strength.
thank you
The ratio of water to cement depends on the composition of the concrete mix, the adopted calculation method, the adopted coefficients, the class of consistency and strength of the concrete mix, the degree of air entrainment.
The technology of mixing the mixture is also important.
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