In this article we will discuss about:- 1. Meaning of No Fines Concrete 2. Mix Proportion of No Fines Concrete 3. Properties 4. Uses.
Meaning of No Fines Concrete:
No fines concrete does not contain fine aggregate (sand). This concrete usually is an agglomeration (i.e., group of particle not arranged in any particular order i.e., it is made of nominally one size coarse aggregate particles. Each panicle has a cement paste coating of about 1.3 mm thickness around it.
Thus large pores exist within the body of the concrete which are responsible for its low strength. However due to large size of pores or voids no capillary movement of water takes place as in the case of aerated concrete. Hence the rate of penetration of water in no fines concrete is low.
For a given type of aggregate, the density of no fines concrete depends mainly on the grading of the aggregate. With one size aggregate, the density is about 10% lower than when a well graded aggregate of the same specific gravity is used. The usual size of aggregate used in no fines concrete is 10 to 20 mm. However 5% over size and 10% under size is permissible, but no material should be smaller than 5 mm.
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A no fines concrete having a low density of the order of 640 kg/m3 can be produced by using light weight aggregate. When normal aggregate is used, its density may vary from 1600 to 2000 kg/m3. Sharp edged aggregate should not be used the sharp edges will be crushed under load.
Mix Proportion of No Fines Concrete:
Generally no fines concrete is produced with a cement aggregate ratio of 1:6 to 1:10, but practical mixes vary rather widely. The variation of cement aggregate ratio for lean mixes is from 1:10 to 1:20. The corresponding cement contents are 130 kg/m3 to 70 kg/m3. Where as in case of normal concrete made with well graded aggregate the water/cement ratio is the controlling factor for the strength development. In case of no fines concrete there is a narrow optimum value of the w/c ratio for any given aggregate.
A higher water- cement ratio than the optimum would make the cement paste drain away from the aggregate particles, while with too little water-cement ratio the paste would not be sufficient adhesive. The optimum water-cement-ratio varies between 0.38 and 0.52 depending upon the cement-content necessary for sufficient coating of aggregate. Actually the strength of no fines concrete is governed by water-cement ratio, cement aggregate ratio and unit weight of concrete. The relation between compressive strength, W/C ratio and A/C ratio is shown in Fig. 22.5.
The compressive strength of no fines concrete generally varies between 1.4 MPa to 14 MPa depending mainly on its density, which is governed by the cement content in the concretes shown in Table 22.7 below:
The actual strength of no fines concrete should be determined by tests. The increase in strength with age is similar as that of ordinary concrete.
No fines concrete develops very little cohesion. Hence form work should not be removed till it has developed sufficient strength to hold the materials together. Moist curing is important, especially in windy or in a dry climate as it has very small thickness of cement paste.
Properties of No Fines Concrete:
Following are the properties of no fines concrete:
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1. No fines concrete compacts very little. Hence the vibration should be applied for a very short period otherwise the cement paste would run off.
2. There is no workability test for no fines concrete. The visual check for ensuring even coating of aggregate particles with cement paste is sufficient.
3. As no fines concrete segregates, it can be dropped from a considerable height and placed in high lifts.
4. The modulus of rupture of no fines concrete is about 30% of its compressive strength. This proportion is higher than the normal weight concrete.
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5. The shrinkage of no fines concrete is much lower than that of normal weight concrete as the contraction is restrained by the large volume of aggregate relative to the paste. However the initial rate of shrinkage is higher as large surface area of the cement paste is exposed to the air. After one month drying, the value of shrinkage is between 120 x 10-6 to 200 x 10. No fines concrete made with river bed gravel showed a drying shrinkage of the order of 200 x 10-6, which is about 50% of the conventional concrete. In case of no fines concrete 50 to 80% of the total shrinkage takes place within about 10 days where as in the case of conventional concrete the drying shrinkage within 10 days is about 20 to 30% only.
6. The thermal movement of no fines concrete is about 70% of that of normal weight concrete. The actual value of coefficient of thermal expansion depends on the type of aggregate used.
7. The thermal conductivity of no fines concrete is low. For light weight aggregate it is found as 0.22 J/m2S°C/m and 0.81 J/m2S°C/m for normal weight concrete. However the high moisture content of the concrete increases the thermal conductivity appreciably.
8. No fines concrete is highly resistant to frost due to the absence of capillaries in it, provided the pores are not saturated. In case, the pores are saturated, freezing would cause a rapid disintegration. High absorption upto 12% by mass makes no fines concrete un suitable for use in foundations and in situations where it may be in contact with water.
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9. To reduce the permeability to air, the external walls of the no fines concrete should be plastered or rendered on both sides. Painting and rendering reduces the sound absorbing properties of no fines concrete, so that where the acoustic properties are considered more important, one side of the wall should not be rendered.
10. Its cost is low as the cement content is as low as 70 to 130 kg/m3.
11. Typical values of modulus of elasticity of no fines concrete are shown in the following Table 22.8.
Uses of No Fines Concrete:
1. No fines concrete normally is not used in reinforced concrete. However if it is essential to use no fines concrete in reinforced concrete, then the reinforcement has to be coated with a thin layer of about 3 mm of cement paste to improve its bond and prevent corrosion. Shot creating may be applied for coating the reinforcement.
2. In spite of its lower weight and strength it can be used for even of many storyed buildings.