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The third method of preparing light concrete is to bypass the fines from traditional concrete. No-fines concrete as the word means is a sort of concrete from which the fine aggregate particle has been neglected. This concrete is prepared up of only coarse aggregate, cement, and water. Very usually only single-sized coarse aggregate, of size moving through 20 mm retained on 10 mm is utilized.
1. No Fines Concrete
No-fines concrete is becoming favored because of the advantages it owns over traditional concrete. The single-sized aggregates produce good no-fines concrete, which in addition to containing big voids and hence light in weight, also provides an architecturally pleasing gaze.
A. Mix Proportion
No fines concrete is prepared with the aggregate/cement ratio from 6:1 to 10:1. Aggregates utilized are usually of size moving through 20 mm and retained on 10 mm. Unlike the traditional concrete, in which strength is mainly influenced by the water/cement ratio, the strength of no-fines concrete, is conditional on the water/cement ratio, aggregate cement ratio, and unit weight of concrete. It represents the relationship of compressive strength, water/cement ratio, and aggregate/cement ratio for no-fines concrete.
The water/cement ratio for sufficient consistency will alter between a little content of 0.38 and 0.52. The water/cement ratio must be chosen with care. If too low a water/cement ratio is adopted, the paste will be so dry that aggregates do not get properly smeared with a paste which results in insufficient adhesion between the particles.
On the other hand, if the water-cement ratio is too high, the paste flows to the bottom of the concrete, particularly when shaken, and fills up the voids between the aggregates at the base and creates that region dense.
This condition also decreases the adhesion between aggregate and aggregate owing to the paste resulting very thin.
No standard method is found, like slump test or compacting factor test for estimating / the consistency of no fines concrete.
Maybe a good, trained visual examination and trial and error method may be the most useful guide for determining the optimum water/cement ratio.
No fines concrete, when traditional aggregates are utilized, may show a density of about 1600 to 1900 kg/m3, but when no fines concrete is prepared by utilizing lightweight aggregate the density may reach about 360 kg/m3.
No-fines concrete does not get any serious issues for compaction. Utilization of mechanical compaction or vibratory methods is not needed. Simple rodding is adequate for full compaction.
No-fines concrete does not provide much side thrust to the formwork as the particles are having point to point contact and concrete does not pour. Therefore, the side of the formworks can be separated in a time gap shorter than for traditional concrete.
However, formwork may be needed to be held for a longer time when utilized as a structural member, as the strength of concrete is comparatively less.
The compressive strength of no-fines concrete can range from 1.4 MPa to about 14 MPa.
The bond strength of no-fines concrete is very less and, therefore, reinforcement is not utilized in junction with no-fines concrete. However, if reinforcement is needed to be utilized in no-fines concrete, it is advisable to smear the reinforcement with cement paste to improve the bond and also protect it from rusting.
B. Drying Shrinkage
The drying shrinkage of no-fines concrete is extensively lower than that of conventional concrete. No-fines concrete made with river gravel may show a drying shrinkage of the order of 200 x 10-6 which is only about 50% of the conventional concrete.
Since there is only a very thin layer of paste existing between aggregates and aggregates, and since the aggregate are having point to point contact, the value of drying shrinkage becomes low. However, the momentum of drying shrinkage is typically much higher than in traditional concrete.
For no fines concrete, 50 to 80% of the total drying shrinkage takes place within about 10 days. The corresponding value for traditional concrete in 10 days is 20 to 30%. Further, all the drying shrinkage will get completed in just over a month.
C. The Thermal Conductivity
The value of the coefficient of thermal conductivity of no-fines concrete is much less than conventional concrete.
D. Application of No-fines Concrete
No fines concrete can be utilized for different uses. It is utilized on a big scale for load caring cast-in-situ exterior walls for single-story and multi-storeyed buildings. This type of concrete has been utilized for temporary structures because of its low initial cost and also for the comfort with which it can be broken and reused as aggregate. Architects consider this an attractive construction material.
Owing to its barely higher thermal insulating possessions, it can be utilized for exterior walls for heat insulation. Because of the rough texture, it provides a good base for plastering. Even if the exterior surface of the no-fines concrete wall is put through rain beating, the interior of the wall will be free from dampness because of low capillary action on the performance of bigvoids. Where sand is not available, no fines concrete should become more popular as a construction material.
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