NO-FINES CONCRETE & ITS MIX PROPORTION

Introduction

No-fines concrete is a concrete containing little or no fine

aggregate. The coarse aggregate should preferably be a

single-size material (nominal maximum sizes 10 mm and

20 mm being the most common). However, blended

aggregates (10 and 7 mm; and 20 and 14 mm) have been

found to perform satisfactorily. Because it is characterised

by uniformly distributed voids, it is not suitable for

reinforced or prestressed concrete construction.

Applications

■ Walls in buildings

Primarily in external and internal walls of low-rise

and multi-storey flats/units.

■ Car park paving

Provides free-draining pavements for light traffic

■ Tennis courts

Using a small nominal aggregate size, eg 5 mm,

a free-draining playing surface is achieved.

■ Drainage layers

Used as drainage layers on civil engineering projects.

20-mm aggregate size is preferred but the surface

finish is poorer than that achieved using 10-mm

aggregate.

■ Levelling courses

Has been used as a lightweight screed for levelling on

floors and roofs.

Mix Proportions

Generally, the cement:aggregate ratio by volume is in the

range 1:6 to 1:8. Leaner mixes (1:8 to 1:10) reduce the

likelihood of the pores being blocked by cement paste.

Thus for drainage layers where lower strength can be

tolerated, 1:10 is preferred. The water-cement ratio needs

to be kept low, eg 0.4–0.45, to ensure the cement paste

coats the aggregates and does not run off.

No-Fines Concrete is a method of producing light concrete by omitting the fines from conventional concrete. No-fines concrete as the term implies, is a kind of concrete from which the fine aggregate fraction has been omitted. This concrete is made up of only coarse aggregate, cement and water. Very often only single sized coarse aggregate, of size passing through 20 mm retained on 10 mm is used. No-fines concrete is becoming popular because of some of the advantages it possesses over the conventional concrete.

The single sized aggregates make a good no-fines concrete, which in addition to having large voids and hence light in weight, also offers architecturally attractive look.

Mix-proportion of No-Fines Concrete

No-fines concrete is generally made with the aggregate/cement ratio from 6 : 1 to 10 :1. Aggregates used are normally of size passing through 20 mm and retained on 10 mm. Unlike the conventional concrete, in which strength is primarily controlled by the water/cement ratio, the strength of no-fines concrete, is dependent on the water/cement ratio, aggregate cement ratio and unit weight of concrete.

The water/cement ratio for satisfactory consistency will vary between a narrow range of 0.38 and 0.52. 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 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 vibrated and fills up the voids between the aggregates at the bottom and makes that portion dense. This condition also reduces the adhesion between aggregate and aggregate owing to the paste becoming very thin.

No standard method is available, like slump test or compacting factor test for measuring the consistency of no-fines concrete. Perhaps a good, experienced visual examination and trial and error method may be the best guide for deciding optimum water/cement ratio.

No-fines concrete, when conventional aggregates are used, may show a density of about 1600 to 1900 kg/m3, but when no-fines concrete is made by using light weight aggregate, the density may come to about 360 kg/m3. No-fines concrete does not pose any serious problem for compaction. Use of mechanical compaction or vibratory methods are not required. Simple rodding is sufficient for full compaction.

No-fines concrete does not give much side thrust to the formwork as the particles are having point to point contact and concrete does not flow. Therefore, the side of the formworks can be removed in a time interval shorter than for conventional concrete. However, formwork may be required to be kept for a longer time, when used as a structural member, as the strength of concrete is comparatively less. The compressive strength of no-fines concrete varies between 1.4 MPa to about 14 MPa. Table 12.5 shows the compressive strength of no-fines concrete.

The bond strength of no-fines concrete is very low and, therefore, reinforcement is not used in conjunction with no-fines concrete. However, if reinforcement is required to be used in no-fines concrete, it is advisable to smear the reinforcement with cement paste to improve the bond and also to protect it from rusting.