Care needed in mixing and using concrete

Press, Volume CXII, Issue 33034, 29 September 1972, Page 14

Care needed in mixing and using concrete

Concrete plays an important part on most home properties even if it is only the garden path. Sometimes the drive has to be widened or lengthened, or a terrace formed.

In the case of large areas it is probably more expedient to get ready-mix, but where only a small quantity is required or work is being carried out progressively, mixing your own may be the best move. Knowing how to mix good concrete is useful knowledge for anyone anxious to do a good job, whether it is a big job or a small one.

The ingredients of concrete are cement, sand and screenings or other coarse aggregate (depending on the type of job and finish desired) and water.

Contract work calls for correct measures of the various ingredients and no estimated quantities, and the same should apply to the small job.

The ingredients are always measured by volume so that if the composition of the ■ concrete being made up is ■52:1 and a bucket is being 'used as a measure then it calls for five buckets of sand and so on.

Don’t prepare a mix until all preparatory work has been completed and you are ready to use it immediately it has been prepared. Much depends on the day but freshly laid concrete can set within half an hour after it has been laid. For special purposes an additive can be put in the mix to delay settling. I For hand mixing (it is still . done today) mix the dry ingredients first, choosing a i perfectly clean, flat solid area. or a wheelbarrow. Initially, deposit the sand on : the mixing area then add [the cement, although it may ! provide a quicker mix if half ■the quantities are added and infixed before being supplemented by the remainder. ; Repeated turning with the ; shovel will eventually give an over-all uniform colour, jfree from streaks, indicating ia good mix. The coarse aggregate should then be added until uniformity of distribution has .been reached. The general availability of pre-mixes which can be specified as to ■ size can, of course, eliminate the latter stage in mixing. Mixing process

Water should not be added until the dry ingredients have been thoroughly mixed and a depression made in the middle of the heap. Shovel the dry mixture from the outside edges into the centre and continue this until the water has been completely absorbed.

Indications of a good batter are that it will just hold together and not spread or flow when flattened with the shoveL The appearance should be one of complete integration not a “soup” or a “crumbly cake.” Remember not to use too much water, which lowers the strength of the concrete. Add it sparingly even if it takes a little longer because once added it cannot be extracted and the only way to overcome the sloppy nature of the mix is to add more cement.

In the more likely advent of using a power-driven mixer add a small volume of water to the bowl first, wetting the internal dimensions thoroughly before adding anything else; if the materials are put in dry they would cake on the beaters and bowl. Using mixers When using a mixer, whether hand or poweroperated, begin by adding water and then a measured quantity of cement. After this has been thoroughly mixed two-thirds of the measured quantity of aggregate should be added. When all the contents of the mixing bowl have been integrated the remainder of the aggregate can then be added and just sufficient additional water to produce the plasticity required. Concrete should be placed with the minimum delay after it has been prepared. After it has been poured into the forms every effort should be made to get it settled and finished off with the minimum of handling. Tamping or vibrating into place is necessary to prevent air pockets and provide consolidation. Screeding off the surface with a straight edged board overlapping the forms and used in a forward sawing motion will help to level the surface, which is finished with a float. For a smooth finish a steel float is used whilst the wooden one will give a rougher surface, on which one is less inclined to slip. As the concrete is close to drying, but while it is still possible to make an impression on the surface, it can be brushed or marked in various ways to provide patterns and textures depending on the circumstances and purpose of the finished product. Water proofers

There are a number of proprietary brands of concrete water-proofing material or hardeners available, but additional cement and a consolidated mix is often the best means of achieving this objective. The curing of concrete, which is simply another way of referring to the hardening process, should be slow for the best results. A warm damp atmosphere is most conducive to good curing. Special care is necessary in hot weather when setting may occur in a much shorter period than usual. Very cold conditions considerably retards hardening and it is better to refrain from carrying out any concrete work under such circumstances unless special precautions are taken. Cement generates heat during setting and the ideal is to conserve it in the concrete. It is not being fussy to cover all freshly laid work with some form of protection from cold, heat and drying winds. Whatever is used should be kept off the concrete by laying batons across it first. Damping down during the following few days will also help in the curing process. To obtain an exposed aggregate finish suitable for such jobs as paths and slabs requires care and time. The concrete should be handled as dry as possible. When the pebbles in the aggregate are firm, some time after the initial set, gently brush away the surface sand and cement, this work being assisted by lightly hosing over from time to time as brushing proceeds. Final cleaning can be done with hydrochloric acid diluted to about 20 per cent and should be thoroughly flushed off when its work is done.