POINTS TO WATCH MAKING CONCRETE

Press, Volume CII, Issue 30028, 12 January 1963, Page 6

POINTS TO WATCH MAKING CONCRETE

(2J.00D concrete requires good cement, good clean aggregates, dean water, and sound techniques, says Mr A. W. Riddolls, reader in agricultural engineering at Lincoln College, in a Canterbury Chamber of Commerce agricultural bulletin on good concrete for the farm.

Mr Raddolls says that every farmer should know how to make good concrete, not only so that he can do a satisfactory concreting job himself, but also so that he can insist that the tradesmen he employs follow sound practices.

"The causes of poor concrete must be either poorquality aggregates or unsound techniques, because the standard of all Portland cement sold in New Zealand is satisfactory, and practically all farms have pure water available,” he says. Both the coarse and fine aggregates must consist of strong, sound, stone particles or crushed rock and must be free of all topsoil, silt, clay, organic matter ,and other impurities. The cement, sand, and coarse aggregate must be correctly proportioned to achieve, the desired strength and soundness in the job. For high strength, resistance to abrasion and corrosion, and water-tightness such as for tanks, fence posts and milking shed floors, the proportions should be one volume of cement to If of sand and 21 of coarse aggregate.

I For general purposes, such as paths, drives and floors, the proportions should be one of cement to 2} of damp sand and 2| of coarse aggregate. For thick work where strength is less important, such as light building foundations and bridge abutments the proportions should be one of cement to 2] of sand and 2} of coarse aggregate. I The shovel is not an accurate measure and this is i one of the reasons for poor concrete. Weighing the cement and measuring the

volume of the coarse aggregate and sand by using a box with a capacity of one cubic foot is a better method. The water-to-cement ratio is the most important factor in concrete strength. The more water in proportion to cement the weaker the concrete. No more water should be added than to produce a plastic workable mix. Excessive water is particularly unsuitable for paths and yards as it comes to the surface and makes a weak surface layer where strength and hardness are most necessary. Mr Riddolls recommends the following quantities to be n xed with one bag of cement:—High strength mix: 28 cubic feet of damp sand, 28 cu. ft. coarse aggregate, 38 gals, water. General purpose: 2} cu. ft damp sand, 3 cu. ft coarse aggregate, 4 gals, water. Thick work: 3| cu. ft. damp sand, 38 cu. ft. coarse aggregate, 4J gals, water. The quantities of water should be regarded as a maximum and if a good plastic mix can be obtained with less water so much the better, he says. While it is possible to buy*, mixes of combined aggregates these have certain disadvantages. Varying proportions and separation out of fine and coarse material in transit are two, and it is not possible to vary the proportions to suit particular jobs. When using river or natural shingle from a pit it is best to separate the coarse and fine material with a three-sixteenths of an inch sieve. There is often too much fine sand, too. When filling the mixer first add half the required water then the coarse aggregate, then the cement followed by the sand. Then slowly add the remaining water. Mix for at least two minutes.

With reinforcing there should be at least threequarters of an inch covering of good sound concrete, preferably an inch if this is possible. If water penetrates it causes the iron to rust and swell, breaking the concrete —a common cause of failure of farm posts.

Important work requiring high strength, watertightness and durability should be cured by keeping the concrete damp for at least a week. Either pour water over the surface, cover the surface with sawdust and spray water on it, or sprinkle with water and lay a poly, thene sheet over the surface. Frequent spraying or covering with polythene will keep a vertical surface damp. In cold weather the concrete should be covered with sacks and boards to keep it warm and to prevent it from freezing. The wanner the concrete the more rapidly it will harden. Mr Riddolls says.