CONCRETE.

Press, Volume LXIX, Issue 20905, 12 July 1933, Page 10

CONCRETE.

WORK AT WAITAKI DAM. CONSTRUCTION PROBLEMS. ••Of ail the materials used ::: mori-e.-n construction practice, one of the most complex and least understood is'cor.crete," said Mr R. H. Packwood. resident engineer at the Waitaki Hydro, in an address delivered last night to the Canterbury gineering Society. Mr J. N. F. Ne'.vbold < president of the society) occupied the chair, and introduced the speaker to a large gathering of members. Modern developments, said the speaker, had doubled the strength of concrete, but there was still much to learn about its durability. The difficulty which usually confronted the construction engineer was that adequate tests required time, and such tests as he could make were often restricted in their application. Considering the many factors which affected concrete, and considering the way in which it was sometimes mixed under the supervision of contractors anxious to "push a job through" quickly, it was remarkable that the product v.-as as good as it was. Then, too, profound effects were produced on a monolith or block of concrete by stresses to which it was subjected; so that figures purporting to give the strength of the finished structure were often misleading. Just as important as strength, perhaps, was weight. In claiming that the ordinary tests did not give accurate results as to either strength or weight, the speaker did not wish to disparage these tests; they wore extremely useful, though not to be relied on to more than a certain extent. The amount of water in the ideal mixture was a matter of quite varied opinion. The Waitaki Hydro. Dealing more particularly with the work in which he was engaged, the speaker said that the quantity of concrete already placed at Waitaki was greater than at any other work in the country; and by the time the scheme was completed it would be more than double that of the next largest in New Zealand. The work was in direct contact by rail with the cement factories of Dunedin; while excavations, both in the river and on its banks, provided the major requirements in the way of aggregates, the rest being obtained at low cost by mechanical excavations. The railway system was organised to minimise the handling of gravel, so that almost complete synchronisation could be obtained between the rate of excavation of gravel and the amount used, thus making the dumping of large reserves unnecessary.

No part of the dam was constructed in water if it could possibly be avoided. When it was necessary to construct under still water, the essential thing was to deposit it slowly at a uniform rate. A temporary wall was built which kept out practically all the water. The protection of the newly-laid concrete from drying winds or direct rays of the sun was of vital importance, as also was adequate protection from frost. All exposed surfaces were therefore covered with a double layer of wet cement bags, which were kept wet for at least ten days, or til! a fresh pour was added. Precautions against freezing however, ai?o included the heating of the sand and r.ggregate with jets of steam and

sjr r.« coJd weather the mixing w.itci v. as also heated by the same means. Freezing was a very effective means of disrupting concrete, but if, on the other hand, the newlvset concrete was overheated, the effect was likewise harmful, the tendency to cracking during the cool-ing-down process being accentuated.