CONCRETE.

Press, Volume LXV, Issue 19675, 19 July 1929, Page 9

CONCRETE.

ITS USE IN BUILDING. ADDRESS BY MR R. A. CAMPBELL. The last of the evening extension lectures being conducted by Lincoln College for farmers was given last night by Mr E. A. _.ampbell, who until recently was Professor of Civil Engineering at Canterbury College, on "The Uses of Concrete on the Farm." The speaker was introduced by Dr. E. E. Alexander, of Lincoln College. Before a man could use concrete on the farm, said Air Campbell, he had to know something about it. It was composed of stone, sand, Portland cement, ana water. Its strength varied with the proportions of the ingredients used. Concrete was an expensive material in New Zealand because cement was expensive. In other countries it could be got at half the price. It as necessary, therefore, to us as little cement as possible. If one filled a box with stone the actual volun of stone would be only about 60 per cent. When one added sand it fitted in between the spaces in the stone, and cement, when added, fitted in between che spaces in the sand. The sum of the three volumes would be considerably more than the volumes of the three separately, as they fitted into the spaces between each other. In order to get one cubic yard of cement one needed .88 cubic yard of stone, .44 cubic yard of sand, and .20 cubic yard of cement, giving a total of 1.52 cubic yards. This made a cubic yard of concrete. A 1-2-4 mix crushed at 20001b per square inch. A bar of concrete of this mix broke at about 2001b per square inch under tension. Contraction and expansion had also to be carefully watched. One had to allow concrete facility for contraction and expansion or put steel into it to prevent cracking. In Christchureh practically every 20 feet there was a crack across the "oncrete gutters. Concrete had almost the same co-efficient of expansion as steel. In 100 feet it would expand two-thirds of an inch for 100 degrees rise in temperature. If the concrete was drying ou< the contraction was about the same as that for a drop in temperature of 100 degrees. This fact had to be continually remembered in concrete construction. In large buildings in America they had to have a complete cut for every 200 feet of frontage to allow for this expansion. It was most important that plain concrete should have room to contract and expand. If steel rods were inserted in the concrete it did not crack in the weakest place. Small cracks occurred, but they were unimportant. In building an ordinary fai wall 0.2 per cent, to 0.4 ier cent, of steel was usually enough to prevent cracking. Particular care should be taken with concrete subject to temperature. It was very hard to build a concrete chimney that would not crack. Concrete was a very bad conductor of heat, and this led to cracking. It was necessary to put steel rings round the vertical steel rods. Speaking of water in concrete, the speaker said it had been proved that after enough water had been added to make the concrete flow, any excess of water weakened the concrete. With the same ingredients it was found that a dry mixture took twice the pressure of a wet mix. By doubling the amount cf water one got a reduction of the strength of the concrete to one-third. In reinforced concrete the concrete had, however, to be wet enough to now round the rods and touch them. ■*■?

was very difficult to say just how wet concrete should be. One could make a very efficient watertank out of reinforced concrete. The best shape was a circular one. The square type of tank required special care from the different stresses involved. The beat concrete was that in which all the spaces were filled up with material of gradually decreasing size. A graded mix with a certain proportion of each size of stone and sand was best. Tests at Canterbury College showed that ungraded material failed at 20001b per square inch, and graded material at 50001b per square inch. Steel in the bottom of a beam added greatly to the strength of a beam, as it stood a stretch that the concrete could not stand alone. Concrete was also a complete protection against rust in the steel. The main principle to remember was that steel was needed where a stretching force was acting, and not needed as a rule under compression. The difficulty in the use of concrete for fencing posts was that it was a heavy material to cart about. There was no reason why posts should not be made quite strong and of fairly reasonable size. "There does not seem to be any reason in my mind why concrete should not be much more extensively used than it is in Canterbury," said Mr Campbell. "In most places you have got the material." Its use in cowbails, for instance, was obvious and desirable. At the conclusion of the address, Mr Campbell was given a hearty vote of thanks.