DRY CONCRETE.

Otago Daily Times, Issue 20702, 27 April 1929, Page 15

DRY CONCRETE.

EXPANSION,WITH HEAT. The extent to which modern concrete structures are affected by temperature,. humidity, stress, and plastic yield ■was. , explained by Dr Oscar'Faber'in a paper read before,' the ' Koyal Institute of British Architects lately, says the Loudon Daily Telegraph. V. Impervious y materials represented by the metals, such ,as steel, he said, changed their - length by change of temperature and - by change of stress. This steel expanded ' approximately .000006 inches per .inch per degree Pahr. rise in temperature, the expansion being practically proportioned to the tenipcraturc change. “ Dry and seasonable concrete expands with temperature rise very much as steel and_ with practically the same coefficient,” he added, “A,reinforced concrete viaduct at Sittingbournd is 2400 feet long, which would represent an expansion .of Si inches. This would clearly be very destructive- if unprovided for. . Actually the viaduct was . divided into 100 feet sections, with, expansion, joints between. . The gaps were found to ho about S-? inch less at 3 p.m., after ’exposure^to a hot siin, as compared with 3 a.m. on a cold,, clear night. Thi sds in close agreement with the previous figure. The force of this expansion is enormous. At one stage in construction a granite pebble got wedged into the joint in the.morning and resisted the,expansion, with the consequence that after mid-day a large piece of deck was blown off with a loud report.” In the case of but seasoned , materials change of humidity produced ' change of length. Timber, however long seasoned, would absorb j.moisture, and expand if .immersed. What was perhaps more remarkable, it would do so if ■, exposed to an atmosphere of greater humidity than that at which -it. had been kept.. Thus, : a timber seasoned in an. atmosphere of SO per cent, dmmidity absorbed moisture from the .air if exposed to air at 60 per cent, humidity, and expanded in doing so. This was remarkable, when it was realised that 60 per cent, humidity air had no free mosture, jhe water existing only as water, vapour.- Hence the importance of "seasoning woods to the humidity of conditions they were likely to be exposed to. afterwards, and of keeping.donyn the humidity of buildings by warming them ■ while wood block floors were being land. “Hence,’ also/’ added Df Faber, "the housewife’s anxiety to ‘air 1 linen sheets, and other porous material, before use, however .dry they may have been after ironing. e ■ A structure composed of seasoned cast blocks was less Itjcely to crack than r _ one of in situ concrete, because while both were subject to contraction’ with drying ,and cooling, the in situ had thd shrinkage in seasoning There was another reason for the ‘ superiority. When concrete sets, the chemical action produced heat and rise of temperature. ■ Dr Faber, mentioned an experiment • made in the building of the aeW 1 Bank of England. “In the bank underpinning,” he said, “ there is a retaining wall ■* eight feet thick of ordinary concrete. A thermometer buried, in the heart Of this indicated a rise of 50 degrees Fahr. in three days (from 50 degrees _to 100 ' degrees (P.), With 'some much richer ■ ■ concrete for strong room construction the temperature rise was approximately. , 100 degrees F(from 50 degrees to 'ls*o degrees F.).”