DRY CONCRETE

Evening Post, Volume CVII, Issue 85, 13 April 1929, Page 20

DRY CONCRETE

EXPANSION WITH HEAT

The extent •to which modem concrete structures are . affected by temperature, humidity, stress,: and plastic yield, was explained by Dr. Oscar Faber in a paper read before, the',Royal Institute of •British .Architects lately, says the London :"Daily Telegraph." Impervious 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 .'.'00006 inches per inch per-(legree.Falir. rise in temperature, theT-expausion-, being .practically proportioned;' to. the temperature 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 Sittingbourne is 2400 ft long, which .would represent an oxpansiou of SJ in. This would clearly be vory destructive of unprovided for. Actually the viaduct was divided into 100 ft sections, with expansion joints between.. The gaps were found to be about 3-Bin. less at 3 p.m., after exposure to a hot sun, as compared with 3 a.m. on a cold, clear night. .This is in close agreement with- the previous figure. . The force of this expansion is enormous. 'At one stage in construction a granite pebblo got wedged into the joint in the morning and resisted tho expansion, ■ with: the consequence that after ..mid-day a. large piece of deck was blown off with.a loud report."

In the case of porus but. seasoned materials • change of '• humidity produced change :• of; length.-■" Timber, however long seasoned," would absorb mois-

ture, and expand if immersed. What was perhaps more remarkable, it would: do" so if. exposed to ail atmosphere of greater humidity than "that at which it had beeii;kept. Thus, a timber seasoned in an'atmosphero of 30 per cent, humidity 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 moisture, the water existing only as water vapour. Hence the importance ■■ of seasoning woods to the humidity of condition's they were likely to be exposed to afterwards, and of keeping down the humidity of buildings by warming them while wood block floors were being laid. :"Hence, also," added Dr. Faber, "the housewife's anxiety to 'air' linen-sheets, and other porous material,, before use, however dry they may have been after ironing. .A -structure . composed., of seasoned precast blocks was less, likely to crack than, one of 'in situ . concrete, . because while both were subject to contraction with drying and cooling, the in situ had; the shrinkage in seasoning additionally. There was another reason for the su-. periority. • When concrete sets, the chemical action produced heat and rise of temperature. ' ■■ '

Dr. Faber. mentioned an "experiment^ "miide in the building of "the now "Bank' of England.- . . ; ''.ln, the bank, underpinning," he said, "there .is .a-retaining wall eight ■feet J thick of ordinary concrete. A thermometer ■-buried in the heart of this indicated a-rise of 50 degrees F. in three days (from 50* degrees to 100 degrees t l.)-.' .With somemuch richer, concrete 'for strong-room construction the '■'. temperature rise was approximately 100 .degrees F (froni 50 degrees to 150 degrees F.).' 3' .:■