No_1_May_and_June__1951 — Page 49

PRESSTRESSED CONCRETE

(Second Section)

A Lecture given at the China Fleet Club under the auspices of the Public Works Department, Hong Kong, on Decem-

ber 11th, 1950, by Dr. Kurt Billig, A.M-Inst.C-E, M.Am-Soc.C.E., Professor of Civil Engineering,

During the last two decades rein- forced concrete shell structures have found a variety of applications in civil engineering. While the quantity and east of the permanent building material employed in such concrete shells is Comparatively small, their complicated shuttering, falsework and high seaf- folding forms a substantial proportion of the total cost involved

and some-

times these expenses arc prohibitive. The application of prestressing pro- cesses to shell structures, Fig. 16, suc- eeeds in reducing considerably the heavy expense of shuttering and seaf- folding.

In this method wires arc spaced along the generating lines to form the skeleton of the structure. The wires are supported on intermediate scaffold- ing trusses a carry 4 fine wire mesh.

Ain, layer of cement mortar

Hongkong University.

ers which form the tensile members of

the shell.

A different method of construction has been followed in the erection of the shell root for the hangar at Kara chi, Fig. 17. There the whole roof was constructed in conventional man- ner, but instead of the main reinfore- ing bars in the tensile ribs between cach two barrels, not-bonded cables were inserted. These cables were post- tensioned after the concrete had har- dened and while it was still supported by the falsework. The shell proper was not directly prestressed. The dour beams carrying the shell roof were also prestressed, and at the time of construction in 1943, these girders of 194 ft. span were the largest prestress- ed concrete beams in existence.

In the mechanical or thermal pre- stressing process, the steel is stretched

CONCRETE SHELL CONSISTING OF OUTER AND INNER COAT OF CEMENT

RENDERING OR GUNITE

FINE AND DENSE WIRE MESH (FLEXIBLE MOULD)

PRE STRESSED HIGH TENSILE

WIRES, SPACED AT SHORT

INTERVALS.

TO STRETHING DEVICE OF MAN WIRES

with expanding rements the rein- foreing steel restrains the expansion whereby the reinforcement is put auto- matically under tension and the con- erete under compression.

Expanding cement consists of three elements: ordinary Portland cement, sulpho-aluminous cement which acts as an expanding agent, and Barium or Strontium compounds which act as stabilising elements and stop expansion by absorbing the principal reagent. Expanding cements do not entirely annul srrinkage but they compensate it by setting up an initial expansion equal to or greater than the shrink- age. The expansion of these cenients can be regulated within wide limits. amounting from 0.2 to 1.2 percent for neat paste cured under water. The period of gradual expansion terminates in 10 to 15 days after mixing; after

STIFFENING DIAPHRAGM

SUPPORTING STRUCTURE

is applied by hand to the barrel form- ed by the wire mesh. The next day when the layer has hardened, one top and one bottom layer is applied in gunite to both sides of the thin skin so that the pre-tensioned wires form the core of the shell. After hardening of the concrete, the wires are released and the shell is precompressed evenly over its cross section. Concentrations of the wires occur at the side stiffen-

LIGHT SCAFFOLDING TO CARRY FIRST COAT OF COMPO

· CONCENTRATION OF PRE STRESSEDD WIRES IN MARGINAL MEMBERS

Fig. 16

or released against the hardened con- crete. The steel is the active part exerting a compression force, and the concrete is the posssive part providing the reaction to that forge. The roles

materials performed by the two

are exchanged in that type of prestressing process which employs expanding cement. In reinforced concrete made

Fig. 18

wards the cement paste becomes stabi- lised. However, the quality of the ex- panding cement at present manufac- turned dees not as yet produce on the steel a tension comparable to that which can be obtained by mechanical pre- stressing

may

The prestressing members

be gripped individually by wedges, set-

A

Fig. 17

47

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Fig. 19