Civil Engineering Reference
In-Depth Information
To all intents then, when the centering was set in place, operations for the
concreting were transferred to outside the building, and new access arrangements
from that quarter had to be provided.
Concreting arcuated rooi ng was not straightforward work. h e basic procedure
for placing concrete is to lay down a succession of horizontal layers, alternately
aggregate (
caementia
) and mortar (
materia
). However the unit to be constructed
was delimited neither horizontally nor vertically—it was inclined at an ever-chang-
ing angle. And the only restraint to the plastic concrete was provided by the shut-
tering, and that was at best (at the springing) “one sided”. In the nature of things
it was impossible to coni ne the plastic concrete at the extrados, that is, unless the
extrados was given a stepped proi le. Such stepped rings were sometimes provided
at the base of domes, where they also performed the statical function of minimis-
ing thrust (v L.C. Lancaster, p. 141, i g 225).
In view of this situation the placing of concrete in arcuated rooi ng is dii cult
to envisage unless some form of compartmentalisation exists to provide a measure
of lateral restraint (and thus dei nition) to the plastic concrete. h us these remarks
bear directly on the question of brick arches and ribs inset into the concrete fabric.
Speaking in this generalised manner it can only be emphasized that the more brick
“ribbing” present, the more feasible was the process of placing the concrete. Inset
brick ribbing was thus very signii cantly a device in the interest of construction,
whatever other function it possessed (cf R. Mainstone, p. 119).
Associated with this is the still open question of how the brick units them-
selves were constructed. In principle were they constructed
pari passu
with the
concrete—i.e. keeping just ahead of the latter? Or were they constructed in their
entirety and the concrete then placed to accord with them? h is is a ramii ed ques-
tion which has received little attention (v R. Mainstone, p. 119).
Inset brickwork also may serve constructional interests in a dif erent context—not
in placing the concrete but in its curing. h is connection has long been noted. h e
process of curing whereby moisture evaporates and a plastic mass becomes solid
causes changes in volumes and pressures. Compartmentalising the mass clearly
reduces these changes quantitatively and localises their occurrence. It is therefore
reasonable to suppose that brick insets into concrete fabric regulated possible dam-
age and distorting to the solidifying mass (cf D.S. Robertson, chap. 15).
Something must now be said concerning the basic proceeding for construct-
ing concrete domes. h is has been let to follow on some statement of the work
entailed. h e question is how access was af orded for the work—and no conve-
niently published discussion of it exists.
On the face of it placing the concrete was in practice a very dii cult operation
to arrange. It was carried out at very dangerous heights above the ground, with a
very coni ned horizontal surface available only at the margins of the area. For the
Placing
Concrete
401, 404
406, 407
383, 412,
413
